DSpace at VNU: Electrochemical Properties of LaNi5-xGax Alloys Used as the Negative Electrodes of Ni-MH Batteries

This article was downloaded by: [Moskow State Univ Bibliote] On: 23 December 2013, At: 08:29 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Analytical Letters Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/lanl20 Electrochemical Properties of LaNi5-xGax Alloys Used as the Negative Electrodes of Ni-MH Batteries Dam Nhan Ba c a c , Luu Tuan Tai Tuan & Tran Quang Huy a b a , Nguyen Phuc Duong , Chu Van d a International Training Institute for Material Science (ITIMS) - Hanoi University of Science and Technology (HUST) , Hanoi , Vietnam b Faculty of Physics - Hanoi University of Science, Vietnam National University (VNU) , Hanoi , Vietnam c Hung Yen University of Technology and Education, Khoai Chau , Hung Yen , Vietnam d National Institute of Hygiene and Epidemiology (NIHE) , Hanoi , Vietnam Accepted author version posted online: 19 Mar 2013.Published online: 25 Jul 2013 To cite this article: Dam Nhan Ba , Luu Tuan Tai , Nguyen Phuc Duong , Chu Van Tuan & Tran Quang Huy (2013) Electrochemical Properties of LaNi5-xGax Alloys Used as the Negative Electrodes of Ni-MH Batteries, Analytical Letters, 46:12, 1897-1909, DOI: 10.1080/00032719.2013.777920 To link to this article: http://dx.doi.org/10.1080/00032719.2013.777920 PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content This article may be used for research, teaching, and private study purposes Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, Downloaded by [Moskow State Univ Bibliote] at 08:29 23 December 2013 systematic supply, or distribution in any form to anyone is expressly forbidden Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/termsand-conditions Analytical Letters, 46: 1897–1909, 2013 Copyright # Taylor & Francis Group, LLC ISSN: 0003-2719 print=1532-236X online DOI: 10.1080/00032719.2013.777920 Electrochemistry Downloaded by [Moskow State Univ Bibliote] at 08:29 23 December 2013 ELECTROCHEMICAL PROPERTIES OF LaNi5-XGaX ALLOYS USED AS THE NEGATIVE ELECTRODES OF Ni-MH BATTERIES Dam Nhan Ba,1,3 Luu Tuan Tai,1,2 Nguyen Phuc Duong,1 Chu Van Tuan,3 and Tran Quang Huy4 International Training Institute for Material Science (ITIMS) - Hanoi University of Science and Technology (HUST), Hanoi, Vietnam Faculty of Physics - Hanoi University of Science, Vietnam National University (VNU), Hanoi, Vietnam Hung Yen University of Technology and Education, Khoai Chau, Hung Yen, Vietnam National Institute of Hygiene and Epidemiology (NIHE), Hanoi, Vietnam The effects of the substitution of nickel by gallium on the structures and the electrochemical properties of LaNi5-xGax (x ¼ 0.1À0.5) alloys were studied systematically The structure of the alloy was tested by X-ray diffraction (XRD) measurements Electrochemical properties and battery parameters were measured by bipotentiostat and battery tester equipment The results showed that when gallium is doped into alloys, the lattice of the LaNi5-xGax is slightly increased but retains the CaCu5 structure Gallium has a low melting temperature When gallium replaces nickel in the LaNi5 alloy, it covers material particles and reduces oxidation process, which leads to a longer lifetime and makes charge/discharge process more stable The shapes of electrochemical impedance spectroscopy measurements of all the LaNi5-xGax samples were similar, and the value increases as the substitution of Ni by Ga increases The cyclic voltammograms of all the LaNi5-xGax samples were similar to the one of pure LaNi5 For the same Ga-doped concentration and experimental conditions, the current density Jmax and charge quantity Q of the samples were increased cycle by cycle of charge/discharge Keywords: Cyclic voltammetry; Electrochemical impedance spectroscopy; Electrochemical properties; LaNi5; Ni-MH batteries INTRODUCTION Nickel-metal hydride (Ni-MH) batteries were discovered in the 1970s, and then launched into the market in the 1990s (Van Vucht, Kuijpers, and Bruning 1970; The Received 13 December 2012; accepted February 2013 Address correspondence to Dam Nhan Ba, Department of Basic Sciences, Hung Yen University of Technology and Education, Khoai Chau, Hung Yen, Vietnam E-mail: damnhanba@gmail com.vn 1897 Downloaded by [Moskow State Univ Bibliote] at 08:29 23 December 2013 1898 D N BA ET AL Economist 2008) These devices have become a clean alternative to the traditional technology of Ni=Cd (Linden and Reddy 2001) In low-weight electronic devices, Ni-MH batteries have been used to replace Ni=Cd ones because of their green advantage as well as a higher energy capacity According to Daniel and Besenhard (2011), hydride