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Enhancement of hydrogen storage by electrophoresis deposition of CNTs into nanoscale pores of silver foams
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 2 2 9 8 e2 3 0 3
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Enhancement of hydrogen storage by electrophoresis deposition of CNTs into nanoscale pores of silver foams B. Khoshnevisan a,*, M. Behpour b, S. Shoaei a a b
Department of Physics, University of Kashan, Kashan, Iran Department of Chemistry, University of Kashan, Kashan, Iran
article info
abstract
Article history:
Hydrogen storage capacity of Ag-CNTs foamed electrodes was studied by chro-
Received 27 June 2011
nopotentiometry method. The CNTs (carbon nanotubes) were deposited inside pores of the
Received in revised form
nanoscale silver foam by electrophoresis deposition (EPD) method and it was believed that
11 October 2011
the interconnections between the CNTs and the Ag frames were increased and were more
Accepted 16 October 2011
stable; therefore, charge transfer process through the electrode became facilitated. XRD,
Available online 21 November 2011
TGA and SEM techniques were employed to examine the purity of the CNTs and the quality of the Ag foam surface.
Keywords:
Hydrogen storage was done by using the Ag-CNTs electrodes as the working elec-
Hydrogen storage
trodes in the electrochemical cell. A set of regulated currents were applied to the cell to
Chemisorptions
produce charge and discharge (C&D) cycles. It was found that the storage capacity
Carbon nanotube
strongly depended on the applied current value and it became optimum corresponding
Electrophoresis deposition
to 4 mA current. In this manner the storage capacity inside the CNTs of the working electrodes approached 5.2%wt, which was quite noticeable in comparison with the other reports (storage capacity of pure silver is negligible). This optimization of the hydrogen storage capacity can be explained as a result of being in harmony with the applied current value and charge transferring ability of the local silver-nanotube junctions. However, re-alignment of the CNTs (inside the pores of the foam) during subsequent C&D cycles caused approaching the optimized capacity to occur after a few initial cycles. Whereas, oxidation of the silver part of the electrode after 30e40 subsequent C&D cycles caused functional instability of the electrode. Finally the effects of heat treatments on the storage capacity were studied, as well. After annealing of the electrode, the capacity increased substantially but with rising the ambient temperature it was reduced. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Nowadays, hydrogen has a preponderant position among the potentially sustainable energy sources. Its storage is of main concern in order to achieve a broad use in multiple
applications. The DOE (U.S. Department of Energy) set some targets of gaseous hydrogen storage percentage for commercial applications; however, the targets have not been achieved yet. On the other hand, the science and technology related to carbon nanostructures in hydrogen storage has evidently
* Corresponding author. Tel.: þ98 9123147358. E-mail address: [email protected] (B. Khoshnevisan). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.10.057
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 2 2 9 8 e2 3 0 3
progressed in recent years. The CNTs in comparison with traditional carbon materials have a higher electron-transferring rate and also, their entangled network is a positive characteristic for the storage purposes [1e4]. Lee et al. [5] have theoretically predicted up to 14% wt capacity for the hydrogen storage in CNTs but some of the experimental results are controversial due to the difficulty of controlling the complicated experimental conditions [6,7]. The values of the reported capacity are very diverse in literature for single walled and multi-walled CNTs, ranging from 0.2wt% to 3.7wt% at different temperatures [8]. Chemisorptions of hydrogen in the CNTs have been under consideration as efficient method for storage at ambient temperature [9]. In our first previous paper [10], we studied hydrogen adsorption in Ag-CNTs electrode, because of high conductivity of the sliver part of the electrode charge transferring process between the deposited CNTs part and the Ag back bone of the electrode was facilitated during storage (charge) and release (discharge) cycles. Additionally, in the second previous paper, we reported our efforts to approach higher storage capacity (up to 1.07 wt%) in the Ag-CNTs electrode by using silver foam with nanometric porosity as a frame for embedded CNTs [11]. In the present paper, we report our attempts to continue the enhancement procedure of the storage capacity by using electrophoresis deposition (EPD) method [12] for dispersing the CNTs inside the pores of the silver foam and therefore making better interconnections between the silver frame and the CNTs to promote electron exchange during electrochemical reactions in the cell. Our results showed that for the prepared electrodes, the discharge capacity went up to around 1500 mAh/gr (z5.2 wt%) when the applied current to the cell was regulated to 4 mA (the inserted currents were scanned from one to 27 mA). The results were confirmed in general by using more than 30 different asprepared Ag-CNTs electrodes. It has to be emphasized that all the mentioned storage percentages were compared to weights
2299
of the loaded CNTs on the silver foams and the results were reproducible and quite comparable with Li-Ion battery capacities [13].
