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electrocatalyst hybrids as asymmetric electrodes for vanadium redox flow battery
Journal of Power Sources 281 (2015) 1e6
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Electrospun carbon nanofibers/electrocatalyst hybrids as asymmetric electrodes for vanadium redox flow battery Guanjie Wei, Xinzhuang Fan*, Jianguo Liu*, Chuanwei Yan Liaoning Engineering Research Center for Advanced Battery Materials, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, 110016, China
h i g h l i g h t s Composite electrodes have been made by electrospinning technique. CNTs/ECNFs show best electrocatalytic activity to VO2þ/VOþ 2 redox couple. Bi/ECNFs present best electrocatalytic activity to V2þ/V3þ redox couple. Hydrogen evolution on the Bi/ECNFs composite electrode is suppressed. CNTs/ECNFs and Bi/ECNFs are used as asymmetric electrodes for VRFB.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 October 2014 Received in revised form 29 December 2014 Accepted 27 January 2015 Available online 28 January 2015
To improve the electrochemical activity of polyacrylonitrile (PAN)-based electrospun carbon nanofibers (ECNFs) toward vanadium redox couples, the multi-wall carbon nanotubes (CNTs) and Bi-based compound as electrocatalyst have been embedded in the ECNFs to make composite electrode, respectively. The morphology and electrochemical properties of pristine ECNFs, CNTs/ECNFs and Bi/ECNFs have been characterized. Among the three kinds of electrodes, the CNTs/ECNFs show best electrochemical activity toward VO2þ/VO2 þ redox couple, while the Bi/ECNFs present the best electrochemical activity toward V2þ/V3þ redox couple. Furthermore, the high overpotential of hydrogen evolution on Bi/ECNFs makes the side-reaction suppressed. Because of the large property difference between the two composite electrodes, the CNTs/ECNFs and Bi/ECNFs are designed to act as positive and negative electrode for vanadium redox flow battery (VRFB), respectively. It not only does improve the kinetics of two electrode reactions at the same time, but also reduce the kinetics difference between them. Due to the application of asymmetric electrodes, performance of the cell is improved greatly. © 2015 Elsevier B.V. All rights reserved.
Keywords: Electrospun carbon nanofibers Carbon nanotubes Bismuth based compound Asymmetric electrodes Vanadium redox flow battery
1. Introduction VRFB has attracted a great deal of interests as a large-scale energy storage device due to its outstanding advantages such as long cycle life, large capacities, flexible design and no crosscontamination [1,2]. Because of the high surface area and low electrical resistivity, the carbon felt (CF) is widely used as electrode material for VRFB. However, the electrochemical reversibility of vanadium redox couples on the CF is very poor. In addition, some previous literature report the reaction kinetics of V2þ/V3þ redox couple is much slower than that of VO2þ/VO2 þ redox couple on carbon electrodes [3,4]. The hydrogen evolution also happens easily
* Corresponding authors. E-mail address: [email protected] (J. Liu). http://dx.doi.org/10.1016/j.jpowsour.2015.01.161 0378-7753/© 2015 Elsevier B.V. All rights reserved.
on carbon electrodes in the acid solution [5]. All of these negative effects limit the energy efficiency of VRFB seriously. Therefore, a lot of electrocatalysts such as Ir, Bi, CuPt3, Nb2O5, Mn3O4, WO3, CNTs, graphene, and reduced graphite oxide have been developed to modify the CF and enhance its electrochemical activity [6e16]. Although some electrocatalysts present good electrocatalytic activity toward vanadium redox couples, they can not still solve the problems mentioned above efficiently. Besides, the adhesion of electrocatalysts on the CF is low with current loading methods, thus, the electrocatalysts may meet the challenge of washing out by flowing electrolyte in VRFB. Therefore, a new composite electrode is developed in this paper. Applying the electrospinning technique and subsequent carbonization process, PAN polymer with electrocatalysts can be made into composite carbon nanofibers easily. With this method, the
