Patent Application
20240429419
Described herein is an electrochemical cell such as a polymer-electrolyte-membrane fuel cell (PEMFC) that incorporates a shunt into the membrane separator that becomes electronically conductive around a well-defined anodic onset potential, thereby preventing excessive anodic potentials at the positive electrode that would otherwise drive deleterious parasitic reactions such as catalyst dissolution or catalyst and carbon oxidation
A fuel cell (FC) is a device that converts energy of a chemical reaction into electrical energy (electrochemical device) without combustion. A polymer-electrolyte-membrane fuel cell (PEMFC; see
oxidation half reaction 15 (cathode 104)
reduction half reaction 45 (cathode 102)
The overall reaction is:
This electrochemical process is a non-combustion process that does not generate airborne pollutants. Therefore, fuel cells are a clean, zero emission, highly efficient source of energy. Fuel cells can have 2-3 times greater efficiency than internal combustion engines and can use abundant or renewable fuels. Fuel cells produce electricity, water, and heat using fuel (H2) 10 and oxygen 20. Water 50 (liquid and vapor) is the only emission when hydrogen 10 is the fuel.
The two half-reactions (15 and 45) normally occur very slowly at the low operating temperature of the fuel cell, and catalysts are used on one or both the anode 104 and cathode 102 to increase the rates of each half reaction. Platinum (Pt) is the typical metal catalyst because it can generate high rates of O2 reduction at the relatively low temperatures of the PEM fuel cells. Kinetic performance of PEM fuel cells is limited primarily by the slow rate of the O2 reduction half reaction (cathode reaction, 45) which is typically more than 100 times slower than the H2 oxidation half reaction (anode reaction, 15). The O2 reduction half reaction is also limited by mass transfer issues.
In operation, the output voltage of an individual fuel cell under load is generally below one volt. Therefore, in order to provide greater output voltage, multiple fuel cells are usually stacked together and are connected in series to create a higher voltage fuel cell stack. Fuel cell stacks can then be further connected in series or parallel combinations to form larger arrays for delivering higher voltages or currents.
As fuel, such as hydrogen 40, flows into a fuel cell on the anode 104 side, a catalyst facilitates the separation of the hydrogen gas fuel into electrons and protons (hydrogen ions). The hydrogen ions pass through the membrane 103 (center of fuel cell) and, again with the help of the catalyst, combine with an oxidant, such as oxygen 20, and electrons 30 on the cathode side, producing water 50. The electrons 30, which cannot pass through the membrane 103, flow from the anode 104 to the cathode 102 through an external circuit containing a motor or other electrical load, which consumes the power generated by the cell.
A catalyst is used to induce the desired electrochemical reactions at the electrodes. The catalyst is often incorporated at the electrode/electrolyte interface by coating a slurry of the electrocatalyst particles to the polymer electrolyte membrane 103 surface. When hydrogen fuel 10 feeds through the anode catalyst/electrolyte interface, electrochemical reaction occurs, generating protons 40 and electrons 30 (15). The electrically conductive anode 104 is connected to an external circuit, which carries electrons by producing electric current. The polymer electrolyte membrane 103 is typically a proton conductor, and protons 40 generated at the anode catalyst migrate through the polymer electrolyte membrane 103 to the cathode 102. At the cathode catalyst interface, the protons 40 combine with electrons 30 and oxygen 20 to yield water 50 (45).
The catalyst is typically a particulate metal such as platinum and is dispersed on a high surface area electronically conductive support. Such catalysts, particularly platinum, are very sensitive to carbon monoxide poisoning. This is a particular concern for the anode catalyst for fuel cells operating on hydrogen, as CO is sometimes present in the hydrogen supply as a fuel contaminant and/or as a result of membrane cross-over from the oxidant supply in applications where air is employed.
The electronically conductive support material in the PEMFC typically includes carbon particles. Carbon has an electrical conductivity (on the order of 10−2 to 1 S/cm, depending on the type) which helps facilitate the passage of electrons from the catalyst to the external circuit.
Proton conductive materials, such as perfluorosulfonic acid (PFSA) membranes (e.g., Nation®), are often added to facilitate transfer of the protons from the catalyst to the membrane interface.
To promote the formation and transfer of the protons and the electrons and to prevent drying out of the membrane 103, the fuel cells are operated under humidified conditions. To generate these conditions, hydrogen fuel 10 and/or oxygen 20 gases are sometimes humidified prior to entry into the fuel cell. In a supported electrocatalyst, carbon is relatively hydrophobic, and as such, the boundary contact between the reactive gases, water and the surface of the solid electrodes made of carbon contributes to electrical contact resistance and ohmic power loss in the fuel cell resulting in lower efficiency of the fuel cell.
Voltage reversal occurs when a fuel cell in a series stack cannot generate sufficient current to keep up with the rest of the cells in the series stack. Several conditions can lead to voltage reversal in a PEM fuel cell, including insufficient oxidant, insufficient fuel, insufficient water, low or high cell temperatures, and problems with cell components or construction. Reversal generally occurs when one or more cells experience a more extreme level of one of these conditions compared to other cells in the stack. Undesirable electrochemical reactions may occur, which may detrimentally affect fuel cell components. Component degradation reduces the reliability and performance of the affected fuel cell, and in turn, its associated stack and array. Groups of cells within a stack can also undergo voltage reversal and even entire stacks can be driven into voltage reversal by other stacks in an array. Aside from the loss of power associated with one or more cells going into voltage reversal, this situation poses reliability concerns.
