A Systematic Study of NiCo Layered Double Hydroxide Grown on Porous Carbon for Hybrid Supercapacitor Electrodes: Growth Method, Morphology and Composition

Supercapacitors have drawn an extensive attention for their high power density and excellent cycling stability to meet a rapidly growing demand for high-capacity energy storage systems. Specifically, metal (e.g., Ni, Co, Fe, Al, Mn, and its combinations) layered double hydroxides (LDHs) have been integrated to supercapacitor electrodes owing to their seamless ion transport through interlayer. LDHs are often hybridized with conductive materials to facilitate charge transport since low conductivity of LDHs hinders the supercapacitors to reach their theoretical capacitance (e.g., ~3,000 F g⁻¹ for pristine NiCo LDH). Although diverse forms of LDH hybridized carbon electrodes are designed, it has not yet been systematically studied how morphology and composition simultaneously affect the performance of supercapacitors.<br/>Accordingly, we have systematically controlled the synthetic method/condition of growing metal LDHs on nitrogen doped porous carbons (NPCs) and studied their effects on electrical performance as hybrid electrodes of supercapacitors. In detail, various metals (i.e., Ni, Co, and NiCo) have been deposited on NPCs by both electrodeposition (i.e., 100, 300, 500, and 1000 mC) and solvothermal method. NPC was chosen as carbon electrodes since it can improve the rate performance as well as structural stability of electrodeposited LDHs owing to its higher conductivity, specific surface area, and hydrophilicity compared to other carbon-based materials with free-standing structure for electrodeposition compatibility. The supercapacitor with electrodeposited NiCo LDH@NPC electrodes exhibited higher specific capacitance than those of either Ni or Co LDH@NPC electrodes, since coexistence of metals shows a synergistic effect on storing higher energy via multiple reduction-oxidation (redox) reactions. Among NiCo LDH@NPC electrodes, the electrode with the total deposited charge of 500 mC (NiCo500 LDH@NPC) showed the highest capacitance of 3,155.52 F g⁻¹. These results might be ascribed to three main factors: (i) multiple redox reactions from Ni-Co coexistence, (ii) shortened hydroxide (OH⁻) ion diffusion length due to the uniform distribution of LDH in nanomesh structure, and (iii) facilitated OH⁻ ion accessibility into active materials owing to broader interlayer of LDH. Furthermore, an asymmetric supercapacitor (NPC||NiCo500 LDH@NPC) achieved energy density of 39.22 Wh kg⁻¹ at a power density of 913.87 W kg⁻¹ and 80.00% retention after 20,000 cycles.