Sandia researchers, from right, Stan Chou, Bryan Kaehr, Jeff Brinker, Ping Lu, and Eric Coker, gather in a lab where improvements on the catalyst molybdenum disulfide, better known as molly, were achieved. (Photo by Randy Montoya)
Hydrogen-powered cars don’t pass carbon into the atmosphere. Unlike gasoline, which does, the combustion of hydrogen with oxygen produces an exhaust of only water. But hydrogen costs more.
So Sandia researchers, seeking to make hydrogen a less expensive fuel, have begun upgrading a plentiful catalyst nearly as cheap as dirt — molybdenum disulfide, “molly” for short — to stand in for platinum, a rare element with the moonlike price of approximately $900 an ounce.
Sandia-induced changes are taking the less-than-$2 an ounce molly from a welterweight outsider in the energy-catalyst field — put crudely, a lazy bum that never amounted to much — to a possible contender with the heavyweight champ.
And the catalyst’s action can be triggered by sunlight, a feature which eventually may provide users an off-the-grid means of securing hydrogen fuel.
A catalyst is necessary to free hydrogen from compounds.
Boosting hydrogen production
The improved catalyst, reported in Oct. 7 Nature Communications, has already released four times the amount of hydrogen ever produced by molly from water, and to Sandia postdoctoral fellow and lead author Stan Chou, this is just the beginning: “We should get far more output as we learn to better integrate molly with, for example, fuel cell systems,” he says.
In Stan’s measured words, “The idea was to understand the changes in the molecular structure of molybdenum disulfide (MoS2), 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 [hydrogen].”
Why were the parts “dead,” one might ask?
The rind of an orange
Visualize an orange slice where only the rind of the orange is useful; the rest — the edible bulk of the orange — must be thrown away. Molly exists as a stack of flat nanostructures, like a pile of orange slices. These layers are not molecularly bolted together like a metal but instead are loose enough to slide over one another — a kind of grease, similar to the structure of graphene, and with huge internal surface areas.
But here’s the rub: 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 excess baggage, a commercial catalyst would require a huge amount of molly. The slender edges would work hard like Cinderella but the stepsister interiors would just hang out, doing nothing.
Stan, who studies two-dimensional materials and their properties, felt the Sandia intent should be to get these stepsisters jobs.
Empowering the center
“There are many ways to do this,” says coauthor Bryan Kaehr, “but the most scalable way is to separate the nanosheets in solution using lithium. With this method, as you pull the material 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 look for. 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.
“The extended test period made possible by the combined skills of our group allowed the reactions to be observed with the amount of detail needed,” says Stan.
Lacking these tools, researchers at other labs had ended their tests before the reaction could complete itself, like a cook taking sugar and water off the stove before syrup is produced, resulting in a variety of conflicting intermediate results.
“Why Stan’s work is impactful is that there was so much confusion as to how this process works and what structures are actually formed,” says Bryan. “He unambiguously showed that this desirable catalytic form is the end result of the completed reaction.”
Says Sandia Fellow and University of New Mexico professor Jeff Brinker, another paper author, “People want a non-platinum catalyst. Molly is dirt cheap and abundant. By making these relatively enormous surface areas catalytically active, Stan established an 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.”
Bryan 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.”
Other paper authors were Ping Lu, Eric Coker, Sheng Liu and Ting Luk, and Kateryna Artyushkova from the University of New Mexico.
The work was supported by DOE’s Office of Science.
Certain measurements were performed at the Sandia/Los Alamos-run Center for Integrated Nanotechnologies (CINT), and computing resources were provided by the National Energy Research Scientific Computing Center (NERSC) and the Texas Advanced Computing Center. CINT and NERSC are DOE Office of Science User Facilities.