- Email: [email protected]
carbon nanofiber nanocomposite on charge storage behaviors
Journal Pre-proof Effects of the composition of reduced graphene oxide/carbon nanofiber nanocomposite on charge storage behaviors Tolendra Kshetri, Duy Thanh Tran, Thangjam Ibomcha Singh, Nam Hoon Kim, Kintak Lau, Joong Hee Lee PII:
S1359-8368(19)34425-7
DOI:
https://doi.org/10.1016/j.compositesb.2019.107500
Reference:
JCOMB 107500
To appear in:
Composites Part B
Received Date: 29 August 2019 Revised Date:
28 September 2019
Accepted Date: 30 September 2019
Please cite this article as: Kshetri T, Tran DT, Singh TI, Kim NH, Lau K-t, Lee JH, Effects of the composition of reduced graphene oxide/carbon nanofiber nanocomposite on charge storage behaviors, Composites Part B (2019), doi: https://doi.org/10.1016/j.compositesb.2019.107500. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier Ltd.
Graphical Abstract
Effects of the Composition of Reduced Graphene Oxide/Carbon Nanofiber Nanocomposite on Charge Storage Behaviors Tolendra Kshetri1, Duy Thanh Tran 1, Thangjam Ibomcha Singh 1, Nam Hoon Kim1*, Kin-tak Lau2 and Joong Hee Lee1,3** 1
Advanced Materials Institute for BIN Convergence Technology (BK21 Plus Global Program),
Department of BIN Convergence Technology, Jeonbuk National University, Jeonju, Jeonbuk, 54896 Republic of Korea 2
Faculty of Science, Engineering & Technology, Swinburne University Technology, Hawthorn,
VIC, 3122, Australia 3
Carbon Composite Research Centre Department of Polymer−Nano Science and Technology,
Jeonbuk National University Jeonju, Jeonbuk 54896, Republic of Korea.
*Corresponding author: Tel.: 82-63-270-2342; Fax: 82-63-270-2341. * E-mail address: [email protected] (Prof. Nam Hoon Kim) ** E-mail address: [email protected] (Prof. Joong Hee Lee)
Abstract: Porous carbon materials with hierarchical features are the important materials to study the electrochemical properties for supercapacitor electrode application. Depending on the synthesis procedures and the carbon precursors used, these carbon materials possess different structures, and they are always associated with different amounts of various oxygen functional groups affecting the wettability, conductivity, and electrochemical performance. In this context, various electrochemical methods are adopted to study the charge storage contribution. Among them,
separation of surface-controlled (capacitive) and diffusion-controlled currents from cyclic voltammetry (CV) curve by using the potential sweep method (PSM) is a preferred method. Herein, three-dimensional “Reduced Graphene Oxide-Carbon Nanofiber” (rGO-CNF) hybrid materials are fabricated by using different rGO:CNF ratios to study the effect of the compositon of rGO and CNF in the charge storage behaviors. The structural, elemental, and electrochemical characterizations of the materials are performed to quantitatively study the surface-controlled (capacitive) and diffusion-controlled charge storage contributions in the rGO-CNF hybrid. It is found that the increase of rGO content in the hybrid material significantly affects the electrochemical properties of the hybrid materials.
KEYWORDS: Reduced graphene oxide; carbon nanofiber; Surface-controlled; Diffusioncontrolled; Potential sweep method
1.
Introduction: In the past few decades, electrochemical capacitor (also called as supercapacitor) has been
rapidly emerging as an electrochemical energy-storage device (EESD) with great promise as a suitable power source for various modern electronic gadgets, electric vehicles, and other applications because of their ability to store and deliver electrical energy at relatively high rates [1-5]. Since the birth of this technology, various carbon-based materials including activated carbon [6], carbon nanotubes (CNTs) [7], carbon nanofibers (CNF) [8], graphene [9,10], and their composites [11–20,], have been extensively employed as the electrode materials in the supercapacitor owing to their high electrical conductivity, desirable porous feature, and high
specific surface area [21-23]. Many studies have exhibited that the specific capacitance of a carbon-based electrode material can be increased by improving the electrical conductivity and specific surface area, and also by tuning the pore size distributions [21]. In this regard, many efforts have been made to enhance the specific surface area by making different carbon-based materials into a hierarchical structure [12,24,25]. However, the nature of the charge storage and the mechanism involved have not been identified well, since the general trend of pure carbon materials follows an electric double layer capacitor (EDLC) nature, which is manifested by a rectangular CV and a triangular galvanostatic charge-discharge (GCD) curves [26,27]. Hence, increasing of specific capacitance in multi-structured carbon composites prepared from various carbon precursors has been attributed to the improvements in electrical conductivity, specific surface area, pore size distributions, and the synergistic effects of the various nanostructures. However, it is also essential to note that, depending on the synthesis route and the nature of the carbon precursors, many carbon materials primarily based on rGO are always associated with surface functional groups, such as carboxylic, lactonic, and phenolic [28,29]. So, it is highly expected that rGO-based carbon materials and other oxygen functional groups containing carbons may also make a significant contribution towards the overall capacitance when they are used as electrodes in supercapacitors. Moreover, a detailed electrochemical study of different charge kinetics to understand the charge storage mechanism in carbon materials is very crucial in the scientific community. Many of the previously reported studies on such carbon materials have not undertaken of such investigations, which are crucial for a proper understanding of the nature of charge storage, and the mechanism associated with it, which would significantly affect the further development of carbon-based electrode materials [2,21,30]. Recently, only a few researchers have started taking
into consideration and investigating the effect of such surface functionalities on the electrochemical performance of the electrodes [29,31]. However, such investigations have been confined to metal-based pseudo-capacitive materials and only a few common carbon materials [31–35]. Therefore, further explorations of various other carbon-based materials are also highly necessary for broadening the outlook on the charge-storage phenomenon. In this context, carbon aerogels are potential candidates for supercapacitor electrodes because of their high specific surface areas, considerable electrical conductivity, stable structures, and controllable pore size distribution [36–38]. rGO-CNF based aerogel is an effective carbon material owing to its porosity and unique hierarchical structures [39]. Herein, we have fabricated rGO-CNF aerogel nanocomposites with different ratios of rGO and CNF as potential candidates for electrode application in supercapacitors. A quantitative investigation of the surface-controlled (capacitive), and the diffusion-controlled charge storage contribution in the rGO-CNF is also carried out from CV analysis by using the PSM. The results show that the content of rGO in rGO-CNF have a high impact on its electrochemical properties, and the involved charge-storage mechanism, suggesting an appropriate ratio of rGO:CNF is crucial to obtain an optimized electrochemical performance for electrode application.
2.
Experimental Methods
2.1
Materials and Methods
2.1.1 Materials: Polyacrylonitrile (PAN, M=150,000) was obtained from Sigma-Aldrich, USA; graphite flake, hydrochloric acid (HCl, 37%), sulfuric acid (H2SO4, 95%), potassium permanganate (KMnO4), , dimethylformamide (DMF, 99%), potassium hydroxide (KOH, 85%),
and hydrogen peroxide (H2O2, 34.5%) were obtained from Samchun, Korea, and used without further treatment or purification. 2.1.2 Synthesis of rGO-CNF aerogel: Preparation of the rGO-CNF aerogels was performed according to the scheme represented in Scheme 1. Firstly, the polymer nanofiber mat was obtained from the PAN/DMF solution by electrospinning technique similar to our previous work [14], and graphene oxide (GO) was synthesized by the chemical oxidation of the graphite flake following the well-known Hummer’s method [40]. Then 0.06 g, 0.05 g, and 0.04 of PAN fiber pieces and 0.04 g, 0.05 g and 0.06 g of GO were respectively mixed and dispersed in 15 ml deionized (DI) water in three separate plastic conical tubes using a tip-sonicator for one hour to obtain homogeneous GO-PAN (1:2), GO-PAN (1:1) and GO-PAN (2:1) gel solutions. The GOPAN gel solutions were frozen in liquid nitrogen to obtain GO-PAN hybrid aerogel after the freeze-drying process. Finally, we obtained the rGO-CNF after a two-step thermal annealing process- stabilization and carbonization processes. During the stabilization process, the GO-PAN aerogel was heated at 280 °C at the heating rate of 1 °C min-1 and remained at 280°C for two hours in the air. After that, the stabilized GO-PAN aerogel was carbonized at 800 °C at the rate of 5 °C min-1 in argon atmosphere for one hour to obtain the rGO-CNF hybrid material.
2.2
Characterization Methods
2.2.1 Material Characterization Field emission scanning electron microscopy (FE-SEM), Carl Zeiss, SUPRA 40 VP, Germany, installed at the Center for University-wide Research Facilities (CURF), Jeonbuk National University was used to analyze the surface morphologies of the prepared samples.
Atomic force microscopy (AFM), Park NX10, Korea was used to get the surface topography image of GO using a non-contact PPP-NCHR 5M cantilever. X-ray diffraction (XRD) patterns of the materials were obtained in the 2θ range from 10° to 80° at the scan rate of 2°/min by using Rigaku diffractometry (Cu-Kα, λ = 1.5406 Ǻ, Japan). The Raman spectra of the materials were attained by using 532 nm laser (0.5 mW) in a Nanofinder-30 instrument (Tokyo Instruments Co., Japan). The elemental compositions of the materials were analyzed from the energy-dispersive X-ray spectroscopy (EDS) obtained from SEM, and X-ray photoelectron spectroscopy (XPS) (Theta Probe AR-XPS System, Thermo Fisher Scientific, UK) in the Busan center of KBSI. Nitrogen adsorption-desorption isotherms of the materials were taken at 77 K to study the Brunauer-Emmett-Teller (BET) specific surface area and the Barrett-Joyner-Halenda (BJH) pore size distributions by using a Micromeritics Tristar 3000 analyzer. To study the electrical conductivities of the materials, the materials were made into a thin film, and the sheet resistance were measured with a four-probe surface resistivity meter (MSTEC) using Keithley DC source and nanovoltmeter. 2.2.2 Electrochemical Characterization A three-electrode electrochemical set-up was used to measure the electrochemical properties of rGO, CNF, and rGO-CNF materials with Hg/HgO as the reference electrode and a platinum (Pt) foil as the counter electrode in a CHI660E, USA potentiostat. An alkaline solution of 6 M potassium hydroxide (KOH) was employed as the electrolyte. For the preparation of the working electrode, a nickel (Ni) foam was cut into six pieces of dimension 1 cm × 2 cm. Then they were washed by ultrasonication with HCl, ethanol, and water consecutively for 15 mins each. The cleaned Ni foams were dried in hot air using a blow dryer. After that, 2 mg of each sample (rGO-CNF (1:2), rGO-CNF (1:1), and rGO-CNF (2:1) were sandwiched between two
cleaned 1 cm × 2 cm sized Ni-foam using a hydraulic pressure to obtain the final working electrode. The cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) techniques were conducted to analyze the electrochemical properties of the materials. The specific capacitance of the rGO-CNF materials was evaluated from the GCD curve by applying the following equation [41]. =
× ∆ (1) × ∆
where, I (A), ∆t, m (g), ν (V/s), and ∆V (V) are the current, the discharged time, the mass of the active electrode material, and the potential window.
Scheme 1. Schematic procedure for preparation of rGO-CNF hybrid. 3.
