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Review

Recent Advances in Non-Precious Transition Metal/Nitrogen-doped Carbon for Oxygen Reduction Electrocatalysts in PEMFCs

by
Meixiu Song
1,
Yanhui Song
1,
Wenbo Sha
1,
Bingshe Xu
1,2,
Junjie Guo
1,* and
Yucheng Wu
1,*
1
Key Laboratory of Interface Science and Engineering in Advanced Materials of Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, China
2
Materials Institute of Atomic and Molecular Science, Shaanxi University of Science & Technology, Xi’an 710021, China
*
Authors to whom correspondence should be addressed.
Catalysts 2020, 10(1), 141; https://doi.org/10.3390/catal10010141
Submission received: 30 December 2019 / Revised: 18 January 2020 / Accepted: 18 January 2020 / Published: 20 January 2020
(This article belongs to the Special Issue Transition Metal Catalysis)
Browse Figures
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Abstract

:
The proton exchange membrane fuel cells (PEMFCs) have been considered as promising future energy conversion devices, and have attracted immense scientific attention due to their high efficiency and environmental friendliness. Nevertheless, the practical application of PEMFCs has been seriously restricted by high cost, low earth abundance and the poor poisoning tolerance of the precious Pt-based oxygen reduction reaction (ORR) catalysts. Noble-metal-free transition metal/nitrogen-doped carbon (M–NxC) catalysts have been proven as one of the most promising substitutes for precious metal catalysts, due to their low costs and high catalytic performance. In this review, we summarize the development of M–NxC catalysts, including the previous non-pyrolyzed and pyrolyzed transition metal macrocyclic compounds, and recent developed M–NxC catalysts, among which the Fe–NxC and Co–NxC catalysts have gained our special attention. The possible catalytic active sites of M–NxC catalysts towards the ORR are also analyzed here. This review aims to provide some guidelines towards the design and structural regulation of non-precious M–NxC catalysts via identifying real active sites, and thus, enhancing their ORR electrocatalytic performance.

1. Introduction

The worldwide demand of energy is rising drastically with the rapid increasing global population and the progressing development of society [1,2,3]. However, there is an inevitable reality that the traditional fossil fuels have been in a danger of drying up with continuous exploitation and utilization, which has resulted in a universal concern about the energy crisis [4,5]. Besides, the urgent environmental issues caused by consuming fossil fuels, such as pollutants emission and global warming, have also severely threatened the future of human society. These problems have spurred intensive researches on the development of sustainable, eco-friendly and high-efficient new energy systems [6,7]. The fuel cell can convert chemical energy directly into electricity, and is regarded as one of the most promising energy technologies, due to its high efficiency and eco-friendliness [8,9,10]. Among various kinds of fuel cells, the proton exchange membrane fuel cells (PEMFCs) have attracted the most attention over the past decades, due to their high energy conversion rate, high reliability, quick startup, low operating temperature and low pollute emission etc. [11,12,13].
However, the widespread application of PEMFCs is greatly hampered by the high-cost and low-performance electrocatalysts towards the oxygen reduction reaction (ORR) at the cathode to accelerate the sluggish reaction kinetics [14,15]. Typically, there are three main obstacles limiting the mass production of ORR catalysts: (i) the high cost. Currently, the most common-used and effective catalysts in PEMFCs are still Pt-based catalysts, of which the scarcity and high cost have resulted in an excessive pricing of PEMFCs [16]; (ii) the low performance. The complex mechanisms and sluggish reaction kinetics of ORRs necessarily lead to high potential demands and low current density outputs [17]. The main research aim is to fabricate suitable catalysts with abundant active sites and high reaction selectivity, and therefore greatly decreasing the ORR energy barrier and raising the conversion efficiency; (iii) the insufficient stability. The high stability of catalysts is the guarantee of the PEMFCs with excellent durability and lifetime. During the long-term operation of PEMFCs, the catalyst would go through Ostwald ripening that causes particle agglomeration and growth, surface oxidation state changes, component migration and loss, etc., which would gradually age and inactivate the catalyst, thus directly leading to the degradation of PEMFCs’ performance [18,19,20,21].
From an economic point of view, some noble metal-based (Au, Ag, and platinum group metals: Pt, Pd, Ru, Rh, Ir and Os) electrocatalysts would definitely increase the manufacturing cost of fuel cells as the scarce and high cost of noble metals [22,23,24]. To solve these problems, numerous efforts have been devoted to exploring non-precious transition metal catalysts, owing to their abundant reserves, economic applicability and their potential catalytic activity comparable to noble metals, while most of the main group metals have shown little potential towards ORR catalysis [25,26]. Particularly, one of the exceptions is the hybrid of transition metal supported on nitrogen-doped carbon (M–NxC, M = non-precious transition metals, generally x = 2 or 4, which corresponds to the bonding of MN2 or MN4, respectively) due to two possible reasons below: (i) The introduction of carbon supports could greatly stabilize and disperse the transition metals, therefore enhancing the ORR catalytic performance; (ii) the synergetic effects between the nitrogen-doped carbon supports and transition metals could significantly increase the number of active sites and boost the catalytic behavior [17,27,28,29,30]. Nevertheless, the reaction kinetics, and in particular, the modulating mechanism of N atoms on the surface state and electronic structure of the catalysts still remains ambiguous.
In this review, we aim to provide an overview of the development of M–NxC ORR catalysts and the summarization of recent advances in this area. Herein, we offer a brief introduction to the reaction of oxygen reduction and its thermodynamic mechanism. We specially pay attention to the research progress in M–NxC ORR catalysts with emphasis on the choice of carbon sources and nitrogen sources, the state-of-the-art Fe–NxC and Co–NxC catalysts, and the analysis of their catalytic sites.

2. Brief Introduction of the ORR Mechanism

As shown in Figure 1, there are three sections in the PEMFC, including an anode where H2 would be oxidized (hydrogen oxidation reaction, HOR), a cathode where O2 would be reduced (ORR), and a proton exchange membrane through which H+ could be transferred from the anode to the cathode [31,32,33]. The reaction on the cathode, the ORR, is a complex process which involves various basic and irreversible intermediate reactions [34,35]. It can be seen from Figure 2 that the process could proceed through two pathways: A direct four-electron way or an indirect two-electron way, varying with the electrode material and pH of the electrolyte. When it comes to the two-electron pathway, the O2 molecules would accept two electrons first to be reduced to H2O2 (in acid and neutral solution) or HO 2 − (in alkaline solution), and then adsorb another two electrons to be transferred into H2O or OH− eventually [36,37]. The generation of H2O2 in electrolyte surely cause damages to catalysts and the proton exchange membrane, thus reducing the lifetime of PEMFCs [38]. Hence, the most ideal mechanism of fuel cells is the four-electron pathway, where oxygen molecules could accept four electrons in succession to be reduced into H2O (in acid solution) or OH- (in alkaline solution), with no detectable intermediates (H2O2) in the final electrolyte [39,40].
However, in most instances, the ORR proceeds via the two-electron pathway, or the parallelism of the two-electron and four-electron way on the electrode surface, as the dissociation energy of the O–O bond in O2 is 498 kJ/mol, which is much higher than that in H2O2 (143 kJ/mol), suggesting a decreasing activation energy of the ORR. The best solution is to develop effective catalysts to selectively reduce the bond energy of O2, therefore boosting the four-electron process [41].

3. Previous M–NxC Catalysts

To explore optimal approaches to produce highly stable and active catalysts with the M–NxC structure, lots of efforts have been devoted to exploring the synthesis strategies and the nature of the catalytic active sites [42,43,44,45,46,47,48,49]. This section reviews the research progress in transition metal catalysts supported on nitrogen-doped carbon with its emphasis on the choice of carbon sources and nitrogen sources, the control of experimental conditions, and the analysis of catalytic sites. Non-pyrolyzed and pyrolyzed transition metal macrocyclic compounds will be firstly discussed, as they are the pioneers of M–NxC catalysts to provide the fundamental research conditions on which the design of the follow-up discussed recent developed M–NxC catalysts are based.

3.1. Non-Pyrolyzed Transition Metal Macrocyclic Compounds

The earliest discovery of M–NxC catalysts should date back to 1964 when cobalt phthalocyanine (CoPc), a kind of transition metal macrocyclic compound, was investigated as a fuel cell cathode catalyst in alkaline electrolyte by R. Jasinski [50]. Since then, macrocyclic compounds composed of various central transition metal atoms (e.g., Fe, Co, Ni, Mn, Cr, etc.) and ligands, such as tetraphenylporphyrin (TPP), tetraazaannulenes (TAA), tetramethoxyphenylporphyrin (TMPP), phthalocyanine (Pc), tetradithiacyelohexeno-tetraazaporphyrin (TDAP) etc., have entered the researcher’s vision and study fields [51,52,53]. Also, their catalytic behaviors were sequentially demonstrated in acidic electrolytes [54,55,56].
Figure 3 shows the typical structure of these compounds where there is usually a chelating group of four nitrogen atoms coordinating with a central metal atom. It has been proven that the catalytic activity of these macrocyclic materials is directly related to the central metal ion and the surrounding ligand structure [57,58]. For a given metal center, the ORR catalytic activity is strongly affected by the nature and electron density of the ligand in macrocyclic compounds [51,59]. Alt et al. [60] experimented with the ORR catalytic behaviors of several Co-ligand compounds in acidic electrolyte, including Pc, TPP, TAA, TMPP, TDAP and TPAP (tetrapyridino-tetraazaporphyrin), with an activity order of TAA > TDAP > MPP > Pc > TPAP > TPP. Similar conclusion was drawn by Song and his partners in 1998 [61]. Besides, K. Wiesener [62] and his colleagues proved that the catalytic performance depends greatly on central metal atoms (Fe, Co, Ni, Mn, Cu etc.) which mainly affect the ORR process, whether for a four-electron or a two-electron process for a given macrocyclic ligand. The ORR activities of various central transition metals coordinating with the Pc ligand followed the order of Fe > Co > Ni > Cu ≈ Mn. With many far previous explorations, transition metal macrocyclic compounds are regarded as a promising kind of cathode catalyst to substitute Pt for ORR. However, these macrocyclic materials are rarely used directly in PEMFCs, due to their poor performance in activity and durability. It was indicated that FePc could facilitate the ORR with a four-electron process, but with a poor stability, while CoPc with a good electrochemical stability were favorable to the two-electron pathway to produce H2O2 [63,64]. They were all limited by theses inherent drawbacks to be used as cathode catalysts in PEMFCs. Therefore, it is of great significance to design a method to raise the stability and activity of transition metal macrocyclic compounds synchronously.

3.2. Pyrolyzed Metal Macrocyclic Compounds

In 1973, Alt et al. [65] found that Co- and Fe-TMPP complexes were more stable in 3 N H2SO4 after a high-temperature treatment. In 1976, Jahnke et al. [66] pointed out in their article that the catalytic activity and stability of various N4-chelates in acidic media could be considerably improved through thermal pretreatment in an inert gas atmosphere (e.g., argon gas). Since then, heat treatment has aroused great research interests, and also been viewed as an effective way to improve the catalytic performance of transition metal macrocyclic compounds [52,67,68,69]. Studies [70,71,72] have revealed that N4-chelates could obtain the highest catalytic activity via a pyrolysis at 500–700 °C, while a temperature of 800 °C is required to achieve a stable catalytic performance in PEMFCs. Moreover, a higher temperature above 1000 °C would lead to an obvious decrease of stability and activity due to (i) the formation and growth of metal particles [73,74] and (ii) a decline in the nitrogen content of catalysts surface [75].
As for the accounts to why thermal treating could improve the macrocyclic compounds’ behavior, there are three main statements [76,77,78]: (i) Heat treatment could improve the dispersity of coordination compounds, therefore boosting the catalytic activity; (ii) heat treatment could lead to the polymerization of macrocyclic compounds, which results in a highest activity at lower temperature; (iii) heat treatment could facilitate the formation of compounds containing M–N4 groups, which are acknowledged to account for the rising activity and long-term stability in low temperature range (500–700 °C). Although these effects would improve the catalysts’ electrocatalytic performance, and could provide reference guidelines to materials design, the material structure and active components of pyrolyzed metal macrocyclic compounds after thermal treating still remain controversial.

