Direct solar-to-hydrogen conversion can be achieved via photoelectrochemical water splitting, and is an attractive approach for the storage of sunlight as a chemical fuel.¹ However, current technologies rely on expensive components such as the noble metal platinum electrocatalyst, making such devices economically inviable. Efforts have therefore recently been turned towards transition metal chalcogenides (TMCs) as potential electrocatalysts for the hydrogen evolution reaction (HER).² MoS₂, in particular, has been studied extensively, with subtle changes in the crystal structure proving to be a pivotal factor for tuning the catalytic activity. Naturally occurring bulk semiconducting 2H-MoS₂, for example, is catalytically inert but can be turned via lithium intercalation into the nanostructured metastable and metallic 1T-MoS₂ polymorph which is a highly efficient HER catalyst.³ However, the nanostructured material is thermodynamically unstable and thus displays the reverse transformation into the less active 2H-phase.⁴

This modest change in crystal structure has therefore sparked interest in the role polymorphic control plays in tuning catalytic activity. Herein, we exploit a solid state route to MoTe₂ which allows for the interconversion between both semiconducting and metallic phases to be carried out in the bulk form. In this way, the effect of polymorphic control on catalytic activity can be accurately investigated without compromising the composition and/or morphology of the materials.⁵

In this work phase purity is essential; therefore powder X-Ray Diffraction and Raman Spectroscopy were used to confirm the absence of impurity phases which may have affected the activity of the catalysts. Likewise, particle morphology and composition were shown to be identical by SEM/EDX and ICP-OES, meaning the only factor affecting the electrochemical performance is the change in crystal structure. Determination of the overpotentials for proton reduction by linear sweep voltammetry proved that 1T’-MoTe₂ is an active HER electrocatalyst, while 2H-MoTe₂ is catalytically inert. This coupled with investigation of the reaction kinetics by Tafel analysis and electrochemical impedance spectroscopy led to the conclusion that the metallic 1T’-MoTe₂ phase exhibits a superior catalytic performance to its semiconducting counterpart. Crucially, gas chromatography also confirmed that hydrogen was indeed produced with a full Faradaic efficiency, thus verifying that 1T’-MoTe₂ is an efficient electrocatalyst for the hydrogen evolution reaction. Therefore, we deem polymorphic control a key factor in the development of future HER electrocatalysts.

1. Roger, I.; Shipman, M. A.; Symes, M. D. Nat. Rev. Chem. 2017, 1, 3.

2. Roger, I.; Moca, R.; Miras, H. N.; Crawford, K. G.; Moran, D. A. J.; Ganin, A. Y.; Symes, M. D. J. Mater. Chem. A 2017, 5 (4), 1472.

3. Voiry, D.; Salehi, M.; Silva, R.; Fujita, T.; Chen, M.; Asefa, T.; Shenoy, V. B.; Eda, G.; Chhowalla, M. Nano Lett. 2013, 13 (12), 6222.

4. Heising, J.; Kanatzidis, M. G. J. Am. Chem. Soc. 1999, 121 (50), 11720.

5. McGlynn, J. C.; Cascallana-Matías, I.; Fraser, J. P.; Roger, I.; Mcallister, J.; Miras, H.; Symes, M. D.; Ganin, A. Y. Energy Technol. 2017.