ORGANIC TRANSFORMATIONS IN IONIC LIQUIDSshodhganga.inflibnet.ac.in/bitstream/10603/9910/11/11_chapter 5.pdf · supercapacitors in [Cmim][HSO 4] synthesized by green technology. 5.A.2

Chapter – 5

Application of TSILs as electrolyte for

Supercapacitor

Section – A

Synthesis and characterization of Ru doped

CuO thin films for supercapacitor based on

[Cmim][HSO4]

Chapter – 5: Application of TSILs as electrolyte for Supercapacitor

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Section - A: Synthesis and characterization of Ru doped CuO thin

films for supercapacitor based on [Cmim][HSO4]

5.A.1 Outline

The nanostructured ruthenium (Ru) doped copper oxide (CuO)

thin films were synthesized by colloidal solution method via spin coating

technique. The prepared undoped and Ru doped CuO films were used

as electrode to measure the specific capacitance in the task specific

Brønsted acidic ionic liquid (BAIL) that is 3-carboxymethyl-1-

methylimidazolium bisulfate, [Cmim][HSO4]. Further, the films were

characterized by Scanning Electron Microscopy (SEM), Fourier

Transform Raman spectroscopy (FT-Raman) and Cyclic Voltammetry

(CV). The Ru doped CuO films exhibited higher specific capacitance,

Csp, (Csp = ratio of average current in CV and a product of scan rate and

mass deposited on the film) with the larger potential window as

compared to undoped CuO film. The highest Csp of 406 Fg-1 was

observed for 15 volume percent of Ru doping concentration. This is the

first successful step towards development of ecofriendly CuO based

supercapacitors in [Cmim][HSO4] synthesized by green technology.

5.A.2 Introduction

5.A.2.1 Ionic liquids - an ideal electrolyte

The constituent components of a supercapacitor, an electrolyte,

are a critical part that governs its overall performance. Common

aqueous electrolytes have narrow potential window, which limits the cell

voltage, a large potential stability window of the electrolyte is quite

important [1]. Good electrolytes should have high conductivity, large

electrochemical windows, excellent thermal and chemical stability, and

negligible evaporation. ILs exhibit intrinsic ionic conductivity, large

electrochemical potential windows, excellent thermal stability,

nonvolatility, nonflammability and nontoxicity, hence have attracted

enormous interest for variety of application [2]. When using IL as an


186

electrolyte medium, it is possible to satisfy the conditions and achieve a

broader range of operational temperatures and conditions relative to

other conventional electrolytic media. This makes ILs promising

materials in various electrochemical devices, such as, batteries, fuel

cells, sensors, and electrochromic windows [3].

5.A.2.2 Story behind electrochemistry of ILs

Welton reported that ILs are not new, and some of the ILs such

as Ethyl ammonium nitrate [C2H5NH3][NO3], which has a melting point

of 12oC [4]. In the late 1940s, scientists Frank Hurley and Tom Weir at

Rice University, who were working on methods of electroplating

aluminum, discovered that RTILs could be prepared by mixing and

warming 1-ethylpyridinium chloride with aluminum chloride. The Air

Force Office of Scientific Research, Washington, D.C., has supported

fundamental research on molten salts and ILs. In the late 1970s and

1980s, Robert A. Osteryoung, chemistry professor at North Carolina

State University; Charles L. Hussey, chemistry professor at the

University of Mississippi; John S. Wilkes, professor of chemistry and

director of the Chemistry Research Center at the U.S. Air Force

Academy, Colorado Springs, Colo.; and others carried out extensive

research on organic chloride-aluminum chloride ambient-temperature

ILs for use as electrolytes in electrical batteries.

The Osteryoung and Wilkes groups discovered the RTILs 1-

butylpyridinium chloride-aluminum chloride and 1-ethyl-3-

methylimidazolium chloride-aluminum chloride ([Emim]Cl- AlCl3),

respectively.

At the North Atlantic Treaty Organization (NATO) meeting Wilkes

pointed out - “The nature of the electrolyte has a huge impact on both

the energy the device can store and the power it can deliver. ILs,

formerly called molten salts, have some properties that make them


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attractive as electrolytes in batteries. ILs are inherently ironically

conductive, they can mitigate self-discharge, and they are virtually

nonvolatile, nonflammable, and less toxic than conventional

electrolytes. In addition, their electrochemical window that is, the

electrochemical potential range over which the electrolyte is not

reduced or oxidized at an electrode is usually much greater than for

aqueous electrolytes. The current generation of batteries containing IL

electrolytes suffers from the requirement of high-temperature operation

and is therefore called thermal batteries. The advent of low-melting ILs

has resulted in some novel battery concepts and promises to spur new

concepts with much higher performance.”

Wilkes was described the work by researchers at the U.S. Air

Force Academy on a dual intercalating molten electrolyte (DIME)

battery. The battery involves the intercalation of the anion of an IL

electrolyte at one carbon electrode and the cation of the same

electrolyte at another carbon electrode. Wilkes group has shown that

RTILs containing 1-ethyl-3-methylimidazolium or 1-ethyl-3-methyl-2-

propylimidazolium cations and anions such as PF6-, BF4

-, AlCl4-, and

CF3SO2O- (trifluoromethanesulfonate or triflate) work in the DIME

battery.

