Improving the capacity and efficiency of the Hydrogen Evolution Reaction (HER) is an enduring challenge of green energy production and artificial photosynthesis. Still, while HER in organisms evolved with time and increasing complexity, artificial HER aims to replicate the same with minimalism and simplicity. A cornerstone of the challenge is to mimic the function of natural hydrogenase enzymes, which catalyse HER in living systems. Indeed, it can be seen that without a catalyst like hydrogenase, HER does not proceed with the speed required for practical applications. However, hydrogenases can be difficult to extract and purify, and can denature under non-natural operations, consequently, inorganic alternatives are used for most applications. The most common example of this is platinum (Pt), which has served as the benchmark catalyst for HER due to its high catalytic efficiency. Nevertheless, because of the scarcity and cost of Pt, a more abundant alternative is needed for cost-effective implementation.

For this, MoS₂, an earth-abundant lamellar solid, has shown prominent HER catalysis nearing the efficiency of platinum. However, experiments using MoS₂ grown on Au(111) indicated that this material is only catalytic on its edge sites. Theoretical studies corroborated these results with the Gibbs free energy of hydrogen adsorption (ΔGH), a measure of HER efficiency, to be feasible for catalysis only at MoS₂ edges; the basal plane of MoS₂ does not appear to participate in catalysis, meaning the bulk of material is catalytically inert. Consequently, the maximization of MoS₂ edges and mimicry of the edge structure has become a significant topic.

Interestingly, recent studies have begun to show enhancement of MoS₂ catalytic efficiency following lithium intercalation and exfoliation… As the lithium-exfoliation reactions increase the availability of basal plane surfaces but not edges, the catalytic improvements are postulated to be basal plane related.

—Chou et al.

MOS₂ exists as a stack of flat nanostructures. These layers are not molecularly bolted together like a metal but instead are loose enough to slide over one another, and with huge internal surface areas.

While the edges of these nanostructures match platinum in their ability to catalyze hydrogen, the relative immense surface area of their sliding interiors are useless because their molecular arrangements are different from their edges. Because of this, a commercial catalyst would require a huge amount of MOS₂.

The idea was to understand the changes in the molecular structure of molybdenum disulfide, so that it can be a better catalyst for hydrogen production: closer to platinum in efficiency, but earth-abundant and cheap. We did this by investigating the structural transformations of MOS₂ at the atomic scale, so that all of the materials parts that were ‘dead’ can now work to make H₂.

—Stan Chou

While there are a number of ways to do this, said co-author Bryan Kaehr, the most scalable way is to separate the nanosheets in solution using lithium. As the material is pulled apart, its molecular lattice changes into different forms; the end product, as it turns out, is catalytically active like the edge structure.

To determine what was happening, and the best way to make it happen, the Sandia team used computer simulations generated by coauthor Na Sai from the University of Texas at Austin that suggested which molecular changes to seek. The team also observed changes with the most advanced microscopes at Sandia, including the FEI Titan, an aberration-corrected transmission electron microscope able to view atoms normally too small to see on most scopes.

Although other researchers had investigated this problem, without the tools used by the Sandia team, they had ended their tests before the reaction could complete itself, resulting in a variety of conflicting intermediate results. In contrast, the Sandia work unambiguously showed that the desirable catalytic form is the end result of the completed reaction.

People want a non-platinum catalyst. Molly is dirt cheap and abundant. By making these relatively enormous surface areas catalytically active, Stan established understanding of the structural relation of these two-dimensional materials that will determine how they will be used in the long run. You have to basically understand the material before you can move forward in changing industrial use.

—co-author Jeff Brinker, Sandia Fellow and University of New Mexico professor

Kaehr cautions that what’s been established is a fundamental proof of principle, not an industrial process.

Water splitting is a challenging reaction. It can be poisoned, stopping the molly reaction after some time period. Then you can restart it with acid. There are many intricacies to be worked out. But getting inexpensive molly to work this much more efficiently could drive hydrogen production costs way down.

—Bryan Kaehr

Other paper authors were Ping Lu, Eric Coker, Sheng Liu and Ting Luk, all from Sandia Labs, and Kateryna Artyushkova from the University of New Mexico.

The work was supported by the Department of Energy’s Office of Science, and through its user facilities at the Sandia/Los Alamos-run Center for Integrated Nanotechnologies and National Energy Research Scientific Computing Center. The Texas Advanced Computing Center also added value.

Resources

• Stanley S. Chou, Na Sai, Ping Lu, Eric N. Coker, Sheng Liu, Kateryna Artyushkova, Ting S. Luk, Bryan Kaehr & C. Jeffrey Brinker (2015) “Understanding catalysis in a multiphasic two-dimensional transition metal dichalcogenide” Nature Communications 6, Article number: 8311 doi: 10.1038/ncomms9311