2.
Experimental details
2.1.
Electrode preparation
1. The used CNTs in this study were synthesized by CVD method with Mo catalyst and they were purified by acidic and thermal treatments [11,14]. Our XRD, TEM and TGA results showed high purity level of the prepared sample (Fig. 1). Apparently, even after the purification process, some nano-particles of the catalyst were still stuck to the CNTs in the TEM images, but the sharp slope of the TGA curve with burning temperature around 550 C indicated that the sample was comprised mostly of the CNTs [14] (Fig. 2). 2. A nano-scaled porous silver foam was used as the frame part of the prepared electrodes (the foam fabrication details will be presented elsewhere), see Fig. 1. Using 2 1 cm2 plate form pieces of the foam for nesting the CNTs inside its nanoscale pores by EPD method caused that the prepared Ag-CNTs electrodes create many interconnections between the CNT particles and the silver frame. This structure of the electrodes not only facilitated charge exchange inside them, but also caused a reasonable functional stability (without using any glue or binder for holding the CNTs). In order to perform the EPD process, we hanged each of the foam plates in a dilute suspension of Penthanol and the CNTs under 80 V potential difference between the plate and a Pt electrode (15 min) [12]. To avoid agglomeration of the CNTs in the suspension, the whole package was put inside a low frequency vibrating bath, as well.
Fig. 1 e Purified CNTs sample, a- XRD pattern of the MWCNTs with indicative characteristics reflections, b- TGA graph shows sharp decreasing slop that indicates the sample has been graphitized well (majority of the CNTs burns around 550 C), c- TEM image of the sample with some trace of the catalyst particles.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 2 2 9 8 e2 3 0 3
Fig. 2 e Surface of the Ag foam plate; a- magnified photo image, b- SEM image with nanoscale pores, c- typical prepared Ag foam plates.
2.2.
Electrochemical system
Chronopotentiometry method (EG&G-model 273 A apparatus) was used to measure the hydrogen storage capacity of the electrodes. The charge and discharge (C&D) cycles were performed in a three electrode cell; the prepared Ag-CNTs electrode as the working electrode, Pt and Ag/AgCl electrodes as counter and reference electrodes and also the electrolyte inside the cell was a 6 M KOH solution (KOH does not have hydrate forms). In this system, a regulated current was inserted between the working and counter electrodes and the potential difference were measured between the working and reference electrodes. After applying the current and reversing it, the electrochemical representation of the hydrogen storage during the C&D cycles (for the working electrode) was: CNT þ xH2 O þ xe 4ðCNT þ xHÞ þ xOH
(1)
where hydrogen ions were intercalated into the CNTs during charging half cycle and were released during discharging half cycle. Previously, we showed that the storage capacity of the silver part of the electrode was negligible and the considerable discharge capacity in the prepared Ag-CNTs electrodes indicated that the used CNTs were capable of adsorbing and desorbing hydrogen in the alkaline medium [11].
3.
Results and discussion
half cycle was completed when the profile became flat and it meant that the potential difference between working and reference electrode remained constant with the time, and the discharging half cycle was finished when the potential profile is dropped to zero. Scientifically the storage capacity is attributed to the discharge capacity and it is expressed in terms of mAh/gr (current (mA) time (hour)/weight of the active material (gram)) [3]. In this study tens of foamed Ag-CNTs electrodes were prepared and data collection was done with several purposes: First; the inserted current of C&D cycles scanned from one to twenty seven (27) mA to find out at which current the storage capacity became maximum (optimized current); second, for the optimized applied current, functional stability of the working electrode during subsequent C&D cycles was under consideration, as well, and finally, we did a few measurements on higher temperatures and also, annealed Ag-CNTs electrodes to see the effects of heat treatments on the storage process.
3.1.
Optimization of the C&D inserted current
Here the applied current was changed from one to twenty seven mA and the optimum discharge capacity occurred at 4 mA. The result was quite reproducible and for 7 different prepared electrodes this optimum capacity was always about 1500 mAh/gr (z5.0%wt). Capacity variation versus the C&D currents is shown in Fig. 4 (for the sake of clearance, the
After applying a current of 1 mA to the cell, the charge and discharge potential profiles were shown in Fig. 3. The charging
Fig. 3 e Charging (left) and discharging (right) potential profiles of the Ag-CNTs electrode under 1 mA cyclic current.
