Nickel-based (oxy)hydroxides materials are among the most used electrocatalysts for alkaline oxygen evolution, and their catalytic activity is intrinsically linked to dynamic redox process that occur under applied bias [1]. In photoelectrochemical systems, these redox processes are influenced by photogenerated charge carriers from the semiconductor substrate too, making static characterization insufficient [2]. Operando techniques capable of tracking oxidation-state changes during operation are required to understand the interplay between semiconductor, cocatalyst, and electrolyte [2].

Titanium-doped hematite (Ti:α-Fe₂O₃) photoanodes functionalized with Nickel–Molybdenum (NiMo) thin films are investigated in alkaline electrolyte (NaOH 0.1 M) by combining operando visible transmission spectroscopy and operando X-ray Absorption Spectroscopy (XAS). The study focuses on tracking the dynamics between Ni²⁺/Ni³⁺ species (nickel hydroxide Ni(OH)₂ and nickel oxy-hydroxide NiOOH, respectively) during photoelectrochemical operation. Operando visible transmission measurements, implemented as Static Visible Absorption Spectroscopy and Fixed Wavelength Transmission Voltammetry, probe potential- and light-induced changes in optical transmittance associated with nickel oxidation. Differential Absorbance spectra shows that the 550–650 nm range provide a sensitive probe of the Ni(OH)₂/NiOOH redox couple, allowing the tracking of oxidation and reduction processes during cyclic voltammetry under dark and illuminated conditions. These measurements reveal that illumination induces an earlier onset of nickel oxidation at intermediate anodic potentials, as well as modified reduction pathways upon reverse scans. Operando XAS, performed at the Ni K-edge using Fixed Energy X-ray Absorption Voltammetry (FEXRAV), provides element-specific confirmation of these trends. XAS directly tracks changes in the nickel oxidation state and demonstrates that photogenerated holes from hematite actively participate in the oxidation of nickel species at intermediate potential. At higher anodic potentials, XAS reveals an inversion of the light-induced effect, with illumination leading to a more reduced nickel steady state compared to dark conditions, indicating a complex, potential-dependent coupling between charge transport and catalyst redox chemistry.

While visible transmission techniques provide a rapid and sensitive probe of global redox-driven optical changes, XAS offers element-specific insight into the underlying oxidation-state evolution. The consistent observation of light-induced early oxidation at intermediate potentials and photoinduced reduction at higher potentials provides key information on the non-trivial role of photogenerated carriers during cocatalyst redox behavior.