4. Recent Developed M–NxC Catalysts

Although the aforementioned non-pyrolyzed and pyrolyzed transition metal macrocyclic compounds have presented impressive ORR electrocatalytic activities, some immanent problems restricting their further development should not be overlooked, including their high cost, complex fabrication process, low activity and stability in acidic electrolyte [79]. Hence, lots of efforts have been devoted to developing a kind of effective catalysts based on cheap and easily available precursors, while the metal macrocyclic compound itself serves as both a nitrogen source and metal donor on the carbon matrix. For the past few decades, researchers attempted to prepare M–NxC catalysts through processing the total mixture of individual metal salt, nitrogen source and carbon support under the high temperature with a certain gas atmosphere [80]. For example, Bouwkamp-Wijnoltz et al. [81] prepared the catalysts from heat-processing the mixture of cobalt acetate, carbon black and various nitrogen donors, among which the best catalytic results comparable to that of heat-treated cobalt porphyrin were obtained with 2,5-dimethylpyrrole. The results of Extended X-ray Absorption Fine Structure (EXAFS) revealed similar active sites (CoN4) in both types of catalyst. This approach provides more possibilities of fabricating a variety of catalysts via the flexible permutation and combination of different metals, N donors and carbon sources. Indeed, it has been acknowledged as one of the most cost-effective and preparation-effortless methods to prepare M–NxC catalysts.

4.1. Choices of Nitrogen and Carbon Sources

As for the introduction of nitrogen and carbon sources, there are various options. First, just for transition metal macrocyclic compounds, such as FePc, the metal, N atoms and carbon support are provided by an individual precursor, which is usually expensive and hard-won in practical applications [82]. Second, the N atoms and carbon matrix in M–NxC catalysts could be introduced by using a kind of precursor containing both nitrogen and carbon, such as polyacrylonitrile (PAN) [83], polyaniline (PANI) [84,85], polypyrrole (PPy) [86], polythiophene (PT) [87], ethylenediamine (EDA) [88], cyanamide (CA) [89] and other organic polymers. Most recently, Liu et al. [90] reported a kind of Co/N-doped cross-linked porous carbon (Co/N-CLPC) catalyst from PAN through a one-step, in-situ synthesis method (Figure 4). Co/N-CLPC showed an excellent ORR catalytic activity with an onset potential of 0.805 V (vs RHE) and a limiting current density of −5.102 mA cm−2, which are comparable to those of commercial Pt/C catalysts.
In most cases, to increase the electrical conductivity or form a certain structural morphology of the catalysts, researchers are pleased to introduce extra carbon supports. For example, Wu et al. [91] synthesized a kind of PANI-M–C catalysts with the short-chain aniline oligomers polymerizing together on the surface of carbon spheres, and then the metal aggregating encapsulated in carbon shells (Figure 5). Liu et al. [92] developed a kind of catalyst with single iron atoms immobilized on the wall of a carbon nanotube (SAICNT), where the pyrrole polymerizes on an organic template (methyl orange) to fabricate a nanotube structure which is favorable to electron transport, and increases the number of active sites (Figure 6). The SAICNT exhibited an ultrahigh ORR activity with a half-wave potential of 0.93 V and a current density of 59.8 mA cm−2 at 0.8 V, which are far better than that of commercial Pt/C. Besides, Huang et al. [93] prepared a trifunctional catalyst via electrodepositing Co ions and PPy onto carbon fibers to form a tube-shaped product with abundant active sites (Figure 7).
The third method to introduce nitrogen and carbon is to use two kinds of materials which contain N atoms and carbon basis, respectively. Generally speaking, the carbon precursors are the same as those used to prepare carbon nanomaterials, such as biomass materials, while the existential forms of carbon supports used for electrocatalysts preparation vary from two-dimension nanosheets (e.g., graphene and its derivatives [1,94,95,96,97]) to three-dimension carbon nanostructures (e.g., carbon nanotubes [98,99], carbon fibers [100,101,102], carbon spheres [103] and porous carbon [3,104]). As there are already some excellent reviews talking about the effects of carbon supports in electrocatalysts, we will not repeat them in this article. Nitrogen sources is an essential part of the M–NxC catalysts so as to provide N atoms chelating with metal ions to form the MN4 active sites. Apart from the aforementioned organic polymers, there are two types of common-used, individual nitrogen sources: (i) some organic monomers or small organic molecule compounds, such as pyrrole, phenanthroline, carbamide, melamine etc.; (ii) some inorganic compounds, such as NH3.
For instance, Liang et al. [105] chose melamine and the ordered porous resin (OPR) as nitrogen and carbon sources, respectively, to prepare the catalysts of Fe–N–CNT-OPC, which displayed an ORR electrocatalytic activity similar to that of the 20% Pt/C catalyst. It was indicated that the Fe–N–CNT-OPC possessed abundant Fe–N active sites, high porosity favorable to the electron and transportation, as well as ample graphitic CNTs to ensure the conductivity (Figure 8). Kim et al. [106] reported a sort of Fe–N–C catalyst from pyrolyzing the mixture of phenanthroline and Ketjenblack carbon supports. According to X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) analyses, the active sites FeNx would prefer to form with pyridinic N stemming from phenanthroline. Compared with organic nitrogen sources, the researches on inorganic compounds containing N atoms used for the electrocatalyst preparation are mainly focused on the utilization of NH3.
In 2016, Park et al. [107] reported a novel catalyst of FexNy/NC nanocomposite with an excellent ORR activity, a long-term catalytic stability, and a direct four-electron pathway in alkaline electrolyte. As show in Figure 9, the mixture of carbon black and Fe salt was heat treated in NH3, thus obtaining the FexNy/NC catalyst directly. Besides, some compounds which could release NH3 under heating conditions, such as urea, have also drawn researchers’ attention. For example, Liu et al. [108] fabricated a sort of Fe/NG/C catalysts with a comparable ORR activity to commercial Pt/C catalysts in 0.1 M KOH. The procedure was simply to thermally decompose the hybrid of graphene oxide, urea, carbon black and iron species, where the N-doping process was accomplished via the decomposition of urea to release NH3. Usually, some porous structure would be generated on carbon supports during the process of NH3 blown off by heating the compounds adhering to the carbon precursor, that is to say the activation function of NH3 [109,110].

4.2. Types of Metal

Apart from the aforementioned heat treatment, with choice of nitrogen and carbon precursors, the transition metal types also have extremely significant impacts on the nature and electrocatalytic performance of the M–NxC catalysts. Generally, there are several non-precious transition metals which are researched in ORR electrocatalysis, including Fe, Co, Ni, Mn, Cu, Zn and so on, among which the most frequently-studied and effective are mainly Fe and Co. The major merit of Fe- and Co-based ORR M–NxC catalysts is not only the low cost but also their excellent behavior in both acidic and alkaline media [111].
The Fe–NxC and Co–NxC catalysts have been widely studied for a long time, since the cobalt phthalocyanine was proven to exhibit a good oxygen reduction activity in alkaline solution in 1964 [50]. In the earliest studies, main attention was focused upon the exploration of the macrocycle compounds of these two metals used in ORR and the related study of performance improvements, active sites, as well as catalytic mechanism. Then, lots of efforts have been devoted into the exploration of inorganic and small organic metal donors, the design of new synthesis routes, and the structure and property optimization of Fe–NxC and Co–NxC.
Recently, researchers are challenging to raise the catalytic activity of Fe–NxC and Co–NxC catalysts in a wide pH range as high as possible via tuning and increasing the active sites [48]. Considerable progresses have been achieved in the synthesis and theoretical analysis of highly active M–NxC catalysts toward ORR in both acidic and alkaline solutions during the past few years.
Very recently, one of the most attractive research issues about M–NxC catalysts would be the design of single-atom M–NxC, which is favorable to realize the utmost utilization of metal sites in catalysts, and the requirements of high catalytic activity. For example, Xia et al. [112] prepared the carbon nanotubes doped with Fe single atoms (Fe–N/CNT) through direct heat treatment of the mixture of the Fe precursor and carbon matrix in the NH3 atmosphere (Figure 10). The Fe single atoms were incorporated with N atoms to form Fe–Nx active sites during the growth process of CNT. As Figure 11 indicated, the single-atom Fe–N/CNT catalyst was more active and stable than the commercial Pt/C catalyst in 0.1 M KOH. In practice, the preparation of Fe or Co single atoms is relatively difficult, as these non-precious transition metal atoms would migrate together and aggregate into nanoparticles or compounds at high temperature. To solve this problem, Li et al. [113] developed a secondary-atom-assisted method to prepare monatomic Fe anchored on porous N-doped carbon nanowires (Fe–NCNWs). The Fe ions were surrounded by secondary atoms (Al, Mg, or Zn) which could ensure Fe ions be transformed into Fe single atoms rather than nanoparticles, and produce porous structure by their pyrolysis under high temperature (Figure 12). Similarly, Yin et al. [114] obtained the Co SAs/N–C catalysts through the pre-carbonation of bimetallic Zn/Co metal-organic frameworks, and then the evaporation of Zn above 800 °C. The prepared Co SAs/N–C showed an excellent catalytic performance towards ORR with a half-wave potential of 0.881 V, which is much higher than that of commercial Pt/C (0.811 V). Overall speaking, this approach makes the preparation of single-atom M–NxC catalysts more facile and reliable. Apart from the fact that the monatomic catalysts have exhibited satisfactory activity towards ORR, recently Zeng’s group [115] found that Fe single atoms together with Fe nanoclusters anchored on nitrogen-doped carbon support (FeAC@FeSA–N–C) displayed superior ORR catalytic activity (Figure 13). Their work provides a novel insight and pathway to the catalysts design.
To increase the active sites in catalysts, some porous organic materials with high surface areas, tunable compositions and various structural topologies, such as metal organic frameworks (MOFs) [116], zeolitic imidazolate frameworks (ZIFs) and covalent organic frameworks (COFs), are used to serve as carbon support after being heat treated. For example, Zhang et al. [117] used an MOF material named NH2-MIL-101@PDA to synthesize the Fe–N–C catalysts with higher ORR activity and stability than those of Pt/C catalyst (Figure 14). According to the XPS results, the Fe–N–C had a high content of N (8.07 at.%) and Fe–Nx (1.22 at.%), which is one of the reasons for high activity. Besides, the large surface area and porous structure are also favorable to the improvement of catalytic behavior. Mao et al. [118] studied the catalytic performance of several Co/N/C catalysts derived from Co-doped ZIF precursors by tuning the experimental conditions, such as the ratio of precursors, reaction temperature and reaction time. The sample of Co/N/C-1000, for which the heating temperature was 1000 °C, displayed a higher half-wave potential of 0.856 V than that of commercial Pt/C in alkaline condition and an outstanding stability in both alkaline and acidic solutions.
Apart from some traditional synthesis methods, some novel and effective approaches are emerging for facile synthesis of M–NxC catalysts. For instance, Peng et al. [119] developed a non-pyrolysis method to prepare pfSAC-Fe catalysts via intermolecular interactions between the Fe-containing COF and the graphene matrix. The prepared catalysts exhibited the superior ORR activity with four times higher than that of commercial Pt/C. This method could also simplify the computational process, as no random structures are involved. Kiciński et al. [120] introduced a permanent magnet to the ORR test system to provide an external magnetic field (Figure 15). This method was proven effective to increase activity and boost the 4e− pathway, as the catalytic performance of Fe–N–C/S was obviously enhanced with an applied magnetic field in the test.
Since the earliest study of M–NxC catalysts, intensive focus and efforts have been attached to the exploration and discussion on the active structure favorable to the ORR. It should be noticed out that the ORR active site of M–NxC catalysts may be an integration of some active components varying with the potential changes, rather than a steadfast concrete structure. Although there are some people favoring of the statements that either some metal-free sites [121] or some carbon layer-coated metal sites [122,123] are the active sites in M–NxC catalysts for the ORR, the most widely accepted theory is still that M–Nx sites are the true active components for ORR catalysis [124,125,126]. However, the debate, about whether M–N2 or M–N4 is the real active sites, never stop. For example, the majority of researchers believe the Fe–N4 groups are the active sites towards ORR [127], while the rest of investigators think that Fe–N2 would play a major part in ORR catalysis [128].
In practice, M–NxC catalysts are usually consisted of M–Nx sites and graphene encapsulated metal nanoparticles simultaneously, which increases the difficulty to identify the true active sits in M–NxC. Most recently, Feng’s group [109] proposed a low-temperature NH4Cl-treatment method to efficiently wipe off the graphene-encapsulated nanoparticles from M–NxC catalysts without destruction of M–Nx groups (Figure 16). This strategy firmly demonstrates the dominant position of M–Nx sites in catalyzing ORR. Wang et al. [129] used first-principles calculation to build a microkinetic model for ORR on single atom Fe–N–C catalysts, and the modeling results indicated that the real active site of single atom Fe–N–C is the Fe(OH)N4 group, rather than inactive FeN4 center, as the latter is covered with an intermediate OH* which is a part of the active component, and the ΔG values along the associative path on the Fe(OH)N4 center are more favorable to ORR catalysis than that of FeN4 center (Figure 17).
Then they applied the conclusion to analyze the mechanism of ORR catalyzed on single atom Fe–N–C (shown in Figure 18), and the result was in good accordance with the previous experimental results. For Co–NxC, several studies have indicated that both the Co–N2 and Co–N4 sites have ideal catalytic performance towards ORR, while the Co–N4 could be converted into Co–N2, which has a stronger interaction with H2O2 under high temperature, therefore boosting the 4e− process of ORR [130,131,132,133]. Zhai et al. [134] prepared a catalyst of cobalt and nitrogen co-doped reduced graphene oxide (Co–N-rGO) with an increasing activity, a four-electron selectivity and a similar stability with commercial Pt/C. They gave some possible active sites in the Co–N-rGO towards ORR, including the edge plane CoN2/C, CoN4/C, and basal plane macrocyclic CoN4/C, via DFT calculations shown in Figure 19. Most recently, Xiao et al. [135] firstly reported a novel binuclear active site structure of Co2N5 with a Co–Co distance of 2.1–2.2 Å. The strategy is to encapsulate the CoN4 and Co particles in the carbon shell through the self-adjusting of the bimetal organic framework. The mechanism of Co2N5 for catalyzing ORR is shown in Figure 20. It was indicated that the ORR catalytic activity of Co2N5 is higher than that of CoN4, which opens a new door to the fabrication of highly effective ORR electrocatalysts.

