Supercapacitors from nitrogen and sulfur co-doped graphene/nickel cobalt sulfide

Abstract

This research aims to develop high performance supercapacitors in term of capacitive efficiency, power density and long-term cycle stability. Supercapacitors are a kind of energy storages device which exhibit high power density when comparing with conventional batteries. However, most of them exhibits inefficient energy capacitance and long-term cycle stability. Thus, this research is focusing on development materials for using as supercapacitor electrodes. Thus, nitrogen and sulfur co-doped graphene and nickel cobalt compounds were synthesized using in situ hydrothermal process. This research is divided into two major parts (i) to sort out the optimum ratio of nickel and cobalt for the synthesis of nickel cobalt carbonate hydroxide and (ii) to optimize the ratio of nitrogen/sulfur co-doped graphene and nickel cobalt sulfide for the synthesis of the composites. At the optimum ratio of nickel and cobalt of 1 : 2 with carbonate hydroxide structure, the specific capacitance of electrode is 950.2 F g-1 at 1 A g-1 and the cycle stability of 178.3% after 3000 charge/discharge cycles at 40 mV s-1. This as-prepared electrode can deliver an energy density in ranging of 42.9-15.8 Wh kg-1, with power density in range of 285.0-2849.9 W kg-1. In addition, charge/discharge mechanism in atomic level is monitored by the time-resolved x-ray absorption spectroscopy (TR-XAS) which indicates that the morphology transformation of cobalt carbonate hydroxide during charge-discharge process. The optimum ratio of nitrogen and sulfur co-doped graphene and nickel cobalt sulfide was found at 25 : 75, when the specific capacitance of 1420.2 F g-1 at 10 mV s-1 and high cycle stability of 110% retention at 10,000 cycle. The results of this research indicate that the nitrogen and sulfur co-doped graphene and nickel cobalt sulfide composite electrodes show surprisingly high performance thus suitable for the supercapacitor electrode application.