Scholarly article on topic 'Synthesis, spectroscopic analysis and electrochemical performance of modified β-nickel hydroxide electrode with CuO'

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Abstract of research paper on Chemical sciences, author of scientific article — B. Shruthi, B.J. Madhu, V. Bheema Raju, S. Vynatheya, B.Veena Devi, et al.

Abstract In the present work, a modified β-nickel hydroxide (β-Ni(OH)2) electrode material with CuO has been prepared using a co-precipitation method. The structure and property of the modified β-Ni(OH)2 with 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 spectroscopy and TG-DTA indicate that the modified β-Ni(OH)2 electrode materials contain intercalated water molecules and anions. A pasted–type 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 pure β-Ni(OH)2 and modified β-Ni(OH)2 electrode with CuO in a 6 M KOH electrolyte. The addition of CuO into β-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. These findings suggest that the modified β-Ni(OH)2 with CuO possesses an enhanced electrochemical response and thus can be recognized as a promising candidate for battery electrode applications.

Academic research paper on topic "Synthesis, spectroscopic analysis and electrochemical performance of modified β-nickel hydroxide electrode with CuO"

Journal of Science: Advanced Materials and Devices xxx (2016) 1—6

Contents lists available at ScienceDirect

Journal of Science: Advanced Materials and Devices

journal homepage: www.elsevier.com/locate/jsamd

Original Article

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

B. Shruthi a, B.J. Madhu b' *, V. Bheema Raju c, S. Vynatheya d, B.Veena Devi a, G.V. Jayashree a, C.R. Ravikumar e

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 09!, India

ARTICLE INFO

ABSTRACT

Article history: Received 16 July 2016 Received in revised form 15 December 2016 Accepted 18 December 2016 Available online xxx

Keywords: Nickel hydroxide Spectroscopic analysis Thermal analysis Electrochemical properties Impedance spectroscopy

In the present work, a modified b-nickel hydroxide (b-Ni(OH)2) electrode material with CuO has been prepared using co-precipitation method. The structure and property of the modified b-Ni(OH)2 with CuO was 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 spectroscopy and TG-DTA studies indicate that the modified b-Ni(OH)2 electrode materials contains intercalated water molecules and anions. A pasted—type electrode is 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 pure b-Ni(OH)2 and modified b-Ni(OH)2 electrode with CuO in 6 M KOH electrolyte. Addition of CuO into b-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 hydroxide 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 with CuO possess better electrochemical properties and thus can be recognized as a promising candidate for the 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/Iicenses/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 [1—4]. 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, reliability, 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

* Corresponding author. E-mail address: bjmadhu@gmail.com (B.J. Madhu). Peer review under responsibility of Vietnam National University, Hanoi.

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 (Ni—MH) cells, the performance of the nickel electrode is to be sufficiently improved to match the superior properties 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(ll)/Ni(lll) electrochemical reaction could be increased by the incorporation of additives. Further, it could inhibit aging effects involving unstable nickel hydroxide species and increase the polarization of the oxygen evolution reaction [6—8].

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

2468-2179/© 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/).

B. Shruthi et al. / Journal of Science: Advanced Materials and Devices xxx (2016) 1—6

It is well known that the nickel hydroxide (Ni(OH)2) exists in two polymorphic forms, namely a-Ni(OH)2 and b-Ni(OH)2, which are transformed into g-NiOOH and b-NiOOH, respectively during charging [5,9]. The b-phase exhibits superior stability compared to the a-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 reported to have beneficial effects such as inhibiting the swelling of the nickel electrode during charging and thus prolong the cycle—life 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 modified b-nickel hydroxide electrodes with CuO

Modified b-nickel hydroxide electrode material with 3% CuO was synthesized using co-precipitation method. The chemical synthesis of b-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 NiSO4 solution by dripping at a flow rate of 10 ml min-1 with 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 60 °C for 48 h.

