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Project Success Stories - The race to cheaper fuel cells

European researchers are working to develop cheaper membranes at the heart of fuel cells, an expensive component of a fast emerging technology.

Energy

Lowering costs is crucial. Scientists and economists believe that, in the long term, hydrogen fuel cells are one of the best candidates to replace fossil fuels in cars. But that is unlikely to happen as long as the technology remains expensive. Currently, the leading technology is the 'Proton exchange membrane fuel cells' (PEMFC). It is one of the least expensive, and even though costs have dropped enormously since fuel cells were first used in earnest for the NASA Gemini programme in the 1960s the technology is still too expensive for mass manufacturing. The PEM is at the heart of these fuel cells. It is possible to use other fuels, like diesel, but hydrogen is the most often cited. Here is how it works. Hydrogen is the most plentiful element in the Universe. Its atom (H) consists of an electron, a single proton and no neutron. The hydrogen molecule (H2) enters the fuel cell and encounters the anode, which splits hydrogen's protons (2H+) from its electrons (2e-). The membrane conducts positively charged ions, or protons (H+), but not electrons, so protons pass through the membrane and on to the cathode, but the electrons must follow an external circuit around the membrane to reach the cathode. As they travel on this circuit the electrons create a current. Once the protons and electrons reach the cathode they react with oxygen to create water. The EU-funded 'Fuel cell membranes based on functional fluoropolymers' (Flupol) project sought to create cheaper membranes using a fluoropolymers - a fluorocarbon-based polymer with multiple strong carbon-fluorine bonds. It is a very challenging task because PEMs must fulfil a laundry list of performance targets. Designing new material for this purpose is complex, according to Justyna Walkowiak, lead researcher on the Flupol project, because you can't focus on only one kind of property. 'They need to exhibit high protonic conductivity, no electronic conductivity, low permeability to fuel and oxidant, low water transport through diffusion and electro-osmosis, oxidative and hydrolytic stability, good mechanical properties, low cost, and the capability of fabrication into membrane electrode assemblies,' she explains. Currently Nafion, the brand name of a membrane by DuPont, is the state of the art. This is a commercially available product that is used in cars and different portable machinery and equipment. There are other brands, too, with a similar design and all of these membranes are polymers. Polymers are a series of linked monomers, and monomers are atoms or small molecules that can form chemical bonds in long chains with other monomers. The most common natural monomer is glucose, and it can bind with other monomers to create polymers like cellulose and starch. There are millions of polymers, both natural and synthetic. All the currently available membranes are based on two or three related monomers, all based on fluorocarbon polymers of one sort or another. Flupol's strategy was to study fluorinated aromatic polymers. In organic chemistry, aromatic compounds include aromatic rings in their chemical structure. These rings are formed by the way in which atoms bind to each other. Not cheap 'What we thought is that aromatic compounds are suitable for processing after polymerisation, and this means we would be able to change the polymer properties after synthesis,' Ms Walkowiak reveals. 'We thought that it might be cheaper to change properties after polymerisation. Right now it is not that cheap, but we think there may be scope to lower those costs in the future.' Ms Walkowiak believes that the two years assigned to the Flupol project was little time to work on such a big topic, but she believes it produced some solid results and offered some clear paths forward. Firstly, Ms Walkowiak developed a new and cost-effective preparation method for an important class of monomers, called monofluorinated styrenic monomers. This was a good result in a challenging area. In polymer science chemists typically will see first if a compound can self polymerise. This is called homo-polymerisation. 'If that does not work, you look at using two monomers to induce co-polymerisation,' says Ms Walkowiak. And if that does not work, you can try 'Termonomer induced copolymerisation' (or terpolymerisation) using three monomers. One of the monomers acts as an active couple to link the other two up. You polymerise the active couple with the first monomer and then incorporate the third, to achieve the desired qualities in the final material. Ms Walkowiak's innovation was to find a terpolymerisation method for preparing monofluorinated styrenic monomers. Secondly, the project found a way to minimise a common problem between certain monomers and styrene, a very important compound in polymer science. Two monomers, FMST and TFMST can bind with styrene, but FMST slows down the reaction, the rate at which the materials bind. That has two costly consequences. First, it takes longer. Second, the slower reaction results in shorter chain bonds, the links between each monomer. So you get less final material because of the shorter links. More time and less material can get very expensive, very fast with some of these materials. Ms Walkowiak found a way to blunt the slowing effect of FMST by combining it with TFMST. 'FMST still slows down the reaction, but not as much,' explains Ms Walkowiak. Finally, the researcher developed a new controlled polymerisation methodology for polymeric materials. 'Conventional free radical polymerisation reactions, like the production of polystyrene, do tend to lack control. One effect of this lack of control is a high degree of branching. Also, as termination occurs randomly, when two chains collide, it is impossible to control the length of individual chains.' explains Ms Walkowiak. 'But using chain transfer agents, or CTAs, you can get a measure of control. 'The important thing is to use the proper CTAs. And my work found the specific CTA required to produce copolymers that contain fluorinated styrene and styrene. Those particular copolymers have never been synthesised before. It is something new. Those copolymers exhibit really interesting properties, which the researcher is not currently at liberty to discuss. 'The investigations on developing these materials are in progress and the results are promising. They could have really very interesting properties regarding the stability of the material,' she confirms. But Ms Walkowiak will reveal that the vital step is the introduction of the fluorinated elements into the copolymers. In all, it is an impressive body of results that significantly adds to our understanding of fluorinated aromatic polymers and their potential application to fuel cell technology and there may be other, unexpected benefits as so often happens in this branch of science. The Flupol project received funding from the Marie Curie programme of the EU's Seventh Framework Programme for research.