MXenes, a family of two-dimensional transition metal carbides, nitrides, and carbonitrides, exhibit exceptional electrical conductivity, mechanical strength, and surface functionality. However, their stability under ambient, aqueous, and thermal conditions remains a critical challenge for long-term utilization. Understanding degradation pathways and developing mitigation strategies are essential to harness their full potential.

Under ambient conditions, MXenes are prone to oxidation, primarily due to their high surface area and reactive termination groups such as -O, -F, and -OH. Exposure to oxygen and moisture accelerates degradation, leading to the formation of metal oxides and carbonaceous byproducts. For instance, Ti3C2Tx, the most studied MXene, undergoes gradual oxidation when stored in air, with visible changes in its electrical and optical properties within days. The oxidation kinetics depend on factors like flake size, defect density, and environmental humidity. Smaller flakes with higher defect concentrations degrade faster due to increased exposure of reactive edges. Studies indicate that even under controlled environments with low humidity (below 30%), Ti3C2Tx films show signs of oxidation within weeks, evidenced by reduced conductivity and the appearance of TiO2 peaks in X-ray diffraction patterns.

Aqueous stability is another major concern, as MXenes are often processed or applied in water-based solutions. In aqueous environments, oxidation is exacerbated by dissolved oxygen and elevated temperatures. The degradation follows a hydrolysis-like mechanism, where water molecules react with MXene layers, leading to the release of carbon dioxide and the precipitation of metal oxide nanoparticles. For example, Ti3C2Tx dispersions in water exhibit rapid degradation at temperatures above 30°C, with complete loss of colloidal stability within hours. The pH of the solution also plays a significant role, with acidic conditions (pH < 4) accelerating oxidation due to proton-assisted cleavage of metal-carbon bonds. Neutral or slightly basic conditions (pH 7–9) offer marginal stability but do not prevent long-term degradation.

Thermal stability is relatively higher compared to ambient and aqueous conditions, but MXenes still decompose at elevated temperatures. In inert atmospheres, Ti3C2Tx remains stable up to approximately 800°C, beyond which it transforms into TiO2 and other carbide phases. In air, however, oxidation begins as low as 200°C, with complete conversion to TiO2 occurring by 500°C. The presence of termination groups influences thermal behavior; -O terminated MXenes exhibit higher thermal stability than -F or -OH terminated counterparts due to stronger metal-oxygen bonds. Differential scanning calorimetry studies reveal exothermic peaks corresponding to oxidation, with energy release varying based on the MXene composition and layer spacing.

To mitigate degradation, encapsulation is a widely explored strategy. Protective coatings of polymers, oxides, or carbon layers can physically shield MXenes from environmental exposure. For instance, polyvinyl alcohol (PVA) coatings reduce oxidation rates by limiting oxygen and water diffusion to the MXene surface. Similarly, atomic layer deposition of Al2O3 creates a conformal barrier that enhances ambient stability for several months. However, encapsulation must balance protection with preserving MXene properties, as thick coatings may impede electrical conductivity or hinder surface reactivity.

Doping with heteroatoms or transition metals is another effective approach to enhance stability. Incorporating aluminum or sulfur into the MXene lattice alters electronic structure and reduces susceptibility to oxidation. Nitrogen-doped Ti3C2Tx, for example, shows improved resistance to aqueous degradation due to passivation of reactive sites. The doping process can be achieved during synthesis or through post-treatment methods such as annealing in reactive atmospheres. The choice of dopant depends on the intended application, as some elements may introduce undesirable changes in electrical or mechanical properties.

Shelf-life studies provide critical insights into long-term storage solutions. MXenes stored in anhydrous organic solvents like dimethyl sulfoxide (DMSO) or N-methyl-2-pyrrolidone (NMP) exhibit extended stability compared to aqueous dispersions. Freeze-drying MXenes into powders and storing them under argon or vacuum can also prolong shelf life, though re-dispersion may require sonication or chemical treatments. Recent work demonstrates that vacuum-sealed MXene films retain their properties for over a year, whereas unsealed films degrade within months. The development of standardized storage protocols is essential for industrial adoption.

Quantitative studies on degradation kinetics reveal activation energies for oxidation, enabling predictive models for shelf life. For Ti3C2Tx in air, the activation energy ranges between 50–70 kJ/mol, depending on termination groups and flake morphology. These values align with diffusion-limited oxidation processes, where environmental permeation through defects dictates degradation rates. Accelerated aging tests at elevated temperatures and humidity levels help extrapolate long-term behavior, though real-time studies remain indispensable for validation.

In summary, MXenes face significant stability challenges under ambient, aqueous, and thermal conditions, driven by oxidation and hydrolysis mechanisms. Encapsulation and doping offer promising mitigation pathways, while shelf-life studies inform optimal storage practices. Future research should focus on refining protective strategies and establishing standardized stability metrics to facilitate broader adoption. The interplay between environmental factors and material properties must be carefully managed to unlock the full potential of MXenes in advanced technologies.