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Electrochemical properties of nickel-metal hydride battery based on directly grown multiwalled carbon nanotubes
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 4 0 ( 2 0 1 5 ) 8 9 3 5 e8 9 4 0
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Electrochemical properties of nickel-metal hydride battery based on directly grown multiwalled carbon nanotubes Fei Xie a, Changkun Dong a,b,*, Weijin Qian a, Ying Zhai a, Li Li a, Detian Li a,b a
Institute of Micro-nano Structures & Optoelectronics, Wenzhou University, Chashan University Town, Wenzhou, Zhejiang 325035, China b Science and Technology on Vacuum & Cryogenics Technology and Physics Laboratory, Lanzhou Institution of Physics, Lanzhou, China
article info
abstract
Article history:
The electrochemical properties of nickel-metal hydride (Ni-MH) batteries with directly
Received 6 March 2015
grown multi-walled carbon nanotubes (MWNTs) have been investigated. MWNTs inter-
Received in revised form
wove through active material powders to benefit the battery in three aspects: enhancing
3 May 2015
the electrode mechanical strength by clutching the active material to the substrate,
Accepted 8 May 2015
improving the electrochemical activity from the increase of the reaction sites, and reducing
Available online 4 June 2015
the charge transfer resistance as conducting bridges. The battery with MWNTs exhibited excellent electrochemical properties with the maximum discharge specific capacity of
Keywords:
360.9 mAh/g, 18% higher than the electrode without MWNTs, and EIS analysis showed that
Carbon nanotube
the charge transfer resistances dropped 37.5%. Meanwhile, the chargeedischarge cycle
Ni/MH battery
stability was improved dramatically with the additions of both MWNTs and PTFE binder,
Direct growth
declining 10.8% only after 220 cycles. This MWNT/Ni foam structure is also very promising
Electrochemical property
for other electrochemical applications.
Electronic conducting bridge
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The Ni-MH batteries are used in a wide range of electric products, including portable electronic equipments, electrical tools, and hybrid electric vehicles (HEVs) due to the superiorities of high power density, low cost, long lifetime, and low environmental pollution [1e3,14,15,19e22,27e30]. The Ni-MH battery consists mainly of the metal hydride (MH) negative
electrode, the Ni(OH)2 positive electrode, the diaphragm, and the electrolyte [15,26]. Research and development efforts have involved every aspects of the battery cell. Ruiz et al. studied the effects of the electrolyte concentration on the battery electrochemical performance, and results showed that the 6 M and 8 M electrodes exhibited quicker activation feature with highest discharge specific capacity (~325 mAh/g) [21]. The electrodes are generally fabricated by the wet-pasting process by mixing the metal hydride (MH) active material powders
* Corresponding author. Institute of Micro-nano Structures & Optoelectronics, Wenzhou University, Chashan University Town, Wenzhou, Zhejiang 325035, China. Tel.: þ86 577 86689067. E-mail address: [email protected] (C. Dong). http://dx.doi.org/10.1016/j.ijhydene.2015.05.054 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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with the polymer binder and water to make the slurry to paste onto the electrode substrate. However, severe oxidation could occur during the drying process, resulting in lower electrode capacity and shorter cycle life. Although various methods have been proposed to prevent the oxidation of the active material, such as fabrication under inert atmospheres, they are either not very effective or too expensive [1]. Ye et al. reported a dry powder roller pressing process to make electrodes with significant improvements on the cycle life and discharging specific capacity [2]. In order to improve the high temperature and time-consuming drying process, Zhong et al. fabricated the dry powder electrodes with the addition of 5 wt.% conductive binder, a mixture of nickel power, graphite powder, and polytetrafluoroethylene (PTFE) powder in the weight ratio 1:1:1 [1]. If enough active material is supplied on the positive electrode, the specific capacity of the Ni-MH battery depends mainly on the performance of the negative electrode [3]. AB5 type alloys are commonly adopted as the active material in the negative electrode [15,20,22]. Sumita et al. investigated AB5 type alloy Mm0.75Ti0.05La0.2Ni3.7Al0.38Co0.3Mn0.5Mo0.02 with the addition of Ti content by radio-frequency induction melting, and showed significant improvement on the discharge capacity [22]. Especially, the improvements on the adhesion and charge transfer conductivity between the active material and the substrate play crucial roles to enhance the battery performances, including the chargeedischarge capacity and cycle life. Since the discovery in 1991 [10], carbon nanotubes (CNTs) have attracted a great deal of attention for electrochemical hydrogen storage due to their unique structural, mechanical, thermal, electronic, and electrochemical properties [4,23e25,28,30,31]. The applications of CNTs on the metal hydride electrode are investigated extensively. Sedlar´ıkova et al. studied the electrochemical properties of Ni/MH batteries with various types of CNTs based negative electrodes [30]. Lv et al. [5] added different amounts of CNTs (0.5, 1, and 2 wt.