Core-shell carbon@Ni₂(CO₃)(OH)₂ particles as advanced cathode materials for hybrid supercapacitor: The key role of carbon for enhanced electrochemical properties

Three-dimensional porous Ni₂(CO₃)(OH)₂ compounds were grown on carbon nanopowder using a facile hydrothermal method for the production of core-shell carbon@Ni₂(CO₃)(OH)₂ compounds. This work successfully overcame the shortcomings related to the low electrical conductivity and poor electrical stability caused by the presence of hollows in the Ni₂(CO₃)(OH)₂ structure. The hollow spaces were filled with carbon powder, which acted as a seed material, yielding an ideal electrode material with a large specific surface area, high electrical conductivity, and good stability. A Ni₂(CO₃)(OH)₂ electrode containing 50 mg of carbon powder could store more energy than a Ni₂(CO₃)(OH)₂ electrode without carbon seed materials. The Ni₂(CO₃)(OH)₂ electrode comprising 50 mg of carbon powder has a considerably high specific capacity (181.7 mAh g⁻¹ at 3 A g⁻¹) and excellent cycling stability (77.9 % capacity retention after 5000 cycles), which is 1.5 times higher than that of the Ni₂(CO₃)(OH)₂ electrode without carbon powder. Moreover, an asymmetric supercapacitor using Ni₂(CO₃)(OH)₂ containing 50 mg of carbon powder as the positive electrode and graphene as the negative electrode exhibits a high energy density of 34.2 Wh kg⁻¹ and a power density of 176.1 W kg⁻¹ at a current density of 2 A g⁻¹. Using a combination of carbon and a Ni₂(CO₃)(OH)₂ nanowire compound to increase the electrochemical property and specific surface area, respectively, a suitable synergistic effect can be obtained, which may pave the way for efficient electrode design for high-performance supercapacitors.