Results and Discussions
3.1. Morphological characterization
The surface morphologies and the structural features of GO, rGO, CNF, and rGO-CNF were analyzed using FE-SEM. The FE-SEM image of the prepared GO is exhibited in Fig. S1 of the supporting information (SI). From the AFM height profile image, we observe that the prepared GO could be exfoliated well into thin sheets with a thickness of ~ 2 nm, which is an
essential factor for synthesis of rGO-CNF aerogel. The FE-SEM images of rGO-CNF (1:2), (1:1), and (2:1) are exhibited in Fig. 1(a, b), (d, e), and (g, h), respectively. From the images, we observe that the rGO and CNF components in the rGO-CNF materials are interconnected, leading to a hierarchical porous structure, which is an important parameter for electrode application. The CNFs were wrapped by the rGO sheets suggesting a strong interaction between the CNF and rGO components. In rGO-CNF (1:2), the CNF content is higher than that of the rGO, whereas, in rGO-CNF (2:1), the rGO content is higher than that of the CNF. In all the rGOCNF materials, there is a strong binding between the rGO sheets and the CNF interconnecting to each other, which makes it possible to form a stable 3D hierarchical rGO-CNF aerogel. Moreover, the rGO sheets serve a crucial role in the formation of 3D rGO-CNF hybrid material by providing a linkage between the individual CNF. Without the support of rGO; the CNFs cannot form a stable three-dimensional structure, because there is no linkage between the individual CNFs, as seen from the FE-SEM image of 3D CNF in Fig. S1. On the other hand, the CNF provides support to rGO sheets and also prevents the rGO sheets from re-stacking, thereby forming a hierarchical porous 3D rGO-CNF aerogel. The change in the ratio between the rGO and CNF components affect the porous feature and electrochemical performance as depicted from the BET and BJH analysis and the electrochemical analyses.
Fig. 1. FE-SEM images (low and high magnification) and EDS spectra of (a,b,c) rGO-CNF (1:2); (d, e, f) rGO-CNF (1:1); and (g, h, i) rGO-CNF (2:1).
3.2. Structural and porous feature characterizations
XRD and Raman spectroscopy techniques were employed to analyze the crystalline and structural properties of the rGO-CNF composite materials [42–44]. The XRD patterns of CNF, rGO, rGO-CNF (1:2), (1:1), and (2:1) are shown in Fig. S3, and Fig. 2. All the prepared carbon materials exhibit two XRD peaks at 2θ of 25.8° and 42.8°, which correspond to the (002) and (101) graphitic plane (PDF# 03-065-6212). From the XRD pattern, we observe that the (002) and (101) peaks of CNF are sharper than those of the rGO, indicating that the CNF is more crystalline than the rGO, as consistent with the electrical conductivity measurement shown in Table 1. The sheet resistances of CNF and rGO are respectively 65.98 ± 2.67 Ω/ and 89.63 ± 2.67 Ω/ . As expected, the (002) and (101) peaks of the rGO-CNF materials become broader when the amount of rGO is increased, thereby decreasing the electrical conductivity. The measured sheet resistance of rGO-CNF (1:2), (1:1), and (2:1) are 66.62 ± 2.67 Ω/ , 71.94 ± 3.67 Ω/ , and 88.81 ± 3.47 Ω/ , respectively. The broad XRD peak of rGO suggests its amorphous
nature due to incomplete reduction and the presence of surface oxygen functionalities, which affect the electrochemical performance of the materials. Further, the structural properties of the rGO-CNF materials were investigated from the Raman spectroscopy. The Raman spectra of rGO-CNF (1:2), (1:1), and (2:1) exhibit the D band (1350 cm-1) and G band (1580 cm-1), which are typical peaks for carbon materials [45]. The D band is the signature for the structural defect associated in the graphitic materials, whereas the G band relates to the stretching mode in the graphitic plane. Therefore, the ratio of the D band intensity to the G band intensity (ID/IG) is crucial parameter to calculate the structural disorder quantitatively [46]. From the Raman spectra (Fig. 2(b)), we observe that the ID/IG ratios for rGO-CNF (1:2), (1:1), and (2:1) are 0.93, 0.94,
and 0.96 respectively suggesting that higher amount of defects are present in the rGO-CNF materials when the rGO content is increased due to incomplete thermal reduction of GO [47]. The porous characteristics of the rGO, CNF, and rGO-CNF materials were analyzed by measuring the BET specific surface area and BJH pore size distribution from the nitrogen adsorption-desorption isotherms (Fig. S2 and Fig. S4). The specific surface area of CNF, rGO, rGO-CNF (1:2), (1:1) and (2:1) are 114.74 m2 g-1, 180.98 m2 g-1, 319.14 m2 g-1, 367.73 m2 g-1, and 260.43 m2 g-1, respectively. It is observed that the specific surface area of rGO-CNF is much higher than that of the individual CNF and rGO components owing to its hierarchical porous characteristic. However, the specific surface area of rGO-CNF (2:1) is lower than that of rGOCNF (1:2) and (1:1) because of the re-stacking of rGO sheets during the freeze-drying, and thermal annealing process as the amount of rGO is higher than the CNF. The BJH pore size distribution curve suggests that the rGO-CNF materials possess mesoporous characteristics (~ 2 nm) and the rGO-CNF (1:1) has higher pore volume than the other synthesized materials. However, for rGO, the pore size is distributed in the range of 20 nm to 60 nm, and the pore volume is also lower than that of the pristine CNF. Therefore, as expected, the rGO-CNF (2:1) has a lower pore volume because of the presence of a higher amount of rGO sheets. The availability of a large volume of mesopores in the rGO-CNF materials is beneficial for increasing the specific capacitance by providing larger accessible sites for adsorption of the electrolyte ions in the charge-discharge process [48].
PDF# 03-065-6212
C (101) Graphene-CNF-2:1 Graphene-CNF-1:1
rGO-CNF (2:1) rGO-CNF (1:1) rGO-CNF (1:2)
(b) Intensity (a.u)
C (002)
Intensity (a.u.)
(a)
ID/IG = 0.96
ID/IG = 0.94 ID/IG = 0.93
Graphene-CNF-1:2
20
40 50 60 2θ (degree)
70
80
500
rGO-CNF (2:1) rGO-CNF (1:1) rGO-CNF (1:2)
C1s
Intensity (a.u)
(c)
30
N1s
1000
1500 2000 2500 Raman shift (cm-1) C-C (284.4)
(d) Intensity (a.u.)
10
O1s
3000
3500
rGO-CNF (1:2) C-C
75.24 %
C-O
16.42 %
C=O
6.62 %
O-C=O 1.72 % C-O (286.3) C=O (287.4) O-C=O (289)
200
400 600 Binding energy (eV) C-C (284.4)
Intensity (a.u.)
(e)
800
282
284 286 288 Binding energy (eV)
(f)
rGO-CNF (1:1)
C-C (284.4)
C-C
74.84 %
C-O
15.91 %
C-O
16.61 %
C=O
6.16 %
C=O
6.33 %
O-C=O 2.03 % C-O (286.3)
O-C=O 2.39 % C-O (286.3)
C=O (287.4)
284
rGO-CNF (2:1)
75.73 %
C=O (287.4) O-C=O (289)
O-C=O (289)
282
290
C-C
Intensity (a.u)
0
286 288 Binding energy (eV)
290
282
284
286 288 Binding energy (eV)
290
Fig. 2. (a-c) XRD patter, Raman spectra, XPS survey spectra; and (d-f) deconvoluted C1s spectra of rGO-CNF (1:2), rGO-CNF (1:1), and rGO-CNF (2:1) respectively.
3.3. Elemental characterization
The elemental analysis of the materials for supercapacitor electrode application is very crucial because the elements present on the electrode materials significantly affect its electrochemical properties [49,50]. The elemental contents of the rGO-CNF materials were determined using EDS and XPS. The EDS and the XPS survey spectra of rGO-CNF (1:2), (1:1), and (2:1) are illustrated in Fig. 1(c, f, i) and Fig. 2(c), respectively, in which the presence of three elements – carbon (C), nitrogen (N), and oxygen (O) was confirmed. The atomic percent (At. %) of each element present in the different materials are listed in Table 1. The EDS and XPS spectra show that the At. % of C and O increases when the rGO content is increased in the rGOCNF, thus affecting the electrical conductivity and electrochemical properties as stated in Table 1. However, the At. % of nitrogen (N) decreases when the rGO content is increased because the N in rGO-CNF is attributed to the PAN polymer, which was used as the precursor for the CNF [51]. Further, to understand the presence of oxygen functionalities in the materials and the bonding between the carbon (C) and oxygen (O), the C1s peak of rGO, CNF, and rGO-CNF (1:2), (1:1) and (2:1) materials were deconvoluted (Fig. (S3(c), (d)) and Fig. 2(d, e, f)). The highresolution C1s peak reveals the presence of C─C, C─O, C═O, and O─C═O. The percentages of each bonding are shown in the figures in which the percentage of C─C decreases as the amount of rGO content increases in rGO-CNF affecting the electrical conductivity and electrochemical properties of the hybrid materials. Among the rGO-CNF materials, rGO-CNF (1:2) has the highest amount of C─C bonding and has better electrical conductivity than the other materials, whereas rGO-CNF (2:1) has the lowest amount of C─C bonds and the lowest electrical conductivity. The presence of oxygen surface functional groups also contributes to the
capacitance of the materials as measured from the electrochemical characterization of the rGOCNF materials with different ratios of rGO and CNF. Table 1. Comparison of elemental composition, sheet resistance, and specific capacitance of CNF, rGO, rGO-CNF (1:2), rGO-CNF (1:1), and rGO-CNF (2:1).
Sheet resistance
Specific
Carbon
Nitrogen
Oxygen
(At. %)
(At. %)
(At. %)
(Ω/ )
CNF
85.64
9.08
5.28
65.98 ± 2.67
105.2
rGO
89.18
-
10.82
89.63 ± 2.67
91.7
rGO-CNF (1:2)
89.55
3.98
6.48
66.62 ± 2.67
192.3
rGO-CNF (1:1)
89.57
3.29
7.14
71.94 ± 3.07
223.5
rGO-CNF (2:1)
90.28
2.44
7.27
88.81 ± 3.47
191.0
Sample
capacitance at 0.5 A g-1 (F g-1)
3.4. Electrochemical characterization The electrochemical performance of the CNF, rGO, rGO-CNF (1:2), (1:1), and (2:1) materials were evaluated in an alkaline aqueous electrolyte (6 M KOH) using the CV, GCD, and EIS techniques. Fig. 4(a, b, c) displays the CV curves of rGO-CNF (1:2), (1:1), and (2:1) at different scan rates. The CV curves of CNF and rGO are shown in Fig. 3(a and c). All the
materials exhibit quasi-rectangular CV curves, indicating that the primary charge storage comes from the electric double-layer mechanism (a surface-controlled phenomenon), as is also supported by the quasi-triangular GCD curve [52]. The charge storage contribution is also anticipated from the surface redox reaction between the oxygen-containing functional groups and the electrolyte solutions via the quasi-reversible redox reaction illustrated in Equation (2) [31]. The detailed analysis of the surface-controlled and diffusion-controlled charge storage contribution is discussed in the following section. In the alkaline electrolyte, the CV curves of rGO, CNF, and rGO-CNF can be analyzed into three characteristic segments, each of which signifies different stage of electrochemical double-layer formation. In the low potential region, the hydrated K+ in the KOH electrolyte could reach the electrode surface rapidly, forming an outer Helmholtz layer [31]. However, in the mid-potential region, an interfacial tension has attained a maximum value as the hydrated K+ has dissipated, and the effect of steric hindrance of the oxygen functionalities improve the ions diffusion into the bulk solution [31]. However, the hydrated K+ ion has been squeezed in the high potential region, and there is a quasi-reversible process represented by Equation (2) [31]. Moreover, the concavity point observed in the CV curve is also expected because of the surface oxygen functionalities on the electrode materials [31]. The concavity point is most significant in rGO-CNF (2:1) due to the higher oxygen contents. >CxO + [K(H2O)n]+ + e- ↔ >CxOK(H2O)n-y
(2)
where n is the number of H2O molecules associated with K+, and y is the number of H2O molecules which the hydrated K+ has lost when squeezed.
