Exploring hydrogen adsorption and release in 2D M2C-MXenes: structural and functional insights.

Abstract

Two-dimensional M₂C-MXenes, characterized by their lightweight nature, tunable surface structures, and strong affinity for hydrogen, hold significant promise for addressing various challenges in hydrogen energy utilization. This study focuses on investigating the hydrogen adsorption and desorption properties, as well as the stability of hydrogenated compounds in 19 pure M₂C-MXenes nanosheets. The results indicate that hydrogen adsorption on M₂C primarily occurs through weak physisorption, with Mn₂C and Fe₂C from the fourth period, and Ag₂C and Cd₂C from the fifth period exhibiting the lowest adsorption energies. In contrast, hydrogen atoms are adsorbed on M₂C primarily through chemisorption, leading to the potential dissociation of H₂molecules into two hydrogen atoms. Among the M₂C-MXenes, Ti₂C, and Zr₂C in thed⁴andd⁵, respectively, demonstrate the most stable hydrogen atom binding. Hydrogen evolution is most facile on Cu₂C and Ag₂C surfaces. Two types of stacking configurations, face-centered cubic and hexagonal close-packed, are observed for hydrogenated M₂C surfaces (e.g. Co₂C and Zr₂C), showing excellent thermodynamic stability. This work elucidates the hydrogen utilization performance of pure M₂C-MXenes nanosheets and guides future research aimed at achieving high hydrogen storage capacities through the functional tuning of MXenes.