Evidence of Molecular Hydrogen Trapped between MXene Layers

MXenes are two-dimensional (2D) transition metal carbides, nitrides and carbonitrides. They possess a variety of different properties including a rare combination of hydrophilicity and conductivity which make MXenes promising materials in a broad range of applications. Even though the MXenes have been explored for hydrogen evolution in hydrogen evolution reaction catalysis, none of the previous studies have detected presence of molecular hydrogen in MXene. Here, using inelastic and elastic neutron-scattering techniques, we find evidence of molecular hydrogen in MXene. Inelastic neutron scattering spectra at 5 K show a peak at 14.6 meV, providing a clear indication of presence of para hydrogen in MXene annealed at 110 oC, and synthesized using 48 % HF. Temperature increase leads to a decrease in the peak intensity with an increase in the peak width due to enhancement on the mobility of the hydrogen molecules in confinement. Presence of molecular hydrogen is further confirmed from the observed mobility of hydrogen in the temperature range of 10-35 K, as captured in a temperature scan of neutron elastic intensity measurements. The role of etching conditions, ion intercalation, and annealing temperature on the removal of the molecular hydrogen trapped in the MXene will be discussed.