Novel redox-active vanadium(v)-dithiolene complexes for efficient supercapacitive energy storage: role of selenium functionalization on pseudocapacitive properties

Abstract

Supercapacitors have been established as efficient storage devices to address the intermittent output issue of renewable energy systems, bridging the gap between batteries and conventional capacitors by offering high power density, rapid charge–discharge rates, and long cycle life; however, achieving superior electrochemical performance remains a key challenge. In this regard, the present work emphasizes the synthesis and characterization of two photo-redox-active anionic V(V)-dithiolene complexes [(THF)₄Li][V(V)(SS–NHC = E)₃] (E = S (1), Se (2)), by reacting dithiolene radical anions [(THF)₂Li(SS-NHC = S˙⁻)] and [(THF)₂Li(SS-NHC = Se˙⁻)] with V(III)Cl₃ in THF, respectively. These complexes were thoroughly characterized using UV-vis-NIR, IR, Raman and EPR spectroscopy, confirming their structural and electronic properties, which are well correlated with their electrochemical energy storage capabilities using cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), electrochemical impedance spectroscopy (EIS) and stability tests. Electrochemical studies reveal that both complexes can exhibit remarkable pseudocapacitive behavior in transition-metal-dithiolene-based materials with excellent charge storage capacity and cycling stability. The unique electronic structures of these vanadium-dithiolene complexes contribute to their efficient redox activity, making them a promising class of supercapacitor material. Moreover, variations in electrolyte composition significantly influenced their performance, highlighting the crucial role of ion–electrode interactions. Impedance analysis further confirmed their low charge transfer resistance, suggesting efficient ion diffusion and active redox processes. The materials exhibited good cycling stability, highlighting the robust redox-active sites and their suitability for long-term energy storage applications. Complex 1 exhibited a slightly better specific capacitance of 82.82 F g⁻¹ (areal 82.82 mF cm⁻²) than complex 2 (76.56 F g⁻¹, areal 76.56 mF cm⁻²) at 5 mV s⁻¹, while complex 2 exhibited excellent capacitive retention of 104.39% after 2000 cycles. Complex 1 showed some degradation over prolonged cycling. This study also highlights the potential of redox non-innocent dithiolene ligands in the design of advanced and next-generation energy storage materials for the first time.