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electrocatalyst will be embedded in the carbon nanofibers and not be washed out by flowing electrolyte. Among the numerous electrocatalysts, CNTs shows high electrocatalytic activity toward VO2þ/ VO2 þ redox couple while the hydrogen evolution on it is easy to happen; Bi metal or its trivalent ion presents high electrocatalytic activity toward V2þ/V3þ redox couple and the overpotential of hydrogen evolution on Bi metal is quite large [17e19]. Therefore, CNTs and Bi-based compound are used as electrocatalyst for positive and negative electrode in the paper, respectively. In this way, the reaction kinetics on two electrodes will be improved at the same time, and the hydrogen evolution on the negative electrode will be also suppressed. 2. Experimental 2.1. Preparation of composite electrodes 12 wt.% of PAN was dissolved in N,N-dimethylformamide (DMF) by stirring at 60 C for 4 h firstly. Then the CNTs and bismuth nitrate were added into the PAN solution, mixing with PAN polymer by mass ratios of 1: 100 and 1: 10 by stirring at room temperature for 1 h to prepare the precursor solution for electrospinning, respectively. The two kinds of precursor solution were made into electrospun nonwoven web consisting of nanofibers by process reported in the previous work, respectively [20]. Then the electrospun nanofibers were pre-oxidized at 280 C for 30 min in air. After that, the stabilized nanofibers were carbonized by heating them to 1000 C at a rate of 5 C min1 and holding for 90 min in nitrogen flow. After this procedure, the CNTs/ECNFs and Bi/ECNFs composite electrode were obtained. For comparison, the pristine
ECNFs without electrocatalyst were also prepared. 2.2. Material characterization The morphology of ECNFs and electrocatalysts were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). For electrochemical measurement, a three-electrode cell was used as reported in the previous work [21]. The electrochemical properties of pristine ECNFs, CNTs/ECNFs and Bi/ECNFs composite electrode were tested by cyclic voltammograms (CV) and electrochemical impedance spectra (EIS). The CV curves were recorded at 2 mV s1 scan rate in 0.1 M VOSO4 þ 2.0 M H2SO4 solution. The EIS was measured by applying an alternating voltage of 5 mV over the frequency ranging from 105 to 102 Hz in 0.1 M VOSO4 þ 2.0 M H2SO4 solution at different potentials. The VRFB single cell performance was tested in the cell structure described in the previous work [21]. The asymmetric electrodes were used by sandwiching them between pristine CF and ion exchange membrane in both half cells. The CNTs/ECNFs acted as positive electrode while the Bi/ECNFs acted as negative electrode. For comparison, the cell just with pristine CF was also tested. 3. Results and discussion 3.1. Morphology of ECNFs Surface morphology of the pre-oxidized nanofibers with Bibased compound is presented in Fig. 1a and b. Comparing the SEM image and Z-contrast image obtained by a backscattering
Fig. 1. Surface morphology of PAN-based nanofibers: SEM image (a) and Z-contrast image (b) of the pre-oxidized nanofibers with Bi-based compound; SEM image of the pristine ECNFs (c) and Bi/ECNFs (d).
G. Wei et al. / Journal of Power Sources 281 (2015) 1e6
detector of the pre-oxidized nanofibers with Bi-based compound, the brighter part indicates the Bi-based compound distributes in the skin region of nanofibers. Fig. 1c and d show morphology of the pristine ECNFs and Bi/ECNFs, respectively. Different to the smooth surface of pristine ECNFs, the surface of Bi/ECNFs is rough and a lot of nanoparticles stand on it. Energy dispersive spectroscopy (EDS) analysis demonstrates the elemental Bi content in the Bi/ECNFs is 0.23 wt.%. Fig. 2 shows the typical morphology of CNTs/ECNFs at different resolutions. As illustrated in Fig. 2a, a single slightly-curved CNT is embedded in the ECNFs and a part of it protrudes from the fiber surface. Fig. 2b and c present the high resolution lattice image of CNTs. Not like the turbostratic carbon structure in carbon nanofibers, the graphitic lattices in the CNTs are quite ordered, presenting the well arranging 2-dimensional graphitic planes clearly. 3.2. Electrochemical properties Fig. 3 shows the CV behavior of VO2þ/VO2 þ and V2þ/V3þ redox couple on the three kinds of electrodes. For the VO2þ/VO2 þ redox couple, values of peak potential separation (DEp) on the pristine ECNFs, Bi/ECNFs, CNTs/ECNFs are 102, 80, 68 mV, respectively. The reaction on the CNTs/ECNFs is quite reversible, indicating the electrochemical activity of the electrode toward VO2þ/VO2 þ redox couple is best. In case of the V2þ/V3þ redox couple, DEp on the pristine ECNFs, Bi/ECNFs are 112, 64 mV, respectively. Due to the serious hydrogen evolution, the anodic peak associated with the oxidation of V2þ to V3þ does not appear on CNTs/ECNFs. The Bi/ ECNFs shows the best electrochemical activity toward V2þ/V3þ
3
