Temperature-Dependent Voltammetry Reveals the Potential-Dependent Formation of Reaction Species and Their Impact on Interfacial Excess Charge during the Ammonia Oxidation Reaction on Pt

The electrocatalytic ammonia oxidation reaction (AOR) has historically garnered significant interest, since ammonia could be used as a carbon-free fuel and convenient H2 carrier. Key challenges hindering its application are the high overpotentials and rapid catalyst poisoning, which remain poorly understood. Here, we study the overpotential-dependent formation of reaction species via temperature-dependent cyclic voltammetry and electrochemical Arrhenius analysis on polycrystalline Pt in conjunction with infrared spectroscopy. Further, we link the evolution of the kinetic parameters to the degree of polarization and excess charge via CO-displacement measurements. Strikingly, with increasing overpotential, we discover the parallel formation of a reaction species (not a direct intermediate) that modulates the electron chemical potential and, thus, the potential of zero charge. Due to this coupling, the parallel formation of this “poisoning” species appears to depolarize the surface with increasing overpotential. To recover the interfacial polarization, the overpotential needs to be increased further, before the activity can be restored and substantial Faradiac current can arise. What emerges is a new kinetic picture, where not only formation but also stabilization of interfacial excess charge are decisive for the catalyst activity. In general, the study demonstrates the utility of performing temperature-dependent cyclic voltammetry for revealing how transient changes in local chemicals modulate the overall inner-sphere kinetics.