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Enhancing the pyridinic N content of Nitrogen-doped graphene and improving its catalytic activity for oxygen reduction reaction
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 1
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Enhancing the pyridinic N content of Nitrogen-doped graphene and improving its catalytic activity for oxygen reduction reaction He Miao a,*, Shihua Li a,b, Zhouhang Wang a,c, Shanshan Sun b, Min Kuang a, Zhaoping Liu b, Jinliang Yuan a a
Faculty of Maritime and Transportation, National Traffic Management Engineering and Technology Research Centre, Ningbo University Sub-centre, Ningbo University, Ningbo 315211, PR China b Key Laboratory of Graphene Technologies and Applications of Zhejiang Province, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Zhejiang 315201, PR China c Jiangsu Province Collaborative Innovation Center for Modern Urban Traffic Technologies, Nanjing 210096, PR China
article info
abstract
Article history:
Nitrogen-doped graphene (NG) has been extensively studied as an effective candidates of
Received 13 June 2017
the oxygen reduction reaction (ORR) catalysts due to its high activities. Whereas, there are
Received in revised form
still some ambiguities or controversies on the contributions of the three N configurations
19 September 2017
(pyridinic N, pyrrolic N and graphitic N) for the ORR catalytic activity of NG. Herein, we
Accepted 24 September 2017
adopt a facile hydrothermal method followed by the annealing treatment at the different
Available online xxx
temperatures to tailor the contents of the three N configurations in NG. Then, the contributions of the N configurations on the ORR catalytic activities can be evaluated. The
Keywords:
annealing treatment can effectively enhance the content of pyridinic N in NG obtained
Nitrogen-doped graphene
from the hydrothermal method, and the pyridinic N content increases obviously with the
Oxygen reduction reaction catalysts
increase of the annealing temperature from 600 C to 800 C. The increase of the pyridinic N
Nitrogen configurations
content causes an improved catalytic activity of NG toward ORR, which confirms that
Active centers
pyridinic N is the active center of NG for ORR. In addition, the stability of NG also can be improved with the increase of the pyridinic N content. Our work will open up new avenues for the developments of the Nitrogen-doped graphene electrocatalysts of practical significance for ORR. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction For the fuel cells and metal air batteries, the oxygen reduction reaction catalysts (ORRCs) which can accelerate the cathode reaction are of great significance [1,2]. Traditionally, Pt-based ORRCs are the best ones, but they still have some
shortcomings including the scarcity, high price and poor stability [3e5]. Therefore, the development of the nonprecious ORRCs remains challenging, and many efforts have been made in this domain [1,6e12]. Recently, N-doped carbon nano-materials including the carbon nanotubes, carbon nanofibers and graphene have been
* Corresponding author. E-mail address: [email protected] (H. Miao). https://doi.org/10.1016/j.ijhydene.2017.09.138 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Miao H, et al., Enhancing the pyridinic N content of Nitrogen-doped graphene and improving its catalytic activity for oxygen reduction reaction, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.138
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considered as the effective ORRC candidates [13e19]. Among them, N-doped graphene (NG) has been investigated extensively and shows the high electrocatalytic activity and stability toward ORR [20]. It is reported that the limiting current density on the N-doped graphene electrode can be three times higher than that on the Pt/C electrode for ORR in alkaline electrolyte [18] besides the better long-term stability. Previously, it was well accepted that enhancing N content in NG could obviously improve its ORR catalytic activity [21,22]. Whereas, some reports indicated that the total N content in NG catalyst did not play an important role in the ORR process, and the content of the active center in NG might be the key parameter determining its ORR catalytic activity [23e25]. In this case, the tremendous efforts have been made to explore the active center of NG [23e28]. Generally, the high catalytic activity of N-doped carbon nano-materials can be related to its unique electronic structure which is caused by the incorporation of nitrogen atoms into the graphitic structure [29]. There are three kinds of N configurations (pyridinic N, pyrrolic N and graphitic N) in NG [30]. NG with the different N configurations demonstrates the diverse catalytic activities toward ORR due to their different electronic structures [31,32]. Nevertheless, there are still some ambiguities or controversies on the contributions of the three N configurations for the ORR catalytic activity of NG [17,19,23e28,33e37]. Most of the studies considered that pyridinic N or/and graphitic N were related to the active centers of NG for ORR. Xia et al. [37] indicated that the pyridine-like nitrogen component in NGs determined the electrocatalytic activity of NGs toward ORR. Luo et al. [28] also revealed that the pyridinic-N tended to be the most active N functional group to facilitate ORR at low overpotential via a four-electron pathway. Nakamura [25] found that the ORR active sites in N-doped graphene were the carbon atoms with Lewis basicity next to pyridinic N. In contrast, Li et al. [27] pointed out that the graphitic N configuration could facilitate the electrons transfer and improve the electrocatalytic activity in the ORR process, and the adjacent carbons should be the main active sites. Also, the studies of Dai et al. demonstrated that the electrondonating quaternary N sites were responsible for ORR [26]. Ruoff et al. [23] found that the electrocatalytic activity of the NG catalyst was dependent on the graphitic N content which determined the limiting current density, while the pyridinic N content improved the onset potential of ORR. Whereas, some reports indicated the catalytic activity of NG was determined by the pyrrolic N. Kurungot et al. [34] revealed that the enhanced proportions of the pyrrolic nitrogen along with the mesoporous structure of graphene were the main reasons for the enhanced catalytic activity towards ORR. It was reported that NG with more planar pyridinic and pyrrolic N exhibited the excellent electronic conductivity, high ORR activity, and good stability [33]. Guo et al. [38] also considered that both pyrrolic N and pyridinic N were the predominant origin of electrochemical activity of NG for ORR. The hydrothermal method which is efficient, simple and easy to scale-up is widely adopted to synthesize NG in the recent years [39e41]. Whereas, NG prepared by this method suffers from the lower contents of pyridinic N and graphitic N which are related to the active centers of NG for ORR in the
mainstream views [40,41]. In this work, N-doped graphene is synthesized by a facile hydrothermal method, and the three N groups can be easily tailored, especially the pyridinic N content increases obviously, by the followed annealing treatment at the different temperatures. Then, the ORR catalytic activity of NG can be obviously improved, and the contribution of the pyridinic N on the ORR catalytic activity of NG is confirmed.
Experimental Material synthesis Graphene oxide (GO) was synthesized from the flake graphite powder using a modified Hummers' method, and the detailed synthetic process was described as the follows: 3 g KNO3 and 3 g graphite were added into 138 ml of concentrated H2SO4 (98%) under stirring. After 10 min, 18 g KMnO4 was slowly added to the mixture, and the reaction flask was then subsequently heated to 40 C. The reaction was carried out for 6 h, during which the mixture became pasty. Subsequently, 240 ml deionized water was gradually poured into the reacted slurry under vigorous stirring. Then, the slurry was heated to 60 C for another 30 min. Afterwards, 600 ml deionized water and 18 ml H2O2 solution (30 wt%) were added in sequence, and the dispersed slurry turned from dark brown to yellow companied by bubbling. For next application, the resulting graphite oxide suspension was repeatedly washed by a large amount of deionized water until the pH value of the washing solution was about 5.0. Nitrogen doped reduced graphene oxide (N-RGO) was synthesized by one-step hydrothermal method with urea as the reducing and doping agents. Typically, a 10 ml of 5 mg/ml GO aqueous dispersion and 1.5 g urea were dispersed in 20 ml deionized water by the ultrasonication for 1 h. Then, the mixture was sealed in a 40 ml Teflon-lined autoclave and heated up to 180 C and maintained at this temperature for 12 h. The autoclave was then cooled to room temperature. The collected sample was immersed in the deionized water to remove the residual for many times and followed by the vacuum freeze-drying. The resulting product was denoted as N-RGO. Then, four N-RGO samples were heated to 600, 700, 800 and 900 C at a heating rate of 2 C/min and annealed for 1 h under high purity Ar flow, respectively. The samples obtained at the annealing temperatures of 600, 700, 800 and 900 C were denoted as N-RGO-600, N-RGO-700, N-RGO-800 and N-RGO-900, respectively. The scheme of N-GRO samples annealing at the different temperatures is shown in Fig. 1.
