## 3 Microstructure and Macroscopic Properties of Catalyst Ink  
### Microstructure of Catalyst Ink  
The CL is generally fabricated by a catalyst ink-based process, as described in the Section 1. A state-of-the-art catalyst ink is often prepared by uniformly mixing catalyst powder, ionomer, and solvent. Although being effectively dispersed, there still exist some undispersed primary and secondary agglomerates due to the complex interplay of interactions between catalyst, ionomer, and solvent. Taking the water/alcohol (1-propanol, isopropanol, or ethanol)-based catalyst ink commonly used as an example, primary catalyst agglomerates surrounded by ionomer (i.e., primary catalyst-ionomer agglomerates), non-absorbed primary ionomer agglomerates, and secondary agglomerates formed by primary agglomerates [21; 51; 52; 53; 54; 55] (Figure 3) may exist. During the coating and drying process, these primary and secondary agglomerates assemble into a porous structure, forming the multiscale structure of CL [56]. Therefore, the multiscale structure of CL and hence its performance and durability would be governed by the size, size distribution, and shape of the various agglomerates, the absorbed ionomer content on the catalyst, or the interface of catalyst! ionomer in the catalyst ink, which are sensitive to the ink formulation and dispersion process [49; 57; 58; 59; 60] (Figure 4).  
To thoroughly investigate the microstructure of the catalyst ink, there are various characterization techniques applied, e.g., Dynamic Light Scattering (DLS), Small Angle Scattering (SAS), \({}^{19}\)F Nuclear Magnetic Resonance (\({}^{19}\)F NMR, always used to investigate the morphology of ionomer in solvent), Transmission Electron Microscope (TEM), Scanning Electron Microscope (SEM), Cryogenic TEM (Cryo-TEM), Cryo-SEM, X-ray computed tomography (X-CT) [26; 61; 62; 63; 64; 65]. The SAS includes Small Angle Neutron Scattering (SANS) and Small Angle X-ray Scattering (SAXS), as well as ultra-small-, and wide-angle X-ray scattering, contrast variation SANS (CV-SANS). The DLS is often used to measure the hydrodynamic radius of the primary and secondary agglomerates in the catalyst ink. One of its primary drawbacks is that the ink needs to be diluted to avoid excessive scattering and guarantee sufficient accuracy, which would lead to a few discrepancies between the DLS results and the real case due to the different concentration scale. [PERSON] et al. found that the decrease in the primary and secondary agglomerates size resulting from dilution approaches 20%, through studying the ink with and without dilution by using a recent DLS apparatus [34]. TEM and SEM need to modify the concentration of catalyst ink as well,  
Figure 4: The effect of the formulation and preparation processes of catalyst ink on its microstructure and macroscopic properties and further on the fabricated CLs.  
Figure 3: Illustration of the microstructure of the water/alcohol-based catalyst ink commonly used [21; 51; 52; 53; 54; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54; 55].  
i.e., consolidation, and merely characterize the local structure. DLS, TEM, and SEM might not truly investigate the microstructure of catalyst ink. Compared to DLS, TEM, and SEM, SAS and X-CT may be more suitable to characterize the microstructure of catalyst ink since the catalyst ink could be investigated without modification.