Enhancement Performance of Carbon Electrode for Supercapacitors by

Quinone Derivatives Loading via Solvent-free Method

Nutcharin Tisawat a, Chanatip Samart a,b, Panichakorn Jaiyong a, Richard A. Bryce c, Khanin

Nuengnoraj d, Narong Chanlek e, Suwadee Kongparakul a,b*

a Department of Chemistry, Faculty of Science and Technology, Thammasat University,

Pathumthani 12120, Thailand

b Bioenergy and Biochemical Refinery Technology Program, Faculty of Science and

Technology, Thammasat University 12120, Thailand

c Division of Pharmacy & Optometry, School of Health Sciences, Faculty of Biology, Medicine

& Health, University of Manchester, Manchester M13 9PT, UK

d School of Bio-Chemical Engineering and Technology, Sirindhorn International Institute of

Technology, Thammasat University, Pathumthani 12120, Thailand

e Synchrotron Light Research Institute (Public Organization), 111 University Avenue, Muang

District, Nakhon Ratchasima 3000, Thailand

*Corresponding Author. E-mail: ksuwadee@tu.ac.th or skongpar@gmail.com (S. Kongparakul)

Tel: +662-564-4440 ext 2418; Fax: +662-564-4483

ABSTRACT

Activated carbon (AC) from coconut shell, surface area of 764.09 cm2/g, was functionalized with

various quinone derivatives (anthraquinone (AQ), 9,10-phenanthrenequinone (PQ) or

tetrachlorohydroquinone (TCHQ)) via a sublimation method for supercapacitor application. The

properties of modified activated carbons were characterized by X-ray diffraction (XRD),

scanning electron microscopy coupled with energy dispersive X-ray (SEM-EDX) spectroscopy,

X-ray photoelectron spectroscopy (XPS) and nitrogen adsorption-desorption. The results showed

a supercapacitor containing AC modified with 16%wt. AQ achieved higher specific capacitance

than other quinone derivatives which performed specific capacitance about 485 F g-1 at a current

density of 1.0 A g-1, resistance of 2.25 Ω and exhibited high cyclability which loss specific

capacitance of 1.2% after 1000 charge-discharge cycles. The experimental data is in good

agreement with the computational results of quinone adsorption on graphene surface; the lowest

interaction energy (IE) of -28.0 kcal mol-1 was obtained for AQ loading model. The modified AC

successfully prepared by a solvent-free method which could be further developed as low-cost

and environmentally friendly electrode materials for high-performance supercapacitors.

Keywords: Supercapacitor, Quinone derivatives, Sublimation, Solvent-free method

1. Introduction

Supercapacitor is an important part of most electronic devices for using as an energy storage.

In addition, they are used for short-term power backup supply which able to rapid charge and

discharge. However, the major drawback of supercapacitor is low charge storage compare to a

traditional battery. The supercapacitor electrodes are usually made of high surface area porous

carbon materials. Previous literatures implemented to reduce their cost based on the research and

development of carbon-based electrodes from biomass and an enhancement of their energy

density including the long-term stability. Various biomass resources such as bamboo1-3, mud-

stone and lignin4, orange-peel5, coconut shell6, lignin7, etc. have been used for porous carbon

preparation and applied as high-performance supercapacitors. It can be prepared by pyrolysis and

activation method, microwave-assisted method, electrospinning and hydrothermal method.

Among these, coconut shell has great potential since it composed of cellulose and hemicellulose

and easily to produce bunch amount of porous carbons. It has the merits of eco-friendly, low

cost, abundant as a sustainable biomass resource. However, those activated carbons derived from

coconut shell always show a specific surface area below 1800 m2 g−1 with micropores size less

than 2 nm leading to low specific capacitance and poor rate capability for energy storage

applications.