Pierre Bonhote, a chemist at the Swiss Federal Institute of

Technology, Lausanne (EPFL), has investigated the use of room-

temperature ionic liquids as electrolytes in dye-sensitized solar cells,

electrochromic devices, and other photoelectrochemical devices.

According to Bonhote - the electrolytes used in these cells should have

low vapor pressures, large electrochemical windows, low viscosity, and

high conductivity. Along with this, they should exhibit thermal stability,

chemical stability in the presence of water and oxygen, and be

compatible with the sealant.


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One report [5] showed that a lithium ion cell with an IL electrolyte

has been performed at a level of practical utility in terms of cell

performance and cycle durability. Both the RTIL [6] and proton-

conducting gelatinous electrolytes templated by RTILs [7] have been

studied as possible solvents in lithium batteries.

Metals that can be obtained from aqueous media in most cases

can also be deposited from ILs, often with superior quality because

hydrogen evolution does not occur. These features and their good ionic

conductivities between 10−3 and 10−2Ω−1 cm−1 make ILs interesting

solvents for low-temperature electrodeposition studies, especially with

respect to elements that cannot be obtained from aqueous solutions

(e.g., silicon, germanium, aluminum, titanium, and plutonium).

5.A.2.3 A newer concept

The specific capacitance of electrode was higher in acidic

electrolyte [8-10]. In comparison with most of ILs in current use, 3-

carboxymethyl-1-methylimidazolium bisulfate [Cmim][HSO4] is halogen

free and environmentally benign as media and catalysts [11].

[Cmim][HSO4] as a new BAIL is first time introduced as an electrolyte

for supercapacitor. It dissolves in water, ethanol and is insoluble in

organic solvents such as diethyl ether, acetone, ethyl acetate,

cyclohexane, chloroform etc. This BAIL can be recovered and recycled

easily without changing its physical and chemical properties [12].

One of the challenging issues in development of CuO based

supercapacitor is to improve its electronic conductivity. Extensive

research work has been focused on enhancing electronic conduction

using metal doping such as gold, silver, aluminium, indium, ruthenium

etc. in the electrode materials [13]. The advantages of Ru over other

metals are high conductivity, large oxidation states and good barrier

against oxygen diffusion that is non corrosive [14]. It can enhance the


189

specific capacitance of the electrode materials. Hence, here new

approach is presented to improve the specific capacitance of the CuO

based supercapacitor by doping the Ru in CuO using colloidal solution

method via spin coating technique. CuO as electrode and [Cmim][HSO4]

as electrolyte has been studied to develop green chemistry approach

for supercapacitor application.

Some of the recent interesting examples of ionic liquids based

supercapacitor are tabulated here.

Year Description Ref. No.

2004 The use of ionic liquids based on 1-buthyl-3-methyl-

imidazolium as electrolytes in an activated

carbon//poly(3-methylthiophene) hybrid super-

capacitor was investigated by Ching et al.

15

2005 Frackowiak et al. studied two phosphonium salts such

as trihexyl (tetradecyl) phosphonium bis

(trifluoromethylsulfonyl) imide (IL1) and trihexyl

(tetradecyl) phosphonium dicyanamide (IL2) as

electrolyte for supercapacitor. To decrease the

viscosity of ILs, a small amount of acetonitrile (from 5

to 25 wt %) was added. Supercapacitor based on

activated carbon (AC) as electrodes and IL1 with 25 wt

% of acetonitrile supplied capacitance values of 100

Fg-1 at a high operating voltage of 3.4 V. Such a

supercapacitor reached a high energy of ~40 Wh/kg

and a good cyclability.

16

2006 Balducci et al. reported a novel and clean

galvanostatic procedure to polymerize poly(3-

methylthiophene) in the 1-ethyl-3-methyl-imidazolium

bis(trifluoro-methanesulfonyl)imide (EMITFSI) IL. The

strategy consists the use of the acid additive

17


190

trifluoromethanesulfonimide (HTFSI) displaying the

same anion of the IL and which provides an acid

proton that is reduced to H2 at the counter electrode

upon the anodic polymer growth on the working

electrode and prevents consumption of the ionic liquid

with great advantage in terms of costs. This procedure

provides a pMeT electrode featuring 250 Fg−1 in

EMITFSI at 60oC, a very interesting result in view of

application of such pMeT in IL-based hybrid

supercapacitors.

2008 Poly(3,4-ethylenedioxythiophene) (PEDOT) were

successfully electropolymerized by Arbizzani et al.

using purified 1-butyl-3-methylimidazolium tetrafluo-

roborate ([Bmim][BF4]) as both the growth medium and

the supporting electrolyte. The electrochemical

performance of the PEDOT thin film was investigated

in 1 mol L−1 H2SO4 solution. It possesses nearly ideal

capacitive property, and its specific capacitance is

about 130 Fg−1. Compared with other conducting

polymers, enhanced cycling lifetime (up to 70,000

cycles), which is close to that of active carbon

materials, was observed on repetitive redox cycling.

