High-Performance Flexible Asymmetric Supercapacitor Based on Nanostructured MnO₂ and Bi₂O₃ Decorated 3D Carbon Nanotube Sponge in an Aqueous Gel-Electrolyte

Supercapacitors have been highly demanded as advanced energy storage devices. A major obstacle is that, compared to commercial battery systems, their energy density is relatively low. Here, we report a flexible asymmetric supercapacitor with high energy and power densities composed of nanostructured MnO 2 nanospheres and Bi 2 O 3 nanoshells decorated on a 3D interconnected carbon nanotube sponge (CNTS) in a neutral hydrogel-electrolyte. Our CNTS@MnO 2 electrode and CNTS@Bi 2 O 3 electrode demonstrate an excellent areal capacitance of 5.54 F·cm –3 and 4.76 F·cm –3, respectively. In 10,000 charge–discharge cycles, they maintain 93% and 80% of their initial capacitance, respectively. A flexible asymmetric supercapacitor composed of the CNTS@MnO 2 anode and the CNTS@Bi 2 O 3 cathode (embedded in a Na 2 SO 4 hydrogel-electrolyte) delivers a high energy density up to 37 Wh·kg –1 at a cell voltage of 1.8 V, 7 times higher than that of traditional electrochemical double-layer capacitors (EDLCs). The long-term cyclic stability and robust mechanical stability profit from the flexible conductive skeleton of CNTS and meet the demands of wearable flexible electronics.