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Cite this: DOI: 10.1039/c4nr06631f Received 10th November 2014, Accepted 17th April 2015 DOI: 10.1039/c4nr06631f www.rsc.org/nanoscale
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Electrochemical doping of three-dimensional graphene networks used as efficient electrocatalysts for oxygen reduction reaction† Zhijuan Wang,a Xiehong Cao,b Jianfeng Ping,b Yixian Wang,b Tingting Lin,a Xiao Huang,b,d Qinglang Ma,b Fuke Wang,*a Chaobin He*a,c and Hua Zhang*b
Three-dimensional graphene networks (3DGNs) doped with a mono-heteroatom of N or B, or dual-heteroatoms of N and B were fabricated, which exhibit excellent oxygen reduction reaction (ORR) performance. Importantly, the onset potential and current density of N and B co-doped 3DGNs are comparable to those of the commercial Pt (30%)/C catalyst.
Platinum (Pt) and Pt-based catalysts toward oxygen reduction reaction (ORR) are critical in fuel cells and lithium/air batteries due to their relatively low overpotential and high current density.1 However, their commercial application has been hampered by the limited Pt resource and their high cost.2 In addition, Pt catalysts suffer from the susceptibility to the timedependent drift, the crossover effect of methanol (CH3OH) and carbon monoxide (CO) poisoning.3 Therefore, numerous efforts have been devoted to reduce the usage of Pt catalysts or even replace them with non-precious metal catalysts,1,4,5 enzymatic electrocatalysts6 or metal-free materials.6–10 Among them, graphene doped with heteroatoms such as nitrogen (N),7–9,11–22 boron (B),8 sulfur (S)23–25 and phosphorus (P)14,19 has attracted great attention due to their exciting electronic and chemical properties as well as their high catalytic efficiency. In particular, N and B are the most studied hetero-
a Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 3 Research Link, Singapore, 117602. E-mail: [email protected], [email protected] b School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798. E-mail: [email protected], [email protected]; http://www.ntu.edu.sg/home/hzhang/ c Department of Materials Science and Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576 d Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergistic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (Nanjing Tech), 30 South Puzhu Road, Nanjing 211816, China † Electronic supplementary information (ESI) available: Details of the N-3DGN, B-3DGN and NB-3DGN fabrication process. Description of characterization. Rotating disk electrode linear sweep voltammograms of 3DGN and Pt (30%)/C in O2-saturated 0.1 M KOH with various rotation rates at a scan rate of 5 mV s−1. Koutecky–Levich plots of 3DGN, Pt (30%)/C, N-3DGN, B-3DGN and NB-3DGN at different electrode potentials. See DOI: 10.1039/c4nr06631f
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atoms for graphene doping as their atomic masses are closest to carbon. In addition, their atomic sizes would be acceptable for doping the carbon lattice of graphene. At the same time, they can tune the electronic properties of graphene due to the electron rich and electron deficient nature of N and B atoms, respectively. At present, the heteroatom doped graphene has been achieved by several different approaches such as chemical vapor deposition (CVD),11,20 thermal annealing8,15,18 and solvothermal synthesis.18 However, the aforementioned methods are effective only for producing singly-doped graphene but not for co-doped graphene. For example, the thermal annealing method was effective for the synthesis of N- or B-doped graphene, but in the presence of both B and N atoms, it produced the covalent B–C–N bonding in graphene instead of B and N co-doped graphene.8 In addition, this method also produced undesired by-products, such as hexagonal boron nitride, which is chemically inert and can reduce the activity of a catalyst. In particular, co-doping with B and N can create a unique electronic structure with a synergistic coupling effect between B and N because of the lower (B (χ = 2.04)) or higher (N (χ = 3.04)) electronegativity compared to C (χ = 2.55),26 which makes such co-doped products much more catalytically active than B- or N-doped graphene catalysts.8 Therefore, an efficient strategy that can be used for both mono-heteroatom doping and multi-heteroatom co-doping of graphene is highly required. In this work, the electrochemical method was demonstrated to successfully dope graphene with either a mono-heteroatom or dual-heteroatoms with high catalytic efficiencies for ORR. To obtain the excellent ORR result, a three-dimensional graphene network (3DGN) prepared by CVD27 was used in this work (Scheme 1). The doped products, i.e., N-doped, B-doped, and N and B co-doped 3DGN, referred to as N-3DGN, B-3DGN and NB-3DGN, respectively, with successful incorporation of either mono- or dual-heteroatoms into graphene lattices were confirmed by X-ray photoelectron spectroscopy (XPS), which also suggested that the formation of by-products could be prevented by our electrochemical doping method. Compared with the existing graphene doping technologies, our electrochemical method shows the obvious advantages, such as mild
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Scheme 1 Left: scheme of the working electrode. Right: the setup of doping 3DGN by the electrochemical method at room temperature. 3DGN, Pt and Ag/AgCl (sat. LiCl in ethanol) are used as WE, CE and RE, respectively.
