Functionalized carbon nanomaterials for advanced anode catalysts of fuel cells

C H A P T E R

7 Functionalized carbon nanomaterials for advanced anode catalysts of fuel cells Youjun Fan, Bo Yang and Chuyan Rong Guangxi Key Laboratory of Low Carbon Energy Materials, College of Chemistry and Pharmaceutical Sciences, Guangxi Normal University, Guilin, P.R. China

7.1 Introduction In order to meet the future demand for energy supply, it is very important to achieve efficient electrochemical energy conversion. Among them, fuel cells have received extensive attention from scientists because they are both environmental friendly and highly efficient energy devices. A fuel cell is a power generation device that directly converts the chemical energy of a fuel and an oxidant into the electrical energy through an electrochemical reaction [1]. As a promising development direction, low-temperature fuel cells that have been studied include several established types such as direct methanol fuel cells (DMFCs), direct ethanol fuel cells (DEFCs), and direct formic acid fuel cells (DFAFCs) [2
5]. DMFCs working with methanol as organic liquid fuel have received significant attention due to their advantages of high energy density, low operation temperature, low pollution emission (DMFCs do not produce unburned hydrocarbons, nitrogen oxides, sulfur dioxide, and particulates that are formed as a result of the combustion of fossil fuels during operation), easy storage and transportation of fuels, and high conversion efficiency [6
11]. Among the types of low-temperature fuel cells, DEFCs have emerged as a way to save fossil fuels consumption and reduce environmental pollution, which can convert chemical energy in ethanol into electrical energy [2]. Ethanol as a liquid and renewable biofuel has the following advantages over methanol. First, it has a higher theoretical energy density, low toxicity, availability, low membrane permeability, and biocompatibility. Second, ethanol is a renewable source that can be produced in large scale from the fermentation of biomass, agricultural products, and greenhouse gases such as carbon dioxide

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[12]. Third, DEFCs operating under alkaline conditions can greatly improve the oxidation kinetics of ethanol and reduce the corrosion risk of the electrocatalytic materials to obtain high durability. Simultaneously, DMFCs and DEFCs have many similarities as alcohol fuel cells. Methanol and Ethanol are ideal combustible materials for fuel cells; their storage and transport are much easier than those of hydrogen [13]. As a new generation of power sources with characteristics of high energy conversion efficiency, low pollutant emission, low toxicity, low membrane permeability, and low operating temperature, alcohol fuel cell systems show great potential in portable devices [14
16]. Moreover, DFAFCs also have attracted considerable interest in recent years as a promising system that can convert chemical energy into electrical energy and heat without a combustion process [17]. Compared with the DMFCs, DFAFCs offer a broader range of advantages, including higher theoretical open-circuit potential, lower crossover of the formic acid fuel through the polymer membrane, nonflammability of formic acid, safe storage and transportation, fast electrooxidation kinetics, and reduced toxicity [18]. Although the energy density of formic acid (2086 W h L21) is lower than that of methanol (4690 W h L21), it can be compensated by using a higher concentration of formic acid because the fuel crossover point is lower [19]. Compared with traditional energy conversion devices, they have been developed as clean and efficient energy sources for backup power systems, automobiles, and mobile applications because of the above-mentioned exceptional performance [20]. In the fuel cell system, the most important part in determining the performance is the membrane electrode assembly (MEA). The MEA consists of three major compartments: the electrolyte, the support for the electrodes, and the electrode catalyst [21,22]. The catalyst in the fuel cell system is a site which provides a surface for the occurrence of fuel electrooxidation at lower activation energy and at a higher rate. The electrochemical properties (e.g., activity, reliability, and durability) of the catalyst mainly depend on the catalyst metal and the support materials. However, catalysts and support materials often suffer from the following problems. First, the electrocatalyst surface is usually heavily poisoned by strong adsorption of carbon monoxide (CO)-like intermediates produced during the fuel cell operation, thus lowering the activity and durability. Then, the catalytic nanoparticles are easily aggregated because of the drive of surface-energy minimization, leading to the lowelectroactivity surface area and durability [23]. Finally, the polarization of the catalyst and the slow reaction kinetics of the anode electrocatalyst are important causes of low cell performance. One of the most realistic approaches toward the improvement of fuel cell performance is the development of advanced anode electrocatalysts that can combine higher electrooxidation activity and enhanced tolerance to CO poisoning. In recent years, much research has been devoted to exploiting various anode electrocatalytic materials for fuel cell applications, for example, nanostructured Pt and Pd, as well as their alloys with other transition metal elements [19,24,25]. On the other hand, the influence of support materials on the valence state, dispersion, and size distribution of catalytic nanoparticles that further affect their catalytic properties and efficiency cannot be overlooked. On the appropriate supports, the catalytic nanoparticles can interact with the supports through the electron transfer, thereby enhance the performance of the catalyst [26,27]. In generally, supports are required to have good dispersibility without aggregation, and to have a high specific surface area that can uniformly adsorb and grow the catalyst. At present, the most common supports for the electrocatalysts are various nanostructured carbon materials with high

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surface area, such as carbon nanotubes (CNTs) [28,29], mesoporous carbon [3,30], and graphene [31,32]. These carbon nanomaterials are widely used as the supports of electrocatalysts due to their cost competitiveness, high electrical conductivity, excellent durability, and thermal stability [33]. However, these carbon nanomaterials are vulnerable to corrosion/oxidation at high potentials ( . 0.7 V vs NHE) under the normal operating condition, resulting in poor durability of electrocatalysts because of the dissolution and aggregation. In addition, the low polarity and high hydrophobicity features of carbon materials have been identified as major drawbacks to limit rate performance of carbon-supported catalysts. Due to their surface inertness, there are few binding sites for anchoring precursor metal ions or metal nanoparticles on the pristine carbon materials, which inevitably results in poor dispersion and aggregation of catalytic nanoparticles. There is no doubt that the surface functionalization of carbon supports plays an important role in the preparation of catalysts for fuel cell applications [34]. To date, many efforts have been made to achieve the appropriate surface functionalization of carbon supports and then explore some advanced anode catalysts that can effectively enhance the electrooxidation kinetics of small organic molecules and reduce the cost [35
37]. In this chapter the development of functionalized carbon nanomaterials for advanced anode catalysts of fuel cells in recent years is reviewed, mainly focusing on the covalent functionalization, the noncovalent functionalization, and the doping functionalization of carbon nanomaterials.

7.2 Covalent functionalization of carbon nanomaterials In the covalent functionalization, the functional units form covalent bonds with the skeleton of carbon materials, which significantly changes the electrocatalytic performance of carbon materials. The most commonly used covalent functionalization pathway is the generation of binding sites (e.g., hydroxyl, carbonyl, and carboxyl groups) through various aggressive oxidation treatments of carbon materials in acidic media [6,25,30,34,38,39]. These oxygen-containing functional groups not only stabilize the dispersion of carbon materials in polar solvents but also provide active sites for the adsorption of metal precursors. The oxygen-containing functional groups and metal precursors can then be further reduced by the addition of some reducing agents (such as ethylene glycol, NaBH4, H2, formic acid, etc.) [40]. For example, according to Eris and coworkers’ work [30], the carbon black support was functionalized with an acidification process, which effectively increases the active sites for anchoring platinum salts or platinum nanoparticles. Two different metal nanoparticles will produce a synergistic effect and have a larger specific surface area, the bimetallic nanoparticles deposited on a covalently functionalized carbon support can further enhance the electrochemical performance of catalyst [41,42]. Yola et al. [43] reported the synthesis of several bimetallic nanoparticles supported on 2aminoethanethiol-functionalized graphene oxide (AETGO) and their catalytic performance against methanol oxidation. In comparison with other counterpart catalysts the prepared rAu-Pt NPs/AETGO catalyst has a large active surface area of 104.7 m2 g21, high electrooxidation activity, and superior CO tolerance. Guo et al. [44] synthesized a CO-tolerant PtRu catalyst using thiol-functionalized carbon nanotube (SH-CNT) as the support, which demonstrated high CO tolerance ability and enhanced methanol oxidation performance.

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Due to the presence of strong interaction between Pt and SH-CNTs, the dispersibility and durability of the catalyst are significantly improved. Yan et al. [45] synthesized a sodium carboxylate functionalized graphene (CS-G) with uniform and ordered sodium carboxylate groups by using a carbon radical reaction process. The bimetallic Pt-Pd nanoparticles can be efficiently bound to these functional groups through in situ electrochemical reduction. It is concluded that the combination of a carbon radical reaction and an electrochemical reduction method is an effective strategy for improving the catalytic activity. The resultant Pt-Pd/CS-G/GCE shows much higher synergistic catalytic activity and more long-term stability toward the electrooxidation of methanol than the Pt-Pd/G/GCE and Pt/CS-G/ GCE electrodes. Eguizabal et al. [46] deposited ultrafine Pt nanoparticles onto the surface of Vulcan XC72 carbon black functionalized with ethylenediamine for methanol oxidation. The presence of functional groups on Vulcan XC72 enhances the dispersion and stability of Pt nanoparticles thanks to the interactions between the Pt nanoparticles and the electron-richnitrogen in the functionalized Vulcan XC72. As seen from the electrochemical results, the prepared electrocatalyst exhibits significantly higher electrochemical active surface area (ECSA) and methanol oxidation activity than the commercial Pt/C. In another case, Huang et al. [47] functionalized single-walled carbon nanotubes (SWCNTs) through a radical reaction, and then deposited Pt NPs on the surface of SWCNTs (Pt/SWCNTs) through a microwave-assisted polyol method. In this method, the azodiisobutyronitrile is also utilized as a functional reagent of SWCNTs, and then a carboxyl functionalized SWCNT is obtained by a hydrolysis process. Fig. 7.1A and B shows the transmission electron microscopy (TEM) images of Pt/SWCNTs at different magnifications. It can be observed that large amounts of Pt NPs are uniformly anchored onto the external walls of functionalized SWCNTs with no aggregation. The high-resolution transmission electron microscopy (HRTEM) image shows that the Pt lattice fringes parallel to the outer surface of the nanocrystals (NCs) have an interfringe distance of 0.25 nm (Fig. 7.1C), and the selected area electron diffraction (SAED) result indicates that the Pt NPs possess a face-centered cubic (fcc) phase (Fig. 7.1D). The electrochemical measurements demonstrate that the asprepared Pt/SWCNTs catalyst has a better electroecatalytic activity, stability, and antipoisoning ability for the oxidation of methanol and ethanol than the commercial Pt/C. Capelo et al. [48] used 4-aminobenzenesulphonic acid-functionalized Vulcan XC-72R as the catalyst support to improve the dispersion of Pt nanoparticles and decrease the resistance of three-phase boundaries for proton exchange membrane (PEM) fuel cell applications. The strategy adopted for a more stable and durable catalyst layer consisted, first, in the introduction of oxygenated groups on the carbon support surface by chemical oxidation and, second, their reaction with the less studied aromatic nitrogen and sulfonic groups. It has been confirmed that the introduction of these groups produces more binding sites, reducing the dissolution and aggregation of the catalyst particles in PEM fuel cell relevant environments. Kim et al. [49] designed the functionalization of the graphitized amorphous carbon blacks by in situ grafting of nitrogen-containing functional groups through a diazotization reaction to provide more sites for the attachment of Pt nanoparticles. This method also has the potential to improve the dispersion of Pt nanoparticles on the graphitized carbon surface without degrading their intrinsic properties. Electrochemical analysis demonstrates that the Pt/functionalized carbon shows improved

