M-N/C Electrocatalysts Derived from MOFs for Oxygen Reduction Reaction

Siyan Yu; Long Zheng; Pengfei Meng; Xiudong Shi; Shijun Liao

doi:10.7536/PC200918

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (10) : 1693-1705 DOI: 10.7536/PC200918

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M-N/C Electrocatalysts Derived from MOFs for Oxygen Reduction Reaction

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Abstract

Benefiting from its high oxygen reduction activity comparable with platinum catalyst, the transition metal and nitrogen co-doped carbon catalyst(M-N/C) has become one of the most important non-noble metal catalysts, and it is recognized as the most promising non-noble metal catalyst, as substitute of precious platinum catalyst, in proton exchange membrane fuel cells in the future. Metal organic frameworks(MOFs), a new class of crystalline porous materials with regular porous structure, high porosity, controllable morphology and size, and tunable ligands, have been regarded as perfect precursors for preparing various high-performance doped carbon catalysts. The catalysts derived from MOF often show superior structure and performance compared with those derived from conventional precursors, making them more prospective for applying in the fuel cells. Actually, it has become one of the most attractive topics to prepare M-N/C catalysts with MOFs as precursors in recent years. In this paper, we systematically introduce the research works on MOFs derived M-N/C catalysts at home and abroad in recent years, including the preparation technologies of MOF derived M-N/C catalysts, the strategies of improving the structure and performance of MOF derived carbon catalysts, as well as the research progress on the characterization technologies of such catalysts. Finally, we indicate the problems and challenges existed in MOF derived M-N/C catalysts, propose possible strategies/measures to solve the problems. The development and application of this type of new catalysts are also prospected.

Contents

1 Introduction

2 General synthesis methods of MOFs materials

3 Progress of MOFs derived carbon catalysts

3.1 Characteristics of MOFs derived carbon catalysts

3.2 General synthetic approach

3.3 Progress of preparation technology of MOFs derived carbon catalyst

4 Studies on improving structure and performance of MOFs derived carbon catalysts

4.1 Regulating the composition of MOFs

4.2 Maintaining the morphology of MOFs-derived carbon

4.3 In-situ growth of MOFs on the substrate

4.4 Pore structure control

4.5 Studies on durability improvement

5 Progress in characterization technology

6 Conclusion and outlook

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Keywords

non-platinum catalyst / M-N/C / MOF / oxygen reduction reaction / atomically

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Siyan Yu, Long Zheng, Pengfei Meng, Xiudong Shi, Shijun Liao. M-N/C Electrocatalysts Derived from MOFs for Oxygen Reduction Reaction. Progress in Chemistry, 2021, 33(10): 1693-1705 DOI:10.7536/PC200918

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1 Introduction

With the increasing shortage of global fossil energy resources and the increasingly serious environmental problems caused by burning a large number of fossil fuels, the development of renewable clean energy and energy technology has been paid more and more attention by countries all over the world. Fuel cell technology and metal-air battery technology can directly and efficiently convert the chemical energy of fuel/metal materials into electrical energy. They have many advantages, such as abundant raw materials, high energy density and environmental friendliness. Therefore, they are known as the two most important new energy technologies in the 21st century, and their research and development have attracted great attention^{[1⇓⇓⇓⇓⇓⇓⇓⇓⇓~11]}. Oxygen reduction reaction (ORR) is an important cathode reaction in fuel cells and metal-air batteries. Due to the slow kinetics of ORR, highly active Pt-based catalysts are required^[6,12,13]. In recent years, low-temperature proton exchange membrane fuel cells and metal-air batteries have entered the early stage of industrialization and practical application. However, the problems of high cost and insufficient durability caused by the use of expensive platinum catalysts still hinder their large-scale commercialization^[4,12,14,15]. The development of high-performance low-platinum and non-platinum catalysts is considered to be the key to the large-scale commercialization of fuel cells and metal-air batteries.

Transition metal and nitrogen Co-doped carbon-based materials (M-N/C, M = Fe, Co, Mn, …) are the most important class of non-noble metal catalysts, which do not use any noble metal and have negligible material cost relative to noble metal catalysts, which is of great significance for reducing the cost of fuel cells and metal-air batteries^{[13,16⇓⇓⇓ ~20]}. More importantly, these catalysts show good catalytic activity under both acidic and alkaline conditions, and are considered to be the most likely non-platinum catalysts to replace Pt catalysts for practical application in fuel cells and metal-air batteries^{[21⇓⇓⇓⇓~26]}.

On the basis of many years of research work, people have realized that the characteristics of precursors (composition, structure, morphology, etc.) Have an important impact on the performance of M-N/C catalysts. Through proper selection and design of the precursor, the structure and performance of the M-N/C material can be adjusted and controlled at the same time, and the catalytic performance of the catalyst is improved. The precursors studied include polymers, macromolecular compounds and biomass materials, and many valuable results have been achieved^[27⇓~29]^[30⇓~32]^[33⇓~35]. However, it is still difficult to effectively control the structure and morphology of the catalyst when using these precursors.

In recent years, it has been found that MOFs are almost the most ideal precursors for the preparation of M-N/C catalysts. MOFs materials contain metal ions (zinc, iron, cobalt, etc.) As central atoms and organic ligands containing N, P, S and other elements. There are more than 6,000 members of the MOFs material family, and their composition, structure and pore size are flexibly controllable, as shown in Figure 1^[36]^[37]. These characteristics bring the diversity of morphology and structure and operability to M-N/C catalyst. First, MOFs-derived M-N/C catalysts can be prepared by pyrolysis and nitrogen-doping/transition-metal-doping techniques. Secondly, in order to maximize the exposure of active sites, researchers have made a lot of efforts in morphology maintenance, pore structure and metal dispersion control, and obtained MOFs-derived M-N/C materials with characteristic geometric morphology and hierarchical porous structure. In addition, the electron density of the material can be adjusted by introducing a second heteroatom source into the MOFs; The introduction of the second metal source can realize the synergistic effect of the metal sites, thereby further improving the oxygen reduction performance of the catalyst. At the same time, reasonable strategies can be taken to improve the stability of the catalyst in the acid medium.

A lot of research work has been done on the preparation technology, composition control, pore size distribution and morphology maintenance control, transition metal dispersion control and durability improvement of M-N/C materials derived from MOFs, and important progress has been made.This paper will briefly introduce the main research progress.

2 General Synthesis Methods of MOFs Material

The typical synthesis method of MOFs is solvothermal method, that is, the coordination reaction between organic ligands and metal ions in solvents^[38]. In addition to solvothermal methods, other methods have been developed, such as microwave-assisted synthesis, ionic liquid-assisted synthesis, ultrasonic-assisted synthesis, electrochemical synthesis and sol-gel synthesis^[39]^[40]^[41]^[42]^[43]. The microwave heating method can heat the inside and the surface of the object to be heated at the same time, and the heating is uniform. Ionothermal synthesis is a method of synthesizing solid materials using ionic liquids instead of water or organic solvents. The ultrasonic-assisted method for synthesizing the MOF material has the characteristics of short reaction time, high yield and the like. In addition, the micro-morphology of the material can be controlled by controlling the ultrasonic reaction time. The reaction of the electrochemical synthesis method can be carried out at normal temperature, thereby saving energy consumption; The reaction raw materials can be basically utilized at 100%; Can be continuously produce. The whole synthesis process of the sol-gel synthesis method does not need the participation of a binder, does not need to be placed in a high pressure environment, and has mild conditions.

