Patent Application
20240141514
An Application Data Sheet is filed concurrently with this specification as part of the present application. Each application that the present application claims benefit of or priority to as identified in the concurrently filed Application Data Sheet is incorporated by reference herein in their entireties and for all purposes.
Anthropogenic carbon dioxide emissions have been linked to climate change. In response to increasing concerns about global greenhouse gas emissions, technologies that can recycle CO2 into high-value products have received growing interest.
Electrochemical reduction of COx (such as CO2, CO, or combinations thereof) combines just three inputs: COx, a source of protons and electricity. These inputs are converted into fuels, chemicals and other products such as methanol, ethanol, carbon monoxide and acetic acid. However, industrial scale electrochemical reactions have been difficult to achieve, in part due to lack of suitable reactors. One issue to be addressed is the potential variability or unreliability of a power source. Interruptions due to power outages or planned shutdowns for maintenance and recovery processes may lead to deterioration of an electrochemical reactor's components. A robust electrochemical reactor that can withstand electrical fluctuations would therefore be advantageous.
The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as art at the time of filing, are neither expressly or impliedly admitted as prior art against the present disclosure.
Provided herein are membrane electrode assemblies (MEAs) for carbon oxide reduction. According to various embodiments, the MEAs are configured to address challenges particular to COx including mitigating the deleterious effects of electrical current fluctuations on the MEA. Bipolar membrane MEAs equipped with an interface region that reduces or prevents these effects are described.
Accordingly, in a first aspect, the present disclosure encompasses a membrane electrode assembly. In some embodiments, the membrane electrode assembly includes a cathode catalyst layer; an anode catalyst layer; and a bipolar membrane disposed between the cathode catalyst layer and the anode catalyst layer, where the bipolar membrane includes an anion-conducting polymer layer, a cation-conducting polymer layer, and an interface region between the anion-conducting polymer layer and the cation-conducting polymer layer, and where the interface region includes nanoparticles.
In some embodiments, the interface region is from about 1 to 10 micrometers thick.
In some embodiments, the interface region is from about 1 to about 5 micrometers thick.
In some embodiments, a ratio of a thickness of the anion-conducting polymer layer to the thickness of the interface region is at least 10:1.
In some embodiments, the nanoparticles are carbon nanoparticles or magnesium oxide nanoparticles.
In some embodiments, the interface region also includes an ion-conducting polymer.
In some embodiments, the nanoparticles are iridium, platinum, titanium or gold.
In some embodiments, the cation-conducting polymer layer is disposed between the anode catalyst layer and the anion-conducting polymer layer.
In some embodiments, the cathode catalyst layer includes a reduction catalyst and an anion-conducting polymer.
In some embodiments, the anode catalyst layer includes an oxidation catalyst and a cation-conducting polymer.
In a second aspect, the present disclosure encompasses a COx reduction reactor. In some embodiments, the membrane electrode assembly includes a cathode catalyst layer having a catalyst configured for reduction of COx; an anode catalyst layer; and a bipolar membrane disposed between the cathode catalyst layer and the anode catalyst layer, where the bipolar membrane includes an anion-conducting polymer layer, a cation-conducting polymer layer, and an interface region between the anion-conducting polymer layer and the cation-conducting polymer layer, and where the interface region includes nanoparticles.
In some embodiments, the interface region is from about 1 to 10 micrometers thick.
In some embodiments, the interface region is from about 1 to about 5 micrometers thick.
In some embodiments, a ratio of a thickness of the anion-conducting polymer layer to the thickness of the interface region is at least 10:1.
In some embodiments, the nanoparticles are carbon nanoparticles or magnesium oxide nanoparticles.
In some embodiments, the interface region also includes an ion-conducting polymer.
In some embodiments, the nanoparticles include iridium, platinum, titanium or gold.
In some embodiments, the cation-conducting polymer layer is disposed between the anode catalyst layer and the anion-conducting polymer layer.
In some embodiments, the cathode catalyst layer also includes an anion-conducting polymer.
In some embodiments, the anode catalyst layer includes an oxidation catalyst and a cation-conducting polymer.
In some embodiments, the COx reduction reactor is a carbon dioxide reduction reactor.
In a third aspect, the present disclosure encompasses a method of electrochemically reducing COx. In some embodiments, the method includes providing a carbon oxide to a COx reduction reactor, and electrochemically reducing the carbon oxide at a cathode of a COx electrolyzer to produce a carbon containing product, where the COx reduction reactor includes a membrane electrode assembly having a cathode catalyst layer; an anode catalyst layer; and a bipolar membrane disposed between the cathode catalyst layer and the anode catalyst layer, where the bipolar membrane includes an anion-conducting polymer layer, a cation-conducting polymer layer, and an interface region between the anion-conducting polymer layer and the cation-conducting polymer layer, and where the interface region includes nanoparticles.
In some embodiments, the COx reduction reactor is subjected to electrical current fluctuations.
In some embodiments, the electrical current fluctuations result from a recovery process, a power interruption, a current reversal, or a power outage.
In some embodiments, the carbon oxide is carbon dioxide.
In some embodiments, the carbon containing product is carbon monoxide, a hydrocarbon, formic acid, an alcohol, or a combination thereof.
In some embodiments, the interface region is from about 1 to 10 micrometers thick.
In some embodiments, the interface region is from about 1 to about 5 micrometers thick.
In some embodiments, a ratio of a thickness of the anion-conducting polymer layer to the thickness of the interface region is at least 10:1.
In some embodiments, the nanoparticles are carbon nanoparticles or magnesium oxide nanoparticles.
In some embodiments, the interface region also includes an ion-conducting polymer.
In some embodiments, the nanoparticles include iridium, platinum, titanium, or gold.
In some embodiments, the cation-conducting polymer layer is disposed between the anode catalyst layer and the anion-conducting polymer layer.
In some embodiments, the anion-conducting polymer layer is disposed between the cathode catalyst layer and the cation-conducting polymer layer.
In some embodiments, the cathode catalyst layer includes a reduction catalyst and an anion-conducting polymer.
In some embodiments, the anode catalyst layer includes an oxidation catalyst and a cation-conducting polymer.
In a fourth aspect, the present disclosure encompasses a membrane electrode assembly. In some embodiments, the membrane electrode assembly includes a cathode catalyst layer; an anode catalyst layer; and a bipolar membrane disposed between the cathode catalyst layer and the anode catalyst layer, where the bipolar membrane includes an anion-conducting polymer layer, a cation-conducting polymer layer, and an interface region between the anion-conducting polymer layer and the cation-conducting polymer layer, where the interface region includes an ion-conducting polymer layer and nanoparticles dispersed in the ion-conducting polymer layer.
In some embodiments, the ion-conducting polymer layer includes a sulfonated fluoropolymer.
In some embodiments, the sulfonated fluoropolymer is a sulfonated tetrafluoroethylene-based fluoropolymer copolymer.
In some embodiments, the nanoparticles are carbon nanoparticles.
In some embodiments, the interface region is from about 1 to 10 micrometers thick.
In some embodiments, the interface region is from about 1 to about 5 micrometers thick.
In some embodiments, a ratio of an ion-conducting polymer in the ion-conducting polymer layer to carbon nanoparticles is from about 2:1 to about 1:0.1 weight percent.
In some embodiments, a ratio of an ion-conducting polymer in the ion-conducting polymer layer to carbon nanoparticles is about 1:1 weight percent.
In some embodiments, a ratio of a thickness of the anion-conducting polymer layer to the thickness of the interface region is at least 10:1.
In some embodiments, the cation-conducting polymer layer is disposed between the anode catalyst layer and the anion-conducting polymer layer.
In some embodiments, the anion-conducting polymer layer is disposed between the cathode catalyst layer and the cation-conducting polymer layer.
In some embodiments, the cathode catalyst layer includes a reduction catalyst and an anion-conducting polymer.
In some embodiments, the ion-conducting polymer in the interface region is the same polymer as the cation-conducting polymer in the cation-conducting polymer layer.
In some embodiments, the ion-conducting polymer in the interface region is the same polymer as the anion-conducting polymer in the anion-conducting polymer layer.
In some embodiments, the interface region includes an anion-conducting polymer and a cation-conducting polymer.
In some embodiments, the ion-conducting polymer in the interface region is a different cation-conducting polymer than the cation-conducting polymer in the cation-conducting polymer layer.
In some embodiments, the ion-conducting polymer in the interface region is a different anion-conducting polymer than the anion-conducting polymer in the anion-conducting polymer layer.
These and other aspects of the disclosure are discussed further below with reference to the drawings.
In the following description, numerous specific details are set forth to provide a thorough understanding of the presented embodiments. The disclosed embodiments may be practiced without some or all of these specific details. In other instances, well-known process operations have not been described in detail to not unnecessarily obscure the disclosed embodiments. While the disclosed embodiments will be described in conjunction with the specific embodiments, it will be understood that it is not intended to limit the disclosed embodiments.
As used herein, the term “about” is understood to account for minor increases and/or decreases beyond a recited value, which changes do not significantly impact the desired function the parameter beyond the recited value(s). In some cases, “about” encompasses+/−10% of any recited value. As used herein, this term modifies any recited value, range of values, or endpoints of one or more ranges.
As used herein, the terms “top,” “bottom,” “upper,” “lower,” “above,” and “below” are used to provide a relative relationship between structures. The use of these terms does not indicate or require that a particular structure must be located at a particular location in the apparatus.
An “electrochemical cell” includes electrolyzers such as CO2 electrolyzers and water electrolyzers. It also includes some forms of CO2 purifiers, particularly those that employ faradaic reactions at an anode and/or a cathode.
A “carbon oxide” (COx) includes carbon dioxide (CO2), carbon monoxide (CO), carbonate ions (CO32−), bicarbonate ions (HCO3−), and any combinations thereof.
A “mixture” contains two or more components and unless otherwise stated may contain components other than the identified components.
A “porosity enhancing agent” is a substance that increases void space in a layer or network. A porosity enhancing agent can increase void space in a polymeric membrane layer.
A “voltage damping agent” is an agent that reduces voltage increase that occurs after current decrease and/or shutoff of a carbon dioxide reduction reactor.
An “ion-conducting polymer” is used herein to describe a polymer electrolyte having greater than about 1 mS/cm specific conductivity for anions and/or cations.
The terms “micrometers” and “microns” and the abbreviations “μm” and “um” are used interchangeably to mean microns. Unless otherwise noted, ranges described herein (e.g., 10 micrometers to 20 micrometers, 0.25-127, between 1 and 90%, etc.) include the endpoints of those ranges.
A membrane electrode assembly (MEA) is described here. It may be used in a COx reduction reactor. COx may be carbon dioxide (CO2), carbon monoxide (CO), CO32− (carbonate ion), HCO3− (bicarbonate ion), or combinations thereof. The MEA contains an anode layer, a cathode layer, electrolyte, and optionally one or more other layers. The layers may be solids and/or soft materials. The layers may include polymers such as ion-conducting polymers.
When in use, the cathode of an MEA promotes electrochemical reduction of COx by combining three inputs: COx, ions (e.g., protons) that chemically react with COx, and electrons. The reduction reaction may produce CO, hydrocarbons, and/or oxygen and hydrogen containing organic compounds such as methanol, ethanol, and acetic acid. When in use, the anode of an MEA promotes an electrochemical oxidation reaction such as electrolysis of water to produce elemental oxygen and protons. The cathode and anode may each contain catalysts to facilitate their respective reactions.
The compositions and arrangements of layers in the MEA may promote high yield of a COx reduction products. To this end, the MEA may facilitate any one or more of the following conditions: (a) minimal parasitic reduction reactions (non-COx reduction reactions) at the cathode; (b) low loss of COx reactants at anode or elsewhere in the MEA; (c) maintain physical integrity of the MEA during the reaction (e.g., prevent delamination of the MEA layers); (d) prevent COx reduction product cross-over; (e) prevent oxidation production (e.g., O2) cross-over; (f) maintain a suitable environment at the cathode/anode for oxidation/reduction as appropriate; (g) provide pathway for desired ions to travel between cathode and anode while blocking undesired ions; and (h) minimize voltage losses.
Polymer-based membrane assemblies such as MEAs have been used in various electrolytic systems such as water electrolyzers and in various galvanic systems such as fuel cells. However, COx reduction presents problems not encountered, or encountered to a lesser extent, in water electrolyzers and fuel cells.
For example, for many applications, an MEA for COx reduction requires a lifetime on the order of about 50,000 hours or longer (approximately five years of continuous operation), which is significantly longer than the expected lifespan of a fuel cell for automotive applications; e.g., on the order of 5,000 hours. Additionally, for various applications a MEA for COx reduction employs electrodes having a relatively large geometric surface area by comparison to MEAs used for fuel cells in automotive applications. For example, MEAs for COx reduction may employ electrodes having geometric surface areas (without considering pores and other nonplanar features) of at least about 500 cm2.
COx reduction reactions may be implemented in operating environments that facilitate mass transport of particular reactant and product species, as well as to suppress parasitic reactions. Fuel cell and water electrolyzer MEAs often cannot produce such operating environments. For example, such MEAs may promote undesirable parasitic reactions such as gaseous hydrogen evolution at the cathode and/or gaseous CO2 production at the anode.
In some systems, the rate of a COx reduction reaction is limited by the availability of gaseous COx reactant at the cathode. By contrast, the rate of water electrolysis is not significantly limited by the availability of reactant:liquid water tends to be easily accessible to the cathode and anode, and electrolyzers can operate close to highest current density possible.
In certain embodiments, an MEA has a cathode layer, an anode layer, and a polymer electrolyte membrane (PEM) between the anode layer and the cathode layer. The polymer electrolyte membrane provides ionic communication between the anode layer and the cathode layer, while preventing electronic communication, which would produce a short circuit. The cathode layer includes a reduction catalyst and a first ion-conducting polymer. The cathode layer may also include an ion conductor and/or an electron conductor. The anode layer includes an oxidation catalyst and a second ion-conducting polymer. The anode layer may also include an ion conductor and/or an electron conductor. The PEM includes a third ion-conducting polymer.
In certain embodiments, the MEA has a cathode buffer layer between the cathode layer and the polymer electrolyte membrane. The cathode buffer includes a fourth ion-conducting polymer.
In certain embodiments, the MEA has an anode buffer layer between the anode layer and the polymer electrolyte membrane. The anode buffer includes a fifth ion-conducting polymer.
In connection with certain MEA designs, there are three available classes of ion-conducting polymers: anion-conductors, cation-conductors, and mixed cation-and-anion-conductors. In certain embodiments, at least two of the first, second, third, fourth, and fifth ion-conducting polymers are from different classes of ion-conducting polymers.
The term “ion-conducting polymer” is used herein to describe a polymer electrolyte having greater than about 1 mS/cm specific conductivity for anions and/or cations. The term “anion-conductor” describes an ion-conducting polymer that conducts anions primarily (although there will still be some small amount of cation conduction) and has a transference number for anions greater than about 0.85 at around 100 micron thickness. The terms “cation-conductor” and/or “cation-conducting polymer” describe an ion-conducting polymer that conducts cations primarily (e.g., there can still be an incidental amount of anion conduction) and has a transference number for cations greater than approximately 0.85 at around 100 micron thickness. For an ion-conducting polymer that is described as conducting both anions and cations (a “cation-and-anion-conductor”), neither the anions nor the cations has a transference number greater than approximately 0.85 or less than approximately 0.15 at around 100 micron thickness. To say a material conducts ions (anions and/or cations) is to say that the material is an ion-conducting material or ionomer. Examples of ion-conducting polymers of each class are provided in the table below.
Some Class A ion-conducting polymers are known by tradenames such as 2259-60 (Pall RAI), AHA by Tokuyama Co, Fumasep® FAA- (fumatech GbbH), Sustanion®, Morgane ADP by Solvay, or Tosflex® SF-17 by Tosoh anion exchange membrane material. Further class A ion-conducting polymers include HNN5/HNN8 by Ionomr, FumaSep by Fumatech, TM1 by Orion, and PAP-TP by W7energy. Some Class C ion-conducting polymers are known by tradenames such as various formulations of Nafion® (DuPont™), GORE-SELECT® (Gore), Fumapem® (fumatech GmbH), and Aquivion® PFSA (Solvay).
Examples of polymeric structures that can include an ionizable moiety or an ionic moiety and be used as ion-conducting polymers in the MEAs described here are provided below. The ion-conducting polymers may be used as appropriate in any of the MEA layers that include an ion-conducting polymer. Charge conduction through the material can be controlled by the type and amount of charge (e.g., anionic and/or cationic charge on the polymeric structure) provided by the ionizable/ionic moieties. In addition, the composition can include a polymer, a homopolymer, a copolymer, a block copolymer, a polymeric blend, other polymer-based forms, or other useful combinations of repeating monomeric units. As described below, an ion conducting polymer layer may include one or more of crosslinks, linking moieties, and arylene groups according to various embodiments. In some embodiments, two or more ion conducting polymers (e.g., in two or more ion conducting polymer layers of the MEA) may be crosslinked.
Non-limiting monomeric units can include one or more of the following:
in which Ar is an optionally substituted arylene or aromatic; Ak is an optionally substituted alkylene, haloalkylene, aliphatic, heteroalkylene, or heteroaliphatic; and L is a linking moiety (e.g., any described herein) or can be —C(R7)(R8)—. Yet other non-limiting monomeric units can include optionally substituted arylene, aryleneoxy, alkylene, or combinations thereof, such as optionally substituted (aryl)(alkyl)ene (e.g., -Ak-Ar— or -Ak-Ar-Ak- or —Ar-Ak-, in which Ar is an optionally substituted arylene and Ak is an optionally substituted alkylene). One or more monomeric units can be optionally substituted with one or more ionizable or ionic moieties (e.g., as described herein).
One or more monomeric units can be combined to form a polymeric unit. Non-limiting polymeric units include any of the following:
in which Ar, Ak, L, n, and m can be any described herein. In some embodiments, each m is independently 0 or an integer of 1 or more. In other embodiments, Ar can include two or more arylene or aromatic groups.
Other alternative configurations are also encompassed by the compositions herein, such as branched configurations, diblock copolymers, triblock copolymers, random or statistical copolymers, stereoblock copolymers, gradient copolymers, graft copolymers, and combinations of any blocks or regions described herein.
Examples of polymeric structures include those according to any one of formulas (I)-(V) and (X)-(XXXIV), or a salt thereof. In some embodiments, the polymeric structures are copolymers and include a first polymeric structure selected from any one of formulas (I)-(V) or a salt thereof; and a second polymeric structure including an optionally substituted aromatic, an optionally substituted arylene, a structure selected from any one of formulas (I)-(V) and (X)-(XXXIV), or a salt thereof.
In one embodiment, the MW of the ion-conducting polymer is a weight-average molecular weight (Mw) of at least 10,000 g/mol; or from about 5,000 to 2,500,000 g/mol. In another embodiment, the MW is a number average molecular weight (Mn) of at least 20,000 g/mol; or from about 2,000 to 2,500,000 g/mol.
In any embodiment herein, each of n, n1, n2, n3, n4, m, m1, m2, or m3 is, independently, 1 or more, 20 or more, 50 or more, 100 or more; as well as from 1 to 1,000,000, such as from 10 to 1,000,000, from 100 to 1,000,000, from 200 to 1,000,000, from 500 to 1,000,000, or from 1,000 to 1,000,000.
Non-limiting polymeric structures can include the following:
or a salt thereof, wherein:
or a salt thereof, wherein:
or a salt thereof, wherein:
In particular embodiments (e.g., of formula (XIV) or (XV)), each of the nitrogen atoms on rings a and/or b are substituted with optionally substituted aliphatic, alkyl, aromatic, aryl, an ionizable moiety, or an ionic moiety. In some embodiments, one or more hydrogen or fluorine atoms (e.g., in formula (XIX) or (XX)) can be substituted to include an ionizable moiety or an ionic moiety (e.g., any described herein). In other embodiments, the oxygen atoms present in the polymeric structure (e.g., in formula XXVIII) can be associated with an alkali dopant (e.g., K+).
In particular examples, Ar, one or more of rings a-i (e.g., rings a, b, f, g, h, or i), L, L1, L2, L3, L4, Ak, R7, R8, R9, and/or R10 can be optionally substituted with one or more ionizable or ionic moieties and/or one or more electron-withdrawing groups. Yet other non-limiting substituents for Ar, rings (e.g., rings a-i), L, Ak, R7, R8, R9, and R10 include one or more described herein, such as cyano, hydroxy, nitro, and halo, as well as optionally substituted aliphatic, alkyl, alkoxy, alkoxyalkyl, amino, aminoalkyl, aryl, arylalkylene, aryloyl, aryloxy, arylalkoxy, hydroxyalkyl, and haloalkyl.
In some embodiments, each of R1, R2, and R3 is, independently, H, optionally substituted aromatic, aryl, aryloxy, or arylalkylene. In other embodiments (e.g., of formulas (I)-(V) or (XII)), R7 includes the electron-withdrawing moiety. In yet other embodiments, R8, R9, and/or R10 includes an ionizable or ionic moiety.
In one instance, a polymeric subunit can lack ionic moieties. Alternatively, the polymeric subunit can include an ionic moiety on the Ar group, the L group, both the Ar and L groups, or be integrated as part of the L group. Non-limiting examples of ionizable and ionic moieties include cationic, anionic, and multi-ionic group, as described herein.
In any embodiment herein, the electron-withdrawing moiety can include or be an optionally substituted haloalkyl, cyano (CN), phosphate (e.g., —O(P═O)(ORP1)(ORP2) or —O—[P(═O)(ORP1)—O]P3—RP2), sulfate (e.g., —O—S(═O)2(ORS1)), sulfonic acid (—SO3H), sulfonyl (e.g., —SO2—CF3), difluoroboranyl (—BF2), borono (B(OH)2), thiocyanato (—SCN), or piperidinium. Yet other non-limiting phosphate groups can include derivatives of phosphoric acid, such as orthophosphoric acid, pyrophosphoric acid, tripolyphosphoric acid, tetrapolyphosphoric acid, trimetaphosphoric acid, and/or phosphoric anhydride, or combinations thereof.
