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Facile synthesis of graphene/N-doped carbon nanowire composites as an effective electrocatalyst for the oxygen reduction reaction Kadumudi Firoz Babu, Balasubramaniyan Rajagopalan, Jin Suk Chung*, Won Mook Choi** School of Chemical Engineering, University of Ulsan, Ulsan 680-749, Republic of Korea
article info
abstract
Article history:
A novel metal-free electrocatalyst for the oxygen reduction reaction (ORR) is one of the
Received 25 February 2015
most important issues in fuel cells. Here, we report a facile method to synthesize reduced
Received in revised form
graphene oxide (rGO) decorated with nitrogen-doped carbon nanowires (rGO-CN) as an
30 March 2015
electrocatalyst for ORR. After the polymerization of polpyrrole nanowires on the rGO
Accepted 2 April 2015
surface (rGO-PPy), the carbonization of rGO-PPy at 800 C affords a unique nanostructured
Available online 22 April 2015
product by the integration of rGO sheets and the N-doped carbon nanowires with high nitrogen content. The morphology of rGO-CN is confirmed by TEM analysis and the
Keywords:
chemical composition and interaction of the prepared samples are analyzed by XPS and
Graphene
FT-IR analysis. The electrocatalytic activity of rGO-CN toward ORR is also evaluated by the
Carbon nanowires
cyclic voltammetry. It is found that the rGO-CN electrode shows superior electrocatalytic
Electrocatalyst
performance toward ORR, compared to rGO and rGO-PPy, which demonstrates the prom-
Oxygen reduction reaction
ising potential of rGO-CN as a carbon-based, metal-free electrocatalyst for enhancing the electrocatalytic property towards ORR. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Fuel cells are considered a potential power source for future vehicles and stationary applications due to their high efficiency, high power density, and clean energy. The performance of fuel cells is largely limited by the sluggish oxygen reduction reaction (ORR), which substantially depends on the activity of the cathode catalyst. Platinum (Pt) and its alloys have long been recognized as the best electrocatalyst for ORR in fuel cells [1]. However, high cost due to the scarcity of Pt and
its insufficient durability limited the mass production and commercialization of electrocatalysts. Thus, considerable effort has been devoted to developing highly stable and cost effective non-precious metal or metal-free electrocatalysts with improved catalytic efficiency for ORR [2,3]. Recently, carbon nanomaterials and their composite materials have been intensively studied for replacing or reducing Pt based materials in fuel cell applications [4,5]. Among them, heteroatom-doped carbon materials for ORR have also been extensively investigated to reduce the cost and enhance the durability of fuel cells [4].
* Corresponding author. Tel./fax: þ82 52 259 2249/1689. ** Corresponding author. Tel./fax: þ82 52 259 1065/1689. E-mail addresses: [email protected] (J.S. Chung), [email protected] (W.M. Choi). http://dx.doi.org/10.1016/j.ijhydene.2015.04.002 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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Graphene, a one-atom-thick layer of graphite, packed into a two-dimensional network of sp2-hybridized carbon, is distinct from other types of carbon materials due to its outstanding thermal, mechanical and electrical properties [6,7]. Since graphene recently served as a worthy support material for anchoring metal/metal oxide nanoparticles and functional nanomaterials [8,9], graphene-based functional composites have been explored as ORR electrocatalysts because of the feasible synergetic effects with enhanced catalytic activity [10,11]. To date, doped graphene with various heteroatoms, such as sulfur (S), nitrogen (N), boron (B), and phosphorous (P), has attracted intense attention due to its reportedly high catalytic activity, high long-term durability and tolerance to poisoning as a metal-free ORR electrocatalyst [12e16]. For example, N-doped graphene, generally consisting of pyridinic N, pyrrole N and graphitic N atoms, has been widely studied as an electrocatalyst for fuel cells, due to its considerable electrochemical activity of ORR with an excellent durability [15,17]. However, its electrochemical activity is strongly affected by N atom content, the limited N content of 2e5% in N-doped graphene still revealed lower catalytic activity compared to Pt-based catalysts [18]. To utilize the outstanding properties of graphene and improve the catalytic efficiency towards ORR, the efficient nanostructure design of graphene-based composites with high content of CeN functional groups is of great interest [19e22]. Herein, we report the facile synthesis of graphene decorated with N-doped carbon nanowires as an efficient ORR electrocatalyst. The proposed method here involves the formation of polypyrrole (PPy) nanowires decorated on reduced graphene oxide (rGO-PPy) by in situ polymerization of pyrrole monomer in the presence of rGO. Subsequently, the synthesized rGO-PPy is annealed at 800 C in an argon atmosphere to afford the N-doped carbon nanowires decorated on rGO (rGOCN) by the carbonization of PPy nanowires. The unique nanostructure of graphene sheets decorated with conductive N-doped carbon wires provides high nitrogen content and high surface area for large amount oxygen access, which improves the electron transfer efficiency and the electrocatalytic activity.
Experimental Synthesis of rGO-CN First, graphene oxide (GO) was prepared by chemical exfoliation of the expanded graphite powder (grade 1721, Asbury Carbon) through a modified Hummers method by the microwave-assisted thermal expansion of graphite powder using the procedure described elsewhere [23]. The obtained GO powder was dispersed in deionized (DI) water for the GO aqueous solution with a concentration of 3.0 mg/ml. The GO solution was then treated with hydrazine hydrate (5 mL) at 40 C for 24 h to obtain rGO. After reduction, rGO was filtered and washed several times with DI water and dried in a vacuum oven. The prepared rGO were redispersed in DMF for rGO solution with a concentration 0.5 mg/ml. To synthesize PPy nanowires on rGO (rGO-PPy), in situ polymerization was performed in the presence of rGO using
pyrrole monomer (0.01 M), ferric p-toluene sulfonate (0.02 M) as an oxidizing agent and 1% polyvinyl pyrrolidone (PVP) as a surfactant at room temperature for 5 h. Then, the resultant composites were washed with ethanol and DI water several times and dried at 60 C in a vacuum oven. The dried rGO-PPy sample was annealed at 800 C under argon atmosphere for 1 h, resulting in rGO-CN by the carbonization of PPy nanowires.
