Journal of Power Sources

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1 Journal of Power Sources 405 (2018) Contents lists available at ScienceDirect Journal of Power Sources j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / j p o w s o u r Formation of electrodes by self-assembling porous carbon fibers into bundles for vanadium redox flow batteries T J. Sun a, L. Zeng a, b, H.R. Jiang a, C.Y.H. Chao c, T.S. Zhao a, a Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR, China b HKUST Jockey Club Institute for Advanced Study, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR, China c Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, China H I G H L I G H T S Electrodes by self-assembling electrospun carbon fibers into bundles are developed. The electrodes exhibit enlarged pores while retaining large speci fic surface areas. The prepared electrodes enable a significant increase in the battery's performance. A R T I C L E I N F O Keywords: vanadium redox flow battery Electrospun porous carbon fiber bundles Mass transport Self-assembly Pore size A B S T R A C T Electrospinning has been employed to fabricate carbonaceous materials with larger surface areas for vanadium redox flow batteries. However, the woven carbon nanofibers prepared with conventional electrospinning methods are plagued by the low porosity and poor permeability, thereby causing a significant mass-transport resistance during the operation of batteries. To tackle this problem, we report a novel method by self-assembling porous carbon fibers into large bundles to form electrodes. This electrode is fabricated by electrospinning polyacrylonitrile and polystyrene binary solutions. Instead of forming single fi bers, the individual fibers are self- assembled into fiber bundles by properly managing the viscosity of the precursor solution. The formation of large fiber bundles significantly enlarges the pore size while retaining large specific surface areas. The single cell with the as-prepared electrodes achieves an energy efficiency of 87.7% at a current density of 100 ma cm 2, which is 15.2% higher than that of the single cell with conventional electrospinning electrodes. The energy efficiency still maintains over 80% at 200 ma cm 2. More importantly, the discharge capacity and electrolyte utilization are nearly doubled. All these results demonstrate that this electrode preparation method is effective to improve the mass transport properties of traditional electrospun electrodes in vanadium redox flow batteries. 1. Introduction Grid-scale energy storage is regarded as an effective way to solve the critical issue of intermittency supply for renewable energy (solar and wind energy) to meet variable needs from users [ 1]. Therefore, in the past decades, various battery systems have been integrated into renewable energy. Especially, redox flow batteries (RFB) show great promise for large-scale energy storage due primarily to the merit of decoupled energy and power because the active species are stored in the external tanks and the power is determined by the pack size [ 2 6], not to mention other attractive characteristics including long life cycle, low operation cost, and adjustable design. Among many types of flow batteries that have been researched since the 1970s, the vanadium redox flow batteries (VRFB) stand out due to the employment of the same element on both negative and positive sides [ 7 9]. In VRFB, V (II)/V (III) and V( Ⅳ)/V( Ⅴ) redox couples are circulated on the negative and positive half-cells, respectively, which can eliminate the cross contamination happened in other RFBs. Even though, the commercialization of the state-of-art VRFBs is still hindered by the high capital cost, the reduction of which requires to operate the battery with high power output and energy efficiency to decrease the size of the power pack. Electrode, one of the most important components in VRFBs, plays a crucial role in deciding the battery performances because it not only provides the active sites for the redox reactions to take place but also greatly affects the ions and electron transport process. Currently, Corresponding author. address: metzhao@ust.hk (T.S. Zhao). Received 23 July 2018; Received in revised form 19 September 2018; Accepted 10 October / 2018 Elsevier B.V. All rights reserved.

