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Electrodeposited reduced graphene oxide as a highly efficient and low-cost electrocatalyst for vanadium redox flow batteries
Accepted Manuscript Electrodeposited reduced graphene oxide as a highly efficient and low-cost electrocatalyst for vanadium redox flow batteries Pooria Moozarm Nia, Ebrahim Abouzari-Lotf, Pei Meng Woi, Yatimah Alias, Teo Ming Ting, Arshad Ahmad, Nurfatehah Wahyuny Che Jusoh PII:
S0013-4686(18)32592-1
DOI:
https://doi.org/10.1016/j.electacta.2018.11.109
Reference:
EA 33110
To appear in:
Electrochimica Acta
Received Date: 2 September 2018 Revised Date:
13 November 2018
Accepted Date: 16 November 2018
Please cite this article as: P.M. Nia, E. Abouzari-Lotf, P.M. Woi, Y. Alias, T.M. Ting, A. Ahmad, N.W. Che Jusoh, Electrodeposited reduced graphene oxide as a highly efficient and low-cost electrocatalyst for vanadium redox flow batteries, Electrochimica Acta (2018), doi: https://doi.org/10.1016/ j.electacta.2018.11.109. 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.
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Electrodeposited reduced graphene oxide as a highly efficient and low-cost electrocatalyst for vanadium redox flow batteries
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Pooria Moozarm Niaa,∗, Ebrahim Abouzari-Lotfa,b,∗, Pei Meng Woic , Yatimah Aliasc , Teo Ming Tingd , Arshad Ahmada,b , Nurfatehah Wahyuny Che Jusohd,e a
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Advanced Materials Research Group, Center of Hydrogen Energy, Institute of Future Energy, Universiti Teknologi Malaysia, Kuala Lumpur, Malaysia b Department of Chemical Engineering, Universiti Teknologi Malaysia, 81310 Johor Bahru, Malaysia c University of Malaya Centre for Ionic Liquids, Department of Chemistry, University of Malaya, Kuala Lumpur 50603, Malaysia d Radiation Processing Technology Division, Malaysian Nuclear Agency, Kajang, Malaysia. e Malaysia–Japan International Institute of Technology, Universiti Teknologi Malaysia, 54100 Kuala Lumpur, Malaysia
Abstract
An electrochemically reduced graphene oxide was grown on carbon felt surface in a simple one-step electrodeposition process of graphene oxide and it was employed as a positive electrocatalyst of vanadium redox flow battery. The electrodeposited graphene-based
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nanocomposite on carbon felt presented outstanding electrochemical redox reversibility toward vanadium redox couple. Based on the cyclic voltammetry data and electrochemical impedance spectroscopy curves, the modified composite electrode possesses a huge amount
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of oxygen functional groups, high surface area, high electrical conductivity and excellent stability in compared to the pristine carbon felt electrode. The prepared electrode showed promising performance for vanadium redox flow battery as the energy efficiency was sig-
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nificantly enhanced by 12% at current density of 60 mA.cm−2 . This superior performance was probably due to the increase in the surface sites and fast electron transfer rate of the reduced graphene oxide.
Keywords: Reduced graphene oxide, Electrodeposition, Vanadium redox flow battery, Electrocatalyst 2010 MSC: 00-01, 99-00
Preprint submitted to Electrochimica Acta
November 13, 2018
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1. Introduction In order to restrict the environmental impacts of the extensively employed fossil fuels, renewable energy sources have been grown and implemented all over the planet. However,
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there has been a recent surge of concern regarding their intermittent nature which makes it crucial to establish large-scale energy storage devices [1, 2]. High-performance energy storage devices are the crusial link between energy production and its consumption. In this regard, electrochemical redox flow batteries and typically vanadium redox flow batteries (VRFBs)
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are represented as one of the most efficient technologies and are highly promising choices for various stationary applications [3, 4, 5, 6]. Fast responsiveness, flexibility and scalability,
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durability, independent sizing of power and energy, high round-trip efficiency and reduced environmental impact are the most appealing features of the VRFB technology which are recently considered for large scale energy storage [7, 8, 9].
The development of more efficient electrode materials is necessary to get VRFBs with improved energy densities and to make them more competitive. The electrodes are the
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core components that provide the active sites for the reaction of redox couples during the charge-discharge process of a battery. Carbon felts (CF) are commercially available and widely used electrodes in the VRFB due to their unique chemical and physical characteristics [10, 11, 12]. However, the surface of CF is inert towards vanadium redox pairs which resulted
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in considerable polarization resistance and energy loss during the battery operation[13, 14]. Expanding specific surface area and increasing the effective active sites are widely considered
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to figure out such issues [15, 16, 17, 18, 19]. Diverse chemical [20], hydrothermal [21] and electrochemical [22, 23, 24] modifications have been used to introduce oxygen-containing functionalities into inert CF surface in order to increase wettability and the number of active sites. Alternatively, designing the composites of the low-cost metals and carbon-based catalysts with higher electrocatalytic performance were proposed [25, 26, 27]. Regarding the low cost metals, it was found that the size and stability have strong effect on electroactivity ∗
corresponding author Email addresses: [email protected] (Pooria Moozarm Nia), [email protected] (Ebrahim Abouzari-Lotf)
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of the catalyst whereas for noble metal composites, gas evolution and their high-cost are the main concerns. Additionally, although prominent electrochemical catalytic activity was reported for functionalized carbon nanotube catalysts due to a huge surface area and large
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number of functionalities [28, 29, 30], their application has been restricted due to severe reaction preparation condition.
Despite some improvements in the electrochemical reactivity, most of the modified carbonbased electrodes showed undesired electrical conductivity. An effective approach to address
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this issue is to introduce an effective electrocatalyst with desirable and tunable electrocatalytic behavior as well as electrical conductivity. To address this, in recent years, graphene-
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based materials have drawn a huge attention in both technological and scientific areas [31, 32, 33]. These materials present significant preferences of ease of processing, safety and low cost as well as remarkable performance in terms of electrical conductivity and electrocatalytic activity [34, 35]. In addition, graphene-based materials show superior electron transfer which promotes the ability for oxidation and reduction of vanadium ions (various states). Such advantages make graphene-based material appealing electrodes for redox flow
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batteries. The reduction of GO became a key research in the current decay to restore the graphitic structure and enhance its conductivity for diverse applications. As the reactivity and conductivity of GO are linked to the oxygen-containing groups, fabrication method of
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reduced graphene oxide (rGO) plays a considerable role in its electrochemical performance. Reduction of GO is often carried out chemically [36]. However, various techniques including plasma [37], hydrothermal [38, 39] and electrochemical reduction [22, 40] are also reported.
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Among all, electrochemical reduction has received a significant attention and opens a door to control the reduction process in both solution and solid state phases. Not only this technique offers the advantages of close monitoring over the reduction process, but can also be performed under mild reaction conditions. In fact, GO will simultaneously electro-deposit and electro-reduce on the surface of the electrode with controllable C/O ratios. This technique also enhances the accessible electrochemical surface area which is responsible for the available pathway for the electron transport. The electrochemical enhancement is mainly attributed to the restoring the graphene-like structure as well as residual functional groups 3
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of OH and COOH remaining even after electro-reduction. It was proposed that the remaining oxygen containing groups on the surface of graphene could serve as potential active sites for the redox reaction of VO2+ /VO+ 2 couple [41, 42]. On the other hand, it is difficult to
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counterbalance between electrocatalytic performance and electron conductivity of GO-based materials, since although elimination of oxygen functional groups can improve the electron conductivity of rGO, it inversely decreases the available active sites and reduces electrocatalytic performance [43]. Recently, various approaches have been used for reduction of GO,
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such as hydrothermal, chemical and electrochemical procedures. The main disadvantage of chemical process is involving hydrazine and other strong oxidants which are of environmental
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concern [44]. For hydrothermal method, it is difficult to control the GO reduction whereas in electrochemical procedure, by controlling various parameters, reduction process can be simply tuned. In a typical comparison with the recently reported hydrothermally reduced GO [25], it is shown that the reduction conversion rate is higher in electrochemical method and the redox reaction reversibility of electrochemically prepared rGO is substantially enhanced. The main intent of this research is to develop low-cost and high performance electrode
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materials for VRFB using a simple, controllable and scalable electrochemical procedure. Electrodeposition and electro-reduction of GO took place at the same time, sheets interacted one by one and rGO was deposited on the CF by controlling current and potential
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window. An electrodeposited material was investigated for the positive electrode of a VRFB battery by means of cyclic voltammetry, impedance spectroscopy and charge-discharge experiments. It was manifested that the significantly enhanced electrochemical ability and
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kinetic reversibility of the developed electrode towards VO2+ /VO+ 2 redox couple as well as remarkably high energy efficiency of the VRFB cell can be related to the unique electrochemical properties of the prepared material. High electron conductivity as well as sufficient functional groups were responsible for the enhanced performance of the VRFBs.
