Cu/Ni Keggin-based viologen complexes with water-assisted 1D proton channels for asymmetric supercapacitors

Abstract

Under the background of global energy transformation, developing high-performance supercapacitors (SCs) has become an important direction for achieving sustainable development. Herein, three Keggin-based Cu/Ni viologen complexes, formulated as [Cu₂(L₁)₄(L₂)(H₂O)₄(Si/GeMo₁₂O₄₀)₂]·8H₂O (1 = Si and 2 = Ge) and [Ni(L₁)₂(L₂)(H₂O)₂(SiMo₁₂O₄₀)]·4H₂O (3) (L₁ = 1-(4-formyl-benzyl)-[4,4′]bipyridinyl-1-ium and L₂ = 4,4′-bipyridinyl), were synthesized and directly used as high-capacity negative electrode materials. Their capacitance performances were significantly boosted owing to the excellent electrical conductivity and potential capacitance capability of the viologen ligand, a three-dimensional supramolecular framework formed through hydrogen bonds and polyoxometalates (POMs) and water molecules forming one-dimensional proton channels. The complexes 1–3 showed high specific capacitances (1618.4 F g⁻¹, 1457.6 F g⁻¹ and 1421.6 F g⁻¹) at 2 A g⁻¹ compared to (HL₁)₄(Si/GeMo₁₂O₄₀)₂·3H₂O (4 = Si and 5 = Ge). An asymmetric supercapacitor (1//AC ASC) assembled with complex 1 and activated carbon (AC) exhibited a significant energy density (9.39 W h kg⁻¹), power density (6000 W kg⁻¹) and enduring cycling stability (78.8%) after 10 000 cycles. Moreover, the response to red diode illumination underscored the practical application potential of complex 1. These experimental results demonstrate that the presented approach is feasible for exploring novel high-performance POM-based pseudo-capacitor materials.