Yet other polymeric units can include poly(benzimidazole) (PBI), polyphenylene (PP), polyimide (PI), poly(ethyleneimine) (PEI), sulfonated polyimide (SPI), polysulfone (PSF), sulfonated polysulfone (SPSF), poly(ether ketone) (PEEK), PEEK with cardo groups (PEEK-WC), polyethersulfone (PES), sulfonated polyethersulfone (SPES), sulfonated poly(ether ketone) (SPEEK), SPEEK with cardo groups (SPEEK-WC), poly(p-phenylene oxide) (PPO), sulfonated polyphenylene oxide (SPPO), ethylene tetrafluoroethylene (ETFE), polytetrafluoroethylene (PTFE), poly(epichlorohydrin) (PECH), poly(styrene) (PS), sulfonated poly(styrene) (SPS), hydrogenated poly(butadiene-styrene) (HPBS), styrene divinyl benzene copolymer (SDVB), styrene-ethylene-butylene-styrene (SEBS), sulfonated bisphenol-A-polysulfone (SPSU), poly(4-phenoxy benzoyl-1,4-phenylene) (PPBP), sulfonated poly(4-phenoxy benzoyl-1,4-phenylene) (SPPBP), poly(vinyl alcohol) (PVA), poly(phosphazene), poly(aryloxyphosphazene), polyetherimide, as well as combinations thereof.
In some embodiments, crosslinking is present within an ion conducting polymer layer and/or between ion conducting polymer layers. Crosslinking within a material can be promoted by use of crosslinking reagents. For instance, the composition can include polymeric units, and a crosslinking reagent can be used to provide crosslinking between polymeric units. For instance, if the polymeric units (P1 and P2) include a leaving group, then a diamine crosslinking reagent (e.g., H2N-Ak-NH2) can be used to react with the polymeric units by displacing the leaving group and forming an amino-containing crosslinker within the composition (e.g., thereby forming P1-NH-Ak-NH—P2). Crosslinkers can be introduced by forming a polymer composition and then exposing the composition to a crosslinking reagent to form crosslinker.
Depending on the functional group present in the material, the crosslinking reagent can include a nucleophilic group (e.g., an amine or a hydroxyl) or an electrophilic group (e.g., a carbonyl). Thus, non-limiting crosslinking reagents can include amine-containing reagents, hydroxyl-containing reagents, carboxylic acid-containing reagents, acyl halide-containing reagents, or others. Further crosslinking reagents can include:
in which Ak is an optionally substituted aliphatic or alkylene; Ar is an optionally substituted aromatic or arylene; L is a linking moiety (e.g., any herein, such as a covalent bond, optionally substituted alkylene, aliphatic, etc.); L3 is an integer that is 2 or more (e.g., 2, 3, 4, 5, 6, or more); and X is halo, hydroxyl, optionally substituted amino (e.g., NRN1RN2, in which each of RN1 and RN2 is, independently, H or optionally substituted alkyl), hydroxyl, carboxyl, acyl halide (e.g., —C(O)—R, in which R is halo), carboxyaldehyde (e.g., —C(O)H), or optionally substituted alkyl. Non-limiting crosslinking reagents can include terephthalaldehyde, glutaraldehyde, ortho-xylene, para-xylene, meta-xylene, or a multivalent amine, such as diamine, triamine, tetraamine, pentaamine, etc., including 1,6-diaminohexane (hexanediamine), 1,4-diaminobutane, 1,8-diaminooctane, propane-1,2,3-triamine, [1,1′: 3′,1″-terphenyl]-4,4″,5′-triamine, and others.
After reacting the crosslinking reagent, the composition can include one or more crosslinkers within the composition. If the crosslinking reagent is bivalent, then a crosslinker can be present between two of any combination of polymeric structures, polymeric units, and ionizable/ionic moieties (e.g., between two polymeric units, between two ionizable/ionic moieties, etc.). If the crosslinking reagent is trivalent or of higher n valency, then the crosslinker can be present between any n number of polymeric units, linking moieties, ionizable moieties, and/or ionic moieties. Non-limiting crosslinkers present in the composition include those formed after reacting a crosslinking reagent. Thus, examples of crosslinkers can include:
in which Ak is an optionally substituted aliphatic or an optionally substituted alkylene, Ar is an optionally substituted aromatic or an optionally substituted arylene, L is a linking moiety (e.g., any herein, such as a covalent bond, optionally substituted alkylene, optionally substituted aliphatic, etc.), L3 is an integer that is 2 or more (e.g., 2, 3, 4, 5, 6, or more), and X′ is a reacted form of X. In some embodiments, X′ is absent, —O—, —C(O)—, or -Ak-, in which RN1 is H or optionally substituted alkyl, and Ak is optionally substituted alkylene, optionally substituted heteroalkylene, optionally substituted aliphatic, or optionally substituted heteroaliphatic.
The polymers described herein include one or more ionizable or ionic moieties. Such moieties can include an anionic or cationic charge, such as in an ionic moiety. Alternatively, an ionizable moiety includes a functional group that can be readily converted into an ionic moiety, such as an ionizable moiety of a carboxy group (—CO2H) that can be readily deprotonated to form a carboxylate anion (—CO2−). As used herein, the terms “ionizable” and “ionic” are used interchangeably.
Moieties can be characterized as an acidic moiety (e.g., a moiety can be deprotonated or can carry a negative charge) or a basic moiety (e.g., a moiety that can be protonated or carry a positive charge). In particular embodiments, the moiety can be a multi-ionic moiety, which can include a plurality of acidic moieties, a plurality of basic moieties, or a combination thereof (e.g., such as in a zwitterionic moiety). Further moieties can include a zwitterionic moiety, such as those including an anionic moiety (e.g., hydroxyl or a deprotonated hydroxyl) and a cationic moiety (e.g., ammonium).
The ionic moieties herein can be connected to the parent structure by way of one or more linking moieties. Furthermore, a single ionic moiety can be extended from a single linking moiety, or a plurality of ionic moieties can have one or more linking moieties therebetween. For instance, the ionic moiety can have any of the following structures: -LA-XA or -LA-(LA′-XA)L2 or -LA-(XA-LA′-XA′)L2 or -LA-XA-LA′-XA′-LA″-XA″, in which each LA, LA′, and LA″ is a linking moiety (e.g., any described herein); each XA, XA′, and XA″ includes, independently, an acidic moiety, a basic moiety, a multi-ionic moiety, a cationic moiety, or an anionic moiety; and L2 is an integer of 1, 2, 3, or more (e.g., from 1 to 20). Non-limiting LA and LA1′ can be —(CH2)L1—, —O(CH2)L1—, —(CF2)L1—, —O(CF2)L1—, or —S(CF2)L1—, in which L1 is an integer from 1 to 3; and XA is any ionizable or ionic moiety described herein.
Non-limiting ionizable or ionic moieties include carboxy (—CO2H), carboxylate anion (—CO2−), guanidinium cation, sulfo (—SO2OH), sulfonate anion (—SO2O−), sulfonium cation, sulfate, sulfate anion, phosphono (e.g., —P(═O)(OH)2), phosphonate anion, phosphate, phosphate anion, phosphonium cation, phosphazenium cation, amino (e.g., —NRN1RN2), ammonium cation (e.g., aliphatic or aromatic ammonium), heterocyclic cation (e.g., including piperidinium, pyrrolidinium, pyridinium, pyrazolium, imidazolium, quinolinium, isoquinolinium, acridinium, quinolinium, isoquinolinium, acridinium, pyridazinium, pyrimidinium, pyrazinium, phenazinium, 1,4-diazabicyclo[2.2.2]octane (DABCO) cation, 4-methyl-1,4-diazoniabicyclo[2.2.2]octan-1-yl (MAABCO) cation), and 1-benzyl-1,4-diazoniabicyclo[2.2.2]octane (BABCO) cation), or a salt form thereof. Such moieties can be associated with one or more counterions. For instance, a cationic moiety can be associated with one or more anionic counterions, and an anionic moiety can be associated with one or more cationic counterions.
Particular moieties herein (e.g., polymeric units, linking moieties, and others) can include an optionally substituted arylene. Such arylene groups include any multivalent (e.g., bivalent, trivalent, tetravalent, etc.) groups having one or more aromatic groups, which can include heteroaromatic groups. Non-limiting aromatic groups (e.g., for Ar) can include any of the following:
in which each of rings a-i can be optionally substituted (e.g., with any optional substituents described herein for alkyl or aryl; or with any ionic moiety described herein); L′ is a linking moiety (e.g., any described herein); and each of R′ and R″ is, independently, H, optionally substituted alkyl, optionally substituted aryl, or an ionic moiety, as described herein. Non-limiting substituents for rings a-i include one or more described herein for aryl, such as alkyl, alkoxy, alkoxyalkyl, amino, aminoalkyl, aryl, arylalkylene, aryloyl, aryloxy, arylalkoxy, cyano, hydroxy, hydroxyalkyl, nitro, halo, and haloalkyl. In some embodiments, L′ is a covalent bond, —O—, —NRN1—, —C(O)—, optionally substituted alkylene, heteroalkylene, or arylene.
Yet other non-limiting arylene can include phenylene (e.g., 1,4-phenylene, 1,3-phenylene, etc.), biphenylene (e.g., 4,4′-biphenylene, 3,3′-biphenylene, 3,4′-biphenylene, etc.), terphenylene (e.g., 4,4′-terphenylene), diphenyl ether, anthracene (e.g., 9,10-anthracene), naphthalene (e.g., 1,5-naphthalene, 1,4-naphthalene, 2,6-naphthalene, 2,7-naphthalene, etc.), tetrafluorophenylene (e.g., 1,4-tetrafluorophenylene, 1,3-tetrafluorophenylene), and the like.
Non-limiting examples of linking moieties for arylene include any herein. In some embodiments, L′ is substituted one or more ionizable or ionic moieties described herein. In particular embodiments, L′ is optionally substituted alkylene. Non-limiting substitutions for L′ can include -LA-XA, in which LA is a linking moiety (e.g., any described herein, such as, -Ak-, —O-Ak-, -Ak-O—, —Ar—, —O—Ar—, or —Ar—O—, in which Ak is optionally substituted alkylene and Ar is optionally substituted arylene), and XA is an acidic moiety, a basic moiety, or a multi-ionic moiety.
Particular chemical functionalities herein can include a linking moiety, either between the parent structure and another moiety (e.g., an ionic moiety) or between two (or more) other moieties. Linking moieties (e.g., L, L1, L2, L3, L4, LA, LA′, LA″, LB′, LB″, L8A, and others) can be any useful multivalent group, such as multivalent forms of optionally substituted aliphatic, heteroaliphatic, aromatic, or heteroaromatic.
In any embodiment herein, the linking moiety (e.g., L, L1, L2, L3, or L4) includes a covalent bond, spirocyclic bond, —O—, —NRN1—, —C(O)—, —C(O)O—, —OC(O)—, —SO2—, optionally substituted aliphatic, alkylene (e.g., —CH2—, —C(CH3)2—, or —CR2—, in which R is H, alkyl, or haloalkyl), alkyleneoxy, haloalkylene (e.g., —CF2— or —C(CF3)2—), hydroxyalkylene, heteroaliphatic, heteroalkylene, aromatic, arylene, aryleneoxy, heterocycle, heterocyclyldiyl, —SO2—NRN1-Ak-, —(O-Ak)L1-SO2—NRN1-Ak-, -Ak-, -Ak-(O-Ak)L1-, —(O-Ak)L1-, -(Ak-O)L1—, —C(O)O-Ak-, —Ar—, or —Ar—O—, as well as combinations thereof. In particular embodiments, Ak is optionally substituted aliphatic, alkylene, or haloalkylene; RN1 is H, optionally substituted alkyl, or aryl; Ar is an optionally substituted aromatic or arylene; and L1 is an integer from 1 to 3.
In other embodiments, L is an optionally substituted C1-6 aliphatic, C1-6 alkylene, or C1-6 heteroalkylene. The use of short linkers could provide more extensive polymeric networks, as shorter linkers could minimize self-cyclization reactions.
In some embodiments, the linking moiety is —(CH2)L1—, —O(CH2)L1—, —(CF2)L1—, —O(CF2)L1—, or —S(CF2)L1— in which L1 is an integer from 1 to 3. In other embodiments, the linking moiety is -Ak-O—Ar-Ak-O-Ak- or -Ak-O—Ar—, in which Ak is optionally substituted alkylene or haloalkylene, and Ar is an optionally substituted arylene. Non-limiting substituted for Ar includes —SO2-Ph, in which Ph can be unsubstituted or substituted with one or more halo.
The polymers described above in the with reference to the Table and formulas (I)-(V) and (X)-(XXXIV), including homopolymers and copolymers thereof and which may be optionally crosslinked and may include any of the linking moieties, arylene groups, and ionic moieties as described above may be used as appropriate in one or more layers of the MEA including a cathode catalyst layer, an anode catalyst layer, a polymer electrolyte membrane (PEM) layer, a cathode buffer layer, and/or an anode buffer layer.
In certain embodiments, the MEA includes a bipolar interface with an anion-conducting polymer on the cathode side of the MEA and an interfacing cation-conducting polymer on the anode side of the MEA. In some implementations, the cathode contains a first catalyst and an anion-conducting polymer. In certain embodiments, the anode contains a second catalyst and a cation-conducting polymer. In some implementations, a cathode buffer layer, located between the cathode and PEM, contains an anion-conducting polymer. In some embodiments, an anode buffer layer, located between the anode and PEM, contains a cation-conducting polymer.
During operation, an MEA with a bipolar interface moves ions through a polymer-electrolyte, moves electrons through metal and/or carbon in the cathode and anode layers, and moves liquids and gas through pores in the layers.
In embodiments employing an anion-conducting polymer in the cathode and/or in a cathode buffer layer, the MEA can decrease or block unwanted reactions that produce undesired products and decrease the overall efficiency of the cell. In embodiments employing a cation-conducting polymer in the anode and/or in an anode buffer layer can decrease or block unwanted reactions that reduce desired product production and reduce the overall efficiency of the cell.
For example, at levels of electrical potential used for cathodic reduction of CO2, hydrogen ions may be reduced to hydrogen gas. This is a parasitic reaction; current that could be used to reduce CO2 is used instead to reduce hydrogen ions. Hydrogen ions may be produced by various oxidation reactions performed at the anode in a CO2 reduction reactor and may move across the MEA and reach the cathode where they can be reduced to produce hydrogen gas. The extent to which this parasitic reaction can proceed is a function of the concentration of hydrogen ions present at the cathode. Therefore, an MEA may employ an anion-conducting material in the cathode layer and/or in a cathode buffer layer. The anion-conducting material at least partially blocks hydrogen ions from reaching catalytic sites on the cathode. As a result, parasitic production of hydrogen gas generation is decreased and the rate of CO or other product production and the overall efficiency of the process are increased.
Another process that may be avoided is transport of carbonate or bicarbonate ions to the anode, effectively removing CO2 from the cathode. Aqueous carbonate or bicarbonate ions may be produced from CO2 at the cathode. If such ions reach the anode, they may decompose and release gaseous CO2. The result is net movement of CO2 from the cathode to the anode, where it does not get reduced and is lost with oxidation products. To prevent the carbonate and bicarbonate ion produced at the cathode from reaching the anode, the polymer-electrolyte membrane and/or a anode buffer layer may include a cation-conducting polymer, which at least partially blocks the transport of negative ions such as bicarbonate or carbonate ions to the anode.
Thus, in some designs, a bipolar membrane structure raises the pH at the cathode to facilitate CO2 reduction while a cation-conducting polymer such as a proton-exchange layer prevents the passage of significant amounts of CO2, negative ions (e.g. bicarbonate, carbonate), hydrogen, and CO2 reduction products (e.g., CO, methane, ethylene, alcohols) to the anode side of the cell.
An example MEA for carbon oxide reduction has a cathode layer and an anode layer separated by an ion-conducting polymer layer that provides a path for ions to travel between the cathode layer and the anode layer. In certain embodiments, the cathode layer includes an anion-conducting polymer and/or the anode layer includes a cation-conducting polymer. In certain embodiments, the cathode layer and/or the anode layer of the MEA are porous. The pores may facilitate gas and/or fluid transport and may increase the amount of catalyst surface area that is available for reaction.
The ion-conducting layer may include two or three sublayers: a polymer electrolyte membrane (PEM), an optional cathode buffer layer, and/or an optional anode buffer layer. One or more layers in the ion-conducting layer may be porous. In certain embodiments, at least one layer is nonporous so that reactants and products of the cathode cannot pass via gas and/or liquid transport to the anode and vice versa. In certain embodiments, the PEM layer is nonporous. Example characteristics of anode buffer layers and cathode buffer layers are provided elsewhere herein.
As illustrated in the magnification inset of a bipolar interface 113 in electrolyzer 103, the cathode 105 includes an anion exchange polymer (which in this example is the same anion-conducting polymer 109 that is in the bipolar layers) electronically conducting carbon support particles 117, and metal nanoparticles 119 supported on the support particles. CO2 and water are transported via pores such as pore 121 and reach metal nanoparticles 119 where they react, in this case with hydroxide ions, to produce bicarbonate ions and reduction reaction products (not shown). CO2 may also reach metal nanoparticles 119 by transport within anion exchange polymer 109.
Hydrogen ions are transported from anode 107, and through the cation-conducting polymer 111, until they reach bipolar interface 113, where they are hindered from further transport toward the cathode by anion exchange polymer 109. At interface 113, the hydrogen ions may react with bicarbonate or carbonate ions to produce carbonic acid (H2CO3), which may decompose to produce CO2 and water. As explained herein, the resulting CO2 may be provided in gas phase and should be provided with a route in the MEA back to the cathode 105 where it can be reduced. The cation-conducting polymer 111 hinders transport of anions such as bicarbonate ions to the anode where they could react with protons and release CO2, which would be unavailable to participate in a reduction reaction at the cathode.
As illustrated, a cathode buffer layer having an anion-conducting polymer may work in concert with the cathode and its anion-conductive polymer to block transport of protons to the cathode. While MEAs employing ion conducting polymers of appropriate conductivity types in the cathode, the anode, cathode buffer layer, and if present, an anode buffer layer may hinder transport of cations to the cathode and anions to the anode, cations and anions may still come in contact in the MEA's interior regions, such as in the membrane layer.
As illustrated in
The delamination problem can be addressed by employing a cathode buffer layer having pores. One possible explanation of its effectiveness is that the pores create paths for the gaseous carbon dioxide to escape back to the cathode where it can be reduced. In some embodiments, the cathode buffer layer is porous but at least one layer between the cathode layer and the anode layer is nonporous. This can prevent the passage of gases and/or bulk liquid between the cathode and anode layers while still preventing delamination. For example, the nonporous layer can prevent the direct passage of water from the anode to the cathode. The porosity of various layers in an MEA is described further at other locations herein.
As an example, an MEA includes a cathode layer including a reduction catalyst and a first anion-conducting polymer (e.g., Sustainion, FumaSep FAA-3, Tokuyama anion exchange polymer), an anode layer including an oxidation catalyst and a first cation-conducting polymer (e.g., PFSA polymer), a membrane layer including a second cation-conducting polymer and arranged between the cathode layer and the anode layer to conductively connect the cathode layer and the anode layer, and a cathode buffer layer including a second anion-conducting polymer (e.g., Sustainion, FumaSep FAA-3, Tokuyama anion exchange polymer) and arranged between the cathode layer and the membrane layer to conductively connect the cathode layer and the membrane layer. In this example, the cathode buffer layer can have a porosity between about 1 and 90 percent by volume, but can additionally or alternatively have any suitable porosity (including, e.g., no porosity). In other examples the cathode buffer layer can have any suitable porosity (e.g., between 0.01-95%, 0.1-95%, 0.01-75%, 1-95%, 1-90%, etc.).
Too much porosity can lower the ionic conductivity of the buffer layer. In some embodiments, the porosity is 20% or below, and in particular embodiments, between 0.1-20%, 1-10%, or 5-10%. Porosity in these ranges can be sufficient to allow movement of water and/or CO2 without losing ionic conductivity. Porosity may be measured as described further below.
In a related example, the membrane electrode assembly can include an anode buffer layer that includes a third cation-conducting polymer, and is arranged between the membrane layer and the anode layer to conductively connect the membrane layer and the anode layer. The anode buffer layer preferably has a porosity between about 1 and 90 percent by volume, but can additionally or alternatively have any suitable porosity (including, e.g., no porosity). However, in other arrangements and examples, the anode buffer layer can have any suitable porosity (e.g., between 0.01-95%, 0.1-95%, 0.01-75%, 1-95%, 1-90%). As with the cathode buffer layer, in some embodiments, the porosity is 20% or below, e.g. 0.1-20%, 1-10%, or 5-10%
In an example, an anode buffer layer may be used in a MEA having a cathode catalyst layer with anion exchange polymer, a cathode buffer layer with anion-exchange polymer, a membrane with cation-exchange polymer, and an anode buffer layer with anion-exchange polymer. In such a structure, the anode buffer layer may be porous to facilitate water transport to the membrane/anode buffer layer interface. Water will be split at this interface to make protons that travel through the membrane and hydroxide that travels to the anode catalyst layer. One advantage of this structure is the potential use of low-cost water oxidation catalysts (e.g., NiFeOx) that are only stable in basic conditions.
