- Email: [email protected]
conversion systems
Electrochemistry Communications 13 (2011) 50–53
Contents lists available at ScienceDirect
Electrochemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e l e c o m
Synthesis of N-doped carbon nanosheets from collagen for electrochemical energy storage/conversion systems Ying-Hui Lee a, Ying-Feng Lee a, Kuo-Hsin Chang a,b, Chi-Chang Hu a,⁎ a b
Department of Chemical Engineering, National Tsing Hua University, Hsin-Chu 30013, Taiwan Department of Chemical Engineering, National Chung Cheng University, Chia-Yi 621, Taiwan
a r t i c l e
i n f o
Article history: Received 26 September 2010 Received in revised form 7 November 2010 Accepted 7 November 2010 Available online 12 November 2010 Keywords: Collagen N-doped Carbon nanosheets Supercapacitors Oxygen reduction
a b s t r a c t This study proposes a simple method for synthesizing carbon nanosheets doped with nitrogen through carbonization of collagen. Collagen, the most abundant protein in mammals, was cross-linked with paraformaldehyde and subsequently heated in vacuum at 800 °C to obtain N-doped carbon nanosheets with a high specific surface area of 695 m2 g−1. With the contribution of N-doped structures, the carbon nanosheets show ideal capacitive behavior with 80% capacitance retention in 0.5 M H2SO4 at 1000 mV s−1. In comparison with a commercial electrocatalyst, 20% Pt on Vulcan XC-72, carbon nanosheets display a positive shift in the onset potential and superior electrocatalytic activity toward the oxygen reduction reaction (ORR). The above excellent electrochemical performances render the N-doped carbon nanosheets a promising material for electrochemical energy storage/conversion systems. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Recently, several efforts have been devoted to change the physicochemical properties of carbonaceous materials by introducing heteroatoms, especially the nitrogen atom, to the graphitic or graphite-like structures. For example, nitrogen-doped carbon materials have been found to enhance the performances of several electrochemical energy storage or conversion systems, such as electric double-layer capacitors (EDLCs) [1–6] and fuel cells [7–12]. The N-doped structures are believed to provide pesudocapacitance contributed from the redox faradaic reactions of these electrochemically active functional groups. Moreover, certain N-doped functional groups can enhance the electronic conductivity of carbon materials for EDLCs [6,13,14]. As a result, N-doped carbon materials are considered to be promising EDLC electrode materials that are used practically as power sources/energy storage devices to supply/deliver pulsed current. More recently, these unique N-containing functional groups have been found to be the electrocatalytic sites for the ORR in fuel cells [7,15–17]. Accordingly, several studies showed the potential of N-doped carbonaceous materials to replace the commonly used yet much more expensive platinum as the electrocatalyst for the ORR. N-doped carbonaceous materials have been prepared through the post-treatment, e.g., ammoxidation, of carbon samples [2,3] or the employment of an N-containing precursor. The latter procedure is preferred because it is easier, more cost-effective, and more controllable
⁎ Corresponding author. Tel./fax: +886 3 573 6027. E-mail address: [email protected] (C.-C. Hu). 1388-2481/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2010.11.010
for the N-doping content [1,4–6,18]. Among many N-containing precursors, biomaterials, such as proteins, have been reported as suitable precursors for synthesizing N-doped carbonaceous materials because of their amino group-bountiful nature [19–21]. Accordingly, collagen [22,23], the most abundant protein in mammals, was utilized as the precursor to prepare N-doped carbon materials in this work. Material analyses confirmed the existence of N-doped structures and characterized the collagen-derived material as carbon nanosheets. The N-doped carbon nanosheets showed a near ideal EDLC behavior with high power performances. Furthermore, they were able to catalyze the ORR with a higher catalytic activity and more positive onset potential in comparison with the commercial electrocatalyst, 20% Pt on Vulcan XC-72. Therefore, the excellent capacitive performance and activity toward the ORR established the potential of collagen-derived N-doped carbon nanosheets for the electrochemical applications. 