2 Fig. 1. Illustration of synthesis procedures of the electrospun porous carbon fiber bundle structure. commercialized carbon materials such as carbon paper and carbon felt are chosen as the electrode materials in VRFBs due to their high electrical conductivity, good chemical stability, and high surface area [ 10, 11]. However, these electrodes show a poor electrochemical activity towards the vanadium redox reactions, thereby leading to a low voltage efficiency and poor rate performance. Thus, tremendous efforts have been taken to modify these carbon materials to improve their electrochemical performance. The surface modifications can be classi- fied as catalyst deposition [ 12 21], surface etching [ 22 26], and heteroatom doping [ 27 29]. Unfortunately, such modifications are mostly carried out on the commercialized electrodes, which means the intrinsic properties of the electrode, including fiber diameter, pore size, and pore shape, are hard to be varied, leading to a limited space for the further enhancement in battery performance. Therefore, a fabrication method which allows to design, fabricate and optimize the electrode starting from the bottom is in urgent demand. Inspired by the success achieved in other energy storage systems such as supercapacitors [ 30, 31], lithium-sulfur batteries [ 32, 33], lithium-ion batteries [ 34, 35], electrospinning method is attracting increasing attention to fabricate the electrodes for VRFBs because it potentially maximize the specific surface area, surface activity, and hydraulic permeability at the same time. Fetyan et al. directly used the free-standing electrospun carbon nano fibers (ECNFs) as electrodes for VRFBs and achieved a 10% higher energy efficiency compared with commercial carbon felts [ 36]. But the batteries can still only be operated at a current density 15 ma cm 2 with an energy efficiency of 59%. There are generally two approaches to further enhance the performance of electrospun electrodes: one is adjusting the surface activity, the other is changing the electrode structures. For the first method, some cata- lysts such as graphite nanopowders (GNPs) [ 37], Bi, nanoparticles [ 38], carbon nanotubes (CNTs) [ 38], CeO 2 [ 39], Mn3 O 4 [ 40], Ni [ 41], and V 2 O 3 [ 42] were embedded into ECNFs to further improve the electrochemical activity of VRFBs. However, ECNFs have intrinsic problems such as low porosity and small pore size due to the nanoscale fibers [ 43]. Therefore, when deploying the electrospun carbon nanofibrous materials as the electrodes in VRFBs, the sluggish electrolyte transport in the electrodes can lead to a large concentration overpotential. Thus, belonging to the second approach, some works focused on optimizing the cell performance by modifying the material structure to improve the mass transport properties. Xu et al. adopted the horizontally-opposed blending electrospinning method to fabricate freestanding ECNFs with expanded pore size and increased porosity to reduce the concentration polarization [ 44]. Liu et al. evaluated the influence of fiber diameter of the polyacrylonitrile (PAN)-derived ECNFs on the battery performances and suggested that the ECNFs made from 12 wt% PAN solution provided sufficient surface area and reasonable permeability. Even though, the improvement of the mass transport properties is still limited and the battery showed poor performances at high current densities which was caused by the heterogeneity of the pore arrangement [ 45 ]. More seriously, the optimal structure of the ECNFs is still unknown and needs much exploration. To address the aforementioned issues, a novel electrode with porous carbon fiber bundles was designed and synthesized by electrospinning bi-component PAN and polystyrene (PS) solutions with high concentrations. Unlike in previous works that PS only act as a sacrificial phase to create porous structures [ 46, 47], our work demonstrates that PS not only acted as a decomposable species to form internal channels inside fi bers but also facilitated the formations of large fiber bundles. More interestingly, the fiber bundle size can be controlled by varying the precursor polymer concentrations. With these efforts, the improved mass transport properties of the electrode were observed in the battery tests. The results demonstrate that forming porous bundle structure was an efficient way to make electrospun carbon materials applicable in VRFBs. 