International Journal of Hydrogen Energy 32 (2007) 2434–2438www.elsevier.com/locate/ijhydene

Microstructure and electrochemical behavior of Cr-addedV2.1TiNi0.4Zr0.06Cr0.152 hydrogen storage electrode alloy

Lu Li, Wenjiao Wang, Xiulin Fan, Xiaofeng Jin, Hai Wang, Yongquan Lei,Qidong Wang, Lixin Chen∗

Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China

Received 11 August 2006; received in revised form 24 November 2006; accepted 25 November 2006Available online 16 January 2007

Abstract

The microstructure and electrochemical behavior of V2.1TiNi0.4Zr0.06Cr0.152 hydrogen storage electrode alloy have been investigated incomparison with V2.1TiNi0.4Zr0.06 alloy. The results show that V2.1TiNi0.4Zr0.06Cr0.152 alloy consists of a V-based solid solution main phaseand a C14-type Laves secondary phase in the form of three-dimensional network, being similar to V2.1TiNi0.4Zr0.06 alloy, the secondary phaseprecipitates along the grain boundaries of the main phase. As compared with V2.1TiNi0.4Zr0.06 alloy, the unit cell volume of each phase in theV2.1TiNi0.4Zr0.06Cr0.152 alloy contracts. It is found that adding Cr restricts the dissolution of vanadium and titanium into the KOH electrolyte,and improves the corrosion resistance of the alloy, thus the cycling stability after 30 cycles increases from 22.34% (V2.1TiNi0.4Zr0.06) to77.96% (V2.1TiNi0.4Zr0.06Cr0.152). Furthermore, V2.1TiNi0.4Zr0.06Cr0.152 alloy has a better high-rate dischargeability and higher exchangecurrent density compared with V2.1TiNi0.4Zr0.06 alloy, but its maximum discharge capacity decreases.� 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

Keywords: Hydrogen storage alloy; Microstructure; Electrochemical property; V-based solid solution

1. Introduction

Along with the rapid development of nickel/metal-hydride(MH) secondary batteries, the demand for novel electrode ma-terials with much higher hydrogen storage capacity is urgent[1]. Recently, vanadium-based solid solution electrode alloyshave been attracting significant attention of scientists becauseof their high electrochemical capacity [2,3]. It was reportedthat these alloys characteristically consist of two phases: aV-based solid solution main phase and a secondary phase, whichwork as a hydrogen storage phase and a microcurrent collector,respectively [4,5]. However, the poor cycle stability of thesealloys in KOH electrolyte keeps them from the industrializa-tion for Ni–MH batteries, which attributes to the dissolutionof vanadium into and the corrosion of vanadium by the al-kali electrolyte [6–10]. In our previous works, V2.1TiNix andV2.1TiNi0.4Zrx alloys have been studied and reported [9–11],

∗ Corresponding author. Tel./fax: +86 571 8795 1152.E-mail address: lxchen@zju.edu.cn (L. Chen).

0360-3199/$ - see front matter � 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.ijhydene.2006.11.027

and V2.1TiNi0.4Zr0.06 was found to have a high discharge ca-pacity of 468.5 mA h/g and a poor capacity retention of 22.34%after 30 charging–discharging cycles. It is well known that mul-ticomponent alloying is an effective method for improving theoverall properties of electrode alloys. Liu et al. [12] reportedthat Cr alloying improved the electrochemical cyclic durabilityof Zr-based AB2 electrode alloys owing to a lower level of Vdissolution. Pan et al. [13] also reported that Cr addition in TiV-based alloys caused positive influence on the cycle stability. Inorder to improve the cycling stability of V2.1TiNi0.4Zr0.06 al-loy further, we chose Cr as an alloying element to inhibit thedissolution of vanadium from alloy into KOH electrolyte [14],and investigated the microstructure and electrochemical behav-ior of the Cr-added V2.1TiNi0.4Zr0.06Cr0.152 hydrogen storageelectrode alloy with V2.1TiNi0.4Zr0.06 as a contrastive alloy inthis paper.

