- Email: [email protected]
gelcast-derived nano-carbon network composite in alkaline solution
Electrochemistry Communications 10 (2008) 922–925
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Electrochemical reduction of oxygen on alumina/gelcast-derived nano-carbon network composite in alkaline solution Jingjun Liu, Hideo Watanabe, Masayoshi Fuji *, Minoru Takahashi Ceramics Research Laboratory, Nagoya Institute of Technology, 10-6-29, Asahigaoka, Tajimi, Gifu 507-0071, Japan
a r t i c l e
i n f o
Article history: Received 12 February 2008 Received in revised form 2 April 2008 Accepted 3 April 2008 Available online 12 April 2008 Keywords: Nano-composite electrode Alkaline solution Electrocatalysis Oxygen reduction Cyclic voltammetry
a b s t r a c t A novel alumina/nano-carbon networks (NCN) composite electrode material was fabricated by combination of gelcasting and reduction–sintering method. The electrocatalytic reduction of dissolved oxygen on this alumina/NCN electrode in 35 wt.% potassium hydroxide solution was investigated using electrochemical methods. The results showed that the electrocatalytic activity of the new electrode was higher than that of a commercial graphite. This higher electrocatalytic activity can be attributed to the presence of pyrolyzed nano-carbon networks in composite matrix. These networks provided very high porosity and electrochemical surface area to alumina ceramic matrix, which are helpful in decreasing the polarization of electrodes by improving the discharge capacity at high rates required by oxygen electrode when large current flows through it. The oxygen reduction reaction mechanism on the nano-composite electrode was discussed. The electrode displayed heterogeneous catalytic activity toward decomposition of intermediate HO 2 due to the presence of alumina particles mixed with pyrolyzed nano-carbon in the composite matrix. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Electrochemical reduction of oxygen is one of the most important electrocatalytic reactions that are widely used in many applications such as fuel cell, metal–air battery, organic electrosynthesis, and electrochemical wastewater treatment [1]. Electrode materials for oxygen reduction have been investigated intensively since the middle of the last the century worldwide [2]. In the past six decades, a variety of new electrode materials with some electrocatalytic activities toward oxygen reduction has been developed, except for the conventional noble metal electrodes. Those electrodes include non-noble metal-based materials (such as transition metal alloy and oxides), and carbon-based materials (such as Fe–C–N systems and nano-carbon composites) [3,4], in which carbon-based electrodes have received an attracting attention in recent years for their preparations and applications, owing to their unique catalytic capability along with remarkable mechanical and chemical inertness, which were found to bear a close analogy with that of platinum electrodes. Jürmann and Tammeveski [5] reported, in their studies of the oxygen reduction behavior of the fabricated multi-walled carbon nanotubes (MWCNTs) modified highly oriented pyrolytic graphite (HOPG) electrodes in KOH solution, that this MWCNTs/HOPG electrodes show a remarkable electrocatalytic activity towards O2 reduction in alkaline media. Collins et al. [6] confirmed that carbon
* Corresponding author. E-mail address: [email protected] (M. Fuji). 1388-2481/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2008.04.008
nanotubes (CNTs) electrodes, owing to their excellent electrical conductivity and superior mechanical strength, had been suitable for oxygen reduction electrodes. Britto et al. [7] discovered that the measured exchange current density on multi-walled carbon nanotubes (MWNTs) electrode was seven times higher than that of graphite electrode. Therefore, it can be foreseen that nano-carbon could be used as additives to improve electrocatalytic activity and electrical properties of nano-composite materials. In this paper, we present a way to fabricate the novel alumina/ NCN nano-composite by a combination of gelcasting and reduction–sintering. The electrochemical reduction behavior of oxygen on the electrode and the electrocatalytic activity toward oxygen reduction in alkaline solution were investigated.
