Design of Multicore–Shell Structured Co3O4–NiO Nanocomposites as High-Performance Cathodes for Zinc-Ion Hybrid Supercapacitors

Mixed-metal oxides (MMOs) hold great promise as cathodes for zinc-ion hybrid supercapacitors (Zn-HSCs); however, the lack of comprehensive insight into how their compositional tuning affects the electrochemical performance hinders the systematic design of MMO-based high-performance materials. To examine the impact of MMOs’ composition on Zn-HSC performance, a series of cathode materials were selected, including carbon (acetylene black), Co3O4, NiO, and Co3O4-NiO nanocomposites with different Co2+:Ni2+ molar ratios (Co–Ni(x:y), where x:y = 1:0.1, 1:0.2, 1:0.3, 1:0.5, and 1:1). Notably, the Co–Ni(1:0.2) nanocomposite exhibited a multicore-shell structure with well-integrated Co3O4 and NiO nanoparticles, combined with a large surface area and an optimized pore architecture. As fabricated Zn-HSCs, assembled with a Zn anode, active material-coated stainless steel cathode, Whatman filter paper separator, and 2 M ZnSO4 electrolyte, offered specific capacitance values at 0.2 A g−1 in the given order: carbon (23 F g−1) < Co–Ni(1:1) (83 F g−1) < Co–Ni(1:0.5) (134 F g−1) < Co3O4 (139 F g−1) < NiO (180 F g−1) < Co–Ni(1:0.1) (253 F g−1) < Co–Ni(1:0.3) (309 F g−1) < Co–Ni(1:0.2) (355 F g−1), emphasizing the superior energy storage capability of the optimized MMO composition over monometallic oxides. Remarkably, the Co–Ni(1:0.2) cathode delivered an impressive energy density of 217 Wh kg−1 and a power density of 525 W kg−1 at 0.2 A g−1, with 90% of initial capacity maintenance over 5000 cycles at 2 A g−1. Thus, the observed superior cathodic performance of the Co–Ni(1:0.2) nanocomposite could be attributed to its optimized Co3O4-NiO composition, enabling synergistic improvements in the cathode’s structural and electrical properties alongside efficient electrode–electrolyte wettability. These results underscore the necessity for compositional engineering of mixed-metal oxides as a strategic direction for developing durable, high-performance cathodes for Zn-HSC applications.