N-doped hollow mesoporous carbon spheres modified with cobalt–nickel hydroxide for supercapacitor applications

Layered double hydroxides (LDHs) have an abundance of redox reaction sites, giving them a high specific capacitance. However, the improvement of intrinsic electrical conductivity remains a great challenge due to the tendency of LDHs to agglomerate, under-exposure of active sites, and under-developed internal pore structure. In this work, cobalt–nickel hydroxide modified N-doped hollow mesoporous carbon spheres (CoNi-LDH/N-HMCS) were designed and prepared as electrode materials. The results showed that N-HMCS was uniformly wrapped by CoNi-LDH nanosheets, which suppressed the volume change and structural collapse of CoNi-LDH during charging and discharging, increased the dispersion of the active material, and exposed more active sites. At the same time, the unique high specific surface area and hollow mesoporous structure of N-HMCS opens up an effective way for electrolyte ion transport, shortens the ion diffusion path, and reduces the diffusion resistance, thus improving the charge/discharge efficiency of the material. In addition, the presence of two different metal ions in CoNi-LDH provides more redox reaction sites and improves electrical charge storage capacity. Satisfactory, the CoNi-LDH/N-HMCS electrode material exhibits an ultra- high mass ratio capacitance with 2244.4 F g⁻¹ (1 A g⁻¹). A hybrid supercapacitor together with CoNi-LDH/N-HMCS as the cathode material achieved a superior gravimetric energy density of 91.98 W kg⁻¹ at a gravimetric power density of 750 W kg⁻¹ and good cycling performance with 99.27 % cycle retention and 95.24 % coulombic efficiency for 10,000 cycles at 3a g⁻¹. In short, CoNi-LDH/N-HMCS composites have excellent energy storage performance and are hope for the development of new hydroxide composite electrodes for advanced energy devices