Defect-Driven Degradation of MXenes in Aqueous Environments and Mitigation Strategies: Insights from First-Principles

MXenes have attracted considerable attention due to their tunable surface chemistry, high electrical conductivity, and ease of solution processing, making them promising candidates for a wide array of applications. The inherent tendency of MXenes to degrade under environmental conditions constrains their compositional diversity and limits certain practical applications. Our computational study shows that degradation of defect-free Ti₃C₂T_x is kinetically limited, whereas common defects markedly lower the activation barriers for water attack. Using ab initio molecular dynamics simulations (AIMD) combined with thermodynamic analysis, we show that titanium vacancies V_Ti act as active sites for the protonation of subsurface carbon atoms, weakening the bonds with and accelerating the release of adjacent Ti atoms. Targeted passivation of these sites by adsorbed metal cations (e.g., Li⁺, Na⁺, K⁺, and Mg²⁺) is predicted to effectively mitigate degradation by suppressing protonation and increasing the barrier for Ti oxidation. This stabilization arises from two synergistic effects: (i) electronic structure modification driven by a strong dipole moment, which markedly shifts the work function, and (ii) steric hindrance that limits water access to reactive defect sites. We also demonstrate that carbon vacancies V_C significantly destabilize adjacent Ti atoms, lowering the energy barrier for the water attack reaction. The substitution of V_C with electronegative species such as O or N does not significantly improve the stability of Ti₃C₂T_x, highlighting the detrimental role of any defects in the carbon sublattice. Because V_C are typically inherited from the precursor phase and cannot be removed during postsynthesis, controlling their concentration during M_n+1AX_n phases synthesis is essential. Our thermodynamic analysis reveals that A-rich (e.g., Al-rich) synthesis conditions substantially increase the formation energy of V_C and V_N defects in a large spectrum of M_n+1AX_n phases, providing a generalizable strategy for defect suppression and improved durability of the resulting MXenes.