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Surface treatment of rare earth-magnesium–nickel based hydrogen storage alloy with lithium hydroxide aqueous solution
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 ) 4 6 2 3 e4 6 2 9
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Surface treatment of rare earth-magnesiumenickel based hydrogen storage alloy with lithium hydroxide aqueous solution Huiping Yuan*, Kang Yang, Lijun Jiang, Xiaopeng Liu, Shumao Wang General Research Institute for Nonferrous Metals, Beijing 100088, China
article info
abstract
Article history:
The alkaline treatments of rare earth-magnesiumenickel based hydrogen storage alloy
Received 10 October 2014
with lithium hydroxide (LiOH) aqueous solutions of various concentrations (1 M, 2 M, 4 M,
Received in revised form
5 M, and 6 M) were investigated. The morphology and composition of the alloy surface and
27 January 2015
the electrochemical characters of the electrode were tested. The discharge capacity and
Accepted 29 January 2015
cycle life of the alloy electrode were effectively improved after the treatment with LiOH
Available online 28 February 2015
solution of 5 M or higher. The samples treated in 5 M LiOH for 1 h and 6 M LiOH for 10 min showed better electrochemical properties than the other samples. The passive oxide and
Keywords:
hydroxide layer formed in LiOH solution increased the charge retention rate and decreased
Rare earth-magnesiumenickel
the high rate dischargeability of the alloy electrode. The high concentration LiOH solution
based
diminished the formation of oxygen containing species on the alloy surface during the
Hydrogen storage alloy
treatment. The LiOH solution can remove the Mg element on the alloy surface effectively.
Surface treatment
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Lithium hydroxide
reserved.
Electrochemical character
Introduction Hydrogen storage alloy used as the negative electrode of nickel-metal hydride (Ni-MH) battery is required to be of good electrocatalytic activity, anti-corrosion in alkaline electrolyte, and cyclic durability. Its performances are not only depended on the alloy composition but also on the surface state [1e3]. Alkaline treatment is an effective method for improving the electrochemical properties of metal hydride electrodes [4e11]. It was reported in previous studies that the high rate dischargeability, cycle life, and charge efficiency were improved after the treatment in alkaline solutions. The Ni rich sub-layer
formed during the alkaline treatment accelerated the H absorption on the alloy surface [4,5]. The surface of the hydrogen storage alloy is oxidized easily in the alkaline solution. The oxide layer affects the electrocatalytic activity seriously. Therefore, the reduced agents, such as H3PO4, NaBH4, and KBH4, were added in the alkaline solutions [6e11]. They can enhance the etching effect of the alkaline treatment and reduce the surface oxidation of the alloy powders. It was also found that the H absorption was accelerated even after the significant oxidation [12,13]. Auger electron spectroscopy (AES) results showed that the alkaline atoms in the surface oxide layers could reduce the work function for the electrons of the alloy powders and help the
* Corresponding author. E-mail address: [email protected] (H. Yuan). http://dx.doi.org/10.1016/j.ijhydene.2015.01.167 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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transportation of the electrons from the alloy surfaces to the H2 and H2O molecules [14,15]. However, there are also some disadvantages of the alkaline treatments. The repeating hydrogenation and dehydrogenation during the alkaline treatment cause the surface microcracks and the pulverization of the alloy powders. The smaller the initial grain size was, the faster the pulverization rate was [5]. It was also found the charge retention rate decreased after the alkaline treatment [8]. The previous studies of the alkaline treatments mostly focused on AB5-type alloys. Their low power characteristics and capacities limit the application of Ni-MH batteries. Recently, AB3e3.8-type rare earth-Mg-Ni based alloys with higher hydrogen storage capacities and fine electrochemical properties have been developed as the alternatives. However, their pulverization and corrosion rates in alkaline electrolyte are faster than AB5-type alloys. It was proved the nickel, cobalt, or copper coating enhanced the catalytic activity, the maximum discharge capacity, and the cyclic stability of the rare earth-Mg-Ni based alloy electrodes [16-19]. The KOH solution containing KBH4 can also improve the cycle life of La0.7Mg0.3Ni2.4Co0.6 hydrogen storage alloy [11]. Recently, Nakatsuji et al. have invented a new method for treating the rare earth-Mg-Ni based hydrogen storage alloy with LiOH solution. This method can effectively remove the oxide and hydroxide precipitations on the alloy surface [20]. It has also been reported that the existence of Li atoms in the surface oxide layer can accelerate the H2 dissociation and the H permeation through the oxide layer [21]. The effect of LiOH solution on the rare earth-Mg-Ni based hydrogen storage alloy was better than that of KOH and NaOH solutions. In this work, the effect of LiOH aqueous solution on the (REMg)2(NiAl)7 hydrogen storage alloy was investigated. To improve the cyclic stability, the charge retention rate, and other electrochemical properties, the alkaline treatments in LiOH solutions of various concentrations were carried out. The morphologies and electrochemical properties of the (REMg)2(NiAl)7 hydrogen storage alloys were tested before and after the treatment.
