Improving fuel cell batteries

Carbon gelsNathalie Job used very simple molecules, resorcinol and formalin which she had polymerised together in a solvent, namely water. In this way she obtained a polymer that has a three-dimensional structure. This solid material soaked with water was dried and pyrolysed. She thus obtained a carbon with a regular structure, very easy to control, composed of interconnected spherical nodules of carbon, resembling pearl necklaces. This was a real solid, the size of whose modules can be modified by changing the pH of the original solution. This signifies that we have at our disposal, a material whose porous qualities can be adapted and reproduced at will.

It is this material that is going to be used as a coating base for the catalyst of the fuel cell battery.

The crucial area is the cathode interface. It is the place where the oxygen in the air will be reduced, a reaction which causes most of the problems. It is also a key area because it is here that the water formed during the reaction must be drained off. The cathode is therefore in fact a complex system that can be broken up into several parts: the membrane or electrolyte (in the case studied Nafion is used) then a catalytic layer (the gel of carbon imbued with platinum) and a diffusion layer (a tissue of carbon fibres) which allows the air to be spread over the entire surface of the catalytic layer. On the other side of the membrane, the anode is attached; this is also composed of a catalytic layer of platinum/carbon and a layer of diffusion. These five layers are pressed together to constitute an electrochemical cell unit, fitted tightly between two electron-collecting plaques of graphite engraved with channels allowing for the gas to be channelled. This procedure is repeated as many times as there are units in the battery. From this we can understand the importance of having a viable catalytic layer which can be controlled and reproduced. The main problem with the current systems is that the membrane-catalytic layer exchange doesn’t work well. Few platinum particles are connected to the membrane. «It is for this reason that a lot of platinum is needed in battery electrodes: it happens that up to nine-tenths are useless as they are not connected to the membrane», explains Nathalie Job. In order to have a sufficient amount of platinum connected, a lot of the metal must be used and this increases the cost of the electrodes. The materials finalized in the laboratory in liege, thanks to the manipulation of the size of the pores, allow not only for an improvement in the movement of matter, gas or liquids, but also allow for a better contact and a greater connectivity between the membrane and the platinum. It is more homogenous.

What are the results? «For the electrodes currently available on the market 0.6 mg of platinum per cm3 of electrode. We have succeeded in coming down to 0.2or 0.3 mg per cm² for equivalent quantities of current» explains Nathalie Job with satisfaction.

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