chapter 4 development of supercapacitor with various forms...

CHAPTER 4

Development of Supercapacitor with Various forms of Carbon

and its Polymeric Derivatives

4.1. Introduction

The development of highly capacitive carbon electrodes which are

chemically modified with a metal oxide yielding superior stability is dealt in

current chapter. The main disadvantage observed for polymer/graphene composite

electrodes discussed in chapter 3 is its instability. A great interest has been

focused on carbon materials for supercapacitive applications due to their easy

availability, processability, relatively low cost and stability. Carbon is an electric

double layer capacitive (EDLC) material which is non faradic but faradic nature

can be induced by chemical modification. This piece of work deals with the

chemical modification on activated carbon, Graphene, carbon nanotubes by the

introduction of faradic components like metal oxide and found an improvement in

the effective capacitance, operating voltage and stability. In this context the

mechanism for energy storage is based on two components; fast faradic reaction

between electrode materials and electrolyte (Aqueous based electrolyte), which

gives rise to the “Pseudo” capacitance and “Double layer interaction” between

electrode and electrolyte. Since it combines both in a single electrode the

developed system is hybrid in nature and the electrode under goes faradic and

non- faradic processes to store charge. Aim of this study is to develop a storage

system which could eliminate the draw backs of graphene/polymer capacitor

without compromising its advantageous features. The reason for the lesser

stability of the latter is due to the presence of polymer which undergoes

irreversible chemical reaction upon charging and discharging. We tried to replace

polymeric part by metal oxide - Nickel oxide (NiO) which can undergo reversible

faradaic reaction for large number of cycles.

Electric double layer capacitors have capacitance value up to thousands of

Farads with the same size as for conventional capacitors. Research efforts have

Development of Supercapacitors with various forms of carbon….

Amrita Centre for Nanosciences and Molecular Medicine 130

been done to increase the specific capacitance of supercapacitor electrodes with

different forms of carbon material like graphite, activated carbon, CNT’s,

graphene etc. By using activated carbon with high specific surface area; specific

capacitance values up to 380 F/g have been reported [1]. Figure 4.1 shows the

schematic illustration of a charged double layer capacitor; the application of

voltage across current collector develops corresponding charge on the electrode

materials then the oppositely charged ions from the electrolyte move towards the

charged surface and forms a double layer [2].

Figure 4.1: Schematic illustration of a charged double layer capacitor. 1 and 2

current collector; 3 and for electrode material; 5-6 seperator; 6-electrolyte; 7-pores

in the electrode material; 8-positive charge ; 9-negative ion; 10-negative charge;

11-positive ion [2].

For the same size of EDLC, the only way to increase the capacitance value

is by considerably increasing the exposed surface area of the electrode; this can be

achieved if porous electrodes are used. The exposed surface area (2000-2500

m2/g) of the carbon to the electrolyte is very high. Electrochemical capacitors

with porous electrodes show higher capacitance value only because of the porosity

of the system.

The key factors determining the performances of electrochemical

capacitors are specific surface area, porosity, properties of electrolyte; these



properties are crucial in achieving high energy and power density. In electric

double layer capacitors (EDLCs), the sizes of cation and anion of the electrolyte

are significant factors in the adsorption of the ions into the pores of the carbon-

based electrodes. EDLCs store the electric charge directly across the double layer

of the electrode [3], since the carbon form has a porous structure allowing the

electrolyte ions to pass through and develop a double layer at exposed surface in

electrolyte. This EDLC is termed as a true capacitance effect since there is no

charge transfer across the interface. In certain cases, surface charges can be

produced due to the surface dissociation, ion adsorption from the electrolyte and

crystal lattice defects. The capacitance contributed by EDLC is similar to that of a

parallel plate capacitor. If an excess or deficit of ions is produced at the electrode

surface, counter ions in the electrolyte build up near the surface of the electrode,

so as to offer electro neutrality [4].

Porosity of the electrode material and size of electrolyte ion is critical in

the power and energy density of the capacitor. Micropores with a diameter of the

order of 1 nm have significant contribution to a high specific surface area, but

access of electrolyte ions in such pores may be difficult. Appropriate distribution

of pore sizes from micropores (< 2 nm) to mesopores (2-50 nm) for a given

surface area and such an electrode can offer high capacitance value, power and

energy density. The ion mobility in micropores could be several times lower than

in the bulk electrolyte. Another factor is the wet-ability of the pore surface by the

electrolyte.

We selected Graphene and carbon nanotubes as the double layer

component and Nickel oxide (NiO) as the faradaic constituent. The attractive

features of graphene are already mentioned in section 3.3. In brief graphene has

high electronic conductivity, low mass density, very high specific surface area

(2630 m2g-1). Graphene consisting of a two-dimensional sheet of covalently

bonded carbon atoms finds a multitude of applications in devices [5]. The very

high in-plane conductivity and surface area makes it an attractive material for use

in double layer supercapacitors [6]; have very high power density, energy density

and long cycle life. Carbon nanotubes (CNT’s) can be either single-walled or

muti-walled. Single walled CNTs conduct better than the muti-walled due to their



efficient ballistic transport mechanism. However, with respect to synthesis, muti-

walled CNTs are promising. Also the aspect ratios of these nano tubes play a

crucial role in their conductivity. Carbon nanotubes are an interesting class of

materials for EDLC with its good mechanical strength and electrical properties

(discovered by Katakabe et al) [7]. CNT based capacitor electrodes are made of

entangled mat of nano tubes which are mesoporous in nature. Since these nano

tubes are interconnected, it offers a continuous charge distribution which uses

almost all of the available surface area for storage. Also the electrolyte interaction

is quite good because of the mesh like structure. The pore diameter has a great

influence on the conductivity of these CNTs as reported by Chmiola et al [8]. The

reported specific capacitance of single-walled CNT is 180 F/g in KOH electrolyte.

The presence of mesopores in electrodes based on CNTs has a significant effect,

due to the central canal and entanglement enabling easy access of ions from

electrolyte.

Transition metal oxides are considered to be the best candidates for

electrochemical capacitors; they have high specific capacitance coupled with very

low resistance resulting in a high specific power, which makes them very

attractive in commercial applications [9]. Among the transition metal oxides,

RuO2, MnO2, NiO are most promising electrode material due to its high specific

capacitance, long cycle life, high conductivity, and good electrochemical

reversibility. Among Transition metal oxides based capacitor RuO2 showed high

specific capacitance ~1340 F/g [10], but the lack of abundance of RuO2 and cost

of the precious metal (Ru) are major disadvantages for commercial manufacture of

RuO2. Most attention has been focused on hydrous manganese oxide and nickel

oxide [11-13] due to the low cost of raw material and the fact that they are more

environmentally friendly than any other transition metal oxide systems.

