Photocorrosion-Limited Maximum Efficiency of Solar Photoelectrochemical Water Splitting

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. We reveal that the desired photocorrosion stability sets a limit of 2.48 V [relative to the normal hydrogen electrode (NHE)] for the highest possible potential of the valence band (VB) edge of a photocorrosion-resistant semiconducting photocatalyst. This limitation excludes semiconducting photocatalysts with band gap less than 2.48 eV for PEC water splitting. We further demonstrate that such a limitation has deep roots in the underlying physics after deducing the relation between the energy position of the VB 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 solar-to-hydrogen (STH) conversion efficiency from common thought 30.7% to approximately 8% for long-term stable single-band-gap 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.