To improve the energy storage performance, an incorporation of organic molecules with

redox kinetics to the inexpensive high-surface-area conductive substrates can store additional

energy by electrochemical reactions. Quinone derivatives are one of the most interesting

molecules according to their redox performance. For example, PQ (9,10-phenanthrenequinone),

PYT (pyrene-4,5,9,10-tetraone) and AQ (anthraquinone) were polymerized and grafted over

Ketjenblack for lithium batteries with high power densities8 or 2,5-bis (pro-2-ny-1-ylamino)

cyclohexa-2,5-diene-1,4-dione (HBU-281) was loaded over AC by physical mixing for

supercapacitor electrode9. Furthermore, the addition of redox active quinone/phenol additives

such as redox reaction of pyrocatechol/o-quinone pair could enhance the capacitance of

pyridinic-N carbonaceous capacitive system which increased the electrode capacitance by up to

512 F g-1 at 1.0 A g-1 and performed an excellent cyclability10.

However, there are many steps and chemical reagents required for this method. It has been

reported that the electrochemical performance of a composite electrode consisting of

carbon/quinone derivative via either electrochemically grafted lithium ion battery or physical

mixing was comparable11. However, the major limitation of carbon materials is charge

accumulate via electric double layer (EDL) mechanism which depends on electrostatic between

electrode and electrolyte interface. The electrochemical properties of carbon electrode for

supercapacitor can be improved by combination of redox material to enhance charge storage via

pseudocapacitance behavior which perform electron transfer through redox reaction12. The

objective of this work is to enhancement the performance of carbon electrode for supercapacitors

by loading of quinone derivatives (anthraquinone (AQ), 9,10-phenanthrenequinone (PQ) or

tetrachlorohydroquinone (TCHQ)) over activated carbon from coconut shell via solvent-free

method. The physical/chemical properties and electrochemical performance will be discussed.

2. Experimental

Materials

Activated carbon (AC) from coconut shell was provided from Carbokarn Co., Ltd. (Thailand).

Anthraquinone (AQ), 9,10-phenanthrenequinone (PQ), tetrachlorohydroquinone (TCHQ) and

polytetrafluoroethylene (PTFE, 60wt% in water) were supplied from Sigma-Aldrich. Sulfuric

acid, AR grade (>98%) was purchased from Acros. All chemicals were used without

purification.

Preparation of quinone/AC.

Quinone derivatives were loaded over activated carbon by a sublimation method. Activated

carbon (0.5 g) was placed in a sealed Schlenk tube and pretreated by vacuum drying at 150C for

4 h. The required amount of quinone was added into an opened-end glass ampule, the topped

with quartz wool and transferred into the Schlenk tube, vacuumed for 20 min, heated up to

desired temperature and sublimed at proper temperature (150-250C) for 5 h. The quinone/AC

products were kept in dry place for further use as a composite electrode. The optimum

sublimation condition of AQ/AC, PQ/AC, and TCHQ/AC are 250C, 220C and 240C for 5h,

respectively using the same pretreatment method as described above (see detail in Supporting

Table S1).

Characterizations

Nitrogen adsorption/desorption isotherms at −196 °C were recorded using a volumetric V-Sorb

2800P instrument (Gold APP Instruments Corporation China). Surface area and the pore

characteristic has been analyzed by the Barrett-Joyner-Halenda (BJH) and Brunauer-Emmett-

Teller (BET) methods, respectively. The surface morphology and elemental distribution were

investigated by scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-

EDX) (JEOL, JSM-5410LV and ISIS300) at accelerating voltage of 20 KV. X-ray diffraction

(XRD) analyses were carried out using an X-ray diffractometer (PANalytical X'Pert Pro) at room

temperature with Cu K𝛼 radiation in the 2 range of 10–50 and a scanning rate of 1.5 min−1. X-

ray photoelectron spectra (XPS) were recorded using (ULVAC-PHI, PHI 500 Versa Probe II)

with Al K X-ray radiation as the X-ray source for excitation. The elemental composition of

quinone modified activated carbons was provided by the analysis of the C1s, O1s, and Cl2p

spectra. The sensitivity of the machine was 0.01–0.5% (atomic percent). Attenuated total

reflection Fourier transform infrared spectrometry (ATR-FTIR, Perkin Elmer, USA) was used

for identifying the functional group of the samples by recording the wavenumbers from 4000 to

600 cm-1.

