There is a fast, simple process for making platinum “nano-raspberries” — microscopic clusters of nanoscale particles. That new process could lead to more practical fuel cells.
The berry-like shape is significant because it has a high surface area, which is helpful in the design of catalysts. For industrial chemists, researchers at the National Institute of Standards and Technology (NIST) figured out when and why the berry clusters clump into larger bunches of “nano-grapes.”
The significance is these nanoparticles can act as catalysts to help convert methanol to electricity in fuel cells. The 40-minute process for making nano-raspberries has several advantages. The high surface area of the berries encourages efficient reactions. In addition, the process uses water, a benign or “green” solvent. And the bunches catalyze methanol reactions consistently and are stable at room temperature for at least eight weeks.
Although the berries consist of platinum, the metal is expensive and used only as a model. The study will actually help guide the search for alternative catalyst materials, and clumping behavior in solvents is a key issue. For fuel cells, nanoparticles often mix with solvents to bind them to an electrode. To learn how such formulas affect particle properties, the NIST team measured particle clumping in four different solvents for the first time. For applications such as liquid methanol fuel cells, catalyst particles should remain separated and dispersed in the liquid, not clumped.
“Our innovation has little to do with the platinum and everything to do with how new materials are tested in the laboratory,” said project leader Kavita Jeerage. “Our critical contribution is that after you make a new material you need to make choices. Our paper is about one choice: What solvent to use. We made the particles in water and tested whether you could put them in other solvents. We found out that this choice is a big deal.”
The NIST team measured conditions under which platinum particles, ranging in size from 3 to 4 nanometers (nm) in diameter, agglomerated into bunches 100 nm wide or larger. They found that clumping depends on the electrical properties of the solvent. The raspberries form bigger bunches of grapes in solvents that are less “polar,” that is, where solvent molecules lack regions with strongly positive or negative charges. (Water is a strongly polar molecule.)
The researchers expected that. What they didn’t expect is the trend doesn’t scale in a predictable way. The four solvents studied were water, methanol, ethanol and isopropanol, ordered by decreasing polarity. There wasn’t much agglomeration in methanol; bunches got about 30 percent bigger than they were in water. But in ethanol and isopropanol, the clumps got 400 percent and 600 percent bigger, respectively. This is a very poor suspension quality for catalytic purposes.
Because the nanoparticles clumped up slowly and not too much in methanol, the researchers concluded the particles could transfer to that solvent, assuming they saw use within a few days, effectively putting an expiration date on the catalyst.