Allotrope-dependent activity-stability relationships of molybdenum sulfide hydrogen evolution electrocatalysts.

Molybdenum disulfide (MoS2) is widely regarded as a competitive hydrogen evolution reaction (HER) catalyst to replace platinum in proton exchange membrane water electrolysers (PEMWEs). Despite the extensive knowledge of its HER activity, stability insights under HER operation are scarce. This is paramount to ensure long-term operation of Pt-free PEMWEs, and gain full understanding on the electrocatalytically-induced processes responsible for HER active site generation. The latter are highly dependent on the MoS2 allotropic phase, and still under debate. We rigorously assess these by simultaneously monitoring Mo and S dissolution products using a dedicated scanning flow cell coupled with downstream analytics (ICP-MS), besides an electrochemical mass spectrometry setup for volatile species analysis. We observe that MoS2 stability is allotrope-dependent: lamellar-like MoS2 is highly unstable under open circuit conditions, whereas cluster-like amorphous MoS3-x instability is induced by a severe S loss during the HER and undercoordinated Mo site generation. Guidelines to operate non-noble PEMWEs are therefore provided based on the stability number metrics, and an HER mechanism which accounts for Mo and S dissolution pathways is proposed.

Local electrochemical activity of transition metal dichalcogenides and their heterojunctions on 3D-printed nanocarbon surfaces.

Transition metal dichalcogenides (TMDs) have shown to be promising catalysts for the electrochemical hydrogen evolution reaction (HER) and 3D-printing enables fast prototyping and manufacturing of water splitting devices. However, the merging of TMDs with complex 3D-printed surfaces and nanostructures as well as their localized characterization remains challenging. In this work, electrodeposition of MoS2 and WS2 and their heterojunctions are used to modify thermally activated 3D-printed nanocarbon structures. Their electrochemical performance for the HER is investigated macroscopically by linear sweep voltammetry and microscopically by scanning electrochemical microscopy. This study demonstrates different local HER active sites of MoS2 and WS2 within the 3D-printed nanocarbon structure that are not solely located at the outer surface, but also in the interior up to ∼150 µm for MoS2 and ∼300 µm for WS2.