Available online at www.sciencedirect.com

International Journal of Hydrogen Energy 29 (2004) 453–458

www.elsevier.com/locate/ijhydene

Impedance of Al-substituted �-nickel hydroxide electrodes

Bing Liu∗;1, Huatang Yuan, Yunshi ZhangInstitute of New Energy Material Chemistry, Nankai University, Tianjin 300071, China

Received 12 February 2002; received in revised form 25 April 2002; accepted 30 April 2002

Abstract

Al-substituted �-nickel hydroxide were used as active materials in pasted nickel hydroxide electrodes for rechargeablealkaline batteries. The electrochemical impedance spectra of �-nickel hydroxide electrodes with di5erent Al contents weremeasured. An equivalent circuit model was applied to simulate the experimental results. Some equivalent circuit parametersfor di5erent electrodes were determined. The reason why the �-nickel hydroxide electrode with 25% Al shows a betterperformance was discussed using these parameters.? 2002 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

Keywords: Nickel hydroxide electrode; Electrochemical impedance; Conductivity; Alkaline secondary battery

1. Introduction

Pasted nickel hydroxide electrodes have been extensivelyused as positive electrodes in rechargeable battery systemssuch as Ni=MH, Ni=H2, Ni=Cd, Ni=Fe and Ni=Zn. It wasfound that the performance of the nickel electrode greatlya5ected that of the overall battery because the batterycapacity is limited by the positive electrode. Thus, improve-ment in the nickel hydroxide electrode is very importantfor these alkaline storage batteries. In the current pastednickel hydroxide electrodes, �-phase nickel hydroxideare generally used as active materials since the �-phasenickel hydroxide has a higher stability in alkaline medium.However, the �-phase nickel hydroxide easily changes to�-phase nickel (oxy)hydroxide (�-NiOOH) when the elec-trode is over charged [1]. The formation of the �-NiOOHresults in swelling of the positive electrode and possiblere-distribution of electrolyte in the battery. The expansion ofthe positive electrode also causes the separator to dry out and@nally accelerates the deterioration of battery cycle life. Incontrast, �-Ni(OH)2 can be reversibly cycled to �-NiOOHphase without any mechanical deformation and mechanical

∗ Corresponding author.E-mail address: liubing28@hotmail.com (B. Liu).

1 Present address: Department of Chemistry, 201 Beury Hall,1901 N. 13th Street, Philadelphia, PA 19122, USA.

constraints [2]. In addition, �-NiOOH has a higher averagenickel oxidation state compared with �-NiOOH, making thecapacity of the alpha=gamma system higher than that ofthe �(II)=�(III) system. For this reason, studies on alpha-phase nickel hydroxide electrodes have attracted much at-tention in recent years. However, �-phase nickel hydrox-ide is not stable and easily reverts to �-phase on standingin alkaline medium. Therefore, stabilization of �-nickel hy-droxide in alkaline medium is very important for its futureapplication.

Some studies on partial substitution for nickel have beencarried out in order to improve the stability of �-phasenickel hydroxide [3,4]. Indira et al., synthesized �-layereddouble hydroxide (LDHs) of nickel with Al, Cr, Mg, Fe andfound that LDH has the highest coulombic eFciency [5].In our previous work, some Al-substituted �-phase nickelhydroxide electrodes were examined with the @ndingthat the �-phase nickel hydroxide electrode with 25% Alexhibited good performance [6–8].

Electrochemical impedance spectroscopy (EIS) has manyadvantages, such as applying small amplitude ac signals tosystems composed of a wide range of materials and provid-ing detailed information on the sub-processes of the system.EIS is particularly suitable for studying the reaction kinet-ics of electrodes. There are many references to EIS studieson nickel plate electrodes [9,10], but little work is reportedon the porous nickel hydroxide electrode in rechargeable

0360-3199/03/$ 30.00 ? 2002 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.doi:10.1016/S0360-3199(02)00058-7

454 B. Liu et al. / International Journal of Hydrogen Energy 29 (2004) 453–458

batteries, especially �-phase nickel hydroxide electrodes.In this work, we studied the characteristics of Al-substituted�-phase nickel hydroxide electrodes using EIS and com-pared the impedance behaviors of di5erent Al contents.

