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This is how physicists contribute to the energy transition

Studying surfaces of solid materials: it may not seem relevant to energy consumption, but it is. Marcel Rost studies how platinum electrodes wear out. Those electrodes are a crucial component in the fuel cells of hydrogen-powered cars. ‘We need to make the switch from fossil fuel energy to hydrogen.’

Rost starts off enthusiastically with the Toyota Mirai, a car that runs on hydrogen gas. ‘Such cars are the future! Driving on electricity is not practical for larger vehicles because the batteries become too heavy. But hydrogen is a possibility. And platinum is the best material for the fuel cell in such a car. There's just one problem: It wears out and is expensive to replace. So the people at Toyota come knocking on our door.’ The Leiden surface physicist works together with electrochemist Marc Koper. Thanks to their combined expertise and the unique microscope Rost built, they are global leaders in this research field.

Platinum in fuel cell wears off

Rost’s research is actually very fundamental. He studies a process called nucleation. ‘The platinum starts out as an almost perfectly flat surface where all the atoms are neatly arranged in a lattice,’ Rost says. ‘But over time, little mounds of atoms grow on the surface. This atomic process that causes the platinum to become rought, is also responsible for the limited life span of the fuel cell.’ They could already observe the formation of those mounds, but now, Rost is one step closer to understanding how they form.

Atoms dancing on the surface

Nucleation occurs by platinum atoms breaking free from the lattice. They roam across the surface and cannot find the hole in the lattice from which they came to return there. ‘We see that the mounds form in a very ordered pattern on the platinum, in islands that get higher and higher. That's crazy! Normally those mounds grow in random places and the islands merge.’ 

When the fuel cell is in use, a layer of platinum oxide forms on the platinum. This is a molecule consisting of one platinum and two oxygen atoms. Because of the arrival of these new atoms, not all of them fit properly in the lattice anymore and tension builds up in the atomic structure. If the tension rises too high, an atom is knocked out. Those loose atoms stick together and form mounds.

From left to right: atoms are pushed out of the lattice. The platinum atoms (blue) and oxygen atoms (red) don't fit properly into the lattice, creating tension until an atom is pushed out.

A spooky model

There are only two possible causes for the regularity in the mound pattern. ‘There are either preferred locations, or there is a repulsive force. We now think that those preferred locations are not immediately there, but quickly form.’ Together with his former PhD candidate Leon Jacobse, Rost developed a model called spoke wheels to explain this. In this model, the platinum atoms form a hexagonal spoke wheel in the lattice. The model can explain all the observations, but there is no hard evidence yet. He laughs: ‘I also call them spook wheels: maybe they exist, but we don't see them.’

Meanwhile, PhD candidate Francesc Valls Mascaro has managed to observe rows of atoms with the microscope, which could just be the spokes of their model. In any case, Rost is thrilled that his work can make a difference: ‘To give our children a future, we have to unplug fossil fuels. And that can only be done with hydrogen. I'm suddenly working on one of the most pressing problems, that's fantastic!’

Read the scientific publications in the Journal of The Electrochemical Society and Angewandte Chemie International Edition.

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