Hierarchical nickel-substituted polyoxometalate/carbon quantum dot/polyaniline ternary electrode for sustainable hybrid supercapacitors

Polyoxometalates (POMs), as multinuclear metal oxide clusters, possess natural multielectron redox properties and high electrical conductivity. Hybrid nanocomposites that integrate redox-active materials, conductive polymers, and carbon-based nanostructures have emerged as promising candidates for achieving synergistic enhancements in electrochemical performance. Hierarchical hybrid nanocomposites of nickel-substituted polyoxometalates (NiPOMs) enriched with carbon quantum dots (CQDs), followed by electrodeposition of polyaniline (PANI), i.e., NiPOM-CQD/PANI, were successfully synthesized through a combination of solvothermal and electrodeposition methods. Comprehensive structural, morphological, and electrochemical analyses confirmed the effective integration of all the components, resulting in a remarkable increase in the electrical conductivity and charge transport kinetics. CQDs, with their outstanding conductivity, high surface area, and rich surface functionalities, serve as effective conductive fillers and interfacial modifiers to facilitate charge transport. Similarly, NiPOMs possess abundant redox-active sites and intrinsic porosity, enabling rapid faradaic reactions. The incorporation of PANI further contributes to enhanced ion diffusion and electrochemical reversibility by providing a conductive matrix. This resulted in an improvement in the specific capacitance from 181 F/g for NiPOM, 342 F/g for NiPOM-CQD, and 425 F/g for NiPOM-CQD/PANI electrodes at 5 mV/s. The fabricated NiPOM-CQD/PANI-based symmetric device demonstrated a specific capacitance of 32 F/g at 0.02 A/g and an energy density of 4.43 Wh/kg at a power density of 500 W/kg.