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Electrochemical and structural characterization of electroless nickel coating on Mg2Ni hydrogen storage alloy
Journal of Alloys and Compounds 580 (2013) S368–S372
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Electrochemical and structural characterization of electroless nickel coating on Mg2Ni hydrogen storage alloy Reiko Ohara a,⇑, Chao-Ho Lan a, Chii-Shyang Hwang b a b
Green Energy & Ecology Research Labs., Industrial Technology Research Institute, Tainan City 734, Taiwan, ROC Department of Materials Science and Engineering, National Cheng Kung University, Taiwan, ROC
a r t i c l e
i n f o
Article history: Available online 19 March 2013 Keywords: Magnesium alloys Electroless nickel coating Hydrogen storage Discharge capacity Impedance spectroscopy
a b s t r a c t Mg2Ni alloys were prepared by high-energy ball milling, followed by an electroless nickel coating, and their electrochemical properties were investigated. The results indicated that the thickness of Ni–P coatings determined the discharge capacity of Ni-coated Mg2Ni alloys. The measuring amount of deposited Ni particles was used to represent the thickness of Ni–P coatings. With an increase in the amount of deposited Ni particles, the discharge capacity reached to a maximum value of 148 mA h g 1 at the deposited Ni particles of 2320 ppm, followed by declining to 106 mA h g 1. From the electrochemical impedance spectroscopy experiments, the charge transfer resistance increased and meanwhile the diffusion coefficient decreased, if the amount of deposited Ni particles was greater than 2320 ppm. According to the X-ray diffraction studies, with an increase in the deposited amount of Ni particles the structure of Ni–P spherical particles, with a diameter of 50–250 nm, changed from nanocrystalline to amorphousness. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction A magnesium-based hydrogen storage alloy is one of the promising negative electrode materials for Ni–MH batteries. For instance, Mg2Ni alloys have a high theoretical discharge capacity close to 1000 mA h g 1, other than advantages of light weight and low cost [1]. However, a poor hydriding reaction rate at room temperature and a short charge/discharge cycle have limited the practical use of Magnesium-based alloy electrodes. A rapid capacity decay rate upon cycling is attributed to Mg oxidation occurring in alkaline solutions. Mg is a highly active metal with the standard electrochemical potential of 2.4 V vs. NHE; even in aqueous solutions, the standard electrochemical potential of Mg still keeps 1.5 V. Thus, Mg dissolves rapidly to form Mg(OH)2 film in an aqueous solution below pH 11 [2]. Till now, many techniques that include surface modifications, alloy element substitution, alloy composites, and oxide additions have been developed to improve the electrochemical hydriding/dehydriding behavior of Mg-based alloys in alkaline solutions [3,4]. Among these techniques, a surface modification of alloy powders with an electroless coating [2,5–10] is an effective and simple way to avoid deterioration and pulverization of Mg-based alloys, and to facilitate electric conductivity and hydrogen diffusion as well. It was reported that the deposited metals act as a protective barrier and a current collector in the electrochemical hydriding/dehydriding performance of Mg-based alloys in alkaline solutions [11]. Regarding to an electroless nickel coating, ⇑ Corresponding author. Tel.: +886 6 693 9265; fax: +886 6 693 9096. E-mail address: [email protected] (R. Ohara). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.03.009
the reduction of nickel basically results from the catalytic dehydrogenation of reducing agents which contains reactive hydrogens. The deposited content is not pure nickel but contains either phosphorus or boron, depending on what type of reducing agents is used. However, the electroless nickel coating process is very complex due to its special self-catalyzed chemical reactions. The electroless coating reaction not only yields Ni–P or Ni–B deposits, it also generates by-products, such as orthophosphite and sodium sulfate. As the concentration of by-products increases, the nickel deposition rate decreases [12]. Most studies related to the electroless nickel coating have focused on the electrocatalytic effect of nickel atoms on the electrochemical hydriding/dehydriding properties of Mg-based alloys. However, in the electroless process nickel atoms are co-deposited with P atoms to form Ni–P deposits on the Mg-based alloy surfaces if hypophosphite is used as a reducing agent. Nowadays there is little fundamental investigation taking into account the impact of the Ni–P deposits on the electrochemical hydriding/dehydriding properties of Mg-based hydrogen storage alloys. In this work, Mg2Ni alloys were synthesized by ball milling and thereby the electroless nickel coating was carried out. The amount of deposited Ni particles was determined to represent the thickness of Ni–P coatings and their electrochemical and structural characteristics on Mg2Ni alloys were investigated. 