Boosting fuel cells
A new process could help fuel cell vehicles become suitable for mass production, as Ellis Davies reports.
Fuel cell cars have been around for 50 years, but have never gone into mass production because of the price and availability of platinum, which is required in large quantities in the fuel cells. Researchers at Chalmers University of Technology, Sweden, and the Technical University of Denmark, have discovered a method that can significantly reduce the need for platinum by using a nanoalloy made up of platinum and yttrium (Pt3Y) in the fuel cells. Using less platinum means that this technique could be suitable for mass production.
Platinum is prominent in today’s fuel cells, and if all cars were to use them, there would not be enough platinum in the world for their manufacture. ‘If we can get the amount of platinum needed down to one tenth of today’s numbers, we would be close to what is used in automotive exhaust catalysts, and then it would be possible to supply such a demand from current platinum production,’ Björn Wickman, Assistant Professor at Chalmers University, told Materials World.
A Pt3Y alloy has been used before, but previous efforts experienced issues with oxidisation. ‘The traditional ways of making nanoparticles using chemical methods have not worked for Pt3Y, since the Y tends to oxidise instead of forming the alloy with platinum,’ said Wickman. The team overcame this by using a single target co-sputter approach in a vacuum chamber – sputtering is an established vacuum deposition technique used for semiconductors, solar cells and other types of coatings. ‘We decorated a platinum sputter target with just the right amount of yttrium foil so that the deposited material has a composition matching Pt3Y. As sputtering has relatively high energy, the platinum and yttrium atoms have enough kinetic energy to move around and form the alloy,’ explained Wickman.
Yttrium changed the catalytic properties of platinum to allow reactions to happen with smaller losses and at a higher rate, meaning that ‘every active site on the Pt3Y catalyst can do more of the desired reaction per unit time compared to pure platinum, which means that for a fuel cell to deliver a certain current, a lower amount of catalyst is needed,’ Wickman highlighted.
Current fuel cells will need to be altered to use the nanoalloy because they have been developed for catalysts prepared using traditional chemical methods, where the nanoparticles are suspended, allowing them to be mixed in a solution with an electrode support material to be sprayed or painted in layers, forming an electrode. ‘It is not possible to combine [this technique] with sputtering and thus a new electrode design is needed in order to incorporate it. However, this may not be as difficult as one might think. There has been a lot of research on alternative electrode designs, for example the type of electrode typically referred to as nanostructured thin film (NSTF) electrodes. These have been identified by the US Department of Energy to be promising for future fuel cells and a large amount of research has been devoted to developing this type of electrode. Most NSTF designs would be well suited to combine with sputtering to deposit the catalyst material,’ elaborated Wickman.
The team believes that scaling the sputtering technique will be relatively easy because it is already used in the mass production of other products. Wickman said that the process would depend on the design, the materials and how the rest of the electrode is made. ‘I’m quite sure it will be some sort of roll-to-roll manufacturing and then it will be quite straightforward to implement the sputter technique on a large scale,’ he said.
Fuel cells that use the Pt3Y alloy could have significant application outside the automotive industry, such as in portable electronics. As the only by-product is water, the team believe that this advance could also aid the development of sustainable energy solutions.