  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.
