Molybdenum sulfides are very attractive noble-metal free electrocatalysts for the hydrogen evolution reaction (HER) from water. Atomic structure and identity of the catalytically active sites have been well established for crystalline molybdenum disulfide (c-MoS 2 ) but not for amorphous molybdenum sulfide (a-MoS x ) which displays significantly higher HER activity compared to its

An electrode made of Au nanoparticles, ca. 13 nm in diameter, displays outstanding catalytic activity for the hydrogen evolution reaction in water. At an overpotential of 200 mV it operates with a catalytic rate TOF of 0.3 s-1, which is among the best performances ever achieved for a Pt-free H2-evolving catalyst.

The spinel MgMn2O4, a cathode material with theoretical capacity of 272 mA h g–1, holds promise for future application in high volumetric magnesium-ion batteries. Atomic-resolution imaging of the structure of the spinel and its surface composition would advance our understanding on its electrochemical properties, mass, and charge transport behavior in electrodes. We observe directly, by aberration-corrected scanning transmission electron microscopy (STEM), the atomic structure of cubic spinel MgMn2O4 for the first time. More importantly, we find that a thin stable surface layer of rocksalt MgMnO2 was grown on a bulk cubic spinel phase. The formation of a rocksalt phase was induced by reconstruction of the spinel phase, i.e., the insertion of Mg into the spinel lattice together with Mg/Mn cation exchange and Frenkel-defect-mediated relocation of Mg cations. This new structural analysis provides a critical step toward understanding and tuning the electrochemical performance of spinel oxide in rechargeable Mg-ion batteries.

The ultrathin two-dimensional nanosheets of layered transition-metal dichalcogenides (TMDs) have attracted great interest as an important class of materials for fundamental research and technological applications. Solution-phase processes are highly desirable to produce a large amount of TMD nanosheets for applications in energy conversion and energy storage such as catalysis, electronics, rechargeable batteries, and capacitors. Here, we report a rapid exfoliation by supercritical fluid processing for the production of MoS 2 and MoSe 2 nanosheets. Atomic-resolution high-angle annular dark-field imaging reveals high-quality exfoliated MoS 2 and MoSe 2 nanosheets with hexagonal structures, which retain their 2H stacking sequence. The obtained nanosheets were tested for their electrochemical performance in a hybrid Mg–Li-ion battery as a proof of functionality. The MoS 2 and MoSe 2 nanosheets exhibited the specific capacities of 81 and 55 mA h g –1 , respectively, at a current rate of 20 mA g –1 .