(339b) Activity and Durability of Wet-Chemical Synthesized Amorphous Molybdenum Sulfide Catalysts for Electrochemical Hydrogen Evolution

Electrochemical water splitting could provide a means of producing hydrogen from renewable resources.¹However, a hydrogen evolution reaction (HER) catalyst composed of inexpensive and abundant materials is required to achieve high efficiency and make electrochemical hydrogen fuel synthesis economically feasible.

Many reports have demonstrated that crystalline molybdenum disulfide (MoS₂) has high catalytic activity for hydrogen evolution.^2-4 More recent studies have shown that another form of molybdenum sulfide, amorphous MoS₃, also exhibits high HER activity.^5,6 However, the properties of amorphous MoS₃ and the origins of its catalytic activity are still poorly understood.

In this study, we developed a simple wet chemical synthesis for an amorphous molybdenum sulfide catalyst.⁷ This synthesis method is advantageous because unlike many procedures used to synthesize MoS₂, it requires no ultra-high vacuum processing, high temperature treatment, or separate sulfidization step and enables catalyst deposition onto many substrates.

To understand the properties and performance of this material, we investigated the catalyst morphology, composition, activity, and durability. As deposited, the catalyst film is rough, nanostructured, and predominantly composed of amorphous MoS₃. The catalyst has high HER activity, with only ~200 mV overpotential required for a current density of 10 mA/cm²_electrode. Characterization after electrochemical testing shows that the material composition changes during catalysis to more closely resemble MoS₂. Electrochemical capacitance measurements were used to determine the catalyst surface area and estimate a turn over frequency. Finally, electrochemical stability tests indicate that although the catalyst performance decreases slightly after extensive reductive cycling, the material remains highly active after 48 hours of testing.

These results suggest that the high activity of this material likely arises from both the inherently favorable surface properties of the molybdenum sulfide material as well as the rough, nanostructured catalyst film morphology. Our improved understanding of this material suggests strategies for designing catalysts with further increased performance.
 
1.    Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q. X.; Santori, E. A.; Lewis, N. S. Chemical Reviews 2010, 110, (11), 6446-6473.
2.    Jaramillo, T. F.; Jorgensen, K. P.; Bonde, J.; Nielsen, J. H.; Horch, S.; Chorkendorff, I. Science 2007, 317, (5834), 100-102.
3.    Chen, Z. B.; Cummins, D.; Reinecke, B. N.; Clark, E.; Sunkara, M. K.; Jaramillo, T. F. Nano Letters 2011, 11, (10), 4168-4175.
4.    Li, Y. G.; Wang, H. L.; Xie, L. M.; Liang, Y. Y.; Hong, G. S.; Dai, H. J. Journal of the American Chemical Society 2011, 133, (19), 7296-7299.
5.    Merki, D.; Fierro, S.; Vrubel, H.; Hu, X. L. Chemical Science 2011, 2, (7), 1262-1267.
6.    Vrubel, H.; Merki, D.; Hu, X. Energy & Environmental Science 2012, 5, 6136-6144.
7.    Benck, J. D.; Chen, Z.; Kuritzky, L. Y.; Forman, A. J.; Jaramillo, T. F. (Submitted) 2012.