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2. Materials and methods 2.1. Chemicals and reagents All reagents were of analytical grade with maximum purity. Deionized water (DI) (resis-
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tivity of 18.2 MΩ.cm at room temperature) was used throughout the study. Nafion solution (5% in ethanol/water mixture) was supplied by DuPont. A 1.5 M solution of V3+ /V4+ in 3 M H2 SO4 was used as electrolyte. Nafion 117 membrane (DuPont, USA) was used for battery tests. The membrane was pretreated by 1 h boiling consecutively in 1 M H2 SO4 and
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deionized water (Milli-Q water). 0.1 M phosphate buffer solutions (PBS) with pH values of 5, 7 and 9 were prepared from 1.0 M mono- and di-potassium phosphate (KH2 PO4 and
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K2 HPO4 ) in DI water. 2.2. Instruments
Electrochemical measurements were carried out using a potentiostat/galvanostat Versa STAT3 employing Versa Studio software. A 1×1 cm2 working electrode (carbon felt), counter (platinum plate) and reference (saturated calomel) electrodes made a three-electrode com-
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partment cell. FTIR Spectra were carried out on a Spectrum GX from Perkin Elmer coupled with an attenuated total reflectance (ATR) with diamond crystal using 16 scans at 4 cm−1 resolutions. Raman spectra were recorded on a LabRAM HR Evo Raman spectrometer
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(Horiba) machine with a 532 nm laser. The morphology of the prepared composite was investigated by Field Emission Scanning Electron Microscopy (FESEM. Quanta 200F) and
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Transmission Electron Microscopy (TEM Philips CM200). X-ray photoelectron spectroscopy (XPS) was carried out with a monochromatic radiation source of Al Kα anode (1486.6 eV) at room temperature and a pressure <1×10−8 bar (PHI Quantera II Scanning XPS Microprobe). The battery performance was studied by employing a multi-channel battery analyzer (BTS8-20A-CDS, MTI-corporation) under constant current densities of 30, 40, 50 and 60 mA.cm−2 . The carbon felt and modified electrodes were separately used as the electrode, and a Nafion 117 was used as a membrane. In order to avoid oxygen and hydrogen evolution, the charge-discharge voltages were 1.72 to 0.7 V. Electrochemical impedance spectroscopy 5
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measurements were recorded by applying an amplitude of 10 mV around open circuit potential in a frequency range of 105 to 10−2 Hz. Equivalent circuit was fitted by using the Zview software.
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2.3. Preparation of GO solution
Graphene oxide was prepared by a modified Hummers’ method [45]. Initially, graphite flakes were oxidized through reacting with a mixture of concentrated phosphoric acid and sulphuric acid (1:9, v/v). The reaction vessel was immersed in an ice bath and a potassium
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chlorate solution was gradually added to the mixture. To fully oxidize graphite flakes into graphite oxide, the solution was continuously stirred for 3 days. Later, hydrogen peroxide
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was added and the solution was stirred for 10 more minutes to ensure a complete oxidization. The GO was then centrifuged in 1 M HCl (4000 rpm, 10 min) and washed with it 3 times. In order to reach pH 7, the solution was repeatedly centrifuged in water and was subsequently filtered. The GO mixture was exfoliated by mild sonication for 5 minutes until achieving a yellow brown suspension. Finally, the product was centrifuged, washed and vacuum-dried.
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The prepared GO powder was exfoliated in a 0.1 M phosphate buffer solution (PBS) under various acidic, neutral and basic conditions as well as at different concentrations of GO and stirred under sonication for 30 minutes to achieve a well-dispersed GO colloidal dispersion. For each run, the mixture was purged with purified N2 for 20 minutes before
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running the electrochemical experiment.
2.4. Electrodeposition and monitoring electrochemical activity
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The prepared GO was deposited and reduced simultaneously on the surface of carbon felt through cyclic voltammetry (CV) by scanning repetitively from 0 to -1.5 V vs. saturated calomel electrode (SCE). To study the pH effect on the performance of the prepared electrodes, GO solution was prepared in alkaline (@pH 9), natural (@pH 7) and acidic (@pH 5) buffer solution. Additionally, for studying the GO concentration effect, diverse GO solutions with different concentrations (10, 20, 30, 40 and 50 mg) were prepared. Scan rate was set to be 10 mV.s−1 and 1, 2, 3, 4 and 5 number of cycles were applied. Resulting modified electrodes were correspondingly named as ED-rGO-1, ED-rGO-2, ED-rGO-3, ED-rGO-4 and 6
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ED-rGO-5. After each electrodeposition, DI water was used to rinse the prepared electrodes to remove the physically adsorbed species and the modified electrode was dried at 60 ◦ C for
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5 hours. 3. Results and discussion 3.1. Electrode preparation
By applying a proper potential window and due to the presence of oxygen-containing
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groups and negatively charged nature of the GO sheets, the sheets were transferred from aqueous solution towards the positive working electrode [22]. As explained in the litera-
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ture[25, 46], the surface of the CF is interact with the oxygen functional groups which are present at the surface GO sheets upon applying proper potential. Chemical bond formation along with physical adsorption between the electrochemically reduced graphene oxide and carbon caused powerful bonding to the CF surface. On the other hand, the removal of hydroxyl and carboxyl groups located at the edges of the GO sheets during the electroreduction
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process, accelerates the recovery of the π-π bonds [47]. Due to the elimination of oxygen containing functional groups of the GO, huge number of rGO sheets are formed at CF surface. In order to reduce the oxygen functional groups and retain the lattice of graphene structure, the electrode was electrochemically cycled between 0 and -1.5 V. Figure. 1a shows the first
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five cycles of electrodeposition process. The electrochemical irreversible reduction of GO is in charge with the cathodic current peak at -1.3 V. For the second cycle, the reduction peak
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decreased and vanished in the subsequent cycles which confirms the reduction of oxygen functional groups. By electrochemical reduction of the oxygen containing functional groups, the sp2 backbone of graphene sheets was regenerated. Based on literature [48], proton is involved in the electroreduction process. So, the following mechanism is proposed for the electrochemical reduction of GO:
GO + aH + + be− −→ graphene + cH2 O
(1)
Due to negative nature of GO, sheets from the solution move towards the positive elec7
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trode because of the existence of the electric field which was created by applying the above mentioned potential [49]. The choice of more negative potentials than -1.5 V may result in hydrogen evolution, leading to the leaching of the material from the surface of the electrode.
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The FESEM image (Figure. 1(b-e)) proves the formation of rGO. Figure. 1b shows the overall overview of the carbon felt . Figure 1c presents a single fiber inside the carbon. The estimated diameter of each single fibre is calculated around 10 µm. After electrodeposition of reduced graphene oxide, a thin layer was formed at the surface of the fiber. The estimated
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diameter of coated rGO is calculated around 15.6 µm. The images clearly confirm the uniform formation of reduced graphene oxide at the surface of carbon felt fibers. The image
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of FESEM shows that the surface of rGO is wrinkled and rough which can be justified as in the course of electrodeposition, with decreasing the electrode potential to negative values, GO was electrochemically reduced to form hydrophob rGO. As can be seen, rGO uniformly covers the surface. TEM image of rGO is shown in Figure. 1f. The transparency of rGO is due to reduction of it by electrochemical way.
