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Catalytic activities enhanced by abundant structural defects and balanced N distribution of N-doped graphene in oxygen reduction reaction
Journal of Power Sources 306 (2016) 85e91
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Catalytic activities enhanced by abundant structural defects and balanced N distribution of N-doped graphene in oxygen reduction reaction Xiaogong Bai a, Yantao Shi a, **, Jiahao Guo a, Liguo Gao b, Kai Wang a, Yi Du a, Tingli Ma b, c, * a b c
State Key Laboratory of Fine Chemicals, School of Chemistry, Dalian University of Technology, Dalian 116024, China School of Petroleum and Chemical Engineering, Dalian University of Technology, Panjin 124221, China Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, Kitakyushu, Fukuoka 808-0196, Japan
h i g h l i g h t s Abundant structural defects on graphene enhance the ORR activity. Balanced N distribution on graphene could also improve the ORR activity. Structural defects and N distribution are adjusted by regulating temperature. NG-1000 shows notable ORR activities in both alkaline and acid media.
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Article history: Received 11 July 2015 Received in revised form 19 October 2015 Accepted 24 October 2015 Available online xxx
N-doped graphene (NG) is a promising candidate for oxygen reduction reaction (ORR) in the cathode of fuel cells. However, the catalytic activity of NG is lower than that of commercial Pt/C in alkaline and acidic media. In this study, NG samples were obtained using urea as N source. The structural defects and N distribution in the samples were adjusted by regulating the pyrolysis temperature. The new NG type exhibited remarkable catalytic activities for ORR in both alkaline and acidic media. © 2015 Elsevier B.V. All rights reserved.
Keywords: N-doping graphene Oxygen reduction reaction N-distribution Structural defects
1. Introduction Toyota's announcement about its mass production of fuel cell vehicles has attracted attention and encouraged scholars to investigate the production cost and durability of the company's vehicles. Pt/C is the cathode catalyst for oxygen reduction reaction (ORR) in fuel cells; however, the commercial application of this catalyst is limited by its high cost and susceptibility to time-dependent drift and carbon monoxide/methanol deactivation [1e3]. To overcome
* Corresponding author. School of Petroleum and Chemical Engineering, Dalian University of Technology, Panjin 124221, China. ** Corresponding author. E-mail addresses: [email protected] (Y. Shi), [email protected] (T. Ma). http://dx.doi.org/10.1016/j.jpowsour.2015.10.081 0378-7753/© 2015 Elsevier B.V. All rights reserved.
these limitations, considerable efforts have been exerted to develop alternative ORR catalysts, such as non-precious metals (Fe and Co) [4e6], metal oxides (sulfides) (Fe3O4, MnO2, and Co1-xS) [7e9], and heteroatom-doped (N-, S-, P-, and B-doped) carbon materials [graphene (GN), carbon nanotube, and carbon aerogels] [10e12]. Among these alternative catalysts, carbon materials are considered the most promising for ORR because of their low cost and stable properties. The carbon-based material GN has elicited significant attention because of its high surface area, superior electrical conductivity, excellent thermal stability, and exceptional mechanical properties [1]. Heteroatom-doped GN materials, specifically N-doped graphene (NG) materials, can tail their electronic property and chemical reactivity to increase their efficiency for ORR. N-doping can also endow GN with more exposed edges, where oxygen
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absorption is facilitated more easier than in basal planes [10]. Thus, NG with abundant exposed edges is desirable to further enhance ORR catalysis. Generally, NG can be synthesized through the post-treatment of GN or graphite oxide (GO) with N-containing sources (urea [13e15], polyaniline [16], polypyrrole [16], melamine [17], dicyandiamide [18], NH3 [19], ammonia water [20], and hydrazine [21]). Among these materials, urea has been widely used to N-dope GN because of its low cost and manufacturing quantities. Although some articles have discussed the relationship between N-doping and ORR activity, the enhancements in catalytic performance are mainly ascribed to one or two specific N species, such as pyridinic, pyrrolic, or graphitic N. A controversy on the role of N species in ORR also exists. For instance, Dai et al. reported that the increased ORR catalytic activity of NG is caused by the presence of pyridinic and pyrrolic N [22,23]. Other reports showed that pyridinic N can change the ORR mechanism from a two-electron-dominated process to a four-electron-dominated one and improve the onset potential for ORR; furthermore, pyrrolic N is favorable for oxygen adsorption and reduction in water through a four-electron mechanism [16,24]. Lin et al. prepared NG with pyridinic N as the dominant dopant by using a chemical vapor deposition method; they found that pyridinic N may not play a key role to improve the ORR catalytic activity [25]. Sun et al. confirmed that the ORR activity is not significantly dependent on the contents of pyridinic and pyrrolic N. Graphitic N seems to be the most important species for ORR and can considerably increase the limiting current and participate in ORR [1,16,19]. In light of these conflicting results, we speculate that all the three types of N species play important roles in ORR, and precisely tuning the proportion of each N species may be more favorable to enhance the ORR catalytic activity of NG. In the present work, we synthesized NG and adjusted its structural defects and N distribution by controlling the post-treatment temperature (900 C, 1000 C, and 1100 C) of GO mixed with urea. Systematical characterizations on morphology and microstructures were conducted, and the catalytic activities in alkaline and acidic media of the samples were evaluated. NG pyrolyzed at 1000 C (NG-1000) exhibited abundant exposed edges and balanced N distribution. Furthermore, NG-1000 showed catalytic activities similar to those of commercial Pt/C for ORR in alkaline medium and demonstrated good ORR catalytic activity in acidic medium. Therefore, this new NG is a promising cathode catalyst for fuel cells. 2. Experimental 2.1. Material preparation GO was purchased from The Sixth Element Material (Changzhou) Ltd. without further post-treatment. For NG synthesis, GO (30 mg) and urea (500 mg) were dispersed in a mixture of water (50 mL) and ethanol (50 mL) by ultrasonic irradiation for 2 h. The solution was transferred to a glass plate and dried at 25 C for 2 days. The dried GO/urea mixture was pyrolyzed at 900 C, 1000 C, and 1100 C in a N2 atmosphere for 2 h to produce NG-900, NG1000, and NG-1100, respectively. Additionally, GO was pyrolyzed at 1000 C in a N2 atmosphere for 2 h to produce a control sample (GN-1000) for comparison. 2.2. Sample characterizations The morphology of NG was observed via scanning electron microscopy (SEM, S-4800) and transmission electron microscopy (TEM, Titan 80-300). X-ray photoelectron spectroscopy (XPS, Thermo Escalab 250Xi) was performed to analyze the chemical