Fig. 4 e Discharge capacity of the Ag-CNTs electrode under different applied currents.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 2 2 9 8 e2 3 0 3
1550
Discharge capacity (mAh/gr)
scanned currents were shown only up to 7 mA). By the way, to make comparison, only the capacity variations of three of the prepared electrodes (versus the C&D applied currents) were shown in Fig. 5. Basically, stability of the inserted C&D current is under the influence of charge transfer rate on surface of the working electrode and obviously, straightening of junctions between the CNTs and the porous silver foam by using EPD technique can facilitate the charge transfer rate from/to the CNTs to/ from the silver (during adsorption/desorption of hydrogen) and consequently, to the external part of electrochemical circuit. However, optimum hydrogen storage occurs when ionic current in the electrolyte, which comes from the cyclic interactions (Equ. 1), is in balance with the charge transferring rate on the surface of the working electrode. Under higher amounts of the applied currents because the system must sustain the current constant, the charge transfer process occurs directly between the silver frame of the electrode and the electrolyte, without intermediating of the deposited CNTs; therefore, the released hydrogen atoms are not adsorbed inside the pours of the Ag-CNTs electrode but they form H2 gas babbles, instead. For more discussion it should be mentioned that based upon the Eqn. 1, in the chemisorptions process the stored hydrogens are mainly in atomic form, therefore, the possibility of a spill-over mechanism for diffusion of the hydrogen atoms on exterior surface of the CNTs cannot be ruled out [15]. For single wall CNTs, ab initio calculations suggest that molecular hydrogen storage is preferred for interior hole region (endo-hydrogenation) whereas atomic hydrogen adsorption is preferred on the external surface (exo-hydrogenations) [16]. In the case of multi wall CNTs (MWCNTs), also the same scenario would be rational and intercalation of hydrogen between carbon layers is not dominant. The intercalation process entails breaking CeC p-bonds and formation of strong CeH bonds by rehybridization of the carbon atoms from sp2 to sp3; therefore, the storage is not reversible and it is distractive for the MWCNTs structure, as well.
1500 1450 1400 1350 1300 1250 1200 0
5
10
15
20
25
30
Number of cycles
Fig. 6 e Small fluctuations of the discharge capacity versus number of the C&D cycles under 4 mA applied current. The first cycle capacity value was about 1250 mAh/gr.
3.2. Subsequent C&D cycles at the optimized applied current Based on the optimized C&D applied current, 4 mA, the effect of the subsequent C&D cycles on the storage capacity were checked and the results showed that after the first couple of cycles the storage capacity reaches its optimum value (attributed to re-alignment of the CNTs during the C&D process), in the subsequent cycles, it would have small fluctuations, Fig. 6. Unfortunately, functional instability was seen for the electrodes after thirty to forty cycles (the CNTs were detached from the foam and deposited) and it was probably because of oxidation of the silver foam (forming Ag2O interface layers on the surface of the electrode’s surface) [11].
3.3.
Heat treatments effects
After rising the ambient temperature up to 70 C, as Fig. 7 shows the discharge capacity reduced at all corresponded C&D currents and this is not surprising because increasing the temperature caused more instability on the silver and CNTs junctions. On the other hand, for more stabilization of the junctions the prepared electrode was annealed at 400 C (at
Fig. 5 e Comparison between discharge capacity of the three as- prepared Ag-CNTs electrodes.
Fig. 7 e Effect of rising ambient temperature up to 70 C on the storage capacity.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 2 2 9 8 e2 3 0 3
Fig. 8 e Effect of annealing of the electrode at 400 C for an hour. The annealed Ag-CNTs electrode showes higher storage capacity when the C&D applied current was 1 mA (right) and 4 mA (left).
higher temperatures the CNTs might be burned) for an hour and the annealed electrode showed substantial increasing on the storage capacity under 1 mA C&D current and an enhancement of about 7% under the optimum current of 4 mA (Fig. 8).
4.
