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Molybdenum sulfides are very attractive noble-metal-free electrocatalysts for the hydrogen evolution reaction (HER) from water. The 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 exhibits significantly higher HER activity compared to its crystalline counterpart. Here we show that HER-active a -MoS x , prepared either as nanoparticles or as films, is a molecular-based coordination polymer consisting of discrete [Mo 3 S 13 ] 2− building blocks. Of the three terminal disulfide (S 2 2− ) ligands within these clusters, two are shared to form the polymer chain. The third one remains free and generates molybdenum hydride moieties as the active site under H 2 evolution conditions. Such a molecular structure therefore provides a basis for revisiting the mechanism of a -MoS x catalytic activity, as well as explaining some of its special properties such as reductive activation and corrosion. Our findings open up new avenues for the rational optimization of this HER electrocatalyst as an alternative to platinum. Molybdenum sulfides are attractive electrocatalysts for the hydrogen evolution reaction. The polymeric structure of amorphous molybdenum sulfide can now be formulated as a coordination polymer based on [Mo 3 S 13 2− ] clusters sharing disulfide ligands.

In arbuscular mycorrhizas (AM), the supply of phosphorus from the fungi is one of the most important benefits to the host plant. Here we describe for the first time the ultrastructure and polyphosphate (poly P) distribution in rapidly frozen and freeze-substituted germ tubes of the AM fungus Gigaspora margarita. At the ultrastructural level, phosphorus distribution was analysed using energy-filtering transmission electron microscopy, and poly P was detected using an enzyme-affinity method. Semithin sections and live cells were also stained with 4',6-diamidino-2-phenylindole, which is not specific but fluoresces yellow when viewed under UV irradiation by binding with poly P. The cryotechnique method showed that extensive elongate ellipsoid vacuoles containing a uniform electron-opaque material occupied most of the cell volume. Combining the results of multiple methods revealed that poly P was localized in a dispersed form in vacuoles and in the outer fungal cell wall. These results show the significant potential of AM fungi for phosphorus storage based on its localization in the extensive complement of vacuoles in thick hyphae. The mechanism of translocation of poly P in tubular vacuoles, and the role of poly P in the cell wall, need to be elucidated.