2. Experimental A collagen solution (5 mg ml−1) was obtained by dissolving collagen powder (Sigma-Aldrich, USA) in an acetic acid solution (0.02 M). With the addition of phosphate buffered saline (PBS, Sigma-Aldrich, USA) and sodium hydroxide, collagen gel was formed after 24-h incubation at 37 °C. After being cross-linked with a 4% paraformaldehyde solution for 15 min, the soaked gel was heated in vacuum at a rate of 4 °C min−1 to 800 °C and kept at this temperature for 6 h. XPS measurement was performed on the Kratos Axis Ultra DLD (Kratos analytical, USA) which employed Al monochromator (hν = 1486.69 eV) irradiation as the photosource. The scanning electron microscopic (SEM) and transmission electron microscopic (TEM)
Y.-H. Lee et al. / Electrochemistry Communications 13 (2011) 50–53
51
images of carbon nanosheets were obtained by Hitachi S-4700I (Hitachi, Japan) and JEM-2010 (JEOL, Japan), respectively. The electrochemical performances of the collagen-derived carbon material were investigated by CHI 633A (CH Instruments, USA) in a three-compartment cell. An Ag/AgCl electrode (Argenthal, 3 M KCl, 0.207 V vs. standard hydrogen electrode at 25 °C) was used as the reference electrode, and platinum wire was employed as the counter electrode. A Luggin capillary was used to minimize errors due to ohmic potential drop in the electrolyte. The potentials for the ORR are referred to a reversible hydrogen electrode (RHE). The material for the working electrode was a mixture consisting of the collagen-derived carbon material and polyvinylidene in the weight ratio of 10:1. The mixture (ca. 1 mg) was homogeneously suspended in N-methyl-2-pyrrolidone and coated on a graphite substrate with an exposed surface area of 1 cm2. The electrolyte for evaluating capacitive performances is a 0.5 M sulfuric acid solution, whereas it is a 0.1 M potassium hydroxide solution for investigating the ORR activity. A commercial electrocatalyst, 20% Pt on Vulcan XC-72 (E-TEK, USA), was also applied (also ca. 1 mg) for the comparison of oxygen reduction activity. 3. Results and discussion XPS analysis was performed to evaluate the nitrogen content of the collagen-derived carbon material. Owing to the presence of amino groups in collagen, the atomic ratio of nitrogen for the carbonized material is about 1%. Fig. 1 reveals the existence of pyridinic-N (398.5 eV, 21.4 at.%), pyrrolic-N (400 eV, 28.1 at%), quaternary-N (401.2 eV, 42.8 at.%), and pyridine-N-oxide (403 eV, 7.7 at.%) [1,5,6]. This result confirmed that the collagen-derived carbon material contains N-doped structures. The surface morphology of the N-doped carbonaceous material was examined by the SEM image, as shown in Fig. 2A. The loose and layered structure characterized the carbonaceous material as carbon nanosheets. The sheet-like morphology was confirmed in the TEM image (Fig. 2B). The above results reveal the formation of a highly porous N-doped carbonaceous material. As shown in Fig. 2C, the porosity of N-doped carbon nanosheets was investigated by the analysis of N2 adsorption/desorption isotherms. In addition to micropores, the hysteresis loop between adsorption and desorption isotherms demonstrates the existence of slit-like mesopores, which agrees well with the morphology. The presence of both micropores and mesopores contributes to the high specific surface area (695 m2 g−1) of the carbon nanosheets. To unravel the capacitive performance of the N-doped carbon nanosheets, cyclic voltammograms at different scan rates were carried
Intensity (a.u.)
Fig. 2. (A) SEM and (B) TEM images, and (C) N2 adsorption/desorption isotherms of the N-doped carbon nanosheets.
408
404
400
396
392
Binding energy (eV) Fig. 1. N 1s core-level XPS spectra of the collagen-derived carbon material. The black line is the raw spectrum; the pink, blue, red, and green lines correspond to pyridine-N, pyrrolic-N, quaternary-N, and pyridine-N-oxide, respectively.