2. Experimental procedure 2.1. Fabrication of alumina/nano-carbon network composite electrodes The composite gelcast electrode material was prepared based on the following three processes: Firstly, alumina powder (AL 160SG-4, Showa Denko, Japan) with a mean particle size (D50) of 0.6 lm in the amount of 5 vol.% was dispersed in distilled water with the addition of dispersing agent. The dispersant used was ammonium polycarboxylate acid (Chukyo, Yushi, Japan), which was added to about 0.36 mass% based on the mass of powder. A gelcasting premix of 20 mass% solution in distilled water was
J. Liu et al. / Electrochemistry Communications 10 (2008) 922–925
prepared consisting of a monomer (MAM, Methacrylamide) and a crosslinking agent (MBAM, N,N’-methylenebisacrylamide) added in mass ratio of 20:1 (MAM:MBAM). This was followed by ballmilling inside a polyethylene container with small zirconia balls for about 48 h to achieve stable slurry. Secondly, the stabilized slurry was screened to remove the milling balls and degassed. The degassed slurry was treated with initiator (APS, ammonium persulfate, 1.03 lL/g) and catalyst (TEMED, N,N’,N’-tetramethyleneethylene-diamine, 0.17 lL/g) to initiate free radical polymerization of the monomer. The final slurry was poured into polyethylene tile mould, which was subsequently allowed to gel for three hours to form testing bars and then dried. Thirdly, the well-dried body was loaded into a gas-tight furnace (Himulti 5000, Fujidenpa Kogyo Company, Japan) for reduction sintering in argon atmosphere at 1700 °C following a procedures [8,9]. The resultant sintered ceramic body consist of alumina particles (99.67 mass%) and pyrolyzed carbon (0.33 mass%) measured by thermogravimetry, and has possessed a satisfactory mechanical strength of about 210 MPa and high electrical conductivity of about 0.37 X cm. This observed high electrical conductivity is attributed to the presence of interconnected nano-carbon networks at interstices and voids in alumina matrix, which was derived from the pyrolysis of gelcast binder made during the polymerization period. The measured specific surface area for the alumina/NCN electrode is 1.5975 m2/g, and it is 484.1 m2/g for the nano-carbon material per unit weight of carbon. Subsequently, the sintered gelcast sample was examined by X-ray diffraction (RINT, Rigaku, Japan) to identify the phase composition involved. 2.2. Electrochemical measurements for the fabricated composite electrodes Electrochemical measurements were performed to study the electrocatalytic reduction of dissolved oxygen for the alumina/ NCN electrodes in 35 wt.% potassium hydroxide by using the electrochemical methods. A HZ-5000 Potentiostat/Galvanostat (HOKUTO DENKO Co., Japan) with a frequency response analyzer (NF, Co., FRA5022) and a conventional three-electrode test cell was used. The working electrode (WE) was the alumina/NCN materials and a platinum sheet was used as the counter electrode (CE), and a saturated calomel electrode (SCE) as the reference. To prepare the test samples, surface polishing was carried out firstly, washed and then cleaned by ultrasonication in ethanol bath. The size of the working area exposed to testing solutions is about 1 cm2. All tests were performed at ambient temperature and were repeated at least three times to get reproducible results. Furthermore, the electrochemical behaviors of the electrode were compared to commercially available graphite electrode (KGR-3, Akechi Ceramics Co. Ltd. Japan).
3. Results and discussion 3.1. Characterization of alumina/NCN composite electrode In XRD patterns of the fabricated alumina/NCN electrode as shown in Fig. 1, typical alumina and graphite peaks with no other miscellaneous peaks have been readily observed. This illustrates that the composition of the electrode material is very pure, except for the presence of alumina particles and pyrolyzed carbon which is derived from the pyrolysis of polymer gel binder occurred in reduction sintering. In addition, it can be noted that a sharp diffraction peak at about 26 ° is attributed to the high graphitization degree of pyrolyzed carbon in the electrode during reduction– sintering process at 1700 °C. On the other hand, some diffraction peaks around 26 °, 42 °, 54 ° in the XRD patterns were distinguish-
923
Fig. 1. XRD pattern of the highly conductive alumina/NCN composite electrode sintered at 1700 °C in argon atmosphere (C: carbon, A: alumina).
