Functionalized graphene oxide-reinforced electrospun carbon nanofibers as ultrathin supercapacitor electrode

Accepted Manuscript

Functionalized graphene oxide-reinforced electrospun carbon nanofibers as ultrathin supercapacitor electrode

http://www.journals.elsevier.com/ journal-of-energy-chemistry/

W.K. Chee , H.N. Lim , Y. Andou , Z. Zainal , A.A.B. Hamra , I. Harrison , M. Altarawneh , Z.T. Jiang , N.M. Huang PII: DOI: Reference:

S2095-4956(17)30126-2 10.1016/j.jechem.2017.04.007 JECHEM 301

To appear in:

Journal of Energy Chemistry

Received date: Revised date: Accepted date:

16 February 2017 13 April 2017 16 April 2017

Please cite this article as: W.K. Chee , H.N. Lim , Y. Andou , Z. Zainal , A.A.B. Hamra , I. Harrison , M. Altarawneh , Z.T. Jiang , N.M. Huang , Functionalized graphene oxide-reinforced electrospun carbon nanofibers as ultrathin supercapacitor electrode, Journal of Energy Chemistry (2017), doi: 10.1016/j.jechem.2017.04.007

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Functionalized graphene oxide-reinforced electrospun carbon nanofibers as ultrathin supercapacitor electrode Chee W.K.a, Lim H.N.a,b,*, Andou Y.c, Zainal Z.a, Hamra A.A.B.a, Harrison I.d, Altarawneh M.e, Jiang Z.T.e, Huang N.M.f

Department of Chemistry, Faculty of Science, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia b

CR IP T

a

Functional Device Laboratory, Institute of Advanced Technology, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia

c

AN US

Graduate School of Life Science and Systems Engineering, Eco-Town Collaborative R&D Center for the Environment and Recycling, Kyushu Institute of Technology, 2-4 Hibikino, Wakamatsu-ku, Kitakyushu-city, Fukuoka 808-0196, Japan

d

Department of Electrical and Electronic Engineering, Faculty of Engineering, University

e

M

of Nottingham, Nottingham NG7 2RD, United Kingdom Surface Analysis and Materials Engineering Research Group, School of Engineering and

Faculty of Engineering, Xiamen University of Malaysia, Jalan Sunsuria,

PT

f

ED

Information Technology, Murdoch University, Murdoch, Western Australia 6150, Australia

Bandar Sunsuria, 43900 Sepang, Selangor Darul Ehsan, Malaysia

AC

CE

*Corresponding author. [email protected].

Abstract

Graphene oxide has been used widely as a starting precursor for applications that cater to the needs of tunable graphene. However, the hydrophilic characteristic limits their application, especially in a hydrophobic condition. Herein, a novel non-covalent surface modification approach towards graphene oxide was conducted via a UV-induced photo-polymerization

1

ACCEPTED MANUSCRIPT technique that involves two major routes; a UV-sensitive initiator embedded via pi-pi interactions on the graphene planar rings, and the polymerization of hydrophobic polymeric chains along the surface. The functionalized graphene oxide successfully achieved the desired hydrophobicity as it displayed the characteristic of being readily dissolved in organic solvent. Upon its addition into a polymeric solution and subjected to an electrospinning process, non-

CR IP T

woven random nanofibers embedded with graphene oxide sheets were obtained. The prepared polymeric nanofibers were subjected to two-step thermal treatments that eventually converted the polymeric chains into a carbon-rich conductive structure. A unique morphology was observed upon the addition of the functionalised graphene oxide, whereby the sheets were

AN US

embedded and intercalated within the carbon nanofibers and formed a continuous structure. This reinforcement effectively enhanced the electrochemical performance of the carbon nanofibers by recording a specific capacitance of up to 140.10 F/g at the current density of 1

M

A/g, which was approximately three folds more than that of pristine nanofibers. It also retained the capacitance up to 96.2% after 1000 vigorous charge/discharge cycles. This

ED

functionalization technique opens up a new pathway in tuning the solubility nature of

CE

PT

graphene oxide towards the synthesis of a graphene oxide-reinforced polymeric structure.

