Electrochemical Kinetics of Multistep Reactions in General and the HOR/HER on Platinum in Particular

While multistep reactions in electrochemistry are ubiquitous, their analytical treatment has been incomplete and at times misleading. Several misconceptions have led to incorrect conclusions, derailing the analysis of electrochemical processes. In this work we present a rigorous application of the Butler-Volmer model and develop a coherent nomenclature and methodology on how to analyze multistep reactions. We discuss how a universal definition of the exchange current density does not exist for multistep reactions, clarify the meaning of the Tafel slope, often misinterpreted in the electrochemical literature. Eventually we discuss the classically proposed HOR/HER mechanisms on platinum, briefly discuss their origin, show how mass transport effects should be correctly analyzed by introduce a new equation that complements the Koutecký-Levich equation in the micropolarization region. We then perform a thorough experimental investigation of the HOR/HER kinetics over a wide range of combination of cations, anions, concentration, and buffers. There we reveal that buffers seem to take directly part in the HOR/HER and discuss the supposed “cation-effect”. Through the analysis of the micropolarization, Tafel region, and kinetics of the H-upd we propose a new mechanistic description of the HOR/HER with substantial experimental agreement and discuss how anions in the interface play a deciding role in the HOR/HER and H-upd on polycrystalline platinum.