2. Experimental Mg2Ni alloys were prepared by mixing appropriate amounts of elemental Mg with Ni powders (325 mesh, >99.8% from Alfa Aesar) in the glove box under an Ar atmosphere. The mechanical alloying was preformed with a high-energy ball mill for 15 h under an Ar atmosphere to avoid the possible oxidation of alloy pow-
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R. Ohara et al. / Journal of Alloys and Compounds 580 (2013) S368–S372 Table 1 Chemical composition and physical condition of coating bath. Component
Concentration (mol L
NiSO46H2O Na3C6H5O72H2O NaH2PO2H2O
0.25 0.4 0–0.5
Temperature pH
80 °C 5
the diffusion of hydrogen in alkaline solutions. Plots of the discharge capacity and the deposited Ni amount as a function of the concentration of NaH2PO2 are presented in Fig. 2. With an increase
1
)
ders. The obtained Mg2Ni alloy powders (<200 mesh) were electroless coated using a coating solution shown in Table 1. The coated alloy powders were filtered and dried at 70 °C for 1 h. The filtrate was collected for quantitative determination of nickel using THERMOM5 Atomic Absorption Spectrophotometer (AA). The amount of nickel deposited on the surface of Mg2Ni alloys is defined as C Cfil, where C represents the amount of nickel before electroless coating and Cfil represents the amount of nickel in the filtrate. The phase structure and the morphology of the coated alloy powders were respectively examined by using a Rigaku Miniflex II X-ray diffractometer (XRD), a JEOL JSM-7001 field-emission scanning electron microscope (FE-SEM) and a JEOL JEM-2010F field-emission transmission electron microscope (FE-TEM) equipped with EDS. The dried powders were mixed with 7.69% of polyvinyl alcohol solution, followed by pasting into nickel foam (5.5 12 mm). The electrodes were dried at 75 °C for 1 h, and then pressed under a pressure of 50 kg cm 2. A Ni(OH)2 counter electrode and a Hg/HgO reference electrode were used to assemble a three-electrode cell in 6 mol L 1 of KOH solution. Charging/discharging electrochemical experiments were conducted in a galvanostatic mode using multi-channel potentio-galvano stat (Maccor 4300 battery test system). The activated electrodes were charged at 100 mA g 1 for 8 h and discharged to 0.6 V vs. Hg/HgO at 25 mA g 1 in 6 mol L 1 of KOH solution. By using an electrochemical analyzer (CHI Model 600B), the electrochemical impedance spectrum (EIS) was measured at room temperature under open-circuit conditions. The AC amplitude of perturbation was set at 5 mV and the frequency was varied from 10 kHz to 0.005 Hz.
Fig. 2. Plots of discharge capacity and deposited Ni amount as a function of NaH2PO2 concentration.
3. Results and discussion 3.1. Discharge capacity The cycle performance of ball-milled Mg2Ni and Ni-coated Mg2Ni electrodes are presented in Fig. 1. Obviously, a Ni-coating effectively improved the discharge capacity of Mg2Ni alloys but the cyclic stability was not satisfactory. It is agreed that a rapid degradation rate of the discharge capacity is resulted from the formation of Mg(OH)2 on the surface of Mg2Ni alloys, which, in turn, hinders
Fig. 3. EIS of Ni-coated Mg2Ni electrodes at 0.5% DOD (a) and equivalent circuit of EIS for Mg2Ni electrode (b).
Table 2 Electrochemical properties of Ni–P coatings.
Fig. 1. Discharge capacities of bare and Ni-coated Mg2Ni alloy electrodes as a function of cycle number.
a b
NaH2PO2 concentration (mol L 1)
Contact resistance (X)a
Charge transfer resistance (X)a
Diffusion coefficient (cm2 s 1 10
0 0.125 0.5
0.19 0.14 0.22
1.66 1.07 2.84
1.20 1.75 1.06
10 b
The charge transfer is determined by a curve fitting method. The calculation of diffusion coefficient is referred to the Cui’s report [17].
)
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of deposited Ni particles was greater than 2320 ppm, the thickness of Ni–P coatings increased linearly. Nevertheless, P atoms were dispersed into Ni atoms, and most of the catalytic sites of Ni atoms were covered with P atoms, leading to a decrease of discharge capacity.