3 MOFs Derived Carbon Catalysts and Their Preparation

3.1 MOFs-derived carbon catalyst

The diversity of metal ions and ligands and the diversity of topology and size in MOFs make them excellent precursors to obtain derived carbons with desired morphology, structure and size. Compare with traditional porous material, these MOF-derived materials have that following advantages: (1) they can effectively inherit the size, morphology and pore structure characteristic of MOFs; (2) The carbon matrix obtained by pyrolysis of organic ligands has high conductivity and high specific surface area, which promotes charge transfer and proton transport; (3) various heteroatom can be doped into that carbon matrix through organic ligand, which can induce charge redistribution in the carbon matrix, thereby enhancing the activity of the catalyst; (4) metal atom can be confined at that node or in the MOF cage to effectively avoid agglomeration/sintering.

3.2 Common preparation technique of carbon derived from MOF

The synthesis process of carbon derived from MOFs usually includes two steps: synthesis of MOFs and post-processing. Pyrolysis is the most important post-treatment method for most catalysts. The reasons are as follows: First, MOF precursors are not tolerant to acid mediators. After being converted into carbon by heat treatment, the carbon is relatively more stable in an acid environment; Second, the M-N_xstructure is considered to be the active site of the ORR, and this structure is usually produced under the complex reaction of the high temperature pyrolysis process^[44⇓~46]. In addition, the inactive metals (such as Zn, Al, or Mg) in the MOF framework are also easily removed after high temperature treatment, while the MOF-derived carbon acquires more pores and hollow structures. Pickling is also an important post-treatment method. The main purpose of acid washing is to remove the low activity metal agglomerate species produced during pyrolysis and to produce more pore structure^[47]. Typically, a secondary heat treatment is required after pickling to remove residual acid^[48⇓~50]. MOF-derived carbon materials with enhanced properties can be obtained by reasonably designing the pyrolysis process of MOF. Both inert and reactive gas (E. G., NH₃, H₂) can be use to enhance that performance of the catalyst during the thermal treatment^[51⇓~53].

3.3 Research Progress of Preparation Technology

3.3.1 Pyrolysis method

The most direct method is to prepare M-N/C catalysts by direct pyrolysis of nitrogen-containing MOFs with single active metal (such as Co, Fe, etc.) As the center. Liu et al. First reported the preparation of M-N/C oxygen reduction catalysts using MOFs^[54]. In this pioneering work, the researchers pyrolyzed a cobalt-centered MOFs:Co(Im)₂ with ordered 3D channels. The unstable metallic cobalt material is removed by acid washing, and the CoN_x component is retained. In acidic medium, the catalyst has an onset potential of 0.83 V (vs RHE) and a half-wave potential of 0.77 V (vs RHE). Researchers believe that the CoN_x sites in the catalyst are the reason for the high activity of the catalyst in acidic media. Zhu et al. prepared a highly efficient Mn-NC catalyst by direct pyrolysis of Mn (II) -based MOF (Mn-TTPBA-4) followed by acid washing and secondary heat treatment^[55]. In alkaline medium, the half-wave potential of the catalyst reached 0. 81 V (vs RHE), and the catalyst had good electrochemical stability.

3.3.2 Nitrogen doping technique

During high temperature pyrolysis, the active nitrogen of MOF-derived carbon will be seriously lost, which limits the formation of porphyrin nitrogen (Por-N₄), and then affects the content of active M-N₄. Therefore, achieving a high level of N-doping, especially porphyrin nitrogen, in the MOF-derived M-N/C catalyst is an effective way to enhance the activity of the catalyst. Typical inorganic nitrogen sources include ammonia and sodium azide^[56]^[57]; Organic nitrogen sources include cyanamide, dicyandiamide, melamine, ethylenediamine and nitrogen-containing polymers (polyacrylonitrile, polypyrrole, polyaniline and polydopamine)^[58]^[59]^[60]^[61]^[62]^[63]^[48]^[64].

Li et al. Synthesized nitrogen-doped carbon nanotubes (NCNT) by direct pyrolysis of Zn-Fe-ZIF and dicyandiamide, which was used as a nitrogen dopant, resulting in a high content of graphite N^[59]. Due to its induction effect on the graphite structure, it is helpful to form NCNTs at lower temperatures. The catalytic activity of the catalyst in alkaline medium was higher than that of commercial Pt/C catalyst. Zheng et al. Developed a two-dimensional porous carbon nanosheet catalyst based on two-dimensional bimetallic Zn/Fe-MOF, and used g-C₃N₄ as nitrogen source for nitrogen doping^[65]. The resulting catalyst had a high single/paired Fe atom loading (about 3.89 wt%) and a high N content (about 10.28 wt%). The catalyst showed excellent durability and ORR activity in both basic and acidic media. Deng et al. Obtained hollow ZIF-8 precursor by using tannic acid as etching agent, and obtained high-density Fe-N₄ active structure by using iron acetylacetonate as Fe precursor and g-C₃N₄ as nitriding agent^[66]. When the catalyst is used for PEMFC, the peak power density is as high as 628 mW·cm^-2. Li et al. Prepared porous nitrogen-doped carbon materials by melamine-assisted co-pyrolysis of Fe-ZIF-8^[22]. The catalyst not only has a high content of N atoms and dispersed Fe-N_x active sites, but also has a rich three-dimensional interconnected mesoporous structure. Yu et al. Synthesized Fe-N/C with ZIF-8 and iron acetylacetonate as precursors, and conducted secondary pyrolysis under NH₃ atmosphere. The obtained catalyst had higher graphitization degree, larger specific surface area/pore volume, and higher total nitrogen/pyridine nitrogen content^[67]. The peak power density of PEMFC constructed with this catalyst is as high as 503 mW·cm^-2.

3.3.3 Transition metal doping technique

For MOFs without active sites, a transition metal can be introduced into the MOFs to prepare the M-N/C catalyst. ZIF-8 is a common zinc-based MOF, and zinc has a relatively low boiling point, so it is easy to be removed during pyrolysis at high temperature. Therefore, M-N/C catalysts are usually prepared by introducing transition metals into ZIF-8 and then heat treating. The specific ways of introducing metal sources include physical mixing, chemical substitution, surface adsorption, guest encapsulation and ion exchange^[68]^[69]^[70,71]^[72,73]^[74].

