Continuous Representation of the Proton and Electron Kinetic Parameters in the pH-Potential Space for Water Oxidation on Hematite

Understanding the mechanisms of multielectron and multiproton electrochemical reactions, particularly in the context of solar-to-fuel water splitting, is an outstanding challenge. Historically, Pourbaix diagrams are used to show the influence of potential and pH on the thermodynamic stability of electrode-electrolyte systems. These diagrams do not carry kinetic or mechanistic information, which often restricts their use to cases in which the thermodynamic limit can be assumed. We introduce and construct from experimental data two new types of diagrams that demonstrate the kinetic variations of electrochemical reactions as a function of pH and potential. These diagrams show the variation of the electron-transfer parameter (alpha) and the proton reaction order (rho) in a wide range of potential and pH. We present alpha(pH, E) and rho(pH, E) for water electrolysis on an iron oxide electrode in the range of pH 7 to 13. In these plots, regions of acidic and basic mechanisms, relationship to surface protonation equilibria, and switching between acidic and basic mechanisms due to electrochemical production of protons can be easily identified. The proton reaction order is zero in the acidic side, while it is nonzero in the basic limit. A larger empirical electron-transfer parameter is observed in the basic compared to the acidic region. These observations are related to the differences in oxidation mechanism between the two regions. We propose the use of such diagrams to gain an expanded and enhanced view of the kinetics of multielectron and multiproton electrochemical reactions.