Patent Application
20230399758
The present invention relates to hydrogen-evolving electrodes and membrane electrode assemblies. The invention has application, for example, in ionomer membrane-based electrolyzer cells operating at greater than about 50° C.
Hydrogen production via electrolysis is a process where a current is applied to an aqueous electrolyte solution and the water is split into its oxygen and hydrogen components. Electrolysis is a mature technology that has its origins in the late 1800s and uses liquid alkaline electrolytes. The introduction of proton-exchange membranes (PEMs), including Chemours 'Nafion® in the 1960's allowed for ionomer membrane-based electrolyzers that were more compact and scalable, while the hydrogen evolved was easier to pressurize. The main drawback to PEM-based electrolyzers is that the cost of the noble-metal anodic oxygen evolution reaction (OER) and cathodic hydrogen evolution reaction (HER) catalysts are too high for widespread adoption. The relatively recent introduction of anion exchange membranes (AEMs) allows for cheaper and more abundant metals to be used in electrochemical reactors that would otherwise corrode at higher pHs.
In either of these cases, high-performance materials and refined electrode architectures are required to allow these reactors to operate at higher current densities within reasonable operating potentials. Catalysts must have adequate concentrations of reactants to satisfy the stoichiometric requirements of operating at specific current densities. In the case of PEM electrolyzers, the catalyst requires solvated hydronium ions to be reduced to hydrogen gas. AEM electrolyzers reduce water molecules that are either pumped to the catalyst or diffuse across the anion-exchange membrane from the anode. Conversely, catalysts and electrodes must also be able to clear the electrocatalytically inert products of these reactions to make room for more reactants.
Objects of the invention are to provide improved electrodes and, particularly, cathodes to promote hydrogen evolution reactions.
Related objects are to provide membrane electrode assemblies and electrolyzers that utilize such electrodes.
Still further related objects of the invention are to provide such electrodes, membrane electrode assemblies and electrolyzers that operate at greater than about 50° C.
The foregoing objects are among those achieved by the invention, aspects of which provide novel electrodes for use with membranes that can operate in liquid-fed ionomer membrane-based electrolyzer cells between at least about 50 and about 95 degrees C. and between 0 and 100% relative humidity. The electrodes can be used in electrochemical cells running in acidic (pH<4) or alkaline (pH>9) environments.
Such electrodes and, particularly, cathodes, can, according to some aspects of the invention, comprise a carbon-based substrate, e.g., of woven cloth or paper, a hydrophobic binder-containing microporous layer, e.g., of polytetrafluoroethylene (PTFE), and a catalyst layer comprising electrocatalysts and binders demonstrating ionic conductivity over a range of dry and wet operating conditions. Such cathodes have a pore structure of between 0.4 μm to 50 μm and, according to related aspects of the invention, of between 0.4 μm and 13 μm.
Related aspects of the invention provide electrodes, e.g., as described above, in which at least one of the microporous layer and the catalyst layer has a defined pore structure and particle size distribution, respectively, that are optimized to minimize the diffusion of water from the catalyst surface and to allow for the efficient clearing of hydrogen gas from the catalyst, thereby, enhancing electrode performance during hydrolysis. More particularly, for example, according to some aspects of the invention, the microporous layer has a pore structure of between 0.4 μm and 50 μm and, according to related aspects of the invention, of between 0.4 μm and 13 μm.
Further aspects of the invention provide membrane electrode assemblies, e.g., with cathodes of the type described above.
Still further aspects of the invention provide electrolyzer cells, e.g., with membrane electrode assemblies and one or more electrodes of the type described above.
Yet still further aspects of the invention provide methods of fabricating cathodes of the types described above.
A more complete understanding of the invention may be attained by reference to the drawings, in which:
Described below are novel electrodes, membrane electrode assemblies and ionomer membrane-based electrolyzers. These capitalize, in some embodiments of the invention, on novel combinations of electrode microporous layer porosity (or pore structure) and catalyst layer porosity profiles, as well in some embodiments on solvent mixes, to promote hydrogen evolution reactions by minimizing the diffusion of water from the catalyst surface and allowing for efficient clearing of hydrogen gas from the catalyst. As used in this application, the terms “porosity” and “pore structure” are used interchangeably to refer to the mean size (or range of mean sizes) of diameters of pores distributed (as a result a mixing and/or other processes) over and through a volume of a material.