out, as shown in Fig. 3A. With its high specific surface area, the N-doped carbon nanosheets possess a nearly ideal double-layer behavior with a specific capacitance of 102 F g−1 at 25 mV s−1, which is comparable to that of activated carbons. In addition, perfect reversibility and stability are observed with varying the scan rate. Only 20% loss in capacitance is found when changing the scan rate from 25 to 1000 mV s−1 (Fig. 3B), indicating that the N-doped carbon nanosheets are a high power material. Previous studies [6,13,14] proposed that with the modified electron donor/acceptor properties, pyridinic-N and pyrrolic-N in carbon materials could induce Faradaic pseudocapacitance, whereas quaternary-N and pyridine-N-oxide could improve the ability of electron transfer in carbon materials. Since the redox peaks in the CV diagrams are not obvious, the contribution of pesudocapacitance from
52
Y.-H. Lee et al. / Electrochemistry Communications 13 (2011) 50–53
A
0.1
0.05
0
-0.05 (1) (2) (3) (4) (5)
-0.1
Current density (mA/cm2)
Current density (A/cm2)
A
2
0
-2
-4 (1) (2)
-6
-8
-0.15
0
0.2
0.4
0.6
0.8
1
-0.2
0
0.2
Potential (V) vs. Ag/AgCl
B
100
Current density (mA/cm2)
Capacitance retention (%)
B
80
60
40
20
0
0
200
400
600
800
0.4
0.6
0.8
1
1.2
Potential (V) vs. R.H.E 0
-1
-2
-4
1000
(1) (2)
-3
-0.2
Scan rate (mV/s) Fig. 3. (A) Cyclic voltammograms of the N-doped carbon nanosheets measured at (1) 100, (2) 250, (3) 500, (4) 750, and (5) 1000 mV s−1 in 0.5 M H2SO4. (B) The dependence of the capacitance retention on the scan rate of CV.
pyridinic-N and pyrrolic-N is not significant. However, the presence of quaternary-N and pyridine-N-oxide which improve electron transfer within the N-doped carbon nanosheets reasonably explains the high capacitance retention appearing at the high scan rate. On the other hand, the high-power performance is also attributed to the porous structure of the nanosheets since the interspaces between nanosheets provide facile accessibility for the electrolyte. In addition to the investigation of N-doped carbon nanosheets as the electrode of EDLCs, the catalytic activity toward the ORR was also measured (Fig. 4A). The significant larger current densities obtained in an air-saturated electrolyte in comparison with the N2-saturated electrolyte confirm that N-doped carbon nanosheets do exhibit the ability to catalyze the reduction of oxygen. In Fig. 4B, the catalytic activity of N-doped carbon nanosheets for the ORR is superior to the commercial electrocatalyst, 20% Pt on Vulcan XC-72, especially in the potential range from 0.2 V to 0.6 V. Furthermore, the onset potential of the ORR for the N-doped carbon nanosheets is 0.95 V; it is even more positive than that for the commercial electrocatalyst Pt, which is 0.83 V. The significant catalytic activity toward the ORR of the N-doped carbon nanosheets mainly comes from the contribution of pyridinic-N and quaternary-N. Previous studies showed that the presence of pyridinic-N and quaternary-N could change the adsorption of the oxygen molecule on carbon materials from end-on to side-on type, thus weakening the bonding of O–O to facilitate the ORR [7,15]. The unique catalytic activity toward the ORR of the N-doped carbon nanosheets is not attributable to the oxygen-/sulfur-containing functional groups. The O-containing functional groups could positively shift the onset potential of the ORR to 0.4 V (vs. RHE) [24], but much more negative than that of the N-doped carbon nanosheets. From the XPS analysis (not shown here), there are no S-containing functional groups
0
0.2
0.4
0.6
0.8
1
Potential (V) vs. R.H.E Fig. 4. (A) Linear sweep voltammograms of (1) 20% Pt on Vulcan XC-72 and (2) the N-doped carbon nanosheets measured at 10 mV s−1 in 0.1 M KOH (dotted line: N2-saturated electrolyte, solid line: air-saturated electrolyte); (B) the i–E curves deduced from the current density difference of the LSVs measured in the air-saturated and N2-saturated electrolytes for (1) 20% Pt on Vulcan XC-72 and (2) the N-doped carbon nanosheets.
