Electrochemical Energy Storage Studies in 3d–3d Mixed-Metal Diphosphonate Oxovanadates: Structure–Property Correlation and Superior Performance of a Copper–Vanadium Electrode in Symmetric Supercapacitors

The versatile redox and coordination chemistry of vanadium make oxo-vanadates promising for electrochemical energy storage studies, especially when stabilized by diphosphonate ligands. Herein, we report a series of newly designed 3d-3d mixed-metal hybrid oxovanadates, viz., (NH₄)₄[H₄{CoII(H₂O)₄}(VV₂O₅)₂(VIVO₂){O₃P-C(O)(CH₂-3-C₅NH₄)-PO₃}₂]·5H₂O (CoV-a), (NH₄)₄[H₄{NiII(H₂O)₄}(VV₂O₅)₂(VIVO₂){O₃P-C(O)(CH₂-3-C₅NH₄)-PO₃}₂]·7H₂O (NiV-a), (NH₄)₆[H₃{CuII(H₂O)₂(CH₃COO)}(VV₂O₅)₂(VIVO₂){O₃P-C(O)(CH₂-3-C₅NH₄)-PO₃}₂]·3H₂O (CuV-a), (NH₄)₅[H₄{ZnII(H₂O)₂(CH₃COO)}(VV₂O₅)₂(VIVO₂){O₃P-C(O)(CH₂-3-C₅NH₄)-PO₃}₂]·4H₂O (ZnV-a), incorporating Co, Ni, Cu, and Zn, respectively, with pyridyl-based diphosphonate ligands. Single-crystal X-ray diffraction reveals that all compounds crystallize in the orthorhombic Pnna space group with an isoreticular sinusoidal wave-like structure. Metal oxidation states were confirmed by bond-valence-sum calculations and multiple spectroscopic studies. Electochemical energy storage studies were performed with binder-free electrodes, fabricated via direct growth of crystalline phases on MWCNT-coated stainless-steel substrates and extensively characterized using powder X-ray diffraction, photoelectron spectroscopy, electron microscopy, and surface area analysis. Electrochemical evaluation through cyclic voltammetry, charge-discharge, stability, and impedance studies demonstrated the superior performance of the vanadocuprate derivative compared to the cobalt, nickel, and zinc analogues. The optimized vanadocuprate electrode was thus employed to prepare a symmetric prototype device, which delivered a high energy density of 12 Wh kg⁻¹ and power density of 2957 W kg⁻¹. Practical applicability was demonstrated by successfully powering LEDs, highlighting the potential of these hybrid oxovanadates as efficient energy storage materials.