Gold nanoparticle embedded paper with

mechanically exfoliated graphite as flexible

supercapacitor electrodes

Soumen Mandal,a,b* Aniruddha Pal,a Ravi Kumar Arun,a and Nripen

Chandaa,b

a CSIR-Central Mechanical Engineering Research Institute,

Durgapur, West Bengal, India.

b Academy of scientific and innovative research, New Delhi,

India.

*Corresponding Author: somandal88@gmail.com.

Abstract

We report a new method for the fabrication of flexible

supercapacitor electrodes wherein mechanically exfoliated

graphite on paper containing gold nanoparticles is used as

supercapacitor electrodes. It has been found that the

supercapacitor electrode made of paper containing gold

nanoparticle with graphite exfoliation yields a capacitance,

1

which is about 4.3 times higher than that of the electrodes

without gold nanoparticles.

Keywords- flexible supercapacitor electrode; paper electronics;

gold nanoparticle; graphite

1. Introduction

Supercapacitors have attracted immense attention as a component

for energy storage in the recent years due to their high

capacitance, exceptional energy density and cycle reliability.1,2

With the increasing demand of power storage requirements within

limited cost, flexible and disposable supercapacitors have come

into existence that have higher charge storage capacity as well

as good flexibility and can be used in micro-nanoscale devices

such as flexible sensors, wearable devices, roll up displays etc.

Supercapacitors consists of two electrodes with very high surface

area and a separator, which contains materials that either

contribute to EDL (electrochemical double layer) or pseudo

capacitance or both.3-5

Whereas commonly used materials for electrodes are activated

2

carbon, carbon nanotubes and exfoliated graphite prepared using

different physiochemical routes, generally electrolytes comprise

of organic or inorganic solvents such as acetonitrile, aqueous

solution of KOH etc.6-10 A number of reports are available which

emphasizes on various techniques for generating few layers of

exfoliated graphite and carbon nanotubes (CNT) 11 on flexible

polymeric substrates for fabrication of supercapacitor electrodes

like microwave exfoliation,12 thermal exfoliation,13 in-situ

reduction of graphene oxide to graphene using laser

irradiation,14 flame induced reduction to graphene.15 Exfoliated

graphite has a very high surface area (2630 m2/gm) whereas

theoretically, single walled CNTs have a surface area of 1300

m2/gm.16,17 However, the exfoliation process of graphite to prepare

supercapacitor electrode material is complex and CNTs are

difficult to be fabricated on polymeric substrates making the

fabrication process expensive. Furthermore, CNTs have a high

areal resistance, 18 which adversely affects the discharge

characteristics and energy density of the supercapacitor by

increasing the equivalent series resistance (ESR).

3

An approach to fabricate supercapacitor electrode using VACNTs

(vertically aligned carbon nanotubes) grown on Si substrate with

sputtered nickel and further transferring the entire system to

polycarbonate film has been reported recently.19 In this work the

sputtered nickel on VACNT was instrumental in reducing the

electrode resistance. This reported work indicated the

feasibility towards enhancing supercapacitor characteristics

employing conducting materials in the electrode matrix. However,

the reported device houses polymeric materials and uses

fabrication procedure like chemical vapour deposition technique

which requires exact process conditions. In order to enhance the

supercapacitor characteristics for a polymer free supercapacitor

electrode with easily exfoliated graphite, we have synthesized

supercapacitor electrodes with gold nanoparticles soaked in paper

substrate and subsequent controlled mechanical exfoliation of

graphite in this work.