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3.2. Structural analysis
The FTIR spectra were carried out to confirm the formation of electrodeposited rGO at the surface of carbon felt. Figure. 2a shows the FT-IR spectra of the CF, pristine graphene oxide and ED-rGOs prepared at different pH of GO solution (@ 5, 7 and 9).
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For the pure GO, the peaks position at 1047, 1393, 1634 and 1734 cm−1 , correspond to C-O-C stretching, -OH bending, sp2 -hybridized C=C group and -C=O (carboxylic acid)
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stretching vibrations, respectively [50, 51]. A broad peak placed at above 3250 cm−1 could be assigned to the O-H functional groups and intercalated water molecules [51]. For EDrGOs, the overall intensity of all oxygen-related peaks decreases due to the partial removal of oxygen-containing functional groups (Figure. 2a) [41]. In details, hydroxyl functional groups were partially eliminated (due to lower required potential for reduction) as depicted in the Figure. 2a. In addition, two peaks in the region of 2860 cm−1 to 2920 cm−1 were emerged. This suggests that a significant conversion of sp3 into sp2 hybridization in GO structure is associated with the elimination of the C-H. On the other side, the reduction 8
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Figure 1: a) Electrodeposition curves for reduction and deposition of GO, FESEM images of b) overall overview of pristine carbon felt, c) a single fibre before electrodeposition process, d) a single fibre after electrodeposition process, e) estimated diameter of electrodeposited rGO and b) TEM image of rGO.
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of C-O stretching vibration peaks of GO is the most typical change observed in IR spectra upon reduction. It can be seen that during the electrochemical reduction at a potential of ∼ -1.3 V vs. SCE, C-O functional groups in GO might be removed. However, for the reduction
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of the O-H and C-O-C functional groups, higher negative potentials are required [40]. As reported, some functional groups at basal sheets of GO are quite stable, which requires a more powerful technique than electrochemical reduction. Thus, some of epoxide or ether groups remained unvaried after electrochemical reduction.
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In the Raman spectroscopy shown in Figure. 2b, two typical characteristic peaks appearing at around 1600 and 1340 cm−1 correspond to the G and D bands in the pristine
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graphene oxide. The G band is related to the E2g phonon of sp2 carbon atoms (the graphene peak), while the D band corresponds to the breathing mode of κ-point phonons of A1g symmetry (defect peak because of intervalley scattering). The appearance of D band indicates the existence of defects in GO caused by oxidation [52]. Meanwhile, the intensity ratio of the D and G bands (ID /IG ) could reflect the size of sp2 domains and the structural disorder of graphene sheets. After electrodeposition of rGO, the ID /IG obviously increases in
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comparison with those of the original GO, suggesting a decrease in the average size of sp2 domains and confirming the formation of GO [53]. The ID /IG of the ED-rGO@pH 9 (1.39) is higher than that of graphene oxide (0.81), suggesting an increase in the number of smaller
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graphene domains upon reduction of graphene oxide. It was also higher than ED-rGO@pH 7 (1.27) and ED-rGO@pH 5 (1.19) in the same test conditions. It is worth mentioning that, by comparing ID /IG values for GO and rGO, it can be found out that the electrochemically
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reduced GO (ID /IG for GO and rGO in electrochemical approach are 0.81 and 1.37, respectively) showed higher reduction conversion in compared with the hydrothermally reduced GO (ID /IG for GO and rGO in hydrothermal approach is 0.78 and 0.95, respectively). Additionally, in order to extensively explorer the reduction process, XPS was carried out to analyze GO and the reduced GO (figure 3a and b). As can be seen in the figure 3a, the C1s XPS spectrum of GO clearly indicates a considerable presence of oxygen and carbon atoms in different functional groups: the non-oxygenated ring C (sp2 ), the C in C-O, carbonyl and the carboxylate carbon bonds. Moreover, figure 3b presents the C1s XPS spectrum 10
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Figure 2: a) FTIR spectra for the bare carbon felt,11 pristine GO and ED-rGOs prepared at different GO solution pH, b) Raman spectra for pristine GO and modified ED-rGOs electrodes at different pH of GO solution.
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of the rGO after electrodeposition process. In compare with GO figure 3a, although it exhibits the same oxygen functionalities, the peak intensities significantly decreased due to electrochemical reduction process. These observations clearly prove the considerable
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reduction of oxygen functional groups due to electrochemical reduction process.
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Figure 3: XPS spectra of C1s of a) GO and b) electrodeposited rGO.
3.3. Electrochemical analysis
Electrochemical impedance spectroscopy (EIS) was employed to evaluate the electron
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transfer between the electroactive species and modified electrode. By measuring the intercept of the semicircle with the real axis in the Nyquist representation, the charge-transfer
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resistance (Rct ) can be obtained (Figure. 5a). Decreasing the charge transfer resistance is supposed to relate with decreasing the interfacial resistance and consequently, led to increasing of electrode conductivity [54]. In order to simulate the electrode behavior, Randles equivalent circuit was employed and the interfacial charge-transfer resistance (Rct ) was measured to be 345 Ω and 76 Ω for the bare CF and ED-rGO-5 modified electrodes, respectively. The obtained data demonstrated that the electrochemically reduced GO/CF provides more favorable electron transfer and causes a significant decrease in Rct in compared with the pristine CF electrode, expressing a much higher electron transfer efficiency. The impedance 12
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Table 1: Electrochemical impedance parameters achieved by fitting equivalent circuit.
Electrode
Rs
Rct
Q
n
14
345
162
0.87
ED-rGO-1
15
284
216
0.96
ED-rGO-2
15
210
284
0.66
ED-rGO-3
14
142
315
0.78
ED-rGO-3
15
112
398
ED-rGO-5
15
76
570
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Pristine CF
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(Ω) (Ω) (µM ho)
0.72
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0. 58
Rs : Solution resistance; Rct : Charge transfer resistance; Q: Constant phase element
parameters in Table. 1 were obtained by fitting the equivalent circuit shown in Figure. 5a (inset) where the fitting error was ≤5%.
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CV was conducted to assess the electrochemical activity of modified electrodes toward −1 the VO2+ /VO+ as scan rate while using 1.5 M VOSO4 solution 2 redox couple at of 10 mV.s
containing 3 M H2 SO4 . From Figure. 4, the anodic peak of pristine CF positioned at 1.41 is ascribed to the oxidation of V4+ ions to V5+ and the reduction peak for pristine CF
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electrode is negligible which indicates that the conversion between V4+ ions and V5+ is not completely reversible. The ED-rGOs were presented more distinct redox peaks than the
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pristine CF. This could be due to abundant rich active sites and improved affinity of the modified electrode to vanadium ions and consequently improved electrocatalytic behavior of the electrode. It can also be observed that higher repetitive number of cycles exhibits the most superior redox ability in compared with the lower number of cycles. This might be due to the presence of more abundant oxygen functional groups on the surface of modified electrode as well as the increased conductivity. As previously reported [55, 56], the residual functional groups play a catalyze role in the redox reaction by generating active sites for the reactions. By having more concentration of the oxygen-containing functional groups on the 13
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electrode surface, better performance could be obtained for the vanadium species reactions. The reaction mechanism is presumed to be as follows [41]:
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R − OH + V O2+ ←→ R − O − V = O+ + H +
(2)
(3)
=O + H + R − OH + V O2+ R − V=O
(4)
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=O R − O − V = O+ + H2 O ←→ R − O − V=O + 2H + + e−
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The mechanism reveals the remarkable role of oxygen containing groups provided by the rGO nanosheets in combination with the CF electrode. The remaining functional groups on the graphene surface catalyze the redox reaction by generating active sites for the positive reaction of VRFB. The existence of these oxygen-containing groups at the surface of the partially reduced GO plays a critical role in its electrocatalytic ability. On the other hand, the more the GO is reduced, the more oxygen-containing functional groups are eliminated
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which causes conversion of sp3 carbons to sp2 , and through this path, considerable restoration takes place in the conjugation of π-electrons, enhancing the conductivity of the rGO. Therefore, it is crucial to make a balance in the reduction extent for maximizing the elec-
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trocatalytic activity of the rGO. The reduction extent should be high-enough to overcome the high electrical resistance of GO, which inhibits the electron transfer, without compromising the functional groups, since most of the them are essential for the electroreduction
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or -oxidation of VO2+ /VO+ 2 ions.
