A structural supercapacitor based on activated carbon fabric and a

solid electrolyte

Richard Reecea, C. Lekakoua*, P.A. Smitha

aDepartment of Mechanical Engineering Sciences, University of Surrey, Guildford,

GU2 7XH, UK

*email of the corresponding author: c.lekakou@surrey.ac.uk

Short biographical notes on all contributors:

Richard Reece obtained his PhD in the Department of Mechanical Engineering at the

University of Surrey. His research focused on the development of different types of structural

supercapacitors and investigations into their electrochemical and structural performance.

Constantina Lekakou is a Reader at the University of Surrey. She completed her PhD in the

Department of Chemical Engineering at Imperial College. London. Her expertise covers the

areas of multi-materials processing and manufacturing of polymer composites as well as the

fabrication, characterisation and analysis of energy storage devices.

Paul Smith is Professor of Composite Materials at the University of Surrey and the Executive

Dean of the Faculty of Engineering and Physical Sciences. He holds a first Degree in

Engineering and a PhD from the University of Cambridge. His research interests focus on the

mechanical behaviour of composite materials.

A structural supercapacitor based on activated carbon fabric and a

solid electrolyte

This paper presents investigations to create a structural supercapacitor with

activated carbon fabric electrodes and a solid composite electrolyte, consisting of

organic liquid electrolyte 1 M TEABF4 in propylene carbonate and an epoxy

matrix where different compositions were considered of 1:2, 1:1 and 2:1 w/w

epoxy: liquid electrolyte. Vacuum-assisted resin transfer moulding was used for

the impregnation of the electrolyte mixture into the electrochemical double layer

capacitor (EDLC) assembly. The best electrochemical performance was exhibited

by the 1:2 w/w epoxy: liquid electrolyte ratio, with a cell equivalent-in-series

resistance of 160 cm2 and a maximum electrode specific capacitance of 101.6

mF g-1 while the flexural modulus and strength were 0.3 GPa and 29.1 MPa,

respectively, indicating a solid EDLC device.

Keywords: solid EDLC; VARTM; activated carbon fibre fabric; solid composite

electrolyte; TEABF4 in propylene carbonate; epoxy; three-point bend test.

Subject classification codes: 7. Composite materials; 8. Functional materials

1. Introduction

Structural composites typically consist of continuous fibre reinforcement and a polymer

matrix [1]. Depending on the type of fibre (mainly: carbon, glass or Kevlar® fibres in

continuous fibre, woven fabric or stitch-boned fabric format) and the type of matrix

(thermoset such as epoxy or polyester or thermoplastic such as poly(ether ether ketone)

(PEEK)), composite materials have a tensile modulus of 20-200 GPa, a tensile strength

of 300-1600 MPa and a density of 1400-1900 kg m-3 [2-3]. Composite materials are

involved in various types of energy harvesting some of which are high power methods

associated with short timescales: for example, wind turbines display continuous series

of power peaks as the wind suddenly changes direction or speed [4] while piezoelectric

energy harvesting related to vibrations [5] and RF energy harvesting [6] display a series

of incoming power pulses. In all these cases, a supercapacitor may be required in

combination with the battery [4-5], where the supercapacitor, with its high power

capability [7-8], may receive the high power peaks above a threshold and so protect the

battery from experiencing high current densities that would have led to fast battery

degradation. As supercapacitors are of high power density, frequently of 20 kW kg-1 [7],

a relatively small mass may be needed which might be integrated in a structural

composite material.

Supercapacitors consist of porous electrodes of large specific surface area, in

intimate contact with the corresponding current collectors, and an electrolyte which

might be liquid, gel or solid [9-12]. Activated carbon fibre (ACF) woven fabrics of

specific pore surface area of around 2000 m2 g-1 are excellent electrodes for high

performance supercapacitors with an aqueous or an organic electrolyte [13-14] and,

hence, are proposed to be investigated for multifunctional composite materials in this

study. Due to the low maximum voltage, Vmax, of aqueous electrolytes, about 1.1 V,

organic electrolytes (Vmax = 2.7 – 3 V) and ionic liquid (IL) electrolytes (Vmax = 3.5 – 4.5

V) are preferred for high voltage applications, such as electric vehicles [15-16]. Solid

electrolytes, although ideal for a structural supercapacitor, display low ion conductivity

[17]. Such investigated electrolytes include gel electrolytes based on a polymer gel

matrix such as polyethylene oxide or polyvinyl alcohol or a silica gel [18-19], Nafion

ionomer membranes [20] or ceramic membranes impregnated with a liquid electrolyte

or doped with ions [21-22].