Experimental The commercial (REMg)2(NiAl)7 hydrogen storage alloy powders with A2B7-type crystal structure of more than 90 wt% and a small amount of AB3, AB5, and A5B19 phases were used. The alloy powders were obtained from Xiamen Tungsten Co., Ltd. To investigate the effect of alkaline treatment, the alloy powders were immersed in LiOH aqueous solutions at 363 K under mechanical stirring. The concentrations of LiOH aqueous solutions were 1 M, 2 M, 4 M, 5 M, and 6 M. The immersing times were 10 min, 20 min, and 60 min. After the completion of this step, the mixture was allowed to stand to settle the hydrogen storage alloy powders and the supernatant LiOH solution was removed. Then, the alloy powder was washed with distilled water and dried in vacuum at 333 K. The metal hydride electrode was made by mixing 200 mg pure alloy powder with 800 mg Ni powder and cold pressing the mixture into a pellet of 16 mm in diameter under 530 MPa pressure. The alloy pellet was sandwiched between two
foamed Ni plates (60 mm 20 mm) with a Ni wire soldered on to form a negative electrode. Electrochemical measurements were carried out in a half-cell consisting of metal hydride electrode as the working electrode, sintered Ni(OH)2/NiOOH as the counter electrode, and 6 M KOH solution as the electrolyte at 298 K. The working electrodes were charged at 60 mA g1 for 7.5 h followed by a 10 min rest and discharged at 60 mA g1 to the cut off potential of 1.0 V. High rate dischargeability (HRD) was described as Ci/Cmax 100%, where Ci was the discharge capacity at the discharge current density i mA g1 and Cmax denoted maximum discharge capacity at 60 mA g1. Charge and discharge current density of 300 mA g1 were conducted to test the cycle lives of the metal hydride electrodes after the activation of 10 cycles. The surface morphology of the alloy powder was observed using scanning electron microscopy (SEM) with a Hitachi-S4800 Unit operating at 10 kV. The surface state and the element distribution of the alloy powder were examined by auger electron spectroscopy (AES). AES depth profiles were measured using a PHI-700 scanning auger nanoprobe with an electron beam of 5 kV and 5 nA. The samples were sputtered with Arþ on an area of 2 mm 2 mm at 3 kV and 2 mA. The oxygen content of the alloy powder was determined by the pulse heating inert gas fusioninfrared absorption method. The magnetization was tested by vibrating sample magnetometer (VSM).
Results and discussion Surface and microstructure characteristics Fig. 1 shows the morphologies of the (REMg)2(NiAl)7 hydrogen storage alloys treated in LiOH aqueous solutions with the concentrations of 2 M, 4 M, 5 M, and 6 M. It is obvious that the alloy surfaces were covered with rod-like and needle-like precipitates. With the increase of the LiOH concentration, the amount of rod-like and needle-like products on the alloy surface increased. When the concentration reached to 6 M, the columnshaped things appeared. During the treating process, some elements dissolved in the alkaline solution, such as Mg ion, light rare-earth metal ions, and complex anions. The hydroxides of the dissolved metals in the solution increased gradually and reprecipitated on the surface of the alloy powder. With the accumulation of the hydroxides, the dissolving speeds of the metal elements went down sharply. The hydroxides served as the barrier for further corrosion [9]. The porous surfaces formed by the corrosion and etching of the alkaline solution are rich in Ni atoms that can improve the reaction activity and raise the oxidation resistance of the alloy electrodes [10]. Fig. 2 shows the energy dispersive spectrometer (EDS) analyses of O contents on the (REMg)2(NiAl)7 alloy surfaces after treated in LiOH solutions of various concentrations. It can be seen from this figure the O content first increases and then decreases with the LiOH concentration for the same treating time. When the alloy powders were treated in 6 M LiOH solution for 10 min and 20 min and in 5 M LiOH solution for 60 min, the O content reversed to decrease. This suggests that the high concentration of 6 M or lower concentration of 5 M with longer treating time can help diminish the oxygen content on the alloy powder. The O contents were also tested
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Fig. 1 e SEM images of the (REMg)2(NiAl)7 alloy powders treated in LiOH aqueous solutions with the concentrations of 2 M (a), 4 M (b), 5 M (c), and 6 M (d) for 1 h.
using infrared (IR) absorption method and listed in Table 1. The results accords well with the EDS analyses. The weight percentage of oxygen element first increases and then decreases with the increase of the LiOH concentration after the alloy was treated for 1 h.
Electrochemical properties The electrochemical properties of the alloy electrodes are depended on the composition and structure of the alloy
surfaces. Fig. 3 shows the cycle life curves of the untreated and treated alloy electrodes after the activation of 10 cycles. The alloy electrodes treated in 2 M and 4 M LiOH aqueous solutions needed much more cycles to be activated. The discharge capacities of the two electrodes were lower than that of the untreated alloy electrode before they have been activated completely. We think the reason is that the insufficient corrosion and the effect of oxygen containing species on the alloy surface. The discharge capacities and cyclic stabilities of the alloy electrodes were improved noticeably after treated in 5 M and 6 M LiOH aqueous solutions for 1 h. The alloy treated in 5 M and 6 M LiOH solutions were corroded effectively. The enriched Ni layer was formed and the oxygen content was lower on the alloy powder. As can be seen in Fig. 4, the alloy electrode treated in 5 M LiOH for 1 h shows lower charge voltage than that of the untreated alloy electrode. It suggests
Table 1 e Oxygen and magnetic material contents of the alloy powders treated in various concentrations of LiOH solutions. Sample
Fig. 2 e EDS weight percentages of O element on the surfaces of the alloy powders treated in various concentrations (1 M, 2 M, 4 M, 5 M, and 6 M) of LiOH aqueous solutions.