Conclusion
Hydrogen storage inside Ag-CNTs foamed electrodes was studied by chronopotentiometery technique. The electrodes were used as working electrode and the major improvement in their preparation were using EPD method to deposit the CNTs inside the nano-scaled pores of the silver foam (with very high specific surface area). The EPD method caused a substantial stabilization on inter-junctions between the CNTs and the silver frame; therefore, better charge transfer rate could be possible through the junctions during the potentiostat/galvanostat measurements and the performance of hydrogen storage was improved. Our studies showed that the amount of discharge capacity depended on cyclic regulated current value and it was optimized about 5.2%wt under the current of 4 mA. This optimized value would be achieved when the ionic current in the electrolyte becomes in balance with the charge transfer rate on the surface of the working electrode. Unfortunately, functional stability of the working electrode was diminished by its oxidation (forming Ag2O interface layers on the surface of the electrode) after thirty to forty C&D cycles. In addition, the measured discharge capacities were affected by rising ambient temperature and there were substantial reductions in them for whole range of applied currents. It can be attributed to the instability of the interjunctions between the CNTs and their silver frame at higher temperatures. In contrary, by annealing the Ag-CNTs electrode at 400 C for an hour, when the inter-junctions became more stable, a huge increasing on the storage capacity was seen under 1 mA applied current and for 4 mA applied current it was about 7% higher than that for the un-annealed electrode). In general, our attempts on fabrication of the Ag-CNTs foamed electrode showed a noticeable enhancement on
chemisorptions storage capacity in comparison with the other reports [17,18].
references
[1] Fu X, Zhang H, Chen Y, Li S, Yi S, Zhou C, et al. The effect of carbon nanotubes on the electrochemical hydrogen storage performance of LaNi5 rare earth alloy. Physica E 2005;25:414. [2] Zhang H, Fu X, Chen Y, Yi S, Li S, Zhu Y, et al. The electrochemical hydrogen storage of multi-walled carbon nanotubes synthesized by chemical vapor deposition using a lanthanum nickel hydrogen storage alloy as catalyst. Physica B 2004;352:66. [3] Fazle Kibria AKM, Mo YH, Park KS, Nahm KS, Yun MH. Electrochemical hydrogen storage behaviors of CVD, AD, and LA grown carbon nanotubes in KOH medium. Int J Hydrogen Energy 2001;26:823. [4] Zu¨ttel A, Sudan P, Mauron Ph, Kiobayashi T, Emmeneger Ch, Schlapbach L. Hydrogen storage in carbon nanostructures. Int J Hydrogen Energy 2002;27:203. [5] Lee SM, Park KS, Choi YC, Park YS, Bok JM, Bae DJ, et al. Hydrogen adsorption and storage in carbon nanotubes. Synth Metal 2000;113:209. [6] Strobel R, Garche J, T Moseley P, Jorissen L, Wolf G. Hydrogen storage by carbon materials. J Power Sources 2006;159:781. [7] Benard P, Chahine R. Storage of hydrogen by physisorption on carbon and nanostructured materials. Scripta Materialia 2007;56:803. [8] Fernandez PS, Castro EB, Real SG, Martins ME. Electrochemical behavior of single walled carbon nanotubesHydrogen storage and hydrogen evolution reaction. Int J Hydrogen Energy 2009;34:8115. [9] Hirscher M, Becher M, Haluska M, Quintel A, Skakalova V. Hydrogen storage in carbon nanostructures. J Alloys Compd 2002;330:654. [10] Khoshnevisan B, Behpour M, Ghoreishi SM, Hemmati M. Absorption of hydrogen in Ag-CNTs electrode. Int J Hydrogen Energy 2007;32:3860. [11] Khoshnevisan B, Behpour M, Kaveh D. Optimization of hydrogen uptake in Ag-CNTs electrodes with chargedischarge cyclic currents. Physica B 2009;404:1733. [12] Boccaccini AR, Cho J, Roether JA, Thomas BJC, Minay EJ, Shaffer MSP. Electrophoretic deposition of carbon nanotubes. Carbon 2006;44:3149.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 2 2 9 8 e2 3 0 3
[13] Liu H, Bo S, Cui W, Li F, Wang C, Xia Y. Nano eSized cobalt oxide/mesoporous carbon sphere composites as negative electrode material for Li-Ion batteries. Electrochemica Acta 2008;53:6497. [14] Moon Jm, An KH, Lee YH, Park YS, Bae DJ, Park Gs. High-Yield purification process of Singlewalled carbon nanotubes. J Phys Chem B 2001;105:5677. [15] Psofogiannakis G, Froudakis GE. Theoretical study of the hydrogen spillover mechanism. J Am Chem Soc 2009; 131:15133.