Current reversal occurs in several scenarios related to hydrogen dynamics in fuel cells. This phenomenon can arise upon local H2 depletion at the anode, causing carbon corrosion on the anode itself. It also happens during the formation of Ha/air fronts at the anode, typically during start/stop events, which results in carbon corrosion on the cathode. Additionally, current reversal can occur in cases of water blockage or mass transport restrictions leading to local H2 depletion at the anode. In these situations, a current reversal takes place in some regions, where protons flow from the positive electrode to the negative electrode. However, it's important to note that this does not constitute a voltage reversal, as the positive electrode maintains a higher voltage than the negative electrode throughout these events. See Zhang et al., J. Power Sources 194:588-600 (2009).
Reversal typically also results in excessive heat generation, which can induce pinholes in the membrane that lead to total cell (and possibly stack) failure. Methods to address voltage or current reversal, e.g., including oxygen evolution catalysts in the positive electrode to maintain voltages below the carbon corrosion potential and drive electrolysis of water, are ultimately insufficient. See e.g., Mandal et al., J. Power Sources 397:397-404 (2018).
Regions of the negative electrode may become depleted of H2 and corrosion results. See
What is needed is a polymer-electrolyte-membrane fuel cell (PEMFC) that incorporates a shunt into the membrane separator that prevents excessive anodic potentials at the positive electrode.
One embodiment described herein is an electrochemical cell comprising: a fuel source; an oxidation source; a positive electrode exposed to an electrolyte membrane; a negative electrode exposed to the electrolyte membrane; and the electrolyte membrane positioned between the positive and negative electrodes, the electrolyte membrane including an electron donor material and a shunt material comprising one or more electroactive polymers that become electronically conductive at a specific shunting onset potential below the electrochemical cell open circuit potential, the one or more electroactive polymers including a diketopyrrolopyrrole (DPP) polymer or a derivative thereof. In one aspect, the DPP polymer includes glycolated side chains. In another aspect, the DPP polymer is DPP-MeOT2.
In another aspect, the specific shunting onset potential is between about 0.5 V and about 1.0 V vs. a standard hydrogen electrode. In another aspect, the electron donor material is a sulfonated tetrafluoroethylene copolymer. In another aspect, the sulfonated tetrafluoroethylene copolymer is Nafion. In another aspect, the shunt material is distributed in discrete domains within the electron donor material. These discrete domains represent one of two important domain sizes to consider. Specifically, the discrete domains of shunt material within the Nafion blend have a size of less than 100 nm. In another aspect, these discrete domains occupy less than 10% of the total volume of the electrolyte membrane. The second important domain size to consider is the overall region where this blend is located. This region size can vary considerably: it could be conservatively less than several centimeters, or when referring to the land/channel dimensions (widths), it could be less than several millimeters, or even less than 0.1 mm.
In another aspect, the electrolyte membrane includes a first region of a blend of the electron donor material and the shunt material and a second region substantially free of the shunt material. In another aspect, the first region forms a percolating network within the electrolyte membrane. In another aspect, the shunt material further includes an additional electron donor selected from the group consisting of a polymer, a viscous salt solution with limited ion mobility, and an ionic liquid.
Another embodiment described herein is an electrochemical cell membrane electrode assembly (MEA) comprising: an ionically conductive material; and a shunt material comprising a diketopyrrolopyrrole (DPP) polymer or a derivative thereof that becomes electronically conductive at a specific shunting onset potential below electrochemical cell membrane open circuit potential, wherein the electrochemical cell membrane includes one or more first regions and one or more second regions, the second regions having a higher shunt material concentration than the one or more first regions. In one aspect, the DPP polymer includes glycolated side chains. In another aspect, the ionically conductive material is Nafion. In another aspect, the one or more second regions are aligned with at least one of a cathode flow field exit, a cathode land region, a flow field entrance, or a flow field exit.
Another embodiment described herein is an electrochemical cell comprising: a cathode adjacent to a cathode flow field; an anode opposite the cathode and adjacent to an anode flow field; and an electrolyte membrane separating the anode from the cathode, the membrane includes a first region of ionically conductive material with a second region of a blend of the ionically conductive material and a shunt material, wherein the shunt material is a diketopyrrolopyrrole (DPP) polymer or a derivative thereof configured to become electronically conductive at a shunting onset potential below open circuit potential of the electrochemical cell, and the second region forms a percolating network within the electrolyte membrane. In one aspect, the DPP polymer includes glycolated side chains. In another aspect, the second region occupies less than 10% of the total volume of the electrolyte membrane. In another aspect, the ionically conductive material is Nafion, and the shunt material further includes an additional electron donor selected from the group consisting of a polymer, a viscous salt solution with limited ion mobility, and an ionic liquid.
As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention.
In this specification and in the claims, reference will be made to several terms which shall be defined to have the following meanings:
As used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise.
Ranges may be expressed herein as from “about” one particular value, and/or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another embodiment. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint.
“Optional” or “optionally” as used herein indicates that the subsequently described event or circumstance may or may not occur, and that the description includes instances where said event or circumstance occurs and instances where it does not.
