Boosting the electrochemical properties of CoCo2O4 porous nanowire arrays by microwave-assisted synthesis for battery–supercapacitor hybrid devices

Abstract

Cobalt oxide (CoCo₂O₄) porous nanowires are synthesized by a microwave assisted hydrothermal process (MHP) followed by annealing. Electrochemical evaluations show that the MHP CoCo₂O₄ nanowire arrays possess higher specific capacity (743.8 C g⁻¹/206.6 mA h g⁻¹ at 1 A g⁻¹), better rate performance (67.7% retention at 15 A g⁻¹) and more excellent cycling performance (89.7% retention after 5000 cycles), compared to those prepared by a conventional hydrothermal process (CHP, 372.1 C g⁻¹/103.4 mA h g⁻¹, 60.3% and 71.2%). It is found that the introduction of microwave assistance led to an improvement in the precursor's purity. More importantly, the crystal structure of this precursor was distorted, which in turn resulted in an expansion in the crystalline lattice of the annealed CoCo₂O₄. Compared to the CHP, the as-synthesized CoCo₂O₄ by the MHP showed a different crystallographic structure and micromorphology, which enlarged the withstand voltage range and improved the electronic conductivity of the electrode. Furthermore, the assembled CoCo₂O₄//microwaved carbon nanotube (m-CNT) battery–supercapacitor hybrid device displays a high energy density of 26.8 W h kg⁻¹ at 775.4 W kg⁻¹ power density with outstanding cycle life (82.3% capacity retention after 8000 cycles). In conclusion, the larger specific surface area, better electronic conductivity, and expanded crystalline lattice and voltage window, resulting from microwave assistance, enhance the electrochemical properties of the MHP CoCo₂O₄.