In another specific example, the membrane electrode assembly includes a cathode layer including a reduction catalyst and a first anion-conducting polymer (e.g., Sustainion, FumaSep FAA-3, Tokuyama anion exchange polymer), an anode layer including an oxidation catalyst and a first cation-conducting polymer, a membrane layer including a second anion-conducting polymer (e.g., Sustainion, FumaSep FAA-3, Tokuyama anion exchange polymer) and arranged between the cathode layer and the anode layer to conductively connect the cathode layer and the anode layer, and an anode buffer layer including a second cation-conducting polymer and arranged between the anode layer and the membrane layer to conductively connect the anode layer and the membrane layer.
An MEA containing an anion-exchange polymer membrane and an anode buffer layer containing cation-exchange polymer may be used for CO reduction. In this case, water would form at the membrane/anode buffer layer interface. Pores in the anode buffer layer could facilitate water removal. One advantage of this structure would be the use of an acid stable (e.g., IrOx) water oxidation catalyst.
In a related example, the membrane electrode assembly can include a cathode buffer layer that includes a third anion-conducting polymer and is arranged between the cathode layer and the membrane layer to conductively connect the cathode layer and the membrane layer. The third anion-conducting polymer can be the same or different from the first and/or second anion-conducting polymer. The cathode buffer layer preferably has a porosity between about 1 and 90 percent by volume but can additionally or alternatively have any suitable porosity (including, e.g., no porosity). However, in other arrangements and examples, the cathode buffer layer can have any suitable porosity (e.g., between 0.01-95%, 0.1-95%, 0.01-75%, 1-95%, 1-90%). In some embodiments, the porosity is 20% or below, and in particular embodiments, between 0.1-20%, 1-10%, or 5-10%.
In an example, a cathode catalyst layer composed of Au nanoparticles 4 nm in diameter supported on Vulcan XC72R carbon and mixed with TM1 (mTPN-1) anion exchange polymer electrolyte (from Orion). Layer is ˜15 um thick, Au/(Au+C)=20 wt %, TM1 to catalyst mass ratio of 0.32, mass loading of 1.4-1.6 mg/cm2 (total Au+C), estimated porosity of 0.56. Anion-exchange polymer layer composed of TM1 and PTFE particles. PTFE is approximately 200 nm in diameter. TM1 molecular weight is 30 k-45 k. Thickness of the layer is ˜15 um. PTFE may introduce porosity of about 8%. Proton-exchange membrane layer composed of perfluorosulfonic acid polymer (e.g., Nafion 117). Thickness is approximately 183 um. Membrane forms a continuous layer that prevents significant movement of gas (CO2, CO, H2) through the layer. Anode catalyst layer composed of Jr or IrOx nanoparticles (100-200 nm aggregates) that is 10 um thick.
U.S. Patent Application Publication No. 2017/0321334, published Nov. 9, 2017 [OPUSP001B] and US Patent Application Publication No. 20190226103, published Jul. 25, 2019 [OPUSP005], which describe various features and examples of MEAs, are incorporated herein by reference in their entireties. All publications referred to herein are incorporated by reference in their entireties as if fully set forth herein.
As indicated above, the cathode of the MEA, which is also referred to as the cathode layer or cathode catalyst layer, facilitates COx conversion. It is a porous layer containing catalysts for COx reduction reactions.
In some embodiments, the cathode catalyst layer contains a blend of reduction catalyst particles, electronically-conductive support particles that provide support for the reduction catalyst particles, and a cathode ion-conducting polymer. In some embodiments, the reduction catalyst particles are blended with the cathode ion-conducting polymer without a support.
Examples of materials that can be used for the reduction catalyst particles include, but are not limited, to transition metals such as V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Au, Ru, Rh, Pd, Ag, Cd, Hf, Ta, W, Re, Ir, Pt, and Hg, and combinations thereof, and/or any other suitable materials. Other catalyst materials can include alkali metals, alkaline earth metals, lanthanides, actinides, and post transition metals, such as Sn, Si, Ga, Pb, Al, Tl, Sb, Te, Bi, Sm, Tb, Ce, Nd and In or combinations thereof, and/or any other suitable catalyst materials. The choice of catalyst depends on the particular reaction performed at the cathode of the CRR.
Catalysts can be in the form of nanoparticles that range in size from approximately 1 to 100 nm or particles that range in size from approximately 0.2 to 10 nm or particles in the size range of approximately 1-1000 nm or any other suitable range. In addition to nanoparticles and larger particles, films and nanostructured surfaces may be used.
If used, the electronically-conductive support particles in the cathode can be carbon particles in various forms. Other possible conductive support particles include boron-doped diamond or fluorine-doped tin oxide. In one arrangement, the conductive support particles are Vulcan carbon. The conductive support particles can be nanoparticles. The size range of the conductive support particles is between approximately 20 nm and 1000 nm or any other suitable range. It is especially useful if the conductive support particles are compatible with the chemicals that are present in the cathode when the CRR is operating, are reductively stable, and have a high hydrogen production overpotential so that they do not participate in any electrochemical reactions.
For composite catalysts such as Au/C, example metal nanoparticle sizes may range from about 1-100 nm, e.g., 2 nm-20 nm and the carbon size may be from about 20-200 nm as supporting materials. For pure metal catalyst such as Ag or Cu, the particles have a broad range from 2 nm to 500 nm in term of crystal grain size. The agglomeration could be even larger to micrometer range.
In general, such conductive support particles are larger than the reduction catalyst particles, and each conductive support particle can support many reduction catalyst particles. In some embodiments, there may be two different kinds of catalysts supported on a catalyst support particle, such as a carbon particle. Catalyst particles of a first type and second catalyst particles of a second type are attached to the catalyst support particle. In various arrangements, there is only one type of catalyst particle or there are more than two types of catalyst particles attached to the catalyst support particle.
Using two types of catalysts may be useful in certain embodiments. For example, one catalyst may be good at one reaction (e.g., CO2→CO) and the second good at another reaction (e.g., CO→CH4). Overall, the catalyst layer would perform the transformation of CO2 to CH4, but different steps in the reaction would take place on different catalysts.
The electronically-conductive support may also be in forms other than particles, including tubes (e.g., carbon nanotubes) and sheets (e.g., graphene). Structures having high surface area to volume are useful to provide sites for catalyst particles to attach.
In addition to reduction catalyst particles and electronically-conductive support particles, the cathode catalyst layer may include an ion conducting polymer. There are tradeoffs in choosing the amount of cathode ion-conducting polymer in the cathode. It can be important to include enough cathode ion-conducting polymer to provide sufficient ionic conductivity. But it is also important for the cathode to be porous so that reactants and products can move through it easily and to maximize the amount of catalyst surface area that is available for reaction. In various arrangements, the cathode ion-conducting polymer makes up somewhere in the range between 30 and 70 wt %, between 20 and 80 wt %, or between 10 and 90 wt %, of the material in the cathode layer, or any other suitable range. The wt % of ion-conducting polymer in the cathode is selected to result in the cathode layer porosity and ion-conductivity that gives the highest current density for COx reduction. In some embodiments, it may be between 20 and 60 wt. % or between 20 and 50 wt. %. Example thicknesses of the cathode catalyst layer range from about 80 nm-300 μm.
In addition to the reduction catalyst particles, cathode ion conducting polymer, and if present, the electronically-conductive support, the cathode catalyst layer may include other additives such as PTFE.
In addition to polymer:catalyst mass ratios, the catalyst layer may be characterized by mass loading (mg/cm2), and porosity. Porosity may be determined by a various manners. In one method, the loading of each component (e.g., catalyst, support, and polymer) is multiplied by its respective density. These are added together to determine the thickness the components take up in the material. This is then divided by the total known thickness to obtain the percentage of the layer that is filled in by the material. The resulting percentage is then subtracted from 1 to obtain the percentage of the layer assumed to be void space (e.g., filled with air or other gas or a vacuum), which is the porosity. Methods such as mercury porosimetry or image processing on TEM images may be used as well.
The catalyst layer may also be characterized by its roughness. The surface characteristics of the catalyst layer can impact the resistances across the membrane electrode assembly. Excessively rough catalyst layers can potentially lead to interfacial gaps between the catalyst and the microporous layer. These gaps hinder the continuous pathway for electron transfer from the current collector to the catalytic area, thus, increasing contact resistances. Interfacial gaps may also serve as locations for water accumulation that is detrimental to mass transport of reactants and products. On the other hand, extremely smooth surfaces may suffer from poor adhesion between layers. Catalyst layer roughness may influence electrical contact resistances and concentration polarization losses. Surface roughness can be measured using different techniques (e.g. mechanical stylus method, optical profilometry, or atomic force microscopy) and is defined as the high-frequency, short wavelength component of a real surface. Arithmetic mean height, Sa, is a parameter that is commonly used to evaluate the surface roughness. Numerically, it is calculated by integrating the absolute height of valleys and peaks on the surface relative to the mean plane over the entire geometric area of the sample. Catalyst layer Sa values between 0.50-1.10 μm or 0.70-0.90 μm may be used in some embodiments.
Examples of cathode catalyst layers for CO, methane, and ethylene/ethanol productions are given below.
The functions, materials, and structures of the components of the cathode catalyst layer are described further below.
A primary function of the cathode catalyst layer is to provide a catalyst for COx reduction. An example reaction is:
CO2+2H++2e−→CO+H2O.
The cathode catalyst layer also has a number of other functions that facilitate COx conversion. These include water management, gas transport, reactant delivery to the metal catalyst, product removal, stabilizing the particulate structure of the metal catalyst, electronic and ionic conduction to the metal catalyst, and mechanical stability within the MEA.
Certain functions and challenges are particular to CRRs and are not found in MEA assemblies for other applications such as fuel cells or water electrolyzers. These challenges include that the cathode catalyst layer of the MEA transports gas (e.g., CO2 or CO) in and gas (e.g., ethylene, methane, CO) or liquid (e.g., ethanol) out. The cathode catalyst layer also prevents accumulation of water that can block gas transport. Further, catalysts for COx reduction are not as developed as catalysts like platinum that can be used in hydrogen fuel cells. As a result, the COx reduction catalysts are generally less stable. These functions, their particular challenges, and how they can be addressed are described below.
The cathode catalyst layer facilitates movement of water to prevent it from being trapped in the cathode catalyst layer. Trapped water can hinder access of COx to the catalyst and/or hinder movement of reaction product out of the cathode catalyst layer.
Water management challenges are in many respects unique to CRRs. For example, compared to a PEM fuel cell's oxygen electrode, a CRR uses a much lower gas flow rate. A CRR also may use a lower flow rate to achieve a high utilization of the input COx. Vapor phase water removal is determined by the volumetric gas flow, thus much less vapor phase water removal is carried out in a CRR. A CRR may also operate at higher pressure (e.g., 100 psi-450 psi) than a fuel cell; at higher pressure the same molar flow results in lower volumetric flow and lower vapor phase water removal. As a result, liquid water in MEA of a CRR is present to be removed. For some MEAs, the ability to remove vapor phase water is further limited by temperature limits not present in fuel cells. For example, CO2 to CO reduction may be performed at about 50° C. and ethylene and methane production may be performed at 20° C.-25° C. This is compared to typical operating temperatures of 80° C. to 120° C. for fuel cells. As a result, there is more liquid phase water to remove.
Properties that affect ability of the cathode catalyst layer to remove water include porosity; pore size; distribution of pore sizes; hydrophobicity; the relative amounts of ion conducting polymer, metal catalyst particles, and electronically-conductive support; the thickness of the layer; the distribution of the catalyst throughout the layer; and the distribution of the ion conducting polymer through the layer and around the catalyst.
A porous layer allows an egress path for water. In some embodiments, the cathode catalyst layer has a pore size distribution that includes pores having sizes of 1 nm-100 nm and pores having sizes of at least 1 micron. This size distribution can aid in water removal. The porous structures could be formed by one or more of: pores within the carbon supporting materials; stacking pores between stacked spherical carbon nanoparticles; secondary stacking pores between agglomerated carbon spheres (micrometer scale); or inert filler (e.g., PTFE) introduced porous with the interface between the PTFE and carbon also creating irregular pores ranging from hundreds of nm to micrometers.
The cathode catalyst layer may have a thickness that contributes to water management. Using a thicker layer allows the catalyst and thus the reaction to be distributed in a larger volume. This spreads out the water distribution and makes it easier to manage.
Ion-conducting polymers having non-polar, hydrophobic backbones may be used in the cathode catalyst layer. In some embodiments, the cathode catalyst layer may include a hydrophobic polymer such as PTFE in addition to the ion-conducting polymer. In some embodiments, the ion-conducting polymer may be a component of a co-polymer that also includes a hydrophobic polymer. In some embodiments, the ion-conducting polymer has hydrophobic and hydrophilic regions. The hydrophilic regions can support water movement and the hydrophobic regions can support gas movement.
The cathode catalyst layer is structured for gas transport. Specifically, COx is transported to the catalyst and gas phase reaction products (e.g., CO, ethylene, methane, etc.) is transported out of the catalyst layer.
Certain challenges associated with gas transport are unique to CRRs. Gas is transported both in and out of the cathode catalyst layer—COx in and products such as CO, ethylene, and methane out. In a PEM fuel cell, gas (O2 or H2) is transported in but nothing or product water comes out. And in a PEM water electrolyzer, water is the reactant with O2 and H2 gas products.
Operating conditions including pressures, temperature, and flow rate through the reactor affect the gas transport. Properties of the cathode catalyst layer that affect gas transport include porosity; pore size and distribution; layer thickness; and ionomer distribution.
In some embodiments, the ionomer-catalyst contact is minimized. For example, in embodiments that use a carbon support, the ionomer may form a continuous network along the surface of the carbon with minimal contact with the catalyst. The ionomer, support, and catalyst may be designed such that the ionomer has a higher affinity for the support surface than the catalyst surface. This can facilitate gas transport to and from the catalyst without being blocked by the ionomer, while allowing the ionomer to conduct ions to and from the catalyst.
The ionomer may have several functions including holding particles of the catalyst layer together and allowing movement of ions through the cathode catalyst layer. In some cases, the interaction of the ionomer and the catalyst surface may create an environment favorable for COx reduction, increasing selectivity to a desired product and/or decreasing the voltage required for the reaction. Importantly, the ionomer is an ion-conducting polymer to allow for the movement of ions through the cathode catalyst layer. Hydroxide, bicarbonate, and carbonate ions, for example, are moved away from the catalyst surface where the COx reduction occurs. In the description below, the ionomer in the cathode catalyst layer can be referred to as a first ion-conducting polymer.
The first ion-conducting polymer can comprise at least one ion-conducting polymer that is an anion-conductor. This can be advantageous because it raises the pH compared to a proton conductor.
In some embodiments, the first ion-conducting polymer can comprise one or more covalently-bound, positively-charged functional groups configured to transport mobile negatively-charged ions. The first ion-conducting polymer can be selected from the group consisting of aminated tetramethyl polyphenylene; poly(ethylene-co-tetrafluoroethylene)-based quaternary ammonium polymer; quaternized polysulfone), blends thereof, and/or any other suitable ion-conducting polymers. The first ion-conducting polymer can be configured to solubilize salts of bicarbonate or hydroxide.
In some embodiments, the first ion-conducting polymer can comprise at least one ion-conducting polymer that is a cation-and-anion-conductor. The first ion-conducting polymer can be selected from the group consisting of polyethers that can transport cations and anions and polyesters that can transport cations and anions. The first ion-conducting polymer can be selected from the group consisting of polyethylene oxide, polyethylene glycol, polyvinylidene fluoride, and polyurethane.
A cation-and-anion conductor will raise pH (compared to a pure cation conductor.) Further, in some embodiments, it may be advantageous to use a cation-and-anion conductor to promote acid base recombination in a larger volume instead of at a 2D interface of anion-conducting polymer and cation conducting polymer. This can spread out water and CO2 formation, heat generation, and potentially lower the resistance of the membrane by decreasing the barrier to the acid-base reaction. All of these may be advantageous in helping avoid the buildup of products, heat, and lowering resistive losses in the MEA leading to a lower cell voltage.
A typical anion-conducting polymer has a polymer backbone with covalently bound positively charged functional groups appended. These may include positively charged nitrogen groups in some embodiments. In some embodiments, the polymer backbone is non-polar, as described above. The polymer may be any appropriate molecular weight, e.g., 25,000 g/mol-150,000 g/mol, though it will be understood that polymers outside this range may be used.
Particular challenges for ion-conducting polymers in CRR's include that CO2 can dissolve or solubilize polymer electrolytes, making them less mechanically stable, prone to swelling, and allowing the polymer to move more freely. This makes the entire catalyst layer and polymer-electrolyte membrane less mechanically stable. In some embodiments, polymers that are not as susceptible to CO2 plasticization are used. Also, unlike for water electrolyzers and fuel cells, conducting carbonate and bicarbonate ions is a key parameter for CO2 reduction.
The introduction of polar functional groups, such as hydroxyl and carboxyl groups which can form hydrogen bonds, leads to pseudo-crosslinked network formation. Cross-linkers like ethylene glycol and aluminum acetylacetonate can be added to reinforce the anion exchange polymer layer and suppress polymer CO2 plasticization. Additives like polydimethylsiloxane copolymer can also help mitigate CO2 plasticization.
According to various embodiments, the ion-conducting polymer may have a bicarbonate ionic conductivity of at least 6 mS/cm, or in some embodiments at least 12 mS/cm, is chemically and mechanically stable at temperatures 80° C. and lower, and soluble in organic solvents used during fabrication such as methanol, ethanol, and isopropanol. The ion-conducting polymer is stable (chemically and has stable solubility) in the presence of the COx reduction products. The ion-conducting polymer may also be characterized by its ion exchange capacity, the total of active sites or functional groups responsible for ion exchange, which may range from 2.1 mmol/g-2.6 mmol/g in some embodiments. In some embodiments, ion-conducting polymers having lower IECs such as greater than 1 or 1.5 mmol/g may be used.
Examples of anion-conducting polymers are given above in above table as Class A ion-conducting polymers. A particular example of an anion-conducting polymer is Orion mTPN1 (also referred to herein as Orion TM1), which has m-triphenyl fluoro-alkyl as backbone and trimethylammonium (TMA+) as cation group. The chemical structure is shown below.
Additional examples include anion exchange membranes produced by Fumatech and Ionomr. Fumatech FumaSep FAA-3 ionomers come in Br— form. Anion exchange polymer/membrane based on polybenzimidazole produced by Ionomr comes in I— form as AF-1-HNN8-50-X.
The as-received polymer may be prepared by exchanging the anion (e.g., I−, Br−, etc.) with bicarbonate.
Also, as indicated above, in certain embodiments the ionomer may be a cation-and-anion-conducting polymer. Examples are given in the above table as Class B ion-conducting polymers.
The metal catalyst catalyzes the COx reduction reaction(s). The metal catalyst is typically nanoparticles, but larger particles, films, and nanostructured surfaces may be used in some embodiments. The specific morphology of the nanoparticles may expose and stabilize active sites that have greater activity.
The metal catalyst is often composed of pure metals (e.g., Cu, Au, Ag), but specific alloys or other bimetallic systems may have high activity and be used for certain reactions. The choice of catalyst may be guided by the desired reaction. For example, for CO production, Au may be used; for methane and ethylene production, Cu may be used. Other metals including Ag, alloys, and bimetallic systems may be used. CO2 reduction has a high overpotential compared to other well-known electrochemical reactions such as hydrogen evolution and oxygen evolution on known catalysts. Small amounts of contaminants can poison catalysts for CO2 conversion. And as indicated above, metal catalysts such as Cu, Au, and Ag are less developed than catalysts such as platinum used in hydrogen fuel cells.
Different metal catalyst materials may be chosen at least in part based on the desired product and MEA operation. For example, a nanowire has a higher selectivity for ethylene production while triangular Cu nanoplates show higher selectivity for methane. Nanocubes show good selectivity for ethylene in an AEM MEA. Gold nanoparticles with a narrow size distribution (e.g., 2-6 nm) and uniform distribution on carbon surface resulted in higher current efficiency and durability.
Metal catalyst properties that affect the cathode catalyst layer performance include size, size distribution, uniformity of coverage on the support particles, shape, loading (characterized as weight of metal/weight of metal+weight of carbon or as mass of particles per geometric area of catalyst layer), surface area (actual metal catalyst surface area per volume of catalyst layer), purity, and the presence of poisoning surface ligands from synthesis.
Nanoparticles may be synthesized by any appropriate method, such as for example, described in Phan et al., “Role of Capping Agent in Wet Synthesis of Nanoparticles,” J. Phys. Chem. A 2018, 121, 17, 3213-3219; Bakshi “How Surfactants Control Crystal Growth of Nanomaterials,” Cryst. Growth Des. 2016, 16, 2, 1104-1133; and Morsy “Role of Surfactants in Nanotechnology and Their Applications,” Int. J. Curr. Microbiol. App. Sci. 2014, 3, 5, 237-260, which are incorporated by reference herein.
In some embodiments, metal nanoparticles are provided without the presence of poisoning surface ligands. This may be achieved by using the ionomer as a ligand to direct the synthesis of nanocrystal catalysts. The surface of the metal nanocatalysts are directly connected with ionically conductive ionomer. This avoids having to treat the catalyst surface to allow ionomer contact with the metal and improves the contact.
The metal catalyst may be disposed on a carbon support in some embodiments. For CO production, examples include Premetek 20 wt % Au supported on Vulcan XC-72R carbon with 4-6 nm Au particle size and 30% Au/C supported on Vulcan XC-72R with 5-7 nm Au particle size. For methane, examples include Premetek 20 wt % Cu supported on Vulcan XC-72R carbon with 20-30 nm Cu particle size. In some embodiments, the metal catalyst may be unsupported. For ethylene production, examples of unsupported metal catalysts include SigmaAldrich unsupported Cu 80 nm particle size and ebeam or sputter deposited thin Cu layer of 10 nm to 100 nm.