“Effective amount” as used herein refers to a composition or property as provided herein is meant such amount as can perform the function of the composition or property for which an effective amount is expressed. The exact amount required often varies from process to process, depending on recognized variables such as the compositions employed, and the processing conditions observed. Thus, it is not possible to specify an exact “effective amount.”
“Substituted conducting polymer” as used herein refers to any chemical variation to a conducting polymer that retains the functionalities of conductivity and hetero atoms. For example, poly-3-butyl-thiophene is a “substituted” polythiophene.
“Electrochemical cell” as used herein refers to a device or a system structured to generate electrical energy from the chemical reactions occurring in it (e.g. fuel cells), or use the electrical energy supplied to it to facilitate chemical reactions in it (e.g. electrolyzer). An electrochemical cell is structured to convert chemical energy into electrical energy, or vice versa. An electrochemical cell may include a fuel cell, electrolyzer, or both.
“Electrolyzer” as used herein is composed of an electrolyte membrane, an anode layer, and a cathode layer separated from the anode layer by the electrolyte membrane. A catalyst material, such as Pt and Ir catalysts, is included in the anode and cathode layers of the electrolyzer. At the anode layer, H2O is hydrolyzed to O2 and H+ (2H2O→O2+4H++4e−). At the cathode layer, H+ combines with electrons to form H2 (4H++4e−→2H2). The electrolyte membrane of the electrolyzer may be a polymer electrolyte membrane (PEM).
“Fuel cell” (FC) as used herein is an electrochemical device that converts chemical energy into electrical energy without combustion.
“PEMFC” as used herein is a type of fuel cell that utilizes a Polymer Electrolyte Membrane (PEM) to carry protons between two catalytic electrode layers, thus generating electrical current. A typical single PEM fuel cell is composed of a PEM, an anode layer, a cathode layer, and GDLs. These components form a membrane electrode assembly (MEA), which is surrounded by two flow field plates. A catalyst material, such as platinum (Pt) catalysts, is included in the anode and cathode layers of the PEM fuel cell. At the anode layer, Pt catalysts catalyze a hydrogen oxidation reaction (HOR, H2→2H++2e−), where H2 is oxidized to generate electrons and protons (H+). At the cathode layer, Pt catalysts catalyze an oxygen reduction reaction (ORR, ½O2+2H++2e−→H2O), where O2 reacts with H+ and is reduced to form water. A PEM typically operates at temperatures up to 100° C.
“Proton Exchange Membrane” as used herein is also known or referred to as polymer electrolyte membrane (PEM). In one or more embodiments, the PEM membrane comprises a shunt material such as one or more “carbon-containing semiconductors” capable of shunting current under certain environmental conditions and preventing the positive electrode from reaching a high potential.
A “carbon-containing semiconductor” as used herein refers to a carbonaceous polymer that is capable of shunting current under specific environmental conditions. Exemplary carbon-containing semiconductors include polyaniline, polypyrrole, polyfuran, thiophene, polythiophene, polyalkyl thiophene, polyacetylene, poly(9,9-dioctylfluorene-alt-benzothiadiazole, poly(phenylenevinylenes), PEDOT, PEDOT: PSS, linear acenes, metal phthalocyanines, perylene derivatives, tetrathiafulvalenes, benzothiophenes, coronenes, porphyrins, rubrenes, thiophenes, C60, graphene, graphene nanoribbons, purified semiconducting carbon nanotubes, or combinations or derivatives thereof.
“Membrane Electrode Assembly” (MEA) as used herein refers to assembly which normally comprises a polymer membrane with affixed/adjacent electrode layers. In some cases, the MEA may also include gas diffusion layer/materials.
“Microporous layer (MPL) as used herein refers to a porous layer located between the catalyst layer (CL) and gas diffusion layer (GDL). The MPL typically contains a hydrophobic agent, polytetrafluoroethylene (PTFE), and carbon black.
“Catalyst layer” (CL) as used herein refers to the active region for the electrochemical reactions in a fuel cell. An effective catalyst layer should have transport channels for protons, electrons, reactants, and products. The catalyst layer is typically made from a catalyst ink containing catalyst powder, binder and solvents applied to a supporting material. “Electrocatalyst” or “catalyst,” as used herein is a metal which is catalytic for fuel cell reactions, typically supported on a catalyst support.
“Supported catalyst” as used herein refers to a catalyst metal dispersed on a support. “Catalyst support” as used herein is a material upon which metal is dispersed, which is typically conductive (e.g., carbon, a conducting polymer, or a metal oxide).
“Electrode,” as used herein, is a supported electrocatalyst in contact with or affixed to a membrane. The electrode may include other materials in addition to the electrocatalyst.
“Metal” as used herein refers to precious metal, noble metal, platinum group metal, platinum, alloys, and oxides of same, and compositions that include transition metals and oxides of same. For example, the metal can be platinum, iridium, osmium, rhenium, ruthenium, rhodium, palladium, vanadium, chromium, or a mixture thereof, or an alloy thereof, specifically, the metal can be platinum. As described herein, the metal can also be alloys or metal oxides effective as catalysts. As used herein, a “metal” acts as a catalyst for the reactions occurring in the fuel cell.