2. Experimental 2.1. The fabrication of carbon fiber bundles Schematic of the synthesis process of porous bundle structure is shown in Fig. 1. The bi-component precursor solutions were prepared by dissolving PAN and PS in N,N dimethylformamide (DMF) with different weight ratios which are listed in Table 1. Then the mixture of precursor solution was stirred at 60 C for 12 h. The well-dissolved precursor solution then was loaded into a plastic syringe equipped with a stainless steel nozzle. Meanwhile, a high voltage of 18 kv was applied between the spinneret tip and the collector. The solution was then ejected at a feeding rate of 1 ml h 1 and electrospun onto a drum collector covered with an aluminum foil. The distance between the tip and the collector was kept at a constant distance (16 cm) and the rotation of the drum was maintained at 4 m min 1. The as-spun web was then pre-oxidized at 250 C for 2 h with a heating rate of 1 C min 1 in air followed with annealing at 1100 C for one hour with a heating rate of 2 C min 1 in the nitrogen atmosphere. After the carbonization process, PS was decomposed to create inner channels in each fiber. All these samples were prepared with a fixed PAN concentration (10 wt%). Table 1 PAN and PS composition in binary component precursor solutions. Composition PAS 0 PAS 3 PAS 5 PAS 8 PAS 10 PAN 10 wt% 10 wt% 10 wt% 10 wt% 10 wt% PS 0 wt% 3 wt% 5 wt% 8 wt% 10 wt% 107

3 Fig. 2. SEM images of electrodes after carbonization. (a1)-(a5) surface morphologies of PAS 0 to PAS 10; (b1) typical single fiber of PAS 0, (b2)-(b5) fiber bundles of PAS 3 to PAS 10; (c1)-(c5) cross-section views of PAS0 to PAS 10. We labeled these samples electrospun from different precursor solutions according to the varied PS concentration as listed in Table 1. For example, the sample made of pure PAN precursor solutions was labeled as PAS 0, others were listed as PAS 3, PAS 5, PAS 8, PAS 10 with PS taking up 3 wt%, 5 wt%, 8 wt% and 10 wt% in the precursor solutions. The preparation of the binary composition solutions, however, was limited by the solubility of the two polymers in DMF solvent thus the largest total polymer concentration was 20 wt% Material characterization The morphologies of different samples after carbonization were determined using a scanning electron microscope (SEM, JEOL 6390). The surface compositions of the electrospinning carbon fibers (PAS 0 - PAS 10) were examined by X-ray photoelectron spectroscopy (XPS) with Mg K α X-ray source. The curve deconvolution of the spectra was carried out following a Shirley-type background subtraction. Raman spectra were collected with the InVia (Renishaw) Raman spectrometer with a 514 nm laser as the excitation source (Notch Filter). The viscosity of the precursor electrospinning solutions was measured using a viscometer (Shanghai Pingxuan). Nitrogen adsorption/desportion isotherms and Brunauer Emmett Teller (BET) surface area were measured with Beckman Coulter SA3100 surface area and pore size analyzer. Mercury intrusion porosimetry was examined with Micromeritics' AutoPore IV 9500 Series Electrochemical properties The electrochemical properties of these samples were evaluated by cyclic voltammetry (CV). A typical three-electrode cell was applied with the electrospinning electrodes as the working electrode; a saturated calomel electrode and the platinum mesh as the as the reference and counter electrode, respectively. The ECNF samples with a fixed area of 0.28 cm 2 and fixed mass of 20 mg were tightly attached to the glass carbon electrode using a polytetrafluoroethylene (PTFE) tubing as a holder. The scanning process for the two half-cells was carried out in 0.1 M V M H 2 SO 4 electrolyte and 0.1 M VO M H 2 SO 4 electrolyte separately on Autolab (PGSTAT30) workstation. The scanning is carried out from 0.6 V to 1.2 V for the positive side and from 0.65 V to 0.3 V for the negative side at a scan rate of 10 mv s Single flow cell test The single cell test was conducted using a potentiostat/galvanostat (Arbin Instrument). The electrospun PAS electrodes were applied to both the positive and negative side of the symmetric cell with an uncompressed thickness of around 700 μm and an active area of 2 2 cm 2. When being assembled into batteries, the electrodes were compressed to the same thickness ( 250 μm) The flow battery set up is a home-made serpentine flow field structure which has been stated in previous papers [ 48]. 