Dodelet et al. used ZIF-8 as the host and iron acetate and 1,10-phenanthroline as the iron and nitrogen precursors^[68]. They were physically mixed by ball milling, and then the catalyst was obtained by secondary pyrolysis. The power density of the catalyst applied to a single cell was equivalent to that of the platinum-based cathode with a loading of 0.3 mgPt·cm^-2. Wu et al. Used chemical substitution method to synthesize iron ZIF precursor, adding Fe and Zn ions and 2-methylimidazole in methanol solution at the same time, and reacting at 60 ℃ to obtain Fe-ZIF^[69]. Atomic iron sites can be generated in a single step of thermal activation, as shown in Figure 2. In the 0.1 mol/L H₂SO₄, the half-wave potential of the catalyst reaches 0.85 V (vs RHE), and the stability of the catalyst is enhanced.

Bao et al. used ammonium ferric citrate (AFC) as an iron precursor to adsorb ZIF-8 in its aqueous solution, and isolated Fe-N sites were formed on the surface of the product after pyrolysis^[70]. Compared with the control samples prepared by physical mixing and guest encapsulation methods, the catalysts prepared by surface adsorption method had higher catalytic activity. Liu et al. Used ZIF-8 with ordered microporous structure as a precursor to uniformly adsorb gas-phase ferrocene (FeCp) molecules under the condition of slow heating, and then carbonized at 800 ℃ under Ar/H₂ mixed gas to prepare porous Fe-N/C with high atomically dispersed iron and high nitrogen content^[71]. The optimized Fe-N/C has a high iron loading of 5.86 wt% and a rich N content of 10.51 at%. Li et al. Used ZIF-8 as a molecular cage to encapsulate iron acetylacetonate (Fe(acac)₃)^[72]. Pyrolysis at 900.d egree. C. in an Ar atmosphere forms Fe-ISAs/CN, as shown in fig. 3. The loading of Fe is as high as 2.16 wt%. The catalyst has high catalytic activity and methanol tolerance.

In ZIF-8, Wang et al introduced a metal precursor FePc (14.66 Å) with a size larger than its cavity (11.66 Å), which broke the confinement effect of the original micropores and promoted the generation of edge active sites and hierarchical pore structures, as shown in Figure 4^[73]. Although the mass fraction of Fe in Fe-N/C catalyst is only 0. 2 wt%, the half-wave potential of Fe-N/C catalyst can still reach 0. 915 V (vs RHE) in alkaline medium.

Yang et al. Synthesized a cationic MOF based on Cd (II) ions as a precursor (exchangeable

ClO 4 -

are contained in the pores). As shown in Figure 5, the iron-containing ion K₂[Fe(CN)₆]^- was introduced into the MOF by ion exchange, and the Fe/N/C catalyst was obtained by high-temperature carbonization^[74].

Jiang et al self-assembled MOF with Fe-TCPP (iron-tetra (4-carboxyphenyl) porphyrin) and H₂-TCPP as mixed ligands and zirconium salt^[23]. Subsequently, N-doped porous carbon (FeSA-N-C) rich in monatomic iron (1.76 wt%) was obtained by pyrolysis, as shown in fig. 6. During heat treatment, the mixed ligand strategy enlarges the distance between adjacent Fe-TCPP ligands, which is conducive to the formation of monatomic iron sites.

4 Study on Improving the Structure and Performance of Carbon Catalysts Derived from MOF

4.1 Modulation of MOFs composition to improve catalyst performance

Different electronegative heteroatoms (nitrogen: 3.04; Phosphorus: 2.19; Sulfur: 2.58; Fluorine: 3.98; Boron: 2.04, etc.) Can effectively adjust the charge distribution on the carbon sites, thereby adjusting the adsorption strength of the intermediate species for oxygen reduction, such as · OOH or · OH, and reducing the reaction energy barrier, thereby improving the catalytic activity^[75]. Therefore, the ORR activity of the derived M-N/C catalyst can also be improved by adding a heteroatom source as a precursor to the MOFs. Zhang et al. First synthesized ZIF-8, impregnated it in Fe(SCN)₃, and then pyrolyzed it to prepare Fe/S-NC co-doped with Fe and S^[76]. In 0. 1 mol/L KOH solution, Fe/S-NC exhibited significantly enhanced ORR activity, good stability and methanol resistance. Zheng et al. Used a two-solvent method to encapsulate ammonium thiocyanate (NH₄SCN) and anhydrous ferric chloride (FeCl₃) with UIO-66-NH₂ as the precursor^[77]. The porous carbon catalyst (Fe/N/SPC) co-doped with iron, nitrogen and sulfur was obtained after pyrolysis, and sulfur doping introduced thiophene sulfur structure with ORR activity, which played an important role in the catalytic ORR reaction. Wei et al. Controlled the growth of ZIF-8 on Fe-PBA to obtain ZIF-8 @ Fe-PBA, which was P-doped with phytic acid as the P source, and then pyrolyzed to obtain 3D-Fe-PNC^[24]. P doping not only improves the ORR activity of Fe-NC catalyst, but also improves its resistance to SO_x, NO_x and PO_x poisoning. Jiang et al. synthesized bimetallic ZIFs precursor by combining the advantages of ZIF-8 and ZIF-67, and obtained CNCo-20 by pyrolysis. After P-doping, the ORR catalytic performance of P-CNCo-20 was further improved, and it also had excellent long-term stability and methanol resistance^[78].

In addition, the doping of the second transition metal can realize the synergistic effect and coupling effect of bimetallic sites, thereby improving the activity of the catalyst^{[79⇓⇓⇓⇓⇓~85]}. Bimetallic MOFs are usually synthesized by introducing other MOFs or adding another metal salt in situ, followed by heat treatment to synthesize bimetallic/carbon-based catalysts with a high density of active sites. Zou et al. introduced Co (II) and Fe (II) by impregnation method using MET-6 as precursor, and obtained Co-Fe @ C catalyst with atomically dispersed bimetallic sites, graded porosity and excellent conductivity after heat treatment at 1000 ℃, as shown in Figure 7^[25].

Kong et al. Synthesized Cu/Fe-MOF-74 by hydrothermal method, and then obtained FeCu @ C catalyst by pyrolysis at 800 ℃ in Ar atmosphere^[86]. Huang et al. Synthesized Fe/Ni-MOFs using nickel acetate tetrahydrate and iron nitrate as metal sources^[87]. Then, the hierarchical porous Fe/Ni-MOFs/NG nanocomposites were prepared using urea as an additive. Long et al. Mixed cobalt nitrate, nickel nitrate and 2-methylimidazole in methanol to obtain Co-Ni-ZIF, and then pyrolyzed in Ar atmosphere to obtain an interconnected porous Co-Ni @ NC-T catalyst with ZIF skeleton^[88]. Zhang et al. Synthesized MOF-74 analogues with Fe and Mn as metal centers, and used melamine as an additional nitrogen source to obtain the structure of N-doped carbon nanotubes (CNT) surrounding Fe-Mn alloy^[89]. The catalyst has excellent ORR activity in an alkaline medium. In addition, the coupling between metal active sites can further improve the ORR performance. Wu et al. Synthesized Fe-Co double sites in N-doped carbon nanotubes (CNTs) by using Zn/Co-MOF as precursor and introducing iron source by two-solvent method^[81]. The structural model of the FeCoN₆ was confirmed by EXAFS characterization. Compared with Co/CNT-C and Fe/CNT-C, (Fe, Co)/CNT with bimetallic sites has higher ORR activity in 0. 1 mol/L KOH solution.