“Ionomer,” as used herein is an ionically conductive polymer (e.g., Nafion®). An ionomer is also frequently used in the electrode layer to improve ionic conductivity.
“Membrane,” 103 as used herein refers to a polymer electrolyte membrane, solid polymer electrolyte, proton exchange membrane, separator, or polymer membrane. The “membrane” is an ionically conductive, dielectric material against which catalytic electrodes are placed or affixed. An exemplary membrane is a sulfonated tetrafluoroethylene-based fluoropolymer-copolymer (e.g., Nafion®), which can be obtained in varying thicknesses, equivalent weights, etc.
“Electrolyte” as used herein is a nonmetallic electric conductor in which current is carried by the movement of ions or a substance that when dissolved in a suitable solvent becomes an ionic conductor. The polymer membrane of a fuel cell can be an electrolyte.
“Oxygen reduction reaction,” “cathode reaction,” or “cathodic process,” as used herein is a reaction in which oxygen gas is reduced in the presence of protons, producing water.
“Hydrogen oxidation reaction,” “HOR,” “anode reaction,” or “anodic process” as used herein is a reaction in which hydrogen gas is converted into protons and electrons.
“Protons,” “W,” “hydrogen ions,” or “positive ions,” as used herein refer to a positively charged portion of hydrogen atom which results from reaction over catalyst material.
“Anode” 104 as used herein is the negative electrode where fuel oxidation reaction occurs. “Cathode” 102 as used herein is the positive electrode where oxidant-reduction reaction occurs.
“Gas diffusion layer,” “GDL,” or “porous backing layer” 101 as used herein refers to a layer adjacent to the electrodes which aides in diffusion of gaseous reactants across the electrode surface; it is typically a carbon cloth or carbon-based/carbon-containing paper (e.g., one manufactured by Toray). The GDL should be electrically conductive to carry electrons through an external circuit. The GDL may be an integral part of the cell, MEA, the flow field, or a combination thereof.
“Bipolar plate” or “BPP” 100 as used herein is the portion of a fuel cell adjacent to the GDL through which electrons pass to an external circuit; it may also contain channels or paths (flow field) to assist in gas distribution and is typically made of graphite or conductive composites. In some embodiments, cooling channels are integrated into the BPP.
“Flow field” as used herein is the scheme for distributing gaseous reactants across the electrode. A flow field may be part of or include a bipolar plate 100 and/or a gas diffusion layer.
“Insulator” or “dielectric” as used herein refers to a material which is not electrically conductive.
“Electrical conductivity” or “electronic conductivity” as used herein refers to the ability of a material to conduct electrons.
“Protonic conductivity” or “ionic conductivity” (IC), as used herein refers to the ability of a material to conduct ions or protons.
“Carbon black” as used herein refers to a conductive aciniform carbon utilized, for example, as a catalyst support.
“Porosity” or “permeability” as used herein refers to macroscopic porosity of an electrode structure (i.e., related to ability of diffusion of gaseous reactants through an electrode layer). “Particulate” as used herein refers to a material of separate particles.
“Polarization curve,” “IV curve,” or “current-voltage curve,” as used herein refers to data/results from electrochemical analysis of MEAs or catalyst materials.
“Current density” as used herein refers to the current flowing per unit electrode area.
An “ionically conductive material” as used herein refers to any material which is effective for the transmitting electrical current. Specifically, the conductive material can be a conducting polymer. For example, perfluorosulfonic acid (PFSA), polyaniline, polypyrrole, polythiophene, polyfuran, poly(p-phenylene-oxide), poly(p-phenylene-sulfide), substituted conducting polymers, or mixtures or derivatives thereof can be used. Specifically, the conducting polymer can include polyaniline, polypyrrole, polyfuran, polythiophene, or mixtures or derivatives thereof. Mixtures of these polymers can include physical mixtures as well as copolymers of the monomers of the respective polymers. As used herein, reference to a polymer also covers a copolymer.
A typical polymer-electrolyte-membrane fuel cell (PEMFC) (
The MEA comprises an anode 104, a cathode 102, and a membrane 103. The anode 104 can be an electrode of the present invention. The electrode should be electrically conducting, porous enough to let reactants diffuse to the metal, and able to carry protons to the membrane. The cathode 102 can also be an electrode. Current technology varies with the manufacturer, but total Pt loading of the electrode has decreased from 4 mg/cm2 to about 0.2 mg/cm2. Pt loadings of 0.5 mg/cm2 are capable to generate about 0.5 amperes per mg Pt.
Membrane/electrode (104/103/102) assembly construction varies greatly, but the following is one of the typical procedures. The supported catalyst/electrode material is first prepared in liquid “ink” form by thoroughly mixing together appropriate amounts of supported catalyst (powder of metal, e.g., platinum dispersed on carbon) and a solution of the membrane material (ionomer) dissolved in a solvent, e.g., alcohols. Once the “ink” is prepared, it is applied to the surface of the membrane, e.g., Nafion®, in several different ways. The simplest method involves painting the catalyst “ink” directly onto a dry, solid piece of membrane. The wet supported catalyst layer and the membrane are heated until the catalyst layer is dry. The membrane is then turned over, and the procedure is repeated on the other side. Supported catalyst layers are thus on both sides of the membrane. The dry membrane/electrode assembly is rehydrated by immersing in dilute acid solution to also ensure that the membrane is in the H+ form needed for fuel cell operation. The final step is the thorough rinsing in distilled water. The membrane/electrode assembly can be assembled into the fuel cell hardware.
