(39f) Nano-Scale Molybdenum Sulfides For Electrocatalytic Hydrogen Evolution

Molecular hydrogen is a potential energy carrier that may play a prominent role in the energy landscape. This fuel can be produced cleanly and renewably if solar or wind energy is coupled to water electrolysis. Unfortunately, the hydrogen evolution reaction (HER) is catalyzed most effectively by Pt group metals [1] ? materials that are expensive and scarce. New materials are needed if this scheme is to ever become viable. By studying different metal surfaces and enzymes that catalyze hydrogen evolution, we have found a necessary criterion, a ?descriptor?, for high catalytic activity ? the binding energy (Gibbs free energy) of atomic hydrogen adsorption must be close to zero [1,2]. Not surprisingly, the nitrogenase and hydrogenase enzymes ? both of which evolve H2 in nature ? satisfy this requirement. In an effort to develop solid-state analogues of the enzymatic active sites, theoretical calculations have guided us to nanoparticulate molybdenum sulfides, which were subsequently synthesized and investigated for hydrogen evolution. We have recently confirmed the catalytic activity of MoS2 nanoparticles for hydrogen evolution [2], and have subsequently aimed to identify relationships between structure and catalytic activity within the MoS2 nanoparticles. In our approach, we combine ultra high vacuum techniques with electrochemical measurements under ambient conditions. MoS2 nanoparticles were first synthesized on Au(111) in ultra-high vacuum, where scanning tunnelling microscopy (STM) was used to investigate their structure, morphology, and surface coverage (see figure). Surface spectroscopy was also employed for chemical characterization. These UHV-prepared samples were then transferred to an electrochemical cell where catalytic activity for hydrogen evolution was measured. By varying particle size and coverage, our measurements have established ? for the first time ? a clear relationship between the structure and electrocatalytic activity of MoS2 nanoparticles.

[1] J.K. Nørskov, T. Bligaard, A. Logadottir, J.R. Kitchin, J.G. Chen, S. Pandelov, U. Stimming, J. Electrochem. Soc. 152, J23 (2005).

[2] B. Hinnemann, P.G. Moses, J.Bonde, K.P. Jørgensen, J.H. Nielsen, S. Horch, I. Chorkendorff, J.K. Nørskov, J. Am. Chem. Soc., 127, 5308 (2005).