A fundamental comprehension and recent progress in advanced Pt‐based ORR nanocatalysts

Today, Pt/C catalysts are widely used in proton exchange membrane fuel cells (PEMFCs). The practical applications of PEMFCs still face many limitations in the preparation of advanced Pt‐based catalysts, including high cost, limited life‐time, and insufficient power density. A kinetically sluggish oxygen reduction reaction (ORR) is primarily responsible for these issues. The development of advanced Pt‐based catalysts is crucial for solving these problems when the large‐scale application of PEMFCs is to be realized. Herein, we demonstrate the design principle of advanced Pt‐based catalysts with an emphasis on theoretical understandings to practical applications. Generally, three main strategies (including strain effect, electronic effect, and ensemble effect) that governing the initial activity of Pt‐based electrocatalysts are elaborated in detail in this review. Recent advanced Pt‐based ORR catalysts are summarized and we present representative achievements to further reveal the relationship of excellent ORR performance based on theoretical mechanisms. Then we focus on the preparation standards of membrane electrode assembles and testing protocols in practice. Finally, we predict the remaining challenges and present our perspectives with regards to design strategies for improving ORR performance of Pt‐based catalysts in the future.


| INTRODUCTION
Currently, because of high-efficiency, stable, and environment-friendly advantages, proton exchange membrane fuel cells (PEMFCs) are regarded as a promising energy transformation to solve the energy issues around power sources of automobiles. [1][2][3] Pt-based catalysts as the most promising cathode electrode materials are always used to catalyze the oxygen reduction reaction (ORR) in process. In 2007, the per-KW cost of 80 kW net PEMFCs systems with high-volume manufacturing of 500,000 units had reached about $45/kW net , with a $15/kW net gap between the long-term cost targets of $30/kW net . 7 However, the high-cost price and scarcity of Pt on the earth are the main bottleneck limitations to their wide commercial applications for industrialization process. [8][9][10] Recently, enormous achievements of advanced Pt-based catalysts have been achieved on theoretical understandings of ORR kinetics, which mainly focus on the following aspects: (i) Understanding the intrinsic mechanisms of ORR kinetics. Due to the complicated the ORR reaction pathway, the reaction kinetics of ORR is limited by adsorption energy and active energy of O 2 molecules on catalyst surface atoms. [11][12][13][14][15] (ii) Demonstrating the effect factors of ligand effect, strain effect, and ensemble effect of Pt-based catalysts toward ORR. Constructing the corresponding relationships between the design principle of catalysts and the reaction mechanism of ORR is beneficial for preparing advanced Pt-based catalysts. [16][17][18][19][20][21][22][23] According to the Sabatier principle, 24 the interactions between catalyst surface/interface atoms and the reactant should be neither too strong nor too weak. Thus, surface engineering strategies based on ligand effect, strain effect, and ensemble effect are widely explored to elaborate the impact of every factor on ORR performance. Usually, implanting a foreign secondary metal into the surface lattice of Pt will induce all the above effects, which is an efficient and facile method for improving the ORR performance. Besides, other synthesis strategies, modification methods, and engineering tactics are also performed to further clarify the relationships between theoretical mechanisms and practical ORR performance, such as, forming Pt skin or skeleton, 25,26 shaping Pt alloy, 27 exposing high-index facets, 28,29 creating multidimensional nanostructure, 25,30 phase hybrid, 31 ordered intermetallic Pt-based alloy, 26 and single atom site catalysts. 32 (iii) Presenting several advanced Pt-based ORR catalysts. Under these theoretical design principles, extensive works have been developed to accommodate the PEMFC targets. The design of these catalysts fully follows the above design principles of advanced Pt-based electrocatalysts mentioned above. (iv) Elaborating the relationships between laboratory ORR performance and practical applications of the single fuel cell. The relationship between electrochemical performance and advanced materials and morphology in ex situ setups is extremely challenging in terms of reproducibility and reliability; thus further efforts are needed to study catalysts under technically relevant conditions. Figure 1 shows the overview of the topics covered in this review. Finally, we present a perspective of further research directions to point out the challenges and strategies for next-generation Ptbased catalysts.

