High-performance nickel oxide–graphene composite as an efficient hybrid supercapacitor

Supercapacitors, thanks to their unique properties, are considered among the main future energy storage systems. However, problems such as low energy density relative to batteries, spontaneous discharge, and low cell voltage have limited their widespread use. In this regard, the development of active and efficient materials is known as a viable solution. Thus, in this study, a hybrid supercapacitor made of nickel oxide and graphene was investigated. Nickel oxide and graphene were synthesized by calcination of nickel hydroxide and electrochemical exfoliation of graphite, respectively. Nickel oxide–graphene composites were synthesized at three levels, including 10, 20 and 30%wt of graphene by a facile hydrothermal-calcination route. The samples were characterized by XRD, FE-SEM, elemental mapping and FTIR tests, and their electrochemical performance was evaluated by electrochemical measurements including CV and EIS tests. The result of the characterization tests confirmed the successful synthesis of nickel oxide, graphene and composites. The results of the electrochemical measurements also showed that the addition of graphene to nickel oxide improved the supercapacitive properties of pure nickel oxide. Improved performance of the composites was attributed to the less aggregation of graphene sheets and their greater conductivity. Based on the results of electrochemical tests, the optimum level of graphene addition was 20%wt and NiO@G20 supercapacitor in 2.0 M KOH medium and a scan rate of 5 \(\frac{{\text{mV}}}{{\text{s}}}\) showed a specific capacitance of 915.40 \(\frac{{\text{F}}}{{\text{g}}}\), energy density of 31.78 \(\frac{{\text{Wh}}}{{\text{kg}}}\) and power density of 2.29 \(\frac{{\text{kW}}}{{\text{kg}}}\). Also, NiO@G20 supercapacitor was able to maintain 96.7% of its initial capacitance after 5000 cycles, which shows its high cycle stability. The high and stable activity of NiO@G20 introduces it as a promising and high-performance material for supercapacitor.