Patent Application
20240401214
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.
The present disclosure relates to electrochemical cells with membrane electrode assemblies for carbon dioxide reduction.
Electrolytic carbon oxide reduction reactors are being developed for capturing and converting carbon oxides to useful chemical products such as carbon monoxide and oxygen. Challenges remain for efficiently operating anodes in carbon oxide electrolyzers, particularly electrolyzers employing membrane electrode assemblies.
Background and contextual descriptions contained herein are provided solely for the purpose of generally presenting the context of the disclosure. Much of this disclosure presents work of the inventors, and simply because such work is described in the background section or presented as context elsewhere herein does not mean that such work is admitted prior art.
This summary is provided to introduce some concepts in simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features or essential features of the claimed subject matter.
Aspects of this disclosure pertain to carbon oxide electrolyzers that may be characterized by the following features: (a) a membrane electrode assembly (MEA) comprising a cathode layer, an anode layer, and one or more polymeric electrolyte layers between and in contact with the cathode layer and the anode layer; and (b) a porous transport layer (PTL) in contact with the anode layer and having a porous structure with an average pore size of about 10 to 50 μm, and an average porosity of about 20 to 50%. In certain embodiments, the carbon oxide electrolyzer is configured to electrolyze carbon dioxide and produce carbon monoxide. In certain embodiments, the carbon oxide electrolyzer is configured to electrolyze carbon dioxide and produce a hydrocarbon, an alcohol, a carboxylic acid, an aldehyde, or any combination thereof.
In some implementations, the carbon oxide electrolyzer includes an anode flow field in contact with the PTL, on a side of the PTL opposite from the anode layer.
In certain embodiments, the carbon oxide electrolyzer is configured to operate at a temperature of about 0 to 80° C., electrolyze anode water having a metal ion concentration of at least about 0.5 mM, and operate at an anode current density of about 1000 mA/cm2 or lower. In some cases, a carbon oxide electrolyzer is configured to electrolyze anode water at a rate of about 1 L/hr or greater.
In certain embodiments, the PTL comprises titanium having a concentration of at least about 50% by weight. In certain embodiments, the PTL comprises niobium having a concentration of at least about 50% by weight. In certain embodiments, the PTL comprises a base material and coating comprising platinum, gold, titanium nitride, or any combination thereof.
In certain embodiments, the PTL has a thickness about 0.2-0.5 mm. In certain embodiments, the PTL has a compressibility, through its thickness, of about 0.7 to 1.3 μm@1 MPa, or about 1.2 to 1.8 μm@2 MPa, or about 1.7 to 2.3 μm@4 MPa. In certain embodiments, the PTL has a flexural modulus of about 100-170 GPa. In certain embodiments, the PTL has a yield strength of about 800 to 900 MPa. In certain embodiments, the PTL has a roughness of about 11 to 16 μm where the PTL contacts the anode layer.
In certain embodiments, the PTL comprises a composite structure.
In certain embodiments, the PTL has a graded structure in which the average pore size and/or the average porosity varies when moving in a direction away from the anode layer of the MEA. IN certain embodiments, the PTL has a graded hydrophobicity in which the PTL's hydrophobicity increases when moving in a direction away from the anode layer of the MEA.
In certain embodiments, the PTL has a mesh structure, and the carbon oxide electrolyzer does not include an anode flow field. In certain embodiments, the PTL comprises of sintered particles having non-spherical shapes.
Any combination of the above features may be implemented together in the carbon oxide electrolyzer aspects of this disclosure.
Aspects of this disclosure pertain to carbon oxide electrolyzers that may be characterized by the following features: (a) a membrane electrode assembly (MEA) comprising a cathode layer, an anode layer, and one or more polymeric electrolyte layers between and in contact with the cathode layer and the anode layer; (b) a microporous layer (MPL) in contact with the anode layer; and (c) a porous transport layer (PTL) in contact with the MPL on a side of the MPL opposite the anode layer, wherein the PTO has a porous structure with an average pore size of about 10 to 50 μm, and an average porosity of about 20 to 50%.
In certain embodiments, the carbon oxide electrolyzer includes an anode flow field in contact with the PTL, on a side of the PTL opposite from the MPL. In certain embodiments, the carbon oxide electrolyzer is configured to electrolyze anode water flow rate of about 1 L/hr or greater. In certain embodiments, the MPL has a roughness of about 11 to 16 μm where the MPL contacts the anode layer.
Any combination of the above features may be implemented together in electrolyzer aspects of this disclosure.
Aspects of this disclosure pertain to methods of operating a carbon oxide electrolyzer, which methods may be characterized by the operations: (I) providing anode water to an anode portion of a carbon oxide electrolyzer comprising: (a) a membrane electrode assembly (MEA) comprising a cathode layer, an anode layer, and one or more polymeric electrolyte layers between and in contact with the cathode layer and the anode layer, and (b) a porous transport layer (PTL) proximate to the anode layer and having a porous structure with an average pore size of about 10 to 50 μm, and an average porosity of about 20 to 50%; (II) electrochemically oxidizing the anode water at the anode layer to produce oxygen; and (III) electrochemically reducing the carbon oxide at the cathode layer to produce a carbon-containing product.
In certain embodiments, the carbon oxide electrolyzer includes an anode flow field in contact with the PTL, on a side of the PTL opposite from the anode layer.
In certain embodiments, the operation of electrochemically reducing the carbon oxide at the cathode layer comprises electrolyzing carbon dioxide to produce carbon monoxide. In certain embodiments, the operation of electrochemically reducing the carbon oxide at the cathode layer comprises electrolyzing carbon dioxide to produce a hydrocarbon, an alcohol, a carboxylic acid, an aldehyde, or any combination thereof.
In certain embodiments, the operation of electrochemically oxidizing the water at the anode layer is performed at a temperature of about 0 to 80° C. and an anode current density of about 1000 mA/cm2 or lower.
In certain embodiments, the anode water has a metal ion concentration of at least about 0.5 mM.
In certain embodiments, the operation of electrochemically oxidizing the anode water at the anode layer comprises flowing the anode water to the anode layer at a flow rate of about 1 L/hr or greater.
