## 1 Introduction
The commonly used substrates include gas diffusion layer (GDL, carbon paper/cloth coated with microporous layer (MPL)), membrane, or decal film. The catalyst ink, substrate, coating process, and solvent evaporation should be elaborately adjusted to form an adherent and continuous CL [35; 36]. For example, different coating techniques have different demands on the viscosity or the choice of solvent [37; 38]. For spray coating, the ink with low solid content, hence low viscosity, is necessary to avoid the blockage of the spraying nozzle [36]. However, for screen printing, slot-die coating, or gravure coating, the viscosity of the catalyst ink needs to be increased [38; 39]. After solvent evaporation, the deposited ink film evolves into a CL. During the solvent evaporation process, the movement, aggregation, or self-assembly of the agglomerates in the ink film occur, determining the final structure of the CL [40; 41]. The solvent evaporation rate is  
Figure 1: Material–structure–properties relation of the CL (T: temperature, RH: relative humidity, P: pressure, ECSA: electrochemical active surface area). primarily impacted by the solvent properties or the drying methods and conditions, e.g., the boiling point and vapor pressure of solvent, temperature, or relative humidity (RH) [40; 41; 42]. Over the last 30 years, considerable effort has been devoted to catalyst ink engineering, development of novel coating methods, as well as optimization of the coating parameters and drying process, to maximize the TPB and minimize the mass transport resistance in CLs. However, the relationship of the recipe and preparation process of catalyst ink or the CL formation process and the multiscale structure of CL and further the performance and durability of PEM fuel cells is still unclear and even inconsistent in some cases. For the time being, there have been some reviews focused on the optimization of the formulation and dispersion techniques of catalyst ink, the characterization and modeling of catalyst ink, the coating techniques, or the underlying interactions between solvent, ionomer, and catalyst [29; 35; 43]. However, there are few reviews focusing on the relationship between the composition and preparation process of the catalyst ink or the CL formation process and the multiscale structure of CL and further the performance and durability of PEM fuel cells. The relation between the multiscale structure of CL and the performance and durability of PEM fuel cells has been summarized in our previous review [44]; hence, this review focuses only on the effect of the formulation and preparation of catalyst ink on its microstructure and macroscopic properties and further on the CL structure as well as the effect of the CL formation process on the multiscale structure of CL. At first, brief overviews on the multiscale structure of CL and the microstructure and macroscopic properties of catalyst ink are presented. Then, it proceeds to the effect of catalyst type, ionomer, and solvent, or their ratios, as well as the order of ingredient mixing and dispersion process on the microstructure and macroscopic properties of catalyst ink and further on the multiscale structure of CL. After this, the CL formation process and its effect on the formation, morphology, or structure of CL are reviewed, including the selection of substrate and coating and drying processes. Finally, several perspectives on future research and development in this area are provided.