Addition of the shunt material such as carbon-containing semiconductors to a Nafion® membrane could be accomplished in a number of ways. Without restricting the scope of the invention, semiconducting filaments, particles, or coated beads could be blended into a Nafion® dispersion, e.g., by sonication, above a percolation threshold prior to solvent-casting of a membrane. Alternatively, membranes could be fabricated with controlled porosity, and carbon-containing semiconductors could be solution cast, or chemically or electrochemically deposited into the pores.
Normal Nafion® will dehydrate (thus lose proton conductivity) when temperature is above ˜80° C. This limitation can be problematic for fuel cells because higher temperatures are desirable for a better efficiency and CO tolerance of the platinum catalyst. Silica and zirconium phosphate can be incorporated into Nafion® water channels through in situ chemical reactions to increase the working temperature to above 100° C.
The membrane/electrode assembly can have a total thickness of about 50 μm (excluding gas diffusion and microporous layers), for example, and conventionally generate more than 0.5 ampere of current for every square cm of membrane/electrode assembly at a voltage between the cathode and anode of 0.7 V, when encased within well engineered components. For example, the membrane is about 15-20 μm and the anode and cathode conductive layers are about 10 μm each. This provides a membrane/electrode assembly of about 40 μm. The gas diffusion layer and microporous layer may be about 300 μm. A five-layer MEA is typically about 640 μm.
The fuel cell can include gas diffusion layers. The layers are generally one next to the anode and another next to the cathode and made of a porous carbon paper or carbon cloth. The layers are made of a material that can conduct the electrons exiting the anode and entering the cathode.
The fuel cell can include flow fields and current collectors 100. Pressed against the outer surface of each backing layer can be a piece of hardware, called a bipolar plate 100, which often serves the dual role of flow field and current collector. The plates are generally made of a lightweight, strong, gas impermeable, electron-conducting material; graphite, metals, or composite plates are commonly used.
The bipolar plates can provide a gas flow field such as channels machined into the plate. The channels carry the reactant gas from the point at which it enters the fuel cell to the point at which the gas exits. The pattern, width, and depth have a large impact on the effectiveness of the distribution of the gases evenly across the active area of the membrane/electrode assembly. The flow field also affects water supply to the membrane and water removal from the cathode.
The bipolar plates can also serve as current collectors. Electrons produced by the oxidation of hydrogen can be conducted through the anode, through the backing layer and through the plate before they can exit the cell, travel through an external circuit, and re-enter the cell at the cathode plate. Typically, many cells are stacked together to provide a higher voltage and the internal bipolar plates transport electrons from one cell to the next.
The polymer electrolyte membrane (PEM) 103 (or proton exchange membrane), carries the necessary protons from the anode 104 to the cathode 102 while keeping the gases (H2/O2) safely separate. The thickness of the membrane in a membrane/electrode assembly can vary with the type of membrane. The thickness of the supported catalyst layers depends on how much metal is used in each electrode. For example, for supported catalyst layers containing about 0.15 mg Pt/cm2, the thickness of the supported catalyst layer can be about 10 μm to about 20 μm. The thickness of the supported catalyst layer can be, for example, about 0.1 to about 50 μm, more specifically on the order of about 20 to about 30 μm. Thicknesses above 50 μm appear to increase the mass transfer problems too much to be effective.
The membrane of the MEA 103 may include a dielectric, ionically conductive material. It is desired that the membrane be sufficiently durable to withstand conditions within an electrochemical cell such as a fuel cell or an electrolyzer. The membrane of the MEA may include an ionomer, specifically a perfluoro sulfonate ionomer or a sulfonated or phosphorylated polybenzimidazole ionomer. More specifically, the membrane may include a sulfonated tetrafluoroethylene copolymer cation exchange ionomer such as Nafion® (Chemours). Nafion® is a perfluorinated polymer (PFSA) that contains small proportions of sulfonic or carboxylic ionic functional groups (e.g., tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octenesulfonic acid copolymer).
In one or more embodiments described herein, the MEA membrane may further include a shunt material. The shunt material may include one or more electroactive polymers capable of or structured to shunt current under certain environmental conditions, prevent the positive electrode from reaching a high potential, or both. Specifically, the shunt material may comprise one or more electroactive polymers that become electronically conductive at a specific shunting onset potential below the electrochemical cell open circuit potential.
In one embodiment, the shunt material may include a diketopyrrolopyrrole (DPP) polymer or a derivative thereof. In a specific aspect, the DPP polymer may include glycolated side chains. In a particular example, the DPP polymer may be DPP-MeOT2.
In at least one embodiment described herein, the MEA membrane may include the shunt material distributed in discrete domains within the electron donor material. These discrete domains may have a size of less than 100 nm and may occupy less than 10% of the total volume of the electrolyte membrane.
In one or more embodiments, the MEA membrane may include the shunt material distributed in a non-uniform manner. The membrane may include one or more first regions and one or more second regions, with the second regions having a higher shunt material concentration than the one or more first regions.
