Hydrogen PEFCs humidification

Often it is necessary to transport excess water from the cathode side to the anode side, where it is used either for humidification of the hydrogen stream in the PEFC or to dilute the methanol fuel. (In order to increase energy and power density, methanol, while being used in solution, will usually be stored as the pure liquid.) Water management needs additional aggregates or devices. This adds to the cost of the fuel cell system and further reduces its efficiency. All these transport limitations give rise to diffusion overpotentials which lead to the rapid breakdown of the cell at high current densities in Fig. 2. 

Fig. 29 Schemes of power systems based on a neat hydrogen/air, PEFC stack, for the case (a) where pre-humidification of the incoming fuel feed stream is required and (b) where satisfactory stack hydration is achievable w/o pre-humidification of the incoming fuel feed stream [61]. 

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