Synthesis of amorphous hydroxyl-rich Co₃O₄ for flexible high-rate supercapacitor

Metal oxides-based materials are promising electrodes for energy storage devices, however, the low rate performance and the high energy consumption in the preparation limit their practical applications. Herein, a novel amorphous hydroxyl-rich Co₃O₄ hierarchical structure flexible electrode is facilely fabricated via electrochemical oxidation of a cobalt-based metal-organic framework. The as-prepared Co₃O₄ hierarchical structure is composed of large microrods and small nanoparticles and possesses abundant mesopores, amorphous, and hydroxyl-rich nature. These characteristics favor rapid electron transfer, complete exposure of the active interface, and sufficient penetration of electrolyte ions within the active material. Benefitting from these advantages, the optimal Co₃O₄ electrode (Co₃O₄-5) expresses a high reversible specific capacitance (C_s) of 226.1 C·g⁻¹ (or 253.2 mC·cm⁻²), which is 2 (or 3.7) times higher than those of highly crystalline hydroxyl-deficient Co₃O₄ electrode. The Co₃O₄-5 electrode also shows excellent rate performance (97% C_s retention after a 6.7-times current increasing), which surpasses the levels of many cobalt oxide-based electrodes. In addition, an asymmetric supercapacitor (ASC) constructed from this Co₃O₄-5 electrode achieves a large energy density of 26.6 Wh·kg⁻¹ (or 0.146 mWh·cm⁻²), outstanding rate capability (1% C_s loss at the 10-fold higher current density), and stable cycle performance (10.1% C_s loss after 20,000 charge-discharge cycles).