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Platinum for a cleaner world The ESRF is helping scientists design fuel cells for green car technology

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.

Jakub drnec activates the fuel cell in the lab

On the beamline, the high energy X-ray beam penetrates the whole fuel cell, which allows scientists to recover the snapshots of the active materials and the water distribution within the cell. This helps to determine the chemical and mechanical connection between the different materials used.

The X-ray diffraction patterns also provide information about the atomic arrangement within the materials. This can be used to track the changes in the atomic structure of the catalyst layer induced by the functioning of the fuel cell. With all this information the team can perfect the fuel cell to become more active and stable.

If we succeed, we will achieve a fuel cell ten times more active than any available in the market today. JAKUB DRNEC

Another reason why hydrogen cars are not readily available has to do with cost. This technology is very expensive, mainly because of the amount of platinum used in the catalyst. The research led by Chattot and Drnec focuses on reducing the amount of platinum in the fuel cell, so that the technology is much more accessible.

The Toyota Mirai is one of the very few hydrogen cars commercially available.

"I expect that in the next 10 years we will use fuel cells much more for transport, in airplanes and trains and other means of heavy transportation, and that gradually it will become standard in cars", explains an optimistic Drnec.

CREDITS

TEXT: Montserrat Capellas Espuny

VIDEOS: Alessandro Zontone and Montserrat Capellas Espuny.

REFERENCES

Chattot, R. et al, Nature Materials, volume 17, 827–833 (2018).

Donald McDonald, Leslie B. Hunt (1982). A History of Platinum and its Allied Metals. Johnson Matthey Plc. pp. 7–8. ISBN 978-0-905118-83-3.

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