AUSTIN, Texas -- In a step toward developing better fuel cells for electric cars and more, The University of Texas at Austin, together with MIT and the Oak Ridge National Laboratory, have taken the first images of individual atoms on and near the surface of nanoparticles key to the eco-friendly energy conversion devices.
Nanoparticles made of platinum and cobalt are known to catalyze some of the chemical reactions behind fuel cells, making those reactions run up to four times faster than if platinum alone is used as the catalyst.
No one, however, understands exactly why. According to Paulo Ferreira, associate professor in Materials Science and Engineering at the university, and Yang Shao-Horn, associate professor in the Department of Mechanical Engineering and Department of Materials Science and Engineering at MIT, “the key to a better understanding has to do with the surface atomic structure and composition of the platinum and cobalt nanoparticles.”
Using a new technique known as aberration-corrected Scanning Transmission Electron Microscopy, Ferreira, Shao-Horn and Larry Allard of Oak Ridge National Laboratory identified specific atomic structures near the surface of such catalyst particles.
“Due to the fact that the aberrations normally present in traditional electron microscopes are corrected, this technique is capable of very high resolutions (less than 1 Ångstrom), allowing specific chemical elements to be imaged at the atomic scale”, said Ferreira, who also directs the Microscopy facility at the Texas Materials Institute.
The work was reported in the Sept. 24 online issue of the Journal of the American Chemical Society.
The researchers analyzed platinum and cobalt nanoparticles in a JEOL 2200 FS Scanning-Transmission Electron Microscope located at Oak Ridge National Laboratory. The particles, treated at MIT in Shao-Horn’s Electrochemical Energy Laboratory, with acid, or treated with acid and then subjected to high heat were found to be more active than platinum alone.
The team found that each, in turn, also had slightly different surface structures. For example, the acid treated nanoparticles form percolated structures with Pt-rich and Co-rich regions. However, in the nanoparticles subjected to heat treatments, the platinum and cobalt atoms formed a "sandwich-like" structure. Platinum atoms covered most of the surface, while the next layer down was composed primarily of cobalt. Successive layers contained mixtures of the two.
The team proposes that these particular nanoparticles are up to four times more active than platinum alone because the platinum atoms on the surface are constrained by the cobalt atoms underneath.
"This modifies the inter-atomic distances between the platinum atoms on the nanoparticle surface," making them more effective in chemical reactions key to fuel cells, Shao-Horn said.
In addition to Ferreira, Shao-Horn and Allard, other members of the research team include: Shuo Chen, first author of the paper and a postdoctoral associate in mechanical engineering at MIT; Wenchao Sheng, a graduate student in chemistry at MIT; and Naoaki Yabuuchi, a research affiliate in mechanical engineering at MIT.
The Department of Energy and the National Science Foundation, through its Materials Research Science and Engineering Center program, funded the work.
