## 6 CL Formation
### Coating Process
In the cone mode, the coulomb repulsive force will increase as the electric field intensity increases, which is beneficial for creating the fine droplets; therefore, the agglomerate size in the CL is reduced and a CL with more uniform pore distribution would be achieved [163]. [PERSON] et al. [67] found that the ionization mode of electrospray has an influence on the morphology of ionomer and then impacts the ionomer film covering catalysts. The CL fabricated by the electrospray with negative ionization mode shows a decreased thickness of ionomer film covering catalysts, which will reduce the local gas transport resistance through the ionomer film. 2. Roll-to-roll manufacturing process: Roll-to-roll manufacturing process is a continuous and scalable production method (Figure 12a). The CL fabrication speed would be increased by over 500 times more than the lab-scale spray coating when using the roll-to-roll coating method [165]. For the roll-to-roll method, catalyst ink is deposited on the substrate at once. Therefore, the ink needs to have a relatively high solid content and relatively high viscosity, different from spray coating, which commonly needs an ink with low solid content and low viscosity. GDE method and DTM are logically more suitable for roll-to-roll production than the CCM method as the GDL and decal substrates are more rigid than the membrane. Compared to decal film, the GDL is more appropriately used as the substrate for roll-to-roll manufacturing process from economic perspective [38; 39; 41]. As mentioned previously, it is often necessary to apply an extra ionomer overlayer on the surface of the CL to improve the interface contact of the CL and membrane when choosing the GDL as substrate. However, this will increase the process steps, decreasing the advantage of the GDE method for mass production. To avoid depositing an extra ionomer layer and improve the interface contact between the CL and membrane, [PERSON] et al. [41] studied the effect of ink coating techniques and drying temperature on the distribution of ionomer through the CL in order to directly create a CL with an ionomer-rich surface during the fabrication process. They observed that increasing the solvent evaporation rate could enrich the ionomer at the top of the CL (facing the membrane) during the roll-to-roll manufacturing process coupled with slot-die coating, improving the performance of the MEA [41]. Slot-die coating and gravure coating are widely used ink coating techniques of the roll-to-roll method (Figure 12b,c). For slot-die coating, the catalyst loading and the homogeneity of the coated ink film could be controlled by adjusting the solid content in ink, the gap height, or the feed ink volume [39]. For gravure coating, the thickness and quality of the CL deposited are affected by the cylinder rotation speed and engraved volume [38]. The engraved volume and the cylinder rotation speed provide a coarse and relatively fine control on the catalyst loading, respectively; the catalyst loading increases as the engraved volume or cylinder rotation speed increase [38]. Increasing the rotation speed or engraved volume also enhance the coating quality and avoid the formation of visible defects, e.g., pinholes or bubbles [38]. Furthermore, it was found that the penetration degree of catalyst into the pores or cracks of MPL could be reduced if using gravure coating due to the relatively high viscosity of the ink used [38]. The slot-die-coated or gravure-coated CLs show significantly larger porosity or thickness than the spray-coated CLs, because of the concentrated ink and the application of the ink at one time [38]. Compared to the gravure-coated CL, it was found that the slot-die-coated CL may have a higher Pt utilization and a better performance at various RH, especially at low RH [173].