2.3. Characterization of modified b-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 = 1.54 A). The FTIR (Infra-red) spectrum (400-4000 cm-1) 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) pure b-Ni(OH)2 electrode with no additives having the composition: 85% b-Ni(OH)2 powder + 10% graphite powder + 5% polytetrafluoroethylene (PTFE) as binder and (ii) Modified b-Ni(OH)2 electrode having the composition: 85% of modified b-Ni(OH)2 with 3% CuO + 10% graphite powder + 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 80 °C temperature for 1 h. Electrode dimension was kept 1 cm x 1 cm covering the rest with insulating teflon tape.

Cyclic voltammetry experiments were undertaken using CH1604D 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 electrolyte 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.

E1S measurements were undertaken using a CH1604D electrochemical 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. 1mpedance spectra were recorded at room temperature. The impedance spectra were recorded at the biasing voltage of 0.1 V and amplitude of 0.025 V.

3. Results and discussion

Structure of modified b-nickel hydroxide with CuO was examined using XRD analysis, with a CuKa source. Fig. 1 displays XRD pattern of representative modified b-nickel hydroxide with CuO. Diffraction peaks can be indexed completely to a crystal phase of b-Ni(OH)2, with lattice parameters, a = 3.130 A and c = 4.630 A, 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)

. p-Ni(OH)2 4 CuO ♦ NiO

26 (degree)

Fig. 1. XRD pattern of modified b-Ni(OH)2 with CuO.

B. Shruthi et al. / Journal of Science: Advanced Materials and Devices xxx (2016) 1—6

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 400—4000 cm-1. Fig. 2 shows the FT-IR spectrum of representative modified b-nickel hydroxide with CuO. The infra-red spectrum confirms that synthesized nickel hydroxide can be regarded as b-form, due to presence of (i) strong and a narrow band around 3655 cm-1 relating to u(OH) stretching vibration, which reveals the presence of hydroxyl groups in free alignment, (ii) band around 512 cm-1 is related to lattice vibration of 5(OH) and (iii) a weak band at 484 cm-1 appearing from lattice vibration of Ni—O, u(Ni—O) [15,16]. Broad and strong band placed around 3430 cm-1 is allocated to O—H stretching vibration of the H2O molecules and of H-bound OH group. Furthermore, peak noticed around 1637 cm-1 is ascribed to bending vibration of H2O molecules [17]. The peaks situated between 800 and 1800 cm-1 could be attributed to existence of anions, which have not possibly been entirely removed during washing. The bands at 1046 cm-1 and 1388 cm-1 are related to stretching vibrations of carbonate anion [8]. The bands at

1041 cm-1 and 1137 cm-1 are related to vibrations of SO2-anion [18]. Detected peak around 1469 cm-1 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 cm-1), can be allocated to stretching mode of (—-CH2) and (—-CH3) groups of residual organic surfactant, while their bending modes (5(—CH2), 5(CH3)) appear in the range 1700—1400 cm-1 [20,21]. Observed bands around 425, 529, and 603 cm-1 corresponds to typical stretching vibrations of Cu—O bond in monoclinic CuO [22]. Further, absence of absorption peak around 610 cm-1 related to infrared active mode of Cu2O confirms the presence of pure CuO within the sample [23].

Modified b-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 cm-1, 444 cm-1, and 300 cm-1, which are attributed to symmetric

o o c ra

Ë </> c ra

'</> c ou o ° CO^tSg CN CO o "t Ui

c ra £ 5

500 1000 1500 2000 2500 3000 3500 4000 Wave number (cm1)

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

stretching of OH- groups, stretching of Ni—O, and E-type vibration of Ni—OH lattice, respectively [24,25]. As compared to well-ordered b-nickel hydroxide, synthesized Ni(OH)2 shows additional Raman peaks (3662 cm-1, 3584 cm-1, 1098 cm-1, 993 cm-1, 622 cm-1 and 486 cm-1) as shown in Fig. 3. The wave number (486 cm-1) corresponding to stretching vibration of Ni——O for the prepared Ni(OH)2 is greater than that for well-ordered b-nickel hydroxide. Analogous results have also been noticed for the extremely disordered Ni(OH)2 [26,27]. The bands observed at 622 cm-1, 993 cm-1, and 1098 cm-1 suggest the existence of adsorbed sulfate ions [26,28]. Band noticed around 3584 cm-1 is allocated to the symmetric stretch of bulk OH- group [26,28]. As compared to well-crystallized b-nickel hydroxide, additional wide band around 3662 cm-1 reveals microstructural disorders of Ni(OH)2, which can be ascribed to stretching of surface OH- group [26,27]. Raman peak round 280 cm-1 is assigned to Raman active optical-phonon Ag mode of monoclinic CuO, and the weaker peaks at 330 and 604 cm-1corresponds to B1g and B2g modes, respectively [29].