able signals from hexagonal structure of graphite (0 0 2), (1 0 0), and (0 0 4), respectively [10]. These diffraction data point out that the pyrolyzed carbon phase in the composite exists in crystalline form rather than in amorphous one. Moreover, there is a broader diffraction peak at 15 ° indicating that the pyrolyzed carbon exists in nano-size structure in the reduction–sintered composite electrode. Considering the packing effect of alumina particles as discussed in the literatures [11], there are a lot of piled pores in nano-scale existing in the matrix in which the residual carbon can occupy. TEM results confirmed that the nano-carbon mainly existed at interstices and voids formed by packing of alumina particles in the matrix [8]. The nano-carbon phase was composed of carbon nano-wires existed in a diameter of about 12 nm and interconnected with the carbon coating on alumina grains, which has a uniform thickness of about 22.5 nm. This structure is contributed to establishing the high electrochemical area for the electrode. 3.2. Measurement of electrochemical surface area The electrochemical surface area was determined for graphite and alumina/NCN electrodes by a potential step method. During the measurement period, the initial potential of the test electrode was its open-circuit potential and held for at least 5 min for stabilization, and then potential steps immediately up to 10 mV and maintains for 250 ms. The resulting chronoampermetric curves obtained are illustrated in Fig. 2. The calculated double layer capacitances for graphite and alumina/NCN electrodes are 1742 lF and 18,200 lF, respectively, according to the equations available in the literature [12]. Pure mercury can be used as a reference to evaluate the real surface area for other materials since its surface is the most smoothly for all the materials, and its double layer capacitance is 20 lF cm2. Based on this reference value, the measured electrochemical surface areas for graphite and alumina/NCN electrodes are 87.1 cm2 and 910 cm2, respectively. The alumina/NCN electrode had much higher porosity and electrochemical surface area than graphite. Some authors [13] reported that the electrochemical surface area of their MWNT electrode fabricated by using poly-tertrafluoroethylene as binder was 497.5 cm2. Compared to this MWNT electrode with a loading amount (>85 wt.%), the nano-composite electrode maybe have higher electrochemical surface area due to the presence of pyrolyzed nano-carbon networks. The electrode with high electrochemical surface area is desirable for decreasing the polarization of electrode in practical applica-
924
J. Liu et al. / Electrochemistry Communications 10 (2008) 922–925
Fig. 2. Chronoampermetric curves for different electrodes: (a) graphite; and (b) the alumina/NCN electrode, in 35 wt.% KOH solution.
tions and improving discharge capacity of oxygen electrode when large current flows through it. The outcomes obtained from the polarization curves for alumina/NCN and graphite electrodes in 35 wt.% KOH confirmed that the polarization resistance of the alumina/NCN electrode is much lower than that obtained on graphite, even though at higher current density. 3.3. Electrocatalytic reduction of oxygen on electrodes The electrocatalytic ability for dissolved oxygen reduction on alumina/NCN electrodes was investigated by using cyclic voltammetry measurements, as shown in Fig. 3. Two characteristic cathodic reduction peaks can be observed at about 420 mV and 810 mV (vs. SCE), similar to that of graphite. However, the higher peak currents measured on the electrode indicate that it has, in some extent, electrocatalytic activity for oxygen reduction reaction. Some authors [5,15] reported that some surface carbon–oxygen functional groups (most probably quinone-type species) on electrodes surface produced by surface oxidation of carbon-based electrodes like MWCNTs and glassy carbon electrodes would in-
Fig. 4. Electrochemical impedance spectroscopy for alumina/NCN composite at different applied potentials: (a) 800 mV; and (b) 420 mV.
