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Electrochemical properties of Ti–Ni–H powders prepared by milling titanium hydride and nickel
international journal of hydrogen energy 35 (2010) 3076–3079
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Electrochemical properties of Ti–Ni–H powders prepared by milling titanium hydride and nickel Xiangyu Zhaoa, Liqun Maa,*, Meng Yanga, Yi Dinga, Xiaodong Shena,b,** a
College of Materials Science and Engineering, Nanjing University of Technology, Nanjing 210009, PR China State Key Laboratory of Material-oriented Chemical Engineering, Nanjing University of Technology, Nanjing 210009, PR China
b
article info
abstract
Article history:
Mechanical alloying has been carried out to synthesize a hydrogen storage alloy by milling
Received 7 April 2009
titanium hydride and nickel. The structure and electrochemical properties such as
Accepted 7 July 2009
discharge capacity, charge-transfer, and hydrogen diffusion of the milled powders were
Available online 8 August 2009
investigated. The results of X-ray diffraction showed that an amorphous phase was formed
Keywords:
0.941 V (vs. Hg/HgO) in the electrolyte of 6 M KOH when the milling periods were 20, 40,
Ti–Ni
and 60 h, respectively. The Ti–Ni–H powders milled for 60 h had a maximum discharge
Mechanical alloying
capacity of 102.2 mAh/g at a discharge current density of 60 mA/g. The results of the linear
Hydrogen storage alloy
polarization showed that the exchange current density decreased as the hydrogen
Linear polarization
concentration within the powders decreased. The electrochemical impedance spectros-
Electrochemical impedance spectra
copy (EIS) demonstrated the same consequence and presented that the hydrogen diffusion
after ball milling. The electrode potentials of the milled powders were 0.989, 0.878 and
decreased by decreasing the hydrogen concentration. ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The Ti–Ni system has been extensively investigated by considering its shape memory characteristics and hydrogen storage performance [1–3]. For hydrogen storage, it is known that both TiNi and Ti2Ni in the Ti–Ni system absorb hydrogen at room temperature. TiNi alloy has a reversible hydrogenation capacity of 1.0 H/M. Ti2Ni alloy absorbs a large amount of hydrogen but desorbs irreversibly due to its poor discharge kinetics [2]. The electrochemical properties such as the discharge capacity [4], the discharge kinetics [5,6] and the discharge plateaus [7] of Ti–Ni alloys were studied previously by synthesizing the alloys with the raw materials of Ti and Ni with high purity. In this work, titanium hydride and nickel were used as raw materials for preparing a Ti–Ni-based alloy by mechanical alloying (MA). Changes of the structure and electrochemical
properties of the milled powders were investigated, emphasizing the discharge properties, the charge-transfer on the alloy surface and the hydrogen diffusion within the bulk alloy.
2.
Experimental
Powders of titanium Hydride (300 mesh, 99.9 at.% pure) and nickel (350 mesh, 99.9 at.% pure) were blended together to form a nominal composition of TiH2 – 50% Ni. They were loaded into a stainless steel container with stainless steel balls (10 mm and 20 mm in diameter) in a glove box under an argon atmosphere. The ball to powder ratio is 20:1. The milling was performed in a planetary mill with a rotation speed of 300 rpm and the milling time was 20, 40 or 60 h. The three milling periods were carried out continuously without interim sampling. The structure of the as-milled powders was studied
* Corresponding author. Tel.: þ86 25 83587243; fax: þ86 25 83240205. ** Corresponding author. College of Materials Science and Engineering, Nanjing University of Technology, Nanjing 210009, PR China E-mail addresses: [email protected] (X. Zhao), [email protected] (L. Ma), [email protected] (X. Shen). 0360-3199/$ – see front matter ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.07.017
3077
international journal of hydrogen energy 35 (2010) 3076–3079
by using the technique of X-ray diffraction (XRD) in a Thermo ARL X’ TRA diffractometer equipped with Cu Ka radiation. The metal-hydride electrodes were prepared by cold pressing a mixture of 0.18 g milled powders and 0.63 g nickel powders into a pellet with 18 mm in diameter under a pressure of 20 MPa. The charge and discharge testing were carried out in a half-cell consisting of the metal-hydride electrode, a Ni(OH)2/NiOOH counter electrode and a Hg/HgO reference electrode in a 6 M KOH solution under BT-2000 testing equipment (Arbin, USA) at 298 K. The open potentials (vs. the Hg/HgO) of the electrodes, before the charge and discharge testing, were obtained when the potential changed less than 1 mV in two minutes. The electrode containing the powders milled for 60 h was charged at 60 mA/g for 4 h followed by a 10 min rest and then discharged at 60 mA/g to the cut-off potential of 0.6 V vs. the Hg/HgO reference electrode. Linear polarization curves of the electrode were measured by scanning the electrode potential with a scan rate of 1 mV/s from 10 to 10 mV (vs. open circuit potential). Electrochemical impedance spectroscopy (EIS) of the electrode was obtained in the frequency range from 10 KHz to 0.001 Hz with an amplitude of 5 mV. These two tests were carried out on a CHI 660B electrochemical workstation at 298 K.