Keywords: Non-covalent functionalization; Functionalized graphene oxide; Electrospinning;

AC

Carbon nanofiber; Supercapacitor electrode

1. Introduction Graphene, a state-of-the-art single-atomic thickness of conjugated sp2 carbon atom, is well known for its exceptionally high electrical conductivity [1], excellent mechanical strength, and thermal conductivity [2]. It is used as a carbon-based nanofiller in polymer

2

ACCEPTED MANUSCRIPT backbones [3] to enhance the mechanical strength, thermal, and electrical conductivity [4–6]. Graphene is a potential active material for supercapacitor applications [7,8]. To effectively inherit the beneficial properties of graphene into polymeric blends or composites, a solution processible route often needs to be developed [5,9]. Ideally, a well-dispersed solution of graphene mixture is the simplest technique to incorporate the graphene structure into the

CR IP T

discontinuous polymeric phase, but the chemical inertness of the graphene serves as the primary barrier in obtaining a fine dispersion with better interaction within a polymeric system.

AN US

Graphene oxide (GO) is a derivative of graphene that is rich in oxygen functional groups [10,11], and possesses excellent dispersibility in polar solvents, consequently having the ability to form hydrogen bonds. It has been widely used as a starting precursor for the synthesis of tunable graphene, in which it can be reduced directly with the use of strong

M

reducing agents such as hydrazine, dimethylhydrazine or sodium borohydride; however with

ED

defects on the graphene structure, which is not favourable, especially for electrical conducting applications [4]. Furthermore, direct reduction of the GO leads to the precipitation

PT

of graphite as a consequence of rapid and irreversible aggregation of the graphene sheets [3]. Therefore, the surface functionalization of the GO sheets is usually carried out by covalent or

CE

non-covalent functionalization approaches in order to tune its characteristics while effectively

AC

preventing the restacking of the graphene layers. Functional groups such as epoxide and hydroxyl on the basal planes, as well as carboxylic acid and other carbonyl groups on the edges of the GO nanosheets were potential sites for chemical functionalization. Thus, GO is the preferred candidate due to the ease of functionalization by targeting the functional groups presence along the surface. Nanostructured materials have unique electrical properties that are contributed by the complex synergy of bulk and interfacial characteristics for an effective energy storage

3

ACCEPTED MANUSCRIPT mechanism [8]. The utilization of an electrospinner to producepolymeric nanofibers has gained a lot of interests amongst researchers [12–15]. Polyacrylonitrile (PAN) is one of the common polymer precursor in obtaining carbon nanofibers mainly because of its high carbon content. Researches were conducted in achieving the dispersion of graphene nanosheets within the polymeric nanofiber structures [16,17], but the high production cost of pristine

CR IP T

graphene, especially graphene nanosheetsthat resulted from the direct dispersion technique, was non-practical. Graphene nanosheets could not be integrated into polymeric nanofibers easily [16,18–21]. Although GO is easily processible due to its hydrophilic characteristic, its

AN US

dispersion in hydrophobic polymeric solution is compromised [18].

The modification of graphene oxide has been conducted vigorously in both covalent and non-covalent pathways. However, the covalent modification technique was less promising because of the structure defects suffered by the graphene sheets upon modification.

M

Herein, a versatile functionalization technique of graphene oxide was reported by utilising the

ED

pi-pi physical interactions without interfering with the structural conformation of the graphene rings. Thefunctionalised graphene oxide was readily dispersed in various solvents,

PT

as well as hydrophobic-based polymeric solution, by purely utilising the hydrophobic functional groups attached within the planar rings to achieve the dissolution. The

CE

electrochemical performance of the fabricated modified graphene/carbon nanofiber paper was

AC

investigated as super capacitor electrodes by directly assembling them into a two-electrode configuration system.

2. Experimental 2.1. Materials

4

ACCEPTED MANUSCRIPT Graphite powder was obtained from Ashbury Graphite Mills Inc.(Code no.: 3061), United States. Sulfuric acid (H2SO4, 95%–98%), phosphoric acid (H3PO4, 85%), charcoal activated powder (Chem-Pur), potassium permanganate (KMnO4, 99.9%), hydrogen peroxide (H2O2, 30%), and hydrochloric acid (HCl, 37%) were purchased from Systerm, Malaysia.1chloromethylnaphthalene,

sodium

diethyldithiocarbamate,

tetrahydrofuran

(Super

CR IP T

Dehydrated, Stabiliser Free), and methyl methacrylate (MMA, 99%) were purchased from Wako Pure Chemical Industries Ltd., Japan. Polyacrylonitrile (MW: 150,000) was obtained

2.2. Synthesis of graphene oxide (GO)

AN US

from Sigma Aldrich, Malaysia.

GO was synthesised via a modified Hummer’s method, as reported in literature [22,23]. 3 g

M

of graphite flakes was oxidised by adding an acid mixture of H2SO4:H3PO4 (9:1) and 18 g of KMnO4. The mixture was stirred continuously for about 5 min to complete the oxidation

ED

process. H2O2 solution was then added to stop the oxidation process, at which time the colour of the solution changed from dark brown to milky yellow. The mixture obtained was then

PT

washed with a 1 M HCl solution using a centrifuge, followed by repeated washing with de-

CE

ionized water until a constant pH of 4–5 was obtained.

