The aerogel, which is a kind of nanostructured foam, has also passed long-term tests in the lab, where the typical operating conditions in a vehicle were simulated.

Schematic representation of the oxygen reduction reaction at the positive electrode (aerogel material). Each oxygen atom from the oxygen molecules fed into the cell captures two electrons, which is followed by the reaction with hydrogen nuclei to form water. Source: PSI. Click to enlarge.

The aerogel, now synthesized and characterized by researchers at Dresden University of Technology and at PSI (Paul Scherrer Institute) forms a three-dimensional network of nanowires and it features a high porosity and large inner surface.

The latter properties facilitate the adsorption of many oxygen molecules onto the catalytically active platinum atoms—a prerequisite for the efficient conversion of oxygen. While catalysts used in commercial fuel cells also exhibit a high degree of porosity and large surfaces, they achieve that only when they consist of platinum nanoparticles on a carbon substrate. The key advantage of the new aerogel is that it combines these assets with an extensive three-dimensional structure, which means there is no need for a support whatsoever.

Because of their excellent properties for many applications in electrochemistry and sensing applications, aerogels have attracted a lot of attention in recent years. Success has been limited to a small group of chemical substances: most aerogels are made of oxides or single metals. However, theoretical considerations had suggested that catalysts made of particular metal alloys would display greater catalytic activity and stability and this had sparked attempts to implement those features in an aerogel catalyst. Synthesizing a bimetal aerogel, however, has proven difficult.

This is the first time that an aerogel made of a metal alloy has ever been synthesized.

—Thomas Justus Schmidt, head of the Electrochemistry Laboratory at the PSI and co-author of the study

The new results confirm the high hopes for these materials. The key to improving the activity of the new aerogel, for instance, is that the alloy with palladium optimizes the bond strength between the platinum atoms and the oxygen-containing species. In other words, the bond is so strong that the oxygen molecules remain adsorbed just long enough for the conversion into water but not too strong as to induce the formation of oxides on the catalyst’s surface.

The fact that the conversion to water is more favorable than the formation of oxides optimizes at each point in time the number of available catalytic centers which in turn leads to oxygen molecules being adsorbed and converted at a considerably high rate.

The researchers do not yet understand the greater stability of the bimetal alloy aerogel compared to monometal aerogels made of pure platinum. The scientists would now like to spend the next three years focusing on this and other questions regarding the new nanomaterial in a follow-up project.

Obviously, the presence of palladium in the aerogel plays a key role here, too, but we don’t know yet exactly what impact this has on the stability of the catalyst. We have just finished the draft for a funding application together with Dresden University of Technology to give the project we have been funding internally up to now a broader financial footing.

—T.J. Schmidt

Resources

• Wei Liu, Paramaconi Rodriguez, Lars Borchardt, Annette Foelske, Jipei Yuan, Anne-Kristin Herrmann, Dorin Geiger, Zhikun Zheng, Stefan Kaskel, Nikolai Gaponik, Rüdiger Kötz, Thomas J. Schmidt and Alexander Eychmüller (2013) Bimetall-Aerogele: hoch effiziente Elektrokatalysatoren für die Sauerstoffreduktion. Angewandte Chemie doi: 10.1002/ange.201303109