## 7 Future Direction and Prospects  
To supply a required performance, rapid electron, proton, gas, and water transport are necessary in the networks of the CL composed of Pt/C, ionomer, and pore. This is closely related to the CL multiscale structure, which is formed during the fabrication process and thus determined by the composition and preparation process of the catalyst ink and the CL formation process. After continuous research and development, the effect of the formulation and preparation processes of catalyst ink on its microstructure and macroscopic properties and then on the CL structure as well as the effect of the substate, coating process, and drying process on the formation, morphology, and structure of the CL has been understood to some degree. However, for the time being, the relationship between the formulation and preparation processes of catalyst ink or the CL formation process and the multiscale structure of CL and further the performance and durability of PEM fuel cell is still unclear. Much more effort should be devoted to thoroughly clear this relationship through investigating the effect of the composition and preparation process of the catalyst ink, the substrate, coating process, and drying process on the morphology and structure of the CL. (1) The catalyst ink microstructure: To uncover how the composition and preparation process of catalyst ink affect the structure of the CL made of the ink, it is necessary to accurately characterize the ink microstructure. The characterization techniques used in the literature have been summarized in Section 3.1. However, because of the opaque, dynamic, and complicated properties, it is difficult to directly characterize the ink microstructure. To date, the characterization techniques used to measure the ink microstructure commonly need to modify its initial concentration, e.g., dilution (e.g., DLS or ELS) or consolidation (e.g., TEM and SEM). Furthermore, some techniques only measure the local microstructure, e.g., TEM or SEM. The results cannot truly assess the microstructure of catalyst ink, a complex system, or are even misleading at times. Hence, it is necessary to introduce or develop some advanced characterization techniques to accurately evaluate the microstructure of catalyst ink without any modification, e.g., XCT, SAS, or CV-SANS. Furthermore, since the macroscopic properties of a catalyst ink are usually determined by its microstructure, the catalyst ink microstructure could be coarsely inferred through testing its macroscopic properties, e.g., the viscosity or rheologic behavior. The microstructure of catalyst ink is primarily governed by the interaction between catalyst, ionomer, and solvent, which could be studied by model and numerical simulations, e.g., molecular dynamic simulation [118] or discrete element method [40]. Therefore, combining the modeling and physical characterization of the ink might allow us understanding the ink microstructure, to optimize the ink formulation and dispersion process. 2. Inconsistent conclusions reported in the literature: Because the microstructure and macroscopic properties of catalyst ink are determined not only by the types of catalyst, ionomer, and solvent(s) as well as the ratio between them, but also by the preparation process. If only varying one of the variables, such as merely changing catalyst or using different dispersion methods, the prepared catalyst inks may show different microstructure or macroscopic properties. The morphology and structure of the CL made from catalyst ink are co-governed by the microstructure or macroscopic properties of the catalyst ink, the substrate, coating process, and drying process. This makes the influence of the composition and preparation of catalyst ink on the multiscale structure of the CL extremely complicated and increases the level of difficulty uncovering their relationship. Finally, the CL structure, hot-pressing process, and the fuel cell assembly will co-determine the performance and durability of PEM fuel cell [167; 170].