## 6 CL Formation
This situation will be more serious when the MPL's surface roughness increases [166]. To improve the interface between the CL and membrane and reduce the interfacial resistance, coating an extra ionomer layer on the top of the GDE was often applied [166; 167]. It has been found that the critical ionomer loading increases as the surface roughness of MPLs increases [166]. Further, the hot press is also necessary for fabricating the GDE-based MEAs, which is not necessary for the CCM-based MEAs [167]. Furthermore, catalyst particles may penetrate the pores or cracks of MPL, reducing the catalyst activation and thereby the performance of MEA. The penetration degree depends on the deposition method due to the difference in the viscosity of the used ink or the drying rates for various coating techniques [38]. 3. Decal transfer method (DTM): CL is deposited on an inert decal substrate first and then transferred to the membrane through the hot press, achieving the CCM, known as DTM (Figure 10c). This method was developed to overcome the drawbacks of the CCM method by avoiding the direct contact of the membrane with the catalyst ink. Although the surface of the decal substrate is similar to the membrane, the CL surface facing the membrane is still uneven but has a lower roughness than that produced by the GDE method [75]. From an economy perspective, direct coating (CCM and GDE) is advantageous; however, decal transfer is not desirable due to the increased production cost caused by the transfer process, the usage of decal substrate, and the waste of catalyst and ionomer due to the incomplete transfer caused by the inappropriate process parameters. To increase the transfer yield and decrease the hot-press temperature simultaneously, considerable efforts have been devoted, e.g., using swelling agents to pre-treat the CL before the hot press [168], pre-depositing a breaking layer composed of carbon, or a carbon and Nafion mixture onto the decal substrate [169], selecting appropriate decal substrate [164], or depositing an extra ionomer layer on the top of CL before the hot press [164]. Nevertheless, it is difficult to completely remove the swelling agents since they usually have a high boiling point, e.g., 1,5-pentanediol [168]; the break layer will increase the interfacial resistance between the CL and GDL. [PERSON] et al. [164] developed an effective low-temperature decal transfer method with complete transfer yield and the improved interfacial contact between the CL and membrane, through investigating the type of decal substrates, hot-pressing condition, as well as an extra Nafion layer on the top of CL (faces the membrane) and the Nafion ionomer loading. It was found that fluorinated ethylene propylene film is a good decal substrate and superior to the commonly used polytetrafluoroethylene film due to its low friction coefficient and low contact angle. If depositing an outer Nafion layer with a Nafion loading of 0.2 mg/cm\({}^{2}\) on the CL before the hot press, the CL can be completely transferred to the membrane at a low hot-pressing temperature of 130 \({}^{\circ}\)C, and the extra Nafion layer could improve the interfacial contact between the CL and membrane, which is similar to the case of GDE. Figure 11: Stylus profilometry traces of Nafion membrane (**a1**), CL deposited on membrane (the surface facing the GDL) (**a2**), GDL (the surface of MPL facing CL) (**b1**), and CL of GDE (the surface facing the membrane) (**b2**) (Reprinted from [75] with permission of John Wiley & Sons); (**c,d**) SEM images of cross-section view of the MEA. Cathode: a good CL/membrane interface due to the ionomer overlayer deposited on GDE; Anode: the observed gaps at the interface of MPL and CL which was deposited on the membrane, due to the high MPL surface roughness (root mean square surface roughness: 15.85 \(\upmu\)m) (Reprinted with permission from [166]. Copyright (2019) American Chemical Society). The CCM-based MEAs, GDE-based MEAs, and DTM-based MEAs generally have different performances due to the different substrates used [167].