Fabrication of a High-Energy Flexible All-Solid-State Supercapacitor Using Pseudocapacitive 2D-Ti3C2Tx-MXene and Battery-Type Reduced Graphene Oxide/Nickel-Cobalt Bimetal Oxide Electrode Materials.

Owing to excellent metallic conductivity, hydrophilic surfaces, and surface redox properties, a two-dimensional (2D) metal carbide of Ti3C2Tx-MXene could serve as a promising pseudocapacitive electrode material for energy storage devices. Meanwhile, the 2D reduced graphene oxide (rGO) combining with the hierarchical cubic spinel nickel-cobalt bimetal oxide (NiCo2O4) nanospikes could control ion diffusion for charge storage, thereby facilitating the improvement of the energy density of a supercapacitor. As per the strategy, the pseudocapacitive 2D Ti3C2Tx was loaded on a flexible acid-treated carbon fiber (ACF) backbone to prepare a Ti3C2Tx/ACF negative electrode by a convenient drop-casting method. Meanwhile, 2D rGO was deposited on ACF by a simple dip-dry process, which was further decorated by the spinel NiCo2O4 nanospikes using a hydrothermal method to obtain a NiCo2O4@rGO/ACF positive electrode. The fabricated Ti3C2Tx/ACF electrode exhibited an excellent specific capacitance of 246.9 F/g (197.5 mF/cm2) at 4 mA/cm2 along with 96.7% capacity retention over 5000 charge/discharge cycles, whereas the NiCo2O4@rGO/ACF electrode showed a specific capacitance of 1487 F/g (458.3 mA h/g) at 3 mA/cm2 with a cycling stability of 88.2% over 10 000 charge/discharge cycles. As a result, a flexible all-solid-state hybrid supercapacitor (FHSC) device using the pseudocapacitive Ti3C2Tx/ACF on the negative side with a widespread voltage window and the battery-type NiCo2O4@rGO/ACF on the positive side with high electrochemical activity delivered an excellent volumetric capacitance of 2.32 F/cm3 (141.9 F/g) at a current density of 5 mA/cm2 with a high-energy density of 44.36 Wh/kg (0.72 mWh/cm3) at a power density of 985 W/kg (16.13 mW/cm3) along with a cycling stability of 90.48% over 4500 charge/discharge cycles. Therefore, the pseudocapacitive 2D Ti3C2Tx/ACF negative electrode could replace carbon-based electrodes and a combination of it with the battery-type NiCo2O4@rGO/ACF positive electrode should be a promising way to step up the energy density of a supercapacitor.