Characterization The morphologies and structures of the samples were observed by the Hitachi S-4800 field emission scanningelectron microscope (SEM) and transmission electronmicroscopy (TEM, JEOL-2100). X-ray photoelectron spectra (XPS) were recorded by an AXISULTARDLD spectroscopy with an Al-Ka X-ray source. Fourier transform infrared (FT-IR) spectra were obtained by a NICOLET 6700 spectroscopy. Raman spectra were recorded by a Renishaw in Via Reflex Raman Spectrometer with 532 nm-wavelength laser.
Please cite this article in press as: Miao H, et al., Enhancing the pyridinic N content of Nitrogen-doped graphene and improving its catalytic activity for oxygen reduction reaction, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.138
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Fig. 1 e Scheme of N-GRO annealing at the different temperatures.
Electrochemical measurements The ORR polarization curves of the samples were measured using the linear sweep voltammetry (LSV) on a CHI 1040B electrochemical workstation and rotating ring-disk electrode (RRDE) (Pine, American) with a three-electrode system in a 0.1 M KOH aqueous solution. A platinum wire and Hg/HgO electrode were used as the counter and reference electrodes, respectively. The working electrode was a catalyst-coated RRDE (Pt ring and glass carbon (GC) disk, disk diameter: 5.6 mm, ring: 6.25e7.92 mm, current collection efficiency: 37%). The preparation of the thin-film electrode was described as the followings: 10.0 mg of sample powder was dispersed in 2.0 mL ethanol plus 60 mL of 5 wt% Nafion solution and ultrasonically blended for 30 min to form a well-dispersed ink. Then, 10 mL of the ink was pipetted on the cleaned glassy carbon electrode and the solvent was evaporated at room temperature. The linear sweep voltammetry (LSV) was scanned from 1.1 to 0.1 V (vs. RHE) at the scan rate of 5 mV s1 with the rotation rate of the ring-disk electrode of 1600 rpm. The potential of Pt ring electrode was held at 1.423 V (vs. RHE). Before all the tests, Hg/ HgO electrode was calibrated with respect to the reversible hydrogen electrode (RHE). The calibration was performed in the 0.1 M KOH electrolyte saturated the high purity hydrogen with a platinum foil (3 3 cm2) as the working electrode. The cyclic voltammetry (CV) was performed at a potential scan rate of 1 mV s1, and the average of the two potentials at which the current crossed zero, which was taken to be the thermodynamic potential for the hydrogen electrode reaction. So, in 0.1 M KOH, E (RHE) ¼ E (Hg/HgO) þ 0.923 V. Prior to measurement, O2 was bubbled directly into the cell for at least 20 min to saturate the solution. During the measurement, O2 atmosphere was maintained. All experiments were conducted at ambient temperature (25 ± 2 C). For evaluating the electrocatalytic activity for ORR under acidic condition, LSV curves were measured in O2-saturated 0.5 M H2SO4 solution at the scan rate of 5 mV s1 with the electrode rotation rate of 1600 rpm. The rotating disk electrode (RDE, 0.196 cm2 geometric area), Pt wire and Ag/AgCl were used as the working, counter and reference electrodes, respectively. Chronoamperometry (CA) technique was applied to assess the durability of the different catalysts toward ORR in 0.1 mol L1 KOH O2-saturated electrolyte at the potential of 0.423 V (vs. RHE) for 45000 s, while the rotation rate of RRDE was fixed at 1600 rpm. In addition, LSV curves of the different catalysts were scanned from 1.1 to 0.1 V (vs. RHE) in
0.1 mol L1 KOH solution saturated with O2 at a scanning rate of 5 mV s1 before and after the aging test.
Results and discussion Fig. 2 shows the SEM images of N-RGO samples annealing at the different temperatures and TEM image of N-RGO-800 as a representative sample. All the samples show the typical graphene sheet structure with thin and wrinkled sheets. The morphologies of the five samples are almost same, indicating the impacts of the annealing treatment at the temperature range of 600e900 C on the morphologies of N-RGO sample are negligible. The FT-IR spectra of N-RGO, N-RGO-600, N-RGO700, N-RGO-800 and N-RGO-900 are shown in Fig. 3 (a). For all the samples, the dominant bands at 3400, 2912, 1565, 1383 and 1074 cm1 can be ascribed to the OeH, CeH, C]C, C]N/CeN and CeO groups, respectively [30,42,43]. Obviously, the main bands of the four samples are very approximate except for the slight difference of the band intensities. Specifically, the peak at 1383 cm1 corresponding to the stretching modes of C]N/ CeN bonds, becomes sharper and sharper with the increase of the annealing temperature, which can be related to the change of the N configurations [44]. Raman scattering is sensitive to electronic structure, and it has been used for distinguishing the ordered or disordered crystal structures of carbon-carbon and carbon-heteroatom bonds [41]. As shown in Fig. 3(b), the Raman spectra of the N-RGO, N-RGO-600, N-RGO-700, N-RGO-800 and N-RGO-900 display two peaks at 1348 and 1590 cm1, corresponding to the D and G bands, respectively [45]. The ratio of the D band to G band intensities (ID/IG) in the Raman spectrum is used as an important parameter to study crystalline or graphite-like carbon structures. The values of ID/IG of N-RGO, N-RGO-600, N-RGO-700, N-RGO-800 and N-RGO-900 are 1.025, 1.024, 1.021, 1.020 and 1.019, respectively, which slightly decreases with the increase of the annealing temperature. This can be related to the slight increase of the graphitization degree of the N-RGO samples at the high temperature. Fig. 4 (a) shows the N2-adsorption isotherms of the different samples. The BET surface areas of NRGO, NRGO-600, NRGO700, NRGO-800 and NRGO-900 are 177.6, 262.3, 280.4, 251.7 and 210.8 m2/g, respectively. Obviously, with the increase of the annealing temperature, the BET surface area increases firstly and then decreases, and it reaches the maximum value at the annealing temperature of 700 C. The proper annealing can
Please cite this article in press as: Miao H, et al., Enhancing the pyridinic N content of Nitrogen-doped graphene and improving its catalytic activity for oxygen reduction reaction, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.138
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Fig. 2 e SEM images of N-RGO (a), N-RGO-600 (b), N-RGO-700 (c), N-RGO-800 (d), N-RGO-900 (e) and TEM image of N-RGO-800 (f).
