Three-dimensional mesoporous graphene-modified carbon felt for high-performance vanadium redox flow batteries

Journal Pre-proof Three-dimensional mesoporous graphene-modified carbon felt for high-performance vanadium redox flow batteries David O. Opar, Rosalynn Nankya, Jihye Lee, Hyun Jung PII:

S0013-4686(19)32148-6

DOI:

https://doi.org/10.1016/j.electacta.2019.135276

Reference:

EA 135276

To appear in:

Electrochimica Acta

Received Date: 6 September 2019 Revised Date:

7 November 2019

Accepted Date: 10 November 2019

Please cite this article as: D.O. Opar, R. Nankya, J. Lee, H. Jung, Three-dimensional mesoporous graphene-modified carbon felt for high-performance vanadium redox flow batteries, Electrochimica Acta (2019), doi: https://doi.org/10.1016/j.electacta.2019.135276. 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.

Submitted to the Electrochimica Acta as a research paper

Three-dimensional mesoporous graphene-modified carbon felt for highperformance vanadium redox flow batteries

David O. Opara, Rosalynn Nankyaa, Jihye Leea and Hyun Junga,b,*

a

Advanced Functional Nanohybrid Material Laboratory, Department of Chemistry, b

Research Center for Photoenergy Harvesting & Conversion Technology, Dongguk University Seoul-Campus, Seoul 04620, Republic of Korea

* To whom all correspondences should be addressed. TEL) +82-2-2260-3212 FAX) +82-2-2290-1368 E-mail) [email protected]

ABSTRACT In our contribution, we study the synthesis of three-dimensional (3D) mesoporous graphenemodified carbon felt (MG-CF) via a facile self-assembly interaction method for the application of mesoporous graphene (MG) as an electrocatalyst for vanadium redox flow batteries (VRFBs). The MG loading on carbon felts (CFs) is systematically varied for optimal performance. Morphological and spectroscopic studies indicate that the MG exhibits a wrinkled configuration over the relatively smooth surface of the pristine CF (P-CF), and shows an increase in sp2/sp3 ratio. Nitrogen adsorption/desorption isotherms exhibit increase in specific surface area and porosity with increasing MG loading. Analysis of cyclic voltammetry and electrochemical impedance spectroscopy measurements reveal MG-CF-4 (4 wt.% MG) possess the best electrochemical performance towards V2+/V3+ and VO2+/VO2+ redox couples, attributing to optimal 3D MG strongly anchoring on the CF, enhancing conductivity, specific surface area, and electrochemical activity. In addition, charge/discharge measurements exhibit a high energy efficiency (EE) of 76.5% at 100 mA cm-2 in MG-CF-4, compared to P-CF (66.2). Moreover, a higher energy efficiency (61.6%) and voltage efficiency (62.6) is obtained at a high current density of 175 mA cm-2 with MG-CF-4 electrode. Furthermore, MG-CF-4 shows excellent stability and rate capability (EE of 74.9% and VE of 77.5 after 100 charge/discharge cycles at 100 mA cm-2), demonstrating superior performance of the modified electrodes during the vanadium ions redox reaction under acidic conditions.

Keywords: Vanadium redox flow battery; carbon felt; mesoporous graphene; electrode modification; electrochemical performance

1. Introduction Over the recent past, the ever-increasing global population has resulted in rapid growth in energy demand. However, the depletion of fossil fuels and increasing concern for environmental pollution on a global scale has resulted in the development of alternative energy sources, such as wind, solar and tidal, because they are abundant, and possess lower environmental impact [1-3]. Despite their great energy potential, renewable energy sources are unstable, dispersed, fluctuant, and unpredictable, and hence pose the threat of grid destabilization through frequency and voltage fluctuations [4]. This calls for the provision of cost effective, safe, reliable, and effective large-scale electrical energy storage systems (EESs) for their optimization [5]. Among the potential EESs, redox flow batteries (RFB) have received significant attention as a promising technology towards large-scale energy storage. Among the currently existing RFBs, vanadium redox flow batteries (VRFBs) have gained tremendous interest, owing to their outstanding features, such as quick response time, flexible design, high cyclic stability (10,000 cycles), minimum self-discharge, low maintenance costs, safety, and high energy efficiency, and hence the most researched aqueous RFB chemistry [68]. In addition, since both electrolytes use different valence states of vanadium as the active species, namely V2+/V3+ and VO2+/VO2+, there is significantly minimal risk of cross contamination between electrolytes [9-11]. Despite their superior features, they suffer from significantly low energy efficiency and electrolyte utilization attributed to ohmic loss, kinetic and concentration polarization, which acts as a barrier towards their wider commercialization. Generally, the electrodes play a crucial role in VRFBs, as they provide the surface where the vanadium ion redox reaction occurs [12, 13], facilitating the process of electron transfer towards the negative and positive half-cell reactions Eqs. 1 and 2 [10, 14], hence directly determining the performance of the VRFB [15, 16]. The surface chemistry and morphology

of electrodes significantly contributes to ohmic loss especially at high current densities and charge transfer polarization resulting from redox reactions at the electrolyte/electrode interface at low current densities [17, 18]. Therefore, to overcome polarization and significantly improve battery’s performance, research has focused on improving the surface chemistry of the electrodes by developing electrocatalysts capable of lowering the redox reaction activation barrier and decrease the electrode overpotential, hence, improve the rate capability, electrolyte utilization and energy efficiency of VRFB, by facilitating efficient operation at high current densities, consequently leading to reduced capital cost and broader VRFB market penetration.

Cathode: VO2+ + H2O

VO2+

+ 2H+ + e-

Eo = +1.00 V (1)

Anode V3+

+ e-

V2+

Eo = -0.26 V (2)

Overall cell reaction; VO2+

+ V3+

+ H2O

VO2+ + V2+ +

2H+

Eo = 1.26 V

(3)

Carbon-based materials, such as carbon felts (CF), offer promising candidates for electrode material in VRFBs, due to their superior properties, which include 3-D network structure, lightweight, high conductivity, mechanical strength, wide operating potential range, chemical stability, and relatively inexpensive [12, 18]. However, CF electrodes suffer from poor electrochemical reversibility, low specific surface area, and inferior reaction kinetics, resulting in low energy efficiency and shorter lifespan [18, 19]. Therefore, this calls for the need to enhance the electrode conductivity and /or surface area by introducing electrocatalysts, consequently improving their electrochemical properties. Over the recent

past, several researchers have demonstrated the positive effects of activating CF electrodes by enhancing redox sites, with the aim of altering the electrochemical surface area, functional groups, and the ratio of sp2/sp3. The electrode activation techniques are broadly divided into carbon-based and metal-based treatment, and include the use of metals, such as Ir, Au, Pd, Bi, and Pt [17, 19, 20], metal oxides PbO2, WO3, CeO2, IrO2, TiO2, Nb2O5, and Mn3O4 [14, 21, 22], carbon nanofibers [23], carbon nanotubes [24], heteroatom doping, such as O and N groups [24-28], graphene nanosheets [29], and graphene oxide [30] on CF surface to fabricate electrodes. Despite the enhancement of electrochemical performance, it is worth mentioning that the application of carbon-based electrodes in single flow cell has been limited to very low current density around 50 mA cm-2, and focused mainly on the positive half-cell [24, 3134]. Graphene, a two-dimensional (2D) material consisting of sp2 hybridized carbon atoms, has attracted considerable research attention over the recent past due to its superior physiochemical properties, which include high theoretical surface area (~2,600 m2 g-1), excellent electrical conductivity (~200,000 cm2 V-1s-1), high thermal conductivity (~5,000 W m-1 K-1), and chemical stability; and offers transport channels that are suitable for VRFB applications [35]. The application of graphitic domain in VRFB has been previously researched with key focus on the application of carbon nanotubes, carbon nanofibers, and reduced graphene oxides as electrocatalysts on CF electrode [23, 36, 37]. Despite the promising results obtained from the application of 2D graphene in VRFB, further improvement can be achieved by using three-dimensional (3D) graphene that overcomes the limitations of 2D graphene, such as irreversible stacking and the agglomeration of nanosheets due to strong π–π interaction, resulting in decreased active surface area [38]. To overcome this challenge, the conversion of 2D graphene nanosheets to 3D porous structures using various techniques, such as hydrothermal process, aerosol spray drying, and thermal

reduction method, among others, has attracted considerable attention over the recent past [39, 40]. In addition to possessing the inherent physiochemical properties from 2D graphene nanosheets, they possess a 3D conductive network, mesoporous and microporous structures, and significantly high specific surface area, making them promising materials for energy applications, lithium-ion batteries, and supercapacitors [41]. However, to the best of our knowledge, the low-cost modification of CF electrodes with 3D wrinkled mesoporous graphene nanosheets for VRFB application at high current density has never been reported. In this research we report the synthesis of 3D mesoporous graphene-modified carbon felt (MG-CF), using a facile self-assembly interaction method that is scalable, cost-effective, and environmentally friendly. We investigated the effect of MG loading by several dip-withdrawdry cycles and determined that optimal loading significantly enhances the electrocatalytic performance. The as-synthesized MG-CFs electrodes show enhanced reversibility and electrochemical activity, as compared to the pristine CF (P-CF) electrode. The optimized electrochemical performance was achieved by MG loading of 4 wt.% (MG-CF-4), exhibiting an energy efficiency (EE) 10% higher compared to P-CF at a high current density of 100 mA cm-2. P-CF and MG-CF-4 exhibited limited current densities of 100 and 200 mA cm-2, respectively. Moreover, MG-CF-4 exhibited an outstanding cycle stability (EE of ~ 75% after 100 charge/discharge cycles at 100 mA cm-2). The observed improvement in electrocatalytic performance exhibited by MG-CFs electrodes can be attributed to abundant 3D wrinkled mesoporous structures, enhanced specific surface area, pore volume, wettability, and electrical conductivity.

2. Experimental 2.1. Materials

Commercially acquired graphite powder (<20 µm) and poly (ethylene glycol)-blockpoly (propylene glycol)-block-poly (ethylene glycol) (PEG-PPG-PEG, Pluronic P-123) were purchased from Sigma Aldrich. Carbon felt with uncompressed thickness of 4.2 mm was purchased from XF30A Toyobo, Japan. 1.6 M V3.5+ was purchased from Oxkem Limited, UK. The 2.0 M H2SO4 supporting electrolyte was purchased from Samchun Chemical, Korea. Potassium permanganate (KMnO4, >99.3%) and Sodium nitrate (NaNO3, >99%) were purchased from Daejung Chemicals, Korea. All chemical reagents were analytical grade, and were used as received, without any further purification.

