- Email: [email protected]
Electrochemical properties of Mg-based hydrogen storage alloys prepared by hydriding combustion synthesis and subsequent mechanical milling (HCS+MM)
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Electrochemical properties of Mg-based hydrogen storage alloys prepared by hydriding combustion synthesis and subsequent mechanical milling (HCS+MM) Yunfeng Zhu, Yicun Wang, Liquan Li College of Materials Science and Engineering, Nanjing University of Technology, 5 Xinmofan Road, Nanjing 210009, PR China
art i cle info
ab st rac t
Article history:
In this paper, a novel method, namely hydriding combustion synthesis (HCS) and
Received 10 October 2007
subsequent mechanical milling (MM) was used to prepare Mg-based hydrogen storage
Received in revised form
electrode alloy. The phase structures and electrochemical properties of the alloys before
22 January 2008
and after MM were characterized by X-ray diffraction (XRD) analysis and galvanotactic
Accepted 2 April 2008
charge–discharge cycle test, respectively. The XRD results showed that the structure of the
Available online 19 May 2008
as-milled alloys was nanocrystallite or amorphous-like state. Electrochemical measure-
Keywords: Mg-based hydrogen storage alloys Hydriding combustion synthesis Mechanical milling Electrochemical properties
ments showed that the discharge capacity was improved greatly for the products of HCS+MM. The HCS product with only 5 h MM showed markedly increased discharge capacity up to 481.5 mAh/g for the first cycle, which was 10 times higher than the HCS product (39.4 mAh/g). The discharge capacity was further increased to 628.3 mAh/g for the HCS product after milling with nickel powder. Besides, the addition of nickel also led to an improved cycling stability of the alloy electrode during cycling in KOH electrolyte. It was indicated that the HCS+MM was promising for preparing Mg-based hydrogen storage electrode alloys. & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Recently, magnesium-based hydrogen storage alloys are considered as one of the most promising hydrogen storage materials due to several advantages: light weight, high gaseous and electrochemical capacity, rich natural resources, low environmental impact and low cost. Mechanical alloying (MA) or mechanical milling (MM) has been used to synthesize amorphous or nanocrystalline magnesium-based alloys which have higher electrochemical capacity than crystalline alloys [1–5]. However, it usually takes a long time and the sample is easily contaminated during the milling process. Hydriding combustion synthesis (HCS) was developed in 1997 by Akiyama et al., which has been regarded recently as an
innovative processing and fabrication to produce magnesium-based hydrogen storage alloys [6–9]. Some advantages of HCS are short time and lower energy requirement of the process and high purity and activity of the product [10]. The HCS product possesses high activity so that the saturated amount of hydrogen can be absorbed at the first cycle without any activation treatment [11]. Very recently, we put forward a novel method that was HCS combined with MM to prepare Mg-based hydrogen storage alloys. The product prepared by HCS+MM exhibited excellent hydrogenation/dehydrogenation properties with high capacity and fast kinetics at lower temperature [12,13]. However, these studies were focused on the hydrogen storage characteristics of the alloys from the gas point of view.
Corresponding author. Tel.: +86 25 83587255; fax: +86 25 86639976.
E-mail address: [email protected] (L. Li). 0360-3199/$ - see front matter & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.04.003
ARTICLE IN PRESS 2966
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33 (2008) 2965 – 2969
In this paper, in order to verify whether the product has well electrochemical properties, the Mg-based hydrogen storage electrode alloys were prepared by HCS+MM firstly, and the phase structure and electrochemical properties of these alloys were investigated.
Experimental
2.1.
Alloy preparation
The HCS products were prepared from commercial magnesium and nickel powders. The magnesium power is 99.9 mass% in purity and o177 mm in particle size and the nickel powder is 99.7 mass% in purity and 2–3 mm in particle size. They were mixed in 2:1 of Mg:Ni molar ratio by an ultrasonic homogenizer in acetone for 1 h. After completely dried, the well-mixed powder was placed directly into the synthesis reactor without compressive treatment. During HCS process, the mixed powder was heated to 850 K at the rate of 7 K/min and held for 1 h under 1.8 MPa hydrogen pressure. Afterwards, the sample was cooled down under hydrogen atmosphere. The synthesis reactor used in this study was the same as reported in Ref. [11]. Parts of the HCS products were mechanically milled with graphite or nickel powder at room temperature by using a planetary-type ball mill with a stainless steel vessel of 100 cm3. To avoid oxidation the vial was cleaned with Ar for three times and finally filled with Ar at 0.1 MPa. The ball-topowder ratio was 40:1 and the milling speed was 400 rev/min.
