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A graphene-based electrocatalyst co-doped with nitrogen and cobalt 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 6 ) 1 e8
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A graphene-based electrocatalyst co-doped with nitrogen and cobalt for oxygen reduction reaction Huijuan Wu a,1, Chaozhong Guo a,b,*,1, Jiaqiang Li a,1, Zili Ma a,c, Qiyun Feng a, Changguo Chen a,** a
College of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, China Research Institute for New Materials Technology, Chongqing University of Arts and Sciences, Yongchuan, Chongqing 402160, China c Huadian Electric Power Research Institute, Hangzhou 310030, China b
article info
abstract
Article history:
The development of low cost, active and stable non-precious metal (Co-N/C) catalysts to
Received 20 May 2016
replace commercial Pt-based catalysts for oxygen reduction reaction (ORR) is a hot topic to
Received in revised form
this day. In this work, we have synthesized a new graphene-based catalyst (Co-N/C-A)
12 September 2016
doped by cobalt and nitrogen atoms with high contents of pyridinic- and pyrrolic-nitrogen
Accepted 12 September 2016
(planar nitrogen), which exhibits ORR electrocatalytic activity with onset and peak po-
Available online xxx
tentials of 0.035 and 0.082 V (versus Hg/HgO) in 0.1 mol l1 KOH solution, respectively. Besides, it has much higher stability and tolerance to methanol compared to the com-
Keywords:
mercial Pt/C catalyst. The overall electron transfer number is calculated to be about 3.8.
Milk powder
The transition-metal (Co) in the precursor can promote the formation of active sites during
Graphene oxide
pyrolysis process and the followed acid-treatment may effectively expose the active sites
Non-precious metal catalyst
on the surface of the catalyst, helping to enhance the ORR activity. It can be also proposed
Oxygen reduction
that pyridinic- and pyrrolic-nitrogen groups may play a key role in the ORR process and serve as the ORR active centers. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Nowadays, high-performance fuel cells rapidly become the ideal power supply because of their high energy conversion efficiency, environmentally friendly and abundant fuel sources [1]. However, the extensive usage of fuel cells has been hampered thanks to the slow kinetics and diversified ways of the oxygen reduction reaction (ORR) [2,3]. Traditionally, the carbon-based Pt catalyst is considered as the most effective
electrocatalyst to catalyze the ORR owing to their high intrinsic activity [4]. However, their large-scale commercial applications are hindered because of high cost and scarcity of metal-Pt resource [5,6]. Apart from these drawbacks, Pt-based electrocatalysts are also subjected to low durability and organic molecule deactivation [7]. Therefore, the development of inexpensive non-Pt electrocatalysts with high ORR activity, long-term stability and good tolerance to organic molecules is of paramount importance so far.
* Corresponding author. College of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, China. ** Corresponding author. E-mail addresses: [email protected] (C. Guo), [email protected] (C. Chen). 1 These authors equally contributed to this work, and they were considered as co-first authors. http://dx.doi.org/10.1016/j.ijhydene.2016.09.074 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Wu H, et al., A graphene-based electrocatalyst co-doped with nitrogen and cobalt for oxygen reduction reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.074
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Since Jasinsky interestingly found that cobalt-phthalo cyanine (CoPc) has exhibited a significant activity towards the ORR in 1964 [8], many researchers are actively focused on non-precious metal-nitrogen-carbon (M-N/C) catalysts for the ORR. For example, Maruyama et al. prepared a cathode catalyst by simple carbonization of hemoglobin in nitrogen atmosphere [9]. An earlier literature [10,11] also reported the preparation of an ORR catalyst from natural organic catalase. However, these kinds of ORR catalysts have poor long-term stability and low ORR electrocatalytic activity [12]. More importantly, some studies adequately demonstrate that the heat-treatment at high temperature can enhance the electrochemical stability and ORR catalytic activity of M-N/C catalysts [13,14], but it can destroy the intrinsic structure of macrocyclic complexes inside the catalyst [15]. Thereafter, it is significantly pointed out that even if there are not chemical bonds between central metal and macrocyclic ligand inside the complex, the prepared M-N/ C catalysts can still exhibit high catalytic activity after heattreatment, which is further confirmed by Gupta et al. [16]. With continuous research, some researchers have prepared highly active ORR catalysts by the simple use of nitrogencontaining compounds, such as melamine, dicyandiamide and urea, to replace macrocyclic complexes [17e19]. Our group has also formed several ORR catalysts from pyrolysis of protein-enriched biomass with outstanding long-term stability, and excellent methanol tolerance [20e22]. However, there is a large controversy about the active sites for ORR of M-N/C catalyst until now. It is largely believed that the nitrogenmodified carbon structure may be the electrocatalytically ORR-active site [23,24], whereas the transition-metal only plays a role in promoting the formation of ORR active site [25,26]. Here we report a new strategy to design the Co-N/C-A catalyst by using cheap milk biomass as the nitrogen source, graphene oxide (GO) as the carbon source and CoCl2$6H2O as the metal source, followed by an acid-treatment process. The structure characterization and electrocatalytic activity towards the ORR in alkaline electrolytes were evaluated in detail. We interestingly found that this catalyst can exhibit excellent ORR activity, high durability and good tolerance to methanol molecule, which may be a promising alternative to the commercial Pt catalysts.
Experimental
we have named it as “N/C”. We further prepared the “GO-900” and “N-900” catalysts through the pyrolysis of GO and milk in same condition, respectively.
Physical characterizations The X-ray diffraction (XRD) analysis was conducted on ShiA) madzu XRD-6000 (Japan) with Cu Ka1 radiation (l ¼ 1.54178 at 4 min1. The scanning electron microscopic (SEM) images were performed on FEI Quanta 200 Scanning Electron Microscope. High-resolution transmission electron microscopy (HR-TEM) was carried out on FEI Tecnai F30 instrument and acceleration voltage is 300 kV. The X-ray photoelectron spectroscopy (XPS) analysis was carried out by using a VG Scientific ESCALAB 220 I XL spectroscopy with an Al Ka (hn ¼ 1486.69 eV) X-ray source.
Electrochemical measurements All electrochemical tests were performed on CHI 660 electrochemical workstation (CH Instruments, USA) at room temperature. The electrolyte was 0.1 mol l1 KOH solution, which was purged by nitrogen or oxygen for 30 min prior to the electrochemical test. A conventional three-electrode cell was adopted, which included an Hg/HgO/1 mol l1 KOH electrode as the reference electrode, and a ring-shaped Pt wire electrode as the counter electrode, respectively. A rotation disk electrode (RDE) with a glass carbon (GC, 4 mm diameter, 0.1256 cm2 geometric area) electrode (5-mm diameter, LKXZ-1, Tianjing Lanlike Electrochemical Instruments, China) was employed as the working electrode. To prepare the modified-GC-RDE working electrode, the doped-carbon catalyst was well-dispersed in the 0.5 wt.% Nafion/ethanol solution. 5.0 ml of 10 mg ml1 dispersion was transferred onto the GC-RDE surface and then dried at room temperature. The mass loading was calculated to be ~400 mg cm2. A commercial Pt/C catalyst (20 wt.% Pt) on the GC electrode was prepared in the same way. Linear sweep voltammetry (LSV) and cyclic voltammetry (CV) experiments were performed over the potential range of 0.3 to 0.7 V at a scan rate of 5 mV s1 in 0.1 mol l1 KOH solution. The number of electron transfer (n) per oxygen molecule was calculated by KouteckyeLevich (KeL) equation [25]: 1 jd ¼ 1 jk þ 1 Bu1=2
(1)
B ¼ 0:62nFCO DO n1=6
(2)
2=3
Catalyst preparations All catalysts were prepared by pyrolysis process in a tube furnace. 0.05 g of GO nanosheets (provided by Jiangshu Nano Co.), 0.05 g of pure milk powder (purchased from local supermarket), and 0.15 g of CoCl2$6H2O were absolutely mixed and milled in a mortar. The obtained sample was then pyrolyzed in flowing N2 at 900 C for 1 h to obtain the “Co-N/C”. In order to further examine the real role of metal in the ORR process, the prepared Co-N/C catalyst was chemically treated with 0.5 mol l1 H2SO4 solution at room temperature to remove metal Co or its compounds. After acid-treatment, the produced sample was hereafter called “Co-N/C-A”. Besides, as a control, a mixture of GO nanosheet and milk powder was also annealed in N2 atmosphere at 900 C for 1 h. Analogously,
where jd was the measured current density; jk was the kinetic current density of the ORR; F was the Faraday constant (96,485 C mol1); CO was the concentration of dissolved oxygen (1.2 106 mol cm3); DO was the diffusion coefficient of oxygen (1.9 105 cm2 s1); n was the kinetic viscosity of the solution (0.01 cm2 s1); and u was the electrode rotation rate (rpm).