Fig. 3 e FT-IR spectra (a) and Raman spectra (b) of N-RGO, N-RGO-600, N-RGO-700, N-RGO-800 and N-RGO-900.
enhance the BET surface area, which had been reported elsewhere [46]. The pore-size distributions of the different N-doped graphene samples are shown in Fig. 4 (b). From the pore size distribution curves, it is noticed that the mesopores with widths of about 2e4 nm exist in all the samples. Moreover, with the increase of the annealing temperature, the numbers of the mesopore increase firstly and then decrease, and the NRGO-600 sample has the highest porosity. XPS characterizations are performed to analyze the elemental compositions and N configurations in NG. Fig. 5 (a) shows the XPS spectra of the N-RGO, N-RGO-600, N-RGO-700, N-RGO-800 and N-RGO-900 samples. From Fig. 3 (a), C1s, O1s and N1s can be recognized at 284.2 eV, 532 eV and 399 eV [47,48], respectively, and the atomic percentages of N1s in all the samples are listed in Table 1. It can be found that about 6.56%, 5.13%, 4.25%, 4.02% and 3.98% nitrogen are introduced into the graphene sheet for N-RGO, N-RGO-600, N-RGO-700, N-RGO-800 and N-RGO-900, respectively. Obviously, the N content in NG decreases with the increase of the annealing temperature. The drop of N content probably results from the elimination of some unstable N moieties at high temperature, which is consistent with the other's results [49]. This substantial change of nitrogen content with annealing temperature implies a self-arrangement or temperature-favored competition between the different CeN bonding configurations [50]. Fig. 5 (b)e(e) demonstrate the de-convoluted N1s regions of N-RGO, N-RGO-600, N-RGO-700, N-RGO-800 and N-RGO-900, respectively. The N1s spectrum can be de-convoluted to three individual peaks, which represent three nitrogen configurations within carbon structures. The binding energy centered at 398.3 eV, 400.2 eV and 401.8 eV can be assigned to the pyridinic N, pyrrolic N, and graphitic N, respectively [13]. Pyridinic-like nitrogen donates one p electron and is sp2 hybridized. Pyrrolic-like nitrogen donates two p-electrons to p
Please cite this article in press as: Miao H, et al., Enhancing the pyridinic N content of Nitrogen-doped graphene and improving its catalytic activity for oxygen reduction reaction, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.138
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Fig. 4 e Nitrogen adsorption/desorption isotherms of the different samples measured at 77 K (a); Pore distributions of the different N-doped graphene catalysts calculated from the desorption branch of N2 adsorption using the Barrett-Joyner-Halenda (BJH) formula (b).
system and is sp3-hybridized. Graphitic nitrogen refers to nitrogen atoms combined into a hexagonal ring by the substitution of carbon atoms. The pyridinic and pyrrolic N are always located at the graphitic edge, whereas quaternary N can be both “edge-N” and “bulk-like-N”. In order to reveal the chemical states of N in detail, the N1s spectra are decomposed into different peaks according to different aforementioned chemical states of N. Based on the simulations of N1s peak in the XPS curves, the atomic percentages of the pyridinic N, pyrrolic N and graphitic N in all the NG samples are calculated and listed in Table 1. Obviously, after the annealing treatment, the pyridinic N content in the NG samples increases from 1.85% (N-RGO) to 2.07% (N-RGO-800). Compared with that of the pyridinic N, the variation of the pyrrolic N content shows an inverse tendency after the annealing treatment, and it deceases from 3.94% (N-RGO) to 1.15% (N-RGO-900). This is because the pyrrolic N is not thermally stable and it easily converts to the other N groups at high annealing temperatures. Additionally, the graphitic N content decreases slightly from 0.77% (N-RGO) to 0.73% (N-RGO-800), and then increases
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to 0.76% (N-RGO-900) with the increase of annealing temperature. These results mean that, with the temperature increasing, the pyrrolic-N is converted to pyridinic-N and graphitic-N species. In another word, the annealing treatment can make the self-arrangement of the different CeN bonding configurations in NG prepared by the hydrothermal reaction. Specially, the content of the pyridinic N which is related to the active centers for ORR in the mainstream views can be enhanced by the annealing treatment at 600e900 C. To investigate the ORR catalytic activities of N-RGO, NRGO-600, N-RGO-700, N-RGO-800 and N-RGO-900, the linear scanning voltammetry (LSV) measurements were performed on the RRDE with the glassy carbon disk coated with the catalyst films, and the obtained electrochemical parameters are listed in Table 1. Fig. 6 (a) shows the disk current density (id) and ring current density (ir) collected on the different NG catalysts during ORR in O2-saturated KOH solution (0.1 mol L1) at a rotation rate of 1600 rpm. As shown in Fig. 6 (a) and Table 1, the ORR onset potentials (the potential under the polarization current of 100 mA) of the NG catalysts are 0.834 (N-RGO), 0.855 (N-RGO-600), 0.862 (N-RGO-700) and 0.876 V (NRGO-800) (vs. RHE), respectively, and shift positively with the increase of the annealing temperature. Whereas, the onset potential of N-RGO-900 is 0.863, which is a little lower than that of N-RGO-800. Comparing with the onset potential, the half-wave potentials (E1/2) and limiting current density (IL) of the NG catalysts show the same variation tendency with the increase of the annealing temperature. From LSV curves of NG catalysts, it can be obviously seen that N-RGO-800 has the highest catalytic activity among all the NG samples. It is well believed that the ring current is an important parameter to evaluate the ORR catalytic activity of ORRCs. As seen from Fig. 6 (a), the ring current density of the NG catalyst increases gradually with increase of the annealing temperature, and NRGO-800 presents the lowest ring current among the five samples at the whole scanning potential. The Tafel plots of the different NG catalysts in the potential region between 0.85 and 0.87 V (vs. RHE) derived from the corresponding LSV curves are shown in Fig. 6(b). The Tafel slopes generally are explained with respect to the coverage degree of adsorbed oxygen [51] and calculated from the Tafel equation [52]. As shown in Fig. 6(b) and Table 1, the Tafel slopes of the NG catalysts are 198.8 mV dec1, 95.3 mV dec1, 94.3 mV dec1, 77.5 mV dec1 and 76.8 mV dec1 for N-RGO, NRGO-600, N-RGO-700, N-RGO-800 and N-RGO-900, respectively. The Tafel slope of the NG catalysts gradually decrease with the increase of the annealing temperature, showing a slight difference with that of the onset potential, half-wave potential or limiting current density. This may be ascribed to the somewhat high specific surface area of N-RGO-900. From the RRDE measurement, the percentages of formed peroxide (HO 2 ) with respect to the total oxygen reduction products (X HO2 ) and the electron transferred numbers (n e ) can be calculated by the disk current (Idisk), the ring current (Iring) and the ring collection efficiency (N) with Eqs. (1) and (2), respectively [53]. Fig. 6 (c) shows the relations of X HO2 and n e with the scanning potential from 0.8 V to 0.1 V (vs. RHE). In the whole scanning potential range, the X HO2 value of the NG catalysts decreases with the increase of the annealing
Please cite this article in press as: Miao H, et al., Enhancing the pyridinic N content of Nitrogen-doped graphene and improving its catalytic activity for oxygen reduction reaction, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.138
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Fig. 5 e XPS spectra of the N-doped graphene catalysts (a), high-resolution N1s spectra of N-RGO (b), N-RGO-600 (c), N-RGO700 (d), N-RGO-800 (e) and N-RGO-900 (f).
temperature, except for that of N-RGO-900 which is between N-RGO-700 and N-RGO-800. Comparing with X HO2 , the n e value of the NG catalysts shows an inverse variation tendency with the annealing temperature. N-RGO-800 has the lowest
percentages of the formed peroxide and highest electron transferred numbers in the whole scanning potential. Specially, X HO2 and n e of N-RGO-800 correspond to 5.55% and 3.89 at 0.5 V (vs. RHE), respectively.
