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Electrochemical characterization of mechanically alloyed LaCaMgNi9 compound

Electrochemical characterization of mechanically alloyed LaCaMgNi9 compound Mater Renew Sustain Energy (2016) 5:12 DOI 10.1007/s40243-016-0076-y OR IGINAL PAPER Electrochemical characterization of mechanically alloyed LaCaMgNi compound 1 1 2 2 • • • S. Chebab M. Abdellaoui M. Latroche V. Paul-Boncour Received: 25 November 2015 / Accepted: 8 June 2016 / Published online: 16 July 2016 The Author(s) 2016. This article is published with open access at Springerlink.com Abstract The performance of a nickel–metal hydride (Ni– safe and efficient storage is a key requirement, but has MH) battery mainly depends on the characteristics of the remained a most challenging issue [1]. negative electrode. The electrochemical characteristics of Hydrogen storage alloys have been extensively studied mechanically alloyed compound LaCaMgNi , including for many years as negative electrode materials of Ni–MH the discharge capacity and the hydrogen diffusion coeffi- batteries [2–6]. Among the various kinds of hydrogen cient, were studied as function of mechanical alloying storage materials for Ni–MH batteries, the AB -type com- (MA) conditions. The electrochemical measurements show pounds display an average maximum discharge capacity of that the LaCaMgNi electrode has a maximum discharge 300 mAh/g. However, none of the currently commercial- capacity of about 150 mAh/g at a discharge rate of C/3. ized electrode alloys, including AB and AB -types, can 5 2 The hydrogen discharge capacity dramatically decreases as meet the demand of power battery owing to the limitation of MA duration exceeds 20 h. their properties, such as low discharge capacity for the AB - type compounds and poor activation capability for the AB - Keywords AB -type alloy  Mechanical alloying  type Laves phase electrode alloy. Therefore, one of the Electrochemical characteristics main challenges in this area is to find new type of electrode materials with higher capacity and longer cycle life. In the recent years, La–Mg–Ni hydrogen storage alloys, Introduction with PuNi -type structure, have been considered as promising candidates owing to the benefit of lower cost, Hydrogen is considered as a promising sustainable energy higher discharge capacity and good electrochemical prop- carrier due to its high energy density and the fact that it can erties compared to AB -type alloys [7–11]. be produced from a variety of renewable sources including Kadir et al. [11, 12] revealed that RMg Ni (R = rare 2 9 biomass and water electrolysis. In addition, hydrogen earth, Ca or Y) alloys keep the PuNi -type rhombohedral combustion does not emit green-house gases to the atmo- structure after hydriding, and that their hydrogen storage sphere which justifies its classification as a clean source of capacity, which could reach 1.7–1.8 % (mass fraction), was energy. To be able to use hydrogen as an energy source, its significantly higher than that of the mischmetal-based AB - type alloys. Chen et al. [7] have prepared by induction melting several kinds of R–Mg–Ni based alloys with PuNi -type & S. Chebab structure. Subsequently, they found that the discharge chebabsafa14@gmail.com capacity of LaCaMgNi alloy could reach 360 mAh/g (1.87 wt%), but the high-rate dischargeability and cyclic Laboratoire des materiaux Utiles, Institut National de stability were poor. Recherche et d’Analyses Physico-chimique, Pole In this work, LaCaMgNi compound was prepared by technologique de Sidi Thabet, 2020 Sidi Thabet, Tunisia mechanical alloying (MA) to avoid inherent problems of ICMPE-CMTR; CNRS-UPEC, 2-8 rue Henri Dunant, 94320 the melting techniques [13]. It is now established that MA Thiais Cedex, France 123 12 Page 2 of 7 Mater Renew Sustain Energy (2016) 5:12 is an efficient process to synthesize a wide variety of type phase with PuNi -type structure (S.G: R-3m) and equilibrium and non-equilibrium structures in solid state. LaNi phase since 10 h of MA. The synthesized alloys exhibit a good compositional and Figure 1 gives the XRD patterns of the samples alloyed microstructural homogeneity with nanometric sized parti- for different times. In the XRD pattern of the un-milled cles due to the high energy impact during milling. sample (S0), all the diffraction lines of the starting ele- Nanostructuring is one of the possible approaches to ments are present. The diffraction peaks of nickel are the improve the hydrogenation properties of these materials most intense while those of the other elements are barely [14, 15]. The aim of this work was to study the nanos- detectable. This fact is mainly related to the low contents of tructuring effect on the electrochemical properties of Ca, Mg and La (5.47, 3.32 and 18.9 wt%, respectively) as LaCaMgNi -type alloy. compared to the Ni one (72.21 wt%). X-ray diffraction patterns of the mechanically alloyed powders reveal a modification of the microstructure of the Experimental details powders during MA process. With increasing MA duration, the originally sharp diffraction lines of the powders show a Elemental La, Ca, Mg and Ni powders (with at least remarkable line broadening and an intensity reduction 99.9 % purity) were mixed with the nominal composition compared to the S0 