Electrochemical studies of aluminum substituted a-Ni(OH)2electrodes

Bing Liu*, Zhang Yunshi, Huatang Yuan, Huabin Yang, Endong Yang

Institute of New Energy Material Chemistry, Nankai University, Tianjin, 300071, People's Republic of China

Abstract

Stabilized a-Ni(OH)2 was used as an active material of pasted positive electrode in Ni/MH battery.

Electrochemical properties such as discharge capacity and discharge potential were improved. Cyclic voltammetricstudies indicated that a-Ni(OH)2electrode exhibited a better reversibility of Ni(OH)2/NiOOH redox reaction andhigher oxygen evolution overpotential than b-Ni(OH)2 electrode. This apparently would increase the chargee�ciency, improve utilization of active material and minimize concurrent oxygen evolution. # 2000 International

Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.

Keywords: a-Ni(OH)2; Stability structure; Reversibility; Cyclic voltammetry

1. Introduction

During the recent years, many studies have been car-

ried out to develop metal hydride (MH) electrodes for

the Ni/MH battery because of its advantages in com-parison with the conventional Ni/Cd battery [1].

Presently, the Ni/MH battery is widely used as a

power source for lap top computer, portable videos,emergency lighting, electric vehicles and other import-

ant applications. However, the performance of the

nickel electrode greatly a�ect that of the overall bat-tery because of the capacity of the battery depend on

the capacity of the positive electrode in Ni/MH bat-

tery. In order to increase further energy density of the

battery, pasted-type nickel electrodes have been devel-oped [2]. This type of nickel electrodes was made by

®lling the pasted-type of nickel hydroxide as an active

material in a porous nickel foam substrate or a porous

nickel ®brous substrate. Such electrodes have simplertechnological conditions, lower costs and higher energydensities [3].

There are two polymorphs about the nickel hydrox-ide denoted a-Ni(OH)2 and b-Ni(OH)2. Usually, theb(II)/b(III) phase represent the classical materials

involved during the electrode cycling. However, the a-Ni(OH)2 has been shown to have better electrochemi-cal properties than the b-Ni(OH)2 [4]. a-Ni(OH)2 is

converted to g-NiOOH at a lower potential than thecorresponding oxidation state compared with b-Ni(OH)2 to b-NiOOH. Since g-NiOOH has a higheraverage nickel oxidation state compared with b-NiOOH, the a/g couple has a higher discharge capacitythan that of b(II)/b(III) couple. Besides, as a-Ni(OH)2and g-NiOOH phase are hydrated, a-Ni(OH)2 can be

cycled to g-NiOOH phase reversibly without any mech-anical deformation and the mechanical constrains thatoccur during cycling are reached [5]. Unfortunately, a-Ni(OH)2 would revert to b-Ni(OH)2 on standing inalkali media. So, the stabilization of a-Ni(OH)2 in

International Journal of Hydrogen Energy 25 (2000) 333±337

0360-3199/00/$20.00 # 2000 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.

PII: S0360-3199(99 )00026-9

* Corresponding author. Tel.: +86-22-2350-3623; fax: +86-

22-2350-2604.

E-mail address: bingliu@tju.edu.cn (B. Liu).

alkali media is an important goal for potential appli-cation.

A lot of studies of the partial substitution of metalion for nickel in the lattice of nickel hydroxide havebeen carried out in order to enhance the stability of

cycling between a-Ni(OH)2 and g-NiOOH. Recently,Faure et al. [6] reported that cobalt substituted a-Ni(OH)2 obtained by precipitation techniques have

been used as positive electrode of Ni/Cd batteries;Ezhov et al. [7] found that zinc additive could stabilizethe thermodynamically unstable a-Ni(OH)2. More

recently, the chemical and physical properties of newmanganese-substituted nickel hydroxide with interla-mellar water prepared by precipitation have been alsoreported [8±10].

In this paper, we have synthesized the aluminumstabilized a-Ni(OH)2 by chemical coprecipitation andincreased the capacity of Ni/MH batteries by using

this compound as electrode material. We have alsoinvestigated the electrochemical behavior of electrodeby cyclic voltammetry.

