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Electrochemical characterization of core-shell carbon-encapsulated magnetic nanoparticles
Materials Letters 63 (2009) 1435–1438
Contents lists available at ScienceDirect
Materials Letters 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 / m a t l e t
Electrochemical characterization of core-shell carbon-encapsulated magnetic nanoparticles M. Bystrzejewski a,⁎, M. Arulepp b, J. Leis c, A. Huczko a, H. Lange a a b c
Department of Chemistry, Warsaw University, 1 Pasteur str., 02-093 Warsaw, Poland Tartu Technologies Ltd, 185 Riia Str., Tartu, 51014, Estonia University of Tartu, 2 Jakobi Str., Tartu, 51014, Estonia
a r t i c l e
i n f o
Article history: Received 20 February 2009 Accepted 18 March 2009 Available online 25 March 2009 Keywords: Nanocomposites Surfaces Electrical properties
a b s t r a c t In this paper we studied the electrochemical behaviour of core-shell carbon-encapsulated magnetic nanoparticles (CEMNPs). CEMNPs have core diameters between 15 and 35 nm and are comprised of Fe, Fe3C and NdC2 nanoparticles encapsulated in crystalline carbon cages. Direct current cyclic voltammetry (CV) studies showed that carbon-encapsulated magnetic nanoparticles are stable in electrolyte environments. The graphitic coating perfectly isolates the encapsulated particles from the electrolyte in a wide range of potentials. CEMNP-based electrodes have low resistance (0.43–1.44 Ω cm2) and posses a specific capacity of 10–40 F g− 1, which depends on the surface area and the crystallinity. It was shown, that CEMNPs are interesting multi-functional materials with a high potential to be used in various electrochemical devices. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The outstanding potential of core-shell nanoparticles stems from the ability of combining the properties of individual materials [1]. This approach is particularly apparent in core-shell magnetic nanoparticles, in which the magnetic nanocrystalline cores are wrapped by various coating materials. The role of the coating material is to isolate the core material against oxidation and agglomeration, and preserve its specific properties. By far, various coating materials have been proposed, e.g. silica, organic polymers, boron nitride, and carbon [2]. Carbon encapsulation seems to be the most desired approach, since such coatings possess high thermal resistivity [3] and have the ability for further chemical functionalization [4]. Core-shell nanostructured materials are very prospective materials in order to improve the performance of electrochemical devices. Depending on the structure, both the core and the coating in such materials may exhibit electrochemical activity [5,6]. Carbon-encapsulated magnetic nanoparticles have a great potential to apply them as novel electrode materials. Graphitic coatings should provide high conductivity, whilst the superparamagnetic properties of encapsulated particles allow to in situ generation of giant magnetic fields using simple external permanent magnets. The hypothetical CEMNP-based electrode with an on–off magnetic switching would affect e.g. the kinetics of electrochemical processes. However, some fundamental questions arise before the electrochemical applications CEMNPs: (i) how much the cores and the shells contribute to the electrochemical performance and (ii) whether the encapsulated metallic nanoparti⁎ Corresponding author. Tel.: +48 22 8220211; fax: +48 22 822 59 96. E-mail address: [email protected] (M. Bystrzejewski). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.03.033
cles can improve the intrinsic conductivity of the CEMNP-based electrodes. 2. Experimental Carbon-encapsulated magnetic nanoparticles were synthesized by thermal radio frequency plasma technique, which is described in detail elsewhere [7]. In brief, Fe–Nd–B magnetic precursor was introduced to the plasma flame simultaneously with methane or acetylene. The as-obtained products were purified to remove the nonencapsulated metals particles (24 h boiling in 3M HCl followed by rinsing with water and ethanol). The mass losses after purification (attributed to removal of non-encapsulated particles) were between 25 and 56 wt.