## 4 Catalyst Ink: Effect of Its Formulation on Its Microstructure and Macroscopic Properties
### Solvent
The Nafion ionomer is likely to self-assemble into a variety of morphologies with different sizes in different dispersion media (Figure 8b), e.g., cylindrical agglomerates with a diameter of 2-5 nm (in glycerol, ethylene glycol (EG) or water/1-propanol mixed solvents with different 1-propanol fractions) [128; 136; 137], less-defined and highly solvated agglomerates with the diameter of \(>\)200 nm (in water/2-propanol mixed solvents with different 2-propanol fractions) [128], random-coil agglomerates with a radii of gyration of \(\sim\)4.1 nm similar to a solution behavior (in N-methylpyrrolidone) [128], or secondary agglomerates due to the hydrogen bond between -SO3- and H3O* (i.e., SO3-\(\cdot\cdot\cdot\)H3O*\(\cdot\cdot\cdot\cdot\)O3S*, in water/1-propanol with 20 wt.% 1-propanol) [26]. The morphology and size of the ionomer agglomerates would affect the ionomer coverage on the catalyst agglomerates or the porosity of CLs. It was believed that the colloid ionomer with fine dispersion or specific size adsorbs on catalyst agglomerates, resulting in the formation of the slightly larger agglomerates, which improves the CL microstructure and enhances the porosity and then the gas transport, further the fuel cell performance [23; 25; 132; 138; 139]. In brief, although the relation between both is still unclear, the physical properties of solvent have a major effect on the morphology and size of the dispersed ionomer. In addition to the morphology and size, the conformation of the ionomer in ink also depends on the dielectric constant and solubility parameter of solvents due to the different affinity of the main chain and side chain with solvent molecules. Due to the high affinity with water molecules, the side chains of Nafion tend to be extended out in the water-rich solvent mixtures, which would increase the coulomb repulsion force between Nafion agglomerates, or between catalyst-Nafion agglomerates, or between both, enhancing the colloidal stability of the ink, whereas the main chains tend to aggregate inside due to the low affinity with the polar water molecules [32]. If the solvent is extremely incompatible with the main chain, e.g., pure water, the Nafion will be poorly dispersed, forming large ionomer agglomerates and resulting in weak interaction with catalyst agglomerates, even losing the binder function and making it difficult to fabricate a complete CL [49]. If the solvent has excellent compatibility with the main chain, e.g., dipropylene glycol [23], the main chains tend to be extended out, and the Nafion ionomer commonly appears to be well dispersed, e.g., the decreased ionomer agglomerate size or approaching solution behavior [32; 128]. Furthermore, the ionomer conformation will influence the interface between ionomer and catalyst, including the coverage of ionomer on carbon or Pt, the thickness of ionomer film, and the interaction between ionomer and Pt [22; 60]. These would affect the local gas transport resistance through the ionomer layer or the interfacial resistance between the ionomer film and Pt [60]. To date, the studies on the effect of solvents on the dispersion and conformation of ionomer mainly focus on Nafion ionomer. However, the results derived from Nafion might not be applicable to the SSC ionomer in some cases, due to the different distance between the sulfonic acid groups and main chain, which likely results in different conformational orientations in the similar dispersion medium. Recently, [PERSON] et al. studied the effect of Hansen solubility parameters of water/diacetone alcohol mixed solvents on the dispersion and conformation of SSC ionomer (Aquivion D79-25 BS). It was found that the Aquivion ionomer always forms solutions in all water/diacetone alcohol mixed solvents, and \(\delta_{\rm H}\) is responsible for the varied affinity between ionomer and solvent with different diacetone alcohol fractions, and the mobility of the main chain declines with increasing diacetone alcohol fraction, different from the case for Nafion [130].