Activity Descriptor Identification for Oxygen Reduction on Nonprecious Electrocatalysts: Linking Surface Science to Coordination Chemistry

摘要

Developing nonprecious group metal based electro-catalysts for oxygen reduction is crucial for the commercial success of environmentally friendly energy conversion devices such as fuel cells and metal-air batteries. Despite recent progress, elegant bottom-up synthesis of nonprecious electrocatalysts (typically Fe-N_x/C) is unavailable due to lack of fundamental understanding of molecular governing factors. Here, we elucidate the mechanistic origin of oxygen reduction on pyrolyzed nonprecious catalysts and identify an activity descriptor based on principles of surface science and coordination chemistry. A linear relationship, depicting the ascending portion of a volcano curve, is established between oxygen-reduction turnover number and the Lewis basicity of graphitic carbon support (accessed via C 1s photoemission spectroscopy). Tuning electron donating/withdrawing capability of the carbon basal plane, conferred upon it by the delocalized π-electrons, (i) causes a downshift of e_g-orbitals (d_z~2) thereby anodically shifting the metal ion's redox potential and (ii) optimizes the bond strength between the metal ion and adsorbed reaction intermediates thereby maximizing oxygen-reduction activity.