The key to sustainable alternative energy sources such as biofuels, hydrogen, synthetic hydrocarbons, and fuel cells is the catalytic processes that drive the energy conversion pathways. Now Ashwin Ramasubramaniam, a faculty member in the Mechanical and Industrial Engineering Department at the University of Massachusetts Amherst, has received a five-year, $750,000 grant from the U.S. Department of Energy (DOE) to study electocatalysts in direct methanol fuels cells and proton exchange membrane hydrogen fuel cells and then suggest revolutionary ways to improve them.
Ramasubramaniam’s project, entitled “Computational Design of Graphene-Nanoparticle Catalysts,” was chosen by the DOE Office of Science Early Career Research Program for the highly competitive award.
“We’re theorists,” says Ramasubramaniam. “We aim to provide fundamental insights into why certain things work, and why certain things don’t work, and what combinations of factors might be expected to work better. So, in this case, our job is to suggest how fuel cells’ electrodes might work better; how they will be more efficient, more economical, and how they will perform better than they do now.”
Widespread adoption of new energy technologies depends on the development of efficient and inexpensive catalysts. Ramasubramaniam’s research will initially focus on improving the catalytic process involving platinum-graphene nanocomposites and making them more economical and effective. Experiments already show that platinum-graphene nanocomposites outperform their predecessors as electrocatalysts in direct methanol fuel cells and hydrogen fuel cells. Why this is so is as yet unclear at a fundamental level. The goal of this project then is to develop and implement computer models for the rational design and evaluation of nanoscale platinum catalysts supported on graphene, a two-dimensional sheet of carbon atoms arranged in a honeycomb lattice.
“In this case, we know that a combination of tiny platinum nanoparticles, used as the catalyst, and flat sheets of graphene, used as support material, is commonly employed to create catalytic reactions in a fuel cell,” says Ramasubramaniam. “We know they work. They are robust. But nobody really knows why. So first we have to study why this combination works so well. Then we have to determine the best possible size for the nanoparticle spheres of platinum and the best possible surface texture of the graphene sheets. And finally, once we know all that, we can suggest more efficient ways to set up those materials in the catalytic reaction process and, going one step further, begin to explore less costly materials to replace platinum.”
One key factor involves the size of platinum particles and how they are dispersed on the graphene sheet.
“If you think about a large sphere of a catalyst,” explains Ramasubramaniam, “the atoms in the interior are much more numerous than on the surface, and for the most part they aren’t doing anything interesting. So, you’ve used a lot of precious metal like platinum, which is in essence doing nothing. On the other hand, if you make this precious metal catalyst into very small spheres, then the number of atoms on the surface is of the same order of magnitude as the number within. And so you are using an expensive material much more efficiently. There is typically an optimal particle size for reactions to occur efficiently.”
So this is the basic strategy Ramasubramaniam is studying for the dispersion of the platinum catalyst on the graphene support sheet. Take one large sphere of platinum catalyst, break it up into successively smaller spheres, and figure out the optimal size for the particular set of reactions of interest.
“The other aspect of enhancing the catalytic qualities of the electrode is that you need to anchor all those tiny platinum nanoparticles to the graphene substrate, so they don’t roam around freely and once again aggregate into large spheres,” says Ramasubramaniam. “The surface of the carbon substrate needs to have defects in it."
In other words, the carbon sheet must be “pock marked” so that the nanoparticles are anchored. Think about how all the marbles in a Chinese Checker board fit snugly into the cavities designed to hold them.
“What we are trying to understand is why this setup works more efficiently than other setups,” Ramasubramaniam notes. “Once we understand that, we hope to suggest strategies for designing the system to improve certain aspects of its performance. For example, instead of platinum, maybe we can use another metal that is less expensive."
He adds that, if platinum simply cannot be dispensed with, one strong possibility is using a core in each sphere that is made of a cheaper metal and then covering it with a thin layer of platinum. That kind of hybrid sphere—a “core-shell particle”—is known to work quite well in other contexts. (July 2013)