| FUNDAMENTAL UNDERSTANDING OF ORR MECHANISM
Due to the complex kinetics of ORR and complicated intermediates during the whole ORR process, it is a great challenge for us to understand the intrinsic ORR mechanism from the atomic or molecular level to design Pt-based electrocatalysts. 33,34 It is generally accepted that the complete electron transfer (4e − ) is in favor of ORR; on the contrary, the two-electron pathway is undesirable because its final product is H 2 O 2 , 35,36 which will destroy the Nafion membrane and cause damage to fuel cells. 37 Theoretical studies reveal that four-electron pathway is preferred to occur on the Pt crystal facet with (111) under the condition of oxygen enrichment. 38 Thus, the elemental steps of ORR are usually presented in the following (*denotes an active site of catalyst) 39  (v) The electroreduction of O 2 will go through two different pathways based on whether the bond-breaking of *OOH or not. The optimized catalyst possesses strong binding energy, which is beneficial for splitting of O-O bond. This review is based on the Pt-based catalysts, so the strong oxygen-binding energies on Pt sites facilitate the form of *O to some extent, namely the fourelectron pathway. Furthermore, the binding free energies of intermediate, such as *OOH, *OH, *OH, derived from complete reduction is shown in Figure 2A. 40,41 It was reported that the binding free energy of intermediates is linearly related to each other. The *OH-binding free energy ΔG OH = 1/2ΔG O and ΔG OH = ΔG OOH -3.2, so the ΔG OH could be used as a descriptor to evaluate the ORR performance. 44 According to the Sabatier principle, the interactions between the catalyst surface atoms and the reactants/intermediates should be neither too strong nor too weak, revealing the presence of maximum catalytic activity to some extent. 24,45 Figure 2B shows the ORR activity plot for different metals as a function of *OH binding free energy. We can clearly see that, in this volcano plot, an ideal ORR catalyst should locate on the peak, whose overpotential is minimum. In other words, for metals that bind oxygen-containing species too strongly, the generation of H 2 O rooting from the further reduction of *OH will control the pace of the reaction, while for metals that bind oxygen-containing species too weakly, the ORR performance will be determined by activity/dissociation of O 2 and deep reduction of *OOH. Among these metals, Pt is the best candidate for ORR, while there is still higher binding energy of about 0.2 eV in comparison with the plot peak. 46 Thus, decreasing the d-band center of Pt-based catalyst is crucial for improving ORR performance.
Under the above discussions, many researchers made enormous efforts to further explore the specific activities of different Pt-based alloy catalysts from the theoretical and experimental aspects. In theory, Figure 2C shows that Ni and Co are the best two promising elements for optimizing the binding free energy of adsorbed oxygen-containing species on Pt. 46 Similarly, enormous literatures 42 relating to Pt-based catalysts (alloying with Ni, Co, Fe, V, Ti etc.) were reported and established an obvious linear relationship between the ORR catalytic activity and metal d-band center under the experimental conditions. The Stamenkovic et al. 43 reported that the transition metals (Ni, Co, Fe, Ti, V) from the fourth-period alloy with Pt would form a "volcano-shaped" relation, focusing on the ORR activity and d-band center for Pt-rich surfaces and Pt-skeleton surfaces. It can be obtained from Figure 2D that the Pt 3 Co alloy catalyst is the best alternative for replacing conventional Pt/C catalyst. In addition, from the positions of Pt 3 Ti and Pt 3 V in the polt, it can be obtained that the d-band center is too far from the Fermi level, the surface possesses lower coverage of OH ad and anions, while the binding energy of O 2 and the intermediates are too weak to ensure a turnover rate of ORR. Thus, the rational design of Pt-based catalysts from the viewpoint of optimizing binding free energy of oxygen-containing species on surface-active sites is of great importance and efficiency for enhancing ORR performance.

| DESIGNING PRINCIPLE OF HIGH-PERFORMANCE PT-BASED ORR ELECTROCATALYSTS
Considering the sluggish kinetics of ORR, rational designing novel-structured Pt-based electrocatalysts with excellent performance is of great importance and significance for fuel cells. Usually, as shown in Figure 3, improving the activity and durability of Pt-based catalysts mainly follows three aspects: (i) increasing the number of Pt active sites; 31 (ii) improving the intrinsic activity of every exposing Pt active sites; 47 (iii) and enhancing the intoxicating behavior of Pt active sites. 48 The previous discussions [49][50][51] demonstrate that modulating the electronic structure and coordination environment of surface Pt active sites are beneficial for downshifting the d-band center of Pt, optimizing the interactions between the oxygencontaining species and surface Pt atoms. So far, enormous works 16,[52][53][54][55][56][57][58] have been made to tune/modify the exposing Pt active sites. From theoretical insight, this review will elaborate the relationships between the ORR performance and Pt-based catalysts from stain effect, ligand effect, and ensemble effect.

