Novel Nanomaterials as Electrocatalysts for Fuel Cells

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Minmin Liu1, 2, Chunmei Zhang1, 3, Wei Chen1 Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, China1; Shanghai University, Shanghai, China2; University of Chinese Academy of Sciences, Beijing, China3

6.1 INTRODUCTION Since energy crisis and environmental pollution problems have become more and more serious, developing environmentally friendly energy storage and conversion devices is a major challenge for the 21st century. At present, structure-controlled preparation and performance of electrode materials are the key factors that limit development of these devices. In recent years, low-cost and durable electrode materials have attracted increasing attention in the research fields of energy storage and conversion. This chapter aims at summarizing fuel cell (FC) electrocatalysts on the basis of novel carbon materials. For FCs, the cathode oxygen reduction reaction (ORR) is a critical step, which is six or more orders of magnitude slower than the anode hydrogen oxidation reaction and thus limits the overall performance of FCs [1,2]. Therefore, designing and fabricating advanced nanostructured electrocatalysts and electrodes with high catalytic performance are of crucial importance for the development of FCs. The sluggish nature of ORR process requires a large amount of noble metale based electrocatalysts (e.g., Pt, Pd, Ru, or Ir) to lower the reaction activation energy and enhance the reaction rate of oxygen reduction. It has been widely accepted that platinum-based materials are the most active catalysts in FCs; however, there exist obvious disadvantages to Pt catalysts, such as high cost, limited stability, and low tolerance toward methanol and carbon monoxide. Besides, conventional Pt nanoparticlesebased catalysts have also been struggling with nanoparticle migration, merger, and detachment during continuous electrochemical operations [3,4]. These phenomena are particularly pronounced in alkaline electrolytes and remain the major challenge in application of these catalysts [4]. Thus, it is necessary to explore novel electrocatalyst materials with low cost but high catalytic efficiency for electrode reactions in FCs, which is still one of the technological bottlenecks and thus one of the most active and competitive research fields for FCs [1]. In recent years, to realize highperformance electrocatalysts with low cost, various nonprecious metal (NPM) electrocatalysts have been developed, including transition metal materials (oxides, carbides, sulfides, and nitrides) [5] and metal-free carbon-based materials (such as graphene [6,7], carbon nanotubes [8e10], and porous organic frameworketemplated porous carbons [11e13]). Nanocarbon-based materials with electroneutrality disruption and charge rearrangement, especially heteroatom-doped, have attracted intense Nanomaterials for Green Energy. https://doi.org/10.1016/B978-0-12-813731-4.00006-0 Copyright © 2018 Elsevier Inc. All rights reserved.

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attention owing to their economic feasibility, tunable surface chemistry, and fast electron transfer performance [14e16]. Compared with proton-exchange membrane FCs (PEMFCs), the advantages of alkaline FCs based on heteroatom-doped carbon catalysts include the nonsensitivity of catalysts to CO poisoning and the multiple choice of cathodic catalyst materials [17,18]. In this chapter, we highlight the recent advances of carbon materials-based electrocatalysts for FCs, including graphene, metale organic frameworks (MOFs), and metalenitrogenecarbon (MeNeC) materials, which can be manufactured by pyrolyzing various nitrogen-containing precursors (such as graphene, MOFs, polymer or organic small molecules, and biomass) [19e23].

6.2 GRAPHENE-BASED FUEL CELL ELECTROCATALYSTS The existence of graphene was predicted decades ago [24], experimentally confirmed by Boehm et al. in 1962 [25], and first stripped and characterized by Novoselov et al. in 2004 [26]. Since then, a great deal of research on graphene has emerged in a large number of fields because of its unique chemical and physical properties. Graphene is a two-dimensional (2-D) carbon crystal, with one-atom thickness, packed in a hexagonal honeycomb [26,27]. Up to now, various methods have been used for graphene synthesis, such as chemical vapor deposition (CVD), the Hummers method, direct liquid exfoliation, epitaxial growth on silicon carbide, arc discharge method, substrate-free gas-phase synthesis method, etc. [28e31]. Electrocatalysis is an acceleration process of the electrochemical interactions of the reactants, intermediates, or products occurring on the electrode surface. Therefore, the electrocatalytic performance is strongly related to the intrinsic properties of the electrode material, e.g., electrical conductivity, surface area, catalytic activity, stability, etc. [32]. For graphene-based electrocatalysts, graphene serves as the supporting material by depositing active components on graphene sheets and protecting the material by wrapping active material inside or metal-free catalysts by doping heteroatoms.

6.2.1 GRAPHENE-SUPPORTED ELECTROCATALYSTS Graphene is an excellent carbon support for noble metal-based electrocatalysts because of the following unique advantages. Firstly, its theoretical specific surface area is up to 2630 m2/g. Secondly, graphene shows not only metal-like conductivity with a fully conjugated sp2 hybridized planar structure and zero band but also excellent mechanical properties and high structural stability. Thirdly, the surface functional groups and lattice defects of chemically synthesized graphene can effectively immobilize and anchor metal nanoparticles. Noble metalebased materials have been widely used as FC electrocatalysts because of their appealing properties, such as high catalytic performance, low operating temperatures, high power densities, etc. [33]. However, the wide application of noble metal catalysts is largely impeded by their limited occurrence in nature, high cost, and poor durability. To solve these problems, one effective method is to control their shape with different exposed surface structures to optimize catalytic activity. The other approach is alloying noble metals with non-noble metals and controlling the component to decrease cost and tune catalytic activity. In addition, depositing noble metals on graphene sheets is also a widely accepted method to lower the cost, improve stability, and enhance catalytic performance of

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catalysts. In recent years, graphene-based nanomaterials with different morphologies (hollow, coreeshell, flower-like, nanodendritic nanostructures, etc.) have been successfully synthesized and investigated as electrocatalysts [34,35]. With the development of synthesis and characterization techniques, highly active metal clusters, even single-atom metals, can be prepared as catalysts. Theoretical and experimental studies have demonstrated that subnanometer-sized metal clusters usually exhibit higher catalytic activities than nanoparticles [36e38]. Recently, single-atom catalysts, as the ultimate small-size limit for metal particles, have attracted much attention because they maximize the efficiency of the exposed atom, which is important for supported noble metal materials [39]. Controlled preparation of stable clusters and single atoms in large scale is still a considerable challenge because the high free energy of metal atoms and clusters render them too mobile, with a natural tendency to sinter [40e42]. However, in practical applications, it is required that metal clusters or single atoms should have high activity, satisfactory durability, and high density of active sites. An effective method to overcome this obstacle is to anchor clusters or atoms on the graphene sheets to lower the free energy and enhance the catalytic activity and stability. Up to now, two methods have been successfully used to prepare graphene-anchored noble metal clusters, one is atomic layer deposition (ALD) and another is a mild self-assembly method. The ALD technique is gaining increasing attention as a controlled technique for the deposition of noble metals because it enables the deposition of uniformly distributed particles ranging from single atoms, to subnanometer-sized clusters, to nanoparticles by precisely controlling ALD cycles [43,44]. In the precisely controlled ALD synthesis process, a substrate is alternately exposed to different reactive precursor gases, and metal materials can be deposited in an atomic layer-by-layer fashion through the self-limiting approach [45]. Pt and Pd single-atom and cluster catalysts supported on graphene have been prepared by this method [45e48]. Sun et al. [45] reported the details of the synthesis process using (methylcyclopentadienyl)-trimethylplatinum (MeCpPtMe) and oxygen as precursors and nitrogen as purge gas, as shown in Fig. 6.1. There is a monolayer of oxygen-containing functional groups on a graphene sheet (Fig. 6.1A). First, MeCpPtMe3 reacts with the absorbed oxygen and forms CO2, H2O, and hydrocarbon fragments. The limited oxygen-containing surface groups of graphene can prevent all of the ligands of MeCpPtMe3 from oxidization and offer the self-limiting growth necessary for ALD, producing a Pt-doped monolayer (Fig. 6.1B). Subsequently, a new oxygen layer could be created on the Pt surface with exposure in oxygen (Fig. 6.1C). The above two processes form an ALD cycle. The Pt deposition can be precisely controlled by tuning the cycle numbers of ALD. The mild self-assembly process is another synthesis method for the formation of single-atom catalysts. This method usually contains three steps (the self-assembly process, the freeze-drying process and carbonization process). The self-assembly process produces the graphene precursor containing trance amount of metals by controlling the amount of metal precursor. The subsequent freeze drying process and carbonization procedure can realize the anchoring of metal atoms on graphene sheets and minimize the restacking of graphene. For example, Zhang et al. [49] prepared ruthenium/ nitrogen-doped graphene (RueN/G) using this method, as shown in Fig. 6.2. Firstly, Ru(NH3)6Cl3 was dispersed in graphene oxide solution by bath sonication. To achieve sufficient isolation of Ru atoms, the loading of Ru was controlled to 0.62%. Secondly, lyophilization process was then performed to remove water without minimizing the restacking of graphene oxide. Finally, RueN/G was obtained after calcinations. What’s more, Au clusters [50,51] and PdePt bimetallic nanoclusters [52] supported on graphene have also been obtained by similar methods.