redox couple. In addition, DEp of V2þ/V3þ redox couple is larger than that of VO2þ/VO2 þ redox couple on the pristine ECNFs, indicating reaction kinetics of the former is slower than that of the latter. However, DEp of VO2þ/VO2 þ redox couple on CNTs/ECNFs is quite similar to that of V2þ/V3þ redox couple on Bi/ECNFs. Therefore, if the CNTs/ECNFs and Bi/ECNFs composite electrode are used as positive and negative electrode for VRFB respectively, not only will the electrochemical activity of two electrodes be improved at the same time, but also the difference between their reaction kinetics can be reduced. All of these aspects contribute to enhance the efficiencies of VRFB. Due to the suppression of hydrogen evolution on the Bi/ECNFs, efficiencies of the battery will be improved in a further step. In order to confirm the CV result and find out the reason for improvement of electrochemical activity, EIS analysis was carried out by applying an alternating voltage of 5 mV over the frequency ranging from 105 to 102 Hz in 0.1 M VOSO4 þ 2.0 M H2SO4 solution at different potentials. The Nyquist plots recorded at different potentials and equivalent circuit for the electrode process are presented in Fig. 4. For all samples, the Nyquist plots consist of two semicircles and a linear part. As analyzed in the previous work, the first semicircle stands for the capacitance and resistance arisen between the nanofibers in electrospun carbon web; the second semicircle stands for the electron transfer step and the linear part represents the diffusion process of vanadium ions [21]. Thus, the Nyquist plots can be fitted with equivalent circuit in Fig. 4c. At the open circuit potential, the electron transfer resistances (Rct) for the pristine ECNFs, Bi/ECNFs, CNTs/ECNFs are 75.0, 38.6, 20.0 U, respectively. At the polarization potential of 0.9 V, values of Rct for
Fig. 2. TEM images of CNTs/ECNFs at low resolution (a), at high resolution (b) and (c).
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Fig. 3. CV curves for VO2þ/VO2 þ (a) and V2þ/V3þ redox couple (b) on the three kinds of electrodes recorded at 2 mV s1 scan rate in 0.1 M VOSO4 þ 2.0 M H2SO4.
the three electrodes are 7.40, 5.01, 2.58 U, respectively. The results indicate the electrocatalysts can surely enhance the electron transfer rate of VO2þ/VO2 þ redox reaction. For the mass transport n of vanadium ions, CPE3 (expressed as CPE ¼ Y1 0 (jw) ) in the equivalent circuit reflects the diffusion process of vanadium ions, and an increase in the prefactor Y0 implies a decrease of the diffusion impedance. The Y0 values for the pristine ECNFs, Bi/ECNFs, and CNTs/ECNFs are 0.096, 0.122, and 0.127, respectively. Obviously, the composite electrodes present higher mass transport rate than the pristine ECNFs. On one hand, the embedding electrocatalysts can catalyze the vanadium redox reaction, thus the electron transfer rate is accelerated; on the other hand, the electroctalysts can provide more active sites for the redox reaction and more vanadium ions adsorb on the electrode, thus the mass transport is also facilitated. Thanks to the improvement in two aspects mentioned above, the electrochemical reversibility of the vanadium redox reaction on the composite electrodes is much higher than that on the pristine ECNFs. 3.3. VRFB single cell performance As shown in Fig. 5a, efficiencies of the cell with asymmetric electrodes are much higher than that of the cell with pristine CF as predicted. The relative parameters are listed in Table 1. Voltage efficiency (VE) and coulombic efficiency (CE) of the cell with asymmetric electrodes increase about 1.2% and 1.3% respectively
Fig. 4. Nyquist plots of the three kinds of electrodes for VO2þ/VO2 þ redox couple at different potentials: (a) At the open circuit potential, (b) At the polarization potential of 0.9 V, (c) Equivalent circuit for the electrode process.
compared with the cell with pristine CF in whole charge current density range. Since the reaction kinetics of V2þ/V3þ redox couple is much slower than that of VO2þ/VO2 þ redox couple on carbon electrodes, the cell performance including VE and CE is controlled by negative reaction. The use of asymmetric electrodes not only improves the kinetics of two electrode reactions at the same time, but also reduces the kinetics difference between them. It means the electrochemical reversibility of the V2þ/V3þ redox couple is improved greatly, which is closed to that of VO2þ/VO2 þ redox couple. According to our previous work, increased electrochemical reversibility of the vanadium redox reaction really contributes to improve VE and CE of the cell [19]. On the other hand, due to the competitive relationship between V2þ/V3þ redox reaction and hydrogen evolution on negative electrode, the suppression of hydrogen evolution on Bi/ECNFs composite electrode also helps to improve VE and CE of the cell. As a result of the VE and CE increase mentioned above, EE of the cell with asymmetric electrodes is
G. Wei et al. / Journal of Power Sources 281 (2015) 1e6
5
Fig. 6. Chargeedischarge curve of the VRFB single cell with different electrodes at a current density of 100 mA cm2.
