Hydrogels that couple nitrogen-enriched graphene with Ni(OH)2 nanosheets for high-performance asymmetric supercapacitors

Nitrogen-enriched graphene coupled with nickel hydroxide nanosheets (Ni(OH)₂/NG) hydrogel is successfully synthesized through a facile one-pot hydrothermal method. Comprehensive investigations reveal that the nitrogen atoms are successfully inserted into graphene and that the nickel hydroxide nanosheets (∼30–50 nm) are anchored on NG homogeneously. With the enhanced electroactivity caused by nitrogen doping and the synergy effect from Ni(OH)₂ nanosheets and NG, the Ni(OH)₂/NG hydrogel electrode displays much better electrochemical properties than two individual electrodes. It features a specific capacitance as high as 896 F g⁻¹ at 0.5 A g⁻¹ and even 504 F g⁻¹ at 12 A g⁻¹ showing a high rate capability (56.3% retention with 24 times higher current density). An asymmetric supercapacitor device on the basic of Ni(OH)₂/NG hydrogel and activated carbon (AC) was assembled and delivered a high energy density of 28.7 W h kg⁻¹ at the power energy density of 0.36 kW kg⁻¹. Such results indicate that the Ni(OH)₂/NG hydrogel could be considered as a promising candidate for electrochemical supercapacitors.