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

Abstract

Tuning\ncrystallinity and surface functionality are supreme for\nextracting maximum charge storage efficiency from electrode materials\nfor energy storage devices. Contextually, suitable organic additives\nwith sluggish precipitants are known to significantly regulate the\nkinetics of reactions and dimensionality of crystals leading to structures\nwith tuned crystallinity and surface functionality. Accordingly, herein,\nextremely uniform sedgelike highly porous Co₃O₄ nanoarrays were synthesized by homogeneous precipitation method\nusing sodium dodecyl sulfate as the organic additive and urea as the\nsluggish precipitant, under hydrothermal condition. The distinctive\nphysicochemical properties of Co₃O₄ were identified\nby powder X-ray diffraction, field-emission scanning electron microscopy,\nhigh-resolution transmission electron microscopy, Brunauer–Emmett−Teller\nsurface area, and UV–vis diffuse reflectance spectroscopy analyses,\nwhich show ∼2 nm crystallinity, uniform sedgelike structure,\npresence of model micro- and mesopores, and signatures of quantum\nconfinement. Thorough electrochemical studies show that the Co₃O₄ nanoarrays sample possess lower electrochemical\nseries resistance of 0.4 Ω, and it offers a very high rate-specific\ncapacitance of 2510 F g^–1 at an applied current\ndensity of 4 A g^–1, and retains ∼42% of capacitance\nat eightfold higher applied current density, when measured in a three-electrode\nassembly. The sedgelike Co₃O₄ was used as a\npositive electrode material, and its compatibility was assessed with\nmicrobelt-like two-dimensional (2D) Bi₂O₃ as\nthe negative electrode material, in a redox ensuing Co₃O₄ || Bi₂O₃ asymmetric supercapacitor\n(ASC) device with a wide operating potential window of 1.4 V. The\nASC device offers very high areal and mass-specific capacitance of\n479 mF cm^–2 & 71 F g^–1, respectively,\nat an applied current density of 6 mA cm^–2 and exhibits\nan excellent rate capacitance of ∼50% at an extremely high\ncurrent density of 48 mA cm^–2. The ASC device also\nretains ∼95% of the areal capacitance after 5000 galvanostatic\ncharge–discharge cycles at an applied current density of 10\nmA cm^–2. The Co₃O₄ || Bi₂O₃ ASC device also offers high energy density of\n∼38.5 Wh kg^–1 at a power density of ∼1225\nW kg^–1 and retains ∼47% of the energy density\nat a very high power density of ∼9473 W kg^–1. Factually, the present study manifests that ideal porosity and\nsurface properties of sedgelike Co₃O₄ nanoarrays\nallow unimpeded OH^– ion diffusion, and the bundled\nstructure provides flake-off resistance/mechanical stability during\nharmonious redox reactions with 2D Bi₂O₃ during\nhigh rate operation of the ASC device. It is proposed that the all-new\nCo₃O₄ || Bi₂O₃ asymmetric\nassimilation will open new avenues in the designing of high rate ASCs\nfor power grid applications.