2. Experimental

The alloy samples were prepared by vacuum magneticinduction melting of vanadium (purity > 99%), titanium

L. Li et al. / International Journal of Hydrogen Energy 32 (2007) 2434–2438 2435

(purity>99%), nickel (purity>99%), zirconium (purity>99%),and chromium (purity > 99%) under argon atmosphere, andeach batch was remelted three times to ensure high homogene-ity. Some of the as-cast ingot was pulverized first by hydrid-ing and then crushed mechanically into powder below 50 �min diameter after dehydriding. The metallographic microstruc-tures and chemical compositions were examined by scanningelectron microscopy (FSEM, SIRION-100) with an energy dis-persive X-ray spectrometer (EDS) after cross-sections of alloyswere polished and etched. The crystal structures and lattice pa-rameters were determined by X-ray powder diffraction (XRD,Rikagu D/Max PC2500) using Cu K� radiation.

Each test electrode was prepared by mixing 100 mg alloypowders with nickel powder in the weight ratio of 1:2 andthen the mixture was cold-pressed (18 MPa) into a pellet of10 mm diameter. The electrochemical properties of each pelletworked as the negative electrode were measured in a trielectrodeopen cell at 298 K with a sintered Ni(OH)2/NiOOH positivecounterelectrode and a Hg/HgO reference electrode by DC-5battery testing instrument.

The negative electrodes were charged at 100 mA/g for 6 hand discharged at 50–400 mA/g with the cut-off potential of−0.70 V vs. Hg/HgO. As the electrodes were activated and thendischarged to half of the maximum capacities (DOD = 50%),their exchange current densities I0 were calculated from theslopes of micropolarization curves, which were determined byscanning the electrode potential at 0.1 mV/s from −5 to +5 mVwith a Solartron SI 1287 potentiostat, and their electrochem-ical impedance measurements were conducted by scanningfrom 100 kHz to 5 mHz at the electrode potential disturbedamplitude of 5 mV with a Solartron SI 1287 potentiostat anda Solartron 1255B frequency spectrograph. The contents ofthe dissolved alloy elements in the KOH electrolyte after 30charging–discharging cycles were measured by the emissionspectrochemical analysis with radio frequency inductivelycoupled plasma (ICP).

3. Results and discussion

3.1. Microstructure

Fig. 1 shows the XRD patterns of the Cr-free V2.1TiNi0.4Zr0.06and Cr-added V2.1TiNi0.4Zr0.06Cr0.152 alloys. It is found thatV2.1TiNi0.4Zr0.06Cr0.152 alloy, being similar to V2.1TiNi0.4Zr0.06alloy, consists of a main phase of V-based solid solution withbcc structure and a secondary phase of C14-type Laves phase.The lattice parameters and unit cell volumes of each phase inthese alloys were determined as presented in Table 1. Compar-ing with V2.1TiNi0.4Zr0.06 alloy, the lattice parameters and unitcell volumes of each phase in V2.1TiNi0.4Zr0.06Cr0.152 alloydecrease, because the atomic radius of Cr is smaller than thatof V, Ti, and Zr.

The SEM micrographs of these as-cast alloys are shown inFig. 2. It can be found that the secondary phase precipitatesalong the grain boundaries of the main phase in the form of athree-dimensional network. Table 1 shows the distribution of

b

a

Fig. 1. XRD patterns of the V2.1TiNi0.4Zr0.06 (a) andV2.1TiNi0.4Zr0.06Cr0.152 (b) alloys.

metallic elements in the main and secondary phases determinedby calculation based on the compositions determined by theEDS analysis. It is found that V is distributed predominantlyinto the bcc main phase while Ti and Ni mainly exist in thesecondary C14 Laves phase. Both V and Cr are dissolved morein the main phase than the secondary phase.