4.2 Study on Morphology Preserving of Carbon Derived from MOF

Earlier studies have shown that pyrolysis at high temperatures (usually above 800 ℃) makes it difficult for MOFs-derived materials to maintain the original skeleton structure and properties of MOFs precursors^[90]. In recent years, it has been found that the size and morphology of MOF precursors can be well maintained by selecting appropriate MOF precursors and reasonable pyrolysis conditions. Using ZIF-8 as the host and ferrocene as the Fe precursor, Deng et al. Successfully prepared a high-performance doped carbon catalyst with high-density active sites by metal vapor deposition^[91]. The catalyst retained the morphology of ZIF-8 and the iron was highly dispersed in the catalyst. The catalyst shows excellent ORR activity and stability in both acidic and alkaline media, and also has excellent PEMFC performance. Dou et al. prepared cobalt and nitrogen Co-doped carbon nanopolyhedra (Co, N-HCNP) with hierarchical porous structure by one-step carbonization of core-shell structure ZIF-8 @ ZIF-67, as shown in fig. 8^[92].

Oh et al. designed a leaf-shaped core-shell hybrid MOF (ZIF-L @ ZIF-67), which was used as a precursor to synthesize cobalt and nitrogen-doped porous carbon materials (Co, N-PCLs)^[93]. The resulting catalyst has a high surface area and a high pore volume while maintaining a thin blade shape. Qian et al. Synthesized ZnO @ ZIF-8 by using MOF-5 as a sacrificial template, impregnated with a metal-ammonia complex, and then pyrolyzed to obtain the M-NC catalyst^[94]. The diameter of the prepared M-NC material shrinks slightly, the three-dimensional cubic morphology is still maintained, and the M-NC material has higher specific surface area and mesoporous/macroporous structure. Electrochemical measurements showed that M-NC catalysts exhibited excellent ORR activity, and the performance of Co-NC in alkaline media was comparable to that of commercial Pt/C catalysts.

Zheng et al. prepared a UIO-66-NH₂ derived porous carbon catalyst (Fe/N/SPC) co-doped with iron, nitrogen, and sulfur by the double solvent diffusion pyrolysis (DSDPM) method^[77]. As shown in fig. 9, the catalyst still maintains a regular octahedral morphology, with a rich mesoporous structure and atomically dispersed active sites. Fe/N/SPC catalyst showed good catalytic activity and stability for ORR in both basic and acidic media. More importantly, the catalyst also has excellent performance in the single cell of the H₂-O₂. Dou et al. Synthesized a kind of Fe/Zn bimetallic ZIF material based on benzimidazole (BIM), and obtained Fe and N-doped hierarchical porous carbon (Fe, N-HPCC) material after pyrolysis^[26]. Fe, N-HPCC basically retains the original cylindrical morphology of its precursor. Electrochemical results showed that the performance of the catalyst in alkaline medium was better than that of commercial Pt/C catalyst. The performance of the catalyst in acidic medium is equivalent to that of a commercial Pt/C catalyst.

4.3 In-situ growth of MOFs on substrate to improve catalyst performance

Generally, MOF precursors are grown in situ on various substrates to improve the dispersion of M-N/C, reduce metal agglomeration and improve the electrocatalytic performance. For example, MOF is grown in situ on graphene or carbon nanotubes. Wang et al. synthesized Co-N_x/C hierarchical porous materials with large/medium/micropore structure and high specific surface area (512 m²·g^-1) by supporting ZIF-67 nanocrystal arrays (ZIF/GO) with graphene oxide and then carbonizing them^[95]. During the carbonization process, the reduced graphene oxide acts as a binder and electrical conductor to connect the ZIF-derived carbon nanoparticles, thus forming a macroporous structure with high electrical conductivity. Meanwhile, the ZIF nanocrystals were transformed into mesoporous/microporous carbon nanoparticles after carbonization and acid washing. Hierarchical macro-/meso/microporous catalysts have exposed active sites, a porous structure, and high electrical conductivity.

Lin et al. prepared the Fe-N-C catalyst by pyrolyzing iron-doped ZIF-8^[96]. To improve the conductivity of the Fe-N-C catalyst, carbon nanotubes (MWCT) were introduced to interconnect the nanoparticles and provide an electron conduction channel, and furfuryl alcohol (FA) was introduced as a second precursor to further improve the intraparticle connectivity/conductivity and intrinsic ORR activity. The improved catalyst enables the power density of the PEMFC to be as high as 820 mW·cm^-2. The ORR half-wave potential in acidic solution is 0.81 V (vs RHE), and the mass activity at 0.8 V is 17.22 A·g^-1.

4.4 Pore structure regulation to improve catalyst performance

The microporous structure can hinder the mass transfer of the catalyst and affect the catalytic activity of ORR. In order to overcome this limitation, researchers can introduce mesopores or macropores into the microporous structure by template method, etching method, surfactant-assisted method and controllable ligand pyrolysis method, so as to obtain catalysts with hierarchically ordered porous structure, which greatly improves the utilization efficiency of catalytic active sites^[97,98]^[99,100]^[101]^[102].

Liang et al. Realized the rapid transformation from ZIF-8 to nitrogen-rich meso-microporous hierarchical porous carbon (NEMC) materials based on the in-situ confined pyrolysis strategy^[103]. As shown in fig. 10, polyvinylpyrrolidone (PVP) forms an amorphous carbon skeleton during pyrolysis, confining the ZIF-8 nanoparticles therein; The shrinkage direction of ZIF-8 in the pyrolysis process was changed by the interfacial force between PVP-derived carbon and ZIF-8, which made it shrink outward along the radial direction, thus forming a uniform mesoporous structure. At the same time, part of the micropores are retained, and finally the three-dimensional nitrogen-rich hierarchical porous carbon material is obtained.

Hu et al. synthesized Fe-doped ordered macroporous/microporous single crystal ZIF-8 (denoted as OMS-Fe-ZIF-8) by combining the two-solvent induced nucleation method and the polystyrene sphere (PS) template pore-forming method, as shown in Figure 11^[104]. Followed by pyrolysis to produce atomically dispersed FeN₄ doped hierarchically ordered porous carbon (FeN₄/HOPC). In the single cell test, the current density of the catalyst was 0.75 A·cm^-2 at 0. 6 V.