Electrochemical measurements

Electrochemical analyses were performed on a potentiostat/galvanostat instrument (VerSa

STAT3, Princeton Applied research) with 1.0M H2SO4 electrolyte at 25C in a three-electrode

system (Ag/AgCl as a reference electrode, Pt as counter electrode, modified carbon composites

as working electrode). The working electrode was fabricated by mixing 80%wt. active material,

10%wt. carbon black and 10wt% polytetrafluoroethylene (PTFE: 60% disperse in H2O),

sonicated the mixture for 10 min and dried at 80C for 2 h. The mixture was grinded in agate

mortar until homogeneous and pressed on stainless steel mesh no.150. Cyclic voltammetry (CV)

was conducted in the potential range of -0.4 – 1.0 V vs. Ag/AgCl and scan rate of 10-125 mV s−1

for ten cycles. The galvanostatic charge-discharge (GCD) was investigated at current density

range of 1-7 A g-1. The electrochemical impedance spectroscopy (EIS, frequency range of 1-105

Hz) and self-discharge were recorded using Autolab M204 (Metrohm). A cyclability test was

conducted at a current density of 1.0 A g−1 for 1000 cycles in 1.0M H2SO4 electrolyte.

Computational modeling

To investigate the molecular interaction between quinones noncovalently bound to pristine

graphene (C96H24), the energies of the optimized models were computed using B97D density

functional with def2-TZVPP basis set. All complex models were optimized at B97D/def2-SVP

level of theory using Gaussian 09 program13. The interaction energy (IE) was calculated

according to Eq (1) with counterpoise correction of basis set superposition error.

IE = Ecomplex + (Egraphene Equinone) …………(1)

3. Results and Discussion

The physical and chemical characteristics of pristine AC and quinone modified AC are

illustrated in Figure 1 and summarized in Table 1. SEM images of AC show irregular shapes of

granular AC including visible pores with a diameter less than 1.0 μm. After sublimation, fine

particles (particle size < 0.5 μm) were deposited and well distributed over the AC surface as

illustrated in SEM-EDX, especially, TCHQ/AC samples where Cl element related to chloride

group in the chemical structure of TCHQ. N2 adsorption-desorption isotherms of AC revealed

type IV isotherm which represented to porous structure and continuous 3D networks. (see

morphology, elementary mapping and N2 adsorption-desorption isotherms in Figures S1-S2,

ESI). The amount of quinone loading significantly affected to the pore characteristic of AC. In

this work, the amount of quinone loading was in the range of 0-30% wt. BET surface area (SBET)

decreased when increased in % quinone loading. According to the IUPAC classification, AQ/AC

samples significantly changed the N2 adsorption-desorption isotherms from type IV isotherm to

type I isotherm which associated with non-porous material behavior14.

Figures 1c-e show XPS survey spectra and high-resolution XPS C1s spectrum of AQ/AC

and Cl2p spectrum of TCHQ/AC. XPS wide scan spectra contain the peaks of carbon (C1s and

Auger peak of C KLL), oxygen (O1s peak and Auger line of O LMM) and chlorine (Cl2p

doublet, Cl2p peak and Auger peak of Cl LMM). All peaks precisely coincide with the binding

energies given in the XPS database15-16. The core level high resolution XPS analyses showed that

the peak of carbon (C) is mainly found at 284.5 eV, 284.9 eV, 286.8 V which are assigned to sp2

C (C=C), sp3 C (C-C) and C–O (hydroxyl and epoxy), respectively17-20. New particular peak of

C=O (carbonyl) at 287.6 eV has been observed after quinones loading which indicated to the

presence of quinone entirely in the quinone/AC composites. The peak of oxygen (O) is mainly

found in O1s which represents O-C=O (531.6 eV), C-O (533.6 eV) and C=O (533.5 eV) which

also supports the results of C1s spectra. The peak of chlorine (Cl) is mainly found in Cl2p which

represents chloride (Cl2p3/2 at 197.8 eV and Cl2p1/2 at 199.1 eV) and organic Cl (Cl2p3/2 at 200.1

eV and Cl2p1/2 at 201.9 eV). It is clearly observed for TCHQ/AC sample, the chloride-containing

composites. The full XPS characterization both wide scan and deconvolution of core level

spectrum of each sample is presented in Figure S3-S6 (ESI).