Untreated 2 M LiOH 1 h 4 M LiOH 1 h 5 M LiOH 1 h 6 M LiOH 1 h 6 M LiOH 20 min 6 M LiOH 10 min
Oxygen content by IR (wt%)
Magnetic material content (wt%)
0.14 0.43 1.14 0.52 0.70 0.63
0.40 0.43 2.51 1.433 1.81 1.22
0.28
0.54
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Fig. 3 e Effect of LiOH concentration on the cycle life of the (REMg)2(NiAl)7 hydrogen storage alloy electrode.
Fig. 5 e Effect of LiOH concentration on the high rate dischargeability of the (REMg)2(NiAl)7 hydrogen storage alloy electrode.
the alkaline treatment enhances the charge efficiency of the alloy electrode [9]. Fig. 5 shows the effect of LiOH treatment on the high rate dischargeability of (REMg)2(NiAl)7 hydrogen storage alloy electrode. The HRD of the alloy electrode deteriorated after the alkaline treatment. In previous studies, the MH electrode exhibited better HRD after the alkaline treatment [3,10,11] due to the addition of reducing agents in the alkaline solution. The reducing agents decreased the amount of oxides and hydroxides on the alloy surfaces, which was in favor of accelerating the diffusion rate of hydrogen atoms and enhancing the electric conductivity. In our study, although the oxidation layer on the alloy surface decreased the high rate dischargeability, the charge retention rate increased after the alkaline treatment (Fig. 6). Based on the electrochemical measurements above, the orders of the discharge capacity, cyclic durability, high rate discharge capability, and charge retention rate of the alloy electrodes treated in LiOH solutions of different concentrations are as follows:
The comprehensive electrochemical performance of the sample treated in 5 M LiOH for 1 h was the best. The alloy powders treated in 4 M LiOH solution for 1 h were most difficult to be activated and showed the lowest high rate discharge capability because of the high oxygen content. However, the charge retention rate was the highest. Nickel is fairly stable in the alkaline solution. It is the only strong magnet in the alloy powder in our experiment. The magnetic material content was calculated by dividing the magnetization of the sample by the magnetization of pure Ni powder. It can be taken as an indication of the corrosion extent by the alkaline solution and the amount of the Ni atoms in the sub-layer of the alloy powders. As is shown in Table 1,
Fig. 4 e Voltage versus time curves of the untreated and treated alloy electrodes during charging and discharging.
Fig. 6 e Effect of LiOH concentration on the charge retention rate of the (REMg)2(NiAl)7 hydrogen storage alloy electrode.
(1) Discharge capacity and cyclic durability: 5M > 6 M > 2M > Untreated ~4 M, (2) High rate discharge capability: Untreated >5M > 2 M > 6M > 4 M, (3) Charge retention rate under open circuit: 4M > 5 Me6M > 2 M > Untreated.
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the magnetic material content of the treated alloy powder significantly increases, when the concentration of LiOH solution is equal to or larger than 4 M. The oxygen content also increases with the magnetic material content. The oxygen containing species decreased the high rate dischargeability and the activation property of the alloy electrode. Therefore, the corrosion extent and the amount of the oxide and hydroxide re-precipitates are two related factors affecting the electrochemical performances of the alloy electrode. The LiOH solutions of 5 M and 6 M can not only corrode the alloy surfaces effectively but also inhibit the formation of too many oxides and hydroxides. The composition and structure changes on the alloy surface enhanced the corrosion resistance and suppressed the decrease of the capacity.
Effect of treating time Fig. 7 shows the AES depth profiles of the alloy powders. Fig. 7(b), (c), and (d) illustrate the relative atomic percentages of Mg, Ni, Al, and rare-earth elements excluding O element before and after the alloy powders were treated in 6 M LiOH solution for 10 min and 1 h. It can be seen that Ni atomic concentration significantly increases from 43% to 58% after a short time treatment of 10 min. Then the Ni atomic concentration increases only a little after treated for 1 h. The Mg element in rare earth-Mg-Ni-based alloy is easily oxidized to MgO because of its exposure to air [22]. The Mg atomic
Fig. 8 e Effect of treating time on the cycle life of the (REMg)2(NiAl)7 hydrogen storage alloy electrode.
concentration is more than 20% on the untreated alloy surface (Fig. 7(b)). After 1 h treatment in 6 M LiOH solution, the Mg content decreases from 22% to less than 5% (Fig. 7(d)). The Nirich layer is in favor of improving the discharge capacity and the cycle life of the alloy electrode. Therefore, the discharge capacity and the cycle life of the alloy electrodes were improved after the treatment (Fig. 8). However, high Ni
Fig. 7 e AES depth profiles of the (REMg)2(NiAl)7 hydrogen storage alloy powder before and after alkaline treatments.
