Shedding light on overpotentials and underpotentials in (photo)electrochemical reactions

The concept of overpotential is central to evaluating the performance of catalysts for electrochemical and photoelectrochemical transformations. This thermodynamic metric is well defined in the context of electrochemical reactions involving conducting electrodes, where the overpotential is the difference between an applied bias potential (which sets the energetics/potential of the electrode's Fermi level) and the equilibrium potential (which is set by the mixture of oxidized and reduced chemical species in the bulk solution and establishes the applied bias potential at which no net current flows, i.e., the open-circuit potential). Nonetheless, there are at least two ways the term overpotential-or in some cases "underpotential"-has been used to describe photoelectrochemical reactions involving semiconducting electrodes, where illumination results in a splitting of the photoelectrode's Fermi level into majority- and minority-carrier quasi-Fermi levels. In one approach, the overpotential/underpotential remains defined as the difference between an applied bias potential and the equilibrium potential. In the alternative approach, the overpotential is defined as the difference between the minority-carrier quasi-Fermi-level potential at the semiconductor surface and the equilibrium potential, thereby accounting for the surface photovoltage generated upon illumination of a semiconductor electrode. This article provides an overview of conceptual differences involving overpotentials and underpotentials in (photo)electrochemical reactions and considers applied electrode potentials in terms of their enthalpic versus entropic contributions.