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[16] Khoshnevisan B, Aghtar M, Yazdani Kachoei M. Different possible hydrogenation in narrow SWCNTs and their electronic characteristics. Int J Hydrogen Energy 2011;36:1053. [17] Chen X, Zhang Y, Gao XP, Pan GL, Jiang XY, Qu JQ, et al. Electrochemical hydrogen storage of carbon nanotubes and carbon nanofibers. Int J Hydrogen Energy 2004;29:743. [18] Rzepka M, Bauer E, Reichenauer G, Schliermann T. Hydrogen storage capacity of catalytically grown carbon nanofibers. J Phys Chem B 2005;109:14979.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Enhancement of hydrogen storage by electrophoresis deposition of CNTs into nanoscale pores of silver foams B. Khoshnevisan a,*, M. Behpour b, S. Shoaei a a b
Department of Physics, University of Kashan, Kashan, Iran Department of Chemistry, University of Kashan, Kashan, Iran
article info
abstract
Article history:
Hydrogen storage capacity of Ag-CNTs foamed electrodes was studied by chro-
Received 27 June 2011
nopotentiometry method. The CNTs (carbon nanotubes) were deposited inside pores of the
Received in revised form
nanoscale silver foam by electrophoresis deposition (EPD) method and it was believed that
11 October 2011
the interconnections between the CNTs and the Ag frames were increased and were more
Accepted 16 October 2011
stable; therefore, charge transfer process through the electrode became facilitated. XRD,
Available online 21 November 2011
TGA and SEM techniques were employed to examine the purity of the CNTs and the quality of the Ag foam surface.
Keywords:
Hydrogen storage was done by using the Ag-CNTs electrodes as the working elec-
Hydrogen storage
trodes in the electrochemical cell. A set of regulated currents were applied to the cell to
Chemisorptions
produce charge and discharge (C&D) cycles. It was found that the storage capacity
Carbon nanotube
strongly depended on the applied current value and it became optimum corresponding
Electrophoresis deposition
to 4 mA current. In this manner the storage capacity inside the CNTs of the working electrodes approached 5.2%wt, which was quite noticeable in comparison with the other reports (storage capacity of pure silver is negligible). This optimization of the hydrogen storage capacity can be explained as a result of being in harmony with the applied current value and charge transferring ability of the local silver-nanotube junctions. However, re-alignment of the CNTs (inside the pores of the foam) during subsequent C&D cycles caused approaching the optimized capacity to occur after a few initial cycles. Whereas, oxidation of the silver part of the electrode after 30e40 subsequent C&D cycles caused functional instability of the electrode. Finally the effects of heat treatments on the storage capacity were studied, as well. After annealing of the electrode, the capacity increased substantially but with rising the ambient temperature it was reduced. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Nowadays, hydrogen has a preponderant position among the potentially sustainable energy sources. Its storage is of main concern in order to achieve a broad use in multiple
applications. The DOE (U.S. Department of Energy) set some targets of gaseous hydrogen storage percentage for commercial applications; however, the targets have not been achieved yet. On the other hand, the science and technology related to carbon nanostructures in hydrogen storage has evidently
* Corresponding author. Tel.: þ98 9123147358. E-mail address: [email protected] (B. Khoshnevisan). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.10.057
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 2 2 9 8 e2 3 0 3
progressed in recent years. The CNTs in comparison with traditional carbon materials have a higher electron-transferring rate and also, their entangled network is a positive characteristic for the storage purposes [1e4]. Lee et al. [5] have theoretically predicted up to 14% wt capacity for the hydrogen storage in CNTs but some of the experimental results are controversial due to the difficulty of controlling the complicated experimental conditions [6,7]. The values of the reported capacity are very diverse in literature for single walled and multi-walled CNTs, ranging from 0.2wt% to 3.7wt% at different temperatures [8]. Chemisorptions of hydrogen in the CNTs have been under consideration as efficient method for storage at ambient temperature [9]. In our first previous paper [10], we studied hydrogen adsorption in Ag-CNTs electrode, because of high conductivity of the sliver part of the electrode charge transferring process between the deposited CNTs part and the Ag back bone of the electrode was facilitated during storage (charge) and release (discharge) cycles. Additionally, in the second previous paper, we reported our efforts to approach higher storage capacity (up to 1.07 wt%) in the Ag-CNTs electrode by using silver foam with nanometric porosity as a frame for embedded CNTs [11]. In the present paper, we report our attempts to continue the enhancement procedure of the storage capacity by using electrophoresis deposition (EPD) method [12] for dispersing the CNTs inside the pores of the silver foam and therefore making better interconnections between the silver frame and the CNTs to promote electron exchange during electrochemical reactions in the cell. Our results showed that for the prepared electrodes, the discharge capacity went up to around 1500 mAh/gr (z5.2 wt%) when the applied current to the cell was regulated to 4 mA (the inserted currents were scanned from one to 27 mA). The results were confirmed in general by using more than 30 different asprepared Ag-CNTs electrodes. It has to be emphasized that all the mentioned storage percentages were compared to weights
2299
of the loaded CNTs on the silver foams and the results were reproducible and quite comparable with Li-Ion battery capacities [13].
