Synthesis spectroscopic analysis and electrochemical performance of modified b nickel hydroxide electrode with cuo

The addition of CuO intob -nickel hydroxide was found to enhance the reversibility of the electrode reaction and also increase the separation of the oxidation current peak of the active

Trang 1

Original Article

Synthesis, spectroscopic analysis and electrochemical performance of

B Shruthia, B.J Madhub,*, V Bheema Rajuc, S Vynatheyad, B.Veena Devia,

G.V Jayashreea, C.R Ravikumare

a Department of Chemistry, Dr Ambedkar Institute of Technology, Bangalore, 560 056, India

b Post Graduate Department of Physics, Government Science College, Chitradurga, 577 501, India

c Department of Chemistry, R.N.S Institute of Technology, Bangalore, 560 098, India

d Materials Technology Division, Central Power Research Institute, Bangalore, 560 080, India

e Department of Chemistry, East West Institute of Technology, Bangalore, 560 091, India

a r t i c l e i n f o

Article history:

Received 16 July 2016

Received in revised form

15 December 2016

Accepted 18 December 2016

Available online 23 December 2016

Keywords:

Nickel hydroxide

Spectroscopic analysis

Thermal analysis

Electrochemical properties

Impedance spectroscopy

a b s t r a c t

In the present work, a modifiedb-nickel hydroxide (b-Ni(OH)2) electrode material with CuO has been prepared using a co-precipitation method The structure and property of the modifiedb-Ni(OH)2with CuO were characterized by X-ray diffraction (XRD), Fourier Transform infra-red (FT-IR), Raman and thermal gravimetric-differential thermal analysis (TG-DTA) techniques The results of the FT-IR spec-troscopy and TG-DTA indicate that the modifiedb-Ni(OH)2electrode materials contain intercalated water molecules and anions A pastedetype electrode was prepared using nickel hydroxide powder as the main active material on a nickel sheet as a current collector Cyclic voltammetry (CV) and Electrochemical impedance spectroscopy (EIS) studies were undertaken to assess the electrochemical behavior of pureb -Ni(OH)2and modifiedb-Ni(OH)2electrode with CuO in a 6 M KOH electrolyte The addition of CuO intob -nickel hydroxide was found to enhance the reversibility of the electrode reaction and also increase the separation of the oxidation current peak of the active material from the oxygen evolution current The modified nickel hydroxide with CuO was also found to exhibit a higher proton diffusion coefficient and a lower charge transfer resistance Thesefindings suggest that the modifiedb-Ni(OH)2with CuO possesses

an enhanced electrochemical response and thus can be recognized as a promising candidate for battery electrode applications

© 2016 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

1 Introduction

In the recent decades, much interest has concentrated on the

development of novel electrode materials for advanced energy

conversion and storage devices[1e4] Due to growing demand for

telecommunication devices, substantial attention is concentrated

on improvement of alkaline battery devices with greater speciﬁc

energies Among them, batteries based on Nickel/Metal hydride

(Ni-MH) materials are treated as one of the favorable candidates

due to their superiority in terms of output power, capacity,

reli-ability, life and cost Nickel hydroxide is extensively used in

rechargeable nickel-based batteries as a positive electrode material

due to its remarkable chemical and thermal stability [5] The

positive nickel electrode strongly inﬂuences the performance of the alkaline batteries In these batteries, the capacity of the negative electrode is greater than that of the positive electrode and hence the cell capacity is limited by the positive electrode For Nickel/ metal hydride (NieMH) cells, the performance of the nickel elec-trode is to be sufﬁciently improved to match the superior proper-ties of the metal hydride negative electrode Thus, the preparation

of a high performance nickel electrode becomes signiﬁcant and essential These objectives could be attained by selecting the proper conditions for the synthesis of high performance active material by using suitable additives that could provide the conductive network

to enhance the utilization of the nickel hydroxide The reversibility

of the Ni(II)/Ni(III) electrochemical reaction could be increased by the incorporation of additives Further, it could inhibit aging effects involving unstable nickel hydroxide species and increase the po-larization of the oxygen evolution reaction[6e8]

* Corresponding author.