A solid electrolyte acts as the solid-state matrix which has fully impregnated the

separator and the porous electrodes and, hence, offers the potential of creating a

structural composite with the additional functionality of supercapacitor. Past attempts to

create structural supercapacitors with both a solid electrolyte and carbon fibre

reinforcement of low porosity yielded supercapacitors of extremely low performance.

Shirshova et al [23] impregnated a glass fibre separator with a polyacrylonitrile gel

electrolyte or a crosslinking polyethylene glycol diglycidylether (PEGDGE) matrix,

both containing 0.1 M LiTFSI in a solvent mixture of ethylene carbonate (EC) and

propylene carbonate (PC). They used carbon fabrics with no activation, low activation

(BET = 21.4 m2 g-1) or high activation (BET = 1100 m2 g-1); data of single fibre

mechanical properties included: Young’s modulus E=204, 207 and 40 GPa and tensile

strength F = 3.3, 4 and 1.1 GPa, respectively, but the fibre specific capacitance in

aqueous 3 M KCl electrolyte was extremely low at: 0.06, 2.6 and 0.1 F g-1, respectively;

the highest electrode specific capacitance in solid electrolyte was observed for the

system ACF/PEGDGE/LiTFSI/IL to be 0.52 mF g-1, while for this system the

compressive modulus was 18 GPa and the compressive strength was 7.5 MPa. The

electrode specific capacitances above are very low when compared with good

supercapacitor performance, for example, 160 F g-1 for an ACF of BET = 2000 m2 g-1

and a liquid electrolyte of 1.5 M TEABF4 in acetonitrile [14]. This indicates not only

poor ion conductivity in the solid electrolyte but also lack of access of the small ions of

the electrolyte (even of the liquid aqueous electrolyte) to the pores of even the highly

activated carbon fabric in reference [23]. Carbon aerogel (CAG)-modified carbon fibres

[24] showed a maximum specific capacitance of 7.4 F g-1 at 9.1 wt% CAG in aqueous 3

M KCl electrolyte [25] and 90.5 mF g-1 for a composite PEGDGE with 10 wt% IL

electrolyte-fibre glass separator; the latter structural supercapacitor had a high in-series

resistance (ESR) of 79 k cm2, a shear modulus of 1.17 GPa and a shear strength of 9.8

MPa [25].

This study starts with ACF fabric electrodes of high performance in liquid

electrolyte TEABF4 in acetonitrile [7], in this case replacing the organic solvent with the

less volatile propylene carbonate (PC). Research focused on developing a composite

electrolyte of TEABF4 in PC and an RTM (resin transfer moulding) epoxy, so that the

low viscosity of this epoxy would contribute to complete filling [26-28] of the porous

electrodes, to their micropores, with the epoxy-electrolyte mixture in vacuum- assisted

resin transfer moulding (VARTM) [29], and provide an integrated structure with good

structural and electrochemical performance. Three different compositions were

investigated for the composite electrolyte: 1:2, 1:1 and 2:1 w/w epoxy: liquid organic

electrolyte.

2. Materials and Methods

Activated carbon fibre plain-woven fabric, ACC-507-15 Kynol® Novoloid Fabric,

manufactured by Gun-ei Chemical Industry Co., was used as the electrode material with

a stated thickness of 0.5 mm, a measured areal density of 12 mg cm-2, and a specific

surface area (BET) of 1500 m2 g-1. 30G01 Toyal Carbo® aluminium carbide composite

foil, manufactured by Toyo Aluminium K.K. was used as electrode current collectors,

with a stated thickness of 30 μm and orthogonal aluminium carbide nano whiskers of

diameter 20-30 nm embedded in the backing aluminium foil. The cellulose paper

TF4060, manufactured by Nippon Kodoshi Corp., was used as separator with a stated

thickness of 60 μm and measured areal density of 2.5 mg cm-2.