The support of the cathode catalyst layer has several functions. It stabilizes metal nanoparticles to prevent them from agglomerating and distributes the catalytic sites throughout the catalyst layer volume to spread out loss of reactants and formation of products. It also forms an electrically conductive pathway to metal nanoparticles. Carbon particles, for example, pack together such that contacting carbon particles provide the electrically conductive pathway. Void space between the particles forms a porous network that gas and liquids can travel through.
In some embodiments, carbon supports developed for fuel cells can be used. Many different types have been developed; these are typically 50 nm-500 nm in size, and can be obtained in different shapes (spheres, nanotubes, sheets (e.g., graphene)), porosities, surface area per volume, electrical conductivity, functional groups (N-doped, O-doped, etc).
The support may be hydrophobic and have affinity to the metal nanoparticle.
Examples of carbon blacks that can be used include:
The anode of the MEA, which is also referred to as the anode layer or anode catalyst layer, facilitates oxidation reactions. It is a porous layer containing catalysts for oxidation reactions. Examples of reactions are:
2H2O→4H++4e−O2 (in acidic environment of proton exchange polymer electrolyte—bipolar membrane); or
4OH−→4e−+O2+2H2O (in basic environment of anion exchange polymer electrolyte)
The oxidation of other materials, such as hydrocarbons to make CO2 or chloride ions to make chlorine gas, or hydrogen gas to make hydrogen ions, may also be performed.
In some embodiments, the anode contains a blend of oxidation catalyst and an anode ion-conducting polymer. There are a variety of oxidation reactions that can occur at the anode depending on the reactant that is fed to the anode and the anode catalyst(s). In one arrangement, the oxidation catalyst is selected from the group consisting of metals and oxides of Ir, Pt, Ni, Ru, Pd, Au, and alloys thereof, IrRu, Par, Ni, NiFe, stainless steel, and combinations thereof. The oxidation catalyst can further contain conductive support particles selected from the group consisting of carbon, boron-doped diamond, and titanium.
The oxidation catalyst can be in the form of a structured mesh or can be in the form of particles. If the oxidation catalyst is in the form of particles, the particles can be supported by electronically-conductive support particles. The conductive support particles can be nanoparticles. It is especially useful if the conductive support particles are compatible with the chemicals that are present in the anode when the CRR is operating and are oxidatively stable so that they do not participate in any electrochemical reactions. It is especially useful if the conductive support particles are chosen with the voltage and the reactants at the anode in mind. In some arrangements, the conductive support particles are titanium, which is well-suited for high voltages. In other arrangements, the conductive support particles are carbon, which can be most useful at low voltages. In general, such conductive support particles are larger than the oxidation catalyst particles, and each conductive support particle can support many oxidation catalyst particles. An example of such an arrangement is shown in
In some embodiments, the MEA has an anode layer comprising oxidation catalyst and a second ion-conducting polymer. The second ion-conducting polymer can comprise one or more polymers that contain covalently-bound, negatively-charged functional groups configured to transport mobile positively-charged ions. The second ion-conducting polymer can be selected from the group consisting of ethanesulfonyl fluoride, 2-[1-[difluoro-[(trifluoroethenyl)oxy]methyl]-1,2,2,2-tetrafluoroethoxy]-1,1,2,2,-tetrafluoro-, with tetrafluoroethylene, tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octenesulfonic acid copolymer, other perfluorosulfonic acid polymers and blends thereof. Examples of cation-conducting polymers include e.g., Nafion 115, Nafion 117, and/or Nafion 211.
There are tradeoffs in choosing the amount of ion-conducting polymer in the anode. It is important to include enough anode ion-conducting polymer to provide sufficient ionic conductivity. But it is also important for the anode to be porous so that reactants and products can move through it easily, and to maximize the amount of catalyst surface area that is available for reaction. In various arrangements, the ion-conducting polymer in the anode makes up approximately 50 wt % of the layer or between approximately 5 and 20 wt %, 10 and 90 wt %, between 20 and 80 wt %, between 25 and 70 wt %, or any suitable range. It is especially useful if the anode can tolerate high voltages, such as voltages above about 1.2 V vs. a reversible hydrogen electrode. It is especially useful if the anode 240 is porous in order to maximize the amount of catalyst surface area available for reaction and to facilitate gas and liquid transport.
In one example of a metal catalyst, Jr or IrOx particles (100-200 nm) and Nafion ionomer form a porous layer approximately 10 μm thick. Metal catalyst loading is approximately 0.5-3 g/cm2.
In some embodiments, NiFeOx or NiOx is used for basic reactions.
The MEAs include a polymer electrolyte membrane (PEM) disposed between and conductively coupled to the anode catalyst layer and the cathode catalyst layer. The polymer electrolyte membrane has high ionic conductivity (greater than about 1 mS/cm), and is mechanically stable. Mechanical stability can be evidenced in a variety of ways such as through high tensile strength, modulus of elasticity, elongation to break, and tear resistance. Many commercially-available membranes can be used for the polymer electrolyte membrane. Examples include, but are not limited to, various Nation® formulations, GORE-SELECT, FumaPEM® (PFSA) (FuMA-Tech GmbH), and Aquivion® (PFSA) (Solvay).
In one arrangement, the PEM comprises at least one ion-conducting polymer that is a cation-conductor. The third ion-conducting polymer can comprise one or more covalently-bound, negatively-charged functional groups configured to transport mobile positively-charged ions. The third ion-conducting polymer can be selected from the group consisting of ethanesulfonyl fluoride, 2-[1-[difluoro-[(trifluoroethenyl)oxy]methyl]-1,2,2,2-tetrafluoroethoxy]-1,1,2,2,-tetrafluoro-, with tetrafluoroethylene, tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octenesulfonic acid copolymer, other perfluorosulfonic acid polymers and blends thereof.
It is important to note that when the polymer electrolyte membrane is a cation conductor and is conducting protons, it contains a high concentration of protons during operation of the CRR, while the cathode operates best when a low concentration of protons is present. It can be useful to include a cathode buffer layer between the polymer electrolyte membrane and the cathode to provide a region of transition from a high concentration of protons to a low concentration of protons. In one arrangement, the cathode buffer layer is an ion-conducting polymer with many of the same properties as the ion-conducting polymer in the cathode. The cathode buffer layer provides a region for the proton concentration to transition from the polymer electrolyte membrane, which has a high concentration of protons to the cathode, which has a low proton concentration. Within the cathode buffer layer, protons from the polymer electrolyte membrane encounter anions from the cathode, and they neutralize one another. The cathode buffer layer helps ensure that a deleterious number of protons from the polymer electrolyte membrane does not reach the cathode and raise the proton concentration. If the proton concentration of the cathode is too high, COx reduction does not occur. High proton concentration is considered to be in the range of approximately 10 to 0.1 molar and low concentration is considered to be less than approximately 0.01 molar.
The cathode buffer layer can include a single polymer or multiple polymers. If the cathode buffer layer includes multiple polymers, the multiple polymers can be mixed together or can be arranged in separate, adjacent layers. Examples of materials that can be used for the cathode buffer layer include, but are not limited to, FumaSep FAA-3, Tokuyama anion exchange membrane material, and polyether-based polymers, such as polyethylene oxide (PEO), and blends thereof. Further examples are given above in the discussion of the cathode catalyst layer.
The thickness of the cathode buffer layer is chosen to be sufficient that COx reduction activity is high due to the proton concentration being low. This sufficiency can be different for different cathode buffer layer materials. In some embodiments, the thickness of the cathode buffer layer is between approximately 200 nm and 100 μm, between 300 nm and 75 μm, between 500 nm and 50 μm, or any suitable range.
In some embodiments, the cathode buffer layer is less than 50 μm, for example between 1-25 μm such between 1-5 μm, 5-15 μm, or 10-25 μm. By using a cathode buffer layer in this range of thicknesses, the proton concentration in the cathode can be reduced while maintaining the overall conductivity of the cell. In some embodiments, an ultra-thin layer (100 nm-1 μm and in some embodiments, sub-micron) may be used. And as discussed above, in some embodiments, the MEA does not have a cathode buffer layer. In some such embodiments, anion-conducting polymer in the cathode catalyst layer is sufficient. The thickness of the cathode buffer layer may be characterized relative to that of the PEM.
Water and CO2 formed at the interface of a cathode buffer layer and a PEM can delaminate the MEA where the polymer layers connect. The delamination problem can be addressed by employing a cathode buffer layer having inert filler particles and associated pores. One possible explanation of its effectiveness is that the pores create paths for the gaseous carbon dioxide to escape back to the cathode where it can be reduced.
Materials that are suitable as inert filler particles include, but are not limited to, TiO2, silica, PTFE, zirconia, and alumina. In various arrangements, the size of the inert filler particles is between 5 nm and 500 μm, between 10 nm and 100 μm, or any suitable size range. The particles may be generally spherical.
If PTFE (or other filler) volume is too high, it will dilute the polymer electrolyte to the point where ionic conductivity is low. Too much polymer electrolyte volume will dilute the PTFE to the point where it does not help with porosity. In many embodiments a mass ratio of polymer electrolyte/PTFE is 0.25 to 2, and more particularly, 0.5 to 1. A volume ratio polymer electrolyte/PTFE (or, more generally, polymer electrolyte/inert filler) may be 0.25 to 3, 0.5 to 2, 0.75 to 1.5, or 1.0 to 1.5.
In other arrangements, porosity is achieved by using particular processing methods when the layers are formed. One example of such a processing method is laser ablation, where nano to micro-sized channels are formed in the layers. Another example is mechanically puncturing a layer to form channels through it. Another example is appropriately tailoring conditions during ultrasonic spray deposition of a layer to make it porous.
In one arrangement, the cathode buffer layer has a porosity between 0.01% and 95% (e.g., approximately between, by weight, by volume, by mass, etc.). However, in other arrangements, the cathode buffer layer can have any suitable porosity (e.g., between 0.01-95%, 0.1-95%, 0.01-75%, 1-95%, 1-90%). In some embodiments, the porosity is 50% or less, e.g., 0.1-50%, 5-50%, 20-50%, 5-40%, 10-40%, 20-40%, or 25%-40%. In some embodiments, the porosity is 20% or below, e.g. 0.1-20%, 1-10%, or 5-10%.
Porosity of the cathode buffer layer or any layer in the MEA may be measured as described above with respect to the catalyst layer, including using mass loadings and thicknesses of the components, by methods such as mercury porosimetry, x-ray diffraction (SAXS or WAXS), and image processing on TEM images to calculate filled space vs. empty space. Porosity is measured when the MEA is completely dry as the materials swell to varying degrees when exposed to water during operation. As described further below, the porosity may be determined using measured loading and thickness of the layer and known density of the material or materials of the layer.
Porosity in layers of the MEA, including the cathode buffer layer, is described further below.
In some CRR reactions, bicarbonate is produced at the cathode. It can be useful if there is a polymer that blocks bicarbonate transport somewhere between the cathode and the anode, to prevent migration of bicarbonate away from the cathode. It can be that bicarbonate takes some CO2 with it as it migrates, which decreases the amount of CO2 available for reaction at the cathode. In one arrangement, the polymer electrolyte membrane includes a polymer that blocks bicarbonate transport. Examples of such polymers include, but are not limited to, Nafion® formulations, GORE-SELECT, FumaPEM® (PFSA) (FuMA-Tech GmbH), and Aquivion® (PFSA) (Solvay). In another arrangement, there is an anode buffer layer between the polymer electrolyte membrane and the anode, which blocks transport of bicarbonate. If the polymer electrolyte membrane is an anion-conductor, or does not block bicarbonate transport, then an additional anode buffer layer to prevent bicarbonate transport can be useful. Materials that can be used to block bicarbonate transport include, but are not limited to Nafion® formulations, GORE-SELECT, FumaPEM® (PFSA) (FuMA-Tech GmbH), and Aquivion® (PFSA) (Solvay). Of course, including a bicarbonate blocking feature in the ion-exchange layer is not particularly desirable if there is no bicarbonate in the CRR.
In another embodiment of the invention, the anode buffer layer provides a region for proton concentration to transition between the polymer electrolyte membrane to the anode. The concentration of protons in the polymer electrolyte membrane depends both on its composition and the ion it is conducting. For example, a Nafion polymer electrolyte membrane conducting protons has a high proton concentration. A FumaSep FAA-3 polymer electrolyte membrane conducting hydroxide has a low proton concentration. For example, if the desired proton concentration at the anode is more than three orders of magnitude different from the polymer electrolyte membrane, then an anode buffer layer can be useful to effect the transition from the proton concentration of the polymer electrolyte membrane to the desired proton concentration of the anode. The anode buffer layer can include a single polymer or multiple polymers. If the anode buffer layer includes multiple polymers, the multiple polymers can be mixed together or can be arranged in separate, adjacent layers. Materials that can be useful in providing a region for the pH transition include, but are not limited to, Nafion, FumaSep FAA-3, Sustainion®, Tokuyama anion exchange polymer, and polyether-based polymers, such as polyethylene oxide (PEO), blends thereof, and/or any other suitable materials. High proton concentration is considered to be in the range of approximately 10 to 0.1 molar and low concentration is considered to be less than approximately 0.01 molar. Ion-conducting polymers can be placed in different classes based on the type(s) of ions they conduct. This has been discussed in more detail above. There are three classes of ion-conducting polymers described in the table above. In one embodiment of the invention, at least one of the ion-conducting polymers in the cathode, anode, polymer electrolyte membrane, cathode buffer layer, and anode buffer layer is from a class that is different from at least one of the others.
In some embodiments, one or more of the layers of the MEA include pores that allow gas and liquid transport. These pores are distinct from ion-conduction channels that allow ion conduction. In many polymer electrolytes (e.g. PFSA), ion conduction occurs through pores lined with stationary charges. The mobile cations hop between the oppositely charged stationary groups that line the ion conduction channel. Such channels may have variable width; for PFSA materials, the ion conduction channel diameter ranges from narrow areas of approximately 10 Å diameter to wider areas of approximately 40 Å diameter. In anion conducting polymer materials, the channel diameters may be larger, up to about a minimum width of 60 Å in the narrow areas of the channel.
For efficient ion conduction, the polymer-electrolyte is hydrated, so the ion conduction channels also contain water. It is common for some water molecules to move along with the mobile ions in a process termed electro-osmotic drag; typically 1-5 water molecules per mobile ion are moved via electro-osmotic drag. The ion-conducting channel structure and degree of electro-osmotic drag can vary with different polymer-electrolytes or ion-conducting materials. While these ion conducting channels allow ions to move along with some water molecules, they do not allow uncharged molecules to move through them efficiently. Nor do they allow bulk water that is not associated with ions to move through them. A solid (i.e., non-porous) membrane of a polymer electrolyte blocks the bulk of CO2 and products of CO2 electrolysis from passing through it. The typical permeability of CO2, water, and H2 through a wet Nafion 117 PFSA membrane at 30° C. are approximately 8.70×106 mol cm−1 s−1·Pa−1, 4.2 (mol/cm-s-bar)×109, and 3.6 (mol/cm-s-bar)×1011. Permeability depends on temperature, hydration, and nature of the polymer-electrolyte material. In ion conduction channels that have variable diameters, uncharged molecules and bulk movement of liquid/gas may be blocked at least at the narrow parts of the channel.
Pores of larger diameter that the ion conduction channels described above allow the passage of bulk liquid and gas, not just ions. The polymer electrolyte membrane layer of the MEA typically does not contain this type of pore because the membrane needs to separate reactants and products at the cathode from reactants and products at the anode. However, other layers of the MEA may have this type of pore, for example, the cathode catalyst layer may be porous to allow for reactant COx to reach the catalyst and for products of COx reduction to move out of the catalyst layer, through the gas distribution layer, and out the flow field of the electrolyzer. As used herein, the term pore refers to pores other than the ion conduction channels in an ionomer. In some embodiments, the pores of anion conducting polymer layer in an MEA have a minimum cross-sectional dimension of at least 60 Å. In some embodiments, the pores of cation conducting polymer layer in an MEA have a minimum cross-sectional dimension of at least 20 Å. This is to distinguish pores that allow gas/liquid transport from the ion conduction channels described above.
It can be useful if some or all of the following layers are porous: the cathode, the cathode buffer layer, the anode and the anode buffer layer. In some arrangements, porosity is achieved by combining inert filler particles with the polymers in these layers. Materials that are suitable as inert filler particles include, but are not limited to, TiO2, silica, PTFE, zirconia, and alumina. In various arrangements, the size of the inert filler particles is between 5 nm and 500 microns, between 10 nm and 100 microns or any suitable size range. In other arrangements, porosity is achieved by using particular processing methods when the layers are formed. One example of such a processing method is laser ablation, where nano to micro-sized channels are formed in the layers. Laser ablation can additionally or alternatively achieve porosity in a layer by subsurface ablation. Subsurface ablation can form voids within a layer, upon focusing the beam at a point within the layer, and thereby vaporizing the layer material in the vicinity of the point. This process can be repeated to form voids throughout the layer, and thereby achieving porosity in the layer. Sublayer by sublayer methods of forming an MEA layer such as ultrasonic spray deposition may be used to form an MEA layer having a controlled porosity. A dry deposit can lead to faster drying of layers and a more porous final deposit. One or more of high substrate temperature, slow deposition rate, high elevation of nozzle from the substrate, and high volatility of deposition ink can be used to make the layer more porous. A wet deposit can lead to slower can lead to slower drying of layers, densification and compaction of several layers for the final deposit. One or more of low substrate temperature, fast deposition rate, low elevation of spray nozzle from the substrate, and low volatility of the deposition ink can be used to make the layer less porous. For example, a room temperature ultrasonic spray deposition may result in a relatively dense layer and a 50° C. ultrasonic spray deposition may result in a relatively porous layer.
In some embodiments, the following conditions may be used to form layers having porosities of at least 1%, e.g., 1-90%, 1-50%, or 1-30% porosity:substrate temperature of at least 40° C.; deposition rate of no more than 0.8 mL/min, e.g., 0.2-0.8 mL/min; elevation of nozzle of at least 50 mm, e.g., 50-75 mm; and solvent volatility of at least 90-100% (e.g., ethanol).
In some embodiments, the following conditions may be used to form layers having non-porous layers or layers having porosities of less than 1%:substrate temperature of less than 40° C.; deposition rate of more than 0.8 mL/min and up to 10 mL/min; elevation of nozzle of less than 50 mm; and lower solvent volatility of at least 90-100% (e.g., 50-90% volatile solvent content such as ethanol or 50-100% intermediate volatility of solvent such as glycol ethers).
The volume of a void may be determined by the laser power (e.g., higher laser power corresponds to a greater void volume) but can additionally or alternatively be determined by the focal size of the beam, or any other suitable laser parameter. Another example is mechanically puncturing a layer to form channels through the layer. The porosity can have any suitable distribution in the layer (e.g., uniform, an increasing porosity gradient through the layer, a random porosity gradient, a decreasing porosity gradient through the layer, a periodic porosity, etc.).
The porosities (e.g., of the cathode buffer layer, of the anode buffer layer, of the membrane layer, of the cathode layer, of the anode layer, of other suitable layers, etc.) of the examples described above and other examples and variations preferably have a uniform distribution, but can additionally or alternatively have any suitable distribution (e.g., a randomized distribution, an increasing gradient of pore size through or across the layer, a decreasing gradient of pore size through or across the layer, etc.). The porosity can be formed by any suitable mechanism, such as inert filler particles (e.g., diamond particles, boron-doped diamond particles, polyvinylidene difluoride/PVDF particles, polytetrafluoroethylene/PTFE particles, etc.) and any other suitable mechanism for forming substantially non-reactive regions within a polymer layer. The inert filler particles can have any suitable size, such as a minimum of about 10 nanometers and a maximum of about 200 nanometers, and/or any other suitable dimension or distribution of dimensions.
As discussed above, the cathode buffer layer preferably has a porosity between about 1 and 90 percent by volume but can additionally or alternatively have any suitable porosity (including, e.g., no porosity). However, in other arrangements and examples, the cathode buffer layer can have any suitable porosity (e.g., between 0.01-95%, 0.1-95%, 0.01-75%, 1-95%, 1-90%, etc.). in some embodiments, the porosity is 20% or below, e.g. 0.1-20%, 1-10%, or 5-10%.
In some embodiments, the cathode buffer layer is porous but at least one layer between the cathode layer and the anode layer is nonporous. This can prevent the passage of gases and/or bulk liquid between the cathode and anode layers while still preventing delamination. For example, the nonporous layer can prevent the direct passage of water from the anode to the cathode.
Porosity of the cathode buffer layer or any layer in the MEA may be measured as described above with respect to the catalyst layer, including using mass loadings and thicknesses of the components, by methods such as mercury porosimetry, x-ray diffraction (SAXS or WAXS), and image processing on TEM images to calculate filled space vs. empty space. Porosity is measured when the MEA is completely dry as the materials swell to varying degrees when exposed to water during operation. Porosity can be determined using the known density of the material, the actual weight of the layer per given area, and the estimated volume of the layer based on the area and thickness. The equation is as follows:
As indicated above, the density of the material is known, and the layer loading and thickness are measured. For example, in a polymer electrolyte layer with a measured loading of 1.69 mg/cm2 made of 42 wt % anion-exchange polymer electrolyte with a density of 1196 mg/cm3 and 58 wt % PTFE with a density of 2200 mg/cm3 and a total layer thickness of 11.44 microns, the porosity is:
As indicated above, the polymer electrolyte layers may have ion conduction channels that do not easily permit the gas/liquid transport. In the calculation above, these ion conduction channels are considered non-porous; that is, the density of the non-porous material above (42 wt % anion-exchange polymer electrolyte) includes the ion conduction channels and is defined by the calculation to be non-porous.