Results and discussion The structural analysis of the catalysts Fig. 1a shows the XRD patterns of the obtained products. A pronounced peak at ~26 and a weak peak at ~43 are assigned
Please cite this article in press as: Wu H, et al., A graphene-based electrocatalyst co-doped with nitrogen and cobalt for oxygen reduction reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.074
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 6 ) 1 e8
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Fig. 1 e The XRD pattern (a) and SEM images of N/C (b), Co-N/C (c), Co-N/C-A (d).
to the 002 and 101 diffraction peaks of graphite, respectively. The 002 diffraction peak at ~26 indicates the presence of a small domain with parallel stacking of graphene sheets, whereas the 101 diffraction peak at ~43 is attributed to the fibrous/spherical structures formed by sp2 hybridized carbons [6]. It is importantly found that the diffraction peaks of Co and CoO have been mainly observed, while those of graphite are very weak in Co-N/C, which suggests that Co and CoO may cover the catalytically active sites exposed on the surface. After acid-treatment, the diffraction peaks of graphite structure can be clearly observed in Co-N/C-A, while those of crystalline Co or CoO can hardly be found due to the dissolution of Co or CoO in H2SO4 solution. According to previous reports [20], the formation of cobalt oxides may be owing to the re-oxidization of metal Co by the oxygen species at hightemperature pyrolysis process. Fig. 1bed depicts high-resolution SEM images of the prepared catalysts. As shown in Fig .1b, the N/C catalyst contains multilayer graphene structures and carbon particles with a size of hundreds of nanometers. The formation of carbon particles with a size of hundreds of nanometers can be attributed to the incomplete decomposition and further birdnesting of milk powder during pyrolysis process. Compared with the SEM images of un-doped GO material [17], there is no major change in the surface morphology during N-doping. For the Co-N/C catalyst in Fig. 1c, it can be clearly shown that many crystal particles are uniformly deposited on the carbon sheets. Associated with the results of XRD, we can conclude that the crystal particles are Co or CoO. After the acidtreatment, the SEM images of the obtained Co-N/C-A catalyst and the Co-N/C catalyst have an obvious distinction. As shown in Fig. 1d, it's hardly to observe the crystal particles on the carbon sheets for the Co-N/C-A catalyst. In contrast, lots of pores are formed on the surface. These results imply that the
cobalt may be basically dissolved in the acid-treatment process, which is consistent with the results of XRD. We further characterize the morphology of the Co-N/C-A catalyst by high-resolution TEM technique. Fig. S1 clearly indicates the wrinkled graphene nanosheets with defect edges, perhaps owing to the doping of nitrogen atoms into the carbon skeleton. In addition, several crooked lattice fringes at the edge of the Co-N/C-A catalyst are found and many nanopores are also formed inside the Co-N/C-A catalyst. To probe the surface compositions and chemical surroundings of N/C and Co-N/C-A catalysts, the XPS analyses were carried out, as in Fig. 2. In Fig. 2a, the full-scan XPS spectra reveals the presence of C, N and O, but the Co 2p peak also appears at the Co-N/C-A catalyst, which suggest that most metallic Co has been removed by acid treatment, whereas residual metallic Co that survived the acid treatment may be encased in the carbon structure [27]. The observed N 1s peak reveals the successful doping of nitrogen into the carbon structure of GO nanosheets. The high-resolution XPS spectra for the C 1s region (Fig. 2b) indicates the presence of different chemical bonds: C]C (284.6 ± 0.1 eV), CeN (285.6 ± 0.2 eV), CeO (286.4 ± 0.1 eV), and OeC]O (288.5 ± 0.1 eV) [28]. The CeN functional groups further confirm the formation of N-doped carbon material inside the prepared catalysts. Besides, high-resolution N 1s XPS spectra are shown in Fig. 2c, and various types of nitrogen-doped species were identified by a curve-fitting method. The N 1s spectra can be deconvoluted into four peaks, which corresponds to the formation of pyridinic-N, pyrrolic-N, quaternary-N and oxidized-N with the binding energies of 398.6 ± 0.1 eV, 400.4 ± 0.1 eV, 401.1 ± 0.1 eV and 403.2 ± 0.2 eV [7], respectively. Table 1 shows the quantitative analysis results of four types of nitrogen-containing groups. Among the four types of N atoms, only the pyridinic-N and pyrrolic-N
Please cite this article in press as: Wu H, et al., A graphene-based electrocatalyst co-doped with nitrogen and cobalt for oxygen reduction reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.074
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Fig. 2 e The full-scanning XPS spectrum (a), XPS spectra for C1s region (b) and N1s region (c) of N/C and Co-N/C-A and XPS spectrum for Co 2p region of Co-N/C-A (d).
have been proved to be active in ORR [29,30]. Particularly, the pyrrolic-N has a lone electron pair in the plane of the carbon matrix, resulting in an increase of electron-donor property for the catalysts and the enhancement of the ORR catalytic activity [31]. By contrast, the pyridinic-N is also considered to significantly increase the ORR stability in the previous reports [32,33]. Therefore, the catalyst with high contents of pyridinicN and pyrrolic-N is positive to the ORR. As shown in Table 1, the Co-N/C-A catalyst with the addition of Co has higher percentage of pyridinic-N and pyrrolic-N with 26.6% and 41.3% than the N/C catalyst with 18.4% and 34.0%. Fig. 2d displays the Co 2p XPS spectra, which is deconvoluted into four peaks. Among them, the peaks centered at ~780.4 eV and ~795.8 eV are from Co 2p3/2 and Co 2p1/2, respectively. Other peaks centered at ~783.7 eV and ~803.5 eV are two shake-up type peaks of Co. This clearly shows the existence of Co2þ in the sample [34]. Above results show that quaternary-N and oxidized-N are largely transferred to pyridinic-N and pyrrolicN with cobalt doping and the followed acid treatment.
Electrocatalytic activity towards the ORR To evaluate the electrocatalytic performance of the prepared catalysts toward the ORR, the linear sweeping voltammetry
(LSV) was used to study the activity of the catalysts in 0.1 mol l1 KOH solution saturated with oxygen. As shown in Fig. 3a, the peak potential (Ep) of GO nanosheets is detected to be 0.23 V, which is more negative than that of GO-900 (Ep ¼ 0.14 V). It demonstrates that the heat-treatment process can make large contribution to effectively generate more ORR active sites [35,36]. Compared with GO-900 and N-900, the N/C catalyst shows a significant improvement of ORR performance with more positive ORR peak and onset potentials [37,38]. These results absolutely suggest that both nitrogen and carbon are indispensable to enhance the ORR activity. Fig. 3b shows the LSV results for the catalyzed electrodes with different catalysts in 0.1 mol l1 N2 or O2-saturated KOH electrolyte. No visible peaks can be observed in N2-saturated electrolyte for the N/C catalyst in the potential range of 0.7 to 0.3 V. On the contrary, when the LSV test has been conducted in O2-saturated KOH electrolyte, a well-defined ORR peak at 0.11 V is obtained, implying that this catalyst possesses excellent ORR catalytic activity in the alkaline electrolyte. In addition, the onset potential and peak potential of the Co-N/C catalyst have positively shifted comparing with those of the N/C catalyst. After acid-treatment, the losses of the onset potential (Eonset) and peak potential for Co-N/C are only 5 and 18 mV, respectively, but the limited ORR current density of Co-
Table 1 e The XPS data for the N 1s region of N/C and Co-N/C-A from Fig. 2. Sample
N/C [%]
Pyridinic-N [%] (398.6 ± 0.1 eV)
Pyrrolic-N [%] (400.4 ± 0.1 eV)
Quaternary-N [%] (401.1 ± 0.1 eV)
Oxidized-N [%] (403.2 ± 0.2 eV)
N/C Co-N/C-A
2.41 2.87
18.4 26.6
34.0 41.3
28.2 15.0
19.4 17.1
Please cite this article in press as: Wu H, et al., A graphene-based electrocatalyst co-doped with nitrogen and cobalt for oxygen reduction reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.074
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Fig. 3 e (a) LSV curves of GO, GO-900, N-900 and N/C in O2-saturated 0.1 mol l¡1 KOH solution at a scan rate of 5 mV s¡1. (b) LSV curves of N/C, Co-N/C and Co-N/C-A in O2-saturated and N/C in N2-saturated 0.1 mol l¡1 KOH solution at a scan rate of 5 mV s¡1. LSV curves of N/C (c) and Co-N/C-A (e) on GC-RDE in O2-saturated 0.1 mol l¡1 KOH solution with various rotation rates at a scan rate of 5 mV s¡1 and KouteckyeLevich plots of N/C (d) and Co-N/C-A (f).