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FIGURE 6.1 Schematic illustration of Pt ALD mechanism on graphene nanosheets. (A) There exists a monolayer of oxygen containing function group on the surface of graphene nanosheets; (B) During the MeCpPtMe3 exposure, some of the precursor ligands react with the adsorbed oxygen, and the limited supply of surface oxygen provides the selflimiting growth necessary for ALD, creating a Pt containing monolayer; (C) The subsequent oxygen exposure forms a new adsorbed oxygen layer on the Pt surface; This two processes (B and C) form a complete ALD cycle, producing one atomic layer of Pt atoms. (D) Through tuning the number of ALD cycles, the Pt deposition can be precisely controlled. Here, ‘*’ represents an active surface species, and Pt-O* represents oxygen molecules (or dissociated oxygen ions) that are adsorbed on the Pt surface. ALD, atomic layer deposition; GNS, graphene nanosheet; MeCpPtMe, (methylcyclopentadienyl)-trimethylplatinum. Reprinted with permission from S.H. Sun, G.X. Zhang, N. Gauquelin, et al., Single-atom catalysis using Pt/graphene achieved through atomic layer deposition, Sci. Rep. 3 (2013) 1775.

FIGURE 6.2 Schematic illustration of synthesis process for ruthenium/nitrogen-doped graphene catalyst. Reprinted with permission from C. Zhang, J. Sha, H. Fei, et al., Single-atomic ruthenium catalytic sites on nitrogen-doped graphene for oxygen reduction reaction in acidic medium, ACS Nano 11 (2017) 6930e6941.

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When the size of the noble metal supported on graphene decreases to the scale of the cluster and even a single atom, the electrocatalytic performance of the catalyst (catalytic activity, stability, antipoisoning ability, etc.) was found to be better than that of graphene alone, nanoparticles supported on graphene, and even commercial Pt/C. For example, by using the ALD method, Cheng et al. [46] prepared dispersed Pt atoms and clusters supported on nitrogen-doped graphene nanosheets (ALDPt/NGN) as electrocatalyst for hydrogen evolution reaction (HER). The electrocatalytic performance of ALDPt/NGN for HER is shown in Fig. 6.3. It can be seen that the ALDPt/NGN catalyst exhibits much higher HER activity than nitrogen-doped graphene nanosheets (NGNs) and commercial Pt/C catalysts (Fig. 6.3A). Meanwhile, the HER activity of ALDPt/NGNs decreases with increasing number of ALD cycles due to the formation of more clusters or nanoparticles under more

FIGURE 6.3 (A) Polarization curves of hydrogen evolution reaction (HER) on different catalysts in 0.5 M H2SO4 at room temperature; the inset shows the enlarged curves around the onset potentials; (B) mass activity comparison of the ALD50Pt/NGNs, ALD100Pt/NGNs, and Pt/C for HER at 0.05 V; (C) durability measurements of the ALD50Pt/ NGNs; (D) scanning transmission electron microscope image of ALD50Pt/NGNs sample after accelerated degradation test; the scale bar is 20 nm. ALD, atomic layer deposition; Pt/NGNs, Pt atoms and clusters supported on nitrogen-doped graphene nanosheets; RHE, reversible hydrogen electrode. Reprinted with permission from N. Cheng, S. Stambula, D. Wang, et al., Platinum single-atom and cluster catalysis of the hydrogen evolution reaction, Nat. Commun. 7 (2016) 13638.

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ALD cycles. With electric current normalized to Pt loading (Fig. 6.3B), the mass activity of ALD50Pt/NGNs (prepared with 50 ALD cycles) at 0.05 V reaches 10.1 A/mg, which is 7.8 times higher than for ALD100Pt/NGNs (2.12 A/mg, prepared with 100 ALD cycles) and 37.4 times higher than for commercial Pt/C (0.27 A/mg), suggesting the increased utilization of Pt for the catalysts with single Pt atom and Pt clusters. Fig. 6.3C shows the polarization curves of HER from ALD50Pt/NGNs recorded initially and after 1000 potential cycles. Fig. 6.3D shows the scanning transmission electron microscope image of ALDPt50/NGNs after the accelerated degradation tests, showing only a slight increase of Pt size without obvious aggregation. Both Fig. 6.3C and D indicate the excellent stability of ALDPt/NGNs catalysts for HER. Both the density functional theory (DFT) and experimental results have shown that the catalytic performance of bulk gold (Au) is poor toward the ORR [53,54]. However, studies have demonstrated that Au clusters with a size less than 2 nm have much higher electrocatalytic activity for ORR than the bulk Au and larger Au nanoparticles (NPs) [55,56]. The durability of Au clusters alone in electrocatalytic reactions is usually restricted by dissolution and aggregation due to their small size and high surface area. As a support, graphene sheets can provide initial nuclear sites and serve as electron donors for the reduction of Au ions and restrain the growth of Au clusters. Yin et al. [57] prepared ultrafine Au clusters supported on reduced graphene oxide (rGO) sheets. In this study, by comparing the electrocatalytic performances of rGO sheets and Au nanoparticles/rGO (Fig. 6.4A), the prepared Au cluster/rGO exhibited more positive onset potential. On the other hand, the Au cluster/rGO showed

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FIGURE 6.4 (A) Linear sweep voltammetry curves of commercial Pt/C, Au/reduced graphene oxide (rGO) hybrid, Au nanoparticle (NP)/rGO hybrid, rGO sheets, and Au clusters in O2-saturated 0.1 M KOH solution at a scan rate of 50 mV/s and rotation rate of 1600 rpm. (B) Chronoamperometric responses of Au/rGO hybrid and commercial Pt/C electrodes in O2-saturated 0.1 M KOH solution at 0.2 V. Reprinted with permission from H. Yin, H. Tang, D. Wang, et al., Facile synthesis of surfactant-free Au cluster/graphene hybrids for high-performance oxygen reduction reaction, ACS Nano 6 (2012) 8288e8297.

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higher stability than commercial Pt/C (Fig. 6.4B). Recently, Wang et al. [58] and Song et al. [51] also prepared rGO-supported Au clusters which exhibited enhanced ORR activity and stability. Zhang et al. [49] reported the synthesis of Ru clusters embedded in nitrogen-doped graphene matrix (RueN/G) through the mild self-assembly method, by controlling the amounts of Ru salts. As shown in Fig. 6.5, the catalyst exhibited excellent electrochemical performance with four-electron transfer number, and higher onset potential, enhanced durability, and higher tolerance against methanol crossover than commercial Pt/C. From the above studies, it can be seen that graphenesupported noble metal atoms or clusters is a type of promising highly active and stable catalyst for FCs, which have a great potential to increase the utilization and reduce the cost of noble metal catalysts. To further reduce the cost of electrocatalysts to the minimum, many studies focus on the design and fabrication of graphene-supported non-noble metal catalysts. The graphene-based non-noble metal materials mainly include three types: metal oxide (sulfide) nanocomposites, transition metal/nitrogen materials, and NPM clusters. Although metal oxides or sulfides are the most pursued alternative electrocatalysts, they suffer from dissolution, agglomeration, and sintering in the electrochemical reactions and result in the degradation of catalysts. The synergetic chemical coupling effects between metal oxides (sulfide) and graphene are favorable to the electrocatalytic performance due to the existence of charge transfer across the grapheneemetal interface [59]. A series of metal oxide (sulfide) nanocrystals grown on graphene, such as Co3O4 [60], MnCo2O4 [61], Mn3O4 [62], Co1
xS [63], Fe3O4 [64], etc., have been studied as electrocatalysts for ORR and other electrode reactions. Chen et al. [65] prepared FeS2 nanoparticles embedded between rGO sheets using the rapid heating and cooling method. The prepared hybrid exhibited improved electrocatalytic activity for HER with outstanding stability. DFT calculations showed that the existence of MoS2 induces p-type doping in graphene, which facilitates hydrogen adsorption on graphene [66]. In other words, MoS2 supported on graphene can enhance electrocatalytic performance for HER. For instance, Li et al. [67] synthesized reduced graphenesupported MoS2 nanoparticles (MoS2/rGO) via a facile solvothermal method. Compared with free MoS2 particles or rGO alone, MoS2/rGO showed smaller overpotential of w0.1 V (Fig. 6.6A) and smaller Tafel slope of 41 mV/decade (Fig. 6.6B). In addition, MoS2/rGO exhibited high stability with negligible loss of cathodic current (Fig. 6.6C). Transition metal/nitrogen supported on carbon materials has attracted increasing attention due to their promising catalytic performance and the utilization of inexpensive, abundant precursor materials. High-temperature pyrolysis process during catalyst synthesis can enhance electrocatalytic activity and stability of MeNeC catalysts (M ¼ Fe, Co, etc.) [68e70]. In alkaline solution, this kind of catalyst shows high catalytic activity, which can compete with commercial Pt/C [71e74]. Remarkably, this kind of catalyst can be used in acidic solutions, which is important for their practical applications [75,76]. For example, the FeeN-modified graphene prepared by Kamiya et al. [77] through a one-pot synthesis method exhibited efficient electrocatalytic activity for ORR in acidic solution with an onset potential of 0.85 V (vs. RHE). Meanwhile, although an increasing number of studies about single-atom catalysis have been reported, most of them have focused on the noble metal atoms or clusters (for example, Au, Pt, Pd). To further reduce the cost and expand the application of catalysts, non-noble metal single-atom catalysts have also attracted increasing attentions (for example, Co [78], Fe [79], Zn [80]). As shown in Fig. 6.7, Fei et al. [81] prepared Co atoms supported on nitrogen-doped graphene (CoeNG) by using a similar