2. Experimental

2.1. Synthesis of aluminum substituted aa-Ni(OH)2

The aluminum substituted a-Ni(OH)2 were obtainedby chemical coprecipitation, which was to add themixed solution containing NiSO4 � 6H2O and

Al2(SO4)3 � 18H2O in the required stoichiometric ratioto 1 M NaOH solution containing appropriate amountof Na2CO3. A green precipitate was obtained which

washed in distilled water and centrifuged several timesuntil the pH value became neutral. The precipitate isdried at 658C.

2.2. Electrodes preparation

Electrode substrates were prepared from 2 � 2 cm

square, thin sheet of nickel foam, to which a nickel rib-bon was spot welded as a current collector. At ®rst,the aluminum stabilized nickel hydroxide powder was

mixed with 10% cobalt oxide powder, then, the appro-priate amount of polytetra¯uoroethylene (PTFE) aqu-eous suspension as a binder was added and annealed

to obtain paste. The paste was incorporated into thenickel foam substrate using a spatula, dried at 608Cfor 1 h, and then pressed at 20 MPa for 1 min toassure good electrical contact between the subject and

the active material.

2.3. Electrochemical measurements

The electrolyte consisted of 6 M KOH+15% LiOH.Charge/discharge experiments were carried out using

one nickel electrode coupling with two metal hydrideelectrodes (Mm(NiCoMnAl)5, provided by our insti-

tute). The electrodes were protected by a separator.Charge was initially carried out at the C/10 rate for150% theoretical capacity, held for 30 min, then the

discharge was performed at the C/5 rate to 0.1 V vsHg/HgO.All cyclic voltammetric studies were performed in a

three-compartment electrolysis cell at 258C using a1287 electrochemical interface with a personal compu-ter. The working electrodes were powder microelec-

trodes with diameter 150 mm. Two nickel sheetcounterelectrodes were placed at the side and theworking electrode was positioned in the center. A Hg/HgO reference electrode was used with a lugging capil-

lary in the region of the working electrode. The work-ing electrodes were activated by repeated potentialscan between 0 and 0.65 V vs Hg/HgO at the 10 mV/s

rate for 10 times prior to the experiments.

2.4. Measurement of structure

The structure were determined using X-ray di�rac-tion (XRD) (D/Max-IIIA X-ray di�ractometer,

Rigaku Ltd, Japan), CuKa radiation and a graphite ®l-ter at 35 kV and 40 mA.

3. Results

3.1. Structure of aluminum substituted aa-Ni(OH)2

The X-ray di�raction patterns of aluminum substi-tuted a-Ni(OH)2 are given in Fig. 1. It can be seen

from Fig. 1 that the XRD patterns of aluminum sub-stituted a-Ni(OH)2 is the same as that of unsubstituteda-Ni(OH)2 reported in literature [11]. They show a lowangle re¯ection close to 0.8 nm, followed by another at

around 0.4 nm. In addition, a-Ni(OH)2 with 20% and

Fig. 1. XRD patterns of a-Ni(OH)2 with di�erent aluminium

content: (1) 10% aluminum; (2) 20% aluminum; (3) 25%

aluminum.

B. Liu et al. / International Journal of Hydrogen Energy 25 (2000) 333±337334

25% aluminum content show broad asymmetric band

in the 0.22 nm to 0.26 nm, this is the typical structureof turbostratic structure. For the a-Ni(OH)2 with 10%aluminum content, the broad band in the 0.22 nm to0.26 nm region is clearly split into two peaks.

Fig. 2 is the XRD patterns of the aged samples (in6 M KOH for 20 days). From Fig. 2, it can be seenthat the sample with 25% aluminum do not show any

structural change after aged, and is an stabilized a-Ni(OH)2. In contrast, the sample contain 10% alumi-num show a distinct tendency to transform into a b-Ni(OH)2 phase with the emergence of a re¯ection at0.46 nm. So, it is clearly the low aluminum contentcannot stabilize the a-Ni(OH)2 structure.

