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.