(704b) Activation Energies of Oxygen Reduction Electrocatalysis on Fe–N–C Catalysts

Single iron atom embedded in nitrogen-doped graphene (Fe–N–C) is one of the promising alternatives to platinum catalysts in oxygen reduction reaction (ORR). However, understanding of ORR on Fe–N–C catalysts at atomic level is lacking due to complexities that lie at solid catalyst/liquid interface, including charge transfer effects. The charge effect is more pronounced in two-dimensional materials than in three-dimensional metals due to their limited density of states. To address these complexities, we have developed an advanced first-principles model that can accurately describe electrochemical reactions. Using this model, we have studied the rate–limiting step of ORR on Fe–N–C catalysts. We discovered a new mechanism that O₂ replacing the preadsorbed H₂O on Fe atom has the largest activation energy. The barrier of this thermal step counterintuitively lowers with decreasing electrode potential, due to the stronger O₂ adsorption and weaker H₂O adsorption when the surface carries more electronic charges. This can be further explained by the fact that O₂ adsorption gains electrons from the surface while H₂O donates electrons to the surface. Our work emphasizes the importance of kinetic information in understanding and designing heterogeneous electrocatalysts.