## 1 Introduction  
With the increased energy crisis, environmental and climatic deterioration, as well as health risks, the use of clean renewable energy becomes important. However, due to the limitation of period and region, it is hard to guarantee the continuous supply for most renewable energy, e.g., solar or wind energy. Therefore, it is necessary to explore clean and efficient carriers to store or transport renewable energy, where hydrogen stands out due to zero-carbon emission. To effectively apply hydrogen in the energy economy, proton exchange membrane (PEM) fuel cells are expected to play a significant role. PEM fuel cells directly convert the chemical energy stored in hydron and oxygen to electrical energy through the oxidation of hydrogen at the anode and the reduction of oxygen at the cathode. If green hydrogen, commonly produced from renewable energy through water electrolysis, could be used as fuel, the by-products include only water and heat, significantly reducing zero-carbon emissions [1, 2]. Furthermore, compared with other energy conversion technologies, e.g., internal combustion engines or other kinds of fuel cells, PEM fuel cells also have the advantages of high energy efficiency and low operation temperature [3, 4]. After more than 30 years of research and development, PEM fuel cells have been successfully applied to stationary and pothale generation systems, as well as to sea, land, air, or rail transportation, and are on their way to commercialization in some fields, e.g., vehicles. However, due to the low operation temperature, the use of expensive platinum (Pt)-based catalysts is necessary to promote the electrode reactions to supply the required performance and durability for practical application [5]. Therefore, to compete with internal combustion engines and Li-ion batteries, the performance and costs must be reduced, while durability should be increased. The catalyst layer (CL), where the electrode reactions occur, is a key component of PEM fuel cells. The CL typically constitutes the continuous and intersecting networks of carbon-supported Pt-based catalysts (supplying active sites and electron transport pathway), ionomers (supplying proton transport pathway and acting as a binder to enhance the mechanical stability of CLs), and pores (supplying reactants and water transport pathway). The electrochemical reactions occur at the intersection region of the three components, mostly referred to as the triple-phase boundary (TPB). Therefore, not only the activity of catalysts, but the accessibility of active sites to gaseous reactants, protons, and electrons, i.e., the formation of TPB, is also essential for improving the performance and durability of PEM fuel cells and thus decreasing the cost. This may be why, although the activity and durability of catalysts have been improved dozens of times in the last decade, the performance and durability of PEM fuel cells have only slightly increased [6; 7; 8]. In brief, the performance, durability, and cost of CLs, and hence PEM fuel cells, are largely dependent on the micro-, meso-, and macro-structures of CLs, which determine the electrochemical surface area (or TPB) as well as the effective transport properties of proton, electron, reactants, water, and heat. One way to optimize the micro-, meso-, and macro-structures of CLs is the optimization of the manufacturing process of the conventional CLs; the other way is fabricating the CLs with ordered structure. Conventional CLs are primarily manufactured by a catalyst ink-based processing method. To create the TPB and the networks for mass transport, supported catalysts and ionomers are typically dispersed in a liquid chemical solvent, referred to as catalyst ink. The catalyst ink is then coated on a substrate surface to form an ink film, and the solvent in the ink film is evaporated through a drying process to form the solid structure of the CLs.