Fig. 5. Efficiencies (a) and EE (b) of the VRFB single cell with asymmetric electrodes and pristine CF.
asymmetric electrodes are larger than that of the cell with pristine CF. Fig. 7 shows the discharge capacity of the cell with different electrodes. At the same current density, the discharge capacity of the cell with asymmetric electrodes is higher than that of the cell with pristine CF, indicating the former shows better utilization of the electrolyte. Taking account of the increased discharge voltage and higher discharge capacity, the cell with asymmetric electrodes also will present a higher discharge energy density compared with the cell with pristine CF [17]. However, capacity retention of the cell with asymmetric electrodes is no better than that of the cell with pristine CF. As reported in previous works, capacity decrease mainly attributes to the cross-diffusion of vanadium ions between two half cells and the resulting self-discharge, which mainly affected by ion exchange membrane, cell structure, and state of charge (SOC) [22e25]. To improve the capacity retention of the cell efficiently, all influence factors should be considered to decrease the vanadium ions cross-diffusion. All in all, due to the application of asymmetric electrodes, the main performance of VRFB such as EE, utilization of the electrolyte and discharge energy density is enhanced greatly.
about 2.3% higher than that of the cell with pristine CF in whole charge current density range. As shown in Fig. 5b, EE of the cell with asymmetric electrodes decreases a little at a charge current density of 50 mA cm2 after testing for 58 cycles (about 127 h), suggesting the properties of asymmetric electrodes are stable. As illustrated in Fig. 6, the VRFB single cell with asymmetric electrodes presents lower overpotential than that with pristine CF during the charge and discharge processes, which helps to improve the output power density of the cell. Due to the reduced overpotential, the charge and discharge capacity of the cell with
Table 1 Efficiencies of VRFB single cell with asymmetric electrodes and pristine CF at different current densities. Current density/mA cm2
50 60 70 80 100
Asymmetric electrodes
Pristine CF
CE/%
VE/%
EE/%
CE/%
VE/%
EE/%
94.9 95.7 96.3 96.8 98.0
91.8 90.2 88.5 86.6 83.6
87.2 86.3 85.2 83.9 82.0
93.0 94.4 95.2 95.8 96.5
91.1 89.0 87.0 85.2 82.5
84.7 84.0 82.8 81.6 79.7
Fig. 7. Discharge capacity of the VRFB single cell with different electrodes at different current densities.
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4. Conclusions In the paper, the CNTs and Bi-based compound have been embedded in the ECNFs to make composite electrode for VRFB, respectively. Among the three kinds of electrodes (pristine ECNFs, CNTs/ECNFs and Bi/ECNFs), the CNTs/ECNFs shows the best electrochemical activity toward VO2þ/VO2 þ redox couple, and the Bi/ ECNFs presents the best electrochemical activity toward V2þ/V3þ redox couple. Because of the low overpotential of hydrogen evolution on CNTs, the side-reaction on the CNTs/ECNFs is quite serious. On the contrary, the Bi/ECNFs can restrain the hydrogen evolution greatly. Due to the performance difference between the two composite electrodes, the CNTs/ECNFs and Bi/ECNFs are used as positive and negative electrode for VRFB, respectively. Compared with the VRFB single cell just with pristine CF, performance of the cell with asymmetric electrodes is improved greatly. Acknowledgments This work is funded by National Basic Research Program of China (No. 2010CB227203). References [1] M. Skyllas-Kazacos, M. Rychcik, R.G. Robins, et al., J. Electrochem. Soc. (1986)