18

2008 Safety is the main concern for energy storage-system

application in hybrid-electrical vehicles (HEVs) and ILs

of low vapor pressure and high thermal stability

represent a strategy to meet this key requisite. The

use of solvent-free ILs in supercapacitors enables the

high cell voltages required for increasing

supercapacitor energy up to the values for power-

assist application in HEVs. In order to exploit the wide

electrochemical stability window of ILs, tailored

19


191

electrode materials and cell configurations were used

by Liu et al. The performance of asymmetric double-

layer carbon supercapacitors (AEDLCs) and

carbon/poly (3-methylthiophene) hybrid

supercapacitors operating with different pyrrolidinium-

based ILs were reported and compared. This study

demonstrates that a design-optimized AEDLC

operating with safe, solvent-free IL electrolyte meets

cycling stability and the energy and power requisites

for power-assisted HEVs at the investigated

temperatures.

2008 The ILs N-butyl-N-methyl-pyrrolidinium

trifluoromethanesulfonate (PYR14Tf) and N-methyl-N-

propyl-pyrrolidinium bis(fluorosulfonyl) imide

(PYR13FSI) are investigated as electropolymerization

media for poly(3-methylthiophene) (pMeT) in view of

their use in carbon/IL/pMeT hybrid supercapacitors.

Data on the viscosity, solvent polarity, conductivity and

electrochemical stability of PYR14Tf and PYR13FSI as

well as the effect of their properties on the

electropolymerization and electrochemical

performance of pMeT, which features >200 Fg−1 at

60oC when prepared and tested in such ILs, are

reported by Arbizzani et al.

20

2009 Polymer - ionic liquid composite electrolytes based on

poly (vinylidenefluoride-co-hexafluoropropylene)

(PVdF - HFP) and RTIL: 2, 3-dimethyl-1-octyl

imidazolium hexafluoro phosphate ([Dmoim][PF6])

were synthesized and studied by Bisoa et al. The

addition of dimethyl acetamide (DMA) and propylene

carbonate (PC), both with high dielectric constant and

21


192

low viscosity, to polymer electrolytes has been found

to result in an enhancement of conductivity by one

order of magnitude. It has been reported that,

composite polymer electrolytes containing IL have

been found to be thermally stable upto 300°C. It was

observed that, motional narrowing observed in the

variation of line width of 1H and 19F NMR peaks with

temperature and it suggests that both cations and

anions are mobile in these electrolytes.

2009 Feng et al studied the molecular dynamics simulations

of the electrical double layers (EDLs) at the interface

of ILs [Bmim][NO3] and planar electrodes. It has been

confirmed from simulations, that a Helmholtz-like

interfacial counter ion layer exists when the electrode

charge density is negative or strongly positive, but the

counter ion layer is not well-defined when the

electrode charge density is weakly positive. The

thickness of the EDL, as inferred from how deep the

charge separation and orientational ordering of the

ions penetrate into the bulk ILs, is about 1.1 nm. The

liquid nature of the IL and the short-range ion-

electrode and ion-ion interactions are found to

significantly affect the structure of the EDL, particularly

at low electrode charge densities. Charge

delocalization of the ions is found to affect the mean

force experienced by the ions and, thus, can play an

important role in shaping the EDL structure. The

differential capacitance of the EDLs is found to be

nearly constant under negative electrode polarization

but increases dramatically with the potential under

positive electrode polarization. Author exhibited that

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the differential capacitance is a quantitative measure

of the response of the EDL structure to a change in

electrode charge density. It was found that the [NO3]-

ion dominates the response of EDL structure to the

change in electrode charge under both positive and

negative electrode polarization, which is qualitatively

different from that in aqueous electrolytes. Detailed

analysis exhibited that the cation-anion correlations

and the strong adsorption of [Bmim]+ ions on the

electrode are responsible for the capacitance-potential

correlation observed here.

2010 Lalia et al reported that, protic ionic liquids (PILs) are

novel electrolyte for carbon-based supercapacitors.

The cyclic voltammograms in three-electrode cells

exhibited reversible redox humps, revealing pseudo-

faradic charge transfer. Oxidative treatment of

activated carbon enriches the surface functionality and

leads to a higher capacitance owing to a stronger

pseudo-faradic contribution. The capacitors using PILs

demonstrated a higher voltage window than with

aqueous H2SO4, while keeping the same values of

capacitance, and being able to operate at lower

temperature. A combination of activated carbons and

PILs holds promise for improving the energy

characteristics of supercapacitors.

23

2010 Properties of capacitors working with the same carbon

electrodes (activated carbon cloth) and three types of

electrolytes: aqueous, organic and ILs were compared.

Capacitors filled with ILs worked at a potential

difference of 3.5V, their solutions in AN and PC were

charged up to the potential difference of 3V, classical

24


194

organic systems to 2.5V and aqueous to 1V. Mysyk et

al showed that, the highest specific energy was

recorded for the device working with ILs, while the

highest power is characteristic for the device filled with

aqueous H2SO4 electrolyte. Aqueous electrolytes led to

energy density of an order of magnitude lower in

comparison to that characteristic of ILs.

2010 Recent advances in the study of ILs based gel polymer

electrolytes have been briefly reviewed in report of their

electrochemical applications, particularly, their

application as electrolytes in supercapacitors. The

incorporation of IL in gel polymer electrolytes, instead

of organic solvents like propylene carbonate, ethylene

carbonate etc., provide added effect in terms of their

thermal, electrical and electrochemical stabilities.