Characterizations The morphology of the samples was characterized using a fieldemission scanning electron microscope (FE-SEM, JEOL, JSM6500F) and a high resolution transmission electron microscope (HR-TEM, Hitachi, H-8100). X-ray diffraction (XRD) analysis was performed on a Rigaku X-ray diffractometer with Cu Ka radiation. The chemical compositions of the prepared samples were analyzed using an X-ray photoelectron spectrometer (XPS, Thermo scientific) with monochromatic Al Ka radiation with hv ¼ 1486.6 eV. Fourier transform infrared (FT-IR) spectra were obtained using a Nicolet IR 200 FT-IR spectrometer (Thermo Scientific). Raman spectra (DXR Raman spectrophotometer, Thermo Scientific) were obtained using 532 nm laser excitation.
Electrochemical measurements Electrochemical measurements of the prepared samples were performed on a multi-channel potentiostat-galvanostat analyzer (VSP, Bio-logic) with a three-electrode system at room temperature, in which Pt wire was used as counter electrode and saturated calomel electrode (SCE) as reference electrode. As the working electrode, the electroactive materials in ethanol (1 mg/ ml) and Nafion in isopropyl alcohol solution (0.5 wt%) as binder were mixed by sonication. Then, 5 ml of this mixture was coated on a glassy-carbon rotating disk electrode (RDE) and dried in air for the electrochemical characterizations. The electrochemical measurements using the modified glassy-carbon electrode were carried out in the O2-purged 0.1 M KOH electrolyte solution at room temperature. The number of electrons transferred per oxygen molecule in the ORR process was determined by the KouteckyeLevich (KeL) equation given below: 1 1 1 ¼ þ j jk Bu0:5 where jk is the kinetic current density and u is the angular velocity of the electrode. B could be determined from the slope of the KeL plots based on the following Levich equation: 2 1 B ¼ 0:2 nF DO2 3 v6 CO2 where n is the number of electrons transferred per oxygen molecule, F is the Faraday constant (96485C mol1), DO2 is the diffusion coefficient of O2 in 0.1 M KOH (1.9 105 cm2 s1), v is the kinetic viscosity (1.0 102 cm2 s1) and CO2 is the bulk concentration of O2 in 0.1 M KOH (1.2 106 mol cm3) [24,25].
Results and discussion A schematic illustration of the fabrication process of the rGOCN is shown in Fig. 1. In a typical procedure, after rGO was
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prepared through the reduction of GO, pyrrole was polymerized in the presence of rGO at room temperature using ferric ptoluene sulfonate as oxidizing agent and polyvinyl pyrrolidone (PVP) as surfactant, resulted in rGO-PPy composites. The prepared rGO-PPy was then annealed at 800 C in an argon atmosphere for 1 h, after which the rGO sheets were fully reduced and the PPy nanowires were transformed into the carbon nanowires by the carbonization. Moreover, the high temperature annealing of N atoms in PPy synthesized the Ndoped carbon nanowires which could then serve as the effective electrocatalyst for ORR. A typical SEM image of the rGO in Fig. 2a shows that the smooth thin sheet structures with wrinkles and folds of the rGO surface are observed due to the flexibility of graphene sheet, which is general morphology of the restacked rGO layers after the reduction. Additionally, after the formation of PPy nanowires and the subsequent carbonization of PPy (Fig. 2b and c), it is observed that the sheet structure of rGO was maintained without further restacking of rGO layers and the uniform deposition of PPy and carbon nanowire are observed on the graphene surface after polymerization and subsequent carbonization. To examine the clear morphology of the deposition of PPy nanowires and carbon nanowires on rGO, TEM was performed as shown in Fig. 2def. The TEM image of rGO sheets (Fig. 2d) shows very clear small folds and wrinkles as shown in the FE-SEM image (Fig. 2a). PPy was polymerized on the rGO surface by adding PVP as the surfactant, which facilitates the formation of PPy nanowires. Fig. 2e shows that the synthesized PPy nanowires were uniformly formed on the rGO surface with a diameter of 17e18 nm. The rGO/PPy nanowire composites were formed owing to p- p interaction between the sp2-carbon atoms of 2D graphene and the electronic structures of the conjugated backbones of PPy. TEM image of rGO-CN in Fig. 2f clearly shows the formation of carbon nanowires on the rGO surface with similar nanostructures after the carbonization process of rGO-PPy, and the diameter of the nanowires was reduced to approximately 13e14 nm. The chemical structures of rGO-PPy and rGO-CN were investigated using FT-IR spectroscopy. Fig. 3a shows the FT-IR spectra of GO, rGO, rGO-PPy and rGO-CN. The characteristic absorption bands of GO observed in FT-IR spectra, which corresponded to the OeH stretching of water as a broad band at 3396 cm1, C]O stretching vibration at 1727 cm1, CeO stretching at 1361 cm1, CeOH stretching at 1228 cm1 and CeO (alkoxy) stretching at 1063 cm1 [26e29]. The peak appearing at 1621 cm1 was assigned to the vibrations of the adsorbed water molecules. FT-IR spectrum of rGO shows the
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retained presence of some OH functional groups at the edges of the rGO sheets, implying the partial reduction of GO by hydrazine hydrate treatment. For rGO-PPy, the characteristic peak located at 1486 cm1 is assigned typical PPy ring vibration, and the peaks at 1220, 1043 and 936 cm1 were assigned to CeN stretching, symmetrical CH in plane bend and CeH ring deformation, respectively. Further broad peak at about 3400 cm1 is attributed to the overlap between NeH stretching of PPy and OeH stretching of rGO. Meanwhile, the peaks at 1647 cm1 is due to the C]O group present in the PVP. These results suggested that PPy nanowires are indeed formed on the rGO surface via interaction such as pep stacking between them [30,31]. In the case of rGO-CN, the peaks were observed at 1572, 1486, 1216 and 985 cm1, corresponding to C]C stretching, C]N stretching, CeN stretching and CeC stretching, respectively. Meanwhile, most of oxygen functional groups had disappeared, implying that all the oxygen functional groups in rGO and PPy nanowires had been converted into N-doped carbon nanowires through the carbonization of PPy nanowires at 800 C. The crystallographic characterization of the samples was examined using XRD analysis. Fig. 3b shows the XRD curves of the rGO, rGO-PPy and rGO-CN. The synthesized rGO exhibits three diffraction peaks centered at 2q of 25.0 , 43.4 and 78.3 corresponding to (002), (100) and (110) planes of graphite, respectively. The d-spacing value from (002) plane in rGO corresponds to 0.35 nm, which higher than the reported dspacing of 0.34 nm of graphite. The broadening and peak shifting compared to graphite indicates the formation of short range ordered stacks in rGO. The slightly larger interlayer spacing than that of graphite results from the residual oxygen functional groups or other structural defects in the rGO sheets [32]. The pattern of rGO-PPy exhibits a broad diffraction peak at 2 ¼ 24.2 for the calculated d-spacing of 0.37 nm, which indicates the pep stacking distance between PPy and rGO sheets [31]. Furthermore, two peaks about at 43 and 78 almost disappear for rGO-PPy, suggesting that the PPy nanowires and rGO have interacted completely. After the carbonization of PPy, the XRD pattern of rGO-CN exhibits the prominent peak located at 25.1 with a calculated d-spacing value of 0.36 nm and a small peak at 43 , which is attributed to the transformation of PPy nanowires to carbon nanowires and the complete removal of oxygen functional groups of rGO sheets [33]. The Raman spectra of the GO, rGO, PPy, rGO-PPy and rGOCN samples are obtained as depicted in Fig. 4a. The Raman spectra of all graphene samples exhibit two prominent peaks at 1596 and 1344 cm1, which are assigned to G and D peaks,
Fig. 1 e Schematic illustration of the fabrication of graphene/N-doped carbon nanowire composites.
