## 5 Catalyst Ink: Effect of Its Preparation on Its Microstructure and Macroscopic Properties
### Dispersion Process  
When adding a catalyst and ionomer into a solvent, the catalyst and ionomer tend to remain as large agglomerates, and the interaction between them is weak. Therefore, the ink dispersion process is necessary and determines the size and size distribution of agglomerates and interaction between catalyst and ionomer, as well as the reproducibility, viscosity, thixotropy, or stability of the ink [29; 156; 157; 158]. The main purpose for the ink dispersion process is to break down the initial large agglomerates, homogeneously disperse catalyst and ionomer in the solvent, and boost the interaction between catalyst and ionomer, which are essential for enhancing the Pt utilization. To date, the typical dispersion methods include ball milling, mechanical stirring, and ultrasonication (tip sonication or bath sonication). Ultrasonication and ball milling are the most widely used ink dispersion methods. For the ink with low viscosity or low solid concentration, tip sonication, bath sonication, or their combination, and ball milling are applicable; however, for a viscous ink or an ink with high solid concentration, ball milling may be more suitable, e.g., ca. 10 wt.% solid concentration [24; 28]. Furthermore, the hydrodynamic cavitation method [24], the planetary mixer [154], or high-pressure homogenization [158] have been applied to disperse catalyst ink recently. Ball milling, mechanical stirring, and ultrasonication as well as their pros and cons have been well summarized in a recent review [29]. This review only pays attention to the effects of the dispersion process on the microstructure and macroscopic properties of the ink and hence the resulting CLs. 1. The effect on the ink microstructure: Appropriate dispersion process could effectively break up the large agglomerates, reduce the size of agglomerates, and improve the dispersion of catalyst and ionomer in solvent, which will maximize TPB, avoid the formation of cracks, as well as improve the pore volume of CL and thus the mass transport [24; 156] (Figure 9b,e). However, if the dispersion process is insufficient, the large agglomerates in the catalyst ink cannot be broken down, leading to an inhomogeneous CL or even causing cracking in the CL (Figure 9a,d) [24; 154]. The large agglomerates in the CL would increase the proton, gas, and water transport resistance. Ionomer molecules mainly adsorb on the exterior surface of the catalyst agglomerates and barely penetrate the catalyst agglomerates, especially the large catalyst agglomerates [156]. Therefore, the Pt inside the large agglomerates cannot access ionomer, increasing the proton transport resistance [156]. Due to the low exteriorsurface-to-volume ratio, a thick ionomer film may be formed outside the large catalyst agglomerates, increasing the gas transport resistance through the ionomer film [156]. Further, the long gas transport distance in the interior of the large agglomerates also has an adverse effect on the gas and water transport [156]. 2 The effect on the reproducibility, viscosity, or thixotropy of ink: The insufficient dispersion process decreases the reproducibility of the ink and hence the CLs made of the ink [159]. As mentioned in Section 3.2, the ink viscosity relies on its microstructure, e.g., the size of agglomerates. Therefore, the dispersion process influences the ink viscosity. For example, the large agglomerates in the catalyst ink are dispersed as the ball milling time or the sonication amplitude increase, increasing the ink viscosity [28]. When the agglomerates are broken down to a certain size or reach a certain hardness, they are not dispersed further as the ball milling time or the sonication amplitude increase; thus, the ink viscosity no longer changes. The viscosity of the ink prepared by ball milling exceeds the case of ultrasonication due to the better dispersion degree [28].