2.2.
Sample testing and analysis
All test electrodes were prepared by first mixing thoroughly 0.1 g alloy powder with 0.4 g carbonyl nickel powder and then cold-pressing the mixture under a pressure of 12 MPa into a pellet of 10 mm diameter and about 1 mm thickness. The discharge capacities of the electrodes were evaluated by the amount of active substance Mg2Ni. The electrochemical measurements were performed in a half-cell consisting of a working electrode (MH electrode), a sintered Ni(OH)2/NiOOH counter electrode and a Hg/HgO reference electrode. The electrolyte was a 6 M KOH solution, controlled at 3071 1C. The discharge capacity and the cycle life of the electrode were determined by the galvanostatic method. Each electrode was charged at 300 mA/g for 3 h followed by 5 min rest and then discharged at 60 mA/g to the cut-off potential of 0.6 V vs. the Hg/HgO reference electrode. For the investigation of high rate dischargeability (HRD), the discharge capacities at several different discharge current densities were tested. The structural change of the mechanical milled HCS product was examined by X-ray diffraction (XRD) using Cu Ka radiation.
3.
Results and discussion
3.1.
Phase structure
Fig. 1(a) shows the XRD pattern of the HCS product. It can be seen that the HCS product was composed of Mg2NiH4 phase
(c)
Intensity (a.u.)
2.
Mg2NiH4 Mg2NiH0.3 Ni
(b)
(a)
20
30
40
50
60
70
2θ (°) Fig. 1 – XRD patterns of (a) the HCS product (b) the HCS product milled with 3 wt% graphite for 5 h (HCS+MM (G)) and (c) the HCS product milled with nickel powder (1:1 in molar ratio) for 40 h (HCS+MM (Ni)).
with monoclinic structure and Mg2NiH0.3 phase with hexagonal structure. The presence of Mg2NiH0.3 phase indicated that the hydrogenation of Mg2Ni during the HCS process was not completed. After milling with 3 wt% graphite for 5 h (denoted as HCS+MM (G)), as shown in Fig. 1(b), the peaks of Mg2NiH4 located at around 241 disappeared, leaving only the sharp peak at 201 and two diffused peaks that can be unambiguously assigned to Mg2NiH0.3, the crystal grain size of which was 9 nm as roughly calculated by Scherrer equation. It was suggested that the solid state amorphization of the hydride phase Mg2NiH4 may occur preferentially because of its brittle nature comparing with Mg2NiH0.3. Consequently, the sample HCS+MM (G) can be described as a nanostructured composite with tiny nanosized crystallites embedded in the amorphous matrix. In the case of the HCS product ball milled with nickel powder in 1:1 molar ratio for 40 h (denoted as HCS+MM (Ni)), as shown in Fig. 1(c), the characteristic peaks of the Mg2NiH4 and Mg2NiH0.3 phase disappeared completely, indicating that the Mg2NiH4+Ni composite after ball milling for 40 h had been transformed into an amorphous-like state. The existence of the sharp diffraction peaks attributed to nickel metal suggested that part of the nickel still remained in its crystal state. Therefore, the sample HCS+MM (Ni) was composed of amorphous phase and unreacted Ni.
3.2.
Electrochemical properties
Fig. 2 shows the discharge curves of the HCS product and asmilled products for the first cycle measured at 303 K. During discharging, the electrode potential of the HCS product dropped rapidly and showed a lower discharge capacity of 39.4 mAh/g. For the sample HCS+MM (G), the discharge potential was shifted to the negative direction and showed clear discharge potential plateaus. A discharge capacity of 481.5 mAh/g was obtained, which was 10 times higher than that of the original HCS product. For the sample HCS+MM
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HCS product HCS+MM (G) HCS+MM (Ni)
-Potential/ (V vs. Hg/HgO)
0.95 0.90 0.85 0.80 0.75 0.70 0.65 0.60 0
100
200 300 400 500 Discharge capacity/ (mAh/g)
600
700
Fig. 2 – Discharge curves of the HCS product and as-milled products for the first cycle measured at 303 K. 700 HCS product HCS+MM (G) HCS+MM (Ni)
Discharge capacity/ (mAh/g)
600 500 400 300 200 100 0 0
2
4
6
8 10 12 Cycle number
14
16
18
20
Fig. 3 – Discharge capacity vs. cycle number of the HCS product and as-milled products measured at 303 K.