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FIGURE 6.5 (A) Linear sweep voltammetry curves of RueN/G-750 with different rotating rates from 400 to 1600 rpm; the inset shows the corresponding KeL plots. (B) Plots of number of electron transferred and H2O2 yield of RueN/G750; inset shows the disk and ring current. (C) Mass and specific activities of RueN/G-750 and commercial Pt/C at 0.70 V. (D) Accelerated degradation test of RueN/G-750 for 10,000 potential cycles. (E) Iet curves of methanol crossover tests from RueN/G-750 and commercial Pt/C at 0.7 V. (F) Iet curves of RueN/G-750 and commercial Pt/C at 0.7 V. All tests were carried out in 0.1 M HClO4. RHE, reversible hydrogen electrode. Reprinted with permission from C. Zhang, J. Sha, H. Fei, et al., Single-atomic ruthenium catalytic sites on nitrogen-doped graphene for oxygen reduction reaction in acidic medium, ACS Nano 11 (2017) 6930e6941.

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FIGURE 6.6 (A) Hydrogen evolution reaction polarization curves of catalysts as indicated. (B) The corresponding Tafel plots on the studied catalysts. (C) Durability test of the reduced graphene-supported MoS2 nanoparticles (MoS2/rGO) hydrid catalyst. RHE, reversible hydrogen electrode. Reprinted with permission from Y. Li, H. Wang, L. Xie, et al., MoS2 nanoparticles grown on graphene: an advanced catalyst for the hydrogen evolution reaction, J. Am. Chem. Soc. 133 (2011) 7296e7299.

FIGURE 6.7 Schematic illustration of the synthetic process of the Co atoms supported on nitrogen-doped graphene catalyst. GO, graphene oxide. Reprinted with permission from H. Fei, J. Dong, M.J. Arellano-Jimenez, et al., Atomic cobalt on nitrogen-doped graphene for hydrogen generation, Nat. Commun. 6 (2015) 8668.

self-assembly method for preparation of noble metal clusters. The prepared catalyst exhibited highly active HER performance with low overpotential of 30 mV and high stability in solution, indicating that such a catalyst is a promising candidate to replace Pt for water splitting applications.

6.2.2 GRAPHENE-WRAPPED ELECTROCATALYSTS In addition to nanocatalysts supported on the surface of graphene, hierarchical architectures that nanoparticles encapsulated in ultrathin graphene shells can also act as electrocatalysts, and such a structure can prevent nanoparticles from agglomeration, corrosion, and dissolution. Compared with

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traditional hollow carbon spheres, ultrathin graphene nanoshell can not only retain the unique properties of graphene but also reduce the diffusion length [82]. The challenge for application of nonnoble-metal electrocatalysts in acidic media is on how to improve the catalytic efficiency and stability. The metalegraphene coreeshell structure with thin graphene layer as shell can effectively improve the stability and activity of the wrapped metal nanoparticles. Bao’s group has given the model catalysts of thin graphene layer protected 3d transition metal catalysts for HER or oxygen evolution reaction (OER) [83,84]. For example, Deng et al. [83] prepared such electrocatalysts by encapsulating CoNi nanoalloys in ultrathin graphene spheres with only one to three layers (CoNieNC), using Ni2þ, Co2þ, and ethylenediaminetetraacetate anions (EDTA4
) as precursors. DFT calculations in this study also demonstrated that the ultrathin graphene shells can enhance electron penetration from CoNi nanoalloy to the graphene surface. The optimized CoNieNC exhibited high HER activity in acidic solution with overpotential of only 142 mV at 10 mA/cm2, which is close to that of commercial Pt/C. In another study, Cui et al. [84] synthesized single-layer grapheneeprotected 3d transition metals (Fe, Co, Ni, and their alloys) through the CVD process in the channels of SBA-15 (Fig. 6.8). The prepared FeNieNC is the best OER catalyst among the studied catalysts, and its activity and stability are superior to commercial IrO2. DFT calculations indicated that the encapsulated 3d transition metal nanoparticles could optimize the electronic structure of the singlelayer graphene surface and immensely promote OER activity. These findings indicate that the ultrathin graphene shelleprotected nanoparticle catalysts are a type of promising electrocatalysts.

SBA-15

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FIGURE 6.8 Schematic illustration of the preparation process of MeNCs from SBA-15 and metal-containing precursors. CVD, chemical vapor deposition; M-NCs, single layer graphene encapsulating 3d transition metals (M¼Fe, Co, Ni, FeCo, FeNi, CoNi). Reprinted with permission from X.J. Cui, P.J. Ren, D.H. Deng, et al., Energy Environ. Sci. 9 (2016) 123e129.

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6.2.3 GRAPHENE-BASED METAL-FREE CATALYSTS Graphene-based metal-free materials are of great importance for the development of electrocatalysts because of their low cost and high durability in strong acid or base electrolyte. Heteroatom-doped graphene is one of the most studied metal-free catalysts, which shows extraordinary electrocatalytic activity, some even close to or exceeding the state-of-the-art commercial noble metal catalysts. Generally, there exist two kinds of chemically heteroatom-doped graphene: surface transfer doping and substitutional doping [85]. The surface transfer doping can’t cause sp3 defects in graphene lattice because foreign agents are adsorbed on the surface of graphene. The substitutional heteroatom-doping can cause carbon atoms in graphene lattice being substituted by single/multiple N, B, S, and P atoms, which may disrupt sp2 network and cause sp3 defect regions. The second type of heteroatom-doping is the main factor that has an effect on the electrocatalytic activity of graphene. The heteroatoms of N, P, S, and P have been used to modify the electronic properties of graphene and create active sites by introduction of charge and spin densities on C atoms near dopants, affecting the adsorption and desorption of reactants, products, and intermediates on the surface of doped graphene, enhancing the electrocatalytic performances for electrode reactions, such as HER, ORR, and OER [86,87]. Recently, carbon materials combing carbon nanotubes and heteroatom-doped graphene have been reported as another important type of metal-free nanocatalyst. Nitrogen-doped graphene/carbon nanotube (NGeNCNT) nanocomposites were synthesized by a facile hydrothermal process at a heating temperature of 180 C using oxidized carbon nanotube, GO, and ammonia as precursors [88]. Compared with nitrogen-doped graphene (NG) and NCNT, NGeNCNT nanocomposites show the most positive onset potential, the largest peak current, and the highest electron transfer number of 3.7, indicating that this kind of metal-free hybrid can effectively improve the electrocatalytic activity for ORR. It was proposed that the enhanced catalytic performance is probably caused by the following two reasons. Firstly, carbon nanotubes (CNTs) can prevent graphene sheets from restacking and increase the basal spacing due to the insertion of CNTs between graphene nanosheets, which is beneficial to increase active sites [89]. Secondly, the combination of graphene with CNT produces a threedimensional (3-D) interpenetrated network structure, which will accelerate the transportation of reactants, ions, and electrons [90,91]. According to the related studies of graphene-based electrocatalysts in recent years, it can be seen that the research trends mainly focus on three aspects. Firstly, researchers seek to prepare tiny metal catalysts supported on graphene which can effectively reduce cost, improve the utilization of precious metals, and improve the active surface area and activity. The size of the metal catalysts can be controlled from nanoparticles with unique surface structures to nanoclusters even to single atom. Secondly, the thinner the graphene layer is, the better the electrochemical property becomes. The ultrathin graphene layer can not only protect the wrapped nanomaterials from dissolution, aggregation, and corrosion but also provide a fast mass transfer. Thirdly, graphene itself can be doped by heteroatom or combined with other carbon materials to prepare high-performance electrocatalysts for FCs.

