Defect engineering of 1T′ MX 2 (M = Mo, W and X = S, Se) transition metal dichalcogenide-based electrocatalyst for alkaline hydrogen evolution reaction

International audienceAbstract The alkaline electrolyzer (AEL) is a promising device for green hydrogen production. However, their energy conversion efficiency is currently limited by the low performance of the electrocatalysts for the hydrogen evolution reaction (HER). As such, the electrocatalyst design for the high-performance HER becomes essential for the advancement of AELs. In this work, we used both hydrogen (H) and hydroxyl (OH) adsorption Gibbs free energy changes as the descriptors to investigate the catalytic HER performance of 1T′ transition metal dichalcogenides (TMDs) in an alkaline solution. Our results reveal that the pristine sulfides showed better alkaline HER performance than their selenide counterparts. However, the activities of all pristine 1T′ TMDs are too low to dissociate water. To improve the performance of these materials, defect engineering techniques were used to design TMD-based electrocatalysts for effective HER activity. Our density functional theory results demonstrate that introducing single S/Se vacancy defects can improve the reactivities of TMD materials. Yet, the desorption of OH becomes the rate-determining step. Doping defective MoS 2 with late 3d transition metal (TM) atoms, especially Cu, Ni, and Co, can regulate the reactivity of active sites for optimal OH desorption. As a result, the TM-doped defective 1T′ MoS 2 can significantly enhance the alkaline HER performance. These findings highlight the potential of defect engineering technologies for the design of TMD-based alkaline HER electrocatalysts

Halogenation effect on physicochemical properties of Ti3C2 MXenes

Halogenated MXenes have been experimentally demonstrated to be promising two-dimensional materials for a wide range of applicability. However, their physicochemical properties are largely unknown at the atomic level. In this study, we applied density functional theory (DFT) to theoretically investigate the halogenation effects on the structural, electronic, and mechanical characteristics of Ti _3 C _2 , which is the most studied MXene material. Three atomic configurations with different adsorption sites for four kinds of halogen terminals (fluorine, chlorine, bromine, and iodine) were considered. Our DFT results reveal that the adsorption site of terminals has a considerable impact on the properties of MXene. This can be ascribed to the different coordination environments of the surface Ti atoms, which change d-orbital splitting configurations of surface Ti atoms and the stabilities of systems. According to the density of states, crystal orbital Hamilton population, and charge analyses, all the considered halogenated MXenes are metallic. The electronic and mechanical properties of the halogenated MXenes are strongly dependent on the electronegativity of the halogen terminal group. The Ti–F bond has more ionic characteristics, which causes Ti _3 C _2 F _2 mechanically behave in a more ductile manner. Our DFT results, therefore, suggest that the physicochemical properties of MXenes can be tuned for practical applications by selecting specific halogen terminal groups