Operando Imaging of Photoelectrochemical Water Oxidation Enhanced by Charge Transfer Relay Across Grain Boundaries

The microstructural properties of grain boundaries (GBs) and defects in semiconductor materials critically influence solar energy conversion. Yet, the precise role of GBs in charge separation and transport remains poorly understood and widely debated, posing a major challenge to the advancement of high-efficiency photoelectrochemical (PEC) devices, especially for water splitting. A fundamental knowledge gap persists in linking carrier dynamics to surface reactions at the semiconductor–liquid interface, necessitating detailed operando and nanoscale investigations. Here, we mapped the charge transfer and catalytic activity of a working BiVO₄ photoanode at the nanoscale using scanning photoelectrochemical microscopy (SPECM). We found that oxygen vacancies within GB interiors and on grain surfaces act synergistically on distinct timescales to trap photogenerated electrons and extend hole lifetimes into the millisecond range, thereby enhancing local water oxidation activity. Contrary to conventional understanding of GBs, this unique microstructural configuration induces a built-in electric field of up to ~3.5 kV/m at GBs, which locally modulates the photocurrent distribution and accounts for approximately 70% of the photoelectrochemical performance. Moreover, while GBs facilitate hole accumulation and linearly tune reaction currents, they do not directly accelerate water oxidation kinetics. These insights underscore the pivotal role of GBs in sustaining long-lived charge carriers, thereby enabling efficient photoelectrochemical water splitting.