crease their electrocatalytic activity for oxygen reduction to hydrogen peroxide in alkaline media. The quinone-modified electrodes showed a high electrocatalytic activity for the two-electron reduction of oxygen as reported by Schiffrin [16,17]. In addition, as reported in the literatures [18], the first current peak in the cathode direction, as observed in cyclic voltammetric curves, is attributed to the reduction reaction of O2 into HO 2 with two-electron transfer. The anodic peak at ca. 0.2 V on the CV curve of alumina/NCN electrode is probably attributed to the hydrogen peroxide oxidation. And this similar peak also appears on the CV curve of the graphite electrode, although the peak is not so obvious in Fig. 3a. The second current peak in the same direction is believed to emerge from further reduction reaction of intermediate HO 2 into OH. However, the second redox reaction of HO 2 () HO is more difficult to take place than the first one, O2 () HO2 , as confirmed by the charge transfer resistances determined in the impedance spectroscopy measurements for the electrode, as depicted in Fig. 4. A larger arc shown in Fig. 4a reveals a slow reaction rate of the second redox reaction due to the higher charge transfer resistance than that of the first redox reaction (Fig. 4b). An electrostatic repulsion between intermediate HO 2 and the negatively charged carbon surface exist at the electrode surface. This should result in a low coverage of peroxide [19], which might affect the second redox reaction rates. This is corroborated by a positive shift of the second reduction peak potential that is readily noticeable in Fig. 3, and the measured peak current is much higher than that measured on graphite elec trode, which involved the reduction reaction of HO 2 ! OH . This behavior reveals that the alumina/NCN electrode might also have, in some extent, heterogeneous catalytic activity toward decomposition of intermediate HO 2 due to existence of metal oxide of alumina particles mixed with pyrolyzed nano-carbon in the composite matrix. Some authors [2] confirmed, in the studies of O2 reduction kinetics on the ground graphite mixed with metal oxides, that the HO 2 can be decomposed by the metal oxides catalytically and the mixture of graphite and metal oxides such as Co3O4, MnO2 etc., has a good electrocatalytic performance for oxygen reduction. 4. Conclusions
Fig. 3. Cyclic voltammetric curves of different electrodes in aerated 35 wt.% KOH solution at scan rate of 2 mV/s (a) graphite; and (b) alumina/NCN composite electrode.
The fabricated novel carbon-based nano-composite electrode has very high electrochemical surface area due to the presence of
J. Liu et al. / Electrochemistry Communications 10 (2008) 922–925
nano-carbon networks existing at interstices and voids in alumina ceramic matrix. It was found that the electrode possesses higher electrocatalytic activity for reduction reaction of dissolved oxygen in alkaline solution than a commercial graphite electrode. In addition, the composite electrode might have, in some extent, heterogeneous catalytic activity toward decomposition of intermediate HO 2. References [1] K. Kinoshita, Electrochemical Oxygen Technology, Wiley, New York, 1992. [2] Z. Wu, Y. Zhou, R. Gao, Acta Chim. Sinica 48 (1990) 988. [3] Arman Bonakdarpour, Kerry Lake, Krystal Stevens, J.R. Dahn, J. Electrochem. Soc. 155 (2008) B108. [4] P. Zelenay, J. Choi, C. Johnston, D. Cao, P. Babu, A. Wieckowski, N. Alonso-Vante, ECS Trans. 3 (2006) 171. [5] G. Jürmann, K. Tammeveski, J. Electroanal. Chem. 597 (2006) 119. [6] P.G. Collins, P. Avouris, Sci. Am. 283 (2000) 62.