In another example, an ion conductive layer without filler is porous. Porosity may be introduced by appropriate deposition conditions, for example. The measured loading of the porous polymer electrolyte layer is 2.1 g/cm2 and the thickness is 19 micrometers. The known density of the polymer electrolyte with ion-conducting channels but without pores is 1196 g/cm3. The porosity is then calculated as:
MEAs for COx reduction may be fabricated using a variety of techniques. In various embodiments, MEAs fabrication employs multiple steps. Small differences in the parameters of the fabrication process can make a large difference in performance.
In certain embodiments, MEA fabrication employs a polymer-electrolyte membrane (e.g., a Nafion PEM) layer and depositing or otherwise forming an anion-exchange polymer electrolyte layer and cathode catalyst layer on the cathode side of the membrane and depositing or otherwise forming an anode catalyst layer on the anode side of the membrane. An alternate route is to fabricate the catalyst layers on to porous gas diffusion layers (e.g., carbon for the cathode or titanium for the anode) and sandwich the membrane (which may include the anion-exchange layer) between catalyst containing porous layers. In certain embodiments, catalyst layers are fabricated by making an ink of the solid catalyst and support particles and polymer electrolyte dispersed in a solvent. The ink may be applied by a variety of methods to the polymer electrolyte membrane or GDL. The solvent subsequently evaporates leaving behind a porous solid catalyst layer.
Imaging methods may be used to characterize the thickness, uniformity, and surface roughness. The thickness should be consistent and controllable, and the uniformity smooth and as defect free as possible.
Various techniques may be employed to form the individual layers of the MEA. Generally, these techniques form the layer on a substrate such as a PEM layer or GDL as mentioned herein. Examples of such techniques include ultrasonic spray deposition, doctor blade application, gravure, screen printing, slot die coating, and decal transfer.
Catalyst inks using anion-exchange polymers are not well studied (particularly for certain polymers) and do not have the same solution structure as typical Nafion-based inks used in fuel cells and electrolyzers. The formulation and steps needed for form a well dispersed and stable catalyst ink were not known. It is believed that Nafion forms micell-like structures that allow relatively easy suspension in aqueous media. Other ion-conducting polymers and particularly some anion-conducting polymers do not form such structures and therefore are more difficult to provide in suspensions.
In certain embodiments, a catalyst layer ink is prepared by mixing metal or metal supported on carbon catalyst with ion-conducting polymer (e.g., an anion-conducting polymer) and dispersing in solvent (alcohol, etc.) by sonicating.
As indicated, certain fabrication techniques utilize doctor blade application, screen printing, decal transfer, electrospinning, etc. Roll-to-roll techniques such as gravure or microgravure or slot die coating may be used for high throughput processing.
In some embodiments, the cathode side of the MEAs is fabricated by first depositing a layer of anion-exchange polymer-electrolyte on top of a cation-exchange polymer electrolyte membrane. Then a second layer of cathode catalyst is applied on top of the anion-exchange layer. This process produces a catalyst coated membrane. Gas diffusion electrodes may be prepared by depositing the catalyst onto a gas diffusion layer. The anion exchange layer can be deposited onto the catalyst layer or the membrane. The layers can then be pressed together inside the electrolysis cell to make a functioning device. Many methods, including doctor blade, gravure or micro gravure, slot die, decal transfer, screen printing, ultrasonic spray deposition and others can be used to fabricate the anion-exchange polymer layer and the cathode catalyst layer. A more detailed description of MEA cathode fabrication using ultrasonic spray deposition follows:
The cathode side of the MEA is fabricated by first forming a solution of polymer-electrolyte (approximately 1-25 wt %) in a suitable solvent, such as ethanol, n-propanol, isopropanol, or other high vapor pressure and/or low boiling point solvent that will evaporate on a reasonable timescale during fabrication. Mixtures of solvents with one or more higher boiling point components can be used. The polymer electrolyte solution is pushed through an ultrasonic spray deposition nozzle at a desired flow rate. The ultrasonic spray deposition nozzle is held at the desired frequency to disperse the polymer-electrolyte solution into small droplets that are then pushed by an air stream onto a polymer-electrolyte membrane substrate. The polymer-electrolyte membrane may be treated with heat, solvent, or other means before deposition. The small droplets of polymer-electrolyte solution land on polymer-electrolyte membrane substrate where the solvent evaporates and leaves behind the entrained polymer-electrolyte. The ultrasonic spray deposition nozzle moves back and forth across the substrate multiple times with the desired pattern with the desired speed to build up a polymer-electrolyte layer on top of the membrane substrate until the desired thickness is reached. This process is then repeated using a solution of catalyst particles, anion-exchange polymer electrolyte and/or other additives, and a suitable solvent or mixture of solvents; this solution is termed the catalyst ink. The catalyst ink is deposited via ultrasonic spray deposition using the same or different fabrication parameters to form the cathode catalyst layer on top of the anion-exchange polymer layer on the cathode side of the MEA.
As indicated, certain applications of MEAs for COx reduction may require relatively large formats. For example, some MEAs have active surface areas (excluding pores) of at least about 500 cm2. And in some other embodiments, MEAs have even larger active surface areas (excluding pores), or e.g., at least about 650 cm2 or 1500 cm2.
To make MEAs with such large active surface areas, an appropriate manufacturing process must be chosen, i.e., a process that can support large volumes of catalyst ink and large surface areas to which the catalyst ink is applied. Scaling up the catalyst ink requires particular methods of dispersing the catalyst particles to ensure good dispersion in large volumes. The ink may be set at a target dispersity, which dynamic light scattering (DLS) can be used to characterize. The ink should be stable within the time range of the layer deposition.
Additionally, humidity and temperature should be tightly controlled. Evaporation rates and processes impact the resulting deposition, so controlling these things within a 1-2 degree temperature window, and roughly 5% RH range is useful.
For ultrasonic spray deposition, thin lines of catalyst ink are laid down by a moving ultrasonic nozzle. The nozzle movement speed and ink flow rate may need to be increased for larger area MEAs. The flow rate and move speed are at least be doubled going from 25 cm2 to a 650 cm2 scale MEA. Water in the solvent is important and adding more water into the ink helps the stacking of droplets be smoother. For example, about 20% water in the formulation may be used for a 650 cm2 MEA.
Catalyst inks are generally relatively less stable, so in certain embodiments, the MEA fabrication time is designed to be relatively short, even when active area is larger. As an example, for a 650 cm2 spray, a deposition time of about 2 hours for the ionomer layer and 1 hour for the catalyst layer may be used. This is relatively fast for such a larger area and can be achieved using the fast flow rate and move speed.
Below are examples of scaling up MEA fabrication. Examples are provided for scaling from 25 cm2 to 650 cm2.
Solvent mixture adjusted (water to alcohol ratio): depending on the size of the spray scale, solvent adjustment from 10% water to 20% water significantly helps with surface uniformity of the surface
Deposition Parameters:
Morphology and thickness: Thickness can be matched from looking at the thickness of fabricated layers in SEM images. Adjustments can be made on characterization data to match the thickness. The morphology is controlled with parameters such as water content and fabrication.
For further scale up, e.g., to 1500 cm2, flow rate and move speed may be further increased, e.g., with ranges being from 0.25-2 mL/min and 30-200 mm/s.
The speed of deposition can be further increased, e.g., to 5-8 mL/min or 5-15 mL/min, by increasing the weight of solids in the solution. In some embodiments, the solution may be greater than 5 wt. %, greater than 10 wt. %, greater than 20 wt. %, or greater than 30 wt. %.
After the MEA is fabricated, additional treatments may be used to increase performance. Examples the types of performance improvement include lifetime and voltage. These improvements may be manifest in MEAs that have structural modifications resulting from the treatments including better adhesion between layers.
Hot pressing: heating the MEA under pressure to bond the layers together. Hot pressing is a step sometimes used in MEA fabrication where the MEA including the membrane and catalyst layers and sometimes GDLs are compressed together for a period of time at a desired temperature.
Hot pressing is used to decrease the interfacial resistance and increase adhesion between layers and can help ‘melt’ layers together to prevent delamination. Example times, temperatures, and pressures are given below:
The temperature of the hot press is typically selected so that it is above the glass transition temperature of the polymer electrolyte, but below the temperature where any materials of the MEA become structurally or chemically damaged. The glass transition temperature is the temperature above which the polymer-electrolyte becomes soft, which may allow for the polymer-electrolyte at layer interfaces to deform and form a better contact with lower ionic transport resistance and better adhesion.
Hydration: soaking the MEA in water or aqueous solutions to wet the polymer-electrolytes prior to cell assembly
Boil Nafion or other polymer electrolyte MEA. This permanently changes the macrostructure of the polymer electrolyte and increases the amount of water in the polymer matrix. This increases ionic conductivity, but also increases water transport number.
Heat to dry. This permanently decrease water content and can reduce the amount of water transported through the polymer electrolyte during operation. Example times and temperatures for heating various MEAs are below.
Water and CO2 formed at the interface of an anion-conducting layer (e.g., a cathode buffer layer) and a cation-conducting membrane (e.g., a PEM) can cause the two layers to separate or delaminate where the polymer layers connect. The reaction at the bipolar interface is depicted in
In addition, it is desirable for the CO2 to return to the cathode of the cell where it can be reduced instead of lost to the anode, so a pathway (e.g., pores) in an anion-exchange layer (e.g., a cathode buffer layer and/or cathode layer) provides both a way to remove water and CO2 from the interface to prevent delamination and return CO2 to the cathode where it can react.
Also depicted in
Various MEA designs contain features that resist delamination and optionally provide a pathway for the reaction products to leave the interface area. In some embodiments, the bipolar interface is flat. But in some designs, the interface is provided with a composition gradient and/or interlocking structures. These are described further below with reference to
Engineering the interface can be used to reduce undesired co-ion leakage through the anion exchange membrane (AEM) and cation exchange membrane (CEM) and improving the mechanical stability of bipolar membrane with better adhesion. Chemical and physical modifications to the interface can be used to achieve these two goals. As described further below, the AEM and CEM layers can be chemically bonded through multiple cross-linking pathways: side chain, backbone, backbone-to-side-chain, and triple cross-linking. In some embodiments, the AEM and CEM layers interpenetrate. This can include one or more of a gradient of anion-exchange and cation-exchange polymers, a mixture of anion-exchange and cation-exchange polymers, and/or protrusions of at least one polymer extending into the other.
There are also different ways to physically modify the interface. Hot-pressing the AEM and CEM close to their respective glass transition temperature can increase the adhesion between the AEM and CEM. In some embodiments, adhesion is improved by increasing the interfacial surface area through electrospinning anion and cation exchange layers. In such embodiments, the anion and cation exchange ionomers to have similar swelling properties to avoid delamination. Adding a small concentration of a third polymer (e.g. PTFE) to the intertwined ionomers could also facilitate water removal from the interface. The surface of both the CEM and AEM can be intentionally roughened through plasma surface treatment, etching, or hot-pressing with a woven or patterned fabric. One or more of these techniques may be used to increase contact between the AEM and CEM.
In some embodiments, the interface includes a gradient. A gradient may be formed, for example, by using two nozzles during spray deposition and adding anion-exchange polymer with the relative amounts of the polymers varied during deposition of the cation-exchange layer. Similarly, cation-exchange polymer may be added during deposition of the anion-exchange layer. Referring for example to
In some embodiments, the layers of the bipolar membrane are melted together. This may be accomplished by choosing an appropriate solvent. For example, Nafion is at least slightly soluble in a water/ethanol mixture. By using that mixture (or another solvent in which the cation-conducting polymer is soluble) as a solvent for the anion-conducting polymer can result in Nafion or other cation-conducting polymer at least slightly dissolvent and melting into the interface. In some embodiments, this results in a thin gradient, e.g., one that extends 0.5-10% into the anion-conducting polymer layer thickness.
In some embodiments, the interface includes a mixture of the polymers.
In some embodiments, the interface includes a third material that physically reinforces the interface. For example,
The third material is generally confined to the interface region 350. While it may be mixed with one or both of the cation conducting polymer and the anion conducting polymer, it does not extend beyond the region 350.
In some embodiments, a third material may be in the form of particulates. The particulates may be in a mixture with other components such as ion-conducting polymers and additives. The particles may of any appropriate shape including spherical, fibrous, tubular, etc. In some embodiments, the particles are nanoparticles. These may or may not be in the form of aggregates. Aggregates include spheroidal aggregates, ellipsoidal aggregates, linear aggregates, branched aggregates, and combinations thereof.
In some embodiments, the particles are or include carbon. Carbon particles include single-walled or multiwall carbon nanotubes, fullerenes, spheroid carbon particles, carbon powders, etc.
In some embodiments, the particles are ceramic nanoparticles including inorganic solids made up of oxides, carbides, carbonates and phosphates. In a particular example, the nanoparticles may be magnesium oxides. In some embodiments, the particles are metal nanoparticles, such as iridium, gold, platinum or titanium. In some embodiments, the particles are polymeric or lipid-based nanoparticles.
In some embodiments, the nanoparticles may be from about 1 to about 250 nanometers in size, or from about 10 to about 100 nanometers. In some embodiments, the nanoparticles are about 50 nanometers. Size refers to average diameter. Agglomerations of particles may have larger diameters, e.g., from 250 to micrometer scale.
Carbon nanoparticles have large specific surface areas and high electrical conductivity due to their sp2-hybridized carbon atomic structures. In some embodiments, they may be covalently or non-covalently functionalized. Covalent functionalization may be direct or indirect. For example, aromatic molecules such as 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) may be utilized to enhance electrical properties or carbon nanotubes, due to coupling of π electrons between the aromatic molecule and the nanotube.
In some embodiments, the third material is or includes one or more porosity enhancing agents. Any particulate third material may be a porosity enhancing agent, with porosity introduced from packing of agglomerated particles. Porosity enhancing agents are further described above with respect to porosity in various layers of the MEA. Porosity may also be introduced to the interface region by any appropriate method including appropriate spray deposition techniques, the use of porogens, etc. A porosity enhancing agent in the interface region may be different from that in a cathode buffer layer.
In some embodiments, the third material is or includes a voltage damping agent. Porosity enhancing agents as discussed above can act as voltage damping agents.
The high surface area of nanoparticles creates the capacity to absorb a discharge current, such as occurs during a power interruption, a recovery process, an unplanned outage, or equipment shutdown. The AEM/PEM interface is a low surface area interface, so introduction of particulate material at the interface beneficially increases surface area for discharge current absorption. When the discharge current is absorbed, any putative voltage increase from the power interruption can be reduced.
In operation of a carbon oxide reduction reactor, a recovery process may be periodically performed, as discussed further below. During a recovery process, cell selectivity may be increased, yet an undesirable irreversible voltage increase may accompany the desirable increased selectivity. The irreversible voltage increase can cause deleterious membrane degradation. Use of a voltage damping agent material such as carbon nanoparticles at the interface of the AEM and PEM facilitates recovery processes, as it reduces or prevents voltage-induced degradation. Thus, a more aggressive recovery process can be performed.
In some embodiments, the third material is or includes a water splitting catalyst. When an electrochemical cell is turned off, an energy intensive water splitting process may occur at the interface of the anion exchange membrane and the proton exchange membrane, leading to membrane decay. This is a reverse reaction, as during normal operation there is a buildup of water and carbon dioxide at the interface. The water is consumed during the reverse reaction. This can cause higher resistance and heat generation, which can degrade the membranes. When a particulate material such as carbon nanoparticles is utilized at the interface, it may act as a catalyst, facilitating the water splitting process and decreasing membrane decay. Similarly, other catalyst materials can perform the same function. Suitable catalysts may include iridium, gold, platinum, or titanium.
In some embodiments, the third material is a mixture composed of particulates dispersed in polymer. The polymer/nanoparticle mixture may be made by dissolving the polymer in a solvent. The dissolved polymer may then be combined with the nanoparticles and aggressively mixed or sonicated to make a stable and well-dispersed mixture which is spray deposited on a membrane. In some embodiments, the membrane surface may be roughened prior to spray deposition of the polymer/nanoparticle interfacial mixture.
In some embodiments, the polymer is an ion-conducting polymer. Useful ion-conducting polymers include those described in the Polymeric Structures section above. In some embodiments, the polymer has a molecular weight of 100 g/mol or higher. In some embodiments, the ion-conducting polymer is an anion-conducting polymer, a cation conducting polymer, or an anion and cation conducting polymer as described in the table of Ion-Conducting Polymers above. In some embodiments, polymer is a cation-conducting polymer such as a sulfonated tetrafluoroethylene-based fluoropolymer copolymer. Examples include Nafion 115, Nafion 117, Nafion 211, Nafion 324, Nafion 350, Nafion 417, Nafion 424, Nafion 438, Nafion 450, Nafion 521 or Nafion 551. Polymers may be dissolved in a suitable solvent for spray application. In certain embodiments, solvents include ethanol, 1-propanol, 2-propanol, water or a mixture thereof. The concentration of polymer in solvent may range from about 1 to about 20 weight percent.
For example, the third material may be a mixture of a cation-conducting sulfonated tetrafluoroethylene-based fluoropolymer copolymer and carbon nanoparticles. The ratio of polymer to nanoparticle may be from about 10:1 to about 1:0.01; or from about 5:1 to about 2:1 weight percent. In some embodiments the ratio is about 1:1 weight percent.
According to various embodiments, the ion-conducting polymer in the interface region may be the same as that of the cation-conducting polymer layer or the anion-conducting polymer layer. In some embodiments, the ion-conducting polymer in the interface region is different from that of the cation-conducting polymer layer and the anion-conducting polymer layer. In some embodiments, the interface region may include a third material (e.g., carbon nanoparticles) that is embedded in both the anion-conducting polymer layer and the cation-conducting polymer layer.
In some embodiments, the interface region 350 may be about 0.1 to about 20 micrometers thick, about 1 to about 10 micrometers thick or about 1 to about 5 micrometers thick. In some embodiments, the interface is 3 micrometers thick. The ratio of the thickness of the anion-conducting polymer layer to the interface may be about 20:1, about 10:1, or about 2:1.
The present disclosure additionally encompasses a method for mitigating effects of electrical current fluctuation. In some embodiments, the method of mitigating effects of electrical current fluctuation on bipolar membrane electrochemical cells includes providing an electrochemical cell having a bipolar membrane, wherein the bipolar membrane includes an anion-conducting polymer layer and a cation-conducting polymer layer and an interfacial layer between the anion-conducting polymer layer and the cation-conducting polymer layer of the bipolar membrane; and wherein the interfacial layer includes nanoparticles such as carbon nanoparticles; and subjecting the electrochemical cell to electrical current fluctuation, whereby degradation of at least one of the anion-conducting polymer layer and the cation-conducting polymer layer caused by electrical current fluctuation is reduced.
In some embodiments, the electrical current fluctuation results from turning the cell on and off. In some embodiments, the electrical current fluctuation results from a power interruption, a current reversal, or a power outage. In some embodiments, degradation of the anion-conducting polymer layer and the cation-conducting polymer layer caused by electrical current fluctuation is reduced. In some embodiments, the bipolar membrane electrochemical cell's lifetime is extended.
Returning to
In some embodiments, the interface region includes additives to facilitate acid-base reactions and prevent delamination. In some embodiments, the additives may facilitate spreading out the acid base recombination a larger volume instead of just at a 2D interface of the anion conducting polymer and cation conducting polymer. This spreads out water and CO2 formation, heat generation, and may lower the resistance of the membrane by decreasing the barrier to the acid-base reaction. These effects can be advantageous in helping avoid build-up of products, heat, and lowering resistive losses in the MEA leading to a lower cell voltage. Further, it helps avoid degrading materials at the interface due to heat and gas production.
