Reactant supply humidity

It was found that the mass transfer behavior of reactants and products of the stack is more complicated compared with a single cell because of the heat exchange, humidity, and reactant supply effects. Some of the produced water was lost by evaporation, while self-humidifying was found to be more efficient at temperatures above 30 °C. Under laboratory conditions, humidification can be lowered if cooling power is improved to compensate for the heat released by the electrode reactions. In applications, however, cooling power is limited and humidification is a necessity. 

Dutta et al. [54] used the unified approach to study mass transport between the channels of a PEM fuel cell with a serpentine flow field. Their model is three-dimensional and allows for multi-species transport. They studied the effect of flow channel width in the serpentine flow field on velocity distribution, gas mixture distribution and reactant consumption. Serpentine flow fields allow for a greater area for diffusion of the supply gases. Their results showed that for low humidity conditions, water transport is dominated by electro-osmotic effects, i.e., water flows from anode to cathode at the side of the cell closer to the gas channel inlet. At the outlet side of the cell, water transport is dominated by back diffusion, and it flows in the opposite direction. Thus the serpentine flow field allows for circulation of the water within the cell. 

Generally speaking, reactant gases with inlet relative humidity (RH) equal to or less than 100% are needed during the cell operation. The supply of reactant humidity is necessary and important because the membrane (e.g., Nafion) requires full hydration in order to maintain good performance and lifetime. It is widely recognized that membrane hydration can be achieved by supplying fully humidified reactant gas streams to both the anode and the cathode. 

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