2.2. Mesoporous graphene coated carbon felt electrodes Briefly, exfoliated graphite oxide (GO) was synthesized using a modified Hummers method, as described in our previous work [39, 42-45]. Thereafter, mesostructured graphene suspension was prepared via a soft-template method using the already prepared GO aqueous suspension (10 mg mL-1), and triblock copolymer (Pluronic, P123) (50 mg mL-1) as the soft template. The two solutions were mixed in the ratio of 1:20, and vigorously stirred for 24 h at room temperature (RT). The electrode was modified with 3D mesoporous graphene by a lowcost dip-withdraw-dry process, as shown in Fig. 1. In a typical synthesis process, commercially acquired CF (P-CF) was cut into pieces, and ultrasonically (Sonics, Vibra Cell, 160 W, 20 kHz) cleaned in ethanol, acetone and deionized (DI) water for 15 min and vacuum dried at 80 °C for 4 h. Then, the pre-treated P-CF with an active area of 6 cm2 was dipped into the GO/P123 mixture, withdrawn, and oven dried at 60 °C. This process of dipwithdraw-dry was repeated several times, until an optimum loading of mesostructured graphene was achieved on both electrodes. Finally, various cycles mesostructured graphene loaded CFs were calcinated at 700 °C for 3 h under Ar atmosphere, with a gas flow of 0.4 L min-1, to obtain MG-CFs. The weight of electrodes before and after modification was

calculated as shown in Eq. 4. The as-prepared electrodes with varying MG concentration were denoted as MG-CF-2 (2 wt.%), MG-CF-4 (4 wt.%), and MG-CF-6 (6 wt.%).


 % =

  



 %

(4)

where, W1 is the initial weight of the CF before modification, and W2 is the final weight of MG-modified CF after annealing at 700 °C.

2.3. Characterization The crystal structures of all the samples were analysed using powder X-ray diffraction (XRD) patterns recorded at a scan rate of 1° min-1 from (5 to 60)° with the Ni-filtered Cu Kα radiation (λ = 1.5418 Å) line, using a Rigaku Ultima IV XRD diffractometer. The XRD patterns were operated at an accelerating voltage of 40 kV and applied current of 30 mA. Xray photoelectron spectroscopy (XPS, Thermo Scientific, Theta probe) measurements to analyse surface chemical elements were acquired using a monochromatized Al Kα X-ray source (1486.6 eV). Morphological and microstructural assessment of the as-prepared electrodes was investigated using field emission-scanning electron microscopy (FE-SEM) by JEOL JSM-7100F microscope at 15 kV. Raman spectra were recorded using a confocal Raman microscope (Nanobase, Xperam 200) with a 532 nm excitation laser from 400 to 2,500 cm-1. Nitrogen adsorption/desorption isotherms measured volumetrically at 77 K were used to investigate the specific surface area and pore size distribution of the samples using Microtrac, BELsorp-mini II. The Barrett-Joyner-Halenda (BJH) method was used to determine pore size distribution from the desorption branch curves, while the BrunauerEmmet-Teller (BET) method was used to determine specific surface areas from the adsorption data. The wettability of the CFs was analysed by contact angle measurement with electrolyte droplet method using a Pendant Drop Tensiometer DSA 100.

2.4. Electrochemical measurement The electrochemical properties of the as-prepared samples were investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The CV and EIS measurements were carried out using an electrochemical workstation (ZIVE-SP2, Korea) at RT (~25 °C). The electrochemical cell was a three-electrode configuration with 0.05 M V3.5+ (equimolar ratio of V3+: V4+ ions) in 1.0 M H2SO4 solution as the electrolyte [31, 46]. The asprepared electrode with a testing area of ~1 cm2 and 4.2 mm thickness was used as the working electrode, Ag/AgCl as the reference electrode, and Pt wire as the counter electrode. The working electrode was held by a PTFE electrode holder at a compression ratio of 28%, with only a small section of the felt in contact with platinum, which was connected to a goldcoated copper wire acting as the current collector. The CV was performed at different scan rates (ν) of (2, 5, 10, and 20) mV s-1 over a fixed potential range between (-1.0 to 0) V and (0.2 to 1.4) V. Electrochemical impedance spectroscopy measurements were performed under open circuit voltage (OCV) over a frequency range of (10 mHz to 100 kHz), applying an amplitude of 10 mV. Electrical conductivity test was performed by dual-probe method at RT. The CFs electrodes with a surface area of 1 cm2 were placed between two strong PTFE plates with bolts and nuts, and uniformly pressed. Conductive stainless-steel plate was inserted at both the bottom and upper surface of the CF, and then EIS technique was used to analyse the resistance.

2.5. VRFB single cell test To evaluate the electrochemical performance of the electrodes with an active area of 6 cm2, and 28% compression rate, a VRFB performance single test cell was designed. The charge/discharge cycles were conducted using a miniature flow cell having copper current

collectors, polytetrafluoroethylene (PTFE) end plate (3 cm thick), PTFE flow frame (0.3 cm thick), modified or carbon felts (4.2 mm thickness) as electrodes, graphite foil (TF6 SGL, USA; 0.06 cm thick) as bipolar plates, and a proton exchange membrane (Nafion 115, DuPoint, USA; 127 μm) as the separator, as shown in Fig. S1 of the Supporting Information (SI). Prior to charge/discharge tests, pristine Nafion 115 was successively pre-treated in 3% H2O2 to remove organic impurities, followed by DI water, and lastly, 1 M H2SO4 to remove metallic impurities. Each treatment was at 80 °C for 1 h, and, the membrane was rinsed three times in DI water after each pre-treatment step. Initially, both the anolyte and catholyte consisted of 10 ml electrolyte solution of 1.6 M V3.5+ in 2.0 M H2SO4 solution; consisting of equal amounts of V3+ and V4+ species, were continuously stirred to prevent concentration polarization and purged with nitrogen gas before each measurement. Based on the above electrolyte concentration and volume, theoretical discharge capacity in this work is 21.5 Ah L-1 [17, 46, 47]. A constant current density of 50 mA cm-2 was applied, and after the first charge cycle, the vanadium ions (V3.5+) were converted into V2+ and VO2+ at the negative and positive electrodes, respectively. The flow of the electrolyte through the unit cell was controlled by a dual channel peristaltic 1400 pump (Thermo Co., USA), at a constant rate of 20 mL min-1. A WBCS3000 workstation (WonA Tech Co., Korea) was used to evaluate the performance curves of the VRFB. Seven current densities of (50, 75, 100 125, 150, 175 and 200) mA cm-2 were applied during cell operation, each within a fixed potential range of (0.8 – 1.6) V under constant current mode, to prevent excessive evolution of O2 and H2 [17, 23] and the degradation of battery components. The discharge capacities and cell efficiencies were calculated using second cycle, and all measurements were carried out at RT (~25 °C). Membrane, temperature, flow rate, and electrolyte concentration were fixed across all cycle tests. The stability and rate capability were analysed from (50 to 200) mA cm-2 with each test having 6 charge/discharge cycles, and then recovery at 50 mA cm-2 and cycle stability at 100

mA cm-2 for 100 cycles. The energy efficiency (EE), coulombic efficiency (CE), and voltage efficiency (VE) of the VRFB cell were calculated from the charge/discharge curves using Eqs. 5-7.

 % =  % =  % =

3.

   

  %

   

  %

   
    


  %

(5) (6) (7)

Results and discussion

3.1. Fabrication of three-dimensional mesoporous graphene on carbon felt Mesoporous graphene was successfully synthesized and characterized in our previous report, including physiochemical and electrochemical performance for supercapacitor application [39, 45]. In this report, we have focused on its electrocatalytic performance for VRFB application. A facile self-assembly interaction method was employed to introduce 3D MG electrocatalyst covalently bonded to the surface of the CF. Initially, P123 solution was vigorously mixed with GO aqueous suspension at ambient conditions, resulting in the formation of a 3D mesostructured graphene due to spontaneous self-assembly interaction between the P123 micelles and GO layers, through van der Waals interactions and relatively weak hydrogen bonding [39]. The P123 micelles played a critical role in preventing neighbouring graphene nanosheets from restacking onto each other, as observed in 2D graphene, which significantly improved the specific surface area and electrocatalytic activity [48]. Upon CF dipping, the high-affinity mesostructured graphene spontaneously selfassembled with the CF through physical interaction. Thereafter, the mesostructured-modified CF was annealed at elevated temperatures (700 °C) in an Ar atmosphere, resulting in the

removal of some degree of oxygen functional groups during the reduction process, and also the removal of P123, which led to enhancement of the rigidity of the 3D mesoporous structure with a multidimensional interconnected network covalently bonding with the entire surface of CFs without the use of the expensive Nafion binder, hence resulting in a strong adherence of the 3D MG to the surface of the CF. Therefore, this effectively increases the pore volume and specific surface area, which are critical for high electrochemical performance. The residual oxygen functional groups, such as -OH groups on the surface of mesoporous graphene, can act as redox active sites through hydrogen bonding, resulting in the formation of C–O–V intermediates, hence additionally facilitate V2+/V3+ and VO2+/VO2+ redox couple reaction.

3.2. Morphological and chemical characterization of the as-prepared MG-CFs Morphological studies including the shape, size, and distribution of the as-prepared samples were observed through FE-SEM analysis, as shown in Fig. 2. SEM images of the PCF reveal a 3D network structure of cross-linked carbon fibres. The carbon fibres in the PCFs have an average diameter of (8 ± 1.3) μm, and exhibit a relatively smooth and clean surface, without any observable defects unfavourable for ionic adsorption of redox species (Fig. 2 a–c). However, MG-CFs show considerable surface modification possessing a wrinkled structure evenly distributed on the CF surface as shown in Fig. 2 d–f, indicating crumpling of MG, and these offer active sites facilitating adsorption of vanadium ions. The nanosheets interconnection with carbon fibres results in the formation of a continuous conductive network facilitating electron and ionic transport, and improves the overall charge transfer kinetics [12]. The nature of MG possessing a 3D architecture implies multidimensional electron flow pathways facilitating the enhancement of mass transport during cycling in MG-CFs electrodes [39].

