Analysis of the TiO₂ Photoanode Process Using Intensity Modulated Photocurrent Spectroscopy and Distribution of Relaxation Times

Photoelectrochemical water splitting offers a promising pathway for green hydrogen production, but its efficiency is limited by electron-hole recombination. Overcoming this challenge requires detailed analysis of the relationship between charge separation and charge transfer kinetics under operando conditions. Here, we applied intensity-modulated photocurrent spectroscopy (IMPS) combined with distribution of relaxation times (DRT) analysis to the photoanodic process under varying light intensities. This approach revealed three distinct applied potential regions: a high-potential region with constant admittance independent of light intensity; a midpotential region strongly influenced by light intensity; and a low-potential region with back electron-hole recombination (BER). Crucially, our analysis demonstrated that what has traditionally been viewed as a single bulk recombination process can be resolved into distinct mechanisms based on light intensity dependence. Additionally, we identified satellite peaks in the slow kinetic regions for the first time. These peaks, influenced by light intensity and reaction conditions, revealed novel insights into surface-trapped hole dynamics. Based on these insights, we propose tailored band bending models for each kinetic scenario and discuss the implications of satellite peaks for reaction bottlenecks. These results offer new perspectives on understanding and optimizing photoelectrochemical systems.