Please cite this article in press as: Miao H, et al., Enhancing the pyridinic N content of Nitrogen-doped graphene and improving its catalytic activity for oxygen reduction reaction, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.138
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Table 1 e Atomic percentages of the different N configurations and electrochemical parameters of the different N-doped graphene catalysts. Samples
Atomic percentages (at. %)
ORR performances
HO N 1s Pyridinic Pyrrolic Graphitic Eonset (V) E1/2 (V) IL (mA/cm2) neTafel slope 2% (0.5 V vs. RHE) (0.5 V vs. RHE) (mV dec1) N N N N-RGO N-RGO-600 N-RGO-700 N-RGO-800 N-RGO-900
6.56 5.13 4.25 4.02 3.98
1.85 1.90 2.02 2.08 2.07
3.94 2.48 1.49 1.21 1.15
0.77 0.75 0.74 0.73 0.76
0.834 0.855 0.862 0.876 0.863
0.698 0.704 0.705 0.725 0.721
2.86 4.63 4.90 5.21 5.28
3.27 3.80 3.84 3.89 3.87
36.30 10.10 7.94 5.55 6.20
198.8 95.3 94.3 77.5 76.8
Fig. 6 e Electrochemical performances of the N-doped graphene catalysts for ORR: LSV curves on RRDE in O2-saturated 0.1 M KOH at a rotation speed of 1600 rpm (a); Tafel curves (b); overall numbers of the transferred electrons (ne-) and contents of the HO¡ 2 species (c); LSV curves on RDE in O2-saturated 0.5 M H2SO4 at a rotation speed of 1600 rpm (d).
n¼
4ID ID þ INR
X HO2 ½% ¼ 100
(1)
2Iring N
Idisk þ
Iring N
(2)
Fig. 6 (d) shows the LSV curves on RDE in O2-saturated 0.5 M H2SO4 at a rotation speed of 1600 rpm. It can be seen clearly that the onset potential and half-wave potential
increase firstly and then decrease, and N-RGO-800 has the most positive onset and half-wave potentials among all the samples. With the increase of the annealing temperature, the variation tendencies of the onset and half-wave potentials in the acid condition (0.5 M H2SO4) are almost same with that in the alkaline condition (0.1 M KOH). In order to clarify the effects of N configurations and BET surface area on the ORR catalytic activity of N-doped graphene, the relationships of the critical ORR polarization
Please cite this article in press as: Miao H, et al., Enhancing the pyridinic N content of Nitrogen-doped graphene and improving its catalytic activity for oxygen reduction reaction, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.138
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parameters of the different NG catalysts (onset potentials, half-wave potentials and limiting current densities) with the contents of three N configurations (pyridinic N, pyrrolic N and graphitic N) and BET surface area are erected and shown in Fig. 7 (a). It can be found from Fig. 7 (a) that, with the increase of the annealing temperature, the variation tendencies of the critical ORR polarization parameters (onset potentials, halfwave potentials and limiting current densities) are not coincident with that of the BET surface area. This means that BET surface area is not the key factors determining the ORR activity of N-doped graphene. Whereas, the onset potential and half-wave potential shift more and more positive, and the limiting current density becomes larger and larger with the increase of the pyridinic N content. Whereas, the above parameters almost show the inverse variation tendency with
that of pyrrolic N and graphitic N. Up till now, as shown in Table 2, the controversies on the active center of the NG catalyst for ORR still exist. Our work confirms that the pyridinic N is related to the active center of the NG catalyst for ORR, and the scheme which describes the mechanism of pyridinic N-dependent ORR is shown in Fig. 7 (b). For evaluating the durability, the stability of N-RGO and NRGO-800 are measured by the chronoamperometric measurement on RRDE at 0.423 V (vs. RHE) in 0.1 mol L1 O2saturated KOH at the rotating rate of 1600 rpm for 45000 s. As can be seen in Fig. 8 (a), the current retention of N-RGO-800 is 84% after 45000s, exhibiting a much better stability than that of N-RGO corresponding to 76%. Furthermore, the LSV curves of N-RGO and N-RGO-800 in an O2-saturated 0.1 mol L1 KOH electrolyte before and after long-term durability test are
Fig. 7 e Relations between Eonset, E1/2, IL and contents of different N groups/BET surface area (a); mechanism scheme of pyridinic N-dependent ORR (b). Please cite this article in press as: Miao H, et al., Enhancing the pyridinic N content of Nitrogen-doped graphene and improving its catalytic activity for oxygen reduction reaction, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.138
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Table 2 e Active centers of the N-doped graphene catalysts for ORR in the recent reports and this work. Names of nitrogen doped graphene N-doped zigzag graphene ribbons (ZGNRs) [35] Polyaniline/RG-O and polypyrrole/RG-O [23] Mesoporous nitrogen-doped graphene (NGE) [34] Pyridinic- and pyrrolic-nitrogen-doped graphene (NG) [33] Nitrogen-doped graphene (NG) with dominance of the pyridinic-N configuration [28] Metal-free three-dimensional (3D) graphene nanoribbon networks (N-GRW) [26] Nitrogen doped graphene (N-graphene) [27] Highly oriented pyrolitic graphite (HOPG) model catalysts [25] Pyridinic nitrogen dominated graphene aerogels with iron incorporation (Fe-NG) [24] N-Graphene (800, 900, 1000) [36] N-RGO (this work)
Active centers of the nitrogen doped graphene Graphitic N next to the edge Graphitic N determines the limiting current density, and pyridinic N improves the onset potential Pyrrolic nitrogen along with the mesoporous structure of graphene The pyridinic and pyrrolic forms which have planar structures Pyridinic N The electron donating quaternary N sites Graphitic N configuration Carbon atoms with Lewis basicity next to pyridinic N Pyridinic N and Fe-Nx sites Quaternary type nitrogen species Pyridinic N
content has better stability than N-RGO with lower pyridinic N content. In another word, enhancing the pyridinic N content is beneficial to the stability of NG catalysts.
Conclusions In summary, the nonprecious ORRCs of nitrogen-doped reduced graphene oxide (N-RGO) was synthesized by a facile hydrothermal method followed by the annealing treatment. It is worth nothing that annealing at the different temperatures can tailor the contents of the different N configuration (pyridinic N, pyrrolic N and graphitic N) in NG catalysts. Specially, the pyridinic N content can be enhanced when the annealing temperature is below 900 C. The increase of the pyridinic N content can improve the ORR catalytic activity as well as stability of the NG catalysts. The pyridinic N is confirmed to be related to the active center of the NG catalyst for ORR. This work may shed light on the further exploration of the N-doped graphene as an excellent ORRC.
Acknowledgement This work was supported by the Key Research Program of the Chinese Academy of Sciences (Grant No. KGZD-EW-T08), National Key Research & Development Plan of China (Contract No. 2016YFC0205800) and K.C. Wong Magna Fund in Ningbo University.
