Molybdenum disulfide is a material that can make flexible sheets only a few atoms thick, about 100,000 times thinner than a human hair. The material is currently being explored in the development of solar cells and an electric generator through piezoelectric effects. A new study has found that by turning it into a nanoporous film, molybdenum disulfide can be used to store energy or to catalyze hydrogen production through hydrogen evolution reaction (HER). Yang Yang of Rice University is the lead author of the paper, which was published in Advanced Materials.

Molybdenum disulfide consists of two single-atom layers of sulfur on either side of a layer of molybdenum that can be prepared in thin, lightweight sheets. When looking straight down onto the sheet, the molecules are highly arranged hexagonally and make for a fairly unreactive material. 

“So much of chemistry occurs at the edges of materials,” senior author James Tour said in a press release. “A two-dimensional material is like a sheet of paper: a large plain with very little edge. But our material is highly porous. What we see in the images are short, 5- to 6-nanometer planes and a lot of edge, as though the material had bore holes drilled all the way through.”

Viewing the layers of the material from the side reveals the molecular sandwich, which is not as orderly and is capable of performing catalytic reactions, such as extracting hydrogen from water through HER. The team needed to increase the amount of “edge” available to perform these reactions, and solved the problem by making very tiny pores throughout the surface. When provided with a current, the porous molybdenum sulfide easily catalyzes the production of the hydrogen.

“Its performance as a HER generator is as good as any molybdenum disulfide structure that has ever been seen, and it’s really easy to make,” Tour continued.

The material has an added bonus of briefly storing energy electrostatically before releasing it abruptly. While conventional batteries store more energy, the porous molybdenum disulfide is a long-lived super capacitor that are able to keep over 80% of their capacity, even after 20,000 charging cycles. Conventional batteries, on the other hand, need to be replaced after a few hundred cycles. If super capacitors could be developed further to increase their storage capacity, energy storage in the future could be much lighter and longer-lived than conventional batteries.

"We see anodization as a route to materials for multiple platforms in the next generation of alternative energy devices," Tour said. "These could be fuel cells, supercapacitors and batteries. And we've demonstrated two of those three are possible with this new material."