Additionally, the peak potential separation (∆E), redox onset potential and ratio of the peak current were evaluated from the electrochemical activity of VO2+ /VO+ 2 redox couples. Summarized data from CVs is tabulated in Table. 2. The ∆E value of the bare CF corresponding to the VO2+ /VO+ 2 redox couples was 505 mv. Additionally, onset potential of both redox peaks for all modified electrodes showed a negative shift as well as a decrease of the peak-to-peak separation (Table. 2). Particularly, the values of ∆E corresponding to the VO2+ /VO+ 2 redox couple on the ED-rGO-5 decreased by 0.213 mV in compared with 14
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Figure 4: CVs of developed electrodes a) in different concentration of GO solution compared with the bare CF at a scan rate of 10 mV.s−1 and b) different electrodeposition cycles in a solution containing 1.5 M V3+ /V4+ in an electrolyte of 3.0 m H2 SO4 .
the corresponding value for CF (Figure. 4). In compared with a recently published paper on hydrothermally reduced GO [25], ∆E of redox couple at the surface of the electrochem-
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ically reduced GO (0.230 mV) is less than that achieved by hydrothermal method (0.37 mV). On the other side, the anodic and cathodic peak currents for the modified electrodes significantly increased compared with the untreated CF electrode. Such increases support the enhancement in the electron transfer kinetics on the surface of ED-rGO electrodes. This
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is in good agreement with the EIS analysis data shown in the Figure. 5a. The reversibility of redox reaction can be calculated by measuring the peak current ratio (Ipa /Ipc ) and the
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peak-to-peak oxidation-reduction potential as summarized in Table. 2. For the bare CF electrode, the value of the Ipa /Ipc was the lowest indicating that it was close to the value in irreversible redox reaction. On the other hand, the electrocatalytic behavior of ED-rGO-5 in terms of redox peak current density and peak-to-peak separation show the best performance. As conclusion, CV response results for rGOs in the Figure. 4 showed not only a strong increases in ipa and ipc currents, but also an increase in the reversibility toward + 2+ the VO2+ /VO+ 2 redox couple. The above results indicate that VO /VO2 is much more
reversible for ED-rGOs than for the pristine carbon felt. 15
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Table 2: Electrochemical characterestics of the VO2+ /VO+ 2 reaction on various electrodes.
Ipc
Ipa /Ipc
Ea
Ec
∆E
0.0034
-8.58E-04
3.963 378
1.018
0.513
0.505
ED-rGO-1/CF
0.00605
-0.00152
3.980263
1.104
0.637
0.595
ED-rGO-2/CF
0.00969
-0.00218
4.444954
1.124
0.529
0.467
ED-rGO-3/CF
0.01373
-0.00408
3.365196
1.123
0.629
0.494
ED-rGO-4/CF
0.0175
-0.00628
2.786624
1.125
0.745
0.38
ED-rGO-5/CF
0.01859
-0.00924
2.011905
1.125
0.833
0.292
Bare
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Ipa
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CF
Ipa : anodic peak current; Ipc : cathodic peak current;
Ea : anodic potential; Ec : cathodic potential; ∆E : peak to peak potential
In order to investigate the influence of pH of GO solution during electrodeposition, EDrGOs were prepared at 5, 7 and 9 pH. As can be seen, although rGO at pH 9 shows better
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performance, the difference between all pH is negligible.
Figure. 6a shows the CV curves of ED-rGO-5 electrode at different scan rates. As given in Figure. 6b, the linear relationship between the square root of the scan rate and the redox current of VO2+ /VO+ 2 is proved, suggesting that the oxidation and reduction behavior of
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VO2+ and VO+ 2 are controlled by a diffusion process [41, 57].
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3.4. Battery performance
In order to further investigations, modified electrodes were studied in VRFB static cell configuration. Charge-discharge studies were carried out between 0.7 and 1.72 V at current density of 30, 40, 50 and 60 mA.cm−2 . Oxygen/hydrogen evaluation was not observed in chosen potential range. As the charge-discharge process is accompanied by a strong color shift from orange to yellow and ochre to blue, solution color represents a simple indication of the state of charge of the battery. Figure. 7a represents a comparison among chargedischarge curve profiles for the first cycle of bare carbon felt at 30 mA.cm−2 and for ED16
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Figure 5: a) Nyquist plots of impedance measured for bare carbon felt and ED-rGO-5/CF in V3+ /V4+
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solution, b) Effect of pH on electrodeposited rGO.
Figure 6: a) Cyclic voltammograms of ED-rGO-5/CF in 1.5 M V3.5+ at different scan rates, b) Peak current Ipc and Ipa as a function of square of scan rate.
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rGO-5 modified electrode at 30, 40, 50 and 60 mA.cm−2 . The cell can be charge-discharged within the chosen potential window at current densities of up to 60 mA.cm−2 , while retaining most of its initial capacity, and achieving an energy efficiency. Remarkable decrease in over-
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potential during the charge-discharge processes could be ascribed to the effect of reduced graphene oxide having large active sites for vanadium redox species and high conductivity for electrons transfer resulting in lower charge and higher discharge voltage [58]. Notably, the ED-rGO-5 displayed a higher discharge capacity of 0.175 Ah, which increased by five
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times compared to the CF electrode (0.031 Ah) at a current density of 60 mA.cm−2 . Figure. 7b represents the energy efficiency obtained during charge-discharge operations
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of the VRFB equipped with bare and modified electrodes. For bare CF electrode, the coulombic efficiency of 59% is noted after 80 cycles. The energy efficiency of the battery with the ED-rGO-5 modified electrode is more than 82% which is significantly higher than the values for CF. The energy efficiency is also very stable with only slight trend of decrease. This result provides evidence for good stability of the modified electrode in the dynamic charge and discharge process of VRFB battery.
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In a similar work, the selectively edge-functionalized graphene nanoplatelets (E-GnP) was used as an electrocatalyst for a VRFB system. It showed an 15% improvement higher than the pristine carbon felt and 5% higher than the prepared rGO-coated CF electrode
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at a current density of 50 mA.cm−2 [16]. In another work, a modified carbon felt was prepared based on phosphonated graphene oxide which showed a 80.2% energy efficiency at 20 mA.cm−2 [12]. In another work, graphene-modified graphite felt was used as electrode
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for vanadium redox flow batteries [1]. It showed an energy efficiency value of up to 95.8% at 25 mA.cm−2 for 20 cycles. It was claimed that 3D cross-linked structure with a suitable amount of well-distributed oxygenated active sites functional groups is responsible for the achieved energy efficiency value. The obtained results clearly confirm the advantages of electrochemically reduced rGO as a promising electrode for flow battery applications by considering the beneficial electrocatalytic activity which enables V4+ /V5+ peaks and supplies more electrons for the electrochemical reaction. In addition to the above favorable electrochemical properties, low-cost, 18
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Figure 7: a) Charge-discharge profile (b) and energy efficiency for over 80 charge-discharge cycles of VRFB
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assembled with the bare CF and ED-rGO-5 electrode at different current densities (30, 40, 50 and 60 mA.cm−2 ).
eco-friendliness and easy synthesis protocols and its inherent flexibility to tune the reduction level make the material more attractive for VRFB as well as other electrochemical
4. Conclusion
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applications.
In summary, reduced graphene oxide electrodeposited on the surface of carbon felt (as
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commercially-used electrode) was fabricated by using a low cost and scalable preparation approach. As compared with carbon felt, the as-prepared electrode significantly enhanced the electrical conductivity and reduced the polarization, presenting significant electrocatalytic
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behavior towards the VO2+ /VO+ 2 redox couples. By employing the developed electrode for the VRFB, it presented an excellent improvement in capacity retention, charge-discharge capacity and energy efficiency. The improvements can be ascribed to the tuning of electron transfer resistance as well as density of functionalities that can accelerate the redox reaction on the surface of the developed materials. The promising outputs propose that the prepared material could be one of the promising candidates for VRFB.