6.3 METALeORGANIC FRAMEWORKeDERIVED ELECTROCATALYSTS In recent years, MOFs have attracted particular attention as alternative precursors and templates for novel nanoporous carbon-based electrocatalysts. Their customizable porous crystalline framework structures can be modularly constructed from transition-metal ions/clusters as nodes and organic

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ligands as struts [92,93]. Inspired by their diverse structures and functions, flexible tunability in compositions, and properties, in principle, MOFs have been tailored for “on demand” applications [94]. Due to their high surface area, large pore volume, and readily self-sacrificial nature, MOFs have been demonstrated as promising precursors of highly nanoporous carbons in electrocatalysts, electrochemical capacitance, sensing applications, and gas adsorption [2,92]. Especially, MOFs-derived electrocatalysts own highly ordered porous structures with an average pore diameter distribution ratio, plentiful permanent open channels, and nanoscaled cavities, offering congenital situations for small molecules to access [23]. Besides, the large surface area is conducive to the dispersion of active components and sites for electrocatalytic reactions. Efficient catalysts with high surface area and uniformly distributed active sites can be formed without adding any carbon support or pore-forming agent. The surface area and pore size of MOFderived carbon catalysts can be tuned by the length of the organic linkers, which will be converted into carbon during thermal activation. Based on systematical investigation of the relationship between precursor structure and catalyst activity, the metaleligand composition can be rationally designed with a wide selection of metalelinker combinations [95]. The preparation and application of MOF-derived carbons as ORR catalysts is briefly summarized in this section. Up to now, several common MOFs, such as MOF-5, ZIF-8, and ZIF-67 (Fig. 6.9) have been successfully transformed into nanoporous carbons with pronounced electrocatalytic properties [93,96]. MOF-5 framework ([Zn4O(bdc)3], bdc¼1,4-benzenedicarboxylate) [97] is one of the most represen˚ ). Xu’s group reported the tative MOFs with a 3-D intersecting channel system (cavity diameter 18 A synthesis of nanoporous carbon materials derived from MOFs for the first time through the decomposition of MOF-5 filled with furfuryl alcohol (FA) [23]. During synthesis, FA was first introduced into the micropores of MOF-5 by a vapor-phase protocol. After carbonization at different decomposition

FIGURE 6.9 Crystal structures of MOF-5 (left), ZIF-67 (middle), and ZIF-8 (right). MOF, metaleorganic framework; ZIFs, zeolitic imidazolate frameworks. Reprinted with permission from W. Chaikittisilp, K. Ariga, Y. Yamauchi, A new family of carbon materials: synthesis of MOF-derived nanoporous carbons and their promising applications, J. Mater. Chem. 1 (2013) 14e19 and A. Phan, C.J. Doonan, F.J. UribeRomo, et al., Synthesis, structure, and carbon dioxide capture properties of zeolitic imidazolate frameworks, Acc. Chem. Res. 43 (2010) 58e67.

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temperatures, nanoporous carbons with different BrunauereEmmetteTeller (BET) surface areas were obtained. For instance, the nanoporous carbon material obtained at the decomposition temperature of MOF-5 (1000 C) possesses a very high specific BET surface area of 2872 m2/g. However, at a lower decomposition temperature of 800 C, the obtained carbon showed a much lower BET surface area of 417 m2/g. An even lower BET surface area of 217 m2/g was obtained at an even lower decomposition temperature of 530 C [23].

6.3.1 ELECTROCATALYSTS PREPARED FROM METALeORGANIC FRAMEWORK-5 Li et al. synthesized metal-free porous N-, P-, and S-doped carbon materials (NPSeCeMOF-5) by using MOF-5 as template [12]. Typically, MOF-5 was added into the methanol solution of dicyandiamide (DCDA), triarylphosphine, and dimethyl sulfoxide, which were used as N, P, and S precursors, respectively. The mixed solution was filtered, dried, and carbonized in ultrapure N2 atmosphere at 900 C for 5 h at a heating rate of 10 C/min. The obtained NPSeCeMOF-5 was collected and washed with dilute HCl solution and distilled water, respectively. The electrochemical studies displayed that the prepared NPSeCeMOF-5 catalyst demonstrated a relatively positive onset potential for ORR in alkaline media, which is very close to that of the commercial Pt/C catalyst. Especially, the NPSe CeMOF-5 showed outstanding methanol tolerance and superior long-term stability than the commercial Pt/C catalyst [12]. The excellent ORR performance of NPSeCeMOF-5 could be attributed to the following reasons: (1) when heteroatoms were doped into the carbon framework of MOF-5, the electroneutrality is broken and a large number of active sites can be generated, which facilitates the adsorption of O2 and can significantly enhance the overall ORR rate; (2) the changed asymmetric spin densities caused by heteroatom-doping can efficiently weaken the OeO bond to generate more active sites and further enhance the ORR activity of the obtained carbon materials; (3) the pore structures of carbon materials are changed. The increase of active sites and mesopores due to the introduction of heteroatom-doping has extraordinary effects on the corresponding electrocatalytic activities [12].

6.3.2 ELECTROCATALYSTS PREPARED FROM ZEOLITIC IMIDAZOLATE FRAMEWORKS Zeolitic imidazolate frameworks (ZIFs), a subfamily of MOFs, are 3-D porous crystalline materials of tetrahedral networks consisting of transition metals (such as Zn2þ and Co2þ) which replace tetrahedrally coordinated atoms (for example, Si) and imidazolate links which replace oxygen bridges [2,98]. Unlike MOFs with carboxyl ligands, some ZIFs not only contain a rich nitrogen source in imidazolate ligands but also show excellent thermal and chemical stability, which makes ZIF analogs an outstanding candidate to fabricate nitrogen-doped porous carbons with high electrocatalytic performances [17]. At the beginning of the decomposition process of the host ZIF frameworks, ZnO will be produced and further reduced to Zn by carbon (or CO) at T > 800 C [99]. At higher temperatures, organic ligands of Zn-based MOFs can be converted to carbon. At the boiling point (908 C), the Zn metal center will be reduced and emitted under inert gas flowing, yielding metal-free nitrogen-doped porous carbon materials at T ¼ 950 C [2]. In earlier studies, FA was basically used as an additional carbon precursor, as mentioned above [23,97,100e102]. Due to the noxiousness, explosivity, and irritation of FA, alternative green carbon precursors have also been developed. For example, Cao’s group fabricated nitrogen-doped porous carbons (carbon-L and carbon-S) derived from glucose-filled

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ZIF [2]. As an alternative carbon precursor, glucose can penetrate the outside surface and/or cavities of solid ZIF-7 and then polymerize and carbonize into nanoporous carbons [103].

6.4 TRANSITION METALeDERIVED METAL (M)eNeC ELECTROCATALYSTS As a substitute, nonprecious metal catalysts (NPMCs) with high catalytic activities have been developed to address the cost and durability issues of precious metal catalysts for ORR. Among the studied NPMCs, low-cost carbon-based catalysts show the greatest potential to substitute precious metals in the future because of their high surface area and functionality and extremely pronounced electrocatalytic properties, especially sufficient stability under harsh environments. Heteroatomdecorated carbon frameworks, especially nitrogen-doped carbon nanocomposites are usually prepared from metal-free NeC or transition metal-derived MeNeC (M ¼ Fe or Co) formulations. These carbon-based materials have been verified to have remarkably enhanced catalytic activity and stability in alkaline and acidic electrolytes, which makes them promising for practical applications [104,105]. The reason is attributed to a great number of metal species (MeNx) introduced into nitrogen-decorated carbon frameworks as active additives, which further enhance the electrocatalytic activity of such materials.

6.4.1 ELECTROCATALYSTS DERIVED FROM NONPYROLYZED MACROCYCLES As one of the most important alternatives to precious metal catalysts for ORR, NPMCs with transition metalenitrogen coordination sites (MeNx, M¼Fe, Co, Ni, etc.) have generally attracted wide attention. Studies on these typical catalysts can be traced back to Jasinski’s pioneering work of discovering a simple transition metalephthalocyanine material (MePc, M¼Co, Ni, Cu) with ORR activity in the 1960s [106]. The high activities of these electrocatalysts originate from the existence of delocalized p-electrons in Pc, which guarantees fast electron transfer during electrochemical reactions [107]. After this pioneering work [106], transition-metal macrocyclic compounds were largely extended to a broader variety of MeN4 macrocyclic components with various metals such as Fe, Co, Ni, Mn, Cu, Zn, and different macrocycles such as (tetraphenyl)porphyrins, phenanthroline, etc. [108]. Due to the most favorable electron occupation of 3d orbital of Fe, it was always found that FeeN4 materials have the best ORR performance, followed by Co- and Ni-based materials [109]. Then, the investigation of metalenitrogen macrocycle analogs sprang up to improve their catalytic activities [110e113].