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FIGURE 7.1 TEM (A, B), HRTEM (C), and SAED (D) images of the Pt/SWCNTs [47].

electrochemical durability than the Pt/graphitized carbon and commercial Pt/C catalysts. This study shows that this simple functionalization of graphitized carbons by the in situ grafting of functional groups onto their surfaces is an effective way to fabricate highly durable catalysts for PEMFCs. Due to the transformation of carbon atoms from planar sp2 hybrid geometry to distorted sp3 hybrid geometry, the covalent functionalization process will inevitably interrupt the inherent structure of carbon materials [50]. Although most functional groups leave a large number of skeleton defects and less defective carbon fragments, it is detrimental to the electrical, mechanical, and optical properties of the material. Therefore, it is important to maintain the structure of carbon materials as much as possible during the design and functionalization. Li et al. [51] reported the synthesis of ionic liquids (ILs)-functionalized CNTs by the 1,3-dipolar cycloaddition reaction. It is found that the obtained bond is sufficiently strong and the structure of the CNTs is also maintained, which facilitates the electrooxidation of ethanol. Raman tests and cyclic voltammetry (CV) studies in K3Fe(CN)6 solution revealed that the IL functionalization effectively promotes the electron transfer ability of the CNTs. On the other hand, functional groups generated during the functionalization process can act as nucleation centers or anchor sites to limit the particle growth, improve the dispersion of metal crystallites, and increase the stability of supported catalytic nanoparticles.

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FIGURE 7.2 TEM images of (A) DMDAAC-GO, (B) Pd/DMDAAC-RGO, (C) Pd/ G, and (D) HRTEM image of Pd/DMDAAC-RGO [53].

Wietecha et al. [52] used N-(trimethoxy-silylpropyl) ethylenediamine triacetic acid (EDTAsilane) modified reduced graphene oxide (EDTA-RGO) as the support and prepared a novel Pt NPs catalyst. They demonstrated that the chelating groups can enhance the catalytic activities and stability of Pt-NPs toward the oxidation of methanol. Zhang et al. [53] fabricated dimethyldiallylammonium chloride (DMDAAC) modified reduced graphene oxide supported Pd nanoparticles (Pd/DMDAAC-RGO) by a polyol microwave heating method. The positively charged groups in DMDAAC can not only interact with the negatively charged groups on the surface of graphene oxide (GO) nanosheets to modify GO but also bind to metal catalysts to enhance the adsorption capacity. After the DMDAAC functionalization, the intrinsic wrinkles of graphene nanosheets (GNS) can be still observed in the TEM image (Fig. 7.2A). As seen from Fig. 7.2A and C, a large number of Pd nanoparticles occupy the surface of the DMDAAC-RGO wafers, and the density distribution is uniform, indicating the strong interaction between Pd nanoparticles and functional RGO nanosheets. HRTEM image (Fig. 7.2D) shown clearly visible crystal lattice fringes reveal the highly crystalline features of Pd NPs. The d-spacings of adjacent fringe for Pd NCs is 0.224 nm that can be indexed to as the (111) crystalline plane of facecentered cubic Pd lattice. Electrochemical tests demonstrate that the Pd/DMDAAC-RGO has better electrocatalytic activity and stability for ethanol oxidation in alkaline media than the conventional carbon-based Pd catalysts. The introduction of nitrogen-containing functional groups on the surface of carbon support can provide active sites for the immobilization of metal nanoparticles and enhance the metal-carbon support interaction. Several studies have demonstrated that carbon nanomaterials modified by nitrogen-containing compounds can significantly improve the

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electrocatalytic efficiency of supported metal nanoparticles. Zhong et al. [54] employed ethylenediamine (ED) functionalized graphene (ED-graphene) as the support material for Pt nanoparticles. In the ED-graphene, the -NH2 groups of ED were grafted to the surface of graphene, which act as the strong adsorption sites for [PtCl6]22. It was found that the composition of the sp3 hybrid orbital in ED-graphene is larger than the component of graphene. The modification of graphene by ED leads to the destruction of the hexagonal lattice structure of graphene, which increases the defects and facilitates the deposition and dispersion of Pt nanoparticles. The activity of the catalysts for formic acid oxidation was measured in a 1.0 M HCOOH 1 0.5 M H2SO4 aqueous solution. The results show that the Pt/ED-graphene catalyst exhibits excellent electrocatalytic performance and long-term stability toward formic acid electrooxidation. Liu et al. [55] fabricated 1H-benzotriazole (BTA) functionalized carbon black (BTA-C) for the immobilization of Pd nanoparticles. The BTA is a bifunctional molecule with a phenyl group and an amino functional group, which could strongly interact with the carbon material. Because of the good effect of BTA functionalization, the as-prepared Pd/BTA-C catalyst has a larger electrochemically active surface area (ECSA) contrasted to the Pd/C. Meanwhile, the electrochemical results in 0.1 M KOH solution indicate that the Pd/BTA-C exhibits higher activity (more than 1.8 times), lower onset potential (negative 90 mV), and better stability than that of the Pd/C counterpart for the ethanol oxidation reaction.

7.3 Noncovalent functionalization of carbon nanomaterials Among various functionalization methods, noncovalent functionalization is more favorable than the covalent one, as the attachment of functionalized molecules would occur by supermolecular interactions such as π-π stacking that preserves the electronic features of carbon nanomaterials and avoids the destruction of their electronic characteristics [56]. Moreover, noncovalent functionalization is also effective to incorporate a multicomponent electrocatalyst system with significantly enhanced synergistic effects [57]. H-bonding and π-π stacking play an important role in noncovalent functionalization and enhance the solubility and assembly without effecting π-π conjugation of the skeleton of carbon nanomaterials [58]. In this context, many molecules including biomacromolecules, aromatic macrocycle/polycycle molecules, polymers, and polyelectrolytes have been widely studied and used as the noncovalent functionalization agents of carbon nanomaterials.

7.3.1 Polymer/polyelectrolyte molecules Polymers/polyelectrolytes have been shown to serve as excellent materials for the noncovalent functionalization of carbon nanomaterials as a result of π-π stacking, electrostatic interaction, and van der Waals interaction between the conjugated polymers/polyelectrolytes and the surface of carbon nanomaterials [59]. The hybrids of conjugated polymers/ polyelectrolytes and carbon nanomaterials can exhibit synergistic properties such as good stability, high mechanical strength, and high electrical conductivity. These are all favorable for the development of fuel cell anode catalysts.

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Poly(diallyldimethylammonium chloride) (PDDA), a positively charged polyelectrolyte, has been found to be attractive for the noncovalent functionalization of graphene sheets. PDDA not only could prevent the aggregation of graphene sheets but also improve their solubility [60]. Furthermore, the PDDA functionalized graphene sheets are positively charged, which are suitable to assemble the multilayer composite films by electrostatic adsorption [31]. Therefore, PDDA has emerged as the most widely used polyelectrolyte that shows outstanding trapping capacity for the negatively charged metal precursor. For example, Li et al. [31] prepared a multilayer film using the electrostatic interaction between the positively charged PDDA-functionalized graphene sheets (PDDA-GS) and the negatively charged poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) through layer-by-layer (LBL) self-assembly techniques. And then, the Pt/[(PDDA-GS/ PEDOT:PSS)n] catalyst was obtained by electrodepositing Pt nanoparticles on the multilayer films. The electrochemical results indicated that the Pt/[(PDDA-GS/PEDOT:PSS)n] showed high electrocatalytic activity toward methanol oxidation and good tolerance toward CO poisoning. In general, the above LBL process allows precise control over the deposition of each component from solution and hence is very useful for the fabrication of hybrid films [61
63]. Analogously, Huang et al. [64] also used an LBL self-assembly technique to form a multilayer film via electrostatic interactions between PDDA functionalized graphene (PDDA-GN) and poly (sodium 4-styrenesulfonate) functionalized graphene (PSS-GN). The functionalized GNs are positively or negatively charged, respectively, which are suitable to assemble multilayer composite films by electrostatic LBL absorption. The electrochemical investigation indicates that the presence of functionalized graphene in the Pt/[PDDA-GN/PSS-GN]n modified electrodes greatly enhances the catalytic activity and stability toward methanol oxidation. One advantage of PDDA functionalized carbon nanomaterials is that the Pt precursor ions can be randomly assembled onto the surface of functionalized materials without employing a surfactant because of the electrostatic interaction between the positively charged PDDA and negatively charged [PtCl6]22. Zhang et al. [65] prepared the PDDAfunctionalized GO through adding PDDA drop by drop into the graphite oxide dispersion under vigorous stirring. With ethylene glycol as the reduction agent, the Pt ions could be microwave reduced and as-obtained Pt nanoparticles were evenly distributed on the PDDA-functionalized graphene. The presence of PDDA prevents the aggregation of GNS, enhances Pt loadings, and increases the graphene conductivity. The CV and chronoamperometry (CA) studies show that the Pt/PDDA-GN catalyst has better electrocatalytic activity and stability than Pt/graphene and Pt/carbon blacks for methanol oxidation. Following the previous work, Zhang et al. [66] synthesized PDDA functionalized graphene supported Co@Pt core-shell bimetallic catalyst using a two-step procedure involving the microwave and replacement methods. The size and morphology of Co@Pt nanoparticles deposited on graphene (G) and PDDA-graphene (PDDA-G) were studied by TEM (Fig. 7.3). As shown in Fig. 7.3A, a large number of Co@Pt nanoparticles occupy majority of the PDDA-G surface with fairly even, densely distribution, indicating a strong interaction between the Co@Pt nanoparticles and the PDDA-G. In contrast, for the Co@Pt/ G catalyst (Fig. 7.3B), low loading amount of Co@Pt nanoparticles are deposited on the graphene surface with a significant amount of agglomeration. The results illustrate that the presence of PDDA on the graphene sheets could help the formation of small,

