Recent progress of Pt-based catalysts for oxygen reduction reaction in preparation strategies and catalytic mechanism

Journal of Electroanalytical Chemistry 848 (2019) 113279

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Review

Recent progress of Pt-based catalysts for oxygen reduction reaction in preparation strategies and catalytic mechanism Shijie Yia, Hao Jianga,b, , Xinjun Baoa, Saiqun Zoua, Jingjing Liaoc, Zejie Zhangc, ⁎

T

⁎⁎

a

Hunan Provincial Key Laboratory of Chemical Power Sources, College of Chemistry and Chemical Engineering, Central South University, Changsha, China Center of Super-Diamond and Advanced Films (COSDAF) & Department of Materials Science and Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China c State Key Laboratory for Powder Metallurgy, Central South University, Changsha, China b

ARTICLE INFO

ABSTRACT

Keywords: Oxygen reduction reaction Reaction mechanism Pt-based electrocatalysts Fuel cell

It goes without saying that the oxygen reduction reaction (ORR) on Pt-based catalyst is one of the most significant cathode reactions in fuel cells. Although high price and low stability, Pt-based catalyst still enjoys its widespread researches for its relatively high activity. Recently, there has been some impressive progress in strategies like alloying, single atoms, morphology and component regulation. This paper aims to introduce the preparation strategies of Pt-based catalyst and further expound its inactivation mechanism and the essence of active sites. It also shows some challenges as well as methods applying in fuel cells, which provides some reference for the practical application in the future.

1. Introduction With the surging growth of global energy demand and its limited fossil fuel, it urges people to pursue clean, sustainable energy and develop effective energy storage system. Meanwhile, fuel cell is regarded as one of ideal energy sources in the future due to its environmentfriendly and high energy efficiency characteristics. Especially, proton exchange membrane fuel cell (PEMFC) is one of the most attractive energy conversion techniques with its high-power density and energy conversion efficiency [1–4]. However, efficient and stable catalysts are needed to boost the oxygen reduction reaction (ORR), which is the key reduction in cathode for PEMFC. Recently, a large number of non-noble metal catalysts like transition metal-nitrogen-carbon materials (M-N-C) [5,6], transition metal oxide [7–9], carbon-based material [10–15] catching the researchers' attention. But their poor performance and stability under acidic conditions of PEMFC makes Pt-based catalyst still in its irreplaceable advantages as an excellent ORR catalyst [16]. Due to the scarcity and high price of Pt, to reduce its amount and to improve its catalytic activity and stability have become a topic with widespread concern. Pt3Ni (111) reached 90 times activity than that of commercial Pt/C, which attracted the researchers' attention into Pt alloy [17]. But the lower stability in dissolving of alloy should be noticed. Recently, molybdenum doped hyperoctahedral Pt3Ni

nanocrystals were prepared by Huang, which exhibited outstanding ORR activity (80 times than commercial Pt/C) as well as the stability [18]. Besides, the single atomic Pt-based catalyst is also treated as the optimal method to maximize atomic utilization, meaning that it is directly anchored by carbon imperfection without heteroatom (such as N, S) [19]. Well-designed topography like core-shell structure can provide more Pt active sites, while more highly active Pt crystal surfaces also can be exposed by some nanodisk [20] and nanowire [21] structures for more electrochemical active area. With some progress in nanotechnology as well as new devices, the ORR catalyzed by Pt electrodes also owns further development, for instance, PtM (M = Ni, Co, Fe, Sn, Cu, Al) alloy materials [22–27], the design and preparation of core-shell structure [28], Pt-based nanomaterials with controllable components, size and morphology and some monatomic Pt-based catalysts [29–31]. The paper gives the priority to the following aspects as for recent development of Pt-based catalysts in ORR: 1) Synthesis methods of various Pt-based catalysts; 2) Researches on ORR active sites of Pt-based catalysts; 3) Researches on the stability and deactivation of Pt-based catalysts. Further, it does not just shows the research progress on synthesis of diversified Pt-based materials, their active sites and stability in recent years, but also gives us a better understanding of the remaining challenges and future directions of Ptbased catalysts for fuel cells.