6.4.2 ELECTROCATALYSTS DERIVED FROM PYROLYZED MACROCYCLES (MeNX/C) The dismal stability of pristine nonpyrolyzed MeN4 organometallic macrocycle complexes in acid solutions limits the development of this type of macrocycle catalysts [114]. To overwhelm this difficulty, various N-containing organic complexes (porphyrin, triazine, their derivatives, etc.) have been used to prepare 2-D covalent organic polymers (COPs) with exactly tuned positions of N heteroatom and holes. Then, it was found that MeNx/C hybrids with strong metalenitrogen covalent bonds can be obtained by pyrolyzing MeN4 macrocycle complexes at high temperatures (>800 C), which can

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further significantly improve their stability and activity for ORR [108]. During the pyrolysis procedure, the peripheral nitrogen-containing ligands can be transformed into pyridinic, pyrrolic, and graphitic nitrogen species bonded to metal centers. Meanwhile, the metal centers will form metal nanoparticles to catalyze the formation of graphitic carbon, which could encapsulate metal nanoparticles to protect the metal centers and further enhance conductivity [20]. Due to the high cost of these metal macrocycles, Yeager proposed a model to form MeNx structures by binding transition metal species and electronically conducting surfaces with nitrogen groups [115]. This model provided an economic way for facilely preparing FeeNeC catalysts with even better activity and stability by high-temperature pyrolyzing of a mixture of inorganic iron salts, nitrogen-containing species, and carbon support [20,116]. Since then, there has been great development in this area [117,118]. Despite the significant progresses achieved by optimizing the constructing processes and precisely selecting suitable precursors containing transition metals, N and C, the electrocatalytic activities of the obtained MeNeC catalysts for ORR still need to be further improved [119,120]. Besides, up to now, although intensive catalysis studies have been carried out by using physical or electrochemical techniques, the nature of the active sites responsible for high activity of MeNeC catalysts is still difficult to explain [121]. It was reported that the electrocatalytic activities are strongly associated with the nature of the macrocyclic ligand and the type of metaleNx edge defects, which always requires advanced synthetic approaches and dexterously controlled chemical compositions [122]. It is generally accepted that the metal center plays an important role in improving ORR activity. Recently, trace doping of transition metals has been proved to significantly enhance ORR catalytic activity of metal-free catalysts. In various MeNeC systems, compared with the commercial Pt/C catalyst, the FeeNeC catalysts display greater ORR electrocatalytic activity due to the strong interactions between Fe centers and N species from the carbon matrix [123]. DFT calculations discovered that the N atoms could get extra electrons due to modified Fe atoms, leading to a decrease of Mulliken charges on central iron atoms of FeeN4. With additional Fe atoms, the highest energy occupied molecular orbital energy of the catalyst rises, resulting in more overlap with the lowest energy occupied molecular orbital of triplet O2 and thus increased ORR activity. Besides, the binding energy of Fe-modified FeeN4 configuration with O2 molecule is a little higher than that of the bare one, which is favorable for the adsorption of O2 molecule on the active sites and promotes the ORR activity further [124,125].

6.4.3 METALeORGANIC FRAMEWORKSeDERIVED MeNeC CATALYSTS Recently, the conversion of metaleorganic coordination materials to nanoporous metal/carbon composites has proved to be an effective method to prepare electrocatalysts for FCs. Due to the ordered structure with M, N, and C components and high surface area, MOF crystals are a class of ideal precursors for preparing MeNeC electrocatalysts. Among them, the preparation of FeeNeC and CoeNeC from MOFs has been extensively studied for high activity and stability [124]. The first example of ORR catalyst derived from MOF was reported by activating cobalt imidazolate frameworks (CoIM) at high temperature, which attracted much attention in this field [95]. There exist abundant CoeN4 moieties within the MOF with each cobalt atom coordinated with four nitrogen atoms from the imidazolate ligand. In the pyrolyzed samples, the single CoIM crystal acts as a catalytic center with high density. Even though the long-range ordered structure was destroyed, the significant fraction of micropores and the high surface area were retained after thermal activation of

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the parent MOF. They all contributed to the high ORR catalytic activity with half-wave potential (E1/2) of 0.68 V and an onset potential of 0.83 V in an acidic medium. Co/N-doped nanoporous carbon composites can be obtained from zeolitic imidazolate frameworks after carbonization [126]. Zhang’s group has also successfully synthesized a hierarchically porous cobalt, nitrogeneco-doped carbon nanoframework (Co, NeCNF) electrocatalysts derived from Zn, Co zeolitic imidazolate frameworks (Zn, CoeZIF) through a silica-protection strategy. First, a Zn, CoeZIF MOF with a typical dodecahedral shape and average size of w120 nm was synthesized using the procedure described for ZIF-8, except that Zn2þ was partially substituted by Co2þ. An mSiO2 shell was then covered onto the surface of this Zn, CoeZIF through the hydrolysis of tetraethyl orthosilicate catalyzed in alkaline environment. The resulted Zn, CoeZIFemSiO2 was subsequently thermaltreated (900 C) in N2 and converted into Co, NeCNFemSiO2. Finally, after the removal of the mSiO2 shell by etching with hydrofluoric (HF) acid yielded Co, NeCNF, which possesses the skeletal structure of ZIF and also a hierarchical pore structure. The resulting Co, NeCNF with high specific surface area displayed comparable and superior ORR catalytic performance to the commercial Pt/C catalysts in acidic and alkaline media, respectively [19]. Yu’s group synthesized highly porous doped carbon nanofibers (CNFs) derived from tellurium nanowires (Te-NWs)eZIF-8 nanofibers via a nanowire-directed templating synthesis strategy (Fig. 6.10A and B). Phosphine introduced into this system can create competent ORR catalytically active sites by a reannealing process with triphenylphosphine under nitrogen flow. The introduction of the P element creates more active sites, which is favorable for activity enhancement of P-doped CNFs for ORR with more positive onset and E1/2 potentials than those of the commercial Pt/C catalyst, as shown in Fig. 6.10C and D. The excellent ORR catalytic performance could be attributed to the complex network structure, hierarchical pores, and high surface area of P-doped CNFs [127]. Wang et al. have also synthesized zeolitic imidazolate framework-67 (ZIF-67)ederived Co/Ndoped nanoporous catalyst (Fig. 6.11AeD), which exhibited excellent ORR activity in both alkaline and acidic electrolytes. The enhanced ORR performance is attributed to combining Co cations with aromatic nitrogen ligands in the MOF structure after pyrolysis which creates the as-proposed active ORR sites (CoeNx). Besides, the rich nanoporosity and ordered graphitic structure are also beneficial for improving the catalytic activity for ORR. It was proved in this work that once the derived catalyst with maximized porosity is obtained by optimizing the pyrolysis temperature and the acid leaching process (Fig. 6.11E), the best ORR performance can be achieved. This work provides guidelines for the optimization of processing conditions and indicates that the well-defined chemical environment of the metal centers in MOF structures can be applied to create catalytic active sites for ORR [128]. In another work, Liu et al. reported a facile synthesis strategy for double-shelled nanocages of microporous nitrogen-doped carboneCoenitrogen-doped graphitic carbon (NCeCoeNGC DSNCs) by templating against coreeshell MOF (ZIF-8eZIF-67). The unusual hollow nanocage structure is formed due to the surface-stabilized contraction of coreeshell ZIF crystals at high temperature. In this study, the pyrolysis temperature is a critical factor in terms of tuning the activity. In detail, when pyrolyzed at 800 C, the sample exhibited the highest current density and the lowest overpotential for ORR due to the formation of CoeNGC shells outside of the robust hollow NC structure. First-principle calculations revealed that high catalytic activity of NCeCoeNGC catalyst is due to the synergistic electron transfer and rearrangement between Co nanoparticles, graphitic carbon, and the doped N species. The NCeCoeNGC with the lowest overpotential for ORR (hORR) suggests the strongest

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bonding of OOH intermediate on carbon structures, which is a vital rate-determining step to achieve pronounced bifunctional electrocatalytic activity [129]. The excellent ORR performance of MOF-derived catalysts might be attributed to the reasons as follows. Firstly, the clearly defined 3-D porous structure of MOFs will be kept after thermal activation. The inherent permanent nanoscaled cavities and open channels will offer congenital conditions for small molecules to access and mass diffusion [130]. Secondly, the porous structure with large numbers of mirco- and mesopores and high surface area will provide more active sites, which is favorable for the four-electron reduction pathway [131]. Thirdly, the initial entities of heteroatoms such as nitrogen elements and transition metals will form uniformly distributed electrocatalytic active sites (e.g., basic N-sites, MeeN4), which will greatly enhance the ORR performance. Furthermore, graphitized carbon

FIGURE 6.10 (A) Transmission electron microscopy (TEM) image of Z8-Te-1000. Inset shows the photograph of the sample. (B) Magnified TEM image of Z8-Te-1000. (C) Cyclic voltammetry (CV) curves of P-Z8-Te-1000 in N2- or O2saturated 0.1 M KOH. (D) Linear sweep voltammetry curves of all the studied catalysts in O2-saturated 0.1 M KOH with a sweep rate of 10 mV/s and electrode rotation speed of 1600 rpm. Reprinted with permission from W. Zhang, Z.Y. Wu, H.L. Jiang, et al., Nanowire-directed templating synthesis of metaleorganic framework nanofibers and their derived porous doped carbon nanofibers for enhanced electrocatalysis, J. Am. Chem. Soc. 136 (2014) 14385e14388.

