Cell surface alterations, mechanism resistance

In order to certify the suitability of bipolar plate base material or coatings with respect to degradation in PEM fuel cells, two main characteristic criteria, interfacial contact resistance between bipolar plate surface and GDL and the corrosion afbnity, emerged from the pool of criteria shown in Table 6.1 as standard characteristics. Since some requirements such as gas permeabihty and mechanical strength are not expected to be affected by fuel cell operation, electric properties are known to be subjected to surface alterations due to, for example, corrosion. 

The conductivity of the membrane depends on the degree of its hydration. However, this is altered by a great many phenomena in the fuel cell. The need to preserve the water content of the electrolyte membrane limits the operating temperature of the PEMFC to below 80°C. Beyond this temperature, the membrane becomes dehydrated, the ionic resistance increases very rapidly and the performances suffer drastically. The thickness of the membrane results from a compromise between low ionic resistance - which requires as thin a membrane as possible - and sufficient mechanical strength. Indeed, it has to be able to withstand pressure variations on both sides of its surface during the operation of the cell. 

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