%. The effect of purification on the CEMNPs content and their magnetic properties was recently studied [8]. Four products were obtained with different content of CEMNPs. The product characteristics involved morphology studies (SEM, TEM), specific surface area (N2 adsorption), Raman spectroscopy (647.1 nm excitation laser) and magnetic studies at room temperature (SQUID). Four tests were performed, in which the metal–carbon molar ratio was changed. It allowed synthesizing the products with different contents of CEMNPs (Table 1). Carbonaceous electrodes with a thickness of 350 ± 15 µm were prepared by pressing the composite consisting 90% of CEMNPs and 10% of PTFE binder. The electrochemical study of disc-shaped carbon electrodes (S = 2.27 cm2) was performed in 6 M KOH aqueous electrolyte solution by using a three-electrode set-up. The carbonaceous electrodes before experiments were vacuum-dried at 180 °C for 24 h. The standard glass cell with a large counter electrode and Hg/ HgO (6 M KOH) reference electrode was used. The electrochemical
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Table 1 CEMNPs sample characteristics and electrochemical parameters for CEMNP-based electrodes. CEMNP sample
Ms [emu g− 1]
I G/ ID
SBET [m2 g− 1]
[F/cm2]
[F/g]
[µF/cm2]
Phase angle [°]
RS [Ω cm2]
1 2 3 4
31.6 39.2 26.8 24.2
0.35 0.56 0.33 0.35
93 108 181 443
0.38 1.48 0.84 1.04
10.4 40.0 22.3 30.1
11.2 37.0 12.3 6.7
75 77 60 73
1.44 0.85 0.63 0.43
CSR
CS
response of CEMNPs electrodes was investigated by cyclic voltammetry (CV) at various scan rates from 2 to 50 mV/s. The constant current charge/discharge cycle was applied to calculate the specific capacitance (Cs) at a current density of 5 mA/cm2 in a potential range from −1.0 V to 0.0 V (vs. Hg/HgO). Electrochemical impedance spectroscopy (EIS) studies at the potential of 0.75 V were performed between 10 mHz and 10 kHz. 3. Results and discussion The synthesised products were extensively characterised elsewhere [7]. Here, we present the results of other investigations, which are directly related to the electrochemical studies. All samples (tests 1–4) have very similar morphology and no significant differences in the particles size and shape were found. Uniform spherical nanoparticles with the size below 100 nm dominate (Fig. 1A). The core-shell structure of CEMNPs is seen on TEM image (Fig. 1B). The metallic core is surrounded by graphitic layers with interlayer spacings of ~0.35– 0.37 nm. The encapsulated nanoparticles have ellipsoidal shapes and their size is in the range of 15–35 nm [7]. CEMNPs are comprised of 3 different metallic phases: bcc Fe, Fe3C and NdC2 [7]. CEMNPs content in the purified product can be derived from the values of saturation magnetization (Ms, Table 1). Obviously, higher Ms values correspond to higher content of CEMNPs. This is fully acceptable, because all samples have the same phase composition. The crystallinity of the carbon phase can be derived from the global intensity ratios of the G and D Raman bands (Table 1). CEMNPs 1, 3, and 4 have similar crystallinity, whilst sample 2 exhibits higher structural ordering. The electrochemical analysis of CEMNPs is a suitable and very sensitive method that can detect the quality of encapsulation, i.e. how well are the magnetic nanoparticles shielded from the electrolyte environment. Cyclic voltammetry studies were applied to investigate the electrochemical properties of CEMNPs. Fig. 2A shows CV curves (after 10 cycles), which are of rectangular shape, which is a characteristic for the typical capacitive behaviour. It should be noticed, that a peak at − 0.67 V appeared on the I–U curve for the electrode composed of CEMNPs 2 material. However, this faradaic reaction
monotonically decreases by each full scan (Fig. 2B). This can be attributed to the metallic impurities, i.e. non-encapsulated magnetic nanoparticles, which are removed from the electrode during subsequent cycles. These particles could be trapped in the pores between the CEMNPs and the applied purification procedure was insufficient to remove them. Small current peaks at +0.2 V (Fig. 2C) were observed only for the electrode composed of CEMNPs 3. They can be ascribed to the adsorbed aromatic hydrocarbons, which could be formed during the synthesis (plasma processing of simple hydrocarbons can also yield larger aromatic structures). Similar signals were observed for the single-wall carbon nanotubes, which were catalytically deposited from hydrocarbons [9]. CV results show that carbon