AC

2.3. Functionalization of GO The surface functionalization of the GO was done using a UV-assisted liquid-phase polymerization technique. Typically, a photo-initiator (iniferter) was synthesised via a coupling reaction of sodium diethyldithiocarbamate and chloromethyl naphthalene, as shown in Reaction 1 [24]. A fixed amount of photo-initiator and 2 mg of GO was then added into a tetrahydroxyfuran (THF) solution and the mixture was sonicated continuously for 2 h to

5

ACCEPTED MANUSCRIPT allow the formation of pi-pi stacking between the naphthalene part of photo-initiator and the GO planar rings. Upon sonication, the mixture was centrifuged for 15 min at 3000 rpm. The supernatant liquid was discarded and 2 mL of THF was added. The mixture was then sonicated for 5 another minutes, followed by centrifugation for 15 min. This step was repeated for three times to ensure that the unreacted initiator was discarded. Lastly, the

CR IP T

sediment was re-dispersed in THF and sonicated for 2 h.The GO/photo-initiator mixture was added with 2 mL of MMA monomers in a quartz flask within an inert atmosphere and subsequently subjected to UV irradiation in a reaction chamber with a fixed temperature of

(Reaction 1)

ED

M

AN US

40 ˚C. Unreacted MMA monomers were removed by precipitation in methanol.

PT

2.4. Synthesis of CNF/rGO

A specific amount of fGO powder and PAN were dissolved in DMF to obtain an PAN/fGO

CE

solution. The polymeric solution was electrospun using an electrospinner set-up (Electroris, Nanolab Malaysia), at which the solution was filled into a syringe connected to a flat tip

AC

needle and subsequently electrospun by applying a static potential of 15 kV. The electrospun fibers were collected on a grounded rotary drum. The fGO/PAN nanofiber was then initially stabilised at 280 ˚C for one hour in open air, followed by a one-hour carbonisation process at 800 ˚C in an inert atmosphere. The synthesised CNF/rGO was kept in a desiccator to avoid any moisture uptake.

6

ACCEPTED MANUSCRIPT 2.5. Materials characterization The surface morphologies of the nanofibers were characterised using a field emission scanning electron microscope (FEI Quanta SEM Model 400F) that was equipped with an energy dispersive X-ray (EDX) feature. The Fourier transform infrared spectroscopy (FT-IR) spectra of the samples were recorded using an attenuated total reflectance (ATR) on a Fourier

CR IP T

transform infrared spectrophotometer (Perkin Elmer 1650). The spectra were recorded over the range of 280–4000 cm-1. Nuclear magnetic resonance (NMR) was carried out via JEOL 450. The molecular weight of the functionalised GO was measured on a TOSOHHLC-8220

TSK gel Super HM-H linear column.

AN US

SEC system with refractive index (RI) and ultraviolet (UV, λ=254 nm) detectors through

M

2.6. Electrochemical performance analysis

The electrochemical properties of the samples were evaluated via a two-electrode system

ED

using an electrochemical analysis system (Versastat 3-400, Princeton Applied Research). The

PT

synthesized CNF was cut into a symmetrical shape with an identical size of 1 cm 2. A filter paper was sandwiched between the prepared CNF and tightly fitted into an electrochemical

CE

cell, as illustrated in Fig. 1. The filter paper was soaked in 6 M of KOH solution overnight prior to usage. The average mass loading of CNF, CNF with 1 mg of fGO (CNF/fGO-1) and

AC

2 mg of fGO (CNF/fGO-2) were 1.8 mg, 1.0 mg and 0.9 mg, respectively. The specific capacitance was calculated from the galvanostatic charging/discharging analysis via the slope of the discharge curve according to Equation 1. Specific capacitance (F/g =

Δ

Equation 1 [25]

7

ACCEPTED MANUSCRIPT where i is the current applied, t is the elapsed discharge time, v is the total working potential

AN US

CR IP T

(minus the IR/voltage drop), and m is the average mass of the electrode materials.

Figure 1. Schematic diagram illustrates the synthesis of functionalized CNF/fGO for

ED

3. Results and discussion

M

electrochemical measurement in a two-electrode configuration.

