## 1 Introduction
To mitigate the mass transport issues caused by the winding transport pathway in conventional CLs, the structure-ordered CL was first sketched by [PERSON] et al. [9], which is constituted of the aligned electronic conductor loaded with Pt nanoparticles perpendicular to the membrane, being covered by ~10 nm thick ionomer layer. This structure is expected to maximize the TPB and boost mass transport by allowing the transport pathway of proton, electron, reactants, and water to be as short as possible. Currently, structure-ordered CLs primarily have four types, based on the ordered electronic conductors (e.g., vertically aligned carbon nanotubes [10; 11] or conductive polymer [12; 13]), ordered catalysts [14; 15; 16], ordered ionomer materials [17], or ordered non-conductive materials (e.g., nanostructured thin film (NSTF) CLs [18]). Pt loading has been successfully decreased to less than 0.125 mg/cm\({}^{2}\) MEA (membrane electrode assembly, MEA) without sacrificing fuel cell performance in some structure-ordered CLs [16; 18; 19]. However, several disadvantages hampering their practical application in PEM fuel cells systems remain, such as complicated and costly fabrication processes, as well as severe water flooding in some cases owing to the hydrophilicity of ultrathin structures. The structure-ordered CLs are usually ultrathin, less than 1 um or even less than 200 nm in some cases [14; 16; 20], which will result in inherently low mass transport resistance versus the conventional CLs with a thickness of several um due to the shorter transport distance. However, most structure-ordered CLs are susceptible to topple over [14; 18], thus losing their ordered structure during the decal or hot-pressing process. Since there is a long way needed for the practical application of structure-ordered CLs, this review focused on conventional CLs (hereafter simply referred to as CLs) practically applied at present. The multiscale structure of CLs is formed during the manufacturing process and is thus influenced by the composition (or formulation), order of ingredient mixing, and preparation/mixing process of catalyst ink, the substrate, and the application (deposition or coating) process and drying process of catalyst ink (Figure 1). Recent studies indicate that the composition and preparation process of the catalyst ink impact the multiscale structure of CLs to a large part, through affecting its microstructure and macroscopic properties [21; 22; 23; 24]. The catalyst infks are biphasic material systems (suspensions) commonly composed of solid catalyst and ionomer as well as solvent. The type of catalyst, ionomer, and solvent, ionomer/carbon (I/C) ratio, or the solid content determine the underlying interactions between the individual constituents. These interactions will influence the microstructure of catalyst ink, e.g., the size and shape of primary catalyst-ionomer agglomerates, conformation, and size of primary ionomer agglomerates, agglomeration, or the interface of catalyst and ionomer [25; 26; 27]. Furthermore, to achieve a well-dispersed ink, an effective dispersion process is necessary to break up the initial large agglomerates into the desired size [28]. The ink formulation and preparation processes influence its macroscopic properties directly or indirectly through impacting its microstructure, e.g., rheology (viscosity or thixotropy), surface/interface tension, or stability, which would influence the coating quality or drying processes and thus the structure of CLs made of the ink [29; 30; 31; 32; 33; 34; 35]. Hence, understanding and tailoring the microstructure and macroscopic properties of the ink by controlling its formulation and preparation process, i.e., catalyst ink engineering, are significantly important in the formation of the optimum CL structure. When the catalyst ink is prepared, the substrate and how the catalyst ink will evolve into the final CL during the coating and drying process also have a critical influence on the CL structure.