2. Experimental

2.1. Electrodes preparation

The Al-substituted �-nickel hydroxide was obtained bychemical precipitation using a previously described method[6]. The aluminum stabilized �-Ni(OH)2 samples were pre-pared at room temperature by adding a mixed solution con-taining NiSO4 ·6H2O and Al2(SO4)3 ·18H2O in the requiredstoichiometric ratio to 1 M NaOH solution containing anappropriate amount of Na2CO3. A green or blue precipi-tate was obtained which was washed in distilled water andcentrifuged several times until the pH became neutral. Theprecipitate was dried at 65

◦C.

2.2. Electrochemical measurements

All cyclic voltammetric studies were performed in athree-compartment electrolysis cell at 25

◦C using a 1287

electrochemical interface with a personal computer. Theworking electrodes were powder microelectrodes of dia-meter 150 �m. Two nickel sheet counter electrodes wereplaced at the side and the working electrode was positionedin the center. A Hg=HgO electrode was used as referenceelectrode. The electrode was scanned between 0 and 0:8 Vat the 10 mV s−1 rate 10 times prior to the experiments.Two electrodes were tested in this work.Electrode A: �-nickel hydroxide with 10 wt% Al content.Electrode B: �-nickel hydroxide with 25 wt% Al content.AC impedance measurements were made using an

EG&G PARC Model 273 potentiostat=galvanostat, a Model5210 Lock-in-ampli@es and a personal microcomputer. Themeasurements were made at open circuit potential with asuperimposed 5 mV sinusoidal voltage in the frequencyrange 10 kHz–10 mHz. An electrochemical impedance sys-tem software (Model 388) was used to collect data. Thedata for the real and imaginary components were analyzedusing the equivalent circuit developed by Boukamp [11].

3. Equivalent circuit analysis

Various circuits may be proposed to represent batteryimpedance and circuit parameters can be determined by @t-ting the data to the circuit. Di5erent portions of the datacan be @tted either with a circle or a line in the impedanceplane. An arc in the impedance plane can be represented bya parallel (RQ) circuit, where Q is a constant phase element(CPE). A line in the impedance plane can be represented bya series RQ circuit.

Fig. 1. Cyclic voltammetric plot of Al-substituted �-Ni(OH)2 withdi5erent Al contents.

Fitting of the data began at the high frequency end withthe data in the impedance form. The points at the highfrequency end were @tted with an arc by partial nonlinearleast-squares @tting. After removing the high frequencyscatter, the data were @tted with a straight line. The CPE cor-responding to the double layer capacitance was subtracted,and the data transferred to the impedance plane. The seriescircuit RQ of the charge-transfer resistance and CPE cor-responding to the Warburg element was subtracted. Uniformscatter around the origin was indicative of the good natureof the @t. A total nonlinear least-squares @t was performedon the data using the subtraction code form the subtraction@le.

4. Results

4.1. Cyclic voltammetric behavior

Fig. 1 shows typical cyclic voltammograms of �-Ni(OH)2with various aluminum contents. In the range of scanningpotentials employed, one large anode oxidation peakappearing at about 504 mV and another small anode oxida-tion peak at about 600 mV for electrode A were recordedprior to oxygen evolution. The pairs of oxidation peaks ob-served may be due to the Ni(II)=Ni(III) and Ni(III)=Ni(IV)reversible reaction processes [12]. Only one oxyhydroxidereduction peak at about 361 mV was observed on the reversesweep. Similar voltammograms were observed for electrodeB, but the anodic oxidation peak and cathodic reduction peakshifted to more positive potentials. The anodic oxidationpeak for the electrode B is about 530 mV. This means thatthe oxidation process is favored for electrode B compared tothat of electrode A. The di5erence between the anodic andcathodic peak position in the electrode B is substantiallyless than that in the electrode A. It means that the reversibil-ity of redox reaction process occurring on the electrode Bis better.