3.
Results and discussion
3.1.
Mechanical alloying
Fig. 1 shows the XRD patterns of the Ti–Ni–H powders as a function of the milling time. The intensities of the diffraction peaks corresponding to the raw materials were obviously decreased by 20 h of milling. In addition to the decrease of the intensity, continued ball milling induced considerable broadening of the peaks. The diffraction peaks of nickel disappear after 40 h of milling, indicating the formation of an alloy between the raw materials. When the milling time is 60 h, the milled product was predominantly amorphous with a few amount of titanium hydride phase.
The open potentials of the electrodes prepared by the milled powders were 0.989, 0.878 and 0.941 V when the milling periods were 20, 40 and 60 h, respectively. Furthermore, a gas flow meter was used to detect the pressure within the milling container at the end of each milling period. It was found that the pressure was almost unchanged after 20 h of milling as compared to that of the non-milled one, while a drastic increase and a slight increase of the pressure occurred when the milling periods were 40 and 60 h, respectively. This means that hydrogen gas was formed within the container after 20 h of milling. Three stages of the milling process can be obtained. The first is the short milling time (below 20 h). In this time range, mechanical milling (MM) leads to a decrease of the stability of titanium hydride and consequently a great many hydrogen atoms adsorb on the powder surface, resulting in the occurrence of hydrogen desorption or hydrogen evolution reaction in the 6 M KOH electrolyte, and thus a large negative open potential of the powders milled for 20 h was presented. In the next time range (from 20 to 40 h), the energy induced by the milling contributes to not only an alloying process (Fig. 1), but also the formation of hydrogen gas caused by the hydrogen desorption during milling and accordingly the hydrogen concentration on the powder surface decreased, resulting in the increases of the open potential of the milled powders and the pressure within the container. For further milling (from 40 to 60 h), part of the hydrogen gas was dissociated and subsequently absorbed by the powders, leading to the decreases of the potential of the powders and the pressure. It is interesting that the powders milled for 60 h are stable in the KOH solution, which means that hydrogen is mainly stored in the obtained amorphous phase instead of hydrogen adsorption on the alloy surface. The long time MA induces to the alloying between titanium hydride and nickel. The electrochemical kinetic performance of the titanium hydride was significant improved by the alloying component nickel.
3.2.
Electrochemical properties
Fig. 2 shows the discharge capacity as a function of the cycle number for the Ti–Ni–H powders milled for 60 h. The capacity 110
titanium hydride nickel 100
Discharge capacity (mAh/g)
60 h
Intensity (arb.unit)
40 h
20 h
90
80
70
Non-milled 60 20
30
40
50
60
70
80
90
2 theta (degree) Fig. 1 – XRD patterns of the Ti–Ni–H powders prepared at several milling periods.
0
10
20
30
40
50
Cycle number (n) Fig. 2 – Discharge capacity vs. cycle number curves of the Ti–Ni–H powders milled for 60 h.
3078
international journal of hydrogen energy 35 (2010) 3076–3079
-1.0 50 % DOD
Potential(Vvs.Hg/HgO)
-0.9 0 % DOD -0.8 95 % DOD -0.7
-0.6 0
20
40
60
80
100
Discharge capacity (mAh/g) Fig. 3 – Discharge curve of the Ti–Ni–H powders milled for 60 h at a discharge current density of 60 mA/g.
