Nitrogen-containing carbon nanotubes as cathodic catalysts for proton exchange membrane fuel cells

Diamond & Related Materials 22 (2012) 12–22

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Nitrogen-containing carbon nanotubes as cathodic catalysts for proton exchange membrane fuel cells Wai Yin Wong a, Wan Ramli Wan Daud a, b,⁎, Abu Bakar Mohamad a, b, Abdul Amir Hassan Kadhum a, b, Edy Herianto Majlan a, Kee Shyuan Loh a a b

Fuel Cell Institute, National University of Malaysia, 43600 UKM Bangi, Selangor, Malaysia Department of Chemical and Process Engineering, National University of Malaysia, 43600 UKM Bangi, Selangor, Malaysia

a r t i c l e

i n f o

Article history: Received 29 September 2011 Received in revised form 16 November 2011 Accepted 17 November 2011 Available online 26 November 2011 Keywords: Proton exchange membrane fuel cell Nitrogen-doped Organic catalyst Oxygen reduction reaction Carbon nanotubes

a b s t r a c t Proton exchange membrane fuel cells (PEMFC) comprise a diverse range of fuel cell thought to have future commercial application and transportation. The introduction of nitrogen into carbon nanostructures has created new pathways for the development of non-noble metal electro-catalysts in fuel cells. This review provides insight into the role of nitrogen inclusion into the carbon nanotubes (CNT) and the possible mechanisms involved in oxygen reduction reaction (ORR) activity. The doping effects of nitrogen into CNT on the surface morphology, electronic structures and electrochemical activity are discussed. Catalyst nanoparticles distribution, chemical composition and the incorporation of a binder play crucial roles in the generation of good catalytic activity and high stability in organic electro-catalysts. Synthesize methods for making nitrogen-containing carbon nanostructures and the resultant oxygen reduction reactivity are compared. Finally, stability issues of the N-CNT electrocatalysts are discussed. © 2011 Elsevier B.V. All rights reserved.

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Role of nitrogen inclusion in CNTs for PEMFC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Enhancement of the oxygen reduction reaction . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Possible mechanism for ORR activity due to nitrogen inclusion in graphitic sheets . . . . . . . . . . 2.3. Enhancement of CNT formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Effects of growth parameter in nitrogen-doped CNT growth . . . . . . . . . . . . . . . . . . . . . . . 3.1. Influence on growth of metal particles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Influence of reactant gas and its nitrogen composition . . . . . . . . . . . . . . . . . . . . . . . 3.3. Influence of reaction temperature and synthesize duration . . . . . . . . . . . . . . . . . . . . . 4. Comparison between synthesis methods for nitrogen-containing carbon nanostructures and the ORR activity . 5. Stability issues of nitrogen-containing CNTs in PEMFC . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1. Introduction

⁎ Corresponding author at: Fuel Cell Institute, National University of Malaysia, 43600 UKM Bangi, Selangor, Malaysia. Tel.: + 60 38921 6405; fax: + 60 38921 6024. E-mail addresses: [email protected] (W.Y. Wong), [email protected] (W.R.W. Daud), [email protected] (A.B. Mohamad), [email protected] (A.A.H. Kadhum), [email protected] (E.H. Majlan), [email protected] (K.S. Loh). 0925-9635/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2011.11.004

Increasing global awareness of the looming energy security crisis caused by the imminent peaking of fossil fuels production from depleting fossil fuel reserves in 2030–2050 [1] and consequently the widening gap between energy demand and production have led governments to refocus their attention on developing or deploying renewable and alternative energy to stem the oncoming energy

W.Y. Wong et al. / Diamond & Related Materials 22 (2012) 12–22

shortage. In addition, the emergence of increasing evidence that greenhouse gas (GHG) emissions from human activities, especially in the supply of energy and transportation, are the main causes of global warming and climate change [2], has also forced governments to limit GHG emissions via the Kyoto Protocol by reducing fossil fuels use and increasing renewable and alternative energy. However, largescale biofuel deployment and global nuclear energy expansion are uncertain because of the conflict between crops for fuel and food, and negative general public opinion after the latest Fukushima Daichi major nuclear disasters respectively [3]. In addition, solar energy is still constrained by costs [3] while wind and wave energy are hindered by low energy extraction efficiency, seasonal variability and weather unpredictability [4]. In the global search for clean energy to solve both energy security and climate change, fuel cell technology using hydrogen fuel has drawn great interest because it is an advanced alternative energy technology that emits only water and therefore is clean, very environmental friendly, green and sustainable [5]. However, further expansion of hydrogen energy is constrained by costs, durability and safety of hydrogen transport and storage [3]. The proton exchange membrane fuel cells (PEMFC) are touted as the most suitable energy conversion device that will replace internal combustion engines in transportation and portable applications in the future [5–6]. The 35% cost reduction of PEMFC for transportation achieved in the past two years is still less than the 50% cost reduction required to reduce PEMFC cost to the target of $30/kW (2015) that will make it more competitive than conventional energy technology [7]. In addition, the PEMFC stack lifetime of 2500 h achieved in 2009 has to be doubled to meet the target of 5000 h [7]. Both the cost and durability targets can be achieved by a greater fundamental understanding of the inter-related and complex processes occurring in the fuel cell stack and balance of plant as well as in the fuel cell vehicle and the portable power system during fuel cell operation [8]. The major advantages of the PEMFC over the other fuel cells are the high power density at low temperature (60–100 °C) [5–6,9–10], the ease of recharging hydrogen fuel cylinders on vehicles from centralized depots [9] and the quick start-up time particularly for transportation [6]. Presently, PEMFCs utilize platinum (Pt) as the electrocatalyst in both hydrogen oxidation and oxygen reduction reactions. Although the noble metal is the best-known catalyst for these reactions, especially for hydrogen oxidation, there are several major drawbacks to large-scale use of the catalyst, especially its high cost and intolerance to the presence of CO in the hydrogen fuel produced from hydrocarbons via steam reforming or from biomass via pyrolysis and steam reforming [5–6,10–12]. The major factor that limits the performance of PEMFCs is the oxygen reduction reaction at the cathode [13]. Even on pure Pt, an over-potential of 300 mV is dissipated for the oxygen reduction reaction because of competing water activation and sluggish kinetics [8,10]. Under an acidic environment, the oxygen reduction may proceed via two routes: a 4-electron reduction to water (O2 + 4e − + 4H + → 2H2O), which is the normal and complete reaction and a 2-electron reduction to hydrogen peroxide (O2 + 2H + → H2O2), which is an incomplete reduction that produces the undesired product, H2O2 [10,14]. The H2O2 may further reduce to water or decompose into oxygen and hydrogen [10,14]. Tremendous efforts have been made all over the world either to reduce the Pt loading or to discover non-noble metal catalysts as a replacement for the Pt that would still give an acceptable catalytic performance. The emergence of nanotechnology has presented a new technology route for replacement of Pt as the electrocatalyst in PEMFCs. Carbon nanostructures such as carbon nanotubes [15] and carbon nanocones, which have been used previously in many applications such as hydrogen storage [16], electrochemical sensing [17], electronic devices, water purification, catalysis and electrodes [18–19], have very promising mechanical and electronic properties of good alternative electroctalysts to Pt. The unique carbon