A solid composite electrolyte was prepared consisting of an organic electrolyte

solution in an epoxy matrix. The organic electrolyte solution was 1 M TEABF4 in PC

(propylene carbonate), both supplied by Sigma Aldrich. The epoxy system comprised

araldite LY564-1 resin and Aradur HY2954 hardener in a ratio of 100:35 parts by

weight, respectively, where this is a low viscosity epoxy system usually employed in

resin transfer moulding (RTM). The composite electrolyte was a mixture of the organic

electrolyte solution and the epoxy matrix at the ratios of 1:2, 1:1, and 2:1 w/w.

Electrochemical double layer capacitors (EDLCs) with only liquid electrolyte 1M

TEABF4 in PC were also fabricated and tested.

115 μm polymer laminated aluminium foil EQ-alf-200-15M was used as casing

material, and 8 mm aluminium tabs EQ-PLiB-ATC8 were used for external circuit

connection (both were supplied by MTI Corporation).

The epoxy mixture of araldite LY564-1 resin and Aradur HY2954 hardener was

prepared at a ratio of 100:35 parts by weight and was mixed with the liquid electrolyte

solution 1 M TEABF4 in PC at the required ratio (1:2, 1:1, and 2:1 w/w). Vacuum-

assisted resin transfer moulding (VARTM) was used in which the first step was to

assemble the EDLC consisting of the outer parts of the casing as external layers, the

current collector foils, the ACF fabrics and the separator in the middle and place the

assembly on a flat glass plate. The whole assembly was then covered with a vacuum bag

as shown in Figure 1. Under the bag two spirals were placed adjacent to the cell to

maintain the vacuum bag over the assembly and allow the free application of vacuum

and the free flow of resin into the EDLC cell. Under vacuum the electrolyte-epoxy

mixture was injected into the EDLC assembly between the current collector foils and it

impregnated the separator and the porous ACF fabric electrodes. Once it was seen on

the other side at the beginning of the vacuum tube, the vacuum tube first and then the

feed tube were clamped. A solid plate of 10 kg was placed on top of the cell and the

epoxy was allowed to cure for 12 hr at room temperature. Upon curing of the epoxy, a

solid composite EDLC was created.

Electrochemical testing of pouch EDLC cells was carried out using a Versastat

MC from Princeton Technology. Electrochemical impedance spectroscopy (EIS)

characterisation of EDLC cells of an area of 16 cm2 was performed in the frequency

range of 10 mHz to 1 MHz. Larger pouch EDLC cells of an area of 72.16 cm2 were

subjected to cyclic voltammetry (CV) tests at scan rates of 0.01 to 1 V s-1 and

galvanostatic charge-discharge (GCD) tests at different currents.

Mechanical three-point bend testing of EDLC beam specimens of 40 mm length

and 15 mm width was undertaken using an Instron 500R universal testing machine,

according to ASTM.D790 [30] at a speed of the moving head of 1 mm/min. The

flexural strength σfm, expressed in MPa, was calculated using the applied load at failure

Pmax in Newtons, and the beam span L, width b, and thickness d in millimetres,

according to the equation:

σ fm=3 Pmax L2b d2 (1)

The flexural strain εf, expressed in millimetres per millimetre, was calculated using the

beam deflection δ, thickness d, and span L, all in millimetres, according to the equation:

ε f =6δdL2 (2)

A nominal flexural modulus, Ef, was obtained by considering the beam span L, width b,

and thickness d in millimetres, and the applied load-time derivative m=dPdt where P is

the load applied to the beam and t is time in seconds.

E f=L3 m604 b d3 (3)

3. Results

Figure 2 presents the Nyquist plots from the EIS data of the tested EDLCs for different

compositions of the composite electrolyte, from 100 wt% liquid electrolyte 1 M

TEABF4/PC to 2:1 w/w solid epoxy:liquid electrolyte 1 M TEABF4/PC. The plots

display an increasing ESR resistance as the epoxy content increases from 12.8 cm-2

for pure liquid electrolyte, to 160 cm-2 for 1:2 w/w epoxy:liquid electrolyte to finally