element and sample states, and Raman spectroscopy (Renishaw inVia) was conducted for microstructure analysis. 2.3. Electrochemical measurements The electrochemical properties of the samples were tested on the CHI 660C electrochemical workstation with a three-electrode system. A glassecarbon (GC) rotating disk electrode (RDE) (diameter, 5 mm; Pine), Pt wire, and Ag/AgCl electrode filled with saturated KCl aqueous solution were used as the working, counter, and reference electrodes, respectively. NG (1.25 mg) and Nafion solution (5 wt%, 25 mL) were mixed ultrasonically in water (355 mL) and isopropanol (120 mL) for more than 1 h. Up to 12 mL of catalyst ink was dipped onto the GC electrode with a loading amount of 152 mg cm2. For comparison, Pt/C (20 wt% Pt, 1 mg) and Nafion solution (20 mL) were dispersed in ethanol (480 mL), whereas 10 mL of Pt/C ink was dipped onto a GC electrode with a loading amount of 102 mg cm2. All electrochemical properties were tested in N2- or O2-saturated 0.1 M KOH or HClO4 solution at room temperature. 3. Results and discussion 3.1. Morphology and structure Fig. 1a illustrates flake-like GO bulks with sizes exceeding 20 mm. Such morphology is common because commercial GO is prepared through simple graphite oxidation rather than exfoliation into individual GN sheets. In the current study, NG-1000 was selected for detailed characterizations. The structure and morphology of NG-1000 were investigated via SEM and TEM. In terms of morphology, NG-1000 is fluffier than pristine GO because of the several single-layered structures in the former (Fig. 1b). The enlarged image in Fig. 1c shows that interlaced and ultrathin films are pronounced. Such structural features provide advanced properties, including large specific area, several exposed active sites, and convenient fluid penetration, which are highly desirable for electrocatalysis. Oxygen adsorption improves with the increasing number of edges [10]. NG-1000 shows a thin (i.e., no more than several nanometers), wrinkled, and silk-veil-like morphology (Fig. 1d and e). The electron diffraction pattern in Fig. 1f displays diffraction points rather than diffraction circles, further revealing that NG-1000 exhibits an exfoliated thin layer [26]. These results suggest that GO can be exfoliated into a thin layer following hightemperature treatment. Raman spectroscopy, a powerful method to evaluate the level of GN defects, was used in this study to characterize the defect sites and band intensity ratio. The ratio of D band to G band intensity (ID/ IG) was utilized to indicate this defect level. Generally, a high ratio shows a high concentration of defects [18]. The Raman spectra (Fig. 2) of GO, GN-1000, NG-900, NG-1000, and NG-1100 display two prominent peaks corresponding to the D band (1350 cm1) arising from the sp3 defect sites and G band (1580 cm1) associated with the emerging sp2-hybridized graphitic carbon atoms [27]. The ID/IG of GN-1000 (2.35), NG-900 (2.93), NG-1000 (3.08), or NG-1100 (2.13) is higher than that of GO (1.87). This result indicates that high-temperature treatment can enhance defect concentration. Additionally, NG-1000 (3.08) contains more defect sites than GN1000 (2.35). This result indicates that N-doping influences the degree of graphitization. Interestingly, the ID/IG of NG-900 (2.93) is lower than that of NG-1000 (3.08) but higher than that of NG-1100 (2.13), thus implying that the NG structural defects can be adjusted by controlling the temperature. XPS measurements were conducted to evaluate the chemical compositions of the samples. The oxygen content decreased significantly after high-temperature sintering, implying that GO
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Fig. 1. (a) SEM image of GO. (b) and (c) SEM images of NG-1000. (d) and (e) TEM images of NG-1000. (f) Electron diffraction pattern of NG-1000.
can be effectively reduced through high-temperature treatment (Table 1 and Fig. 3a). The N contents of NG-900, NG-1000, and NG1100 are much higher than that of GN-1000. This finding proves that N has been successfully doped into GN after pyrolysis of the GO and urea mixture. Fig. 3b shows the high-resolution N1s XPS spectra of NG samples. Generally, the complex N1s peak can be further divided into four primary peaks corresponding to pyridinic, pyrrolic, graphitic, and oxidized N [13,17]. Pyridinic, pyrrolic, and graphitic N can contribute to ORR, and N distribution may affect ORR catalysis [16,19,24,28]. Hence, we analyzed the N distribution of N1s in detail as described in Table 1. As the sintering temperature increased from 900 C to 1100 C, the total atomic content of N in NG decreased from 6.02% to 3.28%, whereas that of graphitic N increased from 24.3% to 31.9% (Table 1). Compared with NG-900 and NG-1100, NG-1000 demonstrated a more balanced N
distribution: 31.8% pyridinic N, 27.2% pyrrolic N, 28.4% graphitic N, and 12.6% oxidized N. NG-1000 exhibited the most efficient ORR catalytic activity among all the samples in both alkaline and acidic media. Thus, we conclude that balanced N distribution improves the catalytic efficiency for ORR. Chemical calculation is also needed to further illustrate the mechanism. 3.2. Electrochemical characterizations 3.2.1. Electrochemical characterizations in alkaline medium Cyclic voltammetry (CV) was used to describe the ORR catalytic activity of NG-1000 in alkaline medium, namely, N2- or O2-saturated 0.1 M KOH solution at a scan rate of 10 mV s1. No obvious peak can be observed in the CV curve for N2-saturated 0.1 M KOH solution, which can provide a clean capacitive CV background
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Fig. 2. Raman spectra of GO, GN-1000, NG-900, NG-1000, and NG-1100.