N/C-A reaches almost twice as much as that of Co-N/C. These results indicate that the existence of metal Co in the heattreatment can promote the formation of active sites, and the followed acid-treatment is a need for the exposure of more active sites for ORR. To investigate the kinetics of the ORR on N/C and Co-N/CA catalysts, we measured the ORR polarization curves at various rotating rates, as shown in Fig. 3c and e. A series of KouteckyeLevich (KeL) plots (j1 vs. u1/2) are also obtained in Fig. 3d and f. The corresponding KeL plots at different electrode potentials show good linearity, and the slopes remain a constant value over the potential range from 0.5 to 0.7 V, indicating a similar electron-transfer number per O2 molecule involved in the O2 reduction. The dependence of n on the electrode potential in N/C and Co-N/C-A catalyzed electrodes is displayed as the insets in Fig. 3d and f. For the N/ C catalyzed electrode, the average n value is 2.5, indicating that the ORR process on the N/C catalyst can undergo a twoand four-electron mixed pathway. However, for the Co-N/C-A catalyzed electrode, the number (n) of electron transfer in the
ORR process is 3.8, indicating that it mainly favors a fourelectron pathway. In practical applications, long-term stability and tolerance to the methanol are important considerations for cathode catalysts of fuel cells. The electrochemical stability of the catalyst was carefully examined by an accelerated aging test (AAT) in O2-saturated 0.1 mol l1 KOH solution over the potential range of 0.7 Ve0.3 V, as shown in Fig. 4. The N/C and Co-N/C-A catalysts exhibit stable ORR performance after the AAT, with the Eonset losses of 10 and 15 mV, respectively. However, the commercial 40% Pt/C catalyst reveals an obvious degradation after the AAT, with an Eonset loss of 80 mV. Above results demonstrate that the Co-N/C-A catalyst has a better long-term stability compared with the commercial Pt-based catalyst for the ORR in alkaline medium. The low stability for Pt-based catalysts may be attributed to the migration or aggregation of Pt nanoparticles during continuous potential cycling [39,40]. To explore the tolerance performance to methanol molecule, the amperometric iet responses of the prepared
Please cite this article in press as: Wu H, et al., A graphene-based electrocatalyst co-doped with nitrogen and cobalt for oxygen reduction reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.074
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Based on above results, this paper can get the following conclusions: (i) the metal Co may not be involved in catalytically active structures as there is no peak of Co-Nx found in the N 1s XPS spectra of Co-N/C-A; (ii) the metal Co may only play a role in promoting the formation of catalytically active structures on the basis of higher ORR activity of Co-N/C-A than N/C; (iii) the doping of Co and acid-treatment can facilitate the large transformation of quaternary- and oxidized-N to pyridinic- and pyrrolic-N active sites for the ORR.
Conclusions
Fig. 4 e LSV curves for N/C, Co/N/C-A and Pt/C before and after AAT in 0.1 mol l¡1 KOH solution on RDE at a rotation rate of 1600 rpm.
catalysts were recorded in Fig. 5. A step-down decrease in cathodic current response upon the successive addition of 5.0 mol l1 methanol can be clearly observed. It is obvious that the chronoamperometric response of the Pt/C catalyst dramatically changes in terms of current density jumping from 0.3 mA cm2 to 3 mA cm2, which is caused by the electrochemical oxidization of methanol molecule. However, for N/C and Co-N/C-A catalysts, the chronoamperometric responses remain fairly balanced tendency while the sequential addition of the methanol solution is performed at an interval of 200 s, meaning that the prepared catalysts possesses strong tolerance to the methanol molecule. Our results indicate that the Co-N/C-A catalyst has a superior long-term stability and good tolerance against the methanol molecule.
A highly active cobalt and nitrogen co-doped carbon-based catalyst has been prepared by the pyrolysis of milk biomass, metallic Co and graphite oxide at 900 C for 1 h, followed by an acid treatment process. Owing to high contents of pyridinic and pyrrolic-N groups, the obtained Co-N/C-A catalyst exhibits excellent electrocatalytic activity for ORR in 0.1 M KOH with onset and peak potentials of 0.035 and 0.082 V (versus. Hg/HgO) and electro-transfer number of 3.8. Compared to Pt/ C, the Co-N/C-A catalyst has a superior stability and tolerance against the methanol molecule. Additionally, doping of Co and the followed acid-treatment facilitate the transformation of quaternary-N and oxidized-N to pyridinic-N and pyrrolic-N, helping to improve the ORR performance. Our study may provide an open mind for easy synthesis of high performance non-precious metal catalysts by design of the composition of the catalyst precursor for a range of significant electrochemical reactions.
Acknowledgments This work was financially supported by the Scientific and Technological Research Program of Chongqing Municipal Education Commission (KJ1501118), the Basic and Frontier Research Program of Chongqing Municipality (cstc2015jcyjA50032, cstc2014jcyjA50038), Chongqing University Scientific Research Training Project (CQU-SRTP-2015501), Sharing Fund of Chongqing University's Large-Scale Euipment (201412150152), and the National Natural Science Foundation of China (21273292).
Appendix A. Supplementary data Supplementary data related to this article can be found online at http://dx.doi.org/10.1016/j.ijhydene.2016.09.074
references Fig. 5 e Amperometric iet curves obtained at N/C, Co/N/C-A and Pt/C in N2-saturated 0.1 mol l¡1 KOH solution under magnetic stirring (600 rpm) and N2-protection over 0e300 s, followed by an immediate introduction of O2. The applied potential is ¡0.1 V and arrows indicate the sequential addition of 5.0 mol l¡1 methanol.