%) into the powder mixture of Ni(OH)2, CoO, and Co in the positive electrode and assembled type-AA batteries by a wetpasting process. Under high rate discharge conditions, the batteries showed improvements on the cycle stability, the discharge voltage plateau, the internal resistance, and the high-rate capability. Zhang et al. [7] made high-power CNT compound Ni-MH batteries by mixing MWNTs into the LaNi5 alloy powder on a mass ratio of 0.5%, 0.8%, and 1%, respectively, in the negative electrodes. The weight of the active material mixture was (17.0 ± 0.2)g for each negative electrode. The battery reached the capacity of 3369 mAh with 3000 mA charging current and 6000 mA discharging current, and the cycle life exceeded 600 times. Wu et al. developed nickel boride-coated carbon nanotube films by the electrophoresis deposition [4]. After treated with nitric acid for 5 h and coated with 45 wt.% of Ni2B, this film showed excellent high rate capability and cycle stability. Tsai et al. [6] coated La-rich AB5 hydrogen storage alloy on buckypapers by the direct current magnetron sputtering, and the maximum discharge capacity of 276 mAh/g was reached. Mazdak Hashempour et al. [8] grown directly MWNTs on the 316 stainless steel by chemical vapor deposition (CVD), and the MWNT film presented strong bonding and low contact resistance with the substrate. Dong et al. [9] synthesized 3D graphene-CNT hybrids by two-
step CVD, and showed that the electron-transfer resistances of the 3D graphene-CNT hybrid, the bare 3D graphene, the MWNT modified glassy carbon, and the glassycarbon electrode were 10.4, 22.8, 34.0, and 76.3 U, respectively. Du et al. [16] synthesized a series of Co/CNT negative electrode based alkaline rechargeable batteries, and the results showed that the Co-CNT composite with weight ratio of 5:1 displayed optimized electrochemical properties. In this work, we fabricated the electrodes by the dry powder roller pressing process after grown MWNTs directly on Ni foam substrates by CVD without extra catalyst layer. The structural and electrochemical properties of this novel battery structure were characterized by scanning electron microscope (SEM), X-Ray Diffraction (XRD), Cyclic voltammetry (CV), and Electrochemical impedance spectroscopy (EIS) on open halfcell systems. Significant improvements on the discharge specific capacity and cycle stability were achieved, benefited mainly from the excellent structural and conductive superiorities.
Experimental In the fabrication of the MWNT grown electrodes, the MWNTs were synthesized directly on the Ni foams of 2.7 2.7 cm2 by CVD at 650 C from source gases acetylene/argon at flow rates of 20/200 sccm for 10 min. In our experiments, the commercial La-rich AB5 alloy powder from Sihui City Double Win Industry CO was employed as the active material, which was mixed with Ni powder on a mass rate of 1:1 to produce the negative electrode. The mixture was hand lapped 10 min in the mortar to ensure uniform hybrid, and was pressed under the pressure of 30 MPa at room temperature to form a composite plate of 0.6 mm thick. The composite plate was then sandwiched with two MWNT grown Ni foams to form the negative electrode A. For the comparison, the electrode B was produced followed the same process using two pure Ni foam substrates. The positive electrode was made from the active material Ni(OH)2 and the conductive agent CoO using the same process. The microstructures were characterized by SEM (JEOL JMS6700F) measurements. All electrodes were pieced together into an open half-cell battery (A and B) based on the electrolyte KOH:NaOH:LiOH with ratio of 6M:1M:0.5M. The chargeedischarge properties were investigated in a battery test station at room temperature. In order to study the discharge capacity and cycle stability, the cells were charged at 0.1 C for 6 h, and discharged at 0.2 C with a cut-off voltage of 0.1 V. After 10 activation cycles, the chargeedischarge currents were doubled. A three-electrode system (battery A and B)with a Hg/ HgO reference electrode, an MH electrode working electrode, and a Ni(OH)2 counter electrode were introduced to conduct the electrochemical property measurements on the electrochemical workstation (German Zahner CO., Zennium). CV performance of the battery was tested in the potential range of 0.6 V to 1 V with five scan rates (5, 10, 20, 50, 100 mV/s). EIS measurements were scanned over a frequency range of 1 MHze1 mHz at equilibrium conditions with a superimposed sinusoidal voltage signal of 5 mV amplitude. The CV and EIS measurements were performed after 10 charging-discharging activation cycles.