Recent studies on poly(ethylene oxide) based polymer

electrolyte plasticized with IL incorporated

poly(vinylidine fluoride-co-hexafluoropropylene) based

gel polymer electrolytes have been summarized in this

review. A special description has been given of

supercapacitors (electrical double layer capacitors),

studied in author’s laboratory, based on multiwalled

carbon nanotubes, activated charcoal powder

electrodes and optimized gel/polymer electrolytes.

Pandey et al observed that PVdF-HEP based gels

were superior electrolytes to develop electrical double

layers capacitors (in terms of higher capacitances and

lower resistive values) over PEO based plasticized

polymer electrolytes.

25


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5.A.3. Results and discussion

5.A.3.1 FT-Raman studies

Fig 5.A.1 (a) exhibited the FT-Raman spectrum of undoped CuO

film recorded over 200 - 4000 cm-1. According to the group theory CuO

belong to the C6-2h space group with two molecules per primitive cell,

one can contribute to the zone center normal modes. The degree of

vibrational freedom is represented by;

BgAgBuAu 254 (5.1)

Where, is degree of vibrational freedom, Au and Bu represent IR

modes; Ag and Bg represent Raman modes. There are six infrared

active modes (3Au+3Bu), three acoustic modes (Au+2Bu), and three

Raman active modes (Ag+2Bg). The spectrum exhibited the peak at

164.28 cm-1, which is attributed to the Ag mode. While the peaks at

339.8 cm-1 and 563.53 cm-1 are assigned to Bg mode. The band at

about 1099.72 cm-1 is assigned to multi-phonon transition [12]. Fig

5.A.1 (b) shows the FT- Raman spectra of Ru doped CuO films

recorded over 200 – 4000 cm-1. Ru doped CuO films also exhibited a

peak of Ag mode and two peaks of Bg mode with small blue shift in

wave number as compared to undoped CuO film. These blue shifts with

respect to concentration of Ru dopant confirm the existence of Ru in

CuO films.


196

(a) (b)

Fig. 5.A.1 (a) FT-Raman spectra of CuO film, (b) FT-Raman spectra of

CuO doped with 5 to 15 % Ru

5.A.3.2 SEM studies

Fig 5.A.2 (a) exhibited SEM image of undoped CuO film. The

sample is highly dense with uniform surface. Fig 5.A.2 (b) exhibited FE-

SEM image of pure CuO film with cluster of nanocrystals of size 23 nm.

Fig 5.A.3 exhibited the SEM image of CuORu15 film. The nanocrystals

are uniformly distributed over all the surface of the film.

(a) (b)

Fig. 5.A.2 (a) SEM image of undoped CuO film (b) FE-SEM image of

undoped CuO films


197

Fig. 5.A.3 SEM image of CuORu15 film

5.A.3.3 Cyclic voltammetry studies

CV is employed to deduce specific capacitance (Csp) of undoped

CuO and Ru doped CuO in 1M [Cmim][HSO4] electrolyte in a

conventional three electrode arrangement of following configuration

SS / thin film/ [Cmim][HSO4] (aq)/ SCE/G

where, thin film (undoped CuO and Ru doped CuO) coated on

steel substrate acts as a working electrode, saturated calomel electrode

(SCE) serving as a reference electrode to which all measured voltage

were referred and G was the graphite which acts as a counter

electrode.

CV was also employed to deduce Csp of undoped CuO and Ru

doped CuO in 1 M Na2SO4 and 1 M [Cmim][HSO4] in distilled water

electrolyte. When the undoped CuO and Ru doped CuO electrode was

swept towards negative potential vs SCE, cathodic current flows and

associated to Cu2+ ↔ Cu1+ reduction process. Similarly, during positive

potential, anodic current flows associated to Cu1+ ↔ Cu2+ oxidation

process. The following reaction represents the redox reaction in the cell:

Cu2+ + e− ↔ Cu+ (5.2)


198

The charge storage mechanism of CuO electrode in aqueous 1 M

Na2SO4 electrolyte has been proposed as follows:

2CuO + 2Na+ +2e− ↔ Cu2O(Na2O) (5.3)

The possible mechanism of charge storage in [Cmim][HSO4]

electrolyte is based on the intercalation/extraction of cations into the

CuO electrode:

When, 1 M [Cmim][HSO4] dissolved in water, following

dissociation reaction takes place.

The possible half cell reaction for charge storage is given below:

N N+

CH3

OH

O

H+

2 CuO + + 2 e-

N N+

CH3

OH

O

OH-

Cu2O

Csp was calculated according to the equation;

i

V×WspC

(5.4)

where, i, V and W represent average current, scan rate and

weight of deposited films.