In certain embodiments, the PTL has a graded structure in which the average pore size and/or the average porosity varies when moving in a direction away from the anode layer of the MEA. In certain embodiments, the PTL has a graded hydrophobicity in which the PTL's hydrophobicity increases when moving in a direction away from the anode layer of the MEA.
In certain embodiments, the PTL has a compressibility, through its thickness, of about 0.7 to 1.3 μm@1 MPa, or about 1.2 to 1.8 μm@2 MPa, or about 1.7 to 2.3 μm@4 MPa. In certain embodiments, the PTL has a flexural modulus of about 100-170 GPa. In certain embodiments, the PTL has a yield strength of about 800 to 900 MPa. In certain embodiments, the MPL has a roughness of about 11 to 16 μm where the MPL contacts the anode layer.
In certain embodiments, the carbon oxide electrolyzer further comprises a microporous layer (MPL) between the anode layer and the PTL, and in contact with the anode layer. In some implementations, the carbon oxide electrolyzer further comprises an anode flow field in contact with the PTL, on a side of the PTL opposite from the MPL.
Any combination of the above features may be implemented together in method aspects of this disclosure.
These and other features of the disclosure will be presented below, sometimes with reference to drawings.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. The terms presented immediately below may be more fully understood by reference to the remainder of the specification. The following descriptions are presented to provide context and an introduction to the complex concepts described herein. These descriptions are not intended to limit the full scope of the disclosure.
An “electrochemical cell” comprises an anode, a cathode, and electrolyte between the anode and cathode. At least one of the anode and cathode can undergo, catalyze, or otherwise support a faradaic reaction. In an electrolytic electrochemical cell, an external circuit applies an electrical potential difference between the anode and cathode, and that potential difference drives the faradaic reaction(s). Examples include electrolyzers such as CO2 electrolyzers and water electrolyzers.
As used herein, the term carbon oxide includes carbon dioxide (CO2), carbon monoxide (CO), carbonate ions (CO32−), bicarbonate ions (HCO3−), and any combinations thereof. Carbonate and bicarbonate ions may be viewed as ions that “carry” or “hold” CO2 in a form that can be dissolved, melted, or otherwise provided in a liquid form, at least temporarily. For convenience, this disclosure frequently refers to carbon dioxide reduction or carbon dioxide electrolyzers, when those of skill in the art will understand that the described principles and details can be applied more generally to carbon oxide reduction or carbon oxide electrolyzers.
A “flow field” is a structure containing passages for delivering a reactant such as water or a gas to an anode or cathode of an electrolyzer. A flow field may distribute the reactant fluid evenly over the active area of an electrolyzer cell. In certain embodiments, a flow field comprises a solid structure that may help support an MEA. A flow field may also facilitate water and heat management, sometimes in conjunction with a porous transport layer. As examples, flow fields may be fabricated from graphite, coated or non-coated metal, polymer-carbon or polymer-metal composites. A flow field may be designed and machined to supply the reactant at a specified rate with minimum pressure drop to the anode or cathode of the electrolyzer. A flow field may contain channels allowing even distribution of the reactant. As examples, the channels may be serpentine, parallel, pin-type, and/or interdigitated.
A mixture contains two or more components and unless otherwise stated may contain components other than the identified components.
Elements, units, or components herein are sometimes described or claimed as “configured to” perform an operation or operations. In such contexts, the phrase “configured to” is used to connote structure by indicating that the element, unit, or component includes structure or structural features and optionally control elements (e.g., processors, stored instructions, circuitry, etc.) that perform in a particular way during operation. As such, an element, unit, or component can be said to be configured to perform in that way even when the specified component is not necessarily currently operational (e.g., is not in use).
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 of 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 term “and/or,” when used in a list of two or more items, means that any one of the listed items can be employed by itself, or any combination of two or more of the listed items, can be employed. For example, if a composition is described as containing components A, B, and/or C, the composition can contain A alone; B alone; C alone; A and B in combination; A and C in combination; B and C in combination; or A, B, and C in combination.
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.
Carbon oxide electrolyzers containing polymer-based membrane electrode assemblies (MEAs) are designed and/or configured to produce oxygen from water at an anode and produce one or more carbon-containing compounds through the electrochemical reduction of carbon dioxide or other carbon oxide at a cathode. Various examples of MEAs and MEA-based carbon oxide electrolyzers are described in the following references: Published PCT Application No. 2017/192788, published Nov. 9, 2017, and titled “REACTOR WITH ADVANCED ARCHITECTURE FOR THE ELECTROCHEMICAL REACTION OF CO2, CO, AND OTHER CHEMICAL COMPOUNDS,” Published PCT Application No. 2019/144135, published Jul. 25, 2019, and titled “SYSTEM AND METHOD FOR CARBON DIOXIDE REACTOR CONTROL,” and Published PCT Application No. 2021/108446, published Jun. 3, 2021, and titled “MEMBRANE ELECTRODE ASSEMBLY FOR COX REDUCTION,” each of which is incorporated herein by reference in its entirety.
The anode side of the carbon dioxide electrolyzer may have an arrangement of structural elements that includes a porous transport layer (PTL). In some embodiments, the elements include a flow field on one side, an anode portion of a membrane electrode assembly (MEA) on the other side, and a PTL between the flow field and the MEA.
In other embodiments, the anode layer of the MEA is supplemented or replaced by an anode structure that has features of a gas diffusion electrode such as a conventional gas diffusion electrode. In some cases, such anode structure has features of a MPL, so the anode itself provides some useful properties of the MPL such as improved mass transport of reactant (water) and product. As examples, the gas diffusion electrode may have a porous or mesh structure. An assembly including an MEA, a gas diffusion anode, and optionally an additional PTL may be pressed (optionally hot pressed) together in a traditional stack arrangement.
In embodiments that employ a microporous layer (MPL), the MPL may serve any one or more of various purposes. Examples of these include:
Aspects of this disclosure pertain to PTLs and associated components on the anode side of CO2 electrolyzer employing a membrane electrode assembly (MEA) such as described herein. In some cases, the CO2 electrolyzer may be characterized by the following operating conditions: (a) a temperature of about 0 to 120° C. or about 0 to 80° C., (b) anode water having a metal ion concentration of at least about 0.5 mM, and (c) an anode current density of up to about 15,000 mA/cm2 or about 1000 mA/cm2 or lower. Examples of anode water composition are presented below in the discussion of MEA embodiments.