coatings in CEMNPs perfectly isolate the encapsulated metallic nanoparticles from the electrolyte environment in a wide range of potentials. CEMNP electrodes are also stable for long cycling times. Specific capacitances (CS) and surface-related capacitances (CSR) evaluated from the CV curves are summarized in Table 1. Relatively low CS values results from low surface area, which is the crucial parameter for efficient charge storage. The higher density of CEMNPs (in comparison to pure carbon materials) also decreases their gravimetric capacitance. Nevertheless, the Cs values are a few times higher than those for carbon blacks and multi-wall carbon nanotubes [10]. The surface-related capacitance for the CEMNPs electrodes varies from 6.7 to 37.0 µF/cm2. The values for samples 1–3 are in a good agreement with the capacitance observed for carbon onions and disordered carbons [10]. Interestingly, the CEMNP electrode 2 has significantly higher surface-related capacitance. This is likely related to its higher crystallinity, because highly graphitized layers in carbon coatings may provide better surface-related capacitance. This has been demonstrated for graphite powders, which have 1.5–4 times higher CSR than carbon materials with lower graphitization degrees [11]. Importantly, the encapsulated particles do not influence the capacitance properties, which are mostly dominated by the carbon shells. The long term stability of CEMNP-based electrodes was further investigated in cycling durability tests. The current and voltage profiles show excellent reproducibility (Fig. 2D). The voltage responses are very sharp and do not have any IR drop. This indicates the low contact resistance of CEMNPs electrodes. The origin of resistance in CEMNP-based electrodes was further investigated by impedance spectroscopy. Nyquist plots acquired between 10 mHz and 1 kHz are shown in Fig. 3A. The slopes are between 60 and 77° (Table 1), which points to the non-ideal supercapacitor behaviour [12]. These deviations are likely linked to the surface roughness and porous structure of the electrodes [10]. In fact, the phase angle is the highest for sample 2, which has the lowest surface roughness. One also cannot exclude the interactions between paramagnetic ions from the electrolyte and the magnetic cores in
Fig. 1. SEM (A) and TEM (B) images of sample 1.
M. Bystrzejewski et al. / Materials Letters 63 (2009) 1435–1438
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Fig. 2. Cyclic voltammograms at v = 5 mV s− 1: A—10th cycle for CEMNP electrodes 1–4, B—CEMNP 2 electrode, cycle numbers noted on figure; C—CEMNP 3 and 4 electrodes, D—cycling durability profile for CEMNP electrode 4.
CEMNPs. The series of resistance (RS) decrease monotonically with increasing frequency (Fig. 3B). At lower frequencies, the electrolyte ions have time to penetrate into the depth of pores and additional surface is accessed [11]. The RS values (Table 1) calculated from the EIS
at 100 Hz show a clear dependence on the BET surface area, whilst not on the content of encapsulated nanoparticles. The estimated resistances are close to those for carbon onions (0.8–1.4 Ω cm2), carbon black (0.8 Ω cm2) and carbon nanotubes (0.7 Ω cm2) [11]. 4. Conclusions Carbon-encapsulated magnetic nanoparticles have interesting electrochemical performance. The carbon coating perfectly isolates the encapsulated nanoparticles from the electrolyte and no faradaic reactions occur in a wide range of potentials. CEMNPs have low resistivity and exhibit very high electrochemical stability. Capacitance properties of CEMNP-based electrodes are similar to other nanocarbon materials (carbon nanoonions and nanotubes). The encapsulated magnetic nanoparticles contribute to neither the capacity nor the conductivity in CEMNP-based electrodes. CEMNP-based electrodes can be easily and reversibly magnetized by external permanent magnets. This allows modulating various electrochemical processes by in situ generated magnetic fields. Acknowledgements This work was supported by the Ministry of Science and Higher Education through the Department of Chemistry, Warsaw University under Grant No. N204 096 31/2160. G. Soucy (University of Sherbrooke) is greatly acknowledged for his assistance in the synthesis process. References
Fig. 3. EIS results for CEMNP electrodes: Nyquist plot (A) and resistance series (B).
