Abraded fluid diffusion layer for an electrochemical fuel cell

Details Abraded fluid diffusion layer for an electrochemical fuel cell Abraded fluid diffusion layer for an electrochemical fuel cell

2001-05-02

2004-04-06

Bell, Bruce F. (Department: 1746)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S006000, C429S006000, C429S006000, C429S006000, C429S047000, C429S047000, C429S047000, C204S284000, C427S115000, C427S289000, C427S290000, C427S307000

Reexamination Certificate

active

06716551

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a fluid diffusion electrode of a solid polymer electrolyte fuel cell, and in particular to a method of abrading a surface of a fluid diffusion layer of the electrode and a product to which the method has been applied.
BACKGROUND OF THE INVENTION
Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly (“MEA”), which comprises an ion exchange membrane, or solid polymer electrolyte disposed between two fluid diffusion electrodes typically comprising a layer of porous, electrically conductive substrate material, such as carbon fiber paper or carbon cloth. The MEA contains a layer of catalyst, typically in the form of finely comminuted platinum, at each membrane/electrode interface to induce the desired electrochemical reaction. In operation the electrodes are electrically coupled to provide a circuit for conducting electrons between the electrodes through an external circuit.
At the anode, the fuel stream moves through the porous anode substrate and is oxidized at the anode catalyst layer. At the cathode, the oxidant stream moves through the porous cathode substrate and is reduced at the cathode catalyst layer to form a reaction product. In fuel cells employing hydrogen as the fuel and oxygen-containing air (or substantially pure oxygen) as the oxidant, the catalyzed reaction at the anode produces hydrogen cations (protons) from the fuel supply. The ion exchange membrane facilitates the migration of protons from the anode to the cathode. In addition to conducting protons, the membrane isolates the hydrogen-containing fuel stream from the oxygen-containing oxidant stream. At the cathode catalyst layer, oxygen reacts with the protons that have crossed the membrane to form water as the reaction product. The anode and cathode reactions in hydrogen/oxygen fuel cells are shown in the following equations:
Anode reaction: H
2
→2H
+
+2
e
−
Cathode reaction: ½O
2
+2H
+
+2
e
−
→H
2
O
In typical fuel cells, the MEA is disposed between two electrically conductive fluid flow field plates or separator plates. Fluid flow field plates have at least one flow passage formed in at least one of the major planar surfaces thereof. The flow passages direct the fuel and oxidant to the respective electrodes, namely, the anode on the fuel side and the cathode on the oxidant side. The fluid flow field plates act as current collectors, provide support for the electrodes, provide access channels for the fuel and oxidant to the respective anode and cathode surfaces, and provide channels for the removal of reaction products, such as water, formed during operation of the cell.
Two or more fuel cells can be electrically connected together in series to increase the overall power output of the assembly. In series arrangements, one side of a given fluid flow field or separator plate can serve as an anode plate for one cell and the other side of the fluid flow field or separator plate can serve as the cathode plate for the adjacent cell. Such a multiple fuel cell arrangement is referred to as a fuel cell stack, and is usually held together in its assembled state by tie rods and end plates.
Conventional fuel cell electrode designs typically comprise a fluid diffusion layer (FDL) and a catalyst layer. The FDL generally comprises an essentially fluid-permeable substrate, and in some cases, a sublayer disposed on one surface of the substrate for providing a base on which a catalyst layer is disposed to form an electrode. The substrate serves as a backing material and structural support for the electrode, and is typically made of an electrically conductive material such as carbon cloth, carbon paper, carbon fiber woven, or carbon fiber non-woven. A hydrophobic polymer such as polytetrafluoro-ethylene (PTFE) is typically applied to the substrate to discourage water (either generated from the electrochemical reaction or from the humidified reactant streams) from accumulating in the electrode. The PTFE-treated substrate is typically sintered so that the hydrophobic polymer melts and coats the substrate.
The sublayer, if present in the FDL, is generally concentrated at the catalyst side of the substrate. The sublayer generally comprises fibers or particles of an electrically conductive material such as carbon or graphite, and may also contain some hydrophobic material such as PTFE. Several types of high surface area carbon particles, both graphitized and non-graphitized, are available for use in the sublayer. The catalyst is typically applied to the substrate surface coated with the sublayer (although such a fluid diffusion layer could be combined with a catalyzed membrane in an MEA). Suitable catalyst materials include precious metals or noble metals such as platinum. The catalyst layer may comprise unsupported catalyst such as platinum black, or include supported catalyst in which catalyst such as platinum is supported on for example, carbon particles.
There is motivation in the fuel cell industry to improve long-term performance and reliability of MEAs while reducing their manufacturing costs. Low cost materials and simplified processing steps are desirable, but the MEA should meet minimum standards of reliability, longevity and performance. For example, the MEA materials should be selected and the MEA manufactured such that the MEA maintains membrane integrity over its designed operating life. Membrane integrity is necessary to maintain fluid isolation of the fuel and oxidant streams during fuel cell operation; a perforation in the membrane can cause reactant transfer leaks (that is, a leakage of one or more reactant through the membrane to the other electrode) which can be detrimental to fuel cell performance and can further damage the cell. Various approaches have been developed to detect membrane perforations and associated reactant transfer leaks; one such approach is described in U.S. Pat. No. 5,763,765, owned by the Ballard Power Systems Inc., the assignee of the present application. In the approach described in the '765 patent, perforations in a membrane are detected by a thermal imaging device that detects heat generated by an exothermic reaction of a pair of reactants which contact each other at a membrane perforation. The localized exothermic reaction appears as a “hotspot” in the thermal image.
SUMMARY OF THE INVENTION
A correlation has been identified between certain surface texture characteristics of the FDLs of an MEA in a solid polymer electrolyte fuel cell and the occurrence of membrane perforations and transfer leaks in operating fuel cells. Examples of such surface texture characteristics include “surface roughness” and “waviness”; in the context of this description, surface roughness relates to the finest (shortest wavelength) irregularities of a surface and waviness relates to the more widely spaced (longer wavelength) deviations of a surface from its nominal (intended) shape that cause the profile of the electrode or FDL of the electrode to vary in thickness.
In one embodiment, a method of manufacturing an FDL for a solid polymer electrolyte fuel cell comprises abrading a surface of the FDL such that the topography of the FDL surface is rendered more uniform, leading to reduced surface roughness and/or waviness. The FDL comprises at least a porous substrate and may also comprise a carbon-containing sublayer on the surface of the substrate. The sublayer provides a support layer for the deposit of catalyst on the substrate. The FDL may also comprise a hydrophobic material such as polytetrafluoroethylene (PTFE).
In the manufacture of such an FDL that does not already comprise hydrophobic material, a hydrophobic material such as PTFE may be applied to the substrate before or after the substrate is abraded. After the hydrophobic material is applied, the substrate is sintered (before or after abrading) so that the hydrophobic material melts and coats on the substrate, thereb

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