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 the performance of Zn-HSCs, 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(<i>x</i> : <i>y</i>), where <i>x</i> : <i>y</i> = 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. The as-fabricated Zn-HSCs, assembled with a Zn anode, an active material-coated stainless steel cathode, a Whatman filter paper separator, and a 2 M ZnSO₄ electrolyte, offered specific capacitance values at 0.2 A g^-1 in the following 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 exhibited 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.