formation takes place by means of a discrete phase transition between a hydrogen-poor (0.1 H per metal atom) solidified solution and the hydrogen-rich hydride (0.6–1 H per metal atom) in these compounds Hydrogen was stored in the crystal lattice of material, and then this material became a clean energy reserve tank with minimal pollution to the environment (Linden and Reddy 2001) This feature has found many applications in science and engineering One of these applications is the negative electrode for Ni-MH rechargeable batteries (Cuevas et al 2001; Daniel and Besenhard 2011) The alloy discharge reaction involves two diffusion processes; one is the diffusion of H atom from alloy bulk to alloy surface, and the other is the diffusion of OH À from solution bulk to alloy surface This former process has been thoroughly investigated (Feng et al 2000; Kadir, Sakai, and Uehara 2000; Kohno et al 2000) Ni-MH batteries are largely used and their production increases rapidly from year to year, and research and development works on these batteries continue to grow (Klebanoff 2012) Especially, in order to improve the quality and to decrease the cost of Ni-MH batteries, many studies on the optimal composition in RT5 compounds have been carried out (Meli, Zuettel, and Schlapbach 1992; Luo et al 1997; Talagan˜is, Esquivel, and Meyer 2011) Long-term cycling leads to severe degradation of the material (Boonstra, Lippits, and Bernards 1989; Park and Lee 1987) To overcome this problem, substitutions have been performed on the Ni positions which leads to pseudo-binary compounds LaNi5ÀxMx (M ¼ Mn, Fe, Co, Ni, Al, Sn, Ge, Si) with improved resistance towards degradation (Bowman et al 2002; Li et al 2008; Shahgaldi et al 2012; Dongliang et al 2012; Prigent, Joubert, and Gupta 2012) In this work, the effects of substitution of Ni by Ga on electrochemical properties of LaNi5-xGax alloys used for Ni-MH batteries will be reported MATERIALS AND METHODS Reagents and MH Electrode Preparation The LaNi5-xGax (x ¼ 0, 0.1, 0.2, 0.3, 0.4, 0.5) samples were prepared by the arc melting method under an argon atmosphere The starting materials (La, Ni, Ga) of purity at least 99.9% were weighted according to their compositions A slight excess of La was added to compensate the weight loss during the arc-melting process The ingots were turned over and re-melted several times to attain good homogeneity Powder samples with an average particle size of about 50 mm were obtained by pulverizing the as-melted compounds in an agate mortar during 30 minutes For the electrochemical measurements, negative electrodes were prepared by mixing LaNi5-xGax powder with nickel and cooper powders at 70:28:2 ratio of weight and then this mixture well with a small amount of 2% polyvinyl alcohol The mixture was scrubbed into porous foamed nickel substrates and finally pressed at a pressure of ton=cm2 and density 0.25 g=cm2 to form a test electrode Before measurements, the MH electrode was modified by immersing it in M LiOH and ELECTROCHEMICAL PROPERTIES OF Ni-MH BATTERIES 1899 M KOH solution for 8–10 h to the accelerated dissociation of H2 on the oxide surface by the presence of Li in the surface region Microstructure Measurements Downloaded by [Moskow State Univ Bibliote] at 08:29 23 December 2013 The crystalline structure and the phase impurity of the samples at room temperature were examined on a D=Max-2500=PC X-ray powder diffractometer (using Cu-Ka radiation, 0.02 per step, 2s per step, 2h ¼ 10 À100 ) The obtained powder XRD patterns were analyzed by means of a Rietveld refinement procedure using X’pert High Score Plus in order to determine the type of structure and the lattice parameters (Rietveld 1969; Pecharsky and Vitalij 2009) Electrochemical Measurements Electrochemical measurements were performed in a three electrode system consisting of the working electrode (WE) as the prepared sample, a counter electrode (CE) of platinum, and a reference electrode (saturated calomel electrode, SCE, Hg=Hg2Cl2, calomel) The electrolyte was M LiOH and M KOH The purpose of the LiOH addition into the M KOH electrolyte is to increase electrochemical activity of the MH electrode (Uchida et al 1999; Uchida 1999; Cui, Luo, and Chuang 2000; Izawa et al 2003; Mohamad et al 2003) In charge-discharge capacity measurements, the electrodes were connected to a potential device called a Bi-Potentiostat 366A The electrodes were fully charged (the over-charged ratio was approximately 30%–50%) at a current density of 50 mA=g, and then discharged at the same current density to a cut-off potential of À0.7 V (versus SCE) The data were transmitted to a computer containing the software for