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

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.