## 4 Catalyst Ink: Effect of Its Formulation on Its Microstructure and Macroscopic Properties
### Solvent
\[\delta_{j,mix}=\sum\varphi_{i}\delta_{j,i}\qquad(j=D,\ P,\ H) \tag{7}\]  
\[\delta_{total,mix}=\sqrt{\delta_{D,mix}{}^{2}+\delta_{P,mix}{}^{2}+\delta_{H, mix}{}^{2}} \tag{8}\]  
where \(\delta_{j,i}\) is \(\delta_{D}\), \(\delta_{P}\), or \(\delta_{H}\) of solvent \(i\), \(\delta_{j,\ mix}\) is \(\delta_{D}\), \(\delta_{P}\), or \(\delta_{H}\) of the mixed solvent. If mixing the solvents with different surface tensions, the surface tension of the mixed solvent is commonly close to the lowest one, as the solvent molecules with low surface tension tend to accumulate at the surface of the mixed solvent. For example, the surface tension value of diacetone alcohol (DAA) is 31 mN/m, lower than that of water (72 mN/m); if mixing DAA into water or even adding only 5 wt.% DAA, the surface tension significantly declines and approaches the value of DAA since DAA molecules favor the solvent/air interface [130]. Hereafter, pure solvents and mixed solvents are collectively called solvents unless otherwise specified. The recent development on the effect of the solvent physical properties on the microstructure and macroscopic properties of the catalyst ink by impacting the interactions between catalyst, ionomer, and solvent is summarized here. 1. Dielectric constant and solubility parameter: The dielectric constant and solubility parameters of a solvent primarily influence the dispersion of catalyst and ionomer or the conformation of ionomer (absorbed ionomer and non-absorbed ionomer) [31; 131; 132]. As the catalyst agglomerates are commonly covered by ionomer in the catalyst ink, solvent affects the dispersion or dispersion stability of catalyst likely through impacting the ionomer coverage on catalyst or the conformation of the adsorbed ionomer [34]. Therefore, it should be mainly through impacting the interaction between solvent and ionomer that the solvent influences the microstructure and macroscopic properties of the ink [127]. Consequently, the studies on the dielectric constant and solubility parameter of solvent primarily focus on the dispersion and conformation of ionomer. Water or water/1-propanol mixed solvent, commonly used as the solvent of the commercial ionomer dispersion solutions as shown in Table 1, are not the good solvent, or the ratio of water and 1-propanol is not optimal. Therefore, to improve the dispersion and conformation of ionomer in catalyst ink, the solvent often needs to be elaborately selected. As described in Section 4.2, the ionomer is composed of main chain and side chains. The main chain is hydrophobic and nonpolar. The side chain is hydrophilic, polar and the terminal -SO3H group can be dissociated into -SO3- and H+ in the protic solvent, e.g., water. It has been reported that Nafion possesses dual solubility parameters, i.e., \(\delta\)1 = 9.7 (cal/cm3)1/2 for main chain and \(\delta\)2 = 17.3 (cal/cm3)1/2 for side chain [133]. Since the main chain and side chain are incompatible, the solvents with different dielectric constant and solubility parameter have different compatibility with the main chain and side chain, resulting in the ionomer agglomerates with different morphology, size, and conformation [26; 32; 134]. When dispersed in the various solvents, Nafion agglomerates with different morphology or size have been observed according to the polarization losses of the fabricated CLs, TEM, DLS, SANS, SAXS, 19F NMR, or molecular dynamic simulation. For example, the Nafion morphology may form a solution (\(\varepsilon>10\)), a colloid (\(3<\varepsilon<10\)), or a precipitate (\(\varepsilon<3\)) in some organic solvents with different dielectric constants [135]. Such morphology variation with a dielectric constant is unsuitable for all cases because the dielectric constant is commonly not the single factor involved.