(418c) Modeling the Theoretical Limits of Solar-to-Hydrogen Efficiency for Photoelectrochemical Water Splitting Based On Realistic Assumptions of Material Performance

Photoelectrochemical
(PEC) water splitting can be used to store solar energy in the form of chemical
bonds, specifically those of hydrogen which can then be used as a fuel.  The
performance of a PEC water splitting device is best evaluated using the
solar-to-hydrogen (STH) efficiency.  Knowledge of practical limits can provide researchers
with a means to assess and guide research directions in the field.  Previous
studies have calculated maximum efficiencies based on solar absorption limits with
the framework used to study photovoltaic (PV) devices.  While these numbers
provide a starting point for understanding the limits of PEC water splitting
devices, these studies neglect additional losses that are specific to PEC
systems and thus can overestimate practical solar conversion efficiencies for
this application.  This work presents results of STH efficiency calculations
for single and dual absorber systems over a wide range of band gaps that take
into account the effects of various system losses including absorption limits,
material defect losses, shunt losses, and reaction overpotentials.  Comparing
maximum STH values for devices with precious vs. non-precious metal catalysts
or minimal vs. significant shunt losses illustrates the need for researchers to
focus on these issues.  Additionally, improvement in performance with the
addition of a small bias is shown by calculating an applied bias photon
conversion efficiency for each device configuration.  Similarly to what has
been accomplished in the PV field, an equivalent circuit diagram has been
proposed to model the additional losses in a PEC system.  These results provide
insight into the intricacies of PEC device functioning as well as define obtainable
efficiency values representative of the current state of materials research in
the field.