doping conditions and easy change of dopants through simple exchange of dopant sources. Most importantly, the doped products obtained from the electrochemical method showed high electrocatalytic activity toward ORR. Among them, NB-3DGN showed the best activity, which is comparable to the commercial Pt(30%)/C. In addition, these doped products exhibited good stability and outstanding durability to CH3OH, suggesting that they are promising potential candidates for replacing Pt-based catalysts. In this work, 3DGN synthesized by CVD was chosen to demonstrate the electrochemical doping of graphene with heteroatoms because of its high specific surface area and excellent electrical and mechanical properties.27 In a typical electrochemical doping process, the 3DGN served as the working electrode directly, which was fabricated by fixing a piece of 3DGN27,28 onto a glass slide with silicon gel, and then placing silver paint on one end of the 3DGN to form a conductive pad. The doping was then carried out at room temperature with Ag/AgCl and Pt wires as reference and counter electrodes, respectively. The electrolyte containing N atom and/or B atom was dissolved in acetonitrile, which was also used as a dopant source. After the experiment, the doped 3DGN was rinsed with water to remove the electrolyte absorbed on its surface. Fig. 1 shows the scanning electron microscopy (SEM) images of 3DGN before and after electrochemical doping. As observed, the 3D network structure remained intact after the electrochemical doping. Usually, graphene can be chemically oxidized by using KMnO4, whose redox potential is even as high as ca. 1.5 V.29 In this work, a potential of 2 V (vs. Ag/AgCl) was applied to elec-
Fig. 1 SEM images of a three-dimensional graphene network (3DGN) before (a) and after (b) electrochemical doping.
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trochemically oxidized graphene films. During the electrochemical doping process, the electric field was precisely tuned through the potentiostat, which induced the adsorption of an electrolyte (dopant source) on the graphene surface. Therefore, the high potential (±2 V) used here is believed to be able to effectively make the cation (e.g., TBA+) or anion (e.g. BF4−) intercalate into the graphene layers with broken C–C bonds.29,30 During the doping process, 5 mV s−1 and 10 cycles were chosen as the scan rate and doping time for each doping process, respectively. Before the electrochemical doping, the cell was bubbled with pure N2 to remove the oxygen. In addition, the whole setup was kept in a desiccator in order to prevent water or oxygen to affect the reaction. Although the exact mechanism is not clear at the moment, it is supported by the XPS results (Fig. 2). Consequently, the electronic structure of graphene can be changed, which induces the band gap opening of graphene. In this work, mono-heteroatom like N or B and dual-heteroatoms of N and B have been successfully doped into graphene (see the detailed experimental procedure in the Electronic Supplementary Information), which were confirmed by XPS measurements. As shown in Fig. 2, the peaks of heteroatom-doped 3DGN at 284 and 532 eV were assigned to C 1s and O 1s, respectively. The O 1s peak might arise from the absorbed water molecules on the surface of the product during washing.31 The absence of a Ni peak in the XPS spectra for 3DGN indicates that the Ni foam was completely removed by the etchant solution containing FeCl3 and HCl during the sample preparation.27,28 The XPS spectra of N-3DGN (Fig. 2a) and B-3DGN (Fig. 2b) show N 1s and B 1s peaks, respectively, and the XPS spectrum of NB-3DGN (Fig. 2c) shows both N 1s and B 1s peaks, suggesting the successful incorporation of
Fig. 2 XPS spectra of (a) 3DGN and N-3DGN, (b) 3DGN and B-3DGN and (c) 3DGN and NB-3DGN. Insets: respective high-resolution N 1s peaks of N-3DGN (a) and NB-3DGN (c); respective high-resolution B 1s peaks of B-3DGN (b) and NB-3DGN (c).
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dopant atoms into the 3DGN. For the N doping, the high resolution N 1s spectra of N-3DGN and NB-3DGN (insets of Fig. 2a and c) show the presence of both pyridinic N (399.0 eV) and quaternary N (402.2 eV).13,32 The elemental analysis revealed that the atomic percentage of N in N-3DGN was about 2.7%, and the atomic ratio between quaternary N and pyridinic N was around 2 : 1. Differently, in NB-3DGN, the atomic percentage of N was 1.7% and the ratio between quaternary N and pyridinic N was 1 : 1. As for the B doping, the atomic percentage of B in B-3DGN and NB-3DGN was similar, both in the range of 2–2.5%. As shown in the insets of Fig. 2b and c, the B 1s peak was found to be centered at around 192 eV with a B atomic percentage of 2.1% in B-3DGN and 2.3% in NB-3DGN. This peak can be assigned to the partially oxidized BC2O nanodomains.23 Note that other common structures such as BN and BC3 were not observed in our products. Raman spectroscopy is a powerful technique to characterize graphitic carbons.13,19,31,32 The typical Raman spectra of 3DGN, N-3DGN, B-3DGN and NB-3DGN are shown in Fig. 3a. The peaks at 1350, 1582 and 2696 cm−1 can be assigned to the D, G and 2D bands of graphene, respectively. The G band is the in-plane vibrational mode and the 2D band corresponds to the double resonance scattering of two transverse optical (TO) phonons around the K-point of the Brillouin zone.19 Both of them are characteristic peaks of graphene. Moreover, the peak of the 2D band is sensitive to the number of graphene layers. The relatively strong and broad 2D band in both 3DGN and the doped products indicate that the 3DGN contains multilayered graphene.31 Unlike the 2D band, the D band does not depend on the number of graphene layers, but strictly depends on the amount of lattice disorder present in the structure. In the Raman spectrum of 3DGN, the weak and broad D band comes from the defects via an intervalley double-resonance Raman process.19 After doping, the intensities of the D band of the products (i.e., N-3DGN, B-3DGN and NB-3DGN) were increased significantly, while the intensities of their G and 2D bands were decreased compared to those of un-doped 3DGN. In general, the intensity ratio of the D to G band (ID/IG) can be used to evaluate the defect density in graphene.31 The ID/IG for 3DGN is 0.29, suggesting that 3DGN is highly crystalline (Fig. 3b).31 However, the ID/IG ratio for N-3DGN, B-3DGN and NB-3DGN was increased to 0.53, 0.41 and 0.96, respectively, indicating that the additional non-sp2 domains were created in
Fig. 3 (a) Raman spectra of 3DGN, N-3DGN, B-3DGN and NB-3DGN. (b) ID/IG and IG/I2D of 3DGN, N-3DGN, B-3DGN and NB-3DGN.
