Fuel cells are an environmentally friendly alternative to diesel and gasoline engines. Unfortunately, fuel cells use a platinum catalyst, one of the world's most expensive metals. Now, researchers from DTU Physics has developed a platinum-yttrium catalyst which is stable and even more active than the existing platinum catalysts. This paves the way for cheaper fuel cells.
Polymer electrolyte membrane fuel cells (PEMFCs) hold promise as a potentially zero-emission alternative source of power for automotive vehicles. Moreover, in comparison to batteries, they provide much longer driving ranges and faster refuelling times. Even so, the widespread uptake of PEMFC's has been hampered by the need for high loadings of platinum catalysts at the cathode, where oxygen reduction takes place. Even using current state-of-the-art technology, it would not be possible to scale up PEMFC production to make a global impact, i.e. to the terawatt level.
Arguably, the most viable route towards decreasing the platinum loading is to employ alloys of platinum with other metals as catalysts for oxygen reduction. Most researchers have focused on alloys of platinum with late transition metals, such as nickel, cobalt, copper or iron. Even so, the long term stability of these catalysts is typically compromised by their tendency to degrade via dealloying.
In 2009, a team of researchers from Center for Individual Nanoparticle Functionality (CINF) at DTU Physics discovered a new alloy for oxygen reduction, platinum-yttrium1. Not only was this new alloy more active than the other alloys of platinum and early transition metals, but it could potentially be much more stable. However, platinum-yttrium was first tested in bulk polycrystalline form, whereas in a real fuel cell it would have to be implemented in nanoparticulate form and this turned out to be a highly challenging task.
Nonetheless, as reported in Nature Chemistry, researchers from DTU Physics have now managed to synthesise nanoparticles of the platinum yttrium alloy. The most active catalysts exhibited a high mass activity of 3.05 A/mg Pt, a factor of five improvement over commercial platinum catalysts measured in the same manner. Moreover, the new catalyst retained 63% of its initial activity in an extended stability test.
"Our results demonstrate a proof-of-principle that platinum-yttrium nanoparticles can be synthesised and are highly active for the oxygen reduction reaction. Furthermore, there is a potential for even higher, record-setting performance if we can optimize the particle size and composition of the Pt-Yt catalyst. The next challenge in order for this promising material to find its way into fuel cell applications will be the development of a chemical synthesis method that allows for production of the catalyst in large quantities, explains Ifan Stephns, Assistant Professor at DTU Physics and one of the researchers involved."
The research has been carried out by researchers from the Center for Individual Nanoparticle Functionality (CINF) in collaboration with the Center for Theoretical Atomic-scale Physics (CAMD) and with DTU CEN and SLAC National Laboratory, USA.
Source and top image: Technical University of Denmark