Correspondence to: H. Jiang, Hunan Provincial Key Laboratory of Chemical Power Sources, College of Chemistry and Chemical Engineering, Central South University, Changsha, China. ⁎⁎ Corresponding author. E-mail addresses: [email protected] (H. Jiang), [email protected] (Z. Zhang). ⁎

https://doi.org/10.1016/j.jelechem.2019.113279 Received 13 April 2019; Received in revised form 11 June 2019; Accepted 1 July 2019 Available online 02 July 2019 1572-6657/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. (a) Schematic illustration of size-selected growth mechanism of the PtNi3 NPs; (b) schematic illustration of the formation process of PtFe-Fe2C Janus-like NPs. (Adapted from ref. [35,37]).

2. Synthesis methods of Pt-based material

assisted thermal reduction. Maillard first synthesized Ni/C nanoparticles, and then PtNi alloys with Ni-rich core and Pt-rich shell were obtained through electrochemical reduction on the differences between chemical potential and oxytropism of Ni and Pt in solutions [38]. Dealloying is one of the most important methods for adjusting the performance of Pt-based catalysts, which refers to regulate their shape and size for optimizing the facet and surface structure in nanoscale by carefully adjusting the electrochemical cycle parameters [39,40]. Abruña optimized the conditions of dealloying to make Pt-rich shell own large specific surface area and control the morphology of ordered Cu3Pt/C intermetallic nanoparticles [41].

2.1. Synthesis methods of Pt alloy Enhancing the mass activity (MA) of Pt-based catalysts is a kind of methods to reduce Pt amount and improve its catalytic efficiency, which shows in the alloying process. Namely, one or more elements are added to synthesize PtM alloys for pursuing lower Pt amount and higher activity in ORR [32,33]. The common method is solvent-assisted thermal reduction, which means that Pt metal salts and alloy metal salts are mixed in aqueous solutions or organic solvents as well as reductants (such as sodium borohydride (NaBH4), 1,2-tetradecanol (TDD), tungsten hexacarbonyl) to generate Pt-based alloys at a certain temperature. For instance, Herranz et al. directly adopted NaBH4 as a reduction agent to prepare PtNi aerogel alloy in aqueous solutions [34]. Besides, the morphology of alloy materials could be regulated by surfactants during the reduction process. For example, The PtNi3 alloy materials with different sizes of 3–10 nm were obtained by adjusting the concentration of oleylamine and oleic acid (Fig. 1a) [35]. Janus-like PtFe-Fe2C NPs were synthesized by using Fe as alloy metal through the reduction process of oleyamine with 320 °C (Fig. 1b) [36]. Furthermore, multimetal Pt alloys also can be synthesized by adding a third metal precursor salt, which exhibited higher ORR activity than that of Pt. As shown in Table 1, heteroatom metal Mo doped PtNi and Rh-doped PtNi alloy presents ultra-high mass activity and specific activity [18,37]. The electrochemical reduction still prevails except for solvent-

2.2. Synthesis method of single atom Pt For most traditional Pt-based catalysts, only a few surface Pt atoms acted as the active site in the catalytic reaction. The utilization of Pt metal in the traditional supported Pt-based catalysts is far lower than the ideal level [45–48]. On the other hand, every atom has an efficient ORR activity in single atom Pt catalyst. If a large amount of usage is adopted, it increases the cost of the catalysts dramatically and is not suitable for large-scale application in industrial production. Therefore, it is of great significance to develop monatomic Pt catalysts for maximizing the catalytic efficiency and reducing the cost. Incipient wetness impregnation (Impregnation method) means that monatomic Pt-based catalysts can be synthesized by putting the carrier in the active metal precursor solutions. On the one hand, the carrier can absorb active substances; on the other hand, the monatomic metal precursors can be captured through the carrier with C defect, O defect, S defect and metal defect after the following dried, calcined and activated process. Finally, the formed single atoms can be stabilized by the charge transfer effect of metal atoms, defect formation and their sites [49]. For example, the defective degree of BP was increased by H2O2 hydro-thermal treatment and Pt was captured by defect C in the secondary hydrothermal process with a large amount of monatomic PtC4 [50]. Wang et al. dissolved the carbon-based materials into PtCl6 solution, in which PtCl6− were adsorbed on the surface of carbon due to its large specific surface area and multi-layer mesoporous structure. Meanwhile, numerous heteroatoms like N, O and PtCl6− ions were

