Surface evolution of electrocatalysts in energy conversion reactions

To develop and improve electrocatalysts for promising energy conversion reactions, an in-depth understanding of their active sites is essential. In real operating conditions, the active sites could be subjected to various structural/chemical evolutions, as revealed recently by post-catalysis/in-situ characterizations. Insightful understanding suggests that multiple interactions between electric field, electrolyte, reactant/intermediate/resultant, and electrocatalyst increase the mutability of the electrocatalyst surfaces, triggering the surface evolution. Hence, the surface evolution causes the deviation of the initial design of active sites from the real ones. It calls for re-identification of active sites responsible for the observed activity and further provides more accurate guidelines for rational electrocatalytic design. This review reveals the origin of surface evolution, summarizes the general forms involving composition leaching, phase transformation, atom rearrangement, and metastable intermediate from typical electrocatalysts. It also proposes control strategies toward the formation or reservation of stable highly-active sites by atomic/defective modulation, interfacial coupling, structural optimization of pre-synthesis, and forming a protective layer. The active sites of typical electrocatalysts are identified and elucidated before and after the surface evolution. The current challenges in developing electrocatalysts addressing the definite structure-performance relationship, advanced characterization techniques, and strategies for more rigorous identification of real active sites after surface evolution are presented.