30 0 % DOD 50 % DOD 95 % DOD
20
Current d ensity(mA/g)
increased first and then decreased continuously. The maximum discharge capacity of the alloy was 102.2 mAh/g after one charge and discharge cycle for activation. The discharge capacity of the alloy was only 75.5 mAh/g after 50 charge and discharge cycles. This may be due to the oxidation of the titanium hydride by the following steps. First, MA decreases the interaction of the metal–hydrogen in the titanium hydride, and thus part of the titanium hydride decomposes to hydrogen and titanium, which is oxidized into titanium oxide during the discharging process. Second, part of the titanium hydride is also oxidized due to the strong oxidation by the alkaline and an increase of the overpotential between the titanium hydride and the amorphous matrix during the discharging process. Third, although it is well known that amorphous phase has high strength and good toughness, the titanium hydride as a brittle phase dispersed in the amorphous matrix leads to the pulverization of the powders during the cycling. This will accelerate the corrosion of the alloy. Accordingly, a drastic decay of the discharge capacity of the alloy occurred by the three factors. Fig. 3 presents the discharge curve of the alloy. It is known that the discharge curve consists of three regions. The first is the metal-hydride region called b-phase, and then the b-phase transfers to the solid solution called a-phase and the two phases coexist. Finally, a drastic decrease of the potential occurs due to the depletion of hydrogen atoms from the electrode surface, indicating the a-phase region occurs. These three regions can be simply expressed by 0%, 50% and 95% depth of discharge (DOD) pointed out in Fig. 3, respectively. Fig. 4 shows the linear polarization curves of the Ti–Ni–H powders milled for 60 h at various depths of discharge. The polarization resistance Rp can be determined from the ratio of the overpotential h to the current density I. The exchange current density I0 can be obtained from the equation which is linearized from Butler–Volmer equation in low overpotential region (<10 mV) [8–10]. The exchange current densities of the alloy were 66.7, 54.1 and 43.6 mA/g at the 0%, 50% and 95% DOD of the alloy, respectively. The corresponding polarization resistances were 385.0, 474.7 and 588.5 mU g, respectively, indicating that the charge-transfer ability on the alloy surface degraded as the hydrogen concentration decreased.
10
0
-10
-20
-30
-10
-5
0
5
10
Overpotential (mV) Fig. 4 – Linear polarization curves of the Ti–Ni–H powders milled for 60 h.
Generally, the fully charged hydrogen storage alloy has a surface layer with saturated metal-hydride phase, which limits the hydrogen dissociation from the water and thus a low exchange current density. When the hydrogen concentration of the alloy decreases, i.e., an increase of DOD, active material without the bondage of the interaction of the metal–hydrogen bond increases, facilitating the chargetransfer reaction on the alloy surface and as a result the I0 increases. On the contrary, the present work shows a reverse result. During the discharging process, the amorphous matrix desorbs hydrogen atoms and thus a positive shift of its potential occurs, resulting in a large overpotential between the amorphous phase and the titanium hydride. Undoubtedly, the increased overpotential accelerates the corrosion of the titanium hydride during the oxidation process. Moreover, the titanium oxide formed on the alloy surface drastically decreases the activity of the alloy for hydrogen absorption/ desorption. In addition, the pulverization of the powders leads to an increase of the coverage of the oxide on the alloy surface. Consequently, the I0 depressed as the DOD increased. The factors for the decay of the discharge capacity vs. cycle number mentioned above were reasonable and could be confirmed here. Fig. 5 shows the EIS curves of the Ti–Ni–H powders milled for 60 h at various depths of discharge. It can be seen that each curve consists of two semicircles and a straight line. The semicircle in high frequency region is related to the contact resistance R2 between the collector and alloy particles. The semicircle in low frequency region is corresponding to the charge-transfer resistance R3 at the interface of electrode/ electrolyte, and the slope is caused by the hydrogen diffusion in the alloy. The equivalent circuit for EIS of the alloy electrode was thus expressed in Fig. 5. According to the equivalent circuit embedded in Fig. 5 and by means of fitting program ZVIEW, the values of R2 were 450.0, 494.4 and 1238.9 mU g at the 0%, 50% and 95% DOD, respectively. The corresponding values of R3 were 420.3, 481.7 and 666.7 mU g, respectively. The values of Warburg impedance Wo of hydrogen diffusion are 387.2, 1327.8 and 4443.5 mU g at the 0%, 50% and 95% DOD,
international journal of hydrogen energy 35 (2010) 3076–3079
of the Ti–Ni–H powders milled for 60 h. The amorphous phase was formed after ball milling. The electrode potentials of the milled powders were 0.989, 0.878 and 0.941 V (vs. Hg/HgO) in the electrolyte of 6 M KOH when the milling periods were 20, 40, and 60 h, respectively. The discharge capacity of the Ti– Ni–H powders milled for 60 h showed a maximum value of 102.2 mAh/g at a discharge current density of 60 mA/g. The linear polarization showed that the exchange current density decreased and the polarization resistance increased as the DOD increased. The electrochemical impedance spectroscopy (EIS) demonstrated the same consequence and showed that the contact resistance increased and the hydrogen diffusion decreased as the DOD increased.