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Acknowledgments The research was supported by Universiti Teknologi Malaysia (Professional Development Research University (Vote no: #04E45 and #17H21)) and and university of Malaya (Re-
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search Grant ST002-2018 and Frontier Research Grant (FG030-17AFR)). Authors would
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like to thank Mahtab Mahdavifar for proofreading the manuscript.
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S0013-4686(18)32592-1
DOI:
https://doi.org/10.1016/j.electacta.2018.11.109
Reference:
EA 33110
To appear in:
Electrochimica Acta
Received Date: 2 September 2018 Revised Date:
13 November 2018
Accepted Date: 16 November 2018
Please cite this article as: P.M. Nia, E. Abouzari-Lotf, P.M. Woi, Y. Alias, T.M. Ting, A. Ahmad, N.W. Che Jusoh, Electrodeposited reduced graphene oxide as a highly efficient and low-cost electrocatalyst for vanadium redox flow batteries, Electrochimica Acta (2018), doi: https://doi.org/10.1016/ j.electacta.2018.11.109. 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.
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Electrodeposited reduced graphene oxide as a highly efficient and low-cost electrocatalyst for vanadium redox flow batteries
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Pooria Moozarm Niaa,∗, Ebrahim Abouzari-Lotfa,b,∗, Pei Meng Woic , Yatimah Aliasc , Teo Ming Tingd , Arshad Ahmada,b , Nurfatehah Wahyuny Che Jusohd,e a
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Advanced Materials Research Group, Center of Hydrogen Energy, Institute of Future Energy, Universiti Teknologi Malaysia, Kuala Lumpur, Malaysia b Department of Chemical Engineering, Universiti Teknologi Malaysia, 81310 Johor Bahru, Malaysia c University of Malaya Centre for Ionic Liquids, Department of Chemistry, University of Malaya, Kuala Lumpur 50603, Malaysia d Radiation Processing Technology Division, Malaysian Nuclear Agency, Kajang, Malaysia. e Malaysia–Japan International Institute of Technology, Universiti Teknologi Malaysia, 54100 Kuala Lumpur, Malaysia
Abstract
An electrochemically reduced graphene oxide was grown on carbon felt surface in a simple one-step electrodeposition process of graphene oxide and it was employed as a positive electrocatalyst of vanadium redox flow battery. The electrodeposited graphene-based
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nanocomposite on carbon felt presented outstanding electrochemical redox reversibility toward vanadium redox couple. Based on the cyclic voltammetry data and electrochemical impedance spectroscopy curves, the modified composite electrode possesses a huge amount
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of oxygen functional groups, high surface area, high electrical conductivity and excellent stability in compared to the pristine carbon felt electrode. The prepared electrode showed promising performance for vanadium redox flow battery as the energy efficiency was sig-
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nificantly enhanced by 12% at current density of 60 mA.cm−2 . This superior performance was probably due to the increase in the surface sites and fast electron transfer rate of the reduced graphene oxide.
Keywords: Reduced graphene oxide, Electrodeposition, Vanadium redox flow battery, Electrocatalyst 2010 MSC: 00-01, 99-00
Preprint submitted to Electrochimica Acta
November 13, 2018
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1. Introduction In order to restrict the environmental impacts of the extensively employed fossil fuels, renewable energy sources have been grown and implemented all over the planet. However,
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there has been a recent surge of concern regarding their intermittent nature which makes it crucial to establish large-scale energy storage devices [1, 2]. High-performance energy storage devices are the crusial link between energy production and its consumption. In this regard, electrochemical redox flow batteries and typically vanadium redox flow batteries (VRFBs)
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are represented as one of the most efficient technologies and are highly promising choices for various stationary applications [3, 4, 5, 6]. Fast responsiveness, flexibility and scalability,
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durability, independent sizing of power and energy, high round-trip efficiency and reduced environmental impact are the most appealing features of the VRFB technology which are recently considered for large scale energy storage [7, 8, 9].
The development of more efficient electrode materials is necessary to get VRFBs with improved energy densities and to make them more competitive. The electrodes are the
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core components that provide the active sites for the reaction of redox couples during the charge-discharge process of a battery. Carbon felts (CF) are commercially available and widely used electrodes in the VRFB due to their unique chemical and physical characteristics [10, 11, 12]. However, the surface of CF is inert towards vanadium redox pairs which resulted
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in considerable polarization resistance and energy loss during the battery operation[13, 14]. Expanding specific surface area and increasing the effective active sites are widely considered
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to figure out such issues [15, 16, 17, 18, 19]. Diverse chemical [20], hydrothermal [21] and electrochemical [22, 23, 24] modifications have been used to introduce oxygen-containing functionalities into inert CF surface in order to increase wettability and the number of active sites. Alternatively, designing the composites of the low-cost metals and carbon-based catalysts with higher electrocatalytic performance were proposed [25, 26, 27]. Regarding the low cost metals, it was found that the size and stability have strong effect on electroactivity ∗
corresponding author Email addresses: [email protected] (Pooria Moozarm Nia), [email protected] (Ebrahim Abouzari-Lotf)
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of the catalyst whereas for noble metal composites, gas evolution and their high-cost are the main concerns. Additionally, although prominent electrochemical catalytic activity was reported for functionalized carbon nanotube catalysts due to a huge surface area and large
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number of functionalities [28, 29, 30], their application has been restricted due to severe reaction preparation condition.
Despite some improvements in the electrochemical reactivity, most of the modified carbonbased electrodes showed undesired electrical conductivity. An effective approach to address
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this issue is to introduce an effective electrocatalyst with desirable and tunable electrocatalytic behavior as well as electrical conductivity. To address this, in recent years, graphene-
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based materials have drawn a huge attention in both technological and scientific areas [31, 32, 33]. These materials present significant preferences of ease of processing, safety and low cost as well as remarkable performance in terms of electrical conductivity and electrocatalytic activity [34, 35]. In addition, graphene-based materials show superior electron transfer which promotes the ability for oxidation and reduction of vanadium ions (various states). Such advantages make graphene-based material appealing electrodes for redox flow
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batteries. The reduction of GO became a key research in the current decay to restore the graphitic structure and enhance its conductivity for diverse applications. As the reactivity and conductivity of GO are linked to the oxygen-containing groups, fabrication method of
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reduced graphene oxide (rGO) plays a considerable role in its electrochemical performance. Reduction of GO is often carried out chemically [36]. However, various techniques including plasma [37], hydrothermal [38, 39] and electrochemical reduction [22, 40] are also reported.
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Among all, electrochemical reduction has received a significant attention and opens a door to control the reduction process in both solution and solid state phases. Not only this technique offers the advantages of close monitoring over the reduction process, but can also be performed under mild reaction conditions. In fact, GO will simultaneously electro-deposit and electro-reduce on the surface of the electrode with controllable C/O ratios. This technique also enhances the accessible electrochemical surface area which is responsible for the available pathway for the electron transport. The electrochemical enhancement is mainly attributed to the restoring the graphene-like structure as well as residual functional groups 3
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of OH and COOH remaining even after electro-reduction. It was proposed that the remaining oxygen containing groups on the surface of graphene could serve as potential active sites for the redox reaction of VO2+ /VO+ 2 couple [41, 42]. On the other hand, it is difficult to
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counterbalance between electrocatalytic performance and electron conductivity of GO-based materials, since although elimination of oxygen functional groups can improve the electron conductivity of rGO, it inversely decreases the available active sites and reduces electrocatalytic performance [43]. Recently, various approaches have been used for reduction of GO,
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such as hydrothermal, chemical and electrochemical procedures. The main disadvantage of chemical process is involving hydrazine and other strong oxidants which are of environmental
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concern [44]. For hydrothermal method, it is difficult to control the GO reduction whereas in electrochemical procedure, by controlling various parameters, reduction process can be simply tuned. In a typical comparison with the recently reported hydrothermally reduced GO [25], it is shown that the reduction conversion rate is higher in electrochemical method and the redox reaction reversibility of electrochemically prepared rGO is substantially enhanced. The main intent of this research is to develop low-cost and high performance electrode
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materials for VRFB using a simple, controllable and scalable electrochemical procedure. Electrodeposition and electro-reduction of GO took place at the same time, sheets interacted one by one and rGO was deposited on the CF by controlling current and potential
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window. An electrodeposited material was investigated for the positive electrode of a VRFB battery by means of cyclic voltammetry, impedance spectroscopy and charge-discharge experiments. It was manifested that the significantly enhanced electrochemical ability and
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kinetic reversibility of the developed electrode towards VO2+ /VO+ 2 redox couple as well as remarkably high energy efficiency of the VRFB cell can be related to the unique electrochemical properties of the prepared material. High electron conductivity as well as sufficient functional groups were responsible for the enhanced performance of the VRFBs.