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Besides, the LiOH solution can effectively remove the Mg element that is unfavorable for the electrochemical performances of the alloy electrode.
Acknowledgments The authors wish to acknowledge the financial supports provided by the Rare Earth and Rare Metal New Materials Research and Development (R & D) and Industrialization Special Fund and the rare earth industry adjustment and upgrade special fund of Ministry of Industry and Information Technology (MIIT).
Fig. 9 e Effect of treating time on the charge retention rate of the (REMg)2(NiAl)7 hydrogen storage alloy electrode.
content in the sub-layer corresponds to the excessive corrosion of the alloy surface and the reduction of the discharge capacity. At the same time, the rare earth oxides and hydroxides were formed in large amounts on the alloy surface after a long time treatment. Therefore, the discharge capacity of the alloy electrode treated for 1 h was lower than that of the samples treated for 10 min and 20 min (Fig. 8). Fig. 9 shows the effect of treating time on the charge retention rate of the (REMg)2(NiAl)7 hydrogen storage alloy electrode. The charge retention rate increases with the treating time. As is shown in Fig. 7(a), the O contents increase with the treating time, which suggests the passive oxidation layers become thicker. It is also shown in Table 1 that the content of oxygen increases with the treating time. Therefore, we think that the oxidation layer formed in LiOH solution restrains the H desorption and can reduce the self-discharge rate of the alloy electrode.
Conclusions The (REMg)2(NiAl)7 hydrogen storage alloys were pretreated in LiOH aqueous solutions of various concentrations (1 M, 2 M, 4 M, 5 M, and 6 M). The morphologies and compositions of the alloy surfaces and the electrochemical characters of the electrodes were tested. The O content first increased and then decreased with the LiOH concentration. The high concentration LiOH solution could inhibit the formation of oxygen containing species during the treatment. The surface passive oxide and hydroxide layer formed in the LiOH solution decreased the activation and high rate discharge capability but increased the charge retention rate of the alloy electrode. The discharge capacity and cycle life of the alloy electrode were improved after the pretreatment in LiOH solution with the concentration higher than 5 M. The samples treated in 5 M LiOH solution for 1 h and 6 M LiOH solution for 10 min showed better electrochemical properties than the other samples. It was attributed to the Ni rich layer on the alloy surface and the appropriate corrosion and etching in the LiOH solution.
references
[1] Zhao XY, Ma LQ, Ding Y, Yang M, Shen XD. Novel surface treatment for hydrogen storage alloy in Ni/MH battery. Int J Hydrogen Energy 2009;34:3506e10. [2] Chen WX. Effect of surface treatments of MlNi4.0Co0.6Al0.4 hydrogen storage alloy on the activation, charge/discharge cycle and degradation of Ni/MH batteries. J Power Sources 2001;92:102e7. [3] Zhao XY, Ding Y, Yang M, Ma LQ. Effect of surface treatment on electrochemical properties of MmNi3.8Co0.75Mn0.4Al0.2 hydrogen storage alloy. Int J Hydrogen Energy 2008;33:81e6. [4] Wu MS, Wu HR, Wang YY, Wan CC. Electrochemical investigation of hydrogen-storage alloy electrode with duplex surface modification. Int J Hydrogen Energy 2004;29:1263e9. [5] Ozu Y, Kuji T, Uchida HH. Pulverization behavior of LaNi5 with alkaline pretreatment. J Alloys Compd 2002;330e2:632e5. [6] Lee HJ, Yang DC, Park CJ, Park CN, Jang HJ. Effect of surface modifications of the LMNi3.9Co0.6Mn0.3Al0.2 alloy in a KOH/ NaBH4 solution upon its electrode characteristics within a Ni-MH secondary battery. Int J Hydrogen Energy 2009;34:481e6. [7] Chen WX, Xu ZD, Tu JP, Li HY, Yuan J, Chen S, et al. Hydrogen adsorption on hydrogen storage alloy surface and electrochemical performances of the MlNi4.0Co0.6Al0.4 alloy electrodes before and after surface treatment. Int J Hydrogen Energy 2001;26:675e81. [8] Kim HK, Yang DC, Jang IS, Park CN, Park CJ, Choi J. Effects of pretreatment of LM-Ni3.9Co0.6Mn0.3Al0.2 alloy powders in a KOH/NaBH4 solution on the electrode characteristics and inner pressure of nickel-metal-hydride secondary batteries. Int J Hydrogen Energy 2009;34:9570e5. [9] Su G, He YH, Liu KY. Effects of pretreatment on MlNi4.00Co0.45Mn0.38Al0.3 