2.
Experimental details
2.1.
Electrode preparation
1. The used CNTs in this study were synthesized by CVD method with Mo catalyst and they were purified by acidic and thermal treatments [11,14]. Our XRD, TEM and TGA results showed high purity level of the prepared sample (Fig. 1). Apparently, even after the purification process, some nano-particles of the catalyst were still stuck to the CNTs in the TEM images, but the sharp slope of the TGA curve with burning temperature around 550 C indicated that the sample was comprised mostly of the CNTs [14] (Fig. 2). 2. A nano-scaled porous silver foam was used as the frame part of the prepared electrodes (the foam fabrication details will be presented elsewhere), see Fig. 1. Using 2 1 cm2 plate form pieces of the foam for nesting the CNTs inside its nanoscale pores by EPD method caused that the prepared Ag-CNTs electrodes create many interconnections between the CNT particles and the silver frame. This structure of the electrodes not only facilitated charge exchange inside them, but also caused a reasonable functional stability (without using any glue or binder for holding the CNTs). In order to perform the EPD process, we hanged each of the foam plates in a dilute suspension of Penthanol and the CNTs under 80 V potential difference between the plate and a Pt electrode (15 min) [12]. To avoid agglomeration of the CNTs in the suspension, the whole package was put inside a low frequency vibrating bath, as well.
Fig. 1 e Purified CNTs sample, a- XRD pattern of the MWCNTs with indicative characteristics reflections, b- TGA graph shows sharp decreasing slop that indicates the sample has been graphitized well (majority of the CNTs burns around 550 C), c- TEM image of the sample with some trace of the catalyst particles.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 2 2 9 8 e2 3 0 3
Fig. 2 e Surface of the Ag foam plate; a- magnified photo image, b- SEM image with nanoscale pores, c- typical prepared Ag foam plates.
2.2.
Electrochemical system
Chronopotentiometry method (EG&G-model 273 A apparatus) was used to measure the hydrogen storage capacity of the electrodes. The charge and discharge (C&D) cycles were performed in a three electrode cell; the prepared Ag-CNTs electrode as the working electrode, Pt and Ag/AgCl electrodes as counter and reference electrodes and also the electrolyte inside the cell was a 6 M KOH solution (KOH does not have hydrate forms). In this system, a regulated current was inserted between the working and counter electrodes and the potential difference were measured between the working and reference electrodes. After applying the current and reversing it, the electrochemical representation of the hydrogen storage during the C&D cycles (for the working electrode) was: CNT þ xH2 O þ xe 4ðCNT þ xHÞ þ xOH
(1)
where hydrogen ions were intercalated into the CNTs during charging half cycle and were released during discharging half cycle. Previously, we showed that the storage capacity of the silver part of the electrode was negligible and the considerable discharge capacity in the prepared Ag-CNTs electrodes indicated that the used CNTs were capable of adsorbing and desorbing hydrogen in the alkaline medium [11].
3.
Results and discussion
half cycle was completed when the profile became flat and it meant that the potential difference between working and reference electrode remained constant with the time, and the discharging half cycle was finished when the potential profile is dropped to zero. Scientifically the storage capacity is attributed to the discharge capacity and it is expressed in terms of mAh/gr (current (mA) time (hour)/weight of the active material (gram)) [3]. In this study tens of foamed Ag-CNTs electrodes were prepared and data collection was done with several purposes: First; the inserted current of C&D cycles scanned from one to twenty seven (27) mA to find out at which current the storage capacity became maximum (optimized current); second, for the optimized applied current, functional stability of the working electrode during subsequent C&D cycles was under consideration, as well, and finally, we did a few measurements on higher temperatures and also, annealed Ag-CNTs electrodes to see the effects of heat treatments on the storage process.
3.1.
Optimization of the C&D inserted current
Here the applied current was changed from one to twenty seven mA and the optimum discharge capacity occurred at 4 mA. The result was quite reproducible and for 7 different prepared electrodes this optimum capacity was always about 1500 mAh/gr (z5.0%wt). Capacity variation versus the C&D currents is shown in Fig. 4 (for the sake of clearance, the
After applying a current of 1 mA to the cell, the charge and discharge potential profiles were shown in Fig. 3. The charging
Fig. 3 e Charging (left) and discharging (right) potential profiles of the Ag-CNTs electrode under 1 mA cyclic current.
Fig. 4 e Discharge capacity of the Ag-CNTs electrode under different applied currents.