20 ml solution containing 1 M V M H 2 SO 4 and another 20 ml solution containing 1 M VO M H 2 SO 4 were employed as anolytes and catholytes which were separately stored in outside reservoirs. The electrolytes were pumped at a flow rate of 1.0 ml s 1. Nafion 212 (Dupont, USA) was used as the membrane. Both the reservoirs were bubbled with nitrogen and sealed before the electrochemical test to avoid the oxidation of the active species. All the tests were carried out at room temperature. 3. Results and discussion 3.1. Morphology The morphologies of the ECNFs were characterized using a scanning electron microscopy. The fibers or fiber bundles interconnect with each other forming the conductive network with inter- fiber pores to facilitate the penetration of electrolytes ( Fig. 2(a1)-(a5)). It can be clearly seen from Fig. 2 (b1)-(b5) that the fibers are self-assembled into fiber bundles when PS was added into the precursor solutions, both the number of fi bers in the fi ber bundles and the diameter of the fiber bundles in- crease with an increase of the PS concentration. Due to the formation of fiber bundles, the pores are gradually enlarged which can be observed from Fig. 2(a1)-(a5). The enlarged pores will significantly enhance the penetration of the electrolyte. PS can also act as the sacrificial phase and can be removed after carbonization to form nanochannels inside each fiber shown in Fig. 2(c1)-(c5). The cross-section view of pure PAN carbon fiber (PAS 0) after carbonization is dense and with few pores examined ( Fig. 2(c1)). While the cross-sections of carbonized PAN-PS fibers exhibit a lotus-root like porous structure. Also, when the PS content increases from 3 wt% to 10 wt%, the size and number of channels inside each fiber increase accordingly ( Fig. 2(c2)-(c5)). The BET surface area increased gradually from PAS 0 (18.43 m 2 g -1 ) to PAS 10 (46.83 m 2 g -1 ) as can be detected from both the BET surface area in 108

4 Table S1 and the nitrogen adsorption/desorption isotherm curves ( Fig. S2(a)). From the pore distribution of BET results, we find that the majority of the pores are less than 10 nm ( Fig. S2(b) ). The mercury intrusion porosimetry provides the pore distribution among fibers. It is shown that when the diameter of fiber bundle increased from PAS 0 to PAS 10, the size of the pores among the fibers also increased. The large pore size of PAS 10 can reach around 24.2 μm while the pore width of PAS 0 was only around 1.6 μm. It is worth noting that there are two peaks appearing for PAS 3 to PAS 10 in Fig. S2(c), while for PAS 0 there is only one peak. These results are consistent with the SEM observation ( Fig. 2) which shows there are large pores among the fi bers and small porous channels inside each fiber for PAS 3 to PAS 10, while only large pores among the fibers for PAS 0 sample. The results of BET analysis and mercury intrusion porosimetry further prove that the addition of PS component can enlarge the pores among the fibers and create internal channels in each fiber to increase the specific surface areas. Therefore, this porous bundle structure can improve the mass transport of the electrolyte to the electrode due to the large pores created among the fiber bundles. In addition, the porous structure with internal channels can provide a su ffi cient specific surface area for the electrochemical reactions to take place Mechanism of the formation of self-assemble fiber bundles It is found that the formation of the self-assembly bundles is strongly related to the viscosity and type of the precursor solutions. It was reported that pure PAN with high concentration could also form bundle structure but the electrospinning process was unstable [ 45]. The SEM image ECNF from PAN 15 wt% ( Fig. S1) exhibits single fibers with several bundles. In our experiments, when PS is added with PAN to form viscous solutions, bundles appear in the structures. When electrospinning PAN/PS binary solutions, the formation of bundles is influenced by the viscosity of the solutions. The viscosities of all these samples with different PS compositions are determined as shown in Fig. 3(a), from which it is seen that the viscosity is increased with an increase of the PS concentration in the mixture. The viscosity of the solution with the highest PS concentration can reach about 4000 mpa s, which is about eight times as that of the pure PAN precursor solution. Due to the great variance between the viscosities of different precursor solutions, the morphologies of the as-spun fi bers show signi ficant