880 k cm-2 for 2:1 w/w epoxy:liquid electrolyte and an increasing loss tangent at low

frequencies (tan = Zre/Zim) while at the same time the capacitance, C, decreases,

where C = (2 f Zim)-1, f is the frequency and Zim is the imaginary impedance

component (ignoring the inductance term of impedance, 2 f Lind, for the majority of

frequencies). In the solid epoxy:liquid electrolyte 1 M TEABF4/PC mixture, the liquid

electrolyte is the only constituent providing conductivity via ion transport (of the TEA+

and BF4- ions), while the epoxy acts as an insulator of electronic transport; hence, as the

proportion of epoxy is raised in the mixture, the resistance increases in the form of three

resistance components: (a) higher solid electrolyte mixture resistance due to higher

proportion of epoxy insulator, (b) higher contact resistance between the ACF fabric and

the current collector foil due to the solid electrolyte mixture interlayer dominated by the

insulating epoxy, and (c) increasing loss tangent at low frequencies due to slower ion

diffusion through the small micropores of the ACF electrode, as the ions have to diffuse

through an increased proportion of solid epoxy matrix of low diffusivity. As a result,

fewer ions reach the walls of micropores, which means that the capacitance at low

frequencies also decreases as the proportion of epoxy is increased (Figure 2(d)).

Given the electrochemical performance of the above structural EDLCs, the ratio

of 1:2 w/w epoxy: electrolyte solution was therefore chosen to proceed with fabricating

a larger 8×9 cm2 cell of 1.73 g ACF fabric mass for both electrodes and 18.5 g total cell

mass, to test in cyclic voltammetry and galvanostatic charge-discharge. Figure 3

presents the voltammograms at different charge-discharge rates from 0.01 V s-1 to 1 V s-

1. The EDLC demonstrates low specific capacitance, reaching a maximum of 0.34 F g-1

and a mean specific capacitance of 0.1 F g-1 at a scan rate of 0.01 V s-1.

Figure 4(a) displays the data from the galvanostatic charge-discharge tests for

the 16 cm2 EDLC with the liquid 1 M TEABF4/PC electrolyte, which yield a cell

specific capacitance of 30.4 F g-1 (i.e. per gram of both ACF fabric electrodes) and an

electrode specific capacitance of 121.5 F g-1 at a current density of 0.0625 mA cm-2 of

cell area. Figure 4(b) displays the GCD data for the 72.16 cm2 EDLC cell with the

composite epoxy: electrolyte 1 M TEABF4/PC at 1:2 w/w, which yield a cell specific

capacitance of 1.15 mF g-1 (i.e. per gram of both ACF fabric electrodes) and an

electrode specific capacitance of 4.6 mF g-1 at the same current density of 0.0625 mA

cm-2. At the lowest tested current density 5.54 A cm-2 of this composite EDLC yields a

maximum cell specific capacitance of 25.4 mF g-1 and an electrode specific capacitance

of 101.6 mF g-1 but the GCD curves are non-linear at current densities below 11 A cm-

2 indicating low energy efficiency in the charge-discharge process. Figure 5(b) shows

that the Ragone plot derived from the discharge phase of the GCD data of Figure 4(b) at

different discharge currents demonstrates a maximum measured energy density of 5

mWh/kg of ACF electrodes mass and a maximum measured power density of 2 W/kg of

ACF electrodes mass. This may be compared with the maximum measured energy and

power density of 32 Wh/kg and 1200 W/kg of ACF electrodes mass for the EDLC with

only 1 M TEABF4/PC liquid electrolyte in Figure 5(a).

The results of the three-point bend tests are presented in Figure 6 in which it can

be seen that the specimens with composite epoxy: liquid electrolyte at 1:2 and 1:1 w/w

ratio show similar behaviour whereas the specimen with 2:1 w/w epoxy: liquid

electrolyte exhibits much better mechanical performance. Table 1 summarises all values

of key performance indicators regarding both electrochemical and mechanical

properties: it can be seen that the specimen with 2:1 w/w epoxy: liquid electrolyte has a

flexural modulus and strength of 24.1 GPa and 1.357 GPa, respectively, whereas the

specimen with 1:2 w/w epoxy: liquid electrolyte has Ef = 0.3 GPa and fm = 29.1 MPa.