(Fig. 4a). Compared with N2-saturated solution, O2-saturated solution showed a higher cathodic current with a peak at ~0.84 V versus RHE, thereby suggesting higher electrocatalytic activity for ORR. Onset potential is an important criterion to evaluate the ORR activity of an electrocatalyst [29]. NG-1000, with a sharply positive onset potential at ~0.99 V versus RHE, also demonstrated a higher electrocatalytic activity than the other samples. To further investigate the ORR activity of the NG samples, linear sweep voltammetry (LSV) measurements were performed in O2saturated 0.1 M KOH solution by using RDE at a scan rate of 10 mV s1. Fig. 4b displays the LSV curves of the different samples. Among these samples, GN-1000 without N-doping showed the lowest onset potential, hence implying that the electrocatalytic activity of this sample is worse than that of the others. By contrast, the ORR onset potentials for NG-900, NG-1000, and NG-1100 all shifted positively. Thus, N-doping may improve the ORR catalytic activity of GN. NG-1000 showed a relatively higher steady current and positive onset potential, which offered the highest ORR activity among the three N-doping samples. Moreover, NG-1000 presented a larger onset potential than commercial Pt/C. This result further confirmed the significance of balanced N distribution, as aforementioned in the Raman spectroscopy and XPS analyses in Figs. 2 and 3, respectively. The Raman spectra indicated the presence of more defects in NG-1000 than in other samples. Therefore, NG1000 can provide more exposed edges for oxygen adsorption and more ORR active sites than the other samples [10]. XPS results showed that pyridinic, pyrrolic, and graphitic N, each of which plays an important role in improving ORR, are evenly distributed in NG-1000 [16,19,24]. The absence of any of these materials may reduce ORR activity. Generally, ORR follows a four- or two-electron reaction pathway. In a four-electron process, O2 can be converted into water without any other side reaction. However, in a two-electron process, peroxide as an intermediate would emerge in ORR [30]. Thus, the four-electron process is more efficient and favorable than the two-
Fig. 3. (a) XPS survey spectra of GO, GN-1000, NG-900, NG-1000, and NG-1100. (b) High-resolution N1s XPS spectra of NG-900, NG-1000, or NG-1100.
electron reaction pathway. The number of electrons transferred per oxygen molecule on NG-1000 can be obtained from the RDE measurements, which should be calculated from the KouteckyeLevich (KeL) equations as follows:
1 1 1 1 1 ¼ þ ¼ þ J JL JK Bu1=2 JK
(1)
B ¼ 0:62nFC0 ðD0 Þ2=3 y1=6
(2)
where J, JL, and JK are the measured, diffusion-limiting, and kinetic-limiting current densities, respectively; u is the rotation speed in rpm; n is the number of electrons transferred during ORR; F is the Faraday constant (96485C mol1); C0 is the bulk concentration of O2; D0 is the diffusion coefficient of O2; and n is the kinetic viscosity of O2 [31,32]. According to Equations (1) and (2), the n of NG-1000 can be obtained from the slope of KeL plots (Fig. 4d). The n of NG-1000 in alkaline medium was
Table 1 Atomic percentages of C, N, O, and N distributions from the XPS survey spectra. Sample
C (at.%)
O (at.%)
GO GN-1000 NG-900 NG-1000 NG-1100
67.59 96.81 88.49 89.51 91.77
32.41 3.19 5.49 5.83 4.95
N (at.%)
6.02 4.66 3.28
N Distribution (%) Pyridinic N
Pyrrolic N
Graphitic N
Oxidized N
40.1 31.8 37.0
26.6 27.2 22.8
24.3 28.4 31.9
9.0 12.6 8.27
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Fig. 4. (a) CV curves of NG-1000 in N2- and O2-saturated 0.1 M KOH solutions at a scan rate of 10 mV s1 (b) LSV curves of GN-1000, NG-900, NG-1000, NG-1100, and Pt/C in O2saturated 0.1 M KOH solution at a scan and rotation rates of 10 mV s1 and 1600 rpm, respectively. (c) LSV curves of NG-1000 in O2-saturated 0.1 M KOH solution at different rotation rates (inset: number of electron transfer as a function of potential). (d) KeL plots for NG-1000 at different potentials. (e) Chronoamperometric curves of NG-1000 and Pt/C in O2saturated 0.1 M KOH solution via the addition of 3 M methanol at a rotation rate of 100 rpm. (f) Chronoamperometric curves of NG-1000 and Pt/C in O2-saturated 0.1 M KOH solution at a rotation rate of 100 rpm for 50000 s.
estimated to be 3.6e3.9 per O2 molecule at different potentials by using the values of C0 ¼ 1.2 103 mol L1, D0 ¼ 1.9 105 cm2 s1, and n ¼ 0.01 cm2 s1 in the 0.1 M KOH solution (Fig. 4c) [31,32]. Therefore, the ORR catalyzed by NG1000 is dominated by an efficient four-electron process with water as the product. For potential use in direct methanol fuel cells, NG-1000 should be tested for its methanol poisoning effect. The catalytic selectivity of NG-1000 in 0.1 M KOH solution added with 3 M methanol was measured by inspecting its chronoamperometric curve. No obvious response was observed in the ORR current of NG-1000 after the addition of 3 M methanol (Fig. 4e). However, an evident decrease was detected in the ORR current of Pt/C when 3 M methanol was
added under the same condition. This observation can be ascribed to the fact that methanol oxidization can be catalyzed by Pt/C and produce CO to poison Pt/C [33,34]. Therefore, NG-1000 improves catalytic selectivity for cathode oxygen reduction and methanol oxidization and also performs an excellent tolerance to methanol in alkaline medium. Stability is a key parameter to evaluate the performance of a catalyst. The currentetime chronoamperometric curves of NG1000 and Pt/C were measured in O2-saturated 0.1 M KOH solution for 40000 s. The ORR current of NG-1000 decreased more slowly than that of commercial Pt/C (Fig. 4f), suggesting that NG1000 demonstrates better stability in alkaline medium than the commercial product.
