Airflow and Water Evaporation

Except in the special case of PEM fuel cells supplied with pure oxygen, it is universally the practice to remove the product water using the air that flows through the cell. The air will also always be fed through the cell at a rate faster than that needed just to supply the necessary oxygen. If it were fed at exactly the stoichiometric rate, then there would 

Water may back-diffuse from the cathode to the anode, if the cathode side holds more water. 

Water may be supplied by externally humidifying the hydrogen supply 

Water will be dragged from the anode to the cathode sides by protons moving through the electrolyte 

Water will be removed by evaporation into the air circuiating over the cathode. 

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