on the N-doped carbon nanosheets because collagen mainly consists of glycine, proline, and hydroproline, which contain no sulfur [22]. 4. Conclusions N-doped carbon nanosheets were obtained simply by carbonization of the collagen gel. The preliminary electrochemical analyses show that the N-doped carbon nanosheets not only display nearly ideal doublelayer responses at high CV scan rates but also possess the superior electrocatalytic activity toward the ORR than a commercial Pt-based electrocatalyst. The contributions of high specific surface area, highly porous architecture, and N-doped structures are responsible for the ideal capacitive behavior; meanwhile, the N-doped structures also act as the active sites of the ORR. The collagen-derived N-doped carbon nanosheets have shown the promising benefits in the applications of EDLCs and fuel cells. Acknowledgement The financial supports of this work, the National Science Council of the ROC Taiwan under contract no. NSC 98-3114-E-007-011 and the boost program of National Tsing Hua University, are gratefully acknowledged. References [1] D. Hulicova-Jurcakova, M. Kodama, S. Shiraishi, H. Hatori, Z.H. Zhu, G.Q. Lu, Adv. Func. Mater. 19 (2009) 1800. [2] K. Jurewicz, Electrochim. Acta 48 (2003) 1491. [3] K. Kang, S. Hong, B. Lee, J. Lee, Electrochem. Commun. 10 (2008) 1105.
Y.-H. Lee et al. / Electrochemistry Communications 13 (2011) 50–53 [4] W. Li, D. Chen, Z. Li, Y. Shi, Y. Wan, J. Huang, J. Yang, D. Zhao, Z. Jiang, Electrochem. Commun. 9 (2007) 569. [5] T. Rufford, D. Hulicovajurcakova, Z. Zhu, G. Lu, Electrochem. Commun. 10 (2008) 1594. [6] M. Seredych, D. Hulicovajurcakova, G. Lu, T. Bandosz, Carbon 46 (2008) 1475. [7] K. Gong, F. Du, Z. Xia, M. Durstock, L. Dai, Science 323 (2009) 760. [8] K.R. Lee, K.U. Lee, J.W. Lee, B.T. Ahn, S.I. Woo, Electrochem. Commun. 12 (2010) 1052. [9] G. Liu, X. Li, P. Ganesan, B.N. Popov, Appl. Catal. B: Environmental 93 (2009) 156. [10] R. Liu, D. Wu, X. Feng, K. Müllen, Angewandte Chemie International Edition 49 (2010) 2565. [11] L. Qu, Y. Liu, J.-B. Baek, L. Dai, ACS Nano 4 (2010) 1321. [12] Y. Shao, S. Zhang, M.H. Engelhard, G. Li, G. Shao, Y. Wang, J. Liu, I.A. Aksay, Y. Lin, J. Mater. Chem. 20 (2010) 7491. [13] E. Frackowiak, Phys. Chem. Chem. Phys. 9 (2007) 1774.
53
[14] D. Hulicova-Jurcakova, M. Seredych, G.Q. Lu, T.J. Bandosz, Adv. Func. Mater. 19 (2009) 438. [15] Z. Shi, J. Zhang, Z. Liu, H. Wang, D. Wilkinson, Electrochim. Acta 51 (2006) 1905. [16] J. Ozaki, T. Anahara, N. Kimura, A. Oya, Carbon 44 (2006) 3358. [17] J. Ozaki, N. Kimura, T. Anahara, A. Oya, Carbon 45 (2007) 1847. [18] R.J. White, M. Antonietti, M.-M. Titirici, J. Mater. Chem. 19 (2009) 8645. [19] T. Iwazaki, R. Obinata, W. Sugimoto, Y. Takasu, Electrochem. Commun. 11 (2009) 376. [20] T. Iwazaki, H. Yang, R. Obinata, W. Sugimoto, Y. Takasu, J. Power Sources 195 (2010) 5840. [21] Y. Kim, Y. Abe, T. Yanagiura, K. Park, M. Shimizu, T. Iwazaki, S. Nakagawa, M. Endo, M. Dresselhaus, Carbon 45 (2007) 2116. [22] A.D. Covington, Chemical Society Reviews 26 (1997) 111. [23] K.E. Kadler, D.F. Holmes, J.A. Trotter, J.A. Chapman, Biochem. J. 316 (1996) 1. [24] K. Matsubara, K. Waki, ECS Transactions 28 (2010) 35.
