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The design of novel and abundant catalytic materials for electrolysis is crucial for reaching carbon neutrality of the global energy system. A deliberate approach to catalyst design requires both theoretical and experimental knowledge not only of the target reactions but also of the supplementary mechanisms affecting the catalytic activity. In this study, we focus on the interplay of hydrogen mobility and reactivity in the hydrogen evolution reaction catalyst MoS2. We have studied the diffusion of atomic hydrogen and water by means of neutron and X-ray photoelectron spectroscopies combined with classical molecular dynamics simulations. The observed interaction of water with single-crystal MoS2 shows the possibility of intercalation within volume defects, where it can access edge sites of the material. Our surface studies also demonstrate that atomic hydrogen can be inserted into MoS2, where it then occupies various adsorption sites, possibly favoring defect vicinities. The motion of H atoms parallel to the layers of MoS2 is fast with D ≈ 1 × 10−9 m2/s at room temperature and exhibits Brownian diffusion behavior with little dependence on temperature, i.e., with a very low diffusion activation barrier.
The diffusion of hydrogen adsorbed inside layered MoS2 crystals has been studied by means of quasi- elastic neutron scattering, neutron spin-echo spectroscopy, nuclear reaction analysis, and X-ray photoelectron spectroscopy. The neutron time-of-flight and neutron spin-echo measurements demonstrate fast diffusion of hydrogen molecules parallel to the basal planes of the two dimensional crystal planes. At room temperature and above, this intra-layer diffusion is of a similar speed to the surface diffusion that has been observed in earlier studies for hydrogen atoms on Pt surfaces. A significantly slower hydrogen diffusion was observed perpendicular to the basal planes using nuclear reaction analysis.
Hydrogen concentrations in ZnO single crystals exposing different surfaces have been determined to be in the range of (0.02–0.04) at.% with an error of ±0.01 at.% using nuclear reaction analysis. In the subsurface region, the hydrogen concentration has been determined to be higher by up to a factor of 10. In contrast to the hydrogen in the bulk, part of the subsurface hydrogen is less strongly bound, can be removed by heating to 550°C, and reaccommodated by loading with atomic hydrogen. By exposing the ZnO(10-10) surface to water above room temperature and to atomic hydrogen, respectively, hydroxylation with the same coverage of hydrogen is observed.
The surface of Mercury is highly space weathered, and the products of space weathering are different than on the Moon. Here we simulate the interaction of solar wind ions with the surface of Mercury by implanting low-iron-bearing (∼3 wt% Fe) olivine slices with 4 keV He+ ions both under ambient conditions and at 450 °C during the irradiation. The sample irradiated under ambient conditions shows a ∼140 nm thick partially amorphous layer on top of the grain with high concentrations of bubbles and cavities. However, the sample irradiated at 450 °C has a 110–140 nm thick polycrystalline layer with 1–5 nm sized nanoparticles of metallic iron and no bubbles or cavities. Mg atoms are preferentially sputtered from the ion-damaged layer present on the sample irradiated under ambient conditions, but no difference in composition is seen between the crystalline olivine and the ion-damaged layer in the sample irradiated at 450 °C. In addition, a thin amorphous film deposited on a micro-electromechanical systems–based chip is heated up to 1200 °C in situ inside a transmission electron microscope for a few milliseconds, and it produced nanoparticles of size 5–30 nm. Our results show that solar wind irradiation of low-iron-bearing olivine in the equatorial regions of Mercury will produce nanoparticles of iron whose sizes will be larger than that on the surface of the Moon and that the size will further increase during impacts of dust particles. Solar-wind-implanted He can diffuse out of regolith grains and can be a source of He observed in the exosphere of Mercury.