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Results and discussions The SEM images of Ni foams after CNT growth are shown in Fig. 1(a) and (b). Dense and tangled MWNTs distributed uniformly on the Ni foam surface with diameters of about 50 nm and lengths of up to 20 mm. Figs. 1(c) and (d) show the cross sections of MH electrodes. The high magnification image (d) shows that the straight long MWNTs interwove through the active material powders after the dry powder rolling process, enhancing clearly the conductive and mechanical performances between the active material and the Ni foam base. The electrochemical CV characteristics of the MWNT/Ni foam substrate were evaluated and compared with the Ni foam substrate. Fig. 2 shows the CV curves of the Ni foam and MWNT/Ni foam electrodes at the scan rate of 20 mV/s. After the MWNT growth, the redox peak current increased about 112%. This implies that the MWNT/Ni foam electrode possesses higher electrochemical reaction activity than the Ni foam electrode due to better conductivity. In addition, the MWNT/Ni foam electrode exhibited higher charge storage capability, which involves the intercalation and deintercalation of protons, mainly ascribed to the pseudocapacitance from the Faradic process. The chargeedischarge curves for batteries A and B under test conditions of 0.2 C charging current density, 0.4 C discharging current density, and 0.1 V cut-off voltage are displayed in Fig. 3. The specific discharge capacities of batteries A and B were 360.9 mAh/g and 303.9 mAh/g, respectively, with the discharging potential plateaus of 1.26 V and 1.23 V correspondingly. Higher discharging potential plateau is associated with better discharge performance and higher discharge
Fig. 2 e The CV curves of Ni foam and MWNT/Ni foam electrodes.
potential [5]. Thus, the directly grown MWNTs improved the battery discharge performances evidently. Fig. 4(a) shows the cycle stability performances for two types of batteries. In the initial stage, the discharge capacities increased quickly as the cycle number until the batteries were fully activated at the 17th and 14th cycles, with the maximum discharge capacities of 360.9 mAh/g and 306.8 mAh/g, respectively, for batteries A and B. The significant improvement of the maximum discharge capacity for battery A was attributed to the formation of conducting network within the electrode [1] and the increase of the electrochemical reaction sites with the presence of MWNTs. The discharge capacity
Fig. 1 e SEM images of MWNT grown Ni foam substrate (a) and uniform surface MWNTs (b). Cross sections of the negative electrode with MWNT interwove through the active material under low (c) and high (d) SEM magnifications.
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Fig. 3 e Chargeedischarge curves for batteries A and B. declines afterward for both batteries were related to the loosing and falling off of the active material [1] and the formation of corrosions [6]. The discharge capacity dropped 41% from its peak value after 500 cycles for the battery A, but lost 56.5% after 300 cycles for the battery B. Apparently, the capacity loss was smaller and the discharge performance was more stable for the battery A. Better cycle stability was associated with stronger adhesion of the active material to the substrate and better electric conductivity from the directly grown MWNTs. By the way, the decline of the discharge capacity could be greatly improved for a sealed unit [1]. In another group of stability test, we added 5% PTFE binder in the active material for both A and B type batteries (Fig. 4(b)). The maximum discharge specific capacities reached 302.9 mAh/g and 302 mAh/g, respectively, after 34 and 35 activation cycles for batteries A and B. The discharge specific capacities declined only 10.8% after 220 cycles for the MWNT grown battery, but dropped quickly from 85th cycle for the pure Ni foam battery. Comparing two groups of tests, it is clear that adding PTFE assisted the improvement of the cycle stability due to the enhancement of the cohesion between the electrode active material. However, with the addition of PTFE, we speculate that the active material particles tended to aggregate into large cluster in the electrolyte after a series of chargeedischarge cycles. Once the active material started
Fig. 5 e Microstructure illustration of MWNT integrated electrode.
falling off, severe discharge capacity decline could happen, causing the quick drop for the battery B. This drawback was overcome effectively by the directly grown MWNT network, which was able to hold the active substances together to combine with the substrate tightly, against the falling off. On the other side, there was no evident improvement on the discharge specific capacity for the MWNT based battery. It is likely that the nonconductive PTFE blocked the charge transfer between the active material and MWNTs. The delay of the battery activation was also related to the weakening of the electric activity from the nonconductive PTFE. Thus, the MWNTs served more like structural supports in this situation. The directly grown MWNTs, which rooted into the Ni foam base, not only improve the charge transfer capability significantly as electronic conducting bridges [1], also enhance the mechanical strength of the electrode. Fig. 5 illustrates the microstructure and the charge transfer mechanism of the MWNT integrated electrode. Beyond the particle-particle-base charge transfer paths (path 1) for regular electrodes, the new particle-MWNT-base paths (path 2) are able to bring down the charge transfer resistance from shorter paths, better conductivity of MWNTs, and less contact resistance from the direct growth. Furthermore, the MWNT network is expected to
Fig. 4 e Cycle stability improvements with MWNT growth. (a) without PTFE, (b) with PTFE.