Thermal analysis of the modified b-nickel hydroxide electrode material bonded with PTFE was studied by TG-DTA. Fig. 4 shows representative TG-DTA curves of modified b-nickel hydroxide

4000 3500 3000 2500 2000 1500 1000 500

—1—I—1—I—1—I—1—I—1—I—1—r 0 100 200 300 400 500 600 700

Wave number (cm- )

Fig. 2. FTIR spectrum of modified b-Ni(OH)2 with CuO.

Temperature ( C)

Fig. 4. TG-DTA curves of modified b-nickel hydroxide electrode with CuO.

B. Shruthi et al. / Journal of Science: Advanced Materials and Devices xxx (2016) 1—6

electrode with CuO bonded with PTFE. Three endothermic peaks were observed in the DTA curve. The first endothermic peak is at around 119 °C due to the elimination of adsorbed water molecules, which is supported by the weight loss of 1.97%. The second endothermic peak observed at 320 °C is related to the endothermic nature of CuO added b-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 402 ° C is attributed to the decomposition of CuO and intercalated anions with a corresponding weight loss of 9.06%. Weight loss observed in the region between 450 °C and 650 °C can be attributed to the loss of intercalated anions and thermal decomposition of PTFE used as a binder for the preparation of electrode [30]. Thus, TG studies shows that modified b-Ni(OH)2 electrode 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 pure b-nickel hydroxide and modified b-nickel hydroxide with CuO electrodes in 6 M KOH electrolyte at a scanning rate of 0.025 V s-1 at potential window of 0—0.7 V vs. Hg/HgO. Observed pair of strong redox peaks in CV curve is owed to Faradaic reactions of b-nickel hydroxide. It is well known that the surface faradic reactions will proceed according to the following reaction [16],

charge

b - Ni(OH)2 < b - NiOOH + H+ + e-

discharge

The anodic peak is due to the oxidation of the b-Ni(OH)2 in to b-NiOOH and the cathodic peak is due to the reverse process. Near the strong oxidation peak (observed at 500 mV) belonging to the b-NiOOH, another typical weak oxidation peak around 452 mV occurs in Fig. 5 corresponding to the pure b-Ni(OH)2 electrode. 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 in Fig. 5.

-0.010-

0.5 0.4 0.3 Potential (V)

Fig. 5. CV curves of pure and modified b-Ni(OH)2 electrodes with CuO at a scanning rate of 0.025 Vs-1.

Generally, the peak potential difference (AEa>c) between the anodic (Epa) and cathodic (Epc) peak potentials is considered as a measure of the reversibility of the redox reaction [32—37]. Smaller DEa c means 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 the Fig. 5. In order to compare the characteristics of the electrodes, CV data in Fig. 5 consisting of anodic nickel hydroxide oxidation peak and cathodic oxyhydroxide reduction peak potentials is tabulated in Table 1. As compared to pure b-Ni(OH)2, the DEac values of modified b-nickel hydroxide with CuO are reduced, which shows that the inclusion of CuO is found to improve the reversibility of the electrode reaction. In the CuO added b-Ni(OH)2 electrode 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 O2 4 OH- [35].

