300

Figure 3. Polarization Curve for Model IB Fuel Cell Stack Installed in Gator™

At maximum power, the fuel cell stack generates over 9.0 kW, of which, approximately 3.0 kW is required to operate the compressor and other ancillary equipment. At low and intermediate loads the efficiency is much higher, allowing vehicle operation for more than 4 hours.

Performance such as this indicates that the technology is available for introduction in specific applications. Utility vehicles are seen as a niche market which will create a greater demand for fuel cells. This demand will be met by industrial scale manufacturing, which in turn will reduce cost and aid fuel cell commercialization in other markets.

fuel cell development for transportation: catalyst development

Narayan Doddapaneni and David Ingersoll Sandia National Laboratories Battery Research Department Albuquerque, NM 87185-0614

Introduction

Fuel cells are being considered as alternative power sources for transportation and stationary applications. The degradation of commonly used electrode catalysts (e.g. Pt, Ag, and others) and corrosion of carbon substrates are making commercialization of fuel cells incorporating present day technologies economically problematic. Furthermore, due to the instability of the Pt catalyst, the performance of fuel cells declines on long-term operation. When methanol is used as the fuel, a voltage drop, as well as significant thermal management problems can be encountered, the later being due to chemical oxidation of methanol at the platinized carbon at the cathode. Though extensive work was conducted on platinized electrodes (1-3) for both the oxidation and reduction reactions, due to the problems mentioned above, fuel cells have not been fully developed for widespread commercial use. Several investigators have previously evaluated metal macrocyclic complexes as alternative catalysts to Pt and Pt/Ru in fuel cells (4). Unfortunately, though they have demonstrated catalytic activity, these materials were found to be unstable on long term use in the fuel cell environment. In order to improve the long-term stability of metal macrocylic complexes, we have chemically bonded these complexes to the carbon substrate, thereby enhancing their catalytic activity as well as their chemical stability in the fuel cell environment. We have designed, synthesized, and evaluated these catalysts for 02 reduction, H2 oxidation, and direct methanol oxidation in Proton Exchange Membrane (PEM) and aqueous carbonate fuel cells. These catalysts exhibited good catalytic activity and long-term stability. In this paper we confine our discussion to the initial performance results of some of these catalysts in Hj/Oj PEM fuel cells, including their long-term performance characteristics as well as CO poisoning effects on these catalysts.

Several metal phthalocyanine polymer complexes (MPc)„ having the structure shown in Figure 1 were synthesized in our laboratory. These catalysts were synthesized by heating a mixture of 3,3',4,4'-benzophenone tetracarboxylic dianhydride, metal chIoride(s), and urea at 200°C for 2 hours. The complexes prepared were (CoPc)„, (PtPc)„, (Pt-RuPc)„ (Pt-MoPc)n, and (RuPc)„. In this study only (CoPc)„ and (Pt-RuPc), were evaluated. A detailed description of the synthetic procedure of the catalysts and their impregnation onto the carbon substrate has been

Experimental previously described (5,6). When the carbon/catalyst mix is heat treated in an inert atmosphere to about 475 °C, a covalent bond appears to form between the carbon substrate and the benzene ring of the complex, as shown in Figure 2. It should be noted that these complexes are soluble in common organic solvents and aqueous solutions of mineral acids, however, after heat treatment they become insoluble.

Figure 1

Figure 1

The membrane electrode assemblies (MEAs) used in this work were prepared by Giner, Inc., Waltham MA. All electrodes, exceept those composed of Pt-black, contain 20% Nafion 117, and were cast onto Teflonized carbon fiber paper. Fuel cells were constructed having cathodes composed of either Pt-black (4 mg Pt/cm2) or 20% Pt/Vulcan (0.5 mg Pt/cm2). The anodes used in these cells consisted of either Pt-black, 10% Pt/Vulcan (0.25 mg Pt/cm2), 20%Pt/VuIcan (0.5 mg Pt/cm2), or 15% (Pt-RuPc)/VuIcan (0.035 mg Pt/cm2 and 0.018 mg Ru/cih ). All MEAs had active areas of 40 cm2 and were evaluated at 80°C with 30 psig

Figure 2.

Figure 2.

H2 and either 30 or 60 psig 02. The electrochemical screening of these complexes for the oxidation of hydrogen and reduction of oxygen in acid media was carried out in our laboratory. In order to provide independent evaluation of the performance of these inexpensive catalysts, PEM fuel cell fabrication and testing was carried out at Giner, Inc.

Results and Discussion

Fuel cells of varying combinations of anodes and cathodes were evaluated using H2 and these results are shown in Figure 3. As seen, the fuel cell having the (Pt-Ru Pc)„/Vulcan anode had slightly lower voltage than the other electrode combinations. However, the initial performance of this system is very promising, considering the unoptimzed nature of this MEA and significantly lower noble metal loadings. At 500 mA/cm2, this fuel cell with a Pt-black cathode exhibited a terminal cell voltage of 650 mV. A fuel cell built with an anode and a cathode both composed of 20% Pt/Vulcan exhibited

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