hydrogen storage alloy powders and the performance of 6 Ah prismatic traction battery. Int J Hydrogen Energy 2012;37:12384e92. [10] Santos DMF, Sequeira CAC, Lobo RFM. Effect of alloys modified by sodium borohydride alkaline solutions on the kinetics of hydrogen evolution reaction at Mm(Ni3.6Co0.7Mn0.4Al0.3)1.15 hydride electrodes. Int J Hydrogen Energy 2010;35:9901e9. [11] Xiao LL, Wang YJ, Liu Y, Song DW, Jiao LF, Yuan HT. Influence of surface treatments on microstructure and electrochemical properties of La0.7Mg0.3Ni2.4Co0.6 hydrogen-storage alloy. Int J Hydrogen Energy 2008;33:3925e9. [12] Uchida HH, Moriai K, Aoyama K, Kondo H, Uchida H. Effect of alkaline pretreatment on the initial activation of LaNi5 and
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LaNi2.5Co2.5 in the H2 gas and electrochemical reactions. J Alloys Compd 1997;253e4:525e8. Wang GL, Wang XY, Miao RR, Cao DX, Sun KN. Effects of alkaline treatment of hydrogen storage alloy on electrocatalytic activity for NaBH4 oxidation. Int J Hydrogen Energy 2010;35:1227e31. Uchida H, Yamashita K, Tabata T, Uchida HH, Iwase M, Katoh Y. Effect of alakaline pretreatments (KOH, NaOH, LiOH) on electrochemical hydriding rate and work function of LaNi2.5Co2.5. J Alloys Compd 1999;293e5:751e5. Uchida H, Yamashita K, Goto M. Comparison of pretreatments by alkaline (Li, Na, K) hydroxides with NH3H2O in the work function and hydriding kinetics of LaNi2.5Co0.5. J Alloys Compd 2002;330e2:622e6. Bai TY, Han SM, Zhu XL, Zhang Y, Li Y, Zhang WC. Effect of duplex surface treatment of electrochemical properties of AB3-type La0.88Mg0.12Ni2.95Mn0.1Co0.55Al0.1 hydrogen storage alloy. Mater Chem Phys 2009;117:173e7. Yang SQ, Liu HP, Han SM. Effects of electroless composite plating NieCueP on the electrochemical properties of
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LaeMgeNi-based hydrogen storage alloy. Appl Surf Sci 2013;271:210e5. Xiang ZY, Han XL, Fan X. Effect of electroless nickel plating on electrochemical properties of LaMgNi3.7Co0.3 hydrogen storage alloy. Electroplat Finish 2012;31:8e11. Ding HL, Han SM, Liu Y, Hao JS, Li Y, Zhang JW. Electrochemical performance studies on cobalt and nickel electroplated LaeMgeNi-based hydrogen storage alloys. Int J Hydrogen Energy 2009;34:9402e8. K. Nakatsuji, H. Ohyama, Hydrogen-absorbing alloy powder, method for treating the surface therefore, negative electrode for alkaline storage battery, and alkaline storage battery. US 2011/0033748, 2011e02e10. Uchida HH, Suzuki K, Kubo S, Kondo H. Improvement of H2 absorption of LaNi5 by LiOH pretreatment. Int J Hydrogen Energy 1999;24:879e83. Liu YF, Pan HG, Yue YJ, Wu XF, Chen N, Lei YQ. Cycling durability and degradation behavior of LaeMgeNieCo-type metal hydride electrodes. J Alloys Compd 2005;395:291e9.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Surface treatment of rare earth-magnesiumenickel based hydrogen storage alloy with lithium hydroxide aqueous solution Huiping Yuan*, Kang Yang, Lijun Jiang, Xiaopeng Liu, Shumao Wang General Research Institute for Nonferrous Metals, Beijing 100088, China
article info
abstract
Article history:
The alkaline treatments of rare earth-magnesiumenickel based hydrogen storage alloy
Received 10 October 2014
with lithium hydroxide (LiOH) aqueous solutions of various concentrations (1 M, 2 M, 4 M,
Received in revised form
5 M, and 6 M) were investigated. The morphology and composition of the alloy surface and
27 January 2015
the electrochemical characters of the electrode were tested. The discharge capacity and
Accepted 29 January 2015
cycle life of the alloy electrode were effectively improved after the treatment with LiOH
Available online 28 February 2015
solution of 5 M or higher. The samples treated in 5 M LiOH for 1 h and 6 M LiOH for 10 min showed better electrochemical properties than the other samples. The passive oxide and
Keywords:
hydroxide layer formed in LiOH solution increased the charge retention rate and decreased
Rare earth-magnesiumenickel
the high rate dischargeability of the alloy electrode. The high concentration LiOH solution
based
diminished the formation of oxygen containing species on the alloy surface during the
Hydrogen storage alloy
treatment. The LiOH solution can remove the Mg element on the alloy surface effectively.
Surface treatment
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Lithium hydroxide
reserved.