sample. This indicates the decrease of LaCaMgNi and charged into carbide vials under con- the crystallite size and the presence of lattice microstrain trolled atmosphere (purified argon in glove box). MA was within the particles. Furthermore, MA for long time results performed with a Fritsch Pulverisette P7 planetary ball mill in the partial amorphization of the alloy. In fact, the at a rotation speed of 400 rpm with a ball to powder ratio increase of MA duration induces more strains and increases equal to 17:1. The MA duration was varied from 4 to 30 h the defect concentration in the crystalline structure of and the samples were labeled Sd (S for sample and d for elemental powders that leads to the destabilization of part duration in hour). of them into amorphous phase. Each powdered sample was sieved to a particle size As we can see, Ni remains up to 20 h of MA while other below 40 lm for X-ray powder diffraction (XRD) analysis. elements are not observed after the first hours of milling. XRD patterns were recorded with a (h–2h) Panaltytical Depending on MA duration, the formation of an AB -type X’Pert pro MPD diffractometer with a copper anti-cathode phase with a PuNi -type structure occurred in coexistence (k = 0.15406 nm) in a 2h range of 10–100. The with LaNi (S.G: P6/mmm). The LaNi -type phase appears CuKa 5 2 XRD data were analyzed by the Rietveld method [16, 17] as an intermediate phase for MA duration ranging from 4 to using FULLPROF program [18]. 8 h and disappears hereafter. Working electrodes were prepared by mixing the alloy powders with black carbon and PTFE in, respectively, 90, 5 and 5 % weight proportions. Electrochemical measure- ments were performed in a conventional three-electrode open-air cell using a VMP biologic potentiostat–galvano- stat. The discharge capacities of the electrodes were determined at room temperature by galvanostatically charge–discharge at, respectively, C/3 and C/6 rates. The Ni(OH) /NiOOH and Hg/HgO electrodes were used as the counter and the reference electrodes, respectively. Cyclic voltammetry was performed for different scan rates in the -0.5 to -1.1 V potential range. The chronoamperometry was applied after 30 cycles of charge–discharge. This method consists in fully charging the electrode and dis- charging it at constant potential (-0.6 V versus Hg/HgO). Experimental results and discussion Structural properties of the alloys Starting from elemental La, Ca, Mg and Ni metals, MA Fig. 1 Evolution of the XRD patterns of samples for increasing MA leads to the formation of a mixture of nanocrystalline AB - duration (4–30 h) 123 Mater Renew Sustain Energy (2016) 5:12 Page 3 of 7 12 Table 1 Phase structural properties and maximum discharge capacities of the samples synthesized by MA at various milling time Sample Phase Space Phase abundance C (mAh/g C (mAh/g Loss of charge after max 10 St group (wt%) of active material) of active material) ten cycles (%) S10 AB R-3m 42.7 (1.6) 130.4 107.8 17.3 LaNi P6/mmm 40.1 (1.4) Ni Fm-3m 17.2 (0.7) S12 AB R-3m 48.4 (1.5) 124.9 120.2 3.51 LaNi P6/mmm 40.9 (1.3) Ni Fm-3m 10.7 (0. 6) S16 AB R-3m 49.8 (1.3) 141.3 122.1 12.7 LaNi P6/mmm 45.6 (1.2) Ni Fm-3m 4.6 (0.5) S20 AB R-3m 52.2 (1.4) 156.1 117.1 24.2 LaNi P6/mmm 43.1 (1.2) Ni Fm-3m 4.7 (0.5) S25 AB R-3m 53.5 (1.7) 138.2 121.5 11.8 LaNi P6/mmm 46.5 (1.3) S30 AB R-3 m 67 (1.8) 121.6 95.4 21.2 LaNi P6/mmm 33 (1.2) The composition and lattice parameters of the phases were calculated by the Rietveld method and are listed in Table 1. As the MA duration increases, the abundance of the LaCaMgNi AB -type phase increases from about 9 3 43–63 wt% whereas that of LaNi increases first from 40 to 47 wt% and then decreases to 36 wt%. The SEM micrograph presented in Fig. 2 shows the morphology of the S30 sample. The powder is composed of large agglomerates of more than 5 lm in size. These agglomerates are made of several smaller (0.1–1 lm) deformed particles welded together. Electrochemical characterizations Activation and charge–discharge cycle stability of the alloys The variation of the measured discharge capacities of the mechanically alloyed samples as a function of the charge– discharge cycle number is shown in Fig. 3. For all samples, Fig. 2 SEM micrography showing the morphology of the S30 sample the maximum discharge capacity was reached at the first cycle, then slightly decreases for next cycles to remain C ¼ C =ðx þ yÞð1Þ activematerial measured stable at higher cycle number. This decrease is due to the corrosion of the active phase in the electrolyte during the where C is the measured discharge capacity (of the measured reaction. whole material), and x and y are, respectively, the LaNi As the alloy contains some phases that do not absorb and the AB -type phase contents. hydrogen, the discharge capacity of the active material The effect of the MA duration on the discharge capac- (C , Table 1) was calculated based on the mea- ities of the active material at the first and the 10th charg- activematerial sured discharge capacity of the hole material (Fig. 3) and ing–discharging cycle is shown in Fig. 4. This figure shows the contributions of the hydrogen absorbing phases only that the discharge capacity increases up to 20 h of milling (AB -type and LaNi phases), as: to reach about 156 mAh/g. Above 20 h, the discharge 3 5 123 % C10 12 Page 4 of 7 Mater Renew Sustain Energy (2016) 5:12 S20) after 20 h of MA. In addition, as samples are formed S10 S20 by two hydrogen absorbing phases (LaNi and AB -type 5 3 phase), it is important to know the contribution of each S12 S25 phase to the total discharge capacity. For a polyphasic S16 S30 alloy, the total measured electrochemical discharge capacity would equal to the sum of the contributions of all phases as: C ðmAh/g of alloy) ¼ x C ð2Þ measured i / where x and C are, respectively, the weight content and i / the electrochemical discharge capacity of the phase / .In this work, as mentioned above as only LaNi and AB -type 5 3 phases are the hydrogen absorbing phases, the total measured electrochemical discharge capacity can be 0 5 10 15 20 25 30 35 expressed as: Cycle number (N) C ðmAh/g of alloy) ¼ xC þyC measured LaNi AB typephase 5 3 Fig. 3 Variation of the discharge capacity versus cycle number of ð3Þ S10–S30 samples where x and y are, respectively, LaNi and AB -type phase 5 3 160 100 weight contents.C and C are, respectively, the LaNi AB 5 3 % C active material discharge capacity of LaNi and AB -type phase. 5 3 To determine the maximum discharge capacity of the obtained AB -type phase, we measured the discharge capacity of LaNi synthesized in the same energetic con- dition that the AB -type one. The obtained value is 180 mAh/g. The discharge capacity of the AB -type phase 130 3 is expressed as below: C ¼½C ðx  C Þ=y ð4Þ AB typephase measured LaNi 3 5 Using expression (4), the maximum discharge capacity of the formed AB -type phase is about 135 mAh/g. Thus, as the discharge capacity of the LaNi is higher than that of AB - 5 3 100 0 type phase, the decrease of the total measured electrochemical 5 101520253035 discharge capacity or the electrochemical discharge capacity MA duration of the active material beyond 20 h of MA (Fig. 4) is attributed Fig. 4 Effect of MA duration on the dischage capacities of the active to the decrease of the LaNi weight content. material at the first cycle and the loss of discharge capacity at the 10th The activation capability is a very important perfor- charging–discharging cycle mance for the practical use of Ni-MH battery. It is usually characterized by the number of charge–discharge cycles capacity decreases with the MA duration to reach values required to reach the greatest discharge capacity at constant comparable to that obtained at lower milling duration. The current density. The smaller the number of cycles, the storage capacity reduction with the MA duration is attrib- better is the activation performance. The examination of uted to the creation of defects in the material during the the variation of discharge capacity as function of the cycle synthesis process. During MA, the powder particles which number (Fig. 3) reveals that the alloys possess good acti- are trapped between the colliding balls are subjected to vation performances, attaining their maximum discharge compressive impact forces. Thus, they are deformed, capacities at the first cycle. fractured and cold-welded leading to defects in the system and also an average particle size reduction [19]. Determination of kinetic parameters of the hydrogen As mentioned before, the discharge capacity of absorption reaction LaCaMgNi alloy could reach 360 mAh/g [7] while the maximum discharge capacity obtained in this work is Cyclic voltametry Figure 5 shows the cyclic voltammo- 156 mAh/g of active material (146.2 mAh/g of sample grams for the S30 sample. The working electrode potential C Discharge capacity (mAh/g) active material Mater Renew Sustain Energy (2016) 5:12 Page 5 of 7 12 was scanned from -1.1 to -0.5 V versus Hg/HgO (1 mol/ a consumption of hydrogen at the surface, while in the L KOH solution) with scan rates of 10, 20, 30, 40 and second time region the current decreases slowly in a linear 50 lV/s. The anodic and cathodic peaks reflect the infor- way. In this case, hydrogen is supplied from the bulk alloy mation related to the discharge and charge processes [20]. proportionally to the concentration gradient of hydrogen The observed anodic peak is attributed to the oxidation of and the current is controlled by the diffusion rate of the desorbed hydrogen atoms at the surface, while the hydrogen atoms [22] with time [23, 24]. cathodic peak is attributed to the reduction of hydrogen As all samples are multi-phases, the calculated coeffi- atoms absorbed by the alloy in the interstitial sites. As we cient of hydrogen diffusion is an average value describing can see in Fig. 5, the anodic peak current increases and its the hydrogen diffusion in the whole sample not in a specific potential shifts towards positive direction with increasing phase. From the slope of the linear region in Fig. 7,itis scan rate. possible to estimate the average coefficient of hydrogen Figure 6 gives the variation of the anodic peak potential diffusion value using the following formula, which is valid of the S30 alloy as function of log (v). This relation is at a large time [23]: linear. In this case, the anodic peak potential can be written 6FD p D logðiÞ¼ log ðC  C Þ t ð6Þ as follows: 0 s 2 2 da 2:303a dE 2:3RT ap ¼ ð5Þ 2:303a d logðiÞ d logðvÞ 2anF D ¼ ð7Þ p dt with R is the gas constant, F is the Faraday constant and where i, D, d, a, C , C and t