B. Liu et al. / International Journal of Hydrogen Energy 29 (2004) 453–458 455

Fig. 2. Nyquist plot of electrode A at steady state.

Fig. 3. Nyquist plot of electrode A after oxidation at 0:50 V.

4.2. Electrochemical impedance spectroscopy ofelectrode A

Fig. 2 shows the Nyquist plot of the electrode A in steadystate. At high frequencies the semicircle was characteris-tic of the charge-transfer resistance acting in parallel withthe double-layer capacitance. At intermediate frequencies astraight line having an angle of 15

◦with the real axis was

seen, being characteristic of semi-in@nite di5usion. At lowerfrequencies, @nite length e5ects were observed and there wasa transition from the 15

◦line toward a vertical line through

the transition region, the angle was about 70◦.

Fig. 3 shows the Nyquist plot for electrode A after holdingthe potential constant for 10 min at 0:50 V. The impedancespectrum consists of a depressed arc of a smaller diameterin the high-frequency range. At intermediate frequencies avery short straight line having an angle of 45

◦with the real

axis was seen, being characteristic of semi-in@nite di5usion.At lower frequencies, @nite length e5ects are observed and

Fig. 4. Nyquist plot of electrode B at steady state.

there is an transition from the 45◦line toward a vertical line

through the transition region, the angle is about 80◦. The

high-frequency arc is probably due to the charge-transferreaction and the inclined line in the low-frequency range isattributable to Warburg impedance associated with protondi5usion. The overall plot subtended an angle of 80

◦with

the real axis within the frequency region studied.

4.3. Electrochemical impedance spectroscopy of theelectrode B

Fig. 4 shows the Nyquist plot for electrode B in steadystate. At high frequencies the semicircle was characteris-tic of the charge-transfer resistance acting in parallel withthe double-layer capacitance. At intermediate frequencies astraight line having an angle of 45

◦with the real axis was

seen, being characteristic of semi-in@nite di5usion. At lowfrequencies, there was a line having an angle of about 55

◦.

After electrode B was held at 0:53 V for 10 min, theNyquist plot shown in Fig. 5 was obtained. The impedancespectrum consists of a depressed arc with a smaller diameterin the high-frequency range. At intermediate frequencies avery short straight line having an angle of 45

◦with the real

axis was seen, being characteristics of the semi-in@nite di5u-sion. At lower frequencies, the @nite length e5ects observedis a line toward a vertical line, the angle is about 70

◦. The

high-frequency arc is probably due to the charge-transferreaction and the inclined line in the low-frequency range isattributed to the di5usion in solution.

Compared to spherical �-Ni(OH)2, Al-substituted�-Ni(OH)2 shows very di5erent impedance spectroscopy.Spherical �-Ni(OH)2 only exhibits a capacitive arc with alarge diameter [13]. This entails that the reaction occurringat �-Ni(OH)2 is under a charge-transfer control. A smallercharge-transfer resistance for the �-Ni(OH)2 was observed.The electrode reaction occurring at �-Ni(OH)2 is underjoint control by the charge-transfer and proton di5usionprocesses.

456 B. Liu et al. / International Journal of Hydrogen Energy 29 (2004) 453–458

Fig. 5. Nyquist plot of electrode B after oxidation at 0:53 V.

From Figs. 2 and 4, it can be seen that electrodes A and Bhave a linear Warburg portion and a capacitive line at lowerfrequencies except for a capacitive semicircle. This is inagreement with the mathematical model for electrochemicalimpedance spectroscopy of porous nickel hydroxide [14].Generally, at higher frequency semicircle is the character-istic of the charge-transfer resistance acting in parallel withthe double-layer capacitance [15]. This means that electrodeA has larger charge-transfer resistance than electrode B. Atlower frequencies a linear Warburg portion due to slow dif-fusion process can be seen. The Warburg slope is interpretedhere as an empirical parameter related qualitatively ratherthan quantitatively to the di5usion resistance where a higherslope signi@es a slower rate of di5usion and a low slopea more rapid rate of di5usion [16]. Thus electrode A hasa smaller proton di5usion resistance than electrode B. Thismay be because electrode A has the higher proton di5usioncoeFcient [8].