Contents lists available at ScienceDirect
Electrochemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e l e c o m
Synthesis of N-doped carbon nanosheets from collagen for electrochemical energy storage/conversion systems Ying-Hui Lee a, Ying-Feng Lee a, Kuo-Hsin Chang a,b, Chi-Chang Hu a,⁎ a b
Department of Chemical Engineering, National Tsing Hua University, Hsin-Chu 30013, Taiwan Department of Chemical Engineering, National Chung Cheng University, Chia-Yi 621, Taiwan
a r t i c l e
i n f o
Article history: Received 26 September 2010 Received in revised form 7 November 2010 Accepted 7 November 2010 Available online 12 November 2010 Keywords: Collagen N-doped Carbon nanosheets Supercapacitors Oxygen reduction
a b s t r a c t This study proposes a simple method for synthesizing carbon nanosheets doped with nitrogen through carbonization of collagen. Collagen, the most abundant protein in mammals, was cross-linked with paraformaldehyde and subsequently heated in vacuum at 800 °C to obtain N-doped carbon nanosheets with a high specific surface area of 695 m2 g−1. With the contribution of N-doped structures, the carbon nanosheets show ideal capacitive behavior with 80% capacitance retention in 0.5 M H2SO4 at 1000 mV s−1. In comparison with a commercial electrocatalyst, 20% Pt on Vulcan XC-72, carbon nanosheets display a positive shift in the onset potential and superior electrocatalytic activity toward the oxygen reduction reaction (ORR). The above excellent electrochemical performances render the N-doped carbon nanosheets a promising material for electrochemical energy storage/conversion systems. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Recently, several efforts have been devoted to change the physicochemical properties of carbonaceous materials by introducing heteroatoms, especially the nitrogen atom, to the graphitic or graphite-like structures. For example, nitrogen-doped carbon materials have been found to enhance the performances of several electrochemical energy storage or conversion systems, such as electric double-layer capacitors (EDLCs) [1–6] and fuel cells [7–12]. The N-doped structures are believed to provide pesudocapacitance contributed from the redox faradaic reactions of these electrochemically active functional groups. Moreover, certain N-doped functional groups can enhance the electronic conductivity of carbon materials for EDLCs [6,13,14]. As a result, N-doped carbon materials are considered to be promising EDLC electrode materials that are used practically as power sources/energy storage devices to supply/deliver pulsed current. More recently, these unique N-containing functional groups have been found to be the electrocatalytic sites for the ORR in fuel cells [7,15–17]. Accordingly, several studies showed the potential of N-doped carbonaceous materials to replace the commonly used yet much more expensive platinum as the electrocatalyst for the ORR. N-doped carbonaceous materials have been prepared through the post-treatment, e.g., ammoxidation, of carbon samples [2,3] or the employment of an N-containing precursor. The latter procedure is preferred because it is easier, more cost-effective, and more controllable
⁎ Corresponding author. Tel./fax: +886 3 573 6027. E-mail address: [email protected] (C.-C. Hu). 1388-2481/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2010.11.010
for the N-doping content [1,4–6,18]. Among many N-containing precursors, biomaterials, such as proteins, have been reported as suitable precursors for synthesizing N-doped carbonaceous materials because of their amino group-bountiful nature [19–21]. Accordingly, collagen [22,23], the most abundant protein in mammals, was utilized as the precursor to prepare N-doped carbon materials in this work. Material analyses confirmed the existence of N-doped structures and characterized the collagen-derived material as carbon nanosheets. The N-doped carbon nanosheets showed a near ideal EDLC behavior with high power performances. Furthermore, they were able to catalyze the ORR with a higher catalytic activity and more positive onset potential in comparison with the commercial electrocatalyst, 20% Pt on Vulcan XC-72. Therefore, the excellent capacitive performance and activity toward the ORR established the potential of collagen-derived N-doped carbon nanosheets for the electrochemical applications. 