A successful oxygen reduction reaction (ORR) catalyst must combine high activity with good long-term stability � a major challenge in the strongly acidic environment of the polymer electrolyte fuel cells (PEFC) cathode. Since the early work of Jasinski in the 1960s (Nature 201, 1212, 1964), recent breakthroughs in the synthesis of high-performance non-precious metal catalysts (NPMCs) make replacement of Pt in ORR electrocatalysts with earth-abundant elements, such as Fe, Co, N, and C, a realistic possibility. Highly promising NPMCs can be obtained by simultaneously heat-treating precursors of nitrogen, carbon, and transition metals at 800-1000�C. The activity and durability of resulting catalysts depend on the selection of precursors, the synthesis chemistry; and especially the catalyst nanostructure. The CNx structures likely play a major role in the ORR active site(s).Apart from the possibility of a direct participation in the active site, the transition metal plays are crucial to the in-situ formation of carbon nanostructures (nanotubes, onion-like structures, graphene) by catalysing the decomposition of the nitrogen/carbon precursor(s) at a high temperature (800-100�C). The formation of different carbon and nitrogen-doped carbon nanostructures can be controlled during the synthesis of M-N-C catalysts. The highly-graphitized carbon nanostructures likely serve as a matrix for the formation of ORR-active groups with improved catalytic activity and durability, containing nitrogen and most probably also metal atoms. Future NPMC synthesis approaches are certain to focus on the precise control of interactions between precursors of the metal and carbon/nitrogen during the heat treatment, with the main purpose of maximizing the number of active sites, optimizing nitrogen doping levels, and generating carbon morphologies capable of hosting active and stable ORR sites. This is evident not only in catalysts developed for PEFC fuel cells but also in materials specifically designed for alkaline fuel cells (Hoon et al., Nat. Commun. 4:1922 doi:10.1038/ncomms2944, 2013).

Carbon-based catalysts for oxygen reduction in polymer electrolyte fuel cells

247th National Meeting (2014)

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