In general, for a planar electrode, the Warburg slope isproportional to 1=CD1=2 and is about 45

◦where C is the

concentration of di5using species and D is the di5usion co-eFcient. The model is satisfactory for simple reactions ona planar electrode but is not adequate for complicated reac-tions and porous electrodes. Karumathilaka and Hampson[17] considered that the Warburg slope of a porous electrodeis between about 22:5

◦and 45

◦based on characteristic of

an electrode with semi-in@nite pores. Warburg slope higherthan 45

◦results from the @nite di5usion e5ects.

As seen from Figs. 3 to 5, after oxidization, �-Ni(OH)2electrodes with di5erent Al contents show the same changetendency. Compared to Fig. 2, Fig. 3 shows smallercharge-transfer and di5usion resistance. The same charac-teristics can be seen in Figs. 4 and 5. From Fig. 1, we cansee that the electrode A has been oxidized to �-NiOOHat a potential of 0:50 V and for the electrode B at 0:53 V.Because �-NiOOH has smaller charge resistance than thatof �-Ni(OH)2, after oxidation it has smaller charge-transfer

resistance. Motupally [10] found that the proton di5usioncoeFcient is larger in NiOOH than in Ni(OH)2. Therefore,after oxidation, there is a greater amount of NiOOH thanbefore polarization, and it has a smaller di5usion resistance.

As seen from Figs. 2–5, the plots at high frequencies inthe complex plane consist of a depressed semicircle withits center below the real axis. Therefore, to @t the dataan equivalent circuit model containing a constant phase ele-ment (CPE) Q should be used. Its impedance is described[9,18] as

ZCPE = 1=Y (j!)n; (1)

where ! is the angular frequency in rad s−1, Y and n areadjustable parameters of CPE. A value of n=1 correspondsto capacitance, n= 0 corresponds to resistance and n= 0:5corresponds to Warburg di5usion.

The plot in Fig. 6 represents the equivalent circuit usedto represent the processes occurring at the two electrodes toassist in analyzing the impedance data. R1, R2, Q1 and Q2

are the ohmic resistance, charge-transfer resistance, constantphase element representing double-layer capacitance andCPE representing Warburg impedance, respectively. Fig. 6represents the equivalent circuit consisting of a semicircleand a Warburg linear portion in the complex plane plot.

Analysis of the experimental data was performed by @t-ting equivalent circuit [13]. It is worth noting that none ofthese equivalent circuit models can represent the true situa-tion in the highly complex porous nickel hydroxide but, inthe absence of an adequate theoretical model, these modelsapproximately represent the electrode processes occurring.

Based on above models, the results are showed in Ta-ble 1. Comparing the results in Table 1, electrode B hasvalue of smaller R2 than electrode A, and has smaller R2

after oxidation. The behavior of the �-Ni(OH)2 electrode isin agreement with that of the porous electrode. Therefore,the electrochemical reaction of �-Ni(OH)2 is controlled bycharge di5usion and proton di5usion.

5. Discussion

The impedance spectra indicate good agreement betweenthe theoretical predications and the experimental data ob-tained with the model shown in Fig. 6.

MacArthur [19] and Zimmerman [20] proposed that dur-ing discharge a proton di5usion from the @lm=electrolyteinterface into the active material and an electron entersacross the conducting substrate=@lm interface. Duringcharging the proton di5uses to the @lm=electrolyte interfaceto react with a hydroxyl ion to form water. Zimmermandescribed the overall process as follows:

H2O → OH−(aq) + H+(s); (2)

H+(s) → H+(s′); (3)

H+(s′) + NiOOH(s′) + e(s′) → Ni(OH)2(s′): (4)

B. Liu et al. / International Journal of Hydrogen Energy 29 (2004) 453–458 457

Fig. 6. Equivalent circuit of the nickel hydroxide electrode for the impedance spectra.