Fig. 3. CV and GCD curves of (a,b) rGO and (c,d) CNF respectively; (e,f) Specific capacitance vs. current density and Nyquist plot of rGO and CNF (inset is the equivalent circuit diagram).
The specific capacitance of the electrode materials were evaluated from the GCD curves using Equation (1); rGO, CNF, rGO-CNF (1:2), (1:1), and (2:1) show specific capacitance of 91.7, 105.2, 192.3, 223.5, and 191.0 F g-1 at 0.5 A g-1, respectively. A significant increase of specific capacitance in the rGO-CNF material compared to the individual rGO and CNF is because of its hierarchical porous feature with high specific surface area and well-developed pore structure which facilitates the movement and diffusion of electrolyte ions [44,53]. From Fig. S6(d), we observe that the rGO-CNF materials have a low rate capability at low current densities from 0.25 A g-1 to 1 A g-1. However, at higher current densities, the rGO-CNF materials maintain good rate capabilities. This characteristic of the materials is attributed to self-discharge or charge leakage due to the presence of surface functionalities and redistribution of electrolyte ions in the porous carbon structure during the discharging process [7,28,52,54]. However, at higher current densities, the rGO-CNF materials exhibit a good rate capability as the capacitive mechanism is more dominant than the pseudocapacitive mechanism. In addition, the rGO-CNF also exhibits high cyclic stability, in which the stability is highest for rGO-CNF (1:2) and lowest for rGOCNF (2:1) signifying that the electrochemical stability decreases as the amount of rGO in rGOCNF increases. From the EIS analysis (Nyquist plot) in Fig. S6(e), we observed that rGO-CNF (2:1) has the highest charge-transfer resistance due to the higher oxygen content which agrees with electrical conductivity measurement. The increase of the slope of the Warburg impedance region in the Nyquist plot indicates an increase of ionic diffusion impedance because there are more oxygen functionalities on the surface of the electrode materials [31].
Fig. 4. (a,b,c) CV curves; (d,e,f) b vs potential (insets are the log(i) vs log(ν) curves) of rGOCNF (1:2), rGO-CNF (1:1), and rGO-CNF (2:1) respectively.
3.4.1 Deconvolution of surface-controlled and diffusion-controlled charge storage contribution From the above electrochemical analysis, it has been noticed that the increase of rGO content in the rGO-CNF materials has a great influence on their electrochemical properties because of the oxygen surface functionalities, which have a pseudocapacitive charge storage contribution in alkaline aqueous electrolyte. It is also expected that the charge storage in rGOCNF has contribution from the diffusion-controlled process [55,56]. Therefore, the quantitative study of the surface-controlled and diffusion-controlled charge storage contribution from the CV curve by using PSM method is also an important step for a better understanding of the charge storage kinetics. In this regard, we analyzed the capacitive and pseudo-capacitive contributions from the CV data measured at different potential sweep rates (5 mV s-1 ~ 200 mV s-1) based on the following power-law equation [57–59]: i = aνb
(3)
which can be re-written as log(i) = blog(ν) + log(a)
(4)
where, i, ν are the current (A) and the potential sweep rate (mV s-1), and a, b are both adjustable parameters. As based on the interrelation between the current and potential sweep rate, the current can be separated into two parts: the diffusion-limited process (when b=0.5) and the surface-limited process (when b=1). To determine the b-value for rGO, CNF, rGO-CNF materials, a curve between log(i) vs. log(ν) in the cathodic scan is plotted at various potentials, and curves were fitted to calculate the slopes which represent the b-values as shown in the inset of Fig. S5(a, d) and Fig. 4(d-f). The
log(i) vs. log(ν) curve of rGO-CNF materials in the anodic scan is also displayed in Fig. S7(a-c). It is observed that the b-values of the materials are between 0.8 and 1 at different potentials which suggests that the measured currents have contributions from both the surface-controlled process (capacitive) and diffusion-controlled process with significant contributions from the capacitive process [55,56,60]. Further, according to the above concepts, the total current response of the CV analysis at a particular potential can be separated into two parts corresponding to the surface-controlled capacitive behavior and the diffusion-controlled charge storage behavior as [33,61]: i(V) = k1ν + k2ν1/2
(5)
For the sake of analytical simplicity, the above equation can be re-written as i(V)/ν1/2 = k1ν1/2 + k2
(6)
where, k1 and k2 are constants independent to potential sweep rate (ν) and the terms k1ν and k2ν1/2 in equation (5) correspond to the surface-controlled process and the diffusion-controlled
process, respectively. Thus, we can find the values of k1 and k2 from the plot of i(V)/ν1/2 vs. ν1/2 at a particular voltage to quantify the current contributions from the surface-controlled and diffusion-controlled processes in rGO, CNF, and rGO-CNF materials. Fig. 5(a-c) shows the i(V)/ν1/2 vs. ν1/2 curve of rGO-CNF (1:2), (1:1), and (2:1), respectively, at a specified potential of
-0.5 V. The surface-controlled and diffusion-controlled current percentages with respect to the different scan rates at a fixed potential (-0.5 V) of the materials are exhibited in Fig. 5(d-f), indicating that the rGO-CNF materials with higher rGO content have higher diffusion-controlled current. Moreover, it is also observed that the surface-controlled capacitive current is more
dominant at higher scan rates. Finally, we also calculated the surface-controlled capacitive currents and diffusion-controlled currents of rGO-CNF (1:2), (1:1), and (2:1) materials at different potentials for two particular scan rates 10 mV s-1 and 200 mV s-1 to determine their respective percentage of contributions in the CV area (Fig. 6). rGO-CNF (1:2) possess the highest capacitive area, whereas rGO-CNF (2:1) has the lowest capacitive area, suggesting that the higher content of rGO increases the diffusion-controlled charge storage contribution.
Fig. 5. (a,b,c) i/ν1/2 vs. ν1/2; and (d,e,f) CV current % vs scan rate of rGO-CNF (1:2), rGO-CNF (1:1), and rGO-CNF (2:1) respectively.
Fig. 6. Percentage of surface-controlled and diffusion-controlled area in CV curve (a,b,c) at 10 mV s-1; (d,e,f) at 200 mV s-1 of respectively.
rGO-CNF (1:2), rGO-CNF (1:1), and rGO-CNF (2:1)
4.
Conclusion In summary, we have fabricated three-dimensional rGO-CNF hybrid materials using
different ratios of GO and PAN fiber. The prepared materials were characterized by different analytical techniques to analyze the structural, elemental contents, and electrochemical properties for supercapacitor electrode application. From the elemental analysis, we found that the rGOCNF materials were associated with surface oxygen functional groups, which greatly influence the electrochemical performance of the materials. CV, GCD, and EIS techniques were used to analyze the electrochemical properties of rGO-CNF materials in which the specific capacitances of rGO-CNF (1:2), (1:1), and (2:1) were calculated as 269.13 F g-1, 316.5 F g-1, and 239.8 F g-1 at a current density of 0.25 A g-1. The high capacitance of rGO-CNF materials has capacitive and pseudo-capacitive contributions. The surface-controlled and diffusion-controlled charge storage contributions in rGO-CNF were also quantitatively separated from the CV curve by using the PSM method which signifies that the increase of rGO content leads to higher diffusioncontrolled charge storage contribution. The pseudocapacitive charge storage contribution in the material was significant at a low scan rate of the CV analysis and low current density of the CD analysis. However, its contributions become less significant at high scan rates and high current density. Therefore, taking account of the surface functionalities towards the electrochemical properties of carbon materials and controlling the contents of surface functionalities is crucial for future characterization of carbon materials for energy storage applications.
Acknowledgment This research was supported by and Basic Science Research Program (2019R1A2C1004983) and X-mind Corps Program (2017H1D8A2030449) through the National Research Foundation (NRF) funded by the Ministry of Science and ICT of Republic of Korea. Reference [1]
Gogotsi Y, Simon P. True performance metrics in electrochemical energy storage. Science 2011;334:917–8.
[2]
Wang F, Wu X, Yuan X, Liu Z, Zhang Y, Fu L, et al. Latest advances in supercapacitors: From new electrode materials to novel device designs. Chem Soc Rev 2017;46:6816–54.
[3]
Nguyen TT, Balamurugan J, Aravindan V, Kim NH, Lee JH. Boosting the energy density of flexible solid-state supercapacitors via both ternary NiV2Se4 and NiFe2Se4 nanosheet arrays. Chem Mater 2019;31:4490–504.
[4]
Das AK, Kim NH, Lee SH, Sohn Y, Lee JH. Facile synthesis of CuCo2O4 composite octahedrons for high performance supercapacitor application. Compos Part B Eng 2018;150:269–76.
[5]
Das AK, Kim NH, Lee SH, Sohn Y, Lee JH. Facile synthesis of porous CuCo2O4 composite sheets and their supercapacitive performance. Compos Part B Eng 2018;150:234–41.
[6]
Kim M-H, Kim K-B, Park S-M, Roh KC. Hierarchically structured activated carbon for ultracapacitors. Sci Rep 2016;6:21182.
[7]
Zhang Q, Cai C, Qin J, Wei B. Tunable self-discharge process of carbon nanotube based supercapacitors. Nano Energy 2014;4:14–22.
[8]
Kim CH, Wee J-H, Kim YA, Yang KS, Yang C-M. Tailoring the pore structure of carbon nanofibers for achieving ultrahigh-energy-density supercapacitors using ionic liquids as
electrolytes. J Mater Chem A 2016;4:4763–70. [9]
Pumera M. Graphene-based nanomaterials for energy storage. Energy Environ Sci 2011;4:668–74.
[10]
Zhang H, Kuila T, Kim NH, Yu DS, Lee JH. Simultaneous reduction, exfoliation, and nitrogen doping of graphene oxide via a hydrothermal reaction for energy storage electrode materials. Carbon 2014;69:66–78.
[11]
Yu D, Goh K, Wang H, Wei L, Jiang W, Zhang Q, et al. Scalable synthesis of hierarchically structured carbon nanotube-graphene fibres for capacitive energy storage. Nat Nanotechnol 2014;9:555–62.
[12]
Fan Z, Yan J, Zhi L, Zhang Q, Wei T, Feng J, et al. A three-dimensional carbon nanotube/graphene sandwich and its application as electrode in supercapacitors. Adv Mater 2010;22:3723–8.