2. Experimental A collagen solution (5 mg ml−1) was obtained by dissolving collagen powder (Sigma-Aldrich, USA) in an acetic acid solution (0.02 M). With the addition of phosphate buffered saline (PBS, Sigma-Aldrich, USA) and sodium hydroxide, collagen gel was formed after 24-h incubation at 37 °C. After being cross-linked with a 4% paraformaldehyde solution for 15 min, the soaked gel was heated in vacuum at a rate of 4 °C min−1 to 800 °C and kept at this temperature for 6 h. XPS measurement was performed on the Kratos Axis Ultra DLD (Kratos analytical, USA) which employed Al monochromator (hν = 1486.69 eV) irradiation as the photosource. The scanning electron microscopic (SEM) and transmission electron microscopic (TEM)
Y.-H. Lee et al. / Electrochemistry Communications 13 (2011) 50–53
51
images of carbon nanosheets were obtained by Hitachi S-4700I (Hitachi, Japan) and JEM-2010 (JEOL, Japan), respectively. The electrochemical performances of the collagen-derived carbon material were investigated by CHI 633A (CH Instruments, USA) in a three-compartment cell. An Ag/AgCl electrode (Argenthal, 3 M KCl, 0.207 V vs. standard hydrogen electrode at 25 °C) was used as the reference electrode, and platinum wire was employed as the counter electrode. A Luggin capillary was used to minimize errors due to ohmic potential drop in the electrolyte. The potentials for the ORR are referred to a reversible hydrogen electrode (RHE). The material for the working electrode was a mixture consisting of the collagen-derived carbon material and polyvinylidene in the weight ratio of 10:1. The mixture (ca. 1 mg) was homogeneously suspended in N-methyl-2-pyrrolidone and coated on a graphite substrate with an exposed surface area of 1 cm2. The electrolyte for evaluating capacitive performances is a 0.5 M sulfuric acid solution, whereas it is a 0.1 M potassium hydroxide solution for investigating the ORR activity. A commercial electrocatalyst, 20% Pt on Vulcan XC-72 (E-TEK, USA), was also applied (also ca. 1 mg) for the comparison of oxygen reduction activity. 3. Results and discussion XPS analysis was performed to evaluate the nitrogen content of the collagen-derived carbon material. Owing to the presence of amino groups in collagen, the atomic ratio of nitrogen for the carbonized material is about 1%. Fig. 1 reveals the existence of pyridinic-N (398.5 eV, 21.4 at.%), pyrrolic-N (400 eV, 28.1 at%), quaternary-N (401.2 eV, 42.8 at.%), and pyridine-N-oxide (403 eV, 7.7 at.%) [1,5,6]. This result confirmed that the collagen-derived carbon material contains N-doped structures. The surface morphology of the N-doped carbonaceous material was examined by the SEM image, as shown in Fig. 2A. The loose and layered structure characterized the carbonaceous material as carbon nanosheets. The sheet-like morphology was confirmed in the TEM image (Fig. 2B). The above results reveal the formation of a highly porous N-doped carbonaceous material. As shown in Fig. 2C, the porosity of N-doped carbon nanosheets was investigated by the analysis of N2 adsorption/desorption isotherms. In addition to micropores, the hysteresis loop between adsorption and desorption isotherms demonstrates the existence of slit-like mesopores, which agrees well with the morphology. The presence of both micropores and mesopores contributes to the high specific surface area (695 m2 g−1) of the carbon nanosheets. To unravel the capacitive performance of the N-doped carbon nanosheets, cyclic voltammograms at different scan rates were carried
Intensity (a.u.)
Fig. 2. (A) SEM and (B) TEM images, and (C) N2 adsorption/desorption isotherms of the N-doped carbon nanosheets.
408
404
400
396
392
Binding energy (eV) Fig. 1. N 1s core-level XPS spectra of the collagen-derived carbon material. The black line is the raw spectrum; the pink, blue, red, and green lines correspond to pyridine-N, pyrrolic-N, quaternary-N, and pyridine-N-oxide, respectively.
