(82h) S-Modified Mxenes for Hydrogen Evolution Reaction - a Computational Approach

Discovering materials with cheap and abundant elements, tunable properties, and platinum like chemical activity is crucial in advancing the hydrogen economy of production and storage­­­­­­­­. In this study, we use density functional theory to capture the compositional and structural effects of sulfur modifications to twelve distinct Mo- and Ti-based 2D MXene materials on their stability and activity for catalyzing the hydrogen evolution reaction (HER). The explored S-modifications include (i) S substitution of the O functional group/termination, (ii) S substitution of the carbon of the MXene , (iii) S intercalation into the MXene material, (iv) S adatom onto the MXene basal plane, and (v) MXene – MoS₂ heterostructure. These computational models are geometric, stoichiometric, and structural in nature and allow for comparison with experimentally synthesized S-treated MXenes, which are tested for HER. We perform structural optimization to evaluate changes in lattice parameters due to the presence of S in the MXenes systems. We identify a direct relationship between hydrogen adsorption energy and the S atomic composition of the MXenes. Furthermore, we demonstrate that S substitution into the C position of the MXene results in a decrease in the theoretical HER overpotential in Mo-based MXenes while the S substitution into the O position results in a decrease in the overpotential in Ti-based MXenes. We find that the Mo-based S-modified MXenes span a larger range in the calculated theoretical HER overpotential as compared to the Ti-based MXenes. Based on our screening study, S modified Ti-based MXenes Ti₃C₂, Ti₂MoC₂, Ti₄C₃, Ti₂Mo₂C₃ and TiMoTiMoC₃ have theoretical overpotentials lower than 0.03 V. Almost all O-term MXenes in this study are predicted to be improved by some form of S treatment. This screening report opens the field to consideration of using S in the design of novel 2D materials to promote a clean hydrogen economy.