The UV-irradiation approach was employed with the presence of a photo-initiator (iniferter)

PT

towards the non-covalent functionalization of graphene oxide. The molecular structure of the synthesized iniferter was validated by 1H NMR spectroscopy (Fig. 2) and the presence of the

CE

–CH2CH3 alkyl group was confirmed by the 1H peaks as magnified in the inset of Fig. 2. The

AC

presence of the functional groups was validated via an FT-IR analysis. From the spectra in Fig. 3, the main peaks correspond to the C=S stretch of sodium diethyldithiocarbamate (1064.55 cm-1) [26] that was detectable at 1070.34 cm-1 respectively, along with the high intensity of C–H stretching vibration at 2977.67 cm-1 that was detected even after the formation of the iniferter (2977.68 cm-1). Furthermore, the strong peak corresponding to the aromatic C–H stretching vibration of 1-chloromethylnaphthalene (771.93 cm-1) was found in

8

ACCEPTED MANUSCRIPT the profile of iniferter, further confirming the success in the synthesis of the iniferter

AN US

CR IP T

molecule.

Figure 2. NMR Spectra of the synthesised iniferter as a photo-initiator; inset is the molecular

AC

CE

PT

ED

M

structure of iniferter as reference.

9

M

AN US

CR IP T

ACCEPTED MANUSCRIPT

Figure 3. FT-IR spectra of (a) 1-chloromethylnaphthalene, (b) sodium

PT

ED

diethyldithiocarbamate, and (c) initiator, indicating the synthesis of photo-initiator.

CE

The iniferter consists of two important fragments: an aromatic group that is responsible for the pi-pi interaction towards the conformational structure of GO and the aliphatic chain,

AC

primarily the sulphide group that tends to form radical active-site upon irradiation by the UV. The formation of the pi-pi interactions of the iniferter towards the surface of graphene oxide was illustrated in Fig. 4 accordingly.

Upon initiation of the UV-light, an immediate

formation of free-radical occurred and recombined reversibly at the aliphatic sulphide group of the iniferter molecule. It then serves as the primary target sight of polymerisation. The polymerisation reaction propagated with the UV irradiation, where the PMMA polymeric chains eventually developed into a macromolecular structure with high molecular weight. As

10

ACCEPTED MANUSCRIPT soon as the UV irradiation was terminated, the polymerisation process was also terminated, at which the sulphide radical species as a dormant eventually recombined towards PMMA chains radical ends, forming a complete polymeric structure. The polymerisation reactions involved were summarised in Fig.5 respectively, at which this iniferter-based polymerisation is known as the living radical polymerisation system. A schematic diagram on the mechanism

PT

ED

M

AN US

CR IP T

of the surface functionalization of graphene oxide was further illustrated in Fig. 6.

pi-pi interactions.

AC

CE

Figure 4. Stacking and anchoring of iniferter molecule onto the surface of graphene oxide via

11

AN US

CR IP T

ACCEPTED MANUSCRIPT

AC

CE

PT

ED

M

Figure 5. UV-assisted PMMA polymerization of iniferter molecules.

Figure 6. Mechanisms of surface functionalization of GO via pi-pi interactions of iniferter.

12

ACCEPTED MANUSCRIPT For the surface functionalization of the graphene oxide, different concentrations of the iniferter (0.5 and 1 mmol respectively) were studied to identify the effects on the degree of polymerization.

Subjected to 30 min of interval, 0.5 mL of the reaction mixture was

extracted from the reaction flask and subjected to an SEC analysis. The results of the SEC are tabulated in Fig. 7 and Fig. 8 respectively. At the initial level of initiator concentration (0.5

CR IP T

mmol), the polymerization smoothly occurred with rapid increment of molecular weight along the reaction time, at which the final molecular weight was recorded at 70,000 upon completion of the reaction. Similarly, a rapid increment in molecular weight was still recorded as the concentration of the iniferter was maximised to two folds (1.0 mmol), at

AN US

which the final molecular weight recorded for the sample of 40,000 for 150 min, indicating that the degree of polymerization was significantly influenced by the initiation concentration. At 0.5 mmol concentration of the iniferter, there were sufficient spaces for continuous

M

polymerization from multiple initiation points, enabling higher molecular weight structures to grow as the reaction time increased. Similarly, the molecular weight increased steadily, but

ED

with a lower extent, as the concentration of the initiator was doubled (1.0 mmol), indicating

PT

excess condition of the initiator. On the other hand, the PDI value of the PMMA with the iniferter indicated large values in spite of the living radical polymerization system under the

CE

iniferter system. The trend of increasing of Mw with time directly indicated a free radical polymerization system, at which the dormant group may not work to control the generation

AC

process. It may indicate that the radical species transfer to that of the GO surface, which then caused the molecular interaction. The successful polymerization of the PMMA molecules was further confirmed via the1H NMR analysis. The signal at the peak attributed to the protons of the ethyl ester unit, methyl group (peak b), as well as the methyl ester group at the chain end (peak c) [27]. Both samples polymerised at the initial concentration of 0.5 mmol

13

ACCEPTED MANUSCRIPT (Fig. 9a) and 1.0 mmol (Fig. 9b) recorded the characteristic peaks of the PMMA molecules.