In the present supercapacitor cell, gold nanoparticles embedded

paper substrate was mechanically exfoliated with graphite layers

and used as electrodes. Pencil lead was used as graphite

4

containing material and was mechanically rubbed on paper in

controlled fashion explained later. The gold nanoparticles

embedded in the paper matrix are used to decrease the resistance

of mechanically exfoliated graphitic layers (pencil lead) as

nanoparticles possess profuse number of free electrons generating

a charge pool. Pencil lead is a mixture of natural graphite and

clay wherein clay is used as a binder. It is a crystalline

material which exhibits fair electrical conductivity. The

crystalline nature of exfoliated graphite layers from pencil lead

on paper has recently been reported using TEM (transmission

electron microscope) images of pencil lead manually rubbed on a

solid substrate.20,21 The separator was a paper soaked in a Ni-Cu

hydroxide complex containing excess KOH. Ni-Cu complex was used

as these transition metal oxides and hydroxides posses high

pseudocapacitance,22 are of low cost, abundant and non-toxic. Use

of Ni hydroxide as a pseudo capacitive material is well known as

it possesses a high value of pseudo capacitance. The presence of

Cu along with Ni enhances the electrochemical activity of the

supercapacitor which may be attributed to the charge imbalance

and imperfections in the Ni-Cu lattice. In one of the earlier

5

studies,23 it has been demonstrated that Ni-Cu double hydroxides

have higher pseudo capacitance when compared to the individual

double hydroxides. The fabricated supercapacitor cell shows high

specific capacitance of the order 790 mF/cm2, which assures the

applicability of these simple devices in micro-nanoscale flexible

electronics.

2. Experimental

The fabrication of supercapacitor electrode employs the following

steps. Filter paper with 10 micron pore size (Whatman) was cut

into 1 X 1 cm pieces (area 1 cm2). Two pieces of the stated size

was dipped into AuNPs solution for 24 hours. The AuNPs were

prepared using 50.0 µl of 0.1M HAuCl4 and 5.0 mg/ml sodium

citrate solutions stirred at 90°C as in the reported

literature.24 The paper pieces were dried in an oven at 60 °C.

Graphitic layers were then exfoliated on the AuNP-paper surface

mechanically using a set up designed for this purpose (Fig. S1).

The set up consist of 2- motion stages that can move in two

directions and is similar to existing CNC (computer numerically

6

controlled) machines used in metal machining. A rectangular

shaped graphitic pencil lead (Flair company, 2B) was fixed on a

cartridge and was attached to the Z axis stage. An electronic

weighing system was rigidly fixed to the X-axis stage. The paper

containing AuNPs was placed on the electronic weighing system,

which rests on the X-axis stage. The Z axis stage was lowered

gradually to the touch the paper surface. This was continued till

the electronic weight showed a mass of 200 gm. The X-axis stage

was now actuated which generated graphitic layers as pencil

markings on the paper substrate. Unlike manual rubbing of pencil

on paper to generate uncontrolled growth of turbostratic

graphitic layers on paper surface, this method uses controlled

pressure (in terms of mass) to generate pencil markings. The set

up is shown in the ESI (Fig. S1).

The separator containing Ni-Cu hydroxide with excess KOH was

prepared using the same procedure which was used for AuNP-paper

preparation wherein Ni-Cu hydroxide with excess KOH was used

instead of AuNPs. Pencil markings were not present and the drying

process was not followed during this step. The detailed procedure

7

for synthesis of Ni-Cu hydroxide in excess KOH can be found in

the ESI. The fabricated electrodes and the separator were pressed

in a hydraulic press at 150 Kg/cm2 pressure for 1 minute. This

leads to a sandwiched structure ready to be tested for

supercapacitor characteristics. The fabricated supercapacitor two

electrode cell has an average thickness of 230 µm. A schematic

for the fabricated supercapacitor cell, electrodes and the

separator layers are shown in Fig. 1. For comparison, a

supercapacitor cell without AuNPs was also fabricated employing

the above procedure wherein the electrodes were not dipped into

AuNPs solution prior to graphite exfoliation.

8

Fig. 1 Figure showing the details of the fabricated

supercapacitor

The fabricated electrodes, the separator and the pencil lead used

in the experiment were subjected to FESEM (field emission

scanning electron microscopy) for material characterization. The

size of AuNPs in solution were found using NanoSight NS500

instrument. The electrical characterization for the electrodes

and the supercapacitor cell was carried out using Parstat 4000

potentiostat. The resistance of the exfoliated graphite layers on

normal filter paper and filter paper containing AuNPs were

estimated using I-V plots for five repeated sets of exfoliation

process at 200 gm weight using the stated set up in Fig. S1.

Cyclic voltammetry (CV) tests were carried out with the

fabricated supercapacitor cell in order to test their

performances. The areal resistance for graphite exfoliated paper

with KOH solution and graphite exfoliated on AuNP-paper with KOH

solution is obtained from the slope of the I-V plot followed by

9

multiplying the value with the surface area (1 cm2 in this

case).20

3.Results and discussions

3.1. Material characterization

The FESEM images for the pencil lead, the paper soaked in Ni-Cu

hydroxide, the paper soaked in AuNPs solution and the paper with

exfoliated graphite containing AuNPs are shown in Fig. 2.