4. Discussion

There are several novel aspects investigated in this study all of which have an impact on

the resulted electrochemical and mechanical performance of the composite

supercapacitors. First of all, this is the first time where the electrolyte 1 M TEABF4

dissolved in PC has been used for a ACF Kynol® fabric. ACF Kynol® fabrics are

phenolic-derived, generally with a very narrow pore size distribution around 1-2 nm

width of slit-type pores [7], which means that acetonitrile (AN) is preferred as the

electrolyte solvent consisting of a small molecule and forming solvated cations

TEA+/AN with dimensions of the van der Waals volume: Lmin = 1.1 nm and Lmax = 1.25

nm [14], which means that they fit very well in the majority of pores and provide a high

energy density and a medium power density of 39 Wh/kg and 6 kW/kg of the two ACF

fabric electrodes, respectively [7]. Replacing AN with the less volatile propylene

carbonate (PC) as the solvent, means that the dimensions of the van der Waals volume

of the solvated cations TEA+/PC are larger: Lmin = 1.96 nm and Lmax = 2.05 nm [31],

closer to the pore slit width of the ACF material, which means that although double

layer capacitance might increase [32], ion transport is much slower and ions might not

have access to all small pores. While PC is a popular non-volatile electrolyte, it is

usually employed as a solvent for TEABF4 for coating-based electrodes which display a

wide pore size distribution [7, 33-34], so ion transport occurs faster through a

hierarchical pore network from macro- to meso- to micro-pores [35-36]. The present

study demonstrated that the liquid electrolyte 1M TEABF4/PC leads to higher electrode

specific capacitance, 121.5 F g-1 than in previous studies [35-36], due to the good

matching between the solvated ion size and the pore slit width. However, it leads to

lower cell power density, 1.2 kW/kg, against 6 kW/kg for the same ACF-based cells

with TEABF4 in AN [7], due to the slower transport of PC-solvated TEA+ ions through

the narrow pores of the ACF fabric electrode.

The composite supercapacitor with 2:1 w/w epoxy:liquid electrolyte matrix

exhibits a flexural modulus and flexural strength of 24.1 GPa and 1.357 GPa,

respectively. This mechanical performance outperforms the mechanical performance of

the other two composite supercapacitors with higher proportion of liquid electrolyte and

is equivalent to that of a low performance carbon reinforced epoxy composite [3].

However, the ESR resistance of this composite cell is very high, 880 k cm2, which

does not recommend it as a supercapacitor device. The best composite supercapacitor

fabricated and tested in this study is the cell with 1:2 w/w epoxy: liquid electrolyte

matrix which provides an ESR resistance of 160 cm2 and a maximum electrode

specific capacitance of 101.6 mF g-1, higher than previously reported for composite

supercapacitors [23, 25] with highly activated carbon electrodes but different liquid

electrolytes and solid matrix (PEGDGE with LiTFSI or IL electrolyte in [23, 25]). The

flexural modulus and flexural strength of this composite supercapacitor were 0.3 GPa

and 29.1 MPa, respectively, which place it in the range of solid devices but not for

advanced structural applications [3].

5. Conclusion

The present study includes investigations to create a structural supercapacitor with a

novel solid composite electrolyte of epoxy and 1M TEABF4 in PC, and activated carbon

fibre fabric electrodes of high specific surface area of 1500 m2 g-1. Different

compositions of the composite electrolyte were considered, including 1:2, 1:1 and 2:1

w/w solid epoxy: liquid 1M TEABF4/PC. Increasing the proportion of epoxy raised the

structural performance of the material, resulting in a flexural modulus of 24.1 GPa and a

flexural strength of 1.357 GPa for the 2:1 w/w epoxy:liquid electrolyte mixture, which

was found equivalent to that of a low performance carbon reinforced epoxy composite

[3]. However, the higher epoxy content was detrimental for the electrochemical

performance of the supercapacitor, raising the device resistance and lowering its

maximum tested capacitance, which yield lower power density and energy density,

respectively. The lowest resistance and highest capacitance were obtained for

supercapacitor with 1:2 w/w epoxy: liquid electrolyte matrix, which exhibited an ESR

resistance of 160 cm2 and a maximum electrode specific capacitance of 101.6 mF g-1

but lower mechanical performance, with Ef = 0.3 GPa and fm = 29.1 MPa.