AN US

CR IP T

The presence of the MMA monomer residues was detected as peaks d and e respectively.

Figure 7. Graphs of intensity vs LgMW for GO functionalised with (a) 0.5 mmol and (b) 1.0

AC

CE

PT

ED

M

mmol photo-initiator.

14

AN US

CR IP T

ACCEPTED MANUSCRIPT

M

Figure 8. Effects of initiator concentration towards the degree of polymerization as the

AC

CE

PT

ED

function of polymerization time.

15

AN US

CR IP T

ACCEPTED MANUSCRIPT

M

Figure 9. 1H NMR Spectra of fGO obtained by the UV-initiated polymerisation of MMA at

ED

150 min in various concentrations of iniferter with CDCl3 as solvent.

PT

The hydrophilicity of the GO is the main reason that caused the occurrence of the agglomeration when dispersed in organic solvents. In order to illustrate the effects of

CE

functionalization, both fGO and the non-modified GO were added separately into chloroform,

AC

at which the non-modified GO, as expected, formed agglomeration immediately upon contact with chloroform solvent. On the contrary, the fGO dispersed readily without the use of physical mixing or agitation to form a stable dispersion system, as shown in Fig. 10. This demonstration signified the success in the functionalization of graphene oxidein enhancing hydrophilicity. The fGO that was synthesised at optimised conditions (0.5 mmol iniferter and UV-polymerised at 150 min) was further incorporated into 10 wt% of PAN polymeric

16

ACCEPTED MANUSCRIPT solution, with DMF as solvent, which will be used as the precursor solution for

AN US

CR IP T

electrospinning process.

Figure 10. Solubility of (a) fGO and (b) GO in CHCl3 solvent. The GO immediately agglomerates when it was into the solvent, whereas the fGO readily dispersed with a no-

ED

M

phase separation observed.

The electrospun PAN polymeric solution resulted in random non-woven fibers with hundreds

PT

of nanometers of width, as shown in Fig. 11(a) [28]. The addition of fGO increased the width

CE

of nanofibers; PAN/ fGO has an average width of 957.35 nm compared to pure PAN with an average width of 476.75 nm) (Fig. 11b). The carbonization process gave rise to a uniform

AC

CNF/fGO membrane (Fig 11c), with an insignificant amount of strayed fGO (Fig. 11d). The strayed fGO might be rather noticeable as the concentration of fGO increased, but the CNF/fGO membrane still possesses a homogenous morphology (Fig. 11e). Fig. 11(f) depicts astray fGO sandwiched among the nanofibers, constricting its movement thus enabled it to work synergistically with the CNF/fGO membrane in enhancing the electrochemical

17

ACCEPTED MANUSCRIPT properties. The EDX analysis portrays a high intensity of carbon compared with a negligible

AC

CE

PT

ED

M

AN US

CR IP T

amount of oxygen in the CNF/fGO membrane (Fig. 11g).

18

ACCEPTED MANUSCRIPT Figure 11. Surface morphology of (a) PAN, (b) PAN/fGO and (c) CNF/fGO; (d) fGO sheets intercalated within the nanofibers, (e) presence of the higher concentration fGO in the nanofibers, (f) high magnification of a single fGO sheet, and (g) EDX spectra of CNF/fGO. The Raman spectra in Fig. 12 illustrate the degree of graphitization upon the carbonization process of the nanofibers. Two fundamental bands were observed at 1349 cm-1 and 1569 cm-1

CR IP T

for CNF, which were attributed to the D-band and G-band, respectively. The calculated intensity ratio of D-band to G-band (ID/IG) was 0.97 for CNF, indicating that the CNF

AC

CE

PT

ED

M

AN US

comprised of graphitic rolls with minimal amount of disordered carbon content [29].

Figure 12. Raman spectra of (1) PAN and (2) CNF.

