Spin State as a Participator for Demetalation Durability and Activity of Fe–N–C Electrocatalysts

While pyrolyzed Fe–N–C single-atom materials have exhibited encouraging oxygen reduction reaction (ORR) activity in proton exchange fuel cells (PEFCs), their activity origin remains a puzzle and their operando degradation greatly restricts practical applications. A major problem lies in that the spin state of the active center has been largely neglected. Herein, we systematically examined the effect of spin state on the demetalation durability and ORR activity of Fe–N–C electrocatalysts, where two kinds of FeN4 clusters, pyridinic-type FeN4C10 and pyrrolic-type FeN4C12, are considered. The computed Pourbaix diagrams and catalytic activity indicate that the low-spin pyridinic-type FeN4 with OH modification [OH–Fe(II)N4C10 (S = 0)] and the medium-spin pyridinic-type FeN4 with O2 modification [O2–Fe(II)N4C10 (S = 1)] have excellent acid durability and high activity toward ORR. Particularly, the thermodynamic limiting potential (0.91 V) predicted on [OH–Fe(II)N4C10 (S = 0)] shows great agreement with the experimentally confirmed ones ranging from 0.86 to 0.96 VRHE. The high-spin pyrrolic-type OH–Fe(III)N4C12 (S = 5/2) also exhibits impressive 4e– ORR activity but has poor stability in acids. The feature importance analysis showed that the catalytic activity and the adsorption strength of oxygenated intermediates are highly correlated with the intrinsic charges and electronic spin moments of the catalytic center. Furthermore, we found that the applied potential may induce a spin crossover in the active center since each spin state responds differently to the electric field. Our studies advance the understanding of the fundamentals of the FeN4 structure–activity correlation and aid the rational design of highly active and stable Fe–N–C electrocatalysts.