E-mail address: bjmadhu@gmail.com (B.J Madhu).

Peer review under responsibility of Vietnam National University, Hanoi.

Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

http://dx.doi.org/10.1016/j.jsamd.2016.12.002

Journal of Science: Advanced Materials and Devices 2 (2017) 93e98

Trang 2

It is well known that the nickel hydroxide (Ni(OH)2) exists in

two polymorphic forms, namelya-Ni(OH)2andb-Ni(OH)2, which

are transformed intog-NiOOH andb-NiOOH, respectively during

charging[5,9] Theb-phase exhibits superior stability compared to

thea-Ni(OH)2 Many studies have revealed that incorporation of

compounds containing transition metal atom, such as cobalt

compounds, into the nickel electrode is an effective approach to

improve active material utilization and cycle life[10,11] The cobalt

compound reduces both the oxidizing and reducing potentials of

the nickel electrode and increases the oxygen evolution potential,

thus the utilization of the active material is improved [12] In

addition, adding cadmium, zinc, manganese, barium, magnesium,

calcium and copper compounds into the nickel electrode are

re-ported to have beneﬁcial effects such as inhibiting the swelling of

the nickel electrode during charging and thus prolong the

cycleelife of rechargeable batteries[13,14]

In the present study, the inﬂuence of Cupric oxide (CuO) on the

structure and electrochemical performance of the nickel hydroxide

electrodes is investigated and the results are reported

2 Experimental

2.1 Preparation of cupric oxide

Cupric oxide (CuO) was prepared by solution combustion

method using stoichiometric composition of Copper nitrate as

oxidizer and urea as a fuel The aqueous solution containing redox

mixture was taken in a pyrex dish and heated in a mufﬂe furnace

maintained at 500 ± 10 C The mixture ﬁnally yields porous

powder

2.2 Synthesis of modiﬁedb-nickel hydroxide electrodes with CuO

Modiﬁed b-nickel hydroxide electrode material with 3% CuO

was synthesized using co-precipitation method The chemical

synthesis ofb-nickel hydroxide in the presence of CuO additive was

achieved in three steps viz (i) addition of the reagents, (ii) digestion

of the precipitate and (iii) drying and grinding of the precipitate

Analar grade potassium hydroxide (KOH) and nickel sulphate

(NiSO4) were used as reagents A solution of 1 M KOH was added to

1 M NiSO4solution by dripping at aﬂow rate of 10 ml min1with

constant stirring The addition of the reagent was terminated when

the pH of the suspension reaches 13 Additive added was 3% CuO

The mixture was left for 24 h for digestion of the precipitate The

separation of the precipitate from the excess reagent was done by

centrifugation at 1500 rpm for 1 h The precipitate was washed

carefully with triple distilled water Barium chloride (BaCl2(1 M)) in

excess was added to wash water, causing precipitation of barium

sulphate (BaSO4) Washing of the precipitate was concluded when

the white precipitate of BaSO4 was no more found in the wash

water The sample was then dried in an air oven at about 60C for

48 h

2.3 Characterization of modiﬁedb-nickel hydroxide electrodes

with CuO

Crystal structure of the modiﬁed nickel hydroxide with CuO was

studied by XRD analysis using Bruker AXS D8 Advance

diffrac-tometer with a Cu Ka source (l ¼ 1.54 Å) The FTIR (Infra-red)

spectrum (400-4000 cm1) of the modiﬁed nickel hydroxide with

CuO was recorded on a Bruker Alpha spectrophotometer in KBr

pellets Raman spectroscopic studies of the synthesized samples

were carried out using BRUKER RFS 27 FT-Raman spectrometer

Thermal gravimetric-differential thermal analysis (TG-DTA) of the

modiﬁed nickel hydroxide electrode with CuO material was carried out by Perkin Elmer STA 6000 thermal analyzer

