Materials Research Bulletin 48 (2013) 422–428

Structure and electrochemical properties of nanometer Cu substituteda-nickel hydroxide

Jie Bao a, Yanjuan Zhu a,*, Zhongju Zhang b, Qingsheng Xu a, Weiren Zhao a, Jian Chen c,Wei Zhang a, Quanyong Han a

a School of Physics and Optoelectronic Engineering, Guangdong University of Technology, WaiHuan Xi Road, No. 100, Guangzhou 510006, Guangdong Province, PR Chinab Guangzhou Tiger Head Battery Group Co., Ltd., 568 Huangpu Road, Guangzhou 510655, Guangdong Province, PR Chinac Instrumentation Analysis and Research Center, Sun Yat-sen University, Guangzhou 510275, Guangdong Province, PR China

A R T I C L E I N F O

Article history:

Received 11 December 2011

Received in revised form 18 August 2012

Accepted 31 October 2012

Available online 9 November 2012

Keywords:

A. Nanostructures

B. Chemical synthesis

C. Electrochemical measurements

D. Electrochemical properties

A B S T R A C T

Nanometer Cu-substituted a-nickel hydroxide was synthesized by means of ultrasonic-assisted

precipitation. Particle size distribution (PSD) measurement, X-ray diffraction (XRD), and high-resolution

transmission electron microscope (HR-TEM) were used to characterize the physical properties of the

synthesized samples. The results indicate that the average particle size of the samples is about 96–

110 nm and the XRD diffraction peaks are anisotropic broadening. The crystal grains are mainly

polycrystal structure with columnar or needle-like morphology, containing many defects. With increase

of Cu content, the shape of primary particles transform from columnar to needle-like. The influences of

doping amounts of Cu on the electrochemical performance were investigated through constant current

charge/discharge and cyclic voltammetric measurements. The specific capacity increases initially and

then decreases with increasing Cu-doping ratio, the electrode C containing 0.9 wt.% Cu shows the

maximum discharge capacity of 310 mAh/g at 0.2 C, and it has the lowest charging voltage, higher

discharge voltage plateau, better cycle performance and larger proton diffusion coefficient than the other

electrodes.

� 2012 Elsevier Ltd. All rights reserved.

Contents lists available at SciVerse ScienceDirect

Materials Research Bulletin

jo u rn al h om ep age: ww w.els evier .c o m/lo c ate /mat res b u

1. Introduction

The alkaline nickel-based batteries are extensively used fortheir high specific energy, environment-friendly characteristic andhigh reliability [1]. Generally, nickel hydroxide used as the activematerial in the positive electrode for nickel-based rechargeablealkaline batteries has two crystal phases, namely a-Ni(OH)2 and b-Ni(OH)2. After fully charged, they can transform into g-NiOOH andb-NiOOH, respectively [2]. Currently, b-Ni(OH)2/b-NiOOH systemis usually utilized in commercial battery, but its theoreticalcapacity is low, and g-NiOOH is easily formed when b-Ni(OH)2 isovercharged which results in a swelling of the positive electrodevolume and accordingly reduces electrode life [3]. On the contrary,the a-Ni(OH)2 not only has a superior theoretical electrochemicalcapacity (433 mAh/g, which is 144 mAh/g larger than that of b-Ni(OH)2), but also exhibits no remarkable change of electrodevolume when over charged [4]. However, pure a-Ni(OH)2 isunstable and can easily transform to b-Ni(OH)2 in strong alkalinesolution [5]. To improve the stability of a-Ni(OH)2, much researchwork has been carried out on partial substitution of nickel ion withother metal additives such as Al [3,4], Y [6], Fe [7], Co [8], Mn [9], Zn

* Corresponding author. Tel.: +86 20 39322265; fax: +86 20 39322265.

E-mail address: YanJuanZhu007@126.com (Y. Zhu).

0025-5408/$ – see front matter � 2012 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.materresbull.2012.10.059

[10] in the nickel hydroxide lattice. The principle is to anchor anionas well as enhance bonding by increasing the positive charge inNiO2 layer, so that the anions and water molecules are maintainedand the lattice constants remain stable.

Cu shows a good effect on improving the electrochemicalperformance of nickel hydroxide. Yu et al. [11] demonstratednanometer Cu-substituted b-Ni(OH)2 synthesized by micro-emulsion method which showed higher utilization of activematerial, better charge efficiency and reversibility of the electro-des. Liu et al. [12] prepared Cu-doped amorphous nano-Ni(OH)2 bycoordination-precipitate method, which has an improved capacitywhen the electrodes were prepared by mixing 80 wt.% nickelhydroxides with other conductive agent (nickel powder). In thiswork, we employed ultrasonic-assisted precipitation to producenanometer Cu-substituted a-Ni(OH)2. The particle size distribu-tion, crystal structure, morphology and electrochemical perfor-mance of the electrode with various Cu contents are investigated.