[1] Yang ZR, Wu J, Zheng XJ, Wang ZJ, Yang RZ. Enhanced catalytic activity for the oxygen reduction reaction with codoping of phosphorus and iron in carbon. J Power Sources 2015;277:161e8. [2] Lu ZS, Xu GL, He CZ, Wang TX, Yang L, Yang ZX, et al. Novel catalytic activity for oxygen reduction reaction on MnN4
Please cite this article in press as: Wu H, et al., A graphene-based electrocatalyst co-doped with nitrogen and cobalt for oxygen reduction reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.074
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[3]
[4]
[5]
[6]
[7]
[8] [9]
[10]
[11]
[12] [13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
embedded graphene: a dispersion-corrected density function theory study. Carbon 2015;84:500e8. Zhang XM, Liu L, Zhao Z, Tu BF, Ou DR, Cui DA, et al. Enhanced oxygen reduction activity and solid oxide fuel cell performance with a nanoparticles-loaded cathode. Nano Lett 2015;15:1703e9. Zhang SM, Chen SL. Enhanced-electrocatalytic activity of Pt nanoparticles supported on nitrogen-doped carbon for the oxygen reduction reaction. J Power Sources 2013;240:60e5. Geng DS, Chen Y, Chen YG, Li YL, Li RY, Sun XL, et al. High oxygen-reduction activity and durability of nitrogen-doped graphene. Energy Environ Sci 2011;4:760e4. Wang CH, Yang CW, Lin YC, Chang ST, Chang SLY. Cobaltiron oxide hybrid catalysis with enhanced catalytic activities for oxygen reduction in anion exchange membrane fuel cell. J Power Sources 2015;277:147e54. Zahoor A, Christy M, Hwang YJ, Lin YR, Kim P, Nahm KS. Improved electrocatalytic activity of carbon materials by nitrogen doping. Appl Catal B 2014;147:633e41. Jasinski R. New fuel cell cathode catalyst. Nature 1964;201:1212e3. Maruyama J, Abe I. Carbonized hemoglobin functioning as a cathode catalyst for polymer electrolyte fuel cells. Chem Mater 2006;18:1303e11. Lai ME, Bergel A. Electrochemical reduction of oxygen on glass carbon: catalysis by catalase. J Electroanal Chem 2000;494:30. Maruyama J, Abe I. Formation of platinum-free fuel cell cathode catalyst with highly developed nanospace by carbonizing catalase. Chem Mater 2005;17:4660e7. Bashyam R, Zelenay P. A class of non-precious metal composite catalysts for fuel cells. Nature 2006;443:63e6. Ahluwalia RK, Wang X, Kwon J, Rousseau A, Kalinoski J, James B, et al. Performance and cost of automotive fuel cell systems with ultra-low platinum loadings. J Power Sources 2011;196:4619e30. Jaouen F, Proietti E, Lefevre M, Chenitz R, Dodelet JP, Wu G, et al. Recent advances in non-precious metal catalysis for oxygen-reduction reaction in polymer electrolyte fuel cells. Energy Environ Sci 2011;4:114e30. Weng LT, Bertrand P, Lalande G, Guay D, Dodelet JP. Surface characterization by time-of-flight SIMS of a catalyst for oxygen electroreduction: pyrolyzed cobalt phthalocyanineon-carbon black. Appl Surf Sci 1995;84:9e21. Gupta S, Tryk D, Bae I, Aldred W, Yeager E. Heat-treated polyacrylonitrile-based catalysts for oxygen electroreduction. J Appl Electrochem 1989;19:19e27. Vikkisk M, Kruusenberg I, Joost U, Shulga E, Kink I, Tammeveski K. Electrocatalytic oxygen reduction on nitrogen-doped graphene in alkaline media. Appl Catal B 2014;147:369e76. Sheng ZH, Shao L, Chen JJ, Bao WJ, Wang FB, Xia XH. Catalyst-free synthesis of nitrogen-doped graphene via thermal annealing graphite oxide with melamine and its excellent electrocatalysis. ACS Nano 2011;5:4350e8. Wang HB, Maiyalagan T, Wang X. Review on recent progress in nitrogen-doped graphene: synthesis characterization, and its potential applications. ACS Catal 2012;2:781e94. Guo CZ, Chen CG, Luo ZL. A novel nitrogen-containing electrocatalyst for oxygen reduction reaction from blood protein pyrolysis. J Power Sources 2014;245:841e5. Guo CZ, Chen CG, Luo ZL. Electrocatalytic activity for the oxygen reduction on the novel carbon material containing nitrogen prepared by using two-step pyrolysis of hemolymph protein. Int J Electrochem Sci 2013;8:8940e50. Guo CZ, Liao WL, Li ZB, Sun LT, Chen CG. Easy conversion of protein-rich enoki mushroom biomass to a nitrogen-doped
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
7
carbon nanomaterial as a promising metal-free catalyst for oxygen. Nanoscale 2015;7:15990e8. Jiao Y, Zheng Y, Jaroniec M, Qiao SZ. Origin of the electrocatalytic oxygen reduction activity of graphene-based catalysts: a roadmap to achieve the best performance. J Am Chem Soc 2014;136:4394e403. Wan K, Long GF, Liu MY, Du L, Liang ZX, Tsiakaras P. Nitrogen-doped ordered mesoporous carbon: synthesis and active sites for electrocatalysis of oxygen reduction reaction. Appl Catal B 2015;165:566e71. Guo CZ, Liao WL, Chen CG. Design of a non-precious metal electrocatalyst for alkaline electrolyte oxygen reduction by using soybean biomass as the nitrogen source of electrocatalytically active center structures. J Power Sources 2014;269:841e7. Ma ZL, Guo CZ, Yin YJ, Zhang YQ, Wu HJ, Chen CG. The use of cheap polyaniline and melamine co-modified carbon nanotubes as active and stable catalysts for oxygen reduction reaction in alkaline medium. Electrochim Acta 2015;160:357e62. Nallathambi V, Lee JW, Kumaragurau SP, Wu G, Popov B. Development of high performance carbon composite catalyst for oxygen reduction reaction in PEM proton exchange membrane fuel cells. J Power Sources 2008;183:34e42. Wang MY, Huang JR, Wang M, Zhang DG, Zhang WM, Li WH, et al. Co3O4 nanorods decorated reduced graphene oxide composite for oxygen reduction reaction in alkaline electrolyte. Electrochem Commun 2013;34:299e303. Byon HR, Suntivich J, Shao HY. Graphene-based non-noblemetal catalysts for oxygen reduction reaction in acid. Chem Mater 2011;23:3421e8. Herranz J, Jaouen F, Lefevre M, Kramm UI, Proietti E, Dodelet JP, et al. Unveiling N-protonation and anion-binding effects on Fe/N/C catalysts for O2 reduction in protonexchange-membrane fuel cells. J Phys Chem C 2011;115:16087e97. Ma YJ, Wang H, Ji S, Goh J, Feng HQ, Wang RF. Highly active Vulcan carbon composite for oxygen reduction reaction in alkaline medium. Electrochim Acta 2014;133:391e8. Luo ZQ, Lim SH, Tian ZQ, Shang JZ, Lai LF, MacDonald B, et al. Pyridinic N doped graphene: synthesis, electronic structure, and electrocatalytic property. J Mater Chem 2011;21:8038e44. Liu SH, Wu JR. Influence of nitrogen and iron precursors on the synthesis of FeNx/carbons electrocatalysts toward oxygen reduction reaction in acid solution. Electrochim Acta 2014;135:147e53. Yuan HL, Wang YQ, Zhou SM, Liu LS, Chen XL, Lou SY, et al. Low-temperature preparation of superparamagnetic CoFe2O4 microspheres with saturation magnetization. Nanoscale Res Lett 2010;5:1817e21. Liu XJ, Li LG, Zhou WJ, Zhou YC, Niu WH, Chen SW. Highperformance electrocatalysts for oxygen reduction based on nitrogen-doped porous carbon from hygrothermal treatment of glucose and dicyandiamide. Chem Electro Chem 2015;2:803e10. Wang RF, Wang K, Wang ZH, Song HH, Wang F, Ji S, et al. Pig bones derived N-doped carbon with multi-level pores as electrocatalyst for oxygen reduction. J Power Sources 2015;297:295e301. Fu XG, Jin JT, Liu YR, Wei ZY, Pan FP, Zhang JY. Efficient oxygen reduction electrocatalyst based on edge-nitrogen rich graphene nanoplatelets: toward a large-scale synthesis. Appl Mater Interfaces 2014;6:3930e6. Wang M, Wang JZ, Hou YY, Shi DQ, Poynton DWSD, Slade RCT, et al. N-doped crumpled graphene derived from vapor phase deposition on PPy on graphene aerogel as an
Please cite this article in press as: Wu H, et al., A graphene-based electrocatalyst co-doped with nitrogen and cobalt for oxygen reduction reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.074
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efficient oxygen reduction reaction electrocatalyst. Appl Mater Interfaces 2015;7:7066e72. [39] Ferrandon M, Wang XP, Kropf AJ, Myers DJ, Wu J, Johnston CM, et al. Stability of irons species in heat-treated polyaniline-iron-carbon polymer electrolyte fuel cell cathode catalysts. Electrochim Acta 2013;110:282e91.