By considering all the factors discussed above we selected Nickel oxide

(NiO) as the faradaic constituent with CNT or graphene as the non faradaic

component to develop a hybrid capacitor. Detailed study on the specific

capacitance, area capacitance and operating voltage for different hybrid capacitors

in both symmetric and asymmetric configurations were carried out and further

best storage system was selected for the integration with solar cell. It was already



reported that the operating voltage of a capacitor depends on the nature of

electrolyte (specifically on decomposition voltage of electrolyte) [14]. Aqueous

solutions are potentially beneficial because of low cost, safety, long lifetime and

low internal resistance due to their better conductivity for storage of excess power

but operating voltage in aqueous electrolyte is lower with respect to organic

medium, which provides voltage ranging from 3 to 5 V [14].

4.2. Activated carbon/NiO Hybrid capacitor

Activated carbons are produced by acid treatment of ball-milled graphite,

which causes exfoliation of carbon due to the negativity of the nitrate ions in the

acid and the intrinsic negative charge of carbon; activation of carbon proceeds by

the introduction of dangled bonds. Generally they have specific surface area of

800-1500 m2/g, which allows many sites for electrolyte interaction for the double

layer formation. Surface area is generally improved by the development of

porosity in the bulk of carbon materials, leading to a porous arrangement inside

the carbon particle. Unfortunately, there is no simple linear association between

the surface area and the capacitance. Indeed, more than the total porous volume, it

is the way this porosity is created i.e., the control of the pore size as well as the

pore size distribution has a great impact on the carbon capacitance. Activated

carbon has very fine pore size and higher specific surface area. Defects are

produced in the carbon due to acid treatment and milling results in activation of

the carbon. Our group has studied on milling time, size of activated carbon and its

influence on the capacitance. We patented this study [15]. I have taken this work

as the base study and extended with CNTs and Graphene.

4.2.1. Synthesis and characterization of Activated carbon (AC)

Wet ball milling method was implemented so as to avoid heat generation

during the milling process. Acetone 99% pure (Nice Chemicals) was used as the

medium. 2 g of milli sized raw carbon in 10 ml of Acetone and the rotational

speed was set to 300 rpm for 4 hrs. Following milling, 1 g of milled carbon was

mixed with 20 ml of Nitric acid (Fischer scientific) and was sonicated for 3 hrs at

60ºC. And then an equal volume of Sulphuric acid 98% pure (Nice Chemicals,

India) was added and sonicated again for 3 hrs. Finally it was washed to remove



traces of acid from the carbon particles. Then, the washed carbon particles were

air dried at 70⁰C in hot air.

4.2.2. Synthesis of Activated carbon (AC) electrode

Electrophoretic deposition: Electrophoretic bath was prepared with 5 mg

of activated carbon with 0.4mM Ni (NO3)2 in iso-propanol and ultrasonicated for

1 hour. Deposition carried out in a two electrode set up, Ti plate as the cathode

and platinum as anode with 15 V for 600 seconds. Activated carbon gets deposited

over cathode because of the Ni2+ ions. When this electrode is annealed in oxygen

atmosphere; Ni2+ coated on activated carbon gets converted to NiO.

4.2.3. Characterization of Activated carbon (AC) electrode

SEM:

Figure 4.2 shows the SEM image of 4 hours ball milled and acid treated

graphite showing particle size ~500 nm.

Figure 4.2: SEM image of 4 hours ball milled EPD deposited activated carbon.

4.2.4. Study on the performance of AC electrode

Cyclic voltammetry:

Cyclic voltammetry was carried out in a three electrode set up with 0.1M

aqueous KOH as the electrolyte, activated carbon coated Ti plate as anode,

platinum wire as cathode and the saturated calomel electrode as the reference

electrode. A potential window of -1.5 to 1.5 V is applied of mass 1 mg for

different scan rates from 10 mV/sec to 100 mV/sec. Capacitance was calculated

from the reduction current (As per equation 3.1), which showed specific



capacitance of 380 F/g (Figure 4.3) and area capacitance of 380 mF/cm2 at 10

mV/sec.

Figure 4.3: Cyclic Voltammetry of activated carbon at different scan rates.

Apart from double layer formation there would be some faradaic reaction

due the traces of Nickel oxide (NiO) present in the activated carbon electrodes.

NiO under goes redox reaction with the OH- ions present in the aqueous 0.1M

KOH in appropriate potential window. During reverse forward scan oxidation

occurs and reduction occurs in reveres scan, the reaction is given below.

NiO + OH- NiOOH + e-

The oxidation and reduction potential of this reaction is +0.5 V/ +0.7 V

and -0.5 V/ -0.7 V [16]. It is clear that at higher scan rate oxidation peak is not

prominent, at high scan rate not all part of the material in electrode will not get

enough time to undergo reaction

4.2.5. Charge-Discharge

Charging and discharging was carried in a three electrode set up with AC

as the working electrode, platinum counter electrode and standard calomel as the

reference electrode. A constant current of 10 μA is applied and the voltage

variation with time is plotted (Figure 4.4). Upon charging the voltage goes up to

0.23 V and a discharging current of 10 μA was applied after 5 seconds and found

that the voltage decays to zero, this experiment shows the charge storage capacity



of AC. From the cycling of CV (Figure 4.5) we could recognise the stability of the

system.

Figure 4.4: Charging and discharging of AC electrode in three electrode system

with charging and discharging current of 10 A/cm2.

Figure 4.5: Cycling of CV 25 times with same AC electrode in three electrode

system.

The main trends observed in this study are: as the ball milling time

increases, the size of the AC reduces and capacitance is observed to increase with

reduction in particle size (Maximum specific capacitance of 1071 Fg-1 and area



capacitance of 0.48 Fcm-2). This effect may be due to the fact that enhancement in

surface area and active pore sites for charge storage and capacitance. However,

the main focus of my work is on other allotropes of carbon (CNT and graphene),

which has the potential to improve capacitance remarkably as discussed below.

Energy density is 61Wh/Kg, while power density is 5.7 kW/Kg.

4.3. Graphene/NiO and CNT/NiO hybrid Supercapacitors

In this system Activated carbon is replaced by CNT’s and Graphene (as

the double layer part) and nickel oxide acts as the faradaic component. Both the

systems Graphene/NiO and CNT/NiO were developed by electrophoretic

deposition followed by annealing and are characterized for its morphology. The

performance like capacitance, operating voltage and stability were studied in

detail.

4.3.1. Synthesis and characterization of Graphene/NiO capacitor electrode

The graphene which we characterized and used for the preparation of

Graphene/Ppy composites (section 3.3.1) was taken as the raw material. An

electrophoretic bath was prepared with 5 mg of graphene with 0.4 mM Nickel

nitrate (Ni(NO3)2) in iso-propanol and is ultrasonicated for 1 hour to get a uniform

dispersion .



Figure 4.6: SEM images of (a) & (b) electrophoretically deposited graphene and

(c) is the Raman spectra of graphene

Deposition was carried out in a two electrode set up. Ti plate was taken as

the cathode and platinum as anode with 15V for different duration and electrode

was annealed for 1 hour in air at 100oC.

