Microscopic Electrode Processes in the Four-Electron Oxygen Reduction on Highly Active Carbon-Based Electrocatalysts

Nitridated carbon (NC) catalysts have attracted considerable interest as promising Pt-free alternatives to standard Pt/C catalysts in the oxygen reduction reaction (ORR). Aiming at a better understanding of the microscopic reaction mechanism and of the nature of the reaction-limiting step, we have investigated the ORR kinetics and in particular the kinetic isotope effects (KIEs) therein for three different NC catalysts with different nitrogen contents. The measurements were performed using ordinary and deuterated water electrolytes, under both alkaline and acidic solutions. From an analysis of the ORR kinetics on the most active NC-I catalyst, including the k(H)/k(D) ratio and the transfer coefficients, and from density functional theory (DFT) based computations, we derive that the initial proton-coupled electron transfer (PCET), whose process is to form OOH* from O-2*, acts as the potential-determining step (PDS) under acidic conditions and the initial electron transfer, whose process is to form (O-2(center dot-))* from O-2*, occurs under alkaline conditions. For the other NC catalysts with higher N concentrations we found significantly lower ORR activities. The DFT calculations support these conclusions and observations, showing that the formation of the OOH* intermediate as a result of the initial PCET to the metastable adsorbed O-2 is the key step for an efficient ORR on NC electrocatalysts under acidic conditions, acting as the PDS. Furthermore, they show that both the configuration of adsorbed O-2 and the N doping content sensitively affect the reaction pathway. This explains the experimental observation that only low nitrogen doping levels support an efficient ORR pathway. The work provides detailed insight into the microscopic mechanism of the ORR on the complex surfaces of NC catalysts and its dependence on the content and configuration of the N dopant atoms.