1057e1058. [2] A. Parasuraman, T.M. Lim, C. Menictas, M. Skyllas-Kazacos, Electrochim. Acta 101 (2013) 27e40. [3] E. Sum, M. Rychcik, M. Skyllas-Kazacos, J. Power Sources 16 (1985) 85e95. [4] E. Sum, M. Skyllas-Kazacos, J. Power Sources 15 (1985) 179e190. [5] E. Agar, C.R. Dennison, K.W. Knehr, E.C. Kumbur, J. Power Sources 225 (2013) 89e94. [6] W.H. Wang, X.D. Wang, Electrochim. Acta 52 (2007) 6755e6762. nchez, C. Blanco, et al., Electrochem. Commun. 13 (2011) [7] Z. Gonz alez, A. Sa 1379e1382. [8] C. Flox, J. Rubio-Garcia, R. Nafria, et al., Carbon 50 (2012) 2372e2374. [9] B. Li, M. Gu, Z. Nie, et al., Nano Lett. 14 (2014) 158e165. [10] K.J. Kim, M.-S. Park, J.-H. Kim, et al., Chem. Commun. 48 (2012) 5455e5457. [11] C. Yao, H. Zhang, T. Liu, et al., J. Power Sources 237 (2013) 19e25. [12] W. Li, J. Liu, C. Yan, Electrochim. Acta 79 (2012) 102e108. [13] M. Park, Y. Jung, J. Kim, et al., Nano Lett. 13 (2013) 4833e4839. [14] H.-M. Tsai, S.-Y. Yang, C.-C.M. Ma, et al., Electroanalysis 23 (2011) 2139e2143. lez, C. Botas, P. Alvarez, [15] Z. Gonza et al., Carbon 50 (2012) 828e834. lez, C. Botas, C. Blanco, et al., J. Power Sources 241 (2013) 349e354. [16] Z. Gonza [17] B. Li, M. Gu, Z. Nie, et al., Nano Lett. 13 (2013) 1330e1335. [18] K. Aghapoor, F. Mohsenzadeh, A. Shakeri, et al., J. Organomet. Chem. 743 (2013) 170e178. [19] G. Wei, C. Jia, J. Liu, C. Yan, J. Power Sources 220 (2012) 185e192. [20] G. Wei, J. Liu, H. Zhao, C. Yan, J. Power Sources 241 (2013) 709e717. [21] G. Wei, X. Fan, J. Liu, C. Yan, J. Power Sources 270 (2014) 634e645. [22] C. Jia, Y. Cheng, X. Ling, et al., Electrochim. Acta 153 (2015) 44e48. [23] B. Schwenzer, S. Kim, M. Vijayakumar, et al., J. Membr. Sci. 372 (2011) 11e19. [24] S. Kim, J. Yan, B. Schwenzer, et al., Electrochem. Commun. 12 (2010) 1650e1653. [25] C. Jia, Q. Liu, C. Sun, et al., ACS Appl. Mater. Interfaces 6 (2014) 17920e17925.
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Electrospun carbon nanofibers/electrocatalyst hybrids as asymmetric electrodes for vanadium redox flow battery Guanjie Wei, Xinzhuang Fan*, Jianguo Liu*, Chuanwei Yan Liaoning Engineering Research Center for Advanced Battery Materials, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, 110016, China
h i g h l i g h t s Composite electrodes have been made by electrospinning technique. CNTs/ECNFs show best electrocatalytic activity to VO2þ/VOþ 2 redox couple. Bi/ECNFs present best electrocatalytic activity to V2þ/V3þ redox couple. Hydrogen evolution on the Bi/ECNFs composite electrode is suppressed. CNTs/ECNFs and Bi/ECNFs are used as asymmetric electrodes for VRFB.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 October 2014 Received in revised form 29 December 2014 Accepted 27 January 2015 Available online 28 January 2015
To improve the electrochemical activity of polyacrylonitrile (PAN)-based electrospun carbon nanofibers (ECNFs) toward vanadium redox couples, the multi-wall carbon nanotubes (CNTs) and Bi-based compound as electrocatalyst have been embedded in the ECNFs to make composite electrode, respectively. The morphology and electrochemical properties of pristine ECNFs, CNTs/ECNFs and Bi/ECNFs have been characterized. Among the three kinds of electrodes, the CNTs/ECNFs show best electrochemical activity toward VO2þ/VO2 þ redox couple, while the Bi/ECNFs present the best electrochemical activity toward V2þ/V3þ redox couple. Furthermore, the high overpotential of hydrogen evolution on Bi/ECNFs makes the side-reaction suppressed. Because of the large property difference between the two composite electrodes, the CNTs/ECNFs and Bi/ECNFs are designed to act as positive and negative electrode for vanadium redox flow battery (VRFB), respectively. It not only does improve the kinetics of two electrode reactions at the same time, but also reduce the kinetics difference between them. Due to the application of asymmetric electrodes, performance of the cell is improved greatly. © 2015 Elsevier B.V. All rights reserved.