0.9
0 % DOD 50 % DOD 95 % DOD
0.8 0.7
37.4 mHZ 1.18 mHZ
-Z" ( ohm)
0.6 0.5 0.4 0.3
R2
R3
Wo
R1
0.2 C2
C1
0.1 0.0 0.5
1.0
1.5
3079
2.0
Z' (ohm)
references Fig. 5 – EIS curves of the Ti–Ni–H powders milled for 60 h. The equivalent circuit for EIS is embedded in the figure. R1: the resistance of electrolyte. R2: contact resistance between collector and alloy particles. R3: charge-transfer resistance at interface of electrode/electrolyte. Ci: capacitor. Wo: Warburg impedance of hydrogen diffusion.
respectively. It is obvious that all of the contact resistance, charge-transfer resistance and Warburg resistance increased as the DOD increased. The result of linear polarization can be demonstrated here in spite of a discrepancy from the corresponding values obtained by EIS. With the increase of DOD, the EIS curves show that the slope line appeared at a relative high frequency region, indicating that the hydrogen diffusion will be the limiting or controlling step during the discharging process. Generally, as the hydrogen concentration increases, the mutual interaction of hydrogen atoms will lower its mobility [11,12]. Moreover, the number of vacant sites available for hydrogen movement increases as the DOD increases. Therefore low hydrogen concentration of the alloy facilitates the hydrogen diffusion. However, a contrary result was obtained here. It can be explained by the oxide layer covered on the alloy surface deteriorates the hydrogen diffusion from the bulk alloy to the alloy surface.
4.
Conclusions
Mechanical alloying has been used to synthesize a Ti–Ni–H hydrogen storage alloy by milling titanium hydride and nickel. X-ray diffraction was performed to study the structure of the milled Ti–Ni–H powders. Charge–discharge test, linear polarization (LP) and electrochemical impedance spectroscopy (EIS) were carried out to investigate the electrochemical properties
[1] Otsuka K, Ren X. Physical metallurgy of Ti–Ni-based shape memory alloys. Prog Mater Sci 2005;50(5):511–678. [2] Wang CS, Lei YQ, Wang QD. Effects of Nb and Pd on the electrochemical properties of a Ti–Ni hydrogen-storage electrode. J Power Sources 1998;70(2):222–7. [3] Drenchev B, Spassov T. Electrochemical hydriding of amorphous and nanocrystalline TiNi-based alloys. J Alloys Compd 2007;441(1–2):197–201. [4] Wang CS, Lei YQ, Wang QD. Studies of electrochemical properties of TiNi alloy used as an MH electrode–I. Discharge capacity. Electrochim Acta 1998;43(21–22):3193–207. [5] Wang CS, Lei YQ, Wang QD. Studies of electrochemical properties of TiNi alloy used as an MH electrode. II. Discharge kinetics. Electrochim Acta 1998;43(21–22):3209–16. [6] Krstajic´ Grgur BN, Mladenovic´ NS, Vojnovic´ MV, Jaksˇic´ MM. The determination of kinetics parameters of hydrogen evolution on Ti–Ni alloys by ac impedance. Electrochim Acta 1997;42(2):323–30. [7] Xu YH, Chen CP, Wang XL, Wang QD. The analysis of the two discharge plateaus for Ti–Ni-based metal hydride electrode alloys. J Power Sources 2002;112(1):105–8. [8] Zheng G, Popov BN, White RE. Application of porous electrode theory on metal hydride electrode in alkaline solution. J Electrochem Soc 1996;143(2):435–41. [9] Zheng G, Popov BN, White RE. Determination of transport and electrochemical kinetic parameters of bare and coppercoated LaNi4.27Sn0.24 electrode in alkaline solution. J Electrochem Soc 1996;143(3):834–9. [10] 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(1): 81–6. [11] Feng F, Han J, Geng M, Northwood DO. Study of hydrogen transport in metal hydride electrodes using a novel electrochemical method. J Electroanal Chem 2000;487(2): 111–9. [12] Feng F, Northwood DO. Hydrogen diffusion in the anode of Ni/MH secondary batteries. J Power Sources 2004;136(2): 346–50.