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(A)

(B)

(C)

(D)

(E)

FIGURE 6.11 TEM images of different metaleorganic framework (MOF)-derived catalysts: (A and B) ZIF-67-900, (C) Co2(bdc)2(dabco)-900, and (D) ZIF-8-900. The arrow in (B) indicates a void enclosed by graphitic layers. The inset in (B) shows a lattice image of a Co particle. (E) Schematic illustration of the synthesis process for the ZIF67-T catalysts. The coordination environment of Co in the product (a proposed structure) is also shown. Reprinted with permission from X.J. Wang, J.W. Zhou, H. Fu, et al., MOF derived catalysts for electrochemical oxygen reduction, J. Mater. Chem. A 2 (2014) 14064e14070.

usually formed via catalysis over transition metals, such as Fe, Co, etc., would further enhance the electrical conductivity and catalytic activities [130]. However, due to the limited size of MOF crystals and the inevitable internal collapse of MOFs in the process of calcination, direct carbonization of a free MOF crystal cannot meet the requirement of consecutive electronic conductivity for ORR [132]. A study from Zhang’s group found that direct calcinations of free MOF cannot achieve electrical conductivity that ORR needs, and therefore, MOF can be grown on conductive substrate to overcome this problem [132]. Similarly, Zhu’s group developed a high surface area and nitrogen-doped carbon material derived from ZIF-8 with in situ growth on CNT. The obtained material showed a 25-mV lower half-wave potential (E1/2) but a comparable limiting current density than the commercial Pt/C catalyst [133]. Except for nitrogen, other heteroatoms, such as boron, fluorine, phosphorus, sulfur, etc., have also been introduced into carbon structures. Recently, Chen et al. has developed a new route to synthesize N and S co-decorated hierarchical carbon layers (S, NeFe/N/CeCNT). The hierarchical structure offers more opportunities for atomically dispersed FeeNx active sites with greatly improved electrical conductivity [105]. Although the ORR mechanism on metal-free materials is still unclear, numerous efforts have been made to analyze the possible reasons for the enhanced ORR catalytic properties of heteroatom-doped carbon catalysts. First, heteroatoms introduced into carbons can change the electronic character

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(mainly electroneutrality) of the carbons and create rational defect structures, which enable a stronger O2 adsorption. Whether the dopants are electron rich (as N, F) or electron deficient (as B, P), breaking the electroneutrality of carbons will create charged sites, which are favorable for O2 adsorption [12,134,135]. In detail, by doping heteroatoms with higher electronegativities than that of C atoms (2.55), such as N (3.04), F (4.0), S (2.58), etc., positively charged carbon atoms could be induced and result in an increase of ORR activity. However, it is a diametrical mechanism for heteroatoms with lower electronegativities than C atom, such as B (2.04) or P (2.19). Asymmetric charge density will be offset due to their opposite properties of positively charged carbon atoms (induced by N or F doping) and negatively charged carbon atoms (induced by B or P doping). Otherwise, it has been reported that asymmetric spin density of carbon atoms caused by heteroatoms is more vital in determining ORR activities of the heteroatom-doped carbon materials. Generally, the carbon atoms that possess the highest spin density are the electrocatalytic active sites. The changed asymmetric spin densities caused by heteroatoms can efficiently weaken the OeO bond to generate more active sites and further enhance the ORR activity of carbon materials [12,136,137]. The ORR performance of heteroatomdoped carbons is also strongly subjected to the doping site and concentration of heteroatoms, which can be elaborately tuned by rationally designed precursor choice and doping procedures (Fig. 6.12A). Theoretical analysis revealed that the origin of this enhancement is intermolecular synergistic

FIGURE 6.12 (A) Typical atomic configuration of different types of doped heteroatoms at different doping sites in the carbon matrix. (B) Free energy diagram of different heteroatom-doped graphene at the equilibrium potential U0. (C) Experimentally determined Tafel plots for different catalysts collected at RDE ¼ 1600 rpm (RDE, rotating disk and DGOOH with charge-transfer coefficient a ¼ 0.5 (red dashed electrode). (D) Volcano plot between jtheory 0 obtained from Tafel plots and density functional theoryederived DGOOH for line). Blue hollow squares are jtheory 0 each doped graphene catalyst. (A) Reprinted with permission from Y. Jiao, Y. Zheng, M. Jaroniec, et al., Design of electrocatalysts for oxygen- and hydrogeninvolving energy conversion reactions, Chem. Soc. Rev. 44 (2015) 2060e2086 and (BeD) reprinted with permission from Y. Jiao, Y. Zheng, M. Jaroniec, et al., Origin of the electrocatalytic oxygen reduction activity of graphene-based catalysts: a roadmap to achieve the best performance, J. Am. Chem. Soc. 136 (2014) 4394e4403.

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catalysis. The DFT calculations indicated that as active sites, the dopants will modify the electron donor properties of carbon atoms nearby and further improve the intermediate adsorption, such as OOH [108]. As shown in Fig. 6.12B, different kinds of heteroatoms and different doping sites in graphene will change the free energy diagram of the ORR reaction process [6]. Fig. 6.12C shows the experimentally determined Tafel plots of different catalysts. From the Volcano plot in Fig. 6.12D, an optimal ORR catalyst should own a higher exchange current density (j0) induced by the free energies of ORR intermediates OOH (DGOOH ), which is closer to the volcano center [138]. Among various heteroatoms-doped graphene catalysts, Negraphene (NeG) displays the lowest change of free energy, signifying its highest activity for catalyzing ORR, which is consistent with numerous experimental results [6,139].

6.5 ELECTROCATALYSTS DERIVED FROM POLYMERS Up to now, the synthesis of a desired carbon material can be achieved by pyrolyzing a suitable polymer precursor; for example, polymer colloidal nanospheres-derived carbon nanospheres with controllable size and tunable architecture, as vibrant materials have been successfully synthesized. And various polymers have been successfully used to synthesize uniform nanospheres under inert gas, such as glucose [140,141], polydopamine (PDA) [21,142e145], phenol-formaldehyde resin [146,147], polyanilinee co-polypyrrole [148], poly(1,8-diaminonaphthalene) (PDAN) [149], 2,6-diaminopyridine [150], poly(phenylacetylene), poly(4-vinylpyridine), poly(3-methylpyrrole), poly(2-methyl-1-vinylimidazole), poly(p-pyridazine-3,6-diyl) [8,14], polyaniline, polypyrrole, and poly(3,4-ethylenedioxythiophene) [151e154]. Since Liu et al. prepared resorcinol/formaldehyde (RF) polymer spheres by the Sto¨ber method, many RF resinederived carbon spheres have been reported [146]. In the related studies, organic solegel of RF resins is analogous to silicate solegel process due to similar morphologies and nanostructures obtained under similar conditions. During the synthesis, RF resins can form fourcoordinate covalently bonded silica-like frameworks through polymerization and can be then converted into carbon spheres after carbonization. Ai et al. prepared monodisperse and size-controlled sp2 carbon-dominant nitrogen-doped carbon submicrometer spheres by employing biomolecule dopamine as the carbon resource due to its self-polymerization and the spontaneous deposition of PDA films on almost any surface in alkaline environment [143,144]. The inherited catechol and NeH groups from starting materials can effectively adsorb transitional metal ions, which enable further introduction of transitional metal-based electroactive substances into the nitrogen-doped carbon matrix (FeeFe3C/ CeN). And compared with the commercial 20 wt% Pt/C, this hybrid material displayed higher stability, better catalytic selectivity, and comparable ORR catalytic activity. Such carbon materials show potential applications in energy conversion and storage technologies that typically proceed in the acidic media [21]. Xing’s group synthesized iron carbide-encapsulated meso-/macroporous nitrogen-doped carbon architecture (Fe3C/NG) by pyrolyzing the environmentally friendly precursors of PDAN and FeCl3 [149]. Zhong et al. have prepared a novel nitrogen-enriched porous nanocarbon (NP) ORR catalyst through a simple procedure of carbonizing a well-defined block copolymer, poly(n-butyl acrylate)block-polyacrylonitrile (PBA-b-PAN). In this study, PAN block acts as carbon source and PBA as a sacrificial block [155]. This work provides a convenient synthetic process for preparing nitrogen-

