## 3 Microstructure and Macroscopic Properties of Catalyst Ink
### Macroscopic Properties of the Catalyst Ink
Therefore, the viscosity and rheology measurements could be used to probe the microstructure of the catalyst ink or investigate the effect of the catalyst ink ingredients and preparation process on its microstructure [52]. When taking the shear force off, the shear-thinning fluid needs to take time to return to the initial viscous state, generally called thixotropy. In addition to visco-elastic behavior, thixotropy is also essential for the coating process, especially for the roll-to-roll coating techniques due to the commonly used ink with a high solid concentration and the fast-coating speed. During the coating process, an appropriate thixotropic response will contribute to the formation of the CL with uniform thickness and flat surface. If the ink viscosity recovery time is too short, it is difficult to obtain a good leveling, forming an uneven surface. However, a long recovery time will cause a hanging phenomenon, making it hard to form an ink film with enough thickness. The microstructure of ink film may not return to the initial ink state before completely drying. The thixotropy of ink could be regulated by varying the dispersion method [28] or adjusting the dielectric constant of solvent or varying I/C ratio [79]. 2. Surface tension: During the coating process, the surface tension of the ink should be minimized, or it should be at least less than the substrate, to boost the ink's wettability on the substrate and avoid coating failures. Hence, low surface tension will be beneficial to decrease the formation of defects, enhance the adhesion of CLs to the substrates, and optimize the interface between the CL and substrate. For example, to allow adequate wetting during the coating process onto carbon cloth, the surface tension value of ink should be lower than 32.5 mN/m (the surface tension value of carbon cloth) [80]. During the spraying process, the surface tension of the ink could impact the spray quality to some extent by affecting the small droplets [68]. During the coating process, the rheological behavior, including viscosity and thixotropy, determines the catalyst ink's response to shear stress, and surface tension of the catalyst ink play a critical role in the coating process (e.g., coating speed or quality) and the quality of the deposited ink film (e.g., uniformity, integrity, thickness, or wettability on substrate) (Figure 4). 3. Stability: Not only the zero-time microstructure and macroscopic properties of a catalyst ink, but also its stability is necessary to be considered to guarantee reproducible coating, which is extremely critical for industrial production. The stability of the catalyst ink includes material stability and colloidal stability. The former includes the oxidation of carbon, ionomer, and organic solvent caused by the catalysis of Pt or dealloying of catalyst during storage, modifying the composition of catalyst ink and as a result, impacting the structure or performance of CL [64,81]. The latter refers to the ability of the agglomerates in the fresh ink to remain dispersion in the solvent. Over time, the well-dispersed agglomerates may collide with each other during random Brownian motion and aggregate together when the collision force is larger than the repulsive force. If the agglomerate size reaches a certain point, sedimentation would take place due to gravity, making the catalyst ink inhomogeneous [34,52]. Zeta potential is commonly used as an indicator of colloidal stability. The zeta potential of particles in the colloidal system is the electric potential at the outside of the stationary layer on the particle surface, i.e., the electric potential at the shear plane of the particle, which could be measured by Electrophoretic Light Scattering (ELS) [63]. Its magnitude (negative or positive) indicates the degree of electrostatic repulsion between neighboring particles with the same charge. In general, the larger its absolute value, the more stable the dispersion.