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Electrochemical properties of Ti–Ni–H powders prepared by milling titanium hydride and nickel Xiangyu Zhaoa, Liqun Maa,*, Meng Yanga, Yi Dinga, Xiaodong Shena,b,** a
College of Materials Science and Engineering, Nanjing University of Technology, Nanjing 210009, PR China State Key Laboratory of Material-oriented Chemical Engineering, Nanjing University of Technology, Nanjing 210009, PR China
b
article info
abstract
Article history:
Mechanical alloying has been carried out to synthesize a hydrogen storage alloy by milling
Received 7 April 2009
titanium hydride and nickel. The structure and electrochemical properties such as
Accepted 7 July 2009
discharge capacity, charge-transfer, and hydrogen diffusion of the milled powders were
Available online 8 August 2009
investigated. The results of X-ray diffraction showed that an amorphous phase was formed
Keywords:
0.941 V (vs. Hg/HgO) in the electrolyte of 6 M KOH when the milling periods were 20, 40,
Ti–Ni
and 60 h, respectively. The Ti–Ni–H powders milled for 60 h had a maximum discharge
Mechanical alloying
capacity of 102.2 mAh/g at a discharge current density of 60 mA/g. The results of the linear
Hydrogen storage alloy
polarization showed that the exchange current density decreased as the hydrogen
Linear polarization
concentration within the powders decreased. The electrochemical impedance spectros-
Electrochemical impedance spectra
copy (EIS) demonstrated the same consequence and presented that the hydrogen diffusion
after ball milling. The electrode potentials of the milled powders were 0.989, 0.878 and
decreased by decreasing the hydrogen concentration. ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The Ti–Ni system has been extensively investigated by considering its shape memory characteristics and hydrogen storage performance [1–3]. For hydrogen storage, it is known that both TiNi and Ti2Ni in the Ti–Ni system absorb hydrogen at room temperature. TiNi alloy has a reversible hydrogenation capacity of 1.0 H/M. Ti2Ni alloy absorbs a large amount of hydrogen but desorbs irreversibly due to its poor discharge kinetics [2]. The electrochemical properties such as the discharge capacity [4], the discharge kinetics [5,6] and the discharge plateaus [7] of Ti–Ni alloys were studied previously by synthesizing the alloys with the raw materials of Ti and Ni with high purity. In this work, titanium hydride and nickel were used as raw materials for preparing a Ti–Ni-based alloy by mechanical alloying (MA). Changes of the structure and electrochemical
properties of the milled powders were investigated, emphasizing the discharge properties, the charge-transfer on the alloy surface and the hydrogen diffusion within the bulk alloy.
2.