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FIGURE 7.3 TEM images of Co@Pt/PDDA-G (A) and Co@Pt/G (B). HRTEM image (C) and EDS spectrum (D) of Co@Pt/PDDA-G [66].

highly concentrated, and uniformly dispersed Co@Pt nanoparticles. Clearly, cobalt nanoparticles interacting strongly with the PDDA-G surface play a key role in keeping a similar and highly dispersed Co@Pt particle size on the supports. The HRTEM image indicates the single-crystalline nature of Co@Pt nanoparticles (Fig. 7.3C), the spacings of adjacent fringe with an interlayer distance of 0.19 nm in the core are indexed to Co(111) crystal planes, and the outer layer with the lattice space of 0.224 nm corresponds to Pt(111) crystal planes. The EDS spectrum (Fig. 7.3D) shows the corresponding peaks of C, Co, Pt, and Cu (the Cu signal comes from the sample holder), confirming the deposition of Co@Pt nanoparticles on the graphene surface. The electrochemical surface area of the Co@Pt/PDDA-G is evaluated to be 105.6 m2 g21Pt, which is 2.8 times that of the commercial Pt/C catalyst (37.8 m2 g21Pt). The current density from the CA tests reaches a constant at 23 mA mg21 for the Co@Pt/PDDA-G catalyst, which is roughly 3.3-fold higher than that of commercial Pt/C catalyst. The results show that the electrocatalytic activity and stability of Co@Pt/ PDDA-G for methanol oxidation are highly better than the widely used Pt supported on PDDA-G sheets, also better than the Co@Pt on unfunctional graphene with the same Pt content on the electrode. Bin et al. [67] deposited Pd NPs on the PDDA-functionalized graphene support through the electrostatic interactions between PdCl22 4 ions and positively charged PDDA-functionalized graphene, and the ethylene glycol reduction method. The synthesized PDDA-functionalized graphene/Pd catalyst exhibited much higher electrocatalytic activity and superior stability for the oxidation of methanol and ethanol in alkaline solution than the Pd NPs/graphene and commercial Pd/C catalysts. Le et al. [18] prepared Pd/PDDA functionalized graphene (PDDA-GN)/transition-metal-substituted polyoxometalate H7PMo11CoO40 xH2O (PMo11Co) by the LBL electrostatic assembly method



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combined with in situ electrodeposition of Pd particles. The electrochemical tests illustrate that the Pd/PDDA-GN/PMo11Co catalyst exhibits enhanced electrocatalytic activity and stability for formic acid oxidation compared to the Pd supported on carbon (Pd/C). Compared to the traditional acid-treated CNTs, noncovalent functionalization through polyelectrolytes or solvents is much more effective for uniformly assembling PtRu NPs on CNTs and enhancing the thermal and structural stability of CNTs. Cheng et al. [7] prepared several PtRu NPs catalysts using PDDA, polyethyleneimine (PEI), 1-aminopyrene (1-AP or AP), and terahydrofuran (THF) functionalized CNTs as the supports and studied the effect of nitrogen-containing groups on the electrocatalytic performance. A comparative study found that nitrogen-containing groups of the functionalizing agents play a critical role in the electrocatalytic activity enhancement of PtRu NPs supported on CNTs. PtRu NPs supported on PEI-CNTs and AP-CNTs and in less extent on PDDA-CNTs exhibited much higher electrocatalytic activity and stability toward methanol oxidation as compared to those on THF-CNTs and AO-CNTs. The results show that the electrocatalytic activity of catalysts strongly depends on the composition and structure of the functionalizing agents and the functionalizing agent having an ammonium or amino group is particularly effective for improving the electrocatalytic activity of PtRu NPs toward the methanol oxidation reaction (MOR). In recent years, polydopamine (PDA)-functionalized carbon materials have been used to increase the electrocatalytic activity and stability of the catalyst, mainly because the PDA has several significant advantages as follows [68]. First, it has multifunctional groups (amino and catechol groups) that are beneficial for the anchoring and dispersion of metal nanoparticles. Then, it is hydrophilic and biocompatible and can accelerate the electron transfer between the electrode and the target. Interestingly, the PDA coating does not ruin the structure of carbon materials, and the thickness of PDA films can be finely controlled on the nanometer scale by adjusting the reaction time, pH, and temperature. Above all, metal ions can be chemical reduced in situ on the surface of PDA-functionalized carbon materials, showing its simplicity, environmental friendliness, and high stability. In fact the PDA is a promising alternative to functionalize carbon materials, combining advantages of both covalent and noncovalent functionalization. Liu et al. [69] used PDA-coated carbon materials including multiwalled CNTs (MWCNTs), carbon nanospheres (CNs), and Vulcan XC-72 carbon black (CB) as the supports to fabricate three Pt-based electrocatalysts of MWNTs/PDA-Pt, CNs/PDA-Pt, and CB/PDA-Pt. The uniform Pt NPs supported on the PDA modified carbon materials (MWCNTs, CNs, and CB) could be readily produced through the chemical reduction of H2PtCl6 by PDA, which was coated on the surface of carbon materials via the self-polymerization of dopamine. The apparent reductive capacity of the PDA sublayer is sufficient to eliminate the need for the addition of an exogenous reducing agent in this procedure. This approach offers an environment-friendly route to the controlled synthesis of Pt-based nanocomposites. The obtained nanocomposites exhibit high activity in the catalytic oxidation of methanol as a model reaction. Ren et al. [70] reported PDA reduced and modified GO composite that can form a stable suspension for several months and thus providing a good platform for anchoring metal NPs. And it is found that the PDA plays an important role in enhancing the dispersion and stability of the catalyst. Examined by CA experiments and electrochemical impedance spectroscopic (EIS), it demonstrates that the Pt/PDA-RGO catalyst has much better electrocatalytic

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FIGURE 7.4 TEM images of acid-purified MWCNTs (A1) and CNTs@PDA (B1), in which the white line indicated the approximate thickness of the PDA layer. TEM images of PtRu/CNTs (A2 and A3) and PtRu/ CNTs@PDA (B2 and B3). The PtRu loading was c. 20 wt.% assessed by ICP-AES [75].

activity and stability toward methanol oxidation. It provides us a facile and environmentally friendly method for the preparation of metal NPs-RGO nanocomposites in large quantities for applications. The amount of Pt is an important factor for limiting the development of fuel cell catalysts. And the direct way to reduce Pt loading is to form alloys with other transition metal elements or to use nonprecious metal oxides [71
74]. Chen et al. [75] reported a one-pot synthesis method for the preparation of high-performance PtRu electrocatalysts supported on the PDA-functionalized MWCNTs (CNT@PDA). Among their work, PtRu alloy catalyst is used as a DMFCs anode catalyst since it shows high catalytic activity for the MOR as well as strong tolerance against CO poisoning, and in particular; the Ru in the PtRu alloy can easily oxidize the CO intermediate to CO2 by the bifunctional mechanism. The prepared CNT@PDA can be stably suspended for several weeks without the addition of a stabilizer, and the polymerization of CNTs is effectively inhibited, thus providing an ideal anchor point for PtRu NPs attachment. The PDA modification could profoundly enhance the dispersity and stability of MWCNTs in water. The typical TEM images of as-prepared materials are shown in Fig. 7.4. In comparison with the acid-purified MWCNTs (Fig. 7.4A1), the PDA coating features can be clearly observed as a thin amorphous-like layer with a thickness of ca. 2
7 nm around the CNT surface (Fig. 7.4B1). In addition, compared to the poor dispersion of PtRu NPs on the MWCNTs (Fig. 7.4A2 and A3), PtRu NPs are homogeneously decorated on the outer surface of CNTs@PDA (Fig. 7.4B2 and B3). Yang et al. [76] fabricated palladium-lead bimetallic alloy nanoparticles anchored onto PDA functionalized MWCNTs (PDA-MWCNTs) by a facile one-step strategy that has high-performance electrocatalysts. The advantage of this catalyst

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is its improved electrochemical activity and enhanced electrochemical stability toward DEFCs due to the synergy between PDA and Pb, as explained by the dispersive effect, bifunctional mechanism and d-band theory. Ye et al. [77] developed a facile and green approach to synthesize flowerlike Pt NCs on PDA functionalized reduced graphene oxide (Pt(F)-PDA/RGO). In this process, the PDA/RGO composites were obtained via the reduction of GO nanosheets by dopamine, followed by simultaneous capping by PDA. Generally, nanoflowers contribute to providing favorable surface areas and active centers because of their porous nanostructures. The kinetic characterization of Pt(F)-PDA/RGO was further discussed by cyclic voltammetry. It is found that the as-prepared Pt(F)-PDA/RGO catalyst shows considerably improved catalytic activity and stability toward methanol electrooxidation, compared with Pt nanoclusters on PDA/RGO (Pt(C)-PDA/RGO) and Pt nanoparticles on pristine graphene sheets (Pt/RGO). Yang et al. [4] fabricated PDA functionalized CNTs-ceria-palladium nanohybrids (Pd-CeO2
x/PDA-CNTs) through LBL synthetic strategy, aiming to integrate the functions of support (PDA-CNTs), metal oxides (CeO2
x), and Pd nanoparticles (NPs). PDA can layered-functionalize the surface of CNTs through intensive covalent and noncovalent binding. For this reason, not only the PDA network structure but also the phenolic hydroxyl group and nitrogen group exist on the surface of the functionalized CNTs that provide an anchoring point for the immobilization of nanoparticles [78]. Hence the CeO2
x and Pd NPs can anchor on the surface of PDA-CNTs by a surfactant-free one-step LBL synthetic approach, respectively. The results show that the sphere-like shape Pd NPs and CeO2
x NPs ranging from 2 to 7 nm are uniformly anchored on the surface of PDA-CNTs. These show that the method for synthesizing LBL is effective for the anchoring of nanoparticles on the supports. As an anode catalyst toward ethanol electrooxidation reaction in DEFCs, the Pd-CeO2
x/PDA-CNTs reveal promoted electrocatalytic performance, accelerated kinetics, and enhanced stability. Poly(ethylenimine) (PEI) is an amino-rich highly hydrophilic cationic polyelectrolyte that has also been used as a functionalizing agent for carbon materials. Cheng et al. [79] prepared highly efficient and CO tolerant PtRu electrocatalysts supported on PEI functionalized MWCNTs (PtRu/PEI-MWCNTs). It has been demonstrated that the PEI functionalized on the surface of MWCNTs via both physisorption and electrostatic adsorption, while the positively charged ammonium cationic groups of PEI provided abundant active sites for the deposition of PtRu nanoparticles. Electrochemical performance studies indicate that the PtRu/PEI-CNTs catalyst exhibits very high catalytic activity and excellent tolerance toward CO poisoning for the MOR in acid solution. Another representative work reported by Kim’s group was the preparation of 3-D structured Pt/xrGO-yPEIfunctionalized MWCNTs (PMWCNT) by the hybridization of rGO and PMWCNT [33]. When the mass ratio of rGO and PMWCNT is different, it has a great influence on the deposition of Pt. The rationality of adding rGO and PMWCNT can be illustrated by the TEM results (Fig. 7.5). In case of Pt/rGO (Fig. 7.5A), relatively high content of Pt nanoparticles is deposited onto rGO with some agglomeration. However, although the surface of MWCNT generally exhibits the chemical inertness and negative charge, the plenty of Pt nanoparticles are preferentially deposited onto the MWCNT surface through the functionalization of cationic PEI (Fig. 7.5B). Interestingly, upon the hybridization of PMWCNT