4.5 Research on Durability Improvement

According to the main degradation mechanism of M-N/C catalyst, there are two ways to improve the durability of the catalyst: ① to enhance the stability of the carbon skeleton; (2) developing metal center catalysts with weak Fenton effect. Kim et al. Synthesized a core-shell (Co @ NC) catalyst derived from a Co-MOF,Zn_xCo_1-_x(C₃H₄N₂) by a one-pot method^[105]. The Co @ NC catalyst has higher ORR onset potential and half-wave potential than the commercial Pt/C catalyst, and has excellent durability in both alkaline and acidic media. Because on the one hand, N-doped carbon has a high resistance to carbon corrosion; On the other hand, the multilayer N-doped carbon support, as a protective shell of the Co nanoparticles, can greatly avoid the leaching of the Co nanoparticles. Du et al. optimize that Co-based catalyst by adjusting the Co/Zn ratio in the precursor, the Co source, and the reaction time^[106]. The optimized monatomic Co/N/C catalyst showed enhanced ORR activity (E_1/2=0.86 V(vs RHE). More importantly, the catalyst has excellent durability after 30,000 cycles in a 0.1 mol·L^-1KOH solution without significant degradation. Even in the more corrosive acidic medium (HClO₄ of the 0.1 mol·L^-1), there is only a loss of 12 mV after 10,000 cycles. The morphology of the catalyst after cycling was observed by TEM, and it was found that there was almost no change. Therefore, the authors believe that the stable carbon skeleton is the key factor to improve the durability of ORR.

Due to the Fenton effect, MOF-derived iron-based catalysts are prone to generate hydrogen peroxide radical species, which cause severe membrane damage and performance degradation in PEMFCs. Therefore, the durability of the catalyst can be improved by using a low Fenton-reactive metal as a center to synthesize the catalyst (such as Cr-N/C, Ir-N/C, Mn-N/C and the like).

Xing et al. First synthesized ZIF-8, then absorbed chromium chloride into ZIF micropores through host-guest interaction, and then pyrolyzed to obtain Cr/N/C SACs^[107]. The combined analysis of electron microscopy and spectroscopic techniques confirmed that the Cr-N₄ configuration was the main source of its high activity. The half-wave potential of Cr/N/C-950 decayed only 15 mV after 20 000 cycles, which was much lower than that of Fe/N/C-950 (31 mV). Chen et al. Prepared a highly efficient oxygen reduction catalyst of uniformly dispersed monatomic Ir-N-C (Ir-SAC)^[108]. HAADF-STEM combined with XAFS characterization confirms the monoatomic dispersion of Ir, and Ir forms a stable Ir-N₄-Ostructure through coordination. Electrochemical tests showed that the catalyst exhibited excellent catalytic activity and stability in acidic medium.The half-wave potential of the catalyst was as high as 0. 864 V, and its performance almost did not decay after 5000 cycles. Wu et al. Used a two-step method combining chemical doping and adsorption: in the first step, the Mn-doped ZIF-8 precursor was carbonized and then leached with an acid solution to prepare a graphitized carbon host with optimal nitrogen doping and microporous structure^[109]; In the second step, Mn and N sources are continuously added, adsorbed into the 3D carbon host, and then thermally activated. The two-step method can gradually increase the density of the active sites of the MOF-derived nitrogen-doped nano-carbon support and prepare the monoatom-dispersed Mn-N-C catalyst with high activity and high stability.

5 Research Progress of Characterization Technology

Although great progress has been made in improving the performance of MOFs-derived M-N/C catalysts, there are still difficulties in understanding the ORR catalytic mechanism of M-N/C materials. Researchers have combined various techniques to characterize the fine structure of active sites in catalysts, reveal the exact nature of active sites, and then understand the mechanism of ORR catalysis. Among the characterization tools for identifying the active sites of M-N/C catalysts, the most common and dominant techniques are X-ray absorption spectroscopy (XAS), Mossbauer spectroscopy (M Mössbauer), and scanning transmission electron microscopy (STEM). Through STEM and EXAFS, Hu et al. Confirmed that there were Fe-N_x active sites near the carbon-coated Fe/Fe₃C nanoparticles (Fe @ C) in the catalyst. Combined with electrochemical characterization and control experiments, they concluded that the Fe @ C particles produced by high temperature pyrolysis could significantly enhance the oxygen reduction activity of the Fe-N_x active sites^[110]. In order to explore the influence of host-guest chemistry on Fe-N chelate structure, Liang et al^[111]. The results showed that the Fe-N structure of the catalyst prepared from the precursor with low concentration of Fe-mIm had a higher chelating number (~ 5), while the Fe-N structure of the catalyst prepared from the precursor with high concentration of Fe-mIm had a lower chelating number (~ 2 ~ 3). In order to accurately characterize the coordination structure of iron clusters, Xiong et al. Confirmed that four Fe-N bonds were formed in the Fe₁ cluster by X-ray fine structure spectroscopy, and no Fe-Fe bond existed, confirming that it was a monatomic Fe structure^[112]. In the Fe₂ and Fe₃ clusters, each iron atom forms three Fe — N bonds and one Fe — Fe bond or two Fe — N bonds and two Fe — Fe bonds, respectively, which confirms the Fe₂ and Fe₃ cluster structure. Wu et al. characterized and analyzed the dispersion state and structural evolution of Fe species in the thermal activation process with the help of atomic resolution STEM and XAS, and found that the FeO_x particles formed in the Fe³⁺ adsorption process would be transformed into atomically dispersed FeN₄ coordination structures in the thermal activation process^[113]. Zhang et al., combined with XAFS and M Mössbauer characterization techniques, proved that there were various coordination structures (FeN_x,x=4,5,6) of ferric ions in Fe-N-C catalyst, and the relative content of various Fe-N_x coordination structures was related to the pyrolysis temperature of Fe-N-C catalyst^[114]. In Argonne National Laboratory, XAS was used to track the geometric and electronic structure changes of the active site Fe atom with temperature during high temperature pyrolysis, revealing the transition process from Fe precursor to Fe₁-N₄ active site^[115].

6 Conclusion and prospect

Because MOFs materials have extremely high specific surface area, regular morphology and pore structure, and high adjustability of composition and structure, doped carbon (M-N/C) catalysts prepared from MOFs materials can well inherit most of the excellent characteristics of MOFs materials,Moreover, the adjustment and high dispersion of the doped metal elements can be realized by adjusting the central atoms, and the doping of the non-metal elements can be adjusted by ligands, so that the preparation of doped carbon-based catalysts with excellent performance by using MOFs as precursors becomes an important way. In recent years, people have made a lot of valuable achievements in preparation technology, doping technology, design and synthesis of MOFs precursors, understanding the structure and mechanism of MOFs-derived catalysts, and understanding the catalytic performance of catalysts. Table 1 summarizes some important research work on MOFs-derived carbon catalysts at home and abroad in recent years. Generally speaking, M-N/C catalysts have higher activity for oxygen reduction in alkaline media, most of which have surpassed commercial Pt/C catalysts, but have lower activity in acidic media. The main reason is that the reduction of the intermediate hydrogen peroxide can not be carried out efficiently under acidic conditions, and the hydrogen peroxide is diffused into the electrolyte, which is not conducive to the ORR reaction. In addition, similar to doped carbon catalysts prepared from other precursors, MOFs-derived carbon catalysts still suffer from insufficient stability and durability, especially in acidic proton exchange membrane fuel cells.The problem of rapid decay is still the most important factor restricting the practical use of this kind of catalyst, and the decay mechanism of this kind of catalyst is not fully understood, which needs further in-depth study. In addition, the relatively high cost of carbon derived from MOFs also needs to be solved urgently.