5. Conclusions

We have introduced the mechanism of ORR briefly and summarized the history of M–NxC catalysts, including the earliest non-pyrolyzed transition metal macrocyclic compounds, then pyrolyzed transition metal macrocyclic compounds, and recent developed M–NxC catalysts with simple and general structures. Besides, we give more detailed information about the currently developed M–NxC catalysts from the perspectives of choice of carbon and nitrogen precursors, the recent emerging Fe–NxC and Co–NxC catalysts, the newly developed method to prepare Fe–NxC and Co–NxC catalysts, and their active sites analyses. The intrinsic structure and catalytic performance of M–NxC catalysts are directly influenced by the choice of precursors. In most recent achievements, some porous organic materials with high surface areas, tunable compositions and various structural topologies, such as MOFs, ZIFs and COFs, have gained many priorities to be the precursors. The size of metals in the M–NxC catalyst is reduced to single atoms, which contributes to an enhanced catalytic performance due to the utmost metal utilization and the low coordination to raise the activity, and the N-containing support’s anchoring effect to improve the stability. The most acknowledged active sites of M–NxC catalysts are M–N2 and M–N4 groups, while there are some trials to design new kinds of active sites, such as the binuclear active sites. Overall speaking, the thorough understandings about the M–NxC catalysts stated in this review are favorable to provide a universal principle of highly effective M–NxC catalysts’ design.
In fact, although the M–NxC catalysts with high ORR activities are demonstrated as one of the most promising substitutes for Pt-based catalysts, their performance is far from the practical requirements in PEMFCs due to the appeal to much higher activity and stability in acidic environments. According to this review, we think there are three main directions in the M–NxC ORR catalysts field:
  • The study of the ORR catalytic mechanism. Understanding the character of M–NxC and the catalytic kinetics in the catalytic process is helpful to us to regulate the catalyst structure and composition accurately, therefore improving the catalytic performance.
  • The study of active sites. Although there are already several researches on the identification of active structures, the true catalytic sites in M–NxC catalyst are still confusing which drastically hinder the development of catalysts with adequate activity and stability. It is suggested that the atom-by-atom structural and chemical analysis in graphene can be achieved by the gentle ADF-STEM, which is a promising pathway to explore the local environment of active sites [136,137].
  • The exploration of new types of active sites. The aforementioned design of binuclear active sites, which are proven to be more positive than traditional M–N2 and M–N4 active sites, is a successful example to search for new kinds of active sites. It has inspired us greatly to enter a newly emerging but promising world to design satisfactory ORR catalysts.