Acknowledgements

We gratefully acknowledge the funding by the UK Defence Science Technology

Laboratory (DSTL) in the form of a PhD studentship for the first author.

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LIST OF TABLES

Table 1. Key performance indicators of the electrochemical and mechanical

performance of composite structural supercapacitors and also the electrochemical

performance of the corresponding supercapacitor with only the liquid electrolyte

LIST OF FIGURES

Figure 1. Schematic of the VARTM process for the impregnation of the electrolyte-

epoxy mixture into the EDLC mounted on a flat glass plate.

Figure 2. Nyquist plots of the EIS data for EDLCs of an area of 16 cm2: (a) electrolyte

1 M TEABF4/PC; (b) epoxy: electrolyte 1 M TEABF4/PC at 1:2 w/w; (c) epoxy:

electrolyte 1 M TEABF4/PC at 1:1 w/w; (d) epoxy: electrolyte 1 M TEABF4/PC at 2:1

w/w.

Figure 3. Cyclic Voltammograms for EDLC cell of 72.16 cm2 with 1:2 w/w epoxy:

electrolyte 1M TEABF4/PC.

Figure 4. Results of the galvanostatic charge-discharge tests for: (a) the EDLC of 16

cm2 with 1M TEABF4/PC electrolyte; (b) the EDLC of 72.16 cm2 with 1:2 w/w epoxy:

electrolyte 1M TEABF4/PC.

Figure 5. Ragone plots from the discharge phase of the galvanostatic charge-discharge

data for: (a) the EDLC with 1M TEABF4/PC electrolyte; (b) the EDLC with 1:2 w/w

epoxy: electrolyte 1M TEABF4/PC.

Figure 6. Mechanical three-point bend test data for: (a) 1:2 and 1: 1 w/w, (b) 2:1 w/w

epoxy: electrolyte 1 M TEABF4/PC composite specimens.

TABLES

Table 1. Key performance indicators of the electrochemical and mechanical performance of composite structural supercapacitors and also the electrochemical performance of the corresponding supercapacitor with only the liquid electrolyte

Epoxy:1M

TEABF4/PC w/w

ESR

cm2)

Cel,sp

@ 10mHz (F g-1)

Cel,sp

@ 0.0625 mA cm-2 (F g-1)

Flexural modulus

Ef

(GPa)

Flexural strength

fm

(MPa)

0:1 12.8 83 121.5

1:2 160 94 x10-3 4.6x10-3 0.3 29.1

1:1 3.2x103 26x10-3 0.6 36.7

2:1 880x103 1.8x10-3 24.1 1357

FIGURES

Figure 1. Schematic of the VARTM process for the impregnation of the electrolyte-

epoxy mixture into the EDLC mounted on a flat glass plate.

(a)

(b)

(c)

(d)

Figure 2. Nyquist plots of the EIS data for EDLCs of an area of 16 cm2: (a) electrolyte

1 M TEABF4/PC; (b) epoxy: electrolyte 1 M TEABF4/PC at 1:2 w/w; (c) epoxy:

electrolyte 1 M TEABF4/PC at 1:1 w/w; (d) epoxy: electrolyte 1 M TEABF4/PC at 2:1

w/w.

Figure 3. Cyclic Voltammograms for EDLC cell of 72.16 cm2 with 1:2 w/w epoxy:

electrolyte 1M TEABF4/PC.

(a)

(b)

Figure 4. Results of the galvanostatic charge-discharge tests for: (a) the EDLC of 16

cm2 with 1M TEABF4/PC electrolyte; (b) the EDLC of 72.16 cm2 with 1:2 w/w epoxy:

electrolyte 1M TEABF4/PC.

(a)

(b)

Figure 5. Ragone plots from the discharge phase of the galvanostatic charge-discharge

data for: (a) the EDLC with 1M TEABF4/PC electrolyte; (b) the EDLC with 1:2 w/w

epoxy: electrolyte 1M TEABF4/PC.

(a)

(b)

Figure 6. Mechanical three-point bend test data for: (a) 1:2 and 1: 1 w/w, (b) 2:1 w/w

epoxy: electrolyte 1 M TEABF4/PC composite specimens.