References

Publishing order | Descend order by publishing year | Descend order by cited within

[1]	Boudghene Stambouli A, Traversa E. Renew. Sustain. Energy Rev., 2002, 6(3): 295.

[2]	Li Y G, Dai H J. Chem. Soc. Rev., 2014, 43(15): 5257.

[3]	Morozan A, Jousselme B, Palacin S. Energy Environ. Sci., 2011, 4(4): 1238.

[4]	Debe M K. Nature, 2012, 486(7401): 43.

[5]	Bruce P G, Freunberger S A, Hardwick L J, Tarascon J M. Nat. Mater., 2012, 11(1): 19.

[6]	Chen Z W, Higgins D, Yu A P, Zhang L, Zhang J J. Energy Environ. Sci., 2011, 4(9): 3167.

[7]	Zheng Y, Jiao Y, Jaroniec M, Jin Y G, Qiao S Z. Small, 2012, 8(23): 3550.

[8]	Lee J S, Tai Kim S, Cao R G, Choi N S, Liu M L, Lee K T, Cho J. Adv. Energy Mater., 2011, 1(1): 34.

[9]	Cheng F Y, Chen J. Chem. Soc. Rev., 2012, 41(6): 2172.

[10]	Rahman M A, Wang X J, Wen C E. J. Electrochem. Soc., 2013, 160(10): A1759.

[11]	Cao R G, Lee J S, Liu M L, Cho J. Adv. Energy Mater., 2012, 2(7): 816.

[12]	Niu W H, Li L G, Liu X J, Wang N, Liu J, Zhou W J, Tang Z H, Chen S W. J. Am. Chem. Soc., 2015, 137(16): 5555.

[13]	Chung H T, Cullen D A, Higgins D, Sneed B T, Holby E F, More K L, Zelenay P. Science, 2017, 357(6350): 479.

[14]	Gong K, Du F, Xia Z, Durstock M, Dai L. Science, 2009, 323(5915): 760.

[15]	Zhou M, Wang H L, Guo S J. Chem. Soc. Rev., 2016, 45(5): 1273.

[16]	Shen M X, Wei C T, Ai K L, Lu L H. Nano Res., 2017, 10(5): 1449.

[17]	Sarapuu A, Kibena-Põldsepp E, Borghei M, Tammeveski K. J. Mater. Chem. A, 2018, 6(3): 776.

[18]	Zheng Y P, Yang D S, Kweun J M, Li C Z, Tan K, Kong F T, Liang C P, Chabal Y J, Kim Y Y, Cho M, Yu J S, Cho K. Nano Energy, 2016, 30: 443.

[19]	Gu W L, Hu L Y, Li J, Wang E K. Electroanalysis, 2018, 30(7): 1217.

[20]	Bezerra C W B, Zhang L, Lee K, Liu H S, Marques A L B, Marques E P, Wang H J, Zhang J J. Electrochimica Acta, 2008, 53(15): 4937.

[21]	Wang X X, Swihart M T, Wu G. Nat. Catal., 2019, 2(7): 578.

[22]	Chen X D, Wang N, Shen K, Xie Y K, Tan Y P, Li Y W. ACS Appl. Mater. Interfaces, 2019, 11(29): 25976.

[23]	Jiao L, Wan G, Zhang R, Zhou H, Yu S H, Jiang H L. Angew. Chem. Int. Ed., 2018, 57(28): 8525.

[24]	Najam T, Shah S S A, Ding W, Wei Z D. J. Phys. Chem. C, 2019, 123(27): 16796.

[25]	Zhao R, Liang Z B, Gao S, Yang C, Zhu B J, Zhao J L, Qu C, Zou R Q, Xu Q. Angew. Chem., 2019, 131(7): 2177.

[26]	Guo Z Y, Zhang Z P, Li Z L, Dou M L, Wang F. Nano Energy, 2019, 57: 108.

[27]	Brüller S, Liang H W, Kramm U I, Krumpfer J W, Feng X L, Müllen K. J. Mater. Chem. A, 2015, 3(47): 23799.

[28]	Chen Y C, Gokhale R, Serov A, Artyushkova K, Atanassov P. Nano Energy, 2017, 38: 201.

[29]	Han A J, Chen W X, Zhang S L, Zhang M L, Han Y H, Zhang J, Ji S F, Zheng L R, Wang Y, Gu L, Chen C, Peng Q, Wang D S, Li Y D. Adv. Mater., 2018, 30(15): 1706508.

[30]	Lee S, Kwak D H, Han S B, Hwang E T, Kim M C, Lee J Y, Lee Y W, Park K W. Electrochimica Acta, 2016, 191: 805.

[31]	Qing X, Shi J J, Ma C Y, Fan M Y, Bai Z Y, Chen Z W, Qiao J L, Zhang J J. J. Power Sources, 2014, 266: 88.

[32]	Kim D W, Li O L, Saito N. Phys. Chem. Chem. Phys., 2014, 16(28): 14905.

[33]	Ma J H, Shi M H, Yao Z Z, Mi H Y, Hoang T K A. J. Solid State Chem., 2019, 276: 139.

[34]	Hu L B, Yu F, Wang F, Yang S C, Peng B H, Chen L, Wang G, Hou J, Dai B, Tian Z Q. Chin. Chem. Lett., 2020, 31(5): 1207.

[35]	Liang H W, Wei W, Wu Z S, Feng X L, Müllen K. J. Am. Chem. Soc., 2013, 135(43): 16002.

[36]	Furukawa H, Cordova K E, O'Keeffe M, Yaghi O M. Science, 2013, 341(6149): 1230444.

[37]	Park K S, Ni Z, Cote A P, Choi J Y, Huang R, Uribe-Romo F J, Chae H K, O'Keeffe M, Yaghi O M. PNAS, 2006, 103(27): 10186.

[38]	Lee Y R, Kim J, Ahn W S. Korean J. Chem. Eng., 2013, 30(9): 1667.

[39]	Zheng N, Richard M. J. Am. Chem. Soc., 2006, 128:12394.

[40]	Jin K, Huang X Y, Pang L, Li J, Appel A, Wherland S. Chem. Commun., 2002,(23): 2872.

[41]	Son W J, Kim J, Kim J, Ahn W S. Chem. Commun., 2008,(47): 6336.

[42]	Mueller U, Schubert M, Teich F, Puetter H, Schierle-Arndt K, PastrÉ J. J. Mater. Chem., 2006, 16(7): 626.

[43]	Tian T, Zeng Z X, Vulpe D, Casco M E, Divitini G, Midgley P A, Silvestre-Albero J, Tan J C, Moghadam P Z, Fairen-Jimenez D. Nat. Mater., 2018, 17(2): 174.