are, respectively, the diffusion 0 s T is the temperature. current (A/g), the diffusion coefficient of hydrogen (cm /s), This equation allows the determination of the charge the density of hydrogen storage, the alloy particle radius transfer coefficient a (Table 2). This parameter indicates (cm), the initial hydrogen concentration in the bulk of alloy the alloy ability to charge and discharge. As the a value (mol/cm), and the hydrogen concentration on the surface of ranges from 0.3 to 0.7, it can be approximated close to 0.5 the alloy particle radius, respectively. Assuming that the [21] which means that the charge and discharge are average particle radius is a = 5 lm (SEM micrograph reversible and the system has the same tendency for the Fig. 2), D can be calculated according to Eq. (3) and the charge as for the discharge process. calculated values are listed in Table 2. As we can notice, the diffusion coefficient of hydrogen D shows the same Chronoamperometry Figure 7 shows the current respon- behavior than the discharge capacity as a function of MA ses expressed in log(i) versus time for the synthesized duration. Its value increases up to 20 h of MA and alloys at 298 K. The semi-logarithmic curves of anode decreases for larger time. current density versus time can be divided into two time regions. In the first one, the current decreases rapidly due to -0,84 y = -0,61561 + 0,052792x R= 0,97041 -0,845 -0,85 -2 -0,855 -4 -0,86 10 µV/s 20 µV/s -6 30 µV/s -0,865 40 µV/s 50 µV/s -8 -0,87 -10 -0,875 -12 -0,88 -5 -4,9 -4,8 -4,7 -4,6 -4,5 -4,4 -4,3 -1,1 -1 -0,9 -0,8 -0,7 -0,6 -0,5 Ewe(V) Log(v) Fig. 5 Cyclic voltammograms of S30 sample at potential scan rate: Fig. 6 Variation of the anodic peak potential of cyclic voltammo- 10, 20, 30, 40 and 50 lV/s grams of S30 sample as function of log(v) I(mA) Epic(V) 12 Page 6 of 7 Mater Renew Sustain Energy (2016) 5:12 Table 2 Charge transfer coefficient ‘a’ and hydrogen diffusion Highlights coefficient ‘‘D’’ of the S10–S30 samples -10 • Synthesis of LaCaMgNi -type alloy by mechanical Sample Charge transfer D (10 9 coefficient ‘a’ cm /s) alloying. • Effect of increasing MA duration on the electrochem- S10 0.23 4.9 ical properties of the obtained alloy. S12 0.49 5.1 • ‘a’ values range from 0.3 to 0.7. S16 0.50 6.3 • ‘D’ and the discharge capacity have the same behavior S20 0.56 6.8 as function of MA duration. S25 0.69 6.2 S30 0.24 5.9 Acknowledgments The financial support was guaranteed by the CMCU project (CMCU PHC Utique N10G1208). Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http:// creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. References 1. Barghi, S.H., Tsotsis, T.T.: Chemisorption, physisorption and hysteresis during hydrogen storage in carbon nanotubes. Int J Hydrogen Energy 39, 1390–1397 (2013) 2. Rogulski, Z., Diubak, J., Karwowska, M., Pytlik, E.: Studies on metal hydride electrodes containing no binder additives. J Power Sour 195, 7517–7523 (2010) 3. Li, S., Zhao, M., Wang, L., Liu, Y., Wang, Y.: Structures and electrochemical characteristics of Ti 6Zr V Mn Ni 0.2 0.07 0.24 0.1 0.33- Fig. 7 Semilogarithmic plots of anodic current–time responses of the Mox (x = 0–0.1) hydrogen storage alloys. Mater Sci Eng B 150, S10–S30 samples 168–174 (2008) 4. Mu, D., Hatano, Y., Abe, T., Watannabe, K.: Degradation kinetics of discharge capacity for amorphous Mg–Ni electrode. J alloys Compd 334, 232–237 (2002) Conclusion 5. Young, K., Nei, J., Huang, B., Fetcenko, M.A.: Studies of off- stoichiometric AB metal hydride alloy: part 2. Hydrogen storage and electrochemical properties. Int J Hydrogen Energy 36, A nanostructured PuNi -type alloy has been synthesized by 11146–11154 (2011) mechanical alloying of a stoichiometric mixture of La, Ca, 6. Miyamura, H., Sakai, T., Kuriyama, N., Oguro, K., Kato, A.: Mg and Ni elements at different milling duration. The Hydrogen storage materials, batteries and electrochemistry. In: Electrochemical Society Proceedings, vol. 92–5, pp 179–198 electrochemical properties of the milled samples have been (1992) investigated by electrochemical methods: chronopoten- 7. Chen, J., Kuriyama, N., Takeshita, H.T., Tanaka, H., Sakai, T.: tiometry, cyclic voltammetry and chronoamperometry. The Hydrogen storage alloys with PuNi3-type structure as metal following conclusions can be drawn: hydride electrodes. Electrochem Solid State Lett 3(6), 249–252 (2000) All samples exhibit limited discharge capacity with a 8. Pan, H.G., Liu, Y.F., Cao, M.X., Zhv, Y.F., Lei, Y.Q., Wang, maximum of about 156 mAh/g after 20 h of MA. Q.D.: An investigation on the structural and electrochemical properties of La Mg (Ni Co ) (x = 3.15–3.80) hydrogen 0.7 0.3 0.85 0.15 x Further, the discharge capacity decreases with increasing storage electrode alloys. J Alloys Compd 351, 228–234 (2003) MA duration. This is attributed to the increase of lattice 9. Liao, B., Li, Y.Q., Chen, L.X., Lu, G.L., Pan, H.G.: A study on defects and the decrease of particle size. the structure and electrochemical properties of La Mg(Ni 2 0.95- The values of the charge transfer coefficient range from M ) (M = Co, Mn, Fe, Al, Cu, Sn) hydrogen storage elec- 0.05 9 trode alloys. J Alloys Compd 376, 186–195 (2004) 0.3 to 0.7 for samples mechanically alloyed between 12 10. Pan, H.G., Liu, Y.F., Gao, M.X., Lei, Y.Q., Wang, Q.D.: Elec- and 25 h