In another exemplary embodiment, the shunt material may be distributed unevenly and/or asymmetrically. The MEA membrane may include different concentrations in particular regions of the membrane. In a non-limiting example, one or more regions of the membrane aligned with at least one of a cathode flow field exit, a cathode land region, a flow field entrance, or a flow field exit may contain greater concentrations of the shunt material than other regions of the membrane.
Described herein is a means of reducing high anodic overpotential at the positive electrode and limiting the rate of carbon consumption and/or Pt oxidation by effectively shorting the membrane with an electronic conductor. Ideally this conductive mechanism is activated only under deleterious conditions such as when the local potential of the positive electrode is greater than under normal operating conditions.
The electrochemical cell incorporates a shunt into the membrane separator that becomes electronically conductive at a particular anodic onset potential, thereby preventing excessive anodic potentials at the positive electrode that would otherwise drive deleterious parasitic reactions such as catalyst dissolution or catalyst and carbon oxidation.
The DPP polymers are electroactive polymers that become electronically conductive when doped. In particular, p-doped DPP polymers may be generated when they are in contact with an electrode at high potential. When DPP polymers are integrated into the membrane and the positive electrode locally experiences a high potential excursion, the DPP polymer may become activated (i.e., p-doped) and thereby electronically conductive.
Once there is a continuous electronic pathway established between the two electrodes, the p-doped DPP polymer has a mediating effect on the locally high positive electrode potential by allowing electrons to be shunted from the negative electrode to the positive electrode. This process is reversible; that is, when the potential drops again (e.g., by drawing sufficient current across the cell), the DPP polymer doping level will decrease and the electronic conductivity will drop such that the membrane again becomes relatively insulating.
In particular, DPP polymers that react in a particular voltage window are desirable such that they are activated above the normal operating potential of the positive electrode but below the potential at which carbon corrosion, catalyst dissolution, or other degradation mechanisms occur at an unacceptable rate. Specifically, the shunting onset potential may be between about 0.5 V and about 1.0 V vs. a standard hydrogen electrode.
There is evidence that DPP polymers may be incorporated into Nafion® or similar proton-conducting membranes used in PEMFCs.
One embodiment described herein is a polymer-electrolyte-membrane fuel cell (PEMFC) system comprising: a fuel source, an oxidation source, a positive electrode exposed to an electrolyte membrane, a negative electrode exposed to the electrolyte membrane; and one or more electrolyte membranes being positioned between the positive electrode and the negative electrode; wherein the electrolytic membrane comprises the shunt material having one or more electroactive polymers that become electronically conductive at a specific shunting onset potential below the electrochemical cell open circuit potential. The electroactive polymers include a DPP polymer or a derivative thereof. While a PEMFC example is given, the membrane including the shunt material is envisioned for an electrolyzer as well.
In some embodiments a “bilayer” membrane is used as the separator, in which the layer adjacent to the positive electrode includes a carbon-containing semiconductor that becomes conductive above a potential V1, and the layer adjacent to the negative electrode includes a carbon-containing semiconductor that becomes conductive above a potential V2, with V1>V2. This is advantageous when the polymer adjacent to the positive electrode is unstable at the potential of the negative electrode, or if the polymer adjacent to the negative electrode is unstable at the potential of the positive electrode.
In some embodiments, one, two, three or more layers are used that include the shunt material such as the carbon-containing semiconductors that become electronically conductive in different potential windows such that all polymers are stable during normal operation of the cell and when the shunt is activated via excessive anodic potential at the positive electrode. In another embodiment, the layers have the same or different volume percentages of the one or more carbon-containing semiconductors. These semiconductors may have overlapping potential windows, the same potential window, or different potential window. In another aspect, the outer layers may have increased volume percentages of the carbon-containing semiconductors and the inner layers may have no or decreased volume percentages of the carbon-containing semiconductors. In one aspect membrane regions localized in proximity to the inlet or outlet may contain greater volume percentages of the carbon-containing semiconductors than other regions of the membrane or other membrane layers.
In some embodiments, the carbon-containing semiconductor becomes electronically conductive at a potential below the open-circuit potential of the cell. This may be advantageous because even the open circuit potential may be high enough to drive degradation reactions at the positive electrode. In this configuration, the fuel cell stack behaves as an electronic resistor when current is not applied (or when a sufficiently low current is applied), and the positive electrode potential of each cell is allowed to exceed the redox potential of the shunt.
The shunt material or carbon-containing semiconductor material or electroactive polymer(s) may be deployed at one or more specific locations in the cell and/or stack. Implementing this may increase overall performance and durability but must be balanced against the challenge of introducing a gradient in the manufacturing process. In a non-limiting example, membranes with a gradient in thiophene concentration are useful, with less at the inlet and more at the outlet (where the chance of H2 starvation is likely higher). Micro-diffraction experiments have shown that the regions close to the outlet of the flow field show more significant Pt size growth, possibly indicating higher potential excursions at the positive electrode in these regions that may be related to H2 starvation. In other systems, H2 starvation may occur close to the inlet.
In a non-limiting example, the shunt material may be included in one or more regions or locations of the MEA, MEA membrane, or both. In these regions or locations, the shunt material may be included in a higher concentration within the blend of the ionically conductive polymer and the shunt material than in other areas of the membrane, at least one other region of the membrane, or the remainder of the membrane. The inclusion of the shunt material within the MEA, MEA membrane may be limited to these region(s) or location(s) such that the remainder or the membrane is substantially or entirely free of the shunt material. As a result, the MEA, MEA membrane may include one or more shunt-free regions and one or more shunt regions, the shunt regions including the shunt material in one or more forms.