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Fig. 6 e CV (a) and EIS (b) curves of the two types of batteries. slow down the inner resistance increase and the active material loss, especially important for long term operation. The XRD pattern analysis showed that the active material components LaNi5 and Ni powder changed during the battery operation. However, the changes were smaller for type A electrodes, which confirmed the slower loss of the active material with the incorporation of MWNTs. Tsai et al. [6] investigated the chargeedischarge performance of carbon nanotube buckypaper/MmNi5 electrode in the open half-cell system, and the maximum discharge capacity was 276 mAh/ g and lost 55% after 200 cycles. Our directly grown MWNT based electrode showed superiority over the buckypaper structure. In another group of test, the active material powder size was reduced from a few hundred microns to sub-microns by the Planetary ball mill (QM-3SP04), resulting in shorter activation of about 5 cycles at the cost of cycle stability degradation. It seems that nanoscale active material powders favor the charge transfer, but are apt to loosen due to the increase of the specific contact surface with the electrolyte. Fig. 6(a) shows the CV curves of batteries A and B at a scan rate of 5 mv/s after the activation. A pair of cathodic and anodic current peaks can be found in both curves. The anodic peak was assigned to the oxidation reaction of metallic nickel. It can be seen that the anodic peak currents varied, 0.181 A (at 0.549 V) and 0.106 A (at 0.523 V) for batteries A and B, respectively. Higher peak current and larger integral peak area from the battery A indicate better electrocatalytic activity [16], favoring the chargeedischarge reactions. This also implies that the contact resistance between the active material and the Ni foam substrate could be reduced with the increases of the surface electrochemical active sites from the interwoven MWNTs [9]. Fig. 6(b) displays EIS Nyquist plots for two types of batteries with the inset showing the equivalent circuit to fit the EIS data. Each curve consists of one semicircle at the highfrequency region and a slope line at the low-frequency region attributed to Warburg hydrogen diffusion (Zw) in the working electrode [6,9,13,15]. The semicircle diameter provides the charge transfer resistance (Rr) information, i.e., the smaller the diameter is, the lower the resistance. The charge transfer resistance mainly consists of the contact resistance between the current collector and the active material, the particle-toparticle resistance, and the interface resistance between the electrode and electrolyte (R1). By fitting the EIS data using an
equivalent circuit, the charge transfer resistances are found to be 0.15 U and 0.24 U, respectively, for batteries A and B. Significant improvement of the electric conductivity benefited mainly from the increase of the accessible surface area and the electronic conducting bridge effect with the addition of MWNTs, agreeing with other investigations [13,17,18].
Conclusions Significant progresses on various electrochemical properties have been made after the direct growth of MWNTs for the electrodes of Ni-MH battery. The redox peak current and the specific discharge capacity increased about 112% and 18%, respectively, attributed to the improvements on electrochemical reaction efficiency and the charge transfer conductivity. EIS analysis showed that the charge transfer resistance of the electrode dropped 37.5% from 0.15 U to 0.24 U with the addition of MWNT layer. After the addition of PTFE binder, the MWNT incorporated cells exhibited excellent discharge stability with the decay of only 10.8% after 220 cycles, implying that the interwoven MWNTs enhanced the adhesion of the active material to the substrate as structural supports. This MWNT/Ni foam structure is also very attractive for other applications, including supercapacitor [11,12] and electrochemical sensor [9].
Acknowledgement This research is partly financially supported by the National Natural Science Foundation of China (11274244, 61125101). The authors thank Prof. Zhi Yang of Shanghai Jiaotong University and Ke Xie of The University of Melbourne for helpful discussions.
references
