Standard Techniques for Reducing Fuel Crossover

There are four principle ways that DMFC designers use to reduce fuel crossover, and there are other ideas that are more at the experimental stage. The four key established methods are the following  

The anode catalyst is made as active as possible, within the bounds of reasonable cost. This results in the methanol reacting properly at the anode and not being available to diffuse through the electrolyte and on to the cathode. 

Thicker electrolytes than what is normal for PEMFCs are used. Clearly this will reduce fuel crossover, though it will also increase the cell resistance. There is a compromise to be found, and in the DMFC that optimum is with a somewhat thicker electrolyte than for hydrogen fuel cells. The thickness membrane normally used by DMFC developer is between 0.15 and 0.20 mm, whereas for a hydrogen PEMFC it would be between 0.05 and 0.10mm.  

In addition to the thickness of the PEM, its composition also has an effect. It has been shown by Ren, Springer and Gottesfeld (2000) that the diffusion and water uptake for llOOEW Nafion is about half that for 1200EW Nafion. 

It should also be noted that fuel crossover reduces as the current from the cell increases. This is linked to points 1 and 2 above - the fuel reacts promptly at the anode and is not made available to crossover. In Ren, Zelanay et al., 2000, it is shown how the crossover equivalent current falls with methanol concentration and with increasing current. These results are summarised in Eigure 6.4. This means that it is helpful if a DMFC is designed to run more or less continuously at quite near its maximum power. To do this, it will usually need to be run in parallel with a supercapacitor or a small rechargeable battery. Such arrangements are very attractive in the case of portable electronics equipment, which will probably be the first application of DMFCs (e.g. studies of such hybrid systems, see Jarvis et al., 1999 and Park et al., 2002). 

---