The functional groups presence within the synthesized nanofibers were analysed, at which the FT-IR spectra of both pure CNF and CNF/fGO were plotted in Fig. 13 respectively. The spectra of both samples were almost similar, with identical peaks, except with the presence of

19

ACCEPTED MANUSCRIPT a new vibrational peak corresponding to the O–H stretching at 3330 cm-1, possibly due to the residual oxygenated group of the fGO, which was intercalated on the surface of the nanofibers. Furthermore, the increase in the intensity of the peak at 1533 cm-1 corresponded

PT

ED

M

AN US

CR IP T

to the stretching vibration of the carbonyl group of PMMA/fGO.

AC

CE

Figure 13. FT-IR spectra of pure (1) CNF and (2) CNF/fGO.

In order to visualize the potential performance of the electrospun CNF/fGO as

supercapacitor active materials, a simple device was fabricated by having a conventional sandwich system, whereby identical pieces of the nanofibers (1 cm2) were separated by a filter paper that acted as a dielectric. The layers of the CNF/fGO-dielectric-CNF/fGO were then encapsulated into a stainless steel electrochemical cell, as shown in Fig. 1 previously.

20

ACCEPTED MANUSCRIPT The cyclic voltammetry (CV) curve of the pure CNF portrayed a skewed rectangular profile, indicating the presence of the resistivity within the electrode system, possibly attributing to the remaining oxygenated functional groups presence in the nanofibers after the carbonisation process (Fig. 14a). However, the CV profiles were tuned to behave towards pseudorectangular behaviour upon the addition of 1mg of fGO (CNF/fGO-1), as shown in Fig. 14(c).

CR IP T

An almost rectangular profile was recorded as the concentration of the fGO was doubled (CNF/fGO-2) (Fig. 14e), illustrating an improved performance upon the adaptation of the capacitive response of the graphene structure within the nanofibers [30].

AN US

A galvanostatic charge/discharge analysis was conducted to simulate the real-field application of a supercapacitor system, whereby the actual performance of the particular system was evaluated by applying a static current [31]. A pure CNF device recorded a specific capacitance of 55.11 F/g at the current density of 1 A/g and the capacitance retained

M

as the function increased in current rates. However, the voltage drop was prominent as the

ED

current rate increased, at which a maximum voltage drop was recorded at 0.24 V at a current density of 5 A/g (Fig. 14b). This was because the insulating oxygenated group presence

PT

within the nanofibers created resistance within the electrode/electrolyte interface. The addition of the fGO enhanced the specific capacitance to 75.19 F/g, compared to the pure

CE

sample, but the voltage leakage remained moderately high as the current rate continued to

AC

increase to the highest, at which the leakage was recorded at approximately 0.15 V, as shown in the inset of Fig. 14(d). As the concentration of the fGO was doubled up, the specific capacitance of the device was achieved at 140.10 F/g at a current rate of 1 A/gof up to 2.5 folds increment compared to pure CNF. The high specific capacitance was retained by 89% (124.93 F/g) even when the current rates increased up to 5 A/g, with only a minimal leakage of 0.099 V respectively (Fig. 14f). The promising performance effectively illustrated the enhancement in capacitive behaviour upon the addition of fGO within the nanostructures, at

21

ACCEPTED MANUSCRIPT which fGO serves as a template with large surface area to accommodate more electrolyte ions

AC

CE

PT

ED

M

AN US

CR IP T

adsorption. This finding complementing to that of the CV curves of CNF/fGO-2 device.

Figure 14. Cyclic voltammetry profiles at various scan rates of (a) pure CNF, (c) CNF/fGO-1, and (e) CNf/fGO-2; galvanostatic charge/discharge analysis of (b) pure CNF, (d) CNF/fGO-1,

22

ACCEPTED MANUSCRIPT and (f) CNF/fGO-2. Insets illustrate the capacitance and leakage drop as the function of current density was applied. The electrochemical impedance spectra of pure CNF, CNF/fGO-1, and CNF/fGO-2 are illustrated in a Nyquist plot in Fig. 15(a). Generally, two important features were shown in the plot: a high frequency arc region corresponding to the resistivity (Rct) caused by the

CR IP T

charge-limiting process at the interface of electrode and electrolyte [32], which can be obtained from the diameter of the semi-circle of the arc [33,34]. The equivalent series resistance (ESR) corresponding to the resistance of the solution along with the internal

AN US

resistance of the electrode could be obtained from the high-frequency region, corresponding to the first X-axis intercept of the Nyquist plot [35,36]. As depicted in the inset of Fig. 15(a), the highest semi-circle arc with the largest diameter was associated to CNF/fGO-2, signifying a higher charge transfer resistance compared to the other samples. The pure CNF and

M

CNF/fGO-1 recorded an almost similar semi-circle diameter, indicating that the system possessed a lower electrode/electrolyte resistance. The Rct value recorded a slight increment