(Ni), the discharge capacity was further increased to 628.3 mAh/g. Besides, there were two distinct discharge potential plateaus located at around 0.83 and 0.72 V (vs. Hg/HgO), respectively. Hence, there must have two different phases capable of discharging, leading to an improved discharge capacity of the composite. The reason still remains unknown now and needs further investigation. Fig. 3 shows the discharge capacity degradation as a function of the cycle number of the HCS product and asmilled products measured at 303 K. It is obvious that the HCS product has very low discharge capacity during the 20 charge/ discharge cycles. It is well known that the crystalline Mg2Ni produced by melting cannot be used as hydrogen storage electrode alloy because it does not have electrochemical hydrogen storage capacity. But Lei et al. [1] developed a kind of Mg–Ni-based amorphous hydrogen storage electrode alloys produced by MA, which had large reversible electrochemical hydrogen storage capacity at room temperature. From XRD analysis, we have known that the HCS product was mainly composed of Mg2NiH4 phase and Mg2NiH0.3 phase.
Even though the HCS product possesses high purity and activity, the magnesium nickel hydride seemed hardly to be discharged/charged at current condition. However, only after 5 h milling of the HCS product with 3 wt% graphite, namely the sample HCS+MM (G), the alloy electrode could be discharged and recharged reversibly during the cycles owing to its special nanocrystalline/amorphous structure. For the sample HCS+MM (Ni), a similar phenomenon was observed, except that the sample had a higher discharge capacity at all cycles as compared with that of the sample HCS+MM (G). Besides, both the samples showed excellent activation performance as the maximum discharge capacity was obtained at the first cycle, which was similar to other kind of Mg-based hydrogen storage electrode alloys. Fig. 4 shows the cycling stability curves for the alloy electrodes of HCS+MM (G) and HCS+MM (Ni) samples. It can be seen that both of the alloy electrodes had a poor cycling stability, especially for the sample HCS+MM (G), for its discharge capacity degraded quickly during the initial several cycles and almost disappeared after 20 charge–discharge cycles. It has been accepted that the rapid degradation in discharge capacity of Mg-based hydrogen storage electrode alloys is mainly caused by the oxidation and corrosion of Mg in KOH alkaline electrolyte [5,14]. Hence, we can attribute the poor cycling stability of the sample HCS+MM (G) to the oxidation and corrosion of Mg during charging/discharging. However, when Ni was added to the sample, an improved cycling stability was observed for the sample HCS+MM (Ni). For example, the capacity retention C/Cmax after 20 cycles was improved from 5.0% for the sample HCS+MM (G) to 27.3% for the sample HCS+MM (Ni). The special structure of the sample HCS+MM (Ni), namely the amorphous phase together with some unreacted Ni, was thought responsible for the improved properties as the amorphous alloys usually have excellent corrosion resistance [15]. In addition, Cui et al. [16] reported that the smaller nickel particles might be inlaid into the surface of larger magnesium alloy particles by cold welding through milling. Based on the XRD analysis, we believe part of
100 HCS+MM (G) HCS+MM (Ni)
80
C/Cmax (%)
1.00
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60
40
20
0 0
2
4
6
8 10 12 Cycle number
14
16
18
20
Fig. 4 – The cycling stability curves for the alloy electrodes of HCS+MM (G) and HCS+MM (Ni) samples measured at 303 K.
