Synthesis of Ni-VO2@C/SO42− composite using cashew leaves for supercapacitor application

Cashew leaves were used to synthesize composites based on vanadium dioxide (VO2@C/SO42−) and on nickel-doped vanadium dioxide (Ni-VO2@C/SO42−) by a green process. The choice of nickel doping is motivated by the fact that the ionic radius of V (IV) and that of Ni2+ are nearly equal (0.72 Å) and also Ni has advantages of low toxicity and cheapness. The structural and morphological properties were analysed by XPS, XRD, Raman, RBS, EDX, SEM and BET. Electrochemical measurements show that the Ni-VO2@C/SO42− composite exhibits a high specific capacitance of 355 F g−1 at a specific current of 1 A g−1 with an excellent Coulombic efficiency of 97 % at 10 A g−1. To evaluate practical applications, a device was assembled in which Ni-VO2@C/SO42− composite and activated carbon (AC) were used as cathode and anode respectively. The device thus prepared displays an energy density of 21 W h kg−1 and an excellent power density of 972 W kg−1 at a cell voltage of 1.6 V and a specific current of 1 A g−1. Stability tests also show an excellent Coulomb efficiency of 98 % and a retention capacity of 88 % at 10 A g−1 after 10,000 cycles. This remarkable electrochemical performance is due to the good synergy between VO2, SO42− and carbon, together with the effect of nickel doping. This finally confirms that this Ni-VO2@C/SO42− composite could be used to develop advanced cathodes for the next generation of supercapacitors.