## 4 Catalyst Ink: Effect of Its Formulation on Its Microstructure and Macroscopic Properties
### Catalyst and Catalyst Content in the Ink  
In this review, the catalysts mainly refer to carbon-supported Pt-based catalysts, excluding non-precious group metal catalysts, as the former is the most promising for commercialization at this time. The structure and surface properties of catalysts and catalyst content influence the microstructure and macroscopic properties of a catalyst ink, e.g., the size of catalyst agglomerates, the ionomer coverage on Pt or carbon, viscosity, rheology, or optimal I/C ratio. 1. The surface area and structure of carbon support and the distribution or loading of Pt on carbon [74,84,85,86]: For Pt/Vulcan, the Vulcan carbon black is a solid carbon support; most Pt nanoparticles are on the external surface of Vulcan. However, for Pt/Ketjen, Ketjen carbon black is a porous carbon support and has a high surface area and abundant internal pore; only \(\sim\)50% or even less than 50% of Pt nanoparticles reside on the external surface [87,88][PERSON] Compared to Pt/Vulcan, Pt/Ketjen has a low Pt surface density. The ionomer exhibits a preferential interaction with Pt relative to carbon through the side chain [50,89][PERSON] Therefore, Pt/Vulcan has a more uniform ionomer coverage than Pt/Ketjen due to higher Pt surface density [53,59,60][PERSON] In the catalyst ink, the Pt/Vulcan-related agglomerates can be effectively stabilized by ionomer via the electro-steric mechanism[PERSON] The viscosity of Pt/Ketjen-based ink is generally higher than that of Pt/Vulcan-based ink [52,53][PERSON] However, [PERSON] et al. recently observed that increasing Pt loading on carbon would lead to reduced density and increased thickness of the adsorbed ionomer layer in the water/ethanol-based catalyst ink, due to the hydrophobic interaction between the main chain of ionomer and carbon [90]. Furthermore, the support [88,91] (e.g., nanopore volume [88]) or Pt loading on carbon [92,93] will impact the optimal I/C ratio; certainly, the optimal I/C ratio also depends on the type of ionomer (discussed more in Section 4.2), Pt loading in the electrode [94], or operation conditions (e.g., RH [95]). Moreover, pure carbon instead of Pt/C was usually used in some studies to investigate the effect of the structure and surface properties of carbon on the ink microstructure [74], which is improper since the surface properties of carbon may be modified during depositing Pt [53]. 2. The amount and charge type of functional groups on carbon [51,85,96]: For example, hydrophobic carbon tends to form large agglomerates in the polar solvent, e.g., water/alcohol mixed solvent, due to hydrophobic interaction [25]; in contrast, increasing the hydrophilicity of carbon through introducing plenty of functional groups tends to form small agglomerates in polar solvents due to an enhanced repulsion force [51]. To improve the dispersion of Pt on carbon, the pristine carbon materials are usually subjected to the oxidation treatment to increase the surface oxygen-containing groups. This makes the carbon surface negatively charged, increasing the columbic repulsive force with the negatively charged ionomer, which is adverse to the adsorption of ionomer on catalyst [97]. The equilibrium constant of ionomer adsorption on catalyst decreases with increasing the amount of the surface oxygen-containing groups [96]. If introducing -SO\({}_{3}\)H onto the carbon surface, a similar phenomenon will be incurred [51]. However, if the surface of the carbon in catalyst contains -NHx or N, the interaction between ionomer and catalyst will be enhanced due to the electrostatic attractive interaction, increasing the coverage and uniformity of ionomer on the catalyst. In brief, the catalyst's structure and surface properties would influence the interaction of catalyst! catalyst, catalyst! ionomer, or catalyst! solvent, which will impact the microstructure and macroscopic properties of the ink made of the catalyst and further the CL structure.