dif- ferences. And the mechanism of the formation of different morpholo- gies is explored as follows. Fig. 3(b)-(c) depict the mechanism to form fibers of the precursor solutions with low viscosity and the formation of fiber bundles of the high viscous solutions. For both the low viscous and high viscous solutions, the polymer solution is firstly held by the surface tension at the tip of the needle. Once the needle is positively charged, the mutual charges will form repulsion force which is against the surface tension. With the increase of applied voltages, the intensity of the electric field increases, resulting in the hemispherical shape of the liquid being elongated to the Taylor cone shape. The jet of charged solution will eject from the tip of the cone when the applied voltage reaches a critical value so that the repulsive electric force overcomes the surface tension. When the charged jet travels in the air, the solvent evaporates leaving behind the charged fibers which will lay on the metal collector. When the polymer solutions are positively charged beyond the critical voltage, several thin polymer jets eject from the tip of the Taylor cone simultaneously. For the solutions with low viscosity, those positively charged jets will repel each other and will land on the collector separately shown in Fig. 3(b). The repulsive force resulted from the mutual charges in the same jet and the adjacent jets represented is denoted as Fr in Fig. 3(b). However, for the jets of high viscosities, those positively charged jets will firstly repel each other and then stick to each other. The repulsive forces cannot overcome attraction between two sticky jets, leading to the fibers intertwining together to form bundles during the traveling process. These fiber bundles will subsequently land on the collector to form the self-assemble fiber bundle mat Chemical structure The surface compositions of different samples were characterized by XPS analysis. It can be seen from the element content table ( Table.2) that pure PAN carbon nanofibers (PAS 0) show the largest nitrogen content (around 6.06 wt%). The samples electrospun with precursor solutions containing PS component show a slightly decreased nitrogen content of 3 wt% - 4 wt%. The possible reason is that part of the nitrogen was lost with the decomposition process of PS during the thermal treatment. The oxygen content, however, does not vary too much for all these samples. The high-resolution O1s and N1s of all these samples were performed to determine the categories and the content of different types of functional groups ( Figs. 4(c) and (e)). The C 1s peak centered at ev was used as the reference and the curve deconvolution of the spectra was performed following a Shirley-type background subtraction. The peaks were fitted with a Gaussian-Lorentzian function. The O1s can be fitted with two peaks corresponding to C=O (531.1 ev) and C-OH (532.5 ev). Moreover, the N1s peak can be deconvoluted into pyridinic N (398 ev), pyrollic N (399.8 ev), quaternary N (401 ev) and N-oxide (403.2 ev) [ 29] as shown in Fig. 4(e) with the corresponding content of the four types of nitrogen being summarized in Fig. 4(f). The pure PAN carbon nanofiber (PAS 0) shows both the largest oxygen content and nitrogen content which has been proved to provide effective reaction sites for the vanadium redox reactions [ 49 51]. Thus, it is expected that the pure PAN carbon nanofiber should have a better electrochemical activity than ECNFs made from Fig. 3. Mechanism for the formation of fiber and fiber bundles. (a) The function of viscosity with PS concentration. (b), (c) formation mechanism of the fiber or fiber bundles from precursor solutions with different viscosities. 109

5 Table 2 Element content of as-prepared samples (PAS 0 to PAS 10). Element content (%) PAS 0 PAS 3 PAS 5 PAS 8 PAS 10 C N O bicomponent PAN/PS solutions. The nitrogen content of the rest of ECNFs (PAS 3 to PAS 10) shows a similar value though there are some small differences on the oxygen functional groups, thus it is expected that these ECNFs should have a similar electrochemical activity. Raman measurement was also conducted to probing the graphitic carbon and the defects of the as-prepared samples. As shown in Fig. 4(b), the G band which centers around 1590 cm 1 is characteristic of highly ordered graphite and the D band at 1350 cm 1 is indicative of the defects and edges in the graphitic domains [ 28, 52]. Since all these as-prepared