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

Abstract

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.