3.2. Electrochemical properties

Fig. 3 shows the discharge capacity at a discharge currentof 50 mA/g vs. cycle number for the V2.1TiNi0.4Zr0.06 andV2.1TiNi0.4Zr0.06Cr0.152 alloys. The V2.1TiNi0.4Zr0.06Cr0.152alloy only reaches the maximum discharge capacity of397.1 mA h/g at the fourth cycle (presented in Table 2), lessthan that of V2.1TiNi0.4Zr0.06 alloy (468.5 mA h/g). However,the cycling capacity degradation of V2.1TiNi0.4Zr0.06Cr0.152alloy is very slow, and its capacity retention after 30 cyclesreaches 77.96%, much higher than that of V2.1TiNi0.4Zr0.06alloy (22.34%). In order to understand this reason, the contentsof the dissolved alloy elements in the KOH electrolyte after 30cycles were measured and shown in Table 3. It is found that thedissolution of V or Ti from V2.1TiNi0.4Zr0.06Cr0.152 alloy intothe KOH electrolyte after 30 charge–discharge cycles clearlyreduces as compared with V2.1TiNi0.4Zr0.06 alloy, which im-plies that Cr element can inhibit the dissolution of V or Ti fromalloy into the KOH electrolyte powerfully. These indicate thatadding Cr into V2.1TiNi0.4Zr0.06 is very helpful in improvingcycling stabilities, because the addition of Cr results in theformation of an oxide layer just like in some Cr-containedAB2-type alloys and V-based alloys with multiphase structure,which remarkably prohibits the dissolution of V and Ti ele-ments due to strengthening the corrosion resistance of the alloyon the surface [15,16]. Furthermore, the alloy containing Crthat would be activated after four charging–discharging cyclesis difficult to activate for the electrochemical reaction in theKOH electrolyte due to the formation of the passive oxide that

2436 L. Li et al. / International Journal of Hydrogen Energy 32 (2007) 2434–2438

Table 1Microstructural characteristics of the V2.1TiNi0.4Zr0.06 and V2.1TiNi0.4Zr0.06Cr0.152 alloys

Sample Phase Lattice parameters (nm) Unit cell volumes (nm3) Composition (at%)

a c V V Ti Ni Zr Cr

V2.1TiNi0.4Zr0.06 bcc 0.3138 — 0.03090 67.83 23.28 5.71 3.18 —C14 0.5058 0.8270 0.21157 22.42 34.09 39.53 3.96 —

V2.1TiNi0.4Zr0.06Cr0.152 bcc 0.3082 — 0.02928 67.26 21.31 5.12 2.94 3.37C14 0.5060 0.8216 0.21036 36.60 33.06 24.92 3.85 1.84

Fig. 2. Scanning electron micrographs of the V2.1TiNi0.4Zr0.06 (a) and V2.1TiNi0.4Zr0.06Cr0.152 (b) alloys.

0 5 10 15 20 25 300

100

200

300

400

500

Dis

ch

arg

e C

ap

acity/m

Ah

g-1

Cycle Number

V2.1TiNi0.4Zr0.06

V2.1TiNi0.4Zr0.06 Cr0.152

Fig. 3. Discharge capacity vs. cycle number for the V2.1TiNi0.4Zr0.06 andV2.1TiNi0.4Zr0.06Cr0.152 alloys discharged at 50 mA/g.

weakens the surface electrocatalytic activity. Therefore addingCr into V-based solid solution alloys is the effective way forimproving the cycling stabilities of the alloys by restrainingthe dissolution of V or Ti elements.

The high-rate dischargeability (HRD) of the V2.1TiNi0.4Zr0.06and V2.1TiNi0.4Zr0.06Cr0.152 alloys is shown in Fig. 4. TheHRD is calculated according to the following formula:

HRDd = Cd/(Cd + C25) × 100%, (1)

wherein Cd is the discharge capacity at the discharge currentdensity Id with the cut-off potential of −0.7 V vs. Hg/HgOreference electrode, C25 is the residual discharge capacityat the discharge current density I = 25 mA/g with the cut-off potential of −0.7 V vs. Hg/HgO reference electrode afterthe alloy electrode has been fully discharged at the largedischarge current density (Id). The HRD of each alloy de-creases with the increase of discharge current density dueto the rise of overpotential. As shown in Table 2, it is alsofound that V2.1TiNi0.4Zr0.06Cr0.152 alloy has a better HRDthan V2.1TiNi0.4Zr0.06 alloy. At the discharged current densityof 400 mA/g, the HRD400 of V2.1TiNi0.4Zr0.06Cr0.152 alloy is62.8%, almost twice that of V2.1TiNi0.4Zr0.06 alloy (32.6%).

