Tetragonal Symmetric Shaped {VIV4O8} Cubane-Encasing Vanado-Phosphate Derivative: Prototype Liquid Configured Device Grade Supercapacitor with Enhanced Performance

Abstract

Vanadate complexes, owing to their versatile redox and coordination properties, are emerging as promising candidates for next-generation electrochemical supercapacitors. Our investigations have examined how vanadium oxidation states, electronic configurations, and solvent interactions dictate charge storage efficiency. Studies with devices based on capsular diphosphonate–oxo–vanadates with linear ligands, Na8[H2(VVO)2(VIV4O8)2{O3P-C6H4-PO3}4⊂2H2O]•34H2O and Na8[H2(VVO)2(VIV4O8)2{O3P-C6H4-C6H4-PO3}4⊂2DMF]•29H2O (Capsular Generation 1), displayed good energy and power densities; but their performance was restricted by capsule length dependency and the strength of their interactions with electrolyte molecules. These effects confined the potential window to ~1.75 V, yielding limited energy and power densities of 5.8 mWh/cm³ and 350 mW/cm³. To overcome these challenges, we turned to polyoxometalate design strategies and developed more open diphosphonate–oxo–vanadate architectures, capable of better regulating solvent interactions. This led to the synthesis of a dumbbell-shaped capsular diphosphonate–oxo–vanadate polyanion, Na16(Me3C-NH3)6[H10(VIV4O8)4{O3P-C6H4-O-C6H4-PO3}8⸦4DMF•4H2O]•44H2O, derived from the bent-shaped bis(4-phenylphosphonic acid) ether ligands. Single-crystal X-ray diffraction revealed a structure consisting of homo-valent vanadium(IV) centers arranged in cubic {V₄O₈} units, interconnected by the diphosphonate ligands. The oxidation state of vanadium was independently confirmed using EPR, XPS, magnetic susceptibility, and spin density analyses. Electrochemical devices fabricated with this new complex (Capsular Generation 2) exhibited substantial improvements in their electrochemical performance. The potential window expanded to 2.6 V, with volumetric energy and power densities reaching 11.24 mWh/cm³ and 977.54 mW/cm³ – nearly double the values of Generation 1 devices. Enhanced cyclic stability was also observed, supported by powder X-ray diffraction and Raman analyses. Mechanistic insights indicated contributions from both surface capacitive and diffusion-controlled processes during charge storage. Practical demonstrations highlighted the rapid charge-discharge capability: after charging for just 10 seconds, the device powered a 21-LED “VNIT” display for 64 seconds and operated a small fan for 10 seconds. These findings establish the significant promise of polyoxovanadates for supercapacitive energy storage. They also demonstrate how subtle ligand-driven structural modifications can profoundly influence electrochemical performance, offering a clear pathway towards lightweight, flexible, and fast-recharging energy storage solutions.