Table 1 The mass specific activity and specific surface activity of various Pt-based alloy materials under 0.1 M HClO4. Catalyst PtFe-Fe2C Pt-Rh-Ni FePt NSs PtNi Mo-Pt3Ni/C Pt54Fe46

Mass activity @0.9 V (A·mg−1 Pt ) 1.50 0.82 0.57 0.86 6.98 NA

Specific activity @0.9 V (mA·cm−2) 3.53 4.38 0.10 3.24 8.20 0.87

Ref. [36] [37] [42] [43] [18] [44]

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Fig. 2. (a) Schematic illustration of the formation of isolated Pt atoms on nitrogen-doped carbon; (b) schematic illustration of the synthesis procedure of A-CoPt-NC. (Adapted from ref. [51,57]).

anchored on the N-doped carbon surface after reduced under ultraviolet irradiation (Fig. 2a), which increased the content of monatomic Pt (3.8 wt%) and inhibited its degradation [51]. Spatial confinement (Localization of electrochemical reaction) is adopted to achieve uniform spatial distribution and atomic dispersion by some porous materials like molecular sieve, Metal-Organic Frameworks (MOFs) [42–54], Covalent-Organic Frameworks (COFs) [55,56], which are acted as cages to encapsulate and anchor the precursors of nuclear metals. And these precursors would be removed in the following process, while their frameworks would be used to form single atoms and to prevent the further reaction. For example, Co-MOFs were selected as the precursor to obtain Co-NC catalyst with core-shell structure through a nitrogen-doped carbonization process [57]. Subsequently, the local amorphous carbon on the carbon shell layer is oxidized by further electrochemical activation to obtain a graphite carbon fault channel containing rich defective structure (Fig. 2b). Thus, the previous dense carbon shell would be opened due to the formation of graphite carbon fault. During the etching process of acid solution into the metal core, numerous positive ions of Pt and Co were captured and anchored in the defective structure to form high-density pairs of adjacent PteCo atomic active sites. Further Spherical aberration electron microscope analysis suggested that the bimetallic atoms were anchored in two-coupling double-vacancy defects with two metal atoms located in the graphite layer gap concentratedly. Besides, the methods of stepwise reduction, atomic layer deposition and coprecipitation have also been reported to synthesize the monatomic Pt-based catalysts [58–60].

shell structures can be obtained by solvothermal method, specifically, the single core was synthesized firstly and then M@Pt core-shell catalyst was formed with supported Pt on its surface. For instance, both Ir@ Pt and Pd@Pt monolayer with 2D ultrathin core-shell showed the enhanced ORR activity [63,64]. Moreover, the utilization rate of Pt can be also improved by 3D Pt-based material with hollow porous structure [65]. Erlebacher et al. utilized the property of hydrophobic and oxygen solubility for the solution and used proton ionic liquid to tailor the geometric and chemical structure of the composite nonporous NiePt alloy, which endowed the catalyst with extremely high mass activity [22]. In addition, exposing more highly active crystal surfaces is also conducive to improve the ORR activity of Pt-based catalysts. Recently, the ultra-thin serrated Pt nanowires with highly active crystal surfaces have been reported by Duan's group, which were obtained by the pyrolysis of Pt/NiO nanowires with core-shell structure and the following electrochemical process to remove metallic nickel [18]. The prepared Pt nanowires owned large ECSA (118 m2·g−1 Pt ) and 13.6 times more than the current of the equivalent mass of Pt-based catalyst (Table 2). Pt film with two-layer atom thick was prepared by dispersing Pt3Fe-NWs in cyclohexane and the following removing of organic substances through pyrolysis, which exposed high crystal surface activity [66]. This structure could protect the internal Pt3Fe alloy, reduce the dissolution of Fe in the process of ORR and further improve the Table 2 The mass specific activity and surface specific activity of various Pt-based alloy materials under 0.1 M HClO4.