3.2. Electrochemical impedance spectroscopy analysis The EIS of the Ni-coating Mg2Ni electrodes at 50% depths of discharge (DOD) and the equivalent circuit are presented in Fig. 3a and b, respectively. Meanwhile, the simulated impedance values (i.e., data fitting lines) derived from the equivalent circuit are also shown in Fig. 3a. As shown in Fig. 3a, in the case of 0 mol L 1 of NaH2PO2, two distorted semicircles appeared at high and middle frequencies whereas a curved line appeared at low frequencies. In contrast, in the cases of 0.125 and 0.5 mol L 1 of NaH2PO2, only one semicircle appeared at high frequencies. The high-frequency distorted semicircle can be related to the electrode processing (i.e., the contact resistances at the boundary of current collectorelectrode and powder–powder) since this loop is independent on NaH2PO2 concentration. The middle-frequency semicircle is identified as the charge transfer resistance at the surface between the Mg2Ni electrode and electrolytes. The low-frequency curved line can be related to hydrogen diffusion in the Mg2Ni alloys. In the absence of NaH2PO2, the hydriding/dehydriding reaction rate of the Mg2Ni alloys is controlled by both charge transfer and hydrogen diffusion. In contrast, in the presence of NaH2PO2 with concentrations of 0.125 and 0.5 mol L 1, the middle-frequency semicircle disappeared. Instead, a linear response appeared over most of frequencies, revealing that the hydriding/dehydriding reaction rate of the Mg2Ni alloys is controlled by hydrogen diffusion. Based on the equivalent circuit, the charge transfer resistance and diffusion coefficient of the Ni-coating Mg2Ni electrodes are summarized in Table 2. At the NaH2PO2 concentration of 0.125 mol L 1, the charge transfer resistance was the lowest whereas the diffusion coefficient was the highest. This can promote the discharge capacity to reach a maximum value because the amount of catalytically active Ni particles on the Mg2Ni surface using 0.125 mol L 1 of NaH2PO2 was higher than those using 0 and 0.5 mol L 1 of NaH2PO2. A more catalytically active Ni surface facilitates a charge transfer reaction and a hydrogen diffusion efficiency. The highest charge transfer resistance appeared at the NaH2PO2 concentration of 0.5 mol L 1. It revealed that the Ni–P coatings became so thick that most of the
Fig. 4. XRD patterns of ball-milled and Ni-coated Mg2Ni alloys (a) ball-milled, (b) 0 mol L 1 NaH2PO2, (c) 0.08 mol L 1 NaH2PO2, (d) 0.125 mol L 1 NaH2PO2, (e) 0.25 mol L 1 NaH2PO2, and (f) 0.5 mol L 1 NaH2PO2.
in NaH2PO2 reductant concentration from 0 to 0.125 mol L 1, the discharge capacity of Ni-coated Mg2Ni alloys abruptly increased to a maximum value of 148 mA h g 1. With a further increase in NaH2PO2 concentration from 0.125 to 0.5 mol L 1, the discharge capacity declined to 106 mA h g 1, even if the deposited amount of Ni on the surface of Mg2Ni alloys increased continuously. In the absence of NaH2PO2, the deposition of Ni is initiated by the dissolution of active Mg metals due to the higher oxidation potential of Mg atoms. Consequently, Ni2+ ions in the electroless bath can be reduced by Mg metals to form the catalytic surface for Mg2Ni alloys [13]. Once NaH2PO2 was added, a catalytic reduction reaction of hypophosphite occurred sequentially on the catalytic surfaces to form Ni–P deposits. It is noted that P atoms do not enter a Ni-rich phase. Instead, P atoms deposit around the catalytic sites of Ni atoms [14–16]. Here, the deposited amount of Ni particles represents the thickness of Ni–P coatings. The discharge capacity increased with increasing thickness of Ni–P coatings when the amount of deposited Ni particles was less than 2320 ppm. When the amount
Fig. 5. SEM images of Ni-coated Mg2Ni alloys (a) 0 mol L NaH2PO2.