ED

upon addition of the fGO (12.78 Ω compared to pure CNF of 10.39 Ω), possibly due to the

PT

PMMA branching chains presence on the surface upon functionalization that affects the electron transfer mechanism, therefore contributed towards the overall resistivity of the

CE

electrode materials. The findings were agreed to the point that further increase in concentration of fGO within the carbon nanofibers significantly increased the Rct value up to

AC

22.45 Ω respectively. On the other hand, the ESR of carbon nanofibers minimally increased upon the addition of a higher amount of fGO, at which the CNF/fGO-2 recorded an ESR value of 0.288 Ω compared to the pure CNF (0.22 Ω). Meanwhile, the CNF/fGO-1 recorded the lowest at 0.17 Ω. All the nanofiber-based supercapacitors also possessed identical vertical behaviours behind the Warburg region, indicating that the total internal structure of the porous carbon electrode was completely wetted by the electrolytes [37].

23

ACCEPTED MANUSCRIPT The cycle performance of the CNF/fGO-2-based supercapacitor is plotted in Fig. 15(b) accordingly. Although a higher value of Rct was recorded for CNF/fGO-2, the system showed a very high capacitance retention of 96.2% even after 1000 continuous charge/discharge cycles at 1 A/g, with a very minimal voltage drop of 0.04 V recorded (0.02 V was recorded at the initial cycle), attributing to the unique intercalation of fGO within the nanofibers that

CR IP T

facilitated the ion transport effectively across the electrode/electrolyte interfaces. Furthermore, the intercalated system prevented the loss of electrode materials due to degradation during the

PT

ED

M

AN US

vigorous influx/outflow of electrolyte ions.

Figure 15. (a) Nyquist plots of pure (i) CNF, (ii) CNF/fGO-1, and (iii) CNF/fGO-2. The inset

CE

magnified the high frequency region of the spectra; (b) Capacitance retention and voltage

AC

drop of CNF/fGO-2 as the function of continuous charge/discharge cycles.

4. Conclusions Graphene oxide-reinforced carbon nanofibers were successfully synthesized via a combination of surface functionalization of graphene oxide and the electrospinning technique. The UV-initiated functionalization of graphene oxide successfully tuned the hydrophilic

24

ACCEPTED MANUSCRIPT nature of graphene oxide with hydrophobicity, which was demonstrated by its ability to dissolve readily in organic solvent, as well as in hydrophobic polymeric solution. A unique morphology was shown by having the functionalized graphene oxide sheets embedded within the nanofiber surface, which significantly contributed towards the overall electrochemical performance of the conductive carbon-rich structure. The fabricated fGO-reinforced CNF

CR IP T

possessed a remarkable performance by recording a specific capacitance of 140.10 F/g, approximately three times higher than that of a pure CNF, along with a high stability capacitance retention of 96.2% after 1000 times of charge/discharge cycles at 1 A/g. This novel technique of functionalization provided an alternative pathway in tuning the solubility

AN US

nature of graphene oxide towards the synthesis of efficiently-reinforced nanofiber structures.

M

Acknowledgments

ED

This research work was supported by IRU-MRUN (9399901).

PT

References

AC

CE

[1] M.D. Stoller, S. Park, Y. Zhu, J. An, R.S. Ruoff, Nano Lett. (8)(2008)3498-3502. [2] K.P. Loh, Q. Bao, P.K. Ang, J. Yang, J. Mater. Chem. (20)(2010)2277-2289. [3] T. Kuila, S. Bose, A.K. Mishra, P. Khanra, N.H. Kim, J.H. Lee, Prog. Mater. Sci. (57)(2012)1061-1105. [4] R. Verdejo, M.M. Bernal, L.J. Romasanta, M.A. Lopez-Manchado, J. Mater. Chem. (21)(2011)33013310. [5] J.R. Potts, D.R. Dreyer, C.W. Bielawski, R.S. Ruoff, Polymer (52)(2011)5-25. [6] D.S. Su, G. Centi, J. Energy Chem. (22)(2013) 151-173. [7] E. Frackowiak, Q. Abbas, F. Béguin, J. Energy Chem. (22)(2013) 226-240. [8] X. Zhang, X. Cheng, Q. Zhang, J. Energy Chem. (25)(2016)967-984. [9] W. Chee, H. Lim, N. Huang, I. Harrison, RSC Adv. (5)(2015)68014-68051. [10] D.R. Dreyer, S. Park, C.W. Bielawski, R.S. Ruoff, Chem. Soc. Rev. (39)(2010)228-240. [11] D.C. Marcano, D.V. Kosynkin, J.M. Berlin, A. Sinitskii, Z. Sun, A. Slesarev, L.B. Alemany, W. Lu, J.M. ACS Nano (4)(2010)4806-4814. [12] Z.-M. Huang, Y.Z. Zhang, M. Kotaki, S. Ramakrishna, Compos. Sci. Technol. (63)(2003)2223-2253. [13] N. Bhardwaj, S.C. Kundu, Biotech. Adv., (28)(2010)325-347. [14] B. Sun, Y.Z. Long, Z.J. Chen, S.L. Liu, H.D. Zhang, J.C. Zhang, W.P. Han, J. Mater. Chem. C (2)(2014)1209.