The membrane may include one or more first regions and second regions. The second regions may include the shunt material. The first regions may be shunt material free or include the shunt material in a lower concentration than the second regions. The second regions may include a higher concentration or elevated amounts/volumes of the shunt material. The second regions may be the shunt regions.
The shunt region(s) may include the shunt material pockets, islands, or discrete areas which may be free of the ionically conductive material, or which may include a blend of the shunt material with the ionically conductive material.
The shunt region(s) may be structured symmetrically, each island including the same volume or weight percentage of the shunt material. Alternatively, the placement and concentration of the shunt material within the shunt region(s) may be random.
The shunt region(s) may extend from the cathode to the anode or an entire width of the membrane between the electrodes.
Furthermore, the one or more shunt regions may include an area structured to have higher ionic conductivity than other regions within the cell, MEA, and/or membrane. Such region may be structured to tolerate a lower concentration of the ionically conductive material or a proton conductor and thus a higher concentration of the shunt material.
The one or more shunt regions may include one or more areas of the MEA, membrane, or both which have increased, elevated, or higher moisture content (during operation of the cell) than other areas of the membrane. Increased moisture content may impart higher proton conductivity through the membrane.
The one or more shunt regions or locations may include an area within the MEA, MEA membrane, or both. The shunt regions or locations may include an area adjacent to, in vicinity of, or aligned with an exit of the cathode flow field. The cathode flow field may include the bipolar plate, GDL or both. Due to water generation in the cathode during operation of the cell, the membrane tends to be wetter near the exit of the cathode flow field, as opposed to the entrance. In this location, the shunt material may be in proximity, close proximity, or be adjacent to the exit of the cathode flow field. The cathode flow field exit is shown schematically in
The location of the shunt material is further schematically illustrated in
In addition, or alternatively, the one or more shunt regions or locations may include one or more land regions of the cathode flow field. The land regions may be rib regions or landings of the BPP. The land regions may include regions of the BPP which are in contact with the MEA, GDL, or both. The shunt material may be included adjacent to, in the vicinity of, around, aligned with, facing, and/or under one or more land regions, landings, or ribs. For example, the shunt material may be included under or facing each rib or land region. Alternatively, the shunt material may be included facing every other rib or land region of the cathode flow field. The inclusion of the shunt material in this location may be structured into a pattern. The shunt material may be included along a portion of a land region or under the entire width of the land region. The cathode under the land regions tends to be wetter or have increased moisture content.
Non-limiting example of the land regions or ribs is shown in
An alternative depiction of the shunt material location is shown in
Alternatively, or in addition, the shunt material may be included in an area adjacent to, in proximity, in close proximity to, or aligned with the exit and/or entrance of the anode flow field 255. In some embodiments, the shunt material may be included facing anode land regions if they coincide with the cathode land regions.
A non-limiting example of a cell 230 having the shunt material 250 incorporated in the proximity of the entrance of the anode flow field 255 is shown in
A further depiction of the anode flow field 255 and the BPP 200 is shown in
Yet, the system may include the shunt material in just one location such as either by the entrance of the anode flow field or being aligned with the exit of the anode flow field. Alternatively, a combination of these locations may include the shunt material.
Additionally, during startup when air is present in the anode flow channels, a H2/air front moves from the entrance of the anode flow field to the exit. Hence, the flow field exit may experience conditions conducive to C corrosion for a longer period of time than the entrance or middle sections of the flow field. A desirable location for the shunt material/region may be near the anode exit location.
Moreover, during shutdown, if air is coming into the anode flow field and creating an H2/air front (although typically a N2 purge may be implemented to avoid the front creation), C corrosion may occur near the anode entrance. Including the shunt material aligned with the anode entrance may be thus desirable.
Various factors may influence the placement of the shunt material. As was discussed above, several of the factors include water accumulation, moisture content, C corrosion, H2 starvation. Additional factors may include placement of the shunt material within the cell such as cell configuration: co-flow, counter flow, or cross flow.
In a co-flow system, the anode and cathode flow field entrances are located near each other, and the overall flow is primarily in the same direction toward exits located near each other. The shunt material/region may be thus located in the vicinity of the flow field exits, the regions of which overlap, under the cathode flow field land region(s), or both.
In a counterflow system, the anode and cathode streams flow in opposite directions. The shunt material/region may be placed in the vicinity of both the cathode and anode flow field exits, under the cathode flow field land region(s), or both.
In a cross flow system, the flow directions are orthogonal. The shunt material/region may be placed in the vicinity of the overlapping cathode and anode exits, specifically in a corner of the cell with the overlapping exits, under the cathode flow field land region(s), or both.
In some aspects, a bilayer or multiple membrane layers are used that include electroactive polymers that become electronically conductive in different potential windows such that all polymers are stable during normal operation of the cell and when the shunt is activated via excessive anodic potential at the positive electrode. This may be advantageous because even the open circuit potential may be high enough to drive degradation reactions at the positive electrode. In this configuration the fuel cell stack behaves as an electronic resistor when current is not applied (or when a sufficiently low current is applied) and the positive electrode potential of each cell may exceed the redox potential of the electroactive polymer.