Fig. 5 illustrates the linear-polarization curves and exchangecurrent densities of the V2.1TiNi0.4Zr0.06 and V2.1TiNi0.4Zr0.06Cr0.152 alloys. The exchange current density I0 is gener-ally an important kinetic parameter for the electrochemicalcharging–discharging reaction, and also represents the rate ofhydriding–dehydriding (charge-transfer rate) at the equilib-rium state, which can be used to evaluate the reversibility ofthe reaction. It can be estimated from the slope of the obtainedpolarization curve since the curve is approximately linear atsmall overpotential by the following formula [17]:

I0 = IdRT/F�, (2)

wherein R is the gas constant, T the absolute temperature,F the Faraday constant, and Id/� is the slope of the po-larization curve. From the curve in Fig. 5, the I0 value ofV2.1TiNi0.4Zr0.06Cr0.152 alloy reaches 159.31 mA/g, which is

L. Li et al. / International Journal of Hydrogen Energy 32 (2007) 2434–2438 2437

Table 2Electrochemical behaviors of the V2.1TiNi0.4Zr0.06 and V2.1TiNi0.4Zr0.06Cr0.152 alloys

Sample Activation cycle number Cmax (mA h/g) HRD400a (%) S30 (%)

V2.1TiNi0.4Zr0.06 2 468.5 32.6 22.34V2.1TiNi0.4Zr0.06Cr0.152 4 397.1 62.8 77.96

aHigh-rate dischargeability at a discharged current density of 400 mA/g.

Table 3The contents of the dissolved alloy elements in the KOH electrolyte after 30 cycles

Sample Content (�g/ml)

V Ti Ni Zr Cr

V2.1TiNi0.4Zr0.06 184.04 44.91 2.23 2.50 —V2.1TiNi0.4Zr0.06Cr0.152 60.00 11.88 1.09 1.38 0.54

0 50 100 150 200 250 300 350 400

20

40

60

80

100

V2.1TiNi0.4Zr0.06

V2.1TiNi0.4Zr0.06 Cr0.152

Hig

h-r

ate

Dis

ch

arg

ea

bili

ty/%

Discharge Current /mAg-1

Fig. 4. High-rate dischargeabilities of the V2.1TiNi0.4Zr0.06 andV2.1TiNi0.4Zr0.06Cr0.152 alloys.

quite larger than 131.01 mA/g for V2.1TiNi0.4Zr0.06 alloy. It in-dicates that Cr enhances the charge-transfer rate on the surfaceof the alloy.

The electrochemical impedance spectrum (EIS) of theV2.1TiNi0.4Zr0.06 and V2.1TiNi0.4Zr0.06Cr0.152 alloys after ac-tivation and 30 cycles (at a half-discharge state) is shown inFig. 6. It can be seen that the EIS of each alloy contains twosemicircles followed by a straight line at the low-frequencyrange. Kuriyama et al. [18] thought the measured high-frequency semicircle which is the first semicircle to the contactresistance between the current collector and the alloy pow-der, the lower-frequency loop to the reaction resistance on thealloy surface, and the impedance between these two semicir-cles to the alloy particle-to-particle resistance. As shown inFig. 6, V2.1TiNi0.4Zr0.06Cr0.152 alloy has the radius of thesmaller semicircle in the low-frequency region when the alloysare activated, and its charge-transfer reaction resistance is lessthan V2.1TiNi0.4Zr0.06 alloy (shown in the magnified section ofFig. 6). This variation is consistent with that of the exchange

-5 -4 -3 -2 -1 0 1 2 3 4 5

-3

-2

-1

0

1

2

3

I0 = 159.31 mAg-1

Cu

rre

nt/

mA

g-1

V2.1TiNi0.4Zr0.06

V2.1TiNi0.4Zr0.06Cr0.152

Overpotential/mV

I0 = 131.01 mAg-1

Fig. 5. Linear-polarization curves and exchange current densities of theV2.1TiNi0.4Zr0.06 and V2.1TiNi0.4Zr0.06Cr0.152 alloys.