Keywords: Electrospun carbon nanofibers Carbon nanotubes Bismuth based compound Asymmetric electrodes Vanadium redox flow battery
1. Introduction VRFB has attracted a great deal of interests as a large-scale energy storage device due to its outstanding advantages such as long cycle life, large capacities, flexible design and no crosscontamination [1,2]. Because of the high surface area and low electrical resistivity, the carbon felt (CF) is widely used as electrode material for VRFB. However, the electrochemical reversibility of vanadium redox couples on the CF is very poor. In addition, some previous literature report the reaction kinetics of V2þ/V3þ redox couple is much slower than that of VO2þ/VO2 þ redox couple on carbon electrodes [3,4]. The hydrogen evolution also happens easily
* Corresponding authors. E-mail address: [email protected] (J. Liu). http://dx.doi.org/10.1016/j.jpowsour.2015.01.161 0378-7753/© 2015 Elsevier B.V. All rights reserved.
on carbon electrodes in the acid solution [5]. All of these negative effects limit the energy efficiency of VRFB seriously. Therefore, a lot of electrocatalysts such as Ir, Bi, CuPt3, Nb2O5, Mn3O4, WO3, CNTs, graphene, and reduced graphite oxide have been developed to modify the CF and enhance its electrochemical activity [6e16]. Although some electrocatalysts present good electrocatalytic activity toward vanadium redox couples, they can not still solve the problems mentioned above efficiently. Besides, the adhesion of electrocatalysts on the CF is low with current loading methods, thus, the electrocatalysts may meet the challenge of washing out by flowing electrolyte in VRFB. Therefore, a new composite electrode is developed in this paper. Applying the electrospinning technique and subsequent carbonization process, PAN polymer with electrocatalysts can be made into composite carbon nanofibers easily. With this method, the
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G. Wei et al. / Journal of Power Sources 281 (2015) 1e6
electrocatalyst will be embedded in the carbon nanofibers and not be washed out by flowing electrolyte. Among the numerous electrocatalysts, CNTs shows high electrocatalytic activity toward VO2þ/ VO2 þ redox couple while the hydrogen evolution on it is easy to happen; Bi metal or its trivalent ion presents high electrocatalytic activity toward V2þ/V3þ redox couple and the overpotential of hydrogen evolution on Bi metal is quite large [17e19]. Therefore, CNTs and Bi-based compound are used as electrocatalyst for positive and negative electrode in the paper, respectively. In this way, the reaction kinetics on two electrodes will be improved at the same time, and the hydrogen evolution on the negative electrode will be also suppressed. 2. Experimental 2.1. Preparation of composite electrodes 12 wt.% of PAN was dissolved in N,N-dimethylformamide (DMF) by stirring at 60 C for 4 h firstly. Then the CNTs and bismuth nitrate were added into the PAN solution, mixing with PAN polymer by mass ratios of 1: 100 and 1: 10 by stirring at room temperature for 1 h to prepare the precursor solution for electrospinning, respectively. The two kinds of precursor solution were made into electrospun nonwoven web consisting of nanofibers by process reported in the previous work, respectively [20]. Then the electrospun nanofibers were pre-oxidized at 280 C for 30 min in air. After that, the stabilized nanofibers were carbonized by heating them to 1000 C at a rate of 5 C min1 and holding for 90 min in nitrogen flow. After this procedure, the CNTs/ECNFs and Bi/ECNFs composite electrode were obtained. For comparison, the pristine
ECNFs without electrocatalyst were also prepared. 2.2. Material characterization The morphology of ECNFs and electrocatalysts were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). For electrochemical measurement, a three-electrode cell was used as reported in the previous work [21]. The electrochemical properties of pristine ECNFs, CNTs/ECNFs and Bi/ECNFs composite electrode were tested by cyclic voltammograms (CV) and electrochemical impedance spectra (EIS). The CV curves were recorded at 2 mV s1 scan rate in 0.1 M VOSO4 þ 2.0 M H2SO4 solution. The EIS was measured by applying an alternating voltage of 5 mV over the frequency ranging from 105 to 102 Hz in 0.1 M VOSO4 þ 2.0 M H2SO4 solution at different potentials. The VRFB single cell performance was tested in the cell structure described in the previous work [21]. The asymmetric electrodes were used by sandwiching them between pristine CF and ion exchange membrane in both half cells. The CNTs/ECNFs acted as positive electrode while the Bi/ECNFs acted as negative electrode. For comparison, the cell just with pristine CF was also tested. 3. Results and discussion 3.1. Morphology of ECNFs Surface morphology of the pre-oxidized nanofibers with Bibased compound is presented in Fig. 1a and b. Comparing the SEM image and Z-contrast image obtained by a backscattering
Fig. 1. Surface morphology of PAN-based nanofibers: SEM image (a) and Z-contrast image (b) of the pre-oxidized nanofibers with Bi-based compound; SEM image of the pristine ECNFs (c) and Bi/ECNFs (d).