The reversible hydrogen electrode (RHE) is described by Equation 1:
where SHE is the standard hydrogen electrode, defined as 0 V 25° C., 1 atm pressure, and 1 M H+ concentration. Here T=temperature in Kelvin, R=the gas constant (8.314 J/mol-K), F is Faraday's constant (96485 C/mol), aH+ is the activity or concentration of protons in the proton-conducting polymer, and pH2 is the partial pressure of hydrogen gas at the catalyst-polymer interface or gas-catalyst-polymer triple-phase boundary.
For a fuel cell operating at 80° C., a hydrogen pressure of 2 atm, and proton concentration of 2 M (i.e., pH=˜−0.3), the potential at the negative electrode of the fuel cell, where H2 gas is oxidized into protons, would be 10.5 mV vs. SHE.
A typical operating potential of the positive electrode would be 0.6 to 0.8 V vs. RHE, or up to ˜0.81 V vs. SHE.
The electroactive polymer poly-3-butyl thiophene (P3BT) is oxidized and becomes increasingly conductive in the potential window 3 to 4.1 V vs. Li metal. See Thomas-Alyea et al., J. Electrochem. Soc. A509 (2004), which is incorporated by reference herein for such teachings. The reversible Li metal potential is-3.04 V vs. SHE; accordingly, P3BT is reversibly oxidized in the potential window-0.04 to 1.06 V vs. SHE. The conductivity increases as the oxidation state of P3BT increases. Id. For example, at an oxidation state of 0.2, which is reached at a potential of approximately 4.03 V vs. Li, or 0.99 V vs. SHE, the polymer conductivity is between 1×10−3 and 1×10−2 S/cm. At the typical operating potential of 0.81 V vs. SHE, the conductivity is below 1×10−4 S/cm.
Assuming a membrane thickness of 20 μm (typical for PEMFC), about 10% volume fraction of P3BT in the membrane 105, 106, and a tortuosity factor of 2, the electronic membrane resistance would be 4 ohm-cm2 at a conductivity of 1×10−2 S/cm and 400 ohm-cm2 at a conductivity of 1×10−4 S/cm.
At the typical operating positive electrode potential of 0.81 V vs. SHE, the membrane 105, 106 is effectively an electronic insulator (vast majority of current is carried by protons). The corresponding membrane electronic resistance in this case is >1000 ohm-cm2. See
Meyers and Darling show that a current density of <10 mA/cm2 would be sufficient to sustain oxygen reduction at the negative electrode. Meyers and Darling, J. Electrochem. Soc. A1432 (2006). Without a shunt, as described herein, the corresponding reaction at the positive electrode may involve carbon corrosion and or oxygen evolution at potentials above 1.5 V vs. SHE. See
The shunt, once activated (105, 107), effectively deactivates regions of the cell where there is H2 depletion in the flow field adjacent to the negative electrode, which may occur during startup, shutdown, or due to local blockage of the flow field or gas diffusion layer by water droplets.
The oxidation (e.g., via doping) of the semiconductor (e.g., P3BT) is expected to occur at high potentials in the presence of sulfonyl groups in the proton conducting membrane. Protons of the polysulfonic acid are displaced by positively charged portions of the thiophene (or other electroactive polymer), maintaining an overall electrically neutral membrane. The protons combine with reduced oxygen at the positive electrode to form water. When the potential of the positive electrode drops, the thiophene groups will be reduced and lose their positive charge and the negative charge of the sulfonyl groups will again be balanced by the positive charge of protons generated from H2 gas at the negative electrode.
It will be apparent to one of ordinary skill in the relevant art that suitable modifications and adaptations to the compositions, formulations, methods, processes, and applications described herein can be made without departing from the scope of any embodiments or aspects thereof. The compositions and methods provided are exemplary and are not intended to limit the scope of any of the specified embodiments. All the various embodiments, aspects, and options disclosed herein can be combined in any variations or iterations. The scope of the compositions, formulations, methods, and processes described herein include all actual or potential combinations of embodiments, aspects, options, examples, and preferences herein described. The exemplary apparata, designs, compositions, and formulations described herein may omit any component, substitute any component disclosed herein, or include any component disclosed elsewhere herein. Should the meaning of any terms in any of the patents or publications incorporated by reference conflict with the meaning of the terms used in this disclosure, the meanings of the terms or phrases in this disclosure are controlling. Furthermore, the foregoing discussion discloses and describes merely exemplary embodiments. All patents and publications cited herein are incorporated by reference herein for the specific teachings thereof.
While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.
This application is a continuation-in-part of U.S. application Ser. No. 18/051,645, filed Nov. 1, 2022, published as U.S. Patent Application Publication No. 2023/0112131, which is a continuation-in-part of U.S. application Ser. No. 16/867,848, filed May 6, 2020, published as U.S. Patent Application Publication No. 2021/0351426, now U.S. Pat. No. 11,515,555 issued Nov. 9, 2022, the disclosures of which are hereby incorporated in their entirety by reference herein.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 18051645 | Nov 2022 | US |
| Child | 18830059 | US | |
| Parent | 16867848 | May 2020 | US |
| Child | 18051645 | US |
