The rise of single-atom catalysts in hematite photoanodes for photoelectrochemical water splitting

The urgent imperative to achieve “carbon peak and carbon neutrality” has spurred a surge of researchers to vigorously advance the development of renewable energy technologies. Under the circumstances, there is a burgeoning interest in developing diverse solar energy utilization methods. Photoelectrochemical (PEC) water splitting, a process that harnesses sunlight, semiconductor materials, and water to transform solar energy into hydrogen energy, has emerged as a promising, environmentally friendly, and cost-effective solution. This process is frequently referred to as the “Holy Grail reaction” of solar energy utilization. Hematite-based photoanodes have attracted significant attention owing to their multitude of advantages: abundance, a suitable band gap, environmental friendliness, high photochemical stability, and exceptional theoretical solar-to-hydrogen (STH) efficiency. However, they still confront several challenges, including insufficient light absorption, limited hole diffusion length, shortened carrier lifetime, slow water oxidation reaction kinetics, and inadequate electrical conductivity. These issues result in significant electron and hole recombination within the bulk, at interfaces, and on surface regions, substantially degrading the PEC water splitting performance. Achieving rapid electron-hole separation and transport among the substrate, semiconductor, co-electrocatalyst, and electrolyte in hematite-based photoanodes is essential for attaining superior STH conversion efficiency. Previous studies have shown that surface states mediated the charge transport process, which is crucial for understanding the PEC water splitting mechanism and enhancing the PEC water splitting performance of hematite-based photoanodes. However, the relationship between the surface/interface structure and surface states of hematite-based photoanodes, and their influence on PEC water splitting, is still not fully understood. Recently, the loading of single-atom catalysts (SACs) onto hematite photoanodes and the exploration of their reaction mechanisms have garnered significant attention from researchers.