[13]
Zhu Y, Li L, Zhang C, Casillas G, Sun Z, Yan Z, et al. A seamless three-dimensional carbon nanotube graphene hybrid material. Nat Commun 2012;3:1225–7.
[14]
Kshetri T, Thanh TD, Singh SB, Kim NH, Lee JH. Hierarchical material of carbon nanotubes grown on carbon nanofibers for high performance electrochemical capacitor. Chem Eng J 2018;345:39–47.
[15]
Yang ZY, Zhao YF, Xiao QQ, Zhang YX, Jing L, Yan YM, et al. Controllable growth of CNTs on graphene as high-performance electrode material for supercapacitors. ACS Appl Mater Interfaces 2014;6:8497–504.
[16]
Kshetri T, Tran DT, Nguyen DC, Kim NH, Lau K, Lee JH. Ternary graphene-carbon nanofibers-carbon nanotubes structure for hybrid supercapacitor. Chem Eng J 2020;380:122543.
[17]
Bose S, Kuila T, Mishra AK, Rajasekar R, Kim NH, Lee JH. Carbon-based nanostructured materials and their composites as supercapacitor electrodes. J Mater Chem 2012;22:767–84.
[18]
Thanh Tran D, Kshetri T, Dinh Chuong N, Gautam J, Van Hien H, Huu Tuan L, et al.
Emerging core-shell nanostructured catalysts of transition metal encapsulated by twodimensional carbon materials for electrochemical applications. Nano Today 2018;22:100– 31. [19]
Thanh TD, Chuong ND, Hien H Van, Kshetri T, Tuan LH, Kim NH, et al. Recent advances in two-dimensional transition metal dichalcogenides-graphene heterostructured materials for electrochemical applications. Prog Mater Sci 2018;96:51–85.
[20]
Kumar S, Saeed G, Kim NH, Lee JH. Fabrication of Co–Ni–Zn ternary Oxide@NiWO4 core-shell nanowire arrays and Fe2O3-CNTs@GF for ultra-high-performance asymmetric supercapacitor. Compos Part B Eng 2019;176:107223.
[21]
Béguin F, Presser V, Balducci A, Frackowiak E. Carbons and electrolytes for advanced supercapacitors. Adv Mater 2014;26:2219–51.
[22]
Zhang LL, Zhao XS. Carbon-based materials as supercapacitor electrodes. Chem Soc Rev 2009;38:2520.
[23]
Gopalsamy K, Balamurugan J, Thanh TD, Kim NH, Lee JH. Fabrication of nitrogen and sulfur co-doped graphene nanoribbons with porous architecture for high-performance supercapacitors. Chem Eng J 2017;312:180–90.
[24]
Dong Q, Wang G, Hu H, Yang J, Qian B, Ling Z, et al. Ultrasound-assisted preparation of electrospun carbon nanofiber/graphene composite electrode for supercapacitors. J Power Sources 2013;243:350–3.
[25]
Ma C, Li Y, Shi J, Song Y, Liu L. High-performance supercapacitor electrodes based on porous flexible carbon nanofiber paper treated by surface chemical etching. Chem Eng J 2014;249:216–25.
[26]
Gogotsi Y, Penner RM. Energy Storage in Nanomaterials − Capacitive, Pseudocapacitive, or Battery-like?. ACS Nano 2018;12:2081-3.
[27]
Brousse T, Belanger D, Long JW. To Be or Not To Be Pseudocapacitive? J Electrochem Soc 2015;162:A5185–9.
[28]
Pandolfo AG, Hollenkamp AF. Carbon properties and their role in supercapacitors. J
Power Sources 2006;157:11–27. [29]
Zuliani JE, Tong S, Jia CQ, Kirk DW. Contribution of surface oxygen groups to the measured capacitance of porous carbon supercapacitors. J Power Sources 2018;395:271–9.
[30]
Liu T, Zhang F, Song Y, Li Y. Revitalizing carbon supercapacitor electrodes with hierarchical porous structures. J Mater Chem A 2017;5:17705–33.
[31]
He Y, Wang X, Lv Z, Huang X, Zhang Y, Li X, et al. Capacitive mechanism of oxygen functional groups on carbon surface in supercapacitors. Electrochim Acta 2018;282:618– 25.
[32]
Dupont MF, Donne SW. Separating the faradaic and non-faradaic contributions to the total capacitance for different manganese dioxide phases. J Electrochem Soc 2015;162:A5096–105.
[33]
Brezesinski T, Wang J, Tolbert SH, Dunn B. Ordered mesoporous α-MoO3 with isooriented nanocrystalline walls for thin-film pseudocapacitors. Nat Mater 2010;9:146–51.
[34]
Dupont MF, Donne SW. Faradaic and non-faradaic contributions to the power and energy characteristics of electrolytic manganese dioxide for electrochemical capacitors. J Electrochem Soc 2016;163:A888–97.
[35]
Dupont MF, Donne SW. Charge storage mechanisms in electrochemical capacitors: Effects of electrode properties on performance. J Power Sources 2016;326:613–23.
[36]
Biener J, Stadermann M, Suss M, Worsley MA, Biener MM, Rose KA, et al. Advanced carbon aerogels for energy applications. Energy Environ Sci 2011;4:656–67.
[37]
Bordjiba T, Mohamedi M, Dao LH. New class of carbon-nanotube aerogel electrodes for electrochemical power sources. Adv Mater 2008;20:815–9.
[38]
Nardecchia S, Carriazo D, Ferrer ML, Gutiérrez MC, Del Monte F. Three dimensional macroporous architectures and aerogels built of carbon nanotubes and/or graphene: Synthesis and applications. Chem Soc Rev 2013;42:794–830.
[39]
Bora A, Mohan K, Doley S, Dolui SK. Flexible asymmetric supercapacitor based on
functionalized reduced graphene oxide aerogels with wide working potential window. ACS Appl Mater Interfaces 2018;10:7996–8009. [40]
Hummers WS, Offeman RE. Preparation of graphitic oxide. J Am Chem Soc 1958;80:1339.
[41]
Laheäär A, Przygocki P, Abbas Q, Béguin F. Appropriate methods for evaluating the efficiency and capacitive behavior of different types of supercapacitors. Electrochem Commun 2015;60:21–5.
[42]
Niu H-J, Zhang L, Feng J-J, Zhang Q-L, Huang H, Wang A-J. Graphene-encapsulated cobalt nanoparticles embedded in porous nitrogen-doped graphitic carbon nanosheets as efficient electrocatalysts for oxygen reduction reaction. J Colloid Interface Sci 2019;552:744–51.
[43]
Niu H-J, Chen H-Y, Wen G-L, Feng J-J, Zhang Q-L, Wang A-J. One-pot solvothermal synthesis of three-dimensional hollow PtCu alloyed dodecahedron nanoframes with excellent electrocatalytic performances for hydrogen evolution and oxygen reduction. J Colloid Interface Sci 2019;539:525–32.
[44]
Shi Y-C, Feng J-J, Lin X-X, Zhang L, Yuan J, Zhang Q-L, et al. One-step hydrothermal synthesis of three-dimensional nitrogen-doped reduced graphene oxide hydrogels anchored PtPd alloyed nanoparticles for ethylene glycol oxidation and hydrogen evolution reactions. Electrochim Acta 2019;293:504–13.
[45]
Antunes EF, Lobo AO, Corat EJ, Trava-Airoldi VJ, Martin AA, Veríssimo C. Comparative study of first- and second-order Raman spectra of MWCNT at visible and infrared laser excitation. Carbon N Y 2006.
[46]
Bokobza L, Bruneel JL, Couzi M. Raman spectroscopy as a tool for the analysis of carbon-based materials (highly oriented pyrolitic graphite, multilayer graphene and multiwall carbon nanotubes) and of some of their elastomeric composites. Vib Spectrosc 2014;74:57–63.
[47]
Kim HG, Oh I-K, Lee S, Jeon S, Choi H, Kim K, et al. Analysis of defect recovery in
reduced graphene oxide and its application as a heater for self-healing polymers. ACS Appl Mater Interfaces 2019;11:16804–14 [48]
Wang Q, Yan J, Wang Y, Wei T, Zhang M, Jing X, et al. Three-dimensional flower-like and hierarchical porous carbon materials as high-rate performance electrodes for supercapacitors. Carbon 2014;67:119-27.
[49]
Oll O, Romann T, Siimenson C, Lust E. Influence of chemical composition of electrode material on the differential capacitance characteristics of the ionic liquid | electrode interface. Electrochem Commun 2017;82:39–42.
[50]
Jaramillo MM, Mendoza A, Vaquero S, Anderson M, Palma J, Marcilla R. Role of textural properties and surface functionalities of selected carbons on the electrochemical behaviour of ionic liquid based-supercapacitors. RSC Adv 2012;2:8439-46.
[51]
Choudhury A, Kim JH, Sinha Mahapatra S, Yang KS, Yang DJ. Nitrogen-enriched porous carbon nanofiber mat as efficient rlexible electrode material for supercapacitors. ACS Sustain Chem Eng 2017;5:2109-18.
[52]
Graydon JW, Panjehshahi M, Kirk DW. Charge redistribution and ionic mobility in the micropores of supercapacitors. J Power Sources 2014;245:822–9.
[53]
Jing X, Wang Q, Zhang M, Fan Z, Wang Y, Yan J, et al. Three-dimensional flower-like and hierarchical porous carbon materials as high-rate performance electrodes for supercapacitors. Carbon N Y 2013;67:119–27.
[54]
Andreas HA. Self-discharge in electrochemical capacitors: a perspective article. J Electrochem Soc 2015;162:A5047–53.
[55]
Wang Y, Song Y, Xia Y. Electrochemical capacitors: mechanism, materials, systems, characterization and applications. Chem Soc Rev 2016;45:5925–50.
[56]
Liu J, Wang J, Xu C, Jiang H, Li C, Zhang L, et al. Advanced energy storage devices: basic principles, analytical methods, and rational materials design. Adv Sci 2018;5:1700322.
[57]
Forghani M, Donne SW. Method Comparison for deconvoluting capacitive and pseudo-
capacitive contributions to electrochemical capacitor electrode behavior. J Electrochem Soc 2018;165:A664–73. [58]
Brezesinski T, Wang J, Polleux J, Dunn B, Tolbert SH. Templated nanocrystal-based porous TiO2 films for next-generation electrochemical capacitors. J Am Chem Soc 2009; 131:1802-9.
[59]
Augustyn V, Come J, Lowe MA, Kim JW, Taberna P-L, Tolbert SH, et al. High-rate electrochemical energy storage through
intercalation pseudocapacitance. Nat Mater
2013;12:518-22. [60]
Wang J, Polleux J, Lim J, Dunn B. Pseudocapacitive contributions to electrochemical energy storage in TiO 2 (anatase) nanoparticles. J Phys Chem C 2007;111:14925–31.
[61]
Zhang X, Luo J, Tang P, Ye X, Peng X, Tang H, et al. A universal strategy for metal oxide anchored and binder-free carbon matrix electrode: a supercapacitor case with superior rate performance and high mass loading. Nano Energy 2017;31:311–21.
Highlights
• A hierarchical three-dimensional rGO-CNF material has been synthesized. • The rGO-CNF (1:1) shows a high specific capacitance of 316.50 F g-1 at 0.25 A g-1. • The rGO-CNF has both surface-controlled and diffusion-controlled charge storage contributions.
