## 4 Catalyst Ink: Effect of Its Formulation on Its Microstructure and Macroscopic Properties
### Ionomer and I/C Ratio  
The ionomer is a critical constituent in CL, which contributes to (i) increasing proton conductivity from the active sites to the membrane, (ii) extending the TPB, (iii) holding catalyst particles together, and (iv) keeping the mechanical integrity of the CL during fabrication and operation. Perfluorinated sulfonic acid (PFSA) random copolymer is one class of the most commonly used ionomers in CLs. PFSA ionomers consist of a hydrophobic polytetrafluoroethylene main chain and hydrophilic side chains terminated with a sulfonic acid group as shown in Figure 7. Such architecture would induce the main chains to aggregate with side chains surrounding in the polar solvent. The physicochemical properties of PFSA ionomer could be modified by varying the side chain length or equivalent weight (EW, ionomer weight per mole of sulfonic acid groups, expressed as grams per mole) [106; 107], which influences the microstructure and macroscopic properties of the catalyst ink by impacting the interaction with catalyst or solvent. Furthermore, the I/C ratio has a vital effect on the microstructure and macroscopic properties of the catalyst ink and further significantly impacts the CL structure. 1. The type of ionomer: According to the side chain length, the ionomers can be categorized into long-side chain (LSC) ionomer, e.g., Nafion produced by DuPont or perfluorimide acid (PFIA) ionomer produced by 3M, and short-side chain (SSC) ionomer, e.g., PFSA ionomer produced by 3M and Aquivion ionomer produced by Solvay-Solexis (Figure 7). The commonly used commercial ionomer dispersion solutions and their related parameters are summarized in Table 1. According to the EW, the ionomers can be categorized into low, middle, and high EW ionomer. 3M PFIA  
Figure 6: Schematics of the distribution of ionomer on carbon and Pt nanoparticles in the conventional case (**left**), the selective distribution of ionomer on only carbon surface through masking Pt nanoparticles (**right**), as well as the oxygen/proton transport in both cases. (Reprinted from Ref. [103] with permission from the Royal Society of Chemistry). ionomer has multiple acid groups per side chain, while the others only have a single acid group at the end of each side chain. Hence, the 3M PFIA ionomer possesses a high ion exchange capacity (IEC). Since it is a newly developed ionomer, there are only a few studies exploring the properties and application of 3M PEIA ionomer [108; 109]; therefore, the LSC ionomers mainly refer to Nafion ionomer in this review. The EW of ionomer depends on the number (m) of -CF\({}_{2}\)- repeat units in the main chain and the molecular weight of the side chain (MW\({}_{\text{sc}}\)), i.e., EW = 50 m + MW\({}_{\text{sc}}\). For a specific EW, compared to a LSC ionomer, a SSC ionomer has a larger m value, i.e., a larger main chain fraction, resulting in higher crystallinity and thus higher glass transition temperature and enhanced thermal stability [110]. Hence, the SSC ionomer can be prepared with a low EW on the premise of the sufficient residual crystallinity (i.e., adequate stability upon water uptake rather than dissolving or becoming gelatinous), leading to higher proton conductivity. EW and the side chain length are two important parameters influencing the physicochemical properties of ionomer. Recently, [PERSON] et al. [111] found that EW is the major factor influencing the performance of PEM fuel cell at the high current region rather than side chain length through systematically analyzing the effect of the side chain length and EW of ionomers on the transport resistances of CL. According to the results [PERSON] et al. reported, the side chain length may determine the performance of PEM fuel cells at the low current region [107]. The better continuity and uniformity of SSC ionomer on Pt and carbon increase Pt utilization [107].