Selective Etching of Ti₃AlC₂ MAX Phases Using Quaternary Ammonium Fluorides Directly Yields Ti₃C₂T<i>_z</i> MXene Nanosheets: Implications for Energy Storage

MXenes have been studied extensively for energy storage and other applications and are synthesized by selectively etching the A element from a MAX phase using concentrated etchants. However, the hazards associated with using toxic chemicals in their synthesis remain a roadblock in scaling up MXene production. To address this challenge, a safer process is demonstrated, consisting of simultaneous etching and exfoliation using a mixture of tetramethylammonium fluoride tetrahydrate (TMAF) and hydrochloric acid (HCl) at room temperature. The large size of the quaternary ammonium ion promoted simultaneous intercalation and exfoliation during the etching of Ti3AlC2, directly resulting in Ti3C2Tz MXene nanosheets without requiring additional delamination steps. The ratio of surface terminal groups on the Ti3C2Tz MXenes also changed with the etching duration and the molarity of the etchant. This method results in MXene yields exceeding 30 wt % of the starting mass of the parent MAX phase. Ti3C2Tz MXenes synthesized by this new method are particularly well-suited for applications in energy storage since these Ti3C2Tz-based electrodes exhibit stable and reversible pseudocapacitive behavior, with specific capacitance values comparable to those reported in the literature. Moreover, these MXenes also exhibit a slower degradation rate in aqueous dispersions as compared to MXenes etched with traditional methods