From Fig. 5 it 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 pure b-Ni(OH)2, the (EOE—Epa) values of the modified b-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 evolution current. Large (Eoe—Epa) value facilitates the electrode to be charged fully before oxygen evolution. Among the studied compositions, the modified nickel hydroxide electrode with CuO is found to possess larger (EOE—Epa), indicating an increased separation of the anodic peak from the oxygen evolution current. Therefore, 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. According to the Randles — Sevcik equation [36], at 25 °C the peak current, ip, in the cyclic voltammogram can be expressed as,

ip = 2.69 x 105 x n2 x A x D2 x Co x u1

where n is the electron number of the reaction (~1 for b-Ni(OH)2), A is the surface area of the electrode (1 cm2), D is the diffusion coefficient, v is the scanning rate, and Co is the initial concentration of the reactant. For an Ni(OH)2 electrode, Co = p/M, where p and M are the density and the molar mass of Ni(OH)2 respectively.

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

Table 1

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

Electrode Epa(V) Epc (V) Eoe (V) D Eac(V) EoE-Epa(V)

Pure b-Ni(OH)2 0.500 0.331 0.550 0.169 0.050

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

B. Shruthi et al. / Journal of Science: Advanced Materials and Devices xxx (2016) 1—6

0.0028-,

0.0024-

0.0020-

0.0016-

0.0012-1

0.0008-

0.0004-

-Pure p-N^OH^

-Modified p-N(OH>2 with CuO

0.0000-

Fig. 8. Proposed equivalent circuit for nickel hydroxide electrodes.

Table 2

Equivalent circuit parameters for pure and modified b-Ni(OH)2 electrodes.

0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16

Electrode R1/U r2/u Y1 n1

Pure b-Ni(OH)2 1.1646 3894 0.0001470 0.82000

Modified b-Ni(OH)2 with CuO 1.1876 1285 0.0001194 0.70464

1/2 „, -1 ,1/2 U (Vs )

Fig. 6. Relationship between ip and v'A for pure and modified b-nickel hydroxide electrodes.

reaction of b-Ni(OH)2 is regulated by diffusion of protons. Using the slope of fitted line in Fig. 6 in Randles — Sevcik equation [36], the proton diffusion coefficients for modified b-nickel hydroxide with CuO is found to be 4.0666 x 10~12 cm2 s~\ which is comparatively greater than that of pure b-nickel hydroxide with 1.44 x 10~12 cm2s31. Thus, the proton diffusion coefficient was found to increase with an insertion of CuO in to b-nickel hydroxide electrode material by co-precipitation method. Observed proton diffusion coefficient values are comparable with the values in literature. Han et al. obtained the proton diffusion coefficient values of 1.93 x 10311 cm2 s31 and 5.50 x 10~13 cm2 s31 for nanometer Ni(OH)2 and spherical Ni(OH)2 respectively [38]. Li et al. obtained the proton diffusion coefficient values of 3.54 x 10311 cm2 s31 and 9.34 x 10~12 cm2 s31 for NO3 intercalated Al-substituted nickel hydroxide and SO4~ intercalated Al-substituted nickel hydroxide respectively [39].

Fig. 7 shows the Nyquist plots of pure b-Ni(OH)2 and modified b-Ni(OH)2 electrode with CuO at the biasing voltage of 0.1 V and an amplitude of 0.025 V. Experimental data was analyzed by fitting equivalent circuit shown in Fig. 8. Parameters R1, R2 and Q1 are

Z'(ohm)

Fig. 7. Electrochemical impedance plots of pure and modified b-nickel hydroxide electrodes.

ohmic resistance, charge-transfer resistance and constant phase element (CPE) respectively. It displays that R1 is in series with parallel connection of Q1 and R2. The impedance of constant phase element (ZCPE) is given by ZCPE = 1/Y (ju)n, where u is angular frequency in rad s-1, Y and n are variable factors of CPE [30]. Equivalent circuit parameters for pure b-nickel hydroxide electrode and modified b-nickel hydroxide electrodes with CuO are tabulated in the Table 2. From the Table 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 modified b—Ni(OH)2 electrode with CuO proceeds more easily than that within the pure b—Ni(OH)2 electrode.

4. Conclusions

A modified b-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 modified b-Ni(OH)2 with 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 hydroxide 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 synthesized by co-precipitation method possess improved electrochemical 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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