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FIGURE 7.5 TEM images of hybridized Pt/rGO-PMWCNT with the different mass ratio of rGO and PMWCNT [33].

with rGO, the deposition and dispersion of Pt nanoparticles onto the PMWCNT surface are different from Pt/rGO and Pt/PMWCNT. This phenomenon may be related to the high affinity of rGO toward Pt nanoparticles. The addition of rGO should have weakened the preferential deposit of Pt nanoparticles onto PMWCNT but led to the broad dispersion of Pt nanoparticles throughout the rGO and PMWCNT. In case of high content of rGO in hybridized Pt/rGO-PMWCNT (Fig. 7.5C), the high affinity of rGO toward Pt nanoparticles can lead to the heterogeneous dispersion of Pt nanoparticles. On the other hand, Pt/1rGO1PMWCNT exhibits a uniform dispersion of Pt nanoparticles (Fig. 7.5D). Therefore, a suitable mass ratio of rGO and PMWCNT can contribute to the uniform dispersion of Pt nanoparticles. The ECSA and durability based on ECSA are also affected by the mass ratio of rGO and PMWCNT, exhibiting the highest ECSA of 32.5 m2 g21 and the least reduction of ECSA after 1200 cycles for the Pt/1rGO-1PMWCNT. Polyaniline (PANI) has been reported as support materials for its promising stability, reversible acid doping/de-doping feature along with charge tunneling properties. Abhishek De et al. [2] prepared CNT-PANI combined structure by the polymerization of aniline on the functionalized CNTs by a chemical method at low temperature. The results show that PANI is successfully coated on the CNTs, and is also relatively uniform when the Pt nanoparticles are deposited, indicating that the functionalized material has good properties. In the electrochemical activity and stability tests, the catalyst also showed excellent catalytic performance toward the electrocatalysis of ethanol in a fuel cell. As one of the polyaniline derivatives, poly(o-methoxyaniline) (POMA) has been extensively investigated due to its good solubility, good electroactivity, excellent electrochromism, and

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thermal stability. Ren et al. [71] fabricated a novel POMA-modified graphene hybrid material (POMA/GE) as efficient catalyst support for PtNi NPs on a glassy carbon (GC) electrode. Electrochemical techniques such as cyclic voltammetry, chronoamperometry, chronopotentiometry, and impedance spectroscopy have been employed to investigate the electrocatalytic activities of the catalyst for methanol oxidation. It is found that the PtNi/ POMA/GE/GC electrode exhibits excellent electrocatalytic activity toward methanol oxidation as compared to the PtNi/GE/GC, PtNi/POMA/GC and PtNi/GC electrodes, showing that POMA/GE is a promising catalyst support material for use in methanol fuel cells. The enhanced performance is proposed to originate from the good dispersion of PtNi NPs on the POMA/GE film with a quasi-three-dimensional (3D) porous structure leading to the increase of the ECSA as well as the synergic effect among the POMA, GE, and PtNi NPs. Mayavan et al. [27] reported the synthesis of PSS-functionalized graphene sheets (PSSG) supported Pt NPs. The experimental results show that the Pt nanoparticles can be uniformly dispersed on the PSS-G support, which should be due to the existence of PSS. The use of PSS as a stabilizer prevents stacking of reduced graphene sheets, binds Pt NPs, and promotes mass transport of reaction species. In addition, the presence of hydrophilic SO2 3 ionic end groups in PSS can facilitate proton transport during methanol oxidation. Analysis in XPS of chemical reduction of Pt on PSS-G revealed a significant change that the XPS S 2p peak shifted to lower binding energy in binding energy (0.7 eV), indicating an effective charge transfer interaction between Pt NPs and sulfurcontaining PSS-G. Luo et al. [80] synthesized a high-performance Pt catalyst using perfluorosulfonic acid (Nafion) functionalized carbon black as the support. The existence of Nafion leads to higher dispersion and utilization of Pt active components due to the increase of electrochemically accessible surface areas, ion channel, and easier charge-transfer at polymer/ electrolyte interfaces. Therefore, Pt/Nafion-C catalyst shows a high ESA, and high activity toward the methanol oxidation. In the synthesis field of electrocatalysts the moderate approach self-assembly strategy has attracted wide attention because it could maintain the surface activation and intrinsic property of carbon nanomaterials [81]. Additionally, this method may also control the density and dispersion of nanoparticles on the surface of carbon nanomaterials. Chen et al. [82] synthesized a noble metal Pt nanoworms (PtNWs)/p-aminothiophenol (pATP)β-cyclodextrin polymer (β-CDP)/reduced graphene oxide (PtNWs/pATP-β-CDP/rGO) nanocomposite using the self-assembly strategy. It has also been found that the amount of PtNW and the degree of dispersion on rGO can be altered by changing the weight ratio of pATP-β-CDP:rGO. The TEM characterization confirms the PtNWs self-assemble onto rGO by using pATP-β-CDP as a linker. Fig. 7.6 shows the TEM images of PtNWs/pATPβ-CDP/rGO composites prepared with different weight ratio of pATP-β-CDP:rGO (0.05, 0.1 and 0.5). Obviously, with the increase of weight ratio of pATP-β-CDP:rGO, the density of PtNWs on pATP-β-CDP/rGO increases, suggesting that the density of PtNWs modified on rGO might be controlled by adjusting the weight ratio of pATP-β-CDP:rGO. Studies of cyclic voltammetry and CA indicate that the as-prepared PtNWs/pATP-β-CDP/rGO catalyst could exhibit both much higher electrocatalytic activity and cycling stability toward methanol oxidation in acid solution than the commercial Pt/C catalyst.

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FIGURE 7.6 TEM images of PtNWs/pATP-β-CDP/rGO with different weight ratio of pATP-β-CDP:rGO of (A) 0.05, (B) 0.1, and (C) 0.5 [82].

FIGURE 7.7 TEM images of functionalized MWCNTs (A), PEDOT-MWCNTs (B), and MnOx-PEDOTMWCNTs (C). TEM images of Pt/MWCNTs (D), Pt/PEDOT-MWCNTs (E), and Pt/MnOx-PEDOT-MWCNTs (F) catalysts. The insets in (D), (E), and (F) are the corresponding size distribution histograms of Pt nanoparticles [83].

Wei et al. [83] reported a novel Pt-based electrocatalyst for DMFCs using MWCNTs supported manganese oxide and poly[3,4-ethylenedioxythiophene (PEDOT) nanocomposite (MnOx-PEDOT-MWCNTs)] as catalyst support for Pt nanoparticles. The results demonstrate that the PEDOT film is homogeneously coated on the surface of MWCNTs, and the nanotubular morphology is retained in this composite (Fig. 7.7A and B). Interestingly, the inclusion of the MnOx to PEDOT-MWCNTs composite matrix generated different micro-structural features (Fig. 7.7C), the MnOx component in the composite is MnOx

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nanowire networks, which are closely combined with the PEDOT layer and homogeneously and densely attached on the surface of MWCNTs. The dispersion of Pt nanoparticles on MWCNTs is characterized by a poor distribution with a large number of aggregates (Fig. 7.7D). The average particle size of Pt nanoparticles is 4.75 6 0.5 nm. It is likely due to the nonuniform defects generated on the surface of MWCNTs by the acid oxidation treatment. However, Pt nanoparticles are evenly deposited on the surface of PEDOT-MWCNTs with no agglomeration (Fig. 7.7E). The average particle size is 2.5 6 0.3 nm, much smaller than that on the MWCNTs. In the case of Pt/MnOx-PEDOTMWCNTs (Fig. 7.7F), the average particle size of Pt nanoparticles is 2.5 6 0.4 nm, which is almost similar to that on the PEDOT-MWCNTs support, indicating that the MnOx cannot influence the size and dispersion of Pt nanoparticles. The electrochemical tests indicate that the methanol electrooxidation activity and stability of the Pt/MnOx-PEDOTMWCNTs are significantly enhanced as compared with the Pt/PEDOT-MWCNTs and Pt/MWCNTs catalysts. Wang et al. [11] prepared an effective electrocatalyst by depositing Pt nanoparticles on the polyindole (PIn)-functionalized MWCNTs (Pt/PIn-MWCNTs) for the MOR. The PInMWCNT composite was synthesized via in situ chemical polymerization of indole on the MWCNT surface. The nitrogen-containing group in PIn allows the Pt nanoparticles to be uniformly anchored on the surface of PIn-MWCNTs. And the prepared catalyst has excellent electrocatalytic activity and long-term durability for methanol oxidation because of the formation of Pt-N bonds.

