J Inorg Organomet Polym (2016) 26:423–430 DOI 10.1007/s10904-016-0329-3

Electrodeposition of Nickel on Glassy Carbon Electrode from Protic Ionic Liquids with Imidazolium Cation B. Meenatchi1 • V. Renuga2 • A. Manikandan3

Received: 20 November 2015 / Accepted: 12 January 2016 / Published online: 21 January 2016 Ó Springer Science+Business Media New York 2016

Abstract The electrodeposition of nickel (Ni) on glassy carbon electrode was performed in a variety of six imidazolium based protic ionic liquids (PILs) like 2-methylimidazolium lactate ([Hmim][lactate]), 1-ethylimidazolium lactate ([Heim][lactate]), 1-butylimidazolium lactate ([Hbim][lactate]), 2-methylimidazolium glycolate ([Hmim][glycolate]), 1-ethylimidazolium glycolate ([Heim][glycolate]), 1-butylim idazolium glycolate ([Hbim] [glycolate]), respectively at various concentrations. While examining the course of the deposition process using cyclic voltammetry, the results demonstrate that all PILs possess high electrochemical stability with good electrochemical window ranging 1.61–3.6 V. In addition, [Hmim][lactate] and [Hmim][glycolate] showed greater cathodic stability, due to the replacement of C-2 acidic proton in imidazolium cation with methyl group. The Ni electrodeposition is found to be proceeding via one step—two electron transfer process that involved the irreversible

reduction of Ni (II) to Ni (0). Also, it was found that by increasing Ni (II) concentration, nickel electrodeposition becomes more facile. Moreover, it was observed that owing to their high ionic conductance, the Ni electrodeposition was more facile in [Hmim][lactate] and [Hmim][glycolate] with diffusion co-efficient 1.8 9 10-5 and 5.1 9 10-7, respectively. The electrodeposited Ni was characterized using scanning electron microscope (SEM) and energy dispersive X-ray (EDX) analysis. The distinctive uniform,sponge and gravel-like morphologies of Ni deposits were confirmed by SEM images and the EDX analysis endorse the presence of Ni element. Keywords Protic ionic liquids  Electrodeposition  Cyclic voltammetry  Electrochemical window

1 Introduction

& B. Meenatchi [email protected] V. Renuga [email protected] A. Manikandan [email protected]; [email protected] 1

Department of Chemistry, Cauvery College for Women, Trichy, Tamilnadu, India

2

Department of Chemistry, National College, Trichy, Tamilnadu, India

3

Department of Chemistry, Bharath Institute of Higher Education and Research, Bharath University, Chennai, Tamilnadu 600 073, India

Nickel (Ni) electrodeposition has significance in versatile surface finishing process. The Ni containing materials are widely used in solar energy generation devices, catalysis, sensors, energy and media storage [1, 2] etc. Furthermore, the self-assembled monolayers (SAM) of n-dodecanethiol, thiol, organothiol and semifluorinated thiol on electrochemically pretreated Ni surface posses improved physicochemical properties like finer chemical stability, resistance to electrochemical oxidation [3–6] etc. Generally, Ni and its alloys are electrodeposited from aqueous solution. However, it is difficult to prevent hydrogen evolution, which leads to the low current deficiency and worse quality of deposit, due to the hydrogen embrittlement. Furthermore, another complexity of the conventional aqueous Ni electrodeposition is associated to the use of organic compounds as inhibitors, levelers or accelerators [7–11] to

