High-Energy-Density Asymmetric Supercapacitor Based on Free-Standing Ti 3 C 2 T X @NiO-Reduced Graphene Oxide Heterostructured Anode and Defective Reduced Graphene Oxide Hydrogel Cathode.

The rational design of an asymmetric supercapacitor (ASC) with an expanded operating voltage window has been recognized as a promising strategy to maximize the energy density of the device. Nevertheless, it remains challenging to have electrode materials that feature good electrical conductivity and high specific capacitance. Herein, a 3D layered Ti 3 C 2 T X @NiO-reduced graphene oxide (RGO) heterostructured hydrogel was successfully synthesized by uniform deposition of NiO nanoflowers onto Ti 3 C 2 T X nanosheets, and the heterostructure was assembled into a 3D porous hydrogel through a hydrothermal GO-gelation process at low temperatures. The resultant Ti 3 C 2 T X @NiO-RGO heterostructured hydrogel exhibited an ultrahigh specific capacitance of 979 F g -1 at 0.5 A g -1 , in comparison to that of Ti 3 C 2 T X @NiO (623 F g -1 ) and Ti 3 C 2 T X (112 F g -1 ). Separately, a defective RGO (DRGO) hydrogel was found to exhibit a drastic increase in specific capacitance, compared to untreated RGO (261 vs 178 F g -1 at 0.5 A g -1 ), owing to abundant mesopores. These two materials were then used as free-standing anode and cathode to construct an ASC, which displayed a large operating voltage (1.8 V), a high energy density (79.02 Wh kg -1 at 450 W kg -1 and 45.68 Wh kg -1 at 9000 W kg -1 ), and remarkable cycling stability (retention of 95.6% of the capacitance after 10,000 cycles at 10 A g -1 ). This work highlights the unique potential of Ti 3 C 2 T X -based heterostructured hydrogels as viable electrode materials for ASCs.