Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 683–689

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Spectrochimica Acta Part A: Molecular andBiomolecular Spectroscopy

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Synthesis, spectroscopic and electrochemical performanceof pasted b-nickel hydroxide electrode in alkaline electrolyte

http://dx.doi.org/10.1016/j.saa.2014.07.0091386-1425/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel./fax: +91 08194234270.E-mail address: bjmadhu@gmail.com (B.J. Madhu).

B. Shruthi a, V. Bheema Raju a, B.J. Madhu b,⇑a Department of Chemistry, Dr. Ambedkar Institute of Technology, Bangalore 560 056, Indiab Post Graduate Department of Physics, Government Science College, Chitradurga 577 501, India

h i g h l i g h t s

� b-Nickel hydroxide (b-Ni(OH)2) wassynthesized using precipitationmethod.� FT-IR and TG–DTA studies show that

the b-Ni(OH)2 contains watermolecules and anions.� Electrochemical performance of

b-Ni(OH)2 was investigated using CVand EIS.� The proton diffusion coefficient (D)

for b-Ni(OH)2 is found to be1.44 � 10�12 cm2 s�1.� EIS studies confirmed that the

electrode reaction processes arediffusion controlled.

g r a p h i c a l a b s t r a c t

Relationship between the anodic peak current (ip) and the square root of the scan rate (t½).

0.009

0.2 0.3 0.4 0.5 0.6 0.70.000

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

i P(A

)

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

a r t i c l e i n f o

Article history:Received 21 February 2014Received in revised form 13 June 2014Accepted 2 July 2014Available online 27 July 2014

Keywords:Nickel hydroxideElectrode materialAlkaline electrolyteElectrochemical propertiesProton diffusion coefficient

a b s t r a c t

b-Nickel hydroxide (b-Ni(OH)2) was successfully synthesized using precipitation method. The structureand property of the b-Ni(OH)2 were characterized by X-ray diffraction (XRD), Fourier Transform infra-red(FT-IR), Raman spectra and thermal gravimetric–differential thermal analysis (TG–DTA). The results ofthe FTIR spectroscopy and TG–DTA studies indicate that the b-Ni(OH)2 contains water molecules andanions. The microstructural and composition studies have been performed using Scanning Electron Micros-copy (SEM) and Energy Dispersive X-ray (EDX) analysis. A pasted-type electrode is prepared using b-Ni(OH)2 powder as the active material on a nickel sheet as a current collector. Cyclic voltammetry (CV)and Electrochemical impedance spectroscopy (EIS) studies were performed to evaluate the electrochemicalperformance of the b-Ni(OH)2 electrode in 6 M KOH electrolyte. CV curves showed a pair of strong redoxpeaks as a result of the Faradaic redox reactions of b-Ni(OH)2. The proton diffusion coefficient (D) for thepresent b-Ni(OH)2 electrode material is found to be 1.44 � 10�12 cm2 s�1. Further, electrochemical imped-ance studies confirmed that the b-Ni(OH)2 electrode reaction processes are diffusion controlled.

� 2014 Elsevier B.V. All rights reserved.

Introduction development of alkaline batteries with higher specific energies in

With the increasing demand for portable electronic devices andelectric vehicle applications, significant attention is focused on the

which battery chemistry plays a vital role [1–5]. In particular, thedevelopment and commercialization of nickel/metal hydride(Ni-MH) technology afford the possibility of producing secondarybatteries with high specific energy. The positive nickel electrodestrongly influences the performance of the alkaline batteries[6–8]. Nickel hydroxide is extensively used in rechargeable

684 B. Shruthi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 683–689

nickel-based batteries as a positive electrode material [9,10]. Nor-mally, it exists in two polymorphic forms, namely a-Ni(OH)2 andb-Ni(OH)2, which are transformed into c-NiOOH and b-NiOOH,respectively, during charging [11–13]. Owing to the instability ofa-Ni(OH)2 in alkaline media, b-Ni(OH)2 is usually used as a precur-sor material in alkaline batteries.

