Synthesis, spectroscopic analysis and electrochemical performance of modified b-nickel hydroxide electrode with CuO

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[r]

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Original Article

Synthesis, spectroscopic analysis and electrochemical performance of

modiﬁed b-nickel hydroxide electrode with CuO

B Shruthia, B.J Madhub,*, V Bheema Rajuc, S Vynatheyad, B.Veena Devia, G.V Jayashreea, C.R Ravikumare

aDepartment of Chemistry, Dr Ambedkar Institute of Technology, Bangalore, 560 056, India bPost Graduate Department of Physics, Government Science College, Chitradurga, 577 501, India cDepartment of Chemistry, R.N.S Institute of Technology, Bangalore, 560 098, India

dMaterials Technology Division, Central Power Research Institute, Bangalore, 560 080, India eDepartment 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 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

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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 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 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 ỵ 10% graphite

powder ỵ 5% polytetrauoroethylene (PTFE) as binder and (ii) Modiﬁed b-Ni(OH)2 electrode having the composition: 85% of

modiedb-Ni(OH)2with 3% CuOỵ 10% graphite powder ỵ 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 h Electrode dimension was kept cm 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 M KOH solution as electro-lyte were used Prior to cyclic voltammetry measurements, the electrodes were activated in 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 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)

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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 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 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 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

T

ransmi

ttan

ce (%)

Wave number (cm-1)

3655 3430

512 484 1637

1388 1469

1041 2853

2924

425 529 603

1046 1137

Fig FTIR spectrum of modiﬁedb-Ni(OH)2with CuO

500 1000 1500 2000 2500 3000 3500 4000

Raman

In

ten

sity (a.u

.)

Wave number (cm-1

)

35

84

300 444

3662

1098

99

3

622

486

28

0 33 60

4

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

We

ig

ht (

m

g)

H

ea

t f

low Endo down (

m

W)

-50

0

50 100

150

200

250

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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 displays the CV curves of pureb-nickel hydroxide and modiﬁed b-nickel hydroxide with CuO electrodes in 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

discharge b NiOOH ỵ H

ỵỵ e (1)

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

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 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 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 n

3

2 A D 2 C

oy

1

2 (2)

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

0.004 Epc

EOE

Current (A

)

Potential (V) Pureβ - Ni(OH)2

Modified β - Ni(OH)2 with CuO

Epa

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

rate of 0.025 V s1

Table

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

Electrode Epa(V) Epc(V) EOE(V) DEac(V) EOEEpa(V)

Pureb-Ni(OH)2 0.500 0.331 0.550 0.169 0.050

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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 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 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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