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Fig. 2 e SEM and TEM images of rGO (a and d), rGO-PPy (b and e) and rGO-CN composites (c and f).
respectively. They are related to the plane vibrations of graphene lattice and the defect sites of graphene lattice, respectively. To investigate the degree of disordered graphite, the D/ G intensity ratios (ID/IG) are calculated and compared. The ID/ IG ratio for GO and rGO are 0.84 and 0.95, respectively, which is typical behavior after the reduction of GO by hydrazine. The broad D and G peaks for rGO-PPy are observed after the polymerization of PPy nanowires on rGO surface, suggesting that the size of the graphene nanocrystals decreases due to the phonon confinement [34]. Additionally, the decreased ID/IG ratio of 0.57 and the G peak is also shifted from 1596 to 1585 cm1 by the PPy introduction, due to the strong p-p interaction of PPy and graphene. The two broad peaks at 934 and 1062 cm1 for rGO-PPy appears due to two small characteristic peaks of pure PPy at 947 and 1060 cm1 related to bipolaron ring deformation and the polaron symmetric CeH in-plane bending vibration, respectively. These results confirm the successful formation of PPy on rGO. For the Raman spectrum of rGO-CN, the two characteristic peaks of PPy located at 934 and 1062 cm1 vanish and the ID/IG ratio increased to 0.99, due to the transformation of PPy to carbon
nanowires and the greater number of defects in rGO-CN, which could facilitate its electrocatalytic property towards ORR. A broad and weak 2D band was observed at 2790 cm1 associated with another peak at 2900 cm1 for rGO-CN due to the structural disorder of rGO [35e37]. This broad and week 2D band is generally found for the GO and rGO samples by the high level of disorder of the graphene layer from the oxidation process. The present Raman results show that graphene layers are well dispersed in the rGO-CN composites, which is consistent with the TEM results. The surface chemical composition of rGO, rGO-PPy and rGO-CN was further analyzed by XPS. The elements of C, O and N were observed in the survey spectrum of all samples depicted in Fig. 4b. The calculated C/O ratios for rGO and rGOPPy are 3.06 and 2.81, respectively, showing that the oxygen content in the rGO-PPy has increased compared to that of rGO. With further carbonization, the C/O ratio of rGO-CN increased to 5.45, indicating that most of the oxygen functionality has been removed in rGO-CN after high temperature annealing. To investigate the nitrogen content in the samples, the C/N ratio is calculated. It is found that the calculated C/N ratio of
Fig. 3 e (a) FT-IR spectra of GO, rGO, PPy, rGO-PPy and rGO-CN and (b) XRD patterns of rGO, rGO-PPy and rGO-CN.
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Fig. 4 e (a) Raman spectra of GO, rGO, Ppy, rGO-PPy and rGO-CN. (b) XPS survey spectra, (c) C1s spectra and (d) N1s spectra of rGO, rGO-PPy and rGO-CN.
rGO sample is 17.51 due to some CeN groups incorporated during the hydrazine reduction. The C/N ratio of rGO-PPy decreases to 4.55 due to the high N atom content in PPy, whereas the C/N ratio of rGO-CN increased to 5.60 by the removal of oxygen functionality after high temperature annealing. The C1s core level spectra for rGO, rGO-PPy and rGO-CN are presented in Fig. 4c. The C1s spectra for rGO and rGO-PPy could be decomposed into five Gaussian peaks with binding energies of 284.7 (CeC), 285.5 (CeN), 286.5 (CeO), 287.4 (C]O) and 288.9 eV (OeC]O). After the polymerization of PPy, the C1s spectra of rGO-PPy presents a weakened peak intensity corresponding to CeC, whereas the peak centered at 285.5 eV notably increases, suggesting the formation of CeN. The C1s spectrum of rGO-CN presents only four peaks with
the absence of the CeO peak, which is related to the significant decrease of oxygen functionality after the high temperature annealing. The N1s core level spectra are further analyzed as shown in Fig. 4d. The N1s spectra of rGO shows two well resolved peaks of pyrazoline groups (400.0 eV) and quartenary N (graphitic N, 401.3 eV) due to the hydrazine treated reduction, respectively [38,39]. The deconvoluted N1s spectra of rGO-PPy splits in to three peaks at, and corresponding to the pyrrolidone N (399.5 eV), pyrrolic N (400.3 eV) and quartenary N (401.3 eV), respectively, after the polymerization of PPy nanowires using the PVP as a surfactant. The N1s spectra of rGO-CN decomposes into three peaks of pyrrolic N, pyridinic N and quaternary N, which shows the different peak combination of N1s spectra, compared to rGO-PPy, by the
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Table 1 e The relative percentage of different N atoms from XPS analysis. Sample
Pyrrolic N (%)
Pyridinic N (%)
Quarternary N (%)
Pyrazoline N (%)
Pyrrolidone N (%)
rGO rGO-PPy rGO-CN
e 76.4 58.8
e e 27.9
48.2 8.1 13.3
51.8 e e
e 14.5 e
