## 8 Concluding Remarks  
Not only the materials used in the CL, but also its structure, play a significant role in the improvement of the performance and durability of PEM fuel cells and consequently lead to cost reduction. The CL structure determines the transport rate and transport probability of proton, electron, gas, and water to or from the active sites. The CL is a complicated system, composed of the continuous and intersecting networks of catalyst (Pt/C), ionomer, and pore, and possesses a multiscale structure, from microscale to mesoscale and then to macroscale. The CL is commonly fabricated through an ink-based process, i.e., homogeneously dispersing catalyst and ionomer in solvent(s), obtaining the catalyst ink, and then coating catalyst ink on a substrate, finally forming a CL after solvent evaporation. The structure of CL is formed during the manufacturing process and is dependent on the microstructure and macroscale properties of the catalyst ink, the substrate, coating process, and drying process. According to the results reported in the literature, the commonly used catalyst ink is composed of the primary catalyst agglomerates surrounded by ionomer, non-absorbed primary ionomer agglomerates, and secondary agglomerates. It is these agglomeratesthat aggregate, assemble, and pack during the drying process, forming the CL. Therefore, their size, size distribution, and shape, as well as the absorbed ionomer content on the catalyst agglomerates and the interface or interaction between catalyst and ionomer in catalyst ink will govern the CL structure. The macroscopic properties of catalyst ink include viscosity, rheology, thixotropy, surface tension, and stability, which will impact the selection of coating techniques, coating speed, coating quality and reproducibility, as well as the uniformity/integrity, thickness and wettability of the deposited ink film, eventually influencing the multiscale structure of CL. The microstructure and macroscopic properties of catalyst ink are sensitive to the formulation and preparation processes. The former includes the types of catalyst, ionomer, and solvent(s) and their ratios; the latter includes the order of ingredient mixing and the dispersion process. The formulation influences the microstructure and macroscopic properties of catalyst ink mainly via affecting the underlying interactions between the individuals. The order of ingredient mixing determines the interaction sequence of the catalyst, ionomer, and solvent, which has an effect on the agglomerate size or the material stability. The dispersion process needs to effectively break up the large agglomerates into well-desired sizes and cannot degrade or decompose the materials. In addition to the catalyst ink, the CL formation process influences the morphology and structure of CL as well, e.g., thickness, uniformity, porosity, cracking, surface roughness, or ionomer distribution. To achieve a well-defined CL, the macroscopic properties of the prepared ink, the structure and physical properties of the substrates, coating methods and coating conditions, as well as drying methods and drying conditions should be rationally controlled and match each other. Based on the selected substrate, there are three pathways to fabricating MEAs, i.e., CCM, GDE, and DTM. Each of them has pros and cons. Compared to CCM and DTM, GDE may be more suitable for application in mass production. Among a variety of coating techniques, spray coating is more suitable for preparing the CL used to assess the performance of the advanced materials in the lab because of the relatively high uniformity of spray-coated CLs and the relatively precise control of catalyst loading. Compared to spray coating, the roll-to roll manufacturing process coupled with slot-die coating or gravure coating shows the advantage in the continuous, scalable production of MEAs.