Fig. 8 e Long-term durability of N-RGO and N-RGO-800: normalized current retention after 45000 s (a); LSV curves before and after 45000 s aging (b).
measured (Fig. 8 (b)). For the N-RGO-800 catalyst, the negative shift of the half-wave potential is 18 mV after the long time aging test. Whereas, the half-wave potential of N-RGO shifts about 32 mV toward the negative potential after the durability test. This means that N-RGO-800 with higher pyridinic N
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Enhancing the pyridinic N content of Nitrogen-doped graphene and improving its catalytic activity for oxygen reduction reaction He Miao a,*, Shihua Li a,b, Zhouhang Wang a,c, Shanshan Sun b, Min Kuang a, Zhaoping Liu b, Jinliang Yuan a a
Faculty of Maritime and Transportation, National Traffic Management Engineering and Technology Research Centre, Ningbo University Sub-centre, Ningbo University, Ningbo 315211, PR China b Key Laboratory of Graphene Technologies and Applications of Zhejiang Province, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Zhejiang 315201, PR China c Jiangsu Province Collaborative Innovation Center for Modern Urban Traffic Technologies, Nanjing 210096, PR China
article info
abstract
Article history:
Nitrogen-doped graphene (NG) has been extensively studied as an effective candidates of
Received 13 June 2017
the oxygen reduction reaction (ORR) catalysts due to its high activities. Whereas, there are
Received in revised form
still some ambiguities or controversies on the contributions of the three N configurations
19 September 2017
(pyridinic N, pyrrolic N and graphitic N) for the ORR catalytic activity of NG. Herein, we
Accepted 24 September 2017
adopt a facile hydrothermal method followed by the annealing treatment at the different
Available online xxx
temperatures to tailor the contents of the three N configurations in NG. Then, the contributions of the N configurations on the ORR catalytic activities can be evaluated. The
Keywords:
annealing treatment can effectively enhance the content of pyridinic N in NG obtained
Nitrogen-doped graphene
from the hydrothermal method, and the pyridinic N content increases obviously with the
Oxygen reduction reaction catalysts
increase of the annealing temperature from 600 C to 800 C. The increase of the pyridinic N
Nitrogen configurations
content causes an improved catalytic activity of NG toward ORR, which confirms that
Active centers
pyridinic N is the active center of NG for ORR. In addition, the stability of NG also can be improved with the increase of the pyridinic N content. Our work will open up new avenues for the developments of the Nitrogen-doped graphene electrocatalysts of practical significance for ORR. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction For the fuel cells and metal air batteries, the oxygen reduction reaction catalysts (ORRCs) which can accelerate the cathode reaction are of great significance [1,2]. Traditionally, Pt-based ORRCs are the best ones, but they still have some
shortcomings including the scarcity, high price and poor stability [3e5]. Therefore, the development of the nonprecious ORRCs remains challenging, and many efforts have been made in this domain [1,6e12]. Recently, N-doped carbon nano-materials including the carbon nanotubes, carbon nanofibers and graphene have been
* Corresponding author. E-mail address: [email protected] (H. Miao). https://doi.org/10.1016/j.ijhydene.2017.09.138 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Miao H, et al., Enhancing the pyridinic N content of Nitrogen-doped graphene and improving its catalytic activity for oxygen reduction reaction, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.138
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considered as the effective ORRC candidates [13e19]. Among them, N-doped graphene (NG) has been investigated extensively and shows the high electrocatalytic activity and stability toward ORR [20]. It is reported that the limiting current density on the N-doped graphene electrode can be three times higher than that on the Pt/C electrode for ORR in alkaline electrolyte [18] besides the better long-term stability. Previously, it was well accepted that enhancing N content in NG could obviously improve its ORR catalytic activity [21,22]. Whereas, some reports indicated that the total N content in NG catalyst did not play an important role in the ORR process, and the content of the active center in NG might be the key parameter determining its ORR catalytic activity [23e25]. In this case, the tremendous efforts have been made to explore the active center of NG [23e28]. Generally, the high catalytic activity of N-doped carbon nano-materials can be related to its unique electronic structure which is caused by the incorporation of nitrogen atoms into the graphitic structure [29]. There are three kinds of N configurations (pyridinic N, pyrrolic N and graphitic N) in NG [30]. NG with the different N configurations demonstrates the diverse catalytic activities toward ORR due to their different electronic structures [31,32]. Nevertheless, there are still some ambiguities or controversies on the contributions of the three N configurations for the ORR catalytic activity of NG [17,19,23e28,33e37]. Most of the studies considered that pyridinic N or/and graphitic N were related to the active centers of NG for ORR. Xia et al. [37] indicated that the pyridine-like nitrogen component in NGs determined the electrocatalytic activity of NGs toward ORR. Luo et al. [28] also revealed that the pyridinic-N tended to be the most active N functional group to facilitate ORR at low overpotential via a four-electron pathway. Nakamura [25] found that the ORR active sites in N-doped graphene were the carbon atoms with Lewis basicity next to pyridinic N. In contrast, Li et al. [27] pointed out that the graphitic N configuration could facilitate the electrons transfer and improve the electrocatalytic activity in the ORR process, and the adjacent carbons should be the main active sites. Also, the studies of Dai et al. demonstrated that the electrondonating quaternary N sites were responsible for ORR [26]. Ruoff et al. [23] found that the electrocatalytic activity of the NG catalyst was dependent on the graphitic N content which determined the limiting current density, while the pyridinic N content improved the onset potential of ORR. Whereas, some reports indicated the catalytic activity of NG was determined by the pyrrolic N. Kurungot et al. [34] revealed that the enhanced proportions of the pyrrolic nitrogen along with the mesoporous structure of graphene were the main reasons for the enhanced catalytic activity towards ORR. It was reported that NG with more planar pyridinic and pyrrolic N exhibited the excellent electronic conductivity, high ORR activity, and good stability [33]. Guo et al. [38] also considered that both pyrrolic N and pyridinic N were the predominant origin of electrochemical activity of NG for ORR. The hydrothermal method which is efficient, simple and easy to scale-up is widely adopted to synthesize NG in the recent years [39e41]. Whereas, NG prepared by this method suffers from the lower contents of pyridinic N and graphitic N which are related to the active centers of NG for ORR in the
mainstream views [40,41]. In this work, N-doped graphene is synthesized by a facile hydrothermal method, and the three N groups can be easily tailored, especially the pyridinic N content increases obviously, by the followed annealing treatment at the different temperatures. Then, the ORR catalytic activity of NG can be obviously improved, and the contribution of the pyridinic N on the ORR catalytic activity of NG is confirmed.