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the crystalline nickel particles must have inlaid into the surface of magnesium nickel hydride produced by HCS during milling. These dispersed nickel particles were helpful for decreasing the electrochemical reaction resistance on the alloy surface and acted as current collector because nickel is known to have good electrocatalytic activity in alkaline electrolyte. Therefore, the sample HCS+MM (Ni) exhibited an improved discharge capacity and cycling stability than the sample HCS+MM (G). Unfortunately, the alloys produced by HCS+MM still showed unsatisfactory cycling stability as compared with that of the commercial RE-based AB5 alloys. Elemental substitution of Mg or Ni by other elements was proved to be an effective way to improve the cycling stability of Mg-based hydrogen storage electrode alloys [17,18]. Thus a combination of elemental substitution in the HCS process and subsequent MM would be promising to prepare novel Mg-based hydrogen storage electrode alloys with good electrochemical properties. Fig. 5 shows the HRD for the alloy electrodes of HCS+MM (G) and HCS+MM (Ni) samples measured at 303 K. The HRD is calculated from the following formula: HRD ¼ Cd =ðCd þ C60 Þ 100%
(1)
where Cd is the discharge capacity with cut-off potential of 0.6 V (vs. Hg/HgO) at the discharge current density Id, C60 is the residual discharge capacity with cut-off potential of 0.6 V (vs. Hg/HgO) at the discharge current density I ¼ 60 mA/g after the alloy electrode has been fully discharged at the large discharge current density under investigation. It can be seen that the HRD of both electrodes decreased with increasing discharge current density and there was little difference in HRD between the two electrodes. Besides, the HRD was lower for practical application. For example, when the discharge current density was 400 mA/g, the HRD of the HCS+MM (G) and HCS+MM (Ni) alloy electrodes were 27.3% and 24.6%, respectively. During charging and discharging, Mg was easily oxidized and corroded and the corrosion products would be precipitated on the electrode surface, which
100 HCS+MM (G) HCS+MM (Ni)
HRD / (%)
80
60
40
20
0 0
100 200 300 Discharge current density/ (mA/g)
400
Fig. 5 – High rate dischargeability HRD for the alloy electrodes of HCS+MM (G) and HCS+MM (Ni) samples measured at 303 K.
33 (2008) 2965 – 2969
increased the reaction polarization. Therefore, the discharge potential of the electrode would increase (more positive) and the discharge process would terminate earlier with cut-off potential of 0.6 V (vs. Hg/HgO), resulting in lower discharge capacity and HRD at large current density.
4.
Conclusions
We successfully prepared Mg-based hydrogen storage electrode alloys by a novel method called hydriding combustion synthesis and subsequent mechanical milling (HCS+MM). XRD analysis indicated that the alloys were mainly composed of nanocrystalline or amorphous phases. Electrochemical measurements showed that the maximum discharge capacity was 481.5 mAh/g for the sample HCS+MM (G) and 628.3 mAh/g for the sample HCS+MM (Ni), respectively. All the alloys showed excellent activation performance. In addition, the sample HCS+MM (Ni) showed an improved cycling stability as compared to the sample HCS+MM (G). The method of HCS+MM has some advantages for preparing Mg-based hydrogen storage alloys, such as short process time, low energy consumption and high activity of the product. The electrochemical properties including the discharge capacity, the cycling stability and HRD should be improved in further studies, such as by elemental substitution in the HCS process.
R E F E R E N C E S
[1] Lei YQ, Wu YM, Yang QM, et al. Electrochemical-behavior of some mechanically alloyed Mg–Ni–based amorphous hydrogen storage alloys. Z Phys Chem 1994;183:379–84. [2] Sun L, Yao P, Liu HK, et al. Mg2Ni hydride electrodes prepared by sintering and subsequent ball milling with Ni powders. J Alloy Compd 1999;293–295:536–40. [3] Kohno T, Yamamoto M, Kanda M. Electrochemical properties of mechanically ground Mg2Ni alloy. J Alloy Compd 1999; 293–295:643–7. [4] Cui N, He P, Luo JL. Synthesis and characterization of nanocrystalline magnesium-based hydrogen storage alloy electrode materials. Electrochim Acta 1999;44:3549–58. [5] Zhang Y, Lei YQ, Chen LX, et al. The effect of partial substitution of Zr for Ti on the electrochemical properties and surface passivation film of Mg35Ti10xZrxNi55 (x ¼ 1, 3, 5, 7, 9) electrode alloys. J Alloy Compd 2002;337:296–302. [6] Akiyama T, Isogai H, Yagi J. Hydriding combustion synthesis for the production of hydrogen storage alloy. J Alloy Compd 1997;252(1–2):L1–4. [7] Saita I, Li LQ, Saito K, et al. Pressure–Composition–Temperature properties of hydriding combustion synthesized Mg2NiH4. Mater Trans 2002;43:1100–4. [8] Saita I, Li LQ, Saito K, et al. Hydriding combustion synthesis of Mg2NiH4. J Alloy Compd 2003;356–357:490–3. [9] Li LQ, Saita I, Akiyama T. Intermediate products during the hydriding combustion synthesis of Mg2NiH4. J Alloy Compd 2004;384:157–64. [10] Li LQ, Akiyama T, Yagi J. Activity and capacity of hydrogen storage alloy Mg2NiH4 produced by hydriding combustion synthesis. J Alloy Compd 2001;316:118–23. [11] Liu DM, Zhu YF, Li LQ. Mechanism of the high activity of Mg2NiH4 produced by hydriding combustion synthesis based
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on the analysis of phase composition, particle characteristic and grain size. Int J Hydrogen Energy 2007;32:2455–60. [12] Liu XF, Zhu YF, Li LQ. Hydriding and dehydriding properties of nanostructured Mg2Ni alloy prepared by the process of hydriding combustion synthesis and subsequent mechanical grinding. J Alloy Compd 2006;425:235–8. [13] Liu XF, Zhu YF, Li LQ. Hydriding characteristics of Mg2Ni prepared by mechanical milling of the product of hydriding combustion synthesis. Int J Hydrogen Energy 2007;32: 2450–4. [14] Liu WH, Lei YQ, Sun DL, et al. A study of the degradation of the electrochemical capacity of amorphous Mg50Ni50 alloy. J Power Sources 1996;58:243–7.
