A pH-Tolerant Nickel-Vanadium Phosphonate Framework for Stable Aqueous Supercapacitor Cycling

We report a hybrid layered metal-phosphonate framework, [Ni(2,2'-bpy)₃]²⁺[(VO(H₂O))₂(VO)₄(μ-O)₂(C₆H₅PO₃)₆]^2-·2H₂O, in which redox-active [(Ni(2,2'-bpy)₃)]²⁺ cations are embedded between anionic vanadium phosphonate layers [(VO(H₂O))₂(VO)₄(μ-O)₂(C₆H₅PO₃)₆]^2- (with 2,2'-bpy = 2,2'-bipyridine). X-ray photoelectron spectroscopy reveals coexisting Ni²⁺, V²⁺, V³⁺, and V⁴⁺/V⁵⁺ species on the surface of the crystals, indicating a highly dynamic redox system. The material exhibits a main photoluminescence peak at 2.25 eV and strong short-range antiferromagnetic coupling between the V⁴⁺ centers. Electrochemical characterization in aqueous media yields capacitances of 79 F/g in Na₂SO₄ (pH 7) and 43 F/g in H₃PO₄ (pH 4) at 1 A/g. In Na₂SO₄ (pH 7), the material reaches about 300 F/g at a scan rate of 1 mV/s and retains capacity over more than 10,000 cycles. The crystals remain chemically stable between pH 2 and pH 10 for at least 1 week. Combined sustainable synthesis, robust redox activity, high capacitance, long cycle stability, and broad pH tolerance make these hybrid metal-phosphonate crystals promising candidates for next-generation aqueous energy-storage applications.