Spin-dependent active centers in Fe–N–C oxygen reduction catalysts revealed by constant-potential density functional theory

Abstract

Iron and nitrogen co-doped carbon (Fe–N–C) catalysts have shown great promise in promoting the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells. Experimental characterization studies, including Mössbauer and X-ray emission spectroscopy, have revealed the crucial role of spin states in Fe–N–C catalysts in ORR catalysis, but comprehensive theoretical understanding in this aspect is still lacking. Herein, using the grand-canonical density functional theory, we systematically investigate the interplay of the oxidation state, spin state, and applied potentials on the catalytic activity of an FeN₄C₁₀ moiety. We have identified two stable spin states of Fe(II)N₄C₁₀ at ORR-relevant potentials, namely, a high-spin state with out-of-plane Fe displacement and an in-plane intermediate-spin state. Our results show that the FeN₄C₁₀ moiety at the two different spin states exhibits distinct abilities to bind ORR intermediates and ORR activities. Our study provides valuable insights into the spin-correlated catalytic performances of Fe–N–C catalysts.