2.4 Fabrication of electrodes and electrochemical testing Following two compositions of the electrode materials were achieved viz (i) pureb-Ni(OH)2electrode with no additives having the composition: 85% b-Ni(OH)2 powder þ 10% graphite powder þ 5% polytetrafluoroethylene (PTFE) as binder and (ii) Modified b-Ni(OH)2 electrode having the composition: 85% of modifiedb-Ni(OH)2with 3% CuOþ 10% graphite powder þ 5% PTFE

as binder Electrodes were fabricated by mixing electrode material with graphite and PTFE solution to form slurry Obtained slurry was pasted onto a Ni sheet Electrode was dried at around 80C tem-perature for 1 h Electrode dimension was kept 1 cm 1 cm covering the rest with insulating teﬂon tape

Cyclic voltammetry experiments were undertaken using CHI604D electrochemical workstation For cyclic voltammetric studies, the test electrode prepared as described above was used as

a working electrode A platinum foil as counter electrode, Hg/HgO electrode as reference electrode and 6 M KOH solution as electro-lyte were used Prior to cyclic voltammetry measurements, the electrodes were activated in 6 M KOH solution After resting for

30 min, CV measurements were recorded All CV studies were undertaken at room temperature

EIS measurements were undertaken using a CHI604D electro-chemical workstation The test electrode prepared as described above was used as a working electrode A platinum foil as counter electrode, Hg/HgO electrode as reference electrode and 6 M KOH solution as electrolyte were used Impedance spectra were recor-ded at room temperature The impedance spectra were recorrecor-ded at the biasing voltage of 0.1 V and amplitude of 0.025 V

3 Results and discussion Structure of modiﬁedb-nickel hydroxide with CuO was exam-ined using XRD analysis, with a CuKasource.Fig 1displays XRD pattern of representative modiﬁedb-nickel hydroxide with CuO Diffraction peaks can be indexed completely to a crystal phase ofb -Ni(OH)2, with lattice parameters, a¼ 3.130 Å and c ¼ 4.630 Å, which are well-matched with the standard values in literature [8] In addition, the XRD pattern revealed the occurrence of monoclinic system of copper oxide with end centered lattice (JCPDS 041-0254)

Fig 1 XRD pattern of modiﬁedb

B Shruthi et al / Journal of Science: Advanced Materials and Devices 2 (2017) 93e98

Trang 3

and cubic system of nickel oxide with face centered lattice (JCPDS

047-1049) within the electrode material Weight fractions of

different phases present in the sample was estimated from the XRD

spectra The 16.18 wt.% of b-Ni(OH)2, 72.80 wt.% of CuO and

11.02 wt.% of NiO are present within the sample

Modiﬁed b-nickel hydroxide with CuO was studied by FT-IR

spectroscopic technique in the range of 400e4000 cm1 Fig 2

shows the FT-IR spectrum of representative modiﬁedb-nickel

hy-droxide with CuO The infra-red spectrum conﬁrms that

synthe-sized nickel hydroxide can be regarded asb-form, due to presence

of (i) strong and a narrow band around 3655 cm1relating toy(OH)

stretching vibration, which reveals the presence of hydroxyl groups

in free alignment, (ii) band around 512 cm1is related to lattice

vibration ofd(OH) and (iii) a weak band at 484 cm1appearing

from lattice vibration of Nie-O,y(Nie-O)[15,16] Broad and strong

band placed around 3430 cm1 is allocated to Oe-H stretching

vibration of the H2O molecules and of H-bound OH group

Furthermore, peak noticed around 1637 cm1 is ascribed to

bending vibration of H2O molecules[17] The peaks situated

be-tween 800 and 1800 cm1could be attributed to existence of

an-ions, which have not possibly been entirely removed during

washing The bands at 1046 cm1 and 1388 cm1 are related to

stretching vibrations of carbonate anion [8] The bands at

1041 cm1and 1137 cm1 are related to vibrations of SO24anion

[18] Detected peak around 1469 cm1 is ascribed to different

vibrational modes of carbonate groups appeared due to adsorption

of atmospheric carbon dioxide[19] Two weak bands observed in

the IR spectra at high frequencies (2853 and 2924 cm1), can be

allocated to stretching mode of (e-CH2) and (e-CH3) groups of

residual organic surfactant, while their bending modes (d(e-CH2),

d(CH3)) appear in the range 1700e1400 cm1 [20,21] Observed

bands around 425, 529, and 603 cm1 corresponds to typical

stretching vibrations of Cue-O bond in monoclinic CuO [22]

