  Photoelectrochemical (PEC) water splitting to generate hydrogen is one of the
most studied methods for converting solar energy into clean fuel because of its
simplicity and potentially low cost. Despite over 40 years of intensive
research, PEC water splitting remains in its early stages with stable
efficiencies far less than 10%, a benchmark for commercial applications. Here,
we revealed that the desired photocorrosion stability sets a limit of 2.48 eV
(relative to the normal hydrogen electrode (NHE)) for the highest possible
potential of the valence band (VB) edge of a photocorrosion-resistant
semiconducting photocatalyst. We further demonstrated that such limitation has
a deep root in underlying physics after deducing the relation between energy
position of the valence band edge and free-energy for a semiconductor. The
disparity between the stability-limited VB potential at 2.48 V and the oxygen
evolution reaction (OER) potential at 1.23 V vs NHE reduces the maximum STH
conversion efficiency to approximately 8% for long-term stable single-bandgap
PEC water splitting cells. Based on this understanding, we suggest that the
most promising strategy to overcome this 8% efficiency limit is to decouple the
requirements of efficient light harvesting and chemical stability by protecting
the active semiconductor photocatalyst surface with a photocorrosion-resistant
oxide coating layer.