Fig.2 FESEM image of (i) Pencil lead (ii) Paper containing Ni-Cu

hydroxide (iii) Paper containing AuNPs (iv) Exfoliated graphite

on AuNPs paper

10

The FESEM images of the paper soaked in AuNPs solution show the

entrapment of gold nanoparticles in the paper fibers (Fig. 2)

having size in the order of 30-40 nm (ESI Fig. S2). The size of

gold nanoparticles in solution has a mean 30 nm. This study

indicates that the gold nanoparticles are embedded as well as

stabilized in the paper matrix without any agglomeration. The

FESEM image of graphite exfoliated on the AuNP-paper shows that

the turbostratic thin layers of graphite rest on the gold

nanoparticles which were embedded in the paper matrix.

3.2. Electrical characterization

The current vs. voltage plots at different scan rates (5, 10, 50

and 100 mV/s), the comparative current vs. voltage plots for the

supercapacitor with electrodes containing AuNPs and without it,

the galvanostatic charge discharge curves at constant currents

(4, 6, 8 and 10 mA/cm2), the impedance spectroscopy for the

supercapacitor containing AuNPs and without it, the cyclic

charge-discharge capacitance retention curve at different

discharge rates and the variation of current-voltage

characteristics with bending are shown in Fig. 3.

11

Fig. 3 (i) Current vs. voltage plots at different scan rates (5,

10, 50 and 100 mV/s) (ii) Comparative current vs. voltage plots

for the supercapacitor with electrodes containing AuNPs and

without it (iii) The galvanostatic charge discharge curves at

constant currents (4, 6, 8 and 10 mA/cm2) (iv) The impedance

spectroscopy for the supercapacitor containing AuNPs and without

AuNPs (v) The cyclic charge-discharge capacitance retention curve

at different discharge rates (vi) Variation of current-voltage

characteristics with bending (0°, 30°, 60° and 90°)

3.3. Calculation of electrical parameters

12

The calculated areal resistance for graphite exfoliated paper

with KOH solution was 110 Ω cm2 whereas the resistance of

graphite exfoliated on AuNP-paper with KOH solution was 19 Ω cm2.

This indicates that the entrapment of gold nanoparticles in the

paper matrix reduces the resistance of graphene exfoliated

electrodes significantly. Furthermore, the resistance of the

electrodes generated using controlled exfoliation of graphite on

AuNPs embedded paper as well as on normal filter paper fabricated

five times using the stated procedure does not show a deviation

of more than 0.5 % in the voltage range of 0-1 V.

The I-V characteristic was used to calculate the specific areal

capacitance at different scan rates using the mathematical

relation stated in equation 1. For paper and thin film based

supercapacitor applications, specific areal capacitance is

preferred over gravimetric capacitance as it is difficult to

determine the exact mass of the electrodes.25

(1)

Where iavg is the average of current amplitude between the anodic

13

and cathodic sweeps (Fig 3 (i)), s is the scan rate and A is the

net effective area (1 cm2). The specific capacitances are 92.5,

145, 550 and 790 mF/cm2 for scan rates 100 mV/s, 50 mV/s, 10 mV/s

and 5 mV/s respectively. The maximum capacitance of 790 mF/cm2

was higher than that of the flexible cellulose paper

supercapacitor reported recently in literature (734 mF/cm2)26

employing graphite/Ni/Co2NiO4-CP prepared by much complex route.

However, the specific capacitance falls very rapidly at higher

scan rate. This may be partially due to increase in resistance of

the electrodes at higher scan rate due to inaccessibility of the

generated ions to the active sites of the AuNPs embedded deep

inside the paper as a result of diffusion limitations27 and

partly due to limited chemical kinetics in ion generation in the

electrolyte. In the voltage window, the linear portion of I-V

curve may be due to resistance of the exfoliated graphite. The

curves show redox peaks confirming the pseudo capacitive nature

of charge transfer. The voltage window lies in the order of 0-1 V

which is the typical window size when Ni and Cu hydroxide based

compounds are used as pseudo capacitive materials9, 23. The peak in

the CV curve Fig 3 (i) near 0.25 V and 0.6 V for scan rate 100

14

mV/s are peaks attributed by Ni, as Ni shows redox peaks of that

order.9 No distinct peaks were observed for copper ion oxidation.

