## 6 CL Formation
### Drying Process
The capillary force compresses the particles, and the pores shrink until the porous structure is strong enough [56]; however, the excessive capillary force may cause cracking, which will be discussed in the last part of this section. A rapid evaporation rate will shorten the first and second drying process, leading to an increased porosity, but having little impact on the pore size [56]. The increased porosity of CL with an increase in evaporation rate, namely, an evaporation-dominated process, was observed in [PERSON]'s studies as well [41]. However, [PERSON] et al. found that the drying rate has an obvious influence on the secondary pore size, which decreases with decrease in the solvent evaporation rate, since the particles in the ink film have enough time to migrate and pack densely [41]. The inconsistent results are likely due to the different drying temperatures, 20-80 \({}^{\circ}\)C in [PERSON]'s studies vs. 20-40 \({}^{\circ}\)C in [PERSON]'s studies. Moreover, it has been found that the evaporation-dominated consolidation leads to the CL with the smaller-particles-rich surface, i.e., the ionomer-rich surface due to the smaller size of the non-adsorbed ionomer agglomerates than catalyst-related agglomerates [41]. During the shrinking process, [PERSON] et al. found that the size of catalyst agglomerates and the thickness of Nafion shell around the catalyst agglomerates will decrease over evaporation, due to the exclusion of the solvent molecules from primary pore and the solvated Nafion ionomer shell [175]. Figure 13: Weight change of the ink film at different drying conditions (20 or 40 \({}^{\circ}\)C, dry—0% RH, humid—80% RH) and the schematics of the structure change from the ink film to the dried CL (Reprinted from [56], Copyright (2016), with permission from Elsevier). Cracking is a common issue during the CL formation, especially when applying a thick ink film on the substrate at one time, e.g., screen printing [66] or slot-die coating [79]. Although the CLs with guided cracks generated by starching the prism-patterned CCM exhibited enhanced water transport at the cathode and thus increased performance at the mass polarization region [177, 178], it was reported that the unguided cracks formed during the CL formation process have an adverse impact on the durability, such as causing flooding at the cracks [179, 180]. Therefore, the cracks should be avoided in the CLs as much as possible during the drying process. Based on the previous research, the formation of cracks is primarily because of the excessively strong drying stress caused by capillary force or the swelling and then shrinking of the membrane. When the solvent molecules at the upper surface evaporate, the particles near the surface are no longer covered by a plane solvent film. Menisci develops between the particles along with the build-up of capillary force, causing capillary shrinkage [181]. If the capillary shrinkage is severe or imbalanced, the cracks will be formed to release the growing tensile stresses. Although the nucleation of cracks is not yet fully understood, there may be several factors promoting cracking. (i) Extremely quick evaporation of the solvent: Too fast solvent vaporization rate, e.g., the solvent with low boiling point, easily causes much difference in the drying speed between the surface and interior, resulting in the increased drying stress and then cracking in the dried CL [42, 66]. (ii) Weak interaction between the ionomer and catalyst [125, 175]: Good affinity between ionomer and catalyst enhances the adhesion strength between particles because ionomer acts as a binder, which could avoid cracking. Furthermore, the deformation of ionomer would dissipate the drying stresses in some degree due to the low Young's modulus of ionomer, decreasing the risk of cracking [74]. (iii) Defects: The defects in the catalyst ink film, e.g., small voids, bubbles, pinholes, uneven film, large catalyst-ionomer agglomerates, or self-organized free ionomer, could induce stress concentration, increasing the risk of cracking [24, 81, 125].