925
[7] P.J. Britto, K.S.V. Santhanam, A. Rubio, J.A. Alonso, P.M. Ajayan, Adv. Mater. 11 (1999) 154. [8] R.L. Menchavez, Masayoshi Fuji, Minoru Takahashi, Adv. Mater. (2008). Accepted. [9] K. Niihara, B.S. Kim, T. Nakayama, T. Kusunose, T. Nomoto, A. Hikasa, T. Sekino, J. Eur. Ceram. Soc. 24 (2004) 3419. [10] A.L. Ocampo, M. Miranda-Hernandez, J. Morgado, J.A. Montoya, P.J. Sebastian, J. Power Sources 160 (2006) 915. [11] G. yi, L. yi, Petrochem. Technol. Appl. 21 (2003) 55. [12] Y. H Liu, Techniques for Electrochemical Measurements, Beijing Aviation College Press, Beijing, 1987. [13] Y. Chu, C. Ma, Y. Zhu, Acta Phys. Chim. Sinica 20 (2004) 331. [15] M. Kullapere, G. Jürmann, T.T. Tenno, J.J. Paprotny, F. Mirkhalaf, K. Tammeveski, J. Electroanal. Chem. 599 (2007) 183. [16] K. Vaik, D.J. Schiffrin, K. Tammeveski, Electrochem. Commun. 6 (2004) 1. [17] K. Vaik, A. Sarapuu, K. Tammeveski, F. Mirkhalaf, D.J. Schiffrin, J. Electroanal. Chem. 564 (2004) 159. [18] E. Yeager, J. Mol. Catal. 38 (1986) 5. [19] M. Kullapere, K. Tammeveski, Electrochem. Commun. 9 (2007) 1196–1201.
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Electrochemical reduction of oxygen on alumina/gelcast-derived nano-carbon network composite in alkaline solution Jingjun Liu, Hideo Watanabe, Masayoshi Fuji *, Minoru Takahashi Ceramics Research Laboratory, Nagoya Institute of Technology, 10-6-29, Asahigaoka, Tajimi, Gifu 507-0071, Japan
a r t i c l e
i n f o
Article history: Received 12 February 2008 Received in revised form 2 April 2008 Accepted 3 April 2008 Available online 12 April 2008 Keywords: Nano-composite electrode Alkaline solution Electrocatalysis Oxygen reduction Cyclic voltammetry
a b s t r a c t A novel alumina/nano-carbon networks (NCN) composite electrode material was fabricated by combination of gelcasting and reduction–sintering method. The electrocatalytic reduction of dissolved oxygen on this alumina/NCN electrode in 35 wt.% potassium hydroxide solution was investigated using electrochemical methods. The results showed that the electrocatalytic activity of the new electrode was higher than that of a commercial graphite. This higher electrocatalytic activity can be attributed to the presence of pyrolyzed nano-carbon networks in composite matrix. These networks provided very high porosity and electrochemical surface area to alumina ceramic matrix, which are helpful in decreasing the polarization of electrodes by improving the discharge capacity at high rates required by oxygen electrode when large current flows through it. The oxygen reduction reaction mechanism on the nano-composite electrode was discussed. The electrode displayed heterogeneous catalytic activity toward decomposition of intermediate HO 2 due to the presence of alumina particles mixed with pyrolyzed nano-carbon in the composite matrix. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Electrochemical reduction of oxygen is one of the most important electrocatalytic reactions that are widely used in many applications such as fuel cell, metal–air battery, organic electrosynthesis, and electrochemical wastewater treatment [1]. Electrode materials for oxygen reduction have been investigated intensively since the middle of the last the century worldwide [2]. In the past six decades, a variety of new electrode materials with some electrocatalytic activities toward oxygen reduction has been developed, except for the conventional noble metal electrodes. Those electrodes include non-noble metal-based materials (such as transition metal alloy and oxides), and carbon-based materials (such as Fe–C–N systems and nano-carbon composites) [3,4], in which carbon-based electrodes have received an attracting attention in recent years for their preparations and applications, owing to their unique catalytic capability along with remarkable mechanical and chemical inertness, which were found to bear a close analogy with that of platinum electrodes. Jürmann and Tammeveski [5] reported, in their studies of the oxygen reduction behavior of the fabricated multi-walled carbon nanotubes (MWCNTs) modified highly oriented pyrolytic graphite (HOPG) electrodes in KOH solution, that this MWCNTs/HOPG electrodes show a remarkable electrocatalytic activity towards O2 reduction in alkaline media. Collins et al. [6] confirmed that carbon
* Corresponding author. E-mail address: [email protected] (M. Fuji). 1388-2481/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2008.04.008
nanotubes (CNTs) electrodes, owing to their excellent electrical conductivity and superior mechanical strength, had been suitable for oxygen reduction electrodes. Britto et al. [7] discovered that the measured exchange current density on multi-walled carbon nanotubes (MWNTs) electrode was seven times higher than that of graphite electrode. Therefore, it can be foreseen that nano-carbon could be used as additives to improve electrocatalytic activity and electrical properties of nano-composite materials. In this paper, we present a way to fabricate the novel alumina/ NCN nano-composite by a combination of gelcasting and reduction–sintering. The electrochemical reduction behavior of oxygen on the electrode and the electrocatalytic activity toward oxygen reduction in alkaline solution were investigated.
