How pH Affects the Oxygen Reduction Reactivity of Fe-N-C Materials

While Fe-N-C materials exhibit great potential for catalyzing the oxygen reduction reaction (ORR), their activity origin, especially the significant activity difference in acidic and alkaline media, remains a long-standing conundrum hindering the development of such catalysts. Here, we show an unanticipated pH-dependent regulation mechanism in Fe-N-C materials via first-principles microkinetic computations that explicitly consider the pH, solvation, and electrode potential effects. We find that, under typical operating potentials, the well-established FeN4 centers of Fe-N-C catalysts, regardless of the pyridinic and pyrrolic-type N-coordination environments, are not adsorbate-free but covered by an intrinsic intermediate *OH at pH = 1 and *O at pH = 13, resulting in FeN4-OH and FeN4-O centers formed in situ. We evaluate the pH- and potential-dependent kinetics and thermodynamics of the real active Fe centers of Fe-N- C catalysts against experimental measurements. We demonstrate that the activity difference of Fe-N-C catalysts is attributed to the *O coordination-induced optimization of the electronic structure and intermediate adsorption over the *OH case. Our work provides the mechanistic insight into the pH effects and paves the way toward a more effective catalyst design.