The role of surface states in electrocatalyst-modified semiconductor photoelectrodes: Theory and simulations

In the last several years, there has been a wealth of studies to clarify the role of thin layers of electrocatalysts on semiconducting photoelectrodes to the efficiency of the oxygen evolution reaction (OER). It has been shown that the addition of a thin oxide overlayer in many cases cathodically shifts the potential of photocurrent onset and/or increases the maximum photocurrent, leading to greater collection efficiencies beneficial for OER. However, the origin of this enhancement is not well understood. Here, we present a model relying on analytical expressions rather than differential equations to investigate the role of surface states in electrocatalyst-modified semiconductor photoelectrodes. Without catalyst overlayer, we find that if surface states are screened, meaning charged surface states are electronically neutralized via nearby solution ions, no Helmholtz potential is generated and photoelectrodes exhibit good performance. In contrast, if the surface states are unscreened, an additional Helmholtz potential forms decreasing the amount of band bending and resulting in poor performance. In the presence of a catalyst overlayer, there is a strong dependence on how the surface states interact with the catalyst. Catalysts in series with surface states can increase the effective rate of transfer from surface states to solution, leading to an increase in total current while catalysts that act in parallel with surface states can increase the open circuit voltage or photovoltage. Both series and parallel catalyst effects operate in tandem in real devices, leading to an increase in current and/or photovoltage, depending on the relevant exchange currents. This model does not only help to understand the role of surface states in charge transfer and ultimately efficiencies in photoelectrochemical systems but also allows facile application for other researchers.