1
NaH2PO2, (b) 0.08 mol L
1
NaH2PO2, (c) 0.125 mol L
1
NaH2PO2, (d) 0.25 mol L
1
NaH2PO2, and (e) 0.5 mol L
1
R. Ohara et al. / Journal of Alloys and Compounds 580 (2013) S368–S372
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63.25nm
Ni
164.56nm
P Mg
Fig. 6. TEM–EDS images of Ni-coated Mg2Ni alloy prepared with 0.5 mol L
catalytic sites of Ni atoms were covered with P to decrease the electrocatalytic performance of Ni atoms. 3.3. Structural and morphological characteristics XRD patterns of ball-milled and Ni-coated Mg2Ni alloy powders are presented in Fig. 4a, b–f, respectively. As shown in Fig. 4a, after 15 h ball-milling characteristic peaks of crystalline Mg2Ni were observed. Diffraction peaks appeared quite broad, revealing that the structure of ball-milled Mg2Ni alloys was poorly crystallized. By using Scherrer equation, approximately 15 nm of the crystallite size of ball-milled Mg2Ni alloys was calculated. As for the Nicoated Mg2Ni alloys (Fig. 4b–f), the characteristic peak of Mg2Ni alloy phase broadened and shifted to 2h = 44.5°. It is noted that new broad Ni peaks were detected in the cases of 0 mol L 1 of NaH2PO2 (Fig. 4b) and 0.08 mol L 1 of NaH2PO2 (Fig. 4c) [5,8]. An increase of NaH2PO2 concentration tended to give a broader peak, excepting that Ni peaks vanished at the NaH2PO2 concentration of 0.25 mol L 1 and 0.5 mol L 1, as shown in Fig. 4d and f, respectively. It is considered that the replacement deposition by Mg contributes to the formation of crystalline Ni particles. Once the Ni–P deposits are formed, P atoms are dispersed into Ni atoms and the crystalline structure of Ni particles can be destroyed to result in amorphization. Additionally, the lattice disorder in the crystalline phase of Ni particles increases with increasing P contents [18– 22]. The morphologies of Ni-coated Mg2Ni alloys with different NaH2PO2 concentrations are presented in Fig. 5. With an increase in NaH2PO2 concentration, the deposited Ni–P particles increased. It is noted that the deposited Ni–P particles will be thick enough to cover the surfaces of Mg2Ni alloys at the NaH2PO2 concentration of 0.5 mol L 1. TEM–EDS images of the Ni-coated Mg2Ni alloys prepared with 0.5 mol L 1 of NaH2PO2 are presented in Fig. 6. It was found that the Ni–P spherical particles with a diameter ranging from of 50 to 250 nm were deposited on the surface of Mg2Ni alloys. 4. Conclusions A Mg2Ni alloy was synthesized by ball-milling and was further modified with an electroless nickel coating by using NaH2PO2 as a
1
NaH2PO2.
reducing agent. The effects of the Ni–P coating thickness on electrochemical characteristics of Mg2Ni alloys were investigated. The thickness of Ni–P coating was represented by the measuring amount of Ni coated on the surface of Mg2Ni alloys. The following conclusions can be drawn: The nickel coatings can effectively improve the discharge capacity of Mg2Ni alloys. Meanwhile, the thickness of Ni–P coatings determined the discharge capacity of Ni-coated Mg2Ni alloys. The discharge capacity decreased with increasing thickness of Ni–P coatings when the amount of deposited Ni particles was greater than 2320 ppm. The reduction of the discharge capacity was caused by the increment of charge transfer resistance and the reduction of diffusion coefficients which were resulted from the formation of amorphous Ni–P coatings together with the reduction of the electrocatalytic activity of Ni particles. The electroless nickel coating was a concurrent reaction of the replacement deposition and the catalytic reduction reaction. The preferential replacement deposition resulted in the formation of crystalline Ni peaks; the lattice disorder in the crystalline phase of Ni particles increased with an increasing concentration of NaH2PO2.
Acknowledgements The authors would like to thank the Bureau of Energy, Ministry of Economic Affairs of Republic of China, for financially supporting. References [1] N. Cui, B. Luan, H.K. Liu, S.X. Dou, J. Power Source 55 (1995) 263–267. [2] R. Ambat, W. Zhou, Surf. Coat. Technol. 179 (2004) 124–134. [3] C. Rongeat, M.H. Grosjean, S. Ruggeri, M. Dehmas, S. Bourlot, S. Marcotte, L. Roué, J. Power Source 158 (2006) 747–753. [4] N. Cui, P. He, J.L. Luo, Electrochim. Acta 44 (1999) 3549–3558. [5] M. Anik, I. Akay, S. Topcu, Int. J. Hydrogen Energy 34 (2009) 5449–5457. [6] J.L. Luo, N. Cui, J. Alloys Comp. 264 (1998) 299–305. [7] T. Abe, T. Tachikawa, Y. Hatano, K. Watanabe, J. Alloys Comp. 330–332 (2002) 792–795. [8] J. Chen, D.H. Bradhurst, S.X. Dou, H.K. Liu, J. Alloys Comp. 280 (1998) 290–293.
