Water-Content Regulation of TEMPO-Based Deep Eutectic Solvent (DES) Electrolyte for Enhanced Solar Redox Flow Battery Performance

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.