Synthesis of nickel hydroxide has been extensively investigateddue to the prominence of nickel hydroxide to the battery industry.Several authors [14–25] have carried out extensive research on thepreparation and physicochemical properties of nickel hydroxide.Cheng Shao-an et al. [14] studied the electrochemical propertiesof the surface modified Ni(OH)2 powder. Microstructural controlof the nickel hydroxide is being researched to obtain optimum per-formance of the NiOOH/Ni(OH)2 electrode [15–19]. Luo et al. [20]synthesized flower-like b-Ni(OH)2 nanoarchitectures through aone-step mild hydrothermal reaction with the aid of ethylenedia-mine in NiCl2 aqueous solution. Bernard et al. [21] investigatedthe relationship between structural defects and electrochemicalreactivity of b-Ni(OH)2. Ramesh and Kamath [22] investigated theeffect of synthesis temperature on the phase selection amongnickel hydroxide and found that low temperature induced the pre-cipitation of a-Ni(OH)2 while at high temperature b-Ni(OH)2 wasformed. Enbo Shangguan et al. [23] have synthesized the high den-sity non-spherical Ni(OH)2 cathode material for Ni-MH batteries.Effect of reaction conditions on size and morphology of ultrasoni-cally prepared Ni(OH)2 powders has been studied by Cabanas-Poloet al. [24]. Recently, a detailed account of the synthesis, character-ization, and electrochemical properties of ultrafine b-Ni(OH)2

nanoparticles have been studied by Mustafa Aghazadeh et al. [25].In the present study, b-nickel hydroxide was synthesized using

the precipitation method. The structure of the synthesized samplewas characterized using XRD, FT-IR, Raman spectra and thermalgravimetric analysis. The microstructural and composition studieshave been performed on b-Ni(OH)2 using Scanning ElectronMicroscopy (SEM) and Energy Dispersive X-ray (EDX) analysisrespectively. The electrochemical performance of the synthesizedsample was tested by cyclic voltammetry and electrochemicalimpedance studies.

Inte

nsity

(a. u

.) (001

) (100

)

(101

)

(102

)

(110

)(1

11)

β -Ni(OH)2

Experimental

Synthesis of b-nickel hydroxide

Nickel hydroxide was synthesized using precipitation method.Analar grade potassium hydroxide (KOH) and nickel sulphate(NiSO4) were used as reagents. Triple distilled water was used forthe solution preparation and washing of the precipitate. A solutionof 1 M KOH was added to 1 M NiSO4 solution by dripping at a flowrate of 10 ml min�1 with constant stirring. The addition of thereagent was terminated when the pH of the suspension reaches13. Then the mixture was allowed to stand for 24 h for digestionof the precipitate. The separation of the precipitate from the excessreagent was done by centrifugation at 1500 rpm for 1 h. The pre-cipitate was washed thoroughly with triple distilled water. Bariumchloride (BaCl2 (1 M)) in excess was added to wash water, causingprecipitation of barium sulphate (BaSO4). The washing of the pre-cipitate was concluded when the white precipitate of BaSO4 wasno more found in the wash water. This nickel hydroxide precipitatewas dried at 60 �C for 48 h.

10 20 30 40 50 60 702θ

Fig. 1. XRD pattern of as-prepared nickel hydroxide.

Characterization of b-nickel hydroxide

Crystal structure of the synthesized nickel hydroxide was deter-mined using X-ray diffraction analysis, with a Cu Ka radiationsource (k = 1.4581 Å) using Bruker AXS D8 Advance diffractometer.

The FT-IR (Infra-red) spectrum (400–4000 cm�1) of the nickelhydroxide was recorded on a Bruker Alpha spectrophotometer inKBr pellets. Raman spectra were obtained using BRUKER RFS 27FT-Raman spectrometer. Thermal gravimetric–differential thermalanalysis (TG–DTA) was carried out by Perkin Elmer STA 6000 ther-mal analyzer. The microstructure of the sample was taken by JEOLModel JSM – 6390LV Scanning Electron Microscope (SEM) andcomposition studies were obtained using JEOL Model JED – 2300Energy Dispersive X-ray Spectrometer (EDS).

Preparation of nickel electrode and electrochemical testing

In the present studies, following composition of the electrodematerial was attained viz. b-Ni(OH)2 (85 wt.%) + graphite(10 wt.%) + PTFE (5 wt.%) as binder. The test electrode was madeby first mixing the prepared sample nickel hydroxide powder withgraphite powder and PTFE solution in the form of slurry. Theresulting slurry was pasted onto a nickel sheet. After being coatedwith the paste, the resulting electrode was dried at 80 �C for 1 h.The backside of the electrode and the wire were insulated withTeflon tape. The electrodes have the following dimensions:1 cm � 1 cm area.

