Structural Water-Induced Proton Storage in Two-Dimensional Vanadyl Phosphate for Energy Storage: Experimental and Theoretical Studies

Electrochemical energy storage owing to the shuttling of protons is an interesting and promising way of deriving sustainable devices. Herein, a systematic study is undertaken to investigate the electrochemical proton storage behavior in two-dimensional vanadyl phosphate (VOPO4). It is demystified that the crystal water in the structure is crucial for enhanced proton storage, which was validated by both experimental and theoretical evaluations. Experimental results reveal that the hydrated form of VOPO4 exhibits a high specific capacitance of 199 F g–1 at a current density of 0.5 A g–1 and retains approximately 72 F g–1 over 3000 cycles at a current density of 3 A g–1, demonstrating good cycling performance. In contrast, dehydrated vanadyl phosphate suffers from rapid capacitance degradation. The crystal water may increase the interlayer spacing and subsequently enable facile proton diffusion similar to the Grotthuss proton insertion mechanism. Density functional theory (DFT) calculations further confirm the active participation of water molecules in proton adsorption and the lowering of energy barriers for ion transport. These findings underscore the pivotal role of structural water in modulating proton transport kinetics and optimizing charge storage efficiency, positioning hydrated vanadyl phosphate as a highly efficient material for next-generation proton-based energy storage systems.