The crystal structures of the as-prepared MG-CFs, P-CF and GO were determined by XRD patterns as shown in Fig. 3 a. Prior to reduction, the (001) diffraction peak of GO appeared at around 12°, corresponding to a d spacing of 0.855 nm [39]. Upon reduction of oxygen functional groups, the (001) GO peak disappeared as evident by its absence in all MG-CFs diffraction profiles, suggesting GO was reduced to few layers’ graphene during thermal treatment resulting in reduced interplane distance [45]. MG-CFs exhibited two typical broad diffraction peaks at around 2θ values of 25.8° and 43.2° matched to the diffraction peaks (002): d spacing 0.344 nm, and (100): d spacing 0.209 nm. Moreover, these two characteristic graphene peaks are also exhibited in P-CF corresponding to graphite diffraction peaks (002) and (100) [49, 50]. Compared with P-CF (2θ = ~25.82°), MG-CFs exhibit a slight shift towards higher 2θ angles (MG-CF-4: 2θ = ~25.92°) and is slightly broader than P-CF, suggesting the sidewalls of the MG-CFs are composed of few-layer graphene nanosheets. No additional peaks are observed, confirming the purity of the asprepared electrodes. Microstructural transformation, such as the disorder and structural defects of the asprepared samples, was further characterized by Raman spectroscopy, as shown in Fig. 3 b. MG phase is evident by the typical well-defined D and G bands centered at around (1,350 and 1,585) cm-1 respectively. Peak intensity increases with increasing MG concentration. The G band corresponds to E2g photon of ordered sp2 carbon bonds, whereas the D band is attributed to the surface functional groups, amorphous irregularities, and structural defects in both surface and edges of the graphene nanosheets, because of lattice vibration [51]. The relative intensity ratio of the D to G bands (ID/IG) is a good measure of the disorder and degree of graphitization, whereby an increasing intensity ratio would suggest an increase in structural defects, such as vacancies, disorders, heteroatoms, and amorphous irregularities through GO reduction, and an increase in sp2 domains [24, 51]. MG modification results in an

increase in ID/IG (Fig. 3 b), as compared to P-CF, suggesting an increase in disordered carbon structures, such as the curvature exhibited by MG structure, in addition to some degree of residual oxygen functional groups in the MG structure ( ~10.12 at.% in the case of MG-CF-4) [39]. Therefore, it can be speculated that the introduction of structural defects sites can act as active sites for redox activity resulting in increased electrochemically active surface area (ECSA) and subsequently, facilitate better electrochemical performance in MG-CF-4 compared to P-CF. The surface chemistry of the electrodes was evaluated using XPS spectra to better understand the oxygen-containing carbon atoms present on the surface of P-CF and MG-CF-4, as shown in Fig. 3 c and d. From the full-survey spectra of P-CF and MG-CF-4 (Fig. S3 a), the O/C atomic ratio decreases upon the introduction of MG, with MG-CF-4 exhibiting a lower O/C atomic ratio (0.11) (Table S1), compared GO (1.29) as reported in our previous works [39, 42, 44]. This suggests a decrease in oxygen content at higher calcination temperatures ascribed to the reduction of the mesostructured graphene precursor during thermal treatment consistent with XRD results, resulting in increasing defect sites consistent with the observed increase in ID/IG in MG-CF-4 (Fig. 3 b) [43, 45]. Furthermore, the reduction of O/C atomic ratio in MG-CF-4 compared GO suggests an increase in conductivity as a result of the introduction of delocalised sp2 domains present on MG, ensuring fast electron transport on the surface of MG-CF-4. On the other hand, P-CF exhibited O/C ratio (0.14) as shown in Table S1. In Fig. 3 d, the deconvoluted high-resolution XPS spectrum of C 1s from MG-CF-4 shows five characteristic carbon peaks at the binding energies of approximately (284.4, 285.3, 286.4, 288.6, and 290.4) eV, corresponding to graphitic carbon (C=C sp2), hybridized carbon (C-C sp3), hydroxyl (C-O), carbonyl (C=O), and carboxyl (-COOH) functional groups, respectively [52]. Analysis of the C 1s peak shows an increase in sp2/sp3 in MG-CF-4 (3.15) compared to P-CF (1.73), indicating the restoration

of C=C bonds in MG-CF-4. The carbon atom in C=C bonds has the potential to form a delocalized π-bond with the unpaired π electrons, resulting in a reduction in energy level, as the π electrons possess a much wider range of motion, consequently enhancing the conductivity of the MG-CF-4. Furthermore, the C=C bonds can act as active sites to facilitate V2+/V3+ and VO2+/VO2+ redox reaction [22]. The presence of some degree of residual oxygen functional groups (such as -OH groups) in MG-CF-4 will provide more active sites for vanadium ion redox reactions as the transfer of electrons is facilitated by C–O–V intermediates formation [53], hence the need to analyse the O 1s spectra. The high-resolution spectrum of O 1s peak from MG-CF-4 shows four characteristic peaks at binding energies of (531.8, 532.3, 533.3, and 534.2) eV, attributed to COOH, C–OH, C–C=O and adsorbed molecular water (H–O–H) functional groups, respectively, indicating the presence of some residual oxygen functional groups that remained after annealing (Fig. S3 b). However, it should be mentioned that, unlike powdered samples, it is surprisingly difficult to measure the changes in surface chemistry of CF fibres using XPS, hence the need for further analysis to better explain the effect of surface electrode modification on electrochemical performance. Nitrogen adsorption-desorption isotherms were used to investigate the porous parameters, such as the specific surface area, pore volume, and pore size distribution of the P-CF and MG-CFs (Fig. 4 a and b, and Table S2). According to the IUPAC nomenclature, the adsorption-desorption isotherms of the MG-CFs are classified under type IV with an evident H2 hysteresis loop characteristic of capillary condensation and evaporation at relative pressures (P/P0) ranging (0.45 to 1.0), indicating that MG-CFs have ink-bottle-shaped mesoporous structures. However, P-CF isotherms can be characterized as type III almost without hysteresis loop; they remain nearly parallel over the entire range of relative pressure, and when P/P0 approaches unity, the isotherms rapidly rise vertically, suggesting the presence of wide macropores [54]. The BET surface area of P-CF is around 3.3 m2 g-1 with

total pore volume (Vt) of 0.007 cm3 g-1, whereas MG-CFs exhibit significantly increasing specific surface area and Vt with increasing MG loading. MG-CF-4 displays a significantly higher specific surface area (26.8 m2 g-1) and mesopore volume 0.035 cm3 g-1 (~ 75% of the total pore volume), indicating enhanced mesopore structures in MG-CFs. The specific surface area, total pore volume, and mesopore volume increase in the order P-CF << MG-CF-2 < MG-CF-4 < MG-CF-6, as shown in Table S2. The average mesopore size of the as-prepared MG-CFs is around 7 nm. The MG-CFs exhibit a tailing pore size distribution of (2 to 50) nm, due to the fabrication of 3D mesoporous graphene structures on the CFs. These mesopore structures may facilitate fast ionic and electronic conducting channels for vanadium ion redox couples, and hence improve electrochemical performance. Although increasing the MG content results in a gradual increase in BET surface area, above 26.8 m2.g-1 (MG-CF-4), the electrochemical performance shows no significant improvement in MG-CF-6, suggesting that there is a maximum effective surface area that can be utilized. However, since BET surface area measurements are known to have limitations regarding the ability of nitrogen to flow through very tiny pores [55], the Randles-Sevcik equation (Eq. 8) was used to estimate the ECSA based on the slope of each electrode (Fig. 5 d), hence to further complement the BET data. From the Randles-Sevcik equation, the ECSA increased gradually in the order P-CF (90.35 cm2) < MG-CF-2 (131.45 cm2) < MG-CF-6 (135.47 cm2) < MG-CF-4 (143.36 cm2), further confirming MG-CF-4 has the optimal MG loading necessary for effective mass transport at the electrolyte-/electrode interface. MG-CF-6 exhibits a decrease in effective surface area ascribed to an excess of MG nanoparticles on the CF surface resulting in aggregation. This is speculated to have negative effects during VRFB operation, as aggregated particles tend to easily detach from the CF surface under a flowing electrolyte, hence, resulting in decrease efficiencies and rate capability.

3.3. Electrochemical measurement To determine the electrochemical properties of the P-CF and MG-CFs electrodes towards the VO2+/VO2+ and V2+/V3+ redox couple, CV and EIS measurements were carried out at RT. Four parameters including redox onset potential, peak-to-peak potential separation (∆E = Epa - Epc), ratio of anodic and cathodic current density (Ipa/Ipc), and redox peak current density values were evaluated to determine the electrochemical behaviour of the as-synthesized electrodes, as shown in Table 1. Fig. 5 a and b show the CVs of the as-prepared electrodes at a scan rate of 5 mV s-1, and display four typical well-defined reversible vanadium ion redox couples V2+/V3+ (Fig. 5 a) and VO2+/VO2+ (Fig. 5 b). According to Fig. 5 a, and Table S3, the anodic and cathodic peaks of the V2+/V3+ redox couples appear at around –(0.37 and -0.64) V, whereas, as shown in Fig. 5 b and Table 1 the redox peak at approximately 1.03 V corresponds to the oxidation of VO2+/VO2+, while the peak at around 0.68 V corresponds to the reduction of VO2+/VO2+ [53]. MG-CF-4 exhibits the lowest onset potential towards oxidation (negative shift vs Ag/AgCl) corresponding to VO2+/VO2+, and highest onset potential towards reduction (positive shift vs Ag/AgCl) corresponding to VO2+/VO2+, suggesting the oxidation and reduction of VO2+ and VO2+, respectively, occur more easily on MG-CFs compared to P-CF. In addition, MG-CF-4 exhibits the highest oxidation and reduction peak current densities for the positive redox couple VO2+/VO2+ (Ipa = 63.2 mA cm-2 and Ipc = 70.9 mA cm-2) and the negative redox couple V2+/V3+ (Ipa = 43.4 mA cm-2 and Ipc = 98.0 mA cm-2) among all the MG-CFs electrodes, whereas the P-CF exhibits the lowest oxidation and reduction peak current densities for the positive redox couple (Ipa = 37.8 mA and Ipc = 30.5 mA) and the negative redox couple (Ipa = 22.5 mA cm-2 and Ipc = 71.3 mA cm2

