Six polyoxotungstate-based transition metal compounds for electrochemical capacitor application and a comparative analysis of factors affecting capacitances

Abstract

Six different polyoxotungstate-based transition metal complexes were synthesized, namely [Cu₅(2,2′-bpy)₅(μ₂-Cl)₂(PO₄)₂(H₂O)₂][HPW₁₂O₄₀]·2H₂O (1), [Cu_1.5(2,2′-bpy)_1.5(inic)₂(H₂O)_1.5]₃[H_1.5PW₁₂O₄₀]₂·16.25H₂O (2), [Cu(2,2′-bpy)₂]₂[SiW₁₂O₄₀]·10H₂O (3), [Zn(phen)₃]₂[PW^VW^VI₁₁O₄₀]·5H₂O (4), [Zn(phen)₂(H₂O)]₂[SiW₁₂O₄₀]·2H₂O (5), and [Zn(2,2′-bpy)₂]₂[SiW₁₂O₄₀] (6) (2,2′-bpy = 2,2′-bipyridine, inic = isonicotinic acid, phen = 1,10-phenanthroline). Compound 1 is based on [HPW₁₂O₄₀]²⁻ anions, which are accommodated within the open channels of a supramolecular network formed by novel Cu–P–Cl coordination clusters. Compound 2 is constructed from [H_1.5PW₁₂O₄₀]^1.5− and novel [Cu_1.5(2,2′-bpy)_1.5(inic)₂(H₂O)_1.5]⁺ coordination fragments, and polyoxoanions are encapsulated within the pores created by the copper coordination fragments, resulting in a unique three-dimensional supramolecular architecture. Compound 3 is a two-dimensional structure formed through the covalent linkage between [SiW₁₂O₄₀]⁴⁻ and [Cu(2,2′-bpy)₂]²⁺. Compound 4 is a supramolecular architecture formed by [PW^VW^VI₁₁O₄₀]⁴⁻ and [Zn(phen)₃]²⁺ coordination fragments, while compound 5 is a supramolecular structure based on POM bi-supported Zn coordination complexes. Compound 6 is a two-dimensional framework structure constituted by [SiW₁₂O₄₀]⁴⁻ and [Zn(2,2′-bpy)₂]²⁺via covalent interactions. In addition, electrochemical measurement results show that the copper-based tungstate compounds 1–3 and zinc-based tungstate compounds 4–6 exhibit different performances and durabilities as electrochemical capacitors (compound 1 shows the highest specific capacitance of 94.0 F g⁻¹ at 1.5 A g⁻¹, whereas compound 6 maintains the best cycling stability with the capacity retention of 80.7% after 1000 cycles at 4 A g⁻¹.). This study contributes to the development of POM-based transition metal complexes with high capacitance by providing insights into the design and synthesis process.