[1] [2] [3] [4]
Burns A, Ow H, Wiesner U. Chem Soc Rev 2006;11:1028–42. Bystrzejewski M, Rummeli MH. Pol J Chem 2007;81:1219–55. Bystrzejewski M, Cudzilo S, Huczko A, Lange H. J Alloys Compd 2006;423:74–6. Seo WS, Lee JH, Sun X, Suzuki Y, Mann D, Liu Z, et al. Nat Mater 2006;5:971–6.
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[5] Joncheray TJ, Audebert P, Schwartz E, Jovanovic AV, Ishaq O, Chavez JL, et al. Langmuir 2006;22:8684–9. [6] Wang S, Xu Q, Liu G. Electroanalysis 2008;20:1116–20. [7] Bystrzejewski M, Huczko A, Lange H, Baranowski P, Soucy G, et al. Nanotechnology 2007;18:145608. [8] Bystrzejewski M, Grabias A, Borysiuk J, Huczko A, Lange H. J Appl Phys 2008;104:54307. [9] M. Arulepp, unpublished data.
[10] Portet C, Yushin G, Gogotsi Y. Carbon 2007;45:2511–8. [11] Pandolfo AG, Hollenkamp AF. J Power Sources 2006;157:11–27. [12] B.E. Conway: in Electrochemical Supercapacitors, Scientific Fundamentals and Technological Applications, Kluwer Academic / Plenum Publications, New York (1999).
Contents lists available at ScienceDirect
Materials Letters 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 / m a t l e t
Electrochemical characterization of core-shell carbon-encapsulated magnetic nanoparticles M. Bystrzejewski a,⁎, M. Arulepp b, J. Leis c, A. Huczko a, H. Lange a a b c
Department of Chemistry, Warsaw University, 1 Pasteur str., 02-093 Warsaw, Poland Tartu Technologies Ltd, 185 Riia Str., Tartu, 51014, Estonia University of Tartu, 2 Jakobi Str., Tartu, 51014, Estonia
a r t i c l e
i n f o
Article history: Received 20 February 2009 Accepted 18 March 2009 Available online 25 March 2009 Keywords: Nanocomposites Surfaces Electrical properties
a b s t r a c t In this paper we studied the electrochemical behaviour of core-shell carbon-encapsulated magnetic nanoparticles (CEMNPs). CEMNPs have core diameters between 15 and 35 nm and are comprised of Fe, Fe3C and NdC2 nanoparticles encapsulated in crystalline carbon cages. Direct current cyclic voltammetry (CV) studies showed that carbon-encapsulated magnetic nanoparticles are stable in electrolyte environments. The graphitic coating perfectly isolates the encapsulated particles from the electrolyte in a wide range of potentials. CEMNP-based electrodes have low resistance (0.43–1.44 Ω cm2) and posses a specific capacity of 10–40 F g− 1, which depends on the surface area and the crystallinity. It was shown, that CEMNPs are interesting multi-functional materials with a high potential to be used in various electrochemical devices. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The outstanding potential of core-shell nanoparticles stems from the ability of combining the properties of individual materials [1]. This approach is particularly apparent in core-shell magnetic nanoparticles, in which the magnetic nanocrystalline cores are wrapped by various coating materials. The role of the coating material is to isolate the core material against oxidation and agglomeration, and preserve its specific properties. By far, various coating materials have been proposed, e.g. silica, organic polymers, boron nitride, and carbon [2]. Carbon encapsulation seems to be the most desired approach, since such coatings possess high thermal resistivity [3] and have the ability for further chemical functionalization [4]. Core-shell nanostructured materials are very prospective materials in order to improve the performance of electrochemical devices. Depending on the structure, both the core and the coating in such materials may exhibit electrochemical activity [5,6]. Carbon-encapsulated magnetic nanoparticles have a great potential to apply them as novel electrode materials. Graphitic coatings should provide high conductivity, whilst the superparamagnetic properties of encapsulated particles allow to in situ generation of giant magnetic fields using simple external permanent magnets. The hypothetical CEMNP-based electrode with an on–off magnetic switching would affect e.g. the kinetics of electrochemical processes. However, some fundamental questions arise before the electrochemical applications CEMNPs: (i) how much the cores and the shells contribute to the electrochemical performance and (ii) whether the encapsulated metallic nanoparti⁎ Corresponding author. Tel.: +48 22 8220211; fax: +48 22 822 59 96. E-mail address: [email protected] (M. Bystrzejewski). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.03.033