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the doped products. Moreover, the intensity ratio of the G to 2D band (IG/I2D) was increased from 0.93 for 3DGN to 1.11, 1.14 and 1.65 for N-3DGN, B-3DGN and NB-3DGN, respectively, further confirming the increased lattice disorder and the decreased overall crystallinity in the doped products.32 It should be noted that NB-3DGN showed the highest ratios of ID/IG and IG/I2D among these three doped products, which can be due to the largest amount of doped domains/defects in the NB-3DGN. The electrocatalytic performance of 3DGN, N-3DGN, B-3DGN and NB-3DGN toward ORR was carried out in 0.1 M KOH solution and the results are shown in Fig. 4a. Their loadings on the glassy carbon electrodes (GCE) are not the same in order to separate these curves from each other. For comparison, the electrocatalytic behavior of the bare GCE was also measured under the same experimental conditions. It is clear that none of the above electrodes exhibited a reduction peak
Fig. 4 (a) Cyclic voltammograms of oxygen reduction on the glassy carbon electrode (GCE), 3DGN, N-3DGN, B-3DGN and NB-3DGN electrodes in O2-saturated 0.1 M aqueous KOH solution at a scan rate of 50 mV s−1. The loadings of N-3DGN, B-3DGN and NB-3DGN on the GCE are not the same in order to separate these curves from each other. (b) Rotating disk electrode (RDE) voltammograms of 3DGN, N-3DGN, B-3DGN, NB-3DGN and Pt(30%)/C at a rotation rate of 1600 rpm. Inset: the onset potential of 3DGN, N-3DGN, B-3DGN, NB-3DGN and Pt(30%)/C. (c–e) RDE linear sweep voltammograms of N-3DGN (c), B-3DGN (d) and NB-3DGN (e) in O2-saturated 0.1 M KOH with various rotation rates at a scan rate of 5 mV s−1. (f ) The dependence of the electron-transfer number for 3DGN, N-3DGN, B-3DGN, NB-3DGN and Pt (30%)/C at various potentials.
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in the N2-saturated KOH solution. However, in the O2-saturated KOH solution, the reduction peaks of oxygen appeared at different potentials for the aforementioned five electrodes. Compared with 3DGN, the doped products i.e., N-3DGN, B-3DGN and NB-3DGN, exhibited positive shifts in the peak potential (i.e., from −386 to 200, 191 and 247 mV, respectively) and higher current density, which can be attributed to the incorporation of various dopants into the graphene structure.14 To gain insight into the electrocatalytic behaviors of 3DGN, N-3DGN, B-3DGN and NB-3DGN toward ORR, their rotating voltammograms were investigated (Fig. 4b). For comparison, the commercially available Pt catalyst, i.e., Pt(30%)/C, was also tested under the same experimental conditions. In a typical experiment, the same amount of 3DGN, N-3DGN, B-3DGN, NB-3DGN and Pt(30%)/C was loaded onto a glassy carbon rotating-disk electrode, respectively, and the geometrical area of the electrode was used here to calculate the current density.16,33 All of the voltammetric curves for different electrodes in the O2-saturated 0.1 M aqueous KOH solution were obtained at the same rotation rate of 1600 rpm. Interestingly, two O2 reduction waves appeared on the 3DGN-modified electrode, which can be documented as the two-electron reduction process of O2 to HO2−, and the further reduction of HO2− to OH− in alkaline medium.34,35 However, only one reduction wave was observed on N-3DGN, B-3DGN, NB-3DGN and Pt(30%)/C modified electrodes. This difference indicates that the catalytic processes of N-3DGN, B-3DGN, NB-3DGN and Pt(30%)/C are different from 3DGN. Moreover, the onset potentials for ORR at 3DGN, N-3DGN, B-3DGN, NB-3DGN and Pt(30%)/C were found at −0.29, −0.1, −0.08, −0.06 and −0.04 V, respectively (Fig. 4b). Obviously, N-3DGN, B-3DGN and NB-3DGN exhibit much better electrocatalytic activities compared to the un-doped 3DGN. Importantly, NB-3DGN showed the highest onset potential that is much closer to that of Pt(30%)/C, indicating that NB-3DGN has the best ORR catalytic properties among these doped 3DGN catalysts. To gain further insight into the electron transfer kinetics of N-3DGN, B-3DGN and NB-3DGN during the ORR, their behaviors in the rotating disk voltammetry at different rotating rates were tested. The current densities ( J) of N-3DGN (Fig. 4c), B-3DGN (Fig. 4d) and NB-3DGN (Fig. 4e) were increased with the rotation rate (ω) increasing from 400 to 2000 rpm. The relationship between J−1 and ω−1/2, i.e., the Koutecky–Levich plots (Fig. S1a, b and c†) can be obtained from the data displayed in Fig. 4c–e, respectively. These plots at different electrode potentials show good linearity and parallelism over the potential range from −0.4 to −0.8 V, which are considered as typical first-order reaction kinetics with respect to the concentration of dissolved O2. The number of transferred electrons (n) (Fig. 4f ) can be obtained from the slope of these plots in Fig. S1a–c,† respectively (see details in the ESI†). Similarly, the transferred electron number of 3DGN and Pt(30%)/C can also be obtained based on their corresponding Koutecky–Levich plots (Fig. S2†). For comparison, the electron-transfer number of ORR at −0.7 V for N-3DGN, B-3DGN and NB-3DGN was calculated to be 3.7, 3.6 and 3.8, respectively, indicating that the
