Crystalline Transition Metal Chalcogenides (c-TMD_s) gained attention as non-precious Hydrogen Evolution Reaction (HER) electrocatalysts thanks to their abundance, activity and because their morphology strongly influences the HER performance. This relationship has been used as design rule to obtain efficient TMD_s catalysts, by either increasing the intrinsic activity of the material through the creation of new HER active sites or by maximizing the effective surface area through morphology and structure control. Amorphous TMDs (a-TMD_s) have in principle higher activity thanks to their disordered structure, however the relation between structure and HER performance is unknown and traditional fabrication techniques grant a limited control over the morphology. Moreover, the poor electrical conductivity of TMD_s related to their large bandgap hinders the maximum HER rate that can be achieved. These two limitations ultimately lead to unsatisfactory HER performances, when these catalysts are self-supported.

In this work, we study the influence of morphology and composition on HER performance for self-supported nano-crystalline TMD_s, focusing on Molybdenum Sulfide (MoS_x) and Tungsten Selenide (WSe_x). We exploit the intrinsic high activity granted by a disordered, metastable structure, leveraging on the HER enhancement granted by the many defective sites of the material, and a precise control over the morphology of the material granted by using Pulsed Laser Deposition (PLD) as synthesis method. Through PLD we fine-tune the morphology of the materials down to the nanoscale, controlling the pore size distribution among the material and the surface area, ranging from compact film to hierarchical nanostructures.

The pristine TMD_s exhibit a short-range ordered, quasi-crystalline structure with crystallites embedded in an amorphous matrix and excess amorphous chalcogen. The shared electrochemical activation process, occurring at the beginning of catalysis, transforms the pristine TMD_s into the actual HER catalysts, modifying the structure and composition of the material. More in detail, sub-stoichiometric oxide/oxysulfide phases with high electrical conductivity and new under-coordinated sites are formed for MoS_x: the incorporation of oxygen in the structure creates a conductive backbone that, we hypothesize, enhances the HER activity of the material and improves the kinetic. For WSe_x, on the other hand, the different oxide phases that are formed upon electrochemical activation have a different effect: while sub-stoichiometric oxyselenide phase exhibits good electrical conductivity, the high resistivity of stoichiometric tungsten trioxide limits the HER kinetics.

By optimizing the morphology of the nanostructures and exploiting their structure, we reach HER performances among state-of-art for TMD_s-based HER catalysts, regardless of their support or crystalline structure. For MoS_x, we achieve a Tafel slope of 35 mV⸱dec^-1 and a -100 mA⸱cm^-2 overpotential (η₁₀₀) of 169 mV, while for WSe_x a -10 mA⸱cm^-2 overpotential (η₁₀) of 190 mV and a Tafel slope of 65 mV⸱dec^-1 are registered. This remarkable performance is obtained by exploiting the short-range ordered structure of the synthesized TMDs and by leveraging on the effect of the oxide layers created during the activation process, whose effect is dependent on the composition of the pristine TMD.

In conclusion, this report shows the importance of morphology and composition on HER performance for TMD_s, as reflected by the relation between structure and intrinsic HER catalytic parameters. This relationship represents a promising design rule to obtain materials which, thanks to their performances, may provide a promising alternative to precious catalysts.