[44]	Sa Y J, Seo D J, Woo J, Lim J T, Cheon J Y, Yang S Y, Lee J M, Kang D, Shin T J, Shin H S, Jeong H Y, Kim C S, Kim M G, Kim T Y, Joo S H. J. Am. Chem. Soc., 2016, 138(45): 15046.

[45]	Li J K, Ghoshal S, Liang W T, Sougrati M T, Jaouen F, Halevi B, McKinney S, McCool G, Ma C R, Yuan X X, Ma Z F, Mukerjee S, Jia Q Y. Energy Environ. Sci., 2016, 9(7): 2418.

[46]	Ramaswamy N, Tylus U, Jia Q Y, Mukerjee S. J. Am. Chem. Soc., 2013, 135(41): 15443.

[47]	Kramm U I, Abs-Wurmbach I, Herrmann-Geppert I, Radnik J, Fiechter S, Bogdanoff P. J. Electrochem. Soc., 2011, 158(1): B69.

[48]	Wu G, More K L, Johnston C M, Zelenay P. Science, 2011, 332(6028): 443.

[49]	Tran T N, Song M Y, Singh K P, Yang D S, Yu J S. J. Mater. Chem. A, 2016, 4(22): 8645.

[50]	Magali F, Jeremy K, Deborah M. J. Phys. Chem. C, 2012, 116:16001.

[51]	Chenitz R, Kramm U I, Lefèvre M, Glibin V, Zhang G X, Sun S H, Dodelet J P. Energy Environ. Sci., 2018, 11(2): 365.

[52]	Carlota D, Miguel P, Sergio R, Francisco P A. Int. J. Hydrogen. Energ., 2019, 166:F3311.

[53]	Singhal R, Kalra V. ChemPhysChem, 2017, 18(2): 223.

[54]	Ma S Q, Goenaga G A, Call A V, Liu D J. Chem. Eur. J., 2011, 17(7): 2063.

[55]	Xiao L, Yang J M, Huang G Y, Zhao Y, Zhu H B. Inorg. Chem. Commun., 2020, 118: 107982.

[56]	Lefèvre M, Proietti E, Jaouen F, Dodelet J P. Science, 2009, 324(5923): 71.

[57]	Deng D H, Yu L, Chen X Q, Wang G X, Jin L, Pan X L, Deng J, Sun G Q, Bao X H. Angew. Chem., 2013, 125(1): 389.

[58]	Wen Z, Ci S, Hou Y, Chen J. Angew. Chem. Int. Edit., 2014, 126:6614.

[59]	Su P P, Xiao H, Zhao J, Yao Y, Shao Z G, Li C, Yang Q H. Chem. Sci., 2013, 4(7): 2941.

[60]	Lee J S, Park G S, Kim S T, Liu M L, Cho J. Angew. Chem. Int. Ed., 2013, 52(3): 1026.

[61]	Choi J Y, Hsu R S, Chen Z W. J. Phys. Chem. C, 2010, 114(17): 8048.

[62]	Zhong R Q, Zhi C X, Wu Y X, Liang Z B, Tabassum H, Zhang H, Qiu T J, Gao S, Shi J M, Zou R Q. Chin. Chem. Lett., 2020, 31(6): 1588.

[63]	Bashyam R, Zelenay P. Nature, 2006, 443(7107): 63.

[64]	Tang F, Lei H T, Wang S J, Wang H X, Jin Z X. Nanoscale, 2017, 9(44): 17364.

[65]	Zheng L, Yu S, Lu X, Fan W, Liao S. ACS Appl. Mater. Inter., 2020, 12:13878.

[66]	Deng Y J, Chi B, Tian X L, Cui Z M, Liu E S, Jia Q Y, Fan W J, Wang G H, Dang D, Li M S, Zang K T, Luo J, Hu Y F, Liao S J, Sun X L, Mukerjee S. J. Mater. Chem. A, 2019, 7(9): 5020.

[67]	Tran T N, Shin C H, Lee B J, Samdani J S, Park J D, Kang T H, Yu J S. Catal. Sci. Technol., 2018, 8(20): 5368.

[68]	Proietti E, Jaouen F, Lefèvre M, Larouche N, Tian J, Herranz J, Dodelet J P. Nat. Commun., 2011, 2(1): 1.

[69]	Zhang H G, Hwang S, Wang M Y, Feng Z X, Karakalos S, Luo L L, Qiao Z, Xie X H, Wang C M, Su D, Shao Y Y, Wu G. J. Am. Chem. Soc., 2017, 139(40): 14143.

[70]	Ye Y F, Cai F, Li H B, Wu H H, Wang G X, Li Y S, Miao S, Xie S H, Si R, Wang J, Bao X H. Nano Energy, 2017, 38: 281.

[71]	Ye G Y, He Q, Liu S Q, Zhao K M, Su Y K, Zhu W W, Huang R J, He Z. J. Mater. Chem. A, 2019, 7(27): 16508.

[72]	Chen Y J, Ji S F, Wang Y G, Dong J C, Chen W X, Li Z, Shen R, Zheng L R, Zhuang Z B, Wang D S, Li Y D. Angew. Chem. Int. Ed., 2017, 56(24): 6937.

[73]	Jiang R, Li L, Sheng T, Hu G F, Chen Y G, Wang L Y. J. Am. Chem. Soc., 2018, 140(37): 11594.

[74]	Huang J W, Chen Y B, Liu X, Huang Y C, Ding Y L, Xu Y, Yao H C, Zhu H B, Yang H. Chem. Commun., 2019, 55(48): 6930.

[75]	Yang N, Li L, Li J, Ding W, Wei Z D. Chem. Sci., 2018, 9(26): 5795.

[76]	Zhang P F, Chen C, Zhang X H, Jiang Z G, Huang J L, Chen J H. Electrochimica Acta, 2019, 298: 570.

[77]	Zheng L, Dong Y Y, Chi B, Cui Z M, Deng Y J, Shi X D, Du L, Liao S J. Small, 2019, 15(4): 1803520.

[78]	Chen Y Z, Wang C M, Wu Z Y, Xiong Y J, Xu Q, Yu S H, Jiang H L. Adv. Mater., 2015, 27(34): 5010.

[79]	Chen L L, Zhang Y L, Dong L L, Yang W X, Liu X J, Long L, Liu C Y, Dong S J, Jia J B. J. Mater. Chem. A, 2020, 8(8): 4369.

[80]	Lu Z Y, Wang B, Hu Y F, Liu W, Zhao Y F, Yang R O, Li Z P, Luo J, Chi B, Jiang Z, Li M S, Mu S C, Liao S J, Zhang J J, Sun X L. Angew. Chem., 2019, 131(9): 2648.

[81]	Wang J, Liu W, Luo G, Li Z J, Zhao C, Zhang H R, Zhu M Z, Xu Q, Wang X Q, Zhao C M, Qu Y T, Yang Z K, Yao T, Li Y F, Lin Y, Wu Y E, Li Y D. Energy Environ. Sci., 2018, 11(12): 3375.

