When it comes to fuel cell catalysis, less is more
It is a chemical minimalist’s dream: use far less of a pricey catalyst and watch the output of your catalytic mechanism increase by two full orders of magnitude. That may sound too good to be true, but it is exactly what Dalhousie University Department of Chemistry Professor Peng Zhang and his colleagues found when they offered their own fresh take on fuel cell catalysts with promising application for automobile industry.
Fuel cells represent the key to hydrogen-powered vehicles, which would run on electricity generated when hydrogen and oxygen combine, a reaction that creates only water vapour as emissions. A platinum catalyst would produce the source hydrogen by reforming it from another fuel, such as natural gas or ethanol, but if any amount of carbon monoxide is present, it will “poison” this initial reaction by binding tightly with the catalyst. Zhang reasoned that this unwanted result could be avoided if platinum atoms were much further apart, so that they became geometrically unavailable for binding with CO.
In order to test that possibility, his team adapted a colloidal synthesis method to generate a gold-platinum alloy from chloride precursor salts. The technique made it possible to nucleate large amounts of gold atoms initially, then deposit platinum atoms intermittently afterward, allowing for a wide range of platinum composition in the emerging alloy.
Using the Canadian Light Source synchrotron in Saskatoon and its partner synchrotron Advanced Photon Source in Chicago, Zhang confirmed the method’s ability to create a significant number of physically isolated platinum atoms.
“Normally the study of these alloys is very challenging, because it’s hard to distinguish these two metals,” he observes, noting that synchrotron analysis revealed that these single atoms represented as much 7% of the total amount of platinum in the final alloy. “None of these platinum atoms would interact directly with the next platinum atom. They’re all surrounded by something different.”
Yet while there was less catalyst available in absolute terms, the researchers compared its performance in a commercial-standard catalyst with an all-platinum version of the same design; the former utterly eclipsed the latter. Zhang credits much of this difference to the drop in CO poisoning of the reaction.
“When we formed this single-atom alloy, we found the binding strength of carbon monoxide to be a lot weaker compared with pure platinum,” he says.
Single atom catalyst. Photo credit: Peng Zhang
These initial investigations, aimed at demonstrating the viability of a single-atom catalyst, employed a gold matrix in order to take advantage of this element’s chemical stability. Future work will apply this principle to more cost-effective alloys, such as nickel and iron, a combination that should be of great interest to automakers.
“Single-atom catalysts are something new and really exciting for chemists,” heconcludes. “This design could solve many problems.”