2.3. The controllable synthesis of Pt based catalysts in size and morphology

Catalyst

The controllable synthesis of Pt based catalyst in size and morphology mainly refers to some methods of reducing the amount of Pt usage, such as the design of core-shell structure, the lower amount of Pt by using the materials with Pt-rich on their surface [61,62]. Generally, for the Pt-based catalyst with core-shell structure, the non-Pt metal is used as core and Pt metal as shell to drop the use of Pt, which shows high ORR activity and improved stability with special cores. These core-

Jagged Pt nanowires Ir@Pt Pt-skin Pt3Fe z-NWs/C Pt3Ni(Pt-skin)/Pd20/C Pt-o-Cu3Pt/C Pt NCs/CNTs Pt-Au-Ni/C

3

Mass activity @0.9 V (A·mg−1 Pt )

Specific activity @0.9 V (mA·cm−2)

Ref.

13.6 0.35 2.87 14.2 0.64 0.167 0.83

11.5 0.9 11.5 16.7 1.73 194 1.1

[21] [63] [66] [67] [68] [69] [70]

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Fig. 3. Structural and compositional characterizations of Pt-skin Pt3Fe zigzag-like nanowires. (a) HAADF-STEM image of Pt-skin Pt3Fe z-NWs; (b, d) the enlarged images of red square in (a); (c) structural demonstration of the z-NWs; (e) STEM-EDS elemental mapping of Pt-skin Pt3Fe z-NWs, clearly showing the presence of a “Pt skin” in around two-atomic-layer thickness on the surface. The crystal orientation of the structure is along the zone axis of (110) face. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) (Adapted from ref. [66]).

stability of catalyst (Fig. 3).

substitution measurement. The adsorption state of O2* was formed with the Pt (hkl) single crystal electrodes adsorbing O2 molecules, and the HO2* was formed after proton electron transfer. Then, a pair of adsorbed O*, OH* would be generated on the adjacent platinum atoms after OeO bond breaking. Finally, H2O was captured through proton electron transfer of OH*. There are significant differences in ORR activity on the single crystal surface of various Pt (hkl) in line with the same intermediate having diversified Gibbs free energy and activation energy on the different crystal surfaces, leading their various existence state and subsequent ORR reactions.

3. Reaction mechanisms for ORR 3.1. The single Pt active site: different crystal face It claims that the ORR on Pt surface involves two types, one is that the formation of H2O or OH− is generated by O molecules going through the 4e− reduction, and the other one is to form peroxides by the 2e− reduction of O2 [71–73]. However, there are still some debates about the ORR path on the Pt-based catalyst surface, especially for the detection of the rate-determining step [74,75]. Recently, in situ electrochemical surface enhanced Raman spectroscopy and density functional theory (DFT) calculation was employed to investigate the ORR mechanism on the single crystal surface of Pt (hkl) [76]. The direct Raman spectrum evidence on these reaction species (such as O2−, OH* and HO2*, * means the intermediate absorbed in the active sites) was obtained by enhanced Raman spectrum (SHINERS) technology of electrochemical shell isolated nanoparticle (Fig. 4). As shown in Fig. 4, gold@SiO2 core@shell nanoparticles were used as substrate to cover Pt (hkl). When the potential was decreased from 1.05 V to 0.15 V, the broad Raman peak around 1150 cm−1 of three low-index Pt (hkl) surface appeared at a similar starting potential, indicating the same intermediate species during the ORR process (Fig. 4e–g). Besides, the intermediates species were measured by a deuterium isotopic