Fig 5.A.4 exhibited CV curves of undoped CuO and Ru doped

CuO films in 1M Na2SO4. The undoped CuO film was swept between

+0.3 to 0 V with respect to SCE at 20 mV/s and CuORu5, CuORu10 and

CuORu15 films were swept between -0.2 to +0.5 V with respect to SCE

at 20 mV/s. For pure CuO the Csp is calculated to be 19 Fg-1. The Ru

N N+

CH3

OH

O

HSO4

-

+ H2O N N+

CH3

OH

O

H+

SO4

--


199

doped CuO films exhibit high current density over large potential

window. The current density increases with respect to the concentration

of Ru in CuO. The calculated Csp 35, 72 and 131 Fg-1 for CuORu5,

CuORu10 and CuORu15 films respectively. The improved Csp attributed

to the increased conductivity with increase in concentration of Ru. The

contribution of any synergetic effect between Ru and CuO, however,

cannot be neglected. This warrants further investigations.

The stability of an electrode material is important for its use in

supercapacitor. The stability of the undoped CuO and CuORu15 are

studied upto 2000 cycles. Fig 5.A.5 (a) exhibited CVs of undoped CuO

film swept between +0.3 to 0 V vs SCE at 40 mV/s up to 2000 cycles in

1M Na2SO4. The inset exhibited the variation of the Csp as a function of

CV cycle number. A slight decrease of Csp with cycle’s number was

observed. It concludes that the undoped CuO film is stable in 1 M

Na2SO4. Fig 5.A.5 (b) exhibited CV of CuORu15 film up to the 2000

cycle in the voltage range of -0.2 to +0.5 V vs SCE at 40 mV/s in 1 M

Na2SO4. The inset exhibited the variation of Csp as a function of CV

cycles. From this study it is concluded that undoped and Ru doped CuO

film is stable up to 2000 cycles in 1 M Na2SO4.

Fig 5.A.4CV curves of undoped and Ru doped CuO films in 1 M Na2SO4


200

(a) (b)

Fig 5.A.5 The CV at different cycle number recorded in 1 M Na2SO4 of

films: - (a) undoped CuO, (b) CuORu15 and the inset figure shows the

specific capacitance vs cycle number

Fig 5.A.6 exhibited CV curves of undoped CuO and Ru doped

CuO films in 1M [Cmim][HSO4]. The pure CuO film was swept between

0 to +0.7 V in 1M [Cmim][HSO4] with respect to SCE at 100 mV/s and

CuORu5, CuORu10 and CuORu15 films were swept between +0.8 to -

0.5 V cycle with respect to SCE at 100 mV/s. The CV plots for pure

CuO is close to rectangular shape with a mirror- image feature. This

indicates excellent reversibility and an ideal capacitive property of the

electrode. For pure CuO the Csp is calculated to be 66 Fg-1 at 100 mV/s.

The higher current density over large potential window in the CV curve

of Ru-doped CuO films exhibited higher Csp: viz. 82, 101 and 160 Fg-1

respectively at 100 mV/s. The enhancement in the Csp with

concentration of Ru doping may be due to the increased conductivity of

the electrode. Also, some distortion of the ideal shape was observed for

all Ru doped CuO films. It is well visible that the different sizes of cation

and anion of IL determined certain irregular shape of the CV. It means

that a suitable matching of pore size of electrode for the positive and


201

negative electrodes with ILs ions size can still induce further

improvement of the supercapacitor performance.

Fig 5.A.6 CV curves of undoped CuO and Ru doped CuO films in 1M

[Cmim] [HSO4]

To evaluate the rate capability of the electrode, the CVs at

different scan rates were recorded in 1M [Cmim][HSO4] and are

exhibited in Fig 5.A.7 (a-d) respectively. The highest Csp of 114, 216,

297 and 406 Fg-1 are observed at 10 mV/s for doped CuO, CuORu5,

CuORu10 and CuORu15 films respectively. The insets of Fig 5.A.7 (a-d)

exhibited the variation of the specific capacitance as a function of scan

rate of undoped CuO, CuORu5, CuORu10 and CuORu15 films

respectively. From these graphs, it is observed that the specific

capacitance decreased with increase in scan rate. The decrease in

capacitance at higher scan rates is attributed to the presence of the

inner active sites, which was not involved in the redox transition

completely, due to the diffusion effect of cations within the electrode.

The CuORu5 film shows acceptable specific capacitance of 122 Fg-1 at

500 mV/s. This indicated the electrode is applicable even at higher scan

rates.


202

(a) (b)

(c) (d)

Fig 5.A.7 CV at different scan rates of films: (a) undoped CuO, (b)

CuORu5, (c) CuORu10, (d) CuORu15 and the inset figure shows the

specific capacitance vs scan rate


supercapacitor. Fig 5.A.8 (a) exhibited CVs of undoped CuO film up to

2000 cycles at 100 mV/s in 1M [Cmim][HSO4]. The inset figure of Fig

5.A.8 (a) exhibited the variation of the Csp of pure CuO film as a function

of CV cycle number. From this plot the film exhibited slight decrease of


203

specific capacitance with cycles number. It concluded that the pure CuO

film is highly stable in acidic [Cmim][HSO4]. Fig 5.A.8 (b) exhibited CV

of CuORu5 film up to the 2000 cycle in the voltage range of 0.8 to -0.5 V

vs. SCE at 100mV/s in 1 M [Cmim][HSO4]. The inset exhibited the

variation of Csp of CuORu5 film in [Cmim][HSO4] as a function of CV

cycles. From this study it could be concluded that Ru doped CuO film is

highly stable in acidic [Cmim][HSO4].

