|Left.High-resolution Transmission Electron Microscopy image of platinum nanoparticles on a fuel cell electrode. Right. Schematics of high-index planes observed on Pt nanoparticles. Credit: ACS, Lee at al. (2009). Click to enlarge.|
A team of researchers from MIT, the Japan Institute of Science and Technology, and Brookhaven National Laboratory have found that changing the surface texture of platinum used in a methanol fuel cell electrode—specifically, creating nano surface steps instead of using a smooth surface—can significantly increase the catalytic activity.
In a paper published online 13 October in the Journal of the American Chemical Society, they show a linear relationship between the intrinsic activity and the amounts of surface steps. Increasing surface steps on Pt nanoparticles of ~2 nm led to enhanced intrinsic activity up to 200% (current normalized to Pt surface area) for electro-oxidation of methanol.
The researchers believe that further development of these surface structures could end up producing far greater increases, yielding more electric current for a given amount of platinum.
The enhanced intrinsic activity for methanol electro-oxidation observed in these Pt/MWNT catalysts can translate to reduction of Pt weight up to 200% for a given fuel cell current output. Of significance, this research suggests that strategies to increasing surface steps such as the n(111) × (111) type on nanoparticles offer promise for finding new highly active electrocatalysts for electro-oxidation of small organic molecules.
—Lee et al. (2009)
One of the intensively debated issues over the last two decades is centered on whether and how nanoparticle activity for CO and methanol electro-oxidation should be size dependent, the researchers note in their paper. The new work shows that the key factor is not the size of the particles, but the details of their surface structure.
MIT Associate Professor of Mechanical Engineering and Materials Science and Engineering Yang Shao-Horn, the corresponding author, suggests that the key factor is the addition of the edges of the steps, which seem to provide a site where it’s easier for atoms to form new bonds. The addition of steps creates more of those active sites. In addition, the team has shown that the step structures are stable enough to be maintained over hundreds of cycles. That stability is key to being able to develop practical and effective direct methanol fuel cells.
Team members also hope to understand whether the steps enhance the other part of the process that takes place in a fuel cell. This study looked at the enhancement of oxidation, but the other side of a fuel cell undergoes oxygen reduction. The researchers expect to have answers to that question in the next few months.
This work was supported in part by the US Department of Energy Hydrogen Initiative program and Toyota Motor Co.
Seung Woo Lee, Shuo Chen, Wenchao Sheng, Naoaki Yabuuchi, Yong-Tae Kim, Tadaoki Mitani, Elio Vescovo and Yang Shao-Horn (2009) Roles of Surface Steps on Pt Nanoparticles in Electro-oxidation of Carbon Monoxide and Methanol. J. Am. Chem. Soc., Article ASAP doi: 10.1021/ja9025648