which indicate a good reversibility of the trochemical Properties of the La0.7Mg0.3Ni2.65 - x electrochemical reaction. For the sample milled for 10 Mn0.1Co0.75Al x (x = 0–0.5) Hydrogen storage alloy elec- and 30 h, the a values are, respectively, 0.232 and 0.242. trodes. J Electrochem Soc 152(2), A326–A332 (2005) 123 Mater Renew Sustain Energy (2016) 5:12 Page 7 of 7 12 11. Kadir, K., Sakai, T., Uehara, I., Kadir, K., Sakai, T., UeharaI, I.: 18. Rodriguez-Carvajal, J.: Recent advances in magnetic structure Structural investigation and hydrogen capacity of YMg Ni and determination by neutron powder diffraction. J Phys 192B,55 2 9 (Y Ca )(MgCa)Ni : new phases in the AB C system (1993) 0.5 0.5 9 2 9 isostructural with LaMg Ni . J Alloys Compd 287, 264–270 19. Wu, Y., Hana, W., Zhou, S.X., Lototsky, M.V., Solberg, J.K., 2 9 (1999) Yartys, V.A.: Microstructure and hydrogenation behavior of ball- 12. Kadir, K., Sakai, T., Uehara, I.: Structural investigation and milled and melt-spun Mg–10Ni–2Mm alloys. J. Alloys Compd hydrogen storage capacity of LaMg Ni and (La Ca )(- 466, 176 (2008) 2 9 0.65 0.35 Mg Ca )Ni of the AB C type structure. J Alloys Compd 20. Tan, Z., Yang, Y., Li, Y., Shao, H.: The performances of La 1.32 0.68 9 2 9 1-- 302, 112–117 (2000) xCexNi (0 B x B 1) hydrogen storage alloys studied by powder 13. Chebab, S., Abdellaoui, M., Latroche, M., Paul-Boncour, V.: microelectrode. J Alloys Compd 453, 79–86 (2008) Structural and hydrogen storage properties of LaCaMgNi -type 21. Bard, A.J., Fulkner, L.R.: Electrochimie: Principe, Methodes and alloy obtained by mechanical alloying. Mater Renew Sust Applications. Masson, Paris (1998) Energy, in press, (2015). http://dx.doi.org/10.1007/s40243-015- 22. Giza, K.: Electrochemical studies of LaNi Co Al hydrogen 4.3 0.4 0.3 0053-x storage. Intermetallics 34, 128–131 (2013) 14. Joseph, B., Iadecola, A., Schiavo, B., Cognigni, A., Olivi, L.: 23. Zheng, G., Popov, B.N., White, R.E.: Electrochemical determi- Local structure of ball-milled LaNi hydrogen storage material nation of the diffusion coefficient of hydrogen through an by Ni K-edge EXAFS. J Solid State Chem 183, 1550 (2010) LaNi Al electrode in alkaline aqueous solution. J Elec- 4.25 0.75 15. Joseph, B., Schiavo, B.: Effects of ball-milling on the hydrogen trochem Soc 142, 2695 (1995) sorption properties of LaNi . J Alloys Compd 480, 912 (2009) 24. Pan, H., Chen, N., Gao, M.: Effects of annealing temperature on 16. Rietveld, H.M.: Line profiles of neutron powder-diffraction peaks structure and the electrochemical properties of La Mg Ni 0.7 0.3 2.45- for structure refinement. Acta cristallgr 22, 151 (1967) Co Mn Al hydrogen storage alloy. J Alloys Compd 397, 0.75 0.1 0.2 17. Rietveld, H.M.: A profile refinement for nuclear and magnetic 306 (2005) structures. J Appl Cryst 2, 65 (1969) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Materials for Renewable and Sustainable Energy Springer Journals

Electrochemical characterization of mechanically alloyed LaCaMgNi9 compound

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Abstract

Mater Renew Sustain Energy (2016) 5:12 DOI 10.1007/s40243-016-0076-y OR IGINAL PAPER Electrochemical characterization of mechanically alloyed LaCaMgNi compound 1 1 2 2 • • • S. Chebab M. Abdellaoui M. Latroche V. Paul-Boncour Received: 25 November 2015 / Accepted: 8 June 2016 / Published online: 16 July 2016 The Author(s) 2016. This article is published with open access at Springerlink.com Abstract The performance of a nickel–metal hydride (Ni– safe and efficient storage is a key requirement, but has MH) battery mainly depends on the characteristics of the remained a most challenging issue [1]. negative electrode. The electrochemical characteristics of Hydrogen storage alloys have been extensively studied mechanically alloyed compound LaCaMgNi , including for many years as negative electrode materials of Ni–MH the discharge capacity and the hydrogen diffusion coeffi- batteries [2–6]. Among the various kinds of hydrogen cient, were studied as function of mechanical alloying storage materials for Ni–MH batteries, the AB -type com- (MA) conditions. The electrochemical measurements show pounds display an average maximum discharge capacity of that the LaCaMgNi electrode has a maximum discharge 300 mAh/g. However, none of the currently commercial- capacity of about 150 mAh/g at a discharge rate of C/3. ized electrode alloys, including AB and AB -types, can 5 2 The hydrogen discharge capacity dramatically decreases as meet the demand of power battery owing to the limitation of MA duration exceeds 20 h. their properties, such as low discharge capacity for the AB - type compounds and poor activation capability for the AB - Keywords AB -type alloy  Mechanical alloying  type Laves phase electrode alloy. Therefore, one of the Electrochemical characteristics main challenges in this area is to find new type of electrode materials with higher capacity and longer cycle life. In the recent years, La–Mg–Ni hydrogen storage alloys, Introduction with PuNi -type structure, have been considered as promising candidates owing to the benefit of lower cost, Hydrogen is considered as a promising sustainable energy higher discharge capacity and good electrochemical prop- carrier due to its high energy density and the fact that it can erties