| Strain effect
The surface strain engineering of Pt-based catalysts has obtained researcher's attention due to its immense potential in modulating the electrocatalytic performance. 56,59,60 Strain effect can tune the lattice fringe of surface Pt atoms to improve the electrocatalytic activity while the foreign metals are introduced into the depths of nanocatalysts greater than 1 nm. 16,[61][62][63][64] Herein, we present the fundamental advances in theoretical modelings to further understand the surface strain effect of Pt-based catalyst on ORR.
The strain effect usually appears combining with ligand effect and synergistic effect through tuning the d-band center of Pt-based catalysts and modulating the binding energies of reactants, intermediates, and products. The nature of strain effect formation is lattice  16,58 that the strain effect is based on the longdistance modulation, within six monolayers of catalyst near-surface, while the ligand effect is limited in the twosurface atomic layer. Usually, the calculations of strain effect are based on the distorted percentage of the Pt lattice between the conventional and advanced Pt-based catalysts. It can be defined as follows: S Pt = (α advanced − α conventional )/α conventional × 100%. 56 Where the α advanced is the lattice value of advanced Pt-based catalyst, the α conventional is the lattice value of pure conventional Pt particle. The change of d-band center in the compressive strain case is illustrated in Figure 4A,B, where Ni atom (stands for small metal atoms) is inserted into the lattice or deposited on a Pt bulk with larger lattice spacing. The d-bandwidth will decrease due to the d states overlap of different interfacial metal atoms. On the contrary, if Pt atoms are deposited onto the Ni surface or inserted into Ni surface lattice, the contracted Pt d-bandwidth increases and the Xd-band center also downshifts ( Figure 4C,D). The recent reports 65 demonstrate that compressive strain will downshift the d-band center, leading to a weakened interaction between active sites and adsorbates, while the tensile strain will upshift the d-band center of Pt, triggering a strong interaction. Please note that this rule is only suitable for the late transition metals with a half-filled d-band.
It is accepted that, for ORR, the compressive strain behavior is beneficial for improving the Pt-based catalytic performance. For example, Du et al. 66 revealed that 52% enhancement and 35% reduction of ORR performance were obtained through depositing 5 nm Pt layers onto the nanofilm under the compressive and tensile conditions, respectively, in comparison with pure nanofilm. On the contrary, Bu et al. 67 documented that suitable tensile strain can also enhance the ORR performance in the calculations of theoretical exploration. Thus, the crosslight about tensile strain effect will limit the ORR performance should be further study.
Tuning the surface strain behavior is an efficient method for improving the ORR activity of Pt-based nanocrystals. Thus, as mentioned above, to understand the intrinsic behavior of long-distance effect as well the exact number of atomic layers, excluding the influence of interfacial reconstructions and nanocatalyst geometries, is fundamental to understand the strain effect in designing the electrocatalysts with some specific functions. As shown in Figure 5, Wang et al. 68 documented a strategy to demonstrate the intrinsic surface strain in the twodimensional nanosheets. They found that the attractive interactions of surface atoms would cause a tensile strain, which was equivalent to lead a force of 100 MPa onto the catalyst's surface and resulting in about 10% contraction stress. In addition, the exact stress value is inversely proportional to the thickness of the nanosheets.