carbonization process [40,41]. These results represent the transformation of PPy nanowires into another type of carbon materials. Furthermore, it is worthwhile to note that the peaks of pyrrolic N and pyrrolidone N of rGO-PPy appears to decrease or removed, while the peaks of pyridinic N of rGO-CN are increased as shown in Table 1. The formation of appreciable amount of pyridinic N in rGO-CN is favorable for the ORR due to its superior ORR activity. Since the N-doped sites provide strong adsorption sites for reduced oxygen species (OOH) [42], the presence of a greater amount of N-doped carbon structures in rGO-CN would facilitate the enhanced electrocatalytic property for ORR. The electrocatalytic activity of rGO-CN was preliminarily evaluated by cyclic voltammetry (CV) with a three-electrode system in 0.1 M KOH solution with O2-saturated. Fig. 5a shows the CV results of rGO, rGO-PPy and rGO-CN in the potential range between 0.2 and 0.9 V vs. SCE at the scan rate of 10 mV s1. A clear cathodic peak located at around 0.45 V of rGO-CN appears in the CV from ORR. The similar cathode peaks for rGO and rGO-PPy appear respectively at around 0.44 and 0.41 V. Despite the small shift to negative potential, the electrode of rGO-CN exhibits higher cathodic current density for the ORR,
suggesting better adsorption and the reduction of oxygen molecules on the surface of rGO-CN. It is great importance in fuel cell to investigate the methanol crossover effect for electrocatalyst, since the permeation of methanol to the cathode would deteriorate the performance of cathode catalysts. The tolerance of rGO-NC towards the methanol crossover is performed in an O2-saturated 0.1 M KOH solution in the presence of methanol (10% vol) (Fig. S1). The CV curve remains unchanged after addition of methanol in to the electrolyte, suggesting a pronounced electrocatalytic activity of the rGONC for oxygen reduction. In addition, the RDE measurements have been performed at a scan rate of 10 mV s1 in O2saturated 0.1 M KOH solution for further study of the ORR kinetics. Linear sweep voltammetry (LSV) of rGO, rGO-PPy and rGO-CN at a rotation of 1600 rpm was performed and the results are shown in Fig. 5b. The ORR onset potentials of rGO, rGO-PPy and rGO-CN are determined to be at 0.17, 0.20 and 0.23 V, respectively. The higher onset potential of rGO-CN electrode compared to the other two electrodes indicates that ORR occurs more easily on the rGO-CN electrode. Moreover, the diffusion limited current at 0.8 V vs. SCE for rGO-CN is more than three times higher than that of rGO and rGO-PPy,
Fig. 5 e ORR activity of rGO, rGO-PPy and rGO-CN. (a) Cyclic voltammetry at 10 mV s¡1 and (b) RDE polarization curves at 10 mV s¡1 and 1600 rpm. (c) RDE polarization curves of rGO-CN at different rotation speeds with 10 mV s¡1 and d) the electron transfer number of rGO-CN at different electrode potentials.
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due to the higher electron accepting capability of the N-doped sites of rGO-CN. These results further confirm the higher electrocatalytic performance of rGO-CN towards the ORR. To further investigate the ORR performance of rGO-CN, LSV was performed with different rotating speeds from 100 to 2500 rpm, as shown in Fig. 5c. For comparison, the LSV curves are also obtained for rGO and rGO-PPy (Fig. S2). The LSV curves of rGO-CN shows the increase of limiting diffusion current density with higher rotating speed, since the diffusion distance is shortened at high rotating speed [24,43]. Compared to rGO and rGO-PPy, the limiting diffusion current density of rGO-CN is recorded to be higher at all rotating speeds. To qualify the ORR process, the number of electrons transferred per oxygen molecule (n) in the ORR process was determined by the K-L equation. Fig. 5d shows the n value of rGO-CN calculated from the slopes of K-L plots at different potentials. The n values of rGO-CN increase with increasing the applied potentials. At the potential of 0.8 V, the n value of rGO-CN is calculated to be 3.08, suggesting that the rGO-CN electron follows a combination of two- and four-electron transfer process. However, the rGO exhibits the n value of 1.96, indicating a two-electron transfer reaction (Fig. S3), which represents that the ORR activity of rGO-CN is significantly enhanced with the presence of N-doped carbon nanowires. The obtained n value of rGO-CN is comparable to that of other reports for the ORR activity using the various carbon based nanomaterials (Table S1). It reveal that the superior ORR activity with more efficient electron transfer have been demonstrated by the greater amount of N-doped active sites in the rGO-CN electrode.
Conclusions We have successfully demonstrated a facile method to synthesize rGO sheets decorated with N-doped carbon nanowires as an electrocatalyst for ORR. The proposed method involves the polymerization of PPy nanowires on rGO sheets, followed by the transformation into N-doped carbons nanowires derived from the carbonization of PPy nanowires. The prepared rGO-CN exhibits the high nitrogen content and a unique nanostructure afforded by the integration of carbon nanowires and graphene sheets, which facilitate the efficient adsorption of oxygen molecules for improved electron transfer efficiency and increased electrocatalytic activity for ORR. The prepared rGO-CN electrode exhibits the enhanced electrocatalyst performance of ORR, compared with rGO and rGO-PPy, which is mainly attributed to the high nitrogen content in rGO-CN, as confirmed by the XPS and FT-IR analyses. The combination of the superior electrical property of graphene and the N-doped carbon nanowires for the effective adsorption of oxygen molecules might be promising for the application of rGO-CN as a noble metal-free ORR electrocatalyst in fuel cells.
Acknowledgment This work is supported by the 2012 Research Fund of University of Ulsan.
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Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2015.04.002.
