high-performance supercapacitor cells with activated

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High-performance Supercapacitor cells withActivated CarbonMWNT nanocompositeelectrodesTo cite this article F Markoulidis et al 2012 IOP Conf Ser Mater Sci Eng 40 012021

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High-performance Supercapacitor cells with Activated

CarbonMWNT nanocomposite electrodes

F Markoulidis1 C Lei

1 C Lekakou

1 EFiggemeier

2 D Duff

2 S Khalil

2

BMartorana3 and ICannavaro

3

1 Mechanical Medical and Aerospace Engineering Faculty of Engineering and

Physical Sciences University of Surrey Guildford GU2 7XH UK 2 Bayer Technology Services GmbH 51368 Leverkusen Germany

3 Centro Ricerche Fiat SCpA Strada Torino 50 10043 Orbassano (TO) Italy

Email foivosmarkoulidissurreyacuk cleisurreyacuk

clekakousurreyacuk danielduffbayercom sofiekhalilbayercom

brunettomartoranacrfit irenecannavarotirocinanticrfit

Abstract The purpose of this work was to investigate and improve the performance of

supercapacitor cells with carbon-based nanocomposite electrodes The electrode structure

comprised activated carbon (AC) four types of multi-wall nanotubes (MWNTs) and two

alternative polymer binders Polyvinyl alcohol (PVA) or Polyvinylidene fluoride (PVDF)

Electrode fabrication involved various stages of mixing and dispersion of the AC powder and

carbon nanotubes rolling and coating of the ACMWNTbinder paste on an aluminium

substrate which also served as current collector The organic electrolyte utilised was 1M

tetraethylammonium tetrafluoroborate (TEABF4) fully dissolved in propylene carbonate (PC)

All devices were of the electrochemical double layer capacitor (EDLC) type incorporating

four layers of tissue paper as separator material The surface topography of the so fabricated

electrodes was investigated with scanning electrode microscopy (SEM) Overall cell

performance was evaluated with a multi-channel potentiostatgalvanostatimpedance analyser

Each supercapacitor cell was subjected to Cyclic Voltammetry (CV) at various scan rates from

001 Vs to 1 Vs Charge-Discharge at a fixed current steps (2 mA) and Electrochemical

Impedance Spectroscopy (EIS) with frequency range from 10 mHz to 1 MHz It was

established that an AC-based supercapacitor with 015ww MWNT content and 30 μm roll-

coated nanocomposite electrodes provided superior energy and power and energy densities

while the cells was immersed in the electrolyte well above those generated by the AC-based

EDLC cells

1 Introduction Electrochemical double layer capacitors (EDLCs) are designed to optimize their supercapacitor

behaviour by using nanomaterials and nanocomposite coatings as electrodes of high surface area and

an appropriate pore size distribution as well as a separator to halve the distance between the electrodes

and hence double the capacitance Activated carbon (AC) powder of large surface area has been the

main electrode material in this study processed into a coating using the typical PVDF(polyvinylidene

fluoride) binder However the large surface area of AC is associated with large porosity which

reduces the conductivity of the activated carbon electrode Hence conductive additives are required

such as carbon black multiwalled carbon nanotubes (MWNTs) and conductive polymer binders

International Conference on Structural Nano Composites (NANOSTRUC 2012) IOP PublishingIOP Conf Series Materials Science and Engineering 40 (2012) 012021 doi1010881757-899X401012021

Published under licence by IOP Publishing Ltd 1

Carbon black is usually added at 5-10 wt [12] to decrease the resistance although its small particle

size creates a compact particle network filling the AC pores and possibly decreasing the overall

specific capacitance PEDOTPSS used as a binder [1] increases the specific capacitance by

contributing pseudocapacitance to the cell but it is not as conductive as the carbon black or MWNTs

As a result the aim of this study is to investigate the use of MWNTs as the conductive additive given

the high conductivity and open nature of their networks Dispersion is a key requirement for carbon

nanotubes (CNTs) whereas the high aspect ratio of these nanomaterials means that percolation

network is formed at very low loadings As a result in contrast to previous studies(15 wt MWNT in

AC in [3]] only 015 wt MWNT in ACPVDF nanocomposite coating has been used in this study