| Ligand effect
Ligand effect usually occurs on the interfaces of different atoms with dissimilar electronegativity (possessing different d-band center) that makes for the electronic charge transfer between them. 68,69 The ligand effect can weaken the difference of electronegativity and make the whole catalyst in the lowest energy state. For Pt-based catalysts, alloying Pt with transition metals (such as Ni, Fe, Co) 2,70-72 is in favor of weakening the d-band center via electron donation ( Figure 6A). In the year 2007, Stamenkovic et al. 73 ( Figure 6B) revealed that the Pt 3 Ni (111) surface is ten times more active toward ORR than Pt (111) surface and ninety times more active than commercial Pt/C catalysts. This is because the Pt 3 Ni (111) surface possesses an unusual electronic structure (d-band center position) and arrangement of surface atoms in the near-surface region. Herein, it is noted that the facet effect is another non-negligible factor for influencing the ORR catalytic performance. As is also shown in Figure 6B, exposed (111)-facets Pt 3 Ni shows an enhanced ORR activity than (110) and (100) facets. However, for Pt-based catalysts, the transition metals will easily be oxidized under an oxygen environment. 74,75 Additionally, the rigorous electrolytes and high potential also accelerate the dissolution of the catalyst surface. 8,71,76,77 Thus introducing another element with low electronegativity than O onto catalyst surface to tune the electronic effect of Pt atoms is a promising effective in keeping the initial activity. Sung Jong Yoo's group 74 used N-containing polymers to modify the PtCo nanoparticles, to modulate the electronic structure of surface Co, and finally resulting in a highly active and durable Pt-based ORR electrocatalyst ( Figure 6C-E). The introduction of N-containing polymers could conduce to the electron donation from Co to Pt, avoiding the further oxidation of surface Co atoms. In addition, this strategy can also improve the stability of PtCo catalysts. It was reported that alloyed Mo-Pd (or Pt) possessed an unrivaled catalytic ORR performance than other alloys, while excessive passivation of surface Mo hindered the exposure of surface-active sites. Thus, Wang et al. 52 introduced S as "active auxiliary" to modulate the surface Mo atoms of ultralong jagged Pt 85 Mo 15 . On the one hand, they found that S modified Pt 85 Mo 15 catalyst possessed an excellent electrocatalytic performance than bare Pt 85 Mo 15 nanowires. On the other hand, the implantation of S element also changed the surface structure of ultralong Pt 85 Mo 15 nanowires, exposing more active sites than initial intact, as shown in Figure 6F-H. Further evidence demonstrated that S element could prevent the further oxidization of Mo and then lead to enough electron transfer from Mo to Pt ( Figure 6J-L).

| Ensemble effect
Ensemble effect also called "dual active sites effect" is based on the Langmuir-Hinshelwood (L-H) principle, 51,78,79 which is different from the electronic transfer behavior existing on the two neighboring active sites of catalyst (inter-)surface. Nanomaterials, including Pt-metal, Pt-oxide, Pt-hydroxide, Pt-sulfide, Pt-Ionic liquids, and so on, possessing rich interfaces usually induce the ensemble effect. Alloying Pt with transition metals not only changes the electronic effect of Pt but also generate an additional impact on adsorbing the interactants. [80][81][82][83] In the practical application of PEMFCs, cathodic electrocatalysts often suffered from the poison phenomenon, which will decrease the performance of fuel cells. 84,85 Considering the preparation technology and toxicity resistance, oxyphilic elements (such as Ru, Mo, Sn) are preferred to alloy with Pt. Figure 7A shows the schematic illustration of poison resistance of typical Pt-Ru alloy based on the L-H pathway. 51,[88][89][90] It can be clearly seen that the implantation of Ru into the catalyst surface can easily adsorb the OH -, which is in favor of the oxidation/ removal of poisonous CO intermediate. Apart from the oxyphilic metals, the oxyphilic hydroxide can also be applied to modify the Pt-based catalysts to increase their toxicity resistance. Huang et al. 86 reported platinum-nickel hydroxide-graphene (Pt/Ni(OH) 2 /rGO) ternary hybrids with long-standing life by enhancing the CO oxidation ability ( Figure 7B). In a basic solution, abundant free OHis suggested to be adsorbed on the defective Ni(OH) 2 and is beneficial for CO oxidation via Langmuir-Hinshelwood (L-H) pathway. The CO stripping experiment ( Figure 7C) of Pt/Ni(OH) 2 /rGO revealed that the introduction of Ni(OH) 2 is in favor of decreasing the oxidation potential of CO. In addition, it is seen from the CV curve of CO oxidation that a new peak emerges for Pt/Ni(OH) 2 /rGO in comparison with bare Pt/rGO catalyst. The first peak can be ascribed to CO oxidation at Pt sites near Ni(OH) 2 , and the second peak can be ascribed to the CO oxidation at Pt sites far from Ni(OH) 2 . This result further demonstrates that the addition of Ni(OH) 2 is beneficial for CO oxidation and removal.