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control the crystal growth rate and the final properties of the deposits. To overcome these obstacles, aqueous solutions are replaced by protic ionic liquids (subset of ionic liquids), which have considerable attention, since these new solvents offer interesting possibilities related to their use as ecologically friendly solvents in both chemical and electrochemical process [12–16]. Compared with aqueous solutions, ionic liquids (ILs) have some unique properties like extremely low vapor pressure, good thermal stability, electrical conductivity as well as electrochemical windows [17, 18]. The negligible hydrogen evolution during electrodeposition is the main advantage of using ILs electrolytes. Hence, there lies a chance of sufficient crack-free and corrosion resistant deposits [19]. In addition, the electodeposition of metals from ILs, is a novel method for the production of nanocrystalline metals and alloys, because of the grain size can be adjusted by varying the electrochemical parameters such as over potential, current density, pulse parameters, bath composition and temperature. Furthermore, ionic liquid electrolytes can be efficiently recycled. The electrodeposition of Ni is successfully achieved in certain room temperature protic ionic liquids (PILs) with imidazolium cation [20, 21]. PILs are formed by the proton transfer reaction from a Bronsted acid to a Bronsted base. Furthermore, the proton conducting materials are used as electrolytes in aqueous batteries, fuel cells, double layer capacitors, dye-sensitized solar cells, and actuators [22–28] etc. In the present study, the electrodeposition of Ni was investigated in a series of six PILs with imidazolium cation like 2-methylimidazolium lactate ([Hmim][lactate]), 1-ethylimidazolium lactate ([Heim][lactate]), 1-butylimidazolium lactate ([Hbim][lactate]), 2-methylimidazolium glycolate ([Hmim][glycolate]), 1-ethylimidazolium glycolate ([Heim][glycolate]), 1-butylimidazolium glycolate ([Hbim][glycolate]) at various concentrations on glassy carbon (GC) electrode. The deposited Ni on the GC electrode was characterized by scanning electron microscope (SEM) and energy dispersive X-ray (EDX) analysis.

2 Experimental Part 2.1 Synthesis of PILs with Imidazolium Cation The equimolar mixture of the precursor hydroxy carboxylic acids (Bronsted acids) and substituted imidazoles (Bronsted bases) in a reacting vessel was stirred for an hour using magnetic stirrer later irradiated using microwave (MW) for appropriate power level and temperature [29]. Scheme 1 shows the formation of Protic ionic Liquids.

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2.2 Electrochemical Measurements Employing each of the six synthesized imidazolium PILs like [Hmim][lactate], [Heim][lactate], [Hbim][lactate], [Hmim][glycolate], [Heim][glycolate], [Hbim][glycolate] as electrolytes and NiSO4 salt as source of Ni (II) ions, the electrodeposition of Ni was carried out by cyclic voltammetry at various concentrations (30, 60 and 90 mM) of Ni(II) solutions. The entire cyclic voltammetry experiment was performed at an atmospheric pressure (since the PILs are stable in air, thereby prevents the oxidation of Ni into NiO), constant temperature (303 K) and scan rate of 20 mV s-1 with the assist of VERSA STAT MC multichannel potentiostat/galvanostat/impedance analyzer. The data acquired from the above experiments were stored by Versa Studio software. A three electrode system comprising of a glassy carbon (GC) working electrode (diameter of 4 mm), calomel reference electrode and platinum counter electrode was employed in all cyclic voltametric experiments (Prior to each experiment, the surface of the electrode was washed with distilled water and acetone). 2.3 Characterization of Nickel Deposits The obtained Ni deposits were characterized using scanning electron microscope (SEM) and their chemical constituents were analyzed by energy dispersive X-ray (EDX) spectroscopy. In both cases VEGA3-TESCAN Microscope was used. Whereas, for the EDX analysis an electron beams acceleration of 20 keV was used for experimental conditions.

3 Results and Discussion 3.1 Electrochemical Stability of PILs The electrochemical window is defined as the potential interval observed between the reduction potential of the organic cationic part and the oxidation potential of the anionic part of pure ionic liquids (ILs). A wide electrochemical window makes PILs as promising electrolytes for electrochemical applications. The cyclic voltammograms (CVs) of six imidazolium based PILs were shown in Fig. 1a–f, and the cathodic and anodic limiting potential of the PILs were summarized in Table 1. The cathodic limiting potential of PILs was mainly due to the reduction of the imidazolium cation and is moderately affected by the nature of the anion. Among the six PILs studied, the PIL containing butyl imidazolium cation shows less cathodic stability, due to lower electrochemical stability of the imidazolium cation. The incorporation of methyl group in