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3.2.2. Electrochemical characterizations in an acidic medium The electrochemical catalytic activity of NG-1000 in an acidic medium (0.1 M HClO4) was also investigated. Compared with N2saturated solution, O2-saturated 0.1 M HClO4 solution reveals a remarkable ORR current peak in its CV curve at ~0.64 V versus RHE (Fig. 5a). The onset potential of NG-1000 is ~0.82 V versus RHE, suggesting that this sample also presents high ORR activity in an acidic medium. The change of ORR catalytic activity in an acidic medium followed the same trend as that observed in alkaline medium. GN1000 showed a weaker ORR activity and lower onset potential in acidic medium than the other catalysts (Fig. 4b). Furthermore, the onset potential of NG-1000 in an acidic medium is more positive than that of NG-900 and NG-1100, indicating that NG-1000 is the most efficient among these three NG catalysts for ORR. The results of Raman spectroscopy and XPS showed that elevated defect degree and balanced N distribution can increase the ORR catalytic activity of NG in an acidic medium. The ORR mechanism of NG-1000 in an acidic medium was also studied by RDE measurements (Fig. 5c). According to KeL equations, the KeL plots (Fig. 5d) can be obtained, and the n of NG-1000 in an acidic medium was estimated to be 3.7e4.0 per O2 molecule at
different potentials (Fig. 5c) by using the values of C0 ¼ 1.26 103 mol L1, D0 ¼ 1.93 105 cm2 s1, and n ¼ 0.01 cm2 s1 in the 0.1 M HClO4 solution [35,36]. This result also suggests that the oxygen reduction with the NG-1000 catalyst follows the four-electron transfer pathway. The comparison in Table 2 shows that the N-doped GN in our work demonstrated different performances in acidic and alkaline solutions compared with the commercial Pt/C because of the varied ORR mechanisms in these two media. In alkaline medium, ORR proceeds as follows: Direct: O2 þ 2H2O þ 4e ¼ 4OH; E ¼ 0.401 V Series: O2 þ H2O þ 2e ¼ HO 2 þ OH ; E ¼ 0.0695 V; HO þ H O þ 2e ¼ 3OH ; E ¼ 0.867 V 2 2
In acidic medium, ORR proceeds as follows: Direct: O2 þ 4Hþ þ 4e ¼ 2H2O; E ¼ 1.229 V Series: O2 þ 2Hþ þ 2e ¼ H2O2; E ¼ 0.695 V; H2O2 þ 2Hþ þ 2e ¼ 2H2O; E ¼ 1.763 V
Fig. 5. (a) CV curves of NG-1000 in N2- and O2-saturated 0.1 M HClO4 solutions at a scan rate of 10 mV s1 (b) LSV curves of GN-1000, NG-900, NG-1000, NG-1100, and Pt/C in O2saturated 0.1 M HClO4 solution at a scan and rotation rates of 10 mV s1 and 1600 rpm, respectively. (c) LSV curves of NG-1000 C in O2-saturated 0.1 M HClO4 solution at different rotation rates (inset: number of electron transfer as a function of potential). (d) KeL plots for NG-1000 at different potentials. (e) Chronoamperometric curves of NG-1000 and Pt/C in O2-saturated 0.1 M HClO4 solution via addition of 3 M methanol at a rotation rate of 100 rpm. (f) Chronoamperometric curves of NG-1000 and Pt/C in O2-saturated 0.1 M HClO4 solution at a rotation rate of 100 rpm for 50000 s.
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Table 2 Summary of ORR electrochemical data for all samples. Sample
GN-1000 NG-900 NG-1000 NG-1100 Pt/C
0.1 M KOH
0.1 M HClO4
Onset potential/V (RHE)
Electron transfer number (n)
Onset potential/V (RHE)
Electron transfer number (n)
0.86 0.94 0.99 0.95 0.98
2.3e2.9 3.4e3.8 3.6e3.9 3.4e3.9 3.7e4.0
0.35 0.76 0.82 0.79 0.93
3.6e3.9 3.7e4.0 3.7e4.0 3.5e4.0
Evidently, the ORR in acidic medium requires more energy than it does in alkaline medium. We conjectured that the activity of NG1000 is not competent to the highly efficient ORR in the acidic media. Moreover, pyrrolic N can be dissolved in acidic solution, thereby leading to a decrease in the ORR catalytic activity. The methanol tolerance and stability of NG-1000 in acidic medium were also investigated using the same procedure. Compared with the performance of Pt/C, NG-1000 showed a stronger methanol tolerance in acidic medium (Fig. 5e). The ORR current of NG1000 obviously decreased (Fig. 5f) at a rate slower than that of Pt/ C. This finding indicates that NG-1000 is more stable than Pt/C in acidic medium. 4. Conclusion NG with abundant defects and balanced N distribution was prepared by regulating the temperature of GO and urea pyrolysis. High-defect concentration and balanced N distribution improved the ORR catalytic activity in both alkaline and acidic media. Among the three NG samples, NG-1000 showed the optimal ORR catalytic activity, stability, and methanol tolerance in both alkaline and acid media. These outstanding electrochemical catalytic performances render NG-1000 as a promising catalyst for ORR in fuel cells. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant No. 51402036, 51273032 and 91333104), and the International Science & Technology Cooperation Program of China (Grant No. 2013DFA51000), and the Fundamental Research Funds for the Central Universities (Grant No. DUT15YQ109). References [1] [2] [3] [4] [5]
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Catalytic activities enhanced by abundant structural defects and balanced N distribution of N-doped graphene in oxygen reduction reaction Xiaogong Bai a, Yantao Shi a, **, Jiahao Guo a, Liguo Gao b, Kai Wang a, Yi Du a, Tingli Ma b, c, * a b c
State Key Laboratory of Fine Chemicals, School of Chemistry, Dalian University of Technology, Dalian 116024, China School of Petroleum and Chemical Engineering, Dalian University of Technology, Panjin 124221, China Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, Kitakyushu, Fukuoka 808-0196, Japan
h i g h l i g h t s Abundant structural defects on graphene enhance the ORR activity. Balanced N distribution on graphene could also improve the ORR activity. Structural defects and N distribution are adjusted by regulating temperature. NG-1000 shows notable ORR activities in both alkaline and acid media.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 July 2015 Received in revised form 19 October 2015 Accepted 24 October 2015 Available online xxx
N-doped graphene (NG) is a promising candidate for oxygen reduction reaction (ORR) in the cathode of fuel cells. However, the catalytic activity of NG is lower than that of commercial Pt/C in alkaline and acidic media. In this study, NG samples were obtained using urea as N source. The structural defects and N distribution in the samples were adjusted by regulating the pyrolysis temperature. The new NG type exhibited remarkable catalytic activities for ORR in both alkaline and acidic media. © 2015 Elsevier B.V. All rights reserved.