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[18] Xia B, Yan Y, Wang X, Lou XW. Recent progress on graphenebased hybrid electrocatalysts. Mater Horiz 2014;1:379e99. [19] Yang S, Feng X, Wang X, Mu¨llen K. Graphene-based carbon nitride nanosheets as efficient metal-free electrocatalysts for oxygen reduction reactions. Angew Chem Int Ed 2011;50:5339e43. [20] Li X-H, Chen J-S, Wang X, Sun J, Antonietti M. Metal-free activation of dioxygen by graphene/g-C3N4 nanocomposites: functional dyads for selective oxidation of saturated hydrocarbons. J Am Chem Soc 2011;133:8074e7. [21] Wang S, Yu D, Dai L, Chang DW, Baek J-B. Polyelectrolytefunctionalized graphene as metal-free electrocatalysts for oxygen reduction. ACS Nano 2011;5:6202e9. [22] Zhang Y, Mori T, Niu L, Ye J. Non-covalent doping of graphitic carbon nitride polymer with graphene: controlled electronic structure and enhanced optoelectronic conversion. Energy Environ Sci 2011;4:4517. [23] Trung NB, Tam TV, Kim HR, Hur SH, Kim EJ, Choi WM. Three-dimensional hollow balls of grapheneepolyaniline hybrids for supercapacitor applications. Chem Eng J 2014;255:89e96. [24] Liu R, Wu D, Feng X, Mu¨llen K. Nitrogen-doped ordered mesoporous graphitic arrays with high electrocatalytic activity for oxygen reduction. Angew Chem Int Ed 2010;122:2619e23. [25] Bard AJ, Faulkner LR. Electrochemical methods: fundamentals and applications. Wiley; 2000. [26] Stankovich S, Piner RD, Nguyen ST, Ruoff RS. Synthesis and exfoliation of isocyanate-treated graphene oxide nanoplatelets. Carbon 2006;44:3342e7. [27] Titelman GI, Gelman V, Bron S, Khalfin RL, Cohen Y, BiancoPeled H. Characteristics and microstructure of aqueous colloidal dispersions of graphite oxide. Carbon 2005;43:641e9. T, Berkesi O, De ka ny I. DRIFT study of deuterium[28] Szabo exchanged graphite oxide. Carbon 2005;43:3186e9. [29] Yang T, Liu L-H, Liu J-W, Chen M-L, Wang J-H. Cyanobacterium metallothionein decorated graphene oxide nanosheets for highly selective adsorption of ultra-trace cadmium. J Mater Chem 2012;22:21909. [30] Sivakkumar SR, Kim WJ, Choi J-A, MacFarlane DR, Forsyth M, Kim D-W. Electrochemical performance of polyaniline nanofibres and polyaniline/multi-walled carbon nanotube composite as an electrode material for aqueous redox supercapacitors. J Power Sources 2007;171:1062e8.
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Facile synthesis of graphene/N-doped carbon nanowire composites as an effective electrocatalyst for the oxygen reduction reaction Kadumudi Firoz Babu, Balasubramaniyan Rajagopalan, Jin Suk Chung*, Won Mook Choi** School of Chemical Engineering, University of Ulsan, Ulsan 680-749, Republic of Korea
article info
abstract
Article history:
A novel metal-free electrocatalyst for the oxygen reduction reaction (ORR) is one of the
Received 25 February 2015
most important issues in fuel cells. Here, we report a facile method to synthesize reduced
Received in revised form
graphene oxide (rGO) decorated with nitrogen-doped carbon nanowires (rGO-CN) as an
30 March 2015
electrocatalyst for ORR. After the polymerization of polpyrrole nanowires on the rGO
Accepted 2 April 2015
surface (rGO-PPy), the carbonization of rGO-PPy at 800 C affords a unique nanostructured
Available online 22 April 2015
product by the integration of rGO sheets and the N-doped carbon nanowires with high nitrogen content. The morphology of rGO-CN is confirmed by TEM analysis and the
Keywords:
chemical composition and interaction of the prepared samples are analyzed by XPS and
Graphene
FT-IR analysis. The electrocatalytic activity of rGO-CN toward ORR is also evaluated by the
Carbon nanowires
cyclic voltammetry. It is found that the rGO-CN electrode shows superior electrocatalytic
Electrocatalyst
performance toward ORR, compared to rGO and rGO-PPy, which demonstrates the prom-
Oxygen reduction reaction
ising potential of rGO-CN as a carbon-based, metal-free electrocatalyst for enhancing the electrocatalytic property towards ORR. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Fuel cells are considered a potential power source for future vehicles and stationary applications due to their high efficiency, high power density, and clean energy. The performance of fuel cells is largely limited by the sluggish oxygen reduction reaction (ORR), which substantially depends on the activity of the cathode catalyst. Platinum (Pt) and its alloys have long been recognized as the best electrocatalyst for ORR in fuel cells [1]. However, high cost due to the scarcity of Pt and
its insufficient durability limited the mass production and commercialization of electrocatalysts. Thus, considerable effort has been devoted to developing highly stable and cost effective non-precious metal or metal-free electrocatalysts with improved catalytic efficiency for ORR [2,3]. Recently, carbon nanomaterials and their composite materials have been intensively studied for replacing or reducing Pt based materials in fuel cell applications [4,5]. Among them, heteroatom-doped carbon materials for ORR have also been extensively investigated to reduce the cost and enhance the durability of fuel cells [4].
* Corresponding author. Tel./fax: þ82 52 259 2249/1689. ** Corresponding author. Tel./fax: þ82 52 259 1065/1689. E-mail addresses: [email protected] (J.S. Chung), [email protected] (W.M. Choi). http://dx.doi.org/10.1016/j.ijhydene.2015.04.002 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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Graphene, a one-atom-thick layer of graphite, packed into a two-dimensional network of sp2-hybridized carbon, is distinct from other types of carbon materials due to its outstanding thermal, mechanical and electrical properties [6,7]. Since graphene recently served as a worthy support material for anchoring metal/metal oxide nanoparticles and functional nanomaterials [8,9], graphene-based functional composites have been explored as ORR electrocatalysts because of the feasible synergetic effects with enhanced catalytic activity [10,11]. To date, doped graphene with various heteroatoms, such as sulfur (S), nitrogen (N), boron (B), and phosphorous (P), has attracted intense attention due to its reportedly high catalytic activity, high long-term durability and tolerance to poisoning as a metal-free ORR electrocatalyst [12e16]. For example, N-doped graphene, generally consisting of pyridinic N, pyrrole N and graphitic N atoms, has been widely studied as an electrocatalyst for fuel cells, due to its considerable electrochemical activity of ORR with an excellent durability [15,17]. However, its electrochemical activity is strongly affected by N atom content, the limited N content of 2e5% in N-doped graphene still revealed lower catalytic activity compared to Pt-based catalysts [18]. To utilize the outstanding properties of graphene and improve the catalytic efficiency towards ORR, the efficient nanostructure design of graphene-based composites with high content of CeN functional groups is of great interest [19e22]. Herein, we report the facile synthesis of graphene decorated with N-doped carbon nanowires as an efficient ORR electrocatalyst. The proposed method here involves the formation of polypyrrole (PPy) nanowires decorated on reduced graphene oxide (rGO-PPy) by in situ polymerization of pyrrole monomer in the presence of rGO. Subsequently, the synthesized rGO-PPy is annealed at 800 C in an argon atmosphere to afford the N-doped carbon nanowires decorated on rGO (rGOCN) by the carbonization of PPy nanowires. The unique nanostructure of graphene sheets decorated with conductive N-doped carbon wires provides high nitrogen content and high surface area for large amount oxygen access, which improves the electron transfer efficiency and the electrocatalytic activity.