In addition, the introduction of oxyphilic species not only enhances the poison resistance but also changes the reaction pathway during the ORR process. Shen et al. 87 report a new concept of utilizing dual active sites for the ORR via engineering Pt-oxide interfaces. Figure 7D shows the TEM image of a typical Pt−Cu−Ni nanoparticle combined with the SnO x , and this catalyst shows a 40% enhancement of specific activity toward ORR, which is ten times higher than pure Pt-Ni-Cu catalyst. Further evidence and density functional theory (DFT) calculations demonstrate that an altered dual-site cascade mechanism wherein the first two steps occur on SnO x sites and the remaining steps occur on adjacent Pt sites, allowing a significant decrease in the energy barrier, as shown in Figure 7E,F.

| RECENT REPRESENTATIVE ADVANCED PT-BASED ELECTROCATALYSTS WITH HIGH ORR PERFORMANCE
Under the guidance of theoretical understandings mentioned above, enormous research breakthroughs were obtained and mainly focused on the surface structure and composition regulation, as well as morphology control at the micro-nano scale. However, these achievements are usually based on the cumulations of strain effect, ligand effect, and ensemble effect. One efficient strategy for increasing the ORR performance is alloying Pt with desired secondary metals, which triggers the strain effect, ligand effect, or both. Figure 8 shows the recent advanced Pt-based ORR catalysts with high activity and stability. 67, It can be seen that Mo-Pt 3 Ni/C, 91 J-Pt NWs/C, 92 PtPb@Pt, 67 PtNi BNSs/C, 94 and PtNi@Pd/C, 16 as well as other Pt-based catalysts with nanoframe structures, 109,110 all show the high mass activities, specific activities, and electrochemical active surface area (ECSA) values. In general, the specific activity stands for the intrinsic activity of catalysts, which can be used to evaluate the catalytic performance of a single Pt active site. The mass activity can be used to predict the real usage of Pt, which is significant for their practical applications. Thus, the high mass activities, specific activities, and ECSA values are the most conventional and the most intuitive standards for evaluating the ORR performance of catalysts. Next, we will further reveal the essential reasons why these Pt-based catalysts possess such high ORR performance based on several typical catalysts from the insight of atomic-level engineering.
In the early age of designing Pt-based electrocatalysts, alloying Pt with other transition metals is a facile and efficient strategy for improving ORR performance. As we know, the oxygen-binding energy is often related to the coupling of adsorbate (O ad ) and d-band position of Pt-based alloys, which are tuned easily by the doped transition metals. Figure 9A-C shows the Mo-Pt 3 Ni/C nanocrystals reported by Huang et al. 91 via surface modification, which possesses enhanced ORR performance after Mo addition. The mass activity is 6.98 A/mg Pt , which is about 70 times higher than commercial Pt/C. The DFT calculations reveal that Mo prefers to deposit on the superficial vertex and edge of particles. Furthermore, Cao 116 documented that Mo could balance the concentration of surface element and increase the vacancy formation energy of transition metals on the catalyst surface, which was beneficial for preventing the dissociation of transition metals. The enhanced ORR performance is attributed to the ligand effect and strain effect. In terms of Pt-M catalysts, Pt-M intermetallic particles also show an enhanced ORR performance. In comparison to Pt-based alloy with atoms being randomly distributed in the Pt fcc lattice, which is usually etched from the alloy due to corrosive environment and high potentials, the ordered intermetallic compounds combine metallic and ionic bonding, which forms an ordered atomic lattice with specific stoichiometry and crystal structure. Several Pt-M intermetallic catalysts, such as PtCo, 118 PtFe, 111 PtCu 119 , and PtNi, 117 are also reported.