compared to AB -type alloys [7–11]. be produced from a variety of renewable sources including Kadir et al. [11, 12] revealed that RMg Ni (R = rare 2 9 biomass and water electrolysis. In addition, hydrogen earth, Ca or Y) alloys keep the PuNi -type rhombohedral combustion does not emit green-house gases to the atmo- structure after hydriding, and that their hydrogen storage sphere which justifies its classification as a clean source of capacity, which could reach 1.7–1.8 % (mass fraction), was energy. To be able to use hydrogen as an energy source, its significantly higher than that of the mischmetal-based AB - type alloys. Chen et al. [7] have prepared by induction melting several kinds of R–Mg–Ni based alloys with PuNi -type & S. Chebab structure. Subsequently, they found that the discharge chebabsafa14@gmail.com capacity of LaCaMgNi alloy could reach 360 mAh/g (1.87 wt%), but the high-rate dischargeability and cyclic Laboratoire des materiaux Utiles, Institut National de stability were poor. Recherche et d’Analyses Physico-chimique, Pole In this work, LaCaMgNi compound was prepared by technologique de Sidi Thabet, 2020 Sidi Thabet, Tunisia mechanical alloying (MA) to avoid inherent problems of ICMPE-CMTR; CNRS-UPEC, 2-8 rue Henri Dunant, 94320 the melting techniques [13]. It is now established that MA Thiais Cedex, France 123 12 Page 2 of 7 Mater Renew Sustain Energy (2016) 5:12 is an efficient process to synthesize a wide variety of type phase with PuNi -type structure (S.G: R-3m) and equilibrium and non-equilibrium structures in solid state. LaNi phase since 10 h of MA. The synthesized alloys exhibit a good compositional and Figure 1 gives the XRD patterns of the samples alloyed microstructural homogeneity with nanometric sized parti- for different times. In the XRD pattern of the un-milled cles due to the high energy impact during milling. sample (S0), all the diffraction lines of the starting ele- Nanostructuring is one of the possible approaches to ments are present. The diffraction peaks of nickel are the improve the hydrogenation properties of these materials most intense while those of the other elements are barely [14, 15]. The aim of this work was to study the nanos- detectable. This fact is mainly related to the low contents of tructuring effect on the electrochemical properties of Ca, Mg and La (5.47, 3.32 and 18.9 wt%, respectively) as LaCaMgNi -type alloy. compared to the Ni one (72.21 wt%). X-ray diffraction patterns of the mechanically alloyed powders reveal a modification of the microstructure of the Experimental details powders during MA process. With increasing MA duration, the originally sharp diffraction lines of the powders show a Elemental La, Ca, Mg and Ni powders (with at least remarkable line broadening and an intensity reduction 99.9 % purity) were mixed with the nominal composition compared to the S0 sample. This indicates the decrease of LaCaMgNi and charged into carbide vials under con- the crystallite size and the presence of lattice microstrain trolled atmosphere (purified argon in glove box). MA was within the particles. Furthermore, MA for long time results performed with a Fritsch Pulverisette P7 planetary ball mill in the partial amorphization of the alloy. In fact, the at a rotation speed of 400 rpm with a ball to powder ratio increase of MA duration induces more strains and increases equal to 17:1. The MA duration was varied from 4 to 30 h the defect concentration in the crystalline structure of and the samples were labeled Sd (S for sample and d for elemental powders that leads to the destabilization of part duration in hour). of them into amorphous phase. Each powdered sample was sieved to a particle size As we can see, Ni remains up to 20 h of MA while other below 40 lm for X-ray powder diffraction (XRD) analysis. elements are not observed after the first hours of milling. XRD patterns were recorded with a (h–2h) Panaltytical Depending on MA duration, the formation of an AB -type X’Pert pro MPD diffractometer with a copper anti-cathode phase with a PuNi -type structure occurred in coexistence (k = 0.15406 nm) in a 2h range of 10–100. The with LaNi (S.G: P6/mmm). The LaNi -type phase appears CuKa 5 2 XRD data were analyzed by the Rietveld method [16, 17] as an intermediate phase for MA duration ranging from 4 to using FULLPROF program [18]. 8 h and disappears hereafter. Working electrodes were prepared by mixing the alloy powders with black carbon and PTFE in, respectively, 90, 5 and 5 % weight proportions. Electrochemical measure- ments were performed in a conventional three-electrode open-air cell using a VMP biologic potentiostat–galvano- stat. The discharge capacities of the electrodes were determined at room temperature by galvanostatically charge–discharge at, respectively, C/3 and C/6 rates. The Ni(OH) /NiOOH and Hg/HgO electrodes were used as the counter and the reference electrodes, respectively. Cyclic voltammetry was performed for different scan rates in the -0.5 to -1.1 V potential range. The chronoamperometry was applied after 30 cycles of charge–discharge. This method consists in fully charging the electrode and dis- charging it at constant potential (-0.6 V versus Hg/HgO). Experimental results and discussion Structural properties of the alloys Starting from elemental La, Ca, Mg and Ni metals, MA Fig. 1 Evolution of the XRD patterns of samples for increasing MA leads to the formation of a mixture of nanocrystalline AB - duration (4–30 h) 123 Mater Renew Sustain Energy (2016) 5:12 Page 3 of 7 12 Table 1 Phase structural properties and maximum discharge capacities of the samples synthesized by MA at various milling time Sample Phase Space Phase abundance C (mAh/g C (mAh/g Loss of charge after max 10 St group (wt%) of active material) of active material) ten cycles (%) S10 AB R-3m 42.7 (1.6) 130.4 107.8 17.3 LaNi P6/mmm 40.1 (1.4) Ni Fm-3m 17.2 (0.7) S12 AB R-3m 48.4 (1.5) 124.9 120.2 3.51 LaNi P6/mmm 40.9 (1.3) Ni Fm-3m 10.7 (0. 