How the Quasi-Eqilibrium Condition Has Obfuscated Electrochemistry in General and the HOR/HER Discussion in Particual (Part 1)

Most reactions in electrocatalysis involve the transfer of multiple electrons. Due to the unlikely transfer of more than one electron at once, such reactions are multistep reactions. However, electrochemical textbooks focus heavily on the one-electron one-step reaction and there seem to be no consensus in the literature on how to treat multistep reactions. We will elucidate how multistep reactions can be treated in a straightforward manner. In this process we explain that the Tafel slope, despite common practice and a recent IUPAC report, cannot be used to determine the reaction mechanism. This misconception stems from the inappropriate use of the quasi-equilibrium condition in the electrochemical literature. We continue to show that the concept of the exchange current density cannot be universally defined for multistep reactions and that it is in almost no cases possible to accurately fit a multistep reaction by a simple Butler-Volmer equation. Finally, we present how the HOR/HER reaction mechanism on Pt can be identified when the Butler-Volmer model is rigorously applied on the two HOR/HER paths dominating the discussion, namely the Tafel-Volmer and Heyrovský-Volmer. Figure 1