Author Contributions

Data Collection and Writing-Original Draft Preparation, M.S.; Writing- Review, Y.S.; References Downloads, W.S.; Writing-Review and Supervision, B.X.; Writing-Review & Editing, Supervision and Project Administration, J.G.; Writing-Review and Supervision, Y.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China, grant number 51501124; the Joint Funds of the National Natural Science Foundation of China, grant number U1810204; Program for the Innovative Talents of Higher Education Institutions of Shanxi (PTIT); Science and Technology Major Project of Shanxi Province, China, grant number 20181102019, and Special Foundation for Youth SanJin Scholars.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Guo, J.J.; Mao, Z.; Yan, X.L.; Su, R.; Guan, P.F.; Xu, B.S.; Zhang, X.F.; Qin, G.W.; Pennycook, S.J. Ultrasmall Tungsten Carbide Catalysts Stabilized in Graphitic Layers for High-Performance Oxygen Reduction Reaction. Nano Energy 2016, 28, 261–268. [Google Scholar] [CrossRef] [Green Version]
  2. Zhu, C.Z.; Fu, S.F.; Xu, B.Z.; Song, J.H.; Shi, Q.R.; Engelhard, M.H.; Li, X.L.; Beckman, S.P.; Sun, J.M.; Du, D.; et al. Sugar Blowing-Induced Porous Cobalt Phosphide/Nitrogen-Doped Carbon Nanostructures with Enhanced Electrochemical Oxidation Performance toward Water and Other Small Molecules. Small 2017, 13, 1700796–1700804. [Google Scholar] [CrossRef]
  3. Song, M.X.; Song, Y.H.; Li, H.; Liu, P.Z.; Xu, B.S.; Wei, H.; Guo, J.J.; Wu, Y.C. Sucrose Leavening-Induced Hierarchically Porous Carbon Enhanced the Hydrogen Evolution Reaction Performance of Pt Nanoparticles. Electrochim. Acta 2019, 320, 134603–134609. [Google Scholar] [CrossRef]
  4. Liu, B.; Zhao, Y.F.; Peng, H.Q.; Zhang, Z.Y.; Sit, C.K.; Yuen, M.F.; Zhang, T.R.; Lee, C.S.; Zhang, W.J. Nickel-Cobalt Diselenide 3D Mesoporous Nanosheet Networks Supported on Ni Foam: An All-pH Highly Efficient Integrated Electrocatalyst for Hydrogen Evolution. Adv. Mater. 2017, 29, 1606521–1606528. [Google Scholar] [CrossRef] [PubMed]
  5. Ravikumar, C.H.; Nair, G.V.; Muralikrishna, S.; Nagaraju, D.H.; Balakrishna, R.G. Nanoflower Like Structures of MoSe2 and MoS2 as Efficient Catalysts for Hydrogen Evolution. Mater. Lett. 2018, 220, 133–135. [Google Scholar] [CrossRef]
  6. Wang, Y.; Chen, Z.X.; Lei, T.; Ai, Y.F.; Peng, Z.K.; Yan, X.Y.; Li, H.; Zhang, J.J.; Wang, Z.M.M.; Chueh, Y.-L. Hollow NiCo2S4 Nanospheres Hybridized with 3D Hierarchical Porous rGO/Fe2O3 Composites toward High Performance Energy Storage Device. Adv. Energy Mater. 2018, 8, 1703453–1703463. [Google Scholar]
  7. Habas, S.E.; Platt, H.A.S.; van Hest, M.F.A.M.; Ginley, D.S. Low-Cost Inorganic Solar Cells: From Ink to Printed Device. Chem. Rev. 2010, 110, 6571–6594. [Google Scholar] [CrossRef] [PubMed]
  8. Wu, J.; Yang, Z.R.; Li, X.W.; Sun, Q.J.; Yang, R.Z. Phosphorus-Doped Porous Carbons as Efficient Electrocatalysts for Oxygen Reduction. J. Mater. Chem. A 2013, 1, 9889–9896. [Google Scholar] [CrossRef] [Green Version]
  9. Steele, B.C.H.; Heinzel, A. Materials for Fuel-Cell Technologies. Nature 2001, 414, 345–352. [Google Scholar] [CrossRef]
  10. Liu, H.S.; Song, C.J.; Zhang, L.; Zhang, J.J.; Wang, H.J.; Wilkinson, D.P. A Review of Anode Catalysis in the Direct Methanol Fuel Cell. J. Power Sources 2006, 155, 95–110. [Google Scholar] [CrossRef]
  11. Shao, M.H.; Chang, Q.W.; Dodelet, J.P.; Chenitz, R. Recent Advances in Electrocatalysts for Oxygen Reduction Reaction. Chem. Rev. 2016, 116, 3594–3657. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  12. Antolini, E. Structural Parameters of Supported Fuel Cell Catalysts: The Effect of Particle Size, Inter-Particle Distance and Metal Loading on Catalytic Activity and Fuel Cell Performance. Appl. Catal. B Environ. 2016, 181, 298–313. [Google Scholar] [CrossRef]
  13. Yun, W.; Chen, K.S.; Mishler, J.; Cho, S.C.; Adroher, X.C. A Review of Polymer Electrolyte Membrane Fuel Cells: Technology, Applications, and Needs on Fundamental Research. Appl. Energy 2011, 88, 981–1007. [Google Scholar]
  14. Nie, Y.; Li, L.; Wei, Z.D. Recent Advancements in Pt and Pt-Free Catalysts for Oxygen Reduction Reaction. Chem. Soc. Rev. 2015, 44, 2168–2201. [Google Scholar] [CrossRef]
  15. Kang, S.Y.; Kim, H.J.; Chung, Y.H. Recent Developments of Nano-Structured Materials as the Catalysts for Oxygen Reduction Reaction. Nano Converg. 2018, 5, 13. [Google Scholar] [CrossRef]
  16. Hung, A.-J.; Chen, Y.-H.; Sung, L.-Y.; Yu, C.-C. Cost Analysis of Proton Exchange Membrane Fuel Cell Systems. AIChE J. 2008, 54, 1798–1810. [Google Scholar] [CrossRef]
  17. Othman, R.; Dicks, A.L.; Zhu, Z.H. Non Precious Metal Catalysts for the PEM Fuel Cell Cathode. Int. J. Hydrogen Energy 2012, 37, 357–372. [Google Scholar] [CrossRef]
  18. El-Kharouf, A.; Chandan, A.; Hattenberger, M.; Pollet, B.G. Proton Exchange Membrane Fuel Cell Degradation and Testing: Review. J. Energy Inst. 2012, 85, 188–200. [Google Scholar] [CrossRef]
  19. Banham, D.; Ye, S.; Pei, K.; Ozaki, J.i.; Kishimoto, T.; Imashiro, Y. A Review of the Stability and Durability of Non-Precious Metal Catalysts for the Oxygen Reduction Reaction in Proton Exchange Membrane Fuel Cells. J. Power Sources 2015, 285, 334–348. [Google Scholar] [CrossRef]
  20. Ioroi, T.; Siroma, Z.; Yamazaki, S.-i.; Yasuda, K. Electrocatalysts for PEM Fuel Cells. Adv. Energy Mater. 2019, 9. [Google Scholar] [CrossRef]
  21. Lin, R.; Cai, X.; Zeng, H.; Yu, Z.P. Stability of High-Performance Pt-Based Catalysts for Oxygen Reduction Reactions. Adv. Mater. 2018, 30. [Google Scholar] [CrossRef] [PubMed]
  22. Hao, Y.J.; Zhang, X.; Yang, Q.F.; Chen, K.; Guo, J.; Zhou, D.Y.; Feng, L.; Slanina, Z. Highly Porous Defective Carbons Derived from Seaweed Biomass as Efficient Electrocatalysts for Oxygen Reduction in Both Alkaline and Acidic Media. Carbon 2018, 137, 93–103. [Google Scholar] [CrossRef]
  23. Wee, J.H.; Lee, K.-Y.; Kim, S.H. Fabrication Methods for Low-Pt-Loading Electrocatalysts in Proton Exchange Membrane Fuel Cell Systems. J. Power Sources 2007, 165, 667–677. [Google Scholar] [CrossRef]
  24. Sharma, S.; Pollet, B.G. Support Materials for PEMFC and DMFC Electrocatalysts-A Review. J. Power Sources 2012, 208, 96–119. [Google Scholar] [CrossRef]
  25. Yu, S.H.; Gu, C.; Hu, S.J.; Zheng, X.S.; Zhu, J.F. Synthesis of Sub-2 nm Fe-Doped NiSe2 Nanowires and Their Surface Confined Oxidation for Oxygen Evolution Catalysis. Angew. Chem. Int. Ed. 2018, 57, 4020–4024. [Google Scholar]
  26. Wu, M.J.; Zhang, E.G.; Guo, Q.P.; Wang, Y.Z.; Qiao, J.L.; Li, K.X.; Pei, P.C. N/S-Me (Fe, Co, Ni) Doped Hierarchical Porous Carbons for Fuel Cell Oxygen Reduction Reaction with High Catalytic Activity and Long-term Stability. Appl. Energy 2016, 175, 468–478. [Google Scholar] [CrossRef]
  27. Stoyanov, S.R.; Titov, A.V.; Král, P. Transition Metal and Nitrogen Doped Carbon Nanostructures. Coord. Chem. Rev. 2009, 253, 2852–2871. [Google Scholar] [CrossRef]
  28. Masa, J.; Xia, W.; Muhler, M.; Schuhmann, W. On the Role of Metals in Nitrogen-Doped Carbon Electrocatalysts for Oxygen Reduction. Angew. Chem. Int. Ed. 2015, 54, 10102–10120. [Google Scholar] [CrossRef]
  29. Jaouen, F.; Proietti, E.; Lefevre, M.; Chenitz, R.; Zelenay, P. Recent Advances in Non-Precious Metal Catalysis for Oxygen-Reduction Reaction in Polymer Electrolyte Fuel Cells. Energy Environ. Sci. 2010, 4, 114–130. [Google Scholar] [CrossRef]
  30. Wu, G.; Zelenay, P. Nanostructured Nonprecious Metal Catalysts for Oxygen Reduction Reaction. Acc. Chem. Res. 2013, 46, 1878–1889. [Google Scholar] [CrossRef]
  31. Zeng, M.; Li, Y.G. Recent Advances in Heterogeneous Electrocatalysts for the Hydrogen Evolution Reaction. J. Mater. Chem. A 2015, 3, 14942–14962. [Google Scholar] [CrossRef]
  32. Liu, M.L.; Zhao, Z.P.; Duan, X.F.; Huang, Y. Nanoscale Structure Design for High-Performance Pt-Based ORR Catalysts. Adv. Mater. 2019, 31. [Google Scholar] [CrossRef]
  33. Liu, S.R.; Pu, Q.S.; Gao, L.; Korzeniewski, C.; Matzke, C. From Nanochannel-Induced Proton Conduction Enhancement to a Nanochannel-Based Fuel Cell. Nano Lett. 2005, 5, 1389–1393. [Google Scholar] [CrossRef]
  34. Arbizzani, C.; Righi, S.; Soavi, F.; Mastragostino, M. Graphene and Carbon Nanotube Structures Supported on Mesoporous Xerogel Carbon as Catalysts for Oxygen Reduction Reaction in Proton-Exchange-Membrane Fuel Cells. Int. J. Hydrogen Energy 2011, 36, 5038–5046. [Google Scholar] [CrossRef]
  35. Ghosh, A.; Chandran, P.; Ramaprabhu, S. Palladium-Nitrogen Coordinated Cobalt Alloy towards Hydrogen Oxidation and Oxygen Reduction Reactions with High Catalytic Activity in Renewable Energy Generations of Proton Exchange Membrane Fuel Cell. Appl. Energy 2017, 208, 37–48. [Google Scholar] [CrossRef]
  36. Seh, Z.W.; Kibsgaard, J.; Dickens, C.F.; Chorkendorff, I.B.; Norskov, J.K.; Jaramillo, T.F. Combining Theory and Experiment in Electrocatalysis: Insights Into Materials Design. Science 2017, 355. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  37. Zou, X.; Zhang, Y. Noble Metal-Free Hydrogen Evolution Catalysts for Water Splitting. Chem. Soc. Rev. 2015, 44, 5148–5180. [Google Scholar] [CrossRef] [PubMed]
  38. Hung, T.-F.; Chen, S.-H.; Tu, M.-H.; Lu, Z.-H.; Chen, C.K.; Liu, R.-S.; Greer, H.F.; Zhou, W.Z.; Lo, M.-Y. Advances in Carbon-Incorporated Non-Noble Transition Metal Catalysts for Oxygen Reduction Reaction in Polymer Electrolyte Fuel Cells. J. Chin. Chem. Soc. 2014, 61, 93–100. [Google Scholar] [CrossRef]
  39. Wu, G.; Santandreu, A.; Kellogg, W.; Gupta, S.; Ogoke, O.; Zhang, H.G.; Wang, H.-L.; Dai, L.M. Carbon Nanocomposite Catalysts for Oxygen Reduction and Evolution Reactions: From Nitrogen Doping to Transition-Metal Addition. Nano Energy 2016, 29, 83–110. [Google Scholar] [CrossRef] [Green Version]
