The electrochemical generation of hydrogen is a key enabling technology for the production of sustainable fuels. Transition metal chalcogenides show considerable promise as catalysts for this reaction, but to date there are very few reports of tellurides in this context, and none of these transition metal telluride catalysts are especially active. Here, we show that the catalytic performance of metallic 1T′-MoTe2 is improved dramatically when the electrode is held at cathodic bias. As a result, the overpotential required to maintain a current density of 10 mA cm−2 decreases from 320 mV to just 178 mV. We show that this rapid and reversible activation process has its origins in adsorption of H onto Te sites on the surface of 1T′-MoTe2. This activation process highlights the importance of subtle changes in the electronic structure of an electrode material and how these can influence the subsequent electrocatalytic activity that is displayed.

The fast ionic, high temperature (HT) phase of LiBH4 can be stabilized by Br¯ substitution. Lithium borohydride bromide compounds, Li­(BH4)1–x Br x , have been synthesized mechanochemically, with and without thermal treatment and the resulting phase behavior determined as a function of composition. Single phase materials exist for 0.29 ≤ x ≤ 0.50 with conductivity 2 orders of magnitude higher than LiBH4 at 313 K. Powder neutron diffraction has been used to resolve the details of the crystal structure of one such compound. These demonstrate that 7Li­(11BD4)0.667Br0.333 retains the HT structure (hexagonal space group P63 mc, a ≈ 4.2 Å, c ≈ 6.7 Å) from 293 to 573 K. The borohydride bromide exhibits considerable static and dynamic disorder, the latter invoking complex rotational motion of the (BH4)¯ anions.

The electrocatalytic hydrogen evolution reaction (HER) is of central importance for the production of H2 from sustainable sources. Currently, Pt is the best electrocatalyst for this transformation, but other materials based on less‐precious elements are now attracting increased attention. Of these alternatives, the molybdenum chalcogenides show particular promise. MoS2 has been explored extensively in this regard, which has highlighted the important role of polymorphism for catalytic activity. However, the conversion into an active polymorph is complex, and the stability of the catalyst under electrochemical conditions is poor. In contrast, MoTe2 has been barely studied as an electrocatalyst for the HER. Herein, we isolate the semiconducting and metallic polymorphs of MoTe2 using an easy solid‐state route and show that interconversion between the two polymorphs of MoTe2 can be achieved without a change in morphology by a simple temperature‐annealing protocol. Although the semiconducting form is a rather poor electrocatalyst for the HER, the metallic 1T′‐MoTe2 polymorph is an efficient and stable electrocatalyst for the HER in 1 m H2SO4. Even in the bulk form, it achieved a low overpotential with a Tafel slope of (78±4) mV dec−1 and full Faradaic efficiency. These findings highlight the importance of polymorphic control in the development of HER catalysts and suggest an efficient route for the discovery of new and improved electrocatalysts.

0.29-0.50 with conductivities two orders of magnitude higher than that of LiBH4 at 313 K. Fast Li ion conductivity is retained in all single phases although the total conductivity is reduced with Br levels exceeding 0.286. The material retains the high-temperature phase (hexagonal space group P6 3mc, powder neutron diffraction) at 293-573 K and exhibits considerable static and dynamic disorder, the latter invoking complex rotational motion of the (BH 4) anions. -(CASCALLANA-MATIAS, I.; KEEN, D. A.; CUSSEN*, E. J.; GREGORY, D. H.; Chem. Mater. 27 (2015) 22, 7780-7787, http://dx.doi.org/10.1021/acs.chemmater.5b03642 ; WestCHEM, Dep. Pure Appl. Chem., Univ. Strathclyde, Glasgow G1 1XL, UK; Eng.) -J. Schramke 06-011