## 7 Future Direction and Prospects
For example, if using dipropylene glycol (DPG)/1-propyl alcohol (NPA) and water mixtures as the solvent, the ionomer agglomerates with moderate size close to that of Pt/C agglomerates in the catalyst ink not only form a connected ionomer network, but also maintain the adequate porosity in CL [23; 25], offering an improved PEM fuel cells performance. However, in another, the PEM fuel cells with a high performance need large Nafion agglomerates in the IPA/water-based catalyst ink, which leads to a thinner ionomer film on the catalyst surface [26]. In the IPA-, dimethyl sulfoxide (DMSO)-, N-Methyl-2-pyrrolidone (NMP)-, ethylene glycol (EG)-, or propylene glycol (PG)-based catalyst ink, the solvent that could make ionomer uniformly distribute will allow for good PEM fuel cells performance, due to the homogeneous coverage on catalyst agglomerates [126; 127]. The reasons why confused conclusions were often reported in the literature are because there are many factors simultaneously influencing the relationship between the formulation and preparation of catalyst ink or the CL formation process and the CL structure and further the performance and durability of the PEM fuel cell. If the other conditions or processes are inconsistent except the main variable in different studies, different or even opposite conclusions may be obtained. 3. Stability of catalyst ink during storing and reversibility of the aged ink: It is impossible in most situations that the prepared catalyst ink is applied immediately or completely consumed in a short time; in other words, the prepared ink needs to be stored before appliance. To guarantee the consistency of the structure and performance of the CLs fabricated at different periods, the composition and microstructure of catalyst ink should be stable. However, it is generally hard to avoid the material degradation, agglomeration, or sedimentation in the catalyst ink during storage. Consequently,the CLs produced in different periods likely show different structures and hence performances due to the altered microstructure and macroscopic properties of the ink over time, which will have a terrible influence on the performance and durability of PEM fuel cell stack. While the material degradation, especially that caused by Pt catalysis, is hardly inhibited and recovered, the agglomeration and sedimentation are able to be inhibited, retarded, or recovered to some degree. To keep it uniform, [PERSON] et al. permanently stirred the catalyst ink with a magnetic stirrer after dispersion [74]. Stirring may prevent sedimentation to some extent but it is unclear if the agglomeration can be inhibited since stirring cannot effectively break down the large agglomerates in the catalyst ink. Thus, it should be taken into account how to effectively and quickly redisperse the aged ink or recover its uniformity when applying it, and the extent of reversibility. 4. Studies on the catalyst ink with high solid concentration: At present, most studies on the composition and preparation process of catalyst ink focus on the cases of low solid concentration. Nevertheless, the roll-to-roll coating method, suitable for continuous, scalable production, always requires a slurry with high solid concentration, which may possess different microstructure and macroscopic properties from the ink with low solid concentration. For example, the thin ink nearly shows Newtonian feature; therefore, its rheology may barely impact the ink deposition process and thus the formation of CL. However, the slurry with high solid concentration always exhibits a high shear-thinning degree, which will increase over the solid concentration. In this case, as the rheology of ink will have an important influence on the ink deposition process and the formation of CL, it is necessary to rationally design the ink rheology by modulating the ink recipe and dispersion process.