  40. Liu, D.D.; Tao, L.; Yan, D.F.; Zou, Y.Q.; Wang, S.Y. Recent Advances on Non-precious Metal Porous Carbon-based Electrocatalysts for Oxygen Reduction Reaction. Chemelectrochem 2018, 5, 1775–1785. [Google Scholar] [CrossRef]
  41. Singh, K.; Razmjooei, F.; Yu, J.-S. Active Sites and Factors Influencing Them for Efficient Oxygen Reduction Reaction in Metal-N Coordinated Pyrolyzed and Non-Pyrolyzed Catalysts: A Review. J. Mater. Chem. A 2017, 5, 20095–20119. [Google Scholar] [CrossRef]
  42. Zhu, C.Z.; Shi, Q.R.; Xu, B.Z.; Fu, S.F.; Wan, G.; Yang, C.; Yao, S.Y.; Song, J.H.; Zhou, H.; Du, D.; et al. Hierarchically Porous M-N-C (M = Co and Fe) Single-Atom Electrocatalysts with Robust MNx Active Moieties Enable Enhanced ORR Performance. Adv. Energy Mater. 2018, 8. [Google Scholar] [CrossRef]
  43. Li, B.; Chen, Y.; Ge, X.M.; Chai, J.W.; Zhang, X.; Hor, T.S.A.; Du, G.J.; Liu, Z.L.; Zhang, H.; Zong, Y. Mussel-Inspired One-Pot Synthesis of Transition Metal and Nitrogen Co-Doped Carbon (M/N-C) as Efficient Oxygen Catalysts for Zn-air Batteries. Nanoscale 2016, 8, 5067–5075. [Google Scholar] [CrossRef] [PubMed]
  44. Wang, J.; Li, L.Q.; Chen, X.; Lu, Y.L.; Yang, W.S.; Duan, X. A Co-N/C Hollow-Sphere Electrocatalyst Derived from A Metanilic CoAl Layered Double Hydroxide for the Oxygen Reduction Reaction, and Its Active Sites in Various pH Media. Nano Res. 2017, 10, 2508–2518. [Google Scholar] [CrossRef]
  45. Yang, Y.; Mao, K.T.; Gao, S.Q.; Huang, H.; Xia, G.L.; Lin, Z.Y.; Jiang, P.; Wang, C.L.; Wang, H.; Chen, Q.W. O-, N-Atoms-Coordinated Mn Cofactors within a Graphene Framework as Bioinspired Oxygen Reduction Reaction Electrocatalysts. Adv. Mater. 2018, 30, e1801732. [Google Scholar] [CrossRef]
  46. Wang, T.; Yang, R.; Shi, N.E.; Yang, J.; Yan, H.Y.; Wang, J.Y.; Ding, Z.; Huang, W.; Luo, Q.; Lin, Y.; et al. Cu,N-Codoped Carbon Nanodisks with Biomimic Stomata-Like Interconnected Hierarchical Porous Topology as Efficient Electrocatalyst for Oxygen Reduction Reaction. Small 2019, 15. [Google Scholar] [CrossRef] [Green Version]
  47. Yang, T.T.; Wang, M.; Ju, X.P.; Zhao, J.S.; Fu, C.G. The Efficient Oxygen Reduction Catalysts Based on the Non-Noble Metal and Conducting Polymers. Int. J. Electrochem. Sci. 2017, 12, 12125–12139. [Google Scholar] [CrossRef]
  48. Gu, W.L.; Hu, L.Y.; Li, J.; Wang, E. Recent Advancements in Transition Metal-Nitrogen-Carbon Catalysts for Oxygen Reduction Reaction. Electroanalysis 2018, 30, 1217–1228. [Google Scholar] [CrossRef]
  49. Peera, S.G.; Balamurugan, J.; Kim, N.H.; Lee, J.H. Sustainable Synthesis of Co@NC Core Shell Nanostructures from Metal Organic Frameworks via Mechanochemical Coordination Self-Assembly: An Efficient Electrocatalyst for Oxygen Reduction Reaction. Small 2018, 14, e1800441. [Google Scholar] [CrossRef]
  50. Jasinski, R. A New Fuel Cell Cathode Catalyst. Nature 1964, 201, 1212–1213. [Google Scholar] [CrossRef]
  51. Veen, J.A.R.V.; Visser, C. Oxygen Reduction on Monomeric Transition Metal Phthalocyanines in Acid Electrolyte. Electrochim. Acta 1979, 24, 921–928. [Google Scholar] [CrossRef]
  52. Vasudevan, P.; Mann, N.; Tyagi, S. Transition Metal Complexes of Porphyrins and Phthalocyanines as Electrocatalysts for Dioxygen Reduction. Transit. Met. Chem. 1990, 15, 81–90. [Google Scholar] [CrossRef]
  53. Shi, C.N.; Anson, F.C. Catalytic Pathways for The Electroreduction of Oxygen by Iron Tetrakis(4-N-methylpyridyl)Porphyrin or Iron Tetraphenylporphyrin Adsorbed on Edge Plane Pyrolytic Graphite Electrodes. Inorg. Chem. 1990, 29, 4298–4305. [Google Scholar] [CrossRef]
  54. Ouyang, J.; Shigehara, K.; Yamada, A.; Anson, F.C. Hexadecafluoro- and Octacyano Phthalocyanines as Electrocatalysts for the Reduction of Dioxygen. J. Electroanal. Chem. Interfacial Electrochem. 1991, 297, 489–498. [Google Scholar] [CrossRef]
  55. Tanabe, H.; Ohno, K. Electrocatalysis of Metal Phthalocyanine Thin Film Prepared by the Plasma-Assisted Deposition on a Glassy Carbon in the Reduction of Carbon Dioxide. Electrochim. Acta 1987, 32, 1121–1124. [Google Scholar] [CrossRef]
  56. Shi, C.; Steiger, B.; Yuasa, M.; Anson, F.C. Electroreduction of O2 to H2O at Unusually Positive Potentials Catalyzed by the Simplest of the Cobalt Porphyrins. Inorg. Chem. 1997, 36, 4294–4295. [Google Scholar] [CrossRef]
  57. Randin, J.P. Interpretation of the Relative Electrochemical Activity of Various Metal Phthalocyanines for the Oxygen Reduction Reaction. Electrochim. Acta 1974, 19, 83–85. [Google Scholar] [CrossRef]
  58. Zagal, J.H. Metallophthalocyanines as Catalysts in Electrochemical Reactions. Coord. Chem. Rev. 1992, 119, 89–136. [Google Scholar] [CrossRef]
  59. Veen, J.A.R.V.; Colijn, H.A. Oxygen Reduction on Transition-Metal Porphyrins in Acid Electrolyte II. Stability. Ber. Der Bunsenges. Phys. Chem. 2010, 85, 700–704. [Google Scholar] [CrossRef]
  60. Alt, H.; Binder, H.; Lindner, W.; Sandstede, G. Metal Chelates as Electrocatalysts for Oxygen Reduction in Acid Electrolytes. J. Electroanal. Chem. 1971, 31, A19–A22. [Google Scholar] [CrossRef]
  61. Song, E.; Shi, C.; Anson, F.C. Comparison of the Behavior of Several Cobalt Porphyrins as Electrocatalysts for the Reduction of O2 at Graphite Electrodes. Langmuir 1998, 14, 4315–4321. [Google Scholar] [CrossRef]
  62. Wiesener, K.; Ohms, D.; Neumann, V.; Franke, R. N4 Macrocycles as Electrocatalysts for the Cathodic Reduction of Oxygen. Mater. Chem. Phys. 1989, 22, 457–475. [Google Scholar] [CrossRef]
  63. Putten, A.V.D.; Elzing, A.; Visscher, W.; Barendrecht, E. Oxygen Reduction on Vacuum-Deposited and Absorbed Transition-Metal Phthalocyanine Films. J. Electroanal. Chem. Interfacial Electrochem. 1986, 214, 523–533. [Google Scholar] [CrossRef] [Green Version]
  64. Kobayashi, N.; Janda, P.; Lever, A.B.P. Cathodic Reduction of Oxygen and Hydrogen Peroxide at Cobalt and Iron Crowned Phthalocyanines Adsorbed on Highly Oriented Pyrolytic Graphite Electrodes. Inorg. Chem. 1992, 31, 5172–5177. [Google Scholar] [CrossRef]
  65. Alt, H.; Binder, H.; Sandstede, G. Mechanism of the Electrocatalytic Reduction of Oxygen on Metal Chelates. J. Catal. 1973, 28, 8–19. [Google Scholar] [CrossRef]
  66. Jahnke, H.; Schonborn, M.; Zimmermann, G. Organic Dyestuffs as Catalysts for Fuel Cells. Top. Curr. Chem. 1976, 61, 133–181. [Google Scholar]
  67. Lefevre, M.; Proietti, E.; Jaouen, F.; Dodelet, J.-P. Iron-Based Catalysts with Improved Oxygen Reduction Activity in Polymer Electrolyte Fuel Cells. Science 2009, 324, 71–74. [Google Scholar] [CrossRef]
  68. Proietti, E.; Jaouen, F.; Lefevre, M.; Larouche, N.; Tian, J.; Herranz, J.; Dodelet, J.-P. Iron-Based Cathode Catalyst with Enhanced Power Density in Polymer Electrolyte Membrane Fuel Cells. Nat. Commun. 2011, 2, 416. [Google Scholar] [CrossRef]
  69. Iliev, I.; Gamburzev, S.; Kaisheva, A. Optimization of the Pyrolysis Temperature of Active Carbon-CoTMPP Catalysts for Air Electrodes in Alkaline Media. J. Power Sources 1986, 17, 345–352. [Google Scholar] [CrossRef]
  70. Faubert, G.; Lalande, G.; Côté, R.; Guay, D.; Dodelet, J.P.; Weng, L.T.; Bertrand, P.; Dénès, G. Heat-treated Iron and Cobalt Tetraphenylporphyrins Adsorbed on Carbon Black: Physical Characterization and Catalytic Properties of These Materials for the Reduction of Oxygen in Polymer Electrolyte Fuel Cells. Electrochim. Acta 1996, 41, 1689–1701. [Google Scholar] [CrossRef]
  71. Lalande, G.; Cote, R.; Tamizhmani, G.; Guay, D.; Dodelet, J.P.; Dignardbailey, L.; Weng, L.T.; Bertrand, P. Physical, Chemical and Electrochemical Characterization of Heat-Treated Tetracarboxylic Cobalt Phthalocyanine Adsorbed on Carbon-Black as Electrocatalyst for Oxygen Reduction in Polymer Electrolyte Fuel-Cells. Electrochim. Acta 1995, 40, 2635–2646. [Google Scholar] [CrossRef]
  72. Bron, M.; Fiechter, S.; Hilgendorff, M.; Bogdanoff, P. Catalysts for Oxygen Reduction from Heat-Treated Carbon-Supported Iron Phenantroline Complexes. J. Appl. Electrochem. 2002, 32, 211–216. [Google Scholar] [CrossRef]
  73. Weng, L.T.; Bertrand, P.; Lalande, G.; Guay, D.; Dodelet, J.P. Surface Characterization by Time-of-Flight SIMS of A Catalyst for Oxygen Electroreduction: Pyrolyzed Cobalt Phthalocyanine-on-Carbon Black. Appl. Surf. Sci. 1995, 84, 9–21. [Google Scholar] [CrossRef]
  74. Alves, M.C.M.; Dodelet, J.P.; Guay, D.; Ladouceur, M.; Tourillon, G. Origin of the Electrocatalytic Properties for Oxygen Reduction of Some Heat-Hreated Polyacrylonitrile and Phthalocyanine Cobalt Compounds Adsorbed on Carbon Black as Probed by Electrochemistry and X-ray Absorption Spectroscopy. J. Phys. Chem. B 1992, 96, 10898–10905. [Google Scholar] [CrossRef]
  75. Li, X.; Liu, G.; Popov, B.N. Activity and Stability of Non-Precious Metal Catalysts for Oxygen Reduction in Acid and Alkaline Electrolytes. J. Power Sources 2010, 195, 6373–6378. [Google Scholar] [CrossRef]
  76. Veen, J.A.R.V.; Baar, J.F.V.; Kroese, K.J. Effect of Heat Treatment on the Performance of Carbon-Supported Transition-Metal Chelates in the Electrochemical Reduction of Oxygen. J. Chem. Soc. Faraday Trans. 1981, 77, 2827–2843. [Google Scholar] [CrossRef]
  77. Scherson, D.A.; Gupta, S.L.; Fierro, C.; Yeager, E.B.; Kordesch, M.E.; Eldridge, J.; Hoffman, R.W.; Blue, J. Cobalt Tetramethoxyphenyl Porphyrin—Emission Mossbauer Spectroscopy and O2 Reduction Electrochemical Studies. Electrochim. Acta 1983, 28, 1205–1209. [Google Scholar] [CrossRef]
  78. Putten, A.V.D.; Elzing, A.; Visscher, W.; Barendrecht, E. Redox Potential and Electrocatalysis of O2 Reduction on Transition Metal Chelates. J. Electroanal. Chem. 1987, 221, 95–104. [Google Scholar] [CrossRef] [Green Version]