Experimental Synthesis of rGO-CN First, graphene oxide (GO) was prepared by chemical exfoliation of the expanded graphite powder (grade 1721, Asbury Carbon) through a modified Hummers method by the microwave-assisted thermal expansion of graphite powder using the procedure described elsewhere [23]. The obtained GO powder was dispersed in deionized (DI) water for the GO aqueous solution with a concentration of 3.0 mg/ml. The GO solution was then treated with hydrazine hydrate (5 mL) at 40 C for 24 h to obtain rGO. After reduction, rGO was filtered and washed several times with DI water and dried in a vacuum oven. The prepared rGO were redispersed in DMF for rGO solution with a concentration 0.5 mg/ml. To synthesize PPy nanowires on rGO (rGO-PPy), in situ polymerization was performed in the presence of rGO using
pyrrole monomer (0.01 M), ferric p-toluene sulfonate (0.02 M) as an oxidizing agent and 1% polyvinyl pyrrolidone (PVP) as a surfactant at room temperature for 5 h. Then, the resultant composites were washed with ethanol and DI water several times and dried at 60 C in a vacuum oven. The dried rGO-PPy sample was annealed at 800 C under argon atmosphere for 1 h, resulting in rGO-CN by the carbonization of PPy nanowires.
Characterizations The morphology of the samples was characterized using a fieldemission scanning electron microscope (FE-SEM, JEOL, JSM6500F) and a high resolution transmission electron microscope (HR-TEM, Hitachi, H-8100). X-ray diffraction (XRD) analysis was performed on a Rigaku X-ray diffractometer with Cu Ka radiation. The chemical compositions of the prepared samples were analyzed using an X-ray photoelectron spectrometer (XPS, Thermo scientific) with monochromatic Al Ka radiation with hv ¼ 1486.6 eV. Fourier transform infrared (FT-IR) spectra were obtained using a Nicolet IR 200 FT-IR spectrometer (Thermo Scientific). Raman spectra (DXR Raman spectrophotometer, Thermo Scientific) were obtained using 532 nm laser excitation.
Electrochemical measurements Electrochemical measurements of the prepared samples were performed on a multi-channel potentiostat-galvanostat analyzer (VSP, Bio-logic) with a three-electrode system at room temperature, in which Pt wire was used as counter electrode and saturated calomel electrode (SCE) as reference electrode. As the working electrode, the electroactive materials in ethanol (1 mg/ ml) and Nafion in isopropyl alcohol solution (0.5 wt%) as binder were mixed by sonication. Then, 5 ml of this mixture was coated on a glassy-carbon rotating disk electrode (RDE) and dried in air for the electrochemical characterizations. The electrochemical measurements using the modified glassy-carbon electrode were carried out in the O2-purged 0.1 M KOH electrolyte solution at room temperature. The number of electrons transferred per oxygen molecule in the ORR process was determined by the KouteckyeLevich (KeL) equation given below: 1 1 1 ¼ þ j jk Bu0:5 where jk is the kinetic current density and u is the angular velocity of the electrode. B could be determined from the slope of the KeL plots based on the following Levich equation: 2 1 B ¼ 0:2 nF DO2 3 v6 CO2 where n is the number of electrons transferred per oxygen molecule, F is the Faraday constant (96485C mol1), DO2 is the diffusion coefficient of O2 in 0.1 M KOH (1.9 105 cm2 s1), v is the kinetic viscosity (1.0 102 cm2 s1) and CO2 is the bulk concentration of O2 in 0.1 M KOH (1.2 106 mol cm3) [24,25].
Results and discussion A schematic illustration of the fabrication process of the rGOCN is shown in Fig. 1. In a typical procedure, after rGO was
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prepared through the reduction of GO, pyrrole was polymerized in the presence of rGO at room temperature using ferric ptoluene sulfonate as oxidizing agent and polyvinyl pyrrolidone (PVP) as surfactant, resulted in rGO-PPy composites. The prepared rGO-PPy was then annealed at 800 C in an argon atmosphere for 1 h, after which the rGO sheets were fully reduced and the PPy nanowires were transformed into the carbon nanowires by the carbonization. Moreover, the high temperature annealing of N atoms in PPy synthesized the Ndoped carbon nanowires which could then serve as the effective electrocatalyst for ORR. A typical SEM image of the rGO in Fig. 2a shows that the smooth thin sheet structures with wrinkles and folds of the rGO surface are observed due to the flexibility of graphene sheet, which is general morphology of the restacked rGO layers after the reduction. Additionally, after the formation of PPy nanowires and the subsequent carbonization of PPy (Fig. 2b and c), it is observed that the sheet structure of rGO was maintained without further restacking of rGO layers and the uniform deposition of PPy and carbon nanowire are observed on the graphene surface after polymerization and subsequent carbonization. To examine the clear morphology of the deposition of PPy nanowires and carbon nanowires on rGO, TEM was performed as shown in Fig. 2def. The TEM image of rGO sheets (Fig. 2d) shows very clear small folds and wrinkles as shown in the FE-SEM image (Fig. 2a). PPy was polymerized on the rGO surface by adding PVP as the surfactant, which facilitates the formation of PPy nanowires. Fig. 2e shows that the synthesized PPy nanowires were uniformly formed on the rGO surface with a diameter of 17e18 nm. The rGO/PPy nanowire composites were formed owing to p- p interaction between the sp2-carbon atoms of 2D graphene and the electronic structures of the conjugated backbones of PPy. TEM image of rGO-CN in Fig. 2f clearly shows the formation of carbon nanowires on the rGO surface with similar nanostructures after the carbonization process of rGO-PPy, and the diameter of the nanowires was reduced to approximately 13e14 nm. The chemical structures of rGO-PPy and rGO-CN were investigated using FT-IR spectroscopy. Fig. 3a shows the FT-IR spectra of GO, rGO, rGO-PPy and rGO-CN. The characteristic absorption bands of GO observed in FT-IR spectra, which corresponded to the OeH stretching of water as a broad band at 3396 cm1, C]O stretching vibration at 1727 cm1, CeO stretching at 1361 cm1, CeOH stretching at 1228 cm1 and CeO (alkoxy) stretching at 1063 cm1 [26e29]. The peak appearing at 1621 cm1 was assigned to the vibrations of the adsorbed water molecules. FT-IR spectrum of rGO shows the
