Electrochemical Activation of MoTe2 for the Hydrogen Evolution Reaction
Authors: McGlynn Jessica C , Ganin Alexey Y 
Published: February 2020

Sunlight is widely distributed geographically and provides enough energy in one hour to supply the world’s energy demands for a full year. However, this form of renewable energy is intermittent; thus current renewable energy technologies must be fed in to the grid or the excess energy is otherwise wasted. Recent efforts have therefore been focused on the conversion of this excess energy into a fuel in the form of hydrogen via water electrolysis. Due to the diffuse nature of solar power, hydrogen evolution reaction (HER) electrocatalysts are typically compared at a current density of 10 mA cm-
2, which roughly equates to a 10% efficient solar-to-fuels device.1 This means electrocatalysts of extremely high surface areas are required, thus making the scarce and expensive platinum electrocatalyst an impractical choice.

Layered transition metal dichalcogenides (TMDCs) can offer a viable alternative due to their high surface area of catalytically active edge and basal plane sites which offer a maximum energy output from the entire surface. Recently, semimetallic 1T’-MoTe2 has emerged as a promising electrocatalyst for the HER based on its ability to accommodate an excess of electrons, making it a prime candidate for investigating the effect of electrochemical activation of TMDCs towards the HER.2,3

With this in mind, we report a novel solid-state nanocrystalline 1T’-MoTe2 electrocatalyst which undergoes an in operando activation as the HER progresses. Continuous cyclic voltammetry under reducing potentials results in a dramatic improvement in electrocatalytic activity after only 100 cycles, thus resulting in a positive shift in overpotential from -320 mV to -178 mV at j = 10 mA cm-
2. Similarly, potentiostatic electrolysis shows a rapid increase in current density with time, while gas chromatography confirms the continuous increase in rate of hydrogen production. This catalytic enhancement is also corroborated by a substantial improvement in turnover frequency and charge transfer resistance, all the while maintaining the original particle morphology, surface area and composition.

Previous speculations attribute this improved performance in similar TMDC materials to substantial changes in crystal structure, morphology and / or composition, but the true mechanism of activation remains unclear.4–6 In this context, we aim to understand the origin of this enhanced catalytic activity, and hence investigate the stability of the nanocrystalline 1T’-MoTe2 after electrochemical activation. Further, we rule out changes in crystal structure by Powder X-Ray Diffraction and Raman Spectroscopy, while HRTEM and ICP-OES exclude changes in morphology and composition as the reason for the improved catalytic activity.

Finally, by proving the electrochemically active surface area remains constant after cycling, while at the same time the charge transfer resistance halves in value and turnover frequency increases, we propose that the in operando activation is a result of changes to the electronic structure upon cycling. Hence, this study offers a new perspective on the path of electrochemical activation of TMDCs towards the hydrogen evolution reaction.

References

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McGlynn, J. C. et al. Molybdenum Ditelluride Rendered into an Efficient and Stable Electrocatalyst for the Hydrogen Evolution Reaction by Polymorphic Control. Energy Technol.
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Seok, J. et al. Active hydrogen evolution through lattice distortion in metallic MoTe 2. 2D Mater.
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Liu, Y. et al. Self-optimizing, highly surface-active layered metal dichalcogenide catalysts for hydrogen evolution. Nat. Energy (2017). doi:10.1038/nenergy.2017.127

Voiry, D. et al. Conducting MoS2 Nanosheets as Catalysts for Hydrogen Evolution Reaction. Nano Lett.
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Roger, I. et al. The direct hydrothermal deposition of cobalt-doped MoS2 onto fluorine-doped SnO2 substrates for catalysis of the electrochemical hydrogen evolution reaction. J. Mater. Chem. A
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Figure 1