Fabrication of a High-Energy Flexible All-Solid-State Supercapacitor Using Pseudocapacitive 2D-Ti 3 C 2 T x -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 Ti 3 C 2 T x -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 (NiCo 2 O 4 ) 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 Ti 3 C 2 T x was loaded on a flexible acid-treated carbon fiber (ACF) backbone to prepare a Ti 3 C 2 T x /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 NiCo 2 O 4 nanospikes using a hydrothermal method to obtain a NiCo 2 O 4 @rGO/ACF positive electrode. The fabricated Ti 3 C 2 T x /ACF electrode exhibited an excellent specific capacitance of 246.9 F/g (197.5 mF/cm 2 ) at 4 mA/cm 2 along with 96.7% capacity retention over 5000 charge/discharge cycles, whereas the NiCo 2 O 4 @rGO/ACF electrode showed a specific capacitance of 1487 F/g (458.3 mA h/g) at 3 mA/cm 2 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 Ti 3 C 2 T x /ACF on the negative side with a widespread voltage window and the battery-type NiCo 2 O 4 @rGO/ACF on the positive side with high electrochemical activity delivered an excellent volumetric capacitance of 2.32 F/cm 3 (141.9 F/g) at a current density of 5 mA/cm 2 with a high-energy density of 44.36 Wh/kg (0.72 mWh/cm 3 ) at a power density of 985 W/kg (16.13 mW/cm 3 ) along with a cycling stability of 90.48% over 4500 charge/discharge cycles. Therefore, the pseudocapacitive 2D Ti 3 C 2 T x /ACF negative electrode could replace carbon-based electrodes and a combination of it with the battery-type NiCo 2 O 4 @rGO/ACF positive electrode should be a promising way to step up the energy density of a supercapacitor.