Keywords: N-doping graphene Oxygen reduction reaction N-distribution Structural defects
1. Introduction Toyota's announcement about its mass production of fuel cell vehicles has attracted attention and encouraged scholars to investigate the production cost and durability of the company's vehicles. Pt/C is the cathode catalyst for oxygen reduction reaction (ORR) in fuel cells; however, the commercial application of this catalyst is limited by its high cost and susceptibility to time-dependent drift and carbon monoxide/methanol deactivation [1e3]. To overcome
* Corresponding author. School of Petroleum and Chemical Engineering, Dalian University of Technology, Panjin 124221, China. ** Corresponding author. E-mail addresses: [email protected] (Y. Shi), [email protected] (T. Ma). http://dx.doi.org/10.1016/j.jpowsour.2015.10.081 0378-7753/© 2015 Elsevier B.V. All rights reserved.
these limitations, considerable efforts have been exerted to develop alternative ORR catalysts, such as non-precious metals (Fe and Co) [4e6], metal oxides (sulfides) (Fe3O4, MnO2, and Co1-xS) [7e9], and heteroatom-doped (N-, S-, P-, and B-doped) carbon materials [graphene (GN), carbon nanotube, and carbon aerogels] [10e12]. Among these alternative catalysts, carbon materials are considered the most promising for ORR because of their low cost and stable properties. The carbon-based material GN has elicited significant attention because of its high surface area, superior electrical conductivity, excellent thermal stability, and exceptional mechanical properties [1]. Heteroatom-doped GN materials, specifically N-doped graphene (NG) materials, can tail their electronic property and chemical reactivity to increase their efficiency for ORR. N-doping can also endow GN with more exposed edges, where oxygen
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absorption is facilitated more easier than in basal planes [10]. Thus, NG with abundant exposed edges is desirable to further enhance ORR catalysis. Generally, NG can be synthesized through the post-treatment of GN or graphite oxide (GO) with N-containing sources (urea [13e15], polyaniline [16], polypyrrole [16], melamine [17], dicyandiamide [18], NH3 [19], ammonia water [20], and hydrazine [21]). Among these materials, urea has been widely used to N-dope GN because of its low cost and manufacturing quantities. Although some articles have discussed the relationship between N-doping and ORR activity, the enhancements in catalytic performance are mainly ascribed to one or two specific N species, such as pyridinic, pyrrolic, or graphitic N. A controversy on the role of N species in ORR also exists. For instance, Dai et al. reported that the increased ORR catalytic activity of NG is caused by the presence of pyridinic and pyrrolic N [22,23]. Other reports showed that pyridinic N can change the ORR mechanism from a two-electron-dominated process to a four-electron-dominated one and improve the onset potential for ORR; furthermore, pyrrolic N is favorable for oxygen adsorption and reduction in water through a four-electron mechanism [16,24]. Lin et al. prepared NG with pyridinic N as the dominant dopant by using a chemical vapor deposition method; they found that pyridinic N may not play a key role to improve the ORR catalytic activity [25]. Sun et al. confirmed that the ORR activity is not significantly dependent on the contents of pyridinic and pyrrolic N. Graphitic N seems to be the most important species for ORR and can considerably increase the limiting current and participate in ORR [1,16,19]. In light of these conflicting results, we speculate that all the three types of N species play important roles in ORR, and precisely tuning the proportion of each N species may be more favorable to enhance the ORR catalytic activity of NG. In the present work, we synthesized NG and adjusted its structural defects and N distribution by controlling the post-treatment temperature (900 C, 1000 C, and 1100 C) of GO mixed with urea. Systematical characterizations on morphology and microstructures were conducted, and the catalytic activities in alkaline and acidic media of the samples were evaluated. NG pyrolyzed at 1000 C (NG-1000) exhibited abundant exposed edges and balanced N distribution. Furthermore, NG-1000 showed catalytic activities similar to those of commercial Pt/C for ORR in alkaline medium and demonstrated good ORR catalytic activity in acidic medium. Therefore, this new NG is a promising cathode catalyst for fuel cells. 2. Experimental 2.1. Material preparation GO was purchased from The Sixth Element Material (Changzhou) Ltd. without further post-treatment. For NG synthesis, GO (30 mg) and urea (500 mg) were dispersed in a mixture of water (50 mL) and ethanol (50 mL) by ultrasonic irradiation for 2 h. The solution was transferred to a glass plate and dried at 25 C for 2 days. The dried GO/urea mixture was pyrolyzed at 900 C, 1000 C, and 1100 C in a N2 atmosphere for 2 h to produce NG-900, NG1000, and NG-1100, respectively. Additionally, GO was pyrolyzed at 1000 C in a N2 atmosphere for 2 h to produce a control sample (GN-1000) for comparison. 2.2. Sample characterizations The morphology of NG was observed via scanning electron microscopy (SEM, S-4800) and transmission electron microscopy (TEM, Titan 80-300). X-ray photoelectron spectroscopy (XPS, Thermo Escalab 250Xi) was performed to analyze the chemical