Further, absence of absorption peak around 610 cm1related to

infrared active mode of Cu2O conﬁrms the presence of pure CuO

within the sample[23]

Modiﬁedb-nickel hydroxide with CuO was studied by Raman

spectroscopy Fig 3 shows Raman spectrum of representative

modiﬁed b-nickel hydroxide with CuO Well-crystallized b-nickel

hydroxide is associated with three Raman peaks around 3584 cm1,

444 cm1, and 300 cm1, which are attributed to symmetric

stretching of OHgroups, stretching of NieeO, and E-type vibration

of NieeOH lattice, respectively [24,25] As compared to well-orderedb-nickel hydroxide, synthesized Ni(OH)2shows additional Raman peaks (3662 cm1, 3584 cm1, 1098 cm1, 993 cm1,

622 cm1and 486 cm1) as shown in Fig 3 The wave number (486 cm1) corresponding to stretching vibration of NieeO for the prepared Ni(OH)2is greater than that for well-orderedb-nickel hy-droxide Analogous results have also been noticed for the extremely disordered Ni(OH)2 [26,27] The bands observed at 622 cm1,

993 cm1, and 1098 cm1suggest the existence of adsorbed sulfate ions [26,28] Band noticed around 3584 cm1 is allocated to the symmetric stretch of bulk OHgroup[26,28] As compared to well-crystallized b-nickel hydroxide, additional wide band around

3662 cm1reveals microstructural disorders of Ni(OH)2, which can

be ascribed to stretching of surface OHgroup[26,27] Raman peak round 280 cm1 is assigned to Raman active optical-phonon Ag

mode of monoclinic CuO, and the weaker peaks at 330 and

604 cm1corresponds to B1gand B2gmodes, respectively[29] Thermal analysis of the modiﬁedb-nickel hydroxide electrode material bonded with PTFE was studied by TG-DTA.Fig 4shows representative TG-DTA curves of modiﬁed b-nickel hydroxide

4000 3500 3000 2500 2000 1500 1000 500

3655

3430

512 484

1637 1388 1469 1041

2853 2924

425 529 603

1046 1137

Fig 2 FTIR spectrum of modiﬁedb

)

300 444

Fig 3 Raman spectrum of modiﬁedb-Ni(OH) 2 with CuO.

0 100 200 300 400 500 600 700 4.0

4.5 5.0 5.5 6.0 6.5 7.0 7.5

Temperature (oC)