SEM:

Figure 4.6 (a) & (b) shows the SEM images of electro-phoretically

deposited graphene. Graphene gets deposited over cathode because the Ni2+ ions

decorate graphene. When this electrode is annealed in oxygen atmosphere; Ni2+

coated on graphene gets converted to NiO. Deposited graphene forms an island

kind of structure decorated by NiO over titanium plate, the thick ness of

deposition was (8~9 µm).

Raman signature:

Figure 4.6(c) shows the Raman spectrum of Graphene, in-planar optical

vibration of carbon atoms is observed with the frequency appearing at

approximately 1583 cm-1. The other Raman modes are: D-mode, is a disorder-

activated Raman mode seen are at 1350 cm-1 (D-mode), second-order Raman

scattering 2680 (2D- or D*-mode), 2950 (D+G-mode), and 4290 cm-1 (2D+G-

mode). Detailed discussion carried out in section 3.3.1.

4.3.2. Mechanism of charge storage

It is claimed that the total capacitance of the pseudocapacitor is contributed

by the double layer and faradaic reaction. Generally, in a perfect formation of

electric double layer, the equilibrium reaction at the negative electrode during

charging is simply presented as

C+K+ C/K+ ------------------- (4.1)

Where C represents the carbon face, K+ is the ion of the electrolyte which forms

double layer at the carbon-electrolyte interface [17]. The process of double layer

formation is due to a simple physical adsorption, by electrostatic forces between

the carbon surface and the ions. In essence, the amount of ions involved in the



construction of a double layer matches the charge density on the electrodes. On

discharging double layer collapses and ions goes back to electrolyte.

Nickel oxide (NiO) traces on the carbon electrodes under goes an

electrochemical reaction with the ions present in the electrolyte. Here OH- ions

present in the aqueous KOH under goes redox reaction with the reactive NiO in

appropriate potential window.

NiO + OH- NiOOH + e- [16] ------------------- (4.2)

By this reaction charges can be admitted or released from the electrode to

the external circuit through electrode/electrolyte interface. Thus the total

capacitance is the sum of both faradaic and non faradaic reaction. Cyclic

voltammogram gives the overall capacitance; ideal double layer capacitance

behaviour of an electrode material is expressed in the form of a rectangular shape

of the voltammogramm. In this type of energy-storage, the phenomenon is solely

electrostatic and current is independent on potential. On the other hand, electrode

materials with faradaic capacitance properties shows a deviation from the

rectangular shape with reversible redox peaks connected with it. The capacitance

can be calculated from the reduction current by

dtdVIC / -------------------------------------- (4.3)

4.3.3. Effect of graphene deposition time on capacitance

The time of electro-phoretic deposition of graphene was varied from 10

minutes to 60 minutes; the developed Hybrid capacitive electrodes were

characterized by cyclic voltammetry to study its charge storage behaviour. Cyclic

voltammetry was carried out in a three electrode set up with 0.1 M aqueous KOH

as the electrolyte, graphene coated Ti plate as anode, platinum wire as cathode and

the saturated calomel electrode as the reference electrode. A potential window of -

1.5 to 1.5 V is applied.



Capacitance is calculated from CV curve and found that initially the area

capacitance increases with graphene deposition time and reaches maximum and

further deposition reduces the area capacitance (Figure 4.7).

Figure 4.7: (a) Cyclic Voltammetry of Graphene/NiO for different duration of

electrophoresis at 10 mV/sec (b) variation in area capacitance with time of

electrophoretic deposition.



This may be explained as: initially the mass of material increases with time

of deposition hence more faradaic sites can undergo reversible reaction but after

optimum time of deposition further deposition reduces the area capacitance. This

reduction may be due to the inaccessibility of initially deposited material to the

electrolyte due to the over deposition i.e. over deposition leads to the reduction of

pores and densification of deposited layer. Typically, reactive ions have to diffuse

from the bulk phase of electrolyte into mesopores and then to the micropores of

electrode to undergo reaction. The external mesopore surface in electrode would

lead to an improved electrolyte transport in pores, promoting the accessibility of

molecules into the active sites for reaction. Over-deposition would close the pores

and reduces the active sites for reaction, hence capacitance reduces.

It was observed that maximum area capacitance obtained with

Graphene/NiO system is for 50 minutes of graphene deposition and the

corresponding maximum area capacitance is 466 mF/cm2. Figure 4.8 shows the

Cyclic Voltammetry of 50 minute deposited sample with scan rate from 10

mV/sec to100 mV/sec, the capacitance is calculated for 10 mV/sec scan rate and is

found to be 466 mF/cm2 and specific capacitance is found to be 1165 F/g.

Figure 4.8: Cyclic Voltammetry carried out for Gr/NiO sample for 50 minutes of

deposition.



Electrochemical impedance analysis: Impedance spectra of Graphene/NiO on Ti

plate were carried out using Autolab potentiostat/galvanostat and data recorded

over a frequency range of 0.1 Hz to 1 MHz with an ac voltage of amplitude 10

mV.

Figure 4.9: Nyquist plot of Graphene/NiO film on Ti plate.

Impedance analysis was carried out applying a constant bias at open circuit

voltage (OCV). The result of an impedance analysis is represented by Nyquist

plot. The presence of an arc in the plot represents an electrochemical interface

existing in the system. Experimental results on interfacial resistance are shown in



figure 4.9, from impedance analysis radius of the circle shows the equivalent

series resistance is nearly 8 .

4.3.4. Cycling stability

The electrochemical stability of graphene/NiO is studied by cycling of CV,

25 cycles of CV is carried out for same electrode. It was found that variation in

reduction current and change in shape of CV is very less, which indicates the

graphene/NiO electrode show better stability compared to polymer systems

(Figure 4.10). Hence this system can be considered to make practical devices.

Figure 4.10: Cycling studies: 25 cycles of CV for same graphene/NiO electrode.

4.3.5. Charge-discharge of single Graphene/NiO electrode

Charge storage capacity of an electrode was carried out in three electrode

cell set up with platinum as counter electrode, standard calomel (SCE) as

reference electrode and carbon deposited Ti plate as working electrode. Figure

4.11 shows the charging and discharging behavior of Graphene/NiO electrode. A

charging current of 5 mA/cm2 is applied and voltage variation with time is noted.

Upon charging the voltage of the electrode (measured with respect to SCE) is

started to increase and reaches maximum (0.57 V) (i.e, 0.814 V wrt S.H.E) and

remains in maximum as long as current is applied. After 100seconds of charging a



forceful discharging is made by applying a discharging current of -100 A/cm2

and voltage variation of electrode with time is noted. It took around 260 seconds

to drain out the complete stored charges. The discharge curve shows initial slow

discharge followed by fast discharge. The current-time plot of electrode is studied

by applying a constant external potential of 0.57 V with respect to SCE and graph

is shown in blue color.

