MXenes represent a class of two-dimensional transition metal carbides, nitrides, and carbonitrides that have gained attention for their potential in hydrogen storage. Among them, Ti₃C₂Tₓ is one of the most studied MXenes due to its unique layered structure, tunable surface chemistry, and promising hydrogen adsorption properties. The material consists of transition metal layers interleaved with carbon or nitrogen layers, terminated with functional groups such as oxygen, fluorine, or hydroxyl, denoted by Tₓ. These terminations influence the material's electronic properties and hydrogen interaction mechanisms.
The layered structure of MXenes provides a high surface area, which is beneficial for hydrogen physisorption. Additionally, the presence of transition metals allows for hydrogen chemisorption through weak metal-hydrogen interactions. The hydrogen binding energy in MXenes typically ranges between 0.1 and 0.6 eV, which is within the ideal range for reversible hydrogen storage at near-ambient conditions. Surface terminations play a critical role in modulating these binding energies. Oxygen-terminated MXenes, for example, exhibit stronger hydrogen interactions compared to fluorine-terminated variants due to differences in electronegativity and charge distribution.
Synthesis of MXenes involves selective etching of the A-layer from MAX phases, typically using hydrofluoric acid or fluoride-containing solutions. The process yields multilayered MXenes, which can be delaminated into single or few-layer sheets via intercalation and sonication. While lab-scale synthesis is well-established, scalability remains a challenge due to the use of hazardous etchants and the need for controlled environments. Recent developments in molten salt etching and electrochemical methods offer potential pathways for more sustainable and scalable production.
Cycling stability is a critical factor for practical hydrogen storage applications. MXenes demonstrate reasonable stability under repeated hydrogen adsorption-desorption cycles, though degradation can occur due to oxidation or structural collapse. Surface passivation and composite formation with polymers or other 2D materials have been explored to enhance stability. Performance under ambient conditions is another area of investigation. While MXenes can adsorb hydrogen at room temperature, their capacity is often limited compared to cryogenic conditions. Current research focuses on optimizing surface terminations and interlayer spacing to improve ambient performance.
When compared to other 2D materials like graphene and boron nitride, MXenes exhibit distinct advantages. Graphene relies primarily on physisorption, resulting in low hydrogen uptake at ambient temperatures unless modified with dopants or defects. Boron nitride shows similar limitations, though its chemical inertness provides better stability. MXenes, in contrast, combine physisorption and chemisorption mechanisms, enabling higher capacities at moderate conditions. However, challenges such as synthesis complexity and long-term durability must be addressed to surpass conventional storage materials like metal hydrides or porous adsorbents.
Quantitative studies report hydrogen storage capacities of MXenes in the range of 1 to 4 wt%, depending on surface modifications and testing conditions. These values are competitive with some metal hydrides but still fall short of DOE targets for vehicular applications. Further improvements may be achievable through defect engineering, hybridization with catalysts, or the development of ternary MXenes with enhanced properties.
In summary, MXenes present a promising avenue for hydrogen storage due to their tunable structure and dual adsorption mechanisms. While challenges in scalability and cycling stability persist, ongoing research into synthesis optimization and material engineering could unlock their full potential. Contrasted with other 2D materials, MXenes offer a unique combination of properties that may bridge the gap between high-capacity and practical hydrogen storage solutions.