13

nanostructures formed by bonded planar sp 2 molecular orbitals that are sandwiched between overlapping unsaturated π molecular orbitals [20], provide enhanced electron transfer abilities that are crucial for any catalytic activity. The application of carbon nanostructures in PEMFC to improve catalytic activity could be achieved via two approaches. Firstly, catalytic activity can be improved by depositing nanosize metal catalysts, such as Pt [21–24], on the porous nanostructure materials to widen the specific surface area for better catalytic performance [19]. Secondly, catalytic activity can also be improved by modifying the carbon nanostructure surface by heteroatom doping [10,25–26], which is believed to add another dimension to the original structure's properties, such as conductivity and catalytic activity [10,19–20,25,27]. For instance, early experiments by Jasinski have identified the catalytic activity of Co-phtalocyanines [25–26], which led to extensive research in non-precious metal catalysts for PEMFC. Recent research has emphasized nitrogen-containing carbon nanoshells, carbon nanotubes (CNTs) and carbon nanofibers (CNFs) [28–34] as substitutes for Pt/C catalysts for oxygen reduction reaction (ORR). Ozaki et al. have managed to achieve a power density of 0.21 W cm − 2 for a nitrogencontaining carbon nanoshell catalyst, which is two-thirds that of the Pt/C catalyst [32]. Despite the discovery of many new carbon nanostructures and attempts to dope nitrogen atoms into them, carbon nanotubes are still believed to possess the greatest potential for commercialization. This review will discuss: (1) the role of nitrogen in CNT for PEMFC, (2) the doping effect of nitrogen in CNT, (3) a comparison between the synthetic methods for creating nitrogen-containing carbon nanostructures and the corresponding ORR activity, and (4) the stability issues of N-CNTs for PEMFC. 2. Role of nitrogen inclusion in CNTs for PEMFC 2.1. Enhancement of the oxygen reduction reaction Nitrogen-doping or nitrogen inclusion in CNTs has recently drawn great interest on the roles of nitrogen dopant or inclusion in enhancing the latter's electronic properties. Nitrogen doping in CNT electrodes has been found to influence significantly oxygen reduction, hydrogen peroxide decomposition and cathechol oxidation reactions [30]. Generally, the introduction of nitrogen as a dopant perturbs the homogenous π-cloud of the carbon nanotubes, changing the electronic surface state of the CNT [35], altering the route of reactions, specifically on the nitrogen surface, and therefore creating an increase in chemical activity. The electron-rich nitrogen atoms substituted into the graphene sheet or CNT are claimed to have n-type dopant activity for electron conductivity [36]. Tunneling microscopy and ab initio calculations have shown that the nitrogen in the CNT increases the Fermi energy at the donor state and allows the electrons to reach the conduction band, causing the material to exhibit metallic properties [36]. Such an increase in the localized density of states at the Fermi level could result in larger current emissions at considerably lower voltages [37], thereby increasing the catalytic activity of the carbon materials in electron-transfer reactions and oxygen reduction reactions [38–39]. Earlier attempts to investigate such influences have correlated the surface morphology determined by using x-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) characterization of N-CNTs [40] with the corresponding catalytic activity [40]. The XPS analysis identified the formation of different species of nitrogen functional groups, pyridinic-N, quaternary-N and pyridineN-oxide in the decomposing carbon during high-temperature synthesis [41]. Pyridinic nitrogen located on the edge of the graphite planes, where it is bonded to two carbon atoms, promotes oxygen reduction reaction by donating one p electron to the aromatic π system. On the other hand, pyridine-N-oxide, where oxygen is bonded to the

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pyridinic-N, has been shown by Liu et al. [41] to be not ORR active sites because the complete loss of pyridine-N-oxide groups after a 100-h stability test does not affect the loss of the catalyst activity. Moreover, Matter et al. [40] had demonstrated that no pyridinic-Noxide is produced at relatively high synthesis temperature (>600 °C), showing the insignificance of this functional groups in contributing to ORR. In contrast, quaternary nitrogen, or ‘graphitic nitrogen’, is bonded to three carbon atoms within a graphite (basal) plane. Terrones et al. showed that, when the overall N content increases within the CNT structures, the number of graphitic walls decreases while the proportion of pyridine-like N increases [36]. The surface morphology of pyridinic nitrogen is modeled in Fig. 1. A recent study by Wiggins-Camacho and Stevenson [42] has shown that the presence of pyridinic-N functional group plays a key role in stabilizing the active site that is responsible for the chemical disproportionation step of peroxide ions during the ORR. Although it was widely known that pyridinic nitrogen might not be the active site for ORR, it is, however, a marker for edge plane exposure, where many electrocatalytic reactions show increased kinetics [19,40]. However, contradictory explanations were reported elsewhere. Liu et al. [41] proposed that quaternary nitrogen could be responsible for the stable ORR active sites of nitrogen-modified carbon composite catalysts, together with the unstable pyridinic-N ORR active sites. Stability tests indicate that, pyridinic-N is susceptible to protonation, forming pyridinic-N-H in the acidic environment of PEMFCs that would decrease the ORR activity with time. This is further corroborated by both Ikeda et al. [43] and Niwa et al. [44] who demonstrated the possibility of enhanced ORR activity on the zigzag-edge of nitrogendoped graphite [43–44]. Sidik et al. [45] showed oxygen reduction on the carbon radical sites adjacent to substituted N through quantum chemical calculations. The role of the cathode in the PEMFC is to reduce the oxygen to water, thus producing an electric current. However, there are at least two competing reactions occurring simultaneously during the