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enriched porous nanocarbons with controlled morphology and nanoporosity. The morphology and porosity can be easily tuned by varying the ratios and lengths of PBA and PAN blocks. With certain PAN/PBA ratios, the original morphology of the bulk precursor can be well reserved during the pyrolysis process [135,155]. Feng and Mulle’s group have synthesized a family of mesoporous NPM catalysts by using silica colloids, ordered mesoporous silica SBA-15, and montmorillonite as templates. The most active mesoporous catalyst was cobaltenitrogen-doped carbon (CoeNeC) fabricated from VB12 and silica colloids and displayed an extraordinary ORR activity in acidic medium with an electron-transfer number larger than 3.95 and a half-wave potential of 0.79 V, which is only w58 mV deviation from Pt/C (Fig. 6.13A and C). Notably, the CoeNeC exhibited an excellent electrochemical stability with half-wave potential negative shifted for only 9 mV after 10,000 potential cycles (Fig. 6.13D). Besides, the authors also prepared FeeNeC catalysts derived from polyanilineeFe complex with templates of silica colloids (12 nm). The remarkable ORR performance of these NPM catalysts could be attributed to their high BET surface area (572 m2/g) (Fig. 6.13B), well-defined mesoporous structures with a narrow pore size distribution, and plentiful homogeneously distributed metaleNx active sites [156]. Then, the same group also prepared nitrogen-doped carbon nanosheets (NDCNs) with uniform and tunable mesoporous structure by applying a templating approach. As shown in Fig. 6.14A, in the preparation procedure, oPD was first polymerized with silica colloid as template in the presence of ammonium peroxydisulphate. After that, the poly-o-phenylenediamine (PoPD)/SiO2 nanocomposite was pyrolyzed in N2 atmosphere and the SiO2 template was subsequently removed with 2.0 M NaOH solution, yielding meso-PoPD. Then, the meso-PoPD was further activated with NH3 to form the final meso-/micro-PoPD catalyst (TEM image is shown in Fig. 6.14B). For such NDCN material with controlled mesoporous structure, the activation condition exhibited an important influence on the electrocatalytic performance for ORR in both acidic and alkaline media. When activated with NH3, the meso-/micro-PoPD owns the BET surface area of 1190 m2/g with a pore diameter of 22 nm (Fig. 6.14C). As a highly efficient metal-free ORR catalyst, the NDCN showed a more positive ORR onset potential of 10 mV and a higher diffusion-limited current density (0.33 mA/cm2) than the commercial Pt/C catalyst in alkaline medium. However, when KOH and CO2 were used as activation agents, the obtained NDCN materials displayed much lower ORR activities than the meso-/microPoPD, although they also showed high BET surface areas of 1278e1348 m2/g (Fig. 6.14D). This is attributed to the great increase of oxygen-containing groups after KOH and CO2 activation, which will introduce abundant defects on the carbon matrix and further reduce charge transport during eletrocatalysis [4]. The pronounced electrocatalytic activity and long-term durability for ORR are attributed to the unique planar mesoporous shells of the metal-free NDCN. The unique planar porous shells can provide abundant highly electroactive and stable catalytic sites, which facilitates the electrolyte/reactant diffusion during the oxygen reduction process [157]. Template-assisted methods such as alumina [9], silica nanospheres with different diameters [158], SBA-15 [159], nickel substrate [160], Te nanowires [161], etc. have been applied to prepare carbon materials with different morphologies, such as carbon nanotubes, coreeshell carbon nanospheres, ordered mesoporous carbon (OMC), 3-D graphene foams, hollow nanofibers, etc., by pyrolyzing processes. Generally, the pyrolysis processes will be followed by a thermal/chemical activation

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(A)

(B) 3 dV/dlog(D)

V/ cm3 g-1

200

800

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10 20 30 40 Pore diameter / nm

0.2

0.4

-2

Δ E1/2

-3 -4 -5

-2 Δ E1/2 = 9 mV

-3 -4 initial 10000 cycles

-6 0.4

1.0

-1

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0.8

VB12/Silica colloid

Current density (mA/cm2)

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(D) 0 VB12/Silica colloid VB12/SBA-15 VB12/MMT VB12/C Pt/C

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400

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FIGURE 6.13 (A and B) Transmission electron microscopy image and N2 sorption isotherms of the as-prepared CoeNeC catalysts (VB12/silica colloid), (C) oxygen reduction reaction (ORR) polarization plots of different CoeNeC catalysts collected at a rotating speed of 1600 rpm with a scan rate of 10 mV/s, and (D) ORR polarization plots of VB12/silica colloid before and after 10,000 potential cycles in O2-saturated electrolyte. RHE, reversible hydrogen electrode. Reprinted with permission from H.W. Liang, W. Wei, Z.S. Wu, et al., Mesoporous metalenitrogen-doped carbon electrocatalysts for highly efficient oxygen reduction reaction, J. Am. Chem. Soc. 135 (2013) 16002e16005.

treatment (e.g., KOH [21], NH3 [162], CO2 [4]) to obtain oxidized carbon surface [163]. BET results showed that the typical product activated by KOH has abundant nanopores with improved micropore volume and larger BET surface area. However, it was also reported that the ORR activities of the products activated by KOH and CO2 are considerably lower than those activated by NH3 [4,17].

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FIGURE 6.14 (A) Schematic illustration of the synthesis process of metal-free meso-/micro-PoPD material. (B) Transmission electron microscopy image of the meso-/micro-PoPD with a scale bar of 30 nm. (C) N2 adsorption/desorption isotherms of meso-PoPD (black) and meso-/micro-PoPD (red). (D) Oxygen reduction reaction polarization plots of such nitrogen-doped carbon nanosheets catalysts. The mass loading is 0.1 mg/cm2 for every material. PoPD, poly-o-phenylenediamine; RHE, reversible hydrogen electrode. Reprinted with permission from H.W. Liang, X.D. Zhuang, S. Bruller, et al., Hierarchically porous carbons with optimized nitrogen doping as highly active electrocatalysts for oxygen reduction, Nat. Commun. 5 (2014) 4973.

6.6 ELECTROCATALYSTS DERIVED FROM BIOMASS With the increase of crude oil prices and the decline in fossil resources, it is essential to create high-end materials derived from renewable resources, especially biowaste, such as sawdust, rice husk, corn cobs, and grass. As advanced materials, these raw materials have been considered “sleeping gold” [164,165]. One of the common synthesis methods for carbon, such as carbon spheres, OMC is the wet processing of phenol-formaldehyde polycondensation, which frequently includes soluble toxic precursors under multiple synthesis steps, thus limiting the extensive application of these carbon materials. One useful method is the replacement of the toxic organic precursors such as phenol, resorcinol,

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or phloroglucinol, etc. with biomass-derived precursors and the elimination of carcinogenic crosslinkers, e.g., formaldehyde [162]. Recently, carbon materials derived from biomass have received extensive attention because biomass is widely available and recyclable. Biomass is a competent carbon raw material and environmental friendly renewable resource for synthesizing valuable carbon materials due to its availability, high quality, and huge amount. Waste biomass derived from agricultural resides and forest byproducts has drawn little attention as a raw material, since only simple combustion has been used to elevate the value of waste biomass. Carbon materials fabricated from waste biomass with inherent porous structures have exposed promising applications in the fields of sorption materials, environment, catalyst, hydrogen storage, electrochemistry, biochemicals, and others [166]. The problem is that, until now, there is still no general and satisfactory technique to produce valuable carbon materials from crude biomass [165]. The conversion of biomass into carbonaceous materials requires the modification of the chemical structure and surface functionality of carbon to meet the desired applications [17]. Conversion methods generally include hydrothermal carbonization (HTC) process, hard- or soft-templating, solid-state synthesis method, mechanochemical assembly, etc. HTC process is an effective technique for the fabrication of activated carbonaceous materials from carbohydrate-rich biomass. Nitrogen-doped carbonaceous materials can be obtained by using nitrogen-containing biomass precursors through hydrothermal treatment, which offers different possibilities for further treatments and applications [167]. HTC process owns advantages of the usage of renewable resources, low toxicological impact of materials and processes, facile instrumentation and techniques, and a high energy and atom economy [20,21,35]. In the past few years, along with hydrothermal process and carbonization mechanism, lots of novel functional carbonaceous materials derived from biomass have been produced via the HTC process [165,168]. Based on the different experimental conditions and reaction mechanisms, HTC processes can be divided into high- and lowtemperature procedures. HTC process proceeds at a temperature between 300 and 800 C, which is obviously beyond the stability of standard organic compounds. Therefore, carbon materials obtained from high-temperature HTC process usually exhibit higher carbon content with some graphitic structures. The high-temperature HTC process is appropriate to manufacture graphite, carbon nanotubes, and activated carbon materials at considerable high temperature and pressure [169,170]. Lowtemperature HTC process is a more environmentally friendly route that generally performs at up to 250 C, including formation processes of dehydration, condensation, polymerization, and aromatization [171]. This process tends to yield uniform colloidal carbonaceous spheres, which are commonly derived from carbohydrate sources, such as sugar, glucose, sucrose, fructose, cyclodextrins, cellulose, and starch, etc. [165]. Song et al. have synthesized Fe, N-co-doped CNF (FeeN/CNF) aerogels as efficient ORR catalysts via a mild template-directed HTC process, using ferrous gluconate, D(þ)glucosamine hydrochloride as precursors, and Te-NWs as templates. The as-synthesized Fe/N-CNF catalysts exhibited the onset potential of 0.88 V with a half-wave potential at 0.78 V (vs. RHE) in an alkaline medium, which is highly comparable to that of the commercial 20 wt% Pt/C catalyst. Additionally, the Fe/NeCNF catalysts displayed greater long-term stability and better methanoltolerance performance than Pt/C catalyst in both alkaline and acidic electrolytes [172].