Electrochemical character
Introduction Hydrogen storage alloy used as the negative electrode of nickel-metal hydride (Ni-MH) battery is required to be of good electrocatalytic activity, anti-corrosion in alkaline electrolyte, and cyclic durability. Its performances are not only depended on the alloy composition but also on the surface state [1e3]. Alkaline treatment is an effective method for improving the electrochemical properties of metal hydride electrodes [4e11]. It was reported in previous studies that the high rate dischargeability, cycle life, and charge efficiency were improved after the treatment in alkaline solutions. The Ni rich sub-layer
formed during the alkaline treatment accelerated the H absorption on the alloy surface [4,5]. The surface of the hydrogen storage alloy is oxidized easily in the alkaline solution. The oxide layer affects the electrocatalytic activity seriously. Therefore, the reduced agents, such as H3PO4, NaBH4, and KBH4, were added in the alkaline solutions [6e11]. They can enhance the etching effect of the alkaline treatment and reduce the surface oxidation of the alloy powders. It was also found that the H absorption was accelerated even after the significant oxidation [12,13]. Auger electron spectroscopy (AES) results showed that the alkaline atoms in the surface oxide layers could reduce the work function for the electrons of the alloy powders and help the
* Corresponding author. E-mail address: [email protected] (H. Yuan). http://dx.doi.org/10.1016/j.ijhydene.2015.01.167 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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transportation of the electrons from the alloy surfaces to the H2 and H2O molecules [14,15]. However, there are also some disadvantages of the alkaline treatments. The repeating hydrogenation and dehydrogenation during the alkaline treatment cause the surface microcracks and the pulverization of the alloy powders. The smaller the initial grain size was, the faster the pulverization rate was [5]. It was also found the charge retention rate decreased after the alkaline treatment [8]. The previous studies of the alkaline treatments mostly focused on AB5-type alloys. Their low power characteristics and capacities limit the application of Ni-MH batteries. Recently, AB3e3.8-type rare earth-Mg-Ni based alloys with higher hydrogen storage capacities and fine electrochemical properties have been developed as the alternatives. However, their pulverization and corrosion rates in alkaline electrolyte are faster than AB5-type alloys. It was proved the nickel, cobalt, or copper coating enhanced the catalytic activity, the maximum discharge capacity, and the cyclic stability of the rare earth-Mg-Ni based alloy electrodes [16-19]. The KOH solution containing KBH4 can also improve the cycle life of La0.7Mg0.3Ni2.4Co0.6 hydrogen storage alloy [11]. Recently, Nakatsuji et al. have invented a new method for treating the rare earth-Mg-Ni based hydrogen storage alloy with LiOH solution. This method can effectively remove the oxide and hydroxide precipitations on the alloy surface [20]. It has also been reported that the existence of Li atoms in the surface oxide layer can accelerate the H2 dissociation and the H permeation through the oxide layer [21]. The effect of LiOH solution on the rare earth-Mg-Ni based hydrogen storage alloy was better than that of KOH and NaOH solutions. In this work, the effect of LiOH aqueous solution on the (REMg)2(NiAl)7 hydrogen storage alloy was investigated. To improve the cyclic stability, the charge retention rate, and other electrochemical properties, the alkaline treatments in LiOH solutions of various concentrations were carried out. The morphologies and electrochemical properties of the (REMg)2(NiAl)7 hydrogen storage alloys were tested before and after the treatment.
Experimental The commercial (REMg)2(NiAl)7 hydrogen storage alloy powders with A2B7-type crystal structure of more than 90 wt% and a small amount of AB3, AB5, and A5B19 phases were used. The alloy powders were obtained from Xiamen Tungsten Co., Ltd. To investigate the effect of alkaline treatment, the alloy powders were immersed in LiOH aqueous solutions at 363 K under mechanical stirring. The concentrations of LiOH aqueous solutions were 1 M, 2 M, 4 M, 5 M, and 6 M. The immersing times were 10 min, 20 min, and 60 min. After the completion of this step, the mixture was allowed to stand to settle the hydrogen storage alloy powders and the supernatant LiOH solution was removed. Then, the alloy powder was washed with distilled water and dried in vacuum at 333 K. The metal hydride electrode was made by mixing 200 mg pure alloy powder with 800 mg Ni powder and cold pressing the mixture into a pellet of 16 mm in diameter under 530 MPa pressure. The alloy pellet was sandwiched between two
foamed Ni plates (60 mm 20 mm) with a Ni wire soldered on to form a negative electrode. Electrochemical measurements were carried out in a half-cell consisting of metal hydride electrode as the working electrode, sintered Ni(OH)2/NiOOH as the counter electrode, and 6 M KOH solution as the electrolyte at 298 K. The working electrodes were charged at 60 mA g1 for 7.5 h followed by a 10 min rest and discharged at 60 mA g1 to the cut off potential of 1.0 V. High rate dischargeability (HRD) was described as Ci/Cmax 100%, where Ci was the discharge capacity at the discharge current density i mA g1 and Cmax denoted maximum discharge capacity at 60 mA g1. Charge and discharge current density of 300 mA g1 were conducted to test the cycle lives of the metal hydride electrodes after the activation of 10 cycles. The surface morphology of the alloy powder was observed using scanning electron microscopy (SEM) with a Hitachi-S4800 Unit operating at 10 kV. The surface state and the element distribution of the alloy powder were examined by auger electron spectroscopy (AES). AES depth profiles were measured using a PHI-700 scanning auger nanoprobe with an electron beam of 5 kV and 5 nA. The samples were sputtered with Arþ on an area of 2 mm 2 mm at 3 kV and 2 mA. The oxygen content of the alloy powder was determined by the pulse heating inert gas fusioninfrared absorption method. The magnetization was tested by vibrating sample magnetometer (VSM).