2. Experimental procedure 2.1. Fabrication of alumina/nano-carbon network composite electrodes The composite gelcast electrode material was prepared based on the following three processes: Firstly, alumina powder (AL 160SG-4, Showa Denko, Japan) with a mean particle size (D50) of 0.6 lm in the amount of 5 vol.% was dispersed in distilled water with the addition of dispersing agent. The dispersant used was ammonium polycarboxylate acid (Chukyo, Yushi, Japan), which was added to about 0.36 mass% based on the mass of powder. A gelcasting premix of 20 mass% solution in distilled water was
J. Liu et al. / Electrochemistry Communications 10 (2008) 922–925
prepared consisting of a monomer (MAM, Methacrylamide) and a crosslinking agent (MBAM, N,N’-methylenebisacrylamide) added in mass ratio of 20:1 (MAM:MBAM). This was followed by ballmilling inside a polyethylene container with small zirconia balls for about 48 h to achieve stable slurry. Secondly, the stabilized slurry was screened to remove the milling balls and degassed. The degassed slurry was treated with initiator (APS, ammonium persulfate, 1.03 lL/g) and catalyst (TEMED, N,N’,N’-tetramethyleneethylene-diamine, 0.17 lL/g) to initiate free radical polymerization of the monomer. The final slurry was poured into polyethylene tile mould, which was subsequently allowed to gel for three hours to form testing bars and then dried. Thirdly, the well-dried body was loaded into a gas-tight furnace (Himulti 5000, Fujidenpa Kogyo Company, Japan) for reduction sintering in argon atmosphere at 1700 °C following a procedures [8,9]. The resultant sintered ceramic body consist of alumina particles (99.67 mass%) and pyrolyzed carbon (0.33 mass%) measured by thermogravimetry, and has possessed a satisfactory mechanical strength of about 210 MPa and high electrical conductivity of about 0.37 X cm. This observed high electrical conductivity is attributed to the presence of interconnected nano-carbon networks at interstices and voids in alumina matrix, which was derived from the pyrolysis of gelcast binder made during the polymerization period. The measured specific surface area for the alumina/NCN electrode is 1.5975 m2/g, and it is 484.1 m2/g for the nano-carbon material per unit weight of carbon. Subsequently, the sintered gelcast sample was examined by X-ray diffraction (RINT, Rigaku, Japan) to identify the phase composition involved. 2.2. Electrochemical measurements for the fabricated composite electrodes Electrochemical measurements were performed to study the electrocatalytic reduction of dissolved oxygen for the alumina/ NCN electrodes in 35 wt.% potassium hydroxide by using the electrochemical methods. A HZ-5000 Potentiostat/Galvanostat (HOKUTO DENKO Co., Japan) with a frequency response analyzer (NF, Co., FRA5022) and a conventional three-electrode test cell was used. The working electrode (WE) was the alumina/NCN materials and a platinum sheet was used as the counter electrode (CE), and a saturated calomel electrode (SCE) as the reference. To prepare the test samples, surface polishing was carried out firstly, washed and then cleaned by ultrasonication in ethanol bath. The size of the working area exposed to testing solutions is about 1 cm2. All tests were performed at ambient temperature and were repeated at least three times to get reproducible results. Furthermore, the electrochemical behaviors of the electrode were compared to commercially available graphite electrode (KGR-3, Akechi Ceramics Co. Ltd. Japan).
