Redox chemistry meets semiconductor defect physics

Understanding how the electronic structure of electrodes influences electrocatalytic reactions has been a longstanding topic in the electrochemistry community, with predominant attention paid to metallic electrodes. In this work, we present a defect physics perspective on the effect of semiconductor band structure on electrochemical redox reactions. Specifically, the Haldane–Anderson model, originally developed to study multiple charge states of transition-metal defects in semiconductors, is extended to describe electrochemical redox reactions by incorporating the solvent effect, inspired by the Holstein model. The solvent coordinate and the actual charge on the redox species in reduced and oxidized states are assumed to be in instant equilibrium, and the transitions between these states are defined by the framework of Green’s function. With these treatments, the charge state transitions are handled in a self-consistent manner, and the implications of charge self-consistency (or, equivalently, charge self-regulation) on electrocatalysis can be investigated explicitly. We first confirm that this self-consistent approach is essential to accurately depict the hybridization effect of band structure by comparing the model-calculated ionization potential and electron affinity, as well as the redox potential of the species, with those obtained from density functional theory calculations. Next, we illustrate that the effect of charge self-consistency is key to obtaining a fuller understanding of the catalytic activities of semiconductor electrodes and the source of asymmetry in reorganization energies, which is often observed in prior ab initio molecular dynamics simulations. In addition, we discuss how band structure impacts redox reactions in the strong coupling limit. Finally, we compare our work with other relevant studies in the literature.