[40] Negro E, Videla AHAM, Baglio V, Arico AS, Specchia S, Koper GJM. Fe-N supported on graphitic carbon nanonetworks grown from cobalt as oxygen reduction catalysts for low-temperature fuel cell. Appl Catal B 2015;166:75e83.
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A graphene-based electrocatalyst co-doped with nitrogen and cobalt for oxygen reduction reaction Huijuan Wu a,1, Chaozhong Guo a,b,*,1, Jiaqiang Li a,1, Zili Ma a,c, Qiyun Feng a, Changguo Chen a,** a
College of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, China Research Institute for New Materials Technology, Chongqing University of Arts and Sciences, Yongchuan, Chongqing 402160, China c Huadian Electric Power Research Institute, Hangzhou 310030, China b
article info
abstract
Article history:
The development of low cost, active and stable non-precious metal (Co-N/C) catalysts to
Received 20 May 2016
replace commercial Pt-based catalysts for oxygen reduction reaction (ORR) is a hot topic to
Received in revised form
this day. In this work, we have synthesized a new graphene-based catalyst (Co-N/C-A)
12 September 2016
doped by cobalt and nitrogen atoms with high contents of pyridinic- and pyrrolic-nitrogen
Accepted 12 September 2016
(planar nitrogen), which exhibits ORR electrocatalytic activity with onset and peak po-
Available online xxx
tentials of 0.035 and 0.082 V (versus Hg/HgO) in 0.1 mol l1 KOH solution, respectively. Besides, it has much higher stability and tolerance to methanol compared to the com-
Keywords:
mercial Pt/C catalyst. The overall electron transfer number is calculated to be about 3.8.
Milk powder
The transition-metal (Co) in the precursor can promote the formation of active sites during
Graphene oxide
pyrolysis process and the followed acid-treatment may effectively expose the active sites
Non-precious metal catalyst
on the surface of the catalyst, helping to enhance the ORR activity. It can be also proposed
Oxygen reduction
that pyridinic- and pyrrolic-nitrogen groups may play a key role in the ORR process and serve as the ORR active centers. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Nowadays, high-performance fuel cells rapidly become the ideal power supply because of their high energy conversion efficiency, environmentally friendly and abundant fuel sources [1]. However, the extensive usage of fuel cells has been hampered thanks to the slow kinetics and diversified ways of the oxygen reduction reaction (ORR) [2,3]. Traditionally, the carbon-based Pt catalyst is considered as the most effective
electrocatalyst to catalyze the ORR owing to their high intrinsic activity [4]. However, their large-scale commercial applications are hindered because of high cost and scarcity of metal-Pt resource [5,6]. Apart from these drawbacks, Pt-based electrocatalysts are also subjected to low durability and organic molecule deactivation [7]. Therefore, the development of inexpensive non-Pt electrocatalysts with high ORR activity, long-term stability and good tolerance to organic molecules is of paramount importance so far.
* Corresponding author. College of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, China. ** Corresponding author. E-mail addresses: [email protected] (C. Guo), [email protected] (C. Chen). 1 These authors equally contributed to this work, and they were considered as co-first authors. http://dx.doi.org/10.1016/j.ijhydene.2016.09.074 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Wu H, et al., A graphene-based electrocatalyst co-doped with nitrogen and cobalt for oxygen reduction reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.074
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Since Jasinsky interestingly found that cobalt-phthalo cyanine (CoPc) has exhibited a significant activity towards the ORR in 1964 [8], many researchers are actively focused on non-precious metal-nitrogen-carbon (M-N/C) catalysts for the ORR. For example, Maruyama et al. prepared a cathode catalyst by simple carbonization of hemoglobin in nitrogen atmosphere [9]. An earlier literature [10,11] also reported the preparation of an ORR catalyst from natural organic catalase. However, these kinds of ORR catalysts have poor long-term stability and low ORR electrocatalytic activity [12]. More importantly, some studies adequately demonstrate that the heat-treatment at high temperature can enhance the electrochemical stability and ORR catalytic activity of M-N/C catalysts [13,14], but it can destroy the intrinsic structure of macrocyclic complexes inside the catalyst [15]. Thereafter, it is significantly pointed out that even if there are not chemical bonds between central metal and macrocyclic ligand inside the complex, the prepared M-N/ C catalysts can still exhibit high catalytic activity after heattreatment, which is further confirmed by Gupta et al. [16]. With continuous research, some researchers have prepared highly active ORR catalysts by the simple use of nitrogencontaining compounds, such as melamine, dicyandiamide and urea, to replace macrocyclic complexes [17e19]. Our group has also formed several ORR catalysts from pyrolysis of protein-enriched biomass with outstanding long-term stability, and excellent methanol tolerance [20e22]. However, there is a large controversy about the active sites for ORR of M-N/C catalyst until now. It is largely believed that the nitrogenmodified carbon structure may be the electrocatalytically ORR-active site [23,24], whereas the transition-metal only plays a role in promoting the formation of ORR active site [25,26]. Here we report a new strategy to design the Co-N/C-A catalyst by using cheap milk biomass as the nitrogen source, graphene oxide (GO) as the carbon source and CoCl2$6H2O as the metal source, followed by an acid-treatment process. The structure characterization and electrocatalytic activity towards the ORR in alkaline electrolytes were evaluated in detail. We interestingly found that this catalyst can exhibit excellent ORR activity, high durability and good tolerance to methanol molecule, which may be a promising alternative to the commercial Pt catalysts.
Experimental
we have named it as “N/C”. We further prepared the “GO-900” and “N-900” catalysts through the pyrolysis of GO and milk in same condition, respectively.
Physical characterizations The X-ray diffraction (XRD) analysis was conducted on ShiA) madzu XRD-6000 (Japan) with Cu Ka1 radiation (l ¼ 1.54178 at 4 min1. The scanning electron microscopic (SEM) images were performed on FEI Quanta 200 Scanning Electron Microscope. High-resolution transmission electron microscopy (HR-TEM) was carried out on FEI Tecnai F30 instrument and acceleration voltage is 300 kV. The X-ray photoelectron spectroscopy (XPS) analysis was carried out by using a VG Scientific ESCALAB 220 I XL spectroscopy with an Al Ka (hn ¼ 1486.69 eV) X-ray source.
Electrochemical measurements All electrochemical tests were performed on CHI 660 electrochemical workstation (CH Instruments, USA) at room temperature. The electrolyte was 0.1 mol l1 KOH solution, which was purged by nitrogen or oxygen for 30 min prior to the electrochemical test. A conventional three-electrode cell was adopted, which included an Hg/HgO/1 mol l1 KOH electrode as the reference electrode, and a ring-shaped Pt wire electrode as the counter electrode, respectively. A rotation disk electrode (RDE) with a glass carbon (GC, 4 mm diameter, 0.1256 cm2 geometric area) electrode (5-mm diameter, LKXZ-1, Tianjing Lanlike Electrochemical Instruments, China) was employed as the working electrode. To prepare the modified-GC-RDE working electrode, the doped-carbon catalyst was well-dispersed in the 0.5 wt.% Nafion/ethanol solution. 5.0 ml of 10 mg ml1 dispersion was transferred onto the GC-RDE surface and then dried at room temperature. The mass loading was calculated to be ~400 mg cm2. A commercial Pt/C catalyst (20 wt.% Pt) on the GC electrode was prepared in the same way. Linear sweep voltammetry (LSV) and cyclic voltammetry (CV) experiments were performed over the potential range of 0.3 to 0.7 V at a scan rate of 5 mV s1 in 0.1 mol l1 KOH solution. The number of electron transfer (n) per oxygen molecule was calculated by KouteckyeLevich (KeL) equation [25]: 1 jd ¼ 1 jk þ 1 Bu1=2
(1)
B ¼ 0:62nFCO DO n1=6
(2)
2=3
Catalyst preparations All catalysts were prepared by pyrolysis process in a tube furnace. 0.05 g of GO nanosheets (provided by Jiangshu Nano Co.), 0.05 g of pure milk powder (purchased from local supermarket), and 0.15 g of CoCl2$6H2O were absolutely mixed and milled in a mortar. The obtained sample was then pyrolyzed in flowing N2 at 900 C for 1 h to obtain the “Co-N/C”. In order to further examine the real role of metal in the ORR process, the prepared Co-N/C catalyst was chemically treated with 0.5 mol l1 H2SO4 solution at room temperature to remove metal Co or its compounds. After acid-treatment, the produced sample was hereafter called “Co-N/C-A”. Besides, as a control, a mixture of GO nanosheet and milk powder was also annealed in N2 atmosphere at 900 C for 1 h. Analogously,
where jd was the measured current density; jk was the kinetic current density of the ORR; F was the Faraday constant (96,485 C mol1); CO was the concentration of dissolved oxygen (1.2 106 mol cm3); DO was the diffusion coefficient of oxygen (1.9 105 cm2 s1); n was the kinetic viscosity of the solution (0.01 cm2 s1); and u was the electrode rotation rate (rpm).
