Modulating Water Oxidation Kinetics on Hematite Photoanodes in Bicarbonate Electrolytes via MOF-Derived NiO Clusters at Grain Boundaries

Developing efficient photoanodes for oxygen evolution in near-neutral media is critical for solar fuel systems; however, sluggish surface kinetics remains a significant bottleneck, particularly at polycrystalline grain boundaries. Here, we demonstrate a grain-boundary engineering strategy using Ni-BDC and NiO modifications on hematite. Both treatments successfully transform grain boundaries from recombination defects into functional pathways for hole extraction, enhancing photocurrent in near-neutral sodium bicarbonate solution. Charge reaction order analysis reveals a transition from high-order kinetics (3–4) on bare hematite to ∼2 after modification, revealing a lowered kinetic barrier for multihole accumulation. This work highlights how in situ-formed nickel-based cocatalysts accelerate charge transport and interfacial kinetics under constrained conditions.