A dye excited by light providing electrons for catalyst molybdenum disulfide. CREDIT: Randy Montoya, Sandia National Laboratories
US researchers seeking to make a cheaper hydrogen fuel for cars have upgraded a 'cheap as dirt' catalyst to stand in for widely-used platinum, which currently has a price of $1,500 a gram.
The research carried out by Sandia National Laboratories, a subsidiary of Lockheed Martin and contractor for the US Department Energy’s National Nuclear Security Administration (NNSA), made changes to elevate the widely available molybdenum disulfide, or "molly", which costs 37-cents-a-gram molly, making it an affordable alternative to costly platinum.
The improved catalyst has already been found to release four times the amount of hydrogen ever produced by molly from water, said Sandia researchers.
Stan Chou, Sandia postdoctoral fellow and lead author of the research, said: "We should get far more output as we learn to better integrate molly with, for example, fuel-cell systems."
An additional benefit is that molly's action can be triggered by sunlight, a feature which Chous said could eventually provide users an off-the-grid means of securing hydrogen fuel.
Chou said: "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 molly at the atomic scale, so that all of the materials parts that were 'dead' can now work to make hydrogen."
Bringing molly's 'dead parts' to life
Molly 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, similar to the structure of graphene, and with huge internal surface areas.
However, while the edges of these nanostructures match platinum in their ability to catalyse 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 , but the interiors would be doing nothing.
Chou, who works on two-dimensional materials and their properties, thought the intent should be to get these interiors working too.
Empowering the center
Bryan Kaehr, co-author, said: "There are many ways to do this 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 co-author Na Sai from the University of Texas, Austin, that suggested which molecular changes to look for. The team also observed changes with 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 microscopes.
Understanding the material
Jeff Brinker, Sandia fellow and University of New Mexico professor, said: "People want a non-platinum catalyst. Molly is dirt cheap and abundant. By making these relatively enormous surface areas catalytically active, Chou 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."
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."
'Green' inorganic photosynthesis
Not requiring electricity to prompt the reaction may be convenient in some circumstances and also keep costs down.
Chou said: "A molly catalyst is essentially a 'green' technology. We used sunlight for the experiment's motive power.” The light is processed through a dye, which harvests the light. A photocatalytic process stores that energy in the chemical bonds of the liberated hydrogen molecule.
"It's a kind of photosynthesis, but using inorganic materials rather than plants," Chou added. "Plants use enzymes powered by sunlight to break up water into hydrogen and oxygen in a delicate process. We're proposing a similar thing here, but in a more rapid reaction and with sturdier components."
Kaehr said, "You could generate hydrogen and use it whenever. Hydrogen doesn't lose charge over time or suffer from conversion inefficiencies as do batteries in a solar car."
Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corp., for the U.S. Department of Energy's National Nuclear Security Administration.