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 Ti3C2Tx is kinetically limited, whereas common defects markedly lower the activation barriers for water attack.. Using ab initio molecular dynamics simulations combined with thermodynamic analysis, we show that VTi defects act as active sites for protonation of subsurface carbon atoms, weakening the bonds with and accelerating release of adjacent Ti atoms. Targeted passivation of these sites by adsorbed metal cations (e.g., Li+, Na+, K+, 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 find that VC defects significantly destabilize adjacent Ti atoms, lowering the energy barrier for water attack reaction. The substitution of VC with electronegative species such as O or N does not significantly improve stability of Ti3C2Tx, highlighting the detrimental role of any defects in carbon sublattice. Because VC are typically inherited from the precursor MAX phase and cannot be removed during post-synthesis, controlling their concentration during MAX phases synthesis is essential. Our thermodynamic analysis reveals that Al-rich conditions substantially raise the formation energy of VC defects in a large spectrum of MAX phases, providing a generalizable strategy for defect suppression and enhanced MXene durability. Our thermodynamic analysis reveals that A-rich (e.g., Al-rich) synthesis conditions substantially raise 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 resulting MXenes.