Uncovering Transition-Metal-Specific Oxygen Evolution Reaction Activity in Isostructural Nickel-Based Zeolitic Imidazolate Framework Cocatalysts on Hematite (α-Fe ₂ O ₃ ) Photoanodes

Hematite (α-Fe 2 O 3 ) is a widely studied oxide photoanode for the oxygen evolution reaction. We decorate α-Fe 2 O 3 photoanodes with a composition-controlled, isostructural series of Ni-based zeolitic imidazolate framework (ZIF) cocatalysts (Ni-, NiCo-, and NiFe-ZIF) that share an identical framework topology. Using a combination of photoelectrochemical (PEC) measurements, potential-resolved photoelectrochemical impedance spectroscopy, time-resolved spectroscopy (near-field heterodyne transient grating, NF-HD-TG), and X-ray photoelectron spectroscopy (XPS), we elucidate how the transition-metal composition reshapes surface states and the dynamics of long-lived surface holes, thereby clarifying the distinct roles of Ni, Co, and Fe in OER-relevant interfacial reactivity. Across this isostructural series, ZIF cocatalyst incorporation shifts surface-state energetics, suppresses recombination channels originating from α-Fe 2 O 3, and accelerates OER-relevant interfacial oxidation. Taken together, these results provide a composition-controlled, isostructural ZIF overlayer platform for probing and tuning the metal-specific contributions of Ni, Co, and Fe to OER-relevant surface reactivity on α-Fe 2 O 3 .