Rational Catalyst Design for Clean Energy Conversion

Abstract

Biomimetic non-precious-metal catalysts based on metal-porphyrin-like (FeN4, CoN4) moieties have been shown competitive with platinum-based catalysts for oxygen reduction reaction (ORR) in fuel cells. However, the exact nature of the active sites in these materials is still not clearly understood, which prevents us from further optimization design. Here we report a first principles study of these metal-nitrogen-carbon materials with varied combinations of M, N, C (M=Mn, Fe, Co, Ni). By calculating the intermediate electrochemical steps in ORR process, each involving one electron transfer, we found that the square-pyramid FeN4-Pyrrole (FeN5) or square-planar CoN4 moieties incorporated graphenes have the best performance in terms of both catalytic activity and chemical stability. Although square-planar NiC4 and square-pyramid CoC2N2-Pyrrole (CoC2N3) also show good activity, carbon sites adjacent to the metal atom can be easily attacked by active oxygen and leading to severe degradation. We also found that the carbon sites adjacent to nitrogen atoms are mostly responsible for H2O2 production, and they are more vulnerable than the nitrogen atoms towards oxidization to cause dissociation of active moieties and degradation.