0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2

0.0

-0.2

-0.4

-0.6

-0.8

-1.0

-1.2

-1.4

-1.6

-1.8

0.3 0.4 0.5 0.6 0.70.00

-0.02

-0.04

-0.06

-0.08

-0.10

-0.12

-0.14

Z'/Ω

After activation After 30 cycles

V2.1TiNi0.4Zr0.06

V2.1TiNi0.4Zr0.06Cr0.152

V2.1TiNi0.4Zr0.06

V2.1TiNi0.4Zr0.06Cr0.152

Z''/Ω

Fig. 6. Electrochemical impedance spectrums of the V2.1TiNi0.4Zr0.06 andV2.1TiNi0.4Zr0.06Cr0.152 alloys.

2438 L. Li et al. / International Journal of Hydrogen Energy 32 (2007) 2434–2438

current densities, and both two measured results are similarto that of HRD. These ascribe to the passive layer formed byadding Cr, because it not only weakens the surface electro-catalytic activity, but also prevents the corrosion of the alloyelements and, namely, reduces creating more fresh surfaceto be oxidated seriously as charge–discharge reaction. Thusthis passive layer is available to heighten the charge-transferrate on the surface. In a word, the kinetic property of theV-based solid solution electrode alloys can be improved by in-troducing Cr into the alloys in the present investigation range.Moreover, it is also seen from Fig. 6 that the charge-transferreaction resistance of each alloy significantly rises after 30charging–discharging cycles, which is reasonable because thesurface passivation is gradually induced by the oxidation ofevery element during charge–discharge cycling.

4. Conclusion

The Cr-added V2.1TiNi0.4Zr0.06Cr0.152 alloys, being similarto the Cr-free V2.1TiNi0.4Zr0.06 alloy, consist of a main phaseof V-based solid solution with bcc structure and a secondaryphase of C14-type Laves phase in the form of three-dimensionalnetwork, and the secondary phase precipitates along the grainboundaries of the main phase. And the lattice parameter andunit cell volume of each phase in V2.1TiNi0.4Zr0.06Cr0.152 alloyare smaller than that of V2.1TiNi0.4Zr0.06 alloy.

The Cr-added V2.1TiNi0.4Zr0.06Cr0.152 alloy reaches themaximum discharge capacity of 397.1 mA h/g at the fourthcycle, less than that of V2.1TiNi0.4Zr0.06 alloy (468.5 mA h/g).However, the added Cr restricts the dissolution of vanadiumand titanium into the KOH electrolyte, improves the corro-sion resistance, thus the cycling stability after 30 cycles hasbeen improved from 22.34% (V2.1TiNi0.4Zr0.06) to 77.96%(V2.1TiNi0.4Zr0.06Cr0.152). Furthermore, V2.1TiNi0.4Zr0.06Cr0.152 alloy has a better HRD and higher exchange currentdensity compared with V2.1TiNi0.4Zr0.06 alloy.

Acknowledgment

This work was supported by the National Natural ScienceFoundation of China (No. 50571089 and No. 50631020).

References

[1] Feng F, Geng M, Northwood DO. Electrochemical behavior ofintermetallic-based metal hydrides used in Ni–MH batteries: a review.Int J Hydrogen Energy 2001;26:725–34.

[2] Tsukahara M, Takahashi K, Mishima T, Sakai T, Miyamura H, KuriyamaN. et al. Metal hydride electrodes based on solid solution type alloyTiV3Nix (0�x �0.75). J Alloys Compd 1995;226:203–7.