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[9] T. Sakai, H. Ishikawa, K. Oguro, C. Iwakura, H. Yoneyama, J. Electrochem. Soc. 134 (1897) 558–562. [10] C.Y. Wang, P. Yao, D.H. Bradhurst, H.K. Liu, S.X. Dou, J. Alloys Comp. 285 (1999) 267–271. [11] K. Naito, T. Matsunami, K. Okuno, J. Appl. Electrochem. 24 (1994) 808–813. [12] Z.H. Xie, G. Yu, T.J. Li, Z.J. Wu, B.N. Hu, J. Coat. Technol. Res. 9 (1) (2012) 107– 114. [13] G.O. Mallory, J.B. Hajdu, Electroless Plating: Fundamental and Applications, AESF, Orlando, 1999. [14] S. Nohara, N. Fujita, S.G. Zhang, H. Inoue, C. Iwakura, J. Alloys Comp. 267 (1998) 76–78.
[15] C. Iwakura, M. Matsuoka, K. Asai, T. Kohno, J. Power Source 38 (1992) 335–343. [16] M. Matsuoka, K.H. Asai, K.A. Asai, Y. Fukumoto, C. Iwakura, J. Alloys Comp. 192 (1993) 149–151. [17] N. Cui, J.L. Luo, K.T. Chuang, J. Electroanal. Chem. 503 (2001) 92–98. [18] J.G. Qian, T. Zhao, Adv. Mater. Res. 393–395 (2012) 40–43. [19] Y.S. Huang, F.Z. Cui, Surf. Coat. Technol. 201 (2007) 5416–5418. [20] I. Okonska, M. Jurczyk, J. Alloys Comp. 475 (2009) 289–293. [21] H.S. Yu, S.F. Luo, Y.R. Wang, Surf. Coat. Technol. 148 (2001) 143–148. [22] P.S. Kumar, P.K. Nair, J. Mater. Process. Technol. 56 (1996) 511–520.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Electrochemical and structural characterization of electroless nickel coating on Mg2Ni hydrogen storage alloy Reiko Ohara a,⇑, Chao-Ho Lan a, Chii-Shyang Hwang b a b
Green Energy & Ecology Research Labs., Industrial Technology Research Institute, Tainan City 734, Taiwan, ROC Department of Materials Science and Engineering, National Cheng Kung University, Taiwan, ROC
a r t i c l e
i n f o
Article history: Available online 19 March 2013 Keywords: Magnesium alloys Electroless nickel coating Hydrogen storage Discharge capacity Impedance spectroscopy
a b s t r a c t Mg2Ni alloys were prepared by high-energy ball milling, followed by an electroless nickel coating, and their electrochemical properties were investigated. The results indicated that the thickness of Ni–P coatings determined the discharge capacity of Ni-coated Mg2Ni alloys. The measuring amount of deposited Ni particles was used to represent the thickness of Ni–P coatings. With an increase in the amount of deposited Ni particles, the discharge capacity reached to a maximum value of 148 mA h g 1 at the deposited Ni particles of 2320 ppm, followed by declining to 106 mA h g 1. From the electrochemical impedance spectroscopy experiments, the charge transfer resistance increased and meanwhile the diffusion coefficient decreased, if the amount of deposited Ni particles was greater than 2320 ppm. According to the X-ray diffraction studies, with an increase in the deposited amount of Ni particles the structure of Ni–P spherical particles, with a diameter of 50–250 nm, changed from nanocrystalline to amorphousness. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction A magnesium-based hydrogen storage alloy is one of the promising negative electrode materials for Ni–MH batteries. For instance, Mg2Ni alloys have a high theoretical discharge capacity close to 1000 mA h g 1, other than advantages of light weight and low cost [1]. However, a poor hydriding reaction rate at room temperature and a short charge/discharge cycle have limited the practical use of Magnesium-based alloy electrodes. A rapid capacity decay rate upon cycling is attributed to Mg oxidation occurring in alkaline solutions. Mg is a highly active metal with the standard electrochemical potential of 2.4 V vs. NHE; even in aqueous solutions, the standard electrochemical potential of Mg still keeps 1.5 V. Thus, Mg dissolves rapidly to form Mg(OH)2 film in an aqueous solution below pH 11 [2]. Till now, many techniques that include surface modifications, alloy element substitution, alloy composites, and oxide additions have been developed to improve the electrochemical hydriding/dehydriding behavior of Mg-based alloys in alkaline solutions [3,4]. Among these techniques, a surface modification of alloy powders with an electroless coating [2,5–10] is an effective and simple way to avoid deterioration and pulverization of Mg-based alloys, and to facilitate electric conductivity and hydrogen diffusion as well. It was reported that the deposited metals act as a protective barrier and a current collector in the electrochemical hydriding/dehydriding performance of Mg-based alloys in alkaline solutions [11]. Regarding to an electroless nickel coating, ⇑ Corresponding author. Tel.: +886 6 693 9265; fax: +886 6 693 9096. E-mail address: [email protected] (R. Ohara). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.03.009