cles can improve the intrinsic conductivity of the CEMNP-based electrodes. 2. Experimental Carbon-encapsulated magnetic nanoparticles were synthesized by thermal radio frequency plasma technique, which is described in detail elsewhere [7]. In brief, Fe–Nd–B magnetic precursor was introduced to the plasma flame simultaneously with methane or acetylene. The as-obtained products were purified to remove the nonencapsulated metals particles (24 h boiling in 3M HCl followed by rinsing with water and ethanol). The mass losses after purification (attributed to removal of non-encapsulated particles) were between 25 and 56 wt.%. The effect of purification on the CEMNPs content and their magnetic properties was recently studied [8]. Four products were obtained with different content of CEMNPs. The product characteristics involved morphology studies (SEM, TEM), specific surface area (N2 adsorption), Raman spectroscopy (647.1 nm excitation laser) and magnetic studies at room temperature (SQUID). Four tests were performed, in which the metal–carbon molar ratio was changed. It allowed synthesizing the products with different contents of CEMNPs (Table 1). Carbonaceous electrodes with a thickness of 350 ± 15 µm were prepared by pressing the composite consisting 90% of CEMNPs and 10% of PTFE binder. The electrochemical study of disc-shaped carbon electrodes (S = 2.27 cm2) was performed in 6 M KOH aqueous electrolyte solution by using a three-electrode set-up. The carbonaceous electrodes before experiments were vacuum-dried at 180 °C for 24 h. The standard glass cell with a large counter electrode and Hg/ HgO (6 M KOH) reference electrode was used. The electrochemical
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Table 1 CEMNPs sample characteristics and electrochemical parameters for CEMNP-based electrodes. CEMNP sample
Ms [emu g− 1]
I G/ ID
SBET [m2 g− 1]
[F/cm2]
[F/g]
[µF/cm2]
Phase angle [°]
RS [Ω cm2]
1 2 3 4
31.6 39.2 26.8 24.2
0.35 0.56 0.33 0.35
93 108 181 443
0.38 1.48 0.84 1.04
10.4 40.0 22.3 30.1
11.2 37.0 12.3 6.7
75 77 60 73
1.44 0.85 0.63 0.43
CSR
CS
response of CEMNPs electrodes was investigated by cyclic voltammetry (CV) at various scan rates from 2 to 50 mV/s. The constant current charge/discharge cycle was applied to calculate the specific capacitance (Cs) at a current density of 5 mA/cm2 in a potential range from −1.0 V to 0.0 V (vs. Hg/HgO). Electrochemical impedance spectroscopy (EIS) studies at the potential of 0.75 V were performed between 10 mHz and 10 kHz. 3. Results and discussion The synthesised products were extensively characterised elsewhere [7]. Here, we present the results of other investigations, which are directly related to the electrochemical studies. All samples (tests 1–4) have very similar morphology and no significant differences in the particles size and shape were found. Uniform spherical nanoparticles with the size below 100 nm dominate (Fig. 1A). The core-shell structure of CEMNPs is seen on TEM image (Fig. 1B). The metallic core is surrounded by graphitic layers with interlayer spacings of ~0.35– 0.37 nm. The encapsulated nanoparticles have ellipsoidal shapes and their size is in the range of 15–35 nm [7]. CEMNPs are comprised of 3 different metallic phases: bcc Fe, Fe3C and NdC2 [7]. CEMNPs content in the purified product can be derived from the values of saturation magnetization (Ms, Table 1). Obviously, higher Ms values correspond to higher content of CEMNPs. This is fully acceptable, because all samples have the same phase