Additional examples of operating conditions of MEA-based CO2 electrolyzers are described in PCT Patent Publication No. 2022031726, published on Feb. 10, 2022, which is incorporated herein by reference in its entirety.
An MEA-containing CO2 electrolyzer that operates in ranges such as these may have any one or more of the characteristics described below for a CO2 electrolyzer or for a component of the anode side of the electrolyzer. On the other hand, the characteristics described below may be implemented on a CO2 electrolyzer that does not necessarily operate in the above ranges.
A PTL design can affect anode water flux, including dissolved ion flux, to the anode catalyst layer. A PTL can promote good liquid access from the bulk anode water to the catalyst. Overall, the components associated with a CO2 electrolyzer anode that impact mass transport, including the PTL, may be configured to provide anode water at a stoichiometry of about 100 or greater. This means that the molar flow rate of water to the anode catalyst is at least 100 times the reaction rate of water oxidation based on total current to anode. For reference, this may translate to an anode water flow rate of about 1 L/hr or greater in a CO2 electrolyzer. Generally, this flow rate is influenced by the operating conditions, the anode water manifold structure, the flow field geometry, and the PTL's geometry and its ability to manage water transport to the catalyst and to remove oxygen from the catalyst layer. In certain embodiments, the anode water is continuously supplied to the anode. In certain embodiments, the anode water is supplied to the anode intermittently or in variable flow rates, e.g., by pulses.
In some embodiments, a PTL employs a mesh structure that is so open that it replaces the function of the flow field, thus eliminating a separate flow field structure.
In some embodiments, a PTL has a porosity of at most about 60%, or at most about 50%, or about 20-50%, or about 35-40%. In some embodiments, a PTL, such as one having any of the porosity ranges listed here, has an average pore diameter about 10-50 μm. In some embodiments, these values are employed in PTLs having uniform (non-graded) porosity and/or pore size. Design considerations for choice of porosity and pore size may balance the ohmic loss and mechanical fragility that larger pore diameters and porosities yield, while providing more open structure for water/ion delivery that the higher porosity yields.
In some embodiments, the PTL comprises a corrosion resistant material. Examples of suitable metals for the PTL include titanium and niobium. In some embodiments, a PTL comprises a composite, layered, or coated material. In some embodiments, a PTL comprises a non-metal such as a metal nitride, e.g., titanium nitride. In some embodiments, a PTL comprises a base material coated with another material. Examples of such structures are described below.
In certain embodiments, the average PTL thickness is about 0.2-0.5 mm prior to installation. Note that in some embodiments, the PTL is generally incompressible, at least over the pressure ranges employed to create a CO2 electrolyzer stack.
Non-Uniform PTL Properties (Properties that can Vary with Depth Along the Flow Path of Water and Oxygen)
In some PTL designs, the hydrophobicity, porosity, or other property of the PTL transitions from the flow field side to the MEA side of the PTL. In some examples, the PTL hydrophobicity transitions from more hydrophobic to less hydrophobic from the flow field side to the MEA side. A graded hydrophobicity in this direction may facilitate the transport of salt-rich anode water to the MEA. The graded hydrophobicity may also facilitate the transport of O2 or other gaseous components out from the catalyst to the backside of the PTL, i.e., the side adjacent to the flow field. As an example, the material comprising the flow field side of the PTL may produce a contact angle of about 100° to 150° while the material comprising the MEA side of the PTL may produce a contact angle of about 90° or less. Note that hydrophobicity is conventionally measured using contact angle, but it may also be measured using permeability or capillary pressure measured with various fluids.
In some embodiments, the average pore size and/or porosity varies from smaller/lower to higher/greater when moving from MEA to the flow field. If a CO2 electrolyzer employs an MPL, the variation across the PTL may be smaller in terms of pore diameter/porosity moving from the cathode layer of the MEA to the flow field. In some embodiments, an MPL has a pore size of about 2 to 5 μm. In some embodiments, an MPL has a porosity of about 10 to 15%.
Layered Construction, which have Separate Layers (e.g., MPL and PTL) Versus a Single Integrated Structure that has Inseparable Regions.
A multi-layered transport structure includes two or more layers, e.g., 2 to 5 layers. Each such layer may have its own composition and physical or chemical features. Examples of such layer-specific features include porosity and hydrophobicity. In designs having an MPL adjacent the MEA, the MPL may, in some embodiments, have average pore sizes of about 2 to 5 μm and porosities of about 10 to 15%. In some designs, a mesh structure (e.g., a wire mesh structure) is provided adjacent to the flow channel. Such mesh structure may have larger pore size and less mass transport resistance than the PTL
In certain embodiments, a PTL has a compressibility (through its thickness) of about 0.7 to 1.3 μm@1 MPa, or about 1.2 to 1.8 μm@2 MPa, or about 1.7 to 2.3 μm@4 MPa. In other examples, a PTL is more compressible. For example, a PTL may exhibit a compressibility of about 10-50 μm@1 MPa, or about 30-60 μm@2 MPa, or about 40-70 μm@4 MPa. In some cases, PTLs with such high compressibility do not exhibit compression set. The flexibility in the thickness direction can act as local springs to maintain PTL compression in the MEA active area and can increase PTL compression tolerances. In certain embodiments, a PTL has a flexural modulus of about 10-170 GPa. For example, the PTL may have a flexural modulus of about 10-150 GPa, about 10-130 GPa, about 10-110 GPa, about 10-90 GPa, about 10-70 GPa, about 10-50 GPa, about 10-30 GPa, about 50-150 GPa, about 75-150 GPa, about 100-150 GPa, about 75-125 GPa, or combinations thereof. In specific examples, the PTL may have flexural modulus of about 170 GPa or less, about 150 GPa or less, about 125 GPa or less, about 100 GPa or less, about 75 GPa or less, about 50 GPa or less, about 25 GPa or less, and/or combinations or ranges thereof. In certain embodiments, a PTL has a yield strength of about 20 to 1300 MPa. This yield strength range can correspond to, at least, titanium powder sintered PTLs and/or titanium fiber sintered PTLs. In certain embodiments, a PTL has a yield strength of about 800 to 1300 MPa. Titanium fiber sintered PTLs may provide greater yield strengths than titanium powder sintered PTLs. Accordingly, in embodiments where the PTL has a yield strength of about 800 to 1300 MPa, such PTLs may correspond to, at least, titanium fiber sintered PTLs. In various examples, the PTL may have a yield strength that is about 20-1300 MPa, about 30-1100 MPa, about 40-900 MPa, about 50-700 MPa, about 60-500 MPa, about 70-300 MPa, about 80-100 MPa, about 50-1250 MPa, about 80-1200 MPa, about 110-1150 MPa, about 130-1100 MPa, about 150-1050 MPa, about 170-1000 MPa, about 190-950 MPa, about 210-900 MPa, and/or combinations thereof.