7.3.2 Aromatic macrocycle/polycycle molecules Porphyrins, the two-dimensional 18 π-electron aromatic macrocycle molecules, possess excellent stability, unique optical, and electronic properties and have been used in various fields. Especially, the porphyrin molecules can not only noncovalently functionalize graphene through π-π stacking but also introduce homogeneous surface functional groups on the graphene surface. Wang et al. [10] developed a novel nanostructured catalyst of Pt nanoparticles supported on 5,10,15,20-tetrakis(1-methyl-4-pyridinio) porphyrin tetra (p-toluenesulfonate) (TMPyP) functionalized graphene (TMPyP-graphene) by the hydrothermal polyol process. Fig. 7.8 shows the TEM micrographs of Pt/TMPyP-graphene and Pt/graphene catalysts. From Fig. 7.8A and B, Pt nanoparticles are evenly dispersed on the surface of the TMPyP-graphene support with no agglomeration. For Fig. 7.8C and D the dispersion of Pt nanoparticles on pristine graphene is characterized by a poor distribution with a large number of aggregates. It is likely due to the nonuniform defects generated on the surface of pristine graphene. When Pt nanoparticles are deposited onto the surface of pristine graphene, the particles tend to be deposited at these localized defect sites, thus resulting in the poor dispersion and extensive aggregation. The electrochemical tests demonstrate that the Pt/TMPyP-graphene catalyst exhibits a much higher electrocatalytic activity and stability than the Pt/graphene and commercial Pt/C catalysts for methanol oxidation, which is of significant importance in improving the efficiency of Pt-based electrocatalysts for DMFCs applications. Similarly, metal phthalocyanines (MPcs), the two-dimensional 18 π-electron aromatic macrocycle molecules with a metal atom located at the central cavity, are of great interest

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FIGURE 7.8 TEM images of Pt/TMPyP-graphene (A, B) and Pt/graphene (C, D) catalysts [10].

due to their excellent electronic properties and potential applications in some fields such as electrical devices, solar cells, and biosensors. In recent years, MPcs-functionalized carbon nanocomposites have been developed for the cathodic electrode materials for fuel cells [84,85]. As for the anodic materials of fuel cells, our group reported the synthesis of several MPcs functionalized carbon nanomaterials with Pt or Pd catalysts. Zhong et al. reported the noncovalent functionalization of graphene with copper phthalocyanine3,40 ,4v,4w-tetrasulfonic acid tetrasodium salt (TSCuPc) [8] or nickel (II) phthalocyaninetetrasulfonic acid tetrasodium salt (TSNiPc) [86] as the promising catalyst supports for Pt nanoparticles. With the assistance of MPcs, Pt nanoparticles are homogeneously deposited on the surface of graphene, and their dispersivity and ECSA are obviously enhanced. Studies of cyclic voltammetry and CA demonstrate that the as-prepared catalysts exhibit much higher electrocatalytic activity and stability for methanol oxidation. In addition, Zeng et al. [87] synthesized a Pd electrocatalyst using copper phthalocyanine-3,40 ,4v,4wtetrasulfonic acid tetrasodium salt (TSCuPc) functionalized MWCNTs as the support. It is found that Pd nanoparticles are uniformly deposited on the surface of TSCuPc-MWCNTs, and their dispersion and ECSA are significantly improved. The electrochemical studies demonstrate that the Pd/TSCuPc-MWCNTs exhibit much higher electrocatalytic activity and stability than the Pd/AO-MWCNTs catalyst for formic acid oxidation.

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Pt-Cu NCs with controllable morphology not only have high surface area and abundant edge/corner atoms but also provide sufficient absorption sites for all absorbing molecules involved over in close range, which can further increase their catalytic activity. However, organic solvents, surfactants, or templates can poison the surface of Pt-Cu NCs, resulting in the blocking of a large number of active sites. Zhang et al. [32] developed a simple and facile aqueous solution method in the synthesis of uniform porous Pt-Cu NCs supported on 1-aminopyrene functionalized graphene nanoplates (p-Pt-Cu/AP-GNPs), without any high boiling point organic solvents, surfactants, stabilizers, and templates. The role of APGNPs in inducing the formation of porous nanostructure was investigated. It is observed that the well-defined porous NCs are supported on the AP-GNPs. In comparison the absence of AP-GNPs yields heavily aggregated particles while other conditions were kept the same. The results indicate the key role of AP-GNPs in determining the morphology of the Pt-Cu products, where AP-GNPs can influence the nucleation and growth process of the NCs. The electrochemical results prove that the as-prepared p-Pt-Cu/AP-GNPs catalyst presents superior electrocatalytic performance for methanol oxidation in alkaline condition. The enhanced electrocatalytic performance of the p-Pt-Cu/AP-GNPs catalyst for methanol oxidation could be attributed to the highly porous structure, the synergistic effects between Pt and Cu atoms and the interaction between porous Pt-Cu NCs and AP-GNPs. Li et al. [88] reported a facile method to prepare Pt nanoparticles supported on noncovalent functionalized GNS that were prepared by exfoliation of expanded graphite in DMF with the assistance of supercritical CO2 and 1-pyrenamine (PA). The introduction of PA provides ideal active sites for the attachment of Pt nanoparticles and improves their dispersion on the GNS. The morphologies and dispersion of Pt particles supported on the GNS and PA-GNS were investigated by TEM. From Fig. 7.9A and B, Pt nanoparticles have a large number of aggregates on GNS, and the distribution is not uniform. For the PA-GNS/Pt (Fig. 7.9C and D), Pt nanoparticles are evenly dispersed on the PA-GNS and the mean size of Pt nanoparticles is smaller (3.0 nm). This can be attributed to a large amount of amine (2NH2) groups introduced by the PA molecules that can act as active sites for the nucleation of Pt nanoparticles and restrict the growth of the particle. The inset of Fig. 7.9C shows an fcc Pt(111) plane with a d-spacing value of 0.22 nm. The size and dispersion of Pt nanoparticles on the supports are highly responsible for their electrocatalytic activity. The PA-GNS/Pt nanocomposite exhibits better electrocatalytic activity and stability toward methanol oxidation than the commercial catalyst JM-C/Pt. Wang et al. [89] developed a novel noncovalent approach to functionalize the carbon nanomaterials. In this method, the triphenylphosphine selenide (PPh3Se) was first noncovalently coupled on the surface of carbon oxides treated with HNO3 and H2O2 via the aromatic triphenylphosphine (PPh3). The resulting PPh3Se and carboxylate-modified carbons were heat-treated to remove the PPh3, and then the carbon materials could be modified by the noncovalent adsorption of selenium. Due to the anchoring effect of selenium atoms, highly dispersed PtRu NPs with narrow size distribution were deposited on the selenium functionalized carbon (Se-C) support and they showed higher electrocatalytic activity and stability toward methanol electrooxidation. Liang et al. [90] prepared water-dispersible 8-hydroxy-1,3,6-pyrene trisulfonic acid trisodium salt (PyS)-functionalized graphene (PyS-graphene) through the π-π interaction

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FIGURE

7.9 Low- and high-magnification TEM images of GNS/Pt (A, B) and PA-GNS/Pt (C, D). The inset of (D) is a high-resolution TEM image of PA-GNS/Pt [88].

between the aromatic PyS and graphene. Using the PyS-graphene as the support, a Pt nanoparticle catalyst could be obtained. The electrochemical results showed that the electrocatalytic activity of Pt/PyS-graphene for the MOR is higher than that of the Pt/ graphene catalyst. The higher electrocatalytic activity of Pt/PyS-graphene could be attributed to the introduction of a negatively charged sulfonic acid ðSO2 3 Þ group to the graphene sheet surface. Due to their nucleophilic nature, sulfonic acid groups not only allow the graphene sheets to be dispersed in water and prevent the aggregation of the graphene sheets but also effectively bind the metal particles. There are many functional processes that do not require the introduction of additional surfactants, which greatly reduces the production complexity. Li et al. [91] fabricated a new Pd NPs electrocatalyst with ILs molecules derived from 3,4,9,10-perylene tetracarboxylic acid (PDIL) noncovalently functionalized MWCNTs (PDIL-MWCNTs) as the support through a facile and surfactant-free synthetic technique. The morphology and structure of as-prepared catalysts were characterized by TEM (Fig. 7.10). From Fig. 7.10A and B, the Pd NPs seriously aggregate and poorly distribute on the surface of MWCNTs with a wide size distribution, and the mean size calculated by the lognormal distribution is 12.5 nm. Though the distribution of Pd NPs is improved on the AO-MWCNTs compared with MWCNTs, the Pd NPs on the AO-MWCNTs ranged from 3 to 9 nm and the average size is 5.6 nm (Fig. 7.10C and D). In contrary, Pd NPs with the quasi-spherical

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FIGURE 7.10

TEM images of Pd/MWCNTs (A, B), Pd/AO-MWCNTs (C, D), and Pd/PDIL-MWCNTs (E, F) (Inset: size distribution histograms and HRTEM images of the Pd NPs crystal structure in detail on MWCNTs, AOMWCNTs, and PDILMWCNTs.) [91].

shape and narrow size distribution are equably attached on the surface of PDIL-MWCNTs (Fig. 7.10E and F), and their average size is about 3.8 nm. In addition, the HRTEM image exhibits clear lattice fringes of the marked regions with the interplanar spacings of 0.226 nm, corresponding to the (111) planes of the fcc Pd for three catalysts [3]. Among three catalysts, the Pd NPs for Pd/PDIL-MWCNTs possess the more perfect crystal structure than that of the Pd/AO-MWCNTs and Pd/MWCNTs. The smaller mean particles size, narrower size distribution, and better distribution of Pd NPs for Pd/PDIL-MWCNTs

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should be attributed to the role of PDIL molecules. Specifically, a large number of imidazolium groups with positive charge was introduced by the PDIL molecules and uniformly immobilized on the surface of MWCNTs, which enhanced the PDIL-MWCNTs dispersion in water and served as the functional groups for the immobilization of Pd precursors on the PDIL-MWCNTs surface through electrostatic interaction and coordination. The results demonstrate that the PDIL-MWCNTs is a well support for Pd NPs and the presence of PDIL is a key factor in controlling the morphology and structure of the Pd NPs. In addition, electrochemical test results show that the Pd/PDIL-MWCNTs exhibit enhanced electrocatalytic activity and improved stability for ethanol electrooxidation compared with the Pd/ MWCNTs and Pd/AO-MWCNTs catalysts, owing to the role of PDIL functional molecules. Ma et al. [92] functionalized RGO with 1,10-dimethyl-4,40- bipyridinium dichloride (methyl viologen, i.e., MV) and then obtained a Pt/MV-RGO catalyst for DMFCs by the deposition of Pt nanoparticles on the MV-RGO. The positively charged functional groups of MV enforced sufficient electrostatic repulsion against the van der Waals attraction of the RGO sheets, which facilitate wetting of RGO with Pt precursors and enable homogeneous dispersion of Pt particles. In addition, the N-containing groups at the surface of the RGO, which were introduced by MV, would also facilitate the dispersion of Pt NPs. The Pt/MV-RGO hybrid exhibits much higher electrocatalytic activity than the Pt/RGO toward methanol oxidation. The enhancement effect of the MV-modification would arise from the better dispersion of Pt and/or synergetic interaction between Pt and MV-RGO hybrid, which favors charge transfer and ion diffusion.