2301
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 2 2 9 8 e2 3 0 3
1550
Discharge capacity (mAh/gr)
scanned currents were shown only up to 7 mA). By the way, to make comparison, only the capacity variations of three of the prepared electrodes (versus the C&D applied currents) were shown in Fig. 5. Basically, stability of the inserted C&D current is under the influence of charge transfer rate on surface of the working electrode and obviously, straightening of junctions between the CNTs and the porous silver foam by using EPD technique can facilitate the charge transfer rate from/to the CNTs to/ from the silver (during adsorption/desorption of hydrogen) and consequently, to the external part of electrochemical circuit. However, optimum hydrogen storage occurs when ionic current in the electrolyte, which comes from the cyclic interactions (Equ. 1), is in balance with the charge transferring rate on the surface of the working electrode. Under higher amounts of the applied currents because the system must sustain the current constant, the charge transfer process occurs directly between the silver frame of the electrode and the electrolyte, without intermediating of the deposited CNTs; therefore, the released hydrogen atoms are not adsorbed inside the pours of the Ag-CNTs electrode but they form H2 gas babbles, instead. For more discussion it should be mentioned that based upon the Eqn. 1, in the chemisorptions process the stored hydrogens are mainly in atomic form, therefore, the possibility of a spill-over mechanism for diffusion of the hydrogen atoms on exterior surface of the CNTs cannot be ruled out [15]. For single wall CNTs, ab initio calculations suggest that molecular hydrogen storage is preferred for interior hole region (endo-hydrogenation) whereas atomic hydrogen adsorption is preferred on the external surface (exo-hydrogenations) [16]. In the case of multi wall CNTs (MWCNTs), also the same scenario would be rational and intercalation of hydrogen between carbon layers is not dominant. The intercalation process entails breaking CeC p-bonds and formation of strong CeH bonds by rehybridization of the carbon atoms from sp2 to sp3; therefore, the storage is not reversible and it is distractive for the MWCNTs structure, as well.
1500 1450 1400 1350 1300 1250 1200 0
5
10
15
20
25
30
Number of cycles
Fig. 6 e Small fluctuations of the discharge capacity versus number of the C&D cycles under 4 mA applied current. The first cycle capacity value was about 1250 mAh/gr.
3.2. Subsequent C&D cycles at the optimized applied current Based on the optimized C&D applied current, 4 mA, the effect of the subsequent C&D cycles on the storage capacity were checked and the results showed that after the first couple of cycles the storage capacity reaches its optimum value (attributed to re-alignment of the CNTs during the C&D process), in the subsequent cycles, it would have small fluctuations, Fig. 6. Unfortunately, functional instability was seen for the electrodes after thirty to forty cycles (the CNTs were detached from the foam and deposited) and it was probably because of oxidation of the silver foam (forming Ag2O interface layers on the surface of the electrode’s surface) [11].
3.3.
Heat treatments effects
After rising the ambient temperature up to 70 C, as Fig. 7 shows the discharge capacity reduced at all corresponded C&D currents and this is not surprising because increasing the temperature caused more instability on the silver and CNTs junctions. On the other hand, for more stabilization of the junctions the prepared electrode was annealed at 400 C (at
Fig. 5 e Comparison between discharge capacity of the three as- prepared Ag-CNTs electrodes.
Fig. 7 e Effect of rising ambient temperature up to 70 C on the storage capacity.
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Fig. 8 e Effect of annealing of the electrode at 400 C for an hour. The annealed Ag-CNTs electrode showes higher storage capacity when the C&D applied current was 1 mA (right) and 4 mA (left).
higher temperatures the CNTs might be burned) for an hour and the annealed electrode showed substantial increasing on the storage capacity under 1 mA C&D current and an enhancement of about 7% under the optimum current of 4 mA (Fig. 8).
4.
Conclusion
Hydrogen storage inside Ag-CNTs foamed electrodes was studied by chronopotentiometery technique. The electrodes were used as working electrode and the major improvement in their preparation were using EPD method to deposit the CNTs inside the nano-scaled pores of the silver foam (with very high specific surface area). The EPD method caused a substantial stabilization on inter-junctions between the CNTs and the silver frame; therefore, better charge transfer rate could be possible through the junctions during the potentiostat/galvanostat measurements and the performance of hydrogen storage was improved. Our studies showed that the amount of discharge capacity depended on cyclic regulated current value and it was optimized about 5.2%wt under the current of 4 mA. This optimized value would be achieved when the ionic current in the electrolyte becomes in balance with the charge transfer rate on the surface of the working electrode. Unfortunately, functional stability of the working electrode was diminished by its oxidation (forming Ag2O interface layers on the surface of the electrode) after thirty to forty C&D cycles. In addition, the measured discharge capacities were affected by rising ambient temperature and there were substantial reductions in them for whole range of applied currents. It can be attributed to the instability of the interjunctions between the CNTs and their silver frame at higher temperatures. In contrary, by annealing the Ag-CNTs electrode at 400 C for an hour, when the inter-junctions became more stable, a huge increasing on the storage capacity was seen under 1 mA applied current and for 4 mA applied current it was about 7% higher than that for the un-annealed electrode). In general, our attempts on fabrication of the Ag-CNTs foamed electrode showed a noticeable enhancement on
chemisorptions storage capacity in comparison with the other reports [17,18].