25

ACCEPTED MANUSCRIPT

PT

AC

CE

Graphical abstract

ED

M

AN US

CR IP T

[15] H. Fan, F. Ran, X. Zhang, H. Song, W. Jing, K. Shen, L. Kong, L. Kang, J. Mater Chem. (23)(2014)684-693. [16] Z.P. Zhou, X.F. Wu, J. Power Sources (222)(2013)410-416. [17] Z.P. Zhou, X.F. Wu, J. Power Sources (262)(2014)44-49. [18] M. Wei, M. Jiang, X. Liu, M. Wang, S. Mu, J. Power Sources (327)(2016) 384-393. [19] T. Zhang, H. Zhao, X.X. Huang, G. Wen, Desalination (379)(2016) 118-125. [20] A. Jha, R. Roy, D. Sen, K.K. Chattopadhyay, Appl. Surf. Sci. (366)(2016) 448-454. [21] X. Li, Y. Yang, Y. Zhao, J. Lou, X. Zhao, R. Wang, Q. Liang, Z. Huang, Carbon (114)(2017)717-723. [22] N. Huang, H. Lim, C. Chia, M. Yarmo, M. Muhamad, Int. J. Nanomed. (6)(2011)3443. [23] W.K. Chee, H.N. Lim, N.M. Huang, Int. J. Energy Res. 39(1)(111-119) [24] Y. Andou, M. Yasutake, H. Nishida, T. Endo, J. photopolym. Sci. Technol. (20)(2007)523-528. [25] M.D. Stoller, R.S. Ruoff, Ener. Env. Sci. (3)(2010)1294-1301. [26] P. Cao, J. Yao, B. Ren, R. Gu, Z. Tian, J. Phys. Chem. B (106)(2002)10150-10156. [27] Z.G. Huaming SUN, Lin YANG, Lingxiang GAO, Front. Chem. China (4)(2009) 89-92. [28] S.-y. Gu, Q.-l. Wu, J. Ren, New Carbon Mater. (23)(2008) 171-176. [29] W. Li, L.-S. Zhang, Q. Wang, Y. Yu, Z. Chen, C.-Y. Cao, W.-G. Song, J. Mater. Chem. (22)(2012)15342-15347. [30] M. Zhi, A. Manivannan, F. Meng, N. Wu, J. Power Sources (208)(2012) 345-353. [31] W.K. Chee, H.N. Lim, I. Harrison, K.F. Chong, Z. Zainal, C.H. Ng, N.M. Huang, Electrochim. Acta (157)(2015) 88-94. [32] Y.-L. Chen, Z.-A. Hu, Y.-Q. Chang, H.-W. Wang, Z.-Y. Zhang, Y.-Y. Yang, H.-Y. Wu, J. Phys. Chem. C (115)(2011)2563-2571. [33] D.P. Dubal, S.H. Lee, J.G. Kim, W.B. Kim, C.D. Lokhande, J. Mater Chem. (22)(2012)3044-3052. [34] M. Hassan, K.R. Reddy, E. Haque, S.N. Faisal, S. Ghasemi, A. I. Minett, V.G. Gomes, Composit. Sci. Tech. (98)(2014)1-8. [35] Y.S. Lim, Y.P. Tan, H.N. Lim, N.M. Huang, W.T. Tan, M.A. Yarmo, C.-Y. Yin, Cer. Inter. (40)(2014)3855-3864. [36] Y. Li, K. Ye, K. Cheng, J. Yin, D. Cao, G. Wang, J. Power Sources (274)(2015)943-950. [37] E.J. Ra, E. Raymundo-Piñero, Y.H. Lee, F. Béguin, Carbon (47)(2009)2984-2992.

26

AN US

CR IP T

ACCEPTED MANUSCRIPT

M

Description:

AC

CE

PT

ED

Carbonization of functionalized graphene oxide with polyacrylonitrile for the formation of graphene oxide-reinforced carbon nanofibers.

27