Exploiting Asymmetric Co States in a Co-N-C Catalyst for an Efficient Oxygen Reduction Reaction

Co-NC catalysts have attracted extensive concerns derived from their high oxygen reduction reaction (ORR) activity, but the catalytic mechanism of Co species with different forms remains controversial. Herein, we prepare Co-NC catalysts with a cobalt nanoparticle-supported and nitrogen-doped carbon structure using the ZIF-67 precursor, in which the Co states in the catalyst present an asymmetric state of an exposed carbon coating (Asy-Co) and a symmetric state of buried carbon (Sy-Co). The acid etching process removed the exposed asymmetric cobalt nanoparticles on the surface. The specific role of cobalt nanoparticles with different forms in the Co-NC catalysts was comprehensively clarified through analyzing the chemical coordination environment by XPS and XAFS. The half-wave potential (E-1/2 = 0.83 V) and onset potential (E-on = 1.04 V) of the Co-NC catalysts obtained after acid etching decreased significantly. Thus, the cobalt species removed by the acid etching process offered confirmed contributions to the catalytic activity. This work puts forward an important reference for the design and exploitation of non-noble metal catalysts using symmetry-derived motifs.