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2. Materials and methods 2.1. Chemicals and reagents All reagents were of analytical grade with maximum purity. Deionized water (DI) (resis-
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tivity of 18.2 MΩ.cm at room temperature) was used throughout the study. Nafion solution (5% in ethanol/water mixture) was supplied by DuPont. A 1.5 M solution of V3+ /V4+ in 3 M H2 SO4 was used as electrolyte. Nafion 117 membrane (DuPont, USA) was used for battery tests. The membrane was pretreated by 1 h boiling consecutively in 1 M H2 SO4 and
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deionized water (Milli-Q water). 0.1 M phosphate buffer solutions (PBS) with pH values of 5, 7 and 9 were prepared from 1.0 M mono- and di-potassium phosphate (KH2 PO4 and
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K2 HPO4 ) in DI water. 2.2. Instruments
Electrochemical measurements were carried out using a potentiostat/galvanostat Versa STAT3 employing Versa Studio software. A 1×1 cm2 working electrode (carbon felt), counter (platinum plate) and reference (saturated calomel) electrodes made a three-electrode com-
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partment cell. FTIR Spectra were carried out on a Spectrum GX from Perkin Elmer coupled with an attenuated total reflectance (ATR) with diamond crystal using 16 scans at 4 cm−1 resolutions. Raman spectra were recorded on a LabRAM HR Evo Raman spectrometer
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(Horiba) machine with a 532 nm laser. The morphology of the prepared composite was investigated by Field Emission Scanning Electron Microscopy (FESEM. Quanta 200F) and
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Transmission Electron Microscopy (TEM Philips CM200). X-ray photoelectron spectroscopy (XPS) was carried out with a monochromatic radiation source of Al Kα anode (1486.6 eV) at room temperature and a pressure <1×10−8 bar (PHI Quantera II Scanning XPS Microprobe). The battery performance was studied by employing a multi-channel battery analyzer (BTS8-20A-CDS, MTI-corporation) under constant current densities of 30, 40, 50 and 60 mA.cm−2 . The carbon felt and modified electrodes were separately used as the electrode, and a Nafion 117 was used as a membrane. In order to avoid oxygen and hydrogen evolution, the charge-discharge voltages were 1.72 to 0.7 V. Electrochemical impedance spectroscopy 5
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measurements were recorded by applying an amplitude of 10 mV around open circuit potential in a frequency range of 105 to 10−2 Hz. Equivalent circuit was fitted by using the Zview software.
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2.3. Preparation of GO solution
Graphene oxide was prepared by a modified Hummers’ method [45]. Initially, graphite flakes were oxidized through reacting with a mixture of concentrated phosphoric acid and sulphuric acid (1:9, v/v). The reaction vessel was immersed in an ice bath and a potassium
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chlorate solution was gradually added to the mixture. To fully oxidize graphite flakes into graphite oxide, the solution was continuously stirred for 3 days. Later, hydrogen peroxide
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was added and the solution was stirred for 10 more minutes to ensure a complete oxidization. The GO was then centrifuged in 1 M HCl (4000 rpm, 10 min) and washed with it 3 times. In order to reach pH 7, the solution was repeatedly centrifuged in water and was subsequently filtered. The GO mixture was exfoliated by mild sonication for 5 minutes until achieving a yellow brown suspension. Finally, the product was centrifuged, washed and vacuum-dried.
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The prepared GO powder was exfoliated in a 0.1 M phosphate buffer solution (PBS) under various acidic, neutral and basic conditions as well as at different concentrations of GO and stirred under sonication for 30 minutes to achieve a well-dispersed GO colloidal dispersion. For each run, the mixture was purged with purified N2 for 20 minutes before
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running the electrochemical experiment.
2.4. Electrodeposition and monitoring electrochemical activity
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The prepared GO was deposited and reduced simultaneously on the surface of carbon felt through cyclic voltammetry (CV) by scanning repetitively from 0 to -1.5 V vs. saturated calomel electrode (SCE). To study the pH effect on the performance of the prepared electrodes, GO solution was prepared in alkaline (@pH 9), natural (@pH 7) and acidic (@pH 5) buffer solution. Additionally, for studying the GO concentration effect, diverse GO solutions with different concentrations (10, 20, 30, 40 and 50 mg) were prepared. Scan rate was set to be 10 mV.s−1 and 1, 2, 3, 4 and 5 number of cycles were applied. Resulting modified electrodes were correspondingly named as ED-rGO-1, ED-rGO-2, ED-rGO-3, ED-rGO-4 and 6
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ED-rGO-5. After each electrodeposition, DI water was used to rinse the prepared electrodes to remove the physically adsorbed species and the modified electrode was dried at 60 ◦ C for
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5 hours. 3. Results and discussion 3.1. Electrode preparation
By applying a proper potential window and due to the presence of oxygen-containing
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groups and negatively charged nature of the GO sheets, the sheets were transferred from aqueous solution towards the positive working electrode [22]. As explained in the litera-
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ture[25, 46], the surface of the CF is interact with the oxygen functional groups which are present at the surface GO sheets upon applying proper potential. Chemical bond formation along with physical adsorption between the electrochemically reduced graphene oxide and carbon caused powerful bonding to the CF surface. On the other hand, the removal of hydroxyl and carboxyl groups located at the edges of the GO sheets during the electroreduction
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process, accelerates the recovery of the π-π bonds [47]. Due to the elimination of oxygen containing functional groups of the GO, huge number of rGO sheets are formed at CF surface. In order to reduce the oxygen functional groups and retain the lattice of graphene structure, the electrode was electrochemically cycled between 0 and -1.5 V. Figure. 1a shows the first
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five cycles of electrodeposition process. The electrochemical irreversible reduction of GO is in charge with the cathodic current peak at -1.3 V. For the second cycle, the reduction peak
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decreased and vanished in the subsequent cycles which confirms the reduction of oxygen functional groups. By electrochemical reduction of the oxygen containing functional groups, the sp2 backbone of graphene sheets was regenerated. Based on literature [48], proton is involved in the electroreduction process. So, the following mechanism is proposed for the electrochemical reduction of GO:
GO + aH + + be− −→ graphene + cH2 O
(1)
Due to negative nature of GO, sheets from the solution move towards the positive elec7
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trode because of the existence of the electric field which was created by applying the above mentioned potential [49]. The choice of more negative potentials than -1.5 V may result in hydrogen evolution, leading to the leaching of the material from the surface of the electrode.
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The FESEM image (Figure. 1(b-e)) proves the formation of rGO. Figure. 1b shows the overall overview of the carbon felt . Figure 1c presents a single fiber inside the carbon. The estimated diameter of each single fibre is calculated around 10 µm. After electrodeposition of reduced graphene oxide, a thin layer was formed at the surface of the fiber. The estimated
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diameter of coated rGO is calculated around 15.6 µm. The images clearly confirm the uniform formation of reduced graphene oxide at the surface of carbon felt fibers. The image
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of FESEM shows that the surface of rGO is wrinkled and rough which can be justified as in the course of electrodeposition, with decreasing the electrode potential to negative values, GO was electrochemically reduced to form hydrophob rGO. As can be seen, rGO uniformly covers the surface. TEM image of rGO is shown in Figure. 1f. The transparency of rGO is due to reduction of it by electrochemical way.
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3.2. Structural analysis
The FTIR spectra were carried out to confirm the formation of electrodeposited rGO at the surface of carbon felt. Figure. 2a shows the FT-IR spectra of the CF, pristine graphene oxide and ED-rGOs prepared at different pH of GO solution (@ 5, 7 and 9).
