Chemists at Rice University, Houston, found an easy and inexpensive way to create flexible films from molybdenum disulfide, a versatile chemical compound with edges that are highly efficient catalysts. The nanoporous films maximize the amount of exposed edge, increasing its potential uses for energy storage and as catalysts for hydrogen evolution reaction (HER), a process used in fuel cells to pull hydrogen from water.
Molybdenum disulfide is not quite as flat as graphene, the atom-thick form of pure carbon, because it contains both molybdenum and sulfur atoms. When viewed from above, it looks like graphene, with rows of ordered hexagons. But seen from the side, three distinct layers are revealed, with sulfur atoms in their own planes above and below the molybdenum.
This crystal structure creates a more robust edge, and the more edge, the better for catalytic reactions or storage, says chemistry professor James Tour. “So much of chemistry occurs at the edges of materials,” Tour says. “A 2D 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, 5-6 nm planes and a lot of edge, as though the material had bore holes drilled all the way through.”
The new film catalyzes the separation of hydrogen from water when exposed to a current. “Its performance as a HER generator is as good as any molybdenum disulfide structure that has ever been seen, and is really easy to make,” Tour adds. While other researchers have proposed arrays of molybdenum disulfide sheets standing on edge, the group took a different approach.
First, they grew a porous molybdenum oxide film onto a molybdenum substrate through room-temperature anodization, an electrochemical process with many uses but traditionally employed to thicken natural oxide layers on metals. The film was then exposed to sulfur vapor at 300°C (572°F) for one hour. This converted the material to molybdenum disulfide without damage to its nanoporous, sponge-like structure.
The films can also serve as supercapacitors, which store energy quickly as static charge and release it in a burst. Though they don’t store as much energy as an electrochemical battery, they have long lifespans and are in wide use because they can deliver far more power than a battery. Researchers built supercapacitors with the films; in tests, they retained 90% of their capacity after 10,000 charge-discharge cycles and 83% after 20,000 cycles.
“We see anodization as a route to materials for multiple platforms in the next generation of alternative energy devices,” Tour says. “These could be fuel cells, supercapacitors, and batteries. And we’ve demonstrated two of those three are possible with this new material.”