Experimental Material synthesis Graphene oxide (GO) was synthesized from the flake graphite powder using a modified Hummers' method, and the detailed synthetic process was described as the follows: 3 g KNO3 and 3 g graphite were added into 138 ml of concentrated H2SO4 (98%) under stirring. After 10 min, 18 g KMnO4 was slowly added to the mixture, and the reaction flask was then subsequently heated to 40 C. The reaction was carried out for 6 h, during which the mixture became pasty. Subsequently, 240 ml deionized water was gradually poured into the reacted slurry under vigorous stirring. Then, the slurry was heated to 60 C for another 30 min. Afterwards, 600 ml deionized water and 18 ml H2O2 solution (30 wt%) were added in sequence, and the dispersed slurry turned from dark brown to yellow companied by bubbling. For next application, the resulting graphite oxide suspension was repeatedly washed by a large amount of deionized water until the pH value of the washing solution was about 5.0. Nitrogen doped reduced graphene oxide (N-RGO) was synthesized by one-step hydrothermal method with urea as the reducing and doping agents. Typically, a 10 ml of 5 mg/ml GO aqueous dispersion and 1.5 g urea were dispersed in 20 ml deionized water by the ultrasonication for 1 h. Then, the mixture was sealed in a 40 ml Teflon-lined autoclave and heated up to 180 C and maintained at this temperature for 12 h. The autoclave was then cooled to room temperature. The collected sample was immersed in the deionized water to remove the residual for many times and followed by the vacuum freeze-drying. The resulting product was denoted as N-RGO. Then, four N-RGO samples were heated to 600, 700, 800 and 900 C at a heating rate of 2 C/min and annealed for 1 h under high purity Ar flow, respectively. The samples obtained at the annealing temperatures of 600, 700, 800 and 900 C were denoted as N-RGO-600, N-RGO-700, N-RGO-800 and N-RGO-900, respectively. The scheme of N-GRO samples annealing at the different temperatures is shown in Fig. 1.
Characterization The morphologies and structures of the samples were observed by the Hitachi S-4800 field emission scanningelectron microscope (SEM) and transmission electronmicroscopy (TEM, JEOL-2100). X-ray photoelectron spectra (XPS) were recorded by an AXISULTARDLD spectroscopy with an Al-Ka X-ray source. Fourier transform infrared (FT-IR) spectra were obtained by a NICOLET 6700 spectroscopy. Raman spectra were recorded by a Renishaw in Via Reflex Raman Spectrometer with 532 nm-wavelength laser.
Please cite this article in press as: Miao H, et al., Enhancing the pyridinic N content of Nitrogen-doped graphene and improving its catalytic activity for oxygen reduction reaction, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.138
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Fig. 1 e Scheme of N-GRO annealing at the different temperatures.
Electrochemical measurements The ORR polarization curves of the samples were measured using the linear sweep voltammetry (LSV) on a CHI 1040B electrochemical workstation and rotating ring-disk electrode (RRDE) (Pine, American) with a three-electrode system in a 0.1 M KOH aqueous solution. A platinum wire and Hg/HgO electrode were used as the counter and reference electrodes, respectively. The working electrode was a catalyst-coated RRDE (Pt ring and glass carbon (GC) disk, disk diameter: 5.6 mm, ring: 6.25e7.92 mm, current collection efficiency: 37%). The preparation of the thin-film electrode was described as the followings: 10.0 mg of sample powder was dispersed in 2.0 mL ethanol plus 60 mL of 5 wt% Nafion solution and ultrasonically blended for 30 min to form a well-dispersed ink. Then, 10 mL of the ink was pipetted on the cleaned glassy carbon electrode and the solvent was evaporated at room temperature. The linear sweep voltammetry (LSV) was scanned from 1.1 to 0.1 V (vs. RHE) at the scan rate of 5 mV s1 with the rotation rate of the ring-disk electrode of 1600 rpm. The potential of Pt ring electrode was held at 1.423 V (vs. RHE). Before all the tests, Hg/ HgO electrode was calibrated with respect to the reversible hydrogen electrode (RHE). The calibration was performed in the 0.1 M KOH electrolyte saturated the high purity hydrogen with a platinum foil (3 3 cm2) as the working electrode. The cyclic voltammetry (CV) was performed at a potential scan rate of 1 mV s1, and the average of the two potentials at which the current crossed zero, which was taken to be the thermodynamic potential for the hydrogen electrode reaction. So, in 0.1 M KOH, E (RHE) ¼ E (Hg/HgO) þ 0.923 V. Prior to measurement, O2 was bubbled directly into the cell for at least 20 min to saturate the solution. During the measurement, O2 atmosphere was maintained. All experiments were conducted at ambient temperature (25 ± 2 C). For evaluating the electrocatalytic activity for ORR under acidic condition, LSV curves were measured in O2-saturated 0.5 M H2SO4 solution at the scan rate of 5 mV s1 with the electrode rotation rate of 1600 rpm. The rotating disk electrode (RDE, 0.196 cm2 geometric area), Pt wire and Ag/AgCl were used as the working, counter and reference electrodes, respectively. Chronoamperometry (CA) technique was applied to assess the durability of the different catalysts toward ORR in 0.1 mol L1 KOH O2-saturated electrolyte at the potential of 0.423 V (vs. RHE) for 45000 s, while the rotation rate of RRDE was fixed at 1600 rpm. In addition, LSV curves of the different catalysts were scanned from 1.1 to 0.1 V (vs. RHE) in
0.1 mol L1 KOH solution saturated with O2 at a scanning rate of 5 mV s1 before and after the aging test.
Results and discussion Fig. 2 shows the SEM images of N-RGO samples annealing at the different temperatures and TEM image of N-RGO-800 as a representative sample. All the samples show the typical graphene sheet structure with thin and wrinkled sheets. The morphologies of the five samples are almost same, indicating the impacts of the annealing treatment at the temperature range of 600e900 C on the morphologies of N-RGO sample are negligible. The FT-IR spectra of N-RGO, N-RGO-600, N-RGO700, N-RGO-800 and N-RGO-900 are shown in Fig. 3 (a). For all the samples, the dominant bands at 3400, 2912, 1565, 1383 and 1074 cm1 can be ascribed to the OeH, CeH, C]C, C]N/CeN and CeO groups, respectively [30,42,43]. Obviously, the main bands of the four samples are very approximate except for the slight difference of the band intensities. Specifically, the peak at 1383 cm1 corresponding to the stretching modes of C]N/ CeN bonds, becomes sharper and sharper with the increase of the annealing temperature, which can be related to the change of the N configurations [44]. Raman scattering is sensitive to electronic structure, and it has been used for distinguishing the ordered or disordered crystal structures of carbon-carbon and carbon-heteroatom bonds [41]. As shown in Fig. 3(b), the Raman spectra of the N-RGO, N-RGO-600, N-RGO-700, N-RGO-800 and N-RGO-900 display two peaks at 1348 and 1590 cm1, corresponding to the D and G bands, respectively [45]. The ratio of the D band to G band intensities (ID/IG) in the Raman spectrum is used as an important parameter to study crystalline or graphite-like carbon structures. The values of ID/IG of N-RGO, N-RGO-600, N-RGO-700, N-RGO-800 and N-RGO-900 are 1.025, 1.024, 1.021, 1.020 and 1.019, respectively, which slightly decreases with the increase of the annealing temperature. This can be related to the slight increase of the graphitization degree of the N-RGO samples at the high temperature. Fig. 4 (a) shows the N2-adsorption isotherms of the different samples. The BET surface areas of NRGO, NRGO-600, NRGO700, NRGO-800 and NRGO-900 are 177.6, 262.3, 280.4, 251.7 and 210.8 m2/g, respectively. Obviously, with the increase of the annealing temperature, the BET surface area increases firstly and then decreases, and it reaches the maximum value at the annealing temperature of 700 C. The proper annealing can
Please cite this article in press as: Miao H, et al., Enhancing the pyridinic N content of Nitrogen-doped graphene and improving its catalytic activity for oxygen reduction reaction, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.138
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Fig. 2 e SEM images of N-RGO (a), N-RGO-600 (b), N-RGO-700 (c), N-RGO-800 (d), N-RGO-900 (e) and TEM image of N-RGO-800 (f).