In some embodiments, a PTL or an MPL comprises small particles, which may collectively form a unitary structure via, for example, sintering. The component particles have various geometric properties such as being flat or irregularly shaped versus being spherical. In some embodiments, the component particles comprise mixes of shapes. In some embodiments, a PTL comprises sintered irregularly shaped particles. In some cases, particles comprising a PTL have shapes with facets. Other shapes may include fibers, and a grate or mesh open framework. In some embodiments, the particles comprising a PTL include agglomerates of particles, such as spherical particles. It has been observed that PTLs made from sintered irregularly shaped particles can have a higher yield strength for similar porosity compared with PTLs made from regularly shaped particles.
The “fineness” or uniformity of texture may impact catalyst performance. In a PTL, the finer the surface facing the MEA, the better the physical contact with the anode catalyst. This can result in lower ohmic resistance, which can provide improved electrolyzer performance, especially when the catalyst loading is low, such as about 0.1 mg/cm2 or lower. In some embodiments, a PTL or an MPL contacting an MEA cathode layer has a roughness of about 11 to 16 μm. On the flow field side of a PTL, some designs employ a wire mesh or similar structure that has a low resistance to mass transport of both anode water and gas. Other structures besides wire meshes may include fibrous structures and sintered particle structures. In some such designs, the mesh or other structure may have a porosity of about 60-80%.
In certain embodiments, the PTL material and/or the MPL material has an average hydrophobicity of about 80° to 150°.
In some implementations, water electrolyzers employ PTLs in an arrangement similar to that described herein. However, while the anode side of a CO2 electrolyzer has certain features in common with the anode side of a water electrolyzer, there are important differences. Among the special considerations for CO2 electrolyzers are the following.
Temperature: CO2 electrolyzer operate at a lower temperature than H2O electrolyzers. This means that electrical resistance and mass transfer resistance are greater in CO2 electrolyzers than in H2O electrolyzers.
Anode water composition: The anode water of CO2 electrolyzers contains ions such as K+ ions, while the anode water of H2O electrolyzers do not. PTLs for CO2 electrolyzers may contain design features that promote uniform and effective delivery of K+ ions to the anode. PTLs with relatively high porosity (e.g., greater than about 50% porosity) used with anode water can exhibit relatively high ionic conductivity and pH at the cathode. Further, such electrolyzers can operate at relatively lower voltages compared to electrolyzers using PTLs with lower porosity.
Chemically aggressive environment: The cathodes of CO2 electrolyzer can produce formic acid and other aggressive chemical species. Thus, in some embodiments, the PTL in a CO2 electrolyzer has a composition that resists liberating some titanium and/or iron from the PTL. In some cases, a PTL comprises a base structure such as titanium or iron and a corrosion resistant coating such as platinum, gold, or titanium nitrides.
High current densities: In some implementations, the current density of an operating CO2 electrolyzer is lower than that of an operating water electrolyzer, even at low operating temperatures. In such cases, a low porosity PTL (e.g., at most about 50% porosity) may be used in a CO2 electrolyzer. Such PTLs may have high mechanical stability and be relatively thin.
Relatively low voltage efficiency: Electrolytic CO2 reduction has a relatively lower voltage efficiency than electrolytic water reduction. This low voltage efficiency can result in higher voltages and higher waste heat generation. Stated another way, CO2 electrolzyers generally operate at lower voltage efficiencies than water electrolyzers. Consequently, CO2 electrolyzers operate at higher voltages and typically produce more heat. Therefore, CO2 electrolyzer cells and their PTLs may be designed to facilitate cooling. In certain embodiments, a PTL's pore diameter/porosity affect the circulation and cooling of a CO2 electrolyzer. Noter that while water circulation at the anode is often the primary mode of cooling for CO2 electrolyzers, the cathode layer's temperature can drive reaction selectively and/or other parameters of interest. While the thermal conductivity of the anode's PTL material strongly contributes to electrolyzer cooling, convective cooling from the water circulation is often the main contributor. The PTL's pore diameter/porosity influence the circulation. The ranges that facilitate water circulation may be above about 20% porosity. Otherwise, porosity and pore size parameters ranges described elsewhere herein will be adequate to facilitate cooling the CO2 electrolyzer.
As indicated, in some embodiments, an MEA-containing CO2 electrolyzer is configured to operate at a temperature of about 0 to 80° C., employ anode water having a metal ion concentration of at least about 0.5 mM, and operate at an anode current density of about 1000 mA/cm2 or lower. A CO2 electrolyzer configured to operate in these ranges may have any one or more of the distinguishing characteristics described in this section or elsewhere in this disclosure.
A PTL or MPL as described herein may be fabricated by various techniques. Mesh and sintered PTL structures can be fabricated by conventional technologies. More exotic PTL or MPL structures may require less conventional processes. For example, additive manufacturing (3D printing) may be employed to produce MPL or PTL structures having graded properties such as graded porosity or graded texture or roughness. In another example, multilayer or multi-component PTL and/or MPL structures may be fabricated by compositing or by attaching elements to one another in a PTL structure by, e.g., diffusion bonding or laser welding.
As explained, a PTL or MPL may have a layered or composite structure. In some cases, such structure may comprise a base material and one or more coatings on such base material.
A coating may help prevent corrosion of PTL (e.g., due to formic acid from the cathode); corrosion results in loss of PTL material and possibly migration of dissolved PTL material to cathode where it could have a negative impact. Additionally, a coating may reduce electrical resistance of the PTL or MPL and/or impart a desired amount of hydrophobicity. Examples of protective coatings may include Pt, Au, and nitrides (e.g., TiNx). In some implementations, the coating is an oxide such as a titanium oxide, which may be stable to a harsh environment such as high pH, high temperature, and/or the presence of hydrogen.