Fig. 1. Molecular model of N-CNTs containing pyridine-like N atoms replacing C atoms [36].

process where water and the undesired hydrogen peroxide are produced. The existence of edge plane due to nitrogen incorporation may aid the formation of water molecules and thus provide better catalytic activity in PEMFC, regardless of whether the pyridinic or quaternary-N is responsible for the reaction. Nitrogen content on the other hand is the most important factor that should be governed to obtain higher catalytic activity. Kim et al. [46], Chen et al. [47] and Wiggins-Chamacho and Stevenson [42] have shown an increase in catalytic activity with increased nitrogen content of up to 4.0– 5.0 at.% displayed in Fig. 2. Chen et al. [47] clearly showed that NCNTs with more defects showed better ORR performance than those with fewer defects, which provides higher selectivity of H2O formation. Although many studies have suggested the possible effect of nitrogen on ORR activity, studies providing real insights into the molecular structure are still lacking. Until recently, many researchers focused on using computational chemistry to study the actual electronic structures of the nitrogen doping on carbon nanotubes and their significant effects on improving electron transfer [19,48–53].

2.2. Possible mechanism for ORR activity due to nitrogen inclusion in graphitic sheets Several experimental studies have shown that the plane in which the oxygen reacts determined the final product of ORR. It has been claimed that oxygen could only be reduced to peroxide when it reacts on the basal plane of a nitrogen-doped carbon [54] via a 2e − reduction process; however, it could possibly be reduced to water molecules when it reacts at the edge [19]. Although this process could be trivial, as the mechanisms involved are not established, it may help to determine how the functionalized carbon reacts in the potential pathway. The elementary process of the redox reaction on the catalyst should be understood before any search of such a mechanism is attempted. Calculations based on density-functional theory (DFT) may provide such information. Numerous studies on first-principle molecular dynamic (FPMD) simulations have been performed in

Fig. 2. Oxygen reduction reaction cyclic voltammograms for N-CNT mat electrodes in (A) 1 M KNO3 (B) 1 M Na2HPO4, pH 6.40 ± 0.03. Scan rate = 20 mV s− 1 [42].

W.Y. Wong et al. / Diamond & Related Materials 22 (2012) 12–22

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order to identify the chemical reaction occurring on the electrode/ water interfaces [55]. Strelko et al. [54] in an early work, described the potential mechanisms of the oxygen reduction reaction through oxygen chemisorption. It was postulated that the reaction is a heterolytic process, initiated by the reduction of oxygen to the superoxide anion O 2 −⁎, which is bound by electrostatic forces on the basic carbon. The adsorbed superoxide then produces OH − counterions in the presence of water and hydroperoxide anions as by-products. The ability of Ncontaining carbons to reduce oxygen is primarily because the positively charged carbon contains larger amounts of OH − counterions per unit surface near the heteroatom surface. O

2−


−

þ H2 O→OH þ HO2


Fig. 4. Bridge model of oxygen adsorption [59].

Zhong et al. [56] reviewed and described the mechanism of oxygen reduction to water. According to the description, the addition of nitrogen to the support can change the electron distribution and promote the dissociation and adsorption of oxygen. Such dissociation is similar to that of dicobalt cofacial porphyrin linked by four atom bridges [57], where the proposed mechanism (Fig. 3) could lead to a direct reduction of O2 to H2O. In fact, this bridge side-on adsorbate configuration was later documented for Pt/C catalysts [58]. Quantum-chemical calculations by Khomenko et al. [59] have shown similar bridge models, as shown in Fig. 4. It was confirmed that such adsorption could only be seen at the edge sides of nitrogen atoms where the two oxygen atoms are bonded to surface atoms. Khomenko [59] identified chemisorption process of oxygen molecules that is solely responsible for the reducing effect proposed by Strelko et al. [54], who explained that through chemisorptions, the O\O bond length in the bridge model would increase due to lower O\O bond orders in comparison to free O2 molecules. As a result, the chemisorbed O2 molecules have comparatively higher degree of activation and are more easily reduced, which allows them to contribute to the catalytic activity in the N-containing carbon structure. Chen et al. [47] adopted this configuration, which is supported by the discovery that the nitrogen would induce charge delocalization and the side-on configuration could cause the weakening of the O\O bonding that facilitates ORR [50]. Such observations were further proven in recent reports by Okamoto [55], whose density functional calculations showed that oxygen molecules are chemisorbed on graphene sheets, although the chemisorptions may be metastable relative to the Ndoped graphene sheet. Furthermore, it was found that the binding interaction between the graphene and oxygen becomes stronger as the number of N atoms bonding with the C_C increases due to a reduced repulsive interaction between the oxygen and the N-doped graphene sheet. Okamoto further described the ORR pathways through FPMD simulations on a model of the catalyst/water interface of N-doped graphene sheets. The simulation indicated that the ORR pathway for the 4e − reduction is as follows:

intermediates and paths in ORR on nitrogen-doped carbon catalyst involving both 2e − and 4e − transfer pathways. Wiggins-Chamacho and Stevenson [42] have further proven through a series of experiments that the O\OH is the key intermediate in the ORR pathways. However, it was observed that the more favorable surface-mediated disproportionation of O\OH − at N-CNT that leads to the desired product of OH − and oxygen, followed the ‘pseudo’-four-electron transfer mechanism proposed by Maldonado and Stevenson [39]. More interestingly, the group proposed that the ORR in N-CNTs is aided by a dual site mechanism in which both the N-doping and residual iron oxide/iron surface phase particles were responsible for the reduction and stabilization effect. However, since the synthesis of N-CNTs often leaves irremovable residual iron, there is a debate on whether N-CNTs by themselves or the iron particles play the actual catalytic role in ORR. There are contradicting opinions on whether disproportionation or further reduction in OOH − occurs in the reaction. More studies should be done to understand further the oxygen reduction mechanism in alkaline and acidic media respectively. Experimentally, the kinetic current and the number of electrons being transferred can be determined by Koutecky–Levich plots constructed from the rotating-ring disk electrode data [60–61] and the correlation shown by Gupta et al. [62]. Tafel analysis was also used to analyze the kinetics of ORR to determine the adsorption condition of oxygen on the NCNT surface [45] but this method gave less accurate results when associated with Langmuir and Temkin adsorption conditions for commercial Pt-based catalysts. Okamoto calculated the reversible potential of each ORR step on the basal plane and edge models and subsequently found that the edge model is expected to show a marginally better catalytic activity due to its higher reversible potential. Therefore, it could be postulated that the most active nanostructures would contain nitrogen with higher edge plane exposure (due to the atomic rearrangement) and that this exposure has a direct correlation with the selectivity of the catalysts for the complete oxygen reduction to water [19].

−

O2ads →O−OHads →Oads þ OHaq →Oads þ H2 Oliq →OHads þ H2 Oliq →2H2 Oliq ;

2.3. Enhancement of CNT formation

which is similar to the mechanism shown in Fig. 2. In retrospect, Okamoto also demonstrated that if a second reduction occurs before the O\OH bond breaks, another path to 2e − reduction will occur, resulting in the unfavorable product, HOOH. Fig. 5 displayed the possible

Numerous studies have explained that the introduction of nitrogen into the graphitic basal plane could enhance the curvature effect and thus promote the formation of curve geometry in carbon nanotubes [63–64].

Fig. 3. Proposed reaction mechanism for the Mo2N/C catalyst [57].

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OOH-aq O2 ads

Oads + Oads

OH-aq + H2Oliq

OOHads

Oads + OH-aq

O2 gas

HOOHaq

Oads + OHads

Oads + H2Oliq

OHads + H2Oliq

2 H2Oliq

OHads + OHads

Fig. 5. Proposed mechanisms for oxygen reduction on nitrogen-doped carbon catalyst. Arrows in solid line corresponds to 4e− reduction and arrows in dotted line correspond to 2e− reduction [55].

Earlier work to investigate such enhancement of curvature conducted through ab initio total energy calculations on graphitic carbon nitride concluded that the energy barriers are reduced when the CN sheets are rolled into tubular forms as compared to graphite sheets [65]. Simulations using a density function based tight binding method (DFTB) suggest that the nitrogen affects the formation of a pentagonal centered hexagonal network from a fully hexagonal network [64]. Nitrogen inclusion stabilizes the pentagon defect [64] in good agreement with the findings of Sjöström et al. [66], who explained that the nitrogen could promote the formation of pentagons, which results in the buckling of basal planes through sp 2-hybridized CNx planes and cross-linked by sp 3-hybridized bonds, hence giving rise to a curved structure. In such defects, it was shown that a pair of nitrogen atoms would prefer to occupy the central pentagon ring due to the lower energy differences (Fig. 6). The possible N\C bonding configurations with nitrogen dopant atoms incorporated in a graphitic network (Fig. 7) were examined by Xu et al. [67] through an XPS study using N2+ ion implantation. In that study, the nanotubes

showed a tendency to be bonded with N through sp 3C atoms, followed by sp 2C atoms [67], which corresponds to the quaternary nitrogen and pyridine-like nitrogen [40] at high CNT synthesis temperatures. Czerw et al. [68] and Ahn et al. [69] correlated this phenomena with the finding that when the nitrogen is saturated with chemical bonding on the tube edge, further reactions could be created with incoming carbon atoms during the synthesis, forming pyridinelike bonding. Srivastava et al. [70] have shown through generalized tightbinding molecular dynamics (GTBMD) that the formation of pentagon-type defects or chemisorptions due to nitrogen incorporation, shown in the top panel of Fig. 8 was more favorable in smaller-diameter nanotubes (8 nm). This finding could be related to the lower strain energy within the structure after the nitrogen incorporation process [64–65] displayed in Figs. 8 and 9. This observation was confirmed by Sumpter et al. [71], who further described that the preferred location of nitrogen tends to fall or adsorb on the zigzag-shaped nanotube edge to create a curvature defect and mediate the growth of CNTs. However, the increasing content of nitrogen at the tube edge or tube end leads to saturation and results in the inhibition of nanotube growth [71], yielding short tubes with smaller diameters and often corrugated or bamboo-like structures [72–74]. Thus, the saturation of nitrogen incorporation, specifically at the tube edge, will significantly increase the curvature effect, which in turn results in a smaller tube radius. However, high nitrogen incorporation would completely inhibit the formation of nanotube structures, as the pyridine-like structure will decrease the stability of the wall [74]. A further detailed study was carried out by Ahn et al. [69] using ab initio calculations to analyze the energetics and kinetics of carbon nanotube growth with nitrogen inclusion. The calculations again showed that when nitrogen is incorporated on the growth edge, the kinetic barrier for the growth can be reduced to zero [69] and the growth rate of a zigzag-type edge can be increased, resulting in the formation of curved nanotubes. Sumpter et al. [71] explained that, with the corrugation of nanotube, the generation of pentagon defects from the chemical bonding of graphene and nitrogen would cause inward bending of the structure toward a defected graphitic dome, causing closure of the tubes [75]. However, as more nitrogen atoms are incorporated, the tube must reopen and start growing [71], resulting again in a bamboo-like configuration [76]. A bamboolike structure is commonly seen in multiwalled carbon nanotubes [77] and it forms a more energetically stable nanotube structure for many applications. 3. Effects of growth parameter in nitrogen-doped CNT growth

Fig. 6. (a) Energy difference between the N-CNT isomers versus number of carbon atoms (b) Position of nitrogen [66].