XPS also performed to investigate the elemental composition and functional groups of

quinone/AC. The result reveals that the atomic concentration of O increased related to the

amount of AQ or PQ over AC. For TCHQ/AC, the elemental content of Cl increased with an

increasing in TCHQ amount. These results suggested to the effective loading of AQ, PQ or

TCHQ on the carbon framework via sublimation method, however, high amount of quinone

loading could be fully filled the into the pore and covered the AC surface which led to non-

porous material as mentioned above.

Figure 1 (a, b) SEM images of pristine activated carbon and quinone/AC, respectively (Insets

show high magnification images of the samples), (c) XPS survey spectra (Inset shows zoom in of

Cl2p region for TCHQ/AC sample), (d) deconvolution of core level spectrum of C1s of AQ/AC,

and (e) deconvolution of core level spectrum of Cl2p of TCHQ/AC, respectively.

Table 1 BET surface area, total pore volume and the elemental composition of pristine AC and

quinone modified AC.

Samples Quinonea SBET Vtot Atomic concentrationb (%)

(%) m2 g-1 cm3 g-1 C1s O1s Cl2p

AC 0 764 0.43 90.89 9.11 0

AQ/AC 3 458 0.26 89.43 10.21 0.35

8 312 0.18 88.20 11.41 0.38

16 254 0.15 87.65 12.09 0.26

27 24 0.03 86.05 13.42 0.53

PQ/AC 2 599 0.35 89.52 10.17 0.32

4 556 0.33 89.00 10.64 0.36

12 547 0.30 88.74 11.05 0.22

28 446 0.25 88.13 11.43 0.45

TCHQ/AC 2 692 0.40 88.89 10.14 0.97

6 637 0.38 87.57 11.96 0.47

12 566 0.31 84.69 13.51 1.80

30 432 0.24 87.95 10.11 1.94

a Weight of quinone (%) in quinone/AC composite by gravimetric calculation.

b Atomic concentration (%) was determined by XPS elemental analysis.

Figure 2 illustrates the FTIR spectra of pristine AC and AC after sublimation with quinone.

All the samples feature two peaks located at 1327 and 1554 cm-1, which are typical vibration

modes of carbonaceous materials. The band between 3500 - 3800 cm-1 attributed to phenolic -

OH, the bands at 2977 cm−1 and 2885 cm−1 attributed to C-H aliphatic stretching of C–H and

δC–H (= stretching and δ =bending) in aromatic structure, respectively. The shoulder at 1554

cm−1 assigned to the C=C stretching vibration in aromatic ring and the other peaks at 1230, 1046,

and 879 are characteristics of C-O, C–OH stretching vibration, and out-of-plane C–H bending

vibration, respectively. The quinone loaded AC samples show a strong band in their main

characteristic peak of quinonyl group (C=O stretching vibration) at 694 cm-1 which assigned to

the ring breathing frequency21-22. The results identified the presence quinone functional groups

over the AC surface.

Figure 2 FTIR spectra of pristine AC and quinone/AC.

From Figure 3, XRD patterns of pristine quinones (AQ, PQ, and TCHQ) clearly showed the

sharp diffraction peaks which indicated crystallinity of substance due to its π-π stacking between

the anthracene ring layers23, on the other hands, XRD pattern of the pristine AC exhibited C(002)

broadened diffraction peak (2θ of 15-30) which attributed to the amorphous carbon structures

and the graphite structure of AC present broad diffraction peak around 2θ of 40-50 which

represent to C (101) and the diffraction peak 2θ of 26.61 referred to oxygen functional group

such as ketone and chromene which typically surface functional groups present in activated

carbons24-25. After AC has been sublimed with quinones, the XRD patterns illustrate both

overlapped broad peak and sharp peak which can be ascribed to quinones/AC. Therefore, this

XRD result suggested that the quinone derivative was preferable to hold inside the porous carbon

with a less-crystalline structure. A similar XRD observation has been reported previously for

loading of 1,5-dichloroanthraquinone (DCAQ) over AC beads and the adsorption of AQ on

carbon nanotube, respectively26-27.