Figure 4.11: Voltage-Time behavior of Graphene/NiO electrode upon charging

(Ic=5 mA/cm2) and discharging (Id=100 µA/cm2), blue line shows the current-time

behavior of electrode on charging at 0.57V w.r.t SCE.

Voltage variation and current variation upon charging of a capacitor is given by

the equation

V=V0 (1- exp-τ/t)

I= I0 exp (-τ/t)

Where τ is time constant, t is time

From these equation, It is clear that at time t=0 I=I0, as time goes current

decays exponentially. Similar behavior is observed with Graphene/NiO electrode.



While charging, maximum current flows initially and it decays exponentially with

time. Maximum initial current indicates that the rate of charge flow is high at the

beginning and these charges are getting stored at the electrode. As the time passes

the number of sites available in electrode for storing charges reduces and the rate

of flow of charges starts to decrease and hence current decreases with time. The

indication of minimum current is the saturation of charges by then capacitor

voltage would have reached its maximum V0. Hence further flow of current would

not lead to the storage in the electrode Further storage can be established if and

only if the charges stored in the electrode are discharged.

4.3.6. Charge-discharge of symmetrical Graphene /NiO capacitor

In this symmetrical model, an electrochemical capacitor is formed by two

similar electrodes in an electrolytic solution separated by a distance. We have two

electrodes capable of storing connected in series hence the overall capacitance C

is determined by the series equivalent circuit consisting of anode capacitance Ca

and cathode capacitance Cc according to the equation [18].

cC

aCC

111 ------------------------------------------ (4.4)

Where Ca is anodic capacitance; C c is the cathodic capacitance.

The oxidation and reduction potential of the reaction 4.2 is +0.5V/+0.7V and -

0.5V/-0.7V [Delichère P, Joiret S, Hugotle Goff A, Bange K and Hetz B,

Electrochromism in Nickel Oxide Thin Films Studied by OMA and Raman

Spectroscopy, J. Electrochem. Soc. 1988, 135, 7, 1856].

Charging voltage of anode and cathode are expected to be same because the

electrodes used are supposed to be identical; From the figure 4.11 the fully

charged state shows voltage of electrode is 0.57V with respect to standard calomel

electrode (section 4.3.5). Also the potential would be higher (0.77V) with respect

to S.H.E.

Electrode capacitance of anode and cathode is supposed to be same and found that

466mF/cm2 (section 4.3.3).



Schematic representation of equivalent circuit of a symmetric capacitor is

included as Figure 4.21(a). Single electrode would be composed of a capacitor

with an effective internal resistance in series. When a symmetric capacitor is

assembled, two electrodes come in series then the total internal resistance (R)

would be double, capacitance would be half (C), voltage capacity adds up.

Figure 4.12: (a) Schematic representation of Equivalent circuit of symmerit

capacitor with cathode and anode. (b) Voltage-Time behavior of Graphene/NiO

symmetrical capacitor upon charging (Ic=5mA/cm2) and discharging (Id=100

µA/cm2), blue line shows the current-time behavior of electrode on charging at

2.2V.

Because of decrease in capacitance and increase in resistance the net

voltage would increase by a factor between1.5 and 2.Thus based on single



electrode voltages the net voltage developed in the symmetrical system is

expected to be between 1.3 to 1.4 V. Parasitic resistances/capacitances due to

interface effect can further increase this voltage and this explains our observation

of our working voltage about 1.6V.

R=R1+R2

C=CaCc/(Ca+Cc),

One of the reported works with MnO2/activated carbon showed 2.2 V in

aqueous K2SO4. [Brousse T, Toupin M, Belanger D.A. A hybrid activated carbon -

manganese dioxide capacitor using an aqueous electrolyte, J. Electrochem. Soc.

2004, 15, A614].

Charge storage capacity of symmetrical capacitor was carried out by

constructing a physical capacitor with two identical graphene/NiO electrodes, with

KOH as electrolyte. Figure 4.12(a) shows schematic representation of equivalent

circuit of a symmetric capacitor.Figure 4.12(b) shows the charging and

discharging behavior of Graphene/NiO symmetrical capacitor; charging current is

5mA/cm2, discharging current applied is 1 mA/cm2. The maximum voltage

developed across this capacitor is 2.2 V. The symmetrical capacitor showed higher

voltage storage capacity of 2.2 V in comparison to single electrode. The increment

in voltage may be due to the contribution from both the electrodes. The charged

capacitor retains its maximum voltage without any fluctuation. Discharging

current of 1mA/cm2 is applied after 100 second and voltage with time variation is

noted. The decay of voltage is found to be almost linear. Around 0.3 V is dropped

by self discharge.

The energy density and power density of the symmetrical electrode is

calculated and found to be 23 Wh/Kg and 1.04 kW/Kg respectively. In conclusion

Graphene/NiO system gave area capacitance maximum 466 mF/cm2, specific

capacitance of 1165 F/g, and energy density of 23 Wh/Kg and Power density of

1.04 KW/Kg.

4.4. Development of Carbon nanotubes/Nickel oxide supercapacitor system

Since the invention of multiwalled/single walled carbon nanotubes

(MWNTs/SWNTs) by Iijima [19], have attracted huge interest because of their

exclusive structural, mechanical and electronic properties and also high chemical



and thermal stability, high elasticity, high tensile strength with some tubes

exhibiting metallic conductivity [20]. Their small size with conductivity implies

that they can also be regarded as the smallest possible electrodes with diameters as

small as less than one nanometer [21-22]. Conceptually, carbon nanotubes can be

consider as all sp2 carbons arranged in graphene sheets, which have been rolled up

to form a hollow tube and can have lengths ranging from tens of nanometers to

several microns [23] Carbon nanotubes (CNTs) have begun to attract vast interest

in electrochemistry because of their small size and good electrochemical

properties. The majority of studies have used ensembles of CNTs to nanostructure

macroscopic electrodes either with randomly dispersed nanotubes or with aligned

carbon nanotubes. We tried to exploit the features like high surface area and

conductivity for the preparation of super capacitor electrodes. Instead of well

aligned CNTs, electrodes with randomly oriented CNT’s (modified with NiO)

having very good porous morphologies were fabricated for super capacitor

application.

4.4.1. Synthesis and characterization of CNT/NiO capacitor electrode

Electro-phoretic deposition of CNT:

5 mg of MWCNT were dispersed in iso-propyl alcohol and 0.4 mM Ni

(NO3)2 were added to this dispersion and ultra sonicated for 1 hour to get a

uniform dispersion. Mechanically polished Ti plate was taken as the substrate for

the deposition; a negative voltage (15 V) is applied to Ti plate with respect to

platinum counter electrode in an electrochemical bath set up and a homogeneous

coating of CNT on Ti plate was obtained which was annealed in air for 3hrs at

100oC. Time of deposition of CNTs was varied from 15 to 85 minutes to get

different thickness and film porosity. CNT gets deposited over cathode because of

the Ni2+ ions decorate CNTs.

