Ligand-directed assembly and transformation of hybrid oxo-vanadates into 3d–3d oxo-vanado-cuprates: structural diversification and superior electrochemical performance in a prototype liquid-configured-device-grade supercapacitor

Abstract

Oxo-vanadates, with their features of varied coordination and oxidation states, can form tunable structures, ranging from zero-dimensional ions or molecules to three-dimensional frameworks. Diphosphonate-functionalized oxovanadates, a recent inclusion into the family of oxovanadates, have formed diverse assemblies, particularly of mixed-valent oxovanadates. With pyridinyl-diphosphonate-functionalized oxovanadates, we have detected that the location of the nitrogen atom on the pyridine ring has strong structure- and symmetric-directing effects. To further explore the reactivity of the pyridinyl-dipshosphonate-functionalized oxo-vanadates, we reacted the mixed-valent oxo-vanadate polyanions [(V^IVO₂)(V^V₂O₅)₂{O₃P-C(O)(CH₂-n-C₅NH₄)-PO₃}₂]¹⁰⁻ (n = 2 or 3) with copper(II) salt. This resulted in the formation of two novel 3d–3d mixed-metal hybrid oxo-vanado-cuprate polyanions: the cube-shaped [(Cu^II)₂(V^IVO₂)₂(V^VO₂)₄{O₃P-C(O)(CH₂-2-C₅NH₄)-PO₃}₄]¹²⁻ (CuV-1) and the sinusoidal one-dimensional chain [{Cu^II(H₂O)₂(CH₃COO)}(V^IVO₂)(V^V₂O₅)₂{HO₃P-C(O)(CH₂-3-C₅NH₄)-PO₃H}₂]⁷⁻ (CuV-2). These polyanions crystallized as (H₃O)₁₂[(Cu^II)₂(V^VO₂)₄(V^IVO₂)₂{O₃P-C(O)(CH₂-2-C₅NH₄)-PO₃}₄]·5.5H₂O (CuV-1a) and (H₃O)₇[{Cu^II(H₂O)₂(CH₃COO)}(V^IVO₂)(V^V₂O₅)₂{HO₃P-C(O)(CH₂-3-C₅NH₄)-PO₃H}₂]·1.56H₂O (CuV-2a), in the monoclinic space group of C2/c and orthorhombic space group of Pnna, respectively. The unique assembly of these mixed-metal polyanions is once again attributed to the strong structure-directing effects of the location of the nitrogen atom on the pyridinyl diphosphonate ligands, which resulted in isostructural oxo-vanadates capable of producing completely different assemblies. The oxidation states of the metal centres were established from bond valence sum (BVS) calculations and confirmed through multiple spectroscopic studies. Electrochemical energy-storage studies of the oxo-vanado-cuprate compounds revealed their enhanced redox activity and conductivity with respect to the parent oxo-vanadate compounds. Improvement was observed across all metrics, including the specific capacitance, energy density, power density, and cyclic stability. Hands-on implementation was performed using a fabricated liquid-type symmetric supercapacitor device to power LEDs and a small fan, making these compounds promising candidates for practical applications in high-performance supercapacitors with improved energy-storage efficiency.