(a) (b)

Fig 5.A.8 The CV at different cycle number recorded in 1M

[Cmim][HSO4] of films: (a) undoped CuO, (b) CuORu15 and the inset

figure exhibited the specific capacitance vs cycle number

5.A.4 Experimental section

5.A.4.1 Materials and methods

The chemicals Copper oxides, ruthenium nitrate, 1-methyl

imidazole and ethyl chloro acetate, were purchased from Sigma Aldrich.

While, absolute ethanol and conc. sulphuric acid were purchased from

Runa, India and used without further purification.


204

5.A.4.2 Instrumental details and their operational conditions

SEM analysis

SEM images were recorded using a scanning electron

microscope (Model JEOL-JSM-6360, Japan) operated at 20 kV.

FT - Raman analysis

The FT - Raman experiments were conducted using Bruker made

(Multi RAM - Germany) with an excitation wavelength (1064 nm) by Nd:

YAG source with Ge detector. In this system quartz filter is used. The

scanning range of FT – Raman was 200 – 4000 cm-1 with resolution 4

cm-1.

Cyclic voltammetry analysis

The cyclic voltammetry (CV) experiments were conducted using

an electrochemical analyzer (CH instruments) and the potential was

swept with respect to SCE.

5.A.4.3 Preparation of ruthenium doped CuO

The colloidal solution method was used to deposit the

nanostructured undoped CuO and Ru doped CuO thin films. The

following synthetic route was followed to obtain monodispersed and

highly stable CuO and Ru doped CuO colloidal solution at low

temperature. The colloidal solution of undoped CuO was prepared by

dissolving 0.05 g of copper acetate in 20 mL dimethylformamide (DMF)

to form 0.0125 M solution (solution A), which was subsequently heated

at 120oC for 20 min with constant stirring using a magnetic stirrer. The

undoped CuO films were deposited on steel substrate by spin coating


205

technique at 3000 rpm. The colloidal solution of Ru doped CuO was

prepared by dissolving 0.028 g ruthenium chloride (RuCl2·6H2O) in 20

mL of DMF to form 0.0125 M solution (solution B). The volume percent

(5, 10 and 15%) of solution B was mixed in solution A to maintain 20 mL

quantity. This solution was heated at 120oC for 20 min with constant

stirring using a magnetic stirrer. The monodispersed colloidal solution of

Ru doped CuO was obtained after 20 min. The Ru doped CuO films

were deposited on steel substrate by spin coating technique at 3000

rpm and the samples were referred as CuORu5, CuORu10 and CuORu15

respectively. The mass deposited on the substrate was in the range of

0.01 - 0.03 mg. The effect of Ru doping on the properties of CuO is

studied in detail. The surface morphology of the films was examined by

analyzing the SEM images. The potential was swept with respect to

SCE in [Cmim][HSO4] electrolyte.

5.A.4.4 Synthesis of [Cmim][HSO4]

The procedure is as described in the chapter 3.8.3.

N NCH3 + ClCH2COOC2H5 Ethanol

60-65oC, 48 h

N N+

CH3

OC2H5

O

N N+

CH3

OC2H5

O

+ 65% H2SO4

60oC, 12 h

N N+

CH3

OH

O

Cl-

Cl-

HSO 4

-

Compound-I

Compound-IICompound-I


206

5.A.5 Conclusion

It concludes the first report on a synthetic strategy to deposit

nanostructured Ru doped CuO thin films by colloidal solution method

via simple and cost effective spin coating technique for supercapacitor

application. Synthesis of Ru doped CuO films as electrode for

supercapacitor is important to replace maximum possible toxic and

expensive RuO2 compound with non-toxic and cheap CuO with

acceptable level of Csp. It is the successful synthesis and presentation

of the first evidence of application of a task-specific ionic liquid i.e.

[Cmim][HSO4] as electrolyte for supercapacitor, advanced over other

ILs and acidic aqueous electrolyte like H2SO4. The highest Csp of

406 Fg-1 is observed at 10 mV/s for CuO film having 15 volume percent

Ru doping concentration with large potential window (+0.8 to -0.5 V)

that is the high power density compared to pure CuO film having

potential window (+0.7 to 0 V). From this study, it concludes that Ru

doped CuO film as electrode and [Cmim][HSO4] as electrolyte were

useful in developing green chemistry approach for better

supercapacitors. Ionic liquid can be frequently used without perturbing

its property by just removing water under reduced pressure. This

recyclable property of ionic liquid gives green approach than other

aqueous electrolyte like H2SO4.


207

5.A.6 References

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8] Frackowiak E, J. Braz, Chem. Soc. 2006, 17, 1074.

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10] Conway BE, Electrochemical Supercapacitor: Scientific

Fundamentals and Technological Application, Kluwer

Academic/Plenum Publisher, New York, 1999.

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179, 858.

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Kim CS, Sekhon SS, Appl, Phys. A, 2009, 96, 661.

