44
As mentioned above the voltage of 1.23 V does not account for any overpotentials
or non-equilibrium phenomena. The minimum overpotential associated
with the oxygen evolution reaction (OER ) is 0.4 V and with the
hydrogen evolution reaction (HER) is 0.1 V 88. Additionally, the irradiation
condition disrupts the equlibrium population of the electrons i.e separate
quasi-Fermi levels have to be introduced for the conduction and valence band
for the electron population. This lowers the effective driving force for the redox
reactions. Typically, the correction due the effect of quasi-Fermi level is
taken as 0.3 eV 88. If all the above mentioned effects are added, it turns
out that the semiconductor should have the bandgap of at least 2 eV. In the
following sections the results and discussions about the screening of materials
for the photoelectrochemical water splitting is based on the above mentioned
details.
Figure 5.1: Schematic of the photon induced water splitting reaction.
5.3 Different methods for the bandgap calculation
As mentioned before, the screening has been performed for the materials available
in the Materials Project Database. The Materials Project Database contains
the subset of compounds present in the experimental database called as
The Inorganic Crystallographic Structures Database (ICSD).92 The ICSD
contains the compounds which have been synthesized experimentally, therefore,
one can expect that the screened materials from the ICSD will be synthesizable
under certain experimental conditions. However, the ICSD does