Core-shell structure plays an important role in ORR catalysts, which are composed of catalytically active Pt shells and cheap cores to utilization. The core not only supports the Pt shell but also tune the electronic structure of Pt shell. Huang et al. 67 presented highly uniform PtPb@Pt (core@shell) nanoplates with dramatically ORR performance, as shown in Figure 9D-J. These nanoplates show a 55 m 2 /g Pt of ECSA values, 7.8 mA/cm 2 of specific activity and 4.3 A/mg Pt of mass activity, of which are attributed to large biaxial tensile strain effect for ORR. In addition, introducing the lanthanide (lanthanum, cerium, samarium, gadolinium, terbium, dysprosium, thulium, or calcium) into the Pt bulk also enhance the ORR performance by tuning the bulk lattice parameters and follows a high peaked "volcano" relation curve ( Figure 9K,L). 120 The Pt-based nanoframe catalyst also shows an excellent ORR activity. As shown in Figure 9M, Yang group reported a 3D Pt 3 Ni nanoframe 25 with a two atomic layer of Pt via PtNi 3 transformation, which exhibits an extraordinary performance toward ORR. This nanoframe structure can expose more surface Pt active sites and mass transfer, as well as stable structure for long-term tests.
Constructing the 1D structure with surface modification strategy, including composition and structure engineering, is beneficial for boosting ORR performance. Huang et al. 92 presented an ultralong jagged Pt nanowires (donates J-Pt NWs) with a diameter of 2-3 nm, which exhibited an outstanding ECSA value (118 m 2 /g Pt ) and mass activity (13.6 A/mg Pt ) for ORR ( Figure 10A-C). This exceptional performance is dominated by ultralong structure and high Pt exposure degree, as well as ligand effect of Pt and Ni. This structure is fabricated by the electrochemical dealloying method, which can induce a Pt-richment on the surface, resulting in a high ORR performance. A stable high-index faceted Pt skin on zigzag-like PtFe nanowires was reported by Guo's group, 96 which selectively exposed active surfaces and judiciously tuning the near-surface composition of electrode materials and showed an enhanced electrocatalytic performance. Further exploration reveals that the combination of 1D structure and exposed high-index facets, as well as Pt-skin surface, is the promoter for optimizing the adsorption energy under the impact of ligand and strain effect. Furthermore, solid nanowires structure is disadvantageous for high utilization of the internal atoms, thus excavating the internal unreacted atoms to fabricate a hollow nanotube is efficient for reducing the usage of Pt. Xia et al. 97 (Figure 10D-F) fabricated a branched PtNi alloy nanocages with a Pt-skin structure, as well as high-index facets exposed on nanocages. And this catalyst shows high specific activity (5.16 mA/cm 2 Pt ) and mass activity (3.52 A/mg Pt ). The experiments and DFT calculations demonstrate that stain and ligand effect are the principal factors for excellent ORR performance. Generally, a hollow structure can usually be presented via nanospheres, nanocages polyhedrons, and nanotubes. The synthesis strategies of hollow structure include galvanic replacement, de-alloy method, and other template sacrifice methods. 25,[121][122][123] Apart from the above-discussed strategies, another method for minimizing the Pt element without compromising the catalytic ORR performance is to reduce the dimension down to clusters and even separately distributed metal atoms, which can greatly increase the atom utilization efficiency, potentially to  [124][125][126] Atomic site catalysts, including single-atomic site catalysts and diatomic site catalysis, are being pursued as economical alternatives to boosting their commercial applications in PEMFCs due to lower Pt usage and high performance triggered by nearby highly stable coordination atoms. Liu's group documented a strategy of fabricating highly active and stable ORR electrocatalyst with ultralow-loading Pt usage by carbonization of Co-containing Zn-based zeolitic imidazolate frameworks ( Figure 10G,H). 126 This catalyst shows an exceptional mass activity of 8.64 A/mg Pt performed by rotating disk electrode (RDE), which attributes to highly dispersed Pt atoms and ligand effect of Co-N-C active node. On the other hand, using carbon materials as the support of ORR catalyst possesses the superiorities of high conductivity and high surface area and enhancing anticorrosion ability. performance of advanced electrocatalysts, as shown in Figure 11. The relationship between electrochemical performance and advanced materials (e.g., electrocatalyst kinetics, conductivity, surface functionality) and morphology (e.g., pore structure, interfaces) in ex situ setups is extremely challenging in terms of reproducibility and reliability; 127-129 thus further efforts are needed to study catalysts under technically relevant conditions. Obtaining reliable data via ex situ electrochemical evaluation of advanced ORR electrocatalysts by RDE voltammetry was highly dependent on the electrode morphology. Once protocols were established to reliably produce uniform electrodes, the expected catalyst activity was reproducibly observed. Thus, the practical performance of ORR catalysts in the fuel cells is different greatly from RDE because of rigorous conditions and difficult preparation technology in MEA. Simultaneously, in the process of automated MEA preparation, gas diffusion media, ionomers, polymer electrolyte membranes (PEMs), and electrode structures are also mainly considered and designed for use in next-generation fuel cells. Noting that in the MEA process, the dissolution of the exchange membrane under the high work potential will cause a decrease in the catalytic performance of the catalyst to the same extent. Thus, the mature preparation technology and rational design principle of MEA are recommended in fuel cells.

