Two-Dimensional Materials for High-Energy Solid-State Asymmetric Pseudocapacitors with High Mass Loadings.

A porous nanostructure and high mass loading are crucial for a pseudocapacitor to achieve a good electrochemical performance. Although pseudocapacitive materials, such as MnO2 and MoS2 , with record capacitances close to their theoretical values have been realized, the achieved capacitances are possible only when the electrode mass loading is less than 1 mg cm-2 . Increasing the mass loading affects the capacitance as electron conduction and ion diffusion become sluggish. Achieving fast ion and electron transport at high mass loadings through all active sites remains a challenge for high-mass-loading electrodes. In this study, 2D MnO2 nanosheets supported on carbon fibers (MnO2 @CF) as well as MoS2 @CF with high mass loadings (6.6 and 7.2 mg cm-2 , respectively) were used in a high-energy pseudocapacitor. These hierarchical 2D nanosheets yielded outstanding areal capacitances of 1187 and 495 mF cm-2 at high current densities with excellent cycling stabilities. A pliable pseudocapacitive solid-state asymmetric supercapacitor was designed using MnO2 @CF and MoS2 @CF as the positive and negative electrodes, respectively, with a high mass loading of 14.2 mg cm-2 . The assembled solid-state asymmetric cell had an energy density of 2.305 mWh cm-3 at a power density of 50 mW cm-3 and a capacitance retention of 92.25 % over 11 000 cycles and a very small diffusion resistance (1.72 Ω s-1/2 ). Thus, it is superior to most state-of-the-art reported pseudocapacitors. The rationally designed nanostructured electrodes with high mass loading are likely to open up new opportunities for the development of a supercapacitor device capable of supplying higher energy and power.