7.3.3 Biomacromolecules The biomacromolecules (e.g., simple saccharides, polysaccharides, deoxyribonucleic acid (DNA), etc.) have also been employed in the noncovalent functionalization of carbon nanomaterials for fuel cell applications. Chitosan (CS), a biomacromolecule with reactive amino and hydroxyl functional groups, has gradually been used to build catalysts due to its superior properties such as excellent film-forming ability, non-toxicity, cheapness, and sensitivity to chemical modification. Its hydrophilicity also promotes electron transfer in the reaction mixture [93]. In addition, chitosan allows good adhesion of the catalyst thin layer on the surface of the working electrode. On the other hand, graphene quantum dots (GQDs) as a chemically inert, water-soluble, low-toxicity, and low-cost nanomaterial displays unique optical and electrical properties due to the quantum confinement and edge effects [94
97]. GQDs have become promising materials for energy storage devices and fuel cells due to their good dispersion in various solvents [98]. Hasanzadeh et al. [99] fabricated a novel metal-free nanocatalyst based on GQDs functionalized by chitosan (GQDCS) and β-cyclodextrin (GQD-β-CD) toward methanol electrooxidation in alkaline solution. The redox behaviors of as-prepared electrode were examined using the electrochemical CV, CA, and EIS techniques. The results indicate that β-CD and CS play an essential role in the electrocatalytic behaviors of GQDs. In general, the CS-GQDs exhibit excellent electrocatalytic activity for the MOR and have great potential for application in the DMFC application. The positively charged functional groups in CS can generate electrostatic attraction with PtCl22 6 , which facilitate good dispersion of Pt nanoparticles. Ekrami-Kakhki et al. [100]

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FIGURE 7.11

TEM images of (A) Pt-CS, (B) Pt-SCO-CS, (C, D) Pt/MV-RGO-CS, (E, F) Pt-SCO/MV-RGO catalysts [100].

studied functionalized graphene with 1,10-dimethyl-4,40-bipyridinium dichloride (methyl viologen) accompanied by chitosan (MV-RGO-CS) for the deposition of PtCl22 6 ions via an electrostatic self-assembly. Since various ABO3/Pt complexes had comparable activity to Pt-Ru alloys, SrCoO3-δ (SCO) was further introduced to form the Pt-SCO/MV-RGO-CS catalyst. Fig. 7.11 presents the TEM images of as-prepared materials. It is found that there is a little agglomeration in the absence of MV-RGO and the dispersion of Pt NPs is more uniform with lower agglomeration in the presence of MV-RGO and CS substrates. The electrostatic attractions between the positively charged functional groups of CS polymer and MV-RGO, and the negatively charged PtCl22 6 led to the uniform dispersion of Pt NPs. The results show that the incorporation of SCO into the Pt catalyst can significantly improve the electrochemical performance of electrode for ethanol electrooxidation. The high electrocatalytic activities of Pt/MV-RGO-CS and Pt-SCO/MV-RGO-CS for ethanol electrooxidation may be due to the good dispersion of catalytic nanoparticles. Carboxymethyl chitosan (CMC) is a biocompatible and biodegradable derivative of chitosan. Compared to the chitosan, CMC has better water solubility and good ability to form films, fibers, and hydrogels. Wu et al. [101] reported a facile strategy to fabricate a CMC functionalized CNTs (CMC-CNTs) and demonstrated its application as a promising

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FIGURE 7.12

TEM images of Pd-Graphene (A) and PdDNA@Graphene (B, C). Their corresponding histograms are provided as the insets. (D) EDS spectrum of PdDNA@Graphene. Inset of (D) is the photograph of ethanol solution containing PdGraphene (left) and PdDNA@Graphene (right) [102].

catalyst support material for DMFCs. The CMC molecular chain, which has a flexible molecular skeleton and many free carboxyl groups, can wrap itself around the CNTs, resulting in carboxylated CNTs with high water solubility. The as-prepared PtRu NPs/ CMC-CNTs nanohybrid has extremely large ECSA and exhibits better electrocatalytic activity and stability than the PtRu NPs/CNTs catalyst for methanol electrooxidation. DNA is a known biological molecule, which has regularly arranged functional groups and well-developed chemistries for different specific modifications. It can well interact with graphitic carbon materials such as CNTs or graphene through π-stacking to provide functional groups while retaining the intact structure of the carbon materials. Guo et al. [102] explored Pd NCs supported on DNA functionalized graphene (Pd-DNA@Graphene) composite as an efficient electrocatalyst toward formic acid oxidation. The morphology and nanostructure of the prepared catalysts were characterized by the TEM technique. As shown in Fig. 7.12A, Pd NCs in the Pd-Graphene are highly agglomerated and have an average size of around 12 nm. While for the Pd-DNA@Graphene (Fig. 7.12B), Pd NCs are uniformly distributed on the DNA@Graphene surface and have an average size as small as 5 nm with relatively narrow size distribution, indicating that the DNA functionalization indeed facilitates the growth of ultrasmall Pd NCs with uniform distribution on graphene. It is noted that both Fig. 7.12A and B exhibit the line-like structures, which are the wrinkles often observed from graphene sheets. From the HRTEM image of PdDNA@Graphene (Fig. 7.12C) a well-defined crystal structure with predominant Pd(111) facets can be observed. The energy-dispersive X-ray spectroscopy results (Fig. 7.12D) of

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Pd-DNA@Graphene show that the Pd-DNA@Graphene is composed of 32 wt.% Pd. For Pd-Graphene, the Pd weight percentage is around 30 wt.% (not shown), suggesting a similar Pd loading for Pd-Graphene and Pd-DNA@Graphene is achieved for a fair comparison. In addition, the use of DNA can promote the good dispersion of the Pd-DNA@Graphene in solution (inset of Fig. 7.12D). The electrocatalytic properties of the Pd-DNA@Graphene were investigated in detail. Compared with the Pd-graphene and commercial Pd/C, the Pd-DNA@Graphene show low oxidation peak potential, high catalytic current density, low charge transfer resistance, and good stability for formic acid oxidation.

7.4 Doping functionalization of carbon nanomaterials In order to overcome the shortcomings of Pt-based catalysts, the Pd catalysts with lower cost, more supply, stronger resistance to intermediate products, and higher catalytic activity have been extensively applied as a substitute to Pt-based catalysts [51,103,104]. However, the poisoning effect induced by the absorption of partial intermediate products and low utilization efficiency for Pd catalysts remain unsolved problems, which will hinder the practical applications of portable fuel cell technology. Another effective approach to enhance the electrocatalytic performance is to research and develop novel carbon materials with excellent physical and chemical properties as the catalyst supports [105
107]. Heteroatom doping is an effective method for tailoring the properties of carbon materials and extending their potential applications in energy conversion devices [108]. In particular, the doping of carbon nanostructures with heteroatoms (e.g., nitrogen, sulfur, phosphorus, boron) can significantly affect their electronic structures, chemical reactivity as well as conductivity, and meanwhile maintain the intrinsic physical/chemical characteristics well [109]. In addition to adjusting the properties of carbon supports the incorporation of heteroatoms is beneficial to immobilization of active metal NPs, which is able to restrain their particle sizes and promote the electrochemical stability [110,111]. Clearly, the heteroatom doping opens up a new way for the functionalization of carbon nanomaterials.

7.4.1 N-doping Among various heteroatoms, the nitrogen atom is undoubtedly deemed to be outstanding atom used to dope the carbon framework of supporting materials because its atomic radius is close to that of the carbon atom [112,113]. There are various synthetic methods for N-doped carbon nanomaterials, and they mainly include chemical vapor deposition (CVD) method [114
120], arc discharge method [121
123], thermal treatment method [124,125], solvothermal method [16,126,127], hydrothermal method [128], and plasma treatment method [129
131]. Wang et al. [114] directly synthesized the N-doped CNTs and reduced graphene oxide composite (NCNTs/RGO) on the carbon cloth (CC) via H2-assisted thermal reduction of GO followed by CVD of NCNTs, and then obtained a Pt-based electrode material through the electrodeposition of Pt nanoparticles on the NCNTs/RGO/CC. As shown in

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FIGURE 7.13 SEM images of (A) Fe nanoparticles on RGO/CC obtained by electrodeposition; (B) NCNTs grown on the RGO/CC from acetonitrile catalyzed by Fe nanoparticles at 750 C via CVD; Pt nanoparticles electrodeposited on (C) NCNTs/CC and (D) NCNTs/RGO/CC [114].

Fig. 7.13A, Fe nanoparticles are deposited with a narrow size distribution in the range from 80 to 100 nm. The homogeneously distributed Fe nanoparticles provide further evidence for the good electrical conductivity between RGO and CC throughout the electrode. Fig. 7.13B represents the NCNTs/RGO/CC electrode, where NCNTs are densely grown on the surface of RGO decorated CC with acetonitrile as carbon precursor at 750 C. Densely populated Pt nanoparticles are uniformly distributed on both NCNTs/CC (Fig. 7.13C) and NCNTs/RGO/CC (Fig. 7.13D) with similar distribution. Furthermore, in the Pt-NCNTs/RGO/CC (Fig. 7.13D), Pt nanoparticles are not only deposited on NCNTs but also on RGO. The electrochemical tests demonstrate that the prepared Pt-NCNTs/ RGO/CC exhibits enhanced mass-specific activity and high poisoning tolerance for methanol oxidation in comparison to the Pt-NCNTs/CC. Wang et al. [124] synthesized nitrogen-doped CNT supporting NiO nanoparticles by a chemical precipitation process coupled with subsequent calcination. The morphology and structure of the composites were characterized by TEM, X-ray diffraction (XRD), and Xray photoelectron spectroscopy (XPS), and the electrochemical performance was evaluated using cyclic voltammetry and chronoamperometric techniques. The results reveal that the evenly dispersed ultrafine NiO nanoparticles supported on nitrogen-doped CNT are obtained after calcination at 400 C. The optimized composite catalysts have high electrocatalytic activity, fast charge-transfer process, excellent accessibility, and stability for MOR in alkaline medium. Liu et al. [125] reported the synthesis of the honeycomb-like nitrogen-doped porous carbons (NPCs) using ILs 1-butyl-3-methylimidazolium dicyanamide (BMIMdca) as the carbon precursor and silica spheres as the templates. The resulting NPCs were used as supports of Pt nanoparticles for methanol oxidation in alkaline medium. The high specific surface area and porous structure of NPCs are beneficial to the uniform dispersion of supported Pt nanoparticles to increase their utilization. The electrocatalytic activity of methanol oxidation on the Pt/NPCs catalyst is higher than that of Pt on commercial Vulcan

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FIGURE 7.14

TEM images of (A) Pd/NGS and (B) Pd/PDIL-NGS. Inset: size distribution histograms and HRTEM images of Pd NPs on NGS and PDIL-NGS [128].