Results and discussion Surface and microstructure characteristics Fig. 1 shows the morphologies of the (REMg)2(NiAl)7 hydrogen storage alloys treated in LiOH aqueous solutions with the concentrations of 2 M, 4 M, 5 M, and 6 M. It is obvious that the alloy surfaces were covered with rod-like and needle-like precipitates. With the increase of the LiOH concentration, the amount of rod-like and needle-like products on the alloy surface increased. When the concentration reached to 6 M, the columnshaped things appeared. During the treating process, some elements dissolved in the alkaline solution, such as Mg ion, light rare-earth metal ions, and complex anions. The hydroxides of the dissolved metals in the solution increased gradually and reprecipitated on the surface of the alloy powder. With the accumulation of the hydroxides, the dissolving speeds of the metal elements went down sharply. The hydroxides served as the barrier for further corrosion [9]. The porous surfaces formed by the corrosion and etching of the alkaline solution are rich in Ni atoms that can improve the reaction activity and raise the oxidation resistance of the alloy electrodes [10]. Fig. 2 shows the energy dispersive spectrometer (EDS) analyses of O contents on the (REMg)2(NiAl)7 alloy surfaces after treated in LiOH solutions of various concentrations. It can be seen from this figure the O content first increases and then decreases with the LiOH concentration for the same treating time. When the alloy powders were treated in 6 M LiOH solution for 10 min and 20 min and in 5 M LiOH solution for 60 min, the O content reversed to decrease. This suggests that the high concentration of 6 M or lower concentration of 5 M with longer treating time can help diminish the oxygen content on the alloy powder. The O contents were also tested
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Fig. 1 e SEM images of the (REMg)2(NiAl)7 alloy powders treated in LiOH aqueous solutions with the concentrations of 2 M (a), 4 M (b), 5 M (c), and 6 M (d) for 1 h.
using infrared (IR) absorption method and listed in Table 1. The results accords well with the EDS analyses. The weight percentage of oxygen element first increases and then decreases with the increase of the LiOH concentration after the alloy was treated for 1 h.
Electrochemical properties The electrochemical properties of the alloy electrodes are depended on the composition and structure of the alloy
surfaces. Fig. 3 shows the cycle life curves of the untreated and treated alloy electrodes after the activation of 10 cycles. The alloy electrodes treated in 2 M and 4 M LiOH aqueous solutions needed much more cycles to be activated. The discharge capacities of the two electrodes were lower than that of the untreated alloy electrode before they have been activated completely. We think the reason is that the insufficient corrosion and the effect of oxygen containing species on the alloy surface. The discharge capacities and cyclic stabilities of the alloy electrodes were improved noticeably after treated in 5 M and 6 M LiOH aqueous solutions for 1 h. The alloy treated in 5 M and 6 M LiOH solutions were corroded effectively. The enriched Ni layer was formed and the oxygen content was lower on the alloy powder. As can be seen in Fig. 4, the alloy electrode treated in 5 M LiOH for 1 h shows lower charge voltage than that of the untreated alloy electrode. It suggests
Table 1 e Oxygen and magnetic material contents of the alloy powders treated in various concentrations of LiOH solutions. Sample
Fig. 2 e EDS weight percentages of O element on the surfaces of the alloy powders treated in various concentrations (1 M, 2 M, 4 M, 5 M, and 6 M) of LiOH aqueous solutions.
Untreated 2 M LiOH 1 h 4 M LiOH 1 h 5 M LiOH 1 h 6 M LiOH 1 h 6 M LiOH 20 min 6 M LiOH 10 min
Oxygen content by IR (wt%)
Magnetic material content (wt%)
0.14 0.43 1.14 0.52 0.70 0.63
0.40 0.43 2.51 1.433 1.81 1.22
0.28
0.54
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Fig. 3 e Effect of LiOH concentration on the cycle life of the (REMg)2(NiAl)7 hydrogen storage alloy electrode.
Fig. 5 e Effect of LiOH concentration on the high rate dischargeability of the (REMg)2(NiAl)7 hydrogen storage alloy electrode.
the alkaline treatment enhances the charge efficiency of the alloy electrode [9]. Fig. 5 shows the effect of LiOH treatment on the high rate dischargeability of (REMg)2(NiAl)7 hydrogen storage alloy electrode. The HRD of the alloy electrode deteriorated after the alkaline treatment. In previous studies, the MH electrode exhibited better HRD after the alkaline treatment [3,10,11] due to the addition of reducing agents in the alkaline solution. The reducing agents decreased the amount of oxides and hydroxides on the alloy surfaces, which was in favor of accelerating the diffusion rate of hydrogen atoms and enhancing the electric conductivity. In our study, although the oxidation layer on the alloy surface decreased the high rate dischargeability, the charge retention rate increased after the alkaline treatment (Fig. 6). Based on the electrochemical measurements above, the orders of the discharge capacity, cyclic durability, high rate discharge capability, and charge retention rate of the alloy electrodes treated in LiOH solutions of different concentrations are as follows:
The comprehensive electrochemical performance of the sample treated in 5 M LiOH for 1 h was the best. The alloy powders treated in 4 M LiOH solution for 1 h were most difficult to be activated and showed the lowest high rate discharge capability because of the high oxygen content. However, the charge retention rate was the highest. Nickel is fairly stable in the alkaline solution. It is the only strong magnet in the alloy powder in our experiment. The magnetic material content was calculated by dividing the magnetization of the sample by the magnetization of pure Ni powder. It can be taken as an indication of the corrosion extent by the alkaline solution and the amount of the Ni atoms in the sub-layer of the alloy powders. As is shown in Table 1,
Fig. 4 e Voltage versus time curves of the untreated and treated alloy electrodes during charging and discharging.