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For the pure GO, the peaks position at 1047, 1393, 1634 and 1734 cm−1 , correspond to C-O-C stretching, -OH bending, sp2 -hybridized C=C group and -C=O (carboxylic acid)
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stretching vibrations, respectively [50, 51]. A broad peak placed at above 3250 cm−1 could be assigned to the O-H functional groups and intercalated water molecules [51]. For EDrGOs, the overall intensity of all oxygen-related peaks decreases due to the partial removal of oxygen-containing functional groups (Figure. 2a) [41]. In details, hydroxyl functional groups were partially eliminated (due to lower required potential for reduction) as depicted in the Figure. 2a. In addition, two peaks in the region of 2860 cm−1 to 2920 cm−1 were emerged. This suggests that a significant conversion of sp3 into sp2 hybridization in GO structure is associated with the elimination of the C-H. On the other side, the reduction 8
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Figure 1: a) Electrodeposition curves for reduction and deposition of GO, FESEM images of b) overall overview of pristine carbon felt, c) a single fibre before electrodeposition process, d) a single fibre after electrodeposition process, e) estimated diameter of electrodeposited rGO and b) TEM image of rGO.
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of C-O stretching vibration peaks of GO is the most typical change observed in IR spectra upon reduction. It can be seen that during the electrochemical reduction at a potential of ∼ -1.3 V vs. SCE, C-O functional groups in GO might be removed. However, for the reduction
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of the O-H and C-O-C functional groups, higher negative potentials are required [40]. As reported, some functional groups at basal sheets of GO are quite stable, which requires a more powerful technique than electrochemical reduction. Thus, some of epoxide or ether groups remained unvaried after electrochemical reduction.
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In the Raman spectroscopy shown in Figure. 2b, two typical characteristic peaks appearing at around 1600 and 1340 cm−1 correspond to the G and D bands in the pristine
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graphene oxide. The G band is related to the E2g phonon of sp2 carbon atoms (the graphene peak), while the D band corresponds to the breathing mode of κ-point phonons of A1g symmetry (defect peak because of intervalley scattering). The appearance of D band indicates the existence of defects in GO caused by oxidation [52]. Meanwhile, the intensity ratio of the D and G bands (ID /IG ) could reflect the size of sp2 domains and the structural disorder of graphene sheets. After electrodeposition of rGO, the ID /IG obviously increases in
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comparison with those of the original GO, suggesting a decrease in the average size of sp2 domains and confirming the formation of GO [53]. The ID /IG of the ED-rGO@pH 9 (1.39) is higher than that of graphene oxide (0.81), suggesting an increase in the number of smaller
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graphene domains upon reduction of graphene oxide. It was also higher than ED-rGO@pH 7 (1.27) and ED-rGO@pH 5 (1.19) in the same test conditions. It is worth mentioning that, by comparing ID /IG values for GO and rGO, it can be found out that the electrochemically
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reduced GO (ID /IG for GO and rGO in electrochemical approach are 0.81 and 1.37, respectively) showed higher reduction conversion in compared with the hydrothermally reduced GO (ID /IG for GO and rGO in hydrothermal approach is 0.78 and 0.95, respectively). Additionally, in order to extensively explorer the reduction process, XPS was carried out to analyze GO and the reduced GO (figure 3a and b). As can be seen in the figure 3a, the C1s XPS spectrum of GO clearly indicates a considerable presence of oxygen and carbon atoms in different functional groups: the non-oxygenated ring C (sp2 ), the C in C-O, carbonyl and the carboxylate carbon bonds. Moreover, figure 3b presents the C1s XPS spectrum 10
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Figure 2: a) FTIR spectra for the bare carbon felt,11 pristine GO and ED-rGOs prepared at different GO solution pH, b) Raman spectra for pristine GO and modified ED-rGOs electrodes at different pH of GO solution.
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of the rGO after electrodeposition process. In compare with GO figure 3a, although it exhibits the same oxygen functionalities, the peak intensities significantly decreased due to electrochemical reduction process. These observations clearly prove the considerable
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reduction of oxygen functional groups due to electrochemical reduction process.
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Figure 3: XPS spectra of C1s of a) GO and b) electrodeposited rGO.
3.3. Electrochemical analysis
Electrochemical impedance spectroscopy (EIS) was employed to evaluate the electron
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transfer between the electroactive species and modified electrode. By measuring the intercept of the semicircle with the real axis in the Nyquist representation, the charge-transfer
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resistance (Rct ) can be obtained (Figure. 5a). Decreasing the charge transfer resistance is supposed to relate with decreasing the interfacial resistance and consequently, led to increasing of electrode conductivity [54]. In order to simulate the electrode behavior, Randles equivalent circuit was employed and the interfacial charge-transfer resistance (Rct ) was measured to be 345 Ω and 76 Ω for the bare CF and ED-rGO-5 modified electrodes, respectively. The obtained data demonstrated that the electrochemically reduced GO/CF provides more favorable electron transfer and causes a significant decrease in Rct in compared with the pristine CF electrode, expressing a much higher electron transfer efficiency. The impedance 12
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Table 1: Electrochemical impedance parameters achieved by fitting equivalent circuit.
Electrode
Rs
Rct
Q
n
14
345
162
0.87
ED-rGO-1
15
284
216
0.96
ED-rGO-2
15
210
284
0.66
ED-rGO-3
14
142
315
0.78
ED-rGO-3
15
112
398
ED-rGO-5
15
76
570
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Pristine CF
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(Ω) (Ω) (µM ho)
0.72
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0. 58
Rs : Solution resistance; Rct : Charge transfer resistance; Q: Constant phase element
parameters in Table. 1 were obtained by fitting the equivalent circuit shown in Figure. 5a (inset) where the fitting error was ≤5%.
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CV was conducted to assess the electrochemical activity of modified electrodes toward −1 the VO2+ /VO+ as scan rate while using 1.5 M VOSO4 solution 2 redox couple at of 10 mV.s
containing 3 M H2 SO4 . From Figure. 4, the anodic peak of pristine CF positioned at 1.41 is ascribed to the oxidation of V4+ ions to V5+ and the reduction peak for pristine CF
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electrode is negligible which indicates that the conversion between V4+ ions and V5+ is not completely reversible. The ED-rGOs were presented more distinct redox peaks than the
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pristine CF. This could be due to abundant rich active sites and improved affinity of the modified electrode to vanadium ions and consequently improved electrocatalytic behavior of the electrode. It can also be observed that higher repetitive number of cycles exhibits the most superior redox ability in compared with the lower number of cycles. This might be due to the presence of more abundant oxygen functional groups on the surface of modified electrode as well as the increased conductivity. As previously reported [55, 56], the residual functional groups play a catalyze role in the redox reaction by generating active sites for the reactions. By having more concentration of the oxygen-containing functional groups on the 13
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electrode surface, better performance could be obtained for the vanadium species reactions. The reaction mechanism is presumed to be as follows [41]:
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R − OH + V O2+ ←→ R − O − V = O+ + H +
(2)
(3)
=O + H + R − OH + V O2+ R − V=O
(4)
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=O R − O − V = O+ + H2 O ←→ R − O − V=O + 2H + + e−
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The mechanism reveals the remarkable role of oxygen containing groups provided by the rGO nanosheets in combination with the CF electrode. The remaining functional groups on the graphene surface catalyze the redox reaction by generating active sites for the positive reaction of VRFB. The existence of these oxygen-containing groups at the surface of the partially reduced GO plays a critical role in its electrocatalytic ability. On the other hand, the more the GO is reduced, the more oxygen-containing functional groups are eliminated
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which causes conversion of sp3 carbons to sp2 , and through this path, considerable restoration takes place in the conjugation of π-electrons, enhancing the conductivity of the rGO. Therefore, it is crucial to make a balance in the reduction extent for maximizing the elec-
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trocatalytic activity of the rGO. The reduction extent should be high-enough to overcome the high electrical resistance of GO, which inhibits the electron transfer, without compromising the functional groups, since most of the them are essential for the electroreduction
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or -oxidation of VO2+ /VO+ 2 ions.
