Importance of the volcano slope to comprehend activity and selectivity trends in electrocatalysis
Kai S. Exner
Abstract
Volcano plots are a powerful tool to comprehend the trends of electrocatalysts from the same class of materials. While initially, it was considered that the volcano slope is constant and that a single elementary reaction step governs the reaction rate at each leg, this well-accepted view is challenged in the present article. Taking the oxygen reduction and nitrogen reduction reactions as case studies, the reasons for a non-constant volcano slope are rationalized on the atomic scale by referring to the mechanistic diversity of electrocatalytic processes and the application of advanced activity descriptors, which go beyond the evaluation of a single free-energy change. Finally, implications of a non-constant volcano slope are discussed on the example of the four-electron and two-electron oxygen reduction reductions, demonstrating that only by the consideration of a varying slope at the volcano legs, the obtained volcano curves are in qualitative agreement with experimental observations.