## 4 Catalyst Ink: Effect of Its Formulation on Its Microstructure and Macroscopic Properties
### Ionomer and I/C Ratio
In addition to the enhanced thermal stability and proton conductivity, SSC ionomers commonly exhibit several advantages compared to Nafion ionomer, less sensitive to the change of solvent (e.g., the water content in dipropylene glycol (DPG) and water mixed solvents [112]), as well as increased main chain mobility, solubility, and dispersibility in the solvent, resulting in relatively weaker clustering tendency and smaller agglomerate size [113]. For example, the SSC ionomer with low EW (IEC = 1.43 meq/g) is better dispersed in polar solvents (e.g., a large ratio of water to alcohol) than the LSC ionomer with high EW (ion-exchange capacity 0.99 meq/g), due to the stronger interaction between sulfonic acid groups and water than alcohol [107]. Therefore, LSC and SSC ionomers might need different solvents or mixed solvents to achieve the optimal dispersion and conformation. In brief, the CLs containing SSC ionomer usually show improved and con  
Figure 7: The chemical structures of 3M PFIA ionomer, Nafion ionomer, 3M PFSA ionomer, and Aquivion ionomer [108; 112]. nected secondary pores, uniform and continuous ionomer coverage on the catalyst, and enhanced ionomer distribution, leading to higher Pt utilization efficiency and improved mass transport [106; 107; 113; 114]. The corresponding PEM fuel cells usually exhibit increased performance, particularly at mass transport polarization region, enhanced durability, and less sensitivity to operation conditions, e.g., RH, operation pressure, or temperature (especially under high operation temperature and low or medium RH due to the effective water trapping ability of the SSC ionomer that usually possesses a low EW) [106; 107; 113; 114; 115; 116]. 2. I/C ratio: If adding a small amount of ionomer in the catalyst ink, i.e., a low I/C ratio, the ionomer molecules adsorb on the catalyst agglomerates, enhancing the columbic repulsion force and steric hindrance of inter-agglomerates [42; 53; 74], inhibiting aggregation, to some extent similar to a stabilizing agent, decreasing the agglomerate size, and thus improving the colloidal stability [53; 54]. When the I/C ratio reaches a certain value, increasing the content of ionomer in the catalyst ink further will not contribute to the size of agglomerates or increase the colloidal stability [34]. It has been reported that the ionomer adsorption on carbon or catalyst follows the Langmuir isotherm for both SSC and LSC ionomer when the ionomer concentration is less than a certain value (at least 1.0 I/C [54]), i.e., there is an adsorption plateau, where the absorbed ionomer content barely increases with increasing I/C ratio [27; 54; 74]. The amount of the absorbed ionomer on catalyst commonly depends on the surface properties of the catalyst [117], EW of ionomer [27], or the dispersion medium [118]. Moreover, I/C ratio impacts the viscosity of catalyst ink; however, the influencing rule alters with the type of catalyst. For example, in the case of Pt/Vulcan, the ink viscosity slightly decreases (I/C ratio \(<\) 0.2) and then increases with the increase in I/C ratio, whereas in the case of Pt/Kejten, the ink viscosity increases over I/C ratio because ionomer flocculates the agglomerates [53]. The I/C ratio in the catalyst ink is a critical parameter impacting the performance and durability of PEM fuel cells, which generally needs to be optimized for different cases to increase Pt utilization and minimize the mass transport resistances. If the I/C ratio is too low to form a continuous network for effective proton transport, the accessibility of Pt to proton is insufficient, decreasing TPB. If the I/C ratio is too high to block the pores of the CL, the blocked pores will hinder the gas and water transport, causing increased local and bulk gas transport resistance or flooding [119].