## 6 CL Formation  
In addition to the composition and preparation process of the catalyst ink, the CL formation process, namely, depositing catalyst ink onto a substrate by ink coating techniques and then drying to form a CL, also regulates the morphology and structure of CL, e.g., thickness [162], uniformity [163], porosity [38; 162], cracking [104], surface roughness [38], ionomer distribution [41]; thereby impacting the performance and durability of PEM fuel cell. To fabricate a well-desired CL, the macroscopic properties of the pre  
Figure 9: The structures of Pt/C-Nafion ionomer and CL with insufficient (**a**,**d**), appropriate (**b**,**e**), and excessive (**c**,**f**) dispersion. (**(**a**–**c**) Modified with permission from [156], Copyright (2019), American Chemical Society; (**d**–**e**) Reprinted with permission from [24], Copyright (2019), American Chemical Society; (**f**) Reprinted from [159], Copyright (2014), with permission from Elsevier). 1. Catalyst coated membrane (CCM) method: Catalyst ink is deposited onto both sides of the membrane, directly forming CLs on the membrane, known as CCM. The MEA is achieved through hot-pressing the CCM sandwiched between two GDLs (Figure 10a). (Industrially, GDLs may be glued to the edges of a frame and then CCM is sandwiched between two GDLs when assembling the fuel cell stack. Contact of CCM with GDLs is realized by stack compression.) The MEA produced by the CCM method commonly has an excellent interfacial contact between the CL and membrane and a higher Pt utilization due to directly applying the CL onto the membrane, resulting in higher power and durability. Membrane swelling or wrinkling occurs during the coating process, due to direct contact with the solvent in the ink. This can be mitigated to some extent a small or moderate scale fabrication (e.g., in the lab-scale fabrication or in the moderate production where many companies use CCM method at present) by mechanically fixing the membrane (e.g., on a vacuum platform or in a die) or shorten the interaction time between the membrane and solvent via heating and selecting spray coating to deposit the ink. However, although the membrane swelling and the solvent penetration could be decreased through selecting the appropriate solvent [165], it is still a challenge to directly deposit the ink onto the membrane in a continuous, scale production, e.g., roll-to-roll manufacturing process. This is attributed to the difficulty to mechanically stabilize the membrane during the coating process unlike the case of lab-scale fabrication. Moreover, the catalyst ink is applied onto the membrane at once during the roll-to-roll coating, which means a long interaction time between the membrane and solvent before complete evaporation. With the improvement of the membranes, the challenges CCM method faces in the roll-to-roll manufacturing process may be solved in the future. 2. Gas diffusion electrode (GDE) method: Catalyst ink is deposited onto the MPL, directly forming the CL on a GDL, obtaining a GDE. The MEA is achieved through hot-pressing the membrane sandwiched between two GDEs (Figure 10b). The membrane swelling or wrinkling could be avoided for the GDE method due to indirect contact with the solvent. However, the GDE method typically offers a limited interfacial contact between the CL and membrane compared to the CCM method, likely resulting from the difference in the surface roughness of MPL and membrane. The MPL has a higher origin surface roughness than the membrane (Figure 11(a1,b1)) [75]. When applying the CL onto the membrane, the CL surface facing the membrane is similar to the membrane surface due to the nearly conformal deposition, forming an excellent contact between the CL and membrane. When applying CL onto MPL, the CL surface facing the membrane is uneven and has an increased roughness compared to the MPL (Figure 11(b2)), making it hard for the membrane to contact the entire CL surface and thus causing gaps (similar to the case shown in Figure 11c,d) [166].