Referring to
With continued reference to
The role of the support layer 32 is to provide an ion-permeable substrate upon which the other, active layers of the electrode 16 are deposited. Though in some embodiments metallic support layers are used, in the illustrated embodiment the support layer comprises carbon. Carbon support materials for layer 32 of the illustrated embodiment have thicknesses less than 400 μm (and, more particularly, preferably of between 20 μm and 50 μm) and are of high inherent hydrophobicity, high conductivity, alkaline stability, and a well-defined pore distribution. Materials with a preferred combinations of those parameters (e.g., as characterized by methods including but not limited to Scanning Electron Microscopy, Cobb titrations and contact angle measurements, electrical conductivity tests, alkaline stability, and Capillary Flow Porometry, though it will be appreciated that the invention can be practiced with materials having other combinations of these and other parameters.
Suitable such substrates 32 can include carbon or graphite-based woven cloths or papers such as those commercially available from AvCarb of AvCarb, Lowell, Massachusetts; SGL Carbon of Germany (under their Sigracet Fuel Cell Component line); and Freudenberg of Germany. The support layer 32 can be fabricated or fashioned from such materials per convention in the art as adapted in accord with the teachings hereof. See,
The role of a microporous layer 34 is to provide an optimized pore structure to facilitate both water transport to the catalytic active sites as well as gas away from the active sites, which can otherwise inhibit mass transport within the ionomer membrane-based electrolyzer cell.
Microporous layer 34 of the illustrated embodiment comprises a carbon and a hydrophobic binder, and is fabricated using formulations and processes of the type known in the art for membrane electrode assembly (MEA) gas diffusion layers (GDLs) based on carbon black and a hydrophobic agent such as PTFE; see, for example, X. L. Wang et al Micro-porous layer with composite carbon black for PEM fuel cells Electrochimica Acta 51 (2006) 4909-4915, the teachings of which are incorporated herein by reference. Illustrated microporous layer 34 is fabricated as discussed below and is additionally characterized by having
With reference to
The role of the catalyst layer 36 of electrode 16 is to enable chemical reactions that, on the cathode side, reduce water to yield hydrogen gas and hydroxide, and, on the anode side, oxidize those hydroxide ions to produce water and oxygen. In the illustrated embodiment, layer 36 is fabricated from mixed aqueous/organic catalyst inks that are formulated from a carbon-supported catalyst, which can be metallic or non-metallic, and one or more ionomeric binders. The metal can be, by way of non-limiting example, platinum. The ionomeric binders can include any of polystyrene, polyphenyl or can be polyfluorene-based, all by way of non-limiting example, additions to and alternatives to which are within the ken of those skilled in the art in view of the teachings hereof. The base solvent is a combination of, but not necessarily solely comprising, water, 1-propanol, 2-propanol, NMP, DMAC, DMSO, DMF, and cyclopentanone. Mixing is done through a combination of high-shear and non-shear mixing to achieve desired particle size distribution. Catalyst layer 36 of the illustrated embodiment have the following characteristics:
Referring to
In some embodiments, electrodes 16 according to the present invention can be processed with a sintering step above 300° C., e.g., of the type known in the art for use in fabrication of ionomer membrane-based electrolyzer electrodes, though this is not a requirement of the invention. See Step 46.
In electrodes 16 according to the invention, porosity of both the substrate 32 with microporous layer 34 as well as the complete electrode 36, is determined, e.g., via capillary flow porometry. Moreover, packing of the catalyst layer 36 onto the microporous layer 34 is preferably achieved by a combination of controlling the hydrophobicity of the MPL as well as the solvent mix in the catalyst ink to limit catalyst falling into the pores of the microporous layer, therefore reducing electrode activity through inaccessible active sites. Confirmation is acquired through Scanning Electron Microscopic analysis of the interface of the catalyst layer and microporous layer.