  79. Wang, J.; Wei, Z.D. Recent Progress in Non-Precious Metal Catalysts for Oxygen Reduction Reaction. Acta Phys. Chim. Sin. 2017, 33, 886–902. [Google Scholar]
  80. Gupta, S.L.; Tryk, D.; Bae, I.; Aldred, W.; Yeager, E.B. Heat-Treated Polyacrylonitrile-Based Catalysts for Oxygen Electroreduction. J. Appl. Electrochem. 1989, 19, 19–27. [Google Scholar] [CrossRef]
  81. Bouwkamp-Wijnoltz, A.L.; Visscher, W.; Veen, J.A.R.V.; Tang, S.C. Electrochemical Reduction of Oxygen: An Alternative Method to Prepare Active CoN4 Catalysts. Electrochim. Acta 1999, 45, 379–386. [Google Scholar] [CrossRef]
  82. Liu, Y.Y.; Yue, X.P.; Li, K.X.; Qiao, J.L.; Wilkinson, D.P.; Zhang, J.J. PEM Fuel Cell Electrocatalysts Based on Transition Metal Macrocyclic Compounds. Coord. Chem. Rev. 2016, 315, 153–177. [Google Scholar] [CrossRef]
  83. Zhong, H.X.; Zhang, H.M.; Liang, Y.M.; Zhang, J.L.; Wang, M.R.; Wang, X.L. A Novel Non-Noble Electrocatalyst for Oxygen Reduction in Proton Exchange Membrane Fuel Cells. J. Power Sources 2007, 164, 572–577. [Google Scholar] [CrossRef]
  84. Wu, G.; Chen, Z.W.; Artyushkova, K.; Garzon, F.H.; Zelenay, P. Polyaniline-Derived Non-Precious Catalyst for the Polymer Electrolyte Fuel Cell Cathode. ECS Trans. 2008, 16, 159–170. [Google Scholar]
  85. Wu, G.; Johnston, C.M.; Mack, N.H.; Artyushkova, K.; Ferrandon, M.; Nelson, M.; Lezama-Pacheco, J.S.; Conradson, S.D.; More, K.L.; Myers, D.J. Synthesis–Structure–Performance Correlation for Polyaniline–Me–C Non-Precious Metal Cathode Catalysts for Oxygen Reduction in Fuel Cells. J. Mater. Chem. 2011, 21, 11392–11405. [Google Scholar] [CrossRef]
  86. Bashyam, R.; Zelenay, P. A Class of Non-Precious Metal Composite Catalysts for Fuel Cells. Nature 2006, 443, 63–66. [Google Scholar] [CrossRef]
  87. Han, S.J.; Jung, H.J.; Shim, J.H.; Kim, H.-C.; Sung, S.-J.; Yoo, B.; Lee, D.H.; Lee, C.; Lee, Y. Non-Platinum Oxygen Reduction Electrocatalysts Based on Carbon-Supported Metal-Polythiophene Composites. J. Electroanal. Chem. 2011, 655, 39–44. [Google Scholar] [CrossRef]
  88. Choi, J.Y.; Hsu, R.S.; Chen, Z.W. Highly Active Porous Carbon-Supported Nonprecious Metal-N Electrocatalyst for Oxygen Reduction Reaction in PEM Fuel Cells. J. Phys. Chem. C 2010, 114, 8048–8053. [Google Scholar] [CrossRef]
  89. Chung, H.T.; Johnston, C.M.; Artyushkova, K.; Ferrandon, M.; Myers, D.J.; Zelenay, P. Cyanamide-Derived Non-Precious Metal Catalyst for Oxygen Reduction. Electrochem. Commun. 2010, 12, 1792–1795. [Google Scholar] [CrossRef]
  90. Liu, Y.J.; Yu, L.M.; Jiang, X.H.; Li, X.; Yan, X.F. In Situ Self-Template Synthesis of Cobalt/Nitrogen-Doped Nanocarbons with Controllable Shapes for Oxygen Reduction Reaction and Supercapacitors. Int. J. Energy Res. 2019, 43, 4217–4228. [Google Scholar] [CrossRef]
  91. Wu, G.; More, K.L.; Johnston, C.M.; Zelenay, P. High-Performance Electrocatalysts for Oxygen Reduction Derived from Polyaniline, Iron, and Cobalt. Science 2011, 332, 443–447. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  92. Liu, D.; Li, J.C.; Shi, Q.R.; Feng, S.; Lyu, Z.Y.; Ding, S.C.; Hao, L.D.; Zhang, Q.; Wang, C.H.; Xu, M.J.; et al. Atomically Isolated Iron Atom Anchored on Carbon Nanotubes for Oxygen Reduction Reaction. ACS Appl. Mater. Int. 2019, 11, 39820–39826. [Google Scholar] [CrossRef] [PubMed]
  93. Huang, N.; Yang, L.; Zhang, M.Y.; Yan, S.F.; Ding, Y.Y.; Sun, P.P.; Sun, X.H. Cobalt-Embedded N-Doped Carbon Arrays Derived In Situ as Trifunctional Catalyst Toward Hydrogen and Oxygen Evolution, and Oxygen Reduction. Chemelectrochem 2019, 6, 4522–4532. [Google Scholar] [CrossRef]
  94. Guo, J.J.; Yang, X.W.; Yao, Y.L.; Wang, X.M.; Liu, X.G.; Xu, B.S. Pt/onion-Like Fullerenes as Catalyst for Direct Methanol Fuel Cell. Rare Met. 2006, 25, 305–308. [Google Scholar] [CrossRef]
  95. Yan, X.L.; Duan, P.; Zhang, F.W.; Li, H.; Zhang, H.X.; Zhao, M.; Zhang, X.M.; Xu, B.S.; Pennycook, S.J.; Guo, J.J. Stable Single-Atom Platinum Catalyst Trapped in Carbon Onion Graphitic Shells for Improved Chemoselective Hydrogenation of Nitroarenes. Carbon 2019, 143, 378–384. [Google Scholar] [CrossRef]
  96. Fan, Y.C.; Ida, S.; Staykov, A.; Akbay, T.; Hagiwara, H.; Matsuda, J.; Kaneko, K.; Ishihara, T. Ni-Fe Nitride Nanoplates on Nitrogen-Doped Graphene as a Synergistic Catalyst for Reversible Oxygen Evolution Reaction and Rechargeable Zn-Air Battery. Small 2017, 13. [Google Scholar] [CrossRef]
  97. Lim, K.H.; Kim, H. Nitrogen-Doped Carbon Catalysts Derived from Ionic Liquids in the Presence of Transition Metals for the Oxygen Reduction Reaction. Appl. Catal. B-Environ. 2014, 158, 355–360. [Google Scholar] [CrossRef]
  98. Liu, Y.Y.; Jiang, H.L.; Zhu, Y.H.; Yang, X.L.; Li, C.Z. Transition Metals (Fe, Co, and Ni) Encapsulated in Nitrogen-Doped Carbon Nanotubes as Bifunctional Catalysts for Oxygen Electrode Reactions. J. Mater. Chem. A 2016, 4, 1694–1701. [Google Scholar] [CrossRef]
  99. Liu, W.J.; Ru, Q.X.; Zuo, S.X.; Yang, S.; Han, J.; Yao, C. Controllable Synthesis of Nitrogen-Doped Carbon Nanotubes Derived from Halloysite-Templated Polyaniline Towards Nonprecious ORR Catalysts. Appl. Surf. Sci. 2019, 469, 269–275. [Google Scholar] [CrossRef]
  100. Zhao, Y.F.; Zhang, J.Q.; Li, K.F.; Ao, Z.M.; Wang, C.Y.; Liu, H.; Sun, K.N.; Wang, G.X. Electrospun Cobalt Embedded Porous Nitrogen Doped Carbon Nanofibers as An Efficient Catalyst for Water Splitting. J. Mater. Chem. A 2016, 4, 12818–12824. [Google Scholar] [CrossRef]
  101. Yin, J.; Qiu, Y.J.; Yu, J. Enhanced Electrochemical Activity for Oxygen Reduction Reaction from Nitrogen-Doped Carbon Nanofibers by Iron Doping. ECS Solid State Lett. 2013, 2, M37–M39. [Google Scholar] [CrossRef]
  102. Huang, X.H.; Yin, X.L.; Yu, X.Q.; Tian, J.; Wu, W. Preparation of Nitrogen-Doped Carbon Materials Based on Polyaniline Fiber and their Oxygen Reduction Properties. Colloids Surf. A-Physicochem. Eng. Asp. 2018, 539, 163–170. [Google Scholar] [CrossRef]
  103. Yan, X.L.; Li, H.; Sun, J.T.; Liu, P.Z.; Zhang, H.X.; Xu, B.S.; Guo, J.J. Pt Nanoparticles Decorated High-Defective Graphene Nanospheres as Highly Efficient Catalysts for the Hydrogen Evolution Reaction. Carbon 2018, 137, 405–410. [Google Scholar] [CrossRef]
  104. Guo, J.J.; Morris, J.R.; Ihm, Y.; Contescu, C.I.; Gallego, N.C.; Duscher, G.; Pennycook, S.J.; Chisholm, M.F. Topological Defects: Origin of Nanopores and Enhanced Adsorption Performance in Nanoporous Carbon. Small 2012, 8, 3283–3288. [Google Scholar] [CrossRef] [PubMed]
  105. Liang, J.; Zhou, R.F.; Chen, X.M.; Tang, Y.H.; Qiao, S.Z. Fe-N Decorated Hybrids of CNTs Grown on Hierarchically Porous Carbon for High-Performance Oxygen Reduction. Adv. Mater. 2014, 26, 6074. [Google Scholar] [CrossRef]
  106. Kim, M.; Yoo, J.M.; Ahn, C.-Y.; Jang, J.-H.; Son, Y.J.; Shin, H.; Kang, J.; Kang, Y.S.; Yoo, S.J.; Lee, K.S.; et al. Rational Generation of Fe-Nx Active Sites in Fe-N-C Electrocatalysts Facilitated by Fe-N Coordinated Precursors for the Oxygen Reduction Reaction. Chemcatchem 2019, 11, 5982–5988. [Google Scholar] [CrossRef]
  107. Park, M.J.; Lee, J.H.; Hembram, K.; Lee, K.R.; Han, S.S.; Yoon, C.W.; Nam, S.W.; Kim, J.Y. Oxygen Reduction Electrocatalysts Based on Coupled Iron Nitride Nanoparticles with Nitrogen-Doped Carbon. Catalysts 2016, 6, 11. [Google Scholar] [CrossRef] [Green Version]
  108. Liu, Q.; Zhang, H.Y.; Zhong, H.W.; Zhang, S.M.; Chen, S.L. N-doped Graphene/Carbon Composite as Non-Precious Metal Electrocatalyst for Oxygen Reduction Reaction. Electrochim. Acta 2012, 81, 313–320. [Google Scholar] [CrossRef]
  109. Chen, M.X.; Zhu, M.Z.; Zuo, M.; Chu, S.Q.; Zhang, J.; Wu, Y.; Liang, H.W.; Feng, X.L. Identification of Catalytic Sites for Oxygen Reduction in Metal/Nitrogen-Doped Carbons with Encapsulated Metal Nanoparticles. Angew. Chem. Int. Ed. 2019, 58. [Google Scholar] [CrossRef] [Green Version]
  110. Li, X.B.; Zou, R.Y.; Niu, Y.Y.; Sun, W.; Tian, X.L. Synthesis of A N-doped Mesoporous Carbon as An Efficient Electrocatalyst for Oxygen Reduction. Int. J. Hydrogen Energy 2018, 43, 21791–21797. [Google Scholar] [CrossRef]
  111. Liu, J.; Jin, Z.; Wang, X.; Ge, J.J.; Liu, C.P.; Xing, W. Recent Advances in Active Sites Identification and Regulation of M-N/C Electro-Catalysts towards ORR. Sci. China-Chem. 2019, 62, 669–683. [Google Scholar] [CrossRef]
  112. Xia, D.S.; Yang, X.; Xie, L.; Wei, Y.P.; Jiang, W.; Dou, M.; Li, X.N.; Li, J.; Gan, L.; Kang, F.Y. Direct Growth of Carbon Nanotubes Doped with Single Atomic Fe-N4 Active Sites and Neighboring Graphitic Nitrogen for Efficient and Stable Oxygen Reduction Electrocatalysis. Adv. Funct. Mater. 2019, 29, 10. [Google Scholar]
  113. Li, J.C.; Xiao, F.; Zhong, H.; Li, T.; Xu, M.J.; Ma, L.; Cheng, M.; Liu, D.; Feng, S.; Shi, Q.R.; et al. Secondary-Atom-Assisted Synthesis of Single Iron Atoms Anchored on N-Doped Carbon Nanowires for Oxygen Reduction Reaction. ACS Catal. 2019, 9, 5929–5934. [Google Scholar] [CrossRef]
  114. Yin, P.Q.; Yao, T.; Wu, Y.E.; Zheng, L.R.; Lin, Y.; Liu, W.; Ju, H.X.; Zhu, J.F.; Hong, X.; Deng, Z.X.; et al. Single Cobalt Atoms with Precise N-Coordination as Superior Oxygen Reduction Reaction Catalysts. Angew. Chem. Int. Ed. 2016, 55, 10800–10805. [Google Scholar] [CrossRef]
  115. Ao, X.; Zhang, W.; Li, Z.S.; Li, J.G.; Soule, L.; Huang, X.; Chiang, W.H.; Chen, H.M.; Wang, C.D.; Liu, M.L.; et al. Markedly Enhanced Oxygen Reduction Activity of Single-Atom Fe Catalysts via Integration with Fe Nanoclusters. ACS Nano 2019, 13, 11853–11862. [Google Scholar] [CrossRef]
  116. Xu, G.; Xu, G.C.; Ban, J.J.; Zhang, L.; Jia, D.Z. Cobalt and Cobalt Oxides N-Codoped Porous Carbon Derived from Metal-Organic Framework as Bifunctional Catalyst for Oxygen Reduction and Oxygen Evolution Reactions. J. Colloid Interface Sci. 2018, 521, 141–149. [Google Scholar] [CrossRef]
