Temperature-Dependent Water Oxidation Kinetics: Implications and Insights

As a vital process for solar fuel synthesis, water oxidation remains a challenging reaction to perform using durable and cost-effective systems. Despite decades of intense research, the understanding of the detailed processes involved is still limited, particularly under photochemical conditions. Recent research has shown that the overall kinetics of water oxidation by a molecular dyad depends on the coordination between the photocharge generation and the subsequent chemical steps. This work explores similar effects by heterogeneous solar water oxidation systems. By varying a key variable, the reaction temperature, we discovered distinctly different behaviors on two model systems, TiO2 and Fe2O3. TiO2 exhibited a monotonically increasing water oxidation performance with rising temperatures across the entire applied potential range, between 0.1 V and 1.5 V vs. the reversible hydrogen electrode (RHE). In contrast, Fe2O3 showed increased performance with temperature at high applied potentials (>1.2 V vs. RHE) but decreased performance at low applied potentials (<1.2 V vs. RHE). This decrease in performance with temperature on Fe2O3 was attributed to increased electron-hole recombination, as confirmed by intensity modulated photocurrent spectroscopy (IMPS). The origin of the differing temperature dependences on TiO2 and Fe2O3 was further ascribed to their different surface chemical kinetics. These results highlight the chemical nature of charge recombination in photoelectrochemical (PEC) systems, where surface electrons recombine with holes stored in surface chemical species. It also indicates that PEC kinetics are not constrained by a single rate determining chemical step, highlighting the importance of an integrated approach to studying the system. Moreover, the results suggest that for practical solar water splitting devices, higher temperatures are not always beneficial for reaction rates, especially under low driving force conditions.