samples were carbonized at the same temperature, the intensity ratio of I D /I G is close to each other which is approximate to 0.85, indicating lots of defects are introduced to these samples. These defects will act as active sites and catalyze the redox reactions [ 53] Electrochemical activity The electrochemical properties were examined by CV measurement at a scan rate of 10 mv s 1. All these samples exhibit the anodic and cathodic peaks towards the VO 2+ /VO + 2 redox couple and the V 2+ / V 3+ redox couple ( Fig. 5). Considering the local heterogeneity of the asprepared samples, we conducted the CV tests using three di fferent electrodes for each sample at different scan rates. The negative and positive side peak potential separations are listed in Table S2. The average peak separation voltage can, therefore, be calculated. To avoid the local heterogeneity issue, we then compared the CV results of all the five samples that are closest to the average peak potential separation. Meanwhile, the weight of each sample was kept the same. The CV curves of PAS 0 to PAS 10 towards both negative and positive sides at the different scan rates were plotted in Fig. S3. From the curves of peak current density versus the square root of scan rate ( Fig. S4), the linearly fitting lines indicate that the CV tests for all the samples are controlled by mass diffusion. The oxidation and reduction peak potential separa- tion of PAS 0 regarding V 2+ /V 3+ and VO 2+ /VO + 2 redox couples are around mv and mv, respectively, at the scan rate of 10 mv s 1. For those electrodes made of PAN and PS component, the average peak potential separation values for the negative and positive electrodes increase to mv and mv, respectively. This result agrees with the XPS analysis that adding PS component could lead to the loss of some oxygen and nitrogen functional groups, thus slightly hampering the electrochemical activities. The peak currents of PAS 3 - PAS 10 samples towards the positive side redox reactions are higher than PAS 0 sample after subtracting the double-layer currents due to an increased speci fic surface area Battery performances Then, the real performances of the as-prepared electrodes are tested in a single cell. Fig. 6(a)-(b) depict the voltage profiles of all these electrodes during the charge and discharge process at current densities of 100 ma cm 2 and 200 ma cm 2. The overpotential is significantly reduced from PAS 0 to PAS 3 and then modestly decreased from PAS 3 to PAS 10, which can be detected from charging and discharge plateaus in Fig. 6(a). Interestingly, although pure PAN carbon nanofiber has richer functional groups and better electrochemical performance, the Fig. 4. Ex-situ X-ray photoelectron spectroscopy and Raman spectra of different samples. (a) Chemical content, (b) Raman spectra; high-resolution spectra of O 1s (c) and N 1s (e); chemical composition of functional groups from curve fitting of O 1s (d) and N 1s (f). 110

6 Fig. 5. CV curves of half cells at a scanning rate of 10 mv s 1. (a) Negative redox V 2+ /V 3+ on PAS 0,3,5,8,10 electrodes; (b) positive redox couple VO 2+ /VO 2 + on PAS 0,3,5,8,10 electrodes. Fig. 6. Battery test of single cell flow battery using different electrodes. (a) Charge-discharge curves of different samples at a current density of 100 ma cm 2. (b) Charge-discharge curves of different samples at a current density of 200 ma cm 2. (c) Charge-discharge profiles of PAS 0 at different current densities. (d) Charge- discharge profiles of PAS 10 at different current densities. 111

7 single battery test with PAS 0 exhibits much worse battery performance than PAS 3 - PAS 10 electrodes. It is known that the battery performance of prepared electrodes can be determined by both electrochemical properties and structural properties. Thus, the enhancement of battery performances of PAS 3 - PAS 10 can be ascribed to more reasonable structures for mass/ion transport, thus leading to reduced activation loss and concentration loss. To be specific, the large over- potential of PAS 0 electrodes is caused by the low utilization of electrode surface area due to the poor mass transport inside the tightly woven fiber networks. From the mercury intrusion porosimetry results, it is found that with the formation of single fibers to fiber bundles from PAS 0 to PAS 10, the pores among fibers or fiber bundles increased from around 1.6 μm to around 24.2 μm. The enlarged pores among fi bers provides pass