2. Experimental

2.1. Preparation of nickel hydroxide

The nanometer Cu-substituted a-nickel hydroxides wereprepared by an ultrasonic-assisted precipitation method at

0

5

10

15

20

25

30

35

Pe

rce

nta

ge

/ %

Particle Size / nm

20 100 400

a1

b1

c1

d1

e1

Fig. 1. Particle diameter of samples a1–e1.

10 20 30 40

2θ/ (degree)

50 60

e1

d1

c1

b1

a1

(110)

(012)

(006)

(003)

Re

lati

ve

in

ten

sit

y (

a. u

.)

Fig. 2. XRD patterns of different Cu-containing Ni(OH)2.

J. Bao et al. / Materials Research Bulletin 48 (2013) 422–428 423

50 8C. An aqueous solution of NaOH containing appropriateamount of anhydrous sodium carbonate and ammonia as buffersolution, and an aqueous solution of mixed CuCl2�2H2O andNiCl2�6H2O with a certain mole ratio were dropped into a motherliquid synchronously under stirring. The drop rates were adjustedto control the pH value at 9.00 � 0.10 by pH meter. The agitationlasted 5 h after dropping was finished; supersonic was employed inall above processes. The suspension was aged for 12 h at 50 8C,followed by filtering and washing three times to neutral and thendried to constant weight at 80 8C. The samples prepared in the Ni2+/Cu2+ mole ratios of 1:0.10, 1:0.15, 1:0.20, 1:0.25 and 1:0.30 aredenoted as a1, b1, c1, d1 and e1, respectively.

2.2. Characterization of the samples

The crystal structure of the samples was determined by X-raydiffraction (XRD) using a D/max-IIIA X-ray diffractometer with CuKa radiation (l = 1.54 A) at 36 kV and 20 mA. Transmissionelectron microscope (TEM) and HR-TEM images were obtainedin a JEM-2010HR transmission electron microscope, respectively.Particle size distribution (PSD) measurement was carried out usinga Nanotrac 150 particle size analyzer.

2.3. Preparation of nickel electrode

The pasted positive electrode was prepared as follows: 8 wt.%samples, 86 wt.% commercial micro-size spherical Ni(OH)2, 3 wt.%nickel powder, 2 wt.% PTFE solution were mixed thoroughly with1 wt.% CMC as binders. The slurry obtained was incorporated intonickel foam (2.5 cm � 2.5 cm). The pasted nickel electrodes weredried at 80 8C and then mechanically pressed to a thickness ofabout 0.5 mm. They were marked as A–E with Cu contents of 0.5,0.7, 0.9, 1.1 and 1.3 wt.%, respectively. A slurry containing 88 wt.%AB5-type metal hydride powder, 2 wt.% CMC as a binder and10 wt.% nickel powder were forced into the same nickel substrates,then dried and compressed to obtain negative electrodes.

2.4. Electrochemical measurements

The prepared positive electrode, Hg/HgO reference electrodeand nickel foam were assembled into a three-electrode system,using 6.0 mol/L KOH and 1.5 g/L LiOH solution as the electrolyte.The cyclic voltammetry (CV) was carried out using an electro-chemical workstation (CHI760D). The testing potential rangesfrom �0.2 V to 0.7 V and the scanning speed ranges from 0.02 V/sto 0.10 V/s.

The prepared positive electrode and negative electrode wereassembled by using polypropylene as the separator to formsimulated batteries. Galvanostatic charge/discharge test wascarried out at 0.2 C rate using a Neware BTS-51800 Battery testingsystem. The electrodes were discharged to 1.0 V at ambienttemperature.

3. Results and discussion

3.1. The particle size, structure and morphology

Fig. 1 shows the particle size distribution of the samples a1–e1.It is found that the samples are nano-scale particles. Thedistribution is narrow and the average particle sizes are 109.1,103.4, 96.4, 100.3 and 105.0 nm, respectively.