[3] Tsukahara M, Takahashi K, Mishima T, Sakai T, Miyamura H, KuriyamaN. et al. Phase structure of V-based solid solutions containing Ti and Niand their hydrogen absorption–desorption properties. J Alloys Compd1995;224:162–7.

[4] Tsukahara M, Takahashi K, Mishima T, Sakai T. Vanadium-based solidsolution alloys with three-dimensional network structure for high capacitymetal hydride electrodes. J Alloys Compd 1997;253–254:583–6.

[5] Iwakura C, Choi WK, Miyauchi R, Inoue H, Miyauchi R, Inoue H.Electrochemical and structural characterization of V–Ti–Ni hydrogenstorage alloys with BCC structure. J Electrochem Soc 2000;147(7):2503–6.

[6] Tsukahara M, Takahashi K, Mishima T, Isomura A, Sakai T. TheTiV3Ni0.56 hydride electrode: its electrochemical and cycle lifecharacterization. J Alloys Compd 1995;231:616–20.

[7] Tsukahara M, Takahashi K, Mishima T, Isomura A, Sakai T. Improvementof cycle stability of vanadium-based alloy for nickel–metal hydride(Ni–MH) battery. J Alloys Compd 1999;287:215–20.

[8] Zhang QA, Lei YQ, Yang XG, Du YL, Wang QD. Phase structures andelectrochemical properties of Cr-added V3TiNi0.56Hf0.24Mn0.15 alloys.Int J Hydrogen Energy 2000;25:977–81.

[9] Guo R, Chen LX, Lei YQ, Liao B, Ying T, Wang QD. The effect ofNi content on the phase structures and electrochemical properties ofV2.1TiNix (x=0.1.0.9) hydrogen storage alloys. Int J Hydrogen Energy2003;28:803–8.

[10] Dai FB, Chen LX, Liu J, Zheng FP, Zhang ZH, Lei YQ. The phasestructures and electrochemical properties of TiV2.1Nix (x = 0.2.0.6)hydrogen storage alloys. Rare Met Mater Eng 2005;34(9):1500–4 [inChinese].

[11] Chen LX, Li L, Wang XH, Dai FB, Zheng FP, Lei YQ. Phase structuresand electrochemical properties of V2.1TiNi0.4Zrx (x=0.0.06) hydrogenstorage electrode alloys. Acta Phys—Chim Sin 2006;22(5):523–7.

[12] Liu BH, Li ZP, Kitani R, Suda S. Improvement of electrochemicalcyclic durability of Zr-based AB2 alloy electrodes. J Alloys Compd2002;330–332:825–30.

[13] Pan HG, Li R, Gao MX, Liu YF, Wang QD. Effects of Cr on the structuraland electrochemical properties of TiV-based two-phase hydrogen storagealloys. J Alloys Compd 2005;404–406:669–74.

[14] Ovshinsky SR, Fetcenko MA, Ross J. A nickel metal hydride batteryfor electric vehicles. Science 1993;260:176–81.

[15] Sun JC, Li S, Ji SJ. Phase composition and electrochemical performancesof the Zr1−xTxCr0.4Mn0.2V0.1Ni1.3 alloys with 0.1�x �0.3. J AlloysCompd 2005;404–406:687–90.

[16] Tsukahara M, Takahashi K, Isomura A, Sakai T. Improvement of thecycle stability of vanadium-based alloy for nickelmetal hydride (Ni–MH)battery. J Alloys Compd 1999;287:215–20.

[17] Notten PHL, Hokkeling P. Double-phase hydride forming compounds:a new class of highly electrocatalytic materials. J Electrochem Soc1991;138(7):1877–85.

[18] Kuriyama N, Sakai T, Miyamura H, Uehara I, Ishikawa H, Iwasaki T.Electrochemical impedance and deterioration behavior of metal hydrideelectrodes. J Alloys Compd 1993;202:183–97.