How Fuel Crossover Occurs

We have already noted that the PEM type of electrode is currently viewed as the only near-term possibility for DMFCs. The use of alkaline electrolytes results in the inevitably fatal problem of carbonate formation. The PEM electrolytes also have a major problem, but steps can be taken to reduce it, and it does not inevitably destroy the cell. 

The problem for PEM electrolytes in connection with using methanol fuel is that of fuel crossover. This was considered in general in Section 3.5, as it occurs to some extent in all fuel cells. However, in the DMFC with a PEM electrolyte it is particularly severe. The reason is that methanol mixes very readily with water, and so spreads into the water 

The loss of methanol is often transposed into a crossover current - the current equivalent to that which would be produced by the methanol, had it reacted properly on the fuel anode. This current, ic, can be used with the useful output current i to give an important figure of merit for a DMFC, which is the fuel utilisation coefficient r)f. This gives the ratio of the fuel that is usefully and properly reacted on the anode to the total fuel supplied, the difference being accounted for by some fuel crossing-over and being lost at the cathode. 

Using the techniques described below, it is possible to bring this figure up to as high as 0.85 or even 0.90 (Gottesfeld, 2002), though 0.80 (or 80%) would probably be a more realistic figure to expect. 

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