-50

0

50 100

150

200

250

Fig 4 TG-DTA curves of modiﬁedb

Trang 4

electrode with CuO bonded with PTFE Three endothermic peaks

were observed in the DTA curve Theﬁrst endothermic peak is at

around 119C due to the elimination of adsorbed water molecules,

which is supported by the weight loss of 1.97% The second

endo-thermic peak observed at 320 C is related to the endothermic

nature of CuO addedb-nickel hydroxide during the decomposition

of Ni(OH)2 into the NiO with the corresponding weight loss of

18.15% The third endothermic peak at around 402C is attributed

to the decomposition of CuO and intercalated anions with a

cor-responding weight loss of 9.06% Weight loss observed in the region

between 450C and 650C can be attributed to the loss of

inter-calated anions and thermal decomposition of PTFE used as a binder

for the preparation of electrode[30] Thus, TG studies shows that

modiﬁedb-Ni(OH)2electrode with CuO bonded with PTFE material

has considerable amount of adsorbed-intercalated H2O molecules

These H2O molecules play substantial role in enhancement of

electrochemical behavior of electrodes since they offer way for

diffusion of proton along molecular chain among layers

Fig 5 displays the CV curves of pureb-nickel hydroxide and

modiﬁed b-nickel hydroxide with CuO electrodes in 6 M KOH

electrolyte at a scanning rate of 0.025 V s1at potential window of

0e0.7 V vs Hg/HgO Observed pair of strong redox peaks in CV

curve is owed to Faradaic reactions ofb-nickel hydroxide It is well

known that the surface faradic reactions will proceed according to

the following reaction[16],

b NiðOHÞ2 charge

The anodic peak is due to the oxidation of theb-Ni(OH)2in tob

-NiOOH and the cathodic peak is due to the reverse process Near the

strong oxidation peak (observed at 500 mV) belonging to theb

-NiOOH, another typical weak oxidation peak around 452 mV occurs

inFig 5corresponding to the pureb-Ni(OH)2electrode Observed

additional weak oxidation peak may be due to the presence of

easily oxidized grains probably contained with excess alkali

incompletely removed during synthesis[31] The excess alkali is

responsible for decrease in the local electrochemical potential

required for the oxidation reaction[31] But for the electrodes with

CuO, no typical peak contributed by the presence of excess alkali is

found inFig 5

Generally, the peak potential difference (DEa,c) between the anodic (Epa) and cathodic (Epc) peak potentials is considered as a measure of the reversibility of the redox reaction[32e37] Smaller

DEa,cmeans more reversible electrode reaction Variation in anodic peak potentials, cathodic peak potentials and oxygen evolution reactions with the addition of CuO by co-precipitation method is shown in theFig 5 In order to compare the characteristics of the electrodes, CV data inFig 5consisting of anodic nickel hydroxide oxidation peak and cathodic oxyhydroxide reduction peak poten-tials is tabulated inTable 1 As compared to pureb-Ni(OH)2, theDEac

values of modiﬁedb-nickel hydroxide with CuO are reduced, which shows that the inclusion of CuO is found to improve the revers-ibility of the electrode reaction In the CuO addedb-Ni(OH)2 elec-trode material, protons can also freely intercalate into the cupric oxide lattice on reduction and out of the lattice on oxidation, allowing facile interconversion of the O24OH[35]

FromFig 5it can be seen that the polarized current is low before the appearance of electrochemical reaction because there are not any free electrons in the electrolyte The presence of polarized current indicates the occurrence of redox reaction As shown in the Fig 5, the strong terminal peak deals with the oxidation peaks of water When nickel hydroxide electrode is being charged, oxygen evolution reaction (OER) is a parasitic side reaction, which has negative effects on the charge efﬁciency and the structure of the electrode Oxygen evolution reaction may contribute signiﬁcantly

to the electrode degradation by generating the internal tensile stress within the pores of the porous pasted nickel electrode and accordingly affect the cyclic performance of the electrodes and batteries In the present studies, as compared to pureb-Ni(OH)2, the (EOEeEpa) values of the modifiedb-nickel hydroxide with CuO are increased, which indicates that the insertion of CuO is found to increase the separation of the anodic peak from the oxygen evo-lution current Large (EOEeEpa) value facilitates the electrode to be charged fully before oxygen evolution Among the studied com-positions, the modified nickel hydroxide electrode with CuO is found to possess larger (EOEeEpa), indicating an increased separa-tion of the anodic peak from the oxygen evolusepara-tion current There-fore, modified nickel hydroxide electrode with CuO can efficiently restrain the oxygen evolution reaction and improve the charge

efﬁciency

It is well established that the electrochemical reaction process of

a nickel hydroxide electrode is limited by proton diffusion through the lattice[20,30,37] Therefore, it is of much importance to study the nickel hydroxide electrode's proton diffusion coefﬁcient Ac-cording to the Randles e Sevcik equation[36], at 25C the peak current, ip, in the cyclic voltammogram can be expressed as,

ip¼ 2:69 105 n3

A D1

where n is the electron number of the reaction (~1 forb-Ni(OH)2), A

is the surface area of the electrode (1 cm2), D is the diffusion

co-efﬁcient,yis the scanning rate, and Cois the initial concentration of the reactant For an Ni(OH)2electrode, Co¼r/M, whererand M are the density and the molar mass of Ni(OH)2respectively

Fig 6displays the relation between the ipandy½ for pureb -nickel hydroxide and modiﬁed b-Ni(OH)2electrodes with CuO A good linear relationship between ipandy½ revealed that electrode

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

-0.010

-0.008

-0.006

-0.004

-0.002

0.000

0.002

Potential (V)

Pureβ - Ni(OH)2 Modified β - Ni(OH)2 with CuO

Fig 5 CV curves of pure and modiﬁedb-Ni(OH) 2 electrodes with CuO at a scanning

1

Table 1

CV characteristics for pure and modiﬁedb-Ni(OH) 2 electrodes.