Coatings on PTLs or MPLs may be produced by any of various techniques such as sputtering, electroplating, etc. Uniformly coating internal pores of a PTL may be challenging.
In some embodiments, a PTL coating has multiple layers. For example, a first layer (an underlayer) of the coating that is only moderately protective, and a second layer (an outer layer) is a more noble and corrosion resistant material. Such structures may use only small amounts of scarce or expensive materials such as platinum. As an example, the underlying structure of the PTL is a stainless steel (e.g., 316 SS) that is clad or PVD coated with titanium (under layer) and platinum (outer layer). In another example, a PTL has a graded composition with more corrosion resistant material closer to the MEA. For example, the PTL may have a stacked structure such as having stainless steel disposed away from the electrode and have titanium closer to the MEA or in the middle, and have titanium coated with platinum adjacent the electrode.
As depicted, the cathode subsystem includes a carbon oxide source 209 configured to provide a feed stream of carbon oxide to the cathode of electrolyzer 203, which, during operation, may generate an output stream 208 that includes product(s) of a reduction reaction at the cathode. The product stream 208 may also include unreacted carbon oxide and/or hydrogen.
The carbon oxide source 209 is coupled to a carbon oxide flow controller 213 configured to control the volumetric or mass flow rate of carbon oxide to electrolyzer 203. One or more other components may be disposed on a flow path from flow carbon oxide source 209 to the cathode of electrolyzer 203. For example, an optional humidifier 204 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 217. In certain embodiments, purge gas source 217 is configured to provide purge gas during periods when current is paused to the cell(s) of electrolyzer 203. 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. Further details of MEA cathode purge processes and systems are described in US Patent Application Publication No. 20220267916, published Aug. 25, 2022, which is incorporated herein by reference in its entirety.
During operation, the output stream from the cathode flows via a conduit 207 that connects to a backpressure controller 215 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 reaction products 208 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 electrolyzer 203. 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 209 upstream of the cathode.
As depicted in
In certain embodiments, the anode water comprises about 10 uM to 100 mM of a salt, or about 1 mM to 50 mM of a salt, or about 10 mM to about 50 mM of a salt. In certain embodiments, the salt comprises alkali metal ions such as ions of potassium, cesium, rubidium, or any combination thereof. In certain embodiments, an anion of the salt is phosphate, sulfate, carbonate, bicarbonate, hydroxide, or any combination thereof. In some examples, the salt comprises (i) an alkali metal cation and (ii) a bicarbonate, hydroxide, or sulfate anion. As further examples, such salt may be present in the anode water at a concentration of about 10 uM to 20 mM. Further, examples of anode water compositions for various carbon oxide electrolyzer configurations are presented in US Patent Application Publication No. 20200240023, published Jul. 30, 2020, which is incorporated herein by reference in its entirety.
During operation, the anode subsystem may provide water or other reactant to the anode of electrolyzer 203, 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 201. For example, a temperature controller may be configured to heat and/or cool the carbon oxide electrolyzer 203 at appropriate points during its operation. In the depicted embodiment, a temperature controller 205 is configured to heat and/or cool anode water provided to the anode water recirculation loop. For example, the temperature controller 205 may include or be coupled to a heater and/or cooler that may heat or cool water in anode water reservoir 219 and/or water in reservoir 221. In some embodiments, system 201 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.
Depending upon the phase of the electrochemical operation, including whether current is paused to carbon oxide reduction electrolyzer 203, certain components of system 201 may operate to control non-electrical operations. For example, system 201 may be 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 electrolyzer 203. Components that may be controlled for this purpose may include carbon oxide flow controller 213 and anode water controller 211.
In addition, depending upon the phase of the electrochemical operation including whether current is paused, certain components of system 201 may operate to control the composition of the carbon oxide feed stream and/or the anode feed stream. For example, water reservoir 221 and/or anode water additives source 223 may be controlled to adjust the composition of the anode feed stream. In some cases, additives source 223 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 205 is configured to adjust the temperature of one or more components of system 201 based on a phase of operation. For example, the temperature of cell 203 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 225a and 225b are configured to block fluidic communication of cell 203 to a source of carbon oxide to the cathode and backpressure controller 215, respectively. Additionally, isolation valves 225c and 225d are configured to block fluidic communication of cell 203 to anode water inlet and outlet, respectively.
The carbon oxide reduction electrolyzer 203 may also operate under the control of one or more electrical power sources and associated controllers. See, block 233. Electrical power source and controller 233 may be programmed or otherwise configured to control current supplied to and/or to control voltage applied to the electrodes in reduction electrolyzer 203. The current and/or voltage may be controlled to execute the current schedules and/or current profiles described elsewhere herein. For example, electrical power source and controller 233 may be configured to periodically pause current applied to the anode and/or cathode of reduction electrolyzer 203. Any of the current profiles described herein may be programmed into power source and controller 233.
In certain embodiments, electric power source and controller 233 performs some but not all the operations necessary to implement desired current schedules and/or profiles in the carbon oxide reduction electrolyzer 203. A system operator or other responsible individual may act in conjunction with electrical power source and controller 233 to fully define the schedules and/or profiles of current applied to reduction electrolyzer 203. For example, an operator may institute one or more current pauses outside the set of current pauses programmed into power source and controller 233.
In certain embodiments, the electrical power source and an optional, associated electrical power controller acts in concert with one or more other controllers or control mechanisms associated with other components of system 201. For example, electrical power source and controller 233 may act in concert with controllers for controlling 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 203, controlling backpressure (e.g., via backpressure controller 215), supplying purge gas (e.g., using purge gas component 217), delivering carbon oxide (e.g., via carbon oxide flow controller 213), humidifying carbon oxide in a cathode feed stream (e.g., via humidifier 204), flow of anode water to and/or from the anode (e.g., via anode water flow controller 211), and anode water composition (e.g., via anode water source 205, pure water reservoir 221, and/or anode water additives component 223).
