Photoelectrochemical (PEC) water oxidation reaction has attracted considerable attention in recent years because of its importance in solar energy conversion. For such a sluggish four-hole and four-proton transfer reaction in a long timescale of ms–s, more and more studies have shown that the accumulation of multiple surface-trapped holes (e.g., high-valent iron oxo on hematite surfaces) or "oxidizing equivalents" is a prerequisite for efficient PEC water oxidation process. Previous PEC water oxidation studies commonly apply high potentials (>1.2 VRHE) to achieve this key. But how to complete multi-hole transfer under low bias (near the onset potential) remains unknown.