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[17] Wu JB, Tu JP, Yu Z, Zhang XB. Electrochemical investigation of carbon nanotubes as additives in positive electrodes of Ni/ MH batteries. J Electrochem Soc 2006;153:A1847e51. [18] Zhao D, Yang Z, Zhang L, Feng X, Zhang Y. Electrodeposited manganese oxide on nickel foamesupported carbon nanotubes for electrode of supercapacitors. Electrochem Solid-State Lett 2011;14:A93e6. [19] Ruiz FC, Castro EB, Real SG, Peretti HA, Visintin A, Triaca WE. Electrochemical characterization of AB2 alloys used for negative electrodes in Ni/MH batteries. Int J Hydrog Energy 2008;33:3576e80. [20] Boussami S, Khaldi C, Lamloumi J, Mathlouthi H, Takenouti H. Electrochemical study of LaNi3.55Mn0.4Al0.3Fe0.75 as negative electrode in alkaline secondary batteries. Electrochim Acta 2012;69:203e8. [21] Ruiz FC, Martı´nez PS, Castro EB, Humana R, Peretti HA, Visintin A. Effect of electrolyte concentration on the electrochemical properties of an AB5-type alloy for Ni/MH batteries. Int J Hydrog Energy 2013;38:240e5. [22] Srivastava S, Upadhyay RK. Investigations of AB5-type negative electrode for nickel-metal hydride cell with regard to electrochemical and microstructural characteristics. J Power Sources 2010;195:2996e3001. [23] Dresselhaus MS, Dresselhaus G, Eklund PC. Science of fullerenes and carbon nanotubes: their properties and applications. Academic Press; 1996. [24] Hu J, Odom TW, Lieber CM. Chemistry and physics in one dimension: synthesis and properties of nanowires and nanotubes. Acc Chem Res 1999;32:435e45. a kova R, Orin a k A. Recent applications of carbon [25] Orin nanotubes in hydrogen production and storage. Fuel 2011;90:3123e40. [26] Santos VEO, Celante VG, Lelis MFF, Freitas MBJG. Chemical and electrochemical recycling of the nickel, cobalt, zinc and manganese from the positives electrodes of spent Ni-MH batteries from mobile phones. J Power Sources 2012;218:435e44. n J, Herna ndez JC. Metal hydride electrodes [27] Soria ML, Chaco and Ni/MH batteries for automotive high power applications. J Power Sources 2001;102:97e104. [28] Pandey SK, Singh RK, Srivastava ON. Investigations on hydrogenation behaviour of CNT admixed Mg2Ni. Int J Hydrog Energy 2009;34:9379e84. [29] Xiong J, Dong X, Dong Y, Hao X, Hampshire S. Dualproduction of nickel foam supported carbon nanotubes and hydrogen by methane catalytic decomposition. Int J Hydrog Energy 2012;37:12307e16. [30] Thomas JE, Andreasen G, Arenillas A, Zubizarreta L, Barath P, M, et al. Effect of carbon support on the kinetic Sedları´kova behaviour of a metal hydride electrode. Electrochim Acta 2009;54:2010e7. [31] Hsieh C-T, Chou Y-W, Lin J-Y. Fabrication and electrochemical activity of Ni-attached carbon nanotube electrodes for hydrogen storage in alkali electrolyte. Int J Hydrog Energy 2007;32:3457e64.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Electrochemical properties of nickel-metal hydride battery based on directly grown multiwalled carbon nanotubes Fei Xie a, Changkun Dong a,b,*, Weijin Qian a, Ying Zhai a, Li Li a, Detian Li a,b a
Institute of Micro-nano Structures & Optoelectronics, Wenzhou University, Chashan University Town, Wenzhou, Zhejiang 325035, China b Science and Technology on Vacuum & Cryogenics Technology and Physics Laboratory, Lanzhou Institution of Physics, Lanzhou, China
article info
abstract
Article history:
The electrochemical properties of nickel-metal hydride (Ni-MH) batteries with directly
Received 6 March 2015
grown multi-walled carbon nanotubes (MWNTs) have been investigated. MWNTs inter-
Received in revised form
wove through active material powders to benefit the battery in three aspects: enhancing
3 May 2015
the electrode mechanical strength by clutching the active material to the substrate,
Accepted 8 May 2015
improving the electrochemical activity from the increase of the reaction sites, and reducing
Available online 4 June 2015
the charge transfer resistance as conducting bridges. The battery with MWNTs exhibited excellent electrochemical properties with the maximum discharge specific capacity of
Keywords:
360.9 mAh/g, 18% higher than the electrode without MWNTs, and EIS analysis showed that
Carbon nanotube
the charge transfer resistances dropped 37.5%. Meanwhile, the chargeedischarge cycle
Ni/MH battery
stability was improved dramatically with the additions of both MWNTs and PTFE binder,
Direct growth
declining 10.8% only after 220 cycles. This MWNT/Ni foam structure is also very promising
Electrochemical property
for other electrochemical applications.