Further characteristics and parameters of preferred electrodes 16 according to the invention are set forth below. Achieving such characteristics and parameters is within the ken of those skilled in the art in view of the teachings hereof:
A more complete understanding of practice of the invention may be attained through study of the examples below.
Example 1—Microporous Layer Fabrication: A woven or paper cloth or graphite material is chosen for coating. The microporous layer ink is prepared by mixing the desired carbon, for example, Soltex Acetylene Black or Vulcan XC-72, with water. Other surfactants can be added to the mix as well, as is within the ken of those skilled in the art in view of the teachings hereof. A series of inks, namely, for example, polyethylene glycol and alkyne diol, with high-shear mixing between 15 and 90 minutes are prepared. A fluorinated hydrocarbon, for example PTFE, is added and the mixture is mixed on a non-shear mixer for 15 to 60 minutes. Other additives such as thickeners or surfactants the use of which is within the ken of those skilled in the art may be used to improve the ability to coat. The mixture is applied to the chosen web, with the web being air-dried in between each coat, until the desired loading is achieved (3 to 50 g/m2). Following completion of coating, the material is heat-treated to 340° C. for between 15 and minutes. This completes the gas diffusion layer (GDL), or microporous layer (MPL)
Example 2—Cathode Fabrication: An already-prepared MPL (such as that prepared in Example 1) is used as a substrate. Catalyst inks are prepared using an aqueous/organic mixture, for example water and 1-propoanol. This mixture contains solvent, catalyst, for example a Platinum or PGM-free metal supported on XC-72, KB-300, or KJ-600, and an ionomer such as Fumion (Fumatech), PiperION-A (Versogen), or Orion CM (Orion). A series of inks is prepared for evaluation using different high-shear mixing times, from 15 to minutes. Following addition of the polymer and/or ionomer, the mixture can be mixed via a non-shear methodology for anywhere from 15 minutes to 16 hours. The ink is applied to one side of the prepared MPL in multiple coats, until the catalyst loading is at the desired loading, which can be from 2.0 mg/cm2 to 12.5 mg/cm2 (either metal or total catalyst depending on catalyst selection). The web and catalyst layer are air-dried in between each layer. A series of electrodes are prepared at different catalyst loadings for evaluation.
Example 3—MPL and GDL Characterization: Catalyst layer thickness is evaluated using cross-section image analysis on a scanning electron microscope (SEM). Cross-section sample preparation and analysis is well-documented. Bubble point and mean pore (diameter) size are evaluated using capillary flow porometry, whereby an increasing pressure of inert gas is used to remove a wetting agent that is introduced to the pores of the electrode. As the pressure is increased, wetting agent is removed from smaller pores. This evaluation is done on the base substrate, the completed microporous layer (before or after sintering), and the completed electrode including catalyst layer.
Example 4—Ionomer membrane-based electrolyzer cell Testing: employs 5 cm2 to 47 cm2 active area single cells and traditional graphite fields. Testing is done with a liquid electrolyte solution delivered to the anode, and in some cases, to the cathode as well.
Described above are novel cathodes, membrane electrode assemblies and ionomer membrane-based electrolyzer cells, as well as methods of fabrication thereof, according to the invention. It will be appreciated that the embodiments discussed above and shown in the drawings are examples of the invention and that other embodiments incorporating changes to those shown here also fall within the scope of the invention.
This application claims the benefit of priority of U.S. Patent Application Ser. No. 63/345,882, filed May 25, 2022, and entitled HYDROGEN EVOLVING ELECTRODES, MEMBRANE ELECTRODE ASSEMBLIES AND ELECTROLYZERS BASED THEREON AND METHODS OF FABRICATION THEREOF, the teachings of which are incorporated herein by reference.
This invention was made with government support under Grant DE-EE0008082, awarded by the Department of Energy. The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 63345882 | May 2022 | US |