treatment and display of results by graphical and data files Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) measurements were performed by using an Autolab 4.9 system Electrochemical impedance spectroscopy was performed on samples with various polarization rates E ¼ À0.9, E ¼ À1.0, E ¼ À1.1 and E ¼ À1.2 (V=SCE); the power AC voltage was a sinusoidal amplitude of mV, and frequencies ranged from MHz to mHz Measurement data were analyzed by FRA software The cyclic voltammetry was applied to re-activate charge-discharge for 50 cycles with a rate of 10 mV=s with a voltage range from À1.4 to À0.7 V=SCE across all of the electrodes The current density Jmax and charge quantity Q of all samples were calculated by the GRES software RESULTS AND DISCUSSION Crytal Structure Analysis X-ray diffraction (XRD) was used to investigate the crystal structure and lattice parameters of synthesized materials Figure shows the XRD patterns of the LaNi5-xGax (x 0.5) system The data confirmed that all the samples were single phase, and crystallized in the hexagonal CaCu5-type structure, the same structure, as does the prototype LaNi5, and no secondary phase was detected within 1% error of measurements Downloaded by [Moskow State Univ Bibliote] at 08:29 23 December 2013 1900 D N BA ET AL Figure The XRD patterns at room temperature of the intermetallic alloys LaNi5-xGax (with x ¼ 0À0.5) (Figure available in color online.) Table represents the lattice parameters and cell volume determined for the LaNi5-xGax samples with x 0.5 by using the Rietveld refinement analysis It can be seen that with the increase of Ga content in the alloys, the lattice parameter a, c and the cell volume V increased linearly as function of content, x The increase of ˚ ) than the lattice parameter can be explained by smaller in atomic radius of Ni (1.24 A ˚ that of Ga (1.35 A) The value of c=a also increased with x, clearly indicates which of the two available crystallographic positions in the crystal structure are involved in the substitution process of Ga for Ni It is well known that in the LaNi5 structure there exist two distinguished layers of atoms The basal layer (z ¼ 0) contains La atoms (1a sites) and Ni atoms (2c sites), and the intermediate layer (z ¼ 1=2) contains only Ni atoms (3 g sites) The observed increase of c=a suggests that replacement of Ni with Ga takes place preferentially within the intermediate layer rather than within the basal or both available layers The results obtained are in good agreement with experimental data for Sn, Ga, Pd, and Rh found in previous literature (Shuang et al 1999; Bowman et al 2002; Prigent et al 2012; Cero´n-Hurtado and Esquivel 2012) This indicates that in the latter system the basal or both available nickel crystallographic positions are involved in the substitution process Table Lattice parameters of the intermetallic alloys LaNi5-xGax (with x Sample LaNi5 LaNi4.9Ga0.1 LaNi4.8Ga0.2 LaNi4.7Ga0.3 LaNi4.6Ga0.4 LaNi4.5Ga0.5 0.5) ˚) a(A ˚) c(A c=a ˚ )3 V(A 5.0125 5.0203 5.0236 5.0285 5.0314 5.0345 3.9838 4.0151 4.0196 4.0241 4.0290 4.0389 0.7948 0.7998 0.8001 0.8003 0.8008 0.8022 86.684 87.637 87.850 88.120 88.329 88.655 ELECTROCHEMICAL PROPERTIES OF Ni-MH BATTERIES 1901 Downloaded by [Moskow State Univ Bibliote] at 08:29 23 December 2013 Galvanostatic Charge-Discharge at Constant Current When hydrogen storage electrode is first charged, the stored hydrogen in the alloy is released gradually after absorption The process in which the freshly formed hydride electrodes are continuously charged and discharged in order to obtain the maximum electrochemical capacity is called activation The activation capability was characterized by the number of charging–discharging cycles required for attaining the greatest discharge capacity through a charging–discharging cycle at a constant current density 50 mA=g The fewer the number of charging–discharging cycles, the better the activation performance This is important for practical use of Ni-MH batteries Figure (Fig 2a and Fig 2b) shows the activation capabilities of the LaNi5-xGax (x ¼ and 0.3, respectively) electrode alloys The LaNi5 alloys display excellent activation performances and can attain their maximum discharge capacities after 5–7 charging–discharging cycles For the substitution of Ni by Ga, the activation of LaNi4.5Ga0.5 alloys needs a bigger number of cycles However, the charging–discharging curve of LaNi5 is unstable, as the charging–discharging cycle could not repeat even in the 10 cycle LaNi5 samples that were Ga-doped had better and more stable charging–discharging cycles Only a few initial charging–discharging cycles of materials were more stable and durable, and