Thin Fuel Cell Electrodes - ACS Publications

9 Thin Fuel Cell Electrodes R. G. HALDEMAN, W. P. COLMAN, S. H. LANGER, and W. A. BARBER

Downloaded by UNIV OF ALABAMA on June 18, 2016 | http://pubs.acs.org Publication Date: January 1, 1969 | doi: 10.1021/ba-1965-0047.ch009

Stamford Research Laboratories, American Cyanamid Co., Stamford, Conn.

An experimental investigation was made of the structure and performance of thin electrodes consisting of platinum and platinum-carbon supported on metal screens. Platinum loadings were in the range 1 to 9 mg./sq. cm. The electrodes were shown to have a very open structure consisting of aggregates of platinum black bonded by Teflon fibrils. Gas adsorption and electrochemical measurements indicate virtually the entire surface area of the platinum black to be available for reaction. The electrodes were tested in both acid- and baseequilibrated matrix cells. Initial polarization and life-testing data were obtained. The effect of temperature and operation on air were studied. These electrodes were capable of sustaining high current densities and show considerable promise for use in lightweight, high performance fuel cell batteries.

J h e r e is considerable current interest i n lightweight high performance electrodes for low temperature hydrogen-oxygen fuel cells. Electrode performance characteristics are of critical importance i n potential application of fuel cells i n space, communications, marine and ground power (12). The present work is directed toward development of electrode structures for "equilibrated matrix" cells since these have compact structure and are capable of operation at high current densities (6). Electrodes suitable for this system should be useful also i n the ion exchange membrane fuel cell and, with some modification, in the free electrolyte fuel cell. The electrode system chosen consisted of catalytic materials spread on thin metal screens and bonded and waterproofed with Teflon. This general type of fuel cell electrode has been investigated previously by 106 Young and Linden; Fuel Cell Systems Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

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Thin Electrodes

Elmore and Tanner ( I ) and b y Grubb (3, 4). Here, w e characterize i n some detail the physical and electrochemical properties of two electrode modifications i n w h i c h platinum is incorporated as a black (Type A ) and as supported on carbon ( Type Β ).


Procedures The initial evaluation work was carried out i n 1-inch diameter (5 sq. cm. active area) matrix cells. L i f e testing was conducted i n 2 X 2 inch cells. Components of these cells were similar to those of plastic cells pre viously described (4), except that for most of the work reported here metal faceplates of nickel or gold-plated nickel were used. F o r the alka line system ( 5N K O H ) asbestos was found to be a suitable matrix mate rial, while for the acid system ( 5 N H S 0 ) glass-fiber paper performed satisfactorily. C e l l matrix thickness was 10 to 20 mils and cell internal re sistance as measured by an a.c. bridge fell within the range 0.2 to 0.4 ohm/ sq. cm. for both base and acid cells. In obtaining polarization data, two minutes at steady potential were required at each load before moving to the next higher current. Product water was removed by flow of sufficient dry hydrogen and oxygen (or air), about equally divided, to remove water as fast as it was formed. Close control of water balance was particularly critical i n life testing, w h i c h was carried out on a continuous basis. Unless otherwise indicated, performance data are given for cells having the same electrode material at anode and cathode. 2

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Electrodes For the equilibrated matrix system electrodes have been developed consisting of a porous layer of platinum black or platinum supported on carbon, mixed with a binder-waterproofing agent (Teflon) and spread uniformly on and supported b y a wire mesh screen. The thickness of the electrode and the catalyst loading can be varied b y using screens of differing mesh and wire diameter. Electrodes of area up to 1 sq. ft. have been prepared. T h e range of composition of the two major electrode variations studied is indicated i n Table I. Preferred compositions are roughly midway in the range indicated. Table I.

Type A Type Β

Thin Fuel Cell Electrodes

Thickness, Mils

Platinum, Mg.l Sq. Cm.