Additionally, the peak potential separation (∆E), redox onset potential and ratio of the peak current were evaluated from the electrochemical activity of VO2+ /VO+ 2 redox couples. Summarized data from CVs is tabulated in Table. 2. The ∆E value of the bare CF corresponding to the VO2+ /VO+ 2 redox couples was 505 mv. Additionally, onset potential of both redox peaks for all modified electrodes showed a negative shift as well as a decrease of the peak-to-peak separation (Table. 2). Particularly, the values of ∆E corresponding to the VO2+ /VO+ 2 redox couple on the ED-rGO-5 decreased by 0.213 mV in compared with 14
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Figure 4: CVs of developed electrodes a) in different concentration of GO solution compared with the bare CF at a scan rate of 10 mV.s−1 and b) different electrodeposition cycles in a solution containing 1.5 M V3+ /V4+ in an electrolyte of 3.0 m H2 SO4 .
the corresponding value for CF (Figure. 4). In compared with a recently published paper on hydrothermally reduced GO [25], ∆E of redox couple at the surface of the electrochem-
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ically reduced GO (0.230 mV) is less than that achieved by hydrothermal method (0.37 mV). On the other side, the anodic and cathodic peak currents for the modified electrodes significantly increased compared with the untreated CF electrode. Such increases support the enhancement in the electron transfer kinetics on the surface of ED-rGO electrodes. This
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is in good agreement with the EIS analysis data shown in the Figure. 5a. The reversibility of redox reaction can be calculated by measuring the peak current ratio (Ipa /Ipc ) and the
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peak-to-peak oxidation-reduction potential as summarized in Table. 2. For the bare CF electrode, the value of the Ipa /Ipc was the lowest indicating that it was close to the value in irreversible redox reaction. On the other hand, the electrocatalytic behavior of ED-rGO-5 in terms of redox peak current density and peak-to-peak separation show the best performance. As conclusion, CV response results for rGOs in the Figure. 4 showed not only a strong increases in ipa and ipc currents, but also an increase in the reversibility toward + 2+ the VO2+ /VO+ 2 redox couple. The above results indicate that VO /VO2 is much more
reversible for ED-rGOs than for the pristine carbon felt. 15
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Table 2: Electrochemical characterestics of the VO2+ /VO+ 2 reaction on various electrodes.
Ipc
Ipa /Ipc
Ea
Ec
∆E
0.0034
-8.58E-04
3.963 378
1.018
0.513
0.505
ED-rGO-1/CF
0.00605
-0.00152
3.980263
1.104
0.637
0.595
ED-rGO-2/CF
0.00969
-0.00218
4.444954
1.124
0.529
0.467
ED-rGO-3/CF
0.01373
-0.00408
3.365196
1.123
0.629
0.494
ED-rGO-4/CF
0.0175
-0.00628
2.786624
1.125
0.745
0.38
ED-rGO-5/CF
0.01859
-0.00924
2.011905
1.125
0.833
0.292
Bare
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Ipa
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CF
Ipa : anodic peak current; Ipc : cathodic peak current;
Ea : anodic potential; Ec : cathodic potential; ∆E : peak to peak potential
In order to investigate the influence of pH of GO solution during electrodeposition, EDrGOs were prepared at 5, 7 and 9 pH. As can be seen, although rGO at pH 9 shows better
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performance, the difference between all pH is negligible.
Figure. 6a shows the CV curves of ED-rGO-5 electrode at different scan rates. As given in Figure. 6b, the linear relationship between the square root of the scan rate and the redox current of VO2+ /VO+ 2 is proved, suggesting that the oxidation and reduction behavior of
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VO2+ and VO+ 2 are controlled by a diffusion process [41, 57].
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3.4. Battery performance
In order to further investigations, modified electrodes were studied in VRFB static cell configuration. Charge-discharge studies were carried out between 0.7 and 1.72 V at current density of 30, 40, 50 and 60 mA.cm−2 . Oxygen/hydrogen evaluation was not observed in chosen potential range. As the charge-discharge process is accompanied by a strong color shift from orange to yellow and ochre to blue, solution color represents a simple indication of the state of charge of the battery. Figure. 7a represents a comparison among chargedischarge curve profiles for the first cycle of bare carbon felt at 30 mA.cm−2 and for ED16
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Figure 5: a) Nyquist plots of impedance measured for bare carbon felt and ED-rGO-5/CF in V3+ /V4+
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solution, b) Effect of pH on electrodeposited rGO.
Figure 6: a) Cyclic voltammograms of ED-rGO-5/CF in 1.5 M V3.5+ at different scan rates, b) Peak current Ipc and Ipa as a function of square of scan rate.
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rGO-5 modified electrode at 30, 40, 50 and 60 mA.cm−2 . The cell can be charge-discharged within the chosen potential window at current densities of up to 60 mA.cm−2 , while retaining most of its initial capacity, and achieving an energy efficiency. Remarkable decrease in over-
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potential during the charge-discharge processes could be ascribed to the effect of reduced graphene oxide having large active sites for vanadium redox species and high conductivity for electrons transfer resulting in lower charge and higher discharge voltage [58]. Notably, the ED-rGO-5 displayed a higher discharge capacity of 0.175 Ah, which increased by five
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times compared to the CF electrode (0.031 Ah) at a current density of 60 mA.cm−2 . Figure. 7b represents the energy efficiency obtained during charge-discharge operations
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of the VRFB equipped with bare and modified electrodes. For bare CF electrode, the coulombic efficiency of 59% is noted after 80 cycles. The energy efficiency of the battery with the ED-rGO-5 modified electrode is more than 82% which is significantly higher than the values for CF. The energy efficiency is also very stable with only slight trend of decrease. This result provides evidence for good stability of the modified electrode in the dynamic charge and discharge process of VRFB battery.
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In a similar work, the selectively edge-functionalized graphene nanoplatelets (E-GnP) was used as an electrocatalyst for a VRFB system. It showed an 15% improvement higher than the pristine carbon felt and 5% higher than the prepared rGO-coated CF electrode
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at a current density of 50 mA.cm−2 [16]. In another work, a modified carbon felt was prepared based on phosphonated graphene oxide which showed a 80.2% energy efficiency at 20 mA.cm−2 [12]. In another work, graphene-modified graphite felt was used as electrode
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for vanadium redox flow batteries [1]. It showed an energy efficiency value of up to 95.8% at 25 mA.cm−2 for 20 cycles. It was claimed that 3D cross-linked structure with a suitable amount of well-distributed oxygenated active sites functional groups is responsible for the achieved energy efficiency value. The obtained results clearly confirm the advantages of electrochemically reduced rGO as a promising electrode for flow battery applications by considering the beneficial electrocatalytic activity which enables V4+ /V5+ peaks and supplies more electrons for the electrochemical reaction. In addition to the above favorable electrochemical properties, low-cost, 18
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Figure 7: a) Charge-discharge profile (b) and energy efficiency for over 80 charge-discharge cycles of VRFB
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assembled with the bare CF and ED-rGO-5 electrode at different current densities (30, 40, 50 and 60 mA.cm−2 ).
eco-friendliness and easy synthesis protocols and its inherent flexibility to tune the reduction level make the material more attractive for VRFB as well as other electrochemical
4. Conclusion
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applications.
In summary, reduced graphene oxide electrodeposited on the surface of carbon felt (as
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commercially-used electrode) was fabricated by using a low cost and scalable preparation approach. As compared with carbon felt, the as-prepared electrode significantly enhanced the electrical conductivity and reduced the polarization, presenting significant electrocatalytic
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behavior towards the VO2+ /VO+ 2 redox couples. By employing the developed electrode for the VRFB, it presented an excellent improvement in capacity retention, charge-discharge capacity and energy efficiency. The improvements can be ascribed to the tuning of electron transfer resistance as well as density of functionalities that can accelerate the redox reaction on the surface of the developed materials. The promising outputs propose that the prepared material could be one of the promising candidates for VRFB.
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Acknowledgments The research was supported by Universiti Teknologi Malaysia (Professional Development Research University (Vote no: #04E45 and #17H21)) and and university of Malaya (Re-
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search Grant ST002-2018 and Frontier Research Grant (FG030-17AFR)). Authors would
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like to thank Mahtab Mahdavifar for proofreading the manuscript.
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