SEM:

Figure 4.13 shows the SEM images of electrophoretically deposited CNTs

on Ti plate. It is clear from SEM that the deposited CNT forms an interconnected

highly porous network structure over the titanium plate and the low magnified

image shows the uniformity of the film. From the image it is obvious that the

CNTs entangled mat formed have interconnected nano tubes which offer a



continuous charge distribution using almost all of the available surface area. Also

the electrolyte interaction is quite good throughout the volume because of the

mesh like structure

Figure 4.13: SEM images of electrophoretically deposited CNTs.

Raman signature:

Raman spectra of CNT, (Witec Raman 300 RA and Spectrophotometer

UHTF) were obtained using a Raman spectrometer at 488 nm. Figure 4.14 shows

the Raman spectra of CNT; which shows peaks at 1352 cm-1, 1575 cm-1 and 2698

cm-1.

Figure 4.14: Raman signature of CNT using a Raman spectrometer at 488 nm.



Raman spectra have been widely used to determine the diameter of

innermost carbon nanotube in multi walled CNT’s and diameter of single walled

nanotubes (SWNT) [24-27].

4.4.2. Effect of CNT deposition time on capacitance

The time of deposition of CNT is varied from 15 minutes to 85 minutes.

The developed Hybrid capacitive electrodes were characterized by cyclic

voltammetry to study its charge storage behaviour. In this system the total

capacitance is contributed by the double-layer and pseudo capacitance. Cyclic

voltammetry of each sample was carried out in a three electrode set up with 0.1 M

aqueous KOH (-1.5 V to 1.5 V) as electrolyte and it was found that as the time of

deposition increases the reduction current starts to increase which in turn shows

the improvement in area capacitance (same trend as what we observed with

graphene).

Initially as the time of deposition increases the available effective area

which can interact with electrolyte increases hence capacitance also increases and

reaches maximum for the optimum time of deposition. After optimum time,

further deposition reduces the porosity hence capacitance. It was found that the

optimum time for CNT deposition is 60 minutes above which the area capacitance

starts to fall. This reduction could be explained may be due to the reduction in

porosity upon over deposition of material. Figure 4.15 shows the CV of the 15

minutes to 85 minutes electro-phoretically deposited CNT electrodes at scan rate

of 10 mV/second and variation of area capacitance with time of electro-phoretic

deposition. The maximum area capacitance obtained is 799 mF/cm2 for 60 minutes

and the specific capacitance for this particular time of deposition is 799 F/g (Area

=0.98 cm2, Mass of CNT =1mg).



Figure 4.15: (a) Cyclic voltammetry of CNT/NiO for different duration of

electrophoresis at 10 mV/sec and (b) variation in area capacitance with time of

electrophoretic deposition.

Cyclic Voltammetry of sample (60 minutes EPD) is carried out for

different scan rate from 100 mV/sec to 10 mV/sec, which is shown in figure 4.16.

The area capacitance is obtained for 10mV/sec scan rate and is found to be 799

mF/cm2 and specific capacitance is found to be 799 F/g. The entangled network of

CNTs provides maximum interaction of electrolyte with electrode; hence total

volume could contribute for capacitance.



Figure 4.16: Cyclic Voltammetry carried out for CNT/NiO sample for 60 minutes

of deposition.

It was observed that area capacitance obtained with Graphene/NiO system

show maximum for 50 minutes of deposition (466 mF/cm2) which is lower than

the maximum area capacitance of CNT/NiO system (799 mF/cm2). The

improvement in area capacitance may be due to the formation of porous CNT

network where as the Graphene electrode shows an island kind of structure.

Porous network would enhance the interaction with electrolyte this could be one

of the reason for the improvement in capacitance. From practical point of view

both area capacitance and specific capacitance are important; higher the area

capacitance lesser would be the size of the supercapacitor for a given capacitance

rating where as higher specific capacitance material reduces the cost of the

device.

Electrochemical impedance analysis:

Impedence spectra of CNT/NiO on Ti plate were carried out using Autolab

potentiostat/galvanostat and data recorded over a frequency range of 0.1 Hz to 1

MHz with an ac voltage of amplitude 10 mV. Figure 4.17 shows the impedance

spectra of electrophoretically deposited CNT film which shows a series resistance

of 4 .



Figure 4.17: Nyquist plot of CNT/NiO film on Ti plate.


The stability of CNT/NiO based capacitor is studied with the help of

cycling of CV for the same sample at constant scan rate.

Figure 4.18: cycling studies: 25 cycles of CV carried out for CNT/NiO electrode.



Here CV is carried at 100mv/sec for 25 times (Figure 4.18) to check the stability

of the electrochemical stability and found that it has very good stability.

4.4.4. Charge-discharge of single CNT/NiO electrode

Charging and discharging of both single electrode and symmetrical

capacitor was carried out. Charge storage capacity of electrode was carried out as

discussed in section 3.2.4, Figure 4.19 shows the charging and discharging

behavior of CNT/NiO electrode; a charging current of 5 mA/cm2 current was

applied and it was found that it reaches maximum voltage of 0.53V and remains

constant. After 100 seconds a discharging current of 500 µA/cm2 was applied and

found that sudden discharge happened from 0.53 V to 0.43 V within a second and

remaining voltage drain out completely in 175 seconds.The current-time plot of

electrode is studied by applying a constant external potential of 0.53 V and graph

is shown in blue line. The plot shows maximum current flows initially and it

decays exponentially with time. Maximum current (225 A) flows initially which

indicates that the rate of charge flow is maximum at the beginning and these

charges are getting stored in the electrode. As the time passes the current starts to

decrease with time. The indication of minimum current is the saturation of charges

in electrode and hence further flow of current would not lead to the voltage

increment of the electrode.

Considering the self discharge CNT/NiO electrode system drops from 0.53

V to 0.49 V (0.04 V) (i.e. around 7% of the total stored voltage); where as the

graphene/NiO system showed self discharge of 0.57 V to 0.49 V (i.e. around 14%

of total voltage). From the impedance spectra the interfacial resistance of CNT

system (4 ) is half of the graphene (8 ) electrode. Due to low interfacial

resistance the voltage drop is less compared to graphene system.

Self-discharge does not only affect the energy efficiency of the EDLC by

loss of charge, but may also affect the life time of it. Reported study says there

are two possible pathways for self-discharge:

(i) Reaction-rate controlled leakage by irreversible decomposition of

electrode material or electrolyte solution.



(ii) Diffusion-limited leakage by a redox couple, giving rise to a shuttle

transport between the two electrodes. This redox species may be an impurity or an

intrinsic redox couple, being produced by reversible conversion of electrode

material or electrolyte solution (e.g. H+/H2). Only if self-discharge is due to an

irreversible electrode reaction (route i), then the electrode material and/or the

electrolyte solution are consumed and reaction products are accumulated inside

the cell, this type of process will influence the capacitors life time. On the other

hand, if self-discharge proceeds by means of a shuttle mechanism (route ii), there

is no net consumption of matter and subsequently no ageing.

