Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Making catalytic electrode – process only
Reexamination Certificate
2001-05-16
2004-03-30
Bell, Mark L. (Department: 1755)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Making catalytic electrode, process only
C429S047000, C429S047000, C429S047000, C429S047000, C429S047000, C427S113000, C427S115000, C427S359000, C427S365000, C427S249100, C427S249200, C029S623100, C029S623500, C029S825000, C029S874000
Reexamination Certificate
active
06713424
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to improved methods for making fluid diffusion layers and electrodes having reduced surface roughness and methods for making membrane electrode assemblies having better reliability and performance. The methods comprise adhering at least one loading material to a porous substrate in a manner such that the surface roughness of the resulting fluid diffusion layer is reduced. The reduced surface roughness may be assessed in terms of average surface roughness (R
a
) or by observation of infrared hot-spots detected.
BACKGROUND OF THE INVENTION
Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. Solid polymer electrolyte fuel cells generally employ a membrane electrode assembly (“MEA”) comprising a solid polymer electrolyte or ion exchange membrane disposed between two electrically conductive electrodes. Such electrodes comprise a fluid diffusion layer and an electrocatalyst. The fluid diffusion layer comprises a substrate with a porous structure having voids therein. The substrate is permeable to fluid reactants and products in the fuel cell.
The electrocatalyst is typically disposed in a layer at each membrane/electrode interface, to induce the desired electrochemical reaction in the fuel cell. The electrocatalyst may be disposed as a layer on the electrode or be part of the electrode in some other way. The electrocatalyst may be disposed on the membrane instead of or in addition to being disposed on the fluid diffusion layer. The electrodes are electrically coupled to provide a path for conducting electrons between the electrodes through an external load.
Fluid reactants may be supplied to the electrodes in either gaseous or liquid form. In electrochemical fuel cells employing hydrogen as the fuel and oxygen as the oxidant, the catalyzed reaction at the anode produces hydrogen cations (protons) and electrons from the fuel. The gaseous reactants move across and through the fluid diffusion layer to react at the electrocatalyst. The ion exchange membrane facilitates the migration of protons from the anode to the cathode while electrons travel from the anode to the cathode via the external load. In addition to conducting protons, the membrane isolates the hydrogen-containing fuel stream from the oxygen-containing oxidant stream. At the cathode electrocatalyst layer, oxygen reacts with the protons that have crossed the membrane and the electrons to form water as the reaction product.
In solid polymer electrolyte fuel cells employing methanol as the fuel supplied to the anode (so-called “direct methanol” fuel cells) and an oxygen-containing oxidant stream such as air (or substantially pure oxygen) as the oxidant supplied to the cathode, methanol and water are oxidized at the anode to produce protons and carbon dioxide. Typically, the methanol is supplied to the anode as an aqueous solution or as a vapor. Gaseous or liquid reactants move across and through the fluid diffusion layer. The protons migrate through the ion exchange membrane from the anode to the cathode, and at the cathode electrocatalyst layer, oxygen reacts with the protons and electrons to form water.
In solid polymer electrolyte fuel cells, the MEA is typically interposed between two separator plates or fluid flow field plates (anode and cathode plates). The plates typically act as current collectors and provide support to the MEA. Fluid flow field plates typically have channels, grooves or passageways formed therein to provide means for access of the fuel and oxidant streams to the porous fluid diffusion layers of the anode and cathode, respectively.
The electrode is electrically conductive to provide a conductive path between the electrocatalyst reactive sites and the current collectors. Materials commonly used as substrates of electrodes or as starting materials to form substrates include carbon fiber paper, woven carbon fabric, optionally filled with carbon particles and a binder, metal mesh or gauze, optionally filled with carbon particles and a binder, and other woven and nonwoven materials.
Typical substrate materials are preformed, highly electrically conductive macroporous sheet materials, which may contain a particulate electrically conductive material and a binder. It has sometimes been found advantageous to coat, impregnate, fill, or otherwise apply porous electrically conductive substrates with materials, such as carbon or graphite materials, in order to reduce porosity or achieve some other object. The material applied to the substrate is referred to herein as “loading material.” When loading material is applied to one side of a substrate to form a layer, the formed layer is frequently referred to as a “sublayer”. The amount of loading material (that is, the material eventually loaded onto the substrate) in an electrode is referred to as the “loading” of loading material and is usually expressed as the mass of material per unit surface area of substrate.
A certain loading of carbon or graphite can improve MEA operational performance. However, if the loading is too high, performance is impaired by interference with diffusion of product or reactant through the fluid diffusion layer. Nonetheless, substrates having larger pores or a higher porosity tend to require higher loadings of carbon or graphite.
A substrate need not be highly electrically conductive and in fact may be an electrical insulator. Such substrates may be filled with electrically conductive materials. Electrodes that are made from filled, poorly electrically conductive webs and methods for making same are disclosed in U.S. Pat. Nos. 5,863,673 and 6,060,190, which are incorporated herein by reference.
A substrate for an electrode typically has a loading material applied to it in order to provide a supporting surface for electrocatalyst, to improve conductivity, and/or to accomplish some other objective. The loading material can be applied by any of the numerous coating, impregnating, filling or other techniques known in the art. The loading material may be contained in an ink or paste that is applied to the substrate. In a typical process for applying a loading material to substrate, the substrate has an ink applied to it, and the ink comprises carbon and/or graphite with a poreformer and a binder (for example, polytetrafluoroethylene) in aqueous solution. After this application, the substrate and the loading material applied to the substrate may or may not be subjected to compaction at an elevated pressure, such as the pressure to which the electrode may be subjected in a fuel cell stack or a higher pressure. The substrate and applied loading material are dried, with the result that the substrate is loaded to a greater or lesser extent with the loading material on its surface and/or within the voids, thus forming a fluid diffusion layer. Binder in the fluid diffusion layer is typically sintered before the electrocatalyst is applied. The final fluid diffusion layer is still permeable to fluid reactants.
U.S. Pat. No. 6,127,059 discloses a gas diffusion layer for use in a solid polymer electrolyte fuel cell that makes use of a membrane electrode assembly of the type in which a catalyst layer is formed on the surface of a solid polymer electrolyte membrane. The gas diffusion layer includes a carbon fiber woven cloth having a surface and a coating of fluororesin containing carbon black on the surface. Preferably, the coating penetrates no more than one-half, more preferably no more than one-third, the thickness of the carbon fiber woven cloth. The carbon fiber woven cloth may be pre-treated with a water-repellent fluororesin (such as polytetrafluoro-ethylene), or with a mixture of a fluororesin and carbon black, to enhance water repellency. U.S. Pat. No. 6,127,059 does not disclose or suggest the step of compacting the carbon fiber woven cloth after applying the coating of fluororesin.
Compaction has been used in other processes of loading a material upon a substrate. Compaction of the wet coated porous substrate tends to push the loading material into the sub
Fong Kelvin Keen-Ven
Gordon John Robert
Haas Herwig Robert
Stumper Jürgen
Ballard Power Systems Inc.
Bell Mark L.
Hailey Patricia L.
McAndrews Held & Malloy Ltd.
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