Fig. 3 e FT-IR spectra (a) and Raman spectra (b) of N-RGO, N-RGO-600, N-RGO-700, N-RGO-800 and N-RGO-900.
enhance the BET surface area, which had been reported elsewhere [46]. The pore-size distributions of the different N-doped graphene samples are shown in Fig. 4 (b). From the pore size distribution curves, it is noticed that the mesopores with widths of about 2e4 nm exist in all the samples. Moreover, with the increase of the annealing temperature, the numbers of the mesopore increase firstly and then decrease, and the NRGO-600 sample has the highest porosity. XPS characterizations are performed to analyze the elemental compositions and N configurations in NG. Fig. 5 (a) shows the XPS spectra of the N-RGO, N-RGO-600, N-RGO-700, N-RGO-800 and N-RGO-900 samples. From Fig. 3 (a), C1s, O1s and N1s can be recognized at 284.2 eV, 532 eV and 399 eV [47,48], respectively, and the atomic percentages of N1s in all the samples are listed in Table 1. It can be found that about 6.56%, 5.13%, 4.25%, 4.02% and 3.98% nitrogen are introduced into the graphene sheet for N-RGO, N-RGO-600, N-RGO-700, N-RGO-800 and N-RGO-900, respectively. Obviously, the N content in NG decreases with the increase of the annealing temperature. The drop of N content probably results from the elimination of some unstable N moieties at high temperature, which is consistent with the other's results [49]. This substantial change of nitrogen content with annealing temperature implies a self-arrangement or temperature-favored competition between the different CeN bonding configurations [50]. Fig. 5 (b)e(e) demonstrate the de-convoluted N1s regions of N-RGO, N-RGO-600, N-RGO-700, N-RGO-800 and N-RGO-900, respectively. The N1s spectrum can be de-convoluted to three individual peaks, which represent three nitrogen configurations within carbon structures. The binding energy centered at 398.3 eV, 400.2 eV and 401.8 eV can be assigned to the pyridinic N, pyrrolic N, and graphitic N, respectively [13]. Pyridinic-like nitrogen donates one p electron and is sp2 hybridized. Pyrrolic-like nitrogen donates two p-electrons to p
Please cite this article in press as: Miao H, et al., Enhancing the pyridinic N content of Nitrogen-doped graphene and improving its catalytic activity for oxygen reduction reaction, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.138
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Fig. 4 e Nitrogen adsorption/desorption isotherms of the different samples measured at 77 K (a); Pore distributions of the different N-doped graphene catalysts calculated from the desorption branch of N2 adsorption using the Barrett-Joyner-Halenda (BJH) formula (b).
system and is sp3-hybridized. Graphitic nitrogen refers to nitrogen atoms combined into a hexagonal ring by the substitution of carbon atoms. The pyridinic and pyrrolic N are always located at the graphitic edge, whereas quaternary N can be both “edge-N” and “bulk-like-N”. In order to reveal the chemical states of N in detail, the N1s spectra are decomposed into different peaks according to different aforementioned chemical states of N. Based on the simulations of N1s peak in the XPS curves, the atomic percentages of the pyridinic N, pyrrolic N and graphitic N in all the NG samples are calculated and listed in Table 1. Obviously, after the annealing treatment, the pyridinic N content in the NG samples increases from 1.85% (N-RGO) to 2.07% (N-RGO-800). Compared with that of the pyridinic N, the variation of the pyrrolic N content shows an inverse tendency after the annealing treatment, and it deceases from 3.94% (N-RGO) to 1.15% (N-RGO-900). This is because the pyrrolic N is not thermally stable and it easily converts to the other N groups at high annealing temperatures. Additionally, the graphitic N content decreases slightly from 0.77% (N-RGO) to 0.73% (N-RGO-800), and then increases
5
to 0.76% (N-RGO-900) with the increase of annealing temperature. These results mean that, with the temperature increasing, the pyrrolic-N is converted to pyridinic-N and graphitic-N species. In another word, the annealing treatment can make the self-arrangement of the different CeN bonding configurations in NG prepared by the hydrothermal reaction. Specially, the content of the pyridinic N which is related to the active centers for ORR in the mainstream views can be enhanced by the annealing treatment at 600e900 C. To investigate the ORR catalytic activities of N-RGO, NRGO-600, N-RGO-700, N-RGO-800 and N-RGO-900, the linear scanning voltammetry (LSV) measurements were performed on the RRDE with the glassy carbon disk coated with the catalyst films, and the obtained electrochemical parameters are listed in Table 1. Fig. 6 (a) shows the disk current density (id) and ring current density (ir) collected on the different NG catalysts during ORR in O2-saturated KOH solution (0.1 mol L1) at a rotation rate of 1600 rpm. As shown in Fig. 6 (a) and Table 1, the ORR onset potentials (the potential under the polarization current of 100 mA) of the NG catalysts are 0.834 (N-RGO), 0.855 (N-RGO-600), 0.862 (N-RGO-700) and 0.876 V (NRGO-800) (vs. RHE), respectively, and shift positively with the increase of the annealing temperature. Whereas, the onset potential of N-RGO-900 is 0.863, which is a little lower than that of N-RGO-800. Comparing with the onset potential, the half-wave potentials (E1/2) and limiting current density (IL) of the NG catalysts show the same variation tendency with the increase of the annealing temperature. From LSV curves of NG catalysts, it can be obviously seen that N-RGO-800 has the highest catalytic activity among all the NG samples. It is well believed that the ring current is an important parameter to evaluate the ORR catalytic activity of ORRCs. As seen from Fig. 6 (a), the ring current density of the NG catalyst increases gradually with increase of the annealing temperature, and NRGO-800 presents the lowest ring current among the five samples at the whole scanning potential. The Tafel plots of the different NG catalysts in the potential region between 0.85 and 0.87 V (vs. RHE) derived from the corresponding LSV curves are shown in Fig. 6(b). The Tafel slopes generally are explained with respect to the coverage degree of adsorbed oxygen [51] and calculated from the Tafel equation [52]. As shown in Fig. 6(b) and Table 1, the Tafel slopes of the NG catalysts are 198.8 mV dec1, 95.3 mV dec1, 94.3 mV dec1, 77.5 mV dec1 and 76.8 mV dec1 for N-RGO, NRGO-600, N-RGO-700, N-RGO-800 and N-RGO-900, respectively. The Tafel slope of the NG catalysts gradually decrease with the increase of the annealing temperature, showing a slight difference with that of the onset potential, half-wave potential or limiting current density. This may be ascribed to the somewhat high specific surface area of N-RGO-900. From the RRDE measurement, the percentages of formed peroxide (HO 2 ) with respect to the total oxygen reduction products (X HO2 ) and the electron transferred numbers (n e ) can be calculated by the disk current (Idisk), the ring current (Iring) and the ring collection efficiency (N) with Eqs. (1) and (2), respectively [53]. Fig. 6 (c) shows the relations of X HO2 and n e with the scanning potential from 0.8 V to 0.1 V (vs. RHE). In the whole scanning potential range, the X HO2 value of the NG catalysts decreases with the increase of the annealing
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Fig. 5 e XPS spectra of the N-doped graphene catalysts (a), high-resolution N1s spectra of N-RGO (b), N-RGO-600 (c), N-RGO700 (d), N-RGO-800 (e) and N-RGO-900 (f).
temperature, except for that of N-RGO-900 which is between N-RGO-700 and N-RGO-800. Comparing with X HO2 , the n e value of the NG catalysts shows an inverse variation tendency with the annealing temperature. N-RGO-800 has the lowest
percentages of the formed peroxide and highest electron transferred numbers in the whole scanning potential. Specially, X HO2 and n e of N-RGO-800 correspond to 5.55% and 3.89 at 0.5 V (vs. RHE), respectively.
