Semiconductor/cocatalyst interfaces strongly influence the efficiency and selectivity of photoelectrochemical photoanodes. In hematite- and BiVO₄-based systems, transition-metal oxyhydroxide overlayers are often described as hole-collecting cocatalysts, yet their actual role under operating conditions involves more complex redox chemistry governed by bias, illumination, catalyst composition, and substrate identity.

Here, we present operando X-ray absorption spectroscopy as an element-selective platform to track redox dynamics at photoanode/cocatalyst interfaces during PEC operation. Using custom PEC cells integrated with synchrotron XAS detection, oxidation-state changes and local structural rearrangements can be monitored while the electrode is biased, illuminated, and exposed to electrolyte.

Three representative approaches are discussed. First, steady-state XAS and fixed-energy X-ray absorption voltammetry reveal light-driven redox changes in catalytic overlayers, including substrate-specific charge trapping and illumination-dependent oxidation in CoFeOx-modified WO₃/BiVO₄ photoanodes¹. Second, potential-modulated X-ray electrochemical impedance spectroscopy synchronizes XAS detection with periodic voltage perturbations, isolating the catalyst redox response and providing access to potential-driven oxidation and reduction kinetics. Third, light-modulated XAS probes transient photoinduced processes by correlating the catalyst absorption signal with modulated illumination. Applied to hematite photoanodes with Ni-based overlayers, this method reveals frequency-dependent Ni oxidation and reduction pathways, highlighting the active role of the cocatalyst in charge transfer, recombination, and back-transfer processes.

Together, these examples show that operando PEC-XAS can move beyond static characterization toward a dynamic, chemically resolved description of working photoanode interfaces. Combining steady-state, potential-modulated, and light-modulated measurements provides mechanistic insight for designing semiconductor/cocatalyst architectures with improved charge selectivity and enhanced solar-driven oxidation performance.

Figure 1: Operando PEC-XAS approaches for probing redox dynamics at photoanode/cocatalyst interfaces. a) Fixed-energy X-ray absorption voltammetry, FEXRAV, records the element-specific fluorescence response during a linear potential sweep, here shown under dark/light conditions. b) X-ray electrochemical impedance spectroscopy, XEIS, follows the catalyst redox response to a sinusoidal potential modulation, allowing frequency-resolved extraction of potential-driven oxidation/reduction dynamics. c) Intensity-modulated XAS, IM-XAS, probes photoinduced redox processes by correlating the X-ray fluorescence signal with a sinusoidally modulated light excitation. In all cases, the incident X-ray energy is kept fixed at the selected absorption-edge energy while the electrical or optical stimulus is varied.