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Platinum Catalyst in Aqueous Solution: the oxygen atoms, in water, are red; the hydrogen molecules are white, and platinum atoms are blue-gray. High-level details of the structure can be seen in the reflections of each atom surface. Photo: Argonne Nationa
Platinum Catalyst in Aqueous Solution: the oxygen atoms, in water, are red; the hydrogen molecules are white, and platinum atoms are blue-gray. High-level details of the structure can be seen in the reflections of each atom surface. Photo: Argonne National Laboratory via Flickr (CC BY-NC-SA).
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Hydrogen car price breakthrough: it's the platinum

The Ecologist

14th October 2015

Hydrogen cars - and the fuel cells that drive them - are about to get a whole lot cheaper thanks to a redesign of the platinum catalyst that makes them work, writes Oliver Tickell. By inserting atom-sized holes into the precious metal's surface, its activity can be trebled.

A catalyst based on this design would be three times more active, so the amount of platinum used for the catalyst in fuel cells could be reduced to one third of what it is now - and the price would decrease accordingly.

European scientists have cracked one of the big puzzles facing the hydrogen car sector: how to make the clean, green vehicles affordable.

And the secret lies in the platinum catalyst that allows the cool, efficient combustion of hydrogen in their 'fuel cell' power houses - converting the 'burn energy' of hydrogen into electricity, not heat.

The technology is wonderful, but not the price. The materials used for the fuel cells are mostly cheap and widely available, except one. At the current price of $32 per gram, The platinum needed to run a typical fuel cell car now clocks in at around $1,100 - just for the metal.

But that's about to change. Scientists at universities and research institutes in France, Germany and the Netherlands have uncovered the secrets that make the platinum tick. And that has led onto a new structural design for the catalyst that hugely increases its efficiency - by inserting atom-sized holes into the metallic surface.

And with that, the cost of the platinum in one car's worth of fuel cells is set to drop to as little as $320, according to a co-author of the just published study, Marcus Pohl of the Technical University of Munich.

Three times as active, a third the amount

"A catalyst based on this design would be three times more active", he says. "So the amount of platinum used for the catalyst in fuel cells could be reduced to one third of what it is now."

Instead of the 30-40 grams of platinum employed in current 'state of the art' fuel cells, he says, only 10-15 grams would be required, "and the price would decrease accordingly" - to around €320 - €400 per car.

That's starting to look comprable to the 3-7 grams of platinum embodied in a typical catalytic converter on a petroleum-driven car. Suddenly, fuel cell vehicles just got a whole lot more realistic.

As well as reducing the cost, the entire fuel cell technology is no considerably more scaleable - it will take a lot longer before it runs into limits in the supply of the essential precious metal as the number of fuel cell vehicles on the roads increases. "The world's annual output would not suffice for all cars using existing technologies", the scientists point out.

What also helps is that fuel cell cars produce no toxic emissions that need cleaning up with a catalytic converter. The end product of hydrogen combustion is pure water, so instead of exhaust fumes, you'll see no more than a light puff of steam on a cold morning.

Now for the science - 'correlating geometric and adsorption properties'

According to the the scientists, it's not the entire surface of the platinum that is catalytically active, but only a few particularly exposed areas of the platinum, the so-called 'active centers'. So the aim is to produce a catalyst that is unusually rich in the 'active centres' in order to increase its activity.

"A common method used in developing catalysts and in modeling the processes that take place on their surfaces is computer simulation", the scientists explain. "But as the number of atoms increases, quantum chemical calculations quickly become extremely complex."

So instead they adopted an altogether different approach. With their new methodology using 'coordination-activity plots' that correlate geometric and adsorption properties, they developed a new parameter, the 'generalized coordination number' (GCN), which counts the immediate neighbors of an atom and the coordination numbers of its neighbors:

"Calculated with the new approach, a typical platinum surface has a GCN value of 7.5. According to the coordination-activity plot, the optimal catalyst should, however, achieve a value of 8.3. The required larger number of neighbors can be obtained by inducing atomic-size cavities into the platinum surface."

To validate their findings, the researchers computationally designed a new type of platinum catalyst for fuel cell applications. The model catalysts were then prepared experimentally using three different synthesis methods. In all three cases, the catalysts showed up to three and a half times greater catalytic activity.

"This work opens up an entirely new way for catalyst development: the design of materials based on geometric rationales which are more insightful than their energetic equivalents", says co-author Federico Calle-Vallejo.

"Another advantage of the method is that it is based clearly on one of the basic principles of chemistry: coordination numbers. This significantly facilitates the experimental implementation of computational designs."

"With this knowledge, we might be able to develop nanoparticles that contain significantly less platinum or even include other catalytically active metals", addsProfessor Aliaksandr S. Bandarenka of the Technical University of Munich. "And in future we might be able to extend our method to other catalysts and processes, as well."

The discovery will also make it much cheaper to build large scale 'hydrogen batteries' to balance the grid, where water is hydrolised to hydrogen and oxygen using surplus power from wind and solar power farms, then the hydrogen is stored for combustion in large banks of fuel cells to return power to the grid as and whan needed.




The paper: 'Finding optimal surface sites on heterogeneous catalysts by counting nearest neighbors' by Federico Calle-Vallejo, Jakub Tymoczko, Viktor Colic, Quang Huy Vu, Marcus D. Pohl, Karina Morgenstern, David Loffreda, Philippe Sautet, Wolfgang Schuhmann, Aliaksandr S. Bandarenka is published in  Science, October 9., 2015 - DOI : 10.1126/science.aab3501.

The research was funded by the European Union's Fuel Cells and Hydrogen (FCH) Initiative, the Netherlands Organization for Scientific Research (NWO), the German Research Council (via SFB 749, the Cluster of Excellence Nanosystems Initiative Munich (NIM) and Ruhr Explores Solvation (RESOLV)) and the Helmholtz Energy Alliance.

It was carried out by scientists from the Technical University of Munich and the Ruhr University Bochum (Germany), the Ecole normale superieure (ENS) de Lyon, Centre national de la recherche scientifique (CNRS), Universite Claude Bernard Lyon 1 (France) and Leiden University (Netherlands).

 

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