Experimental
Powders of titanium Hydride (300 mesh, 99.9 at.% pure) and nickel (350 mesh, 99.9 at.% pure) were blended together to form a nominal composition of TiH2 – 50% Ni. They were loaded into a stainless steel container with stainless steel balls (10 mm and 20 mm in diameter) in a glove box under an argon atmosphere. The ball to powder ratio is 20:1. The milling was performed in a planetary mill with a rotation speed of 300 rpm and the milling time was 20, 40 or 60 h. The three milling periods were carried out continuously without interim sampling. The structure of the as-milled powders was studied
* Corresponding author. Tel.: þ86 25 83587243; fax: þ86 25 83240205. ** Corresponding author. College of Materials Science and Engineering, Nanjing University of Technology, Nanjing 210009, PR China E-mail addresses: [email protected] (X. Zhao), [email protected] (L. Ma), [email protected] (X. Shen). 0360-3199/$ – see front matter ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.07.017
3077
international journal of hydrogen energy 35 (2010) 3076–3079
by using the technique of X-ray diffraction (XRD) in a Thermo ARL X’ TRA diffractometer equipped with Cu Ka radiation. The metal-hydride electrodes were prepared by cold pressing a mixture of 0.18 g milled powders and 0.63 g nickel powders into a pellet with 18 mm in diameter under a pressure of 20 MPa. The charge and discharge testing were carried out in a half-cell consisting of the metal-hydride electrode, a Ni(OH)2/NiOOH counter electrode and a Hg/HgO reference electrode in a 6 M KOH solution under BT-2000 testing equipment (Arbin, USA) at 298 K. The open potentials (vs. the Hg/HgO) of the electrodes, before the charge and discharge testing, were obtained when the potential changed less than 1 mV in two minutes. The electrode containing the powders milled for 60 h was charged at 60 mA/g for 4 h followed by a 10 min rest and then discharged at 60 mA/g to the cut-off potential of 0.6 V vs. the Hg/HgO reference electrode. Linear polarization curves of the electrode were measured by scanning the electrode potential with a scan rate of 1 mV/s from 10 to 10 mV (vs. open circuit potential). Electrochemical impedance spectroscopy (EIS) of the electrode was obtained in the frequency range from 10 KHz to 0.001 Hz with an amplitude of 5 mV. These two tests were carried out on a CHI 660B electrochemical workstation at 298 K.
3.
Results and discussion
3.1.
Mechanical alloying
Fig. 1 shows the XRD patterns of the Ti–Ni–H powders as a function of the milling time. The intensities of the diffraction peaks corresponding to the raw materials were obviously decreased by 20 h of milling. In addition to the decrease of the intensity, continued ball milling induced considerable broadening of the peaks. The diffraction peaks of nickel disappear after 40 h of milling, indicating the formation of an alloy between the raw materials. When the milling time is 60 h, the milled product was predominantly amorphous with a few amount of titanium hydride phase.
The open potentials of the electrodes prepared by the milled powders were 0.989, 0.878 and 0.941 V when the milling periods were 20, 40 and 60 h, respectively. Furthermore, a gas flow meter was used to detect the pressure within the milling container at the end of each milling period. It was found that the pressure was almost unchanged after 20 h of milling as compared to that of the non-milled one, while a drastic increase and a slight increase of the pressure occurred when the milling periods were 40 and 60 h, respectively. This means that hydrogen gas was formed within the container after 20 h of milling. Three stages of the milling process can be obtained. The first is the short milling time (below 20 h). In this time range, mechanical milling (MM) leads to a decrease of the stability of titanium hydride and consequently a great many hydrogen atoms adsorb on the powder surface, resulting in the occurrence of hydrogen desorption or hydrogen evolution reaction in the 6 M KOH electrolyte, and thus a large negative open potential of the powders milled for 20 h was presented. In the next time range (from 20 to 40 h), the energy induced by the milling contributes to not only an alloying process (Fig. 1), but also the formation of hydrogen gas caused by the hydrogen desorption during milling and accordingly the hydrogen concentration on the powder surface decreased, resulting in the increases of the open potential of the milled powders and the pressure within the container. For further milling (from 40 to 60 h), part of the hydrogen gas was dissociated and subsequently absorbed by the powders, leading to the decreases of the potential of the powders and the pressure. It is interesting that the powders milled for 60 h are stable in the KOH solution, which means that hydrogen is mainly stored in the obtained amorphous phase instead of hydrogen adsorption on the alloy surface. The long time MA induces to the alloying between titanium hydride and nickel. The electrochemical kinetic performance of the titanium hydride was significant improved by the alloying component nickel.