Figure 4.19: Voltage-time behavior of CNT/NiO electrode upon charging (Ic=5

mA/cm2) and discharging (Id=500 µA/cm2), blue line shows the Current-time

behavior of electrode on charging at 0.57 V.

4.4.5. Charge-discharge of CNT/NiO symmetrical capacitor

Charge storage capacity of symmetrical capacitor was carried out by

constructing a physical capacitor with two identical CNT/NiO electrodes, with

KOH as electrolyte. Figure 4.20 shows the charging and discharging behavior of

CNT/NiO symmetrical capacitor; Charging current of 5 mA/cm2 is applied and



voltage variation noted (Figure 4.20). The maximum voltage developed across this

capacitor is 2.2 V. The symmetrical capacitor showed higher voltage storage

capacity of 2.2 V in comparison to single electrode (0.53 V w.r.t SCE). The

increment in voltage may be due to the contribution from both the electrodes.

Figure 4.20: Voltage-time behavior of CNT/NiO symmetrical capacitor upon

charging (Ic=5 mA/cm2) and discharging (Id=5 mA/cm2), blue line shows the

Current-time behavior of electrode on charging at 2.2 V.

When a symmetrical CNT/KOH/CNT capacitor is fabricated it was found

that the maximum voltage capacity is changed from 0.53 V (0.77 V wrt .S.H.E) to

2.2 V, and the capacitance would be the half of the single electrode capacitance ~

400 mF/cm2 as per (equation 4.4), similar to that of symmetrical graphene/NiO

system. The energy and power density were calculated as per section 3.2.4: and

found that the energy density is 21 Wh/kg and power density is 2.75 kW/Kg. From

the above study CNT system showed better capacitance, power density with very

good stability. The energy density is found to be almost equal to the energy



density of graphene/NiO system. Considering factors like area capacitance and

stability, CNT system showed better promise over graphene system.

It was already reported that the operating voltage of a capacitor depends on

the nature of electrolyte (specifically on decomposition voltage of electrolyte) [14,

18]. Aqueous based electrolyte show lesser operating voltage compared to organic

based electrolyte hence, the electrical energy accumulated in EC can be

significantly enhanced by the selection of an organic medium where the

decomposition potential of the electrolyte varies from 3 V to 5 V. In the

Graphene/NiO and CNT/NiO systems the chemically modified carbon electrodes

under goes an electrochemical reaction with the ions present in the aqueous KOH

electrolyte and the operating voltages of these systems are found to be in ~2 to 2.3

V range. Previous chapter showed the development of Graphene/Ppy and

Graphene/PEDOT capacitor electrodes in organic LiClO4 electrolyte. Chemical

modification of graphene with conducting polymer yielded a supercapacitor with

moderate area capacitance and very high specific capacitance. As far as size of the

device is concerned, area capacitance plays a more important role than specific

capacitance. The above discussed system [CNT/NiO] shows higher area

capacitance because of its interwoven network compared to graphene/NiO system.

Hence, we expected that replacement of graphene with CNT in the Graphene/Ppy

system would enhance high area capacitance and operating voltage of the system.

4.5. Synthesis of CNT/Ppy composite electrode supercapacitor in organic

electrolyte

We carried out our study with Ti electrode modified with entangled

network of CNTs and carried out electro polymerization of pyrrole on top of it to

make CNT/Ppy composite with higher operating voltage and good area

capacitance. Electrophoresis of CNT was carried out as discussed in section 3.3.1.

and was taken as the template for polymerization. Electro-polymerization was

carried out in an electrochemical bath containing 0.1M pyrrole in acetonitrile 1

mA /cm2 is applied for different duration from 300 seconds to 3000 seconds. The

morphology of the electrode was characterized by SEM and its performance was

studied by Cyclic voltammetry, charge discharge etc.



4.5.1. Characterization of CNT/Ppy composite electrode

SEM:

Figure 4.21 shows the SEM images of CNT/Ppy composite on Ti plate. From the

images, we can observe that each CNTs are surrounded by polymer.

Figure 4.21: SEM image of CNT/Ppy composite for 2300 seconds of deposition.

Effect of polymerization time in CNT/Ppy system on capacitance:

The charge storage behaviour of synthesized hybrid electrodes was

characterized by cyclic voltammetry. The time of polymerization varied from 300

seconds to 3000 seconds and CV of each sample is carried out in 0.1M LiClO4 in

acetonitrile in a three electrode set up with SCE as reference electrode. It was

found that as the time of deposition increases the area capacitance is found to be

increasing up to an optimum time and further polymerization reduces the area

capacitance; this trend is similar to what we observed with all earlier discussed

systems. Mechanism of charge storage in similar system is discussed in chapter 3

(section 3.2.2).

The nature of the CV curves (Figure 4.22(a)), shows enhancement in

capacitance with increasing polymerization time up to a maximum optimum value

and thereafter decreasing (Figure 4.22(b)). A plot of area capacitance versus time

of polymerization (or layer thickness) shows a gradual increase of capacitance up

to a maximum (2300 seconds) and thereafter it reduces. The above behavior could

be explained based on the polymerization process as in chapter 3. In all systems

the optimum time depends on the nature of carbon coated substrate.



Figure 4.22: (a) Cyclic Voltammetry of CNT/Ppy for different duration of

electrophoresis at 10 mV/sec, (b) Variation in area capacitance with time of



electrophoretic deposition and (c) Cyclic Voltammetry carried out for CNT/Ppy

sample for 2300 seconds of deposition.

The optimum time of polymerization for graphene coated Ti substrate is

1500 seconds where as on carbon coated Ti plate showed maximum area

capacitance at 2300 seconds of polymerization. This indirectly implies the

porosity of CNT coated Ti plate is more than graphene coated plate.

At very long times of polymerization, the oligomers compact the porous

network in a way that the 3-D network is lost and replaced by equivalent thin film

or 2-D morphology. This considerably lowers the faradiac reaction sites and leads

to a decrease in capacitance. In composite, polymer utilizes the extremely high

specific surface area of CNT network and this mode of polymerization is

beneficial by exposing maximum surface sites for faradaic redox reactions with

electrolyte. Figure 4.22(c) shows the CV of optimum time of polymerization

(2300 seconds) at different scan rates. The area capacitance is found to be

422mF/cm2 and specific capacitance for this particular time of deposition (2300

sec) is 508 F/g.

The mechanism of charge storage is already been discusse in section 3.2.2.

In brief, poly (pyrrole) (Ppy) [28] undergoes oxidation by the incorporation of

anion from the electrolyte (ClO4–) into the polymeric chain while discharging and

neutralization by release of the same. Oxidation is analogous to discharging and

neutralization is corresponding to charging [29].