Similar results were demonstrated by Zhang et al where authors

have inferred that copper shows very small peaks when Ni-Cu

double hydroxides are used for pseudo capacitor applications. 23

A plausible explanation for the peak of Ni is due to reversible

reactions of Ni(II) to Ni(III) while formation of nickel

oxyhydroxide.

The comparative I-V characteristics for the supercapacitor cells

with electrodes containing AuNPs and electrodes devoid of AuNPs

(only containing exfoliated graphite) in Fig 3(ii) shows that the

supercapacitor electrode devoid of AuNPs has a specific

capacitance of 182 mF/cm2 as compared to 790 mF/cm2 for

supercapacitor electrode containing AuNPs at 5mV/s scan rate.

Thus the specific capacitance is enhanced by 4.3 times on

application of AuNPs. The enhanced capacitance may be due to

reduced ESR (equivalent series resistance) of the device on use

of AuNPs in the electrode matrix.

The galvanostatic charge discharge curve (Fig. 3(iii)) at

15

constant current was used to verify the capacitance found from

the I-V plots. The mathematical relationship in equation 1 holds

for this case wherein the scan rate is replaced by the slope of

the charging curve and iavg is replaced by the constant discharge

currents. The specific capacitance calculated were 810.2 mF/cm2,

502 mF/cm2, 173.1 mF/cm2 and 76.8 mF/cm2 for currents 4 mA, 6 mA,

8mA and 10 mA respectively which closely resembles the

capacitance found from the I-V plots. A close observation of the

galvanostatic charge discharge curves reveal that, the discharge

profiles have two distinct parts viz. a stiff decrease which is

due to internal resistance (ohmic region) of the supercapacitor

cell and a gradual discharge which is due to the change in energy

of the capacitor. The stiff discharge region can be used to

estimate the ESR (equivalent series resistance) of the

supercapacitor by taking the ratio of the voltage drop to the

fixed discharge current. The estimated values of the ESR were

23.5 Ω, 17.4 Ω, 10.8 Ω and 6.4 Ω for discharge currents of 4 mA,

6 mA, 8 mA and 10 mA respectively.

The EIS (electrochemical impedance spectroscopy) was used to

16

exactly determine the ESR and figure of merit of the device.28

The impedance at higher frequency limit of the device in the

Nyquist plot gives the ESR value of the device (Fig. 3(iv)). For

our fabricated cell the ESR was calculated as 5.38 Ω which

closely resembles the estimated values of ESR from galvanostatic

charge discharge curves. The ESR for the supercapacitor cell

without containing AuNPs was found to be 13.1 Ω showing that the

AuNPs were effective in reducing the ESR of the device by more

than two-folds. The high value of ESR may be due to the

insulating clay material present in pencil lead, which gets into

electrode along with graphite during mechanical exfoliation. The

real and imaginary parts of the impedance were equal at

characteristic frequency 0.12 Hz bearing an impedance value of Z=

22 Ω for AuNPs-graphite supercapacitor. The characteristic

frequency was 0.14 Hz with Z=24.9 Ω for the supercapacitor

without AuNPs. The Nyquist plots for the supercapacitor cell with

and without AuNPs were almost isotropic. The difference in shape

of the Nyquist plots might be due to local electron density

variations as a result of silver nanoparticles in AuNPs-graphite

electrodes. The characteristic frequency was in the order of

17

microwave exfoliated graphite supercapacitor (0.29 Hz) fabricated

on rigid substrates.12 The characteristic frequency was used to

find the AC capacitance of supercapacitor cell whose specific

capacitance was calculated previously using equation 2.

(2)

Where C is the AC capacitance, f0 is the characteristic frequency

and Zim is imaginary part of impedance at characteristic

frequency (Real part of impedance Zre equals to Zim).

