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

Abstract

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