Consequently, several theoretical concepts, including the eg orbital occupancy, bandgap energy model, and binding energy-based scaling relations, have been developed and used to establish correlations between active site (electronic) structure and electrocatalytic activity. However, these descriptors usually stem from calculations that do not explicitly consider the electrode potential. Consequently, they fail to account for the potential-dependent surface charging effects at semiconductor-electrolyte interfaces and thus cannot mimic the impact of charge accumulation/depletion or the formation of localized electronic states, known as polarons. As polarons are closely associated with potential-dependent changes in the geometry and in the electronic structure, traditional constant charge DFT approaches cannot capture these intricate effects influencing semiconductor electrocatalysis. Polarons and defects have also been associated with (photo)electrocatalytic activity, and recent experimental studies have highlighted a strong correlation between semiconductor electrocatalytic activity and polaron formation. For example, Co3O4 can undergo oxidation at a potential of 1.5VRHE to form high-valence Co 4+ states, which revert to pristine Co species at the open circuit potential. This potentialdependent Co 4+ formation was identified as the driving force for the 5-Hydroxymethylfurfural oxidation activity on Co3O4. Another experimental study demonstrated band edge shifts in ultrathin ZnO films as a function of the potential, resulting in the formation of new (polaronic) electronic states and changes in charge transfer kinetics. Also the reduction of tetrabromobenzoquinone was observed when the Fermi level was located near the conduction band (CB) edge. These and other experimental results explicitly demonstrate that the electronic structure, polaron formation, and related electrocataltyic activity of semiconductors can efficiently be controlled through the application of an electrode potential. Hence, it is essential to include electrode potential driven changes in the geometric and electronic structure in computational studies of semiconductor electrochemistry. Besides computational and electrochemical experiments, insight on potentialdependent changes in the charge distribution and electronic structure under operation conditions can be obtained through in situ/operando spectroscopic techniques such as in situ X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge structure spectroscopy (XANES), electron paramagnetic resonance (EPR) spectroscopy, and ultraviolet-visible (UV-Vis) spectroscopy. For example, monolayer Ni(OH)2 was probed with operando XANES to demonstrate that the Ni 2+ state can be oxidized to Ni and Ni 4+ at more positive potentials. These changes in the Ni oxidation state under reaction conditions further promote oxygen vacancy (VO) formation in monolayer Ni(OH)2 and markedly improve the oxygen evolution (OER) activity. Recently, microcell-based in situ electronic/electrochemical measurements for a MoS2 semiconductor electrode demonstrated that the conductivity of the MoS2 surface enhances with increasing surface charge concentration and the enhanced conductivity is responsible for high electrocatalytic hydrogen evolution (HER) activity. On Fe2O3 hematite, high photoelectrochemical water splitting activity arises from its capability to form multiple hole polarons on the electrode surface under light illumination and the application of oxidizing electrode potential. While all these in situ studies highlight that the electrode potential can qualitatively change the electronic structure, control polaron formation, and quantitatively impact the catalytic activity of semiconductor electrodes, the state of a catalyst, the exact nature of active sites, and reaction mechanisms under operation conditions remain debated. Resolving these questions requires detailed atomic-scale insight on the potential-dependent changes in the semiconductor (electronic) structure and their influence on electrocatalytic performance. For example, the activity of acidic hydrogen evolution reaction (HER) has been identified to correlate with the presence of defects and polarons. For 2H-MoS2, it has been observed that sulphur (S) vacancies can improve the HER performance by introducing polaron gap states near the Fermi level. The HER performance has also been linked to oxygen vacancy densities and subsequent polaron formation in TiO1.23. However, the interplay of the electrode potential on polaron/defect formation and its correlation with HER activity remains an unresolved question.