6) S16 AB R-3m 49.8 (1.3) 141.3 122.1 12.7 LaNi P6/mmm 45.6 (1.2) Ni Fm-3m 4.6 (0.5) S20 AB R-3m 52.2 (1.4) 156.1 117.1 24.2 LaNi P6/mmm 43.1 (1.2) Ni Fm-3m 4.7 (0.5) S25 AB R-3m 53.5 (1.7) 138.2 121.5 11.8 LaNi P6/mmm 46.5 (1.3) S30 AB R-3 m 67 (1.8) 121.6 95.4 21.2 LaNi P6/mmm 33 (1.2) The composition and lattice parameters of the phases were calculated by the Rietveld method and are listed in Table 1. As the MA duration increases, the abundance of the LaCaMgNi AB -type phase increases from about 9 3 43–63 wt% whereas that of LaNi increases first from 40 to 47 wt% and then decreases to 36 wt%. The SEM micrograph presented in Fig. 2 shows the morphology of the S30 sample. The powder is composed of large agglomerates of more than 5 lm in size. These agglomerates are made of several smaller (0.1–1 lm) deformed particles welded together. Electrochemical characterizations Activation and charge–discharge cycle stability of the alloys The variation of the measured discharge capacities of the mechanically alloyed samples as a function of the charge– discharge cycle number is shown in Fig. 3. For all samples, Fig. 2 SEM micrography showing the morphology of the S30 sample the maximum discharge capacity was reached at the first cycle, then slightly decreases for next cycles to remain C ¼ C =ðx þ yÞð1Þ activematerial measured stable at higher cycle number. This decrease is due to the corrosion of the active phase in the electrolyte during the where C is the measured discharge capacity (of the measured reaction. whole material), and x and y are, respectively, the LaNi As the alloy contains some phases that do not absorb and the AB -type phase contents. hydrogen, the discharge capacity of the active material The effect of the MA duration on the discharge capac- (C , Table 1) was calculated based on the mea- ities of the active material at the first and the 10th charg- activematerial sured discharge capacity of the hole material (Fig. 3) and ing–discharging cycle is shown in Fig. 4. This figure shows the contributions of the hydrogen absorbing phases only that the discharge capacity increases up to 20 h of milling (AB -type and LaNi phases), as: to reach about 156 mAh/g. Above 20 h, the discharge 3 5 123 % C10 12 Page 4 of 7 Mater Renew Sustain Energy (2016) 5:12 S20) after 20 h of MA. In addition, as samples are formed S10 S20 by two hydrogen absorbing phases (LaNi and AB -type 5 3 phase), it is important to know the contribution of each S12 S25 phase to the total discharge capacity. For a polyphasic S16 S30 alloy, the total measured electrochemical discharge capacity would equal to the sum of the contributions of all phases as: C ðmAh/g of alloy) ¼ x C ð2Þ measured i / where x and C are, respectively, the weight content and i / the electrochemical discharge capacity of the phase / .In this work, as mentioned above as only LaNi and AB -type 5 3 phases are the hydrogen absorbing phases, the total measured electrochemical discharge capacity can be 0 5 10 15 20 25 30 35 expressed as: Cycle number (N) C ðmAh/g of alloy) ¼ xC þyC measured LaNi AB typephase 5 3 Fig. 3 Variation of the discharge capacity versus cycle number of ð3Þ S10–S30 samples where x and y are, respectively, LaNi and AB -type phase 5 3 160 100 weight contents.C and C are, respectively, the LaNi AB 5 3 % C active material discharge capacity of LaNi and AB -type phase. 5 3 To determine the maximum discharge capacity of the obtained AB -type phase, we measured the discharge capacity of LaNi synthesized in the same energetic con- dition that the AB -type one. The obtained value is 180 mAh/g. The discharge capacity of the AB -type phase 130 3 is expressed as below: C ¼½C ðx  C Þ=y ð4Þ AB typephase measured LaNi 3 5 Using expression (4), the maximum discharge capacity of the formed AB -type phase is about 135 mAh/g. Thus, as the discharge capacity of the LaNi is higher than that of AB - 5 3 100 0 type phase, the decrease of the total measured electrochemical 5 101520253035 discharge capacity or the electrochemical discharge capacity MA duration of the active material beyond 20 h of MA (Fig. 4) is attributed Fig. 4 Effect of MA duration on the dischage capacities of the active to the decrease of the LaNi weight content. material at the first cycle and the loss of discharge capacity at the 10th The activation capability is a very important perfor- charging–discharging cycle mance for the practical use of Ni-MH battery. It is usually characterized by the number of charge–discharge cycles capacity decreases with the MA duration to reach values required to reach the greatest discharge capacity at constant comparable to that obtained at lower milling duration. The