Figure 3 XRD pattern of pristine AC, pristine quinone derivatives and quinone/AC.

Electrochemical properties of the quinone/AC composites

The effect of quinone types and the amount of quinones (2-30%wt) on electrochemical

properties was carried out employing a three-electrode system in 1.0 M H2SO4 electrolyte. Cyclic

voltammetry (CV) and galvanostatic charge/discharge (GCD) have been carried out at a scan rate

of 0.1 V s-1 and a current density of 1.0 A g-1, respectively.

Figure 4 displays a series of voltammograms at various scan rates from 0.01 V s-1 to

0.125 V s-1 and a series of galvanostatic charge–discharge curves at various current density from

1.0 A g-1 to 7 A g-1. Generally, an increasing in the scan rate refer to an increasing in the

electronic field which will alter both faradaic and non-faradaic processes. From Figure 4(a), the

shift of voltammograms has been observed with the increase of scan rate where cathodic peak

shifted towards the negative potential and the anodic peak shifted towards positive potential. The

results implied to a diffusion controlled redox process. The anodic current (Ipa) to cathodic

current (Ipc) linearly increased with an increasing in the scan rate as shown in Figure 4(c),

moreover, the ratio of Ipa /Ipc was in the range of 0.91-1.17 which not unity and corresponds to a

quasi-reversible system28. The GCD profiles by variation of current density, Figure 4(b),

illustrates the symmetric charge-discharge at high currents with low IR drops which indicated to

the excellent coulombic efficiency and the low IR drops indicated to the minimal diffusion29. The

comparison of capacity based on active quinone loading (mAh per gram of quinone loading),

Figure 4(d), revealed that AQ/AC performed higher capacity, however, the electrode tends to

degrade earlier at high currents compare to the rate capabilities of the PQ/AC and TCHQ/AC

electrodes. The high active surface area particularly exhibited the great capacity response with

excellent rate stability which allows for the shortened electron pathways from the electrolyte to

the current collector. As presented in Table 1, SBET of AQ/AC quite lower than that of PQ/AC

and TCHQ/AC. Furthermore, as the scan rate increases, the diffusion of electrolyte ion into the

internal structure of porous electrode became more difficulty (diffusion limitation) which led to

the capacity decay. Therefore, the active species loading, and surface area of the electrode are the

important factors to improve the electrode performance.

Figure 4 (a) Cyclic voltammetric evolution of quinone/AC at varying scan rates from 0.01 to

0.125 V s-1 (where a1 is AQ/AC, a2 is PQ/AC and a3 is TCHQ/AC), (b) Galvanostatic charge–

discharge curves of quinone/AC at varying current density from 1.0 to 7.0 A g-1 (where b1 is

AQ/AC, b2 is PQ/AC and b3 is TCHQ/AC), (c) Dependence of peak current on square root of

scan rate from cyclic voltammograms of (a1)-(a3), and (d) Comparison of capacity of quinone/AC

samples at different current density.

Figure S7 (ESI) clearly shows redox peak for AQ/AC electrodes which informed to

pseudocapacitor behavior, whereas the CV of PQ/AC and TCHQ/AC show weak redox peak.

Figure 5(a) illustrates the AQ-composite electrode consisting 16 wt% of AQ loading reached

oxidation peak current of 0.009 A, 3 times larger than pristine activated carbon which presents a

quasi-rectangular shape, the electrical double-layer (EDL) characteristic. Figures 5(b) shows

cyclic voltammetry (CV) curves of AQ/AC at a constant current of 1.0 A g-1. At 16 wt% of AQ

loading exhibited longer discharge time than the other ratios and achieved highest specific

capacitance about 485 F g-1 whilst 28 wt% of PQ loading, 30 wt% of TCHQ loading and pristine