Incorporation of nitrogen into CNT as the electrode material is believed to provide the advantage of enhanced oxygen reduction reactivity in PEMFC due to the presence of an extra lone pair of electrons, which facilitates the reductive O2 adsorption [78]. However, heteroatomic doping of graphitic carbon lattices with nitrogen

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Fig. 7. Schematic representation of possible N\C bonding configurations with a nitrogen dopant atom inserted in a graphitic network: (a) substitutional N-3sp2C C− 1 N1; (b) pyridine-like N-2sp2C C− 1 N1; (c) pyridine-like N-2sp2C C− 2 N1; (d) bridge-like N-4sp3C C− 2 N1; (e) N-3sp3C C− 2 N1 [69].

In the process of CNT or N-CNT growth, pretreatment with catalysts, such as Fe, for CNT formation is vital to control the nanoparticle catalyst distribution and the resulting nanotubes [82]. Terrado et al. reported the effects of the pretreatment time, temperature, catalyst

film thickness and the reductive gas environment on the nanoparticle distribution [83]. According to Terrado, it is necessary to form high density catalyst nanoparticles to aid the growth of a densely packed CNT forest. It was shown that hydrogen present solely as a reductive gas could produce denser and smaller nanoparticles, as a hydrogen atmosphere may favor film breakage by reducing metal oxidation, which may increase the atom mobility and contribute to the formation of smaller particles [83]. This is in good agreement with Ting et al., who obtained a smaller diameter (41 nm) for the H2-pretreated Fe catalyst compared to the NH3-pretreated Fe catalyst (53 nm) [84]. However, there exists contradiction from earlier studies that

Fig. 8. (Top panel) Chemisorptions-type N incorporation at reconstruction site corresponding to point B in Fig. 4(a); (Bottom panel) N substitution at the reconstruction site corresponding to a point beyond C in Fig. 4(a), representing a large diameter tube [70].

Fig. 9. (a) Strain energy of a curved graphene sheet with and without a vacancy as a function of curvature (b) Surface reconstructions formed at the vacancy site corresponding to points B (left) and C (right) in (a). Fixed edge carbon atoms are black [70].

has a great effect on the physical properties [79] such as the hardness of the doped CNT [30]. Nitrogen doping involves a combination of various activation conditions, primarily the reactant gas composition [30, 80–81], reaction time and temperature [30]. 3.1. Influence on growth of metal particles

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CNTs could be obtained only when NH3 was used as a pretreatment gas with a Ni catalyst [85]. The reason given was that the nitride layer formed from ammonia would suppress the passivation of the catalyst and enhance the growth of CNTs. In another study, it was shown that NH3 is critical to the alignment of nanotubes at pretreatment temperatures of 800–950 °C [86–88]. Exposure to higher temperatures would result in the coalescence of neighboring nanoparticles and thus result in larger diameters [83]. Terrado et al. has identified the optimal temperature and time for catalyst nanoparticle formation to be 750 °C for 30 min [83]. The growth of CNTs as a function of catalyst film thickness was reported by Dahn and Liu [89], where the CNT height plateau is reached when the Fe film thickness is approximately 2.2–3.5 nm and under a flow of ethylene, whereas there is no significant CNT growth with film thicknesses below 1.2 nm. This finding was similar to that of Bronikowski [90], who reported a CNT height plateau at Fe film thicknesses between 2 and 3 nm. Although many articles have discussed the optimization of growth catalysts for CNT formation, very few experimental studies evaluated the effect of catalyst selection on N-CNT formation [13,91]. Dommele et al. [91] conducted experiments using different precursors for N-CNT growth with Fe, Co or Ni catalysts at 923 K. It was found that N-CNTs grown from iron appeared to have a bamboo-like shape [92] regardless of the precursor and growth temperature. In contrast, the morphology of N-CNTs obtained with Ni or Co catalysts showed straight tubes with compartments that had shapes like stacked cups. Interestingly, this finding was correlated with the thermodynamic stabilities of the different metal-carbides, in which a ‘pulsating’ growth from Fe might result from the greater stability of Fe-carbides compared to Ni-carbides and Co-carbides [92]. Despite these findings, there are works showing that Co could provide better catalysis of the formation of N-CNTs. However, to our knowledge, Fe is more commonly used for N-CNT growth. 3.2. Influence of reactant gas and its nitrogen composition Optimization of the reactant gas composition for the formation of nitrogen-doped CNTs is essential for effective enhancement of the electrochemical activity in fuel cells. Many methods have been used to produce N-CNTs with controllable compositions [80,84,93]. CVD-based processes are the most promising method exploited to date. There have also been attempts to use an aerosol method to control the morphology and structure of the N-CNTs as well [94], specifically based on the type of nanotubes formed and the dimensions of the nanotubes. This section provides an overview of the influence of the type of reactant gas and the nitrogen composition on the N-CNTs formed. Studies on the influence of the nitrogen precursor's gas feed composition on the nitrogen content in CNTs have been widely reported [30,41,63,80–81]. XPS characterization shows a linear increase in the nitrogen doping of N-CNTs with NH3 feed stream composition [30,80–81]. Nevertheless, nitrogen doping level shows a slight depression at approximately 15% NH3 composition in the presence of a pyridine precursor [30]. It was presumed that this effect is due to the gasification of carbon, which causes an inhibitory effect. Similar trends were found by Shalagina et al. [95], who further explained the NH2∙ radicals that are generated from the decomposition of ammonia would attack the formed CNFs and thus hasten their gasification. A linear correlation was observed for the relationship between reduction character and the amount of pyridine-like nitrogen in the nanotubes. Although studies have reported that such a structure is responsible for oxygen reduction activity, such influences have not been directly correlated to date [39]. Nxumulo et al. [96] studied the effect of varying the N-source concentration on the production of Ndoped CNTs in a CVD reactor. Using ferrocene/aniline (or melamine [73]) as a precursor, it was found that compartmentalized N-doped CNT with a bamboo-like structure were formed. XPS data revealed the N observed at high aniline concentration (25%) to be sp 3

