“Ethanol is one of the most ideal reactants for fuel cells,” notes Brookhaven chemist Radoslav Adzic. “It’s easy to produce, renewable, non-toxic, relatively easy to transport, and it has a high energy density. In addition, with some alterations, we could re-use the infrastructure that’s currently in place to store and distribute gasoline.” However, the fuel cells require oxidation of ethanol and current catalysts provide slow reaction speeds and form unwanted products, he adds.
The electrocatalyst provides several crucial advantages, says Adzic. It oxidizes ethanol at 60°C, a good match for direct ethanol fuel cells, which typically operate at 60°C to 80°C. Moreover, it works at a positive potential of only 0.3 V while conventional catalysts require 0.7 V, which precludes their use in fuel cells. Reaction rates are significantly higher than for other catalysts. “At the mild conditions the catalyst shows around 100 times greater activity than platinum and also much greater activity than platinum-ruthenium,” he notes. And it produces mainly carbon dioxide while other catalysts largely yield acetaldehyde and acetic acid.
Figure 1 -- Carbon cleaver:
“The ability to split the carbon-carbon bond and generate CO2 at room temperature is a completely new feature of catalysis,” Adzic claims. “There are no other catalysts that can achieve this at practical potentials.”
The catalyst consists of platinum and rhodium atoms on carbon-supported tin dioxide nanoparticles (Figure 1).
Right now, the material generates relatively small amounts of acetaldehyde and acetic acid but there’s still room for fine-tuning of its structure, according to Adzic. Plus, the catalyst should yield higher CO2 levels, perhaps only CO2, at 80°C, but tests at that temperature haven’t been run yet.
The main limitation is the price of platinum and rhodium — minimizing the loading of the metals and recycling can address this, Adzic believes. “Cost won’t be the reason for not applying the catalyst,” he says.
Catalyst poisoning is a potential problem. Regenerating the material by removing acetaldehyde is possible but won’t be easy. “Poisoning won’t be a ‘showstopper,’” he declares.
For the last two years the researchers have tested the material using 1-cm2 electrodes. They now will be sending catalyst to the Los Alamos National Laboratory, Los Alamos, N.M., for trials on a fuel cell either with 25-cm2 or 50-cm2 electrode area. Tests should be completed by August, says Adzic.
Within a year researchers may know if the catalyst suits fuel cells for portable electronics, notes Adzic. It’ll take two to three years before they can tell whether the material can handle automotive applications, which require much higher oxidation rates, he adds.
Scale-up of the catalyst won’t be an issue, Adzic believes. The remaining challenges revolve around boosting yields of CO2, assessing catalyst stability and reducing the amounts of platinum and rhodium, he says.
Fuel cell makers already have contacted the researchers but no detailed discussions have taken place yet.
The value of the catalyst may extend beyond fuel cells, Adzic points out. “Splitting the C-C bond is a major problem in organic synthesis. This hasn’t been done efficiently. Rhodium can split it in the gas phase at high temperatures, while this catalyst can do it at ambient temperature.”