## 4 Catalyst Ink: Effect of Its Formulation on Its Microstructure and Macroscopic Properties
### Solvent
Furthermore, it is necessary to specially summarize the impact of the water/alcohol (methanol, ethanol, 1-propanol, or 2-propanol) mixed solvents most commonly used. The morphology, size, and conformation of the ionomer agglomerates dispersed will differ with the water fraction or the type of alcohol, altering the CLs' structure and their performance [25; 30; 32; 49; 57; 131; 137; 140]. While 1-propanol and 2-propanol have similar physical properties, the corresponding inks possess different microstructure or macroscopic properties likely due to the different symmetry of polarity, i.e., 1-propanol--non-symmetric polarity and 2-propanol--symmetric polarity. In the inks containing water/propanol mixed solvents with the same ratio, the ink containing water/1-propanol has smaller agglomerates than the case of water/2-propanol [57]. Furthermore, the ink containing 2-propanol exhibits a stronger shear-thinning behavior and a higher viscosity than that containing 1-propanol [57]. It was reported that cracks [57; 58; 141] or larger ionomer patches [49] are more likely formed in the CLs made of the ink containing water/2-propanol. 2. Viscosity: The solvent viscosity has an effect on the ink viscosity and stability, coating process, and drying process [42; 126]. The ink containing a viscous solvent is commonly more stable than that containing a thin solvent due to the decreased Brownian motion of agglomerates and thus decreasing their collision chance [42]. The ink for brush coating, blade coating, or print coating usually needs the relatively viscous solvent, while the ink for spray coating needs the solvent with low viscosity. Moreover, the solvent viscosity may impact the sedimentation and stacking of agglomerates during solvent evaporation due to the different fluidity in the solvents with various viscosity and then affect the pore size and pore volume of the CL. In the ink containing a thin solvent, agglomerates have a tendency to be packed densely, forming many small pores in the CLs, whereas in the ink containing a viscous solvent, agglomerates tend to be packed loosely due to the low fluidity, forming many pores but less small pores in CLs [126]. 3. Boiling point and vapor pressure: The higher the boiling point or the lower the vapor pressure, the smaller the solvent evaporation rate at the special temperature or RH, influencing the coating and drying processes. A fast evaporation rate, e.g., methanol, will make the ink unstable during coating process in which the catalyst ink is exposed to air, e.g., brush coating, screen printing, or gravure and flexographic printing. Fast evaporation could also lead to surface cracking in the dried CLs [42]. A low solvent evaporation rate could increase the ink stability during the coating process, increasing the drying time in return [66]. Sometimes it is difficult to completely remove the solvents with a high boiling point, e.g., glycerol [142], or N-methyl-2-pyrrolidone [143]. The residual solvent will block the pores of CLs, decreasing the gas and water transport. Therefore, it is necessary to select the solvents with an appropriate window of boiling point or vapor pressure to guarantee the sufficient stability of ink during the manufacturing process and the complete evaporation of the solvent. 4. Surface tension: The surface tension of solvent determines the surface tension of the ink to some extent. However, the addition of catalyst and ionomer will change the surface tension [33]. Therefore, the surface tension of the ink is determined by solvent, ionomer, and catalyst. Compared to modifying catalyst or ionomer, controlling the physical properties of solvent is an easier pathway and could provide much more opportunity and possibility to tailor the microstructure and macroscopic properties of the ink. The physical properties of the dispersion medium could be finely altered by varying the type of solvent or through using the mixed solvent and varying their ratio.