3. Results and discussion 3.1. Characterization of alumina/NCN composite electrode In XRD patterns of the fabricated alumina/NCN electrode as shown in Fig. 1, typical alumina and graphite peaks with no other miscellaneous peaks have been readily observed. This illustrates that the composition of the electrode material is very pure, except for the presence of alumina particles and pyrolyzed carbon which is derived from the pyrolysis of polymer gel binder occurred in reduction sintering. In addition, it can be noted that a sharp diffraction peak at about 26 ° is attributed to the high graphitization degree of pyrolyzed carbon in the electrode during reduction– sintering process at 1700 °C. On the other hand, some diffraction peaks around 26 °, 42 °, 54 ° in the XRD patterns were distinguish-
923
Fig. 1. XRD pattern of the highly conductive alumina/NCN composite electrode sintered at 1700 °C in argon atmosphere (C: carbon, A: alumina).
able signals from hexagonal structure of graphite (0 0 2), (1 0 0), and (0 0 4), respectively [10]. These diffraction data point out that the pyrolyzed carbon phase in the composite exists in crystalline form rather than in amorphous one. Moreover, there is a broader diffraction peak at 15 ° indicating that the pyrolyzed carbon exists in nano-size structure in the reduction–sintered composite electrode. Considering the packing effect of alumina particles as discussed in the literatures [11], there are a lot of piled pores in nano-scale existing in the matrix in which the residual carbon can occupy. TEM results confirmed that the nano-carbon mainly existed at interstices and voids formed by packing of alumina particles in the matrix [8]. The nano-carbon phase was composed of carbon nano-wires existed in a diameter of about 12 nm and interconnected with the carbon coating on alumina grains, which has a uniform thickness of about 22.5 nm. This structure is contributed to establishing the high electrochemical area for the electrode. 3.2. Measurement of electrochemical surface area The electrochemical surface area was determined for graphite and alumina/NCN electrodes by a potential step method. During the measurement period, the initial potential of the test electrode was its open-circuit potential and held for at least 5 min for stabilization, and then potential steps immediately up to 10 mV and maintains for 250 ms. The resulting chronoampermetric curves obtained are illustrated in Fig. 2. The calculated double layer capacitances for graphite and alumina/NCN electrodes are 1742 lF and 18,200 lF, respectively, according to the equations available in the literature [12]. Pure mercury can be used as a reference to evaluate the real surface area for other materials since its surface is the most smoothly for all the materials, and its double layer capacitance is 20 lF cm2. Based on this reference value, the measured electrochemical surface areas for graphite and alumina/NCN electrodes are 87.1 cm2 and 910 cm2, respectively. The alumina/NCN electrode had much higher porosity and electrochemical surface area than graphite. Some authors [13] reported that the electrochemical surface area of their MWNT electrode fabricated by using poly-tertrafluoroethylene as binder was 497.5 cm2. Compared to this MWNT electrode with a loading amount (>85 wt.%), the nano-composite electrode maybe have higher electrochemical surface area due to the presence of pyrolyzed nano-carbon networks. The electrode with high electrochemical surface area is desirable for decreasing the polarization of electrode in practical applica-
924
J. Liu et al. / Electrochemistry Communications 10 (2008) 922–925
Fig. 2. Chronoampermetric curves for different electrodes: (a) graphite; and (b) the alumina/NCN electrode, in 35 wt.% KOH solution.
tions and improving discharge capacity of oxygen electrode when large current flows through it. The outcomes obtained from the polarization curves for alumina/NCN and graphite electrodes in 35 wt.% KOH confirmed that the polarization resistance of the alumina/NCN electrode is much lower than that obtained on graphite, even though at higher current density. 3.3. Electrocatalytic reduction of oxygen on electrodes The electrocatalytic ability for dissolved oxygen reduction on alumina/NCN electrodes was investigated by using cyclic voltammetry measurements, as shown in Fig. 3. Two characteristic cathodic reduction peaks can be observed at about 420 mV and 810 mV (vs. SCE), similar to that of graphite. However, the higher peak currents measured on the electrode indicate that it has, in some extent, electrocatalytic activity for oxygen reduction reaction. Some authors [5,15] reported that some surface carbon–oxygen functional groups (most probably quinone-type species) on electrodes surface produced by surface oxidation of carbon-based electrodes like MWCNTs and glassy carbon electrodes would in-
Fig. 4. Electrochemical impedance spectroscopy for alumina/NCN composite at different applied potentials: (a) 800 mV; and (b) 420 mV.