composition. The crystallinity of the carbon phase can be derived from the global intensity ratios of the G and D Raman bands (Table 1). CEMNPs 1, 3, and 4 have similar crystallinity, whilst sample 2 exhibits higher structural ordering. The electrochemical analysis of CEMNPs is a suitable and very sensitive method that can detect the quality of encapsulation, i.e. how well are the magnetic nanoparticles shielded from the electrolyte environment. Cyclic voltammetry studies were applied to investigate the electrochemical properties of CEMNPs. Fig. 2A shows CV curves (after 10 cycles), which are of rectangular shape, which is a characteristic for the typical capacitive behaviour. It should be noticed, that a peak at − 0.67 V appeared on the I–U curve for the electrode composed of CEMNPs 2 material. However, this faradaic reaction
monotonically decreases by each full scan (Fig. 2B). This can be attributed to the metallic impurities, i.e. non-encapsulated magnetic nanoparticles, which are removed from the electrode during subsequent cycles. These particles could be trapped in the pores between the CEMNPs and the applied purification procedure was insufficient to remove them. Small current peaks at +0.2 V (Fig. 2C) were observed only for the electrode composed of CEMNPs 3. They can be ascribed to the adsorbed aromatic hydrocarbons, which could be formed during the synthesis (plasma processing of simple hydrocarbons can also yield larger aromatic structures). Similar signals were observed for the single-wall carbon nanotubes, which were catalytically deposited from hydrocarbons [9]. CV results show that carbon coatings in CEMNPs perfectly isolate the encapsulated metallic nanoparticles from the electrolyte environment in a wide range of potentials. CEMNP electrodes are also stable for long cycling times. Specific capacitances (CS) and surface-related capacitances (CSR) evaluated from the CV curves are summarized in Table 1. Relatively low CS values results from low surface area, which is the crucial parameter for efficient charge storage. The higher density of CEMNPs (in comparison to pure carbon materials) also decreases their gravimetric capacitance. Nevertheless, the Cs values are a few times higher than those for carbon blacks and multi-wall carbon nanotubes [10]. The surface-related capacitance for the CEMNPs electrodes varies from 6.7 to 37.0 µF/cm2. The values for samples 1–3 are in a good agreement with the capacitance observed for carbon onions and disordered carbons [10]. Interestingly, the CEMNP electrode 2 has significantly higher surface-related capacitance. This is likely related to its higher crystallinity, because highly graphitized layers in carbon coatings may provide better surface-related capacitance. This has been demonstrated for graphite powders, which have 1.5–4 times higher CSR than carbon materials with lower graphitization degrees [11]. Importantly, the encapsulated particles do not influence the capacitance properties, which are mostly dominated by the carbon shells. The long term stability of CEMNP-based electrodes was further investigated in cycling durability tests. The current and voltage profiles show excellent reproducibility (Fig. 2D). The voltage responses are very sharp and do not have any IR drop. This indicates the low contact resistance of CEMNPs electrodes. The origin of resistance in CEMNP-based electrodes was further investigated by impedance spectroscopy. Nyquist plots acquired between 10 mHz and 1 kHz are shown in Fig. 3A. The slopes are between 60 and 77° (Table 1), which points to the non-ideal supercapacitor behaviour [12]. These deviations are likely linked to the surface roughness and porous structure of the electrodes [10]. In fact, the phase angle is the highest for sample 2, which has the lowest surface roughness. One also cannot exclude the interactions between paramagnetic ions from the electrolyte and the magnetic cores in
Fig. 1. SEM (A) and TEM (B) images of sample 1.