The calculated AC capacitance using equation 2 (Miller’s

capacitance) was found to be 60.3 mF/cm2 for AuNP-graphite

supercapacitor cell and 45.5 mF/cm2 for graphite supercapacitor

cell. The Miller’s capacitance (AC capacitance) is deemed to

differ from the normal DC capacitance due to non-uniform

(fibrous) nature of paper based electrode where the paper fibres

lead to limited ion diffusion into electrodes. However it shall

be noted that the Miller’s capacitance for supercapacitor cell

containing AuNP was about 1.3 times higher than the

supercapacitor cell devoid of AuNP. The capacitance retention of

18

the device does not degrade by more than 10% at higher limits of

discharge current after 5000 cycles (Fig 3 (v)). Degradation of

specific capacitance was higher for 8 mA (about 10 %) as compared

to 5% and 8% for discharge currents 4 mA and 6 mA respectively

after 5000 cycles.

The bending test (I-V characteristics) for fabricated

supercapacitor cell reveals that the current sweep at acute

bending angle does not change significantly, indicating nominal

change in the capacitance values (Fig. 3 (vi)). However at higher

bending angles (90°), the current sweep decreases indicating

decrease in the capacitance of the device. This decrease in

capacitance is possibly due to lesser ion diffusion into paper

matrix as a result of dislocation of the cellulose fibres from

their original position on bending. Further at higher bending

angle, a number of peaks could be found indicating fracture of

turbostratic graphite layers leading to different rate of ion

diffusion at localized positions in the electrodes. For bending

at acute angles there is a gradual peak shift as the bending

angle increases. This might be due to higher ion diffusion length

19

on bending. Due to higher diffusion length, more potential is to

be applied for same amount of current flow. This may be the

reason for peak shifts towards higher voltage on bending.

The energy density (E) and the power density (P) can be

calculated using the equations 3 and 4.6

(3)

(4)

Where C is the capacitance per unit area, V is the voltage window

(1 volt minus the resistive drop in the galvanostatic discharge

curve) and Δt is the discharge time in hours. The energy (E)

calculated using equation 3 is in joules/cm2 and by dividing it

by 3600, the areal energy density in Wh/cm2 is obtained. The

maximum areal energy and power density were found to be 83.2

µWh/cm2 and 1.576 mW/cm2 respectively. In order to compare these

values with current literature, the volumetric energy and power

densities were calculated. The dimension of our fabricated

20

supercapacitor cell is (1 cm X 1 cm X 230 µm). The energy density

and the power density are found to be 3.617 mWh/cm3 and 68.514

mW/cm3 respectively. The volumetric energy and power densities

for previously reported flexible supercapacitors in Feng et. al26

were 2.48 mWh/cm3 and 0.79 Wh/cm3 respectively whereas in Tao et

al29 the values were 6.16 mWh/cm3 and 40 mWh/cm3 respectively.

Thus considering simplicity in fabrication procedure, good energy

and power density, flexibility, disposability and

biodegradability of these devices, the performance

characteristics are excellent when compared to its flexible

counterparts fabricated via relatively complex routes.

4.Conclusion

In this article, we demonstrate fabrication of a flexible

supercapacitor cell using gold nanoparticles embedded in the

paper with controlled mechanically exfoliated graphite

electrodes. The embedded AuNPs in the paper matrix are

instrumental in reducing the ESR of the mechanically exfoliated

graphite used as electrode and enhancing the specific capacitance

of the device by about 4.3 times. Very high specific capacitance

of 790 mF/cm2 at scan rate 5 mV/sec is found for these simple

21

paper pencil based devices. The described process of controlled

mechanical exfoliation of graphite to fabricate simple devices

opens up new vistas in organic electronics at much lower cost and

simplicity when compared to other methods of graphite

exfoliation. The synthesis method is reliable and uses simple

paradigm for fabrication of flexible supercapacitor. The device

has good performance with bending for acute angles. The device is

tested for reliability up to 5000 cycles. The maximum volumetric

energy and power density are 3.617 mWh/cm3 and 68.514 mW/cm3

which is comparable with flexible supercapacitors fabricated

previously using more complex procedures. The capacitance as well

as the energy density of the fabricated device is high which

assures its applicability in flexible paper based electronic

devices.

Funding Acknowledgement

The work was funded by CSIR- 12th FYP Grant No- ESC0112 of Govt.

Of India.

22

† Electronic Supplementary Information (ESI) available: Set up

for controlled exfoilation of graphite on paper, Synthesis of Ni-

Cu hydroxide.

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