Results and discussion The structural analysis of the catalysts Fig. 1a shows the XRD patterns of the obtained products. A pronounced peak at ~26 and a weak peak at ~43 are assigned
Please cite this article in press as: Wu H, et al., A graphene-based electrocatalyst co-doped with nitrogen and cobalt for oxygen reduction reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.074
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Fig. 1 e The XRD pattern (a) and SEM images of N/C (b), Co-N/C (c), Co-N/C-A (d).
to the 002 and 101 diffraction peaks of graphite, respectively. The 002 diffraction peak at ~26 indicates the presence of a small domain with parallel stacking of graphene sheets, whereas the 101 diffraction peak at ~43 is attributed to the fibrous/spherical structures formed by sp2 hybridized carbons [6]. It is importantly found that the diffraction peaks of Co and CoO have been mainly observed, while those of graphite are very weak in Co-N/C, which suggests that Co and CoO may cover the catalytically active sites exposed on the surface. After acid-treatment, the diffraction peaks of graphite structure can be clearly observed in Co-N/C-A, while those of crystalline Co or CoO can hardly be found due to the dissolution of Co or CoO in H2SO4 solution. According to previous reports [20], the formation of cobalt oxides may be owing to the re-oxidization of metal Co by the oxygen species at hightemperature pyrolysis process. Fig. 1bed depicts high-resolution SEM images of the prepared catalysts. As shown in Fig .1b, the N/C catalyst contains multilayer graphene structures and carbon particles with a size of hundreds of nanometers. The formation of carbon particles with a size of hundreds of nanometers can be attributed to the incomplete decomposition and further birdnesting of milk powder during pyrolysis process. Compared with the SEM images of un-doped GO material [17], there is no major change in the surface morphology during N-doping. For the Co-N/C catalyst in Fig. 1c, it can be clearly shown that many crystal particles are uniformly deposited on the carbon sheets. Associated with the results of XRD, we can conclude that the crystal particles are Co or CoO. After the acidtreatment, the SEM images of the obtained Co-N/C-A catalyst and the Co-N/C catalyst have an obvious distinction. As shown in Fig. 1d, it's hardly to observe the crystal particles on the carbon sheets for the Co-N/C-A catalyst. In contrast, lots of pores are formed on the surface. These results imply that the
cobalt may be basically dissolved in the acid-treatment process, which is consistent with the results of XRD. We further characterize the morphology of the Co-N/C-A catalyst by high-resolution TEM technique. Fig. S1 clearly indicates the wrinkled graphene nanosheets with defect edges, perhaps owing to the doping of nitrogen atoms into the carbon skeleton. In addition, several crooked lattice fringes at the edge of the Co-N/C-A catalyst are found and many nanopores are also formed inside the Co-N/C-A catalyst. To probe the surface compositions and chemical surroundings of N/C and Co-N/C-A catalysts, the XPS analyses were carried out, as in Fig. 2. In Fig. 2a, the full-scan XPS spectra reveals the presence of C, N and O, but the Co 2p peak also appears at the Co-N/C-A catalyst, which suggest that most metallic Co has been removed by acid treatment, whereas residual metallic Co that survived the acid treatment may be encased in the carbon structure [27]. The observed N 1s peak reveals the successful doping of nitrogen into the carbon structure of GO nanosheets. The high-resolution XPS spectra for the C 1s region (Fig. 2b) indicates the presence of different chemical bonds: C]C (284.6 ± 0.1 eV), CeN (285.6 ± 0.2 eV), CeO (286.4 ± 0.1 eV), and OeC]O (288.5 ± 0.1 eV) [28]. The CeN functional groups further confirm the formation of N-doped carbon material inside the prepared catalysts. Besides, high-resolution N 1s XPS spectra are shown in Fig. 2c, and various types of nitrogen-doped species were identified by a curve-fitting method. The N 1s spectra can be deconvoluted into four peaks, which corresponds to the formation of pyridinic-N, pyrrolic-N, quaternary-N and oxidized-N with the binding energies of 398.6 ± 0.1 eV, 400.4 ± 0.1 eV, 401.1 ± 0.1 eV and 403.2 ± 0.2 eV [7], respectively. Table 1 shows the quantitative analysis results of four types of nitrogen-containing groups. Among the four types of N atoms, only the pyridinic-N and pyrrolic-N
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Fig. 2 e The full-scanning XPS spectrum (a), XPS spectra for C1s region (b) and N1s region (c) of N/C and Co-N/C-A and XPS spectrum for Co 2p region of Co-N/C-A (d).
have been proved to be active in ORR [29,30]. Particularly, the pyrrolic-N has a lone electron pair in the plane of the carbon matrix, resulting in an increase of electron-donor property for the catalysts and the enhancement of the ORR catalytic activity [31]. By contrast, the pyridinic-N is also considered to significantly increase the ORR stability in the previous reports [32,33]. Therefore, the catalyst with high contents of pyridinicN and pyrrolic-N is positive to the ORR. As shown in Table 1, the Co-N/C-A catalyst with the addition of Co has higher percentage of pyridinic-N and pyrrolic-N with 26.6% and 41.3% than the N/C catalyst with 18.4% and 34.0%. Fig. 2d displays the Co 2p XPS spectra, which is deconvoluted into four peaks. Among them, the peaks centered at ~780.4 eV and ~795.8 eV are from Co 2p3/2 and Co 2p1/2, respectively. Other peaks centered at ~783.7 eV and ~803.5 eV are two shake-up type peaks of Co. This clearly shows the existence of Co2þ in the sample [34]. Above results show that quaternary-N and oxidized-N are largely transferred to pyridinic-N and pyrrolicN with cobalt doping and the followed acid treatment.
Electrocatalytic activity towards the ORR To evaluate the electrocatalytic performance of the prepared catalysts toward the ORR, the linear sweeping voltammetry
(LSV) was used to study the activity of the catalysts in 0.1 mol l1 KOH solution saturated with oxygen. As shown in Fig. 3a, the peak potential (Ep) of GO nanosheets is detected to be 0.23 V, which is more negative than that of GO-900 (Ep ¼ 0.14 V). It demonstrates that the heat-treatment process can make large contribution to effectively generate more ORR active sites [35,36]. Compared with GO-900 and N-900, the N/C catalyst shows a significant improvement of ORR performance with more positive ORR peak and onset potentials [37,38]. These results absolutely suggest that both nitrogen and carbon are indispensable to enhance the ORR activity. Fig. 3b shows the LSV results for the catalyzed electrodes with different catalysts in 0.1 mol l1 N2 or O2-saturated KOH electrolyte. No visible peaks can be observed in N2-saturated electrolyte for the N/C catalyst in the potential range of 0.7 to 0.3 V. On the contrary, when the LSV test has been conducted in O2-saturated KOH electrolyte, a well-defined ORR peak at 0.11 V is obtained, implying that this catalyst possesses excellent ORR catalytic activity in the alkaline electrolyte. In addition, the onset potential and peak potential of the Co-N/C catalyst have positively shifted comparing with those of the N/C catalyst. After acid-treatment, the losses of the onset potential (Eonset) and peak potential for Co-N/C are only 5 and 18 mV, respectively, but the limited ORR current density of Co-
Table 1 e The XPS data for the N 1s region of N/C and Co-N/C-A from Fig. 2. Sample
N/C [%]
Pyridinic-N [%] (398.6 ± 0.1 eV)
Pyrrolic-N [%] (400.4 ± 0.1 eV)
Quaternary-N [%] (401.1 ± 0.1 eV)
Oxidized-N [%] (403.2 ± 0.2 eV)
N/C Co-N/C-A
2.41 2.87
18.4 26.6
34.0 41.3
28.2 15.0
19.4 17.1
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Fig. 3 e (a) LSV curves of GO, GO-900, N-900 and N/C in O2-saturated 0.1 mol l¡1 KOH solution at a scan rate of 5 mV s¡1. (b) LSV curves of N/C, Co-N/C and Co-N/C-A in O2-saturated and N/C in N2-saturated 0.1 mol l¡1 KOH solution at a scan rate of 5 mV s¡1. LSV curves of N/C (c) and Co-N/C-A (e) on GC-RDE in O2-saturated 0.1 mol l¡1 KOH solution with various rotation rates at a scan rate of 5 mV s¡1 and KouteckyeLevich plots of N/C (d) and Co-N/C-A (f).