In the depicted embodiment, a voltage monitoring system 234 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. The voltage determined in this way can be used to control the cell voltage during a current pause, inform the duration of a pause, etc. In certain embodiments, voltage monitoring system 234 is configured to work in concert with power supply 233 to cause reduction cell 203 to remain within a specified voltage range. For example, power supply 233 may be configured to apply current and/or voltage to the electrodes of reduction cell 203 in a way that maintains the cell voltage within a specified range during a current pause. If, for example during a current pause, the cell's open circuit voltage deviates from a defined range (as determined by voltage monitoring system 234), 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.
In certain embodiments, a control system is configured to control the flow rate of one or more feed streams (e.g., a cathode feed stream such as a carbon oxide flow and an anode feed stream) in concert with a current schedule. For example, the flow of carbon oxide or a purge gas may be turned on, turned off, or otherwise adjusted when current applied to an MEA cell is reduced, increased, or paused.
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. 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.
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.
In various embodiments, an MEA contains an anode layer, a cathode layer, electrolyte, and optionally one or more other layers. The layers may be solids and/or gels. Any one or more of the MEA 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., hydrogen ions, hydroxide ions, or bicarbonate ions) that chemically react with the COx ions, and electrons. The reduction reaction may produce CO, hydrocarbons, and/or hydrogen and oxygen-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.
During operation of an MEA, ions move through one or more polymer layers that serve as the electrolyte, while electrons flow from an anode, through an external circuit, and to a cathode. In some embodiments, liquids and/or gas move through or permeates the MEA layers. This process may be facilitated by pores in one or more layers of the MEA.
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 to the anode or elsewhere in the MEA; (c) physical integrity of the MEA during the reaction (e.g., the MEA layers remain affixed to one another); (d) prevent COx reduction product cross-over; (e) prevent oxidation product (e.g., O2) cross-over; (f) a suitable environment at the cathode for the reduction reaction; (g) a pathway for desired ions to travel between cathode and anode while blocking undesired ions; and (h) low voltage operation.
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, which are often on the order of 5,000 hours. And for various applications, an MEA for COx reduction employs electrodes having a relatively large surface area by comparison to MEAs used for fuel cells in automotive applications. For example, MEAs for COx reduction may employ electrodes having 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 the 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 that would produce a short circuit. The cathode layer includes a reduction catalyst and, optionally, an ion-conducting polymer (sometimes called an ionomer). The cathode layer may also include an electron conductor and/or an additional ion conductor. The anode layer includes an oxidation catalyst and, optionally, an ion-conducting polymer. The anode layer may also include an electron conductor and/or an additional ion conductor. The PEM comprises an 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 comprises an ion-conducting polymer.
The ion-conducting polymers in the PEM, the cathode, the anode, and the cathode buffer layer, if present, may each be different from one another in composition, conductivity, molecular weight, or other property. In some cases, two or more of these polymers are identical or substantially identical. For example, the ion-conducting polymer in the cathode and cathode buffer layer may be identical.
In certain embodiments, the MEA has an anode buffer layer between the anode layer and the polymer electrolyte membrane. The anode buffer layer also comprises an ion-conducting polymer, which may have the same properties as any of the other ion-conducting polymers (e.g., the ion-conducting polymer in the anode). Or the ion-conducting layer of the anode buffer layer may be different from every other ion-conducting layer in the MEA.
In connection with certain MEA designs, the polymers in the layers may be chosen from among 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 ion-conducting polymers are from different classes of ion-conducting polymers.
In certain embodiments, an MEA has a bipolar interface, which means that it has one layer of anion-conducting polymer in contact with a layer of cation-conducting polymer. One example of an MEA with a bipolar interface is an anion-conducting cathode buffer layer adjacent to (and in contact with) a cation-conducting PEM. In certain embodiments, an MEA contains only anion-conducting polymer between the anode and the cathode. Such MEAs are sometimes referred to as “AEM only” MEAs. Such MEAs may contain one or more layers of anion-conducing polymer between the anode and the cathode.
The term “ion-conducting polymer” or “ionomer” is used herein to describe a polymer that conducts ions (anions and/or cations) is to say that the material is an ion-conducting material or ionomer. In certain embodiments, an MEA contains one or more ion-conducting polymers having a specific conductivity of about 1 mS/cm or greater 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 of about 0.85 or greater at around 100 micrometers 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 of about 0.85 or greater at about 100 micrometers 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 have a transference number greater than about 0.85 or less than about 0.15 at about 100 micrometers thickness. Examples of ion-conducting polymers of each class are provided in the below Table 1.
Examples of polymeric structures that can include an ionizable moiety or an ionic moiety and be used as ion-conducting polymers (ionomers) 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:
Further non-limiting polymeric structures can include one or more of the following:
or a salt thereof, wherein:
Yet other polymeric structures include the following:
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(cpichlorohydrin) (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 certain embodiments, the MEA includes a bipolar interface having 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.
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 decreases and the rate of production of CO or other carbon-containing product increases.
Another reaction that may be avoided is reaction of carbonate or bicarbonate ions at the anode to produce CO2. Aqueous carbonate or bicarbonate ions may be produced from CO2 at the cathode. If such ions reach the anode, they may react with hydrogen ions to produce 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 anode and/or an anode buffer layer may include a cation-conducting polymer, which at least partially blocks the transport of negative ions such as bicarbonate 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 and CO2 reduction products (e.g., bicarbonate) to the anode side of the cell.
An example MEA 300 for use in COx reduction is shown in
The ion-conducting layer 360 may include two or three sublayers: a polymer electrolyte membrane (PEM) 365, an optional cathode buffer layer 325, and/or an optional anode buffer layer 345. 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 365 is nonporous. Example characteristics of anode buffer layers and cathode buffer layers are provided elsewhere herein. In certain embodiments, the ion-conducting layer includes only a single layer or two sublayers.
In some embodiments, a carbon oxide electrolyzer anode contains a blend of oxidation catalyst and an 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, PtIr, Ni, NiFe, stainless steel, and combinations thereof. The oxidation catalyst can further contain conductive support particles such as carbon, boron-doped diamond, titanium, and any combination thereof.