J Inorg Organomet Polym (2016) 26:423–430 Scheme 1 Synthesis of Protic ionic liquids

425 O

R1 N

+

N

OH

H3C

R2

O

MW Irradiation

OH

R

+

N H

N

Protic ionic liquids

O R1 N

N R2

+

-

OH

R

Lactic acid

O

H3C

O

OH

MW Irradiation

OH

+

R1 N

N H R2

Glycolic acid

O

-

OH

Protic ionic liquids

where R1-H, C2H5- or C4H9-. R2-H, CH3

acidic C-2 hydrogen atom of imidazolium cation may influence the cathodic stability. The values of cathodic limit potential of these PILs were scattered ranging from -0.91 to -2.5 V and increased in the order: [Hbim][glycolate] \ [Hbim][lactate] \ [Heim][glycolate] \ [Heim] [lactate] \ [Hmim][glycolate] \ [Hmim][lactate]. This tendency suggested that introduction of methyl group to replace acidic C-2 hydrogen atom of imidazolium cation lightly increases the cathodic stability and when the length of alkyl chain on the imidazolium ring is expanded, wider electrochemical windows were observed. In addition, the anodic limiting potential of PILs was mainly attributed to the oxidation of the lactate and glycolate anion. The electrochemical windows for the six PILs were scattered between 1.61 and 3.6 V. Furthermore, the additional peaks were observed at 0.6 and 0 V in Fig. 1c, f, due to the electro-oxidation of the reduced species (1-butyl imidazole in both cases). Hence, the PILs [Hbim][lactate] and [Hbim][glycolate] posses lower electrochemical window of 1.80 and 1.61 V, respectively, than rest of the other PILs (where there was no additional peaks). 3.2 Electrodeposition of Nickel from PILs The cyclic voltammograms of NiSO4 in each of [Hmim] [lactate], [Heim][lactate], [Hbim][lactate], [Hmim][glycolate], [Heim][glycolate], [Hbim][glycolate] were recorded using glassy carbon (GC) at different concentrations (30, 60 and 90 mM) and presented in Figs. 2, 3, 4, 5, 6, and 7.

The cathodic and anodic peaks probably correspond to the electrochemical reactions shown in Eq. (1). At room temperature, the cathodic and anodic peaks were attributed to the bulk reduction of Ni2? to the metal and stripping of the deposited nickel respectively. Ni2þ þ 2e ! Ni Ni  2e ! Ni2þ

ð1Þ

From Figs. 2, 3, 4, 5, 6, and 7, it was noted that the integrated electric charges of the oxidation peaks were smaller comparing to reduction ones. It revealed that the electrodeposited Ni during the cathodic scan could not be stripped completely during the anodic scan, because the concentration of Ni (II) close to the electrode increased up to its saturated concentration rapidly resulting in the hindrance of the anodic dissolution [30]. Furthermore, the ratio of anodic to cathodic charges is lesser than one (Ipa/ Ipc \ 1) indicating an incomplete anodic stripping of Ni deposits and a large potential peak separation i.e. difference between the cathodic and anodic peak potential (DE = Epa - Epc) was observed for all cases (Table 2). All these characteristics indicate the electrode reaction of Ni (II) to Ni (0) is irreversible and it proceed via follows one step—two electron transfer process [31]. It was noted from the cyclic voltammograms (Figs. 2, 3, 4, 5, 6, 7) corresponding to the electrodeposition of Ni in PILs [Hmim][laclate], [Heim][laclate], [Hbim][laclate], [Hmim][glycolate], [Heim][glycolate], [Hbim][glycolate] that, the reduction potential was shifted positively and the reduction peak current increased with the increasing of the

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Fig. 1 Cyclic voltammogram of PILs (where a–f are Cyclic voltammogram of [Hmim][lactate], [Heim][lactate], [Hbim][lactate], [Hmim][glycolate], [Heim][glycolate], [Hbim][glycolate], respectively)

