Researchers at the US Department of Energy’s (DOE’s) Ames Laboratory have discovered a method for making smaller, more efficient intermetallic nanoparticles for fuel cell applications, and which also use less of the expensive precious metal platinum. A paper on the work is published in the Journal of the American Chemical Society.
The researchers succeeded by overcoming some of the technical challenges presented in the fabrication of the platinum-zinc nanoparticles with an ordered lattice structure, which function best at the small sizes in which the chemically reactive surface area is highest in proportion to the particle volume.
|Schematic representation of the synthesis route to PtZn/MWNT@mSiO2. Click to enlarge.|
Tremendous endeavors have been devoted to the investigation of intermetallic nanomaterials, particularly Pt‐based, as fuel cell electrocatalysts with the aim of decreasing Pt usage, increasing poisoning tolerance and improving the catalysts activities and stabilities. A great many scientific efforts have been devoted to the preparation of Pt‐based alloys and intermetallic compounds … in the electro‐oxidation of methanol or formic acid and electro‐reduction of oxygen.
Among aforementioned intermetallic compounds, PtZn iNPs have been proven as active catalysts toward formic acid and methanol electrooxidation. … the development of a new synthetic strategy to obtain well‐defined and small PtZn iNPs is highly desired. To the best of our knowledge, there is no general method available for the synthesis of small iNPs.—Qi et al.
The surface-to-volume ratio is important in getting the most out of an intermetallic nanoparticle, said corresponding author Wenyu Huang, Ames Laboratory scientist and assistant professor of Chemistry at Iowa State University. “The smaller the particle, the more surface there is, and more surface area increases the catalytic activity.”
But the high temperature of the annealing process necessary to form intermetallic nanoparticles often defeats the goal of achieving a small size.
High-temperature annealing can cause the particles to aggregate or clump, and produces larger sizes of particles that have less available surface and aren’t as reactive. So, just the steps necessary to produce them can defeat their ultimate chemical performance.—Wenyu Huang
To prevent aggregation from occurring during the heating process, Huang’s research group first used carbon nanotubes as a support for the PtZn nanoparticles, and then coated them with a sacrificial mesoporous silica shell for the high-temperature annealing to form the intermetallic structures. A chemical etching process then removes the silica shell afterward.
The resulting final product of uniform 3.2 nm platinum-zinc particles not only yielded twice the catalytic activity per surface site, that surface area saw ten times the catalytic activity of larger particles containing the same amount of platinum.
The discovery was made possible in part by the capabilities of a new Titan scanning electron microscope at Ames Laboratory’s Sensitive Instrument Facility, jointly funded by the Department of Energy and Iowa State University.
The work was funded by the National Science Foundation, Iowa State University, Ames Laboratory Directed Research and Development (LDRD) funds, and the US Department of Energy’s Office of Science. Computational work was supported by the Laboratory Computing Resource Center and the Center for Nanoscale Materials, both at Argonne National Laboratory.
Zhiyuan Qi, Chaoxian Xiao, Cong Liu, Tian-Wei Goh, Lin Zhou, Raghu Maligal-Ganesh, Yuchen Pei, Xinle Li, Larry A. Curtiss, and Wenyu Huang (2017) “Sub-4 nm PtZn Intermetallic Nanoparticles for Enhanced Mass and Specific Activities in Catalytic Electrooxidation Reaction” Journal of the American Chemical Society 139 (13), 4762-4768 doi: 10.1021/jacs.6b12780