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Fig. 5 (a) Chronoamperometric response of N-3DGN, B-3DGN and NB-3DGN at −0.25 V in O2-saturated 0.1 M KOH solution with a rotation rate of 1600 rpm. The loading of N-3DGN, B-3DGN and NB-3DGN on the GCE is not the same in order to separate these three curves from each other. (b) Chronoamperometric response plot of current density vs. time obtained at the Pt(30%)/C, N-3DGN, B-3DGN and NB-3DGN electrodes at −0.25 V in O2-saturated 0.1 M KOH with a rotation rate of 1600 rpm. The final concentration of CH3OH in the KOH solution is 3 M. The arrow indicates the addition of CH3OH into the O2-saturated 0.1 M aqueous KOH solution.
ORR with the aforementioned three catalysts is dominated by a four-electron process (Fig. S3†). However, the ORR on 3DGN (n = 1.8) was approximately a two-electron process (Fig. S3†). In addition, over a wide potential range from −0.4 to −0.8 V, the ORR on N-3DGN, B-3DGN and NB-3DGN is also dominated by a four-electron process, which is similar to that of Pt(30%)/C (Fig. 4f ). Differently, the n value for 3DGN (1.5–2.2) is much lower over the same potential range. The stability and tolerance to the crossover effect of CH3OH are two important properties for evaluating a new electrocatalyst. The stability of N-3DGN, B-3DGN and NB-3DGN was tested based on their current–time chronoamperometric responses for ORR. As shown in Fig. 5a, these three types of doped 3DGN exhibited high stability, which can be attributed to the strong covalent bonding between the active sites and the graphitic lattice. In addition, their tolerance to the crossover effect of CH3OH should also be considered since CH3OH can permeate through the polymer membrane from the anode to the cathode, which can severely affect the performance of the catalyst in the cathode. As shown in Fig. 5b, the current density of Pt(30%)/C sharply decreased immediately after the addition of CH3OH, while the current density of N-3DGN, B-3DGN and NB-3DGN decreased to ∼85%, 70% and 82% of their initial values after 3000 s, respectively. Therefore, N-3DGN, B-3DGN and NB-3DGN showed good tolerance to the crossover effect of CH3OH. The excellent selectivity towards ORR and remarkable tolerance against CH3OH poisoning of N-3DGN, B-3DGN and NB-3DGN make them promising metal-free catalysts for ORR in fuel cells and other catalytic applications.
Conclusions In summary, a simple electrochemical method has been successfully developed to dope graphene with mono- and dualheteroatoms to prepare efficient electrocatalysts for ORR. The
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successful doping was confirmed by the XPS and Raman analysis. The resulting products in this work, i.e., N-3DGN, B-3DGN and NB-3DGN, exhibited improved electrocatalytic activities toward ORR. Impressively, NB-3DGN showed the highest onset potential, which is comparable to the commercial Pt(30%)/C. Importantly, these products showed high stability and good tolerance to the crossover effect of CH3OH. Our electrochemical method might be used to incorporate other foreign atoms into graphene sheets for the preparation of novel mono- or multi-heteroatoms doped graphene for catalysis application.
Acknowledgements This work was financially supported by the Science and Engineering Research Council (grant 1123004024) of Agency for Science, Technology and Research of Singapore, MOE under AcRF Tier 2 (ARC 26/13, no. MOE2013-T2-1-034) and AcRF Tier 1 (RG 61/12, RGT18/13 and RG5/13), and Start-Up grant (M4080865.070.706022) in NTU. This research is also conducted by NTU-HUJ-BGU Nanomaterials for Energy and Water Management Programme under the Campus for Research Excellence and Technological Enterprise (CREATE), that is supported by the National Research Foundation, Prime Minister’s Office, Singapore. X. H. thanks the financial support from the National Natural Science Foundation of China (51322202) and Natural Science Foundation of Jiangsu Province (BK20130927).