). This suggests that the MG-CFs electrodes have significantly enhanced electron transfer

kinetics towards vanadium redox reaction and remarkably reduced polarization as compared to P-CF in both half-cells (Table S3), resulting in a reduced charging voltage and enhanced

discharge voltage suitable for VRFB operation. However, MG-CF-6 does not exhibit any improvement in peak current densities or onset redox potential in either half-cell, suggesting MG-CF-4 possesses optimum MG concentration over CFs. It is noteworthy, according to Fig. 5 a, hydrogen evolution reaction (HER), a typical phenomenon in the voltage region of the negative redox couple and almost inevitable due its redox potential being very close to that of V3+/V2+ [56, 57] as observed in both P-CF and MG-CFs electrodes, was stronger in MG-CF4. HER results to performance decrease and if the generated bubbles are unable to detach from the CF surface, then there is a risk of the evolved hydrogen bubbles blocking active sites for V3+/V2+ [33, 58], restricting the diffusion of electrolyte [59] and or peeling of the carbon fibres resulting in loss of active edge sites and ECSA [60]. During full cell chargedischarge test, presence of side reactions such as the irreversible and competitive HER, will result to increased concentration overpotential which ultimately leads to a reduction in coulombic efficiency, electrolyte utilization and capacity fading as observed in Fig. 7 e especially at higher current densities, attributed to electrolyte and charge imbalance [61]. However, it can be speculated that presence of a mesoporous structures in MG-CFs could facilitate the rapid flow of generated hydrogen ions from the active sites in MG-CF-4 as compared to P-CF, preventing further decrease in electrocatalytic activity, resulting to superior electrochemical performance in MG-CFs consistent with CV, EIS and chargedischarge test results. Furthermore, when the electrodes were tested in VRFB single cell test, the cut off voltage was fixed between 0.8-1.6 V, to further prevent excessive HER [17, 23, 62]. Therefore, under these set conditions applied in VRFB single cell test, HER was significantly minimized hence the observed superior electrolyte utilization, energy and voltage efficiencies in MG-CFs compared to P-CF (Fig. 7). Sun et al., reported a strong correlation between ECSA and HER on the negative carbon electrode and concluded that despite an increase in ECSA resulting in an increase in HER as observed in our modified

electrodes, the overall electrochemical performance is improved by using high porosity and high surface area electrodes [63], consistent with our results. Despite graphene being an excellent surface for HER, graphene-based electrocatalysts have also been successfully applied as negative electrodes in VRFB applications [30, 34, 64]. Deng et al., reported the use of reduced graphene oxides-graphite felt as an excellent negative electrode hampering hydrogen evolution and consequently enhancing electrochemical performance [62]. Similar findings were reported by Park et al., who employed the use of rGO and edge-functionalized graphene nanoplatelets as electrocatalysts for both the positive and negative electrode [52]. Further on-going work is based on enhancing the hydrophilic properties of our modified negative electrodes by heteroatom doping and increase in oxygen functional groups to facilitate preferential bonding of V3+/V2+ ions to the introduced functional groups over H+ ions, which will significantly enhance redox reactions at the negative electrodes and consequently supress the occurrence of HER [65, 66]. The redox reaction kinetics and reversibility of the flow cell were evaluated by analysing the ∆E and the ratio of Ipa/Ipc. P-CF reveals the highest ∆E in the positive (0.583 V) and negative half-cell (0.469 V) compared to MG-CFs electrodes, suggesting the redox reaction has a high activation resistance, irreversible and highly polarized [67]. MG-CF-4 exhibits the lowest ∆E of (0.355 and 0.274) V in the positive and negative half-cell, respectively, indicating a quasi-reversible process with significantly reduced polarization and enhanced redox kinetics due to 3D mesoporous graphene modification. This suggests a decrease in charge transfer resistance (Rct) and mass transport resistance. Further increase in MG loading (MG-CF-6) shows no improvement in ∆E. Furthermore, Ipa/Ipc, measures redox reversibility [23, 52], and in the positive half-cell, MG-CFs are much closer to unity (MG-CF-2 = 0.90, MG-CF-4 = 0.89, MG-CF-6 = 0.91) than P-CF (1.24), suggesting MG modification increases the reversibility of the stable vanadium ion redox reaction.

The mass transfer performances of the P-CF and MG-CFs electrodes were estimated by plotting the values of anodic and cathodic peak current density (Ip) versus the square root of scan rate (ν1/2) based on the Randles-Sevcik equation, as shown in Fig. 5 c (inset) and d [52]. The ν1/2 is almost proportional to Ip, in all the electrodes, suggesting that the VO2+/VO2+ and V2+/V3+ redox reaction are diffusion-controlled processes [20]. Moreover, MG-CFs display a higher/larger slope for both reduction and oxidation processes compared to P-CF; for example, MG-CF-4 is approximately (1.4 and 2.1) times steeper for the positive half-cell, compared with P-CF, suggesting higher ECSA and significantly reduced polarization, hence faster mass transfer kinetics on the surface of MG-CFs [68]. According to Eq. 8, ip is the anodic peak current (A), α is the transfer coefficient (0.5), n is the number of exchanged electrons, A is the ECSA (cm2), D is the diffusion coefficient (cm2 s-1), ν is the scan rate (V s1

), and C0 is the initial concentration of the active species (mol cm-3). The value of D of the

VO2+/VO2+ species was taken as 1.0 × 10-6 cm2 s-1 [9]. The extent of overpotential was further analysed by potting a graph of redox peak potential separation versus scan rate (Fig. 5 e), which exhibits a good correlation between increasing scan rate and corresponding increase in ∆E. This demonstrates, an increase in overpotential at higher scan rates, attributed to increase in cell polarization. Notably, MG-CFs demonstrates remarkably reduced electrode polarization, and, significantly improved reversibility as evident by exhibiting the smallest ∆E at all analysed scan rates. 





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To further determine the redox kinetics and reduced polarization of the MG-CFs, EIS test was performed on the as-prepared electrodes. Fig. 5 f shows the Nyquist plots for the asprepared electrodes, which display a semi-circular shape and a sloping linear line in the highfrequency and low-frequency regions, respectively. This indicates the redox mechanism is a combination of both ionic charge transfer processes that occur at the electrolyte/electrode

interface, and diffusion-controlled (Warburg, W) process within the porous electrode [12, 69]. Furthermore, looking at the Nyquist plots, the x-axis intercept represents ohmic resistance (Rs), which comprises electrode, electrolyte, and contact resistance; whereas the radius of the semicircle is proportional to the kinetic polarization of the redox process, and the diameter represents the Rct [12, 67, 69]. This implies that a larger radius indicates a higher Rct, resulting in slower electron transfer during redox. Therefore, the Nyquist plots are fitted with an equivalent circuit as shown in Fig. 5 f (inset), whereby CPE is the constant phase element; double-layer capacitance at the electrode/electrolyte interface. The MG-CFs electrodes exhibit remarkably lower Rct values compared to P-CF (Table 1), suggesting inferior reaction kinetics on P-CF [70]. On the other hand, presence of mesoporous graphene significantly enhances the conductivity by proving a highly conductive matrix for electron transfer attibuted to the presence of delocalised sp2 structure. Furthermore, it increases the ECSA of the modified electrodes resulting in enhanced charge transfer kinetics, smaller polarization value and consequently, improved energy efficiency in VRFB, which is consistent with CV results. Charge transfer resistance is a good indicator of the number of active or catalytic sites at the electrode interface [71], therefore, difference in ECSA and conductivity between P-CF and MG-CFs electrodes resulted in the observed difference in ohmic resistance, further confirming the successful modification of CF with mesoporous graphene. Moreover, increasing the MG content results in a decrease in Rct, as shown in Table 1, suggesting an accelerated electron-transfer process at the interface of electrode and electrolyte. However, beyond the optimum MG concentration, MG-CF-6 exhibits a slight increase in Rct, suggesting an excess of MG modification results in a decrease in conductivity and active surface area, consistent with CV and BET results. To visually investigate the effect of surface treatment on the wettability properties of the P-CF and MG-CFs, the sessile drop method was used to measure the contact angles when the

electrolyte (0.05 M V3.5+ + 1.0 M H2SO4) and CF surface interact at RT. Figure S7 shows that P-CF exhibits a significantly larger contact angle (143.4°) compared to MG-CF-4 (126.9°), implying MG modification results in the increased surface energy and wettability property due to the presence of hydroxyl functional groups on MG-CFs, making it more hydrophilic, hence reduced mass transfer property. P-CF exhibits very low surface energy and poor wettability consistent with the observed relatively smooth surface morphology (Fig. 2 a–c), and very low redox peak current densities compared to MG-CFs (Fig. 5 a and b). To further evaluate the effect of surface modification on the conductivity of the electrodes, the electrical conductivity test was performed by dual-probe method. The Rs of MG-CFs (0.78 Ω MG-CF2, 0.60 Ω MG-CF-4, and 0.73 Ω MG-CF-6) is relatively smaller compared to P-CF (0.91 Ω), ascribed to reduced physical polarization for MG-CF electrodes, therefore, facilitating an increase in conductivity due to the predominant sp2 C=C bonds, hence better electrochemical performance consistent with the CV and EIS results. Therefore, the observed remarkable decrease in redox peak potential separation, increase in redox peak current densities, and, enhancement of onset potential, ECSA, wettability and electron and ionic transport of MGCFs, can be ascribed to improved electrical conductivity, increased number of active sites and presence of 3D network of interconnected mesoporous structures favourable for electron transport and ionic diffusion as a result of MG modification. Fig. 6 a and b show the proposed electrochemical mechanism for improved electrochemical activity. A comparison of the SEM images of P-CF and MG-CFs (Fig. 2) clearly shows the presence of MG on the surface of MG-CFs electrodes. Therefore, it is expected that more ECSA is available on the MG-CFs surface, resulting in the adsorption of an increased amount of vanadium ions. Initially, during the oxidation of VO2+ to VO2+, absorption of VO2+ from the catholyte to the electrode surface occurs, followed by ionic exchange that occurs between VO2+ ions and H+ ions from the -OH functional groups present

on the CF electrodes. Thereafter, oxygen atom from the C–O group is transferred to VO2+ ion accompanied by electron transfer via the C–O–V bond, resulting in the formation of VO2+. Eventually, VO2+ undergoes ionic exchange with H+ ion from the electrolyte, and it diffuses back into the electrolyte from the electrode’s surface (Fig. 6 a) [18]. For the reduction process of VO2+ to VO2+, the reverse reaction proceeds with the help of hydroxyl functional groups [35, 53]. Moreover, C=C bonds can also act as active sites to facilitate V2+/V3+ and VO2+/VO2+ redox reaction (Fig. 6 b) [22]. During the oxidation of VO2+ to VO2+, VO2+ ions and -OH groups from the bulk electrolyte bond with each of the C atoms in the C=C bond on the surface of MG-CFs. Thereafter, the V atom substitutes for the H atom to form a new bond with the O atom, and consequently, H+ is released back into the electrolyte. Finally, VO2+ ion is released back into the electrolyte. Therefore, the synergistic effect between the presence of hydroxyl groups and C=C domain, in addition to the enhanced electrical conductivity and increased specific surface area of the MG-CFs, results in enhanced electrochemical performance.