4-8 4-8

7-10 0.5-4.0

Carbon, Mg.l Sq. Cm. None 8-12

Support Screen 1.0-3.5 1.0-3.5

N i or T a N i or T a

Young and Linden; Fuel Cell Systems Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

FUEL CELL SYSTEMS

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In Type A no extender is used. Thus, with screens in the range 50 to 100 mesh and 0.002 to 0.004 inch wire diameter, platinum loadings are typically i n the range 7 to 10 mg./sq. cm. and electrode thickness 0.004 to 0.008 inch. Resistivity is nearly that of the support screens. In Type Β electrodes, platinum is deposited, usually by borohydride reduction, on a carbon or graphite extender. Electrodes of this type may contain 0.5 to 4 mg. Pt/sq. cm. electrode area and have the same thickness and resistivity characteristics as Type A electrodes. Nickel screens are used for base cells, tantalum screens for acid cells.


Characterization of Electrode Structures The electrode structures were investigated by electron microscopy and by surface area measurements using electrochemical and adsorption techniques. Microscopy. Type A electrodes were examined by electron micros copy. In a typical study, material from the surface of an electrode was extracted by the gelatin stripping method. After the stripped gelatin film was vacuum coated with silica and the gelatin removed, the silica layer remaining was a partial replica of the original surface and a medium for entrapping the stripped fragments of material that were originally a part of the electrode surface. Figure 1 shows that the platinum black of the

Figure 1. Electronmicrograph (10,000%) of silica replica of sur face of Tijpe A electrode showing Ft aggregates at A; Teflon latex particles at B; and Teflonfibrilsat C



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electrode was i n the form of loosely packed aggregates ( A ) with fibrils ( C ) of Teflon interspersed. These are believed to b i n d the platinum aggregates together as well as to provide water repellancy. The field also shows the presence of Teflon latex particles ( B ) . This study reveals a high degree of dispersion of platinum aggregates and indicates that the Teflon does not occlude or cover any significant fraction of the platinum surface. This conclusion is consistent with surface area measurements. Surface Area Studies. Surface area studies were made on a typical Type A electrode both by electrochemical and low temperature krypton adsorption measurements ( B E T method ). A galvanostatic oxidation technique was used i n the electrochemical measurements (11). As shown i n Table II, surface area values obtained by the two methods on this electrode are i n approximate agreement, indicating that essentially the same platinum surface is available for electrochemical reaction as for gas adsorption. Further, krypton adsorption measurements indicate roughly the same surface area for platinum in the electrode as i n the black. Also, the electrochemical surface area of the electrode had not changed after 1200 hours of continuous operation i n a fuel cell. Table II.

Surface Area of Type A Electrodes Area: Sq. M. I Gram Platinum ElectroKrypton chemical adsorption

Initial After 1200 hours on test Pt black

33 33 ..

26 28

Performance of Type A Electrodes Comprehensive studies were made with the platinum black-metal screen electrodes, since these were found to be capable of sustaining very high current densities at low polarization. A c i d System. Figure 2 shows typical polarization curves for 50-mesh tantalum screen electrodes containing 9 mg. Pt/sq. cm. and 25 wt. % Teflon as used on both sides of hydrogen-oxygen and hydrogen-air cells. Measurements were made at 30° and 70° C . The electrolyte consisted of 5N H S 0 in a glass fiber matrix. The polarization curve for hydrogenoxygen at 30° C . indicates a working potential of about 0.74 volt at 200 ma./sq. cm. and 0.65 volt at 400 ma./sq. cm. Similar curves have been found to be approximately linear to current densities of more than 1000 ma./sq. cm. As indicated i n Figure 2, an increase i n temperature of about 40° C . produces a relatively small increase i n cell working potential. Thus, at 200 ma./sq. cm. the increase is about 30 mv., most of which can be accounted for by decrease in the internal resistance of the cell, and indicates a rather small effect of temperature on electrode activity. 2

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FUEL CELL SYSTEMS

110

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MATRIX C E L L - 5N H S0 2

4

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Thin Fuel Cell Electrodes - ACS Publications

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