Ideal membrane humidification

The polarization voltage of the cathode side is determined by reaction kinetics, transport of water and oxygen across the cell and by proton transport across the CCL. In this section, we will assume an ideal membrane humidification. We start with the model of the CCL. 

In this section, we re-formulate the model equations for the case of constant oxygen stoichiometry and obtain the criterion of ideal membrane humidification. 

The solutions (6.119-1.122) do not contain the water management parameter r and p. Furthermore, the exponential shapes of and j/J are governed by a single parameter, the oxygen stoichiometry A. Eqs. (6.119), (6.120) with p (6.122) and k (6.121) coincide with the solutions (6.43), (6.44) in Section 6.3.2, where ideal membrane humidification is assumed. This means that the current, which obeys (6.115) does not produce any significant non-uniformity of membrane resistance along z. Equation (6.115) is thus the condition of ideal membrane humidification. 

So far the models in this chapter assume that the membrane is weU humidified. However, such an ideal water management is difficult to achieve, especially in fuel cell stacks. A PEFC needs water to maintain the membrane in a wet state and it may suffer from excess of water. Due to insufficient humidification of the air flow, close to the channel inlet the membrane can dry up, thus limiting local current density. Close to the outlet, the excess of liquid water produced in the cell may retard oxygen transport to the catalyst sites (flooding). 

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