Charging process:

Ppy x+: xClO4- + xe- Ppy + xClO4 ----------- (4.5)

Discharging process:

Ppy + xClO4- Ppy x+: xClO4

- + xe- ------ (4.6)

Apart from this faradaic reaction there is also exists the possibility of

lithium intercalation and de-intercalation [30].




The stability of CNT/Ppy based capacitor is studied with the help of

cycling of CV for the same sample at constant scan rate. Here CV is carried at

100 mv per second for 25 times (Figure 4.23) to check the stability of the

electrode and found that it has poorer stability when compared with graphene,

CNT/NiO systems.

Figure 4.23: Cycling studies: 25 cycles of CV carried out for CNT/Ppy electrode.

4.5.3. Charge-discharge of single CNT/Ppy electrode

Charging and discharging of both single electrode and symmetrical

capacitor was carried out. Figure 4.24 shows the charging and discharging

behavior of CNT/Ppy electrode; a charging current of 2 mA/cm2 current was

applied and found that it reaches maximum voltage of 1.1 V. This system showed

twice the voltage compared with aqueous system but it was found that the voltage

drops from 1.1 V to 0.9 V while charging itself. This voltage drop is an indirect

indication of instability of the system. After 100 seconds a discharging current of

1mA/cm2 was applied and found that gradual discharge takes place within 100

seconds. The current-time plot of electrode is studied by applying a constant

external potential of 1.1 V and graph is shown in blue line. The plot shows that



maximum current (6.5 mA/cm2) flows initially and it decays exponentially with

time.

Figure 4.24: Voltage-Time behavior of CNT/NiO electrode upon charging (Ic=2

mA/cm2) and discharging (Id=1 mA/cm2), blue line shows the Current-time

behavior of electrode on charging at 1.1 V.

4.5.4. Charge-discharge of CNT/Ppy symmetrical supercapacitor

Charge storage capacity of symmetrical capacitor was studied by

constructing a physical capacitor with two indistinguishable CNT/Ppy electrodes,

with organic LiClO4 as electrolyte. Figure 4.25 shows the charging and

discharging behavior of CNT/Ppy symmetrical capacitor; charging current is

5mA/cm2, discharging current applied is 10 A/cm2. The final maximum stable

voltage developed across this capacitor is 2.2 V.



Figure 4.25: Voltage-Time behavior of CNT/Ppy symmetrical capacitor upon

charging (Ic=5 mA/cm2) and discharging (Id=10 A/cm2), blue line shows the

Current-time behavior of electrode on charging at 7 V.

The symmetrical capacitor initially showed higher voltage storage capacity

~8V in comparison to single electrode (1.1V). Once after reaching the maximum

voltage, the voltage fluctuation occurs across the supercapacitor which could be

due the instability of the polymer. Apart from the voltage fluctuation, the self

discharge is found to be very high; the voltage finally settles down to 2-3 V. This

voltage decay shows the unsteadiness of the system. After 100 seconds a

discharging current of 10 A/cm2 was applied and found that high self discharge

occurs and finally settles to 2-3 V then a gradual decay within 200 seconds. Even

though the maximum developed voltage is high (7-8 V), it is too unstable to use

for a practical application.



4.6. Coupling Stability with voltage-Development of an asymmetric battery

type hybrid capacitor

A Symmetric capacitor uses similar electrodes in series in an electrolyte to

form a capacitor whereas an asymmetric capacitor includes two different types of

material in a single capacitor structure [31-33]. This system as we developed in

the below section refers to an “asymmetric” capacitor device, in which the

electrode materials used as cathode and anode are different. The phrase

asymmetric device derives from the US Patent 6,222,723 titled “Asymmetric

Electrochemical Capacitor and Method of Making” by Razoumov et al [34]. An

asymmetric system has two electrodes of different capacitors separated by an

electrolyte. Each capacitor is formed by either EDLC or by faradaic reaction. Our

aspect of study includes two electrochemical supercapacitor cells (composed of

different materials and compatible electrolyte packed separately) stacked in series.

The experimental evaluation was made on a CNT/Ppy electrode (electrode 1)

combined with a highly reversible CNT/NiO hybrid capacitor electrode (electrode

2). Since the first electrode and electrolyte is different from second electrode and

electrolyte they are packed in different compartments and connected by common

platinum electrode (as in figure 4.26). We were able to combine the properties of

both the systems, i.e. high operating voltage and high stability in a single system.

From the development and optimization of all earlier discussed

supercapacitor systems, CNT/NiO system showed higher area capacitance with

very good stability where as CNT/Ppy symmetrical capacitor showed very high

operating voltage with instability. We tried to couple the advantages of both CNT

based (renders stability) electrode and CNT-polymer (yields high voltage)

composite electrode through an asymmetric battery type hybrid capacitor

construction, i.e., one electrode is CNT and other one is CNT/Ppy. Since

CNT/NiO electrode is compatible with KOH (aqueous electrolyte phase) and

CNT/Ppy electrode is compatible with LiClO4 (an organic electrolyte), we

designed new symmetrical system as shown in figure 4.26, both the systems are

internally connected by a platinum electrode.



Figure 4.26: Schematic illustration of asymmetric capacitor.

Upon charging electrode 1, CNT/Ppy in LiClO4 will undergo oxidation by

absorbing ClO4- ion and releasing one electron (equation 4.7). This electron would

flow through external circuit to electrode 2: (CNT/NiO in KOH); there K + ions

will come and form a double layer by absorbing this electron (equation 4.8).

Platinum 1 electrode in KOH would undergo condensation reaction as per

equation 3 releasing 4 electrons; these electrons would flow to platinum 2. We

know that the stability of polymer is less upon oxidation the color of solution

starts to change to blue indicates presence of polymer in solution, the oxidized

polymer (equation 4.10) reduces back to polymer by accepting one electron and

get deposited on platinum 2 wire which is observable on platinum 2. Upon

discharging the opposite reactions are taking place.



When Electrode 1 is +ve w.r.t Electrode 2:

Electrode 1: CNT/ Ppy in LiClO4

Ppy + ClO4- [Ppy+: ClO4

-] +e- ---------------- (4.7)

Electrode 2: CNT/ NiO in KOH

CNT/NiO + K+ + e- [CNT/NiO] - : K+ ------------- (4.8)

Platinum 1: Pt in KOH

4(OH-) O2 + 2H2O + 4e- -------------------- (4.9)

Platinum 2: Pt in LiClO4

[Ppy+: ClO4-] in LiClO4

- solution+ e- Ppy + ClO4- ----- (4.10)

When electrode 2 is +ve w.r.t to electrode 1:

Electrode 1: CNT/ Ppy in LiClO4

[Ppy+: ClO4-] +e- Ppy + ClO4

- ------------- (4.11)

Electrode 2: CNT/ NiO in KOH

[CNT/NiO]- : K+ CNT/NiO + K+ + e- ------------- (4.12)

Platinum 2: Pt in LiClO4

Ppy (deposited on Pt) [Ppy+: ClO4-] to solution + e- ---- (4.13)

Platinum 1: Pt in KOH

O2 + 2H2O + 4e- 4(OH-) -------------------- (4.14)

Apart from these reactions there is possibility of Li intercalation (equivalent to

equation 4.11) and de-intercalation (equivalent to equation 4.7) and also reversible

reaction NiO with OH- ions in KOH which is shown in equation 4.15, 4.16. The

reaction 4.16 (NiOOH + e- NiO + OH-) is co-synonymous with equation 4.8



while charging, while reverse of reaction 4.16 (NiO + OH- NiOOH + e-) is

co-synonymous with equation 4.12 while discharging.

