Polyoxometalate/ZIF-67 composite with exposed active sites as aqueous supercapacitor electrode

Abstract

Polyoxometalates (POMs) owing to their distinctive structural and highly stable redox properties are emerging as advanced electrode material for energy storage applications. The imminent concern with POMs is their long-term stability in aqueous electrolytes and conductivity which can be effectively tackled by structural engineering at molecular level. Here, we report smart assembly of polyoxometalates (POMs) and zeolitic imidazole framework (ZIF-67) as promising tool for structural modulation. The complementing dimensions of POM and ZIF-67 results in successful encapsulation of POM which not only enhances the active sites for charge storage, but also prevents the dissolution of POMs. Resulting nanocomposites were investigated in three different electrolytes where ammonium decavanadate intercalated nanocomposite (ZADV) is found to exhibit excellent electrochemical performance in 1 M Na2SO4 owing to its ordered structure with enhanced number of active sites. Further, to meet the demand of high energy density supercapacitor for modern applications, structure modulated POM (positive potential electrode) is integrated with a negative potential material “loofah sponge derived carbon (LSC)” that extends the potential from −1 to 0.6 V. The resulting integrated electrode ([email protected]) displays a maximum specific capacitance of 250.1F g−1 @ 0.8 A g−1 with good columbic efficiency. Interestingly, [email protected]||[email protected] works well upto 1.6 V, delivers energy density of 19.7 Wh kg−1 @ 586 W kg−1 with excellent cycling stability (89% after 5000 cycles @ 1.5 A g−1). This work is a keystone for the upcoming studies on enhancing the performance of polyoxometalate based electrodes with improved active sites and energy density.