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Cite this: DOI: 10.1039/c4nr06631f Received 10th November 2014, Accepted 17th April 2015 DOI: 10.1039/c4nr06631f www.rsc.org/nanoscale
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Electrochemical doping of three-dimensional graphene networks used as efficient electrocatalysts for oxygen reduction reaction† Zhijuan Wang,a Xiehong Cao,b Jianfeng Ping,b Yixian Wang,b Tingting Lin,a Xiao Huang,b,d Qinglang Ma,b Fuke Wang,*a Chaobin He*a,c and Hua Zhang*b
Three-dimensional graphene networks (3DGNs) doped with a mono-heteroatom of N or B, or dual-heteroatoms of N and B were fabricated, which exhibit excellent oxygen reduction reaction (ORR) performance. Importantly, the onset potential and current density of N and B co-doped 3DGNs are comparable to those of the commercial Pt (30%)/C catalyst.
Platinum (Pt) and Pt-based catalysts toward oxygen reduction reaction (ORR) are critical in fuel cells and lithium/air batteries due to their relatively low overpotential and high current density.1 However, their commercial application has been hampered by the limited Pt resource and their high cost.2 In addition, Pt catalysts suffer from the susceptibility to the timedependent drift, the crossover effect of methanol (CH3OH) and carbon monoxide (CO) poisoning.3 Therefore, numerous efforts have been devoted to reduce the usage of Pt catalysts or even replace them with non-precious metal catalysts,1,4,5 enzymatic electrocatalysts6 or metal-free materials.6–10 Among them, graphene doped with heteroatoms such as nitrogen (N),7–9,11–22 boron (B),8 sulfur (S)23–25 and phosphorus (P)14,19 has attracted great attention due to their exciting electronic and chemical properties as well as their high catalytic efficiency. In particular, N and B are the most studied hetero-
a Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 3 Research Link, Singapore, 117602. E-mail: [email protected], [email protected] b School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798. E-mail: [email protected], [email protected]; http://www.ntu.edu.sg/home/hzhang/ c Department of Materials Science and Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576 d Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergistic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (Nanjing Tech), 30 South Puzhu Road, Nanjing 211816, China † Electronic supplementary information (ESI) available: Details of the N-3DGN, B-3DGN and NB-3DGN fabrication process. Description of characterization. Rotating disk electrode linear sweep voltammograms of 3DGN and Pt (30%)/C in O2-saturated 0.1 M KOH with various rotation rates at a scan rate of 5 mV s−1. Koutecky–Levich plots of 3DGN, Pt (30%)/C, N-3DGN, B-3DGN and NB-3DGN at different electrode potentials. See DOI: 10.1039/c4nr06631f
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atoms for graphene doping as their atomic masses are closest to carbon. In addition, their atomic sizes would be acceptable for doping the carbon lattice of graphene. At the same time, they can tune the electronic properties of graphene due to the electron rich and electron deficient nature of N and B atoms, respectively. At present, the heteroatom doped graphene has been achieved by several different approaches such as chemical vapor deposition (CVD),11,20 thermal annealing8,15,18 and solvothermal synthesis.18 However, the aforementioned methods are effective only for producing singly-doped graphene but not for co-doped graphene. For example, the thermal annealing method was effective for the synthesis of N- or B-doped graphene, but in the presence of both B and N atoms, it produced the covalent B–C–N bonding in graphene instead of B and N co-doped graphene.8 In addition, this method also produced undesired by-products, such as hexagonal boron nitride, which is chemically inert and can reduce the activity of a catalyst. In particular, co-doping with B and N can create a unique electronic structure with a synergistic coupling effect between B and N because of the lower (B (χ = 2.04)) or higher (N (χ = 3.04)) electronegativity compared to C (χ = 2.55),26 which makes such co-doped products much more catalytically active than B- or N-doped graphene catalysts.8 Therefore, an efficient strategy that can be used for both mono-heteroatom doping and multi-heteroatom co-doping of graphene is highly required. In this work, the electrochemical method was demonstrated to successfully dope graphene with either a mono-heteroatom or dual-heteroatoms with high catalytic efficiencies for ORR. To obtain the excellent ORR result, a three-dimensional graphene network (3DGN) prepared by CVD27 was used in this work (Scheme 1). The doped products, i.e., N-doped, B-doped, and N and B co-doped 3DGN, referred to as N-3DGN, B-3DGN and NB-3DGN, respectively, with successful incorporation of either mono- or dual-heteroatoms into graphene lattices were confirmed by X-ray photoelectron spectroscopy (XPS), which also suggested that the formation of by-products could be prevented by our electrochemical doping method. Compared with the existing graphene doping technologies, our electrochemical method shows the obvious advantages, such as mild
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Scheme 1 Left: scheme of the working electrode. Right: the setup of doping 3DGN by the electrochemical method at room temperature. 3DGN, Pt and Ag/AgCl (sat. LiCl in ethanol) are used as WE, CE and RE, respectively.
