Nanoscale design of molybdenum sulfides for more efficient electro- and photoelectrocatalytic hydrogen evolution

Abstract

This thesis treats the electro- and photoelectrochemical hydrogen evolution from water splitting. Water splitting stores ~ 237 kJ/(mol H2O) most of which may be released in a fuel cell. As MoS2 is stable and an excellent H2 evolution catalyst, it is a sustainable alternative to the archetypical H2 evolution catalyst Pt. MoS2 is primarily limited by its low electron conduction and high stability and therefore its expression of the inactive basal planes. The focus of this thesis is the synthesis of amorphous MoSx from either electrodeposition or precipitation as this material circumvents the issues above. These catalysts were studied on titania protected silicon photocathodes. These showed an excellent activity and demonstrated the potential of these electrodes for sustainable H2 production without the use of Pt. Electrodes using MoS2 as both the protection layer and the catalyst were also investigated. The full potential of these electrodes could not be realized due to time constraints, given the severe limitations in the electron transport across the protection layer. Regardless, these electrodes showed superior stability to the titania protected photoelectrodes. It was attempted to improve the electron transport through the catalyst layers by synthesizing composites of carbon nanotubes and MoSx. The syntheses included both electro-co-deposition and electrodeposition on a pre-fabricated nanotube film. No improvement could consistently be detected in these experiments. Hence, MoS3 was coated directly onto carbon nanotubes. These H2 evolution catalysts showed an improvement compared to the MoS2 analogue, but not compared to the best electrodeposited samples. Doping with Co or Fe increased the activity, however further experiments should clarify how much the activity could be improved. The electrochemical oxidation behavior of MoSx was investigated compared to that of MoS2, and MoS3. It was found that the MoSx demonstrated a unique oxidation behavior indicating that the active sites were not MoS2 or MoS3-like - further experiments are needed to clarify the relation between oxidation behavior and H2 evolution. The H2 evolution on a few selected chevrel phases was also investigated. This hitherto for this reaction uninvestigated class of catalysts showed a decent H2 evolution activity in spite of problems with obtaining pure samples. This class of materials are an interesting contribution to the list of active molybdenum sulfide based H2 evolution catalysts. This demonstrated the potential of molybdenum sulfides as electro- and photoelectrochemical catalysts for the H2 evolution reaction.