[82]	Wang J, Huang Z Q, Liu W, Chang C R, Tang H L, Li Z J, Chen W X, Jia C J, Yao T, Wei S Q, Wu Y E, Li Y D. J. Am. Chem. Soc., 2017, 139(48): 17281.

[83]	Duan X D, Ren S S, Pan N, Zhang M D, Zheng H G. J. Mater. Chem. A, 2020, 8(18): 9355.

[84]	Guo X Y, Gu J X, Lin S R, Zhang S L, Chen Z F, Huang S P. J. Am. Chem. Soc., 2020, 142(12): 5709.

[85]	Zhang N, Zhou T P, Ge J K, Lin Y, Du Z Y, Zhong C A, Wang W J, Jiao Q Y, Yuan R L, Tian Y C, Chu W S, Wu C Z, Xie Y. Matter, 2020, 3(2): 509.

[86]	Qiao Y, Ni Y Y, Kong F T, Li R J, Zhang C Q, Kong A G, Shan Y K. Chem. Asian J., 2019, 14(15): 2676.

[87]	Qin X L, Huang Y, Wang K, Xu T T, Wang Y L, Wang M Y, Zhao M, Gao Q. Ind. Eng. Chem. Res., 2019, 58(24): 10224.

[88]	Long J L, Li R, Gou X L. Catal. Commun., 2017, 95: 31.

[89]	Dong Z, Liu G L, Zhou S C, Zhang Y Y, Zhang W L, Fan A X, Zhang X, Dai X P. ChemCatChem, 2018, 10(23): 5475.

[90]	Liu B, Shioyama H, Akita T, Xu Q. J. Am. Chem. Soc., 2008, 130(16): 5390.

[91]	Deng Y J, Chi B, Li J, Wang G H, Zheng L, Shi X D, Cui Z M, Du L, Liao S J, Zang K T, Luo J, Hu Y F, Sun X L. Adv. Energy Mater., 2019, 9(13): 1802856.

[92]	Hu Z W, Zhang Z P, Li Z L, Dou M L, Wang F. ACS Appl. Mater. Interfaces, 2017, 9(19): 16109.

[93]	Park H, Oh S, Lee S, Choi S, Oh M. Appl. Catal. B: Environ., 2019, 246: 322.

[94]	Chai L L, Zhang L J, Wang X, Hu Z Y, Xu Y W, Li T T, Hu Y, Qian J J, Huang S M. Carbon, 2020, 158: 719.

[95]	Wei J, Hu Y X, Wu Z X, Liang Y, Leong S, Kong B, Zhang X Y, Zhao D Y, Simon G P, Wang H T. J. Mater. Chem. A, 2015, 3(32): 16867.

[96]	Zhang C, Wang Y C, An B, Huang R, Wang C, Zhou Z, Lin W. Adv. Mater., 2017, 29: 1604556.1.

[97]	Kone I, Xie A, Tang Y, Chen Y, Liu J, Chen Y M, Sun Y Z, Yang X J, Wan P Y. ACS Appl. Mater. Interfaces, 2017, 9(24): 20963.

[98]	Sun T T, Li Y L, Cui T T, Xu L B, Wang Y G, Chen W X, Zhang P P, Zheng T, Fu X Z, Zhang S L, Zhang Z D, Wang D S, Li Y D. Nano Lett., 2020, 20(8): 6206.

[99]	Gao L Q, Xiao M L, Jin Z, Liu C P, Ge J J, Xing W. J. Energy Chem., 2019, 35: 17.

[100]

Tian P F, Zang J B, Song S W, Zhou S Y, Gao H W, Xu H Q, Tian X Q, Wang Y H. J. Power Sources, 2020, 448: 227443.

[101]

Meng Z H, Cai S C, Wang R, Tang H L, Song S Q, Tsiakaras P. Appl. Catal. B: Environ., 2019, 244: 120.

[102]

Feng L, Yuan S, Zhang L L, Tan K, Li J L, Kirchon A, Liu L M, Zhang P, Han Y, Chabal Y J, Zhou H C. J. Am. Chem. Soc., 2018, 140(6): 2363.

[103]

Lai Q X, Zhao Y X, Liang Y Y, He J P, Chen J H. Adv. Funct. Mater., 2016, 26(45): 8334.

[104]

Qiao M F, Wang Y, Wang Q, Hu G Z, Mamat X, Zhang S S, Wang S Y. Angew. Chem. Int. Ed., 2020, 59(7): 2688.

[105]

Peera S G, Balamurugan J, Kim N H, Lee J H. Small, 2018, 14(19): 1800441.

[106]

Sun W, Du L, Tan Q, Zhou J G, Hu Y F, Du C Y, Gao Y Z, Yin G P. ACS Appl. Mater. Interfaces, 2019, 11(44): 41258.

[107]

Luo E G, Zhang H, Wang X, Gao L Q, Gong L Y, Zhao T, Jin Z, Ge J J, Jiang Z, Liu C P, Xing W. Angew. Chem. Int. Ed., 2019, 58(36): 12469.

[108]

Xiao M L, Zhu J B, Li G R, Li N, Li S, Cano Z P, Ma L, Cui P X, Xu P, Jiang G P, Jin H L, Wang S, Wu T P, Lu J, Yu A P, Su D, Chen Z W. Angew. Chem., 2019, 131(28): 9750.

[109]

Li J Z, Chen M J, Cullen D A, Hwang S, Wang M Y, Li B Y, Liu K X, Karakalos S, Lucero M, Zhang H G, Lei C, Xu H, Sterbinsky G E, Feng Z X, Su D, More K L, Wang G F, Wang Z B, Wu G. Nat. Catal., 2018, 1(12): 935.

[110]

Jiang W J, Gu L, Li L, Zhang Y, Zhang X, Zhang L J, Wang J Q, Hu J S, Wei Z D, Wan L J. J. Am. Chem. Soc., 2016, 138(10): 3570.

[111]

Lai Q X, Zheng L R, Liang Y Y, He J P, Zhao J X, Chen J H. ACS Catal., 2017, 7(3): 1655.

[112]

Ye W, Chen S M, Lin Y, Yang L, Chen S J, Zheng X S, Qi Z M, Wang C M, Long R, Chen M, Zhu J F, Gao P, Song L, Jiang J, Xiong Y J. Chem, 2019, 5(11): 2865.

[113]

Li J Z, Zhang H G, Samarakoon W, Shan W T, Cullen D A, Karakalos S, Chen M J, Gu D M, More K L, Wang G F, Feng Z X, Wang Z B, Wu G. Angew. Chem. Int. Ed., 2019, 58(52): 18971.

[114]

Liu W G, Zhang L L, Liu X, Liu X Y, Yang X F, Miao S, Wang W T, Wang A Q, Zhang T. J. Am. Chem. Soc., 2017, 139(31): 10790.

[115]

Li J, Jiao L, Wegener E, Richard L L, Liu E, Zitolo A, Sougrati M T, Mukerjee S, Zhao Z, Huang Y, Yang F, Zhong S, Xu H, Kropf A J, Jaouen F, Myers D, Jia Q. J. Am. Chem Soc., 2019, 142:1417.