Electronic conducting bridge
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The Ni-MH batteries are used in a wide range of electric products, including portable electronic equipments, electrical tools, and hybrid electric vehicles (HEVs) due to the superiorities of high power density, low cost, long lifetime, and low environmental pollution [1e3,14,15,19e22,27e30]. The Ni-MH battery consists mainly of the metal hydride (MH) negative
electrode, the Ni(OH)2 positive electrode, the diaphragm, and the electrolyte [15,26]. Research and development efforts have involved every aspects of the battery cell. Ruiz et al. studied the effects of the electrolyte concentration on the battery electrochemical performance, and results showed that the 6 M and 8 M electrodes exhibited quicker activation feature with highest discharge specific capacity (~325 mAh/g) [21]. The electrodes are generally fabricated by the wet-pasting process by mixing the metal hydride (MH) active material powders
* Corresponding author. Institute of Micro-nano Structures & Optoelectronics, Wenzhou University, Chashan University Town, Wenzhou, Zhejiang 325035, China. Tel.: þ86 577 86689067. E-mail address: [email protected] (C. Dong). http://dx.doi.org/10.1016/j.ijhydene.2015.05.054 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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with the polymer binder and water to make the slurry to paste onto the electrode substrate. However, severe oxidation could occur during the drying process, resulting in lower electrode capacity and shorter cycle life. Although various methods have been proposed to prevent the oxidation of the active material, such as fabrication under inert atmospheres, they are either not very effective or too expensive [1]. Ye et al. reported a dry powder roller pressing process to make electrodes with significant improvements on the cycle life and discharging specific capacity [2]. In order to improve the high temperature and time-consuming drying process, Zhong et al. fabricated the dry powder electrodes with the addition of 5 wt.% conductive binder, a mixture of nickel power, graphite powder, and polytetrafluoroethylene (PTFE) powder in the weight ratio 1:1:1 [1]. If enough active material is supplied on the positive electrode, the specific capacity of the Ni-MH battery depends mainly on the performance of the negative electrode [3]. AB5 type alloys are commonly adopted as the active material in the negative electrode [15,20,22]. Sumita et al. investigated AB5 type alloy Mm0.75Ti0.05La0.2Ni3.7Al0.38Co0.3Mn0.5Mo0.02 with the addition of Ti content by radio-frequency induction melting, and showed significant improvement on the discharge capacity [22]. Especially, the improvements on the adhesion and charge transfer conductivity between the active material and the substrate play crucial roles to enhance the battery performances, including the chargeedischarge capacity and cycle life. Since the discovery in 1991 [10], carbon nanotubes (CNTs) have attracted a great deal of attention for electrochemical hydrogen storage due to their unique structural, mechanical, thermal, electronic, and electrochemical properties [4,23e25,28,30,31]. The applications of CNTs on the metal hydride electrode are investigated extensively. Sedlar´ıkova et al. studied the electrochemical properties of Ni/MH batteries with various types of CNTs based negative electrodes [30]. Lv et al. [5] added different amounts of CNTs (0.5, 1, and 2 wt.%) into the powder mixture of Ni(OH)2, CoO, and Co in the positive electrode and assembled type-AA batteries by a wetpasting process. Under high rate discharge conditions, the batteries showed improvements on the cycle stability, the discharge voltage plateau, the internal resistance, and the high-rate capability. Zhang et al. [7] made high-power CNT compound Ni-MH batteries by mixing MWNTs into the LaNi5 alloy powder on a mass ratio of 0.5%, 0.8%, and 1%, respectively, in the negative electrodes. The weight of the active material mixture was (17.0 ± 0.2)g for each negative electrode. The battery reached the capacity of 3369 mAh with 3000 mA charging current and 6000 mA discharging current, and the cycle life exceeded 600 times. Wu et al. developed nickel boride-coated carbon nanotube films by the electrophoresis deposition [4]. After treated with nitric acid for 5 h and coated with 45 wt.% of Ni2B, this film showed excellent high rate capability and cycle stability. Tsai et al. [6] coated La-rich AB5 hydrogen storage alloy on buckypapers by the direct current magnetron sputtering, and the maximum discharge capacity of 276 mAh/g was reached. Mazdak Hashempour et al. [8] grown directly MWNTs on the 316 stainless steel by chemical vapor deposition (CVD), and the MWNT film presented strong bonding and low contact resistance with the substrate. Dong et al. [9] synthesized 3D graphene-CNT hybrids by two-
step CVD, and showed that the electron-transfer resistances of the 3D graphene-CNT hybrid, the bare 3D graphene, the MWNT modified glassy carbon, and the glassycarbon electrode were 10.4, 22.8, 34.0, and 76.3 U, respectively. Du et al. [16] synthesized a series of Co/CNT negative electrode based alkaline rechargeable batteries, and the results showed that the Co-CNT composite with weight ratio of 5:1 displayed optimized electrochemical properties. In this work, we fabricated the electrodes by the dry powder roller pressing process after grown MWNTs directly on Ni foam substrates by CVD without extra catalyst layer. The structural and electrochemical properties of this novel battery structure were characterized by scanning electron microscope (SEM), X-Ray Diffraction (XRD), Cyclic voltammetry (CV), and Electrochemical impedance spectroscopy (EIS) on open halfcell systems. Significant improvements on the discharge specific capacity and cycle stability were achieved, benefited mainly from the excellent structural and conductive superiorities.