the reduction of nickel basically results from the catalytic dehydrogenation of reducing agents which contains reactive hydrogens. The deposited content is not pure nickel but contains either phosphorus or boron, depending on what type of reducing agents is used. However, the electroless nickel coating process is very complex due to its special self-catalyzed chemical reactions. The electroless coating reaction not only yields Ni–P or Ni–B deposits, it also generates by-products, such as orthophosphite and sodium sulfate. As the concentration of by-products increases, the nickel deposition rate decreases [12]. Most studies related to the electroless nickel coating have focused on the electrocatalytic effect of nickel atoms on the electrochemical hydriding/dehydriding properties of Mg-based alloys. However, in the electroless process nickel atoms are co-deposited with P atoms to form Ni–P deposits on the Mg-based alloy surfaces if hypophosphite is used as a reducing agent. Nowadays there is little fundamental investigation taking into account the impact of the Ni–P deposits on the electrochemical hydriding/dehydriding properties of Mg-based hydrogen storage alloys. In this work, Mg2Ni alloys were synthesized by ball milling and thereby the electroless nickel coating was carried out. The amount of deposited Ni particles was determined to represent the thickness of Ni–P coatings and their electrochemical and structural characteristics on Mg2Ni alloys were investigated. 2. Experimental Mg2Ni alloys were prepared by mixing appropriate amounts of elemental Mg with Ni powders (325 mesh, >99.8% from Alfa Aesar) in the glove box under an Ar atmosphere. The mechanical alloying was preformed with a high-energy ball mill for 15 h under an Ar atmosphere to avoid the possible oxidation of alloy pow-
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R. Ohara et al. / Journal of Alloys and Compounds 580 (2013) S368–S372 Table 1 Chemical composition and physical condition of coating bath. Component
Concentration (mol L
NiSO46H2O Na3C6H5O72H2O NaH2PO2H2O
0.25 0.4 0–0.5
Temperature pH
80 °C 5
the diffusion of hydrogen in alkaline solutions. Plots of the discharge capacity and the deposited Ni amount as a function of the concentration of NaH2PO2 are presented in Fig. 2. With an increase
1
)
ders. The obtained Mg2Ni alloy powders (<200 mesh) were electroless coated using a coating solution shown in Table 1. The coated alloy powders were filtered and dried at 70 °C for 1 h. The filtrate was collected for quantitative determination of nickel using THERMOM5 Atomic Absorption Spectrophotometer (AA). The amount of nickel deposited on the surface of Mg2Ni alloys is defined as C Cfil, where C represents the amount of nickel before electroless coating and Cfil represents the amount of nickel in the filtrate. The phase structure and the morphology of the coated alloy powders were respectively examined by using a Rigaku Miniflex II X-ray diffractometer (XRD), a JEOL JSM-7001 field-emission scanning electron microscope (FE-SEM) and a JEOL JEM-2010F field-emission transmission electron microscope (FE-TEM) equipped with EDS. The dried powders were mixed with 7.69% of polyvinyl alcohol solution, followed by pasting into nickel foam (5.5 12 mm). The electrodes were dried at 75 °C for 1 h, and then pressed under a pressure of 50 kg cm 2. A Ni(OH)2 counter electrode and a Hg/HgO reference electrode were used to assemble a three-electrode cell in 6 mol L 1 of KOH solution. Charging/discharging electrochemical experiments were conducted in a galvanostatic mode using multi-channel potentio-galvano stat (Maccor 4300 battery test system). The activated electrodes were charged at 100 mA g 1 for 8 h and discharged to 0.6 V vs. Hg/HgO at 25 mA g 1 in 6 mol L 1 of KOH solution. By using an electrochemical analyzer (CHI Model 600B), the electrochemical impedance spectrum (EIS) was measured at room temperature under open-circuit conditions. The AC amplitude of perturbation was set at 5 mV and the frequency was varied from 10 kHz to 0.005 Hz.
Fig. 2. Plots of discharge capacity and deposited Ni amount as a function of NaH2PO2 concentration.
3. Results and discussion 3.1. Discharge capacity The cycle performance of ball-milled Mg2Ni and Ni-coated Mg2Ni electrodes are presented in Fig. 1. Obviously, a Ni-coating effectively improved the discharge capacity of Mg2Ni alloys but the cyclic stability was not satisfactory. It is agreed that a rapid degradation rate of the discharge capacity is resulted from the formation of Mg(OH)2 on the surface of Mg2Ni alloys, which, in turn, hinders
Fig. 3. EIS of Ni-coated Mg2Ni electrodes at 0.5% DOD (a) and equivalent circuit of EIS for Mg2Ni electrode (b).