can serve as an electrode of a battery The effect of the substitution of Ni by Ga on the course of the hydrogen storage capacity of LaNi5-xGax (x ¼ 0À0.5) electrodes as a function of the number of cycles is presented in Figure For LaNi5 alloy, there is a fast increase in capacity in the first few cycles; the highest capacity Cmax of electrode was observed at the 7th cycle All the Ga-doped electrodes reach their highest capacity Cmax near at the same time after about the 10th cycles; from the 12th cycle on the discharge capacity is almost saturated Compared with the alloy original LaNi5, Ga-doped alloys had a slightly lower capacity but prolonged lifetime and a more stable charge-discharge process This can be explained by, since Ga has a low melting temperature, when arc molten, Ga will melt first, sneak and cover the LaNi5-xGax particles which then makes LaNi5-xGax crystals smaller and less oxygen in the Figure Charge and discharge potential curves of alloys: (a) LaNi5 and (b) LaNi4.7Ga0.3 (Figure available in color online.) Downloaded by [Moskow State Univ Bibliote] at 08:29 23 December 2013 1902 D N BA ET AL Figure Discharge capacities vs cycle number of LaNi5-xGax (x ¼ 0À0.5) alloys (Figure available in color online.) charge-discharge process All of this leads to battery’s longer lifetime However, covering LaNi5-xGax particles also reduces the battery’s capacity This is in good agreement with the results obtained previously The substitution of Ni by Mn, Cu, Sn, and In (Drasner and Blazˇina 2003, 2004; Chen et al 2008; Prigent et al 2011, 2012) makes the material’s ability absorption decrease but the lifetime and performance of the batteries is increased enough to be used as negative electrode for Ni-MH rechargeable batteries Figure The electrochemical impedance spectra (EIS) of electrodes with various different polarization potentials: (a) LaNi5 and (b) LaNi4.5Ga0.5 (Figure available in color online.) ELECTROCHEMICAL PROPERTIES OF Ni-MH BATTERIES 1903 Downloaded by [Moskow State Univ Bibliote] at 08:29 23 December 2013 Electrochemical Impedance Spectroscopy The electrochemical impedance spectroscopy (EIS) is an effective method characterizing the electrochemical performance of MH electrode Figure shows typical Nyquist impedance spectra of electrode material LaNi5-xGax (x ¼ and 0.5) at different polarization potentials (À1.2 to À0.9 V vs SCE) in the whole frequency range (105 to 10À2 Hz) As shown in this figure, the shapes of the electrochemical impedance spectra are similar, with only one semicircle, and no apparent linear response appears in the low frequency region for these electrodes It is similar to the case with substitution of Ni by Ge and Sn, as reported by Witham (1997) It has been suggested that the loop in the impedance spectra is a characteristic of the charge transfer reaction The diameter of the loop increases apparently with increasing the Ga concentration in the alloys On the other hand, the diameters of semicircles are smaller when the polarization potential increases The diameter of semicircle corresponds to the charge transfer resistance, Rct It means charge transfer reaction is realized at high applied polarization In order to see more clearly the influence of Ga content substituted for Ni on the electrochemical impedance spectrum of alloy electrodes, we have calculated the preliminary charge transfer resistance Rct and double layer capacitance Cdl parameters of the electrode material by FRA software and used the equivalent circuit method Figure shows that when the same voltage was applied to the samples and the increased Ga content was substituted for Ni, the charge transfer resistance of the material electrodes increased (Figure 5a), and, inversely, the double layer capacitance was decreased (Figure 5b) A similar increase was reported by Pan et al (1999) The obtained results suggest that there is a variation in lattice parameters of samples with increasing Ga content substituted for Ni; both parameters a and c increased with the increasing Ga-doped proportion This change in crystal structure makes the conductivity and charge transfer more difficult In addition, the decrease of Cdl also shows that the density of conductive ions in the charge double layer is smaller and it leads to the possibility of charge exchange at the peripheral layers of electrolytes and the electrode surface is decreased This result is in Figure The dependence of (a) Rct and (b) Cdl in LaNi5-xGax alloys on (x) concentration (Figure available in color online.) 