M. Bystrzejewski et al. / Materials Letters 63 (2009) 1435–1438
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Fig. 2. Cyclic voltammograms at v = 5 mV s− 1: A—10th cycle for CEMNP electrodes 1–4, B—CEMNP 2 electrode, cycle numbers noted on figure; C—CEMNP 3 and 4 electrodes, D—cycling durability profile for CEMNP electrode 4.
CEMNPs. The series of resistance (RS) decrease monotonically with increasing frequency (Fig. 3B). At lower frequencies, the electrolyte ions have time to penetrate into the depth of pores and additional surface is accessed [11]. The RS values (Table 1) calculated from the EIS
at 100 Hz show a clear dependence on the BET surface area, whilst not on the content of encapsulated nanoparticles. The estimated resistances are close to those for carbon onions (0.8–1.4 Ω cm2), carbon black (0.8 Ω cm2) and carbon nanotubes (0.7 Ω cm2) [11]. 4. Conclusions Carbon-encapsulated magnetic nanoparticles have interesting electrochemical performance. The carbon coating perfectly isolates the encapsulated nanoparticles from the electrolyte and no faradaic reactions occur in a wide range of potentials. CEMNPs have low resistivity and exhibit very high electrochemical stability. Capacitance properties of CEMNP-based electrodes are similar to other nanocarbon materials (carbon nanoonions and nanotubes). The encapsulated magnetic nanoparticles contribute to neither the capacity nor the conductivity in CEMNP-based electrodes. CEMNP-based electrodes can be easily and reversibly magnetized by external permanent magnets. This allows modulating various electrochemical processes by in situ generated magnetic fields. Acknowledgements This work was supported by the Ministry of Science and Higher Education through the Department of Chemistry, Warsaw University under Grant No. N204 096 31/2160. G. Soucy (University of Sherbrooke) is greatly acknowledged for his assistance in the synthesis process. References
Fig. 3. EIS results for CEMNP electrodes: Nyquist plot (A) and resistance series (B).
[1] [2] [3] [4]
Burns A, Ow H, Wiesner U. Chem Soc Rev 2006;11:1028–42. Bystrzejewski M, Rummeli MH. Pol J Chem 2007;81:1219–55. Bystrzejewski M, Cudzilo S, Huczko A, Lange H. J Alloys Compd 2006;423:74–6. Seo WS, Lee JH, Sun X, Suzuki Y, Mann D, Liu Z, et al. Nat Mater 2006;5:971–6.
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[5] Joncheray TJ, Audebert P, Schwartz E, Jovanovic AV, Ishaq O, Chavez JL, et al. Langmuir 2006;22:8684–9. [6] Wang S, Xu Q, Liu G. Electroanalysis 2008;20:1116–20. [7] Bystrzejewski M, Huczko A, Lange H, Baranowski P, Soucy G, et al. Nanotechnology 2007;18:145608. [8] Bystrzejewski M, Grabias A, Borysiuk J, Huczko A, Lange H. J Appl Phys 2008;104:54307. [9] M. Arulepp, unpublished data.
[10] Portet C, Yushin G, Gogotsi Y. Carbon 2007;45:2511–8. [11] Pandolfo AG, Hollenkamp AF. J Power Sources 2006;157:11–27. [12] B.E. Conway: in Electrochemical Supercapacitors, Scientific Fundamentals and Technological Applications, Kluwer Academic / Plenum Publications, New York (1999).