Understanding the Oxidation Degradation Mechanism to Enable Preparation of Ambient Ultra Stable Ti <sub>3</sub>C <sub>2</sub>T <sub>x</sub>-MXene

Although MXene shows amazing application potential, it has always been subject to its own ambient instability, and preparation of MXene with excellent stability remains a persistent challenge because of oxidative degradation mechanism was not fully understood. Herein, we report a oxidative degradation mechanism hypothesis of Ti3C2Tx-MXene and point out the key factors affecting its stability are strong reduc ing Ti atoms inherent in Ti3C2Tx and the exposure of reducing Ti atoms caused by desorption of surface adsorbed HF acid. Based on the mechanism understanding, ambient ultrastable Ti3C2Tx (US-Ti3C2Tx) was successfully obtained and can be durably stored without being oxidatively degraded even if dispersed in water, while the conventional Ti3C2Tx aqueous dispersion can only be stored for a few days. Furthermore, the US-Ti3C2Tx membrane showed better seawater desalination ability than conventional Ti3C2Tx, and more importantly, it can avoid the mixing of harmful elements into desalinated water caused by instability of conventional Ti3C2Tx.