ways for the electrolyte. The mass transport resistance is therefore significantly reduced from PAS 0 to PAS 10 and an increased surface area can be accessed by the electrolyte, therefore the discharge capacity is nearly doubled from PAS 0 (5.4 Ah L 1 ) to PAS 10 (10.6 Ah L 1 ) at a current density of 100 ma cm 2. The difference between the performances of the batteries with PAS 0 and PAS 10 electrodes can be compared from the charge-discharge curves in Fig. 6(c)-(d) at various current densities. The PAS 0 electrodes with dense thin fibers can only be charged and discharged at the current density up to 120 ma cm 2 while PAS 10 which have the largest porous bundles can be operated at a current density up to 250 ma cm 2. Fluctuations are observed at the final discharge stages of the PAS 0 due to the large concentration polarization arising from the depletion of the reactant as well as poor mass transport of the active species. Fig. 7(a)-(b) show the coulombic efficiency (CE), voltage efficiency (VE) and energy efficiency (EE) of single cell battery equipped with the as-prepared electrodes. The CEs of all these tests are above 97%, indicating good airtightness of the set-up. The VEs of VRFBs assembled with these samples are gradually improved from PAS 0 to PAS 10 which are consistent with the charge-discharge profiles. The EE which is the product of CE and VE also exhibits an increased trend from PAS 0 to PAS 10. The EE of the PAS 10 (87.7%) is enhanced by 15.2% at a current density of 100 ma cm 2 compared with PAS 0 (72.5%). The single cell assembled with PAS 10 electrodes exhibits the best performances, with CE, VE, and EE achieving 99.2%, 80.8% and 80.1% at a current density of 200 ma cm 2. In addition to the charge-discharge performance, polarization curves are conducted to display the voltage losses coming from the activation overpotential, ohmic resistance, and concentration overpotential. As shown in Fig. 8, the polarization curves of all these batteries with different electrodes are tested from the full charge state. At Fig. 8. Polarization curves of single cell VRFB with different electrodes. high current densities, the concentration overpotential dominates. It is found that due to enhanced mass transport properties of the bundle structure, the concentration overpotential comes into effect at much larger current densities for PAS 10, thus the limiting current densities are greatly increased. The limiting current density for PAS 10 can reach as high as around 1300 ma cm 2 which is about 2.6 times of that of PAS 0 (500 ma cm 2 ). It is therefore confirmed that the enhancement of the transport properties of the ECNFs can significantly improve the VRFB performance. 4. Conclusions In summary, porous carbon fiber bundle mats by electrospinning bicomponent precursor solutions were synthesized and used as electrodes in VRFB. The formation of the bundle structure can significantly enlarge the pore size of the electrodes compared with the traditional dense electrospun fiber electrode, thus enhancing the transport prop- erties of the electrode. The nanochannels inside the fibers can provide additional specific surface areas. Mercury intrusion porosimetry results show that the pores among fibers can be enlarged from around 1.6 μm to 24.2 μm with the BET surface area being increased from Fig. 7. (a) Coulombic efficiencies and voltage efficiencies of the single cell employing different electrodes. (b) Energy efficiency of the single cells with different electrodes. 112

8 m 2 g -1 to m 2 g -1. As concluded from the CV results, the porous bundle electrodes (PAS 3 to PAS 10) showed a slightly decreased electrochemical activities towards both sides compared with the pure PAN carbon fiber electrode (PAS 0). However, benefiting from the re- markably improved mass transport properties, the porous bundle carbon fiber electrodes (PAS 10) exhibit significantly enhanced battery performance. Compared with PAS 0, the energy efficiency and dis- charge capacity of PAS 10 were increased by 15.2% and 100%, respectively, at a current density of 100 ma cm 2. In addition, an energy efficiency of 80.1% was achieved with PAS 10 electrode at a current density of 200 ma cm 2. The limiting current density of PAS 10 electrode was almost 2.6 times of the PAS 0 electrode due to reduced concentration overpotentials. Furthermore, the mechanism of the formation of bundle structure was explored. 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