Strategies to improve cold start performance

The above results indicate that the produced water initially diffuses into the membrane. After the membrane is saturated, water starts to accumulate in the cell and freeze. At temperatures below —20°C the accumulating water starts freezing immediately in the catalyst layer. It is therefore not possible to control the amount of water freezing in the locations where the ice does not affect the oxygen diffusion. This indicates that one solution to improve the cold start is to dry the 

This chapter details the general idea of water transport phenomena in PEM fuel cells ranging from the nano-scale in the catalyst layer to the millimeter scale in the separator channels. Different from a review paper, the chapter is written largely as a summary of work performed by the authors. Some of the results still need to be proven to be generally applicable, but the readers will become aware of the phenomena and be able to consider the mechanisms involved from the point of view of the authors. 

The major findings presented in the chapter may be summarized as follows  

Although serpentine channels are most commonly used, straight channels may give better performance with lower stoichiometric ratio, if an optimum water distribution structure is established as discussed in Section 1.3. Porous separators also have the potential to enable the operation of cells without flooding at high current densities with smaller stoichiometric ratios. Here, attention must be paid to the cooling due to the low thermal conductivity of the porous separators. 

The design details of GDLs are very important to ensure a smooth and efficient removal of liquid water from the catalyst layer to the channels and from the region under the lands, where liquid water tends to accumulate. A gradient distribution of the wettability of the GDL fibers in the through plane direction may be effective for the water removal. Anisotropic orientations of the GDL fibers also offer good possibilities for improvement of water removal characteristics. 

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