XC-72R carbon (Pt/C). The results indicated that the NPCs are a potential candidate for application as electrocatalyst supports in alkaline DMFCs. Li et al. [128] synthesized the ILs functionalized nitrogen-doping graphene nanosheets (PDIL-NGS) with few layers through an effective one-pot hydrothermal method with GO as raw material, urea as reducing-doping agents, and ILs derived from 3,4,9,10-perylene tetracarboxylic acid as functional molecules. Then, the Pd NPs was anchored on the PDILNGS by a facile and surfactant-free synthetic technique. The detailed characterizations indicate that the PDIL molecules not only can functionalize NGS through π-π stacking with no affecting the nitrogen doping but also prevent the agglomeration of NGS. As presented in Fig. 7.14A and B, the NGS appears to obviously aggregate with more layers, while the PDIL-NGS is transparent and thin with few layers. In the case of Pd/NGS (Fig. 7.14A), all the Pd NPs with average diameters of 8.6 nm are randomly attached on NGS and some of them appear to aggregate. However, for the Pd/PDIL-NGS (Fig. 7.14B), the quasi-spherical Pd NPs with narrow size distribution and the average size of about 4.2 nm are uniformly distributed on the surface of PDIL-NGS. In addition, from the HRTEM results of two catalysts, the Pd NPs for Pd/PDIL-NGS possess the more perfect crystal structure than that of the Pd/NGS. More importantly, due to introducing a large number of ILs groups, the processing performance and the electrocatalytic activity are significantly enhanced [51]. The Pd/PDIL-NGS catalyst exhibits better kinetics, more superior electrocatalytic performance, higher poisoning tolerance, and electrochemical stability than the other catalysts (e.g., Pd/NGS, Pd/RGO, Pd/C) toward ethanol electrooxidation. Huang et al. [132] demonstrated a combined hydrothermal self-assembly, freeze-drying, and thermal annealing method for the fabrication of a hybrid catalyst made from nanosized Pt particles and 3D nitrogen-doped graphene nanoribbons (N-GNRs). The resulting 3D architecture has a large surface area, interconnected porous networks, extremely small sizes of Pt NPs, uniform nitrogen distribution, and good electrical conductivity, which is very desirable for the electrocatalysis of the MOR. As the anode electrocatalysts, the Pt/NGNRs not only allow a rapid rate of diffusion of the methanol electrolyte but also provide many electroactive sites. Therefore the Pt/N-GNR architecture achieves remarkable electrocatalytic properties for methanol oxidation, including excellent electrocatalytic activity, strong poison tolerance, and superior long-term stability, all of which are superior to Pt/ Vulcan XC-72 (Pt/C), Pt/carbon nanotube (Pt/CNT), and Pt/undoped GNR (Pt/GNR)

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FIGURE 7.15 TEM images of as-prepared PdBi/ NG (A, C) and PdBi/RGO (B, D). The insets in (A) and (B) are the corresponding particle size distributions [134].

catalysts. In addition, Zhao et al. [133] constructed a 3D N-doped graphene aerogel with porous structures and uniformly distributed PtRu NPs (N-GA/PtRu) through a simple, rapid, and eco-friendly method. The N-GA/PtRu exhibits an exceptional performance (high catalytic activity, good CO tolerance, and excellent stability) toward the methanol electrochemical oxidation reaction. It is worth noting that the N-GA/PtRu can be directly used as the anode of DMFCs by simple physical pressing without the need for any binders or additives. Xu et al. [134] reported a highly reliable and surfactant-free strategy to prepare nitrogen-doped graphene (NG) supported network-like palladium-bismuth nanoparticles. As can be seen from Fig. 7.15A and C, most PdBi nanoparticles are well dispersed on the surface of NG and interconnected with each other into network-like structure, which benefits for supplying with massive surface-active areas. Same as the PdBi/NG, the obtained PdBi nanoparticles are also evenly dispersing on the rough surface of RGO (Fig. 7.15B and D). However, after attentive observations of particle size distributions, it can be easily found that the dimensions of obtained PdBi nanoparticles (4.64 nm) with supports of NG are smaller than those of PdBi nanoparticles (4.94 nm) with supports of RGO. Therefore, the as-prepared Pd1Bi1/NG network-like electrocatalysts exhibit much higher electrocatalytic activities for formic acid oxidation than the Pd1Bi1/RGO, Pd1Bi1 and commercially

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available Pd/C catalysts in terms of mass activity (1.69, 4.33, and 15.5 times higher, respectively). The significantly enhanced performances are related to the electron transport between Bi and N, bifunctional effect between Pd, Bi, and NG hybrids as well as the welldispersed network-like structure on the surface of NG.

7.4.2 S-doping It has been found that the introduction of sulfur atoms can also maintain the electrical conductivity of carbon nanomaterials and significantly improve the dispersion and electrochemical performance of catalytic nanoparticles [135
139]. However, S-doping is more difficult to happen compared to N-doping because of the sulfur atomic size and its different binding behavior [140]. Therefore very few studies have focused on sulfur-doped carbon nanomaterials as the anode catalyst support of fuel cells. Fan et al. [141] developed a novel approach to synthesize sulfur-doped MWCNTs (S-MWCNTs) as a highly efficient support material for Pt nanoparticle catalysts. The S-MWCNTs were synthesized by annealing PEDOT functionalized MWCNTs at 800 C under a nitrogen atmosphere. Fig. 7.16 shows the TEM images of Pt nanoparticles supported on S-MWCNTs or AO-MWCNTs. As seen from Fig. 7.16A and B, Pt nanoparticles with smaller size are homogeneously deposited on the S-MWCNT support and there is no FIGURE 7.16

TEM images of Pt/S-MWCNT (A, B) and Pt/AO-MWCNT (C, D) catalysts. The insets in (B) and (D) are the corresponding size distribution histograms of catalytic nanoparticles [141].

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evident agglomeration. Moreover, most of the Pt nanoparticles of this catalyst are distributed in the range of 1.4
3.4 nm, and its average particle size is statistically obtained to be 2.37 nm by measuring the diameter of 200 nanoparticles (the inset of Fig. 7.16B). However, the Pt/AO-MWCNT catalyst has a poor dispersion of Pt nanoparticles on the AOMWCNTs with obvious agglomeration (Fig. 7.16C and D). Most of the Pt nanoparticles of this catalyst are distributed in the range of 2.0
5.25 nm, and its average particle size is measured to be 3.88 nm (the inset of Fig. 7.16D). The doping of sulfur into MWCNTs could obviously improve the dispersion of supported Pt nanoparticles and enhance the electron transfer interaction between Pt nanoparticles and S-MWCNT support. The electrochemical results show that the as-prepared Pt/S-MWCNTs exhibit much higher electrocatalytic activity, long-term durability, and CO-tolerance ability for the MOR compared to the undoped MWCNT supported Pt and commercial Pt/C catalysts, demonstrating that the S-MWCNT hybrid derived from PEDOT-functionalized MWCNTs is a promising electrocatalytic material for fuel cell applications. Niu et al. [142] reported the synthesis of small palladium nanoparticles (Pd NPs) inside sulfur-doped carbon microsphere (S-CMS). In this method, the highly dispersed Pd NPs encapsulated in S-CMS with an architectural feature like the plum pudding model were gained by a simple one-pot hydrothermal synthesis using reduced glutathione (r-GSH) as both reducing and capping agents, followed by a carbonization procedure. Compared to commercial Pd/C, the synthesized Pd NPs inside S-CMS were found to provide a larger effective surface for the MOR. The mass activity of methanol oxidation on the Pd NPs inside S-CMS is 5.9 times higher than that of the Pd/C, which is due to the small size of Pd nanoparticles and their interaction with the heteroatom-modified CMS coating. Moreover, the proposed encapsulated Pd NPs also remained more stable during continuous start-stop operation.

7.4.3 Other heteroatom-doping Besides nitrogen and sulfur, other heteroatoms (e.g., boron, phosphorus) can also be incorporated into carbon nanomaterials to form carbon hybrids with synergy or multiple functionalities. For instance, Sun et al. [55] prepared boron-doped graphene (BG) by thermally annealing the mixture of GO and boric acid, and using it as the support of Pt catalyst toward the MOR. The composition, structure, and morphology of as-obtained materials were characterized by TEM, inductively coupled plasma mass spectrometry, Raman spectroscopy, XRD, and XPS. The results show that the boron atoms are doped into the graphene network in the form of BC2O and BCO2 bonds, which lead to the increase in defect sites and promote the subsequent deposition of Pt nanoparticles. Therefore the Pt/BG catalyst has a smaller average particle size (2.4 nm) and narrower size distribution than the graphene supported Pt (Pt/G, 2.8 nm) catalyst, as shown in Fig. 7.17. The cyclic voltammetry and CA tests indicate that the Pt/BG catalyst shows prominent electrochemical activity and stability for the MOR. The enhanced activity of Pt/ BG is mainly attributed to the electronic interaction between BG and Pt nanoparticles, which lowers the d-band center of Pt and thus weakens the absorption of the poisoning intermediate CO.

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FIGURE 7.17 TEM images of Pt/G (A) and Pt/BG (B) catalysts [55].