Table 1Equivalent circuit parameters for di5erent electrode

Electrode R1=� R2=� Y1 n1 Y2 n2

A 5:1 × 10−1 918.35 2:5 × 10−4 0.48 1:82 × 10−3 0.90B 70.13 287.64 6:1 × 10−5 0.64 1:08 × 10−3 0.75A at 0:5 V 59.32 371.21 1.22×10−4 0.47 3:96 × 10−3 0.91B at 0:53 V 286.60 200.02 8:68 × 10−4 0.53 8:69 × 10−4 0.43

Reaction (2) represents the formation of a proton at thecatalytic site s at the electrode=electrolyte interface. Reaction(3) involves di5usion of the proton from site s into theelectrode to the charge transfer site s′. Reaction (4) is thecharge-transfer process involving the reduction of one ofthe higher valence species of active material in the lattice,represented here as simply NiOOH. It is well known that theNiOOH is an n-type semiconductor with high-charge phaseand Ni(OH)2 is a low conductive p-type semiconductor withthe low charge.

Both structures of �-Ni(OH)2 and �-Ni(OH)2 con-sist of brucite-type layers well ordered along the C-axis(�-Ni(OH)2) or randomly stacked along the C-axis(� − Ni(OH)2) with, for the latter, water molecules andanionic species within the Van der Waals gap resultingin a c-spacing of 0:75 nm, compared to 0:48 nm for the�-Ni(OH)2. Pure �-Ni(OH)2 is a pure divalent materialand nitrate intercalation occurs more due to hydroxyl ionvacancies rather than because of the exigencies of chargecompensation [21]. Consequently, its bonding strength withthe brucite layer is also poor. This accounts for the poorstability of �-Ni(OH)2 in alkali media. Introducing Al intothe nickel hydroxide lattice, with the purpose of enhancingthe intercalated anion content successfully stabilizes the�-Ni(OH)2 under alkali media. The charge excess due toAl3+ is compensated by the insertion of carbonate betweenthe hydroxide slabs. The anions strongly anchor the positivecharged brucite layers and stabilize the structure in a varietyof stressed conditions. Thus, it is easy for proton to transferin the slab of Al-substituted �-Ni(OH)2 and has higher con-ductive than �-Ni(OH)2. It is also evident the conduction isgreater with more Al3+ added, shown in the above Figures,

indicating that the Al-substituted �-Ni(OH)2 have higherconductivity after oxidation. It is known that nickel hydrox-ide has considerably lower conductivity after oxidation. Itis known that nickel hydroxide has considerably lower con-ductivity in the Ni steady state than in the oxidized state.The increased conductivity of the active mass possibly al-lows the electron exchange to become faster, so the R2 de-crease after oxidation for both of electrodes A and B.

From Figs. 2–5, it can be seen that Warburg-type linechanges less obviously after oxidation for both electrodes Aand B. The decrease in di5usion control is possibly a resultof the increase in the proton di5usion coeFcient [17].

As mentioned above,Q1 andQ2 in the equivalent circuit inFig. 6 are similar to a capacitance and Warburg impedance.Accordingly, Q1 and Q2 can be represented by capacitanceCd1 and Warburg impedance W , respectively.

The total impedance, Z , of any network is given by [22]

Z = Rs − (j=!Cs); (5)

where Rs, Cs are the e5ective series resistance and capac-itance, respectively. For the equivalent circuit shown inFig. 6, the series resistance Rs may be expressed by

Rs = R1 + (R2 + �√!)=((1 +

√!Cdl�)

2

+!2C2dl(R2 + �

√!)2 (6)

The capacitance is given by

1=!Cs = ((�=√!)(1 +

√!C�)

+!C(R2 + �=√!)2=(1 +

√!C�)2

+!2C(R2 + �=√!)2); (7)

where � is the Warburg coeFcient.