[Ppy+: ClO4-] +Li+ +e- [Ppy: Li+] + ClO4

- --------- (4.15)

NiO + OH- NiOOH + e- ------------------- (4.16)

Two various mechanisms of charge storage are realized in this asymmetric

electrochemical supercapacitor The use of a non-polarizable electrode raises the

voltage of a single cell and increases the working voltage window [35]. Energy

density depends on the square of the voltage (1/2 CV2) and the energy density of

asymmetric capacitor become 3-4 times, comparing to the symmetric one [36].

Here the asymmetric electrodes (CNT/NiO and CNT/Ppy) in asymmetric

(KOH and LiClO4) electrolyte system and are connected by platinum as common

electrode. One of electrolyte used is organic and it can go up to the 3-5 V. Since

both systems are connected in series the effective voltage would be the sum of the

voltage of organic and aqueous system. Series arrangement also increases the

internal resistance that could be the one of the reason for the higher self discharge.

The working voltage after self discharge is 4.5 V

4.6.1. Charging and discharging of asymmetric capacitor

The charging and discharging of the systems were carried by applying a

constant current and its voltage variation with time was noted and plotted below.

The charging current of 1 mA/cm2 is applied and found that voltage rises

maximum to 7-7.3 V and a discharging current is applied after 400 seconds and

found that a system shows self discharge of voltage to 4.5 V followed by a gradual

decay. From the graph it is evident that the system is fairly stable. No fluctuation

in voltage is found while charging. We were able to obtain a highly stable

supercapacitor, which can charge up to 7 V and can finally store 4.1 V (after the

self discharge) as shown in Figure 4.27. Energy density and power density is

calculated 390 Wh/kg, 36 W/Kg. Thus, we were able to develop a super capacitor

having high voltage and stability in asymmetric configuration.



Figure 4.27: Charge-discharge of CNT-CNT/Ppy asymmetric capacitor, charging

current of 1 mA/cm2 applied and found that it reaches maximum 7.3 V and

remains constant. A discharging current of 10 µA/cm2 is applied and found an

initial voltage drop to 4.1 V and remaining voltage drops in 1000 seconds.

Table 4.1: Shows the capacitance and operating voltage variation with electrode.

Table summarizes the capacitance, energy and power density of all systems

discussed above.



4.7. Integration of CNT/KOH capacitor with DSSC

Here, CNT/KOH super capacitor with maximum area capacitance,

moderate energy and power density was selected to combine with DSSC and we

made practical integrated module.

The output of the back side illuminated solar cell is fed to the CNT based

symmetric supercapacitor and photo charging and discharging was studied (figure

4.28(a)). Around 80% of Voc is found to be stored in supercapacitor. The

equivalent circuit model of this integrated structure is a solar cell in parallel with a

capacitor. When illumination starts, the voltage across the supercapacitor rises to

maximum. As long as illumination is on, the voltage remains at the maximum and

when illumination turns off, the voltage initially reduces and stabilizes. The

internal resistance of the supercapacitor causes an initial IR drop and around 20%

of Voc drops; remaining 80% gets stored in the supercapacitor. Figure 4.28(b)

shows the voltage variation in DSSC/CNT based supercapacitor-integrated

structure. Solar cell is illuminated for 50 seconds and it was found that Voc =0.57

V and ~0.45 V got stored in the supercapacitor.

Figure 4.28: (a) Photo-charging of CNT/KOH symmetric capacitor from back

side illuminated DSSC, Voltage variation across the capacitor and (b) Voltage

variation while discharging current of 10 µA/cm2 current applied.

The supercapacitor retained this voltage until a discharging current

applied. A discharging current of 10 µA/cm2 is applied after 400 seconds; then the

voltage across the capacitor has dropped to 0 V in 65 seconds.



In order to utilize the maximum storage efficiency of a capacitor we need

to have more than three DSSC. We have already shown the operating voltage of

CNT/KOH capacitor as 2.3 V. Similarly, symmetric CNT/LiClO4 capacitor and

asymmetric battery type hybrid capacitor requires very high solar voltage source

to charge it completely. Since single DSSC (Voc is < 0.7 V) can’t utilize the

maximum storage efficiency of this capacitor, we need to develop a practical

DSSC panel with three or more DSSC cells in series. Integration efforts were

directed by combining the best performing super capacitor with DSSC showing

almost 80% of energy storage. It was already stated that the integration is not only

limited to DSSC but can be extended to any energy generating units. Very high

voltage capacity capacitors were developed which can be integrated with high

efficiency solar cell for the maximum energy conversion and storage.

This study shows the easy method of development of high capacitive

composite supercapacitor electrodes using cheap raw materials like carbon and

polymers. Prospects for further study include an efficient design of an integrated

solar cell/storage panel with associated electronic circuitry and investigations into

effective packaging and circuitry schemes.

4.8. Conclusion

In conclusion we have developed different energy storage systems

with chemically modified carbon allotropes; each system has its own advantages

and drawbacks. The super capacitor was developed with chemically modified

carbon nanotube (CNT) and Graphene (with metal oxide (NiO)). These electrodes

show good stability, very high area capacitance (Upto 800 mF/cm2) and an

operating voltage of 2-2.3 V in aqueous KOH electrolyte. The operating voltage

mainly depends upon decomposition voltage of electrolyte. Normally, aqueous

electrolyte gives lesser voltage compared to organic electrolyte. Symmetric

capacitor with composite electrode of Ppy and CNT in organic electrolyte show

area capacitance of 420 mF/cm2 and very high charging voltage of 6-7 V, but

shows very high self discharge and lesser stability compared to CNT-NiO/KOH or

graphene-NiO/KOH system. We combined advantages of both systems by

developing an asymmetric battery-type hybrid capacitor in two-electrolyte system

and found that it can store voltage of 4.1- 4.3 V even after self discharge with



good stability. These studies helped us to develop stable supercapacitors with very

high operating voltage. We can select the optimized system to couple with a high

efficiency solar cell to provide maximum energy transfer and storage.

Integration efforts were directed by combining the best performing super

capacitor with DSSC showing almost 80% of energy storage. Very high voltage

capacity capacitors were developed which can be integrated with high efficiency

solar cell for the maximum energy conversion and storage.



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