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[15] Horikiri H, Kato A, Inoue A, et al. New Mg-based amorphous alloys in Mg–Y-Misch metal systems. Mater Sci Eng A 1994;179–180:702–6. [16] Cui N, He P, Luo JL. Magnesium-based hydrogen storage materials modified by mechanical alloying. Acta Mater 1999;47(14):3737–43. [17] Nohara S, Hamasaki K, Zhang SG, et al. Electrochemical characteristics of an amorphous Mg0.9V0.1Ni alloy prepared by mechanical milling. J Alloy Compd 1998;280:104–6. [18] Wang MH, Zhang LZ, Zhang Y, et al. The effects of partial substitution of Fe and Co for Ni in the Mg1.75Al0.25Ni electrode alloy on electrochemical performances. Int J Hydrogen Energy 2006;31:621–5.
33 (2008) 2965 – 2969
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Electrochemical properties of Mg-based hydrogen storage alloys prepared by hydriding combustion synthesis and subsequent mechanical milling (HCS+MM) Yunfeng Zhu, Yicun Wang, Liquan Li College of Materials Science and Engineering, Nanjing University of Technology, 5 Xinmofan Road, Nanjing 210009, PR China
art i cle info
ab st rac t
Article history:
In this paper, a novel method, namely hydriding combustion synthesis (HCS) and
Received 10 October 2007
subsequent mechanical milling (MM) was used to prepare Mg-based hydrogen storage
Received in revised form
electrode alloy. The phase structures and electrochemical properties of the alloys before
22 January 2008
and after MM were characterized by X-ray diffraction (XRD) analysis and galvanotactic
Accepted 2 April 2008
charge–discharge cycle test, respectively. The XRD results showed that the structure of the
Available online 19 May 2008
as-milled alloys was nanocrystallite or amorphous-like state. Electrochemical measure-
Keywords: Mg-based hydrogen storage alloys Hydriding combustion synthesis Mechanical milling Electrochemical properties
ments showed that the discharge capacity was improved greatly for the products of HCS+MM. The HCS product with only 5 h MM showed markedly increased discharge capacity up to 481.5 mAh/g for the first cycle, which was 10 times higher than the HCS product (39.4 mAh/g). The discharge capacity was further increased to 628.3 mAh/g for the HCS product after milling with nickel powder. Besides, the addition of nickel also led to an improved cycling stability of the alloy electrode during cycling in KOH electrolyte. It was indicated that the HCS+MM was promising for preparing Mg-based hydrogen storage electrode alloys. & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Recently, magnesium-based hydrogen storage alloys are considered as one of the most promising hydrogen storage materials due to several advantages: light weight, high gaseous and electrochemical capacity, rich natural resources, low environmental impact and low cost. Mechanical alloying (MA) or mechanical milling (MM) has been used to synthesize amorphous or nanocrystalline magnesium-based alloys which have higher electrochemical capacity than crystalline alloys [1–5]. However, it usually takes a long time and the sample is easily contaminated during the milling process. Hydriding combustion synthesis (HCS) was developed in 1997 by Akiyama et al., which has been regarded recently as an
innovative processing and fabrication to produce magnesium-based hydrogen storage alloys [6–9]. Some advantages of HCS are short time and lower energy requirement of the process and high purity and activity of the product [10]. The HCS product possesses high activity so that the saturated amount of hydrogen can be absorbed at the first cycle without any activation treatment [11]. Very recently, we put forward a novel method that was HCS combined with MM to prepare Mg-based hydrogen storage alloys. The product prepared by HCS+MM exhibited excellent hydrogenation/dehydrogenation properties with high capacity and fast kinetics at lower temperature [12,13]. However, these studies were focused on the hydrogen storage characteristics of the alloys from the gas point of view.