• The surface-controlled and diffusion-controlled charge storage contributions have been deconvoluted.
Conflict of Interest ☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
S1359-8368(19)34425-7
DOI:
https://doi.org/10.1016/j.compositesb.2019.107500
Reference:
JCOMB 107500
To appear in:
Composites Part B
Received Date: 29 August 2019 Revised Date:
28 September 2019
Accepted Date: 30 September 2019
Please cite this article as: Kshetri T, Tran DT, Singh TI, Kim NH, Lau K-t, Lee JH, Effects of the composition of reduced graphene oxide/carbon nanofiber nanocomposite on charge storage behaviors, Composites Part B (2019), doi: https://doi.org/10.1016/j.compositesb.2019.107500. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier Ltd.
Graphical Abstract
Effects of the Composition of Reduced Graphene Oxide/Carbon Nanofiber Nanocomposite on Charge Storage Behaviors Tolendra Kshetri1, Duy Thanh Tran 1, Thangjam Ibomcha Singh 1, Nam Hoon Kim1*, Kin-tak Lau2 and Joong Hee Lee1,3** 1
Advanced Materials Institute for BIN Convergence Technology (BK21 Plus Global Program),
Department of BIN Convergence Technology, Jeonbuk National University, Jeonju, Jeonbuk, 54896 Republic of Korea 2
Faculty of Science, Engineering & Technology, Swinburne University Technology, Hawthorn,
VIC, 3122, Australia 3
Carbon Composite Research Centre Department of Polymer−Nano Science and Technology,
Jeonbuk National University Jeonju, Jeonbuk 54896, Republic of Korea.
*Corresponding author: Tel.: 82-63-270-2342; Fax: 82-63-270-2341. * E-mail address: [email protected] (Prof. Nam Hoon Kim) ** E-mail address: [email protected] (Prof. Joong Hee Lee)
Abstract: Porous carbon materials with hierarchical features are the important materials to study the electrochemical properties for supercapacitor electrode application. Depending on the synthesis procedures and the carbon precursors used, these carbon materials possess different structures, and they are always associated with different amounts of various oxygen functional groups affecting the wettability, conductivity, and electrochemical performance. In this context, various electrochemical methods are adopted to study the charge storage contribution. Among them,
separation of surface-controlled (capacitive) and diffusion-controlled currents from cyclic voltammetry (CV) curve by using the potential sweep method (PSM) is a preferred method. Herein, three-dimensional “Reduced Graphene Oxide-Carbon Nanofiber” (rGO-CNF) hybrid materials are fabricated by using different rGO:CNF ratios to study the effect of the compositon of rGO and CNF in the charge storage behaviors. The structural, elemental, and electrochemical characterizations of the materials are performed to quantitatively study the surface-controlled (capacitive) and diffusion-controlled charge storage contributions in the rGO-CNF hybrid. It is found that the increase of rGO content in the hybrid material significantly affects the electrochemical properties of the hybrid materials.
KEYWORDS: Reduced graphene oxide; carbon nanofiber; Surface-controlled; Diffusioncontrolled; Potential sweep method
1.
Introduction: In the past few decades, electrochemical capacitor (also called as supercapacitor) has been
rapidly emerging as an electrochemical energy-storage device (EESD) with great promise as a suitable power source for various modern electronic gadgets, electric vehicles, and other applications because of their ability to store and deliver electrical energy at relatively high rates [1-5]. Since the birth of this technology, various carbon-based materials including activated carbon [6], carbon nanotubes (CNTs) [7], carbon nanofibers (CNF) [8], graphene [9,10], and their composites [11–20,], have been extensively employed as the electrode materials in the supercapacitor owing to their high electrical conductivity, desirable porous feature, and high
specific surface area [21-23]. Many studies have exhibited that the specific capacitance of a carbon-based electrode material can be increased by improving the electrical conductivity and specific surface area, and also by tuning the pore size distributions [21]. In this regard, many efforts have been made to enhance the specific surface area by making different carbon-based materials into a hierarchical structure [12,24,25]. However, the nature of the charge storage and the mechanism involved have not been identified well, since the general trend of pure carbon materials follows an electric double layer capacitor (EDLC) nature, which is manifested by a rectangular CV and a triangular galvanostatic charge-discharge (GCD) curves [26,27]. Hence, increasing of specific capacitance in multi-structured carbon composites prepared from various carbon precursors has been attributed to the improvements in electrical conductivity, specific surface area, pore size distributions, and the synergistic effects of the various nanostructures. However, it is also essential to note that, depending on the synthesis route and the nature of the carbon precursors, many carbon materials primarily based on rGO are always associated with surface functional groups, such as carboxylic, lactonic, and phenolic [28,29]. So, it is highly expected that rGO-based carbon materials and other oxygen functional groups containing carbons may also make a significant contribution towards the overall capacitance when they are used as electrodes in supercapacitors. Moreover, a detailed electrochemical study of different charge kinetics to understand the charge storage mechanism in carbon materials is very crucial in the scientific community. Many of the previously reported studies on such carbon materials have not undertaken of such investigations, which are crucial for a proper understanding of the nature of charge storage, and the mechanism associated with it, which would significantly affect the further development of carbon-based electrode materials [2,21,30]. Recently, only a few researchers have started taking
into consideration and investigating the effect of such surface functionalities on the electrochemical performance of the electrodes [29,31]. However, such investigations have been confined to metal-based pseudo-capacitive materials and only a few common carbon materials [31–35]. Therefore, further explorations of various other carbon-based materials are also highly necessary for broadening the outlook on the charge-storage phenomenon. In this context, carbon aerogels are potential candidates for supercapacitor electrodes because of their high specific surface areas, considerable electrical conductivity, stable structures, and controllable pore size distribution [36–38]. rGO-CNF based aerogel is an effective carbon material owing to its porosity and unique hierarchical structures [39]. Herein, we have fabricated rGO-CNF aerogel nanocomposites with different ratios of rGO and CNF as potential candidates for electrode application in supercapacitors. A quantitative investigation of the surface-controlled (capacitive), and the diffusion-controlled charge storage contribution in the rGO-CNF is also carried out from CV analysis by using the PSM. The results show that the content of rGO in rGO-CNF have a high impact on its electrochemical properties, and the involved charge-storage mechanism, suggesting an appropriate ratio of rGO:CNF is crucial to obtain an optimized electrochemical performance for electrode application.
2.
Experimental Methods
2.1
Materials and Methods
2.1.1 Materials: Polyacrylonitrile (PAN, M=150,000) was obtained from Sigma-Aldrich, USA; graphite flake, hydrochloric acid (HCl, 37%), sulfuric acid (H2SO4, 95%), potassium permanganate (KMnO4), , dimethylformamide (DMF, 99%), potassium hydroxide (KOH, 85%),
and hydrogen peroxide (H2O2, 34.5%) were obtained from Samchun, Korea, and used without further treatment or purification. 2.1.2 Synthesis of rGO-CNF aerogel: Preparation of the rGO-CNF aerogels was performed according to the scheme represented in Scheme 1. Firstly, the polymer nanofiber mat was obtained from the PAN/DMF solution by electrospinning technique similar to our previous work [14], and graphene oxide (GO) was synthesized by the chemical oxidation of the graphite flake following the well-known Hummer’s method [40]. Then 0.06 g, 0.05 g, and 0.04 of PAN fiber pieces and 0.04 g, 0.05 g and 0.06 g of GO were respectively mixed and dispersed in 15 ml deionized (DI) water in three separate plastic conical tubes using a tip-sonicator for one hour to obtain homogeneous GO-PAN (1:2), GO-PAN (1:1) and GO-PAN (2:1) gel solutions. The GOPAN gel solutions were frozen in liquid nitrogen to obtain GO-PAN hybrid aerogel after the freeze-drying process. Finally, we obtained the rGO-CNF after a two-step thermal annealing process- stabilization and carbonization processes. During the stabilization process, the GO-PAN aerogel was heated at 280 °C at the heating rate of 1 °C min-1 and remained at 280°C for two hours in the air. After that, the stabilized GO-PAN aerogel was carbonized at 800 °C at the rate of 5 °C min-1 in argon atmosphere for one hour to obtain the rGO-CNF hybrid material.
2.2
Characterization Methods
2.2.1 Material Characterization Field emission scanning electron microscopy (FE-SEM), Carl Zeiss, SUPRA 40 VP, Germany, installed at the Center for University-wide Research Facilities (CURF), Jeonbuk National University was used to analyze the surface morphologies of the prepared samples.
Atomic force microscopy (AFM), Park NX10, Korea was used to get the surface topography image of GO using a non-contact PPP-NCHR 5M cantilever. X-ray diffraction (XRD) patterns of the materials were obtained in the 2θ range from 10° to 80° at the scan rate of 2°/min by using Rigaku diffractometry (Cu-Kα, λ = 1.5406 Ǻ, Japan). The Raman spectra of the materials were attained by using 532 nm laser (0.5 mW) in a Nanofinder-30 instrument (Tokyo Instruments Co., Japan). The elemental compositions of the materials were analyzed from the energy-dispersive X-ray spectroscopy (EDS) obtained from SEM, and X-ray photoelectron spectroscopy (XPS) (Theta Probe AR-XPS System, Thermo Fisher Scientific, UK) in the Busan center of KBSI. Nitrogen adsorption-desorption isotherms of the materials were taken at 77 K to study the Brunauer-Emmett-Teller (BET) specific surface area and the Barrett-Joyner-Halenda (BJH) pore size distributions by using a Micromeritics Tristar 3000 analyzer. To study the electrical conductivities of the materials, the materials were made into a thin film, and the sheet resistance were measured with a four-probe surface resistivity meter (MSTEC) using Keithley DC source and nanovoltmeter. 2.2.2 Electrochemical Characterization A three-electrode electrochemical set-up was used to measure the electrochemical properties of rGO, CNF, and rGO-CNF materials with Hg/HgO as the reference electrode and a platinum (Pt) foil as the counter electrode in a CHI660E, USA potentiostat. An alkaline solution of 6 M potassium hydroxide (KOH) was employed as the electrolyte. For the preparation of the working electrode, a nickel (Ni) foam was cut into six pieces of dimension 1 cm × 2 cm. Then they were washed by ultrasonication with HCl, ethanol, and water consecutively for 15 mins each. The cleaned Ni foams were dried in hot air using a blow dryer. After that, 2 mg of each sample (rGO-CNF (1:2), rGO-CNF (1:1), and rGO-CNF (2:1) were sandwiched between two
cleaned 1 cm × 2 cm sized Ni-foam using a hydraulic pressure to obtain the final working electrode. The cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) techniques were conducted to analyze the electrochemical properties of the materials. The specific capacitance of the rGO-CNF materials was evaluated from the GCD curve by applying the following equation [41]. =
× ∆ (1) × ∆
where, I (A), ∆t, m (g), ν (V/s), and ∆V (V) are the current, the discharged time, the mass of the active electrode material, and the potential window.
Scheme 1. Schematic procedure for preparation of rGO-CNF hybrid. 3.
