Operando spectroscopic ellipsometry enables direct quantification of dynamic degradation rates in photoelectrochemical cells

Abstract

The instability of photoelectrodes remains a critical barrier to the practical implementation of photoelectrochemical (PEC) systems for solar fuel production. A comprehensive understanding of photocorrosion mechanisms under operating conditions is essential yet challenging to achieve due to the difficulty of real-time, spatially resolved monitoring. In this study, we introduce an operando-spectroscopic ellipsometry (SE)-based method for the real-time, quantitative analysis of photoelectrode surface degradation. Using atomic layer deposited TiO₂ thin films with different crystalline phases as model systems, we demonstrate continuous monitoring of degradation rates under operando conditions across acidic, alkaline, and near-neutral pH environments, both with and without illumination. PEC-operando-SE reveals that photocorrosion dynamics are strongly influenced by crystallinity, illumination, and electrolyte conditions. Notably, under illumination, degradation rates vary significantly with TiO₂ crystallinity; slower charge transport in amorphous films accelerates photocorrosion compared to crystalline counterparts. When combined with complementary techniques such as electrochemical impedance spectroscopy, operando-SE offers synergistic insights into surface degradation mechanisms and holds strong potential for guiding the development of more stable PEC materials.