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Table 1 e Atomic percentages of the different N configurations and electrochemical parameters of the different N-doped graphene catalysts. Samples
Atomic percentages (at. %)
ORR performances
HO N 1s Pyridinic Pyrrolic Graphitic Eonset (V) E1/2 (V) IL (mA/cm2) neTafel slope 2% (0.5 V vs. RHE) (0.5 V vs. RHE) (mV dec1) N N N N-RGO N-RGO-600 N-RGO-700 N-RGO-800 N-RGO-900
6.56 5.13 4.25 4.02 3.98
1.85 1.90 2.02 2.08 2.07
3.94 2.48 1.49 1.21 1.15
0.77 0.75 0.74 0.73 0.76
0.834 0.855 0.862 0.876 0.863
0.698 0.704 0.705 0.725 0.721
2.86 4.63 4.90 5.21 5.28
3.27 3.80 3.84 3.89 3.87
36.30 10.10 7.94 5.55 6.20
198.8 95.3 94.3 77.5 76.8
Fig. 6 e Electrochemical performances of the N-doped graphene catalysts for ORR: LSV curves on RRDE in O2-saturated 0.1 M KOH at a rotation speed of 1600 rpm (a); Tafel curves (b); overall numbers of the transferred electrons (ne-) and contents of the HO¡ 2 species (c); LSV curves on RDE in O2-saturated 0.5 M H2SO4 at a rotation speed of 1600 rpm (d).
n¼
4ID ID þ INR
X HO2 ½% ¼ 100
(1)
2Iring N
Idisk þ
Iring N
(2)
Fig. 6 (d) shows the LSV curves on RDE in O2-saturated 0.5 M H2SO4 at a rotation speed of 1600 rpm. It can be seen clearly that the onset potential and half-wave potential
increase firstly and then decrease, and N-RGO-800 has the most positive onset and half-wave potentials among all the samples. With the increase of the annealing temperature, the variation tendencies of the onset and half-wave potentials in the acid condition (0.5 M H2SO4) are almost same with that in the alkaline condition (0.1 M KOH). In order to clarify the effects of N configurations and BET surface area on the ORR catalytic activity of N-doped graphene, the relationships of the critical ORR polarization
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parameters of the different NG catalysts (onset potentials, half-wave potentials and limiting current densities) with the contents of three N configurations (pyridinic N, pyrrolic N and graphitic N) and BET surface area are erected and shown in Fig. 7 (a). It can be found from Fig. 7 (a) that, with the increase of the annealing temperature, the variation tendencies of the critical ORR polarization parameters (onset potentials, halfwave potentials and limiting current densities) are not coincident with that of the BET surface area. This means that BET surface area is not the key factors determining the ORR activity of N-doped graphene. Whereas, the onset potential and half-wave potential shift more and more positive, and the limiting current density becomes larger and larger with the increase of the pyridinic N content. Whereas, the above parameters almost show the inverse variation tendency with
that of pyrrolic N and graphitic N. Up till now, as shown in Table 2, the controversies on the active center of the NG catalyst for ORR still exist. Our work confirms that the pyridinic N is related to the active center of the NG catalyst for ORR, and the scheme which describes the mechanism of pyridinic N-dependent ORR is shown in Fig. 7 (b). For evaluating the durability, the stability of N-RGO and NRGO-800 are measured by the chronoamperometric measurement on RRDE at 0.423 V (vs. RHE) in 0.1 mol L1 O2saturated KOH at the rotating rate of 1600 rpm for 45000 s. As can be seen in Fig. 8 (a), the current retention of N-RGO-800 is 84% after 45000s, exhibiting a much better stability than that of N-RGO corresponding to 76%. Furthermore, the LSV curves of N-RGO and N-RGO-800 in an O2-saturated 0.1 mol L1 KOH electrolyte before and after long-term durability test are
Fig. 7 e Relations between Eonset, E1/2, IL and contents of different N groups/BET surface area (a); mechanism scheme of pyridinic N-dependent ORR (b). Please cite this article in press as: Miao H, et al., Enhancing the pyridinic N content of Nitrogen-doped graphene and improving its catalytic activity for oxygen reduction reaction, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.138
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Table 2 e Active centers of the N-doped graphene catalysts for ORR in the recent reports and this work. Names of nitrogen doped graphene N-doped zigzag graphene ribbons (ZGNRs) [35] Polyaniline/RG-O and polypyrrole/RG-O [23] Mesoporous nitrogen-doped graphene (NGE) [34] Pyridinic- and pyrrolic-nitrogen-doped graphene (NG) [33] Nitrogen-doped graphene (NG) with dominance of the pyridinic-N configuration [28] Metal-free three-dimensional (3D) graphene nanoribbon networks (N-GRW) [26] Nitrogen doped graphene (N-graphene) [27] Highly oriented pyrolitic graphite (HOPG) model catalysts [25] Pyridinic nitrogen dominated graphene aerogels with iron incorporation (Fe-NG) [24] N-Graphene (800, 900, 1000) [36] N-RGO (this work)
Active centers of the nitrogen doped graphene Graphitic N next to the edge Graphitic N determines the limiting current density, and pyridinic N improves the onset potential Pyrrolic nitrogen along with the mesoporous structure of graphene The pyridinic and pyrrolic forms which have planar structures Pyridinic N The electron donating quaternary N sites Graphitic N configuration Carbon atoms with Lewis basicity next to pyridinic N Pyridinic N and Fe-Nx sites Quaternary type nitrogen species Pyridinic N
content has better stability than N-RGO with lower pyridinic N content. In another word, enhancing the pyridinic N content is beneficial to the stability of NG catalysts.
Conclusions In summary, the nonprecious ORRCs of nitrogen-doped reduced graphene oxide (N-RGO) was synthesized by a facile hydrothermal method followed by the annealing treatment. It is worth nothing that annealing at the different temperatures can tailor the contents of the different N configuration (pyridinic N, pyrrolic N and graphitic N) in NG catalysts. Specially, the pyridinic N content can be enhanced when the annealing temperature is below 900 C. The increase of the pyridinic N content can improve the ORR catalytic activity as well as stability of the NG catalysts. The pyridinic N is confirmed to be related to the active center of the NG catalyst for ORR. This work may shed light on the further exploration of the N-doped graphene as an excellent ORRC.
Acknowledgement This work was supported by the Key Research Program of the Chinese Academy of Sciences (Grant No. KGZD-EW-T08), National Key Research & Development Plan of China (Contract No. 2016YFC0205800) and K.C. Wong Magna Fund in Ningbo University.
Fig. 8 e Long-term durability of N-RGO and N-RGO-800: normalized current retention after 45000 s (a); LSV curves before and after 45000 s aging (b).
measured (Fig. 8 (b)). For the N-RGO-800 catalyst, the negative shift of the half-wave potential is 18 mV after the long time aging test. Whereas, the half-wave potential of N-RGO shifts about 32 mV toward the negative potential after the durability test. This means that N-RGO-800 with higher pyridinic N
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