Operando spectroscopy of redox dynamics in Hematite/NiMo photoanodes

This work investigates the redox behavior of nickel catalyst layers deposited on titanium-doped hematite photoanodes. The study focuses on the interaction between photoanode (titanium-doped hematite) and cocatalyst, (nickel-molybdenum) a key aspect for solar-to-chemical energy conversion, particularly for hydrogen production. Nickel oxidation state changes dynamically under potential bias and illumination; therefore, operando measurements are required to track the behavior of the system and to correlate photo-electrochemical processes with redox variations. Electrochemical, optical, and X-ray spectroscopic techniques are combined to study how illumination affects nickel redox dynamics. Static absorption spectra and operando visible spectroscopy reveal potential-dependent variations consistent with nickel redox processes, and reveal that illumination modifies both the redox dynamics and their kinetics. Fixed Wavelength Transmission Voltammetry identifies new oxidation and reduction pathways under illumination, suggesting modified reaction dynamics driven by photogenerated holes. X-ray Absorption Near-Edge Spectroscopy (XANES) and operando Fixed Energy X-ray Absorption Voltammetry (FEXRAV) confirm the illumination-dependent behavior: an early photoinduced oxidation at intermediate potentials, and a light-induced reduction at lower anodic potentials. Overall, the results highlight the coupling between photogenerated carriers and cocatalyst states at the hematite–nickel–electrolyte interfaces. The data show that illumination modifies both the dynamics and kinetics of nickel redox reactions, providing new insight into the operando behavior of Ni-based catalysts in photoelectrochemical systems.