Photocorrosion-limited maximum efficiency of solar photoelectrochemical water splitting

TL;DR

This paper reveals a fundamental photocorrosion stability limit of 2.48 eV for semiconductors in PEC water splitting, constraining maximum efficiency to about 8%, and suggests decoupling light absorption and stability as a solution.

Contribution

It identifies a physics-based stability limit for photocorrosion-resistant semiconductors and quantifies its impact on maximum PEC water splitting efficiency.

Findings

Maximum stable VB potential is 2.48 V vs NHE.

Efficiency limit for stable PEC water splitting is approximately 8%.

Decoupling light harvesting and stability can overcome efficiency barriers.

Abstract

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.