3.2. The active site of Pt doped with heteroatoms Pt-based alloy catalysts, namely the integrated Pt with other transition metal materials (such as Fe, Co, Ni, Cu), which boost the ORR by reducing the energy of Pt in the d-band center, decreasing the bond energy of Pt and oxygen intermediate species [77–80]. Guo et al. synthesized a class of Janus-like PtFe-Fe2C NPs by a one-pot approach and the following carbonizing treatment [34]. DFT calculation showed a high energy barrier in the conversion process of O2* + 4(H+ + e−) into OOH* + 3(H+ + e−) for PtFe-Fe2C NPs (Fig. 5), which is not conducive to ORR due to the increased intermediate barrier of Fe2C in the process of OH* + H2O + (H+ + e−) into 2H2O. However, the energy barrier decreases at each step of ORR for the interface of PtFe||Fe2C, especially in the process of O* + H2O + (2H+ + e−) 4

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Fig. 4. (a) Model of shell-isolated nanoparticles (Au@SiO2 NPs, SHINs) at Pt (111) surface and the mechanism of the ORR process revealed by the EC-SHINERS method; (b) TEM image of Au@SiO2 nanoparticles; (c) SEM image of a Pt (111) single-crystal electrode surface modified with SHINs; (d) 3D-FDTD simulations of four SHINs NPs with a model of a 2 × 2 array on a Pt substrate; EC-SHINERS spectra of ORR at (e) Pt (110), (f) Pt (111) and (g) Pt (100) surfaces in 0.1 M NaClO4 solution (pH ≈ 10.3) saturated with O2. The arrows represent the potential scanning direction, and all the potentials are relative to the RHE. (Adapted from ref. [76]).

converting into OH* + H2O + (H+ + e−) with the energy barrier of −6.11 eV. It is suggested that the free H+, e− could be easily obtained by monatomic O on the interface, which agrees with the impressive properties of materials under acidic conditions. In addition, the relaxation of local interface electro lattice coupling can compensate the coulomb repulsive potential of electron transfer between H and O. The obtained energy (−16.36 eV) in the PtFe||Fe2C interface also evidences the efficient ORR performance in the whole process [36]. The introduction of Fe atom in the third layer under two layers of Pt atoms and Ir core in the core-shell Ir@Pt alloy can also jointly reduce the bond energy between Pt and O on the surface [63]. It is indicated that the extended single-crystal Pt3Ni (111) is the most active ORR crystal surface due to the synergistic effect of strain, ligand and overall effect and its closely central position of the Pt dvalence band (Fig. 6). Experiment and DFT calculation proved that Pt site with surface indentation and Pt-based catalyst with other point defect and planar defect also enjoyed high ORR activity [19]. Moreover,

Pt-based or PtNi alloy with different morphologies and components usually exhibit various ORR catalytic properties. And the structural distortion had an essential effect on the ORR activity for Pt-based catalysts in the long cycle [81]. 3.3. The active site of Pt single atom The electrocatalytic oxygen reduction enjoys 2e− or 4e− catalytic pathways in general. With the lack of 2e− by-product hydrogen peroxide, the 4e− ORR path has a higher energy conversion efficiency and stability of catalyst in the application of fuel cells. The choice of 2e− or 4e− depends on whether OeO bond breaks in the ORR process, and the broken OeO bond needs continuous active sites to absor the same O2 molecule. Therefore, the isolated single-atom Pt-based catalyst generally tends to adopt 2e− pathway to catalyze ORR [42]. The synthesized single-atom Pt-based catalyst, on the one hand, can maximize the atomic utilization rate and the exposure of active sites to improve the 5