references
[1] Fu X, Zhang H, Chen Y, Li S, Yi S, Zhou C, et al. The effect of carbon nanotubes on the electrochemical hydrogen storage performance of LaNi5 rare earth alloy. Physica E 2005;25:414. [2] Zhang H, Fu X, Chen Y, Yi S, Li S, Zhu Y, et al. The electrochemical hydrogen storage of multi-walled carbon nanotubes synthesized by chemical vapor deposition using a lanthanum nickel hydrogen storage alloy as catalyst. Physica B 2004;352:66. [3] Fazle Kibria AKM, Mo YH, Park KS, Nahm KS, Yun MH. Electrochemical hydrogen storage behaviors of CVD, AD, and LA grown carbon nanotubes in KOH medium. Int J Hydrogen Energy 2001;26:823. [4] Zu¨ttel A, Sudan P, Mauron Ph, Kiobayashi T, Emmeneger Ch, Schlapbach L. Hydrogen storage in carbon nanostructures. Int J Hydrogen Energy 2002;27:203. [5] Lee SM, Park KS, Choi YC, Park YS, Bok JM, Bae DJ, et al. Hydrogen adsorption and storage in carbon nanotubes. Synth Metal 2000;113:209. [6] Strobel R, Garche J, T Moseley P, Jorissen L, Wolf G. Hydrogen storage by carbon materials. J Power Sources 2006;159:781. [7] Benard P, Chahine R. Storage of hydrogen by physisorption on carbon and nanostructured materials. Scripta Materialia 2007;56:803. [8] Fernandez PS, Castro EB, Real SG, Martins ME. Electrochemical behavior of single walled carbon nanotubesHydrogen storage and hydrogen evolution reaction. Int J Hydrogen Energy 2009;34:8115. [9] Hirscher M, Becher M, Haluska M, Quintel A, Skakalova V. Hydrogen storage in carbon nanostructures. J Alloys Compd 2002;330:654. [10] Khoshnevisan B, Behpour M, Ghoreishi SM, Hemmati M. Absorption of hydrogen in Ag-CNTs electrode. Int J Hydrogen Energy 2007;32:3860. [11] Khoshnevisan B, Behpour M, Kaveh D. Optimization of hydrogen uptake in Ag-CNTs electrodes with chargedischarge cyclic currents. Physica B 2009;404:1733. [12] Boccaccini AR, Cho J, Roether JA, Thomas BJC, Minay EJ, Shaffer MSP. Electrophoretic deposition of carbon nanotubes. Carbon 2006;44:3149.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 2 2 9 8 e2 3 0 3
[13] Liu H, Bo S, Cui W, Li F, Wang C, Xia Y. Nano eSized cobalt oxide/mesoporous carbon sphere composites as negative electrode material for Li-Ion batteries. Electrochemica Acta 2008;53:6497. [14] Moon Jm, An KH, Lee YH, Park YS, Bae DJ, Park Gs. High-Yield purification process of Singlewalled carbon nanotubes. J Phys Chem B 2001;105:5677. [15] Psofogiannakis G, Froudakis GE. Theoretical study of the hydrogen spillover mechanism. J Am Chem Soc 2009; 131:15133.
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[16] Khoshnevisan B, Aghtar M, Yazdani Kachoei M. Different possible hydrogenation in narrow SWCNTs and their electronic characteristics. Int J Hydrogen Energy 2011;36:1053. [17] Chen X, Zhang Y, Gao XP, Pan GL, Jiang XY, Qu JQ, et al. Electrochemical hydrogen storage of carbon nanotubes and carbon nanofibers. Int J Hydrogen Energy 2004;29:743. [18] Rzepka M, Bauer E, Reichenauer G, Schliermann T. Hydrogen storage capacity of catalytically grown carbon nanofibers. J Phys Chem B 2005;109:14979.