Chemistry: electrical current producing apparatus – product – and – Having earth feature
Reexamination Certificate
2001-09-13
2003-12-23
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Having earth feature
C429S006000, C429S006000
Reexamination Certificate
active
06667127
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to fluid diffusion layers for fuel cells and in particular to gas diffusion layers for solid polymer electrolyte fuel cells. Further, it relates to the use of carbonized polymers containing pyrrolidone groups in the manufacture of fluid diffusion layers.
BACKGROUND OF THE INVENTION
Fuel cells convert reactants, namely fuel and oxidants, to generate electric power and reaction products. Fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. A catalyst typically induces the desired electrochemical reactions at the electrodes. Preferred fuel cell types include solid polymer electrolyte fuel cells that comprise a solid polymer electrolyte and operate at relatively low temperatures.
During normal operation of a solid polymer electrolyte fuel cell, fuel is electrochemically oxidized at the anode catalyst, typically resulting in the generation of protons, electrons, and possibly other species depending on the fuel employed. The protons are conducted from the reaction sites at which they are generated, through the electrolyte, to electrochemically react with the oxidant at the cathode catalyst. The catalysts are preferably located at the interfaces between each electrode and the adjacent electrolyte.
A broad range of fluid reactants can be used in solid polymer electrolyte fuel cells and may be supplied in either gaseous or liquid form. For example, the oxidant stream may be substantially pure oxygen gas or a dilute oxygen stream such as air. The fuel may be, for example, substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream, or an aqueous liquid methanol mixture in a direct methanol fuel cell. Reactants are directed to the fuel cell electrodes and are distributed to catalyst therein through fluid diffusion layers. In the case of gaseous reactants, these layers are referred to as gas diffusion layers.
Solid polymer electrolyte fuel cells employ a membrane electrode assembly (“MEA”) which comprises the solid polymer electrolyte or ion-exchange membrane disposed between the two electrodes. Each electrode comprises an appropriate catalyst, preferably located next to the solid polymer electrolyte. The catalyst may, for example, be a metal black, an alloy or a supported metal catalyst such as platinum on carbon. The catalyst may be disposed in a catalyst layer, and a catalyst layer typically contains ionomer, which may be similar to that used for the solid polymer electrolyte (for example, Nafion®). The catalyst layer may also contain a binder, such as polytetrafluoroethylene. The electrode may also contain a substrate (typically a porous, electrically conductive sheet material) that may be employed for purposes of mechanical support and/or reactant distribution, thus serving as a fluid diffusion layer.
The MEA is typically disposed between two plates to form a fuel cell assembly. The plates act as current collectors and provide support for the adjacent electrodes. The fuel cell assembly is typically compressed to ensure good electrical contact between the plates and the electrodes, in addition to good sealing between fuel cell components. A plurality of fuel cell assemblies may be combined in series or in parallel to form a fuel cell stack. In a fuel cell stack, a plate may be shared between two adjacent fuel cell assemblies, in which case the plate also serves as a separator to fluidly isolate the fluid streams of the two adjacent fuel cell assemblies.
Flow fields are employed for the purpose of directing reactants across the surfaces of the fluid diffusion electrodes or electrode substrates. Flow fields are disposed on each side of the MEA and comprise fluid distribution channels. The channels provide passages for the distribution of reactants to the electrode surfaces and also for the removal of reaction products and depleted reactant streams. The flow fields may be incorporated in the current collector/support plates on either side of the MEA (in which case the plates are known as flow field plates) or, alternatively, may be integrated into the fluid distribution layers of the electrodes.
The fluid distribution layers in such fuel cells may therefore have several functions, typically including: to provide access of the fluid reactants to the catalyst, to provide a pathway for removal of fluid reaction products, to serve as an electronic conductor between the catalyst layer and an adjacent flow field plate, to serve as a thermal conductor between the catalyst layer and an adjacent flow field plate, to provide mechanical support for the catalyst layer, and to provide mechanical support and dimensional stability for the ion-exchange membrane.
Preferably, the fluid distribution layers are thin, lightweight, inexpensive, and readily prepared using mass production techniques (for example, reel-to-reel processing techniques). Materials which have been employed in fluid distribution layers for solid polymer electrolyte fuel cells include commercially available carbon fiber paper and woven and/or non-woven carbon fabrics. However, the mechanical and/or electrical properties of these materials alone may not be adequate to meet all the requirements for fuel cell applications.
Consequently, appropriate fillers and/or coatings have been employed in the art to improve one or more of these properties. For instance, the electrical conductivity of a carbonaceous web might be increased by filling with an electrically conductive filler such as graphite particles plus a binder. (Carbonaceous in this context simply means containing carbon.) Alternatively, the stiffness of a carbonaceous web might be increased by impregnating the web with a suitable amount of curable polymer and then curing the polymer. Further, both stiffness and conductivity might be increased by impregnating the web with a carbonizable polymer, followed by carbonization of the polymer-impregnated web in an inert atmosphere. “Carbonization” is defined herein as increasing the proportion of carbon by heating to temperatures of 600° C. or greater in a nonoxidizing environment. During carbonization, carbon proportion increases as hydrogen, oxygen and nitrogen are evolved. The carbonization product remaining after carbonization can provide both mechanical support and additional electrical pathways throughout the web. Carbonization product means the reaction product after carbonization. Specific examples of webs filled with a binder include the thin, highly porous, non-woven carbon fiber web products of Technical Fibre Products Ltd., which typically comprise non-woven carbon fiber webs bound with a styrene-acrylic binder. While offering many desirable features, such products may not be sufficiently stiff for handling purposes nor of sufficient electrical conductivity for desired fuel cell performance. The styrene-acrylic binder present in the web is neither conductive nor carbonizable. However, both stiffness and conductivity of such webs may be suitably improved by impregnating with a carbon particle filler and a phenol-formaldehyde resin. The resin is then cured and carbonized leaving behind a substantial amount of carbonization product and resulting in a stiffer, more conductive web.
Phenol-formaldehyde resins have been commonly used in many impregnation applications. The phenol-formaldehyde resin is typically diluted in a carrier solvent and then used as an impregnant for fluid diffusion layers. However, a disadvantage of phenol-formaldehyde resins is that the uncured resin and by-products during curing are toxic as are certain preferred carrier solvents. Thus appropriate precautions must be taken when using these materials. Water can be used as a carrier solvent to some extent. However, in situations where one wishes to also add a conductive filler to the fluid diffusion layer, the preferable method of application is by applying an ink to the web, where the ink comprises the conductive filler, the resin, and the carrier solvent. Such an ink usually requires a relatively large amount of carrier solvent. If wa
Beattie Paul D.
Chiem Bien
Davis Michael Todd
Erickson Lynn C.
Fong Kelvin Keen-Ven
Ballard Power Systems Inc.
Chaney Carol
McAndrews Held & Malloy Ltd.
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