1904 D N BA ET AL agreement with the previous studies The doped Ga increases the material’s impedance but the lifetime and performance of the batteries is increased enough to be used as negative electrode for Ni-MH rechargeable batteries Downloaded by [Moskow State Univ Bibliote] at 08:29 23 December 2013 Cyclic Voltammetry Cyclic voltammetric measurements of the negative electrode were performed in the potential range of À1.4 to À0.7 V at sweep rate of mV=s The cyclic voltammetry curves of the MH electrodes are illustrated in Figure As shown in this figure, we can see that charge and discharge cyclic characteristics of the LaNi5 and LaNi4.5Ga4.5 compounds have similar formats The cyclic voltammetry are continuous, with no wave or peak expression of side effects during the test from the beginning to the end of cycle It was in good agreement with some reference data (Ananth et al 2009; van Druten et al 2000) This suggests that the samples are clean, have high structural uniformity, and contain no impurities in electrolytic dissociation solution For the same charge potential value and experiment conditions, current density increases with each cycle in all the samples The increase of charge current density represents good quality of electrode materials with increased charge=discharge cycle performance To see more clearly the influence of Ga-doped concentration on the charge-discharge process, the GRES software was used to calculate the current density (Jmax) and charge quantity (Q) of each sample Figure shows the activity capability of electrodes through charge=discharge cycles characterized by the maximal discharge current density Jpmax (Figure 7a) and the maximal charge current density Jnmax(Figure 7b) During the hydrogen storage process of negative electrode, these current densities increase when the number of charge=discharge cycles increases The increase of the maximal current densities shows well the increase of activity of materials due to the increasing number of cycles The increased rate of the discharge current density is higher than that of the charge current For initial cycles, the maximal current density Jmax is very low and then increases rapidly to the increase of the Figure Cyclic voltammetry (CV) curves of the alloy electrodes: (a) LaNi5 and (b) LaNi4.5Ga0.5 (Figure available in color online.) Downloaded by [Moskow State Univ Bibliote] at 08:29 23 December 2013 ELECTROCHEMICAL PROPERTIES OF Ni-MH BATTERIES 1905 Figure Variation of the charge density Jcmax (a) and discharge density Jdmax (b) as a function of number of cycles (Figure available in color online.) charge=discharge cycles This explains why it is necessary to activate a few cycles before the use starts This fast increase of current densities for initial cycles is explained mainly by the necessity of adsorbing process on the electrode surface to ease the charge=discharge process When the number of charge=discharge cycles increases, the hydrogen uncovered area of electrodes decreases which leads to the decrease of current densities From cycle 20 the increase rate reduces which means the electrodes are more stable This increase is mainly due to the diffusion of hydrogen atoms into material particles The change of the charge quantity Q of electrode materials at the same scanning rate is illustrated in Figure The results show that for the same Ga-doped Figure Variation of the charge quantity Qc and Qd as a function of number of cycles (Figure available in color online.) Downloaded by [Moskow State Univ Bibliote] at 08:29 23 December 2013 1906 D N BA ET AL concentration, the values of both charge quantity Qd and Qc increased by the number of cycles This indicates that the amount of hydrogen atoms absorbed in the electrode material increased cycle by cycle In general, an oxidation reaction of hydrogen on the MH electrode surface consists of two processes The first one is a charge transferring process at the interface of electrode=electrolyte and the other one is a diffusion process of hydrogen atoms from the inside of the electrode to its surface If the hydrogen atom diffusion is much faster than that of the rate of charge transfer process, the charge quantity Q will rapidly raise up with increasing numbers of cycles The curves in Figure indicate that in initial cycles, the charge quantity Q increases strongly which shows its oxidation reaction is controlled by hydrogen atom diffusion From cycle 20, charge quantity Q increases more slowly with increasing number of cycles, suggesting that the oxidation reaction is controlled mainly by charge transfer process on the electrode surface or, at least, by mixture of diffusion and charge transfer processes To evaluate the performance of the electrode materials, we have calculated the performance between the discharge quantity Qd and charge