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(H$_2$O))$_2$(VO)$_4$(μ-O)2(C$_6$H$_5$PO$_3$)$_6$]$^{2–}$·2H$_2$O, in which redox-active [(Ni(2,2’-bpy)$_3$)]$^{2+}$ cations are embedded between anionic vanadium phosphonate layers [(VO(H$_2$O))$_2$(VO)$_4$(μ-O)$_2$(C$_6$H$_5$PO$_3$)$_6$]$^{2–}$ (with 2,2’-bpy = 2,2’-bipyridine). X-ray photoelectron spectroscopy reveals coexisting Ni$^{2+}$, V$^{2+}$, V$^{3+}$, and V$^{4+}$/V$^{5+}$ 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 V$^{4+}$ centers. Electrochemical characterization in aqueous media yields capacitances of 79 F/g in Na$_2$SO$_4$ (pH 7) and 43 F/g in H$_3$PO$_4$ (pH 4) at 1 A/g. In Na$_2$SO$_4$ (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.