3D hierarchical Co3O4@Co3S4 nanoarrays as cathode materials for asymmetric pseudocapacitors

Three-dimensional (3D) hierarchical Co3O4@Co3S4 nanoarrays (NAs) were synthesized via a stepwise hydrothermal method involving precipitation and in situ sulfurization of Co3O4 nanoneedle arrays (NNAs). By controlling both anion exchange and Ostwald ripening reactions during the sulfurization process, 3D hierarchical Co3O4@Co3S4 NAs with tailored Co3S4 nanostructures have been fabricated as electrode materials for electrochemical capacitor applications. Owing to an interconnected matrix within the 3D architecture, the as-prepared Co3O4@Co3S4 NAs exhibit excellent electrical conductivity, high specific capacity and high cycling stability. It can deliver a high capacitance of 1284.3 F g(-1) at 2 mV s(-1) and maintain a capacitance retention of 93.1% after 5000 cycles. Moreover, a flexible solid-state asymmetric supercapacitor (ASC) composed of Co3O4@Co3S4 NAs as the positive electrode and activated carbon (AC) as the negative electrode exhibited an energy density of 1.5 mW h cm(-3) and a power density of 6.1 W cm(-3) at a high operating voltage of 1.6 V. Our results not only present the 3D hierarchical nanostructure of Co3O4@Co3S4 NAs, but they also demonstrate the potential of electrodes for future generation supercapacitors.