Activating Pseudocapacitive Charge Storage of Molten-Salt-Synthesized MXenes in Mild Aqueous Electrolytes

Traditional MXenes, synthesized by an HF-based wet chemical process, are promising pseudocapacitive materials in strong acidic aqueous electrolytes, but they show poor performance in neutral or alkaline aqueous electrolytes due to the lack of pseudocapacitive activity. In this work, we demonstrate that molten-salt-synthesized MXenes (MS-MXenes) with -O and -Cl surface groups can exhibit a high pseudocapacitive behavior in both AlCl3 and acetate-buffered aqueous electrolytes. MS-Ti2CTx MXene achieves a high specific capacitance of 318 C g(-1) in a 1 M AlCl3 electrolyte and 280 C g(-1) in a 1 M acetate buffer electrolyte. Furthermore, the mild acidity of AlCl3 and acetate electrolytes suppresses hydrogen evolution and enables more negative cutoff potentials of -1.6 and -1.4 V versus Hg/Hg2SO4, respectively. Most of the charge storage and release occur at potentials below -1 V versus Hg/Hg2SO4, making MS-MXene carbides suitable for negative electrodes. By pairing with a MnO2 positive electrode, the asymmetric supercapacitor delivers a high voltage of 2.1 V and an energy density of 37 Wh kg(-1) together with high cycling stability in a 1 M acetate aqueous electrolyte. Our findings demonstrate the potential of MXenes as negative electrode materials in mild aqueous electrolytes, opening avenues for their practical implementation in advanced energy storage devices.