out, as shown in Fig. 3A. With its high specific surface area, the N-doped carbon nanosheets possess a nearly ideal double-layer behavior with a specific capacitance of 102 F g−1 at 25 mV s−1, which is comparable to that of activated carbons. In addition, perfect reversibility and stability are observed with varying the scan rate. Only 20% loss in capacitance is found when changing the scan rate from 25 to 1000 mV s−1 (Fig. 3B), indicating that the N-doped carbon nanosheets are a high power material. Previous studies [6,13,14] proposed that with the modified electron donor/acceptor properties, pyridinic-N and pyrrolic-N in carbon materials could induce Faradaic pseudocapacitance, whereas quaternary-N and pyridine-N-oxide could improve the ability of electron transfer in carbon materials. Since the redox peaks in the CV diagrams are not obvious, the contribution of pesudocapacitance from
52
Y.-H. Lee et al. / Electrochemistry Communications 13 (2011) 50–53
A
0.1
0.05
0
-0.05 (1) (2) (3) (4) (5)
-0.1
Current density (mA/cm2)
Current density (A/cm2)
A
2
0
-2
-4 (1) (2)
-6
-8
-0.15
0
0.2
0.4
0.6
0.8
1
-0.2
0
0.2
Potential (V) vs. Ag/AgCl
B
100
Current density (mA/cm2)
Capacitance retention (%)
B
80
60
40
20
0
0
200
400
600
800
0.4
0.6
0.8
1
1.2
Potential (V) vs. R.H.E 0
-1
-2
-4
1000
(1) (2)
-3
-0.2
Scan rate (mV/s) Fig. 3. (A) Cyclic voltammograms of the N-doped carbon nanosheets measured at (1) 100, (2) 250, (3) 500, (4) 750, and (5) 1000 mV s−1 in 0.5 M H2SO4. (B) The dependence of the capacitance retention on the scan rate of CV.
pyridinic-N and pyrrolic-N is not significant. However, the presence of quaternary-N and pyridine-N-oxide which improve electron transfer within the N-doped carbon nanosheets reasonably explains the high capacitance retention appearing at the high scan rate. On the other hand, the high-power performance is also attributed to the porous structure of the nanosheets since the interspaces between nanosheets provide facile accessibility for the electrolyte. In addition to the investigation of N-doped carbon nanosheets as the electrode of EDLCs, the catalytic activity toward the ORR was also measured (Fig. 4A). The significant larger current densities obtained in an air-saturated electrolyte in comparison with the N2-saturated electrolyte confirm that N-doped carbon nanosheets do exhibit the ability to catalyze the reduction of oxygen. In Fig. 4B, the catalytic activity of N-doped carbon nanosheets for the ORR is superior to the commercial electrocatalyst, 20% Pt on Vulcan XC-72, especially in the potential range from 0.2 V to 0.6 V. Furthermore, the onset potential of the ORR for the N-doped carbon nanosheets is 0.95 V; it is even more positive than that for the commercial electrocatalyst Pt, which is 0.83 V. The significant catalytic activity toward the ORR of the N-doped carbon nanosheets mainly comes from the contribution of pyridinic-N and quaternary-N. Previous studies showed that the presence of pyridinic-N and quaternary-N could change the adsorption of the oxygen molecule on carbon materials from end-on to side-on type, thus weakening the bonding of O–O to facilitate the ORR [7,15]. The unique catalytic activity toward the ORR of the N-doped carbon nanosheets is not attributable to the oxygen-/sulfur-containing functional groups. The O-containing functional groups could positively shift the onset potential of the ORR to 0.4 V (vs. RHE) [24], but much more negative than that of the N-doped carbon nanosheets. From the XPS analysis (not shown here), there are no S-containing functional groups
0
0.2
0.4
0.6
0.8
1
Potential (V) vs. R.H.E Fig. 4. (A) Linear sweep voltammograms of (1) 20% Pt on Vulcan XC-72 and (2) the N-doped carbon nanosheets measured at 10 mV s−1 in 0.1 M KOH (dotted line: N2-saturated electrolyte, solid line: air-saturated electrolyte); (B) the i–E curves deduced from the current density difference of the LSVs measured in the air-saturated and N2-saturated electrolytes for (1) 20% Pt on Vulcan XC-72 and (2) the N-doped carbon nanosheets.
