Abstract
Deep eutectic solvents (DESs) are promising electrolyte media for solar redox flow batteries, but their high viscosity and sluggish ion transport can limit redox-species diffusion, interfacial charge transfer, and photocharging performance. This study aimed to regulate a TEMPO/VCl3-based DES electrolyte by controlled water addition for a Yb,Er-doped TiO2–g-C3N4 photoanode solar redox flow battery. DES electrolytes containing 0, 5, 10, and 15 wt% added water were evaluated by physicochemical, electrochemical, and photoelectrochemical measurements, while full-cell tests compared pristine DES with the half-cell-selected 10 wt% electrolyte. While water incorporation systematically enhanced bulk redox-species transport, the optimal photoelectrochemical performance was achieved at 10 wt% water, rather than 15 wt%. Although the 15 wt% electrolyte exhibited the highest bulk diffusivity, the 10 wt% composition provided the most favorable balance between mass transport and interfacial charge transfer kinetics. The Raman spectra supported water-induced reorganization of the bulk DES hydrogen-bonding network. Full-cell tests confirmed that the 10 wt% water-containing electrolyte delivered higher and more stable photocharging responses than pristine DES over 20 cycles. These results indicate that moderate water regulation is an effective strategy for improving DES-based solar redox flow batteries.