Polyvinyl alcohol (PVA) has also been investigated as a binder

2 Materials and Experimental Procedures

The activated carbon (AC) powder (Sigma Aldrich) has a surface area of 1000 m2g and a particle size

distribution of 10-15 greater than 74 tm and 70-75 greater than 10 tm The carbon nanotubes

were Baytubesreg (Bayer Technology Services) of 4 nm inner diameter and 13 nm outer diameter and

length greater than 1 tm Different types of these Baytubesreg have been tried the main difference

being their powder density C70P of 45-95 kgm3 and purity gt95 wt C150P of 130-150 kgm

3 and

purity gt95 wt C150PW same as C150W but washed C150HP of 140-230 kgm

3 and purity gt99 wt

The MWNTs were dispersed in l-methyl-2-

pyrrolidinone (NMP) by first sonicating for half hour

and then vigorously stirring for 1 hour using a

Wiggenhauser homogenizer (Fig1) The MWNT solution was then added into the ACbinder-NMP solution The ingredients were mixed to form a slurry

which was coated onto aluminium foil using a Film

Applicator The coating was then dried in an oven at

120oC for 4 hours (Fig1)

Fig1 MWNT slurry in the homogeniser

and nanocomposite coating on Al foil

Fig2 Capacitor cell assembly cell tested as just sealed and cell immersed in the electrolyte

Capacitor cells were prepared using 1 M TEABF4PC electrolyte as the organic electrolyte has

relatively large voltage window to Vmax = 33V (used up to 3 V in this study) Lens tissue was used

as separator The cells were tested using a VersaSTAT MC Potentiostat and were subjected to

impedance spectroscopy cyclic voltammetry and galvanostatic charge-discharge After the cell

fabrication and sealing cells were tested as just sealed or in some cases the cell was immersed in the

electrolyte contained in the ldquobagrdquo with a feed-tube to replenish the electrolyte supply if needed


2

3 Results and Discussion Fig3 presents the Nyquist plots of the capacitor cells with the different types of MWNTs in their

carbonaceous nanocomposite electrodes where all of them have an electrode coating thickness of 120

tm whereas the standard AC electrode has a thickness of 30 tm The smaller thickness of the AC

electrode reduces its resistance substantially hence in a way this is an unfair comparison

Nevertheless MWNTs added at 015 wt to the electrode reduce the resistance in almost all cases

even if their corresponding electrodes are four times thicker than the AC-based reference electrode

The low density MWNT powder C70P leads to the highest resistance cell whereas the high density

MWNT powder C150HP seems to lead to the lowest resistance cell On the other hand the AC-based

cell has the highest capacitance

Fig3 Impedance plots of capacitor cells with ACMWNTPVDF nanocomposite electrodes for

different types of MWNTs (Baytubesreg) frequency 1Mz-10mHz

5

Fig4 presents the Nyquist plots for various electrode thicknesses (30 60 120 tm) where it is clear

that the smallest electrode thickness of 30 tm in the cell immersed in the electrolyte has the highest

performance including the smallest resistance highest specific capacitance and smallest loss (most

vertical line)


3

Fig4 Impedance plots of capacitor cells with ACMWNTbinder nanocomposite electrodes of different thickness with

PVDF or PVA binder cells just sealed or immersed in the electrolyte frequency 1Mz-10mHz

The excellent performance of the ACMWNT 015 wtPVDF-based cell is confirmed in the CV tests (Figure5) where the specific capacitance of the electrode of 30 tm thickness immersed in the electrolyte doubles and more than triples that of the AC-based cell at the rates of 001 and 01 Vs respectively The charge-discharge data in Fig6 display a large voltage drop for the AC-based cell due to its high equivalent in series resistance (ESR) which is really reduced when 015 wt MWNTs is added while the capacitance of the AC015 wt MWNTPVDF cell is significantly higher than that of the AC-based cell at 20 and 50 A

The SEM micrographs in Fig7 show a large amount of macropores in the AC carbon coatingWhen 015 wt MWNTs is added the carbon particles are laced with a MWNT network which apartfrom increasing conductivity reduces the macropore size and seems to increase the specific surface area which results in a significant increse of capacitance


4

Fig5 Cyclic voltammetry results at 001 Vs and 01 Vs


5

Fig6 Charge-discharge results for different currents

(a) (b) (c)