XRD patterns of the samples are presented in Fig. 2. Diffractionpeaks of five samples are in accordance with those of the standardsubstance Ni(OH)2�0.75H2O (JCPDS38-0715), indicating that theyare a-Ni(OH)2. Samples in the diffraction angles of 34–388appear asymmetrically broad diffraction peaks, which are the

characteristic of the turbostratic disorder in a-Ni(OH)2 [13]. Fig. 2also exhibits that, the intensity of the first two peaks correspond to(0 0 3) and (0 0 6) planes from sample a1 to e1 are graduallyincreasing, while the FWHMs of the peaks reduce, but for the(1 1 0) diffraction peak, the half-width is broadening by degrees.Peaks of all samples at about 118, 238 and 608 show thecharacteristic of anisotropic broadening, this may be due to thedirectional growth of the crystallite as well as the defects andstacking faults in the crystal structures [14]. The high-resolutionTEM pictures of Figs. 3 and 4 confirm the existence of these defects.

The crystal constants a and c calculated from the (0 0 3) and(1 1 0) planes are listed in Table 1. As shown in Table 1, theconstant a which reflects the hexagonal symmetry of a-Ni(OH)2

reduces slightly with increasing Cu content, while c parameterwhich reflects the layered stacking structure increases obviously.The increase of Cu content has significant effect on layer spacingrather than lattice array. It may be attributed to part of Cu2+

replaced Ni2+ in the growth of nickel hydroxide crystal, anotherpart of Cu2+ occupied the tetrahedral interstitial sites of a-Ni(OH)2,resulting in excessive positive charge existed in the hydroxideslabs. To balance the charge neutralization, the anions and watermolecules were inserted in the crystal layer to compensate thepositive charge; hence the layer spacing and defects increase [15].The increase of layer spacing and defects is beneficial to protons

Fig. 3. TEM images of samples c1: (1) TEM image, (2 and 3) HR-TEM images and (4) SAED pattern.

J. Bao et al. / Materials Research Bulletin 48 (2013) 422–428424

intercalation and deintercalation during the redox process, thereby enhances the diffusion rate of protons and reduces polarization,further improves effectively the electrochemical performance[9,16].

Figs. 3 and 4 show the TEM, HR-TEM images of sample c1 and e1with the selected area electron diffraction (SAED) patterns. It canbe observed from Fig. 3(1) that, sample c1 is well dispersed withcolumnar structure, its average length is about 100–200 nm, 50–100 nm for the diameter, aspect ratio is approximately 2:1. Samplee1, however, part of which is needle-like, appears someagglomeration, as seen in Fig. 4(1). The high dispersivity of samplec1 is related to the adding of ultrasonic in preparation. Thecavitation of ultrasonic wave [17] makes the primary crystalparticles become small and preserves the particles from aggregat-ing, while the directional growth of the crystallite makes thecrystal particle become columnar. With the increasing Cu content,the stacks speed along c-axis increases, making the primaryparticles change from columnar to needle-like. Moreover, accord-ing to the two stages of grain formation: nucleation and graingrowth, a larger proportion of Cu2+ in reactants will increase thesolution supersaturation. So the formation of a-Ni(OH)2 crystalnucleus and the crystal growth speed up, while too fast crystalgrowth may lead to poor crystallinity as shown in Fig. 3(2). Besides,numerous microscopic bubbles generated by the cavitation ofultrasonic on the surface of crystal nucleus play a role in micro-stirring of the reaction solution, so the chance of nuclei capturinggroups increases greatly and nucleation get much faster.While large number of nucleation quickly reduce the solution

supersaturation, which will in turn inhibit the further growth ofthe grains. Therefore, the primary particles become much moresmaller, the high activity of small particles aggravates theagglomeration and the secondary particles become larger, leadingto the average particle size of sample e1 to be larger than that ofsample c1 (Fig. 1).

Fig. 3(2) and (3) exhibits that the high-resolution TEM picturesof different columnar crystal grains are variant. Some grains showdifferent crystallinity (as seen in Fig. 3(2)), and the arrangement ofcrystal plane was not straight, with a lot of defects: dislocations (asseen the arrows in III area of Fig. 3(2)) and vacancies (as seen thecircles in III area of Fig. 3(2)). While the smaller radial grains showmuch neater crystal plane arrangement, along with many structuredefects as well (as seen in Fig. 3(3)). As mentioned, these defectsare beneficial to the improvement of the electrodes’ electrochemi-cal performance. The SAED was operated on sample c1, manyfigures were obtained and the results are similar to Fig. 3(4), theyhave many polycrystalline diffraction rings, indicating that samplec1 is mainly with polycrystalline structure.

Fig. 4(2) and (3) shows the HR-TEM images of the circle andsquare selection in Fig. 4(1). As seen in Fig. 4(2), the interplanarspacing of the needle-like grains is basically the same, the crystalplanes bend along the grain appearance. Obviously, there areamounts of dislocation defects in the grains. Fig. 4(3) displays that,the crystallization of the blocky-shaped particles is not consistenteverywhere and the crystal orientation are inconsistent, indicatingthe grains are polycrystalline structure. The SAED of the circle inFig. 4(1) is shown in Fig. 4(4), which appears only two

Fig. 4. TEM images of samples e1: (1) TEM image, (2 and 3) HR-TEM images and (4) SAED pattern.