  117. Zhang, X.K.; Huang, X.B.; Hu, W.H.; Huang, Y.M. A Metal-Organic Framework-Derived Fe-N-C Electrocatalyst with Highly Dispersed Fe-Nx towards Oxygen Reduction Reaction. Int. J. Hydrogen Energy 2019, 44, 27379–27389. [Google Scholar] [CrossRef]
  118. Sun, W.; Du, L.; Tan, Q.; Zhou, J.G.; Hu, Y.F.; Du, C.Y.; Gao, Y.Z.; Yin, G.P. Engineering of Nitrogen Coordinated Single Cobalt Atom Moieties for Oxygen Electroreduction. ACS Appl. Mater. Inter. 2019, 11, 41258–41266. [Google Scholar] [CrossRef]
  119. Peng, P.; Shi, L.; Huo, F.; Mi, C.X.; Wu, X.H.; Zhang, S.J.; Xiang, Z.H. A Pyrolysis-Free Path toward Superiorly Catalytic Nitrogen-Coordinated Single Atom. Sci. Adv. 2019, 5, 7. [Google Scholar] [CrossRef] [Green Version]
  120. Kicinski, W.; Sek, J.P.; Matysiak-Brynda, E.; Miecznikowski, K.; Donten, M.; Budner, B.; Nowicka, A.M. Enhancement of PGM-free Oxygen Reduction Electrocatalyst Performance for Conventional and Enzymatic Fuel Cells: The Influence of An External Magnetic Field. Appl. Catal. B Environ. 2019, 258, 14. [Google Scholar] [CrossRef]
  121. Lee, S.H.; Kim, J.; Yoon, S.H.; Peck, D.H.; Kim, S.K.; Jung, D.H. Preparation of Chestnut-Like Carbon and Its Application for Electrodes with High Specific Capacitance. Appl. Catal. B Environ. 2014, 158–159, 308–313. [Google Scholar] [CrossRef]
  122. Deng, J.; Ren, P.J.; Deng, D.H.; Yu, L.; Yang, F.; Bao, X.H. Highly Active and Durable Non-Precious-Metal Catalysts Encapsulated in Carbon Nanotubes for Hydrogen Evolution Reaction. Energy Environ. Sci. 2014, 7, 1919–1923. [Google Scholar] [CrossRef]
  123. Strickland, K.; Miner, E.; Jia, Q.Y.; Tylus, U.; Ramaswamy, N.; Liang, W.T.; Sougrati, M.-T.; Jaouen, F.; Mukerjee, S. Highly Active Oxygen Reduction Non-Platinum Group Metal Electrocatalyst without Direct Metal-Nitrogen Coordination. Nat. Commun. 2015, 6, 7343. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  124. Kramm, U.I.; Herrmann-Geppert, I.; Behrends, J.; Lips, K.; Fiechter, S.; Bogdanoff, P. On an Easy Way to Prepare Metal Nitrogen Doped Carbon with Exclusive Presence of MeN4-type Sites Active for the ORR. J. Am. Chem. Soc. 2016, 138, 635–640. [Google Scholar] [CrossRef]
  125. Zitolo, A.; Ranjbar-Sahraie, N.; Mineva, T.; Li, J.K.; Jia, Q.Y.; Stamatin, S.; Harrington, G.F.; Lyth, S.M.; Krtil, P.; Mukerjee, S.; et al. Identification of Catalytic Sites in Cobalt-Nitrogen-Carbon Materials for the Oxygen Reduction Reaction. Nat. Commun. 2017, 8, 957. [Google Scholar] [CrossRef]
  126. Chung, H.T.; Cullen, D.A.; Higgins, D.; Sneed, B.T.; Holby, E.F.; More, K.L.; Zelenay, P. Direct Atomic-Level Insight into the Active Sites of a High-Performance PGM-free ORR Catalyst. Science 2017, 357, 479–483. [Google Scholar] [CrossRef] [Green Version]
  127. Zeng, X.J.; Shui, J.L.; Liu, X.F.; Liu, Q.T.; Li, Y.C.; Shang, J.X.; Zheng, L.R.; Yu, R.H. Single-Atom to Single-Atom Grafting of Pt1 onto Fe-N4 Center: Pt1@Fe-N-C Multifunctional Electrocatalyst with Significantly Enhanced Properties. Adv. Energy Mater. 2018, 8. [Google Scholar] [CrossRef]
  128. Shen, H.J.; Gracia-Espino, E.; Ma, J.Y.; Tang, H.D.; Mamat, X.; Wagberg, T.; Hu, G.Z.; Guo, S.J. Atomically FeN2 Moieties Dispersed on Mesoporous Carbon: A New Atomic Catalyst for Efficient Oxygen Reduction Catalysis. Nano Energy 2017, 35, 9–16. [Google Scholar] [CrossRef]
  129. Wang, Y.; Tang, Y.J.; Zhou, K. Self-Adjusting Activity Induced by Intrinsic Reaction Intermediate in Fe-N-C Single-Atom Catalysts. J. Am. Chem. Soc. 2019, 141, 14115–14119. [Google Scholar] [CrossRef]
  130. Chen, L.Y.; Liu, X.F.; Zheng, L.R.; Li, Y.C.; Guo, X.; Wan, X.; Liu, Q.T.; Shang, J.X.; Shui, J.L. Insights into the Role of Active Site Density in the Fuel Cell Performance of Co-N-C Catalysts. Appl. Catal. B-Environ. 2019, 256, 117849. [Google Scholar] [CrossRef]
  131. Zhang, X.L.; Lu, Z.S.; Yang, Z.X. The Mechanism of Oxygen Reduction Reaction on CoN4 Embedded Graphene: A Combined Kinetic and Atomistic Thermodynamic Study. Int. J. Hydrogen Energy 2016, 41, 21212–21220. [Google Scholar] [CrossRef]
  132. Bruller, S.; Liang, H.W.; Kramm, U.I.; Krumpfer, J.W.; Feng, X.L.; Mullen, K. Bimetallic Porous Porphyrin Polymer-Derived Non-Precious Metal Electrocatalysts for Oxygen Reduction Reactions. J. Mater. Chem. A 2015, 3, 23799–23808. [Google Scholar] [CrossRef] [Green Version]
  133. Chen, P.Z.; Zhou, T.P.; Xing, L.L.; Xu, K.; Tong, Y.; Xie, H.; Zhang, L.D.; Yan, W.S.; Chu, W.S.; Wu, C.Z.; et al. Atomically Dispersed Iron-Nitrogen Species as Electrocatalysts for Bifunctional Oxygen Evolution and Reduction Reactions. Angew. Chem. Int. Ed. 2017, 56, 610–614. [Google Scholar] [CrossRef] [PubMed]
  134. Zhai, L.F.; Kong, S.Y.; Zhang, H.Y.; Tian, W.J.; Sun, M.; Sun, H.Q.; Wang, S.B. Facile Synthesis of Co-N-rGO Composites as An Excellent Electrocatalyst for Oxygen Reduction Reaction. Chem. Eng. Sci. 2019, 194, 45–53. [Google Scholar] [CrossRef]
  135. Xiao, M.L.; Zhang, H.; Chen, Y.T.; Zhu, J.B.; Gao, L.Q.; Jin, Z.; Ge, J.J.; Jiang, Z.; Chen, S.L.; Liu, C.P.; et al. Identification of Binuclear Co2N5 Active Sites for Oxygen Reduction Reaction with More than One Magnitude Higher Activity than Single Atom CoN4 Site. Nano Energy 2018, 46, 396–403. [Google Scholar] [CrossRef]
  136. Guo, J.J.; Lee, J.; Contescu, C.I.; Gallego, N.C.; Pantelides, S.T.; Pennycook, S.J.; Moyer, B.A.; Chisholm, M.F. Crown Ethers in Graphene. Nat. Commun. 2014, 5, 5389–5394. [Google Scholar] [CrossRef] [Green Version]
  137. Zhang, H.X.; Liu, W.W.; Zhang, Z.H.; Li, M.F.; Xu, B.S.; Guo, J.J. Direct Imaging of A Single Ni Atom Cutting Graphene to Form A Graphene Nanomesh. Phys. Chem. Chem. Phys. 2018, 20, 26814–26818. [Google Scholar] [CrossRef]
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Figure 1. Schematic diagram of the proton exchange membrane fuel cells (PEMFC).
Figure 1. Schematic diagram of the proton exchange membrane fuel cells (PEMFC).
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Figure 2. The structure of the cathode in PEMFCs and the oxygen reduction reaction (ORR) pathways on the cathode.
Figure 2. The structure of the cathode in PEMFCs and the oxygen reduction reaction (ORR) pathways on the cathode.
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Figure 3. The structures of phthalocyanine (Pc), tetraphenylporphyrin (TPP) and tetramethoxyphenylporphyrin (TMPP).
Figure 3. The structures of phthalocyanine (Pc), tetraphenylporphyrin (TPP) and tetramethoxyphenylporphyrin (TMPP).
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Figure 4. Schematic illustration for the fabrication of nanocarbons. Reproduced with permission from Xuefeng Yan, Xia Li, Xiaohui Jiang, et al., International Journal of Energy Research; published by John Wiley and Sons, 2019.
Figure 4. Schematic illustration for the fabrication of nanocarbons. Reproduced with permission from Xuefeng Yan, Xia Li, Xiaohui Jiang, et al., International Journal of Energy Research; published by John Wiley and Sons, 2019.
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Figure 5. Schematic diagram of the synthesis of PANI-M–C catalysts. (A) Mixing of high-surface area carbon with aniline oligomers and transition-metal precursor (M: Fe and/or Co). (B) Oxidative polymerization of aniline by the addition of ammonium persulfate (APS). (C) First heat treatment in an N2 atmosphere. (D) Acid leaching. Reproduced with permission from Gang Wu, Karren L. More, Christina M. Johnston, Piotr Zelenay, Science; published by the American Association for the Advancement of Science, 2011.
Figure 5. Schematic diagram of the synthesis of PANI-M–C catalysts. (A) Mixing of high-surface area carbon with aniline oligomers and transition-metal precursor (M: Fe and/or Co). (B) Oxidative polymerization of aniline by the addition of ammonium persulfate (APS). (C) First heat treatment in an N2 atmosphere. (D) Acid leaching. Reproduced with permission from Gang Wu, Karren L. More, Christina M. Johnston, Piotr Zelenay, Science; published by the American Association for the Advancement of Science, 2011.
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Figure 6. (a) Schematic illustration of synthesis process of the single iron atoms immobilized on the wall of a carbon nanotube (SAICNT) catalyst; (b) Scanning electron microscopy (SEM) image of Fe-modified polypyrrole (PPy); (c) Transmission electron microscopy (TEM) and (d) High-resolution TEM images of SAICNT-900; (e) Energy-dispersive X-ray spectroscopy (EDS) elemental mapping photos of SAICNT-900 (C, N and Fe); (f) High-angle annular dark-field imaging-scanning transmission electron microscope (HAADF-STEM) images of the SAICNT-900 catalyst. Reprinted with permission from Liu et al. (2019) [92]. Copyright 2019, American Chemical Society.
Figure 6. (a) Schematic illustration of synthesis process of the single iron atoms immobilized on the wall of a carbon nanotube (SAICNT) catalyst; (b) Scanning electron microscopy (SEM) image of Fe-modified polypyrrole (PPy); (c) Transmission electron microscopy (TEM) and (d) High-resolution TEM images of SAICNT-900; (e) Energy-dispersive X-ray spectroscopy (EDS) elemental mapping photos of SAICNT-900 (C, N and Fe); (f) High-angle annular dark-field imaging-scanning transmission electron microscope (HAADF-STEM) images of the SAICNT-900 catalyst. Reprinted with permission from Liu et al. (2019) [92]. Copyright 2019, American Chemical Society.