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Fig. 5. (a) Gibbs free-energy diagrams for ORR in acid media on the Fe2C||FePt interface structure and (b) the corresponding local structures of different intermediates of ORR mechanism. (Adapted from ref. [36]).

interactions and charge transfer between single atoms and carriers and enhance the catalytic performance; on the other hand, which can be used to boost the essential activity of the active sites and to reduce some side reactions. By means of DFT calculation, Xu et al. reported that a single platinum atom stably anchored by four carbon atoms of adjacent carbon ion (PteC4) was the main active center for ORR [40]. While, Wang et al. proved that electrons in a structure like PtN4 were transferred from Pt to the surrounding N, which made Pt with more charge comparing to Pt in its block [51]. Also, two kinds of PteN4 configurations were discussed in Fig. 7, the pyridine-type PteN4 shows more overlap areas between the p orbitals of N and the d orbitals of Pt, suggesting the former is more stable than the latter. The negatively charged N adjacent to Pt can reject the adsorption of OH− on the Pt active site to further expose new Pt active sites and to improve ORR kinetics. When PteN4 active site induced with Co atoms and vacancy, and the (PteCo)@N8V4 (N8 indicates the number of N surrounding the defect and V4 means the number of C vacancy) active sites can be formed. The synergetic effect of the atomic Pt and Co in (PteCo)@N8V4 boosts the d orbital and charge polarization of the active sites, and further affect the dissociation energy of the OeO bond towards the 4e− ORR [57].

internal Fe atoms from being dissolved in the acidic solution during long-term operation [83]. One possible reason for the poor stability of non-Pt catalysts is that the by-product (H2O2) of 2e− in the ORR process can corrode metals and carbon-based materials in practical applications of fuel cells [84,85]. PteCo core-shell catalyst with ultra-low Pt content was prepared by taking Co or bimetallic Co/Zn Zeolite imidazolate frameworks (ZIFs) as the precursors [86]. The stability would be improved due to the synergistic catalysis between PteCo core-shell NPs and Co-NC species. This is because PteCo NPs with core-shell structure owns a relatively high ORR activity and promotes the degradation of H2O2 generated by the non-Pt catalytic active sites. Moreover, the Co-NC layer was generated from ZIFs after carbonization would grow in Pt (100) rather than Pt (111), which is conducive to expose (111) surfaces with higher activity, to decrease Pt dissolution on 100 surfaces and to improve the activity and stability of PteCo core shell. The fuel cell assembled by this kind of catalyst has excellent catalytic performance under barometric pressure of one oxygen or air, high voltage and high current. The ORR activity of these two catalysts is 1.08 A·mg−1 Pt and 1.77 A·mg−1 Pt respectively. When Pt-based catalyst is long time applied in fuel cells, Pt-based oxide would be formed with cathode environment of low pH and high oxygen content. O atoms can be induced the outer layer through exchanging position easily, and thus internal transition metals would gradually dissolve and Pt-based active sites on the surface would dissipate. Hence, the electrochemical activity area shall be decreased with Pt dissolving during the process. Stamenkovic et al. synthesized Ni@Au@PtNi core-shell materials by introducing Au atomic layer (> 3) into Ni@PtNi core-shell to decrease the possibility of O diffusion from the surface to the interior as well as the dissolution of Pt on the surface [87]. The catalytic activity of Ni@Au@PtNi (the content of Au < 5 wt%) decreases < 10%, but its stability improved a lot. Comparing to obviously larger particles in Pt/C and Au without layers of

3.4. The reasons of inactivation and stability analysis The normal commercial Pt/C catalyst usually displays serious agglomeration and the growth of nanoparticles after durability testing, which decreasing the ECSA of catalytic materials. 1D NWs material owns many binding points with carbon carrier to drop the possibility of material movement, agglomeration and degradation [82]. Meanwhile, high-index facets (HIFs) with high specific surface free energy were constructed on the Pt3Fe nanowires (10 nm in diameter and several hundred nm in length). The Pt surface is almost 0.5 nm thickness and is equivalent to 2 layers of atoms. These two layers of Pt can protect the 6

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Fig. 6. Relationship between catalyst distortion and ORR activity. (Adapted from ref. [19]).