Results and Discussions
3.1. Morphological characterization
The surface morphologies and the structural features of GO, rGO, CNF, and rGO-CNF were analyzed using FE-SEM. The FE-SEM image of the prepared GO is exhibited in Fig. S1 of the supporting information (SI). From the AFM height profile image, we observe that the prepared GO could be exfoliated well into thin sheets with a thickness of ~ 2 nm, which is an
essential factor for synthesis of rGO-CNF aerogel. The FE-SEM images of rGO-CNF (1:2), (1:1), and (2:1) are exhibited in Fig. 1(a, b), (d, e), and (g, h), respectively. From the images, we observe that the rGO and CNF components in the rGO-CNF materials are interconnected, leading to a hierarchical porous structure, which is an important parameter for electrode application. The CNFs were wrapped by the rGO sheets suggesting a strong interaction between the CNF and rGO components. In rGO-CNF (1:2), the CNF content is higher than that of the rGO, whereas, in rGO-CNF (2:1), the rGO content is higher than that of the CNF. In all the rGOCNF materials, there is a strong binding between the rGO sheets and the CNF interconnecting to each other, which makes it possible to form a stable 3D hierarchical rGO-CNF aerogel. Moreover, the rGO sheets serve a crucial role in the formation of 3D rGO-CNF hybrid material by providing a linkage between the individual CNF. Without the support of rGO; the CNFs cannot form a stable three-dimensional structure, because there is no linkage between the individual CNFs, as seen from the FE-SEM image of 3D CNF in Fig. S1. On the other hand, the CNF provides support to rGO sheets and also prevents the rGO sheets from re-stacking, thereby forming a hierarchical porous 3D rGO-CNF aerogel. The change in the ratio between the rGO and CNF components affect the porous feature and electrochemical performance as depicted from the BET and BJH analysis and the electrochemical analyses.
Fig. 1. FE-SEM images (low and high magnification) and EDS spectra of (a,b,c) rGO-CNF (1:2); (d, e, f) rGO-CNF (1:1); and (g, h, i) rGO-CNF (2:1).
3.2. Structural and porous feature characterizations
XRD and Raman spectroscopy techniques were employed to analyze the crystalline and structural properties of the rGO-CNF composite materials [42–44]. The XRD patterns of CNF, rGO, rGO-CNF (1:2), (1:1), and (2:1) are shown in Fig. S3, and Fig. 2. All the prepared carbon materials exhibit two XRD peaks at 2θ of 25.8° and 42.8°, which correspond to the (002) and (101) graphitic plane (PDF# 03-065-6212). From the XRD pattern, we observe that the (002) and (101) peaks of CNF are sharper than those of the rGO, indicating that the CNF is more crystalline than the rGO, as consistent with the electrical conductivity measurement shown in Table 1. The sheet resistances of CNF and rGO are respectively 65.98 ± 2.67 Ω/ and 89.63 ± 2.67 Ω/ . As expected, the (002) and (101) peaks of the rGO-CNF materials become broader when the amount of rGO is increased, thereby decreasing the electrical conductivity. The measured sheet resistance of rGO-CNF (1:2), (1:1), and (2:1) are 66.62 ± 2.67 Ω/ , 71.94 ± 3.67 Ω/ , and 88.81 ± 3.47 Ω/ , respectively. The broad XRD peak of rGO suggests its amorphous
nature due to incomplete reduction and the presence of surface oxygen functionalities, which affect the electrochemical performance of the materials. Further, the structural properties of the rGO-CNF materials were investigated from the Raman spectroscopy. The Raman spectra of rGO-CNF (1:2), (1:1), and (2:1) exhibit the D band (1350 cm-1) and G band (1580 cm-1), which are typical peaks for carbon materials [45]. The D band is the signature for the structural defect associated in the graphitic materials, whereas the G band relates to the stretching mode in the graphitic plane. Therefore, the ratio of the D band intensity to the G band intensity (ID/IG) is crucial parameter to calculate the structural disorder quantitatively [46]. From the Raman spectra (Fig. 2(b)), we observe that the ID/IG ratios for rGO-CNF (1:2), (1:1), and (2:1) are 0.93, 0.94,
and 0.96 respectively suggesting that higher amount of defects are present in the rGO-CNF materials when the rGO content is increased due to incomplete thermal reduction of GO [47]. The porous characteristics of the rGO, CNF, and rGO-CNF materials were analyzed by measuring the BET specific surface area and BJH pore size distribution from the nitrogen adsorption-desorption isotherms (Fig. S2 and Fig. S4). The specific surface area of CNF, rGO, rGO-CNF (1:2), (1:1) and (2:1) are 114.74 m2 g-1, 180.98 m2 g-1, 319.14 m2 g-1, 367.73 m2 g-1, and 260.43 m2 g-1, respectively. It is observed that the specific surface area of rGO-CNF is much higher than that of the individual CNF and rGO components owing to its hierarchical porous characteristic. However, the specific surface area of rGO-CNF (2:1) is lower than that of rGOCNF (1:2) and (1:1) because of the re-stacking of rGO sheets during the freeze-drying, and thermal annealing process as the amount of rGO is higher than the CNF. The BJH pore size distribution curve suggests that the rGO-CNF materials possess mesoporous characteristics (~ 2 nm) and the rGO-CNF (1:1) has higher pore volume than the other synthesized materials. However, for rGO, the pore size is distributed in the range of 20 nm to 60 nm, and the pore volume is also lower than that of the pristine CNF. Therefore, as expected, the rGO-CNF (2:1) has a lower pore volume because of the presence of a higher amount of rGO sheets. The availability of a large volume of mesopores in the rGO-CNF materials is beneficial for increasing the specific capacitance by providing larger accessible sites for adsorption of the electrolyte ions in the charge-discharge process [48].
PDF# 03-065-6212
C (101) Graphene-CNF-2:1 Graphene-CNF-1:1
rGO-CNF (2:1) rGO-CNF (1:1) rGO-CNF (1:2)
(b) Intensity (a.u)
C (002)
Intensity (a.u.)
(a)
ID/IG = 0.96
ID/IG = 0.94 ID/IG = 0.93
Graphene-CNF-1:2
20
40 50 60 2θ (degree)
70
80
500
rGO-CNF (2:1) rGO-CNF (1:1) rGO-CNF (1:2)
C1s
Intensity (a.u)
(c)
30
N1s
1000
1500 2000 2500 Raman shift (cm-1) C-C (284.4)
(d) Intensity (a.u.)
10
O1s
3000
3500
rGO-CNF (1:2) C-C
75.24 %
C-O
16.42 %
C=O
6.62 %
O-C=O 1.72 % C-O (286.3) C=O (287.4) O-C=O (289)
200
400 600 Binding energy (eV) C-C (284.4)
Intensity (a.u.)
(e)
800
282
284 286 288 Binding energy (eV)
(f)
rGO-CNF (1:1)
C-C (284.4)
C-C
74.84 %
C-O
15.91 %
C-O
16.61 %
C=O
6.16 %
C=O
6.33 %
O-C=O 2.03 % C-O (286.3)
O-C=O 2.39 % C-O (286.3)
C=O (287.4)
284
rGO-CNF (2:1)
75.73 %
C=O (287.4) O-C=O (289)
O-C=O (289)
282
290
C-C
Intensity (a.u)
0
286 288 Binding energy (eV)
290
282
284
286 288 Binding energy (eV)
290
Fig. 2. (a-c) XRD patter, Raman spectra, XPS survey spectra; and (d-f) deconvoluted C1s spectra of rGO-CNF (1:2), rGO-CNF (1:1), and rGO-CNF (2:1) respectively.
3.3. Elemental characterization
The elemental analysis of the materials for supercapacitor electrode application is very crucial because the elements present on the electrode materials significantly affect its electrochemical properties [49,50]. The elemental contents of the rGO-CNF materials were determined using EDS and XPS. The EDS and the XPS survey spectra of rGO-CNF (1:2), (1:1), and (2:1) are illustrated in Fig. 1(c, f, i) and Fig. 2(c), respectively, in which the presence of three elements – carbon (C), nitrogen (N), and oxygen (O) was confirmed. The atomic percent (At. %) of each element present in the different materials are listed in Table 1. The EDS and XPS spectra show that the At. % of C and O increases when the rGO content is increased in the rGOCNF, thus affecting the electrical conductivity and electrochemical properties as stated in Table 1. However, the At. % of nitrogen (N) decreases when the rGO content is increased because the N in rGO-CNF is attributed to the PAN polymer, which was used as the precursor for the CNF [51]. Further, to understand the presence of oxygen functionalities in the materials and the bonding between the carbon (C) and oxygen (O), the C1s peak of rGO, CNF, and rGO-CNF (1:2), (1:1) and (2:1) materials were deconvoluted (Fig. (S3(c), (d)) and Fig. 2(d, e, f)). The highresolution C1s peak reveals the presence of C─C, C─O, C═O, and O─C═O. The percentages of each bonding are shown in the figures in which the percentage of C─C decreases as the amount of rGO content increases in rGO-CNF affecting the electrical conductivity and electrochemical properties of the hybrid materials. Among the rGO-CNF materials, rGO-CNF (1:2) has the highest amount of C─C bonding and has better electrical conductivity than the other materials, whereas rGO-CNF (2:1) has the lowest amount of C─C bonds and the lowest electrical conductivity. The presence of oxygen surface functional groups also contributes to the
capacitance of the materials as measured from the electrochemical characterization of the rGOCNF materials with different ratios of rGO and CNF. Table 1. Comparison of elemental composition, sheet resistance, and specific capacitance of CNF, rGO, rGO-CNF (1:2), rGO-CNF (1:1), and rGO-CNF (2:1).
Sheet resistance
Specific
Carbon
Nitrogen
Oxygen
(At. %)
(At. %)
(At. %)
(Ω/ )
CNF
85.64
9.08
5.28
65.98 ± 2.67
105.2
rGO
89.18
-
10.82
89.63 ± 2.67
91.7
rGO-CNF (1:2)
89.55
3.98
6.48
66.62 ± 2.67
192.3
rGO-CNF (1:1)
89.57
3.29
7.14
71.94 ± 3.07
223.5
rGO-CNF (2:1)
90.28
2.44
7.27
88.81 ± 3.47
191.0
Sample
capacitance at 0.5 A g-1 (F g-1)
3.4. Electrochemical characterization The electrochemical performance of the CNF, rGO, rGO-CNF (1:2), (1:1), and (2:1) materials were evaluated in an alkaline aqueous electrolyte (6 M KOH) using the CV, GCD, and EIS techniques. Fig. 4(a, b, c) displays the CV curves of rGO-CNF (1:2), (1:1), and (2:1) at different scan rates. The CV curves of CNF and rGO are shown in Fig. 3(a and c). All the
materials exhibit quasi-rectangular CV curves, indicating that the primary charge storage comes from the electric double-layer mechanism (a surface-controlled phenomenon), as is also supported by the quasi-triangular GCD curve [52]. The charge storage contribution is also anticipated from the surface redox reaction between the oxygen-containing functional groups and the electrolyte solutions via the quasi-reversible redox reaction illustrated in Equation (2) [31]. The detailed analysis of the surface-controlled and diffusion-controlled charge storage contribution is discussed in the following section. In the alkaline electrolyte, the CV curves of rGO, CNF, and rGO-CNF can be analyzed into three characteristic segments, each of which signifies different stage of electrochemical double-layer formation. In the low potential region, the hydrated K+ in the KOH electrolyte could reach the electrode surface rapidly, forming an outer Helmholtz layer [31]. However, in the mid-potential region, an interfacial tension has attained a maximum value as the hydrated K+ has dissipated, and the effect of steric hindrance of the oxygen functionalities improve the ions diffusion into the bulk solution [31]. However, the hydrated K+ ion has been squeezed in the high potential region, and there is a quasi-reversible process represented by Equation (2) [31]. Moreover, the concavity point observed in the CV curve is also expected because of the surface oxygen functionalities on the electrode materials [31]. The concavity point is most significant in rGO-CNF (2:1) due to the higher oxygen contents. >CxO + [K(H2O)n]+ + e- ↔ >CxOK(H2O)n-y
(2)
where n is the number of H2O molecules associated with K+, and y is the number of H2O molecules which the hydrated K+ has lost when squeezed.