on the N-doped carbon nanosheets because collagen mainly consists of glycine, proline, and hydroproline, which contain no sulfur [22]. 4. Conclusions N-doped carbon nanosheets were obtained simply by carbonization of the collagen gel. The preliminary electrochemical analyses show that the N-doped carbon nanosheets not only display nearly ideal doublelayer responses at high CV scan rates but also possess the superior electrocatalytic activity toward the ORR than a commercial Pt-based electrocatalyst. The contributions of high specific surface area, highly porous architecture, and N-doped structures are responsible for the ideal capacitive behavior; meanwhile, the N-doped structures also act as the active sites of the ORR. The collagen-derived N-doped carbon nanosheets have shown the promising benefits in the applications of EDLCs and fuel cells. Acknowledgement The financial supports of this work, the National Science Council of the ROC Taiwan under contract no. NSC 98-3114-E-007-011 and the boost program of National Tsing Hua University, are gratefully acknowledged. References [1] D. Hulicova-Jurcakova, M. Kodama, S. Shiraishi, H. Hatori, Z.H. Zhu, G.Q. Lu, Adv. Func. Mater. 19 (2009) 1800. [2] K. Jurewicz, Electrochim. Acta 48 (2003) 1491. [3] K. Kang, S. Hong, B. Lee, J. Lee, Electrochem. Commun. 10 (2008) 1105.
Y.-H. Lee et al. / Electrochemistry Communications 13 (2011) 50–53 [4] W. Li, D. Chen, Z. Li, Y. Shi, Y. Wan, J. Huang, J. Yang, D. Zhao, Z. Jiang, Electrochem. Commun. 9 (2007) 569. [5] T. Rufford, D. Hulicovajurcakova, Z. Zhu, G. Lu, Electrochem. Commun. 10 (2008) 1594. [6] M. Seredych, D. Hulicovajurcakova, G. Lu, T. Bandosz, Carbon 46 (2008) 1475. [7] K. Gong, F. Du, Z. Xia, M. Durstock, L. Dai, Science 323 (2009) 760. [8] K.R. Lee, K.U. Lee, J.W. Lee, B.T. Ahn, S.I. Woo, Electrochem. Commun. 12 (2010) 1052. [9] G. Liu, X. Li, P. Ganesan, B.N. Popov, Appl. Catal. B: Environmental 93 (2009) 156. [10] R. Liu, D. Wu, X. Feng, K. Müllen, Angewandte Chemie International Edition 49 (2010) 2565. [11] L. Qu, Y. Liu, J.-B. Baek, L. Dai, ACS Nano 4 (2010) 1321. [12] Y. Shao, S. Zhang, M.H. Engelhard, G. Li, G. Shao, Y. Wang, J. Liu, I.A. Aksay, Y. Lin, J. Mater. Chem. 20 (2010) 7491. [13] E. Frackowiak, Phys. Chem. Chem. Phys. 9 (2007) 1774.
53
[14] D. Hulicova-Jurcakova, M. Seredych, G.Q. Lu, T.J. Bandosz, Adv. Func. Mater. 19 (2009) 438. [15] Z. Shi, J. Zhang, Z. Liu, H. Wang, D. Wilkinson, Electrochim. Acta 51 (2006) 1905. [16] J. Ozaki, T. Anahara, N. Kimura, A. Oya, Carbon 44 (2006) 3358. [17] J. Ozaki, N. Kimura, T. Anahara, A. Oya, Carbon 45 (2007) 1847. [18] R.J. White, M. Antonietti, M.-M. Titirici, J. Mater. Chem. 19 (2009) 8645. [19] T. Iwazaki, R. Obinata, W. Sugimoto, Y. Takasu, Electrochem. Commun. 11 (2009) 376. [20] T. Iwazaki, H. Yang, R. Obinata, W. Sugimoto, Y. Takasu, J. Power Sources 195 (2010) 5840. [21] Y. Kim, Y. Abe, T. Yanagiura, K. Park, M. Shimizu, T. Iwazaki, S. Nakagawa, M. Endo, M. Dresselhaus, Carbon 45 (2007) 2116. [22] A.D. Covington, Chemical Society Reviews 26 (1997) 111. [23] K.E. Kadler, D.F. Holmes, J.A. Trotter, J.A. Chapman, Biochem. J. 316 (1996) 1. [24] K. Matsubara, K. Waki, ECS Transactions 28 (2010) 35.