AC provided specific capacitance about 161 F g-1,143 F g-1, and 98 F g-1, respectively as shown

in Figure S7 (ESI). The quinone/AC samples with high specific capacitance for each quinone

loading have been selected and further characterized for cyclability testing and electrochemical

impedance, as presented in Figure 5(c) – (d). After 1,000 charge-discharge cycles, AQ/AC

performed highest cyclability with capacitance loss of 1.2%. It can be implied to the advantage

of employing AQ group with pristine carbon that works primarily by EDL characteristic. EIS

measurement in Figure 5(d) shows a semicircle which ascribed to the charge transfer resistance

and constant phase element of the electrodes, and a sloping line following the semicircle which

reflected to the diffusion of ions in the porous electrode. The results demonstrated that all

quinone/AC electrodes possess lower resistances than pristine AC electrode which usually come

from pore tortuosity of AC structure30. From Table 2, the bulk solution resistance (Rs) values are

slightly different with related to a similar diffusion resistance, however, the charge transfer

resistance at electrode-electrolyte (Rct) of quinone/AC was lower than that of pristine AC where

AQ/AC showed lowest Rct value. This result revealed the improvement of conductivity by

quinone loading over pristine AC and demonstrate good properties for supercapacitor

application. Therefore, a possibility of physicochemical transport model was (i) electric double

layer (EDL) formation at the electrode/electrolyte interface, (ii) charge transport in the electrode,

and (iii) ion electrodiffusion. Figure 3(e) shows Ragone plot of all quinone/AC electrodes31. The

energy density in terms of the amount of working material varied from 2 to 8 Wh kg-1

corresponding to a power density from 111 to 265 W kg-1 (detail in Table S2, ESI). The

comparison of quinone/AC electrode characteristics from this work compare to various energy

deliveries or storage systems is presented in Figure S8 (ESI).

Figure 5 (a) CV of AQ/AC electrodes at a scan rate of 0.01 V s-1, (b) GCD of AQ/AC electrodes

at a current density of 1.0 A g-1, (c) Cyclability of supercapacitor, (d) Nyquist plots (The inset

shows the equivalent circuit of a supercapacitor containing double layer capacitor C , charge

transfer resistance Rct, series resistance Rs, and Warburg element W), and (e) Ragone plot of

quinone/AC supercapacitors at various amount of quinones.

Table 2 Resistance value of composite electrode.

Sample Rs (Ω) Rct (Ω)

AC 0.93 16.04

AQ/AC (16%wt.) 1.08 2.25

PQ/AC (28%wt.) 1.45 4.79

TCHQ/AC (30%wt.) 0.98 6.37

At acidic media, two protonation reaction directly affords hydroquinone QH2 as product

(Q + 2H+ +2e- QH2). The quinone derivatives undergo reversible two-electron reduction

during charge-discharge process where AQ was reduced into anthrahydroxyquinone (AHQ), PQ

was reduced into 9,10-phenanthrenehydroquinone (PHQ), and TCHQ was oxidized into

tetrachloro benzoquinone (TCBQ). Therefore, quinone and hydroxyl could enhance the carbon

materials capacity. The theoretical capacity (mAh g of quinone-1) can be calculated by Faraday’s

law32 as presented in Figure 6(a). It was found that the capacity of quinone/AC electrodes were

lower than the theoretical capacity where AQ/AC performed higest capacity with capacity loss of

2.8% for 1000 cycles. The results implied to the stability of AQ/AC electrode was better than

that of the other electrodes.

Self-discharge is an essentially characteristic to measure the voltage drop in the charged

capacitor after a period with no load condition. The prepared electrodes were charged to 1.0 V

with constant current of 10 mA and the charging source was disconnected, and recorded the

voltage drop during the discharge time in the open circuit system. In particular, there is three

self-discharge mechanisms in supercapacitors which are self-discharge due to overvoltage, self-

discharge due to faradaic reactions and self-discharge due to ohmic leakage33. In Figure 6(b),

AQ/AC and AC show the voltage decay whilst voltages of PQ/AC and TCHQ/AC initially

slightly increased after discharged and then slowly voltage decay due to non-uniformly charged

and time dependent charge re-distribution in the porous electrodes. The quinone/AC electrodes

overcome the self- discharge due to faradaic reactions according to the presence of quinone

moieties which are chemically active molecules via redox reaction over the potential range of

study. These redox processes causing self-discharge are diffusion controlled. According to the

experimental results, the equivalent circuit model is presented in Figure 6(c) where C is double

layer capacitor, Rct is charge transfer resistance, Rs is series resistance, and W is Warburg

element).