Experimental In the fabrication of the MWNT grown electrodes, the MWNTs were synthesized directly on the Ni foams of 2.7 2.7 cm2 by CVD at 650 C from source gases acetylene/argon at flow rates of 20/200 sccm for 10 min. In our experiments, the commercial La-rich AB5 alloy powder from Sihui City Double Win Industry CO was employed as the active material, which was mixed with Ni powder on a mass rate of 1:1 to produce the negative electrode. The mixture was hand lapped 10 min in the mortar to ensure uniform hybrid, and was pressed under the pressure of 30 MPa at room temperature to form a composite plate of 0.6 mm thick. The composite plate was then sandwiched with two MWNT grown Ni foams to form the negative electrode A. For the comparison, the electrode B was produced followed the same process using two pure Ni foam substrates. The positive electrode was made from the active material Ni(OH)2 and the conductive agent CoO using the same process. The microstructures were characterized by SEM (JEOL JMS6700F) measurements. All electrodes were pieced together into an open half-cell battery (A and B) based on the electrolyte KOH:NaOH:LiOH with ratio of 6M:1M:0.5M. The chargeedischarge properties were investigated in a battery test station at room temperature. In order to study the discharge capacity and cycle stability, the cells were charged at 0.1 C for 6 h, and discharged at 0.2 C with a cut-off voltage of 0.1 V. After 10 activation cycles, the chargeedischarge currents were doubled. A three-electrode system (battery A and B)with a Hg/ HgO reference electrode, an MH electrode working electrode, and a Ni(OH)2 counter electrode were introduced to conduct the electrochemical property measurements on the electrochemical workstation (German Zahner CO., Zennium). CV performance of the battery was tested in the potential range of 0.6 V to 1 V with five scan rates (5, 10, 20, 50, 100 mV/s). EIS measurements were scanned over a frequency range of 1 MHze1 mHz at equilibrium conditions with a superimposed sinusoidal voltage signal of 5 mV amplitude. The CV and EIS measurements were performed after 10 charging-discharging activation cycles.
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Results and discussions The SEM images of Ni foams after CNT growth are shown in Fig. 1(a) and (b). Dense and tangled MWNTs distributed uniformly on the Ni foam surface with diameters of about 50 nm and lengths of up to 20 mm. Figs. 1(c) and (d) show the cross sections of MH electrodes. The high magnification image (d) shows that the straight long MWNTs interwove through the active material powders after the dry powder rolling process, enhancing clearly the conductive and mechanical performances between the active material and the Ni foam base. The electrochemical CV characteristics of the MWNT/Ni foam substrate were evaluated and compared with the Ni foam substrate. Fig. 2 shows the CV curves of the Ni foam and MWNT/Ni foam electrodes at the scan rate of 20 mV/s. After the MWNT growth, the redox peak current increased about 112%. This implies that the MWNT/Ni foam electrode possesses higher electrochemical reaction activity than the Ni foam electrode due to better conductivity. In addition, the MWNT/Ni foam electrode exhibited higher charge storage capability, which involves the intercalation and deintercalation of protons, mainly ascribed to the pseudocapacitance from the Faradic process. The chargeedischarge curves for batteries A and B under test conditions of 0.2 C charging current density, 0.4 C discharging current density, and 0.1 V cut-off voltage are displayed in Fig. 3. The specific discharge capacities of batteries A and B were 360.9 mAh/g and 303.9 mAh/g, respectively, with the discharging potential plateaus of 1.26 V and 1.23 V correspondingly. Higher discharging potential plateau is associated with better discharge performance and higher discharge
Fig. 2 e The CV curves of Ni foam and MWNT/Ni foam electrodes.
potential [5]. Thus, the directly grown MWNTs improved the battery discharge performances evidently. Fig. 4(a) shows the cycle stability performances for two types of batteries. In the initial stage, the discharge capacities increased quickly as the cycle number until the batteries were fully activated at the 17th and 14th cycles, with the maximum discharge capacities of 360.9 mAh/g and 306.8 mAh/g, respectively, for batteries A and B. The significant improvement of the maximum discharge capacity for battery A was attributed to the formation of conducting network within the electrode [1] and the increase of the electrochemical reaction sites with the presence of MWNTs. The discharge capacity
Fig. 1 e SEM images of MWNT grown Ni foam substrate (a) and uniform surface MWNTs (b). Cross sections of the negative electrode with MWNT interwove through the active material under low (c) and high (d) SEM magnifications.
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Fig. 3 e Chargeedischarge curves for batteries A and B. declines afterward for both batteries were related to the loosing and falling off of the active material [1] and the formation of corrosions [6]. The discharge capacity dropped 41% from its peak value after 500 cycles for the battery A, but lost 56.5% after 300 cycles for the battery B. Apparently, the capacity loss was smaller and the discharge performance was more stable for the battery A. Better cycle stability was associated with stronger adhesion of the active material to the substrate and better electric conductivity from the directly grown MWNTs. By the way, the decline of the discharge capacity could be greatly improved for a sealed unit [1]. In another group of stability test, we added 5% PTFE binder in the active material for both A and B type batteries (Fig. 4(b)). The maximum discharge specific capacities reached 302.9 mAh/g and 302 mAh/g, respectively, after 34 and 35 activation cycles for batteries A and B. The discharge specific capacities declined only 10.8% after 220 cycles for the MWNT grown battery, but dropped quickly from 85th cycle for the pure Ni foam battery. Comparing two groups of tests, it is clear that adding PTFE assisted the improvement of the cycle stability due to the enhancement of the cohesion between the electrode active material. However, with the addition of PTFE, we speculate that the active material particles tended to aggregate into large cluster in the electrolyte after a series of chargeedischarge cycles. Once the active material started
Fig. 5 e Microstructure illustration of MWNT integrated electrode.