crease their electrocatalytic activity for oxygen reduction to hydrogen peroxide in alkaline media. The quinone-modified electrodes showed a high electrocatalytic activity for the two-electron reduction of oxygen as reported by Schiffrin [16,17]. In addition, as reported in the literatures [18], the first current peak in the cathode direction, as observed in cyclic voltammetric curves, is attributed to the reduction reaction of O2 into HO 2 with two-electron transfer. The anodic peak at ca. 0.2 V on the CV curve of alumina/NCN electrode is probably attributed to the hydrogen peroxide oxidation. And this similar peak also appears on the CV curve of the graphite electrode, although the peak is not so obvious in Fig. 3a. The second current peak in the same direction is believed to emerge from further reduction reaction of intermediate HO 2 into OH. However, the second redox reaction of HO 2 () HO is more difficult to take place than the first one, O2 () HO2 , as confirmed by the charge transfer resistances determined in the impedance spectroscopy measurements for the electrode, as depicted in Fig. 4. A larger arc shown in Fig. 4a reveals a slow reaction rate of the second redox reaction due to the higher charge transfer resistance than that of the first redox reaction (Fig. 4b). An electrostatic repulsion between intermediate HO 2 and the negatively charged carbon surface exist at the electrode surface. This should result in a low coverage of peroxide [19], which might affect the second redox reaction rates. This is corroborated by a positive shift of the second reduction peak potential that is readily noticeable in Fig. 3, and the measured peak current is much higher than that measured on graphite elec trode, which involved the reduction reaction of HO 2 ! OH . This behavior reveals that the alumina/NCN electrode might also have, in some extent, heterogeneous catalytic activity toward decomposition of intermediate HO 2 due to existence of metal oxide of alumina particles mixed with pyrolyzed nano-carbon in the composite matrix. Some authors [2] confirmed, in the studies of O2 reduction kinetics on the ground graphite mixed with metal oxides, that the HO 2 can be decomposed by the metal oxides catalytically and the mixture of graphite and metal oxides such as Co3O4, MnO2 etc., has a good electrocatalytic performance for oxygen reduction. 4. Conclusions
Fig. 3. Cyclic voltammetric curves of different electrodes in aerated 35 wt.% KOH solution at scan rate of 2 mV/s (a) graphite; and (b) alumina/NCN composite electrode.
The fabricated novel carbon-based nano-composite electrode has very high electrochemical surface area due to the presence of
J. Liu et al. / Electrochemistry Communications 10 (2008) 922–925
nano-carbon networks existing at interstices and voids in alumina ceramic matrix. It was found that the electrode possesses higher electrocatalytic activity for reduction reaction of dissolved oxygen in alkaline solution than a commercial graphite electrode. In addition, the composite electrode might have, in some extent, heterogeneous catalytic activity toward decomposition of intermediate HO 2. References [1] K. Kinoshita, Electrochemical Oxygen Technology, Wiley, New York, 1992. [2] Z. Wu, Y. Zhou, R. Gao, Acta Chim. Sinica 48 (1990) 988. [3] Arman Bonakdarpour, Kerry Lake, Krystal Stevens, J.R. Dahn, J. Electrochem. Soc. 155 (2008) B108. [4] P. Zelenay, J. Choi, C. Johnston, D. Cao, P. Babu, A. Wieckowski, N. Alonso-Vante, ECS Trans. 3 (2006) 171. [5] G. Jürmann, K. Tammeveski, J. Electroanal. Chem. 597 (2006) 119. [6] P.G. Collins, P. Avouris, Sci. Am. 283 (2000) 62.
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