3.2.
Electrochemical properties
Fig. 2 shows the discharge capacity as a function of the cycle number for the Ti–Ni–H powders milled for 60 h. The capacity 110
titanium hydride nickel 100
Discharge capacity (mAh/g)
60 h
Intensity (arb.unit)
40 h
20 h
90
80
70
Non-milled 60 20
30
40
50
60
70
80
90
2 theta (degree) Fig. 1 – XRD patterns of the Ti–Ni–H powders prepared at several milling periods.
0
10
20
30
40
50
Cycle number (n) Fig. 2 – Discharge capacity vs. cycle number curves of the Ti–Ni–H powders milled for 60 h.
3078
international journal of hydrogen energy 35 (2010) 3076–3079
-1.0 50 % DOD
Potential(Vvs.Hg/HgO)
-0.9 0 % DOD -0.8 95 % DOD -0.7
-0.6 0
20
40
60
80
100
Discharge capacity (mAh/g) Fig. 3 – Discharge curve of the Ti–Ni–H powders milled for 60 h at a discharge current density of 60 mA/g.
30 0 % DOD 50 % DOD 95 % DOD
20
Current d ensity(mA/g)
increased first and then decreased continuously. The maximum discharge capacity of the alloy was 102.2 mAh/g after one charge and discharge cycle for activation. The discharge capacity of the alloy was only 75.5 mAh/g after 50 charge and discharge cycles. This may be due to the oxidation of the titanium hydride by the following steps. First, MA decreases the interaction of the metal–hydrogen in the titanium hydride, and thus part of the titanium hydride decomposes to hydrogen and titanium, which is oxidized into titanium oxide during the discharging process. Second, part of the titanium hydride is also oxidized due to the strong oxidation by the alkaline and an increase of the overpotential between the titanium hydride and the amorphous matrix during the discharging process. Third, although it is well known that amorphous phase has high strength and good toughness, the titanium hydride as a brittle phase dispersed in the amorphous matrix leads to the pulverization of the powders during the cycling. This will accelerate the corrosion of the alloy. Accordingly, a drastic decay of the discharge capacity of the alloy occurred by the three factors. Fig. 3 presents the discharge curve of the alloy. It is known that the discharge curve consists of three regions. The first is the metal-hydride region called b-phase, and then the b-phase transfers to the solid solution called a-phase and the two phases coexist. Finally, a drastic decrease of the potential occurs due to the depletion of hydrogen atoms from the electrode surface, indicating the a-phase region occurs. These three regions can be simply expressed by 0%, 50% and 95% depth of discharge (DOD) pointed out in Fig. 3, respectively. Fig. 4 shows the linear polarization curves of the Ti–Ni–H powders milled for 60 h at various depths of discharge. The polarization resistance Rp can be determined from the ratio of the overpotential h to the current density I. The exchange current density I0 can be obtained from the equation which is linearized from Butler–Volmer equation in low overpotential region (<10 mV) [8–10]. The exchange current densities of the alloy were 66.7, 54.1 and 43.6 mA/g at the 0%, 50% and 95% DOD of the alloy, respectively. The corresponding polarization resistances were 385.0, 474.7 and 588.5 mU g, respectively, indicating that the charge-transfer ability on the alloy surface degraded as the hydrogen concentration decreased.
10
0
-10
-20
-30
-10
-5
0
5
10
Overpotential (mV) Fig. 4 – Linear polarization curves of the Ti–Ni–H powders milled for 60 h.