doping conditions and easy change of dopants through simple exchange of dopant sources. Most importantly, the doped products obtained from the electrochemical method showed high electrocatalytic activity toward ORR. Among them, NB-3DGN showed the best activity, which is comparable to the commercial Pt(30%)/C. In addition, these doped products exhibited good stability and outstanding durability to CH3OH, suggesting that they are promising potential candidates for replacing Pt-based catalysts. In this work, 3DGN synthesized by CVD was chosen to demonstrate the electrochemical doping of graphene with heteroatoms because of its high specific surface area and excellent electrical and mechanical properties.27 In a typical electrochemical doping process, the 3DGN served as the working electrode directly, which was fabricated by fixing a piece of 3DGN27,28 onto a glass slide with silicon gel, and then placing silver paint on one end of the 3DGN to form a conductive pad. The doping was then carried out at room temperature with Ag/AgCl and Pt wires as reference and counter electrodes, respectively. The electrolyte containing N atom and/or B atom was dissolved in acetonitrile, which was also used as a dopant source. After the experiment, the doped 3DGN was rinsed with water to remove the electrolyte absorbed on its surface. Fig. 1 shows the scanning electron microscopy (SEM) images of 3DGN before and after electrochemical doping. As observed, the 3D network structure remained intact after the electrochemical doping. Usually, graphene can be chemically oxidized by using KMnO4, whose redox potential is even as high as ca. 1.5 V.29 In this work, a potential of 2 V (vs. Ag/AgCl) was applied to elec-
Fig. 1 SEM images of a three-dimensional graphene network (3DGN) before (a) and after (b) electrochemical doping.
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trochemically oxidized graphene films. During the electrochemical doping process, the electric field was precisely tuned through the potentiostat, which induced the adsorption of an electrolyte (dopant source) on the graphene surface. Therefore, the high potential (±2 V) used here is believed to be able to effectively make the cation (e.g., TBA+) or anion (e.g. BF4−) intercalate into the graphene layers with broken C–C bonds.29,30 During the doping process, 5 mV s−1 and 10 cycles were chosen as the scan rate and doping time for each doping process, respectively. Before the electrochemical doping, the cell was bubbled with pure N2 to remove the oxygen. In addition, the whole setup was kept in a desiccator in order to prevent water or oxygen to affect the reaction. Although the exact mechanism is not clear at the moment, it is supported by the XPS results (Fig. 2). Consequently, the electronic structure of graphene can be changed, which induces the band gap opening of graphene. In this work, mono-heteroatom like N or B and dual-heteroatoms of N and B have been successfully doped into graphene (see the detailed experimental procedure in the Electronic Supplementary Information), which were confirmed by XPS measurements. As shown in Fig. 2, the peaks of heteroatom-doped 3DGN at 284 and 532 eV were assigned to C 1s and O 1s, respectively. The O 1s peak might arise from the absorbed water molecules on the surface of the product during washing.31 The absence of a Ni peak in the XPS spectra for 3DGN indicates that the Ni foam was completely removed by the etchant solution containing FeCl3 and HCl during the sample preparation.27,28 The XPS spectra of N-3DGN (Fig. 2a) and B-3DGN (Fig. 2b) show N 1s and B 1s peaks, respectively, and the XPS spectrum of NB-3DGN (Fig. 2c) shows both N 1s and B 1s peaks, suggesting the successful incorporation of
Fig. 2 XPS spectra of (a) 3DGN and N-3DGN, (b) 3DGN and B-3DGN and (c) 3DGN and NB-3DGN. Insets: respective high-resolution N 1s peaks of N-3DGN (a) and NB-3DGN (c); respective high-resolution B 1s peaks of B-3DGN (b) and NB-3DGN (c).
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dopant atoms into the 3DGN. For the N doping, the high resolution N 1s spectra of N-3DGN and NB-3DGN (insets of Fig. 2a and c) show the presence of both pyridinic N (399.0 eV) and quaternary N (402.2 eV).13,32 The elemental analysis revealed that the atomic percentage of N in N-3DGN was about 2.7%, and the atomic ratio between quaternary N and pyridinic N was around 2 : 1. Differently, in NB-3DGN, the atomic percentage of N was 1.7% and the ratio between quaternary N and pyridinic N was 1 : 1. As for the B doping, the atomic percentage of B in B-3DGN and NB-3DGN was similar, both in the range of 2–2.5%. As shown in the insets of Fig. 2b and c, the B 1s peak was found to be centered at around 192 eV with a B atomic percentage of 2.1% in B-3DGN and 2.3% in NB-3DGN. This peak can be assigned to the partially oxidized BC2O nanodomains.23 Note that other common structures such as BN and BC3 were not observed in our products. Raman spectroscopy is a powerful technique to characterize graphitic carbons.13,19,31,32 The typical Raman spectra of 3DGN, N-3DGN, B-3DGN and NB-3DGN are shown in Fig. 3a. The peaks at 1350, 1582 and 2696 cm−1 can be assigned to the D, G and 2D bands of graphene, respectively. The G band is the in-plane vibrational mode and the 2D band corresponds to the double resonance scattering of two transverse optical (TO) phonons around the K-point of the Brillouin zone.19 Both of them are characteristic peaks of graphene. Moreover, the peak of the 2D band is sensitive to the number of graphene layers. The relatively strong and broad 2D band in both 3DGN and the doped products indicate that the 3DGN contains multilayered graphene.31 Unlike the 2D band, the D band does not depend on the number of graphene layers, but strictly depends on the amount of lattice disorder present in the structure. In the Raman spectrum of 3DGN, the weak and broad D band comes from the defects via an intervalley double-resonance Raman process.19 After doping, the intensities of the D band of the products (i.e., N-3DGN, B-3DGN and NB-3DGN) were increased significantly, while the intensities of their G and 2D bands were decreased compared to those of un-doped 3DGN. In general, the intensity ratio of the D to G band (ID/IG) can be used to evaluate the defect density in graphene.31 The ID/IG for 3DGN is 0.29, suggesting that 3DGN is highly crystalline (Fig. 3b).31 However, the ID/IG ratio for N-3DGN, B-3DGN and NB-3DGN was increased to 0.53, 0.41 and 0.96, respectively, indicating that the additional non-sp2 domains were created in
Fig. 3 (a) Raman spectra of 3DGN, N-3DGN, B-3DGN and NB-3DGN. (b) ID/IG and IG/I2D of 3DGN, N-3DGN, B-3DGN and NB-3DGN.
