MoS2-Nano Layers Deposited Onto Multi Walled Carbon Nanotubes As a Hydrogen Evolving Catalyst for PV-Hybrid Electrolyzers: A Structure-Activity Correlation Study

Sunlight is a nearly inexhaustible energy source, but its large-scale use requires an efficient storage solution. One promising storage concept is the direct conversion of light into chemical energy, for example by producing hydrogen through solar-powered water electrolysis. In order to provide the required minimum photo­voltage of 1.23 V, photoelectrochemical cells can be made from suitable semiconducting materials that form a semiconductor/liquid junction. Another option is the application of photovoltaic devices (buried junctions) whose front- and back-contacts are functionalized with electrocatalysts for the hydrogen and oxygen evolution reactions (HER, OER). In both cases the develop­ment of low cost, but highly efficient electro-catalysts is a challenging task. Especially for the HER, alternative materials are in demand in order to avoid the current need for expensive noble metal catalysts in acidic electrolytes.

In this contribution we investigated the structural and electrochemical properties of MoS₂ nano-layers as an earth abundant catalyst for electrochemical hydrogen evolution. The MoS₂ nano layers were prepared by impregnating multi walled carbon nanotubes (MWCNT) by a wet chemical process. The electrochemical activity towards the HER was investigated by cyclic voltammetry and electrochemical mass spectroscopy analysis (EMS). The absence of any H₂S signal in EMS shows that no or only marginal corrosion occurs under hydrogen evolution conditions in acidic electrolytes. From DFT calculations it has been predicted that the edges of the hexagonal layered MoS₂ are the active centers for the HER. In order to verify this prediction, we studied samples with different nanotube diameters and for comparison also commercial MoS₂ powders with different particle sizes. As a descriptor for the ratio between edges and basal planes we used the current waves which are due to oxidation processes at the edges and planes and correlate it with the activity for the HER. The same was done with the E_2g and A_1g Raman signals which also reflect the amount of edges and basal planes in the MoS₂. In the light of this analysis the influence of the crystalline structure on the catalytic stability and activity will be discussed.

Finally, in order to demonstrate the concept of solar water splitting we deposited MWCNT-MoS₂ as a catalyst for the HER onto a a-Si/µc-Si triple-junction solar cell. The “superstrate geometry” of these solar cells enables illumination from the transparent glass substrate side so that no shadowing or scattering effects caused by the catalyst or by  gas bubbles influence the performance. RuO₂ or Co₂O₃OER catalysts were used for these ‘artificial leafs’ and the efficiency under AM 1.5 illumination was determined.

The figure shows transmission electron spectroscopy analysis which reveals the formation of thin MoS₂ layers wrapping multi walled carbon nano tubes.