Corresponding author. Tel.: +86 25 83587255; fax: +86 25 86639976.
E-mail address: [email protected] (L. Li). 0360-3199/$ - see front matter & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.04.003
ARTICLE IN PRESS 2966
I N T E R N AT 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
33 (2008) 2965 – 2969
In this paper, in order to verify whether the product has well electrochemical properties, the Mg-based hydrogen storage electrode alloys were prepared by HCS+MM firstly, and the phase structure and electrochemical properties of these alloys were investigated.
Experimental
2.1.
Alloy preparation
The HCS products were prepared from commercial magnesium and nickel powders. The magnesium power is 99.9 mass% in purity and o177 mm in particle size and the nickel powder is 99.7 mass% in purity and 2–3 mm in particle size. They were mixed in 2:1 of Mg:Ni molar ratio by an ultrasonic homogenizer in acetone for 1 h. After completely dried, the well-mixed powder was placed directly into the synthesis reactor without compressive treatment. During HCS process, the mixed powder was heated to 850 K at the rate of 7 K/min and held for 1 h under 1.8 MPa hydrogen pressure. Afterwards, the sample was cooled down under hydrogen atmosphere. The synthesis reactor used in this study was the same as reported in Ref. [11]. Parts of the HCS products were mechanically milled with graphite or nickel powder at room temperature by using a planetary-type ball mill with a stainless steel vessel of 100 cm3. To avoid oxidation the vial was cleaned with Ar for three times and finally filled with Ar at 0.1 MPa. The ball-topowder ratio was 40:1 and the milling speed was 400 rev/min.
2.2.
Sample testing and analysis
All test electrodes were prepared by first mixing thoroughly 0.1 g alloy powder with 0.4 g carbonyl nickel powder and then cold-pressing the mixture under a pressure of 12 MPa into a pellet of 10 mm diameter and about 1 mm thickness. The discharge capacities of the electrodes were evaluated by the amount of active substance Mg2Ni. The electrochemical measurements were performed in a half-cell consisting of a working electrode (MH electrode), a sintered Ni(OH)2/NiOOH counter electrode and a Hg/HgO reference electrode. The electrolyte was a 6 M KOH solution, controlled at 3071 1C. The discharge capacity and the cycle life of the electrode were determined by the galvanostatic method. Each electrode was charged at 300 mA/g for 3 h followed by 5 min rest and then discharged at 60 mA/g to the cut-off potential of 0.6 V vs. the Hg/HgO reference electrode. For the investigation of high rate dischargeability (HRD), the discharge capacities at several different discharge current densities were tested. The structural change of the mechanical milled HCS product was examined by X-ray diffraction (XRD) using Cu Ka radiation.
3.
Results and discussion
3.1.
Phase structure
Fig. 1(a) shows the XRD pattern of the HCS product. It can be seen that the HCS product was composed of Mg2NiH4 phase
(c)
Intensity (a.u.)
2.
Mg2NiH4 Mg2NiH0.3 Ni
(b)
(a)
20
30
40
50
60
70
2θ (°) Fig. 1 – XRD patterns of (a) the HCS product (b) the HCS product milled with 3 wt% graphite for 5 h (HCS+MM (G)) and (c) the HCS product milled with nickel powder (1:1 in molar ratio) for 40 h (HCS+MM (Ni)).
