Photoelectrochemical (PEC) water oxidation is gaining increasing attention as a sustainable pathway for solar-driven hydrogen production. Developing efficient photoanodes capable of catalyzing the inherently sluggish water oxidation reaction is essential for enabling practical solar-assisted water splitting. Beyond the Oxygen Evolution Reaction (OER), alternative oxidation pathways leading to value-added chemicals are also being explored, as they are energetically less demanding and can enhance overall process efficiency. 

Hematite (Fe₂O₃) stands out as a promising photoanode material due to its chemical stability, earth abundance and suitable bandgap (1.9-2.1 eV) corresponding to a theoretical Solar-to-Hydrogen efficiency of 16.8% [1]. Its application, however, remains hindered by intrinsically low electrical conductivity and pronounced bulk electron-hole recombination. Cation doping with Ti(IV) or Sn(IV), the latter frequently introduced via thermal diffusion from the FTO substrate during annealing above 750°C, can partially mitigate these limiting factors [2]. Despite extensive research efforts, the beneficial role of dopants in hematite photoanodes, as well as the kinetic processes governing bulk charge-transport and interfacial charge-transfer reactions, are still not fully understood [3].

In this work, we employ a comprehensive set of electrochemical techniques, including Linear Sweep Voltammetry (LSV), Cyclic Voltammetry (CV), Electrochemical Impedance Spectroscopy (EIS), and especially Intensity Modulated Photocurrent Spectroscopy (IMPS), to investigate Ti and Sn-doped hematite photoanodes for OER and Methanol Oxidation Reaction (MOR). IMPS, a powerful in-operando technique for PEC technology, enables us to extract characteristic time constants associated with charge-transfer and recombination pathways. A central focus of this work is the adoption of a physically consistent IMPS model, which is essential for the accurate interpretation of complex spectral features and for distinguishing between competing kinetic processes [4]. Structural and morphological characterization via XRD and SEM complete the electrochemical analysis. 

Our results show that under back illumination, photocurrent generation is mainly limited by inefficient electron collection, whereas under front illumination the dominant bottleneck arises from slow hole-transfer kinetics during the OER. The introduction of Ti or Sn dopants mitigates these effects. Additionally, EIS analysis reveals that, unlike the OER, the MOR proceeds without the involvement of surface states typically active at the hematite/electrolyte interface.

By integrating a rigorous IMPS modeling framework with complementary photoelectrochemical methods, this work provides a coherent mechanistic picture of the processes limiting hematite-based photoanodes. The insights gained offer valuable guidelines for the rational design of improved, earth-abundant materials for solar-driven chemical transformations.