As examples, 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. The conductive support particles may be compatible with the chemicals that are present in an electrolyzer anode when the electrolyzer 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 some embodiments, such conductive support particles are larger than the oxidation catalyst particles, and each conductive support particle can support one or more oxidation catalyst particles. In one arrangement, the oxidation catalyst is iridium ruthenium oxide. Examples of other materials that can be used for the oxidation catalyst include, but are not limited to, those listed above. It should be understood that many of these metal catalysts can be in the form of oxides, especially under reaction conditions.
As mentioned, in some embodiments, an anode layer of an MEA includes an ion-conducting polymer. In some cases, this polymer contains one or more covalently bound, negatively charged functional groups configured to transport mobile positively charged ions. Examples of the second ion-conducting polymer include ethanesulfonyl fluoride, 2-[1-[difluoro-[(trifluorocthenyl)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. Commercially available examples of cation-conducting polymers include e.g., Nafion 115, Nafion 117, and/or Nafion 211. Other examples of cationic conductive ionomers described above are suitable for use in anode layers. Further examples of polymer backbones and pendant groups suitable for use in the ion-conducting polymer of the anode layer are describes above.
There may be tradeoffs in choosing the amount of ion-conducting polymer in the anode. For example, an anode may include enough anode ion-conducting polymer to provide sufficient ionic conductivity, while being porous so that reactants and products can move through it easily. An anode may also be fabricated 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 about 10 and 90 wt %, or about 20 and 80 wt %, or about 25 and 70 wt % of the total anode mass. As an example, the ion-conducting polymer may make up about 5 and 20 wt % of the anode. In certain embodiments, the anode may be configured to tolerate relatively high voltages, such as voltages above about 1.2 V vs. a reversible hydrogen electrode. In some embodiments, an anode 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, Ir 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 is used for basic reactions.
In some embodiments, the MEA and/or the associated cathode layer is designed or configured to accommodate gas generated in situ. Such gas may be generated via various mechanisms. For example, carbon dioxide may be generated when carbonate or bicarbonate ions moving from the cathode toward the anode encounter hydrogen ions moving from the anode toward the cathode. This encounter may occur, for example, at the interface of anionic and cationic conductive ionomers in a bipolar MEA. Alternatively, or in addition, such contact may occur at the interface of a cathode layer and a polymer electrolyte membrane. For example, the polymer electrolyte membrane may contain a cationic conductive ionomer that allows transport of protons generated at the anode. The cathode layer may include an anion conductive ionomer.
Left unchecked, the generation of carbon dioxide or other gas may cause the MEA to delaminate or otherwise be damaged. It may also prevent a fraction of the reactant gas from being reduced at the anode.
The location within or adjacent to an MEA where a gas such as carbon dioxide is generated in situ may contain one or more structures designed to accommodate such gas and, optionally, prevent the gas from reaching the anode, where it would be otherwise unavailable to react.
In certain embodiments, pockets or voids are provided at a location where the gas is generated. These pockets or voids may have associated pathways that allow the generated gas to exit from the MEA, optionally to the cathode where, for example, carbon dioxide can be electrochemically reduced. In certain embodiments, an MEA includes discontinuities at an interface of anionic and cationic conductive ionomer layers such as at such interface in a bipolar MEA. In some embodiments, a cathode structure is constructed in a way that includes pores or voids that allow carbon dioxide generated at or proximate to the cathode to evacuate into the cathode.
In some embodiments, such discontinuities or void regions are prepared by fabricating in MEA in a way that separately fabricates anode and cathode structures, and then sandwiches to the two separately fabricated structures together in a way that produces the discontinuities or voids.
In some embodiments, and MEA structure is fabricated by depositing copper or other catalytic material onto a porous or fibrous matrix such as a fluorocarbon polymer and then coating the resulting structure with an anionic conductive ionomer. In some embodiments, the coated structure is then attached to the remaining MEA structure, which may include an anode and a polymer electrolyte membrane such as a cationic conductive membrane.
In some embodiments, a cathode has a porous structure and the/or an associated cathode buffer layer that has a porous structure. The pores may be present in an open cell format that allows generated carbon dioxide or other gas to find its way to the cathode.
In some MEAs, an interface between an anion conducting layer and a cation conducting layer (e.g., the interface of a cathode buffer layer and a PEM) includes a feature that resists delamination caused by carbon dioxide, water, or other material that may form at the interface. In some embodiments, the feature provides void space for the generated material to occupy until as it escapes from an MEA. In some examples, natural porosity of a layer such as an anion conducting layer provides the necessary void space. An interconnected network of pores may provide an escape route for carbon dioxide or other gas generated at the interface. In some embodiments, an MEA contains interlocking structures (physical or chemical) at the interface. In some embodiments, an MEA contains discontinuities at the interface. In some embodiments, an MEA contains of a fibrous structure in one layer adjacent the interface. A further discussion of interfacial structures between anion and cation conducting layers of MEAs is contained in Published PCT Application No. 2021/108446, published Jun. 3, 2021, and titled “MEMBRANE ELECTRODE ASSEMBLY FOR COX REDUCTION,” which is incorporated herein by reference in its entirety.
A primary function of the cathode catalyst layer is to provide a catalyst for COx reduction. An example reaction is Reaction 4:
The cathode catalyst layer may also have 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 carbon oxide electrolyzers 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 may be designed or configured to prevent accumulation of water that can block gas transport. Further, catalysts for COx reduction are sometimes less stable than catalysts like platinum that can be used in hydrogen fuel cells. These functions, their particular challenges, and how they can be addressed are described below.
Water management challenges are in many respects unique to COx electrolyzers. For example, compared to a PEM fuel cell's oxygen electrode, a COx electrolyzer uses a much lower gas flow rate. A COx electrolyzer 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 COx electrolyzer. A COx electrolyzer 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. 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 even 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 some pores having sizes of about 1 nm to 100 nm and other pores having sizes of at least about 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 structures (e.g., support particles); stacking pores between stacked carbon nanoparticles; secondary stacking pores between agglomerated carbon spheres (micrometer scale); or inert filler (e.g., PTFE) introduced pores with the interface between the PTFE and carbon also creating irregular pores ranging from hundreds of nm to micrometers.
The thickness of cathode catalyst layer may contribute 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. In certain embodiments, the cathode layer thickness is about 80 nm-300 μm.
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 may be structured for gas transport. During operation, 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 COx electrolyzers. 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. Example values of these parameters are provided elsewhere herein.
