Assembling 2D MXenes into Highly Stable Pseudocapacitive Electrodes with High Power and Energy Densities

Electrochemical capacitors (ECs) that store charge based on the pseudocapacitive mechanism combine high energy densities with high power densities and rate capabilities. 2D transition metal carbides (MXenes) have been recently introduced as high-rate pseudocapacitive materials with ultrahigh areal and volumetric capacitances. So far, 20 different MXene compositions have been synthesized and many more are theoretically predicted. However, since most MXenes are chemically unstable in their 2D forms, to date only one MXene composition, Ti₃ C₂ T_x , has shown stable pseudocapacitive charge storage. Here, a cation-driven assembly process is demonstrated to fabricate highly stable and flexible multilayered films of V₂ CT_x and Ti₂ CT_x MXenes from their chemically unstable delaminated single-layer flakes. The electrochemical performance of electrodes fabricated using assembled V₂ CT_x flakes surpasses Ti₃ C₂ T_x in various aqueous electrolytes. These electrodes show specific capacitances as high as 1315 F cm^-3 and retain ≈77% of their initial capacitance after one million charge/discharge cycles, an unprecedented performance for pseudocapacitive materials. This work opens a new venue for future development of high-performance supercapacitor electrodes using a variety of 2D materials as building blocks.