| PRACTICAL APPLICATIONS OF SINGLE FUEL CELL
The US DOE has documented a series of operating protocols and targeted ORR electrocatalysts for transportation applications in 2020, as shown in Tables 1 and 2. Under the guidance of this protocol, enormous advanced Pt-based catalysts were designed precisely, as mentioned above. Apart from the design of advanced catalysts, the loading usage of Pt, pore structure of carbon, and preparation of catalyst-coated layer are also the key factors for high-performance of MEA. Gasteiger and coworkers reported a benchmark to test the MA of Pt cathode electrocatalysts in MEA by contrasting corresponding activity and stability under the conditions (0.9 V vs.RHE). 128 To make clear the intrinsic mechanism, the Kongkanand group developed a high-power density development of the cathode electrocatalysts as a supplement to verify the Gasteiger's conclusion. 129 They found that the O 2 transport resistance serious affected the availability of Pt active sites. In addition, the carbon support also limits the performance of MEA by constructing the pore distribution. 128

| CONCLUSIONS AND PERSPECTIVE
PEMFCs as a next-generation energy conversion mode attracted people's attention in 1840. However, high production costs, immature production process, and rare reserves of Pt resource make it impossible for commercialized production. Thus, from the viewpoint of electrocatalysts, an in-depth understanding of the ORR mechanisms on Pt surface is necessary to better design the advanced Pt-based catalysts with high ORR performance. In addition, we have described the influence factors (strain effect, ligand effect, and ensemble effect) from the theoretical understandings to practical applications (reported advanced Pt-based ORR catalysts). Especially, the highest mass activity has enhanced to 13.6 A/mg PGM for PtNi catalysts, which reveals that there is a large room to improve for Pt-based ORR catalysts. However, for MEA applications, there are still big gaps to skip.
In the future, the design of advanced Pt-based ORR electrocatalysts should follow these principles to obtain excellent performance. (i) Narrowing the structure to one dimension of Pt-based electrocatalyst, the 1D nanostructure can reduce the embedded points at the interface between the electrode and catalyst to take full advantage of the catalyst. At the same time, the 1D shape possesses excellent electronic transmission, high utilization of Pt, and stable structure. While the 1D nanostructure still faces the same difficulties in dispersing, length increasing, and MEA tests. [130][131][132][133][134] (ii) Engineering high-indexed facets, nanocrystals with high-indexed facets will possesses abundant active sites compared with low indexed facets due to amounts of stepped atoms on the surface. 93,135,136 The undercoordinated surface Pt atoms make it possible to optimize the adsorption/desorption energy of intermediates, which will accelerate the reaction kinetics. However, the unstable structure of Pt-based high-indexed facets facile reconstructed is a key obstacle for further practical applications because of its high surface energy and amounts of undercoordinated surface atoms. [137][138][139] (iii) Constructing the hollow nanostructure, the subnanometer-thick walls of hollow nanostructured Pt-based catalysts not only reduces the usage of Pt weigh but also increase the utilization of Pt. 29,[140][141][142] In addition, the high mass transport and electronic excursion and atomic contraction effect rooting from the fold surface are all beneficial for enhanced ORR performance. [143][144][145] Furthermore, collaborative adoptions of these abovementioned will lead to exponential growth toward ORR performance. For example, Xia and coworkers reported hollow PtNi nanobranched nanostructure 94 showed a 3.52 A/mg Pt of mass activity, which is 14 times higher as compared with a commercial platinum on carbon (Pt/C) catalyst. Thus, rational design and engineering of the structure of Pt-based catalysts at the atomic level based on the theoretical principle is promising for generating next-generation ORR catalysts toward PEMFCs.