3.2. Discharge curves of the electrodes

Fig. 3 shows representative discharge curves for

electrodes with di�erent aluminum content and b-Ni(OH)2 at a rate of 0.2 C. As seen from dischargecurves in Fig. 3, the discharge capacity of a-Ni(OH)2electrodes are much higher than that of b-Ni(OH)2electrode. It also can be seen discharge potentials for

all the electrodes have an initial fall, and then followed

by a more stable region. In addition, a-Ni(OH)2electrodes have a higher discharge potential plateauthan b-Ni(OH)2 electrode. The discharge potential

plateau of the a-Ni(OH)2 electrode with 25% alumi-num content is 100 mV higher than that of b-Ni(OH)2electrode.

3.3. Cyclic voltammetric behavior of the electrodes

Fig. 4 shows the typical cyclic voltammograms of a-Ni(OH)2 with various aluminum content. In the rangeof scanning potentials employed, a split anode oxi-dation peak for the electrode with 10% aluminum con-

tent, appearing at about 504 mV was recorded prior tooxygen evolution. One oxyhydroxide reduction peak atabout 361 mV was observed on the reverse sweep.

Similar voltammograms have also been observed forthe electrodes with 20% and 25% aluminum content,but the anodic oxidation peak corresponding to Ni(II)oxidation reaction and cathodic reduction peak corre-

sponding to Ni(III) reduction reaction shift to morepositive potentials. In order to conveniently compare

Fig. 2. XRD patterns of aged a-Ni(OH)2 with di�erent alumi-

num content: (1) 10% aluminum; (2) 20% aluminum; (3)

25% aluminum.

Fig. 3. Constant current discharge curves of electrodes: (1) a-Ni(OH)2 with 10% aluminum; (2) b-Ni(OH)2; (3) a-Ni(OH)2with 25% aluminum.

Fig. 4. Cyclic voltammograms of a-Ni(OH)2 with di�erent

aluminum content: (1) 10% aluminum; (2) 20% aluminum;

(3) 25% aluminum.

Table 1

Results of the cyclic voltammetry of di�erent electrodes

Electrode Eanodic/mV Ecathodic/mV DEa,c/mV DOP/mV

1a 504 361 143 158

2b 480 378 102 120

3c 537 405 132 92

4d 638 202 436 22

a a-Ni(OH)2 with 10% aluminum content.b a-Ni(OH)2 with 20% aluminum content.c a-Ni(OH)2 with 25% aluminum content.d 4, b-Ni(OH)2 [12].

B. Liu et al. / International Journal of Hydrogen Energy 25 (2000) 333±337 335

the characteristics of the electrodes, cyclic voltam-metric results in Fig. 3 are tabulated in Table 1.

In Table 1, the di�erence between the anodic andcathodic peak positions, DEa,c, is taken as an estimateof the reversibility of the redox reaction [13]. Oxygen

evolution is a parasitic reaction during charge of nickelelectrode. To compare the e�ect of aluminum stabil-ized a-Ni(OH)2 on oxygen evolution reaction, the

di�erence between the oxidation peak potential andthe oxygen evolution potential (DOP) on the returnsweep required to produce 1.5� 10ÿ5 A of anodic cur-

rent is also estimated from voltammograms.The results in Fig. 4 and Table 1 illustrated that

aluminum substituted a-Ni(OH)2 allow the electrode tocharge at a signi®cantly less positive potential (504 mV,

480 mV, 537 mV) instead of 638 mV. In addition, thecharge process appear to occur more reversibly (DEa,c

is 143 mV, 102 mV, 132 mV instead of 436 mV).

Moreover, oxygen evolution overpotential shift to amore positive value (DOP is 155 mV, 120 mV, 92 mVinstead of 22 mV). Thus, these results indicate clearly

that aluminum substituted a-Ni(OH)2 electrodes allowthe charge process to occur more easily and morereversibly, suggesting that much more active material

can be utilized during charge. Besides, due to theincrease in the oxygen evolution overpotential and thedecrease of the oxidation peak potential of nickel hy-droxide, the charge e�ciency of the electrode could

markedly be improved, indicating that the electrodehas greater discharge capacity. These results are ingood agreement with those obtained by Corrigan et al.

who have reported that coprecipitation of cobalt ormanganese in nickel hydroxide thin ®lms decrease theoxidation potential with regard to that of the unsubsti-

tuted nickel hydroxide [13].Furthermore, the increase of oxygen evolution over-

potential is bene®cial to reduce the internal pressure ofthe battery.