Examples of additives that facilitate acid-base reactions include molecules that are both proton and anion acceptors, such as hydroxide containing ionic liquids with 1-butyl-3-methylimidazolium hydroxide being a specific example. Other ionic liquids may also be used, including those having one of the following ionic groups: N,N,N,N-tetraalkylammonium (e.g., N,N,N,N-tetramethylammonium, N,N-dimethyl-N,N-dipropylammonium, or N-methyl-N,N,N-tri-C1-12 alkylammonium), N,N,N-trialkylammonium-1-yl (e.g., N,N,N-trimethylammonium-1-yl, N-methyl-N,N-dipropylammonium-1-yl, or N,N,N-tri-C1-12 alkylammonium-1-yl), N,N,N-trialkyl-N-alkoxyalkylammonium (e.g., N,N,N-trimethyl-N-alkoxyalkylammonium, N-methyl-N,N-diethyl-N-methoxyethylammonium, or N,N,N-tri-C1-12 alkyl-N—C1-6 alkoxy-C1-6 alkylammonium), N,N-dialkyl-N-alkoxyalkylammonium-1-yl (e.g., N,N-dimethyl-N-alkoxyalkylammonium-1-yl or N,N-di-C1-12 alkyl-N—C1-6 alkoxy-C1-6 alkylammonium-1-yl), N,N-dialkylpyrrolidinium (e.g., N,N-dimethylpyrrolidinium, N-methyl-N-ethylpyrrolidinium, or N-methyl-N—C1-12 alkylpyrrolidinium), N-alkylpyrrolidinium-1-yl (e.g., N-methylpyrrolidinium-1-yl or N—C1-12 alkylpyrrolidinium-1-yl), N,N-dialkylpiperidinium (e.g., N,N-dimethylpiperidinium, N-methyl-N-ethylpiperidinium, or N-methyl-N—C1-12 alkylpiperidinium), N-alkylpiperidinium-1-yl (e.g., N-methylpiperidinium-1-yl or N—C1-12 alkylpiperidinium-1-yl), N,N,4-trialkylpiperidinium (e.g., N,N,4-trimethylpiperidinium, N,4-dimethyl-N-ethylpiperidinium, or N-methyl-N,4-di-C1-12 alkylpiperidinium), N,4-dialkylpiperidinium-1-yl (e.g., N,4-dimethylpiperidinium-1-yl or N,4-di-C1-12 alkylpiperidinium-1-yl), N,N,3,5-tetraalkylpiperidinium (e.g., N,N,3,5-tetramethylpiperidinium, N,3,5-trimethyl-N-ethylpiperidinium, or N-methyl-N,3,5-tri-C1-12 alkylpiperidinium), N,3,5-trialkylpiperidinium-1-yl (e.g., N,3,5-trimethylpiperidinium-1-yl or N,3,5-tri-C1-12 alkylpiperidinium-1-yl), N,N,2,6-tetraalkylpiperidinium (e.g., N,N,2,6-tetramethylpiperidinium, N,2,6-trimethyl-N-ethylpiperidinium, or N-methyl-N,2,6-tri-C1-12 alkylpiperidinium), N,2,6-trialkylpiperidinium-1-yl (e.g., N,2,6-trimethylpiperidinium-1-yl or N,2,6-tri-C1-12 alkylpiperidinium-1-yl), N,N-dialkylazepanium (e.g., N,N-dimethylazepanium, N-methyl-N-ethylazepanium, or N-methyl-N—C1-12 alkylazepanium), N-alkylazepanium-1-yl (e.g., N-methylazepanium-1-yl or N—C1-12 alkylazepanium-1-yl), N,N-dialkylmorpholinium (e.g., N,N-dimethylmorpholinium, N-methyl-N-ethylmorpholinium, or N-methyl-N—C1-12 alkylmorpholinium), N-alkylmorpholinium-4-yl (e.g., N-methylmorpholinium-4-yl or N—C1-12 alkylmorpholinium-4-yl), N1,N3-dialkylimidazolium (e.g., N1,N3-dimethylimidazolium, N1-ethyl-N3-methylimidazolium, or N1-C1-12 alkyl-N3-methyl-imidazolium), N3-alkylimidazolium-1-yl (e.g., N3-methylpiperidinium-1-yl or N3-C1-12 alkylpiperidinium-1-yl), 1-alkyl-1-azabicyclo[2.2.2]octane (e.g., 1-methyl-1-azabicyclo[2.2.2]octane or 1-C1-12 alkyl-1-azabicyclo[2.2.2]octane), or 1-azoniabicyclo[2.2.2]octan-1-yl, in which each of these can be optionally substituted (e.g., substituted on a ring with one or more alkyl and/or substituted on an alkyl with one or more heteroatoms).
In some embodiments, an ionomer different from that of the anion-conductive polymer layer and the cation-conductive polymer layer may be used. For example, a relatively high conductivity anion-exchange material such as Sustanion may be used. Such anion-exchange material may not be selective enough to use as a cathode buffer layer but can be used at the interface.
In particular examples, an ionomer may be used at the interface that has a higher ion exchange capacity than at least one of the ionomers of the bipolar membrane. Such an ionomer may not be suitable for the layers of the bipolar membrane, for example, due to propensity to swelling or lack of stability, but can be added at the interface. In particular examples, an ionomer that improves adhesion and physical contact may be used. A polymer at the interface that goes into both layers may be used to improve adhesion. An ionomer at the interface can itself have multiple sublayers. In one example, a third ionomer may have center region having higher void space disposed between denser regions.
In some embodiments, an ionomer used at the interface is an anion exchange ionomer that is different from the anion-conducting polymer of the anion conducting polymer layer and may be referred to as an interface AEM to distinguish it from the bulk AEM of the anion conducting polymer layer. In some such embodiments, the interface AEM has lower water uptake than the anion-conducting polymer layer to match the water uptake of PFSA or other cation-conducting polymer. This can help prevent delamination at the interface while maintaining a higher ion exchange capacity (IEC). Both the higher IEC and lower water uptake of the interface ionomer may help minimize cation crossover from the anode side. Lower water uptake can result from smaller ion conduction channels in the interface ionomer than in the anion-conducting polymer of the bipolar membrane. Higher IEC can result from a higher concentration of cation functional groups on the interface ionomer. One or both of these characteristics may be present in the interface ionomer and can restrict cations from the cathode.
In particular embodiments, when forward bias is applied across a bipolar membrane, ion recombination occurs at the interface to form products such as water. An interfacial layer should be mechanically robust during ion recombination (i.e., exhibit good adhesion between AEM and CEM of the bipolar membrane) while minimizing undesired co-ion leakage through the AEM and CEM. In some embodiments, an interface AEM has a thickness of 0.1%-10% of the bulk AEM thickness, with examples of bulk AEM thickness being between 5-80 μm. The interface AEM1 and 90 percent by volume may be kept relatively low to avoid additional ohmic resistances across the bipolar membrane. The water uptake of the interface AEM can be between 0%-25% to circumvent membrane delamination due to a mismatch of swelling properties between the adjacent AEM and CEM. In some interface AEM can have an ion exchange capacity (IEC) in the range of 2.5-3.0 mmol/g. In some such embodiments, the IEC of the bulk AEM is lower than that of the interface AEM and may be 1.5-2.5 mmol/g. A high density of positively-charged functional groups (i.e. high IEC) at the interface serves to electrostatically repel undesired co-ion (e.g. H+ or K+) transport to the bulk AEM via the Donnan exclusion effect.
Additional examples of materials that may be present at the interface include block copolymers having different charged groups (e.g., both cation and anion stationary charge groups), cation-and-anion conducting polymers, resin material, ion donors such as oxides including graphene oxide, catalysts for acid/base recombination, catalysts that react H2 and O2 diffusing from the anode and cathode, water splitting catalysts, CO2 absorbing material, and H2 absorbing material.
In some embodiments, the anion-conducting polymer and the cation-conducting polymer of the bipolar membrane have the same backbone, with different stationary charge groups. As an example, Orion ionomers may be used with different stationary charge groups. The ionomers are more compatible and less apt to delaminate.
In the examples above, the interface 313 may be a three-dimensional volume having thickness that is between 1% and 90% of the overall thickness of the bipolar membrane, or between 5% and 90%, or between 10% and 80%, or between 20% and 70%, or between 30% and 60% of the overall thickness of the bipolar membrane. In some embodiments, it less than half the overall thickness, including between 0.1% and 30%, 5% and 25%, 5% and 20%, 5% and 15% or 0.5% to 5%.
Any of the bipolar interfaces described above may be hot pressed. Particularly between the anion-exchange and cation exchange membrane layers, hot pressing can soften the polymer electrolytes and allow them to meld together.
In some embodiments, the bipolar AEM/PEM interface includes a relatively smooth PEM layer in contact with a rougher AEM layer. For example, a PEM arithmetic mean height (Sa) in such embodiments can range from near 0 to 0.2 μm. The AEM layer in contact with the PEM layer can have higher roughness, and in some embodiments have an Sa in the 0.2 to 0.5 μm range, in the 0.4 to 1.5 μm range, or in the 0.6 to 1 μm range. The roughness of the AEM with the PEM can create a discontinuous interface. The Sa of the AEM layer in contact with the PEM can be lowered to near 0 to 0.2 μm or near 0 to 1 μm through changes to fabrication parameters, such as treatment with solvent that partially dissolves the polymer electrolyte before evaporating to leave behind a smoother surface or hot pressing. The AEM layer may be substantially continuous and non-porous, or it may contain pores with typical porosity ranges can be 0.1 to 90%, 1-20%, and 5-15% that allow for gas and/or water movement.
In another embodiment, the surface of the PEM membrane may be roughened to an Sa of 5 to 10 μm, 1 to 5 μm, 0.2 to 1 μm, or 0.4 to 0.6 μm. In some such embodiments, the AEM layer in contact with the PEM membrane may have Sa near 0 to 1 μm or near 0 to 0.2 μm, or it may be rougher, with a Sa in the 2 to 5 μm range, in the 0.4 to 1.5 μm range, or in the 0.6 to 1.0 μm range. The AEM may be substantially continuous and non-porous, or it may contain pores with typical porosity ranges of 0.1 to 90%, 1-20%, and 5-15% that allow for gas and/or water movement.
In some embodiments, a cross-linker may be added to covalently cross-link the two polymers of the bipolar membrane. A crosslinker can be used at an interface between ion-conducting polymer layers. Each layer can include one or more polymers, in which each polymer can be characterized by a backbone and a side chain attached to the backbone. A crosslinking reaction can occur at the interface, as well as between a crosslinker and (i) two or more side chains, (ii) two or more backbones, or (iii) a combination of two or more side chain(s) and backbone(s).
The crosslinker can be bivalent, trivalent, tetravalent, or other higher valency. In this way, the crosslinker can react with any number of reactive groups present at the interface within the cation-conducting, anion-conducting, or mixed cation-and-anion-conducting polymer layers. In some embodiments, the crosslinker includes:
Akx]L3 or Ar
x]L3 Ar
L-x]L3,
in which Ak is an optionally substituted aliphatic, alkylene, cycloaliphatic, or cycloalkylene; Ar is an optionally substituted aromatic, arylene, heteroaromatic, or heteroarylene; L is a linking moiety (e.g., any herein); L3 is an integer that is 2 or more; and X is halo, hydroxyl, optionally substituted amino (e.g., NRN1RN2, in which each of RN1 and RN2 is, independently, H or optionally substituted alkyl), carboxyl, acyl halide (e.g., —C(O)—R, in which R is halo), carboxyaldehyde (e.g., —C(O)H), or optionally substituted alkyl.
Non-limiting crosslinkers can include terephthalaldehyde, glutaraldehyde, ortho-xylene, para-xylene, meta-xylene, or a multivalent amine, such as diamine, triamine, tetraamine, pentaamine, etc., including 1,6-diaminohexane (hexanediamine, DHA), N,N′-dimethyl-1,6-hexanediamine, N,N,N′,N′-tetramethyl-1,6-hexanediamine (TMHDA), 1,3-diaminopropane, N,N′-dimethyl-1,3-propanediamine, N,N,N′,N′-tetramethyl-1,3-propanediamine, 1,4-diaminobutane, N,N′-dimethyl-1,4-butanediamine, N,N,N′,N′-tetramethyl-1,4-butanediamine, 1,8-diaminooctane, N,N′-dimethyl-1,8-octanediamine, N,N,N′,N′-tetramethyl-1,8-octanediamine, propane-1,2,3-triamine, [1,1′:3′,1″-terphenyl]-4,4″,5′-triamine, 1,3,5-triazine-2,4,6-triamine (melamine), and others.
In some embodiments, a crosslinker is used for crosslinking between side chain groups of the first and second polymer layers. The side chain group can include a reactive group that is either present within the material or installed in any useful manner.
For instance, if a polymer layer includes an ionic or ionizable side chain group (e.g., —SO2OH, —CO2H, etc.), then this group can be converted to provide a reactive group (e.g., a halo or a leaving group). In one non-limiting embodiment, as seen in step (i) of Scheme I above, the first polymer (1) includes an ionic side chain group (—SO2OH), which is converted to a reactive group (—SO2Cl in (2)) by use of thionyl chloride. The second polymer, in turn, can also include a reactive side chain group(s) (e.g., a halide, such as —Br, haloalkyl, or another leaving group), as in (4). By using a crosslinker, crosslinks are formed between reactive groups. As seen in step (ii) of Scheme I above, the reactive groups in the first polymer (2) and the second polymer (4) are reacted with a crosslinker that is a multivalent amine (3). In this way, crosslinks (5) are formed at the interface and between the side chain groups. In one instance, the polymer layers can be crosslinked based on the formation of two or more covalent bonds (e.g., an N—S covalent bond, N—C covalent bond, or C—C covalent bond).
In other embodiments, a crosslinker forms crosslinks between backbone(s) of the first and second polymer layers.
In one non-limiting embodiment, as seen in Scheme II above, the first polymer (6) and the second polymer (8) include an aryl backbone. Then, a crosslinker is used to react with backbone groups. If the crosslinker is a multivalent hydroxyalkyl as in (7), then the two polymer layers can be crosslinked by way of a be an acid-catalyzed in the presence of a proton source, such as an organic acid (e.g., trifluoromethanesulfonic acid, sulfuric acid, methanesulfonic acid, trifluoroacetic acid, p-toluenesulfonic acid, etc.). In one instance, the hydroxyalkyl crosslinker can be a tertiary alcohol, which is protonated by a Bronsted acid, lose water as a byproduct, and form a tertiary carbocation intermediate. This intermediate, in turn, can readily react with the π electrons of the aromatic backbone based on electrophilic substitution. In this method, aromatic backbone from the polymer layers could be grafted with a crosslinker to from a high-density polymer matrix.
In yet other embodiments, a crosslinker is used for crosslinking between side chain group(s) of the first polymer layer and backbone(s) of the second polymer layer. For instance, the side chain group can be converted into a nucleophile, and the backbone can include an electrophile.
In one non-limiting embodiment, as seen in Scheme III above, the first polymer (1) includes an ionic side chain group (—SO2OH), which is converted to a reactive group (—SO2Cl in (2)) by use of thionyl chloride and then aminated to provide a reactive nucleophilic group (e.g., a sulfonamide, such as —SO2NRN1RN2 in (10)). The second polymer (8) can include an aryl backbone, which can be reacted with a multivalent crosslinker. For instance, the crosslinker (11) can be a hydroxyhaloalkyl, which can react by way of an acid-catalyzed Friedel-Crafts alkylation reaction to provide an alkylated polymer (12). Finally, the first polymer (10) having a nucleophilic group can be reacted with the second polymer (12) having an electrophilic group to provide a crosslinked polymer (13). Alternative chemistries, reactive groups, electrophiles, and nucleophiles can be used to provide reactive pairs in the first and second polymers that can react at the interface.
In certain embodiments, a polymer electrolyte membrane and an adjoining cathode buffer layer or other anion-conducting polymer layer may have relative thickness that facilitate the fabrication and/or operating performance of an MEA.
Partial layers of an MEA may include an anion-conducting polymer layer (AEM), which may be a cathode buffer layer, and a polymer electrolyte membrane (PEM), which may be cation-conducting polymer layer (e.g., a proton exchange polymer layer) or an anion-conducting polymer layer. In some embodiments, the PEM is relatively thicker than the anion-conducting polymer layer. For example, the PEM may be 120 micrometers compared with about 10-30 or 10-20 micrometers thick for the AEM. The PEM may provide mechanical stability to the AEM.
In some cases, anion-conducting polymers are substantially less conductive than cation-conducting polymers. Therefore, to provide the benefits of a cathode buffer layer (e.g., anion-conducting polymer layer) without substantially increasing the overall resistance of the MEA, a relatively thin cathode buffer is used. However, when a cathode buffer layer becomes too thin, it becomes difficult to handle during fabrication of the MEA and in other contexts. Therefore, in certain embodiments, a thin cathode buffer layer is fabricated on top of a relatively thicker PEM layer such as a cation-conducting polymer layer. The anion-conducting polymer layer may be fabricated on the PEM layer using, for example, any of the fabrication techniques described elsewhere herein.
In various embodiments, the polymer electrolyte membrane layer is between about 20 and 200 micrometers thick. In some embodiments, the polymer electrolyte membrane layer is between about 60 and 120 micrometers thick. In some embodiments, a thin polymer electrolyte membrane layer is used, being between about 20 and 60 micrometers thick. In some embodiments, a relatively thick polymer electrolyte layer is used, between about 120 and 200 micrometers thick.
In some embodiments, a thinner cathode buffer layer is used with a thinner polymer electrolyte membrane. This can facilitate movement of the CO2 formed at the interface back to cathode, rather than to the anode. In some embodiments, a thicker cathode buffer layer is used with a thicker polymer electrolyte membrane. This can result in reducing cell voltage in some embodiments.
Factors that can influence the thickness of a cathode buffer layer include the ion selectivity of the anion-conducting polymer, the porosity of the anion-conducting polymer, the conformality of the anion-conducting polymer coating the polymer electrolyte membrane.
Many anion-conducting polymers are in the range of 95% selective for anions, with about 5% of the current being cations. Higher selectivity anion-conducting polymers, with greater than 99% selectivity for anions can allow for a reduction in a significant reduction in thickness while providing a sufficient buffer.
Mechanical strength of an anion-conducting layer can also influence its thickness, with mechanical stable layers enabling thinner layers. Reducing porosity of an anion-conducting polymer may reduce the thickness of the anion-conducting layer.
In some implementations, a cathode buffer layer or other anion-conducting polymer layer that abuts the polymer electrolyte membrane is between about 5 and 50 micrometers, 5 and 40 micrometers, 5 and 30 micrometers, 10 and 25 micrometers, or 10 and 20 micrometers thick. Using a >99% selective polymer can allow the cathode buffer layer to be reduced to between 2 and 10 microns in some embodiments.
In some cases, the ratio of thicknesses of the polymer electrolyte membrane and the adjoining anion-conducting polymer layer is between about 3:1-90:1 with the ratios at the higher end used with highly selective anion-conducting polymer layers. In some embodiments, the ratio is 2:1-13:1, 3:1-13.1, or 7:1-13.1.
In certain embodiments, a relatively thinner PEM improves some aspects of the MEA's performance. Referring to
In some embodiments, a thin PEM may have a thickness of 10 micrometers to 50 micrometers, 30 micrometers to 50 micrometers, or 25 micrometers to 35 micrometers. In some such embodiments, the AEM may have a similar thickness to the PEM, such as 5 micrometers to 50 micrometers, 5 micrometers to 30 micrometers, or 10 micrometers to 20 micrometers. The ratio of PEM:AEM thicknesses could be 1:2 to 1:1 when PEMs with thicknesses of 10-30 micrometers are used, 1:2 to 2:1 when PEM thickness is 30-50 micrometers, or 1:1 to 3:1 when PEM thickness is 20-35 micrometers. As described further below, AEMs in these thickness ranges may be useful for water management.
Commercially available anion exchange membranes and cation exchange membranes typically have known thicknesses. For example, Nafion® membranes have the following dry thicknesses:
Such known thickness can be used to determine thickness ratios. For example, if an AEM has a thickness between approximately 200 nm and 100 μm, between 300 nm and 75 μm, between 500 nm and 50 μm as described above in the discussion of the cathode buffer layer, the PEM:AEM thickness ratio can be determined as follows:
An AEM may have a thickness that aids in water management, as discussed further below.
As described above, one of the key challenges in a COx electrolyzer is managing water in the cathode due to the need to have water present to hydrate the polymer-electrolyte and/or participate in the COx reduction reaction but not so much water that it blocks the transport of COx to the cathode catalyst. Water can transport predominantly by two methods in a polymer electrolyte system: by electro-osmotic drag and by diffusion. Through diffusion, water will move from areas of high concentrations to low concentrations, the rate of water transport depends on the diffusion coefficient that is an inherent property of the polymer electrolyte material. Electro-osmotic drag is the movement of water molecules with ions as they travel through the polymer-electrolyte. Water in a cation exchange membrane system will transport along with the movement of cations from the anode to the cathode, whereas water moves in the opposite direction with anions in an anion exchange membrane system.
With a bipolar membrane (including a cation exchange membrane and an anion exchange membrane), the net movement of water from the anode to the cathode can be managed by changing the thickness of the anion-exchange and cation-exchange polymer electrolyte layers and/or their material properties.
In some embodiments, an AEM may have a thickness between 5 and 80 micrometers, 5 and 50 micrometers, 5 and 40 micrometers, or 5 and 30 micrometers. As described below, relatively thick AEMs can aid in water management and in preventing delamination, which prolongs lifetime. However, the thickness also contributes to higher voltages and lower efficiencies. Thus, in some embodiments, the AEM may be no more than 50 microns thick.
The tables below show net water transported from the anode to the cathode of the COx electrolyzer per ionic charge moved through the polymer-electrolyte when the thickness of the anion-exchange polymer electrolyte layer and cation-exchange membrane thickness are varied. When the anion-exchange polymer-electrolyte layer thickness increases, the net movement of water from the anode to the cathode decreases. Increasing the molecular weight of the anion-exchange polymer, which reduces the diffusion coefficient of water through the anion-exchange layer has a similar effect of decreasing the net movement of water per ionic charge from the anode to the cathode of the device.
Nafion 115 (PFSA cation exchange membrane thickness 127 microns)
Nafion 212 (PFSA cation exchange membrane thickness 50.8 microns)
Thus, in some embodiments, the ratio of the cation exchange membrane thickness:anion exchange membrane thickness (i.e., the PEM:AEM ratio) in a bipolar MEA is no more than 7:1, 5:1, 3:1, 2:1, 1.5:1, 1:1, or 1:1.5.
Nafion 115 (PFSA cation exchange membrane thickness 127 microns) with anion-exchange polymer electrolyte layers with different molecular weights.
Thus, in some embodiments, the molecular weight of the anion exchange polymer electrolyte may be at least 50 kg/mol, at least 60 kg/mol, at least 70 kg/mol, at least 80 kg/mol, or at least 90 kg/mol.
In some embodiments, the AEM polymer may be crosslinked to decrease water movement from the anode to the cathode.
As depicted, the cathode subsystem includes a carbon oxide source 509 configured to provide a feed stream of carbon oxide to the cathode of reduction reactor 503, which, during operation, may generate an output stream that includes product(s) of a reduction reaction at the cathode. The product stream may also include unreacted carbon oxide and/or hydrogen. See 508.