hybridized N (corresponding to quaternary nitrogen); however, at low concentrations of approximately 15% aniline, only an sp 2 signal (corresponds to pyridine-like nitrogen) was seen [73,96–97]. Recent studies by Koos et al. used benzaylamine as a nitrogen precursor, and showed a decrease in growth rate with an increasing precursor amount due to nitrogen saturation at the growth end of the tube edge [98]. Liu et al. [41,80] adapted melamine as a nitrogen precursor and used ethylene as carbon precursor for the growth of NCNTs and reported an increase in growth rate with increasing melamine. It was claimed that the atomic nitrogen introduced by melamine would abstract considerable amounts of hydrogen atoms from ethylene and thus retard the etching of carbon growth and enhance the growth rate of nanotubes [41,80], which is supported by Lee et al. [93]. Liu et al. [80] also concluded that increasing the amount of melamine would reduce the wall thickness to below 10 nm, where the nanotubes growth rate increases to 14 μm min − 1 when 2000 mg melamine is utilized. With increased melamine dosage, bamboo-like structures became more apparent. Raman spectroscopy has shown an increase in the intensity ratio of the D-band to the Gband with increases in melamine amounts, implying that there are more defects and disordered structure on the surface of the CNTs. Regarding structure formation, XPS measurements have shown that increases in nitrogen concentration would give rise to an increase of N\sp 3 bonds that degrade the crystallinity of the bambooshaped CNTs [97]. In addition to the nitrogen composition and the use of different precursors to study N-CNTs formation, a newly published paper by Nxumalo et al. [99] has surprisingly shown that different isomers of methylimidazole as precursors for N-CNT formation provide tubes with different individual bamboo compartment distances and different morphologies and characteristics, including N content. Thus far, there is no standard rule governing the whole synthetic process that enables us to compare all of the different studies, as different precursors and compositions will lead to different physio-chemical properties of N-containing CNTs. Hence, researchers are still striving to find the best precursor and reactant composition in order to produce the most stable and active nitrogen-containing CNTs to be used in various applications.

3.3. Influence of reaction temperature and synthesize duration The most commonly used synthetic method for generating NCNTs by CVD uses the temperature range of 700–1100 °C, depending on the precursors and growth catalyst involved [95]. This section will give a thorough review of the effect of synthesis temperature and reaction duration on the N-CNTs formed. Many groups have reported that an elevation in temperature reduces the amount of nitrogen incorporated into the CNT during the growth process [91, 99–101]. Tang et al. [101] studied the reaction temperature and N concentration within carbon nanotubes using the CVD aerosol method with dimethylformamide as the N precursor. It was suggested that the precursors, which initially contain = C-N units linked by sp 2 carbons, may be catalytically converted to sp 2 bonded CNx nanotubes. Hence, a high reaction temperature would lead to breakage of the C\N bond and to a decrease of the nitrogen content in the nanotubes. Tang et al. [101] has also shown the chemical instability of nitrogen-incorporated CNTs after exposure to air for 1 week, which resulted in nitrogen loss. Dommele et al. [91], on the other hand, correlate the above situation with the thermodynamic stability. The formation of metal carbides is more stable than the formation of metal nitrides at higher temperatures, which in turn results in low nitrogen incorporation at higher growth temperatures. Kim et al. [46] correlate this phenomenon with the slower diffusion rate of the nitrogen species compared to the carbon species during high temperature synthesis of CNF where the participation of the carbon species becomes dominant at the critical temperature of 640–680 °C.

W.Y. Wong et al. / Diamond & Related Materials 22 (2012) 12–22

There is no actual theory relating such occurrences because of many factors, including the precursors used [101] and the kinetics or thermodynamics [91] of the precursors that might influence the N-CNT formation process. It is worthwhile to note that an increase in growth temperature causes the pyridine-like nitrogen/quaternary-like nitrogen ratio to decrease [91]; such a decrease commonly correlated with a reduced catalytic activity due to fewer active sites for the oxygen reduction reaction in fuel cells [40]. However, various groups [41,43–44,47] have shown that quaternary-like nitrogen would greatly affect the catalytic activity the most within the nanotubes. Discussions of such influence on N-CNT formation have been vague, and there were even fewer articles discussing the effect of synthesis duration. Shalagina et al. [95] studied the effect of the reaction time based on the reaction temperature and ammonia concentration on the ethylene conversion of the 65Ni–25Cu–Al2O3 catalyst for nanofiber formation. It was shown that the conversion of ethylene abruptly dropped from 100% to 10% after 2–6 h of reaction, and this effect was claimed to be result of the deactivation of the reaction mixture. Increasing the reaction temperature and ammonia concentration in the feed accelerates the deactivation process. Higher temperatures would increase the metal particles' mobility and may thus cause agglomeration, generating larger particles as well as graphene shells and blocking the growth of CNFs during the decomposition of ethylene. Higher ammonia concentrations cause extremely fine dispersion of metal particles, resulting in the formation of relatively small particles compared to the critical size for N-CNF nucleation that leads to earlier deactivation. Hence, tailoring the temperature and synthesis duration could produce carbon nanotubes that are optimal in all aspects. 4. Comparison between synthesis methods for nitrogen-containing carbon nanostructures and the ORR activity In PEMFC, the real purpose of nitrogen inclusion into the carbon support is to improve the electrocatalysis at electrodes. It was proven experimentally that solely nitrogen-doped carbon nanostructures without the presence of metal could promote the oxygen reduction reaction (ORR), giving activity comparable to that of commercialized Pt/C catalyst at the cathode in PEMFC [31,40,88,102–105]. The discussion herein focuses on the efforts conducted by researchers to

