**4. Conclusion**

Mass transport limitations in PEME and PEMFC may be due to several factors. Poor control of humidification levels within the cell can result in substantial losses in potential. In addition, good electrical and ionic conduction must be achieved between the electro catalyst layer and the membrane and diffusion layers. This will enable better utilization of the catalyst and limit cell losses through mass transport.

In addition, the feed provided to the PEMFC or PEME can greatly attribute to cell losses. If there are impurities present in the feed, it may affect the electrocatalyst performance or conductivity of the electrolyte. In both cases, substantial potential losses may be achieved, which may or may not be reversible, depending on the impurity present. In order to mitigate these effects, there is an on going effort to develop more tolerant electrocatalyst and membranes for these systems.