The carbon oxide source 509 is coupled to a carbon oxide flow controller 513 configured to control the volumetric or mass flow rate of carbon oxide to reduction reactor 503. One or more other components may be disposed on a flow path from flow carbon oxide source 509 to the cathode of reduction reactor 503. For example, an optional humidifier 504 may be provided on the path and configured to humidify the carbon oxide feed stream. Humidified carbon oxide may moisten one or more polymer layers of an MEA and thereby avoid drying such layers. Another component that may be disposed on the flow path is a purge gas inlet coupled to a purge gas source 517. In certain embodiments, purge gas source 517 is configured to provide purge gas during periods when current is paused to the cell(s) of reduction reactor 503. In some implementations, flowing a purge gas over an MEA cathode facilitates recovery of catalyst activity and/or selectivity. Examples of purge gases include carbon dioxide, carbon monoxide, hydrogen, nitrogen, argon, helium, oxygen, and mixtures of any two or more of these.
In various embodiments, a CO2 purifier (not shown in
During operation, the output stream from the cathode flows via a conduit 507 that connects to a backpressure controller 515 configured to maintain pressure at the cathode side of the cell within a defined range (e.g., about 50 to 800 psig, depending on the system configuration). The output stream may provide the reduction products 508 to one or more components (not shown) for separation and/or concentration.
In certain embodiments, the cathode subsystem is configured to controllably recycle unreacted carbon oxide from the outlet stream back to the cathode of reduction reactor 503. In some implementations, the output stream is processed to remove reduction product(s) and/or hydrogen before recycling the carbon oxide. Depending upon the MEA configuration and operating parameters, the reduction product(s) may be carbon monoxide, hydrogen, hydrocarbons such as methane and/or ethylene, oxygen-containing organic compounds such as formic acid, acetic acid, and any combinations thereof. In certain embodiments, one or more components, not shown, for removing water from the product stream are disposed downstream form the cathode outlet. Examples of such components include a phase separator configured to remove liquid water from the product gas stream and/or a condenser configured to cool the product stream gas and thereby provide a dry gas to, e.g., a downstream process when needed. In some implementations, recycled carbon oxide may mix with fresh carbon oxide from source 509 upstream of the cathode. Not shown in
As depicted in
During operation, the anode subsystem may provide water or other reactant to the anode of reactor 503, where it at least partially reacts to produce an oxidation product such as oxygen. The product along with unreacted anode feed material is provided in a reduction reactor outlet stream. Not shown in
Other control features may be included in system 500. For example, a temperature controller may be configured to heat and/or cool the carbon oxide reduction reactor 503 at appropriate points during its operation. In the depicted embodiment, a temperature controller 505 is configured to heat and/or cool anode water provided to the anode water recirculation loop. For example, the temperature controller 505 may include or be coupled to a heater and/or cooler that may heat or cool water in anode water reservoir 519 and/or water in reservoir 521. In some embodiments, system 500 includes a temperature controller configured to directly heat and/or cool a component other than an anode water component. Examples of such other components in the cell or stack and the carbon oxide flowing to the cathode.
In certain embodiments, system 500 is configured to adjust the flow rate of carbon oxide to the cathode and/or the flow rate of anode feed material to the anode of reactor 503. Components that may be controlled for this purpose may include carbon oxide flow controller 513 and anode water controller 511.
Certain components of system 500 may operate to control the composition of the carbon oxide feed stream and/or the anode feed stream. For example, water reservoir 521 and/or anode water additives source 523 may be controlled to adjust the composition of the anode feed stream. In some cases, additives source 523 may be configured to adjust the concentration of one or more solutes such as one or more salts in an aqueous anode feed stream.
In some cases, a temperature controller such controller 505 is configured to adjust the temperature of one or more components of system 500 based on a phase of operation. For example, the temperature of cell 503 may be increased or decreased during break-in, a current pause in normal operation, and/or storage.
In some embodiments, a carbon oxide electrolytic reduction system is configured to facilitate removal of a reduction cell from other system components. This may be useful with the cell needs to be removed for storage, maintenance, refurbishment, etc. In the depicted embodiments, isolation valves 525a and 525b are configured to block fluidic communication of cell 503 to a source of carbon oxide to the cathode and backpressure controller 515, respectively. Additionally, isolation valves 525c and 525d are configured to block fluidic communication of cell 503 to anode water inlet and outlet, respectively.
The carbon oxide reduction reactor 503 may also operate under the control of one or more electrical power sources and associated controllers. See, block 533. Electrical power source and controller 533 may be programmed or otherwise configured to control current supplied to and/or to control voltage applied to the electrodes in reduction reactor 503. Any of the current profiles described herein may be programmed into power source and controller 533.
In certain embodiments, electric power source and controller 533 performs some but not all the operations necessary to implement control profiles of the carbon oxide reduction reactor 503. A system operator or other responsible individual may act in conjunction with electrical power source and controller 533 to fully define the schedules and/or profiles of current applied to reduction reactor 503. In certain embodiments, electric power source and controller 533 controls operation of a carbon oxide purifier disposed upstream of carbon oxide source 509.
In certain embodiments, the electrical power source and controller acts in concert with one or more other controllers or control mechanisms associated with other components of system 500. For example, electrical power source and controller 533 may act in concert with controllers for controlling the purification of carbon oxide, the delivery of carbon oxide to the cathode, the delivery of anode water to the anode, the addition of pure water or additives to the anode water, and any combination of these features. In some implementations, one or more controllers are configured to control or operate in concert to control any combination of the following functions: applying current and/or voltage to reduction cell 503, controlling backpressure (e.g., via backpressure controller 515), supplying purge gas (e.g., using purge gas component 517), delivering carbon oxide (e.g., via carbon oxide flow controller 513), humidifying carbon oxide in a cathode feed stream (e.g., via humidifier 504), flow of anode water to and/or from the anode (e.g., via anode water flow controller 511), and anode water composition (e.g., via anode water source 505, pure water reservoir 521, and/or anode water additives component 523).
In the depicted embodiment, a voltage monitoring system 534 is employed to determine the voltage across an anode and cathode of an MEA cell or across any two electrodes of a cell stack, e.g., determining the voltage across all cells in a multi-cell stack. In certain embodiments, voltage monitoring system 534 is configured to work in concert with power supply 533 to cause reduction cell 503 to remain within a specified voltage range. If, for example the cell's voltage deviates from a defined range (as determined by voltage monitoring system 534), power supply may be configured to apply current or voltage to the electrodes to maintain the cell voltage within the specified range.
An electrolytic carbon oxide reduction system such as that depicted in
Among the various functions that may be controlled by one or more controllers are: applying current and/or voltage to a carbon oxide reduction cell, controlling backpressure on an outlet from a cathode on such cell, supplying purge gas to a cathode inlet, delivering carbon oxide to the cathode inlet, humidifying carbon oxide in a cathode feed stream, flowing anode water to and/or from the anode, and controller anode feed composition. Any one or more of these functions may have a dedicated controller for controlling its function alone. Any two or more of these functions may share a controller. In some embodiments, a hierarchy of controllers is employed, with at least one master controller providing instructions to two or more component controllers. For example, a system may comprise a master controller configured to provide high level control instructions to (i) a power supply to a carbon oxide reduction cell, (ii) a cathode feed stream flow controller, and (iii) an anode feed stream flow controller. For example, a programmable logic controller (PLC) may be used to control individual components of the system.
A controller may be integrated with electronics for controlling operation the electrolytic cell before, during, and after reducing a carbon oxide. The controller may control various components or subparts of one or multiple electrolytic carbon oxide reduction systems. The controller, depending on the processing requirements and/or the type of system, may be programmed to control any of the processes disclosed herein, such as delivery of gases, temperature settings (e.g., heating and/or cooling), pressure settings, power settings (e.g., electrical voltage and/or current delivered to electrodes of an MEA cell), liquid flow rate settings, fluid delivery settings, and dosing of purified water and/or salt solution. These controlled processes may be connected to or interfaced with one or more systems that work in concert with the electrolytic carbon oxide reduction system.
A controller may include any number of processors and/or memory devices. The controller may contain control logic such software or firmware and/or may execute instructions provided from another source. In various embodiments, a controller comprises electronics having various integrated circuits, logic, memory, and/or software that receive instructions, issue instructions, control operations described herein. The integrated circuits may include chips in the form of firmware that store program instructions, digital signal processors (DSPs), chips defined as application specific integrated circuits (ASICs), and/or one or more microprocessors, or microcontrollers that execute program instructions (e.g., software). Program instructions may be instructions communicated to the controller in the form of various individual settings (or program files), defining operational parameters for carrying out a process on one or more components of an electrolytic carbon oxide reduction system. The operational parameters may, in some embodiments, be part of a recipe defined by process engineers to accomplish one or more processing steps during generation of a particular reduction product such as carbon monoxide, hydrocarbons, and/or other organic compounds.
The controller, in some implementations, may be a part of or coupled to a computer that is integrated with, coupled to the system, otherwise networked to the system, or a combination thereof. For example, the controller may utilize instructions stored remotely (e.g., in the “cloud”) and/or execute remotely. The computer may enable remote access to the system to monitor current progress of electrolysis operations, examine a history of past electrolysis operations, examine trends or performance metrics from a plurality of electrolysis operations, to change parameters of current processing, to set processing steps to follow a current processing, or to start a new process. In some examples, a remote computer (e.g., a server) can provide process recipes to a system over a network, which may include a local network or the internet. The remote computer may include a user interface that enables entry or programming of parameters and/or settings, which are then communicated to the system from the remote computer. In some examples, the controller receives instructions in the form of data, which specify parameters for each of the processing steps to be performed during one or more operations.
The controller may be distributed, such as by comprising one or more discrete controllers that are networked together and working towards a common purpose, such as applying current to an MEA cell and other process controls described herein. An example of a distributed control system for such purposes includes one or more processors on a system for electrolytically reducing a carbon oxide and one or more processors located remotely (such as at the platform level or as part of a remote computer) that combine to control a process.
Controllers and any of various associated computational elements including processors, memory, instructions, routines, models, or other components are sometimes described or claimed as “configured to” perform a task or tasks. In such contexts, the phrase “configured to” is used to denote structure by indicating that the component includes structure (e.g., stored instructions, circuitry, etc.) that performs a task or tasks during operation. As such, a controller and/or associated component can be said to be configured to perform the task even when the specified component is not necessarily currently operational (e.g., is not on).
Controllers and other components that are “configured to” perform an operation may be implemented as hardware—for example, circuits, memory storing program instructions executable to implement the operation, etc. Additionally, controllers and other components “configured to” perform an operation may be implemented as hardware that is manipulated by software and/or firmware (e.g., an FPGA or a general-purpose processor executing software) to operate in manner that is capable of performing the recited task(s). Additionally, “configured to” can refer to one or more memories or memory elements storing computer executable instructions for performing the recited task(s). Such memory elements may include memory on a computer chip having processing logic.
Non-computation elements such as reactors such electrolyzers, membrane assemblies, layers, and catalyst particles may also be “configured” to perform certain functions. In such contexts, the phrase “configured to” indicate that the referenced structure has one or more features that allow the function to be performed. Examples of such features include physical and/or chemical properties such as dimensions, composition, porosity, etc.
As described above, in some embodiments, a bipolar interface including a region having one or more of a particulate third material, a porosity enhancing agent, a voltage damping agent, and/or a catalyst may be particularly advantageous in the context of a recovery process.
In certain embodiments, a sequence of operations includes temporarily deviating from normal operating conditions to flow water or other liquid to the cathode and/or to flow a gas to the cathode under non-standard conditions. It has been found that flowing water to the cathode and/or flowing a gas (e.g., a gas other than the normal carbon oxide reactant) to cathode can facilitate a recovery in performance of a carbon oxide electrolyzer. This alternate sequence of operations is sometimes referred to as a “recovery process” or a “recovery sequence.”
A recovery process may be performed after a carbon oxide electrolyzer has been in service, operating under normal conditions, for a period of time such as a few thousand hours. After a recovery process is completed, an electrolyzer may transition back to normal operation. A recovery process may be performed repeatedly over the service life of an electrolyzer or over the life of one or more of its components such as its associated MEA(s), gas diffusion layer(s) (GDL), and flow field(s). For example, a recovery process may be performed every 1000 to 10,000 hours of service life.
While many embodiments disclosed herein are presented as procedures for recovering lost performance of a carbon oxide electrolyzer, some embodiments pertain to protecting a carbon oxide electrolyzer from the detrimental effects of some unanticipated event such as loss of power to the electrolyzer. A carbon oxide electrolyzer may be placed in a protection mode when an unexpected event is determined to be occurring or likely to occur soon. If unmitigated, such unexpected events could damage the electrolyzer or infrastructure supporting the electrolyzer.
In some implementations, any of the operations, or any combination of such operations, described herein for performing recovery may also be employed for the protection of a carbon oxide electrolyzer.
In some embodiments, an electrolyzer and/or associated control system implements a protection mode by (a) determining that an unexpected and potentially detrimental event is occurring or will likely occur in the future and such unexpected event will, if unmitigated, likely damage or degrade the carbon oxide electrolyzer; and (b) performing one or more protective operations on the carbon oxide electrolyzer that reduce the likelihood that the electrolyzer will be damaged or degraded if the unexpected event continues to occur or does in fact occur in the future.
Examples of unexpected events that may trigger the protective operations include sudden decrease or loss of an input material such as anolyte or carbon oxide (e.g., CO2) gas decrease or loss of heating or cooling, and loss of power to the electrolyzer. A substantial decrease or loss input material may require adjusting the power to electrolyzer to produce open circuit voltage or no current. Loss of power to the electrolyzer may cause the electrolyzer to discharge from operating voltage to an uncontrolled voltage, such as open circuit voltage or zero voltage either rapidly or gradually.
Examples of protective operations to mitigate the impact of the unexpected event include applying a relatively low current density to the electrolyzer, transitioning the electrolyzer voltage to open circuit voltage and reducing or ramping down the current applied to the electrolyzer. Any of these protective operations may be applied for a limited time such as only while the unexpected event continues to occur or until the likelihood of such event occurring is substantially reduced.
In some embodiments, the protective operation reduces electrolyzer current density to a relatively small (in comparison to normal operation) forward current density of about 1-50 mA/cm2 or about 5-25 mA/cm2 (e.g., about 10 mA/cm2), or about 0.3% to 20% of the current density in normal operating conditions.
In some embodiments, the protective operation ramps down current to the electrolyzer. A ramp may have any form or slope. In some cases, the average ramp rate from full current (normal operation) to a final current is about 0.1 to 1 mA/cm2 per minute, or about 1 to 10 mA/cm2 per minute. In some cases, the ramping is stepped. The number of steps, the time duration of the steps, and the magnitude of the current density changes of the steps may vary. As an example, a ramp may have about 2 to 50 steps, or about 5 to 30 steps. As a further example, the duration of the steps may be about 1 to 100 seconds, or about 5 to 50 seconds. As a further example, the current magnitude of the steps may be about 0.1 to 10 mA/cm2 or about 0.5 to 5 mA/cm2.
In one example, a step profile reduces current density to an electrolyzer from a normal operating value (e.g., about 300 mA/cm2 to 2 A/cm) via a sequence of steps, each having a much smaller value (e.g., about 10, 9, 8, 7, 6, 5, 4, 3, 2, 1 mA/cm2) and each having a defined duration (e.g., about 30 seconds each), and then sets the final current output to maintain the electrolyzer at open circuit for about 5-10 minutes.
In some embodiments, an electrolyzer returns from recovery or protective mode to normal operating conditions via a current ramp. Such a return ramp may have any of the characteristics just identified for ramping current down but in the opposite direction, i.e., from low current density to higher current density.
In one example, a recovery process includes the following sequence: pause electrical current to the electrolyzer, then flow water over the cathode, and then restart flow of electrical current to the electrolyzer. In another example, a recovery process includes the following sequence: pause electrical current to the electrolyzer, then flow gas over the cathode, then flow water over the cathode, then again flow gas over the cathode, and finally restart normal operation by flowing electrical current through the cell. Included below are a few further examples of recovery sequences.
In some examples, a recovery operation comprises contacting the cathode with water while no current flows to the cathode. In some implementations, a relatively small amount of current flows while water is present in the cathode. In some cases, this current flows in the reverse direction (anodic at the carbon oxide reduction cathode). As an example, no more than about 1 mA/cm2 current flows to the cathode in the reverse direction while water is present. In some examples, during a portion of the recovery process, water flows over the cathode, rather than quiescently contacting the cathode. Examples of recovery operations involving water flow or water contact include examples 1, 2, 3, and 4.
Example Recovery Sequence 1
Normal operation, optionally including pulsing electrical current or voltage;
Turn off or significantly reduce electrical current;
Introduce water to the cathode by flowing or otherwise contacting the cathode;
Turn on or increase current to normal operating level and reestablish normal operation (e.g., flow reactant gas at normal operating pressure and flow rate).
In some embodiments, a recovery operation comprises stopping the current, flowing water to the cathode, and then drying the cathode. The drying operation is optionally performed with the reactant gas, a modified reactant gas, or a different gas such as an inert gas.
Example Recovery Sequence 2
Normal operation, optionally including pulsing electrical current or voltage;
Turn off or significantly reduce electrical current;
Introduce water to the cathode by flowing or otherwise contacting the cathode;
Dry the cathode (e.g., by flowing gas through the cathode);
Turn on or increase current to normal operating level and reestablish normal operation (e.g., flow reactant gas at normal operating pressure and flow rate).
In some embodiments, a recovery operation comprises flowing water to the cathode while no gas (reactant or other gas) flows to the cathode.
Example Recovery Sequence 3
Normal operation, optionally including pulsing electrical current or voltage;
Turn off or significantly reduce electrical current;
Release backpressure and stop flow of the reactant gas to the cathode;
Introduce water to the cathode;
Dry the cathode (e.g., by flowing gas through the cathode);
Turn on or increase current to normal operating level and reestablish normal operation (e.g., flowing the reactant gas at normal operating pressure and flow rate).
Note that stopping the gas flow and releasing or reducing the backpressure can occur in either order. If the process employs a different gas than the reactant, it may be necessary to first release the gas pressure and then stop the flow.
Example Recovery Sequence 4
Normal operation, optionally including pulsing electrical current or voltage;
Turn off or significantly decrease cell current;
Release backpressure and stop flow of the reactant gas to the cathode;
Flow gas under alternative conditions to cathode;
Stop gas flow;
Introduce water to the cathode by flowing or otherwise contacting the cathode;
Stop contacting the cathode with water;
Flow drying gas to cathode;
Flow reactant gas to cathode;
Turn on or increase current to normal operating level and reestablish normal operation.
In some embodiments, the recovery process comprises stopping the flow of current, followed by flowing a gas, which may be the reactant gas or a gas other than the reactant gas. In a case, where the gas is not the reactant gas, the gas flows for a period of time before restarting flow of the reactant gas and turning the current back on. In some embodiments, the gas other than the reactant gas comprises air, an oxidative gas, an inert gas, a combination thereof, or a modified composition of the reactant gas. In some such embodiments, water is not flowed to the cathode at least part of the time while the gas flows. Examples of recovery operations involving gas flow without water contact for at least part of time gas flows include examples 5, 6, and 7.
Example Recovery Sequence 5
Normal operation, optionally including pulsing electrical current or voltage;
Turn off or significantly reduce electrical current;
Release backpressure of the reactant gas and turn off the reactant gas to the cathode;
Flow a gas other than the reactant gas to the cathode for a period of time;
Reintroduce the reactant gas at normal operating pressure and flow rate;
Turn on or increase current to reestablish normal operation.
In some embodiments, the recovery process comprises stopping the flow of current, followed by flowing the reactant gas at reduced pressure and/or reduced flow rate for a period of time, then increasing the pressure and/or flow rate of the reactant gas to normal operating pressure, and finally turning the current back to normal level. In some such embodiments, water is not flowed to the cathode.
Example Recovery Sequence 6
Normal operation, optionally including pulsing electrical current or voltage;
Turn off or significantly reduce electrical current;
Flow gas (optionally the reactant gas) to the cathode under alternative conditions such as reduced flow rate;
Turn on or increase current to reestablish normal operation.
Example Recovery Sequence 7
Normal operation, optionally including pulsing electrical current or voltage;
Turn off or significantly reduce electrical current;
Reduce pressure of the reactant gas to the cathode for a period of time;
Increase pressure of the reactant gas to normal operating pressure;
Turn on or increase current to reestablish normal operation.
Example Recovery Sequence 8
Normal operation, optionally including pulsing electrical current or voltage;
Ramp down electrical current to the electrolyzer and optionally apply a low reverse current;
Flow gas (optionally the reactant gas) to the cathode under alternative conditions such as reduced flow rate and/or pressure (while applying a low current, which may be the low reverse current);
Ramp up positive electrical current;
Reestablish full normal operating conditions including full positive current and full flow of reactant gas.
Example Recovery Sequence 9
Normal operation, optionally including pulsing electrical current or voltage;
Apply a low reverse current (by ramping or direct transition);
Flow gas (optionally the reactant gas) to the cathode under alternative conditions such as reduced flow rate and/or pressure (while applying a low current, which may be the low reverse current);
Ramp up positive electrical current;
Reestablish full normal operating conditions including full positive current and full flow of reactant gas.
Example Recovery Sequence 10
Normal operation, optionally including pulsing electrical current or voltage;
Adjust power source or associated circuit to short the electrolyzer or otherwise reach a potential below open circuit voltage;
Flow gas (optionally the reactant gas) to the cathode under alternative conditions such as reduced flow rate and/or pressure (while holding the electrolyzer at open circuit voltage or in a short circuit state);
Reestablish full normal operating conditions including normal operating electrolyzer potential and full flow of reactant gas.
Example Protection Sequence 11
Normal operation, optionally including pulsing electrical current or voltage;
Determine likely occurrence of an unexpected, detrimental event;
Transition electrolyzer to a protective mode (examples of protective mode operating conditions include applying a relatively low current density to the electrolyzer, transitioning the electrolyzer voltage to open circuit voltage, reducing or ramping down the current applied to the electrolyzer);
Determine that that detrimental event is no longer a threat;
Reestablish full normal operating conditions.