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the doped products. Moreover, the intensity ratio of the G to 2D band (IG/I2D) was increased from 0.93 for 3DGN to 1.11, 1.14 and 1.65 for N-3DGN, B-3DGN and NB-3DGN, respectively, further confirming the increased lattice disorder and the decreased overall crystallinity in the doped products.32 It should be noted that NB-3DGN showed the highest ratios of ID/IG and IG/I2D among these three doped products, which can be due to the largest amount of doped domains/defects in the NB-3DGN. The electrocatalytic performance of 3DGN, N-3DGN, B-3DGN and NB-3DGN toward ORR was carried out in 0.1 M KOH solution and the results are shown in Fig. 4a. Their loadings on the glassy carbon electrodes (GCE) are not the same in order to separate these curves from each other. For comparison, the electrocatalytic behavior of the bare GCE was also measured under the same experimental conditions. It is clear that none of the above electrodes exhibited a reduction peak
Fig. 4 (a) Cyclic voltammograms of oxygen reduction on the glassy carbon electrode (GCE), 3DGN, N-3DGN, B-3DGN and NB-3DGN electrodes in O2-saturated 0.1 M aqueous KOH solution at a scan rate of 50 mV s−1. The loadings of N-3DGN, B-3DGN and NB-3DGN on the GCE are not the same in order to separate these curves from each other. (b) Rotating disk electrode (RDE) voltammograms of 3DGN, N-3DGN, B-3DGN, NB-3DGN and Pt(30%)/C at a rotation rate of 1600 rpm. Inset: the onset potential of 3DGN, N-3DGN, B-3DGN, NB-3DGN and Pt(30%)/C. (c–e) RDE linear sweep voltammograms of N-3DGN (c), B-3DGN (d) and NB-3DGN (e) in O2-saturated 0.1 M KOH with various rotation rates at a scan rate of 5 mV s−1. (f ) The dependence of the electron-transfer number for 3DGN, N-3DGN, B-3DGN, NB-3DGN and Pt (30%)/C at various potentials.
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in the N2-saturated KOH solution. However, in the O2-saturated KOH solution, the reduction peaks of oxygen appeared at different potentials for the aforementioned five electrodes. Compared with 3DGN, the doped products i.e., N-3DGN, B-3DGN and NB-3DGN, exhibited positive shifts in the peak potential (i.e., from −386 to 200, 191 and 247 mV, respectively) and higher current density, which can be attributed to the incorporation of various dopants into the graphene structure.14 To gain insight into the electrocatalytic behaviors of 3DGN, N-3DGN, B-3DGN and NB-3DGN toward ORR, their rotating voltammograms were investigated (Fig. 4b). For comparison, the commercially available Pt catalyst, i.e., Pt(30%)/C, was also tested under the same experimental conditions. In a typical experiment, the same amount of 3DGN, N-3DGN, B-3DGN, NB-3DGN and Pt(30%)/C was loaded onto a glassy carbon rotating-disk electrode, respectively, and the geometrical area of the electrode was used here to calculate the current density.16,33 All of the voltammetric curves for different electrodes in the O2-saturated 0.1 M aqueous KOH solution were obtained at the same rotation rate of 1600 rpm. Interestingly, two O2 reduction waves appeared on the 3DGN-modified electrode, which can be documented as the two-electron reduction process of O2 to HO2−, and the further reduction of HO2− to OH− in alkaline medium.34,35 However, only one reduction wave was observed on N-3DGN, B-3DGN, NB-3DGN and Pt(30%)/C modified electrodes. This difference indicates that the catalytic processes of N-3DGN, B-3DGN, NB-3DGN and Pt(30%)/C are different from 3DGN. Moreover, the onset potentials for ORR at 3DGN, N-3DGN, B-3DGN, NB-3DGN and Pt(30%)/C were found at −0.29, −0.1, −0.08, −0.06 and −0.04 V, respectively (Fig. 4b). Obviously, N-3DGN, B-3DGN and NB-3DGN exhibit much better electrocatalytic activities compared to the un-doped 3DGN. Importantly, NB-3DGN showed the highest onset potential that is much closer to that of Pt(30%)/C, indicating that NB-3DGN has the best ORR catalytic properties among these doped 3DGN catalysts. To gain further insight into the electron transfer kinetics of N-3DGN, B-3DGN and NB-3DGN during the ORR, their behaviors in the rotating disk voltammetry at different rotating rates were tested. The current densities ( J) of N-3DGN (Fig. 4c), B-3DGN (Fig. 4d) and NB-3DGN (Fig. 4e) were increased with the rotation rate (ω) increasing from 400 to 2000 rpm. The relationship between J−1 and ω−1/2, i.e., the Koutecky–Levich plots (Fig. S1a, b and c†) can be obtained from the data displayed in Fig. 4c–e, respectively. These plots at different electrode potentials show good linearity and parallelism over the potential range from −0.4 to −0.8 V, which are considered as typical first-order reaction kinetics with respect to the concentration of dissolved O2. The number of transferred electrons (n) (Fig. 4f ) can be obtained from the slope of these plots in Fig. S1a–c,† respectively (see details in the ESI†). Similarly, the transferred electron number of 3DGN and Pt(30%)/C can also be obtained based on their corresponding Koutecky–Levich plots (Fig. S2†). For comparison, the electron-transfer number of ORR at −0.7 V for N-3DGN, B-3DGN and NB-3DGN was calculated to be 3.7, 3.6 and 3.8, respectively, indicating that the