3.4. VRFB single cell evaluation VRFB single cell tests were carried out to analyse the cyclic performance and long-term cycle stability of the MG-CF-4 and P-CF electrodes. The EE, CE, and VE were evaluated under various current densities (50, 75, 100 125, 150, 175 and 200 mA cm-2). The CE, corresponding to capacity loss, gradually increases with increasing applied current density (Fig. 7 c), which is attributed to increased net vanadium ion convective crossover through the membrane at low current densities resulting in reduced CE. On the other hand, with increasing current density, vanadium permeation decreases, resulting in improved CE [72, 73]. The VE varies significantly between P-CF and MG-CF-4 electrodes, which suggests that VE reveals a true electrocatalytic measure on the effect of surface modifications on CFs.

MG-CF-4 electrode displays a significantly higher VE (79.9%) at 100 mA cm-2, compared to P-CF (68.0%) at the same current density, suggesting enhanced transfer of electrons as a result of reduced overpotentials. Furthermore, VE gradually reduces at higher current densities, with P-CF exhibiting the highest gradient (Fig. 7 c), ascribed to increasing ohmic resistance and overpotential [74]. The energy loss in the VRFB system was assessed by analysing the EE. Over the range of the current density, P-CF shows a faster loss in EE and VE compared to MG-CF-4 electrode-based VRFB with greater efficiency variation apparent at higher current densities (Fig. 7 c and d). The EE of VRFBs based on MG-CF-4 reaches 83.3% at 50 mA cm-2 and 53.3% at 200 mA cm-2 (limiting current density), whereas P-CF shows a lower EE value of 79.4% at 50 mA cm-2 and 66.2% at 100 mA cm-2 (limiting current density). MG-CF-4 exhibits the best EE at all tested current densities (Fig. 7 d), ascribed to optimum amount of mesoporous graphene fabrication on CF. Fig. 7 a shows that at a high current density of 100 mA cm-2, MG-CF-4 displays a remarkably larger specific discharge capacity (10.9 Ah L-1, 51% of theoretical capacity), compared to P-CF showing the least specific discharge capacity (5.2 Ah L-1, 24% of theoretical capacity) suggesting severe polarization in P-CF and enhanced electrolyte utilization in MG-CF-4. Moreover, MG-CF-4, reveals superior discharge capacities at all current densities (Fig. 7 e), ascribed to remarkably reduced overpotentials. The observed increase in specific discharge capacity in MG-CF-4 compared to P-CF, can be equated to an improvement in electrolyte utilization, which efficiently promotes capital cost reduction of stack size. However, with increasing charge/discharge cycles, the specific discharge capacity was observed to gradually decrease ascribed to increasing concentration overpotential due to differential rates of vanadium ion permeation across the membrane from one half-cell to the other and side reactions due to hydrogen evolution and chemical oxidation of V2+ [61]. In addition, MG-CF-4 exhibits significantly lower overpotentials during charge (109.3 mV) and

discharge cycles (216.5 mV), compared to P-CF at 100 mA cm-2 (Fig. 7 b), validating the enhancement of electrochemical performance as a result of MG fabrication. Furthermore, MG-CF-4 shows the lowest charge and highest discharge voltage plateau, suggesting lower activation polarization, hence higher energy and voltage efficiency. Moreover, the initial charge voltage increases, whereas, discharge voltage decreases with increasing current density ascribed to increasing kinetic and ohmic polarization, respectively, consequently, resulting in reduced charge and discharge specific capacities. In Fig. 7 b, the ‘tail’ region of the discharge curves deviates from the pseudo-linearity appearing over the main part of the discharge curve, representing mass transport loss. It is more predominant during discharging, as opposed to charging, as the charging profile is observed to maintain a quasi-linear behaviour, suggesting relatively minor mass transport polarization [75]. To assess the rate capability and long-term stability of the as-prepared electrodes, the current density was changed abruptly from (200 to 50) mA cm-2, and tested under 6 cycles to analyse recovery, followed by 100 charge/discharge cycles at a high current density of 100 mA cm-2 to analyse long term stability under harsh acidic environment. The EE of MG-CF-4 is fully recovered when current density was abruptly decreased from 200 to 50 mA cm-2 (Fig. 7 d), indicating a superior rate capability that is attributed to the presence of electrochemically robust MG that enhanced the specific surface area of the electrode, and improved the transfer rate of oxygen and electron [52, 53]. On the other hand, P-CF shows a relatively lower EE and a much inferior rate capability, as shown in Fig. 7 d. The limiting current density values of (100 and 200) mA cm-2 for P-CF and MG-CF-4, respectively, suggest MG-CF-4 electrode has a significantly improved mass-transport limitation. MG-CF-4 electrode exhibits excellent cycle stability exhibiting 18.6% and 17.6% higher VE and EE compared to P-CF (Fig. 7 f), suggesting the enhanced robustness of the MG-CF-4 electrode under an acidic environment, hence a potential candidate for long-term cycling performance.

The physiochemical and structural stability of the electrodes was finally investigated by comparing their surface morphology prior to and after cycle stability test, as shown in Figs. S2 b and c. It is evident that cycling results in structural change of the P-CF, whereby sections of the fibre surface are being peeled-off, possibly due to electrochemical oxidation and/or ionic intercalation/de-intercalation process, consequently resulting in loss of active edge sites and ECSA [76], hence affecting its electrocatalytic performance and integrity [77]. This eventually leads to loss of electrochemical performance, and increased resistance consistent with the EIS tests conducted on P-CF and MG-CF-4 after 100 charge/discharge cycles (Fig. S6). P-CF exhibits a significantly increased charge transfer resistance of from (8.73 to 144.0) Ω compared to MG-CF-4 of from (1.24 to 6.89) Ω. Parasitic hydrogen evolution reaction during charging is also a possible reason for the corrugation observed in CFs, as hydrogen gas bubbles tend to block the active sites in P-CF, consequently decreasing the electrochemical performance [17]; whereas in MG-CF-4 electrode, it can be speculated that the presence of a 3D mesoporous structure facilitates the flow of hydrogen gas bubbles outside the electrode, hence hampering the effect of hydrogen evolution. Table 2 summarizes several papers that have previously reported on the electrochemical performance of graphene and nitrogen doped carbon-based materials for VRFBs, in comparison with our present material. It is observed that most reported graphene-based materials exhibited EE of <80% at a current density of 50 mA cm-2, whereas in our work, MG-CF-4 electrode exhibits superior EEs of (83.3 and 53.3) % at current densities of (50 and 200) mA cm-2, respectively, indicating enhanced electrochemical performance compared with that of most of the graphene-based electrodes previously reported.

4. Conclusions

In summary, 3D MG-CFs were synthesized to act as an advanced electrode for VRFB applications through a facile self-assembly interaction method. For optimal electrocatalytic performance, the amount of MG-modified on CF was systematically varied. Among all the MG-CFs electrodes, MG-CF-4 (4 wt.% MG) showed the best electrochemical performance towards the V2+/V3+ and VO2+/VO2+ redox couples. This can be attributed to optimum MG loading on the CF leading to enhanced specific surface area and conductivity, hence, reduced activation polarization, decreased electrode overpotential, accessibility of more redox active sites, and rapid electron transfer for vanadium redox reaction. Furthermore, charge/discharge measurements using MG-CF-4 electrode exhibited an excellent EE of 76.5% at a high current density of 100 mA cm-2 compared to P-CF (66.2%) and specific discharge capacity was enhanced by 110%. Moreover, MG-CF-4 displayed excellent cycle stability at 100 mA cm-2 for 100 cycles, with an EE and VE of 17.6% and 18.6 % higher than P-CF, and without observable decay. Therefore, it is observed that MG-CF-4 outperforms P-CF with smaller peak potential separations, higher peak current densities, reduced charger transfer resistances, lower discharge/charge overpotentials, enhanced charge/discharge specific capacities and superior voltage and energy efficiencies. This work clearly demonstrates the effectiveness of 3D MG-CFs towards enhancing the electrocatalytic behaviour of V2+/V3+ and VO2+/VO2+ redox couples and provides valuable insights into the applicability of MG-CF as an advanced positive and negative electrode material for high-performance VRFB.

Acknowledgements The authors of this research are grateful for the financial support by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education (No. NRF 2016R1D1A1B01009640) and the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy

(MOTIE) of the Republic of Korea (No. 20194030202320). The first author acknowledges the Study and Research at Dongguk University (SRD) scholarship provided for the PhD program.

References [1] B.V. Mathiesen, H. Lund, K. Karlsson, 100% Renewable energy systems, climate mitigation

and

economic

growth,

Applied

Energy,

88

(2011)

488-501.

https://doi.org/10.1016/j.apenergy.2010.03.001. [2] P. Leung, X. Li, C. Ponce de León, L. Berlouis, C.T.J. Low, F.C. Walsh, Progress in redox flow batteries, remaining challenges and their applications in energy storage, RSC Advances, 2 (2012) 10125-10156. https://doi.org/10.1039/C2RA21342G. [3] D.M. Kabtamu, J.-Y. Chen, Y.-C. Chang, C.-H. Wang, Electrocatalytic activity of Nbdoped hexagonal WO3 nanowire-modified graphite felt as a positive electrode for vanadium redox flow batteries, Journal of Materials Chemistry A, 4 (2016) 11472-11480. https://doi.org/10.1039/C6TA03936G.