element and sample states, and Raman spectroscopy (Renishaw inVia) was conducted for microstructure analysis. 2.3. Electrochemical measurements The electrochemical properties of the samples were tested on the CHI 660C electrochemical workstation with a three-electrode system. A glassecarbon (GC) rotating disk electrode (RDE) (diameter, 5 mm; Pine), Pt wire, and Ag/AgCl electrode filled with saturated KCl aqueous solution were used as the working, counter, and reference electrodes, respectively. NG (1.25 mg) and Nafion solution (5 wt%, 25 mL) were mixed ultrasonically in water (355 mL) and isopropanol (120 mL) for more than 1 h. Up to 12 mL of catalyst ink was dipped onto the GC electrode with a loading amount of 152 mg cm2. For comparison, Pt/C (20 wt% Pt, 1 mg) and Nafion solution (20 mL) were dispersed in ethanol (480 mL), whereas 10 mL of Pt/C ink was dipped onto a GC electrode with a loading amount of 102 mg cm2. All electrochemical properties were tested in N2- or O2-saturated 0.1 M KOH or HClO4 solution at room temperature. 3. Results and discussion 3.1. Morphology and structure Fig. 1a illustrates flake-like GO bulks with sizes exceeding 20 mm. Such morphology is common because commercial GO is prepared through simple graphite oxidation rather than exfoliation into individual GN sheets. In the current study, NG-1000 was selected for detailed characterizations. The structure and morphology of NG-1000 were investigated via SEM and TEM. In terms of morphology, NG-1000 is fluffier than pristine GO because of the several single-layered structures in the former (Fig. 1b). The enlarged image in Fig. 1c shows that interlaced and ultrathin films are pronounced. Such structural features provide advanced properties, including large specific area, several exposed active sites, and convenient fluid penetration, which are highly desirable for electrocatalysis. Oxygen adsorption improves with the increasing number of edges [10]. NG-1000 shows a thin (i.e., no more than several nanometers), wrinkled, and silk-veil-like morphology (Fig. 1d and e). The electron diffraction pattern in Fig. 1f displays diffraction points rather than diffraction circles, further revealing that NG-1000 exhibits an exfoliated thin layer [26]. These results suggest that GO can be exfoliated into a thin layer following hightemperature treatment. Raman spectroscopy, a powerful method to evaluate the level of GN defects, was used in this study to characterize the defect sites and band intensity ratio. The ratio of D band to G band intensity (ID/ IG) was utilized to indicate this defect level. Generally, a high ratio shows a high concentration of defects [18]. The Raman spectra (Fig. 2) of GO, GN-1000, NG-900, NG-1000, and NG-1100 display two prominent peaks corresponding to the D band (1350 cm1) arising from the sp3 defect sites and G band (1580 cm1) associated with the emerging sp2-hybridized graphitic carbon atoms [27]. The ID/IG of GN-1000 (2.35), NG-900 (2.93), NG-1000 (3.08), or NG-1100 (2.13) is higher than that of GO (1.87). This result indicates that high-temperature treatment can enhance defect concentration. Additionally, NG-1000 (3.08) contains more defect sites than GN1000 (2.35). This result indicates that N-doping influences the degree of graphitization. Interestingly, the ID/IG of NG-900 (2.93) is lower than that of NG-1000 (3.08) but higher than that of NG-1100 (2.13), thus implying that the NG structural defects can be adjusted by controlling the temperature. XPS measurements were conducted to evaluate the chemical compositions of the samples. The oxygen content decreased significantly after high-temperature sintering, implying that GO
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Fig. 1. (a) SEM image of GO. (b) and (c) SEM images of NG-1000. (d) and (e) TEM images of NG-1000. (f) Electron diffraction pattern of NG-1000.
can be effectively reduced through high-temperature treatment (Table 1 and Fig. 3a). The N contents of NG-900, NG-1000, and NG1100 are much higher than that of GN-1000. This finding proves that N has been successfully doped into GN after pyrolysis of the GO and urea mixture. Fig. 3b shows the high-resolution N1s XPS spectra of NG samples. Generally, the complex N1s peak can be further divided into four primary peaks corresponding to pyridinic, pyrrolic, graphitic, and oxidized N [13,17]. Pyridinic, pyrrolic, and graphitic N can contribute to ORR, and N distribution may affect ORR catalysis [16,19,24,28]. Hence, we analyzed the N distribution of N1s in detail as described in Table 1. As the sintering temperature increased from 900 C to 1100 C, the total atomic content of N in NG decreased from 6.02% to 3.28%, whereas that of graphitic N increased from 24.3% to 31.9% (Table 1). Compared with NG-900 and NG-1100, NG-1000 demonstrated a more balanced N
distribution: 31.8% pyridinic N, 27.2% pyrrolic N, 28.4% graphitic N, and 12.6% oxidized N. NG-1000 exhibited the most efficient ORR catalytic activity among all the samples in both alkaline and acidic media. Thus, we conclude that balanced N distribution improves the catalytic efficiency for ORR. Chemical calculation is also needed to further illustrate the mechanism. 3.2. Electrochemical characterizations 3.2.1. Electrochemical characterizations in alkaline medium Cyclic voltammetry (CV) was used to describe the ORR catalytic activity of NG-1000 in alkaline medium, namely, N2- or O2-saturated 0.1 M KOH solution at a scan rate of 10 mV s1. No obvious peak can be observed in the CV curve for N2-saturated 0.1 M KOH solution, which can provide a clean capacitive CV background
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Fig. 2. Raman spectra of GO, GN-1000, NG-900, NG-1000, and NG-1100.
