Facile growth of homogeneous Ni（OH）2 coating on carbon nanosheets for high-performance asymmetric supercapacitor applications

The growth of a Ni（OH）2 coating on conductive carbon substrates is an efficient way to address issues related to their poor conductivity in electrochemical capacitor applications. However, the direct growth of nickel hydroxide coatings on a carbon substrate is challenging, because the surfaces of these systems are not compatible and a preoxidation treatment of the conductive carbon substrate is usually required. Herein, we present a facile preoxidation-free approach to fabricate a uniform Ni（OH）2 coating on carbon nanosheets （CNs） by an ion-exchange reaction to achieve the in situ transformation of a MgO/C composite to a Ni（OH）2/C one. The obtained Ni（OH）2/CNs hybrids possess nanosheet morphology, a large surface area （278 m2/g）, and homogeneous elemental distributions. When employed as supercapacitors in a three-electrode configuration, the Ni（OH）JCNs hybrid achieves a large capacitance of 2,218 F/g at a current density of 1.0 A/g. Moreover, asymmetric supercapacitors fabricated with the Ni（OH）2/CNs hybrid exhibit superior supercapacitive performances, with a large capacity of 198 F/g, and high energy density of 56.7 Wh/kg at a power density of 4.0 kW/kg. They show excellent cycling stability with 93% capacity retention after 10,000 cycles, making the Ni（OH）2/CNs hybrid a promising candidate for practical applications in supercapacitor devices.