[4] S. Chu, A. Majumdar, Opportunities and challenges for a sustainable energy future, Nature, 488 (2012) 294-303. https://doi.org/10.1038/nature11475. [5] C. Wang, H. Xie, L. Zhang, Y. Gong, G. Pastel, J. Dai, B. Liu, E.D. Wachsman, L. Hu, Universal Soldering of Lithium and Sodium Alloys on Various Substrates for Batteries, Advanced Energy Materials, 8 (2018) 1701963. https://doi.org/10.1002/aenm.201701963. [6] M. SkyllasKazacos, F. Grossmith, Efficient vanadium redox flow cell, Journal of the Electrochemical Society, 134 (1987) 2950-2953. https://doi.org/10.1149/1.2100321. [7] C. Jia, Q. Liu, C.-J. Sun, F. Yang, Y. Ren, S.M. Heald, Y. Liu, Z.-F. Li, W. Lu, J. Xie, In situ x-ray near-edge absorption spectroscopy investigation of the state of charge of allvanadium redox flow batteries, ACS applied materials & interfaces, 6 (2014) 17920-17925. https://doi.org/10.1021/am5046422. [8] M. Rychcik, M. Skyllas-Kazacos, Characteristics of a new all-vanadium redox flow battery, Journal of Power Sources, 22 (1988) 59-67. https://doi.org/10.1016/03787753(88)80005-3. [9] C. Ding, H. Zhang, X. Li, T. Liu, F. Xing, Vanadium Flow Battery for Energy Storage: Prospects and Challenges, The Journal of Physical Chemistry Letters, 4 (2013) 1281-1294. https://doi.org/10.1021/jz4001032. [10] L. Li, S. Kim, W. Wang, M. Vijayakumar, Z. Nie, B. Chen, J. Zhang, G. Xia, J. Hu, G. Graff, J. Liu, Z. Yang, A Stable Vanadium Redox-Flow Battery with High Energy Density for Large-Scale Energy Storage, Advanced Energy Materials, 1 (2011) 394-400. https://doi.org/10.1002/aenm.201100008. [11] M. Ulaganathan, V. Aravindan, Q. Yan, S. Madhavi, M. Skyllas-Kazacos, T.M. Lim, Recent Advancements in All-Vanadium Redox Flow Batteries, Advanced Materials Interfaces, 3 (2016) 1500309. https://doi.org/10.1002/admi.201500309.

[12] P. Han, Y. Yue, Z. Liu, W. Xu, L. Zhang, H. Xu, S. Dong, G. Cui, Graphene oxide nanosheets/multi-walled carbon nanotubes hybrid as an excellent electrocatalytic material towards VO 2+/VO 2+ redox couples for vanadium redox flow batteries, Energy & Environmental Science, 4 (2011) 4710-4717. https://doi.org/10.1039/C1EE01776D. [13] Z. Li, W. Dai, L. Yu, L. Liu, J. Xi, X. Qiu, L. Chen, Properties investigation of sulfonated poly (ether ether ketone)/polyacrylonitrile acid–base blend membrane for vanadium redox flow battery application, ACS applied materials & interfaces, 6 (2014) 18885-18893. https://doi.org/10.1021/am5047125. [14] Z. He, L. Dai, S. Liu, L. Wang, C. Li, Mn3O4 anchored on carbon nanotubes as an electrode reaction catalyst of V (IV)/V (V) couple for vanadium redox flow batteries, Electrochimica Acta, 176 (2015) 1434-1440. https://doi.org/10.1016/j.electacta.2015.07.067. [15] W. Wang, Q. Luo, B. Li, X. Wei, L. Li, Z. Yang, Recent progress in redox flow battery research and development, Advanced Functional Materials, 23 (2013) 970-986. https://doi.org/10.1002/adfm.201200694. [16] M.-S. Park, N.-J. Lee, S.-W. Lee, K.J. Kim, D.-J. Oh, Y.-J. Kim, High-energy redoxflow batteries with hybrid metal foam electrodes, ACS applied materials & interfaces, 6 (2014) 10729-10735. https://doi.org/10.1021/am5025935. [17] B. Li, M. Gu, Z. Nie, Y. Shao, Q. Luo, X. Wei, X. Li, J. Xiao, C. Wang, V. Sprenkle, W. Wang, Bismuth Nanoparticle Decorating Graphite Felt as a High-Performance Electrode for an

All-Vanadium

Redox

Flow

Battery,

Nano

Letters,

13

(2013)

1330-1335.

https://doi.org/10.1021/nl400223v. [18] K.J. Kim, M.-S. Park, Y.-J. Kim, J.H. Kim, S.X. Dou, M. Skyllas-Kazacos, A technology review of electrodes and reaction mechanisms in vanadium redox flow batteries, Journal

of

Materials

Chemistry

https://doi.org/10.1039/C5TA02613J.

A,

3

(2015)

16913-16933.

[19] W.H. Wang, X.D. Wang, Investigation of Ir-modified carbon felt as the positive electrode of an all-vanadium redox flow battery, Electrochimica Acta, 52 (2007) 6755-6762. https://doi.org/10.1016/j.electacta.2007.04.121. [20] C. Flox, J. Rubio-Garcia, R. Nafria, R. Zamani, M. Skoumal, T. Andreu, J. Arbiol, A. Cabot, J.R. Morante, Active nano-CuPt3 electrocatalyst supported on graphene for enhancing reactions at the cathode in all-vanadium redox flow batteries, Carbon, 50 (2012) 2372-2374. https://doi.org/10.1016/j.carbon.2012.01.060. [21] B. Li, M. Gu, Z. Nie, X. Wei, C. Wang, V. Sprenkle, W. Wang, Nanorod niobium oxide as powerful catalysts for an all vanadium redox flow battery, Nano letters, 14 (2013) 158165. https://doi.org/10.1021/nl403674a. [22] C. Yao, H. Zhang, T. Liu, X. Li, Z. Liu, Carbon paper coated with supported tungsten trioxide as novel electrode for all-vanadium flow battery, Journal of Power Sources, 218 (2012) 455-461. https://doi.org/10.1016/j.jpowsour.2012.06.072. [23] M. Park, Y.J. Jung, J. Kim, H. Lee, J. Cho, Synergistic effect of carbon nanofiber/nanotube composite catalyst on carbon felt electrode for high-performance allvanadium

redox

flow

battery,

Nano

Lett,

13

(2013)

4833-4839.

https://doi.org/10.1021/nl402566s. [24] S. Wang, X.

Zhao, T. Cochell, A. Manthiram, Nitrogen-Doped Carbon

Nanotube/Graphite Felts as Advanced Electrode Materials for Vanadium Redox Flow Batteries,

The

Journal

of

Physical

Chemistry

Letters,

3

(2012)

2164-2167.

https://doi.org/10.1021/jz3008744. [25] J. Jin, X. Fu, Q. Liu, Y. Liu, Z. Wei, K. Niu, J. Zhang, Identifying the Active Site in Nitrogen-Doped Graphene for the VO2+/VO2+ Redox Reaction, ACS Nano, 7 (2013) 47644773. https://doi.org/10.1021/nn3046709.

[26] Y. Shao, X. Wang, M. Engelhard, C. Wang, S. Dai, J. Liu, Z. Yang, Y. Lin, Nitrogendoped mesoporous carbon for energy storage in vanadium redox flow batteries, Journal of Power Sources, 195 (2010) 4375-4379. https://doi.org/10.1016/j.jpowsour.2010.01.015. [27] J. Ryu, M. Park, J. Cho, Catalytic effects of B/N-co-doped porous carbon incorporated with ketjenblack nanoparticles for all-vanadium redox flow batteries, Journal of The Electrochemical Society, 163 (2016) A5144-A5149. https://doi.org/10.1149/2.0191601jes. [28] P. Huang, W. Ling, H. Sheng, Y. Zhou, X. Wu, X.-X. Zeng, X. Wu, Y.-G. Guo, Heteroatom-doped electrodes for all-vanadium redox flow batteries with ultralong lifespan, Journal of Materials Chemistry A, 6 (2018) 41-44. https://doi.org/10.1039/C7TA07358E. [29] M. Park, I.-Y. Jeon, J. Ryu, H. Jang, J.-B. Back, J. Cho, Edge-halogenated graphene nanoplatelets with F, Cl, or Br as electrocatalysts for all-vanadium redox flow batteries, Nano Energy, 26 (2016) 233-240. https://doi.org/10.1016/j.nanoen.2016.05.027. [30] P. Han, H. Wang, Z. Liu, X. Chen, W. Ma, J. Yao, Y. Zhu, G. Cui, Graphene oxide nanoplatelets as excellent electrochemical active materials for VO2+/VO2+ and V2+/V3+ redox couples for a vanadium redox flow battery, Carbon, 49 (2011) 693-700. https://doi.org/10.1016/j.carbon.2010.10.022. [31] M. Ulaganathan, A. Jain, V. Aravindan, S. Jayaraman, W.C. Ling, T.M. Lim, M.P. Srinivasan, Q. Yan, S. Madhavi, Bio-mass derived mesoporous carbon as superior electrode in all vanadium redox flow battery with multicouple reactions, Journal of Power Sources, 274 (2015) 846-850. https://doi.org/10.1016/j.jpowsour.2014.10.176. [32] W. Li, J. Liu, C. Yan, Reduced graphene oxide with tunable C/O ratio and its activity towards vanadium redox pairs for an all vanadium redox flow battery, Carbon, 55 (2013) 313-320. https://doi.org/10.1016/j.carbon.2012.12.069. [33] W. Li, J. Liu, C. Yan, The electrochemical catalytic activity of single-walled carbon nanotubes towards VO2+/VO2+ and V3+/V2+ redox pairs for an all vanadium redox flow

battery,

Electrochimica

Acta,

79

(2012)

102-108.