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Chapter – 5

Application of TSILs as electrolyte for

Supercapacitor

Section – B

Supercapacitor based on CuO and different task-specific ionic liquids (TSILs)

Chapter - 5: Application of TSILs as electrolyte for Supercapacitor

209

Section - B: Supercapacitor based on CuO and different task-specific ionic liquids (TSILs)

5.B.1 Outline

A large number of different classes of ionic liquids (ILs) have been

investigated. Examples includes Imidazolium, Pyrolidinium, Phosphonium,

Quarternary ammonium and Trizolium salts. In this work, we mainly

focused on Imidazolium based electrolyte for supercapacitor. The [Bmim]

as a cation and chloride (Cl-), hydroxide (OH-) and hydrogen sulfate

(HSO4-) anion based ILs were studied as electrolyte for copper oxide (CuO)

based supercapacitor. Here, we observed that, due to different anion

structure the ILs exhibited different properties such as ionic conductivity,

potential window, pH and electrochemical stability etc. Supercapacitor

properties of CuO films in [Bmim][Cl], [Bmim][OH] and [Bmim][HSO4] were

studied by comparing with known aqueous electrolyte such as Na2SO4.

5.B.2 Results and discussion

5.B.2.1 FT-Raman studies

The FT-Raman studies of CuO are discussed in 5.A.3.1.

5.B.2.2 SEM studies

The SEM studies of CuO are discussed in 5.A.3.2.

5.B.2.3 Cyclic voltammetry studies

Cyclic voltammetry (CV) was employed to deduce specific

capacitance of CuO films in 0.1 M Na2SO4, [Bmim][ILs] {[Bmim][OH],

[Bmim][Cl], [Bmim][HSO4]} as electrolyte in a conventional three electrode

arrangement of following configuration

SS / thin film/ [Bmim][ILs] (aq)/ SCE/G


210

where, thin film of CuO coated on steel substrate acts as a

working electrode, saturated calomel electrode (SCE) serving as a

reference electrode to which all measured voltage were referred and

G was the graphite which acts as a counter electrode. CV was

employed to deduce Csp of CuO in 0.1 M Na2SO4 and 0.1 M

[Bmim][ILs] in distilled water electrolyte. When CuO electrode was

swept towards negative potential vs SCE, cathodic current flows and

associated to Cu2+ ↔ Cu1+ reduction process. Similarly, during

positive potential, anodic current flows associated to Cu1+ ↔ Cu2+

oxidation process. The following reaction represents the redox

reaction in the cell:

Cu2+ + e− ↔ Cu+ (5.10)

The charge storage mechanism of CuO electrode in

aqueous 0.1 M Na2SO4 electrolyte has been proposed as follows:

2CuO + 2Na+ +2e− ↔ Cu2O(Na2O)

To evaluate the rate capability of the CuO electrode, the CVs

at different scan rates were recorded in 0.1M Na2SO4, [Bmim][ILs]

are shown in Fig 5.B.3 (a-d) respectively. The inset shows the

variation of the specific capacitance as a function of scan rate of

CuO. From these graphs, it is observed that the specific capacitance

decreased with increase in scan rate. The decrease in capacitance

at higher scan rates is attributed to the presence of the inner active

sites, which is not involved in the redox transition completely, due to

the diffusion effect of cations within the electrode. Csp of CuO are 97

Fg-1 at 20 mV/s in aqueous Na2SO4, 177 Fg-1 at 20 mV in

[Bmim][OH], 254 Fg-1 at 20 mV/s in [Bmim][Cl] and 213 Fg-1 at 20

mV/s in [Bmim][HSO4] electrolytes. The highest Csp is observed for

[Bmim][Cl] with respect to other ILs. The second highest Csp is found

for [Bmim][HSO4], while, [Bmim][OH] shows less Csp. Here, we

conclude that Csp as well as stability did not affected by pH.


211

(a) (b)

(c) (d)

Fig 5.B.3 The CVs at different scan rates were recorded for CuO electrode

in 0.1M (a) Na2SO4, (b) [Bmim][OH], (c) [Bmim][Cl], (d) [Bmim][HSO4] and

the inset figure shows the specific capacitance vs cycle number


supercapacitor. The stability of CuO was studied upto 2000 cycle in

Na2SO4 and [Bmim][ILs]. Fig 5.B.4 (a) exhibited CVs of CuO film

swept between +0.6 to -0.6 V vs SCE at 40 mV/s up to 2000 cycles

in 0.1M Na2SO4. The inset exhibited the variation of the Csp as a

function of CV cycle number. A slight decrease of Csp with cycle’s


212

number was observed. It concluded that the CuO film was stable in

0.1 M Na2SO4. Fig 5.B.4 (b) exhibited CV of CuO film up to the

2000 cycle in the voltage range of +0.4 to -0.6 V vs SCE at 40 mV/s

in 0.1 M [Bmim][OH]. The inset exhibited the variation of Csp as a

function of CV cycles. From this study it is concluded that CuO film

was stable up to 2000 cycles in 0.1 M [Bmim][OH]. Fig 5.B.4 (c)

exhibited CV of CuO film up to the 2000 cycle in the voltage range of

+0.9 to -0.3 V vs SCE at 40 mV/s in 0.1 M [Bmim][Cl]. The inset

exhibited the variation of Csp as a function of CV cycles. From this

study it is concluded that CuO film was stable up to 2000 cycles in

0.1 M [Bmim][Cl]. Fig 5.B.4 (d) exhibited CV of CuO film up to the

2000 cycle in the voltage range of +0.7 to -0.4 V vs SCE at 40 mV/s

in 0.1 M [Bmim][HSO4]. The inset exhibited the variation of Csp as a

function of CV cycles. From this study it is concluded that CuO film

was stable up to 2000 cycles in 0.1 M [Bmim][HSO4].

