Quantitative Evaluation of a Quasi-Fermi Level of the Hole at a Photoanode/Electrolyte Interface

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Abstract
Valence band maximum (VBM) position does not necessarily indicate the driving force for oxidation reactions on photoanodes because it simply reflects an electrostatic potential. Rather, the quasi-Fermi level of the hole at the surface of photoanodes must be considered. Here, we report a protocol to quantitatively evaluate the quasi-Fermi level of the holes at the surface of photoanodes using redox species with various redox potentials. The quasi-Fermi level of these holes is estimated by correlating the photocurrent derived from redox species oxidation to the separately measured electrode potential on a stable model glassy carbon electrode. Using this protocol, the quasi-Fermi levels of holes on the surface of CdS and CdSe model photoanodes were found to be limited to merely 0.5 V vs. standard hydrogen electrode; this is fairly negative from the VBM position. The estimation of a quasi-Fermi level of the hole and the partial photocurrent density of the CdS and CdSe photocorrosion further enabled predictions of the photocorrosion kinetics. The predicted kinetics of the photocorrosion showed that the quasi-Fermi levels of hole at the surface of these photoanodes are pinned by the facile kinetics of the photocorrosion. This methodology offers a quantitative understanding of the oxidation capability of photoanodes.

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