Electrode E pa (V) E pc (V) E OE (V) DE ac (V) E OE E pa (V)

Modiﬁedb-Ni(OH) 2 with CuO 0.424 0.284 0.543 0.140 0.119

Trang 5

reaction ofb-Ni(OH)2is regulated by diffusion of protons Using the

slope ofﬁtted line inFig 6in Randles e Sevcik equation[36], the

proton diffusion coefﬁcients for modiﬁedb-nickel hydroxide with

CuO is found to be 4.0666 1012cm2s1, which is comparatively

greater than that of pure b-nickel hydroxide with 1.44 1012

cm2s1 Thus, the proton diffusion coefﬁcient was found to increase

with an insertion of CuO in tob-nickel hydroxide electrode material

by co-precipitation method Observed proton diffusion coefﬁcient

values are comparable with the values in literature Han et al

ob-tained the proton diffusion coefﬁcient values of 1.93 1011cm2s1

and 5.50 1013cm2 s1for nanometer Ni(OH)2and spherical

Ni(OH)2 respectively[38] Li et al obtained the proton diffusion

coefﬁcient values of 3.54 1011cm2s1and 9.34 1012cm2s1

for NO3intercalated Al-substituted nickel hydroxide and SO24

intercalated Al-substituted nickel hydroxide respectively[39]

Fig 7shows the Nyquist plots of pureb-Ni(OH)2and modiﬁedb

-Ni(OH)2electrode with CuO at the biasing voltage of 0.1 V and an

amplitude of 0.025 V Experimental data was analyzed byﬁtting

equivalent circuit shown inFig 8 Parameters R1, R2 and Q1 are

ohmic resistance, charge-transfer resistance and constant phase element (CPE) respectively It displays that R1 is in series with parallel connection of Q1and R2 The impedance of constant phase element (ZCPE) is given by ZCPE¼ 1/Y (ju)n, where uis angular frequency in rad s1, Y and n are variable factors of CPE [30] Equivalent circuit parameters for pureb-nickel hydroxide electrode and modiﬁedb-nickel hydroxide electrodes with CuO are tabulated

in theTable 2 From theTable 2, it can be seen that, the charge transfer resistance R2 is reduced after CuO is incorporated The presence of CuO grains enhance the effectiveness of the current collection process and further improves the charge transfer process

on the electrode and electrolyte interface This implies that the electrochemical reaction within the modiﬁedbeNi(OH)2electrode with CuO proceeds more easily than that within the pure

beNi(OH)2electrode

4 Conclusion

A modifiedb-nickel hydroxide electrode material with CuO has been prepared using co-precipitation method The results of the FTIR spectroscopy and TG-DTA studies indicate that the modifiedb -Ni(OH)2with CuO contains water molecules and anions Addition of CuO into nickel hydroxide by co-precipitation method is found to enhance the reversibility of the electrode reaction and also increase the separation of the oxidation current peak of the active material from the oxygen evolution current Further, modified nickel hy-droxide with CuO is also found to exhibit higher proton diffusion coefficient and lower charge transfer resistance These findings suggest that the modified b-Ni(OH)2 electrode with CuO synthe-sized by co-precipitation method possess improved electro-chemical properties and thus can be recognized as a promising candidate for the battery electrode applications

Acknowledgements Authors wish to acknowledge the Sophisticated Test and Instrumentation Centre (STIC), CUSAT, Cochin for TG-DTA analysis and SAIF, IIT, Madras for Raman analysis

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