Example Recovery Sequence 12
Normal operation, optionally including pulsing electrical current or voltage;
Adjust power source or associated circuit to reach and maintain open circuit voltage;
Flow gas (optionally the reactant gas) to the cathode under alternative conditions such as reduced flow rate and/or pressure (while holding the electrolyzer at open circuit voltage or in a short circuit state);
Reestablish full normal operating conditions including normal operating electrolyzer potential and full flow of reactant gas.
Process Parameters Associated with Recovery Processes
Various operations associated with a recovery process are discussed in sequence below.
As mentioned, a carbon oxide electrolyzer may operate normally for a period of time before a recovery sequence is executed. Normal operation may include a set of normal operating conditions as described elsewhere herein. Such conditions may include (a) normal reactant gas flow, which may be characterized by normal levels of a reactant gas pressure and flow rate or flow velocity at the cathode, (b) a reactant gas composition, (c) a set temperature or temperature profile, (d) an electrical current or voltage magnitude, optionally with a non-constant waveform, or (e) any combination thereof. In some embodiments, during normal operation, the electrical current or voltage has a pulsed or paused profile in which the current magnitude at the electrolyzer is periodically temporarily decreased or increased.
Normal operation may comprise converting a carbon oxide in the reactant gas to a carbon-containing product. In some embodiments, the carbon oxide is CO2 and/or CO and the carbon-containing reduction product comprises COx a hydrocarbon, and/or an organic oxygen-containing compound. Typically, during normal operation, liquid (e.g., water) is not introduced to the cathode via the carbon oxide inlet or other source outside the MEA. However, liquid in the form of mist or droplets may, during normal operation, contact the cathode along with the inlet gas.
In various embodiments, an electrolyzer operates normally for a period of time prior to a recovery sequence. For example, an electrolyzer may operate under normal conditions for a period of at least about 100 hours before executing a recovery sequence. In some cases, the period of normal operation lasts for at least about 1000 hours, or at least about 2000 hours, or at least about 5000 hours, or at least about 10,000 hours before executing a recovery sequence. After the recovery sequence, an electrolyzer may return to normal operation for an extended period such as at least about 100 hours. The ensuing period of normal operation may continue uninterrupted by another recovery process or by terminating operation of the electrolyzer. In some implementations, a recovery process is performed periodically during the life of the electrolyzer, but instances of the recovery process are separated by minimum periods of normal operation, such as periods of at least about 100 hours or at least about 500 hours, or at least about 1000 hours.
In certain implementations, the recovery sequence stops the flow of electrical current to the electrolyzer, reduces the magnitude of the current density, or reverses the direction of the current at the cathode. The reduction in current may be significant, which means that it does not detrimentally affect the electrolyzer or any of its components such as a cathode catalyst layer. For example, the current should not corrode or otherwise degrade catalyst components such as metals, carbon support material, or polymers. As an example, the current is reduced by at least about 50%. In some examples, a reduced current density at the cathode has a magnitude of at most about 100 mA/cm2 of planar cathode surface area. This current density may apply when gas and/or water is delivered to the cathode from outside the MEA.
In some cases, the recovery sequence employs, at least temporarily, application of electrical current in the reverse direction (i.e., an anodic current flows at the cathode side of the cell). A small anodic current (at the cathode side) may assist in the recovery of performance at the electrolyzer cathode during a subsequent normal operation. It may refresh the catalyst surface.
In some embodiments, current or voltage ramping is applied to a carbon oxide electrolyzer for either recovery for protection. As discussed above in connection with the discussion of protection mode, a ramp may have any of form or slope. In some cases, the average ramp rate from full current (normal operation) to a final current is about 20 mA/cm2 per minute or less, or about 1 to 10 mA/cm2 per minute, or about 0.5 to 1 mA/cm2 per minute. In some cases, the ramping is stepped. The number of steps, the time duration of the steps, and the magnitude of the current density changes of the steps may vary. As an example, a ramp may have about 2 to 50 steps, or about 5 to 30 steps. As a further example, the duration of the steps may be about 1 to 100 seconds, or about 5 to 50 seconds. As a further example, the current magnitude of the steps may be about 0.1 to 10 mA/cm2 or about 0.5 to 5 mA/cm2.
In certain embodiments, the duration of current stoppage in a recovery sequence is about 5 minutes to days (e.g., 10 days). In certain embodiments, the duration of current stoppage is about 10 minutes to about 300 minutes, or about 15 minutes to 60 minutes.
When changing the current from the normal operation at the beginning of the recovery process, the current may be reduced by a sudden stoppage (a single step change), ramping down, and/or multiple steps.
In certain embodiments in which a reverse current is applied, the duration of reverse current applied to the cathode is about 5 seconds to 60 minutes or about 5 minutes to 60 minutes.
In some embodiments, a power source for powering a carbon oxide electrolyzer is short circuited during recovery or protection mode. A short circuit may occur when the electronic resistance is not large enough in the circuit to impede current flow between the anode and cathode. In such cases, the potential or potentials of the anode and cathode equalize; in other words, the cell voltage is 0 volts. When shorted, the electrolyzer discharges from a normal operating state or from open circuit voltage. During shorting of the electrolyzer, the cell voltage transitions to a level below the open circuit voltage.
The short circuit condition may be held for a prescribed period during recovering or protection mode. In some embodiments, short circuit mode is held for about 30 minutes or less, or about 10 minutes or less, or about 5 minutes or less, or about 1 to 5 minutes. After exiting the short circuit condition, the electrolyzer may return to a normal operating voltage, and optionally to full normal operation.
A short circuit and associated discharge process in which current flows in the reverse direction may provide an oxidation condition on a catalyst (e.g., a gold catalyst) that increases the catalytic effect by, e.g., generating extra active surface area and/or by removing potential impurities or intermediates. A short circuit may also deplete ionic species such as K+, H+, COOH−, HCO3−, CO3(2−), OH−, or any combination of positive and/or negative ions. A short circuit may also cause water electrolysis at an MEA interface (e.g., between a PEM and AEM in a bipolar MEA) and/or at the cathode to provide a drier condition for better carbon oxide (e.g., CO2) mass transport. A short circuit may temporarily change the cathode local environment such as its pH or ionic concentrations to thereby improve the CO2 reduction selectivity. A change from lower to higher pH or a change from lower to higher concentrations of potassium may improve selectivity for a particular reaction as the CO generation reaction.
In certain embodiments, a system applies a short circuit when the selectivity of CO2 reduction to CO is below a certain target value and/or it is desired to extend the electrolyzer life by, e.g., a couple of hundred hours. In certain embodiments, a system applies a short circuit for periodic (e.g., recurring) recovery on the reaction selectivity to reach a certain decay rate target within a certain time range. For example, a system may perform a recovery operation whenever the selectivity decays to certain value, e.g., about 90% or less. Another approach performs a recovery operation every time an electrolyzer operates normally for a defined period (e.g., about 200 to 500 hours). Another approach performs a recovery operation every time an electrolyzer exhibits a threshold drop in the selectivity (e.g., about 10% or more).
In some embodiments, a carbon oxide electrolyzer is allowed to reach or is maintained at an open circuit voltage during recovery or protection. Open circuit voltage refers to the voltage difference between the anode and cathode when no net electrochemical reaction is taking place at the anode or cathode. This may result when no current flows between the anode and cathode. An open circuit potential may be achieved when a switch in the circuit containing the electrolyzer and a power supply opens by breaking a conductive path between the anode and/or cathode. An open circuit potential may also be achieved by employing a very high impedance element in the circuit including the power supply and electrolyzer. Such a resistor will have the effect of completely shutting off or nearly shutting off all current flow between the anode and the cathode of the electrolyzer.
In some cases, setting an electrolyzer to OCV or allowing to reach OCV while maintaining similar CO2 gas flow rate at the cathode may provide a relative dry condition thereby allowing improved CO2 mass transport. Setting an electrolyzer to OCV or allowing to reach OCV may flush out potential intermediates or impurities on the catalyst surface when no electrochemical reactions are occurring. In some cases, setting an electrolyzer to OCV or allowing to reach OCV temporarily changes the cathode local environment such as pH or ionic concentrations and thereby improve the CO2 reduction selectivity for, e.g., CO production.
Various scenarios and applications may benefit from setting an electrolyzer to OCV or allowing to reach OCV. In some cases, OCV is used when a modest recovery in selectivity is desired and/or the electrolyzer voltage should be maintained at a relatively high level as by comparison to a short circuit condition.
As indicated, in some embodiments, a reverse current is applied to a carbon oxide electrolyzer during recovery or protection. Forward current is current applied to a carbon oxide electrolyzer during normal operation. Electrons are supplied from a power source to the cathode allowing reduction to occur, and electrons are withdrawn from the anode allowing oxidation to occur. During reverse current, the flow of electrons is reversed so that the electrode that serves as a cathode during normal operation serves as an anode during application of reverse current, and the electrode that serves as an anode during normal operation serves as the cathode during application of reverse current.
Typically, a reverse current is maintained below a level at which carbon and/or other material in the cathode catalyst oxidizes or corrodes. In some embodiments, a reverse current has a magnitude of about −100 mA/cm2 or less or about −5 to ˜100 mA/cm2. In some embodiments, a reverse current is applied for a time duration of about 100 minutes or less, or about 50 minutes or less, or about 30 seconds to 20 minutes. As a further example, the reverse current may be no greater than about 1 mA/cm2 of cathode surface area or not greater than about 0.5 mA/cm2. In some cases, the reverse current flows is maintained at or below a level in which the cell voltage does not exceed about 1.25 V (for reactant gas) and 2.5 V (for oxidizing gases such as air), or does not exceed about 0.5 V (for reactant gases) and 2 V (for oxidizing gases such as air). In certain embodiments employing a reverse current, such current is limited to no more than about 0.6 Coulombs/cm2 of cathode surface area.
After the finishing application of a reverse current, the electrolyzer may return to a normal operating current, and optionally to full normal operation. In certain embodiments, a final value of the reverse current is achieved by ramping to the final value. A ramp rate and/or stepped ramp procedure as described herein for protection mode or for achieving a reduced current value may be employed.
Among the potential benefits of exposing a carbon oxide electrolyzer to reverse current are those described herein for applying a short circuit. Similarly, exposing a carbon oxide electrolyzer to a reverse current may find applications similar to those for applying a short circuit.
Types of Gases Flowed to the Cathode (Pre-Contact with Water)
In some implementations of a recovery process, a gas flows to the cathode for a period of time after the electrical current is stopped or reduced. Sometimes this gas is referred to herein as a “recovery gas.” In some cases, the recovery gas has the same composition as the carbon oxide reactant that flows during normal operation, optionally at a different pressure and/or flow rate than employed in normal operation. For example, the gas flowed during normal operation and during the recovery process contains carbon dioxide or carbon monoxide at a defined concentration. In some cases, a recovery gas that flows to the cathode during a recovery process has composition that is different from that of the reactant gas. In some cases, compared to the reactant gas, the recovery process gas has a lower concentration of carbon oxide reactant. In some cases, the recovery process gas contains an inert gas that is not present in (or is present at a different concentration in) the normal process gas. Examples of inert gases include noble gases (e.g., Ar, He, or Kr) or nitrogen. In some cases, the recovery process gas is or contains air. In some cases, the recovery gas contains an oxidative gas such as oxygen. In some cases, the oxidative gas is simply air, which may contain about 21% oxygen. In other cases, the oxidative gas is oxygen or other oxidizer provided apart from air. For example, oxygen produced at the electrolyzer anode, during normal operation, may be used as an oxidative recovery gas. In some implementations, the recovery gas is humidified. In some embodiments, component gases include carbon dioxide, air, water, an inert gas, or any combination thereof.
In some examples, the recovery gas is 100% or pure reactant gas. In some examples, the recovery gas is 100% or pure inert gas. In some examples, the recovery gas comprises a reactant gas and an inert gas in any ratio. In some examples, the recovery gas comprises an oxidative gas and an inert gas in any ratio. In some examples, the recovery gas is a humidified gas having water vapor present in a concentration of about 0-2% by volume. In some cases, a humidified gas comprises a reactant gas, an inert gas, an oxidative gas, or any combination thereof.
Gas Pressure at Cathode (Pre-Contact with Water)
In some embodiments, after current stoppage or reduction, the pressure of a recovery gas flowing to the cathode may be at a level up to normal operating pressure of the electrolyzer cell. In some embodiments, after current stoppage or reduction, the cathode gas back pressure is reduced to, e.g., 0 psig. Cathode gas back pressure may be controlled by a pressure regulator located downstream from the cathode in the gas flow path. After reducing the cathode gas back pressure, the recovery gas may be present and optionally flowing under a pressure of about 0-600 psig, or about 0-400 psig, or about 0-50 psig.
Gas Flow Rate Going Through the Cathode (Pre-Contact with Water)
In certain embodiments in which a recovery gas flows after reducing or stopping the electrical current, the gas may be flowed at a rate of about 0 to 50 sccm/cm2 of planar cathode surface area, or about 10 to 30 sccm/cm2 of planar cathode surface area. Note that the flow rate values presented herein are provided on a per surface area of cathode (e.g., per cm2 of planar cathode surface). As a single example, the gas flow rate may be about 500 sccm for an electrolyzer having 25 cm2 of cathode surface area. The gas flow rate may scale linearly or non-linearly with surface area of the cathode. The flow rate values presented here may be instantaneous flow rates or average flow rates.
Duration of Gas Flowing at Cathode (Pre-Contact with Water)
In recovery sequence embodiments in which a recovery gas flows to the cathode, that gas may flow for a period of time after the electrical current is stopped or reduced. In certain embodiments, the duration of the gas flowing to or residing at the cathode is about 30 minutes to 10 days, or about 1 hour to 2 days. The duration of gas flow in a recovery sequences may be at least partially dependent on the flow rate of the gas (e.g., the average flow rate in cases where the flow rate varies). As an example, low or zero flow for a few days and high flow for a few minutes or hours may have similar effects. As a further example, a gas flow and/or exposure time range could be 30 minutes to even days. But at a flow rate of 50 sccm/cm2, the maximum contact/flow time may be about 5 hours.
In some embodiments, the recovery gas is flowed to the cathode before contacting the cathode with water or other liquid. The recovery gas flow may be stopped before contact with the liquid.
As indicated, various recovery processes involve contacting the cathode with a liquid such as water. It should be understood that when referring to water herein, the recovery process may employ water over a wide range of purities. In some embodiments, the water is deionized water such as deionized water having a resistivity of at least about 1 megaohm, or at least about 10 megaohm, or at least about 18 megaohm. In some embodiments, the water includes one or more dissolved solutes or suspended components. Examples of dissolved solutes include bicarbonates, carbonates, sulfates, hydrogen sulfates, formates, acetates, and halides. As examples, the solutes may be metal (e.g., sodium, potassium, or cesium) or ammonium salts of these anions. In certain embodiments, recovery process water comprises a bicarbonate at a concentration of at most about 10 mM bicarbonate ion, or at most about 5 mM bicarbonate ion, or at most about 2 mM bicarbonate ion. In some embodiments, the recovery process water has a composition that matches or is similar to that of anode water used during normal operation. Examples of anode water compositions are presented in PCT Patent Application No. PCT/US2019/063471, filed Nov. 26, 2019, which is incorporated herein by reference in its entirety.
The water optionally flows during contact with the cathode. In some embodiments, the water flow rate to the cathode is up to about 20 ml/min per cm2 of planar cathode surface area. As an example, the water flow rate is about 2-10 ml/min per cm2 of planar cathode surface area. In some embodiments, the flow rate is limited based on pump and associated hardware. Note that water provided to the cathode as part of a recovery process is typically provided from outside MEA, such as via the carbon oxide inlet to the cathode, as opposed to being provided from the anode via the MEA.
In certain embodiments, the duration of water flowing to the cathode is about 1-100 minutes. In certain embodiments, the duration of water flowing to the cathode is about 2-50 minutes, or about 5-15 minutes.
Drying Steps (Gas Flowing) after Water Flowing
A drying operation may be performed when no water contacts the cathode. Drying may be performed after water contact but before, or possibly during an initial period of, resumption of normal operation.
Drying may be performed with a gas of any composition that removes water from the cathode. Such gas may be referred to herein as a “drying gas.” Examples of gases that may be present in a drying gas are air, the reactant gas, and inert gases. Examples of the inert gases include the noble gases (e.g., He, Ar, Kr) and nitrogen. If the reactant gas is used, it is optionally used at a concentration that is different from its use during normal operation. For example, carbon dioxide may be present at 50% (molar) in the reactant and be present at 20% (molar) in the drying gas.
The physical mechanism by which the drying gas removes water from the cathode may include pushing or otherwise forcing water out of the cathode by contacting the cathode with gas at a pressure and/or velocity sufficient to remove liquid water.
Water may also or alternatively be removed by evaporation into the drying gas. To this end, the relative humidity of the drying gas entering the cathode may be controlled to facilitate evaporation. While, in some embodiments, the drying gas may have a very low relative humidity (e.g., about 0 to 100%), in other embodiments, it may have a higher value, which may be desirable to ensure that the MEA does not lose moisture to the point where it dries and its performance degrades.
In certain embodiments, the duration of drying the cathode is about 0 to 500 minutes, or about 2 to 100 minutes, or about 5 to 30 minutes.
In some embodiments, the drying operation continues until no further liquid water (e.g., water droplets or mist) is present downstream from the cathode. In some embodiments, the drying operation continues until the humidity of the drying gas entering the cathode is approximately the same as the humidity of the drying gas exiting the cathode.
If the drying is not conducted with the reactant gas, the recovery process transitions from flowing drying gas to flowing reactant gas. This may involve reestablishing the normal operating gas pressure at the cathode by, e.g., adjusting a setting on the pressure regulator downstream from the cathode. If the drying gas is the reactant gas, then the transition from flowing the drying gas to flowing the reactant gas need not occur, or optionally it occurs but represents only a change in the gas flow rate, pressure, composition, or a combination thereof.
Current Ramp Rate after Recovery
In some embodiments, at the conclusion of the recovery operation, electrical current is resumed directly to the original value or with some ramping steps or step increase.
In some embodiments, a recovery process is performed with a voltage or current scan. A voltage or current scan may be performed repeatedly in alternating directions, between two endpoints. A voltage scan may be performed in the manner of cyclic voltammetry. In some cases, a voltage or current scan is performed in a recovery process while the current is otherwise stopped. In some cases, a voltage or current scan is performed while gas flows to the cathode, but water does not contact the cathode. For example, a voltage or current scan may be performed during a gas drying operation in any of recovery sequence examples 2-4, above. In some examples, a voltage or current scan is performed during a gas contact operation that occurs prior to contact with water. See recovery sequence examples 3 and 4, above. In some examples, a voltage or current scan is performed during a gas contact operation that is not associated with a water contact operation. See recovery sequence examples 5-7, above.
A voltage or current scan may have various effects on an electrolyzer cell or its cathode. Examples include:
A voltage scan may be characterized by an initial cathode voltage E0 (V), an upper cathode voltage limit V1 (V), a lower cathode voltage limit V2 (V), and a scan rate S (mV/s). In some embodiments, E0 has a range of about −1V to 1.2V vs. RHE. In some embodiments, E1 has a range of about −1V to 1.2V. In some embodiments, E2 has a range of about −1V to 1.2V. In some embodiments, S has a range of about −10000 to −0.1 mV/s, or about 0.1 to 10000 mV/s. The scan direction can be positive or negative, with positive meaning the first sweep is towards the positive direction, while negative meaning the first sweep is towards the negative direction. The scan rates in the positive and negative directions may be the same or different. The number of scans (n) can be in the range from about 1 to 1000.
In certain embodiments, the temperature of an electrolyzer cell remains unchanged during a recovery operation. In certain embodiments, the temperature of an electrolyzer cell changes during a recovery operation. The temperature change may be driven by a temperature controller, water flowing to the cathode, gas flowing to the cathode, or any combination thereof. In some embodiments, the electrolyzer cell temperature increases during the recovery operation. In some embodiments, the electrolyzer cell temperature decreases during the recovery operation, e.g., by about 5-25° C.
The ability of interface components in accordance with certain embodiments of the invention to mitigate the effects of electrical interruptions is illustrated in
The top plot shows CO selectivity, the middle plot voltage, and the bottom plot resistance. Over the course of time, an electrochemical cell without an interface (control) of at least one porosity enhancing agent gradually records an increased resistance. However, when an interface composed of a carbon and Nafion mixture (CNI) is tested under the same conditions, it does not build up resistance. There is a clear mitigation of voltage decay.
In
The make-up of the interface was a 1:1 ratio by weight of carbon nanoparticles and Nafion, applied 3 um thick. The Nafion component was a 5% by weight polymer in solution. Without being bound to any particular mechanism, the increased resistance demonstrated by the cell without an interface may be due to decomposition of the AEM and PEM materials. Notably, the selectivity remains high throughout the process.
Although omitted for conciseness, embodiments of the system and/or method can include every combination and permutation of the various system components and the various method processes, wherein one or more instances of the method and/or processes described herein can be performed asynchronously (e.g., sequentially), concurrently (e.g., in parallel), or in any other suitable order by and/or using one or more instances of the systems, elements, and/or entities described herein.
Although the foregoing embodiments have been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be practiced within the scope of the appended claims. It should be noted that there are many alternative ways of implementing the processes, systems and apparatus of the present embodiments. Accordingly, the present embodiments are to be considered as illustrative and not restrictive, and the embodiments are not to be limited to the details given herein.
| Number | Date | Country | |
|---|---|---|---|
| 63379374 | Oct 2022 | US |