Table 2 Electrochemical properties of Ni–P coatings.
Fig. 1. Discharge capacities of bare and Ni-coated Mg2Ni alloy electrodes as a function of cycle number.
a b
NaH2PO2 concentration (mol L 1)
Contact resistance (X)a
Charge transfer resistance (X)a
Diffusion coefficient (cm2 s 1 10
0 0.125 0.5
0.19 0.14 0.22
1.66 1.07 2.84
1.20 1.75 1.06
10 b
The charge transfer is determined by a curve fitting method. The calculation of diffusion coefficient is referred to the Cui’s report [17].
)
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R. Ohara et al. / Journal of Alloys and Compounds 580 (2013) S368–S372
of deposited Ni particles was greater than 2320 ppm, the thickness of Ni–P coatings increased linearly. Nevertheless, P atoms were dispersed into Ni atoms, and most of the catalytic sites of Ni atoms were covered with P atoms, leading to a decrease of discharge capacity.
3.2. Electrochemical impedance spectroscopy analysis The EIS of the Ni-coating Mg2Ni electrodes at 50% depths of discharge (DOD) and the equivalent circuit are presented in Fig. 3a and b, respectively. Meanwhile, the simulated impedance values (i.e., data fitting lines) derived from the equivalent circuit are also shown in Fig. 3a. As shown in Fig. 3a, in the case of 0 mol L 1 of NaH2PO2, two distorted semicircles appeared at high and middle frequencies whereas a curved line appeared at low frequencies. In contrast, in the cases of 0.125 and 0.5 mol L 1 of NaH2PO2, only one semicircle appeared at high frequencies. The high-frequency distorted semicircle can be related to the electrode processing (i.e., the contact resistances at the boundary of current collectorelectrode and powder–powder) since this loop is independent on NaH2PO2 concentration. The middle-frequency semicircle is identified as the charge transfer resistance at the surface between the Mg2Ni electrode and electrolytes. The low-frequency curved line can be related to hydrogen diffusion in the Mg2Ni alloys. In the absence of NaH2PO2, the hydriding/dehydriding reaction rate of the Mg2Ni alloys is controlled by both charge transfer and hydrogen diffusion. In contrast, in the presence of NaH2PO2 with concentrations of 0.125 and 0.5 mol L 1, the middle-frequency semicircle disappeared. Instead, a linear response appeared over most of frequencies, revealing that the hydriding/dehydriding reaction rate of the Mg2Ni alloys is controlled by hydrogen diffusion. Based on the equivalent circuit, the charge transfer resistance and diffusion coefficient of the Ni-coating Mg2Ni electrodes are summarized in Table 2. At the NaH2PO2 concentration of 0.125 mol L 1, the charge transfer resistance was the lowest whereas the diffusion coefficient was the highest. This can promote the discharge capacity to reach a maximum value because the amount of catalytically active Ni particles on the Mg2Ni surface using 0.125 mol L 1 of NaH2PO2 was higher than those using 0 and 0.5 mol L 1 of NaH2PO2. A more catalytically active Ni surface facilitates a charge transfer reaction and a hydrogen diffusion efficiency. The highest charge transfer resistance appeared at the NaH2PO2 concentration of 0.5 mol L 1. It revealed that the Ni–P coatings became so thick that most of the
Fig. 4. XRD patterns of ball-milled and Ni-coated Mg2Ni alloys (a) ball-milled, (b) 0 mol L 1 NaH2PO2, (c) 0.08 mol L 1 NaH2PO2, (d) 0.125 mol L 1 NaH2PO2, (e) 0.25 mol L 1 NaH2PO2, and (f) 0.5 mol L 1 NaH2PO2.
in NaH2PO2 reductant concentration from 0 to 0.125 mol L 1, the discharge capacity of Ni-coated Mg2Ni alloys abruptly increased to a maximum value of 148 mA h g 1. With a further increase in NaH2PO2 concentration from 0.125 to 0.5 mol L 1, the discharge capacity declined to 106 mA h g 1, even if the deposited amount of Ni on the surface of Mg2Ni alloys increased continuously. In the absence of NaH2PO2, the deposition of Ni is initiated by the dissolution of active Mg metals due to the higher oxidation potential of Mg atoms. Consequently, Ni2+ ions in the electroless bath can be reduced by Mg metals to form the catalytic surface for Mg2Ni alloys [13]. Once NaH2PO2 was added, a catalytic reduction reaction of hypophosphite occurred sequentially on the catalytic surfaces to form Ni–P deposits. It is noted that P atoms do not enter a Ni-rich phase. Instead, P atoms deposit around the catalytic sites of Ni atoms [14–16]. Here, the deposited amount of Ni particles represents the thickness of Ni–P coatings. The discharge capacity increased with increasing thickness of Ni–P coatings when the amount of deposited Ni particles was less than 2320 ppm. When the amount
Fig. 5. SEM images of Ni-coated Mg2Ni alloys (a) 0 mol L NaH2PO2.