https://doi.org/10.1016/j.electacta.2012.06.109. [34] G. Wei, C. Jia, J. Liu, C. Yan, Carbon felt supported carbon nanotubes catalysts composite electrode for vanadium redox flow battery application, Journal of Power Sources, 220 (2012) 185-192. https://doi.org/10.1016/j.jpowsour.2012.07.081. [35] W. Li, Z. Zhang, Y. Tang, H. Bian, T.-W. Ng, W. Zhang, C.-S. Lee, GrapheneNanowall-Decorated Carbon Felt with Excellent Electrochemical Activity Toward VO2+/VO2+ Couple for All Vanadium Redox Flow Battery, Advanced Science, 3 (2016) 1500276. https://doi.org/10.1002/advs.201500276. [36] M.Z. Jelyani, S. Rashid-Nadimi, S. Asghari, Treated carbon felt as electrode material in vanadium redox flow batteries: a study of the use of carbon nanotubes as electrocatalyst, Journal of Solid State Electrochemistry, 21 (2017) 69-79. https://doi.org/10.1007/s10008016-3336-y. [37] Z. González, C. Botas, C. Blanco, R. Santamaría, M. Granda, P. Álvarez, R. Menéndez, Thermally reduced graphite and graphene oxides in VRFBs, Nano Energy, 2 (2013) 13221328. https://doi.org/10.1016/j.nanoen.2013.06.014. [38] Z. Wen, X. Wang, S. Mao, Z. Bo, H. Kim, S. Cui, G. Lu, X. Feng, J. Chen, Crumpled Nitrogen-Doped Graphene Nanosheets with Ultrahigh Pore Volume for High-Performance Supercapacitor,

Advanced

Materials,

24

(2012)

5610-5616.

https://doi.org/10.1002/adma.201201920. [39] J. Lee, R. Nankya, A. Kim, H. Jung, Fine-tuning the pore size of mesoporous graphene in a few nanometer-scale by controlling the interaction between graphite oxide sheets, Electrochimica Acta, 290 (2018) 496-505. https://doi.org/10.1016/j.electacta.2018.09.110. [40] W.M.A. El Rouby, Crumpled graphene: preparation and applications, RSC Advances, 5 (2015) 66767-66796. 10.1039/C5RA10289H.

[41] M. Xiao, T. Kong, W. Wang, Q. Song, D. Zhang, Q. Ma, G. Cheng, Interconnected Graphene Networks with Uniform Geometry for Flexible Conductors, Advanced Functional Materials, 25 (2015) 6165-6172. https://doi.org/10.1002/adfm.201502966. [42] M. Kang, D.H. Lee, J. Yang, Y.M. Kang, H. Jung, Simultaneous reduction and nitrogen doping of graphite oxide by using electron beam irradiation, RSC Advances, 5 (2015) 104502-104508. https://doi.org/10.1039/C5RA20199C. [43] M. Kang, D.H. Lee, Y.-M. Kang, H. Jung, Electron beam irradiation dose dependent physico-chemical and electrochemical properties of reduced graphene oxide for supercapacitor,

Electrochimica

Acta,

184

(2015)

427-435.

https://doi.org/10.1016/j.electacta.2015.10.053. [44] J. Yang, M.R. Jo, M. Kang, Y.S. Huh, H. Jung, Y.-M. Kang, Rapid and controllable synthesis of nitrogen doped reduced graphene oxide using microwave-assisted hydrothermal reaction

for

high

power-density

supercapacitors,

Carbon,

73

(2014)

106-113.

https://doi.org/10.1016/j.carbon.2014.02.045. [45] R. Nankya, J. Lee, D.O. Opar, H. Jung, Electrochemical behavior of boron-doped mesoporous graphene depending on its boron configuration, Applied Surface Science, 489 (2019) 552-559. https://doi.org/10.1016/j.apsusc.2019.06.015. [46] L. Wei, T.S. Zhao, L. Zeng, X.L. Zhou, Y.K. Zeng, Copper nanoparticle-deposited graphite felt electrodes for all vanadium redox flow batteries, Applied Energy, 180 (2016) 386-391. https://doi.org/10.1016/j.apenergy.2016.07.134. [47] N. Yun, J.J. Park, O.O. Park, K.B. Lee, J.H. Yang, Electrocatalytic effect of NiO nanoparticles evenly distributed on a graphite felt electrode for vanadium redox flow batteries,

Electrochimica

Acta,

https://doi.org/10.1016/j.electacta.2018.05.039.

278

(2018)

226-235.

[48] R. Wang, C. Xu, J. Sun, L. Gao, H. Yao, Solvothermal-Induced 3D Macroscopic SnO2/Nitrogen-Doped Graphene Aerogels for High Capacity and Long-Life Lithium Storage,

ACS

Applied

Materials

&

Interfaces,

6

(2014)

3427-3436.

https://doi.org/10.1021/am405557c. [49] G. Wei, W. Su, Z. Wei, M. Jing, X. Fan, J. Liu, C. Yan, Effect of the graphitization degree for electrospun carbon nanofibers on their electrochemical activity towards VO2+/VO2+

redox

couple,

Electrochimica

Acta,

199

(2016)

147-153.

https://doi.org/10.1016/j.electacta.2016.03.154. [50] M.G. Hosseini, S. Mousavihashemi, S. Murcia-López, C. Flox, T. Andreu, J.R. Morante, High-power positive electrode based on synergistic effect of N- and WO3 -decorated carbon felt

for

vanadium

redox

flow

batteries,

Carbon,

136

(2018)

444-453.

https://doi.org/10.1016/j.carbon.2018.04.038. [51] L.G. Cançado, A. Jorio, E.H.M. Ferreira, F. Stavale, C.A. Achete, R.B. Capaz, M.V.O. Moutinho, A. Lombardo, T.S. Kulmala, A.C. Ferrari, Quantifying Defects in Graphene via Raman Spectroscopy at Different Excitation Energies, Nano Letters, 11 (2011) 3190-3196. https://doi.org/10.1021/nl201432g. [52] M. Park, I.-Y. Jeon, J. Ryu, J.-B. Baek, J. Cho, Exploration of the Effective Location of Surface Oxygen Defects in Graphene-Based Electrocatalysts for All-Vanadium Redox-Flow Batteries,

Advanced

Energy

Materials,

5

(2015)

1401550.

https://doi.org/10.1002/aenm.201401550. [53] B. Sun, M. Skyllas-Kazacos, Chemical modification of graphite electrode materials for vanadium redox flow battery application—part II. Acid treatments, Electrochimica Acta, 37 (1992) 2459-2465. https://doi.org/10.1016/0013-4686(92)87084-D. [54] S.J. Gregg, K.S.W. Sing, H. Salzberg, Adsorption surface area and porosity, Journal of The Electrochemical Society, 114 (1967) 279C-279C. https://doi.org/10.1149/1.2426447.

[55] C.U. Pittman, W. Jiang, Z.R. Yue, C.A. Leon y Leon, Surface area and pore size distribution of microporous carbon fibers prepared by electrochemical oxidation, Carbon, 37 (1999) 85-96. https://doi.org/10.1016/S0008-6223(98)00190-0. [56] S. Mehboob, G. Ali, H.-J. Shin, J. Hwang, S. Abbas, K.Y. Chung, H.Y. Ha, Enhancing the performance of all-vanadium redox flow batteries by decorating carbon felt electrodes with

SnO2

nanoparticles,

Applied

Energy,

229

(2018)

910-921.

https://doi.org/10.1016/j.apenergy.2018.08.047. [57] L. Wei, T.S. Zhao, L. Zeng, Y.K. Zeng, H.R. Jiang, Highly catalytic and stabilized titanium nitride nanowire array-decorated graphite felt electrodes for all vanadium redox flow batteries,

Journal

of

Power

Sources,

341

(2017)

318-326.

https://doi.org/10.1016/j.jpowsour.2016.12.016. [58] A. Fetyan, G.A. El-Nagar, I. Derr, P. Kubella, H. Dau, C. Roth, A neodymium oxide nanoparticle-doped carbon felt as promising electrode for vanadium redox flow batteries, Electrochimica Acta, 268 (2018) 59-65. https://doi.org/10.1016/j.electacta.2018.02.104. [59] A.A. Shah, H. Al-Fetlawi, F.C. Walsh, Dynamic modelling of hydrogen evolution effects in the all-vanadium redox flow battery, Electrochimica Acta, 55 (2010) 1125-1139. https://doi.org/10.1016/j.electacta.2009.10.022. [60] P. Mazur, J. Mrlik, J. Pocedic, J. Vrana, J. Dundalek, J. Kosek, T. Bystron, Effect of graphite felt properties on the long-term durability of negative electrode in vanadium redox flow

battery,

Journal

of

Power

Sources,

414

(2019)

354-365.

https://doi.org/10.1016/j.jpowsour.2019.01.019. [61] Q. Luo, L. Li, W. Wang, Z. Nie, X. Wei, B. Li, B. Chen, Z. Yang, V. Sprenkle, Capacity Decay and

Remediation

of Nafion-based

All-Vanadium

Redox

ChemSusChem, 6 (2013) 268-274. https://doi.org/10.1002/cssc.201200730.

Flow

Batteries,

[62] Q. Deng, P. Huang, W.-X. Zhou, Q. Ma, N. Zhou, H. Xie, W. Ling, C.-J. Zhou, Y.-X. Yin, X.-W. Wu, X.-Y. Lu, Y.-G. Guo, A High-Performance Composite Electrode for Vanadium Redox Flow Batteries, Advanced Energy Materials, 7 (2017) 1700461. https://doi.org/10.1002/aenm.201700461. [63] C.-N. Sun, F.M. Delnick, L. Baggetto, G.M. Veith, T.A. Zawodzinski, Hydrogen evolution at the negative electrode of the all-vanadium redox flow batteries, Journal of Power Sources, 248 (2014) 560-564. https://doi.org/10.1016/j.jpowsour.2013.09.125. [64] O. Di Blasi, N. Briguglio, C. Busacca, M. Ferraro, V. Antonucci, A. Di Blasi, Electrochemical investigation of thermically treated graphene oxides as electrode materials for

vanadium

redox

flow

battery,

Applied

Energy,

147

(2015)

74-81.

https://doi.org/10.1016/j.apenergy.2015.02.073. [65] Y.-S. Chou, K.-T. Jeng, S.-C. Yen, Characterization and electrochemical properties of graphite felt-based electrode modified using an ionomer impregnation approach for vanadium redox

flow

battery,

Electrochimica

Acta,

251

(2017)

109-118.

https://doi.org/10.1016/j.electacta.2017.08.064. [66] J. Langner, M. Bruns, D. Dixon, A. Nefedov, C. Wöll, F. Scheiba, H. Ehrenberg, C. Roth, J. Melke, Surface properties and graphitization of polyacrylonitrile based fiber electrodes affecting the negative half-cell reaction in vanadium redox flow batteries, Journal of Power Sources, 321 (2016) 210-218. https://doi.org/10.1016/j.jpowsour.2016.04.128. [67] A.J. Bard, L.R. Faulkner, J. Leddy, C.G. Zoski, Electrochemical methods: fundamentals and applications, wiley New York1980. [68] M. Park, J. Ryu, Y. Kim, J. Cho, Corn protein-derived nitrogen-doped carbon materials with oxygen-rich functional groups: a highly efficient electrocatalyst for all-vanadium redox flow

batteries,

Energy

&

Environmental

https://doi.org/10.1039/C4EE02123A.

