PNNL

Abstract

The systematic improvement of Fe-N-C materials for fuel cell applications has proven challenging, due in part to an uncomplete atomistic understanding the oxygen reduction reaction (ORR) under electrochemical conditions. Herein, a multilevel computational approach, which combines ab initio molecular dynamics simulations and constant potential density functional theory calculations, is used to assess proton-coupled electron transfer (PCET) processes and adsorption thermodynamics of key ORR intermediates. These calculations indicate that the potential-limiting step for ORR on Fe-N-C materials is the formation of the FeIII-OOH intermediate. They also show that for a proper description of the ORR energetics the ligation of the Fe center must be accurately described. In particular, reliable predictions of the onset potential and the redox potential of the key PCET resuction of the FeIII-OH intermediate to yield FeII and desorbed H2O require an axial H2O coadsorbed to the iron center throughout the catalytic cycle. These findings highlight the importance of solvent-substrate interactions and surface charge effects for understanding PCET reaction mechanisms and transition metal redox couples under realistic electrochemical conditions.