Nickel-cobalt based nanostructures for supercapacitor electrodes: From hydroxides, oxides to sulfides and phosphides

Nickel‑cobalt-based nanostructures have attracted considerable attention as electrode materials for supercapacitors due to their bimetallic synergistic effects, abundant redox-active sites, and tunable nanostructures. This review systematically summarizes recent advances in four typical nickel‑cobalt-based materials, namely nickel‑cobalt hydroxides, oxides, sulfides, and phosphides, for supercapacitor applications. The crystal structures and energy storage mechanisms of these materials are discussed in detail. Nickel‑cobalt layered double hydroxides (LDHs) mainly rely on reversible Faradaic reactions involving Ni 2+ /Ni 3+ and Co 2+ /Co 3+ redox couples. Spinel-type NiCo 2 O 4 and NiCo 2 S 4 generally exhibit higher electrical conductivity than their single-metal counterparts, owing to their mixed-valence states and open crystal structures, while Nickel‑cobalt phosphides (NiCoP) enables rapid electron transport because of its metal-like electrical conductivity. In addition, this review summarizes how different synthesis strategies, including co-precipitation, hydrothermal/solvothermal synthesis, electrodeposition, microwave-assisted synthesis, and template-assisted synthesis, regulate the morphology, crystallinity, and electrochemical performance of nickel‑cobalt-based materials. The effects of carbon-based composites (e.g., graphene, carbon nanotubes (CNTs), and MXene), heterostructure construction, and defect engineering on specific capacitance, rate capability, and cycling stability are also systematically discussed. Despite substantial progress, nickel‑cobalt-based materials still face challenges, including limited intrinsic electrical conductivity, structural degradation during repeated charge-discharge processes, insufficient cycling stability, and relatively low device-level energy density, which hinder their practical application. Future research should focus on optimizing the intrinsic properties of nickel‑cobalt hydroxides, developing structurally stable nanostructures and multifunctional composite systems, and exploring high-voltage electrolytes and asymmetric device configurations to achieve supercapacitors with higher energy density and longer cycle life.