Fig. 3. CV and GCD curves of (a,b) rGO and (c,d) CNF respectively; (e,f) Specific capacitance vs. current density and Nyquist plot of rGO and CNF (inset is the equivalent circuit diagram).
The specific capacitance of the electrode materials were evaluated from the GCD curves using Equation (1); rGO, CNF, rGO-CNF (1:2), (1:1), and (2:1) show specific capacitance of 91.7, 105.2, 192.3, 223.5, and 191.0 F g-1 at 0.5 A g-1, respectively. A significant increase of specific capacitance in the rGO-CNF material compared to the individual rGO and CNF is because of its hierarchical porous feature with high specific surface area and well-developed pore structure which facilitates the movement and diffusion of electrolyte ions [44,53]. From Fig. S6(d), we observe that the rGO-CNF materials have a low rate capability at low current densities from 0.25 A g-1 to 1 A g-1. However, at higher current densities, the rGO-CNF materials maintain good rate capabilities. This characteristic of the materials is attributed to self-discharge or charge leakage due to the presence of surface functionalities and redistribution of electrolyte ions in the porous carbon structure during the discharging process [7,28,52,54]. However, at higher current densities, the rGO-CNF materials exhibit a good rate capability as the capacitive mechanism is more dominant than the pseudocapacitive mechanism. In addition, the rGO-CNF also exhibits high cyclic stability, in which the stability is highest for rGO-CNF (1:2) and lowest for rGOCNF (2:1) signifying that the electrochemical stability decreases as the amount of rGO in rGOCNF increases. From the EIS analysis (Nyquist plot) in Fig. S6(e), we observed that rGO-CNF (2:1) has the highest charge-transfer resistance due to the higher oxygen content which agrees with electrical conductivity measurement. The increase of the slope of the Warburg impedance region in the Nyquist plot indicates an increase of ionic diffusion impedance because there are more oxygen functionalities on the surface of the electrode materials [31].
Fig. 4. (a,b,c) CV curves; (d,e,f) b vs potential (insets are the log(i) vs log(ν) curves) of rGOCNF (1:2), rGO-CNF (1:1), and rGO-CNF (2:1) respectively.
3.4.1 Deconvolution of surface-controlled and diffusion-controlled charge storage contribution From the above electrochemical analysis, it has been noticed that the increase of rGO content in the rGO-CNF materials has a great influence on their electrochemical properties because of the oxygen surface functionalities, which have a pseudocapacitive charge storage contribution in alkaline aqueous electrolyte. It is also expected that the charge storage in rGOCNF has contribution from the diffusion-controlled process [55,56]. Therefore, the quantitative study of the surface-controlled and diffusion-controlled charge storage contribution from the CV curve by using PSM method is also an important step for a better understanding of the charge storage kinetics. In this regard, we analyzed the capacitive and pseudo-capacitive contributions from the CV data measured at different potential sweep rates (5 mV s-1 ~ 200 mV s-1) based on the following power-law equation [57–59]: i = aνb
(3)
which can be re-written as log(i) = blog(ν) + log(a)
(4)
where, i, ν are the current (A) and the potential sweep rate (mV s-1), and a, b are both adjustable parameters. As based on the interrelation between the current and potential sweep rate, the current can be separated into two parts: the diffusion-limited process (when b=0.5) and the surface-limited process (when b=1). To determine the b-value for rGO, CNF, rGO-CNF materials, a curve between log(i) vs. log(ν) in the cathodic scan is plotted at various potentials, and curves were fitted to calculate the slopes which represent the b-values as shown in the inset of Fig. S5(a, d) and Fig. 4(d-f). The
log(i) vs. log(ν) curve of rGO-CNF materials in the anodic scan is also displayed in Fig. S7(a-c). It is observed that the b-values of the materials are between 0.8 and 1 at different potentials which suggests that the measured currents have contributions from both the surface-controlled process (capacitive) and diffusion-controlled process with significant contributions from the capacitive process [55,56,60]. Further, according to the above concepts, the total current response of the CV analysis at a particular potential can be separated into two parts corresponding to the surface-controlled capacitive behavior and the diffusion-controlled charge storage behavior as [33,61]: i(V) = k1ν + k2ν1/2
(5)
For the sake of analytical simplicity, the above equation can be re-written as i(V)/ν1/2 = k1ν1/2 + k2
(6)
where, k1 and k2 are constants independent to potential sweep rate (ν) and the terms k1ν and k2ν1/2 in equation (5) correspond to the surface-controlled process and the diffusion-controlled
process, respectively. Thus, we can find the values of k1 and k2 from the plot of i(V)/ν1/2 vs. ν1/2 at a particular voltage to quantify the current contributions from the surface-controlled and diffusion-controlled processes in rGO, CNF, and rGO-CNF materials. Fig. 5(a-c) shows the i(V)/ν1/2 vs. ν1/2 curve of rGO-CNF (1:2), (1:1), and (2:1), respectively, at a specified potential of
-0.5 V. The surface-controlled and diffusion-controlled current percentages with respect to the different scan rates at a fixed potential (-0.5 V) of the materials are exhibited in Fig. 5(d-f), indicating that the rGO-CNF materials with higher rGO content have higher diffusion-controlled current. Moreover, it is also observed that the surface-controlled capacitive current is more
dominant at higher scan rates. Finally, we also calculated the surface-controlled capacitive currents and diffusion-controlled currents of rGO-CNF (1:2), (1:1), and (2:1) materials at different potentials for two particular scan rates 10 mV s-1 and 200 mV s-1 to determine their respective percentage of contributions in the CV area (Fig. 6). rGO-CNF (1:2) possess the highest capacitive area, whereas rGO-CNF (2:1) has the lowest capacitive area, suggesting that the higher content of rGO increases the diffusion-controlled charge storage contribution.
Fig. 5. (a,b,c) i/ν1/2 vs. ν1/2; and (d,e,f) CV current % vs scan rate of rGO-CNF (1:2), rGO-CNF (1:1), and rGO-CNF (2:1) respectively.
Fig. 6. Percentage of surface-controlled and diffusion-controlled area in CV curve (a,b,c) at 10 mV s-1; (d,e,f) at 200 mV s-1 of respectively.
rGO-CNF (1:2), rGO-CNF (1:1), and rGO-CNF (2:1)
4.
Conclusion In summary, we have fabricated three-dimensional rGO-CNF hybrid materials using
different ratios of GO and PAN fiber. The prepared materials were characterized by different analytical techniques to analyze the structural, elemental contents, and electrochemical properties for supercapacitor electrode application. From the elemental analysis, we found that the rGOCNF materials were associated with surface oxygen functional groups, which greatly influence the electrochemical performance of the materials. CV, GCD, and EIS techniques were used to analyze the electrochemical properties of rGO-CNF materials in which the specific capacitances of rGO-CNF (1:2), (1:1), and (2:1) were calculated as 269.13 F g-1, 316.5 F g-1, and 239.8 F g-1 at a current density of 0.25 A g-1. The high capacitance of rGO-CNF materials has capacitive and pseudo-capacitive contributions. The surface-controlled and diffusion-controlled charge storage contributions in rGO-CNF were also quantitatively separated from the CV curve by using the PSM method which signifies that the increase of rGO content leads to higher diffusioncontrolled charge storage contribution. The pseudocapacitive charge storage contribution in the material was significant at a low scan rate of the CV analysis and low current density of the CD analysis. However, its contributions become less significant at high scan rates and high current density. Therefore, taking account of the surface functionalities towards the electrochemical properties of carbon materials and controlling the contents of surface functionalities is crucial for future characterization of carbon materials for energy storage applications.
Acknowledgment This research was supported by and Basic Science Research Program (2019R1A2C1004983) and X-mind Corps Program (2017H1D8A2030449) through the National Research Foundation (NRF) funded by the Ministry of Science and ICT of Republic of Korea. Reference [1]
Gogotsi Y, Simon P. True performance metrics in electrochemical energy storage. Science 2011;334:917–8.
[2]
Wang F, Wu X, Yuan X, Liu Z, Zhang Y, Fu L, et al. Latest advances in supercapacitors: From new electrode materials to novel device designs. Chem Soc Rev 2017;46:6816–54.
[3]
Nguyen TT, Balamurugan J, Aravindan V, Kim NH, Lee JH. Boosting the energy density of flexible solid-state supercapacitors via both ternary NiV2Se4 and NiFe2Se4 nanosheet arrays. Chem Mater 2019;31:4490–504.
[4]
Das AK, Kim NH, Lee SH, Sohn Y, Lee JH. Facile synthesis of CuCo2O4 composite octahedrons for high performance supercapacitor application. Compos Part B Eng 2018;150:269–76.
[5]
Das AK, Kim NH, Lee SH, Sohn Y, Lee JH. Facile synthesis of porous CuCo2O4 composite sheets and their supercapacitive performance. Compos Part B Eng 2018;150:234–41.
[6]
Kim M-H, Kim K-B, Park S-M, Roh KC. Hierarchically structured activated carbon for ultracapacitors. Sci Rep 2016;6:21182.
[7]
Zhang Q, Cai C, Qin J, Wei B. Tunable self-discharge process of carbon nanotube based supercapacitors. Nano Energy 2014;4:14–22.
[8]
Kim CH, Wee J-H, Kim YA, Yang KS, Yang C-M. Tailoring the pore structure of carbon nanofibers for achieving ultrahigh-energy-density supercapacitors using ionic liquids as
electrolytes. J Mater Chem A 2016;4:4763–70. [9]
Pumera M. Graphene-based nanomaterials for energy storage. Energy Environ Sci 2011;4:668–74.
[10]
Zhang H, Kuila T, Kim NH, Yu DS, Lee JH. Simultaneous reduction, exfoliation, and nitrogen doping of graphene oxide via a hydrothermal reaction for energy storage electrode materials. Carbon 2014;69:66–78.
[11]
Yu D, Goh K, Wang H, Wei L, Jiang W, Zhang Q, et al. Scalable synthesis of hierarchically structured carbon nanotube-graphene fibres for capacitive energy storage. Nat Nanotechnol 2014;9:555–62.
[12]
Fan Z, Yan J, Zhi L, Zhang Q, Wei T, Feng J, et al. A three-dimensional carbon nanotube/graphene sandwich and its application as electrode in supercapacitors. Adv Mater 2010;22:3723–8.
[13]
Zhu Y, Li L, Zhang C, Casillas G, Sun Z, Yan Z, et al. A seamless three-dimensional carbon nanotube graphene hybrid material. Nat Commun 2012;3:1225–7.