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Figure 7. (a) The fabrication processes and schematic structure for Cobalt/Metal Matrix Composites (Co/MMCs). (b) SEM image of Co/MMCs. High-resolution transmission electron microscopy (HRTEM) images of the components of Co/MMCs with different morphologies: (c) nanorod, (d) hollow nanowire, (f) nanowire, and (i) nanoparticle. (e) The partially enlarged image of the surface area of a Co metal nanoparticle shown in (d), (g) partially enlarged image of (f). (h) Selected area electron diffraction (SAED) of the metal particle in (e). (j) dark-field high-resolution TEM images of the nanoparticle shown in (i). The red bar represents to 1 μm, green bar to 100 nm, and blue bar to 50 nm. Reproduced with permission from Niu Huang, Liu Yang, Mingyi Zhang, et al., ChemElectroChem; published by John Wiley and Sons, 2019.
Figure 7. (a) The fabrication processes and schematic structure for Cobalt/Metal Matrix Composites (Co/MMCs). (b) SEM image of Co/MMCs. High-resolution transmission electron microscopy (HRTEM) images of the components of Co/MMCs with different morphologies: (c) nanorod, (d) hollow nanowire, (f) nanowire, and (i) nanoparticle. (e) The partially enlarged image of the surface area of a Co metal nanoparticle shown in (d), (g) partially enlarged image of (f). (h) Selected area electron diffraction (SAED) of the metal particle in (e). (j) dark-field high-resolution TEM images of the nanoparticle shown in (i). The red bar represents to 1 μm, green bar to 100 nm, and blue bar to 50 nm. Reproduced with permission from Niu Huang, Liu Yang, Mingyi Zhang, et al., ChemElectroChem; published by John Wiley and Sons, 2019.
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Figure 8. Synthetic procedure of the Fe–N–CNT-OPC material. Reproduced with permission from Ji Liang, Ruifeng Zhou, Xuemin Chen, et al., Advanced Materials; published by John Wiley and Sons, 2014.
Figure 8. Synthetic procedure of the Fe–N–CNT-OPC material. Reproduced with permission from Ji Liang, Ruifeng Zhou, Xuemin Chen, et al., Advanced Materials; published by John Wiley and Sons, 2014.
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Figure 9. Illustration of FexNy/nanocomposite (NC) synthesis. Open Access.
Figure 9. Illustration of FexNy/nanocomposite (NC) synthesis. Open Access.
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Figure 10. (a): Schematic illustration of NH3-promoted catalytic growth of Fe–N/CNT-1. (b–d): SEM, TEM and scanning transmission electron microscope-electron energy loss spectroscopy (STEM-EELS)elemental mapping of the Fe–N/CNT-1 catalyst, respectively. (e–g): TEM, HRTEM, and STEM-EELS elemental mapping of the Fe–N/CNT-2 catalyst, respectively. (h–i): Aberration-corrected HAADF-STEM images of different regions of the Fe–N/CNT-2 catalyst marked in (g). Reproduced with permission from Feiyu Kang, Lin Gan, Jia Li, et al., Advanced Functional Materials; published by John Wiley and Sons, 2019.
Figure 10. (a): Schematic illustration of NH3-promoted catalytic growth of Fe–N/CNT-1. (b–d): SEM, TEM and scanning transmission electron microscope-electron energy loss spectroscopy (STEM-EELS)elemental mapping of the Fe–N/CNT-1 catalyst, respectively. (e–g): TEM, HRTEM, and STEM-EELS elemental mapping of the Fe–N/CNT-2 catalyst, respectively. (h–i): Aberration-corrected HAADF-STEM images of different regions of the Fe–N/CNT-2 catalyst marked in (g). Reproduced with permission from Feiyu Kang, Lin Gan, Jia Li, et al., Advanced Functional Materials; published by John Wiley and Sons, 2019.
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Figure 11. ORR activity measurement of the two Fe–N–CNT catalysts. (a) Linear sweep voltammetry (LSV) curves in O2-saturated 0.1 M KOH and (b) Calculated H2O2 yield and electron transfer number based on rotating ring-disc electrode (RRDE) tests. Reproduced with permission from Feiyu Kang, Lin Gan, Jia Li, et al., Advanced Functional Materials; published by John Wiley and Sons, 2019.
Figure 11. ORR activity measurement of the two Fe–N–CNT catalysts. (a) Linear sweep voltammetry (LSV) curves in O2-saturated 0.1 M KOH and (b) Calculated H2O2 yield and electron transfer number based on rotating ring-disc electrode (RRDE) tests. Reproduced with permission from Feiyu Kang, Lin Gan, Jia Li, et al., Advanced Functional Materials; published by John Wiley and Sons, 2019.
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Figure 12. Schematic for achieving single-atom iron catalyst and traditional Fe–N–C catalyst. Reprinted with permission from Li et al. (2019) [113]. Copyright 2019 American Chemical Society.
Figure 12. Schematic for achieving single-atom iron catalyst and traditional Fe–N–C catalyst. Reprinted with permission from Li et al. (2019) [113]. Copyright 2019 American Chemical Society.
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Figure 13. (a) Illustration of the synthesis process of FeAC@FeSA–N–C. (b) SEM image, (c) TEM image, and (d–f) HAADF-STEM image of FeAC@FeSA–N-C. (g) HAADF-STEM image and corresponding EDS element mapping images of FeAC@FeSA–N–C. Reprinted with permission from Ao et al. (2019) [115]. Copyright 2019, American Chemical Society.
Figure 13. (a) Illustration of the synthesis process of FeAC@FeSA–N–C. (b) SEM image, (c) TEM image, and (d–f) HAADF-STEM image of FeAC@FeSA–N-C. (g) HAADF-STEM image and corresponding EDS element mapping images of FeAC@FeSA–N–C. Reprinted with permission from Ao et al. (2019) [115]. Copyright 2019, American Chemical Society.
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Figure 14. Schematic illustration of the synthesis of Fe–N–C. Reproduced with permission from Xiangkun Zhang, Xiaobo Huang, Wenhui Hu, Yongmin Huang, International Journal of Hydrogen Energy; published by Elsevier, 2019.
Figure 14. Schematic illustration of the synthesis of Fe–N–C. Reproduced with permission from Xiangkun Zhang, Xiaobo Huang, Wenhui Hu, Yongmin Huang, International Journal of Hydrogen Energy; published by Elsevier, 2019.
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Figure 15. Schematic representation of the utilized electrochemical cell. Reproduced with permission from Wojciech Kiciński, Jakub P. Sęk, Edyta Matysiak-Brynda, et al., Applied Catalysis B: Environmental; published by Elsevier, 2019.
Figure 15. Schematic representation of the utilized electrochemical cell. Reproduced with permission from Wojciech Kiciński, Jakub P. Sęk, Edyta Matysiak-Brynda, et al., Applied Catalysis B: Environmental; published by Elsevier, 2019.
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Figure 16. Schematic illustration of the removal of graphene-encapsulated M-NPs from M–N–C ORR catalysts by a low-temperature NH4Cl-treatment process. Reproduced with permission from Xinliang Feng, Haiwei Liang, Yuen Wu, et al., Angewandte Chemie International Edition; published by John Wiley and Sons, 2019.
Figure 16. Schematic illustration of the removal of graphene-encapsulated M-NPs from M–N–C ORR catalysts by a low-temperature NH4Cl-treatment process. Reproduced with permission from Xinliang Feng, Haiwei Liang, Yuen Wu, et al., Angewandte Chemie International Edition; published by John Wiley and Sons, 2019.
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Figure 17. Free energy diagrams of ORR along (a) associative and (b) OOH* dissociative paths on Fe(OH)N4 at 0 and 0.76 V. Reprinted with permission from Ref. [129]. Copyright 2019 American Chemical Society.
Figure 17. Free energy diagrams of ORR along (a) associative and (b) OOH* dissociative paths on Fe(OH)N4 at 0 and 0.76 V. Reprinted with permission from Ref. [129]. Copyright 2019 American Chemical Society.
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Figure 18. Mechanistic scheme for ORR on single atom Fe–N–C. Reprinted with permission from Wang et al. (2019) [129]. Copyright 2019, American Chemical Society.
Figure 18. Mechanistic scheme for ORR on single atom Fe–N–C. Reprinted with permission from Wang et al. (2019) [129]. Copyright 2019, American Chemical Society.
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Figure 19. Possible catalytic active sites on Co–N-rGO. Reproduced with permission from Linfeng Zhai, Shiyu Kong, Huayang Zhang, et al., Chemical Engineering Science; published by Elsevier, 2019.
Figure 19. Possible catalytic active sites on Co–N-rGO. Reproduced with permission from Linfeng Zhai, Shiyu Kong, Huayang Zhang, et al., Chemical Engineering Science; published by Elsevier, 2019.
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Figure 20. (a,b) Schematic energy profiles for the ORR pathway on different CoxNy active sites in acidic media. (c) Proposed mechanism for the ORR on the Co2N5 active site. Reproduced with permission from Meiling Xiao, Hao Zhang, Yongting Chen, et al., Nano Energy; published by Elsevier, 2018.
Figure 20. (a,b) Schematic energy profiles for the ORR pathway on different CoxNy active sites in acidic media. (c) Proposed mechanism for the ORR on the Co2N5 active site. Reproduced with permission from Meiling Xiao, Hao Zhang, Yongting Chen, et al., Nano Energy; published by Elsevier, 2018.
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Song, M.; Song, Y.; Sha, W.; Xu, B.; Guo, J.; Wu, Y. Recent Advances in Non-Precious Transition Metal/Nitrogen-doped Carbon for Oxygen Reduction Electrocatalysts in PEMFCs. Catalysts 2020, 10, 141. https://doi.org/10.3390/catal10010141

AMA Style

Song M, Song Y, Sha W, Xu B, Guo J, Wu Y. Recent Advances in Non-Precious Transition Metal/Nitrogen-doped Carbon for Oxygen Reduction Electrocatalysts in PEMFCs. Catalysts. 2020; 10(1):141. https://doi.org/10.3390/catal10010141

Chicago/Turabian Style

Song, Meixiu, Yanhui Song, Wenbo Sha, Bingshe Xu, Junjie Guo, and Yucheng Wu. 2020. "Recent Advances in Non-Precious Transition Metal/Nitrogen-doped Carbon for Oxygen Reduction Electrocatalysts in PEMFCs" Catalysts 10, no. 1: 141. https://doi.org/10.3390/catal10010141

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