Sedgelike Porous Co₃O₄ Nanoarrays as a Novel Positive Electrode Material for Co₃O₄ || Bi₂O₃ Asymmetric Supercapacitors

Abstract

Tuning crystallinity and surface functionality are supreme for extracting maximum charge storage efficiency from electrode materials for energy storage devices. Contextually, suitable organic additives with sluggish precipitants are known to significantly regulate the kinetics of reactions and dimensionality of crystals leading to structures with tuned crystallinity and surface functionality. Accordingly, herein, extremely uniform sedgelike highly porous Co3O4 nanoarrays were synthesized by homogeneous precipitation method using sodium dodecyl sulfate as the organic additive and urea as the sluggish precipitant, under hydrothermal condition. The distinctive physicochemical properties of Co3O4 were identified by powder X-ray diffraction, field-emission scanning electron microscopy, high-resolution transmission electron microscopy, Brunauer–Emmett−Teller surface area, and UV–vis diffuse reflectance spectroscopy analyses, which show ∼2 nm crystallinity, uniform sedgelike structure, presence of model micro- and mesopores, and signatures of quantum confinement. Thorough electrochemical studies show that the Co3O4 nanoarrays sample possess lower electrochemical series resistance of 0.4 Ω, and it offers a very high rate-specific capacitance of 2510 F g–1 at an applied current density of 4 A g–1, and retains ∼42% of capacitance at eightfold higher applied current density, when measured in a three-electrode assembly. The sedgelike Co3O4 was used as a positive electrode material, and its compatibility was assessed with microbelt-like two-dimensional (2D) Bi2O3 as the negative electrode material, in a redox ensuing Co3O4 || Bi2O3 asymmetric supercapacitor (ASC) device with a wide operating potential window of 1.4 V. The ASC device offers very high areal and mass-specific capacitance of 479 mF cm–2 & 71 F g–1, respectively, at an applied current density of 6 mA cm–2 and exhibits an excellent rate capacitance of ∼50% at an extremely high current density of 48 mA cm–2. The ASC device also retains ∼95% of the areal capacitance after 5000 galvanostatic charge–discharge cycles at an applied current density of 10 mA cm–2. The Co3O4 || Bi2O3 ASC device also offers high energy density of ∼38.5 Wh kg–1 at a power density of ∼1225 W kg–1 and retains ∼47% of the energy density at a very high power density of ∼9473 W kg–1. Factually, the present study manifests that ideal porosity and surface properties of sedgelike Co3O4 nanoarrays allow unimpeded OH– ion diffusion, and the bundled structure provides flake-off resistance/mechanical stability during harmonious redox reactions with 2D Bi2O3 during high rate operation of the ASC device. It is proposed that the all-new Co3O4 || Bi2O3 asymmetric assimilation will open new avenues in the designing of high rate ASCs for power grid applications.