Figure 6 (a) Theoretic capacity of quinones by Faraday’s law, (b) self-discharge of electrodes,

and (c) model of equivalent circuit.

To confirm the stability of quinone molecules physically interacted on the carbon surface,

the quantum chemical study based on density functional theory (DFT) has been carried out (see

detail in Supplementary Table S3) using the graphene (Gr) model of 120 atoms in size. For all

models studied, the interaction energy (IE) for TCHQ/Gr is -19.6 kcal mol-1 and its weighted IE

regarding its molar mass is -23.3 kcal mol-1. Both AQ/Gr and PQ/Gr have the lower IE of -28

kcal mol-1 with the staggered conformation of AQ with respect to carbon atoms of Gr sheet

(Figure 7). From our computational results, we suggest that either AQ or PQ could be better

stabilized on the Gr surface than TCHQ. The same amount of IE for AQ/Gr and PQ/Gr is due to

the same fashion of aromaticity and the charge-redistribution process of PQ compared with AQ.

However, PQ slightly differs from AQ only at the position of the two carbonyl groups. This

could affect the preferred coordinated position of the co-cations, resulting in the different

electrochemical performance as reported in previous literature34. Our model of AQ/Gr well

agreed with our experimental data from which the highest performance and long-term stability of

electrode. We highlight strong correlation between the computed interaction energy of

quinone/carbon framework and the performance of a fabricated electrode.

Figure 4 Optimized configuration of anthraquinone (AQ) on graphene (Gr) surface: (a) top view

and (b) side view.

4. Conclusions

Activated carbon (AC) from coconut shell, surface area of 764.09 cm2 g-1, was successfully

functionalized with various quinone derivatives (AQ, PQ or TCHQ) via sublimation method for

supercapacitor application. High loading amount of quinone effected to the pore structure of AC.

The supercapacitor containing AC modified AQ (AQ/AC) achieved higher specific capacitance

than other quinone derivates loaded over AC. At 25% wt. of AQ loaded over AC, the

supercapacitor performed specific capacitance about 485 F g-1 at current density of 1.0 A g-1,

resistance of 2.25 Ω and exhibited high cyclability which loss specific capacitance 1.18% after

1000 charge-discharge cycles, however rapidly decay of self-discharge has been observed. Our

experimental data is in good agreement with the computational results of quinone adsorption on

graphene surface; the lowest interaction energy (IE) of -28.0 kcal mol-1 was obtained for AQ

loading model. Hence, the modified AC successfully prepared by a solvent-free method which

could be further developed as low-cost and environmentally friendly electrode materials for

high-performance supercapacitors.

Supporting information.

Supplementary Figures S1–S8

Supplementary Tables S1–S3

AUTHOR INFORMATION

Corresponding Author

(S.K.) E-mail: ksuwadee@tu.ac.th

Author Contributions

NT conceived the experiments, synthesized and characterized the composites based on

discussion with SK. CS and SK provided instrument and laboratory equipment. NC characterized

XPS. PJ and RB simulated the molecular modeling. KN measured EIS and SDC. SK wrote the

manuscript, responded to the reviewer, revised and finalized the manuscript.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge the financial supported by Energy Promotion and

Conservation Fund and National Science and Technology Development Agency (P-17-50509)

and Thammasat University Research Fund under the TU Research Scholar, contract no.

2/49/2561. Instrument support from Center of Scientific Equipment for Advanced Science

Research, Office of Advanced Science and Technology, Thammasat University. XPS

measurement from the SUT-NANOTEC-SLRI joint research facility at the Synchrotron Light

Research Institute (Public Organization). The assistance given by IT Services and the use of the

Computational Shared Facility at The University of Manchester; the Science Cloud Project at

Department of Computer Science, Faculty of Science and Technology, Thammasat University

and National e-Science Infrastructure Consortium.

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