Noble metals of the periodic table are exceptional because they resist corrosion. That makes them ideal for a wide range of applications. One of these elements, platinum, has been used since the early days of civilisation and it is now taking a specially relevant role in the quest to reduce vehicle emissions.
Platinum is rare and that makes it very expensive. Today it is hard to believe that when Europeans first found platinum in Central America, back in the XV century, they thought that it was an impurity of gold and discarded it.
Despite its high cost, platinum is used mostly to transform toxic gases into harmless ones in vehicle catalysts. Of the 218 tonnes of platinum sold in 2014, 98 tonnes were used for vehicle emissions control devices (45%), 74.7 tonnes for jewellery (34%), 20.0 tonnes for chemical production and petroleum refining (9.2%), and 5.85 tonnes for electrical applications such as hard disk drives (2.7%). The remaining 28.9 tonnes went to various other minor applications, such as medicine, anticancer drugs, oxygen sensors and turbine engines.
Platinum has been known for more than a decade as one of the best catalysts for fuel cell oxygen reduction reaction. However it is difficult to meet high activity and stability in long-term operation. At the ESRF, scientists have been studying platinum in car catalysts powered by fossil fuels for the last two decades.
Today, in a collaboration between the ESRF and the Université Grenoble Alpes (UGA), researchers have designed a catalyst with an activity exceeding expectations. Unlike other catalysts where platinum is mixed with non-noble elements to increase activity (in detriment of long-term stability), UGA uses the non-noble element to carve the catalyst at the nanoscale before removing it. The resulting material is almost pure platinum.
"During my PhD we came up with this new design for a catalyst. Surprisingly, it was very active and stable but we didn't know why. So we came to the ESRF to find the answers", Raphaël Chattot, scientist at UGA.
Defects are key
The UGA team took their catalyst to the ESRF, where they studied it with Jakub Drnec on the high-energy beamline ID31. "The ESRF is a perfect facility to investigate the fine structure of nanoparticles thanks to high intensity X-rays", says Chattot.
Jakub Drnec (left) and Raphaël Chattot discuss the set up on the beamline for the experiment. On the left, the tube where the X-rays travel, on the far right, the detector (square) that will provide the data.
The results showed that the defects in the catalyst made it perform better. In fact, the more defects that were introduced, the better. The team then tried to maximize the initial amount of structural defects within the nanomaterial. Stability tests under simulated fuel cell environment showed the sustainability of the approach.
From the catalyst to the fuel cell
One of the main reasons why the hydrogen car is not on the road is because while materials work well under lab conditions, when they are placed in a fuel cell in working conditions, they don't perform in the same way. To be sure that the catalyst would work in a car, researchers need to first study it in the fuel cell. This means testing it in a more complicated environment, where gas, solid and liquids come together. The key is that all the materials ultimately work together to provide power. At the ESRF, scientists can study why unfortunately most catalysts do not perform in a working fuel cell.