Hydrogen Evolution Reaction for Vacancy-Ordered i-MXenes and the Impact of Proton Absorption into the Vacancies

Journal article, 2021

A study of the hydrogen evolution reaction (HER) for Mo-, W-, and Nb-based MXenes is presented, where W1.33C MXene with ordered vacancies is chosen for further investigation. The electrochemical measurements show that if the W1.33C MXene is subjected to high cathodic potentials, it greatly improves the activity and onset potential for the HER. The enhancement continues to improve independent of whether the potential is kept fixed at a certain cathodic potential or if the potential is scanned repeatedly. Interestingly, the improvement disappears if the material is subjected to anodic potential. Based on these observations, the hydrogen interaction with the MXene surface as well as in the vacancies is investigated by means of first-principles calculations. These show that the adsorption energy of hydrogen is sensitive to both surface coverage and vacancy occupancy, and that, for certain structures with hydrogen in the vacancies, thermoneutral values of hydrogen adsorption can be obtained. Based on the calculations it is argued that under high cathodic potentials, protons can transfer to the vacancies and stay there in a metastable state as hydrogen atoms, while at anodic potential the process is reversed. The first-principles results provide a rationale for the strongly enhanced HER activity observed experimentally on W1.33C MXene.