Science,

7

(2014)

3727-3735.

[69] L. Yue, W. Li, F. Sun, L. Zhao, L. Xing, Highly hydroxylated carbon fibres as electrode materials

of

all-vanadium

redox

flow

battery,

Carbon,

48

(2010)

3079-3090.

https://doi.org/10.1016/j.carbon.2010.04.044. [70] H.R. Jiang, W. Shyy, M.C. Wu, L. Wei, T.S. Zhao, Highly active, bi-functional and metal-free B4C-nanoparticle-modified graphite felt electrodes for vanadium redox flow batteries,

Journal

of

Power

Sources,

365

(2017)

34-42.

https://doi.org/10.1016/j.jpowsour.2017.08.075. [71] Y. Xiang, W.A. Daoud, Cr2O3-modified graphite felt as a novel positive electrode for vanadium

redox

flow

battery,

Electrochimica

Acta,

290

(2018)

176-184.

https://doi.org/10.1016/j.electacta.2018.09.023. [72] X. Wei, Z. Nie, Q. Luo, B. Li, B. Chen, K. Simmons, V. Sprenkle, W. Wang, Nanoporous Polytetrafluoroethylene/Silica Composite Separator as a High-Performance AllVanadium Redox Flow Battery Membrane, Advanced Energy Materials, 3 (2013) 1215-1220. https://doi.org/10.1002/aenm.201201112. [73] E. Agar, A. Benjamin, C.R. Dennison, D. Chen, M.A. Hickner, E.C. Kumbur, Reducing capacity fade in vanadium redox flow batteries by altering charging and discharging currents, Journal

of

Power

Sources,

246

(2014)

767-774.

https://doi.org/10.1016/j.jpowsour.2013.08.023. [74] Q. Wang, Z.G. Qu, Z.Y. Jiang, W.W. Yang, Experimental study on the performance of a vanadium redox flow battery with non-uniformly compressed carbon felt electrode, Applied Energy, 213 (2018) 293-305. https://doi.org/10.1016/j.apenergy.2018.01.047. [75] A.M. Pezeshki, J.T. Clement, G.M. Veith, T.A. Zawodzinski, M.M. Mench, High performance electrodes in vanadium redox flow batteries through oxygen-enriched thermal activation,

Journal

of

Power

https://doi.org/10.1016/j.jpowsour.2015.05.118.

Sources,

294

(2015)

333-338.

[76] N. Pour, D.G. Kwabi, T. Carney, R.M. Darling, M.L. Perry, Y. Shao-Horn, Influence of Edge- and Basal-Plane Sites on the Vanadium Redox Kinetics for Flow Batteries, The Journal of Physical Chemistry C, 119 (2015) 5311-5318. https://doi.org/10.1021/jp5116806. [77] H. Liu, Q. Xu, C. Yan, Y. Qiao, Corrosion behavior of a positive graphite electrode in vanadium

redox

flow

battery,

Electrochimica

Acta,

56

(2011)

8783-8790.

https://doi.org/10.1016/j.electacta.2011.07.083. [78] Z. He, L. Shi, J. Shen, Z. He, S. Liu, Effects of nitrogen doping on the electrochemical performance of graphite felts for vanadium redox flow batteries, International Journal of Energy Research, 39 (2015) 709-716. https://doi.org/10.1002/er.3291. [79] X. Wu, H. Xu, Y. Shen, P. Xu, L. Lu, J. Fu, H. Zhao, Treatment of graphite felt by modified Hummers method for the positive electrode of vanadium redox flow battery, Electrochimica Acta, 138 (2014) 264-269. https://doi.org/10.1016/j.electacta.2014.06.124. [80] W. Li, J. Liu, C. Yan, Modified multiwalled carbon nanotubes as an electrode reaction catalyst for an all vanadium redox flow battery, Journal of Solid State Electrochemistry, 17 (2013) 1369-1376. https://doi.org/10.1007/s10008-013-2006-6. [81] W. Li, J. Liu, C. Yan, Multi-walled carbon nanotubes used as an electrode reaction catalyst for VO2+/VO2+ for a vanadium redox flow battery, Carbon, 49 (2011) 3463-3470. https://doi.org/10.1016/j.carbon.2011.04.045.

Figure captions Fig. 1. Schematic illustrating the synthesis process for MG modified carbon felt electrode. Fig. 2. FE-SEM images of (a)–(c) P-CF, and (d)–(f) MG-CF-4 under different magnification, showing surface morphology differences. Fig. 3. (a) XRD patterns, and (b) Raman spectra, of the as-prepared samples. Deconvoluted high-resolution XPS spectra showing the C 1s peaks of (c) P-CF, and (d) MG-CF-4.

Fig. 4. (a) N2 adsorption-desorption isotherms, and (b) BJH pore size distributions of P-CF and the as-prepared MG-CFs. Fig. 5. Cyclic voltammograms of the P-CF and MG-CFs electrodes prepared at different MG concentration; (a) V2+/V3+ redox couple, and (b) VO2+/VO2+ redox couples in 0.05 M V3.5+ + 1.0 M H2SO4 electrolyte at the scan rate of 5 mV s-1, respectively, and (c) MG-CF-4 electrode in 0.05 M V3.5+ + 1.0 M H2SO4 electrolyte solution at different scan rates range (2–20) mV s1

. Inset: plot of anodic peak current density (Ipa) versus square root of scan rate. (d) Plot of

anodic (Ipa) and cathodic (Ipc) peak current density versus the square root of scan rate for VO2+/VO2+ redox couples. (e) Plot of peak potential separation versus scan rate for VO2+/VO2+ redox couples. (f) Nyquist plots of P-CF and MG-CFs electrodes used as working electrodes in 0.05 M V3.5+ + 1.0 M H2SO4 electrolyte at open-circuit voltage (Inset shows the equivalent circuit of fit). Fig. 6. Proposed catalytic mechanism for the electrochemical reaction of VO2+ /VO2+ redox couple on the MG-CFs electrodes. Fig. 7. Electrochemical cycling performance of VRFB cells employing P-CF and MG-CF-4 electrodes. (a) Specific discharge capacity, and (b) charge/discharge voltage profiles at the current density of 100 mA cm-2. (c) VE and CE at different current densities (50 to 200 mA cm-2). (d) EE and (e) discharge capacity, as a function of cycle numbers at different charge/discharge current densities ranging (50 to 200 mA cm-2). (f) EE for 100 charge/discharge cycles at a current density of 100 mA cm-2.

Table captions Table 1. Electrochemical properties obtained from cyclic voltammetry results for the asprepared samples at a scan rate of 5 mV s-1 for the positive half-cell and fitting data of the Nyquist plots.

Table 2. Comparison of CE, VE, and EE of MG-CF-4 with previously reported graphite felt (GF) or carbon felt (CF) electrodes modified by various carbon nanomaterials for VRFB application.

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Table 1 Ipa P-CF MG-CF-2 MG-CF-4 MG-CF-6

-2

(A cm ) 0.0378 0.0593 0.0632 0.0586

-Ipc

-2

(A cm ) 0.0305 0.0661 0.0709 0.0646

Epa (V) 1.177 1.043 1.032 1.039

Epc (V) 0.594 0.665 0.677 0.668

Ipa/-Ipc

∆E (V)

1.237 0.897 0.891 0.907

0.583 0.378 0.355 0.370

Rct (Ω) 8.13 1.53 1.24 1.43

Table 2 Electrolyte Modification concentration MG

1.6 M V3.5+ + 2 M H2SO4

Mesoporous activated carbon

1.7 M V3.5+ + 4 M H2SO4

N-doped GF Graphene nanosheet Modified Hummers method CNF/CNT Graphene oxide Acid treated MWCNTs N-doped CNT SWCNT Carboxylic MWCNTs MWCNT

1.2 M VOSO4 + 3 M H2SO4 2 M V3+, V4+ + 3 M H2SO4 0.5 M VOSO4 + 1 M H2SO4 2 M V3+, V4+ + 3 M H2SO4 1.5 M V3+, V4+ + 2 M H2SO4 1.5 M V3+, V4+ + 2 M H2SO4 1 M V3+, V4+ + 2 M H2SO4 1.5 M V3+, V4+ + 2 M H2SO4 1.5 M VOSO4 + 2 M H2SO4 1.5 M V3+ + 2 M H2SO4

Electrode Cut-off Current size density Voltage (mA/cm2) (cm2) (V) 6 50 (175) 0.8–1.6 10 70 50 (150)

25 9 5

CE (%)

VE (%)

EE (%)

92.4 (98.4)

90.1 (62.6)

83.3 This work (61.6)

0.5–1.5

91.8

92.6

85.0

[31]

0.7–1.7

98.5

75.6

74.5

[78]

0.8–1.65

96.1 (97.0)

89.4 (67.8)

85.9 [52] (65.8)

0.8–1.7

97.2

73.4

71.3

[79]

0.8–1.6

97.2

67.5

65.6

[23]

0.8–1.7

-

-

82

[32]

0.8–1.7

-

-

66.7

[80]

0.8–1.7

81.3

94.7

77.0

[24]

0.8–1.7

92.2

96.8

89.3

[33]

0.75– 1.65

93.9

87.3

82

[34]

0.7–1.7

98.6

76.1

75.0

[81]

Ref.

12 80 100 20 80 10 20 50 70

5 6.25 9 5 6.25 28 6.25

HIGHLIGHTS • Self-assembly interaction to modify electrode with 3D mesoporous graphene. • Significant enhancement of V2+/V3+ and VO2+/VO2+ redox reversibility and kinetics. • Excellent performance attributed to 3D wrinkled mesoporous graphene structures. • Increased specific surface area, pore volume, and electrical conductivity. • MG-CF-4 (4 wt.% MG) improves VRFB EE from (66.2 to 76.5) % at 100 mA cm-2.

Declaration of interests ☒ 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

None.