## 6 CL Formation
### Coating Process  
The ink coating techniques commonly used include spray coating, brush coating, doctor blades coating, screen printing, inkjet-print coating, slot-die coating, or gravure coating [29; 38; 170]. Spray coating is the coating technique commonly used in small or moderate scale and usually used in the lab for evaluating the performance of the advanced materials. At the end of 2014, Toyota released its first generation of PEM fuel cell vehicles to the market; at the end of 2020, it released its second generation of PEM fuel cell vehicles with improved performance to the market, including the increased driving range (has increased the world record distance to 1360 km with one hydrogen fill recently) or increased stack volume power density (5.4 kW/L). In recent years, the other vehicle companies, e.g., Honda and Hyundai, have released their version of PEM fuel cell vehicles to the market. Recently, the application of PEM fuel cells in heavy-duty vehicles has also attracted significant attention [171; 172]. As the demand for the application of PEM fuel cells increases and is expected to increase substantially in the coming years, the cost of MEAs production needs to be reduced further and the MEAs production yield needs to be increased. These need to reduce the process steps of MEAs and the reliance on manual labor (the small/moderate-scale coating techniques often need highly trained labor and are labor-intensive), as well as realize the continuous production. Therefore, roll-to-roll coating method attracted attention and has been applied to deposit CLs recently. The working principles, related to coating parameters, or pros and cons of various coating techniques have been well summarized in recent reviews [29; 170]. Therefore, in this review, the attention is only paid to the spray coating and the roll-to-roll manufacturing process coupled with slot-die coating or gravure coating. A brief description on their development and the effect of the relevant coating parameters on the morphology or structure of CLs is presented as follows. 1. Spray coating: Spray coating works mainly through applying high energy (e.g., high-pressure gas turbulence or ultrasonic vibration) to break the large catalyst ink droplets into tiny droplets finally deposited onto the substrate. It has several merits, such as relatively high uniformity of the CLs and relatively precise control on the catalyst loading since the CL is deposited layer by layer. However, the spray coating is a slow process, only suitable for depositing CLs on a small or moderate scale [165]. Therefore, spray coating is more suitable for fabricating the CLs in the lab used to evaluate the performance of the advanced materials due to the relatively excellent control on the CL uniformity and the catalyst loading. The most used spray coating method is the gas-assisted spray. To form a fine ink mist with well-defined droplets size, ultrasonic spray and electrospray were developed [104; 163; 162]. Compared to the CL deposited by the gas-assisted spray, the electrospray-coated CL is much more porous and has an increased ECSA value due to the fine ink droplets formed by the coulomb repulsion [162]. During spray coating, some spraying parameters influence the morphology and structure of the CL and hence the performance of the cell, such as the nozzle height [104], electrospray mode [67], or spraying voltage [163]. While the thickness and porosity are independent of the ultrasonic nozzle height, the CL surface may crack when the distance between the nozzle and substrate is too short (e.g., 3.5 cm) due to the presence of excess liquid ink on the substrate [104]. When the injection rate is fixed and the intensity of the electric field (determined by the working distance and the spraying voltage) is within a certain range, a stable cone jet mode can be formed, where the large ink droplets can be effectively broken into fine droplets due to the sufficiently strong coulomb repulsive force [163].