(a) (b)


213

(c) (d)

Fig 5.B.4 The CV at different cycle number recorded for CuO in 1M

(a) Na2SO4, (b) [Bmim][OH], (c) [Bmim][Cl], (d) [Bmim][HSO4], and

the inset figure shows the specific capacitance vs cycle number

5.B.3 Experimental

5.B.3.1 Materials and methods

The chemicals copper oxide, ruthenium nitrate, 1-methyl

imidazole and 1-chloro butane were purchased from Sigma Aldrich.

While, potassium bromide, dichloromethane, absolute ethanol and

conc. sulphuric acid were purchased from Runa, India and used

without further purification.

5.A.3.2 Instrumental details and their operational conditions

SEM analysis

SEM images were recorded using a scanning electron

microscope (Model JEOL-JSM-6360, Japan) operated at 20 kV.


214

FT - Raman analysis

The FT - Raman experiments were conducted using Bruker

made (Germany – Multi RAM) with an excitation wavelength (1064

nm) by Nd: YAG source with Ge detector. In this system quartz filter

is used. The scanning range of FT – Raman was 200 – 4000 cm-1

with resolution 4 cm-1.

Cyclic voltammetry analysis

The cyclic voltammetry (CV) experiments were conducted

using an electrochemical analyzer (CH instruments) and the potential

was swept with respect to SCE.

5.A.3.3 Preparation of CuO films

The colloidal solution method was used to deposit the

nanostructured CuO thin film. The following synthetic route was

followed to obtain monodispersed and highly stable CuO colloidal

solution at low temperature. The colloidal solution of CuO was

prepared by dissolving 0.05 g of copper acetate in 20 mL

dimethylformamide (DMF) to form 0.0125 M solution (solution A),

which was subsequently heated at 120oC for 20 min. with constant

stirring using a magnetic stirrer. The CuO film was deposited on steel

substrate by spin coating technique at 3000 rpm. The surface

morphology of the film was examined by analyzing the SEM images.

The potentials were swept with respect to SCE in Na2SO4,

[Bmim][OH], [Bmim][Cl] and [Bmim][HSO4] electrolyte and referred

as Na2SO4, [Bmim][OH], [Bmim][Cl] and [Bmim][HSO4] respectively.

5.B.3.4 Synthesis of ionic liquids

A mixture of 1-methylimidazole (0.2 mol) and 1-chlorobutane

(0.3 mol) was charged into a 150 mL round bottom flask. The mixture

was stirred at 65 -70oC for 48 h in presence of ethanol to get viscous


215

yellow colored liquid i.e. [Bmim][Cl]. The solvent is removed from the

resulting yellow colored liquid under reduced pressure. Finally, the

product was washed with ethyl acetate (3 x 10 mL) to remove polar

impurities and dried in vacuum desiccator at reduced pressure to

remove the volatile solvents [1].

The [Bmim][OH] was prepared by introducing solid potassium

hydroxide (25 mmol) in to a solution of [Bmim][Cl] (25 mmol) in

anhydrous methylene chloride (10 mL). Then, the mixture was stirred

vigorously at room temperature for 10 h. The precipitated KCl was

filtered off, and the filtrate was evaporated to leave the crude

[Bmim][OH] as a viscous liquid that was washed with ether (3 x 10

mL) and dried in vacuum desiccator at reduced pressure to remove

the volatile solvents [2].

The [Bmim][HSO4] derived from [Bmim][Cl] were obtained by

a dropwise addition of one equivalent of concentrated sulphuric acid

(98%) to solution of the corresponding [Bmim][Cl] in anhydrous

methylene chloride. The reactions proceed at 60 - 65oC for 12 h with

vigorous stirring. Then, the mixture was dried in vacuum by a rotary

evaporator to remove the HCl and solvent to obtain the viscous clear

[Bmim][HSO4] [3].


216

5.B.4 Conclusion

We concluded that, this is the first comparative study of

imidazolium based ionic liquids with same cation that is [Bmim] and

different anions such as chloride (Cl-), hydroxide (OH-) and hydrogen

sulfate (HSO4-) as electrolyte for CuO based supercapacitor. The pH

of the electrolyte is varied with different anions such as basic (-OH),

neutral (-Cl) and acidic (-HSO4). The highest Csp was observed for

[Bmim] [Cl]. The largest stability was also observed for [Bmim][Cl].

[Bmim][HSO4] exhibited relatively lower Csp and stability than [Bmim]

[Cl]. While, [Bmim][OH] exhibited lowest Csp as well as stability.

Here, we conclude that Csp and stability did not get affected by

change in pH.


217

5.B.5 References

1] Dupont J, Consorti CS, Suarez PAZ, de Souza RF, Org,

Synth., 2004, 10, 184; 2002, 79, 236.

2] Ranu BC, Banerjee S, Org. Lett., 2005, 7 (14) 3049.

3] Wang W, Shao L, Cheng W, Yang J, He M, Catal. Commun.,

2008, 9, 337.

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