Cyclic voltammetry (CV) and AC impedance measurementswere carried out using CHI604D electrochemical workstation. Forcyclic voltammetric studies, the test electrode prepared asdescribed above was used as a working electrode. The platinum foilwas used as a counter electrode; Ag/AgCl electrode was used as areference electrode and 6 M KOH solution was used as an electro-lyte. Prior to CV studies the electrodes were activated in 6 M KOHsolution. After resting for 30 min, the cyclic voltammograms wereobtained. All measurements were carried out at room temperature.

Electrochemical impedance spectroscopy (EIS) is an effectivetechnique for analyzing the internal structures and structuralchange during cycling [26]. An AC impedance study of b-nickelhydroxide electrode was carried out at different applied DCpotentials.

Results and discussion

Fig. 1 represents the XRD pattern of the as prepared nickelhydroxide. All of the diffraction peaks can be indexed entirely toa (space group: P3m1) crystal phase of b-Ni(OH)2, with the latticeconstants of a = 3.130 Å and c = 4.630 Å, which are well matchedwith the reported standard values (JCPDS card 74-2075). No other

B. Shruthi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 683–689 685

peaks for the impurities such as a-Ni(OH)2 or other phases areobserved in the pattern. In general, the broadening of XRD peaksmay result from small grain sizes or structural micro distortionsin the crystal [24,27]. Anomalous broadening of the (10 l) reflectionlines (l – 0) in the diffraction pattern cannot be attributed tocrystallite size alone. Structural defects such as stacking faults,growth faults, and/or proton vacancies also play a significant rolein explaining this broadening [13,22]. The (101) line shows arelationship with electrochemical activity, probably due to theexistence of stacking faults in the crystalline lattice of nickelhydroxide powders [9,28–31]. It has been reported that the lessordered nickel hydroxide materials characterized by an FWHM ofthe (101) line of 0.9 (2h) or more exhibit a better specific capacity[9]. The FWHM of the (101) diffraction line of the as-preparednickel hydroxide sample is 1.966, which indicates that the samplepossess a high density of structural disorder [32,33].

Fig. 2 shows the FT-IR spectrum of the as-prepared nickelhydroxide. The FT-IR spectrum confirms that the as-preparednickel hydroxide can be considered as b form, due to the existenceof (i) a narrow and strong band at 3643 cm�1 relating to the t(OH)stretching vibration, which indicates hydroxyl (OH) groups in afree configuration, (ii) the strong narrow band at 514 cm�1

corresponding to tNi-OH vibration, and (iii) a band around469 cm�1 resulting from the Ni–O stretching lattice vibrationmode, t(Ni–O) [3,27,34–37].

The broad and intense band centered at 3433 cm�1 is assignedto the O–H stretching vibration of the interlayer water moleculesand of the H-bound OH group. Furthermore, the other peakobserved at 1635 cm�1 is assigned to the bending vibration ofwater molecules. It can be seen from the strength of these twobands (1635 cm�1, 3433 cm�1) that the synthesized nickel hydrox-ide contains water molecules. The peaks located between the 800–1800 cm�1 could be assigned to the presence of anions, which havenot probably been completely eliminated during the washing stage[35,36]. These anions may be responsible for the structural microdistortions in the present sample through the electrostatic attrac-tions between positively charged Ni(OH)2 and negatively chargedanionic species. The two low bands at about 1486 cm�1 and1384 cm�1 can be assigned to the various vibrational modes ofthe carbonate ions originating from the adsorption of atmosphericCO2 due to the open system used for synthesis [38]. The band atabout 1042 cm�1 corresponds to the vibration of SO2�

4 [39].The Raman spectrum of the synthesized sample is shown in

Fig. 3. Well-crystallized b-Ni(OH)2 is characterized by three Ramanpeaks at 3570 cm�1, 445 cm�1, and 310 cm�1, which is due to the

Fig. 2. FT-IR spectrum of the prepared nickel hydroxide.

symmetric stretching of the hydroxyl groups, the Ni–O stretching,and an E-type vibration of the Ni–OH lattice, respectively [40,41].In comparison with the well crystallized b-Ni(OH)2, the present syn-thesized nickel hydroxide shows more Raman peaks (3634 cm�1,3554 cm�1, 1094 cm�1, 998 cm�1, 654 cm�1, 481 cm�1) as dis-played in Fig. 3. The wave number (481 cm�1) of the Ni–O stretchingvibration for the synthesized nickel hydroxide is higher than that forthe well-ordered b-Ni(OH)2 (445 cm�1). Similar results have alsobeen observed for the highly defected nickel hydroxide [42,43].Observed bands at 654 cm�1, 998 cm�1, and 1094 cm�1 suggestthe presence of some adsorbed sulfate species [42,44]. In the OHstretching frequency region, band at 3554 cm�1 is ascribed to thesymmetric stretch of the bulk hydroxyl group [42,43]. In compari-son with the well-ordered b-Ni(OH)2, the additional broad band atabout 3634 cm�1 reveals microstructural disorders of the nickelhydroxide [42]. This band can be attributed to the symmetricstretch of the surface hydroxide group [42,44].