[14]
Kshetri T, Thanh TD, Singh SB, Kim NH, Lee JH. Hierarchical material of carbon nanotubes grown on carbon nanofibers for high performance electrochemical capacitor. Chem Eng J 2018;345:39–47.
[15]
Yang ZY, Zhao YF, Xiao QQ, Zhang YX, Jing L, Yan YM, et al. Controllable growth of CNTs on graphene as high-performance electrode material for supercapacitors. ACS Appl Mater Interfaces 2014;6:8497–504.
[16]
Kshetri T, Tran DT, Nguyen DC, Kim NH, Lau K, Lee JH. Ternary graphene-carbon nanofibers-carbon nanotubes structure for hybrid supercapacitor. Chem Eng J 2020;380:122543.
[17]
Bose S, Kuila T, Mishra AK, Rajasekar R, Kim NH, Lee JH. Carbon-based nanostructured materials and their composites as supercapacitor electrodes. J Mater Chem 2012;22:767–84.
[18]
Thanh Tran D, Kshetri T, Dinh Chuong N, Gautam J, Van Hien H, Huu Tuan L, et al.
Emerging core-shell nanostructured catalysts of transition metal encapsulated by twodimensional carbon materials for electrochemical applications. Nano Today 2018;22:100– 31. [19]
Thanh TD, Chuong ND, Hien H Van, Kshetri T, Tuan LH, Kim NH, et al. Recent advances in two-dimensional transition metal dichalcogenides-graphene heterostructured materials for electrochemical applications. Prog Mater Sci 2018;96:51–85.
[20]
Kumar S, Saeed G, Kim NH, Lee JH. Fabrication of Co–Ni–Zn ternary Oxide@NiWO4 core-shell nanowire arrays and Fe2O3-CNTs@GF for ultra-high-performance asymmetric supercapacitor. Compos Part B Eng 2019;176:107223.
[21]
Béguin F, Presser V, Balducci A, Frackowiak E. Carbons and electrolytes for advanced supercapacitors. Adv Mater 2014;26:2219–51.
[22]
Zhang LL, Zhao XS. Carbon-based materials as supercapacitor electrodes. Chem Soc Rev 2009;38:2520.
[23]
Gopalsamy K, Balamurugan J, Thanh TD, Kim NH, Lee JH. Fabrication of nitrogen and sulfur co-doped graphene nanoribbons with porous architecture for high-performance supercapacitors. Chem Eng J 2017;312:180–90.
[24]
Dong Q, Wang G, Hu H, Yang J, Qian B, Ling Z, et al. Ultrasound-assisted preparation of electrospun carbon nanofiber/graphene composite electrode for supercapacitors. J Power Sources 2013;243:350–3.
[25]
Ma C, Li Y, Shi J, Song Y, Liu L. High-performance supercapacitor electrodes based on porous flexible carbon nanofiber paper treated by surface chemical etching. Chem Eng J 2014;249:216–25.
[26]
Gogotsi Y, Penner RM. Energy Storage in Nanomaterials − Capacitive, Pseudocapacitive, or Battery-like?. ACS Nano 2018;12:2081-3.
[27]
Brousse T, Belanger D, Long JW. To Be or Not To Be Pseudocapacitive? J Electrochem Soc 2015;162:A5185–9.
[28]
Pandolfo AG, Hollenkamp AF. Carbon properties and their role in supercapacitors. J
Power Sources 2006;157:11–27. [29]
Zuliani JE, Tong S, Jia CQ, Kirk DW. Contribution of surface oxygen groups to the measured capacitance of porous carbon supercapacitors. J Power Sources 2018;395:271–9.
[30]
Liu T, Zhang F, Song Y, Li Y. Revitalizing carbon supercapacitor electrodes with hierarchical porous structures. J Mater Chem A 2017;5:17705–33.
[31]
He Y, Wang X, Lv Z, Huang X, Zhang Y, Li X, et al. Capacitive mechanism of oxygen functional groups on carbon surface in supercapacitors. Electrochim Acta 2018;282:618– 25.
[32]
Dupont MF, Donne SW. Separating the faradaic and non-faradaic contributions to the total capacitance for different manganese dioxide phases. J Electrochem Soc 2015;162:A5096–105.
[33]
Brezesinski T, Wang J, Tolbert SH, Dunn B. Ordered mesoporous α-MoO3 with isooriented nanocrystalline walls for thin-film pseudocapacitors. Nat Mater 2010;9:146–51.
[34]
Dupont MF, Donne SW. Faradaic and non-faradaic contributions to the power and energy characteristics of electrolytic manganese dioxide for electrochemical capacitors. J Electrochem Soc 2016;163:A888–97.
[35]
Dupont MF, Donne SW. Charge storage mechanisms in electrochemical capacitors: Effects of electrode properties on performance. J Power Sources 2016;326:613–23.
[36]
Biener J, Stadermann M, Suss M, Worsley MA, Biener MM, Rose KA, et al. Advanced carbon aerogels for energy applications. Energy Environ Sci 2011;4:656–67.
[37]
Bordjiba T, Mohamedi M, Dao LH. New class of carbon-nanotube aerogel electrodes for electrochemical power sources. Adv Mater 2008;20:815–9.
[38]
Nardecchia S, Carriazo D, Ferrer ML, Gutiérrez MC, Del Monte F. Three dimensional macroporous architectures and aerogels built of carbon nanotubes and/or graphene: Synthesis and applications. Chem Soc Rev 2013;42:794–830.
[39]
Bora A, Mohan K, Doley S, Dolui SK. Flexible asymmetric supercapacitor based on
functionalized reduced graphene oxide aerogels with wide working potential window. ACS Appl Mater Interfaces 2018;10:7996–8009. [40]
Hummers WS, Offeman RE. Preparation of graphitic oxide. J Am Chem Soc 1958;80:1339.
[41]
Laheäär A, Przygocki P, Abbas Q, Béguin F. Appropriate methods for evaluating the efficiency and capacitive behavior of different types of supercapacitors. Electrochem Commun 2015;60:21–5.
[42]
Niu H-J, Zhang L, Feng J-J, Zhang Q-L, Huang H, Wang A-J. Graphene-encapsulated cobalt nanoparticles embedded in porous nitrogen-doped graphitic carbon nanosheets as efficient electrocatalysts for oxygen reduction reaction. J Colloid Interface Sci 2019;552:744–51.
[43]
Niu H-J, Chen H-Y, Wen G-L, Feng J-J, Zhang Q-L, Wang A-J. One-pot solvothermal synthesis of three-dimensional hollow PtCu alloyed dodecahedron nanoframes with excellent electrocatalytic performances for hydrogen evolution and oxygen reduction. J Colloid Interface Sci 2019;539:525–32.
[44]
Shi Y-C, Feng J-J, Lin X-X, Zhang L, Yuan J, Zhang Q-L, et al. One-step hydrothermal synthesis of three-dimensional nitrogen-doped reduced graphene oxide hydrogels anchored PtPd alloyed nanoparticles for ethylene glycol oxidation and hydrogen evolution reactions. Electrochim Acta 2019;293:504–13.
[45]
Antunes EF, Lobo AO, Corat EJ, Trava-Airoldi VJ, Martin AA, Veríssimo C. Comparative study of first- and second-order Raman spectra of MWCNT at visible and infrared laser excitation. Carbon N Y 2006.
[46]
Bokobza L, Bruneel JL, Couzi M. Raman spectroscopy as a tool for the analysis of carbon-based materials (highly oriented pyrolitic graphite, multilayer graphene and multiwall carbon nanotubes) and of some of their elastomeric composites. Vib Spectrosc 2014;74:57–63.
[47]
Kim HG, Oh I-K, Lee S, Jeon S, Choi H, Kim K, et al. Analysis of defect recovery in
reduced graphene oxide and its application as a heater for self-healing polymers. ACS Appl Mater Interfaces 2019;11:16804–14 [48]
Wang Q, Yan J, Wang Y, Wei T, Zhang M, Jing X, et al. Three-dimensional flower-like and hierarchical porous carbon materials as high-rate performance electrodes for supercapacitors. Carbon 2014;67:119-27.
[49]
Oll O, Romann T, Siimenson C, Lust E. Influence of chemical composition of electrode material on the differential capacitance characteristics of the ionic liquid | electrode interface. Electrochem Commun 2017;82:39–42.
[50]
Jaramillo MM, Mendoza A, Vaquero S, Anderson M, Palma J, Marcilla R. Role of textural properties and surface functionalities of selected carbons on the electrochemical behaviour of ionic liquid based-supercapacitors. RSC Adv 2012;2:8439-46.
[51]
Choudhury A, Kim JH, Sinha Mahapatra S, Yang KS, Yang DJ. Nitrogen-enriched porous carbon nanofiber mat as efficient rlexible electrode material for supercapacitors. ACS Sustain Chem Eng 2017;5:2109-18.
[52]
Graydon JW, Panjehshahi M, Kirk DW. Charge redistribution and ionic mobility in the micropores of supercapacitors. J Power Sources 2014;245:822–9.
[53]
Jing X, Wang Q, Zhang M, Fan Z, Wang Y, Yan J, et al. Three-dimensional flower-like and hierarchical porous carbon materials as high-rate performance electrodes for supercapacitors. Carbon N Y 2013;67:119–27.
[54]
Andreas HA. Self-discharge in electrochemical capacitors: a perspective article. J Electrochem Soc 2015;162:A5047–53.
[55]
Wang Y, Song Y, Xia Y. Electrochemical capacitors: mechanism, materials, systems, characterization and applications. Chem Soc Rev 2016;45:5925–50.
[56]
Liu J, Wang J, Xu C, Jiang H, Li C, Zhang L, et al. Advanced energy storage devices: basic principles, analytical methods, and rational materials design. Adv Sci 2018;5:1700322.
[57]
Forghani M, Donne SW. Method Comparison for deconvoluting capacitive and pseudo-
capacitive contributions to electrochemical capacitor electrode behavior. J Electrochem Soc 2018;165:A664–73. [58]
Brezesinski T, Wang J, Polleux J, Dunn B, Tolbert SH. Templated nanocrystal-based porous TiO2 films for next-generation electrochemical capacitors. J Am Chem Soc 2009; 131:1802-9.
[59]
Augustyn V, Come J, Lowe MA, Kim JW, Taberna P-L, Tolbert SH, et al. High-rate electrochemical energy storage through
intercalation pseudocapacitance. Nat Mater
2013;12:518-22. [60]
Wang J, Polleux J, Lim J, Dunn B. Pseudocapacitive contributions to electrochemical energy storage in TiO 2 (anatase) nanoparticles. J Phys Chem C 2007;111:14925–31.
[61]
Zhang X, Luo J, Tang P, Ye X, Peng X, Tang H, et al. A universal strategy for metal oxide anchored and binder-free carbon matrix electrode: a supercapacitor case with superior rate performance and high mass loading. Nano Energy 2017;31:311–21.
Highlights
• A hierarchical three-dimensional rGO-CNF material has been synthesized. • The rGO-CNF (1:1) shows a high specific capacitance of 316.50 F g-1 at 0.25 A g-1. • The rGO-CNF has both surface-controlled and diffusion-controlled charge storage contributions.
• The surface-controlled and diffusion-controlled charge storage contributions have been deconvoluted.
Conflict of Interest ☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.