Fig. 7. Optimized geometrical structures for (a) pyridine-type PteN4 and (b) pyrrole-type Pt-N4; the unit of bond length is Å. Partial density of states for (c) pyridine-type PteN4 and-(d)-pyrrole-type PteN4, the dashed line at zero energy represents the Fermi level. (Adapted from ref. [51]).

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Fig. 8. (a) The electrochemically active surface areas, (b) the specific activity @0.95 V, (c) mass activity at 0.95 V, (d) TEM images of different catalyst before and after 10,000 CV cycles. (Adapted from ref. [87]).

Ni@PtNi, there were no significant changes both in CV and TEM image of Ni@Au@PtNi after 10,000 cycles due to the Ostwald ripening effect (Fig. 8). In addition to the introduction of Au layer to reduce corrosion, doping a small number of atoms is also a good way to improve stability. It is found that the reason for the morphological damage of octahedral PtNi alloy is the migration of Pt on the surface instead of the dissolution of Ni as commonly believed [88]. Further, it is believed that the doping of Rh was beneficial to the morphological stability and limited the migration of Pt atoms on the surface [37]. In addition, the modified carbon carrier can also enhance the stability of Pt/C catalyst [89,90]. The CeO2/MWCNT (CeO2 modified MWCNT) exhibited better corrosion resistance than that of commercial XC-72, which can protect Pt NPs from degrading easily in ORR process because of the strong interaction between CeO2 and Pt [91]. With CeO2/MWCNT decreasing by 14.7% in ECSA and 9.02% in mass activity and XC-72 by 42.8% and 21.4% respectively in the accelerated aging experiment, CeO2/MWCNT showed higher stability than that of XC-72.

higher atomic utilization. It is worth noting that the stability of the above materials is still difficult to meet the needs of practical applications, and the complexity of the preparation is difficult to achieve real commercialization. Therefore, the priority should be given to develop a new, simple and cheap method. The theoretical calculation is adopted to research the relationship between their structure and catalytic performance in the active sites of Pt-based materials. Meanwhile, some in-situ characterization methods can be used to study the ORR process on the surface of Pt-based materials, such as in-situ electron microscope, in-situ synchrotron radiation so as to testify the catalytic reaction mechanism gained by theoretical calculation. There are still some debates on the actual inactivation reasons with the various catalytic mechanisms on the surface of different Pt-based catalysts. Nowadays, advanced characterization equipments (such as in-situ characterization methods) and theoretical researches are still desired to explore the reasons for catalytic inactivation. The accurate and correct inactivation mechanism can boost the long-term durability of Pt-based catalysts in practical application. Rotating disk electrode (RDE) was still regarded as the main testing method nowadays. Although some ORR catalysts in many works have shown better performance than the current target at 0.9 V, these catalysts have not been converted into real fuel cell equipment. The differences from different equipments make it difficult to match the actual performance of fuel cells and RDE performance. It is suggested that some new evaluation systems are needed to make up these deficiencies, such as the design of gas diffusion electrode, the standardized evaluation for battery testing technology, and so on. Therefore, more efforts

4. Summary and outlook The past decade saw considerable efforts to develop many important Pt-based ORR catalytic materials. It can be seen above that Ptbased alloy enjoys higher performance and can reduce the usage of Pt, while Pt content is still high with the lower proportion of non-Pt metal in alloy. At present, there are still few products with low Pt content satisfying the requirements given by the U.S. department of energy. And the attention is also paid to some Pt monatomic catalysts with its 8

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should be devoted to developing Pt-based catalysts with higher efficiency and stability in fuel cell systems, and to reduce the cost of Ptbased catalysts for the commercialization of fuel cell vehicles.

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