Thermal behavior of the as-prepared b-Ni(OH)2 was investi-gated by TG–DTA analysis. Fig. 4 depicts the typical TG–DTA curveof the as-prepared b-Ni(OH)2 sample. The b-Ni(OH)2 sample under-went a three-step weight loss due to dehydration, decompositionand removal of intercalated anions. The three endothermic peaksat 117.23 �C, 269.9 �C and 373.49 �C on the DTA curve are indica-tive of three successive stages of these physico-chemical changesduring the heat treatment. The first region is below 200 �C, whichis related to the evaporation of the adsorbed and intercalatedwater molecules associated with the Ni(OH)2. XadsH2O deposit.Dehydration reaction:

NiðOHÞ2 � XadsH2O! NiðOHÞ2 þ XH2O ð1Þ

Correspondingly, TG curve shows a weight loss with 2.99 wt.%.The amount of water plays an important role in the crystal struc-ture and the electrochemical properties of Ni(OH)2.

It has been reported that decomposition of Ni(OH)2 into NiOoccurs between 200 �C and 350 �C [22]. In the present studies,the second region is between 200 �C and 360 �C, where the samplesdecompose to NiO (Eq. (2)). Thus the endothermic peak with themaximum located at around 269.9 �C corresponds to the endother-mic behavior of Ni(OH)2 during the decomposition into NiO. In fact,this peak is associated with the loss of water produced by dehydr-oxylation of the hydroxide layers:

NiðOHÞ2 ! NiOþH2O ð2Þ

The theoretical weight loss corresponding to the decompositionreaction (Eq. (2)) is 19.43%, and the practical weight loss corre-sponding to this reaction, which can be estimated from the secondweight loss steps of TG curve is 9.03 wt.%. The deviations betweenthe practical and theoretical weight loss may be attributed to theSO2�

4 and CO2�3 adsorbed in these materials, as revealed by IR anal-

ysis. The third region is between 360 �C and 600 �C, where theintercalated anions are removed. The weight loss due to intercala-tion of anions was observed to be 4.75 wt.%. Thus the endothermicpeak with the maximum located at around 373.49 �C correspondsto the endothermic behavior during the removal of intercalatedanions.

The total weight loss at temperature of 600 �C is about16.77 wt.%, which is close to those for b-nickel hydroxide [45,46],but much lower than those (30 wt.%) for a-nickel hydroxide[47,48]. The TG analysis shows that the synthesized nickel hydrox-ide has adsorbed/intercalated water molecules, which is in agree-ment with the results of IR and Raman spectrum as discussedabove (see Figs. 2 and 3). These water molecules may play signifi-cant role in the enhancement of the rate-capacity performance ofthe electrodes because they provide the passage of proton diffusionalong the molecular chain between the layers [46,49]. From theabove investigations on TG–DTA, Raman, IR and XRD data, it can

300 400 500 600 700 800 900 1000 1100 1200

Ram

an In

tens

ity (a

. u.)

Wavenumber / cm-13000

3100 3200 3300 3400 3500 3600 3700 3800 3900 4000

Ram

an In

tens

ity (a

. u.)

Wavenumber / cm-1

(a) (b)

Fig. 3. Raman spectrum of the as-prepared nickel hydroxide: (a) frequency region of Ni–O lattice vibrational modes; (b) OH group stretching vibrations.

0 100 200 300 400 500 600 700

13.0

13.5

14.0

14.5

15.0

15.5

16.0

Temperature (oC)

Wei

ght (

mg)

Hea

t flo

w E

ndo

dow

n (m

W)

0

−20

−40

−60

−80

20

40

60

80

Fig. 4. TG and DTA curves of the as-prepared nickel hydroxide.

686 B. Shruthi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 683–689

be concluded that the as prepared nickel hydroxide is b-nickelhydroxide.

