Tuning the Surface Chemistry of MXene to Improve Energy Storage: Example of Nitrification by Salt Melt

Abstract

The unique properties of 2D MXenes, such as metal-like electrical conductivity and versatile surface chemistry, make them appealing for various applications, including energy storage. While surface terminations of 2D MXene are expected to have a key influence on their electrochemical properties, the conventional HF-etching method limits the surface functional groups to -F, -OH, and -O. In this study, O-free, Cl-terminated MXenes (noted as Ti3C2Clx) are first synthesized by a molten salt (FeCl2) etching route. Then, a substitution of surface termination from Cl- to N- is performed via post-thermal treatment of Ti3C2Clx in Li3N containing molten salt electrolytes. While the Cl-terminated pristine material does not show electrochemical activity, the surface-modified, N-containing Ti3C2Tx exhibits a unique capacitive-like electrochemical signature in sulfuric acid aqueous electrolyte with rate performance—more than 300 F g−1 (84 mAh g−1) at 2 V s−1. These results show that control of the MXene surface chemistry enables the preparation of high-performance electrodes in a previously not accessed limit of energy storage.