ZnNiCo hydroxide/graphene-carbon nanotube hydrogel on surface-modified Ni foam as a battery-type electrode for hybrid supercapacitors

Highlights: • Ni foam is functionalized with cationic polyelectrolyte to improve interfacial bonding with electroactive materials. • The ZnNiCo hydroxide/graphene-CNT hydrogel electrode on surface-modified Ni foam demonstrates battery-like characteristics. • The synergy of multiple metal compositions and its close contact with Ni foam contribute to a high capacity. -- Abstract: Ternary transition metal compounds are regarded as potential electrode materials for supercapacitors (SCs) due to the synergistic effect of multiple metal ions. However, their sluggish redox reaction kinetics, poor conductivity and weak interfacial contact with the current collector lead to insufficient capacity utilization. Herein, a three-dimensional (3D) conductive graphene-carbon nanotube hydrogel (GCH) network is constructed on cationic polyelectrolyte-functionalized Ni foam to bridge ZnNiCo hydroxide nanoneedles, which increases electrolyte-accessible areas and improves ionic/electronic conductivity. The surface modification for Ni foam favors intimate interfacial bonding with active materials, resulting in efficient charge transfer at their interface and good structural stability. Moreover, metal cations in ZnNiCo hydroxide provide multiple oxidation states for redox reactions and high electronic conductivity. Beneficial from close interfacial contact with Ni foam, the synergy of different metal cations and 3D porous structure, the binder-free ZnNiCo/GCH electrode exhibits a high capacity, favorable rate capability, and cycling stability. The fabricated ZnNiCo/GCH//GCH hybrid supercapacitor realizes a high energy density of 0.41 mWh cm⁻².