19

develop nitrogen-doped carbon nanostructures with high electrocatalytic activity for oxygen reduction reaction. Table 1 shows a summary of the previous research done on this subject. A comparison of the ORR activity in basic medium between nondoped CNFs and N-doped CNFs was carried out by Maldonado and Stevenson [39] using ferrocene as a growth catalyst with either xylene or pyridine as a precursor. A positive potential shift for Ndoped CNFs (approximately 70 mV) in voltammetric responses implies that this process is more kinetically facile for O2 reduction. The electrochemical data of that study indicated that the ORR proceeds via 2e − reduction to produce peroxide ions, but it would also act as a regenerative process with decomposition to oxygen. This hypothesis concerning the catalytic effect of nitrogen active sites in carbon nanostructures was proven. Matter et al. examined the effects of different metals and their supports on the surface morphology of NCNFs and their ORR activity [88,106]. The structures formed are similar regardless of the support used. Fe- and Co-catalyzed samples show the highest ORR activity. Calculations from RDE data revealed that Fe-catalyzed samples completely reduced O2 to water [88, 106] because of the high value of n ≈ 4.0 (n is an indicator for number of electrons transferred in the mass transfer limited region [62]). Although the Co-catalyzed samples [107] showed high activity, their selectivity towards water formation was rather low. This is because Co is part of an active site for hydrogen peroxide formation. Wei et al. [108] showed an increase in ORR activity when a small amount of cobalt was incorporated into the nitrogen-containing carbon structure, but the mechanism of the oxygen reduction was not clear but there is the possibility of a chemical interaction between nitrogen, carbon and cobalt. Gong et al. [102] revealed that the vertically aligned nitrogen-containing carbon nanotubes (VA-NCNTs) produced by pyrolysis of iron (II) phtalocyanine could be an effective ORR electrocatalyst, even after complete removal of residual Fe. The sample showed much higher electrocatalytic activity, lower overpotential, a smaller crossover effect and better long-term stability than commercially available platinum-based electrodes. In fact, many studies have shown that despite the loss of metal during high-temperature pyrolysis, the ORR activity is not reduced [80,109–110]. Thus it was deduced that the metal is used to catalyze the nitrogen incorporation into the carbon material.

Table 1 Nitrogen-doping methods, nitrogen concentration on carbon nanostructure materials and their corresponding ORR activity. No. Catalyst Precursors

Preparation method

1

N-CNT

CVD on silicon wafer at 800 °C. 2.70–2.91

%N

ORR performance

Reference

Fc-NCNT shows lower overpotential than FePc-NCNT with respect to ORR. Higher H2O selectivity in Fc-NCNT than FePc-NCNT

[47]

2

N-CNF

1.5

ORR peak potential of 555 mV compared to 750 mV for 20wt.% Pt/Vulcan carbon catalysts.

[40]

3

N-CNF

3.0 (for Fe catalyzed sample) 4.0

ORR peak potential (Fe catalyzed samples) of 785 mV compared to 880 mV for commercial 20% Pt/VC catalysts. H2O selectivity decrease in order of Fe, Co, Ni catalyzed sample. N-doped CNF electrodes show significant catalytic activity towards ORR in neutral to basic pH. About 100 fold enhancement for H2O2 decomposition for N-doped CNFs. Exposed edge plane defects and nitrogen doping enhanced ORR activity. Higher reduction potential for N-CNT (lower overpotential) of about − 0.15 V, with current density of 4.1 mA cm− 2 compared to 1.1 mA cm− 2 for Pt/C electrode OCV 0.78 V and maximum power density 0.21 W cm− 2 during fuel cell test for 3% CoPc at 1000 °C (synthesis condition), about 2/3 of commercialized 10% Pt/C catalyst. In alkaline medium with 0.1 M potassium hydroxide concentration, the current density is 4.71 mA cm− 2 at − 0.4 V using RRDE test. ORR through 4e− transfer.

[87]

4

N-CNF

5

Pyridine, ferrocene/ FePc Acetonitrile, iron (II) acetate Acetonitrile, Fe/Ni/Co particles Pyridine, ferrocene

Pyrolysis on quartz boat with Vulcan carbon as support at 900 °C. Pyrolysis at 900 °C on silica/ MgO support.

N-CNT

NH3, FePc

Pyrolysis

4.0–6.0

6

N-CNS

Furan resin, Fe/Ni/Co-Pc

Carbonization at 600–1000 °C for 1 h.

1.0–4.0

7

N-CNT

N-CNT

9

N-CNT

Floating catalyst CVD on carbon paper substrate at 950 °C Floating catalyst CVD on silicon wafer at 950 °C for 15 min. CVD at 800 °C

8.4

8

Ethylene, melamine, ferrocene Melamine, ethylene, ferrocene Acetonitrile, Co/Al

Floating catalyst CVD method at 800 °C.

1.4–7.7

3.0

Higher nitrogen content in NCNT gives higher ORR. NCNT with 7.7%wt nitrogen content shows comparable onset current density to Pt-based catalyst in alkaline solution using RRDE test. NCNT shows 4 electron transfers in acidic and alkaline media. Catalytic activity of NCNT in alkaline media is higher than in acidic media.

[39]

[101] [32]

[102]

[103]

[104]

