Among the many strategies proposed to enhance the photoelectrochemical (PEC) performance of contemporary photoelectrodes, the integration of semiconductor/cocatalyst heterostructures has emerged as one of the most effective. Co-catalyst layers, such as Ni-based oxyhydroxides interfaced with hematite (Fe₂O₃), are often credited with boosting quantum efficiencies by accelerating interfacial charge transfer through holes accumulation. Yet, a growing body of work shows that this apparent simplicity masks a far richer and more nuanced interfacial chemistry whose behavior depends strongly on the precise microscopic nature of the semiconductor/metal-oxide junction¹. Understanding the redox dynamics at these buried interfaces under operating conditions is therefore essential for steering photogenerated carriers toward targeted reaction pathways.

To address these challenges, we have focused on the operando characterization of Fe₂O₃/Ni interfaces, exploiting operando X-ray absorption spectroscopy (XAS) as a direct, element-specific probe of interfacial charge transfer, catalytic turnover, and transient oxidation processes in functioning PEC cells. While indirect techniques such as IMPS² and TAS³ provide valuable phenomenological information, operando PEC-XAS offers uniquely powerful insight by resolving oxidation-state changes and local structural rearrangements in real time, under the same potential, illumination, and electrolyte conditions used during PEC operation.

Here, we summarize our recent studies that illustrate how operando PEC-XAS reveals the diversity and complexity of redox processes at Fe₂O₃/Ni interfaces:

1. Specific molecular interactions during biomass-derived oxidation.
   Operando PEC-XAS allowed us to follow the oxidation state evolution of Ni centers in contact with organic molecules such as HMF, demonstrating that the Ni layer does not merely act as a generic hole-accepting catalyst, but enables alternative oxidation pathways through molecular-level interactions⁴.
2. Bias-dependent inversion of photoinduced redox processes at Ni cocatalysts.
   By tracking Ni oxidation states under simultaneous dark and illuminated conditions, we identified a potential-dependent regime in which photogenerated holes no longer accumulate in the Ni layer. Instead, competition between dark electrochemical oxidation and photoinduced charge transfer leads to an inversion of the expected redox sequence.
3. Development of a frequency-resolved operando technique to probe transient photoinduced processes.
   Building on the principles established for purely electrochemical systems (e.g., the dark analogue demonstrated for Co-based catalysts by Farnoush et al., Adv. Energy Mater. 2025), we extended operando PEC-XAS into the frequency domain. This method enables selective probing of short-lived oxidation events at the Fe₂O₃/Ni interface by synchronizing XAS detection with periodic potential or light perturbations. The resulting time- and frequency-dependent XAS response provides access to interfacial charge-transfer kinetics—including hole injection, NiOOH formation, and back-transfer pathways—that are otherwise hidden in steady-state measurements.

Together, these studies demonstrate how operando PEC-XAS enables a mechanistic understanding of the Fe₂O₃/Ni interface during real PEC operation, capturing substrate-specific chemistry, competing dark and photodriven redox processes, and transient charge-transfer events. This integrated operando framework not only reveals the true dynamic behavior of the cocatalyst layer but also provides actionable design principles for next-generation semiconductor/cocatalyst assemblies tailored for selective and efficient solar-driven oxidation reactions.