6.6.1 ELECTROCATALYSTS WITH GLUCOSE AS PRECURSOR Glucose is a common small molecule serving as an alternative carbon precursor because it can be absorbed onto the external surface and/or penetrate into the voids of the templates, followed by

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polymerization and carbonization into nanoporous carbons with controllable morphologies [2,161]. However, in the absence of any template, the glucose precursor tends to convert into carbon colloid spheres by homogeneous nucleation and growth after polymerization and carbonization [140,161]. The carbon spheres derived from glucose are generally obtained under hydrothermal conditions at 160e180 C, which is higher than the normal glycosidation temperature, resulting in aromatization and carbonization [140,173].

6.6.2 ELECTROCATALYSTS WITH GELATIN AS PRECURSOR Gelatin is a highly economical animal derivative composed of various proteins with a high average molecular weight (c. 50,000 to 80,000) [174]. Gelatin is generally extracted from the boiled bones and connective tissues of animals with an average nitrogen content of 16% [175]. Due to its natural abundance and being a sustainable resource with high solubility in polar solvents, gelatin could be a potential precursor for nitrogen-doped carbon catalysts. It is accepted that the solegel process owns the advantage of accomplishing homogeneous mixing of the components on the atomic scale. By using gelatin as the precursor, uniformly distributed metal particles in the carbon matrix can be achieved through a solegel process owing to the high biocompatibility and coordinating capability (carboxyl and amide groups with metal ions) of gelatin [174]. Based on this, Zhang’s group has synthesized homogeneous metal/nitrogen-containing polymer composite by using gelatin biomolecule and iron nitrate as precursors through a solegel method. The obtained catalyst exhibited higher catalytic activity and better durability for ORR than the commercial Pt/C catalysts, which relates well with the porosity, surface area, and the content of active FeeNx/C (D1 þ D3) species. In detail, most of Fe ions have been converted into FeeN4/C (D1) and NeFeN2þ2/C (D3) active sites which were identified by Mo¨ssbauer spectroscopy [174].

6.6.3 ELECTROCATALYSTS WITH TANNIC ACID AS PRECURSOR Tannic acid (TA, C76H52O46) is a renewable and ubiquitous natural polyphenol with a molecular weight ranging from 500 to 3500 (world production: B220,000 ton per year). The TA molecule contains inherent pyrogallol- and catechol-like sites that can, in principle, function for both hydrogenbonding and coordination interactions [22,126,162]. Hence, TA can be coordinated with different metal ions to form stable TAemetal complexes (e.g., TAeFe3þ) [22,126,176,177]. In addition, as a carbon precursor, TA can be transformed into carbon materials [126,162,178e180]. For example, Wei et al. have fabricated high-performance Fe3C/FeeNeC catalysts by coating a ironetannin source onto cellulose fibers, followed by grinding with DCDA, carbonization, and acid etching [22]. Dai’s group synthesized OMC and NieOMC with uniform and controllable mesopores, large pore volumes and high-surface areas through mechanochemical assembly of TAemetal complexes with triblock co-polymers as surfactants [162]. The coordination polymerization between biomass-derived polyphenols and metal ions provides a versatile substitute over traditional phenol-formaldehyde polycondensation, avoiding the usage of carcinogenic crosslinkers (formaldehyde) and toxic organic precursors such as phenol, resorcinol, or phloroglucinol. Meanwhile, the mechanochemical synthesis runs in solid conditions, which can assure faster crosslinking polymerization in absence of F127 mesophases. The solid state assembly technique not only uses precursors with no or low solubility in solvents but also avoids solvent wastes, such as water, ethanol, or tetrahydrofuran. Besides TA, tannin

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derivatives, such as ellagic acid, gallotannins, quercetin, and lignin with low molecule weight are also promising precursors for porous carbon catalyst production by using the solid state assembly technique. Besides, other transition metal ions with vacant orbital that can coordinate to catechol are potential metal species for the current assembly [162].

6.6.4 ELECTROCATALYSTS WITH OTHER BIOMASS AS PRECURSORS Other biomasses such as waste seaweed biomass [181] and bacterial cellulose [182] have been used for preparing carbon nanomaterials. For example, the Yao and Yang’s group [181,182] reported a simple and scalable synthesis of a 3-D Fe2N-based nanoaerogel catalyst derived from waste seaweed biomass for ORR. Fe3þ cations can coordinate with four a-L-guluronate blocks of sodium alginate (SA) and be immobilized into a novel “egg-box” structure of alginate, forming seaweed hybrid aerogels via a freeze-drying and calcination processes. An excellent ORR performance was observed due to the synergistic effect of 3-D porous aerogel support and core/shell-structured Fe2NeNeAC NPs [181]. Nitrogen-doped nanoporous carbon nanosheets were also synthesized through a simple and low-cost synthesis strategy by employing plant Typha orientalis as the carbon source without adding organic solvent [17].

6.7 ELECTROCATALYSTS SYNTHESIZED BY COMBINED VERSATILE METHODS Versatile methods can be combined to synthesize nitrogen-doped novel carbon materials. For instance, by coupling HTC and the hard- or soft-template method, various carbonaceous nanostructures with controllable special morphology have been synthesized. For example, well-defined ultralong CNFs have been prepared from glucose using the HTC process by employing ultrathin Te-NWs as templates. The diameter of CNFs can be easily controlled by just fine-tuning the reaction time or the ratio of the Te-NWs and glucose [183]. The as-prepared carbonaceous materials typically own inherent porous structures with controllable morphology and surface functionality (numerous reactive oxygencontaining functional groups). Surface modification of these porous carbonaceous materials results in high reactivity, which will extend their application in the environment, as catalysts, and in electrochemistry [165]. Yang et al. reported a new procedure for constructing nitrogen-doped hollow porous carbon derived from the hard template of ZIFemetaleTA coreeshell composites. ZIF-8 crystals are coated with a shell of secondary material containing TA coordination polymer (or a metal-impregnated RF polymer). The authors also studied the evolution of hollow structures. During the decomposition and carbonization processes, KeTA layer was first carbonized at a relatively lower temperature. Then, the ZIF-8 core decomposed to give ZnO/NCeC with the increase of temperature, which is the genesis of capsule formation. As the calcination temperature increases to 700 C, the density is significantly reduced, indicating the complete formation of a hollow capsule core. At this point, the powder X-ray diffraction pattern showed that the ZnO peaks disappear owing to the reduction of ZnO to amorphous Zn metal by carbon. When the temperature rises up to 800 C and then 900 C, the Zn metal vaporizes and escapes from the material, resulting in the formation of nitrogendoped hollow porous carbon (NHPC) [13].

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The transferred electron number (n) per oxygen molecule is a key factor to evaluate the catalysis efficiency of cathode materials, which is dependent on the N/C ratio for biomass-derived nitrogendoped carbon materials (NC). Generally, accompanying the nitrogen doping, nitrogenecarbon groups mainly including pyridinic, pyrrolic, and graphitic nitrogen are formed [184]. It is possible to achieve the four-electron pathway with higher pyridinic nitrogen content. Pyridinic nitrogen with lone electron pairs can weaken the OeO bond via side-on adsorption of oxygen molecules while the graphitic nitrogen can enhance the electron transfer from the carbon electronic bonds to oxygen antibonding orbitals [185]. An nitrogen-doped carbon with fullerene-like carbon shell derived from chitin was obtained through pyrolyzing fallen ginkgo leaves at 900 C. The chitin-derived NC exhibited better ORR activity than the commercial Pt/C catalyst via an effective four-electron pathway [186].

6.8 CONCLUSIONS AND OUTLOOK This chapter summarizes the recent developments of advanced novel carbon nanomaterials derived from different precursors as effective catalysts for FCs. Low-cost nonprecious metal carbon nanomaterials have especially attracted increasing attention in recent years for FC technology. Currently, the structures and properties of these novel carbon-based catalysts can be elaborately tuned through optimizing synthetic pathways and synthesis conditions, including precursors, templates, reaction temperature, and other parameters which are critical factors strongly associated with the catalytic performance. Moreover, future work should be devoted to the better understanding of the involved reaction mechanisms and factors that determine the exact structure of active sites of these excellent carbon catalysts. Due to the difficulty of understanding catalysis mechanisms by experimental studies on metal-free catalysts, more theoretical calculation analysis should be applied to the study of the nature of active sites responsible for high activities of heteroatom-doped carbon and metaleNeC catalysts. Hopefully, metal-free catalysts have the potential to surpass the catalytic performance of the state-of-the-art Pt catalyst.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 21575134, 21633008, 21773224, 21275136) and National Key Research and Development Plan (2016YFA0203200).

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