4. Discussion

Improvements in nickel hydroxide electrodes require

[14]: (1) the shift of the nickel hydroxide redox reactionto less anodic potentials; (2) more facile nickel hydrox-ide redox interconversion; and (3) higher oxygen evol-ution overpotential. As seen from Fig. 4 and Table 1,

the aluminum-substituted a-Ni(OH)2 can meet theabove requirements. In Fig. 3, the a-Ni(OH)2 electro-des gave a greater discharge capacity and thus pre-

sented a higher utilization of active material. The halfdischarge potential, which is an important parameterof judging battery performance, have been improved

markedly; especially with the higher aluminum content,the half discharge potential signi®cantly increased.Both structures of a-Ni(OH)2 and b-Ni(OH)2 consist

of brucite-type layers well ordered along the C-axis(b-Ni(OH)2) or randomly stacked along the C-axis(a-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 b-Ni(OH)2. Pure a-Ni(OH)2 is a pure divalent ma-terial and nitrate intercalation occurs more on accountof hydroxyl ion vacancies rather than due to the exi-

gencies of charge compensation [15]. Consequently, itsbonding strength with the brucite layer is also poor.This account for the poor stability of a-Ni(OH)2 in

alkali media. Introducing Al in the nickel hydroxidelattice, with the purpose of enhancing the intercalatedanion content successfully stabilizes the a-Ni(OH)2structure under alkali media. The charge excess due to

Al3+ are compensated by the insertion of carbonatebetween the hydroxide slabs. The anions stronglyanchor the positive charged brucite layers and stabilize

the structure in a variety of stressful conditions.However, sample with lower anion content, namelywith 10% aluminum content is less stable in alkali

media. This can be seen from Fig. 2 clearly.The redox reaction taking place at the nickel hy-

droxide electrode during discharge and charge can be

represented as

NiOOH� H2O� e, Ni�OH�2 � OHÿ �1�

As seen from Fig. 3, the discharge capacity of alumi-

num stabilization a-Ni(OH)2 positive electrodes havegreater discharge capacity than the theoretical capacityof b-Ni(OH)2 (289 mAh gÿ1) As a consequence, it can

be concluded that the aluminum stabilization a-Ni(OH)2 has greater discharge capacity than the theor-etical capacity of b-Ni(OH)2. The reasons might be thedischarge process as the speci®c discharge capacity

depends signi®cantly on the phase composition. Thedischarge capacity, as a rule, is higher for electrodescontaining g-NiOOH, which is caused by a higher

nickel oxidation state; in g-NiOOH it can reach valuesclose to 3.7 while b-NiOOH it only slightly exceeds 3.0[16]. While in the case of aluminum stabilized a-Ni(OH)2, the phase transformation during charge/dis-charge cycle is a-Ni(OH)2 \ g-NiOOH, the exchangeelectrons per nickel atom are more than one. So, thesplit anodic peak in the cyclic voltammograms may be

due to a Ni(II)/Ni(III), Ni(III)/Ni(IV) reaction process[17]. Thus, the electrode exhibits the greater dischargecapacity.

From Fig. 4 and Table. 1, it is seen that aluminumstabilized a-Ni(OH)2 have the low oxidation potentialof Ni(OH)2/NiOOH, and higher oxygen evolution po-

tential and thus enhanced di�erence between the oxi-dation potential of Ni(OH)2/NiOOH and the oxygenevolution potential. This is undoubtedly signi®cant to

B. Liu et al. / International Journal of Hydrogen Energy 25 (2000) 333±337336

promote full oxidation of Ni(II) species during chargeand increase the electrode capacity.

5. Conclusion

The aluminum-stabilized a-Ni(OH)2 has the samestructure as that of a-Ni(OH)2. With more than 10%

aluminum content, the structure is stable and cannottransform to b-Ni(OH)2 in alkali media. Due toexchange of more than one electron during charge/dis-

charge cycle, the electrode has greater discharge ca-pacity and higher utilization of active material. Theelectrode reactions occurring at aluminum stabilized a-Ni(OH)2 have better redox reversibility, and the oxy-

gen evolution overpotential shifts more positive value.In addition, the aluminum-stabilized a-Ni(OH)2 is ben-e®cial in decreasing the internal pressure of Ni/MH

battery.

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