N/C-A reaches almost twice as much as that of Co-N/C. These results indicate that the existence of metal Co in the heattreatment can promote the formation of active sites, and the followed acid-treatment is a need for the exposure of more active sites for ORR. To investigate the kinetics of the ORR on N/C and Co-N/CA catalysts, we measured the ORR polarization curves at various rotating rates, as shown in Fig. 3c and e. A series of KouteckyeLevich (KeL) plots (j1 vs. u1/2) are also obtained in Fig. 3d and f. The corresponding KeL plots at different electrode potentials show good linearity, and the slopes remain a constant value over the potential range from 0.5 to 0.7 V, indicating a similar electron-transfer number per O2 molecule involved in the O2 reduction. The dependence of n on the electrode potential in N/C and Co-N/C-A catalyzed electrodes is displayed as the insets in Fig. 3d and f. For the N/ C catalyzed electrode, the average n value is 2.5, indicating that the ORR process on the N/C catalyst can undergo a twoand four-electron mixed pathway. However, for the Co-N/C-A catalyzed electrode, the number (n) of electron transfer in the
ORR process is 3.8, indicating that it mainly favors a fourelectron pathway. In practical applications, long-term stability and tolerance to the methanol are important considerations for cathode catalysts of fuel cells. The electrochemical stability of the catalyst was carefully examined by an accelerated aging test (AAT) in O2-saturated 0.1 mol l1 KOH solution over the potential range of 0.7 Ve0.3 V, as shown in Fig. 4. The N/C and Co-N/C-A catalysts exhibit stable ORR performance after the AAT, with the Eonset losses of 10 and 15 mV, respectively. However, the commercial 40% Pt/C catalyst reveals an obvious degradation after the AAT, with an Eonset loss of 80 mV. Above results demonstrate that the Co-N/C-A catalyst has a better long-term stability compared with the commercial Pt-based catalyst for the ORR in alkaline medium. The low stability for Pt-based catalysts may be attributed to the migration or aggregation of Pt nanoparticles during continuous potential cycling [39,40]. To explore the tolerance performance to methanol molecule, the amperometric iet responses of the prepared
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Based on above results, this paper can get the following conclusions: (i) the metal Co may not be involved in catalytically active structures as there is no peak of Co-Nx found in the N 1s XPS spectra of Co-N/C-A; (ii) the metal Co may only play a role in promoting the formation of catalytically active structures on the basis of higher ORR activity of Co-N/C-A than N/C; (iii) the doping of Co and acid-treatment can facilitate the large transformation of quaternary- and oxidized-N to pyridinic- and pyrrolic-N active sites for the ORR.
Conclusions
Fig. 4 e LSV curves for N/C, Co/N/C-A and Pt/C before and after AAT in 0.1 mol l¡1 KOH solution on RDE at a rotation rate of 1600 rpm.
catalysts were recorded in Fig. 5. A step-down decrease in cathodic current response upon the successive addition of 5.0 mol l1 methanol can be clearly observed. It is obvious that the chronoamperometric response of the Pt/C catalyst dramatically changes in terms of current density jumping from 0.3 mA cm2 to 3 mA cm2, which is caused by the electrochemical oxidization of methanol molecule. However, for N/C and Co-N/C-A catalysts, the chronoamperometric responses remain fairly balanced tendency while the sequential addition of the methanol solution is performed at an interval of 200 s, meaning that the prepared catalysts possesses strong tolerance to the methanol molecule. Our results indicate that the Co-N/C-A catalyst has a superior long-term stability and good tolerance against the methanol molecule.
A highly active cobalt and nitrogen co-doped carbon-based catalyst has been prepared by the pyrolysis of milk biomass, metallic Co and graphite oxide at 900 C for 1 h, followed by an acid treatment process. Owing to high contents of pyridinic and pyrrolic-N groups, the obtained Co-N/C-A catalyst exhibits excellent electrocatalytic activity for ORR in 0.1 M KOH with onset and peak potentials of 0.035 and 0.082 V (versus. Hg/HgO) and electro-transfer number of 3.8. Compared to Pt/ C, the Co-N/C-A catalyst has a superior stability and tolerance against the methanol molecule. Additionally, doping of Co and the followed acid-treatment facilitate the transformation of quaternary-N and oxidized-N to pyridinic-N and pyrrolic-N, helping to improve the ORR performance. Our study may provide an open mind for easy synthesis of high performance non-precious metal catalysts by design of the composition of the catalyst precursor for a range of significant electrochemical reactions.
Acknowledgments This work was financially supported by the Scientific and Technological Research Program of Chongqing Municipal Education Commission (KJ1501118), the Basic and Frontier Research Program of Chongqing Municipality (cstc2015jcyjA50032, cstc2014jcyjA50038), Chongqing University Scientific Research Training Project (CQU-SRTP-2015501), Sharing Fund of Chongqing University's Large-Scale Euipment (201412150152), and the National Natural Science Foundation of China (21273292).
Appendix A. Supplementary data Supplementary data related to this article can be found online at http://dx.doi.org/10.1016/j.ijhydene.2016.09.074
references Fig. 5 e Amperometric iet curves obtained at N/C, Co/N/C-A and Pt/C in N2-saturated 0.1 mol l¡1 KOH solution under magnetic stirring (600 rpm) and N2-protection over 0e300 s, followed by an immediate introduction of O2. The applied potential is ¡0.1 V and arrows indicate the sequential addition of 5.0 mol l¡1 methanol.
