Electrochemical effect of carbon nanospheres on an AB5 alloy

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 ) 1 4 9 7 8 e1 4 9 8 2

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Electrochemical effect of carbon nanospheres on an AB5 alloy D.J. Cuscueta a,*, H.L. Corso b, A. Arenillas c, P.S. Martinez b, A.A. Ghilarducci a, H.R. Salva a a

CNEA, Instituto Balseiro-UNCu, CONICET, Av. Bustillo 9500, 8400 San Carlos de Bariloche, Argentina CNEA, Av. Bustillo 9500, 8400 San Carlos de Bariloche, Argentina c Instituto Nacional del Carbo´n, INCAR-CSIC, Apartado 73, 33080 Oviedo, Spain b

article info

abstract

Article history:

In this paper, the effect of the addition of carbon nanospheres (CNE) by high energy

Received 26 August 2011

mechanical milling to the active material of the negative electrode of a NickeleMetal

Accepted 13 December 2011

Hydride (NieMH) battery is studied. The hydrogen storage alloy (MH) is an AB5-type

Available online 20 January 2012

(Lm0.95Ni3.8Co0.3Mn0.3Al0.4), where Lm is a mixture of rare earths, and it was prepared by melting the constituent elements in an electric induction furnace. The CNE were synthe-

Keywords:

sized by two-stage polymerization of furfuryl alcohol, drying and carbonization, and were

NieMH batteries

subsequently doped with different proportions of Ni (0e20%) by the method of wet

AB5 alloy

impregnation with Nickel nitrate hexahydrate. The behavior of electrochemical cells

Carbon nanospheres

consisting of a working electrode with 45% of MH, 5% of CNE pure and doped with different

Niedoped

proportions of Ni and 50% carbon Teflon acting as electrical and mechanical support,

Electrochemical capacity

a counter electrode of Ni mesh, and a reference electrode Hg/HgO was studied. The experiments covered electrochemical activation and cycle stability, rate capability and electrochemical impedance spectroscopy. The results show that the addition of CNE improves the electrochemical performance, and particularly the CNEs pure and doped with 20% of Ni present the best electrochemical performance in discharge capacity, rate capability and impedance spectroscopy. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

In recent years, NickeleMetal Hydride (NieMH) rechargeable batteries are used as power sources of various portable devices such as digital cameras or cell phones. They have advantageously replaced the nickelecadmium battery especially for environmental reasons. Although the capacity of a NieMH battery is limited by the nickel electrode, this research is basically oriented to improve the performance of cycling behavior, rate capability and kinetic behavior during charge and discharge of the metal hydride electrode. Many works have been published [1e4] where metal hydrides have been doped with different materials in order to

improve the performance of NieMH batteries. Generally, the addition of dopants has a catalytic effect that enhances kinetics reaction, modifies the charge and discharge parameters or improves the cycling behavior or rate capability. In this work, an AB5-type hydride forming alloy was used to test it with the addition of carbon nanospheres (CNE), pure or doped with different percentages of Nickel. The analyzed samples were prepared by mechanical milling of the alloy mixed with different CNE. They were characterized by X-ray diffraction to determine the phases present. Cell electrodes were prepared to characterize their electrochemical behavior by testing the capacity during cycling, the rate capability, electrochemical impedance and cell potential.

* Corresponding author. E-mail address: [email protected] (D.J. Cuscueta). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.12.101

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2.

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Experimental

The active material of the negative electrode is an AB5-type alloy of composition Lm0.95Ni3.8Co0.3Mn0.3Al0.4, where Lm is a mixture of rare earths. It was prepared by melting the constituent elements inside a Quartz capsule under Ar atmosphere in a LEPEL electric induction furnace. The CNE were synthesized by two-stage polymerization of furfuryl alcohol, drying and carbonization. They were subsequently doped with different proportions of Ni (0e20%) by the method of wet impregnation with Nickel nitrate. Detailed description of the synthesis of these carbon materials can be found elsewhere [5,6]. The different active materials (alloy only, alloy with pure CNE and alloy with Ni doped CNE) were mixed in a SPEX8000D high energy mill using 3 steel balls of 1 g each, with a balls/ material mass ratio of 10:1. The alloy mass used in each milling was 300 mg and the alloy/CNE mass ratio was also 10:1. The composition and the label of the different studied samples are presented in Table 1. The working negative electrodes were prepared with 100 mg of sample mixed with equal amount of teflonized carbon black (Vulcan XC-72 with 33 wt% PTFE) as mechanical support, and the mixture was then pressed into a cylindrical matrix under a pressure of 300 MPa at room temperature. A Ni mesh was used as counter electrode, Hg/HgO as reference electrode and a solution of KOH 6 M as electrolyte. The electrochemical measurements were carried out at room temperature, by charging the cells with a current of 150 mA/g for 144 min, then 10 min of rest and subsequently discharging them at 150 mA/g up to the cut off potential of 0.6 V. The measurements were performed using a potentiostat designed and built in our laboratories, except for the electrochemical impedance spectroscopy measurements (EIS), which were carried out on an Autolab equipment with an excitation of 6 mV rms from 10 kHz to 2 mHz.