with monoclinic structure and Mg2NiH0.3 phase with hexagonal structure. The presence of Mg2NiH0.3 phase indicated that the hydrogenation of Mg2Ni during the HCS process was not completed. After milling with 3 wt% graphite for 5 h (denoted as HCS+MM (G)), as shown in Fig. 1(b), the peaks of Mg2NiH4 located at around 241 disappeared, leaving only the sharp peak at 201 and two diffused peaks that can be unambiguously assigned to Mg2NiH0.3, the crystal grain size of which was 9 nm as roughly calculated by Scherrer equation. It was suggested that the solid state amorphization of the hydride phase Mg2NiH4 may occur preferentially because of its brittle nature comparing with Mg2NiH0.3. Consequently, the sample HCS+MM (G) can be described as a nanostructured composite with tiny nanosized crystallites embedded in the amorphous matrix. In the case of the HCS product ball milled with nickel powder in 1:1 molar ratio for 40 h (denoted as HCS+MM (Ni)), as shown in Fig. 1(c), the characteristic peaks of the Mg2NiH4 and Mg2NiH0.3 phase disappeared completely, indicating that the Mg2NiH4+Ni composite after ball milling for 40 h had been transformed into an amorphous-like state. The existence of the sharp diffraction peaks attributed to nickel metal suggested that part of the nickel still remained in its crystal state. Therefore, the sample HCS+MM (Ni) was composed of amorphous phase and unreacted Ni.
3.2.
Electrochemical properties
Fig. 2 shows the discharge curves of the HCS product and asmilled products for the first cycle measured at 303 K. During discharging, the electrode potential of the HCS product dropped rapidly and showed a lower discharge capacity of 39.4 mAh/g. For the sample HCS+MM (G), the discharge potential was shifted to the negative direction and showed clear discharge potential plateaus. A discharge capacity of 481.5 mAh/g was obtained, which was 10 times higher than that of the original HCS product. For the sample HCS+MM
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HCS product HCS+MM (G) HCS+MM (Ni)
-Potential/ (V vs. Hg/HgO)
0.95 0.90 0.85 0.80 0.75 0.70 0.65 0.60 0
100
200 300 400 500 Discharge capacity/ (mAh/g)
600
700
Fig. 2 – Discharge curves of the HCS product and as-milled products for the first cycle measured at 303 K. 700 HCS product HCS+MM (G) HCS+MM (Ni)
Discharge capacity/ (mAh/g)
600 500 400 300 200 100 0 0
2
4
6
8 10 12 Cycle number
14
16
18
20
Fig. 3 – Discharge capacity vs. cycle number of the HCS product and as-milled products measured at 303 K.
(Ni), the discharge capacity was further increased to 628.3 mAh/g. Besides, there were two distinct discharge potential plateaus located at around 0.83 and 0.72 V (vs. Hg/HgO), respectively. Hence, there must have two different phases capable of discharging, leading to an improved discharge capacity of the composite. The reason still remains unknown now and needs further investigation. Fig. 3 shows the discharge capacity degradation as a function of the cycle number of the HCS product and asmilled products measured at 303 K. It is obvious that the HCS product has very low discharge capacity during the 20 charge/ discharge cycles. It is well known that the crystalline Mg2Ni produced by melting cannot be used as hydrogen storage electrode alloy because it does not have electrochemical hydrogen storage capacity. But Lei et al. [1] developed a kind of Mg–Ni-based amorphous hydrogen storage electrode alloys produced by MA, which had large reversible electrochemical hydrogen storage capacity at room temperature. From XRD analysis, we have known that the HCS product was mainly composed of Mg2NiH4 phase and Mg2NiH0.3 phase.
Even though the HCS product possesses high purity and activity, the magnesium nickel hydride seemed hardly to be discharged/charged at current condition. However, only after 5 h milling of the HCS product with 3 wt% graphite, namely the sample HCS+MM (G), the alloy electrode could be discharged and recharged reversibly during the cycles owing to its special nanocrystalline/amorphous structure. For the sample HCS+MM (Ni), a similar phenomenon was observed, except that the sample had a higher discharge capacity at all cycles as compared with that of the sample HCS+MM (G). Besides, both the samples showed excellent activation performance as the maximum discharge capacity was obtained at the first cycle, which was similar to other kind of Mg-based hydrogen storage electrode alloys. Fig. 4 shows the cycling stability curves for the alloy electrodes of HCS+MM (G) and HCS+MM (Ni) samples. It can be seen that both of the alloy electrodes had a poor cycling stability, especially for the sample HCS+MM (G), for its discharge capacity degraded quickly during the initial several cycles and almost disappeared after 20 charge–discharge cycles. It has been accepted that the rapid degradation in discharge capacity of Mg-based hydrogen storage electrode alloys is mainly caused by the oxidation and corrosion of Mg in KOH alkaline electrolyte [5,14]. Hence, we can attribute the poor cycling stability of the sample HCS+MM (G) to the oxidation and corrosion of Mg during charging/discharging. However, when Ni was added to the sample, an improved cycling stability was observed for the sample HCS+MM (Ni). For example, the capacity retention C/Cmax after 20 cycles was improved from 5.0% for the sample HCS+MM (G) to 27.3% for the sample HCS+MM (Ni). The special structure of the sample HCS+MM (Ni), namely the amorphous phase together with some unreacted Ni, was thought responsible for the improved properties as the amorphous alloys usually have excellent corrosion resistance [15]. In addition, Cui et al. [16] reported that the smaller nickel particles might be inlaid into the surface of larger magnesium alloy particles by cold welding through milling. Based on the XRD analysis, we believe part of
100 HCS+MM (G) HCS+MM (Ni)
80
C/Cmax (%)
1.00
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60
40
20
0 0
2
4
6
8 10 12 Cycle number
14
16
18
20
Fig. 4 – The cycling stability curves for the alloy electrodes of HCS+MM (G) and HCS+MM (Ni) samples measured at 303 K.
