Metal Carbides in Fuel Cell Cathode

This chapter is devoted, in particular, to metal carbides in fuel cell cathode as potential alternative materials to conventional carbon support owing to their bifunctional capability, catalysts and catalyst supports. 

Nonprecious metal alloys, carbides, or oxides may hold the solution to the cost and chemical instability problems. WC and WO are the most studied systems as low-cost alternatives to a Pt anode because of their excellent stability in acidic media. They also have high CO tolerance because CO does not readily adsorb onto their surfaces. " WC is particularly attractive because its electron density states near the Fermi level are similar to those of Yang and Wang obtained a high CD (0.9 A/cmO from a H2-air PEM fuel cell with a WC anode (0.48 mg WC/cm ). The CD was limited by the WC anode in contrast to a typical PEM fuel cell with a Pt anode, which is cathode (ORR) limited and can reach over 2 A/cm under the same test conditions. Nevertheless, this represents a CD increase of two orders of magnitude over the previously reported WC anode catalysts. ... 

Normally, the kinetics of ORR and OER occurring at the cathode of fuel cells, including direct methanol fuel cells (DMFCs) is very slow. In order to speed up the ORR kinetics to reach a practical usable level in a fuel cell, ORR catalyst is needed at the air cathode. Platinum (Pt)-based materials are the most practical catalysts used in PEM technology. These Pt-based catalysts are too expensive to make fuel cells commercially viable, and hence extensive research over the past several decades has been focused on development of alternative catalysts. These alternative electrocatalysts include noble metals and allo37S, carbon materials, quinone and its derivatives, transition metal macrocyclic compounds, transition metal chalcogenides, transition metal carbides and transition metal oxides. In this chapter, we focus on both noble and nonnoble electrocatalysts being used in air cathodes and the kinetics and mechanisms O2 reduction/oxidation reaction (both ORR and OER), catal37zed by them. 

Recently, 3M Corporation [97] successfully conducted fuel cell durability tests using Fe-C-N/TiC as the cathode catalyst. This TiC-suppoited Fe-C-N catalyst showed a stable performance over 1000 hours at a cell voltage of 0.6 V, whieh is the longest durability of non-noble metal eatalysts yet reported in a real fuel eell environment. Beside TiC, some conductive and electrochemically stable materials, including carbides, silicides, nitrides, and their combinations, could also be used as catalyst supports for the ORR. 

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