3.

Results and discussion

3.1.

Characterization of materials

Fig. 1 e SEM image of AB5 alloy.

micrograph shows two different areas by contrast, indicating the inhomogeneity in the composition produced during melting of the constituent elements. The overall elemental analysis was performed in a large area of the sample (180 mm  120 mm) and some punctual analysis (1 mm  1 mm) were also done to determine the specific composition in different zones. Unlike the observed in Fig. 1, the elemental analysis revealed a similar composition in the analyzed points, although it differs from the target composition. In general, a higher concentration is detected for all elements, except for Ni, that shows a reduction from 56 wt % to 32 wt % of the measured value with respect to the target one. Fig. 2 shows the X-ray diffractograms of the AB5 and AB5M samples, where the peaks positions of the found phases are indicated for reference. The LaNi3.94Al1.06 phase, which has hexagonal structure and belongs to the P6/mmm space group, appears with higher intensity peaks. The LaNi phase has orthorhombic structure and belongs to the Cmcm space group. Fig. 2 also shows that the AB5 alloy is dominated by the LaNi3.94Al1.06 but also presents a different phase, identified as

Fig. 1 shows a SEM image obtained by backscattered electrons from the AB5 sample before milling and hydriding. The

Table 1 e Composition and label of samples. AB5 AB5M NE0 NE10 NE15 NE20 AB5NE0 AB5NE10 AB5NE15 AB5NE20

AB5 alloy milled AB5 alloy Nanospheres without Ni Nanospheres with 10 wt% of Ni Nanospheres with 15 wt% of Ni Nanospheres with 20 wt% of Ni AB5 milled with NE0 AB5 milled with NE10 AB5 milled with NE15 AB5 milled with NE20

Fig. 2 e AB5 and AB5M diffractograms.

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Fig. 3 e Diffractograms of NE0 and AB5NE0.

Fig. 5 e Detail of a CNE doped with 10 wt% of Ni added on an AB5 particle.

the LaNi intermetallic. This confirms the inhomogeneities presented in Fig. 1. As there were no more peaks after the mechanical milling, it is probably that no more phases were created during the high-energy milling. However, the increase in the background indicates the amorphous state of the material, making it difficult to distinguish the low intensity peaks from the background. Figs. 3 and 4 show the comparison between only nanospheres (NE0 and NE20) and after milling together with the AB5 alloy. It is observed that NE0 only shows the carbon background, while in NE20 are clearly distinguished 3 peaks of the Ni-doping phase. The diffractograms of the mixtures show the predominance of the AB5 alloy peaks, but also no new phases associated to new peaks. Fig. 5 shows a TEM image of the AB5NE10 sample. On the edges of the alloy particle, characterized by an irregular and dark shape, can be seen some spheres attached. The arrow indicates a zoom of the sphere, where the added Ni particles can be distinguished. Fig. 5 confirms that the mixing process by mechanical milling has joined the AB5 alloy particles and nanospheres without totally breaking the structure of carbon spheres.

It was studied the electrochemical behavior of cells consisting of a working electrode prepared with 45 wt% of MH, 5 wt% of CNE pure and doped with different proportions of Ni and 50 wt % of teflonized carbon, which acts as electrical and mechanical support, a counter electrode of Ni mesh, and a Hg/HgO reference electrode. Fig. 6 shows the first electrochemical charge-discharge cycles of the cells. The maximum discharge capacity, 170.4 mAh/g, was reached by the AB5NE20 electrode at cycle 6. Fig. 6 also shows that all samples with the addition of nanospheres improved the electrochemical activation, attaining its nominal capacity at cycle 3. It is important to note that the specific discharge capacity of electrodes with nanospheres was calculated considering the total weight of material, i.e. the alloy and the nanospheres. Fig. 7 shows that the rate capability behavior is similar for all electrodes, with a strong reduction of capacity for rates higher than 0.1C, followed by a plateau capacity up to 1C and then a constant decrease of capacity for higher rates, showing the inability of the electrodes to deliver the stored energy at

Fig. 4 e Diffractograms of NE20 and AB5NE20.