1
NaH2PO2, (b) 0.08 mol L
1
NaH2PO2, (c) 0.125 mol L
1
NaH2PO2, (d) 0.25 mol L
1
NaH2PO2, and (e) 0.5 mol L
1
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63.25nm
Ni
164.56nm
P Mg
Fig. 6. TEM–EDS images of Ni-coated Mg2Ni alloy prepared with 0.5 mol L
catalytic sites of Ni atoms were covered with P to decrease the electrocatalytic performance of Ni atoms. 3.3. Structural and morphological characteristics XRD patterns of ball-milled and Ni-coated Mg2Ni alloy powders are presented in Fig. 4a, b–f, respectively. As shown in Fig. 4a, after 15 h ball-milling characteristic peaks of crystalline Mg2Ni were observed. Diffraction peaks appeared quite broad, revealing that the structure of ball-milled Mg2Ni alloys was poorly crystallized. By using Scherrer equation, approximately 15 nm of the crystallite size of ball-milled Mg2Ni alloys was calculated. As for the Nicoated Mg2Ni alloys (Fig. 4b–f), the characteristic peak of Mg2Ni alloy phase broadened and shifted to 2h = 44.5°. It is noted that new broad Ni peaks were detected in the cases of 0 mol L 1 of NaH2PO2 (Fig. 4b) and 0.08 mol L 1 of NaH2PO2 (Fig. 4c) [5,8]. An increase of NaH2PO2 concentration tended to give a broader peak, excepting that Ni peaks vanished at the NaH2PO2 concentration of 0.25 mol L 1 and 0.5 mol L 1, as shown in Fig. 4d and f, respectively. It is considered that the replacement deposition by Mg contributes to the formation of crystalline Ni particles. Once the Ni–P deposits are formed, P atoms are dispersed into Ni atoms and the crystalline structure of Ni particles can be destroyed to result in amorphization. Additionally, the lattice disorder in the crystalline phase of Ni particles increases with increasing P contents [18– 22]. The morphologies of Ni-coated Mg2Ni alloys with different NaH2PO2 concentrations are presented in Fig. 5. With an increase in NaH2PO2 concentration, the deposited Ni–P particles increased. It is noted that the deposited Ni–P particles will be thick enough to cover the surfaces of Mg2Ni alloys at the NaH2PO2 concentration of 0.5 mol L 1. TEM–EDS images of the Ni-coated Mg2Ni alloys prepared with 0.5 mol L 1 of NaH2PO2 are presented in Fig. 6. It was found that the Ni–P spherical particles with a diameter ranging from of 50 to 250 nm were deposited on the surface of Mg2Ni alloys. 4. Conclusions A Mg2Ni alloy was synthesized by ball-milling and was further modified with an electroless nickel coating by using NaH2PO2 as a
1
NaH2PO2.
reducing agent. The effects of the Ni–P coating thickness on electrochemical characteristics of Mg2Ni alloys were investigated. The thickness of Ni–P coating was represented by the measuring amount of Ni coated on the surface of Mg2Ni alloys. The following conclusions can be drawn: The nickel coatings can effectively improve the discharge capacity of Mg2Ni alloys. Meanwhile, the thickness of Ni–P coatings determined the discharge capacity of Ni-coated Mg2Ni alloys. The discharge capacity decreased with increasing thickness of Ni–P coatings when the amount of deposited Ni particles was greater than 2320 ppm. The reduction of the discharge capacity was caused by the increment of charge transfer resistance and the reduction of diffusion coefficients which were resulted from the formation of amorphous Ni–P coatings together with the reduction of the electrocatalytic activity of Ni particles. The electroless nickel coating was a concurrent reaction of the replacement deposition and the catalytic reduction reaction. The preferential replacement deposition resulted in the formation of crystalline Ni peaks; the lattice disorder in the crystalline phase of Ni particles increased with an increasing concentration of NaH2PO2.
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