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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 Ti3C2Tx 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 VTi 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 VC significantly destabilize adjacent Ti atoms, lowering the energy barrier for the water attack reaction. The substitution of VC with electronegative species such as O or N does not significantly improve the stability of Ti3C2Tx, highlighting the detrimental role of any defects in the carbon sublattice. Because VC are typically inherited from the precursor phase and cannot be removed during postsynthesis, controlling their concentration during Mn+1AXn phases synthesis is essential. Our thermodynamic analysis reveals that A-rich (e.g., Al-rich) synthesis conditions substantially increase the formation energy of VC and VN defects in a large spectrum of Mn+1AXn phases, providing a generalizable strategy for defect suppression and improved durability of the resulting MXenes. 

MXenes are versatile 2D materials demonstrating outstanding electrochemical and physical properties, but their practical use is limited, because of fast degradation in an aqueous environment. To prevent the degradation of MXenes, it is essential to understand the atomistic details of the reaction and to identify active sites. In this letter, we provided a computational analysis of the degradation processes at the interface between MXene basal planes and water using enhanced sampling ab initio molecular dynamics simulations and symbolic regression analysis. Our results indicate that the reactivity of Ti sites toward the water attack reaction depends on both local coordination and chemical composition of the MXene surfaces. Decreasing the work function of the Ti3C2Tx surfaces and avoiding Ti sites that are loosely anchored to the subsurface (e.g., O-coordinated) can improve surface stability. The developed computational framework can be further used to investigate other possible culprits of the degradation reaction, including the role of defects and edges.