The microstructural observations and composition studies havebeen performed on b-Ni(OH)2 electrode using Scanning ElectronMicroscope (SEM) and Energy Dispersive X-ray (EDX) analyzer.SEM image (Fig. 1S) shows that b-Ni(OH)2 electrode material isflaky and appears as aggregates of irregular tabular shapes. Fur-ther, Fig. 5 displays the EDX pattern of b-Ni(OH)2 electrode mate-rial. The EDX pattern shows the presence of 76.83 wt.% of Ni and20.32 wt.% of O within the b-Ni(OH)2 electrode material.

Fig. 6 shows the cyclic voltammogram of nickel electrodes usingb-Ni(OH)2 electrode material in 6 M KOH electrolyte at the scanrate of 50 mV s�1 at the potential window of �1 – +1 V vs. Ag/AgCl.For b-Ni(OH)2 electrode, at a scan rate of 50 mV s�1, one anodicnickel hydroxide oxidation peak and one cathodic oxyhydroxidereduction peak were noticed on the CV curves. A pair of strongredox peaks are due to the Faradaic reactions of b-Ni(OH)2. Forb-Ni(OH)2 electrode material, it is well known that the surfacefaradaic reactions will proceed according to the following reaction[37],

b-NiðOHÞ2 þ OH� ¢charge

dischargeb-NiOOHþH2Oþ e� ð3Þ

The anodic peak is due to the oxidation of the b-Ni(OH)2 tob-NiOOH and the cathodic peak is due to the reverse process.The electrochemical energy storage in the nickel hydroxide elec-trodes is associated with the reversible insertion of hydrogen into the nickel hydroxide/oxihydroxide. The insertion of the hydro-gen takes place during discharge and the inverse process occursduring the charge. It has been suggested that both charge anddischarge processes are controlled by the diffusion of the protons[50–52]. XRD studies indicated that the present b-Ni(OH)2 samplepossess high density of structural disorder. Structural disorder innickel hydroxide can provide a better path for the diffusion of pro-tons within the NiO layers and can help lower the free energy byincreasing the entropy contribution, which can in turn increasethe electrochemical reaction rate [31]. An increase in the currentat the end of anodic peak sweep is due to the oxygen evolutionreaction (OER). The corresponding reaction is given below:

2OH� ! H2Oþ 12

O2 " þ2e� ð4Þ

The polarized current is low before the appearance of electro-chemical reaction because there are not free electrons in the elec-trolyte. The presence of polarized current indicates the occurrenceof redox reaction. As shown in Fig. 6, a strong terminal peak dealswith the oxidation peaks of water. When nickel hydroxide elec-trode is being charged, oxygen evolution reaction is a parasitic sidereaction, which has negative effects on the charge efficiency andthe structure of the electrode. It can be seen that the oxidation cur-rent peak of the active material is separated perfectly from theoxygen evolution current (Fig. 6).

Normally, the average of the anodic and cathodic peak poten-tials (Erev) can be taken as an estimate of the reversible potentialfor nickel hydroxide electrodes, and the potential difference (DEa,c)between the anodic (Ea) and cathodic (Ec) peak potentials is a mea-sure of the reversibility of the redox reaction [53,54]. At a scan rateof 50 mV s�1, the values of Erev and DEa,c for the present sample arefound to be 0.269 V and 0.163 V respectively.

Fig. 7 shows a series of recorded voltammetric scans (at a scanrate of 50 mV s�1) of b-Ni(OH)2 electrode for 10 cycles. It can beseen that the positions of the oxidation and reduction peak of bothelectrodes did not change with increasing number of cycles. There-fore, it can be inferred that b-Ni(OH)2 electrode possess stablecycle and the structural changes did not occur during charge/dis-charge process for 10 cycles.

Fig. 5. EDX pattern of b-Ni(OH)2 sample.

1.0 0.5 0.0 -0.5 -1.0

-0.08

-0.06

-0.04

-0.02

0.00

Cur

rent

(A)

Potential, V vs. Ag/AgCl

Fig. 6. Cyclic voltammogram of the b-Ni(OH)2 at a scan rate of 50 mV s�1.

1.0 0.5 0.0 -0.5 -1.0

-0.08

-0.06

-0.04

-0.02

0.00

10 cycles at 50 mVs-1

Cur

rent

(A)

Potential, V vs. Ag/AgCl

Fig. 7. Cyclic voltammograms of the b-Ni(OH)2 electrode at a scan rate of 50 mV s�1

for 10 cycles.