(Fig. 4a). Compared with N2-saturated solution, O2-saturated solution showed a higher cathodic current with a peak at ~0.84 V versus RHE, thereby suggesting higher electrocatalytic activity for ORR. Onset potential is an important criterion to evaluate the ORR activity of an electrocatalyst [29]. NG-1000, with a sharply positive onset potential at ~0.99 V versus RHE, also demonstrated a higher electrocatalytic activity than the other samples. To further investigate the ORR activity of the NG samples, linear sweep voltammetry (LSV) measurements were performed in O2saturated 0.1 M KOH solution by using RDE at a scan rate of 10 mV s1. Fig. 4b displays the LSV curves of the different samples. Among these samples, GN-1000 without N-doping showed the lowest onset potential, hence implying that the electrocatalytic activity of this sample is worse than that of the others. By contrast, the ORR onset potentials for NG-900, NG-1000, and NG-1100 all shifted positively. Thus, N-doping may improve the ORR catalytic activity of GN. NG-1000 showed a relatively higher steady current and positive onset potential, which offered the highest ORR activity among the three N-doping samples. Moreover, NG-1000 presented a larger onset potential than commercial Pt/C. This result further confirmed the significance of balanced N distribution, as aforementioned in the Raman spectroscopy and XPS analyses in Figs. 2 and 3, respectively. The Raman spectra indicated the presence of more defects in NG-1000 than in other samples. Therefore, NG1000 can provide more exposed edges for oxygen adsorption and more ORR active sites than the other samples [10]. XPS results showed that pyridinic, pyrrolic, and graphitic N, each of which plays an important role in improving ORR, are evenly distributed in NG-1000 [16,19,24]. The absence of any of these materials may reduce ORR activity. Generally, ORR follows a four- or two-electron reaction pathway. In a four-electron process, O2 can be converted into water without any other side reaction. However, in a two-electron process, peroxide as an intermediate would emerge in ORR [30]. Thus, the four-electron process is more efficient and favorable than the two-
Fig. 3. (a) XPS survey spectra of GO, GN-1000, NG-900, NG-1000, and NG-1100. (b) High-resolution N1s XPS spectra of NG-900, NG-1000, or NG-1100.
electron reaction pathway. The number of electrons transferred per oxygen molecule on NG-1000 can be obtained from the RDE measurements, which should be calculated from the KouteckyeLevich (KeL) equations as follows:
1 1 1 1 1 ¼ þ ¼ þ J JL JK Bu1=2 JK
(1)
B ¼ 0:62nFC0 ðD0 Þ2=3 y1=6
(2)
where J, JL, and JK are the measured, diffusion-limiting, and kinetic-limiting current densities, respectively; u is the rotation speed in rpm; n is the number of electrons transferred during ORR; F is the Faraday constant (96485C mol1); C0 is the bulk concentration of O2; D0 is the diffusion coefficient of O2; and n is the kinetic viscosity of O2 [31,32]. According to Equations (1) and (2), the n of NG-1000 can be obtained from the slope of KeL plots (Fig. 4d). The n of NG-1000 in alkaline medium was
Table 1 Atomic percentages of C, N, O, and N distributions from the XPS survey spectra. Sample
C (at.%)
O (at.%)
GO GN-1000 NG-900 NG-1000 NG-1100
67.59 96.81 88.49 89.51 91.77
32.41 3.19 5.49 5.83 4.95
N (at.%)
6.02 4.66 3.28
N Distribution (%) Pyridinic N
Pyrrolic N
Graphitic N
Oxidized N
40.1 31.8 37.0
26.6 27.2 22.8
24.3 28.4 31.9
9.0 12.6 8.27
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Fig. 4. (a) CV curves of NG-1000 in N2- and O2-saturated 0.1 M KOH solutions at a scan rate of 10 mV s1 (b) LSV curves of GN-1000, NG-900, NG-1000, NG-1100, and Pt/C in O2saturated 0.1 M KOH solution at a scan and rotation rates of 10 mV s1 and 1600 rpm, respectively. (c) LSV curves of NG-1000 in O2-saturated 0.1 M KOH solution at different rotation rates (inset: number of electron transfer as a function of potential). (d) KeL plots for NG-1000 at different potentials. (e) Chronoamperometric curves of NG-1000 and Pt/C in O2saturated 0.1 M KOH solution via the addition of 3 M methanol at a rotation rate of 100 rpm. (f) Chronoamperometric curves of NG-1000 and Pt/C in O2-saturated 0.1 M KOH solution at a rotation rate of 100 rpm for 50000 s.
estimated to be 3.6e3.9 per O2 molecule at different potentials by using the values of C0 ¼ 1.2 103 mol L1, D0 ¼ 1.9 105 cm2 s1, and n ¼ 0.01 cm2 s1 in the 0.1 M KOH solution (Fig. 4c) [31,32]. Therefore, the ORR catalyzed by NG1000 is dominated by an efficient four-electron process with water as the product. For potential use in direct methanol fuel cells, NG-1000 should be tested for its methanol poisoning effect. The catalytic selectivity of NG-1000 in 0.1 M KOH solution added with 3 M methanol was measured by inspecting its chronoamperometric curve. No obvious response was observed in the ORR current of NG-1000 after the addition of 3 M methanol (Fig. 4e). However, an evident decrease was detected in the ORR current of Pt/C when 3 M methanol was
added under the same condition. This observation can be ascribed to the fact that methanol oxidization can be catalyzed by Pt/C and produce CO to poison Pt/C [33,34]. Therefore, NG-1000 improves catalytic selectivity for cathode oxygen reduction and methanol oxidization and also performs an excellent tolerance to methanol in alkaline medium. Stability is a key parameter to evaluate the performance of a catalyst. The currentetime chronoamperometric curves of NG1000 and Pt/C were measured in O2-saturated 0.1 M KOH solution for 40000 s. The ORR current of NG-1000 decreased more slowly than that of commercial Pt/C (Fig. 4f), suggesting that NG1000 demonstrates better stability in alkaline medium than the commercial product.
