## 4 Catalyst Ink: Effect of Its Formulation on Its Microstructure and Macroscopic Properties
### Solvent
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Ionomer Dispersion** & **Content (wt.\%)** & **Solvent** & **EW (g/mol)** & **IEC (meq/g)** & **Viscosity (mP s, 25\({}^{+}\)C)** \\ \hline \multirow{8}{*}{Nafion PFSA} & D-520 & 5–5.4 & Water (45 wt.\%), 1-propanol (48 wt.\%) & & & 10–40 (40 s\({}^{-1}\)) \\ \cline{2-7}  & D-1020 & 10–12 & Water (87–90 wt.\%) & 890–970 & 1.03–1.12 & 2–10 \\ \cline{2-7}  & D-2020 & 20–22 & Water (34 wt.\%), 1-propanol (44 wt.\%) & & & 50–500 \\ \cline{2-7}  & D-521 & 5–5.4 & Water (45 wt.\%), 1-propanol (48 wt.\%) & & 10–40 \\ \cline{2-7}  & D-1021 & 10–12 & Water (87–90 wt.\%) & 970–1050 & 0.95–1.03 & 2–10 \\ \cline{2-7}  & D-2021 & 20–22 & Water (34 wt.\%), 1-propanol (44 wt.\%) & & & 50–500 \\ \hline \multirow{2}{*}{3M PFSA} & 725 EW & 15 & \multirow{2}{*}{725} & \multirow{2}{*}{1.38} & \multirow{2}{*}{50–300(20\({}^{-}\)C, 1 s\({}^{-1}\))} \\ \cline{2-7}  & 800 EW & 20 & & & 800 & 1.25 \\ \hline \multirow{2}{*}{3M PFSA} & \multirow{2}{*}{-} & \multirow{2}{*}{Powder} & \multirow{2}{*}{620} & \multirow{2}{*}{1.61} & \multirow{2}{*}{—} \\ \cline{2-7}  & & & & & 700–740 & 1.35–1.43 & \textless{}25 \\ \cline{2-7}  & D79–258B & 25 & & & 790 \(\pm\) 20 & 1.23–1.30 & \textless{}25 \\ \cline{2-7}  & D83-24B & 24 & \(>\)99\% water & 810–850 & 1.17–1.23 & 5–15 \\ \cline{2-7}  & D98–258B & 25 & 99\% water, free of ethers \% & 940–1020 & 0.98–1.06 & \textless{}25 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The commonly used commercial ionomer dispersion solutions and their related parameters [109; 121; 122; 123; 124]. parameters include (cal/cm\({}^{3}\))\({}^{1/2}\) (old) and MPa\({}^{1/2}\) (commonly used now), and 1 MPa\({}^{1/2}\) is equal to 2.0455 (cal/cm\({}^{3}\))\({}^{1/2}\)[129]. Hildebrand solubility parameter describes the global interaction, limiting its application, e.g., unsuitable for the case with polar or H-bonding interactions. In 1966, [PERSON] divided the total Hildebrand solubility parameter value into three parameters by taking the effects of dispersion force, polar force, and H-bonding into account, i.e., \(\delta_{D}\), \(\delta_{P}\), and \(\delta_{H}\)[129]. The relation of [PERSON] solubility parameter (\(\delta\)) and [PERSON] solubility parameter (\(\delta_{total}\)) can be described via Equation (3). \[\delta=\delta_{total}=\sqrt{\delta_{D}{}^{2}+\delta_{P}{}^{2}+\delta_{H}{}^{2}} \tag{3}\]  
The materials with similar values of \(\delta_{D}\), \(\delta_{P}\), and \(\delta_{H}\) are miscible [129]. Compared to Hildebrand solubility parameter, Hansen solubility parameter is a better indication of the interaction degree between polymer and solvent or predicting the polymer solubility in a solvent. In addition to pure solvents, the mixtures of two or more miscible solvents, e.g., the commonly used water/alcohol (1-propanol, 2-propanol, or ethanol) mixed solvents, are often used to disperse catalyst and ionomer as well. The physical properties of the dispersion medium will be easily modified by varying the ratio of the mixed solvents. The dielectric constant of the mixed solvents (\(\varepsilon_{mix}\)) can be calculated via Equations (4) and (5) [26,130]. \[\rho_{mix}=\sum x_{i}\varepsilon_{i}{}^{1/2}M_{i} \tag{4}\]  
\[\varepsilon_{mix}=\left(\frac{\rho_{mix}}{\sum x_{i}M_{i}}\right)^{2} \tag{5}\]  
where \(\rho_{mix}\) is the molar dielectric polarization of the mixed solvent, \(x_{i}\), \(\varepsilon_{i}\), and \(M_{i}\) are the molar fraction, dielectric constant, and molar mass of solvent \(i\), respectively. [PERSON] solubility parameter of the mixed solvent (\(\delta_{mix}\)) can be calculated via Equation (6) [26,130]. \[\delta_{mix}=\sqrt{\frac{\sum x_{i}E_{coh,i}}{\sum x_{i}V_{m,i}}} \tag{6}\]  
where \(E_{coh,i}\) and \(V_{m,i}\) are the cohesive energy and molar volume of solvent \(i\), respectively. [PERSON] solubility parameter of the mixed solvent (\(\delta_{total,mix}\)) can be calculated via Equations (7) and (8) [130].