Hole Storage Layers in Photoelectrodes for Mitigating Recombination and Driving Stable PEC Water Splitting

Photoelectrochemical (PEC) water splitting has garnered increasing attention as a viable and eco-friendly approach for converting solar energy into hydrogen fuel. However, the practical efficiency is limited by rapid carrier recombination and inefficient hole utilization. Unlike previous reviews on hole transport layers (HTLs) or general interface engineering, this review provides a systematic overview of the applications of hole storage layers (HSLs) in PEC water splitting. The core principles and key performance metrics of PEC water splitting are first outlined, and the fundamental differences between HSLs (store and release mechanism) and conventional HTLs (continuous charge transport) are clarified. Subsequently, recent advances in HSLs for mainstream photoanodes (e.g., Ta₃N₅, BiVO₄, Fe₂O₃) and emerging photocathodes (e.g., Cu₂O, a-Si, Sb₂Se₃) are summarized. The main functions of the HSL strategy are discussed: such as storing holes temporarily to inhibit recombination, balancing the oxidation/reduction kinetics of water, and protecting the semiconductor from photocorrosion. Finally, it summarizes and outlines the strategies for the future development of HSL. It is hoped that this review will provide useful guidance for designing efficient and stable photoelectrodes for PEC water splitting, addressing carrier recombination and hole utilization to accelerate large-scale solar-to-hydrogen conversion.