[1] Yang ZR, Wu J, Zheng XJ, Wang ZJ, Yang RZ. Enhanced catalytic activity for the oxygen reduction reaction with codoping of phosphorus and iron in carbon. J Power Sources 2015;277:161e8. [2] Lu ZS, Xu GL, He CZ, Wang TX, Yang L, Yang ZX, et al. Novel catalytic activity for oxygen reduction reaction on MnN4
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[3]
[4]
[5]
[6]
[7]
[8] [9]
[10]
[11]
[12] [13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
embedded graphene: a dispersion-corrected density function theory study. Carbon 2015;84:500e8. Zhang XM, Liu L, Zhao Z, Tu BF, Ou DR, Cui DA, et al. Enhanced oxygen reduction activity and solid oxide fuel cell performance with a nanoparticles-loaded cathode. Nano Lett 2015;15:1703e9. Zhang SM, Chen SL. Enhanced-electrocatalytic activity of Pt nanoparticles supported on nitrogen-doped carbon for the oxygen reduction reaction. J Power Sources 2013;240:60e5. Geng DS, Chen Y, Chen YG, Li YL, Li RY, Sun XL, et al. High oxygen-reduction activity and durability of nitrogen-doped graphene. Energy Environ Sci 2011;4:760e4. Wang CH, Yang CW, Lin YC, Chang ST, Chang SLY. Cobaltiron oxide hybrid catalysis with enhanced catalytic activities for oxygen reduction in anion exchange membrane fuel cell. J Power Sources 2015;277:147e54. Zahoor A, Christy M, Hwang YJ, Lin YR, Kim P, Nahm KS. Improved electrocatalytic activity of carbon materials by nitrogen doping. Appl Catal B 2014;147:633e41. Jasinski R. New fuel cell cathode catalyst. Nature 1964;201:1212e3. Maruyama J, Abe I. Carbonized hemoglobin functioning as a cathode catalyst for polymer electrolyte fuel cells. Chem Mater 2006;18:1303e11. Lai ME, Bergel A. Electrochemical reduction of oxygen on glass carbon: catalysis by catalase. J Electroanal Chem 2000;494:30. Maruyama J, Abe I. Formation of platinum-free fuel cell cathode catalyst with highly developed nanospace by carbonizing catalase. Chem Mater 2005;17:4660e7. Bashyam R, Zelenay P. A class of non-precious metal composite catalysts for fuel cells. Nature 2006;443:63e6. Ahluwalia RK, Wang X, Kwon J, Rousseau A, Kalinoski J, James B, et al. Performance and cost of automotive fuel cell systems with ultra-low platinum loadings. J Power Sources 2011;196:4619e30. Jaouen F, Proietti E, Lefevre M, Chenitz R, Dodelet JP, Wu G, et al. Recent advances in non-precious metal catalysis for oxygen-reduction reaction in polymer electrolyte fuel cells. Energy Environ Sci 2011;4:114e30. Weng LT, Bertrand P, Lalande G, Guay D, Dodelet JP. Surface characterization by time-of-flight SIMS of a catalyst for oxygen electroreduction: pyrolyzed cobalt phthalocyanineon-carbon black. Appl Surf Sci 1995;84:9e21. Gupta S, Tryk D, Bae I, Aldred W, Yeager E. Heat-treated polyacrylonitrile-based catalysts for oxygen electroreduction. J Appl Electrochem 1989;19:19e27. Vikkisk M, Kruusenberg I, Joost U, Shulga E, Kink I, Tammeveski K. Electrocatalytic oxygen reduction on nitrogen-doped graphene in alkaline media. Appl Catal B 2014;147:369e76. Sheng ZH, Shao L, Chen JJ, Bao WJ, Wang FB, Xia XH. Catalyst-free synthesis of nitrogen-doped graphene via thermal annealing graphite oxide with melamine and its excellent electrocatalysis. ACS Nano 2011;5:4350e8. Wang HB, Maiyalagan T, Wang X. Review on recent progress in nitrogen-doped graphene: synthesis characterization, and its potential applications. ACS Catal 2012;2:781e94. Guo CZ, Chen CG, Luo ZL. A novel nitrogen-containing electrocatalyst for oxygen reduction reaction from blood protein pyrolysis. J Power Sources 2014;245:841e5. Guo CZ, Chen CG, Luo ZL. Electrocatalytic activity for the oxygen reduction on the novel carbon material containing nitrogen prepared by using two-step pyrolysis of hemolymph protein. Int J Electrochem Sci 2013;8:8940e50. Guo CZ, Liao WL, Li ZB, Sun LT, Chen CG. Easy conversion of protein-rich enoki mushroom biomass to a nitrogen-doped
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
7
carbon nanomaterial as a promising metal-free catalyst for oxygen. Nanoscale 2015;7:15990e8. Jiao Y, Zheng Y, Jaroniec M, Qiao SZ. Origin of the electrocatalytic oxygen reduction activity of graphene-based catalysts: a roadmap to achieve the best performance. J Am Chem Soc 2014;136:4394e403. Wan K, Long GF, Liu MY, Du L, Liang ZX, Tsiakaras P. Nitrogen-doped ordered mesoporous carbon: synthesis and active sites for electrocatalysis of oxygen reduction reaction. Appl Catal B 2015;165:566e71. Guo CZ, Liao WL, Chen CG. Design of a non-precious metal electrocatalyst for alkaline electrolyte oxygen reduction by using soybean biomass as the nitrogen source of electrocatalytically active center structures. J Power Sources 2014;269:841e7. Ma ZL, Guo CZ, Yin YJ, Zhang YQ, Wu HJ, Chen CG. The use of cheap polyaniline and melamine co-modified carbon nanotubes as active and stable catalysts for oxygen reduction reaction in alkaline medium. Electrochim Acta 2015;160:357e62. Nallathambi V, Lee JW, Kumaragurau SP, Wu G, Popov B. Development of high performance carbon composite catalyst for oxygen reduction reaction in PEM proton exchange membrane fuel cells. J Power Sources 2008;183:34e42. Wang MY, Huang JR, Wang M, Zhang DG, Zhang WM, Li WH, et al. Co3O4 nanorods decorated reduced graphene oxide composite for oxygen reduction reaction in alkaline electrolyte. Electrochem Commun 2013;34:299e303. Byon HR, Suntivich J, Shao HY. Graphene-based non-noblemetal catalysts for oxygen reduction reaction in acid. Chem Mater 2011;23:3421e8. Herranz J, Jaouen F, Lefevre M, Kramm UI, Proietti E, Dodelet JP, et al. Unveiling N-protonation and anion-binding effects on Fe/N/C catalysts for O2 reduction in protonexchange-membrane fuel cells. J Phys Chem C 2011;115:16087e97. Ma YJ, Wang H, Ji S, Goh J, Feng HQ, Wang RF. Highly active Vulcan carbon composite for oxygen reduction reaction in alkaline medium. Electrochim Acta 2014;133:391e8. Luo ZQ, Lim SH, Tian ZQ, Shang JZ, Lai LF, MacDonald B, et al. Pyridinic N doped graphene: synthesis, electronic structure, and electrocatalytic property. J Mater Chem 2011;21:8038e44. Liu SH, Wu JR. Influence of nitrogen and iron precursors on the synthesis of FeNx/carbons electrocatalysts toward oxygen reduction reaction in acid solution. Electrochim Acta 2014;135:147e53. Yuan HL, Wang YQ, Zhou SM, Liu LS, Chen XL, Lou SY, et al. Low-temperature preparation of superparamagnetic CoFe2O4 microspheres with saturation magnetization. Nanoscale Res Lett 2010;5:1817e21. Liu XJ, Li LG, Zhou WJ, Zhou YC, Niu WH, Chen SW. Highperformance electrocatalysts for oxygen reduction based on nitrogen-doped porous carbon from hygrothermal treatment of glucose and dicyandiamide. Chem Electro Chem 2015;2:803e10. Wang RF, Wang K, Wang ZH, Song HH, Wang F, Ji S, et al. Pig bones derived N-doped carbon with multi-level pores as electrocatalyst for oxygen reduction. J Power Sources 2015;297:295e301. Fu XG, Jin JT, Liu YR, Wei ZY, Pan FP, Zhang JY. Efficient oxygen reduction electrocatalyst based on edge-nitrogen rich graphene nanoplatelets: toward a large-scale synthesis. Appl Mater Interfaces 2014;6:3930e6. Wang M, Wang JZ, Hou YY, Shi DQ, Poynton DWSD, Slade RCT, et al. N-doped crumpled graphene derived from vapor phase deposition on PPy on graphene aerogel as an
Please cite this article in press as: Wu H, et al., A graphene-based electrocatalyst co-doped with nitrogen and cobalt for oxygen reduction reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.074
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efficient oxygen reduction reaction electrocatalyst. Appl Mater Interfaces 2015;7:7066e72. [39] Ferrandon M, Wang XP, Kropf AJ, Myers DJ, Wu J, Johnston CM, et al. Stability of irons species in heat-treated polyaniline-iron-carbon polymer electrolyte fuel cell cathode catalysts. Electrochim Acta 2013;110:282e91.
[40] Negro E, Videla AHAM, Baglio V, Arico AS, Specchia S, Koper GJM. Fe-N supported on graphitic carbon nanonetworks grown from cobalt as oxygen reduction catalysts for low-temperature fuel cell. Appl Catal B 2015;166:75e83.
Please cite this article in press as: Wu H, et al., A graphene-based electrocatalyst co-doped with nitrogen and cobalt for oxygen reduction reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.074