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retained presence of some OH functional groups at the edges of the rGO sheets, implying the partial reduction of GO by hydrazine hydrate treatment. For rGO-PPy, the characteristic peak located at 1486 cm1 is assigned typical PPy ring vibration, and the peaks at 1220, 1043 and 936 cm1 were assigned to CeN stretching, symmetrical CH in plane bend and CeH ring deformation, respectively. Further broad peak at about 3400 cm1 is attributed to the overlap between NeH stretching of PPy and OeH stretching of rGO. Meanwhile, the peaks at 1647 cm1 is due to the C]O group present in the PVP. These results suggested that PPy nanowires are indeed formed on the rGO surface via interaction such as pep stacking between them [30,31]. In the case of rGO-CN, the peaks were observed at 1572, 1486, 1216 and 985 cm1, corresponding to C]C stretching, C]N stretching, CeN stretching and CeC stretching, respectively. Meanwhile, most of oxygen functional groups had disappeared, implying that all the oxygen functional groups in rGO and PPy nanowires had been converted into N-doped carbon nanowires through the carbonization of PPy nanowires at 800 C. The crystallographic characterization of the samples was examined using XRD analysis. Fig. 3b shows the XRD curves of the rGO, rGO-PPy and rGO-CN. The synthesized rGO exhibits three diffraction peaks centered at 2q of 25.0 , 43.4 and 78.3 corresponding to (002), (100) and (110) planes of graphite, respectively. The d-spacing value from (002) plane in rGO corresponds to 0.35 nm, which higher than the reported dspacing of 0.34 nm of graphite. The broadening and peak shifting compared to graphite indicates the formation of short range ordered stacks in rGO. The slightly larger interlayer spacing than that of graphite results from the residual oxygen functional groups or other structural defects in the rGO sheets [32]. The pattern of rGO-PPy exhibits a broad diffraction peak at 2 ¼ 24.2 for the calculated d-spacing of 0.37 nm, which indicates the pep stacking distance between PPy and rGO sheets [31]. Furthermore, two peaks about at 43 and 78 almost disappear for rGO-PPy, suggesting that the PPy nanowires and rGO have interacted completely. After the carbonization of PPy, the XRD pattern of rGO-CN exhibits the prominent peak located at 25.1 with a calculated d-spacing value of 0.36 nm and a small peak at 43 , which is attributed to the transformation of PPy nanowires to carbon nanowires and the complete removal of oxygen functional groups of rGO sheets [33]. The Raman spectra of the GO, rGO, PPy, rGO-PPy and rGOCN samples are obtained as depicted in Fig. 4a. The Raman spectra of all graphene samples exhibit two prominent peaks at 1596 and 1344 cm1, which are assigned to G and D peaks,
Fig. 1 e Schematic illustration of the fabrication of graphene/N-doped carbon nanowire composites.
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Fig. 2 e SEM and TEM images of rGO (a and d), rGO-PPy (b and e) and rGO-CN composites (c and f).
respectively. They are related to the plane vibrations of graphene lattice and the defect sites of graphene lattice, respectively. To investigate the degree of disordered graphite, the D/ G intensity ratios (ID/IG) are calculated and compared. The ID/ IG ratio for GO and rGO are 0.84 and 0.95, respectively, which is typical behavior after the reduction of GO by hydrazine. The broad D and G peaks for rGO-PPy are observed after the polymerization of PPy nanowires on rGO surface, suggesting that the size of the graphene nanocrystals decreases due to the phonon confinement [34]. Additionally, the decreased ID/IG ratio of 0.57 and the G peak is also shifted from 1596 to 1585 cm1 by the PPy introduction, due to the strong p-p interaction of PPy and graphene. The two broad peaks at 934 and 1062 cm1 for rGO-PPy appears due to two small characteristic peaks of pure PPy at 947 and 1060 cm1 related to bipolaron ring deformation and the polaron symmetric CeH in-plane bending vibration, respectively. These results confirm the successful formation of PPy on rGO. For the Raman spectrum of rGO-CN, the two characteristic peaks of PPy located at 934 and 1062 cm1 vanish and the ID/IG ratio increased to 0.99, due to the transformation of PPy to carbon
nanowires and the greater number of defects in rGO-CN, which could facilitate its electrocatalytic property towards ORR. A broad and weak 2D band was observed at 2790 cm1 associated with another peak at 2900 cm1 for rGO-CN due to the structural disorder of rGO [35e37]. This broad and week 2D band is generally found for the GO and rGO samples by the high level of disorder of the graphene layer from the oxidation process. The present Raman results show that graphene layers are well dispersed in the rGO-CN composites, which is consistent with the TEM results. The surface chemical composition of rGO, rGO-PPy and rGO-CN was further analyzed by XPS. The elements of C, O and N were observed in the survey spectrum of all samples depicted in Fig. 4b. The calculated C/O ratios for rGO and rGOPPy are 3.06 and 2.81, respectively, showing that the oxygen content in the rGO-PPy has increased compared to that of rGO. With further carbonization, the C/O ratio of rGO-CN increased to 5.45, indicating that most of the oxygen functionality has been removed in rGO-CN after high temperature annealing. To investigate the nitrogen content in the samples, the C/N ratio is calculated. It is found that the calculated C/N ratio of
Fig. 3 e (a) FT-IR spectra of GO, rGO, PPy, rGO-PPy and rGO-CN and (b) XRD patterns of rGO, rGO-PPy and rGO-CN.
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Fig. 4 e (a) Raman spectra of GO, rGO, Ppy, rGO-PPy and rGO-CN. (b) XPS survey spectra, (c) C1s spectra and (d) N1s spectra of rGO, rGO-PPy and rGO-CN.