Fig7 SEM micrographs of (a) AC+5wtPVDF (b) AC+015wtC150HP+5wtPVDF) (c)

AC+015wtC150PW+5wtPVA

4 Conclusions Novel EDLC supercapacitors have been presented based on ACMWNTpolymer binder

nanocomposite electrodes where the addition of just 015 wt MWNTs increases not only

conductivity but also specific capacitance two- to more than three-fold in comparison to ACpolymer

binder electrodes-based cells Such dramatic increase of capacitance (as well as conductivity as

expected) was attributed to the MWNT network lacing the AC particle surface and creating more

pores It must be noted that the MWNTs used in this study are of the long and thin type where their

high aspect ratio and length contribute greatly to the high conductivity of the MWNT network while

they also create a well entangled network that greatly contributes to the increase of the specific

capacitance However the dispersion process of such MWNTs is critical for the improved

performance of the supercapacitor device

Acknowledgements

The research is part of the FP7 project AUTOSUPERCAP funded by the European Community under

the Green Car Programme

References [1] CLei PWilson and CLekakou J Power Sources 196(18) 2011 7823-27

[2] PKossyrev J Power Sources 201 2012 347-52

[3] P-LTaberna GChevallier PSimon DPlee and TAubert MatResBull 41(3) 2006 478-484


6

High-performance Supercapacitor cells with Activated

CarbonMWNT nanocomposite electrodes

F Markoulidis1 C Lei

1 C Lekakou

1 EFiggemeier

2 D Duff

2 S Khalil

2

BMartorana3 and ICannavaro

3

1 Mechanical Medical and Aerospace Engineering Faculty of Engineering and

Physical Sciences University of Surrey Guildford GU2 7XH UK 2 Bayer Technology Services GmbH 51368 Leverkusen Germany

3 Centro Ricerche Fiat SCpA Strada Torino 50 10043 Orbassano (TO) Italy

Email foivosmarkoulidissurreyacuk cleisurreyacuk

clekakousurreyacuk danielduffbayercom sofiekhalilbayercom

brunettomartoranacrfit irenecannavarotirocinanticrfit

Abstract The purpose of this work was to investigate and improve the performance of

supercapacitor cells with carbon-based nanocomposite electrodes The electrode structure

comprised activated carbon (AC) four types of multi-wall nanotubes (MWNTs) and two

alternative polymer binders Polyvinyl alcohol (PVA) or Polyvinylidene fluoride (PVDF)

Electrode fabrication involved various stages of mixing and dispersion of the AC powder and

carbon nanotubes rolling and coating of the ACMWNTbinder paste on an aluminium

substrate which also served as current collector The organic electrolyte utilised was 1M

tetraethylammonium tetrafluoroborate (TEABF4) fully dissolved in propylene carbonate (PC)

All devices were of the electrochemical double layer capacitor (EDLC) type incorporating

four layers of tissue paper as separator material The surface topography of the so fabricated

electrodes was investigated with scanning electrode microscopy (SEM) Overall cell

performance was evaluated with a multi-channel potentiostatgalvanostatimpedance analyser

Each supercapacitor cell was subjected to Cyclic Voltammetry (CV) at various scan rates from

001 Vs to 1 Vs Charge-Discharge at a fixed current steps (2 mA) and Electrochemical

Impedance Spectroscopy (EIS) with frequency range from 10 mHz to 1 MHz It was

established that an AC-based supercapacitor with 015ww MWNT content and 30 μm roll-

coated nanocomposite electrodes provided superior energy and power and energy densities

while the cells was immersed in the electrolyte well above those generated by the AC-based

EDLC cells

1 Introduction Electrochemical double layer capacitors (EDLCs) are designed to optimize their supercapacitor

behaviour by using nanomaterials and nanocomposite coatings as electrodes of high surface area and

an appropriate pore size distribution as well as a separator to halve the distance between the electrodes

and hence double the capacitance Activated carbon (AC) powder of large surface area has been the

main electrode material in this study processed into a coating using the typical PVDF(polyvinylidene

fluoride) binder However the large surface area of AC is associated with large porosity which

reduces the conductivity of the activated carbon electrode Hence conductive additives are required

such as carbon black multiwalled carbon nanotubes (MWNTs) and conductive polymer binders


Published under licence by IOP Publishing Ltd 1

















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high-performance supercapacitor cells with activated

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