Unveiling the non-innocence of vanadium dopant in TiO₂ nanocrystals for advanced energy storage and smart windows

Vanadium doped TiO2 NCs stand out as a promising candidate for energy storage applications due to its high electrical conductivity and redox properties. However, the thermodynamical behavior of the material under working conditions has not been explored and the reasons for its superior performance remain unlocked. This study explores the use of a combination of advanced in situ spectroscopy techniques, including x-ray absorption spectroscopy (XAS), spectro-electrochemistry (SEC), and electrochemical impedance spectroscopy (EIS) to provide unprecedented insights into the intricate electrochemical reaction mechanisms within these nanocrystals. Density functional theory calculations and EIS reveal the active role of substitutional V ions in the TiO2 anatase network as electron donors, enhancing surface charge and carrier density and improving pseudocapacitive properties. Cyclic voltammetry and in situ SEC reveal that V-doped TiO2 NCs exhibit significantly improved charge storage capacities, particularly in the pseudo-capacitance storage mechanism. In situ SEC and XAS analyses indicate that a more effective reduction of Ti4+ ions occurs during the electrochemical process in doped NCs, leading to higher charge capacitance and faster processes. Furthermore, in situ XAS measurements of the V K-edge revealed that the vanadium ions, beyond improving the redox behavior of the host, also actively participate in the reduction process. The significant changes in the V K-edge XANES and extended x-ray absorption fine structure spectra observed under reduction conditions can be ascribed to a change in the structure and oxidation state of the vanadium ions during the electrochemical reaction.