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Fig. 5 (a) Chronoamperometric response of N-3DGN, B-3DGN and NB-3DGN at −0.25 V in O2-saturated 0.1 M KOH solution with a rotation rate of 1600 rpm. The loading of N-3DGN, B-3DGN and NB-3DGN on the GCE is not the same in order to separate these three curves from each other. (b) Chronoamperometric response plot of current density vs. time obtained at the Pt(30%)/C, N-3DGN, B-3DGN and NB-3DGN electrodes at −0.25 V in O2-saturated 0.1 M KOH with a rotation rate of 1600 rpm. The final concentration of CH3OH in the KOH solution is 3 M. The arrow indicates the addition of CH3OH into the O2-saturated 0.1 M aqueous KOH solution.
ORR with the aforementioned three catalysts is dominated by a four-electron process (Fig. S3†). However, the ORR on 3DGN (n = 1.8) was approximately a two-electron process (Fig. S3†). In addition, over a wide potential range from −0.4 to −0.8 V, the ORR on N-3DGN, B-3DGN and NB-3DGN is also dominated by a four-electron process, which is similar to that of Pt(30%)/C (Fig. 4f ). Differently, the n value for 3DGN (1.5–2.2) is much lower over the same potential range. The stability and tolerance to the crossover effect of CH3OH are two important properties for evaluating a new electrocatalyst. The stability of N-3DGN, B-3DGN and NB-3DGN was tested based on their current–time chronoamperometric responses for ORR. As shown in Fig. 5a, these three types of doped 3DGN exhibited high stability, which can be attributed to the strong covalent bonding between the active sites and the graphitic lattice. In addition, their tolerance to the crossover effect of CH3OH should also be considered since CH3OH can permeate through the polymer membrane from the anode to the cathode, which can severely affect the performance of the catalyst in the cathode. As shown in Fig. 5b, the current density of Pt(30%)/C sharply decreased immediately after the addition of CH3OH, while the current density of N-3DGN, B-3DGN and NB-3DGN decreased to ∼85%, 70% and 82% of their initial values after 3000 s, respectively. Therefore, N-3DGN, B-3DGN and NB-3DGN showed good tolerance to the crossover effect of CH3OH. The excellent selectivity towards ORR and remarkable tolerance against CH3OH poisoning of N-3DGN, B-3DGN and NB-3DGN make them promising metal-free catalysts for ORR in fuel cells and other catalytic applications.
Conclusions In summary, a simple electrochemical method has been successfully developed to dope graphene with mono- and dualheteroatoms to prepare efficient electrocatalysts for ORR. The
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successful doping was confirmed by the XPS and Raman analysis. The resulting products in this work, i.e., N-3DGN, B-3DGN and NB-3DGN, exhibited improved electrocatalytic activities toward ORR. Impressively, NB-3DGN showed the highest onset potential, which is comparable to the commercial Pt(30%)/C. Importantly, these products showed high stability and good tolerance to the crossover effect of CH3OH. Our electrochemical method might be used to incorporate other foreign atoms into graphene sheets for the preparation of novel mono- or multi-heteroatoms doped graphene for catalysis application.
Acknowledgements This work was financially supported by the Science and Engineering Research Council (grant 1123004024) of Agency for Science, Technology and Research of Singapore, MOE under AcRF Tier 2 (ARC 26/13, no. MOE2013-T2-1-034) and AcRF Tier 1 (RG 61/12, RGT18/13 and RG5/13), and Start-Up grant (M4080865.070.706022) in NTU. This research is also conducted by NTU-HUJ-BGU Nanomaterials for Energy and Water Management Programme under the Campus for Research Excellence and Technological Enterprise (CREATE), that is supported by the National Research Foundation, Prime Minister’s Office, Singapore. X. H. thanks the financial support from the National Natural Science Foundation of China (51322202) and Natural Science Foundation of Jiangsu Province (BK20130927).
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