Fig. 6 e Activation and cyclic stability.

3.2.

Electrochemical characterization

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Fig. 9 e Impedance at 75% of SOC.

Fig. 7 e Rate capability behavior.

the required rate. While all the electrodes have a similar behavior, the AB5NE0 one presents the highest capacity at all discharge rates. Fig. 8 shows the discharge potential referred to the Hg/HgO electrode. It shows that the constant potential zone, where the chemical reaction of dehydriding occurs, is similar for all electrodes except for the AB5NE10 one, which is slightly lower. The discharge capacity obtained for each electrode is superior to that obtained in the activation and cycle stability studies because this study was performed at a C/10 discharge rate, while the other at C/2. Fig. 9 shows the results of electrochemical impedance spectroscopy (EIS) at a SOC of 75%. The details in the figure show an enlargement of the high frequencies zone. The frequency response corresponds to a curve that is determined by the superposition of many physical processes occurring during the hydridring/dehydriding of the electrode. Wang et al. [7] propose that the impedance of the negative electrode is controlled by a series of 3 processes: the charge transfer reaction, the hydrogen transfer between the absorbed and adsorbed state and the hydrogen diffusion in the bulk of the electrode. The first two processes are visualized by semicircles, while the diffusion is determined by a straight line at about 45 .

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The resistance from the origin of the graph (0,0) to the start of the impedance curves is due to the contact resistance and electrolyte of the cell. The analysis of Fig. 9 shows that the AB5NE20 electrode presents best behavior at low frequencies, followed by the AB5NE0, while for higher frequencies, the AB5 electrode presents the lower contact resistance, possibly due to the contact through metal particles being better than contact through carbon particles such as nanospheres.

4.

Conclusions

The electrochemical behavior of carbon nanospheres added on an AB5-type hydrogen storage alloy for negative electrodes of NieMH batteries have been studied. The samples were crystallographic and metallographically characterized before and after high-energy milling. The maximum electrochemical discharge capacity of 170.4 mAh/g was obtained for the alloy with nanospheres doped with 20 wt% of Ni in the 6th charge-discharge cycle. All samples doped with nanospheres showed an improved behavior in activation, reaching its nominal capacity in the 3rd cycle. Although all samples show similar behavior in rate capability, the AB5NE0 material presented higher discharge capacity for all discharge rates. During the electrochemical impedance measurements, the AB5NE20 followed by the AB5NE0 electrodes show better performance at low frequencies, while AB5 presented the lower contact resistance.

Acknowledgments

Fig. 8 e Discharge potential against reference electrode.

The authors wish to express the financial support provided by the Agencia Nacional de Promocio´n Cientı´fica y Tecnolo´gica de Argentina, through the Project PAE-PICT-2007-02164. Also, they are thankful to Dr. S. Moreno for TEM images, and Dr. H.A. Peretti for the manuscript revision.

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references [4] [1] Visintin A, Peretti HA, Ruiz F, Corso HL, Triaca WE. J Alloys Compd 2007;428:244e51. [2] Thomas JE, Humana RM, Zubizarreta L, Arenillas A, Mene´ndez JA, Corso HL, et al. Energy Fuels 2010;24:3302e6. [3] Zubizarreta L, Arenillas A, Corso HL, Moreno MS, Pis JJ. Hybrid materials 2009. First international conference on

[5] [6] [7]

multifunctional, hybrid and nanomaterials; 15e19 March 2009. Tours, France. Zubizarreta L, Corso HL, Arenillas A, Moreno MS, Peretti HA, Mene´ndez JA, et al. Carbon 2009, The annual world conference on carbon, Biarritz, France; June 14e19, 2009. Zubizarreta L, Mene´ndez JA, Pis JJ, Arenillas A. Int J Hydrogen Energy 2009;34:3070e6. Zubizarreta L, Arenillas A, Pis JJ. Appl Surf Sci 2008;254: 3993e4000. Wang CJ. Electrochem Soc 1998;145:1801e12.