20

W.Y. Wong et al. / Diamond & Related Materials 22 (2012) 12–22

On the other hand, Ozaki and coworkers [32] compared nitrogencontaining nanoshells prepared with metal phtalocyanines to those prepared with metal-acetylacetones. The ORR activities are higher for nanoshells prepared with the former metal catalyst. A TEM study showed that the Co-catalyzed sample carbonized at 800 °C has the highest ORR activity. From TEM micrograph, incompletely developed graphene stacking around the cobalt particles, which created many defects and exposed edge sites [88,106] was seen on the sample that is said to be responsible to the increase in catalytic activity. The hypothesis of active sites caused by the co-existence of N4-metal structure was rejected, however, as it was shown in the experiment that the carbons prepared with non-nitrogen complexes also showed enhanced catalytic activities. Recently, Chen et al. [47] examined the effects of using catalysts containing the same metal atom but different molecular structures of ferrocene (Fc) and iron (II) phtalocyanines (FePc) on surface structure and ORR activity. TEM characterization showed that the Fc-NCNTs had more rugged surfaces and thinner graphitic walls compared to FePc-NCNTs. RRDE analysis shows a 2-step reduction process to produce water, which is in agreement with results of Zhang et al. [111], where selectivity towards water production was much higher for Fc-NCNTs. Hence, tailoring the surface structure of NCNTs using catalysts with different structures improved ORR activity. Despite these findings, many groups are striving to produce novel N-CNTs with high nitrogen content. It is noteworthy, as seen in the summarized table below, that the average percentage of nitrogen incorporated into the carbon nanomaterials is approximately only 4.0 at.%. Possible over inclusion of nitrogen would cause saturation on the carbon nanotubes, resulting in the formation of unstable [112] pyridine-like structure with subsequent structure collapse or inhibition of nanotube formation, as described by Villalpando-Paez et al. [74]. Table 1 shows clearly that most nitrogen-containing carbon nanostructure growth is catalyzed by iron metal. Iron-catalyzed CNTs have been shown to exhibit bamboo-like structures [71–73,77,91–92] with enhanced stability due to compartmentalization [76]. Other important information reported in this table is that the precursors used, such as pyridine, acetonitrile and ammonia, are in fact common ligands [113] that coordinate with the metals. They are all monodentate ligands that are simple Lewis bases [113]. Perhaps by examining similar properties of Lewis bases, novel nitrogen precursors could be found that would produce high ORR activity in an NCNT electrode for suitable use in fuel cell technology. Recently, the use of melamine has attracted great attention among researchers in N-CNT synthesis due to its considerably high stability [47,72,76,80]. It is therefore suggested that suitable candidates for nitrogen precursors may be identified by exploring the use of Lewis bases of nitrogen-containing aromatic compounds.

determined that even the best catalyst that produced comparably small amounts of peroxide relative to Pt/C (approximately 3% at 0.5 V vs. RHE) still showed significant instability. Attempts to utilize heat treatment to improve the stability seem effective, but the material showed poor catalytic activity as a trade-off [13]. In combination with the growth parameter, it is vital to optimize these synthesis conditions to obtain the optimal activity and stability, which is being conducted actively by many researchers at present. Dodelet and coworkers [112] have compared the stability and activity when NH3 and Ar are used as carrier gases in the pyrolysis of Cl-FeTMPP, where the former gas is used as a nitrogen precursor together with Cl-FeTMPP. The polarization curves showed that pyrolysis in NH3 produced an active but unstable catalyst, while in the latter case; the catalyst showed more activity but was less stable. This finding is worth noting although the study did not emphasize CNT synthesis. The study provides information on the influence of the nitrogen sources on the stability of the CNT formed. It should be noted as well that at higher concentrations of NH3, the N-CNTs produced are less wellaligned due to defects in nitrogen incorporation [84], as shown in Fig. 10, which could have implications for stability. A recent study by Feng et al. [116] has surprisingly displayed better electrocatalytic activity and stability than Pt/C in neutral pH electrolyte. It was claimed that the ORR of N-doped CNT follows the fourelectron pathway, negligible HOO − oxidation was recorded by RRDE and no significant loss in electrochemical activity was observed after a continuous potentiodynamic sweep for 60,000 cycles at room temperature. There seems to be a consensus relating the ORR pathway and the stability of the catalysts. From our point of view, the incorporation of a metal binder into NCNTs could help to enhance the stability of the electrode, as described through the first principle calculation. The reason is that the N-CNTs possess a high amount of surface nucleation sites that allow for anchorage and high dispersion of the metal on the support surface

5. Stability issues of nitrogen-containing CNTs in PEMFC Despite the extensive work on proton exchange membrane fuel cells since the early 1990s, there remains some unresolved issues, such as the high manufacturing cost and low stability. The use of CNTs has significantly improved the stability of Pt-based catalysts in comparison to the use of conventional carbon black support. It is claimed that CNTs show a greater graphitization degree and exhibit higher electrochemical corrosion resistance in comparison to Pt/C, which enhances the stability of Pt/CNT. The discovery of nitrogen-doped CNTs as alternative candidates for ORR electrocatalysts, has made the stability issue more critical. As explained earlier, non-noble metal catalysts often catalyze a twoelectron process of O2 reduction, producing H2O2 as a side product [13]. It was suggested by Schulenburg et al. that catalyst active sites could be degraded by the oxidation of nitrogen atoms by H2O2 [114]. However, Dodelet [115], in an attempt to correlate the amount of peroxide released and the stability using an Fe-based catalyst,

Fig. 10. CNTs grown on NH3 treated catalysts at C2H2/H2/NH3: (a) 20%:80%:0% (b) 20%/ 0%/80% [84].

W.Y. Wong et al. / Diamond & Related Materials 22 (2012) 12–22

[35]. This is demonstrated in the reports regarding the role of nitrogen functionalities on increasing the stability of Pt or Pt-Ru supported on N-doped carbon materials [117–118]. However, whether or not metal and nitrogen will both serve as active sites due to the incorporation of the binder, or even whether either metal or nitrogen will play a role in the oxygen reduction reactivity, remains unclear. A change in the reaction mechanism may also take place due to these changes as proposed by Wiggins-Camacho and Stevenson [42] in their recent report. Moreover, it was shown earlier that the longterm exposure of metal to air could corrode the electrode [119–121] subsequently reducing the efficiency of the fuel cell. This may be another issue that needs to be closely monitored during the search for a novel organic catalyst with good stability, good catalytic activity and long-term durability. 6. Conclusion Nitrogen incorporation into carbon nanotubes was shown to be an effective organic catalyst in PEMFC, providing a long-term solution to the synthesis of these catalysts by replacing the scarce and expensive platinum cathode. Further investigation on achieving targeted ORR activity (reduction in overpotential below 300 mV, as is standard for the Pt/C catalyst) and high stability for long-term use in PEMFCs are of the utmost importance at the current stage. Improvement on the activity and stability of the catalyst can be achieved through tailoring the synthesis conditions and compositing with active metal nanoparticles. More advances in theoretical studies should be employed to understand the nanoscale level of electronic activity within the nitrogen-carbon nanostructures that could aid in the enhancement of ORR activity in PEMFCs.

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