Ni (II) concentration. These results showed that the deposition of Ni becomes more facile by increasing Ni (II) concentration. In addition, the current loop was observed in voltammograms (Figs. 4, 5, 6), due to the requirement of nucleation overpotential during the negative scan. Furthermore, the cyclic voltammogram of Ni electrodeposition in [Heim][glycolate] showed current loop at -0.5 and -0.2 V for 30 and 60 mM Ni (II) concentration, respectively. The decreased nucleation overpotential for electrodeposition of Ni at higher concentration (60 mM) was due to increased

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mobility of the electroactive species at higher concentration. For an irreversible charge transfer process, the transfer coefficient and diffusion co efficient can be calculated with reference to Eqs. (2) and (3) [32] respectively. Ep  Ep=2 ¼ 1:857RT=a na F ð2Þ ip ¼ 0:496nFCAD1=2 ða na F=RTÞ1=2 v1=2

ð3Þ

where Ep is the cathodic peak potential in V, Ep/2 is the cathodic half peak potential in V, R is the gas constant,

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427

Table 1 Electrochemical stability of PILs Protic ionic liquids

Conductance (mS, cm-1)

Cathodic limiting potential (V)

Anodic limiting potential (V)

Electrochemical window (V)

[Hmim][lactate]

6.64

-2.49

1.0

3.49

[Heim][lactate]

5.51

-1.75

0.8

2.55

[Hbim][lactate]

4.15

-1.25

0.55

1.80

[Hmim][glycolate]

5.29

-2.5

1.1

3.6

[Heim][glycolate]

4.49

-1.76

0.9

2.66

[Hbim][glycolate]

3.61

-0.91

0.7

1.61

Fig. 2 Nickel electrodeposition in [Hmim][lactate] (Color figure online) Fig. 4 Nickel electrodeposition in [Hbim][lactate] (Color figure online)

Fig. 3 Nickel electrodeposition in [Heim][lactate] (Color figure online)

Fig. 5 Nickel figure online)

electrodeposition

in

[Hmim][glycolate]

(Color

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Fig. 6 Nickel figure online)

electrodeposition

in

[Heim][glycolate]

(Color

8.314 J K-1 mol-1, T is the absolute temperature in K, a is transfer coefficient, na is the number of exchanged electrons, F is the Faraday constant, 96,485 C mol-1. According to Eqs. (2) and (3) and data obtained from Figs. 2, 3, 4, 5, 6, 7, the average transfer coefficient and diffusion coefficient can be calculated and displayed in Table 2. The diffusion co-efficient of Ni (II) in various ionic liquids increases in the order [Hbim][glycolate] \ [Hbim]

Fig. 7 Nickel figure online)

electrodeposition

in

[Hbim][glycolate]

(Color

[lactate] \ [Heim][glycolate] \ [Heim][lactate] \ [Hmim] [glycolate] \ [Hmim][lactate]. The diffusion co-efficient of Ni (II) in [Hmim][lactate] and [Hmim][lactate] was found to be comparatively higher than other ionic liquids, which could be explained in terms of ionic conductivity of ionic liquids. The ionic conductivity (r) correlates to diffusion coefficient (D) in reference to Nernst–Einstein equation (Eq. 4), where n and q implies the number of carrier ions and electric charge, respectively. R, T, and F stand for the

Table 2 Electrodeposition of nickel in six PILs PILs

Concentration (mM)

Epc (V)

Epa (V)

Ipc (A)

Ipa (A)

Ep/2 (V)

|Ep - Ep/2| (V)

DEp (V)

