## 6 CL Formation
### Drying Process  
If the formulation and preparation processes of the catalyst ink, as well as the coating process are fixed, the drying process of the ink film deposited on the substrate will determine the morphology and structure of the CL. There are several influencing factors that should be considered, e.g., the drying techniques, evaporation conditions, solvent physical properties, or solid concentration in the ink. The drying methods that have been used to dry the CLs previously include freeze drying, vacuum drying, heated substrate drying, oven drying, or forced-air oven drying [41; 174]. It was reported that compared to the vacuum-dried CL or oven-dried CL, the freeze-dried CL shows the improved porosity and ionomer distribution, enhancing the gas transport [174]. However, the freeze drying technique is not wildly used to dry the CL since the drying process is complicated, costly, and dangerous due to the usage of liquid nitrogen. There are primarily two major factors influencing the evaporation rate, i.e., the physical properties of the solvent used (the boiling point and vapor pressure, as discussed previously) and the evaporation condition (temperature, ambient pressure, or RH). The solid concentration of catalyst ink also has an influence on the drying dynamic and thus leads to the different microstructures of the dried CLs. When dripping a droplet of ink (1 \(\upmu\)L) onto the substrate, the ink with a relatively high solid concentration (e.g., 1.59 wt.%) has a smaller contact area (spherical crown, diameter 6 mm, height 70.3 \(\upmu\)m) on the substrate, resulting in a smaller surface exposed to air and hence a lower solvent evaporation rate (total evaporation time 28 s), than that with a low solid concentration (e.g., 0.2 wt.%, spherical crown, diameter 8.3 mm, height 37 \(\upmu\)m, total evaporation time 18 s). After the solvent is completely evaporated, the resulting CL surface  
Figure 12: (**a**) Schematic of the roll-to-roll manufacturing process coupled with slot-die coating, (Reprinted from [165], Copyright (2020), with permission from Elsevier), (**b**) illustration of slot-die coating (Adapted from [39], Copyright (2019), with permission from Elsevier), (**c**) illustration of gravure coating (Adapted from [38]). becomes rougher and the distribution of catalyst and ionomer in the resulting CLs becomes more inhomogeneous with decreasing the solid concentration [40]. Although the structure evolution of CLs during the drying process is still unclear, there are some processes possibly dictating the distribution of materials or governing the formation of the CL structure, e.g., solvent evaporation, internal particles flow (or migration) in the ink film due to concentration and temperature gradients induced by solvent evaporation, sedimentation resulting from gravitational force, agglomeration, assembly, packing, or shrinking [41; 175; 176]. To understand the drying process, [PERSON] et al. monitored the drying process through combining in-situ visualization using atmospheric SEM with weight test and studied the effect of drying rate (drying temperature or humidity) on the porosity and pore size of the CLs [56]. It was found that there are two packing steps occurring in the ink film with solvent evaporation, i.e., sedimentation, and then shrinking. In the initial period, the solid concentration in the ink film increases and the ink film thickness decreases rapidly over evaporation [56] (Figure 13). When the solid concentration reaches a critical value, the particles in the ink film contact each other and form a network, i.e., a solvent-saturated CL, and then the drying process enters the second stage. The porous structure is formed gradually, and the thickness of CL slightly decreases over solvent evaporation in the second period, which needs a longer time than the first stage [56] (Figure 13). During this stage, the capillary force works between the particles near the drying front that moves from the surface to the inside during evaporation.