falling off, severe discharge capacity decline could happen, causing the quick drop for the battery B. This drawback was overcome effectively by the directly grown MWNT network, which was able to hold the active substances together to combine with the substrate tightly, against the falling off. On the other side, there was no evident improvement on the discharge specific capacity for the MWNT based battery. It is likely that the nonconductive PTFE blocked the charge transfer between the active material and MWNTs. The delay of the battery activation was also related to the weakening of the electric activity from the nonconductive PTFE. Thus, the MWNTs served more like structural supports in this situation. The directly grown MWNTs, which rooted into the Ni foam base, not only improve the charge transfer capability significantly as electronic conducting bridges [1], also enhance the mechanical strength of the electrode. Fig. 5 illustrates the microstructure and the charge transfer mechanism of the MWNT integrated electrode. Beyond the particle-particle-base charge transfer paths (path 1) for regular electrodes, the new particle-MWNT-base paths (path 2) are able to bring down the charge transfer resistance from shorter paths, better conductivity of MWNTs, and less contact resistance from the direct growth. Furthermore, the MWNT network is expected to
Fig. 4 e Cycle stability improvements with MWNT growth. (a) without PTFE, (b) with PTFE.
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Fig. 6 e CV (a) and EIS (b) curves of the two types of batteries. slow down the inner resistance increase and the active material loss, especially important for long term operation. The XRD pattern analysis showed that the active material components LaNi5 and Ni powder changed during the battery operation. However, the changes were smaller for type A electrodes, which confirmed the slower loss of the active material with the incorporation of MWNTs. Tsai et al. [6] investigated the chargeedischarge performance of carbon nanotube buckypaper/MmNi5 electrode in the open half-cell system, and the maximum discharge capacity was 276 mAh/ g and lost 55% after 200 cycles. Our directly grown MWNT based electrode showed superiority over the buckypaper structure. In another group of test, the active material powder size was reduced from a few hundred microns to sub-microns by the Planetary ball mill (QM-3SP04), resulting in shorter activation of about 5 cycles at the cost of cycle stability degradation. It seems that nanoscale active material powders favor the charge transfer, but are apt to loosen due to the increase of the specific contact surface with the electrolyte. Fig. 6(a) shows the CV curves of batteries A and B at a scan rate of 5 mv/s after the activation. A pair of cathodic and anodic current peaks can be found in both curves. The anodic peak was assigned to the oxidation reaction of metallic nickel. It can be seen that the anodic peak currents varied, 0.181 A (at 0.549 V) and 0.106 A (at 0.523 V) for batteries A and B, respectively. Higher peak current and larger integral peak area from the battery A indicate better electrocatalytic activity [16], favoring the chargeedischarge reactions. This also implies that the contact resistance between the active material and the Ni foam substrate could be reduced with the increases of the surface electrochemical active sites from the interwoven MWNTs [9]. Fig. 6(b) displays EIS Nyquist plots for two types of batteries with the inset showing the equivalent circuit to fit the EIS data. Each curve consists of one semicircle at the highfrequency region and a slope line at the low-frequency region attributed to Warburg hydrogen diffusion (Zw) in the working electrode [6,9,13,15]. The semicircle diameter provides the charge transfer resistance (Rr) information, i.e., the smaller the diameter is, the lower the resistance. The charge transfer resistance mainly consists of the contact resistance between the current collector and the active material, the particle-toparticle resistance, and the interface resistance between the electrode and electrolyte (R1). By fitting the EIS data using an
equivalent circuit, the charge transfer resistances are found to be 0.15 U and 0.24 U, respectively, for batteries A and B. Significant improvement of the electric conductivity benefited mainly from the increase of the accessible surface area and the electronic conducting bridge effect with the addition of MWNTs, agreeing with other investigations [13,17,18].
Conclusions Significant progresses on various electrochemical properties have been made after the direct growth of MWNTs for the electrodes of Ni-MH battery. The redox peak current and the specific discharge capacity increased about 112% and 18%, respectively, attributed to the improvements on electrochemical reaction efficiency and the charge transfer conductivity. EIS analysis showed that the charge transfer resistance of the electrode dropped 37.5% from 0.15 U to 0.24 U with the addition of MWNT layer. After the addition of PTFE binder, the MWNT incorporated cells exhibited excellent discharge stability with the decay of only 10.8% after 220 cycles, implying that the interwoven MWNTs enhanced the adhesion of the active material to the substrate as structural supports. This MWNT/Ni foam structure is also very attractive for other applications, including supercapacitor [11,12] and electrochemical sensor [9].
Acknowledgement This research is partly financially supported by the National Natural Science Foundation of China (11274244, 61125101). The authors thank Prof. Zhi Yang of Shanghai Jiaotong University and Ke Xie of The University of Melbourne for helpful discussions.
references
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