current density. The smaller the number of cycles, the storage capacity reduction with the MA duration is attrib- better is the activation performance. The examination of uted to the creation of defects in the material during the the variation of discharge capacity as function of the cycle synthesis process. During MA, the powder particles which number (Fig. 3) reveals that the alloys possess good acti- are trapped between the colliding balls are subjected to vation performances, attaining their maximum discharge compressive impact forces. Thus, they are deformed, capacities at the first cycle. fractured and cold-welded leading to defects in the system and also an average particle size reduction [19]. Determination of kinetic parameters of the hydrogen As mentioned before, the discharge capacity of absorption reaction LaCaMgNi alloy could reach 360 mAh/g [7] while the maximum discharge capacity obtained in this work is Cyclic voltametry Figure 5 shows the cyclic voltammo- 156 mAh/g of active material (146.2 mAh/g of sample grams for the S30 sample. The working electrode potential C Discharge capacity (mAh/g) active material Mater Renew Sustain Energy (2016) 5:12 Page 5 of 7 12 was scanned from -1.1 to -0.5 V versus Hg/HgO (1 mol/ a consumption of hydrogen at the surface, while in the L KOH solution) with scan rates of 10, 20, 30, 40 and second time region the current decreases slowly in a linear 50 lV/s. The anodic and cathodic peaks reflect the infor- way. In this case, hydrogen is supplied from the bulk alloy mation related to the discharge and charge processes [20]. proportionally to the concentration gradient of hydrogen The observed anodic peak is attributed to the oxidation of and the current is controlled by the diffusion rate of the desorbed hydrogen atoms at the surface, while the hydrogen atoms [22] with time [23, 24]. cathodic peak is attributed to the reduction of hydrogen As all samples are multi-phases, the calculated coeffi- atoms absorbed by the alloy in the interstitial sites. As we cient of hydrogen diffusion is an average value describing can see in Fig. 5, the anodic peak current increases and its the hydrogen diffusion in the whole sample not in a specific potential shifts towards positive direction with increasing phase. From the slope of the linear region in Fig. 7,itis scan rate. possible to estimate the average coefficient of hydrogen Figure 6 gives the variation of the anodic peak potential diffusion value using the following formula, which is valid of the S30 alloy as function of log (v). This relation is at a large time [23]: linear. In this case, the anodic peak potential can be written 6FD p D logðiÞ¼ log ðC  C Þ t ð6Þ as follows: 0 s 2 2 da 2:303a dE 2:3RT ap ¼ ð5Þ 2:303a d logðiÞ d logðvÞ 2anF D ¼ ð7Þ p dt with R is the gas constant, F is the Faraday constant and where i, D, d, a, C , C and t are, respectively, the diffusion 0 s T is the temperature. current (A/g), the diffusion coefficient of hydrogen (cm /s), This equation allows the determination of the charge the density of hydrogen storage, the alloy particle radius transfer coefficient a (Table 2). This parameter indicates (cm), the initial hydrogen concentration in the bulk of alloy the alloy ability to charge and discharge. As the a value (mol/cm), and the hydrogen concentration on the surface of ranges from 0.3 to 0.7, it can be approximated close to 0.5 the alloy particle radius, respectively. Assuming that the [21] which means that the charge and discharge are average particle radius is a = 5 lm (SEM micrograph reversible and the system has the same tendency for the Fig. 2), D can be calculated according to Eq. (3) and the charge as for the discharge process. calculated values are listed in Table 2. As we can notice, the diffusion coefficient of hydrogen D shows the same Chronoamperometry Figure 7 shows the current respon- behavior than the discharge capacity as a function of MA ses expressed in log(i) versus time for the synthesized duration. Its value increases up to 20 h of MA and alloys at 298 K. The semi-logarithmic curves of anode decreases for larger time. current density versus time can be divided into two time regions. In the first one, the current decreases rapidly due to -0,84 y = -0,61561 + 0,052792x R= 0,97041 -0,845 -0,85 -2 -0,855 -4 -0,86 10 µV/s 20 µV/s -6 30 µV/s -0,865 40 µV/s 50 µV/s -8 -0,87 -10 -0,875 -12 -0,88 -5 -4,9 -4,8 -4,7 -4,6 -4,5 -4,4 -4,3 -1,1 -1 -0,9 -0,8 -0,7 -0,6 -0,5 Ewe(V) Log(v) Fig. 5 Cyclic voltammograms of S30 sample at potential scan rate: Fig. 6 Variation of the anodic peak potential of cyclic voltammo- 10, 20, 30, 40 and 50 lV/s grams of S30 sample as function of log(v) I(mA) Epic(V) 12 Page 6 of 7 Mater Renew Sustain Energy (2016) 5:12 Table 2 Charge transfer coefficient ‘a’ and hydrogen diffusion Highlights coefficient ‘‘D’’ of the S10–S30 samples -10 • Synthesis of LaCaMgNi -type alloy by mechanical Sample Charge transfer D (10 9 coefficient ‘a’ cm /s) alloying. • Effect of increasing MA duration on the electrochem- S10 0.23 4.9 ical properties of the obtained alloy. S12 0.49 5.1 • ‘a’ values range from 0.3 to 0.7. 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Materials for Renewable and Sustainable EnergySpringer Journals

Published: Jul 16, 2016

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