458 B. Liu et al. / International Journal of Hydrogen Energy 29 (2004) 453–458

The expression for the Warburg coeFcient includes termscontaining the concentrations of the reactants and products,as well as their di5usion coeFcients. The charge transferresistance, R2, is related to the concentration term throughthe exchange current density, since

R2 = (RT=nFI0); (9)

where R is the gas constant, T is the absolute tempera-ture, F is Faraday constant, n is the number of electronsinvolved in the relation and I0 is the exchange current den-sity. Theoretical analysis of electrodes can also be carriedout based on the above considerations. According to theresults in Table 1, the values of R2 with high Al contentand after oxidation apparently decrease. It can be knownfrom Eq. (9) that the exchange current density graduallyincreases. Hence, 25 wt% Al and oxidized �-Ni(OH)2can promote transfer reaction with a high exchange cur-rent density of the electrode reaction. From Table 1, itis clear that n1 and n2 are di5erent. So the impedancespectroscopy is very complicated for Al-substituted�-Ni(OH)2.

6. Conclusion

Studies of electrochemical impedance spectra ofAl-substituted �-Ni(OH)2 electrodes were performed.The equivalent circuit was used to @t experimental data.The reaction occurring at the Al-substituted �-nickel hy-droxide was controlled by charge transfer and Warburgdi5usion.

The behavior of the Al-substituted �-nickel hydroxide isthe same as that of porous electrode. The electrode with 25%Al content has a larger exchange current density, and thus,during charge=discharge process, the active material of elec-trode reaction was fully used, exhibiting higher utilizationand higher discharge capacity.

After oxidation, the Al-substituted �-nickel hydroxideelectrode showed smaller charge-transfer resistance and

di5usion resistance, meaning the Al-substituted �-nickelhydroxide electrode is more active after oxidation.

References

[1] Bode H, Dehmelt K, Witte J. Electrochim Acta 1966;11:1079.[2] Delmas C, Faure C, Borthanieu Y. Mater Sci Eng

1992;B13:89.[3] Faure C, Delmas C, Willman P. J Power Sources 1991;35:263.[4] Demourgues LG, Braconnier JJ, Delmas C. J Power Sources

1993;45:281.[5] Indira L, Dixit M, Kammath PV. J Power Sources 1994;52:93.[6] Liu B, Wang X-Y, Yuan Y-T, Zhang Y-S, Song D-Y, Zhou

Z-X. J Appl Electrochem 1999;29:855.[7] Liu B, Yuan Y-T, Zhang Y-S, Zhou Z-X, Song D-Y. J Power

Sources 1999;79:277.[8] Liu B, Yuan Y-T, Zhang Y-S, Yang H-B, Yang E-D. Int J

Hydrogen Energy 2000;25:333.[9] Viswannathan VV, Salkind AJ, Kelley JJ, Ocherman JB. J

Appl Electrochem 1995;25:716.[10] Motupallly S, Streinz CC, Weidner JW. J Electrochem Soc

1995;142:1401.[11] Boukamp BA. Equivalent circuit users manual, 2nd, ed. The

Netherlands: University of Twente, 1989.[12] Armstrong RD, Charles EA. J Power Sources 1989;25:89.[13] Wang XY, Yan J, Yuan YT, Zhang YS. Int J Hydrogen Energy

1999;24:973.[14] Yuan AB, Chen SA, Zhang JQ. Acta Physico-chimica

Sinica(China) 1998;14:804.[15] Musilova, Jindra J, Mrha J, Novak P, Garche J, Wiesener K.

J Power Sources 1987;21:67.[16] Reid MA, Loyselle PL. J Power Sources 1988;27:285.[17] Karunathilaka SAGR, Hampson NA. J Appl Electrochem

1981;11:365.[18] Brug GL, Van Den Eden ALG, Sluyters-Rehbach M. J

Electroanal Chem 1984;176:275.[19] MacArthur DM. J Electrochem Soc 1970;117:422.[20] Zimmerman AH, E5a PK. J Electrochem Soc 1984;131:70.[21] Faure C, Delmas C, Fouassier. J Power Sources 1991;35:279.[22] Murugesamoorthi KA, Srinivasan S, Appleby AJ. J Appl

Electrochem 1991;21:95.