Generally, the fully charged hydrogen storage alloy has a surface layer with saturated metal-hydride phase, which limits the hydrogen dissociation from the water and thus a low exchange current density. When the hydrogen concentration of the alloy decreases, i.e., an increase of DOD, active material without the bondage of the interaction of the metal–hydrogen bond increases, facilitating the chargetransfer reaction on the alloy surface and as a result the I0 increases. On the contrary, the present work shows a reverse result. During the discharging process, the amorphous matrix desorbs hydrogen atoms and thus a positive shift of its potential occurs, resulting in a large overpotential between the amorphous phase and the titanium hydride. Undoubtedly, the increased overpotential accelerates the corrosion of the titanium hydride during the oxidation process. Moreover, the titanium oxide formed on the alloy surface drastically decreases the activity of the alloy for hydrogen absorption/ desorption. In addition, the pulverization of the powders leads to an increase of the coverage of the oxide on the alloy surface. Consequently, the I0 depressed as the DOD increased. The factors for the decay of the discharge capacity vs. cycle number mentioned above were reasonable and could be confirmed here. Fig. 5 shows the EIS curves of the Ti–Ni–H powders milled for 60 h at various depths of discharge. It can be seen that each curve consists of two semicircles and a straight line. The semicircle in high frequency region is related to the contact resistance R2 between the collector and alloy particles. The semicircle in low frequency region is corresponding to the charge-transfer resistance R3 at the interface of electrode/ electrolyte, and the slope is caused by the hydrogen diffusion in the alloy. The equivalent circuit for EIS of the alloy electrode was thus expressed in Fig. 5. According to the equivalent circuit embedded in Fig. 5 and by means of fitting program ZVIEW, the values of R2 were 450.0, 494.4 and 1238.9 mU g at the 0%, 50% and 95% DOD, respectively. The corresponding values of R3 were 420.3, 481.7 and 666.7 mU g, respectively. The values of Warburg impedance Wo of hydrogen diffusion are 387.2, 1327.8 and 4443.5 mU g at the 0%, 50% and 95% DOD,
international journal of hydrogen energy 35 (2010) 3076–3079
of the Ti–Ni–H powders milled for 60 h. The amorphous phase was formed after ball milling. The electrode potentials of the milled powders were 0.989, 0.878 and 0.941 V (vs. Hg/HgO) in the electrolyte of 6 M KOH when the milling periods were 20, 40, and 60 h, respectively. The discharge capacity of the Ti– Ni–H powders milled for 60 h showed a maximum value of 102.2 mAh/g at a discharge current density of 60 mA/g. The linear polarization showed that the exchange current density decreased and the polarization resistance increased as the DOD increased. The electrochemical impedance spectroscopy (EIS) demonstrated the same consequence and showed that the contact resistance increased and the hydrogen diffusion decreased as the DOD increased.
0.9
0 % DOD 50 % DOD 95 % DOD
0.8 0.7
37.4 mHZ 1.18 mHZ
-Z" ( ohm)
0.6 0.5 0.4 0.3
R2
R3
Wo
R1
0.2 C2
C1
0.1 0.0 0.5
1.0
1.5
3079
2.0
Z' (ohm)
references Fig. 5 – EIS curves of the Ti–Ni–H powders milled for 60 h. The equivalent circuit for EIS is embedded in the figure. R1: the resistance of electrolyte. R2: contact resistance between collector and alloy particles. R3: charge-transfer resistance at interface of electrode/electrolyte. Ci: capacitor. Wo: Warburg impedance of hydrogen diffusion.
respectively. It is obvious that all of the contact resistance, charge-transfer resistance and Warburg resistance increased as the DOD increased. The result of linear polarization can be demonstrated here in spite of a discrepancy from the corresponding values obtained by EIS. With the increase of DOD, the EIS curves show that the slope line appeared at a relative high frequency region, indicating that the hydrogen diffusion will be the limiting or controlling step during the discharging process. Generally, as the hydrogen concentration increases, the mutual interaction of hydrogen atoms will lower its mobility [11,12]. Moreover, the number of vacant sites available for hydrogen movement increases as the DOD increases. Therefore low hydrogen concentration of the alloy facilitates the hydrogen diffusion. However, a contrary result was obtained here. It can be explained by the oxide layer covered on the alloy surface deteriorates the hydrogen diffusion from the bulk alloy to the alloy surface.
4.
Conclusions
Mechanical alloying has been used to synthesize a Ti–Ni–H hydrogen storage alloy by milling titanium hydride and nickel. X-ray diffraction was performed to study the structure of the milled Ti–Ni–H powders. Charge–discharge test, linear polarization (LP) and electrochemical impedance spectroscopy (EIS) were carried out to investigate the electrochemical properties
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