Co₃O₄/NiCo₂O₄ Perforated Nanosheets for High-Energy-Density All-Solid-State Asymmetric Supercapacitors with Extended Cyclic Stability

To develop new-generation electrode materials for high-performance all-solid-state asymmetric supercapacitors (ASSASCs), herein, Co3O4/NiCo2O4 perforated nanosheets are synthesized using a trisodium-citrate-assisted chemical precipitation by the hydrothermal method, at an elevated pH medium. The physicochemical analyses of Co3O4/NiCo2O4 by powder X-ray diffraction (PXRD), Fourier transform (FT)-Raman, X-ray photoelectron spectroscopy (XPS), field-emission scanning electron microscopy (FESEM), and high-resolution transmission electron microscopy (HRTEM) methods show ideal properties of a supercapacitor electrode material, such as discrete phases due to Co3O4 and NiCo2O4, small-size crystallites, and uniquely perforated nanosheets with an overlapped architecture. The preliminary charge storage efficiency study of Co3O4/NiCo2O4 in a three-electrode setup shows pseudocapacitive charge storage phenomena, significant kinetic reversibility, high specific capacitance and good rate capacitance, very low charge transfer resistance, and bias-potential-independent total series resistance. Further, the 1.8 V Co3O4/NiCo2O4∥N-rGO ASSASC device, fabricated using Co3O4/NiCo2O4 and nitrogen-doped reduced graphene oxide (N-rGO) as the positive and negative electrode materials, respectively, delivers excellent capacitance (areal- and mass-specific) and rate capacitance at extreme reaction conditions. The ASSASC device also delivers high energy density, excellent rate energy density at elevated power density conditions, and ∼93.8% capacitance retention, after 10 000 successive galvanostatic charge/discharge (GCD) cycles. The outstanding efficiency of the Co3O4/NiCo2O4∥N-rGO ASSASC device is ascribed to the superior conductivity of Co3O4/NiCo2O4 perforated nanosheets augmented by N-rGO, abundant electrolyte channels in the overlapped perforated nanosheets of Co3O4/NiCo2O4, and improved electromechanical stability offered by the “OH– ion-buffering-reservoir”-like behavior of Co3O4/NiCo2O4.