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the crystalline nickel particles must have inlaid into the surface of magnesium nickel hydride produced by HCS during milling. These dispersed nickel particles were helpful for decreasing the electrochemical reaction resistance on the alloy surface and acted as current collector because nickel is known to have good electrocatalytic activity in alkaline electrolyte. Therefore, the sample HCS+MM (Ni) exhibited an improved discharge capacity and cycling stability than the sample HCS+MM (G). Unfortunately, the alloys produced by HCS+MM still showed unsatisfactory cycling stability as compared with that of the commercial RE-based AB5 alloys. Elemental substitution of Mg or Ni by other elements was proved to be an effective way to improve the cycling stability of Mg-based hydrogen storage electrode alloys [17,18]. Thus a combination of elemental substitution in the HCS process and subsequent MM would be promising to prepare novel Mg-based hydrogen storage electrode alloys with good electrochemical properties. Fig. 5 shows the HRD for the alloy electrodes of HCS+MM (G) and HCS+MM (Ni) samples measured at 303 K. The HRD is calculated from the following formula: HRD ¼ Cd =ðCd þ C60 Þ 100%
(1)
where Cd is the discharge capacity with cut-off potential of 0.6 V (vs. Hg/HgO) at the discharge current density Id, C60 is the residual discharge capacity with cut-off potential of 0.6 V (vs. Hg/HgO) at the discharge current density I ¼ 60 mA/g after the alloy electrode has been fully discharged at the large discharge current density under investigation. It can be seen that the HRD of both electrodes decreased with increasing discharge current density and there was little difference in HRD between the two electrodes. Besides, the HRD was lower for practical application. For example, when the discharge current density was 400 mA/g, the HRD of the HCS+MM (G) and HCS+MM (Ni) alloy electrodes were 27.3% and 24.6%, respectively. During charging and discharging, Mg was easily oxidized and corroded and the corrosion products would be precipitated on the electrode surface, which
100 HCS+MM (G) HCS+MM (Ni)
HRD / (%)
80
60
40
20
0 0
100 200 300 Discharge current density/ (mA/g)
400
Fig. 5 – High rate dischargeability HRD for the alloy electrodes of HCS+MM (G) and HCS+MM (Ni) samples measured at 303 K.
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increased the reaction polarization. Therefore, the discharge potential of the electrode would increase (more positive) and the discharge process would terminate earlier with cut-off potential of 0.6 V (vs. Hg/HgO), resulting in lower discharge capacity and HRD at large current density.
4.
Conclusions
We successfully prepared Mg-based hydrogen storage electrode alloys by a novel method called hydriding combustion synthesis and subsequent mechanical milling (HCS+MM). XRD analysis indicated that the alloys were mainly composed of nanocrystalline or amorphous phases. Electrochemical measurements showed that the maximum discharge capacity was 481.5 mAh/g for the sample HCS+MM (G) and 628.3 mAh/g for the sample HCS+MM (Ni), respectively. All the alloys showed excellent activation performance. In addition, the sample HCS+MM (Ni) showed an improved cycling stability as compared to the sample HCS+MM (G). The method of HCS+MM has some advantages for preparing Mg-based hydrogen storage alloys, such as short process time, low energy consumption and high activity of the product. The electrochemical properties including the discharge capacity, the cycling stability and HRD should be improved in further studies, such as by elemental substitution in the HCS process.
R E F E R E N C E S
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