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3.2.2. Electrochemical characterizations in an acidic medium The electrochemical catalytic activity of NG-1000 in an acidic medium (0.1 M HClO4) was also investigated. Compared with N2saturated solution, O2-saturated 0.1 M HClO4 solution reveals a remarkable ORR current peak in its CV curve at ~0.64 V versus RHE (Fig. 5a). The onset potential of NG-1000 is ~0.82 V versus RHE, suggesting that this sample also presents high ORR activity in an acidic medium. The change of ORR catalytic activity in an acidic medium followed the same trend as that observed in alkaline medium. GN1000 showed a weaker ORR activity and lower onset potential in acidic medium than the other catalysts (Fig. 4b). Furthermore, the onset potential of NG-1000 in an acidic medium is more positive than that of NG-900 and NG-1100, indicating that NG-1000 is the most efficient among these three NG catalysts for ORR. The results of Raman spectroscopy and XPS showed that elevated defect degree and balanced N distribution can increase the ORR catalytic activity of NG in an acidic medium. The ORR mechanism of NG-1000 in an acidic medium was also studied by RDE measurements (Fig. 5c). According to KeL equations, the KeL plots (Fig. 5d) can be obtained, and the n of NG-1000 in an acidic medium was estimated to be 3.7e4.0 per O2 molecule at
different potentials (Fig. 5c) by using the values of C0 ¼ 1.26 103 mol L1, D0 ¼ 1.93 105 cm2 s1, and n ¼ 0.01 cm2 s1 in the 0.1 M HClO4 solution [35,36]. This result also suggests that the oxygen reduction with the NG-1000 catalyst follows the four-electron transfer pathway. The comparison in Table 2 shows that the N-doped GN in our work demonstrated different performances in acidic and alkaline solutions compared with the commercial Pt/C because of the varied ORR mechanisms in these two media. In alkaline medium, ORR proceeds as follows: Direct: O2 þ 2H2O þ 4e ¼ 4OH; E ¼ 0.401 V Series: O2 þ H2O þ 2e ¼ HO 2 þ OH ; E ¼ 0.0695 V; HO þ H O þ 2e ¼ 3OH ; E ¼ 0.867 V 2 2
In acidic medium, ORR proceeds as follows: Direct: O2 þ 4Hþ þ 4e ¼ 2H2O; E ¼ 1.229 V Series: O2 þ 2Hþ þ 2e ¼ H2O2; E ¼ 0.695 V; H2O2 þ 2Hþ þ 2e ¼ 2H2O; E ¼ 1.763 V
Fig. 5. (a) CV curves of NG-1000 in N2- and O2-saturated 0.1 M HClO4 solutions at a scan rate of 10 mV s1 (b) LSV curves of GN-1000, NG-900, NG-1000, NG-1100, and Pt/C in O2saturated 0.1 M HClO4 solution at a scan and rotation rates of 10 mV s1 and 1600 rpm, respectively. (c) LSV curves of NG-1000 C in O2-saturated 0.1 M HClO4 solution at different rotation rates (inset: number of electron transfer as a function of potential). (d) KeL plots for NG-1000 at different potentials. (e) Chronoamperometric curves of NG-1000 and Pt/C in O2-saturated 0.1 M HClO4 solution via addition of 3 M methanol at a rotation rate of 100 rpm. (f) Chronoamperometric curves of NG-1000 and Pt/C in O2-saturated 0.1 M HClO4 solution at a rotation rate of 100 rpm for 50000 s.
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Table 2 Summary of ORR electrochemical data for all samples. Sample
GN-1000 NG-900 NG-1000 NG-1100 Pt/C
0.1 M KOH
0.1 M HClO4
Onset potential/V (RHE)
Electron transfer number (n)
Onset potential/V (RHE)
Electron transfer number (n)
0.86 0.94 0.99 0.95 0.98
2.3e2.9 3.4e3.8 3.6e3.9 3.4e3.9 3.7e4.0
0.35 0.76 0.82 0.79 0.93
3.6e3.9 3.7e4.0 3.7e4.0 3.5e4.0
Evidently, the ORR in acidic medium requires more energy than it does in alkaline medium. We conjectured that the activity of NG1000 is not competent to the highly efficient ORR in the acidic media. Moreover, pyrrolic N can be dissolved in acidic solution, thereby leading to a decrease in the ORR catalytic activity. The methanol tolerance and stability of NG-1000 in acidic medium were also investigated using the same procedure. Compared with the performance of Pt/C, NG-1000 showed a stronger methanol tolerance in acidic medium (Fig. 5e). The ORR current of NG1000 obviously decreased (Fig. 5f) at a rate slower than that of Pt/ C. This finding indicates that NG-1000 is more stable than Pt/C in acidic medium. 4. Conclusion NG with abundant defects and balanced N distribution was prepared by regulating the temperature of GO and urea pyrolysis. High-defect concentration and balanced N distribution improved the ORR catalytic activity in both alkaline and acidic media. Among the three NG samples, NG-1000 showed the optimal ORR catalytic activity, stability, and methanol tolerance in both alkaline and acid media. These outstanding electrochemical catalytic performances render NG-1000 as a promising catalyst for ORR in fuel cells. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant No. 51402036, 51273032 and 91333104), and the International Science & Technology Cooperation Program of China (Grant No. 2013DFA51000), and the Fundamental Research Funds for the Central Universities (Grant No. DUT15YQ109). References [1] [2] [3] [4] [5]
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