As the world tries to move into the “hydrogen economy,” there has to be a way to make the chemical less expensive as a fuel source.
There is a new process in development that could make hydrogen a less expensive fuel for cars by using a catalyst nearly dirt cheap, said researchers at Sandia National Laboratories. The catalyst is molybdenum disulfide, “molly,” to stand in for platinum, a rare element with the moonlike price of $1,500 a gram.
Sandia-induced changes to elevate the 37-cents-a-gram molly from being an outsider in the energy-catalyst field to a possible contender.
The improved catalyst has already released four times the amount of hydrogen ever produced by molly from water.
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 said.
An additional benefit is molly’s action can end up triggered by sunlight, a feature which eventually may provide users an off-the-grid means of securing hydrogen fuel.
Hydrogen fuel is desirable because, unlike gasoline, it doesn’t release carbon into the atmosphere when burned. The combustion of hydrogen with oxygen produces an exhaust of only water.
“The idea was to understand the changes in the molecular structure of molybdenum disulfide (MOS), so that it can be a better catalyst for hydrogen production: closer to platinum in efficiency, but earth-abundant and cheap,” Chou said. “We did this by investigating the structural transformations of MOS? at the atomic scale, so that all of the materials parts that were ‘dead’ can now work to make hydrogen.”
Visualize an orange slice where only the rind of the orange is useful; the rest — the edible bulk of the orange — must end up thrown away. Molly exists as a stack of flat nanostructures, like a pile of orange slices. These layers do not molecularly bolt 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.
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.
“There are many ways to do this,” said co-author 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.
“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,” Kaehr said. “He unambiguously showed that this desirable catalytic form is the end result of the completed reaction.”
“People want a non-platinum catalyst. Molly is dirt cheap and abundant,” said Sandia Fellow and University of New Mexico professor Jeff Brinker, another paper author. “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.”
Kaehr said they established 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.”