B. Shruthi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 683–689 687

In Fig. 8 cyclic voltammograms of b-Ni(OH)2 sample at differentscan rates are presented. The shape of the curve indicates that theobserved capacitance characteristic is distinct from that of theelectric double layer capacitor, which would produce a CV curvethat is usually close to an ideal rectangular shape. It can be seenthat the shape of CV curves b-Ni(OH)2 is not significantly influ-enced by increasing the scan rates. This indicates improved masstransportation and electron conduction within the material. It iswell established that the electrochemical process of nickel hydrox-ide electrode is limited by proton diffusion through the lattice[22,37,55]. According to the Randles–Sevcik equation [55], at25 �C the peak current, ip, in the cyclic voltammogram can beexpressed as

ip ¼ 2:69� 105 � n32 � A� D

12 � Co � t1

2 ð5Þ

where n is the electron number of the reaction (�1 for b-Ni(OH)2), Ais the surface area of the electrode, D is the diffusion coefficient of H+,t is the scanning rate, and Co is the initial concentration of the reac-tant. For b-Ni(OH)2 electrode, Co = q/M, where q and M are the den-sity and the molar mass (92.7 g/mol) of Ni(OH)2 respectively.

1.0 0.5 0.0 -0.5 -1.0

-0.14

-0.12

-0.10

-0.08

-0.06

-0.04

-0.02

0.00

0.02

Cur

rent

(A)

Potential, V vs. Ag/AgCl

0.05 Vs-1

0.1 Vs-1

0.2 Vs-1

0.3 Vs-1

0.4 Vs-1

0.5 Vs-1

Fig. 8. Cyclic voltammograms of the b-Ni(OH)2 electrode at various scan rates.

0 5 10 15 20 25 30 35

0

2

4

6

8

10

-Z″ (

Ohm

)

Z′ (ohm)

500mV

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

0

5

10

15

20

25

30

-Z″ (

Ohm

)

Z′ (ohm)

300mV

Fig. 9. Electrochemical impedance spectra of b-Ni(OH)2 electrode: (a) at 500 mV; (b) at 300 mV.

688 B. Shruthi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 683–689

Fig. 2S(a) shows the relationship between the anodic peak cur-rent (ip) and the square root of the scan rate (t½) and Fig. 2S(b)shows the relationship between the cathodic peak current (ip)and the square root of the scan rate (t½) for b-Ni(OH)2 electrode.A good linear relationship between ip and t½ confirms that theelectrode reaction of b-Ni(OH)2 is controlled by proton diffusion.Using the slope of the fitted line and Eq. (5), the proton diffusioncoefficient (D) for b-Ni(OH)2 electrode material is found to be1.44 � 10�12 cm2 s�1.

Fig. 9 represents the electrochemical impedance spectra ofb-Ni(OH)2 electrode at steady state. At an applied DC potential of500 mV, electrode was fully charged and oxygen evolution wascommencing on the electrode. This charge transfer reaction [56]would be expected to give rise to a semicircle in the impedanceplane; however, in Fig. 9(a), only the first part of the semicircle isobserved due to its large diameter. As the potential was loweredbelow the oxygen evolution potential, the reduction process couldbegin, and at around 300 mV, spectrum (Fig. 9(b)) showing adepressed charge-transfer semicircle and a Warburg-type linewere recorded. The presence of a Warburg line indicated that theelectrode reaction process was diffusion controlled.

Conclusions

b-Nickel hydroxide was successfully prepared by using precipi-tation method. The structure and property of the synthesizednickel hydroxide were characterized by XRD, IR, Raman spectra,TG–DTA, SEM and EDX analysis. The results demonstrate that thesynthesized b-nickel hydroxide materials have an irregular tabularshape, a high density of structural disorder and contain watermolecules and anions, which could improve the electrochemicalperformance of the nickel hydroxide electrode. The synthesizedb-nickel hydroxide has a relatively good structural stability in alka-line KOH medium. CV curves showed a pair of strong redox peaksas a result of the Faradaic redox reactions of b-Ni(OH)2. The protondiffusion coefficient (D) for b-Ni(OH)2 electrode material is foundto be 1.44 � 10�12 cm2 s�1. Further, electrochemical impedancestudies confirmed that the electrode reaction processes are diffu-sion controlled.

Acknowledgements

Authors wish to acknowledge the STIC, CUSAT, Cochin for SEM,EDX and TGDTA analysis and the SAIF, IIT, Madras for Ramanstudies.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.saa.2014.07.009.

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