quantity Qc according to the following formula: HcÀd ¼ Qd :100% Qc Calculation results are shown in Figure For Ga-doped LaNi5 cases, the performance of batteries was higher than for LaNi5 In initial cycles, the performance of all the electrodes has achieved more than 50% rate, and the performance increased when the number of activated cycles increases From cycle 20 onward, the performance of the material increases more slowly After 50 cyclic voltammetric cycles, Figure Variation of charge-discharge performance Hd-c as a function of number of cycles (Figure available in color online.) Downloaded by [Moskow State Univ Bibliote] at 08:29 23 December 2013 ELECTROCHEMICAL PROPERTIES OF Ni-MH BATTERIES 1907 the performance of the electrode has reached 90% rate With Ga replaced for Ni, the performance of electrodes was higher It shows that the Ga-doped LaNi5 alloy enhances the performance and stability of the electrodes We can see in Figure and 8, compared with the original material LaNi5, when we substitute a part Ni by Ga, the current density and charge quantity of the materials decreased This is consistent with the results of measuring the discharge capacity of the materials shown in Figures and As the doping concentration increased, the capacity of the materials decreased However, when increasing the doping concentration, prolonged lifetime and charge-discharge performance of materials were higher (Figure 9) The initial cycle, the current density and charge quantity and capacity were small and increased strongly with the number of cycles, but after some period of training, their value increased slowly and gradually stabilize The electrochemical properties of LaNi5-xGax showed that the material can be used as the negative electrode in Ni-MH rechargeable batteries CONCLUSIONS We have investigated the crystallographic and the electrochemical properties of the intermetallic compounds LaNi5-xGax (x 0.5) The XRD patterns provide evidence of the single phase and crystallization in the hexagonal CaCu5-type structure in all the samples The lattice parameters calculated by the Rietveld method, both a and c, slightly increased with increasing Ga concentration Comparing with the original LaNi5 alloy, Ga-doped alloys had a slightly lower capacity but prolonged lifetime, a more stable charge-discharge process, and better performance The Nyquist plots of electrodes LaNi5-xGax in the vicinity of equilibrium potential (À1.2 to À0.9 V vs SCE) were similar They have only a semicircle shape, like pure LaNi5 This demonstrates that, after doping, the electric conductivity and charge transfer of negative electrodes were not changed The cyclic voltammetry was studied in the different polarized potential, and the results indicate that voltammograms were similar For the same Ga-doped concentration and experimental conditions, the maximal current density Jmax and charge quantity Q increased with each cycle in all samples REFERENCES Ananth, M., M Ganesan, N Renganathan, and S Lakshmi 2009 Influence of rare earth content on electrode kinetics in misch metal-based AB5 MH alloys – cyclic voltammetric investigations Int J Hydrogen Energ 34: 356–362 Boonstra, A H., G J M Lippits, and T N M Bernards 1989 Degradation processes in a LaNi5 electrode J Less Common Metals 155: 119–131 Bowman, R C., C A Lindensmith, S Luo, T B Flanagan, and T Vogt 2002 Degradation behavior of LaNi5ÀxSnxHz (x ¼ 0.20À0.25) at elevated temperatures J Alloy Compd 330–332: 271–275 Cero´n-Hurtado, N M., and M R Esquivel 2012 Characterization of LaNi4.70Al0.30 handled in air and 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Pe´richon, and A Percheron-Gue´gan 2000 Electrocatalytic hydrogenation of Alkenes on LaNi5 electrodes J Electroanal Chem 487: 31–36 Van Vucht, J H N., F A Kuijpers, and H C A M Bruning 1970 Reversible roomtemperature absorption of large quantities of hydrogen by intermetallic compounds Philips Res Rep 25: 133–140 Witham, C 1997 Electrochemical properties of LaNi5ÀxGex alloys in Ni-MH batteries J Electrochem Soc 144(11): 3758  ... the material’s ability absorption decrease but the lifetime and performance of the batteries is increased enough to be used as negative electrode for Ni-MH rechargeable batteries Figure The electrochemical. .. charge=discharge cycles increases The increase of the maximal current densities shows well the increase of activity of materials due to the increasing number of cycles The increased rate of the discharge current... Electrochemistry Downloaded by [Moskow State Univ Bibliote] at 08:29 23 December 2013 ELECTROCHEMICAL PROPERTIES OF LaNi5-XGaX ALLOYS USED AS THE NEGATIVE ELECTRODES OF Ni-MH BATTERIES Dam Nhan Ba,1,3 Luu