Phosphorus is an element of the nitrogen group, which has the same number of valence electrons as nitrogen and can also be doped into the carbon networks. Recent studies have shown that phosphorus-doping can effectively improve the electrocatalytic activities of Pt and Pd catalysts. In Xin’s work [143] the acid-treated CNTs were annealed at 1000 C under H2/He atmosphere with sodium hypophosphite (NaH2PO2) to obtain P-doped CNTs (P-CNTs). Low-loading Pd nanoparticles with high dispersion and narrow size distribution were uniformly deposited on the P-CNTs and served as efficient anode catalysts for HCOOH oxidation. Surface analysis shows that the decreased Pd 3d electron density caused by electron transfer from Pd to P-CNTs enhances the HCOOH oxidation performance. Liu et al. [144] synthesized P-doped multiwalled CNTs (P-MCNTs) using FeMo/Al2O3 catalyst by the thermolysis of a toluene solution containing 7.5 wt.% of ferrocene and 5 wt. % of triphenylphosphine, and then Pt nanoparticles were dispersed on the P-MCNTs support for DMFCs. Due to the higher dispersion and the improvement of intrinsic activity of Pt nanoparticles, the Pt/P-MCNTs exhibit much higher electrocatalytic activity and longer-term stability for methanol oxidation than the Pt/MCNTs in acidic medium, indicating a highly potential application of Pt/P-MCNTs in DMFCs and meaning the possibility of the development of an interesting new class of support materials in energy conversions and fuel cells.

7.4.4 Co-doping Recent studies indicated that the dual-doping of two different heteroatoms into carbon nanomaterials frameworks could provide much more electroactive sites due to the synergistic effects, rendering further enhanced catalytic performance [145,146]. Zhang et al. [147] used 1,3,4-thiadiazole-2,5-dithiol as N and S precursors to synthesize nitrogen/sulfur dual-doped graphene (NS-G) via a thermal treatment process. Then, a Pd/NS-G catalyst was prepared by depositing Pd NPs on this composite support. As shown in Fig. 7.18A, the lamellar structure features of NS-G sheets can be clearly observed, which is similar to the previously reported graphene or GO materials. Under close inspection, the NS-G sheets are decorated uniformly by numerous small Pd NPs with sizes below 10 nm (Fig. 7.18B and C). Due to the unique structural features and the strong synergistic effects the prepared Pd/NS-G hybrid exhibits excellent electrocatalytic properties toward both

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FIGURE 7.18 Typical FE-SEM (A, B) and TEM (C) images of the Pd/NS-G hybrid. The insets in (C) are corresponding SAED pattern and Pd particle size distribution of Pd/NS-G [147].

formic acid and methanol electrooxidation, such as higher anodic peak current densities and more exceptional catalytic stability than those of Pd/Vulcan XC-72R and Pd/undoped graphene catalysts. Sun et al. [148] synthesized boron/nitrogen co-doped graphene (BNG) by the two-step thermal annealing of graphene in the presence of melamine and boric acid, which was served as novel support to enhance the catalytic properties of noble metal catalysts for the MOR. The results show that the BNG support has more defect sites, so that Pt nanoparticles with an average size of 2.3 nm are uniformly anchored on the surface of BNG support. The Pt/BNG catalyst demonstrates outstanding activity and improved stability toward the MOR, which is mainly attributed to the synergetic effects of boron and nitrogen codoping into graphene support. The codoping of boron and nitrogen produces more oxygencontaining species, thereby accelerating the methanol oxidation by the so-called bifunctional mechanism. In addition, the boron doping weakens the adsorption energy of poisoning intermediates on Pt surface by reducing the d-band center of Pt, promoting the oxidative removal of poisoning intermediates and methanol oxidation.

7.5 Theoretical understanding of catalytic activity for functionalized carbon nanomaterials The development of fuel cell technologies is inseparable from the improvement of anode electrocatalysts. Although significant advances have been achieved for the anode electrocatalysts, two main problems, namely insufficient performance (e.g., low reaction kinetics, poor stability) and high cost, remain unresolved. In order to overcome the issues, numerous efforts have been devoted to developing various functionalized nanocarbon supports to obtain the high dispersion and electrochemically active surface area of catalytic nanoparticles. The enhanced electrocatalytic properties of metal nanoparticles supported on functionalized carbon nanomaterials may be due to the following aspects: (1) The suitable functionalization of carbon nanomaterials can significantly improve their solubility in aqueous and organic solvents and prevent their aggregation, which contributes to the formation of high-performance catalysts. (2) After the functionalization, a large number of functional groups or defect sites can be formed on the surface of carbon nanomaterials.

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The formation of functional groups and defect sites will effectively destroy the conjugated structure of carbon nanomaterials and activate the support materials, thereby improving the stability and activity of the catalyst. (3) The functionalized carbon nanomaterials can provide more active sites that are beneficial to the attachment of metal nanoparticles, and help to improve their uniform dispersion on the supports. (4) The presence of strong electron transfer interaction between catalytic particles and functionalized carbon supports can further increase the electrooxidation activity and the tolerance to CO. To optimizing the catalyst performance, theoretical modeling and calculation are very useful to achieve a fundamental understanding of the affecting factors. The obtained results can be used to guide the design and fabrication of new electrocatalysts. Zhu et al. [149] synthesized N-doped bamboo-shaped CNTs (BCNTs) by CVD for the dispersion of Pt nanoparticles. Density functional theory (DFT) calculations were applied to study the activity enhancement mechanism of the Pt/BCNTs for methanol oxidation. According to the XPS results, two main nitrogen types in the doped CNTs, substitutional N (sN) and pyridinic N (pN), were proposed as the DFT models to study the methanol decomposition reaction on the different CNTs-based electrocatalysts. The DFT results indicate that the reaction pathways in terms of energetic favorability follow the order of: pN-CNT . Pt-sNCNT . Pt-pN-CNT . Pt3-sN-CNT. Reaction energies are much more exothermic over the N-doped CNTs compared to pure CNTs for methanol decomposition, suggesting that the addition of nitrogen can efficiently improve the catalytic performance of CNTs toward methanol electrooxidation. Moreover, Huang et al. [150] fabricated a hybrid electrocatalyst consisting of strongly coupled worm-shape Pt NCs and nitrogen-doped low-defect graphene (N-LDG) sheets toward the MOR. They adopted the DFT calculations to unveil the possible locations of N dopants in graphene plane and explain the formation mechanism of worm-like Pt NCs. It is found that the formation energies (Ef) of N dopants positioned near pyridinic and pyrrolic structures are mostly lower than those positioned in pristine graphene, which means that the doping reactions are more likely to take place at defective sites in carbon skeletons. This result correlates well with the reported N-doped graphene nanomaterials, implying that the N atoms tend to gather together to create a number of Nrich areas on the pristine graphene surface. Accordingly, the N atoms are responsible for the firm connection with Pt, thereby the existence of N-riched areas could lead to the yield of worm-shaped NCs.

7.6 Challenges and perspectives The employment of functionalized carbon nanomaterials as the supports of anode catalysts can significantly improve their electrocatalytic performance. A variety of strategies such as covalent connection, noncovalent modification, heteroatom-doping, etc., have been developed to achieve different types of functionalized carbon nanomaterials. Although these engineering processes have demonstrated their feasibility in preparing effective anode catalysts, there are still technical and commercial challenges to overcome before they are feasible for practical applications. Several challenges can be identified as follows. (1) It is difficult to balance the contradiction between the modification of carbon materials and the protection of their internal structure and properties. A higher

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surface-functionalization level of carbon materials undoubtedly makes them more vulnerable to be excessively oxidized, even resulting in a structural collapse. In the process of covalent functionalization, new groups will be introduced into the carbon nanomaterials, which are favorable for the formation of the catalyst, but at the same time it will also destroy the original structure of carbon nanomaterials, resulting in the weakening or loss of some special properties. As for the noncovalent functionalization, it can effectively preserve the original structure of carbon nanomaterials, but the functionalization of carbon nanomaterials through van der Waals forces or hydrogen bonds with weaker forces is unstable. Additionally, the doping method is accompanied by the destruction of the carbon nanomaterial structure, and also involves high-temperature or high-pressure treatment to destroy the strong C 5 C bond so that the dopant replaces the carbon atom to form a defect structure. When used as a support, the doping sites at the defects can effectively anchor the metal particles by electrostatic interaction. (2) The electrocatalytic performance of carbon-based anode materials is limited by the size, shape, microstructure, and dispersion of supported active metal NPs. (3) The underlying mechanisms of enhancement, including the improved performance of catalytic nanoparticles on functionalized carbon nanomaterials and the electronic interactions between nanocatalysts and carbonbased composites, are still not clear. To overcome the above challenges, continued efforts are required for the functionalized carbon nanomaterials to become practically viable. Research directions may be suggested as follows: (1) Exploring the rational synthesis methods with the possibility of introducing anchor sites on the carbon surface without affecting the original electronic network. (2) Developing more efficient engineering technologies for the construction of carbon-based composites with more advanced architectural design. In particular, diverse advanced strategies and techniques for the fabrication of carbon-based anode electrocatalysts with different compositions, architectures, and morphologies must be optimized to overcome the physical and chemical factors that limit electrocatalytic performance. (3) Further combining theoretical calculations and experimental results to investigate the performance enhancement mechanism of catalytic nanoparticles on functionalized carbon nanomaterials. Under such development trends, we believe that the rapid development of carbon nanomaterials as supports will drive the development of high-performance fuel cells.

7.7 Conclusion Fuel cells play an important role in modern life and are of great significance in solving the energy crisis and reducing pollutant emissions. The development of new types of catalysts is the key to solving the problem of fuel cell dynamics. In this chapter, several functionalization methods (e.g., covalent functionalization, noncovalent functionalization, and doping functionalization) of carbon nanomaterials for advanced anode catalysts of fuel cells in recent years are reviewed with a detailed discussion in terms of their synthesis, characterization, and support catalyst preparation as well as performance validation. Due to its unique advantages the important role of noncovalent functionalization for carbon nanomaterials is emphasized in the performance improvement of anode catalysts. The heteroatoms doped in carbon nanomaterials can effectively enhance the electrochemical

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performance of supported metal nanoparticles; the doping functionalization of carbon nanomaterials for the fabrication of anode catalysts is also emphasized in this chapter. To give the reader a complete picture about the functionalized carbon nanomaterials for anode catalysts and their associated research and development, the progress in research and development, and the challenges currently facing are also described and discussed. To promote the research and development on this topic, several research directions required more attention that are also presented in this chapter.

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