Fig. 6 e Effect of LiOH concentration on the charge retention rate of the (REMg)2(NiAl)7 hydrogen storage alloy electrode.
(1) Discharge capacity and cyclic durability: 5M > 6 M > 2M > Untreated ~4 M, (2) High rate discharge capability: Untreated >5M > 2 M > 6M > 4 M, (3) Charge retention rate under open circuit: 4M > 5 Me6M > 2 M > Untreated.
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the magnetic material content of the treated alloy powder significantly increases, when the concentration of LiOH solution is equal to or larger than 4 M. The oxygen content also increases with the magnetic material content. The oxygen containing species decreased the high rate dischargeability and the activation property of the alloy electrode. Therefore, the corrosion extent and the amount of the oxide and hydroxide re-precipitates are two related factors affecting the electrochemical performances of the alloy electrode. The LiOH solutions of 5 M and 6 M can not only corrode the alloy surfaces effectively but also inhibit the formation of too many oxides and hydroxides. The composition and structure changes on the alloy surface enhanced the corrosion resistance and suppressed the decrease of the capacity.
Effect of treating time Fig. 7 shows the AES depth profiles of the alloy powders. Fig. 7(b), (c), and (d) illustrate the relative atomic percentages of Mg, Ni, Al, and rare-earth elements excluding O element before and after the alloy powders were treated in 6 M LiOH solution for 10 min and 1 h. It can be seen that Ni atomic concentration significantly increases from 43% to 58% after a short time treatment of 10 min. Then the Ni atomic concentration increases only a little after treated for 1 h. The Mg element in rare earth-Mg-Ni-based alloy is easily oxidized to MgO because of its exposure to air [22]. The Mg atomic
Fig. 8 e Effect of treating time on the cycle life of the (REMg)2(NiAl)7 hydrogen storage alloy electrode.
concentration is more than 20% on the untreated alloy surface (Fig. 7(b)). After 1 h treatment in 6 M LiOH solution, the Mg content decreases from 22% to less than 5% (Fig. 7(d)). The Nirich layer is in favor of improving the discharge capacity and the cycle life of the alloy electrode. Therefore, the discharge capacity and the cycle life of the alloy electrodes were improved after the treatment (Fig. 8). However, high Ni
Fig. 7 e AES depth profiles of the (REMg)2(NiAl)7 hydrogen storage alloy powder before and after alkaline treatments.
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Besides, the LiOH solution can effectively remove the Mg element that is unfavorable for the electrochemical performances of the alloy electrode.
Acknowledgments The authors wish to acknowledge the financial supports provided by the Rare Earth and Rare Metal New Materials Research and Development (R & D) and Industrialization Special Fund and the rare earth industry adjustment and upgrade special fund of Ministry of Industry and Information Technology (MIIT).
Fig. 9 e Effect of treating time on the charge retention rate of the (REMg)2(NiAl)7 hydrogen storage alloy electrode.
content in the sub-layer corresponds to the excessive corrosion of the alloy surface and the reduction of the discharge capacity. At the same time, the rare earth oxides and hydroxides were formed in large amounts on the alloy surface after a long time treatment. Therefore, the discharge capacity of the alloy electrode treated for 1 h was lower than that of the samples treated for 10 min and 20 min (Fig. 8). Fig. 9 shows the effect of treating time on the charge retention rate of the (REMg)2(NiAl)7 hydrogen storage alloy electrode. The charge retention rate increases with the treating time. As is shown in Fig. 7(a), the O contents increase with the treating time, which suggests the passive oxidation layers become thicker. It is also shown in Table 1 that the content of oxygen increases with the treating time. Therefore, we think that the oxidation layer formed in LiOH solution restrains the H desorption and can reduce the self-discharge rate of the alloy electrode.
Conclusions The (REMg)2(NiAl)7 hydrogen storage alloys were pretreated in LiOH aqueous solutions of various concentrations (1 M, 2 M, 4 M, 5 M, and 6 M). The morphologies and compositions of the alloy surfaces and the electrochemical characters of the electrodes were tested. The O content first increased and then decreased with the LiOH concentration. The high concentration LiOH solution could inhibit the formation of oxygen containing species during the treatment. The surface passive oxide and hydroxide layer formed in the LiOH solution decreased the activation and high rate discharge capability but increased the charge retention rate of the alloy electrode. The discharge capacity and cycle life of the alloy electrode were improved after the pretreatment in LiOH solution with the concentration higher than 5 M. The samples treated in 5 M LiOH solution for 1 h and 6 M LiOH solution for 10 min showed better electrochemical properties than the other samples. It was attributed to the Ni rich layer on the alloy surface and the appropriate corrosion and etching in the LiOH solution.
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