G. Wei et al. / Journal of Power Sources 281 (2015) 1e6
detector of the pre-oxidized nanofibers with Bi-based compound, the brighter part indicates the Bi-based compound distributes in the skin region of nanofibers. Fig. 1c and d show morphology of the pristine ECNFs and Bi/ECNFs, respectively. Different to the smooth surface of pristine ECNFs, the surface of Bi/ECNFs is rough and a lot of nanoparticles stand on it. Energy dispersive spectroscopy (EDS) analysis demonstrates the elemental Bi content in the Bi/ECNFs is 0.23 wt.%. Fig. 2 shows the typical morphology of CNTs/ECNFs at different resolutions. As illustrated in Fig. 2a, a single slightly-curved CNT is embedded in the ECNFs and a part of it protrudes from the fiber surface. Fig. 2b and c present the high resolution lattice image of CNTs. Not like the turbostratic carbon structure in carbon nanofibers, the graphitic lattices in the CNTs are quite ordered, presenting the well arranging 2-dimensional graphitic planes clearly. 3.2. Electrochemical properties Fig. 3 shows the CV behavior of VO2þ/VO2 þ and V2þ/V3þ redox couple on the three kinds of electrodes. For the VO2þ/VO2 þ redox couple, values of peak potential separation (DEp) on the pristine ECNFs, Bi/ECNFs, CNTs/ECNFs are 102, 80, 68 mV, respectively. The reaction on the CNTs/ECNFs is quite reversible, indicating the electrochemical activity of the electrode toward VO2þ/VO2 þ redox couple is best. In case of the V2þ/V3þ redox couple, DEp on the pristine ECNFs, Bi/ECNFs are 112, 64 mV, respectively. Due to the serious hydrogen evolution, the anodic peak associated with the oxidation of V2þ to V3þ does not appear on CNTs/ECNFs. The Bi/ ECNFs shows the best electrochemical activity toward V2þ/V3þ
3
redox couple. In addition, DEp of V2þ/V3þ redox couple is larger than that of VO2þ/VO2 þ redox couple on the pristine ECNFs, indicating reaction kinetics of the former is slower than that of the latter. However, DEp of VO2þ/VO2 þ redox couple on CNTs/ECNFs is quite similar to that of V2þ/V3þ redox couple on Bi/ECNFs. Therefore, if the CNTs/ECNFs and Bi/ECNFs composite electrode are used as positive and negative electrode for VRFB respectively, not only will the electrochemical activity of two electrodes be improved at the same time, but also the difference between their reaction kinetics can be reduced. All of these aspects contribute to enhance the efficiencies of VRFB. Due to the suppression of hydrogen evolution on the Bi/ECNFs, efficiencies of the battery will be improved in a further step. In order to confirm the CV result and find out the reason for improvement of electrochemical activity, EIS analysis was carried out by applying an alternating voltage of 5 mV over the frequency ranging from 105 to 102 Hz in 0.1 M VOSO4 þ 2.0 M H2SO4 solution at different potentials. The Nyquist plots recorded at different potentials and equivalent circuit for the electrode process are presented in Fig. 4. For all samples, the Nyquist plots consist of two semicircles and a linear part. As analyzed in the previous work, the first semicircle stands for the capacitance and resistance arisen between the nanofibers in electrospun carbon web; the second semicircle stands for the electron transfer step and the linear part represents the diffusion process of vanadium ions [21]. Thus, the Nyquist plots can be fitted with equivalent circuit in Fig. 4c. At the open circuit potential, the electron transfer resistances (Rct) for the pristine ECNFs, Bi/ECNFs, CNTs/ECNFs are 75.0, 38.6, 20.0 U, respectively. At the polarization potential of 0.9 V, values of Rct for
Fig. 2. TEM images of CNTs/ECNFs at low resolution (a), at high resolution (b) and (c).