Ipa/Ipc

PIL1

30

-1.98

1.9

-0.0015

0.0005

-1.75

0.23

0.08

0.33

60

-1.90

2.3

-0.0019

0.0008

-1.65

0.25

0.4

0.42

90 30

-1.85 -2.25

3.0 2.2

-0.0210 -0.0009

0.0008 0.0007

-1.70 -2.10

0.15 0.15

1.15 0.05

0.04 0.78

60

-2.20

2.0

-0.0013

0.0005

-2.0

0.09

0.2

0.39

90

-1.98

1.5

-0.0015

0.0011

-1.85

0.13

0.48

0.73

30

-2.40

1.5

-0.0001

0.00003

-2.25

0.15

0.90

0.3

60

-2.35

1.40

-0.0002

0.00002

-2.15

0.20

0.95

0.1

90

-2.0

1.0

-0.0003

0.00007

-1.80

0.20

1.0

0.2

30

-2.3

1.5

-0.0001

0.00001

-2.15

0.15

0.8

0.1

60

-2.20

1.2

0.00014

0.00003

-2.0

0.20

1.0

0.21

90

-2.1

1.0

-0.0002

0.00006

-1.95

0.15

1.1

0.3

30

-0.65

2.45

-0.00008

0.00007

-0.5

0.15

1.8

0.88

60

-0.55

2.85

0.0001

0.00005

-0.4

0.15

2.3

0.5

90

-0.20

3.1

-0.00015

0.00006

-0.1

0.1

2.9

0.4

30

-0.3

3.0

-0.0004

0.0003

-0.2

0.10

2.7

0.75

60

-0.25

3.65

-0.0006

0.0004

-0.15

0.10

3.4

0.67

90

-0.2

4.45

-0.0007

0.0003

-0.1

0.05

4.25

0.43

PIL2

PIL3

PIL4

PIL5

PIL6

Average transfer coefficient

Average diffusion coefficient

0.12

1.8 9 10-5

0.21

2.35 9 10-7

0.13

1.06 9 10-8

0.15

5.1 9 10-7

0.19

1.6 9 10-7

0.3

3.6 9 10-9

Where PIL1, PIL2, PIL3, PIL4, PIL5, PIL6 are [Hmim][lactate], [Heim][lactate], [Hbim][lactate], [Hmim][glycolate], [Heim][glycolate], [Hbim][glycolate], respectively

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429

Fig. 8 SEM images (a, b and c) and EDX (d) results of Nickel electrodeposits

gas constant, temperature in K, and Faraday constant, respectively.
ð4Þ r ¼ Dnq2 F2 =RT As the ionic conductivity of ionic liquids increases, diffusion coefficient increased thereby increasing the diffusion of Ni (II) ions towards cathode and facilitates the electrodeposition process. Thus, the Ni electrodeposition was more facile in in [Hmim][lactate] and [Hmim][glycolate], since, their ionic conductivity was noted higher when compared to others (shown in Table 2), therefore, diffusion of Ni (II) ions increased towards cathode. 3.3 Morphological and Elemental Composition Analysis of Ni Electrodeposits The surface morphologies of Ni electrodeposits obtained from Ni(II) solution with various concentrations (30, 60,

90 mM) were illustrated in Fig. 8a–c. From the Fig. 8a, it makes clear that the uniform and compact deposits were obtained for 30 mM Ni(II) concentration. Indeed, some of the particles begin to grow into sponge and gravel-like structure in various dimensions and sizes since the concentration increased to 60 and 90 mM, respectively, as shown in Fig. 8b, c. The EDX analysis (Fig. 8d) confirms the presence of Ni without any other impurities on glassy carbon electrode.

4 Conclusion Using six imidazolium PILs as electrolytes, the Ni electrodeposition on GC electrode was carried out successfully without employing additives. The PILs of [Hmim][lactate] and [Hmim][glycolate] comparatively possess higher electrochemical window of 3.49 and 3.6 V than rest of the

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PILs, due to their high cathodic stability of 2-methyl imidazolium cation. The reduction of Ni (II) to Ni (0) during the electrodeposition process was noted irreversible and proceed via one step—two electron transfer process. In all the mentioned cases, the electrodeposition was facilitated by increasing the Ni (II) concentration. The Ni electrodeposition was observed more facile in [Hmim][lactate] and [Hmim][glycolate] with diffusion co-efficients 1.8 9 10-5 and 5.1 9 10-7 respectively. Sponge and gravel-like morphologies of Ni deposits depending on various Ni(II) concentration was confirmed by SEM analysis and their elemental composition were confirmed by EDX analysis.

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