rGO sample is 17.51 due to some CeN groups incorporated during the hydrazine reduction. The C/N ratio of rGO-PPy decreases to 4.55 due to the high N atom content in PPy, whereas the C/N ratio of rGO-CN increased to 5.60 by the removal of oxygen functionality after high temperature annealing. The C1s core level spectra for rGO, rGO-PPy and rGO-CN are presented in Fig. 4c. The C1s spectra for rGO and rGO-PPy could be decomposed into five Gaussian peaks with binding energies of 284.7 (CeC), 285.5 (CeN), 286.5 (CeO), 287.4 (C]O) and 288.9 eV (OeC]O). After the polymerization of PPy, the C1s spectra of rGO-PPy presents a weakened peak intensity corresponding to CeC, whereas the peak centered at 285.5 eV notably increases, suggesting the formation of CeN. The C1s spectrum of rGO-CN presents only four peaks with
the absence of the CeO peak, which is related to the significant decrease of oxygen functionality after the high temperature annealing. The N1s core level spectra are further analyzed as shown in Fig. 4d. The N1s spectra of rGO shows two well resolved peaks of pyrazoline groups (400.0 eV) and quartenary N (graphitic N, 401.3 eV) due to the hydrazine treated reduction, respectively [38,39]. The deconvoluted N1s spectra of rGO-PPy splits in to three peaks at, and corresponding to the pyrrolidone N (399.5 eV), pyrrolic N (400.3 eV) and quartenary N (401.3 eV), respectively, after the polymerization of PPy nanowires using the PVP as a surfactant. The N1s spectra of rGO-CN decomposes into three peaks of pyrrolic N, pyridinic N and quaternary N, which shows the different peak combination of N1s spectra, compared to rGO-PPy, by the
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Table 1 e The relative percentage of different N atoms from XPS analysis. Sample
Pyrrolic N (%)
Pyridinic N (%)
Quarternary N (%)
Pyrazoline N (%)
Pyrrolidone N (%)
rGO rGO-PPy rGO-CN
e 76.4 58.8
e e 27.9
48.2 8.1 13.3
51.8 e e
e 14.5 e
carbonization process [40,41]. These results represent the transformation of PPy nanowires into another type of carbon materials. Furthermore, it is worthwhile to note that the peaks of pyrrolic N and pyrrolidone N of rGO-PPy appears to decrease or removed, while the peaks of pyridinic N of rGO-CN are increased as shown in Table 1. The formation of appreciable amount of pyridinic N in rGO-CN is favorable for the ORR due to its superior ORR activity. Since the N-doped sites provide strong adsorption sites for reduced oxygen species (OOH) [42], the presence of a greater amount of N-doped carbon structures in rGO-CN would facilitate the enhanced electrocatalytic property for ORR. The electrocatalytic activity of rGO-CN was preliminarily evaluated by cyclic voltammetry (CV) with a three-electrode system in 0.1 M KOH solution with O2-saturated. Fig. 5a shows the CV results of rGO, rGO-PPy and rGO-CN in the potential range between 0.2 and 0.9 V vs. SCE at the scan rate of 10 mV s1. A clear cathodic peak located at around 0.45 V of rGO-CN appears in the CV from ORR. The similar cathode peaks for rGO and rGO-PPy appear respectively at around 0.44 and 0.41 V. Despite the small shift to negative potential, the electrode of rGO-CN exhibits higher cathodic current density for the ORR,
suggesting better adsorption and the reduction of oxygen molecules on the surface of rGO-CN. It is great importance in fuel cell to investigate the methanol crossover effect for electrocatalyst, since the permeation of methanol to the cathode would deteriorate the performance of cathode catalysts. The tolerance of rGO-NC towards the methanol crossover is performed in an O2-saturated 0.1 M KOH solution in the presence of methanol (10% vol) (Fig. S1). The CV curve remains unchanged after addition of methanol in to the electrolyte, suggesting a pronounced electrocatalytic activity of the rGONC for oxygen reduction. In addition, the RDE measurements have been performed at a scan rate of 10 mV s1 in O2saturated 0.1 M KOH solution for further study of the ORR kinetics. Linear sweep voltammetry (LSV) of rGO, rGO-PPy and rGO-CN at a rotation of 1600 rpm was performed and the results are shown in Fig. 5b. The ORR onset potentials of rGO, rGO-PPy and rGO-CN are determined to be at 0.17, 0.20 and 0.23 V, respectively. The higher onset potential of rGO-CN electrode compared to the other two electrodes indicates that ORR occurs more easily on the rGO-CN electrode. Moreover, the diffusion limited current at 0.8 V vs. SCE for rGO-CN is more than three times higher than that of rGO and rGO-PPy,
Fig. 5 e ORR activity of rGO, rGO-PPy and rGO-CN. (a) Cyclic voltammetry at 10 mV s¡1 and (b) RDE polarization curves at 10 mV s¡1 and 1600 rpm. (c) RDE polarization curves of rGO-CN at different rotation speeds with 10 mV s¡1 and d) the electron transfer number of rGO-CN at different electrode potentials.
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due to the higher electron accepting capability of the N-doped sites of rGO-CN. These results further confirm the higher electrocatalytic performance of rGO-CN towards the ORR. To further investigate the ORR performance of rGO-CN, LSV was performed with different rotating speeds from 100 to 2500 rpm, as shown in Fig. 5c. For comparison, the LSV curves are also obtained for rGO and rGO-PPy (Fig. S2). The LSV curves of rGO-CN shows the increase of limiting diffusion current density with higher rotating speed, since the diffusion distance is shortened at high rotating speed [24,43]. Compared to rGO and rGO-PPy, the limiting diffusion current density of rGO-CN is recorded to be higher at all rotating speeds. To qualify the ORR process, the number of electrons transferred per oxygen molecule (n) in the ORR process was determined by the K-L equation. Fig. 5d shows the n value of rGO-CN calculated from the slopes of K-L plots at different potentials. The n values of rGO-CN increase with increasing the applied potentials. At the potential of 0.8 V, the n value of rGO-CN is calculated to be 3.08, suggesting that the rGO-CN electron follows a combination of two- and four-electron transfer process. However, the rGO exhibits the n value of 1.96, indicating a two-electron transfer reaction (Fig. S3), which represents that the ORR activity of rGO-CN is significantly enhanced with the presence of N-doped carbon nanowires. The obtained n value of rGO-CN is comparable to that of other reports for the ORR activity using the various carbon based nanomaterials (Table S1). It reveal that the superior ORR activity with more efficient electron transfer have been demonstrated by the greater amount of N-doped active sites in the rGO-CN electrode.
Conclusions We have successfully demonstrated a facile method to synthesize rGO sheets decorated with N-doped carbon nanowires as an electrocatalyst for ORR. The proposed method involves the polymerization of PPy nanowires on rGO sheets, followed by the transformation into N-doped carbons nanowires derived from the carbonization of PPy nanowires. The prepared rGO-CN exhibits the high nitrogen content and a unique nanostructure afforded by the integration of carbon nanowires and graphene sheets, which facilitate the efficient adsorption of oxygen molecules for improved electron transfer efficiency and increased electrocatalytic activity for ORR. The prepared rGO-CN electrode exhibits the enhanced electrocatalyst performance of ORR, compared with rGO and rGO-PPy, which is mainly attributed to the high nitrogen content in rGO-CN, as confirmed by the XPS and FT-IR analyses. The combination of the superior electrical property of graphene and the N-doped carbon nanowires for the effective adsorption of oxygen molecules might be promising for the application of rGO-CN as a noble metal-free ORR electrocatalyst in fuel cells.
Acknowledgment This work is supported by the 2012 Research Fund of University of Ulsan.
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Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2015.04.002.
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