Abstract
AbstractA 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.
Highlights
Compared to many other 2D materials MXenes stands out, since they are both hydrophilic and conducting
The activity of the MXene materials toward hydrogen evolution reaction (HER) was evaluated in 0.5 m H2SO4 and the results are shown in Figure 1, where we have added results from a polycrystalline Pt and a bare glassy carbon electrode, for comparison
It was shown that changing the potential limit from −0.5 to −0.75 VRHE leads to a significant increase in HER activity and a shift in the anodic direction of the onset potential by about 0.2 V was observed
Summary
Compared to many other 2D materials MXenes stands out, since they are both hydrophilic and conducting. Pan used density functional theory (DFT) to calculate the H adsorption free energy, ∆Gad, and predicted that the hypothetical W2C should even have a HER activity rivaling that of Pt as the coverage dependent ∆Gad approaches thermoneutral.[12] Most recently, we realized the first W-based MXene, W1.33C, derived from a new i-MAX phase,[9] and initial HER analysis showed a clear catalytic activity toward HER.[9] Given these early indications, along with the need to identify new and cheap/abundant materials for HER, we have chosen to conduct a more detailed experimental and theoretical investigation into such properties. The value of the onset potential was found to change
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