Table 1Crystal constants a and c of different Cu-containing Ni(OH)2.

Samples a1 b1 c1 d1 e1

a (A) 3.083 3.079 3.078 3.077 3.072

c (A) 22.500 22.611 22.776 22.878 23.252

0 50 100 150 200 250 300 350 400 450 500

A 0.5Wt.%Cu

B 0.7Wt.%Cu

C 0.9Wt.%Cu

D 1.1Wt.%Cu

E 1.3Wt.%CuVolt

age/

V

Capacity/ (mAh/g)

1.0

1.1

1.2

1.3

1.4

1.5

1.6

Fig. 5. Charge and discharge curves of electrodes at 0.2 C rate.

J. Bao et al. / Materials Research Bulletin 48 (2013) 422–428 425

polycrystalline diffraction rings, the diameter of the first rings isabout 56.7 nm, slightly less than that (60.7 nm) of the first rings inFig. 3(4). As the interplanar spacing is inversely proportional to thediffraction ring radius [18], the crystal cell of sample c1 is slightlysmaller than that of sample e1, which is consistent with the resultsin Table 1. Different from that of sample e1, it’s obviously observedthat the selection electron diffraction spectrum of sample c1 hasfour polycrystalline diffraction rings (Fig. 3(4)) which can be

5 10 15 20 25 30 35 40

200

250

300

350

Dis

charg

e ca

paci

ty/(

mA

h/g

)

Cycle number

A

B

C

D

E

Fig. 6. Cyclic performance of samples A–E at 0.2 C discharge rate.

J. Bao et al. / Materials Research Bulletin 48 (2013) 422–428426

mainly interpreted as, sample c1 has much larger primary grainsthan that of sample e1. Because, under the condition of constantincident electronic number in the selected area, the larger thecrystal size is, the larger the diffraction area is, and the stronger thediffraction intensity is, so the third and fourth diffraction ring ofsample c1 can be seen as well.

3.2. Charging/discharging performance of the samples

Fig. 5 plots the typical charge-discharge profiles of theelectrodes A–E at 0.2 C. As seen in Fig. 5, the specific dischargecapacity increases firstly and then decreases, the electrode Ccontaining 0.9 wt.% Cu has the largest specific discharge capacity of310 mAh/g, while the electrode E with 1.3 wt.% Cu has the minimumcapacity of 294 mAh/g. The electrode C has the highest specificdischarge capacity, the lowest charge potential platform and a

-0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

Cu

rren

t/A

Potential/V

0.02V/s

0.04V/s

0.06V/s

0.08V/s

0.10V/s

A

-0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

Cu

rren

t/A

Potential/V

0.02V/s

0.04V/s

0.06V/s

0.08V/s

0.10V/s

C

-0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

Cu

rren

t/A

Potential/V

0.02V/s

0.04V/s

0.06V/s

0.08V/s

0.10V/s

E

Fig. 7. Cyclic voltammograms of elec

higher discharge potential platform, meanwhile, the oxygenevolution reaction at the latest. The reasons may be caused by that,more anions get into the crystal interlayer resulting in more defectswhich provide a good channel for the proton diffusion in NiO2 layer[19]. More defects are conductive to the electrolyte into the ofNi(OH)2, so the conductivity increases. The defects reach to arelatively proper degree for the 0.9 wt.% Cu-containing electrode, ifthe defects continue to increase, it would destroy the original crystalstructure and block the improvement of the discharge capacity. Inaddition, the copper does not participate in electrochemical reactionwith increase of Cu content. The reducing active ingredients ofelectrode will also affect the discharge capacity. Further, from amorphological point of view, sample c1 is more columnar grainswith uniform particle size, these columnar grains can fully fill thegap in spherical Ni(OH)2 when mixed with spherical Ni(OH)2,accordingly reduce contact resistance and increase the active

-0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

Cu

rren

t/A

Potential/V

0.02V/s

0.04V/s

0.06V/s

0.08V/s

0.10V/s

B

-0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

Cu

rren

t/A

Potential/V

0.02V/s

0.04V/s

0.06V/s

0.08V/s

0.10V/s

D

trodes A–E at various scan rates.

0.14 0.16 0.18 0.20 0.22 0.24 0.26 0.28 0.30 0.32

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

0.11

0.12C

urr

ent/

A

Square root of scan rate/(V/s)1/2

A

B

C

D

E

Fig. 8. Relationship between the anodic peak current and the square root of scan

rate for electrodes A–E.