In some embodiments, the ionomer-catalyst contact is minimized. For example, 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 of a cathode layer may have multiple 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 that allows 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 certain embodiments, an ion-conducting polymer of a cathode comprises 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.
Various anion-conducting polymers are described above. Many of these have aryl groups in their backbones. Such ionomers may be used in cathode catalyst layers as described herein. In some embodiments, an ion-conducting polymer can comprise one or more covalently bound, positively charged functional groups configured to transport mobile negatively charged ions. Examples of such ion-conducting polymers include 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, an ion-conducting polymer in a cathode comprises at least one ion-conducting polymer that is a cation and an anion-conductor. Examples of such ion-conducting polymer include polyethers that can transport cations and anions and polyesters that can transport cations and anions. Further examples of such ion-conducting polymer include polyethylene oxide, polyethylene glycol, polyvinylidene fluoride, and polyurethane.
During use in an electrolyzer, a cation and anion conductor may raise the local 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.
In certain embodiments, an 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 have 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 COx electrolyzers include CO2 dissolving in and/or solubilizing the polymers, 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.
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.
Further examples of polymer backbones and pendant groups suitable for use in the ion-conducting polymer of the anode layer are described above.
There are tradeoffs in choosing the amount of cation-conducting polymer in the cathode. A cathode may include enough cathode ion-conducting polymer to provide sufficient ionic conductivity, but be sufficiently 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 about 10 to 90 wt %, about 20 to 80 wt %, or about 30 to 70 wt % of the material in the cathode layer.
In certain embodiments, metal catalysts have one or more of the properties presented above. In general, a metal catalyst catalyzes one or more COx reduction reactions. The metal catalyst may be in the form of 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.
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, TI, 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 reaction performed at the cathode of the COx electrolyzer.
The metal catalyst may be composed of pure metals (e.g., Cu, Au, Ag), but alloys or bimetallic systems may be used for certain reactions. In some embodiments, a metal catalyst comprises a dopant. Examples of dopants include boron, nitrogen, and hydrogen. In some cases, the metal catalyst comprises boron-doped copper. The concentration of dopant may be substantially uniform throughout the metal particle or it may vary as a function of distance from particle surface. For example, the dopant concentration may decrease with distance from the particle surface.
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. 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.
Different metal catalyst materials may be chosen at least in part based on the desired product and MEA operation. For example, the 1D nanowire may have a higher selectivity for ethylene production while triangular Cu nanoplates may have higher selectivity for methane. Nanocubes may show good selectivity for ethylene in an AEM MEA.
Additional details of certain suitable cathode metal catalysts are presented in U.S. patent application Ser. No. 18/053,945, filed Nov. 9, 2022, which is incorporated herein by reference in its entirety.
As explained above, support structures may be particles. But more generally, they may have many different shapes such as spheres, polygons (e.g., triangles), nanotubes, and sheets (e.g., graphene)). Structures having high surface area to volume are useful to provide sites for catalyst particles to attach. Support structures may also be characterized by their porosity, surface area per volume, electrical conductivity, functional groups (N-doped, O-doped, etc.), and the like. Various characteristics of particulate support structures are presented above.
If present, a support of the cathode catalyst particles may have any of various functions. It may stabilize metal nanoparticles to prevent them from agglomerating and distribute the catalytic sites throughout the catalyst layer volume to spread out loss of reactants and formation of products. A support may also provide 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.
The support may be hydrophobic and have affinity to the metal nanoparticle.
In many cases, the conductive support particles are compatible with the chemicals that are present in the cathode during operation, are reductively stable, and have a high hydrogen production overpotential so that they do not participate in any electrochemical reactions. In certain embodiments, conductive support particles are larger than the reduction catalyst particles, and each conductive support particle can support many reduction catalyst particles.
Additional details of certain suitable support materials for cathode metal catalysts are presented in U.S. patent application Ser. No. 18/053,945, filed Nov. 9, 2022, which is incorporated herein by reference in its entirety.
In certain embodiments, a cathode layer has a porosity of about 15 to 75%. Porosity of the cathode layer may be determined by various techniques. 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 occupied 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. In some embodiments, porosity is determined directly by a method such as mercury porosimetry or image analysis of TEM images.
The cathode layer may also be characterized by its roughness. The surface characteristics of the cathode layer can impact the resistances across the membrane electrode assembly. Excessively rough cathode layers can potentially lead to interfacial gaps between the catalyst and a current collectors or other electronically conductive support layer such as a microporous layer. These gaps hinder 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. Cathode 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. Cathode layer Sa values between 0.50-1.10 μm or 0.70-0.90 μm may be used in some embodiments.
Additional details of certain suitable cathode catalyst layer properties are presented in U.S. patent application Ser. No. 18/053,945, filed Nov. 9, 2022, which is incorporated herein by reference in its entirety.
MEAs may include a polymer electrolyte membrane (PEM) disposed between and conductively coupled to the anode catalyst layer and the cathode catalyst layer. In certain embodiments, a polymer electrolyte membrane has high ionic conductivity (e.g., 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 Nafion® 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-[(trifluorocthenyl)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.
When the polymer electrolyte membrane is a cation conductor (e.g., it conducts protons), it may contain a high concentration of protons during operation of the CRR, while a cathode may operate better when a low concentration of protons is present. A cathode buffer layer may be provided 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, a cathode buffer layer is an ion-conducting polymer with many of the same properties as the ion-conducting polymer in the cathode. A cathode buffer layer may provide a region for the proton concentration to transition from a 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 may encounter anions from the cathode, and they may neutralize one another. The cathode buffer layer may help 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. A high proton concentration may be a concentration in the range of about 10 to 0.1 molar and low proton concentration may be a concentration of less than about 0.01 molar.
A 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 general, 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.
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 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 in layers of the MEA, including the cathode buffer layer, is described further below.
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 μm, between 10 nm and 100 μm, 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. The volume of a void is preferably 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.
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. As a person skilled in the art will recognize from the previous detailed description and from the figures and claims, modifications and changes can be made to the preferred embodiments of the invention without departing from the scope of this invention defined in the following claims.
| Number | Date | Country | |
|---|---|---|---|
| 63505243 | May 2023 | US |
