Mass Transport Effects

The end of the pseudolinear region of the current/potential curve (Figs. 4.3, 4.6 and 4.7) is marked by the onset of mass transport limitations that cannot be described by the simple functional shape of Eq. (4.3). An adequate treatment is based on the fluid dynamics of the two-phase flow problem of air entering and water leaving the porous cathode media (compare Fig. 4.1). Experimentally, the use of artificial air (a mix of 21 vol% oxygen in helium) and pure oxygen in comparison with normal air allows the identification and location of mass transport losses in the catalyst layer, in the substrate material, or in the flow field. The mass transport effects can be ascribed to one or more of the following limitations: reduced partial pressure of oxygen in air, in addition to a so-called nitrogen blanket effect; limited diffusion of oxygen in the catalyst layer; and blocking of gas access by water droplets formed in the flow field or inside the electrode.

Establishing the total water balance of a single cell can also provide valuable information on the water transport inside an MEA from anode to cathode. This net crossover of water is the sum of electro-osmotic drag associated with the migration of protons and (Fickian) back diffusion of water along a concentration gradient. The ratio of transported water molecules and transported protons varies for different membrane materials and has a strong impact on mass transport behavior of the MEA. Compare the discussion in Section 4.3.1.2.

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