J. Bao et al. / Materials Research Bulletin 48 (2013) 422–428 427

material utilization in electrodes. However, sample e1, with moreagglomeration and decreasing activity, can not matched well assample c1 when mixed with spherical Ni(OH)2, so the specificdischarge capacity of electrode E is the minimum.

The cycle performance of electrodes A–E at 0.2 C rate is shownin Fig. 6. The decay rate Rd is introduced for qualitativecharacterization of the electrodes’ cycle stability. It is seen that,after 40 cycles, the Rd values of electrodes A–E are 12.4%, 11.5%,4.4%, 9.5% and 12.7%. It shows that electrode C has not only a higherspecific discharge capacity, but also a better cycling performancethan the others.

3.3. CV characteristics

Fig. 7 shows the typical C–V curves of electrodes A–E at variousscan rates. As shown in the figure, all the oxidation peak potentialsincrease and the reduction peak potentials decrease with increaseof scanning speed, the relationship between the anodic peakcurrent IP and the scan rate y is shown in Fig. 8. The linearrelationship between IP and y1/2 indicates that the electrodereaction is controlled by proton diffusion [20,21]. For the electrodesystem controlled by diffusion process, according to Randle–Sevickequation [22], at room temperature (25 8C), anodic peak current IP

and the scan rate y meet the following relationship:

IP ¼ ð2:69 � 105Þ � S � n3=2ðD0yÞ1=2C0 (2.1)

where n is the electronic number of reaction (the value is about 1for nickel hydroxide), S is the real surface area of the electrode(cm2), D0 is the diffusion coefficient (cm2/s), and C0 is the protonconcentration (mol/cm3).

Let ðIP=ffiffiffiffi

ypÞ ¼ k, Eq. (2.1) become

D0 ¼ k2

ð2:69 � S � C0Þ2 � 1010(2.2)

Table 2Proton diffusion coefficients of electrodes A–E.

Electrode A B C D E

k 0.355 0.377 0.427 0.411 0.350

D0 (�10�10 cm2/s) 1.12 1.26 1.62 1.50 1.09

k is the slope of the fitted line in Fig. 8; D0 is the proton diffusion coefficient of

electrodes A–E.

The proton diffusion coefficient D0 of each electrode can becalculated from S, C0, the slope of the fitted line (in Fig. 8) andEq. (2.2). The results are tabulated in Table 2. As can be seen fromTable 2, the proton diffusion coefficient of the electrodes increasesinitially and subsequently decreases, the electrode C has thelargest proton diffusion coefficient of 1.62 � 10�10 cm2/s, which is44.6% larger than that of electrode A. The variation of D0 agrees withthe discharge capacity results as shown in Fig. 5, but different fromthat of c value as tabulated in Table 1. Copper ions and anionsembedded in the crystal layer will increase with addition of Cu-coping amount, making the layer spacing improve, that is, the c

value increase, and the agglomeration aggravates as well (asshown in Fig. 4(1)). If the crystal spacing increases to a certaindegree, the original structure will be broken as a result of thesevere crystal lattice distortion. Instead, the resistance increasesand the proton diffusion become difficult. So the electrode Econsisting of 1.3 wt.% Cu has the maximum c value, while the D0

value and discharge capacity is the smallest. So a proper amount ofdoping can effectively enhance the proton diffusion coefficient andimprove their specific discharge capacity and other electrochemi-cal properties.

4. Conclusion

(1) Nanometer Cu-substituted a-nickel hydroxide was preparedby ultrasonic-assisted precipitation. The crystal morphology iscolumnar or needle-like, mainly with polycrystalline structure.The interlamellar spacing and the defects will increase as aresult of the increasing Cu content, and the primary particleschanged from columnar to needle-like.

(2) The doping amount of Cu has effects on proton diffusioncoefficient, discharge capacity and cycle performance of theelectrodes, when the doping amount is 0.9 wt.%, the electrodereaches a maximum discharge capacity of 310 mAh/g (0.2 C)with an optimal charging efficiency and cycle performance, theproton diffusion coefficient gets to the greatest.

(3) There is a good synchrony relationship between the electro-chemical properties and the diffusion coefficient of thematerial, while the electrochemical properties and the layerspacing are not exactly corresponding relationship due to someother factors like crystals agglomeration. So a moderateamount of doping can effectively improve the proton diffusionrate, thus the electrode performance is promoted.

Acknowledgements

This work was supported by the National Natural ScienceFoundation of China (No. 10774030), and by the Science andTechnology Program of Guangzhou City of China (No.12C232111916).

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