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Fig. 3. CV curves for VO2þ/VO2 þ (a) and V2þ/V3þ redox couple (b) on the three kinds of electrodes recorded at 2 mV s1 scan rate in 0.1 M VOSO4 þ 2.0 M H2SO4.
the three electrodes are 7.40, 5.01, 2.58 U, respectively. The results indicate the electrocatalysts can surely enhance the electron transfer rate of VO2þ/VO2 þ redox reaction. For the mass transport n of vanadium ions, CPE3 (expressed as CPE ¼ Y1 0 (jw) ) in the equivalent circuit reflects the diffusion process of vanadium ions, and an increase in the prefactor Y0 implies a decrease of the diffusion impedance. The Y0 values for the pristine ECNFs, Bi/ECNFs, and CNTs/ECNFs are 0.096, 0.122, and 0.127, respectively. Obviously, the composite electrodes present higher mass transport rate than the pristine ECNFs. On one hand, the embedding electrocatalysts can catalyze the vanadium redox reaction, thus the electron transfer rate is accelerated; on the other hand, the electroctalysts can provide more active sites for the redox reaction and more vanadium ions adsorb on the electrode, thus the mass transport is also facilitated. Thanks to the improvement in two aspects mentioned above, the electrochemical reversibility of the vanadium redox reaction on the composite electrodes is much higher than that on the pristine ECNFs. 3.3. VRFB single cell performance As shown in Fig. 5a, efficiencies of the cell with asymmetric electrodes are much higher than that of the cell with pristine CF as predicted. The relative parameters are listed in Table 1. Voltage efficiency (VE) and coulombic efficiency (CE) of the cell with asymmetric electrodes increase about 1.2% and 1.3% respectively
Fig. 4. Nyquist plots of the three kinds of electrodes for VO2þ/VO2 þ redox couple at different potentials: (a) At the open circuit potential, (b) At the polarization potential of 0.9 V, (c) Equivalent circuit for the electrode process.
compared with the cell with pristine CF in whole charge current density range. Since the reaction kinetics of V2þ/V3þ redox couple is much slower than that of VO2þ/VO2 þ redox couple on carbon electrodes, the cell performance including VE and CE is controlled by negative reaction. The use of asymmetric electrodes not only improves the kinetics of two electrode reactions at the same time, but also reduces the kinetics difference between them. It means the electrochemical reversibility of the V2þ/V3þ redox couple is improved greatly, which is closed to that of VO2þ/VO2 þ redox couple. According to our previous work, increased electrochemical reversibility of the vanadium redox reaction really contributes to improve VE and CE of the cell [19]. On the other hand, due to the competitive relationship between V2þ/V3þ redox reaction and hydrogen evolution on negative electrode, the suppression of hydrogen evolution on Bi/ECNFs composite electrode also helps to improve VE and CE of the cell. As a result of the VE and CE increase mentioned above, EE of the cell with asymmetric electrodes is
G. Wei et al. / Journal of Power Sources 281 (2015) 1e6
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Fig. 6. Chargeedischarge curve of the VRFB single cell with different electrodes at a current density of 100 mA cm2.
Fig. 5. Efficiencies (a) and EE (b) of the VRFB single cell with asymmetric electrodes and pristine CF.
asymmetric electrodes are larger than that of the cell with pristine CF. Fig. 7 shows the discharge capacity of the cell with different electrodes. At the same current density, the discharge capacity of the cell with asymmetric electrodes is higher than that of the cell with pristine CF, indicating the former shows better utilization of the electrolyte. Taking account of the increased discharge voltage and higher discharge capacity, the cell with asymmetric electrodes also will present a higher discharge energy density compared with the cell with pristine CF [17]. However, capacity retention of the cell with asymmetric electrodes is no better than that of the cell with pristine CF. As reported in previous works, capacity decrease mainly attributes to the cross-diffusion of vanadium ions between two half cells and the resulting self-discharge, which mainly affected by ion exchange membrane, cell structure, and state of charge (SOC) [22e25]. To improve the capacity retention of the cell efficiently, all influence factors should be considered to decrease the vanadium ions cross-diffusion. All in all, due to the application of asymmetric electrodes, the main performance of VRFB such as EE, utilization of the electrolyte and discharge energy density is enhanced greatly.
about 2.3% higher than that of the cell with pristine CF in whole charge current density range. As shown in Fig. 5b, EE of the cell with asymmetric electrodes decreases a little at a charge current density of 50 mA cm2 after testing for 58 cycles (about 127 h), suggesting the properties of asymmetric electrodes are stable. As illustrated in Fig. 6, the VRFB single cell with asymmetric electrodes presents lower overpotential than that with pristine CF during the charge and discharge processes, which helps to improve the output power density of the cell. Due to the reduced overpotential, the charge and discharge capacity of the cell with
Table 1 Efficiencies of VRFB single cell with asymmetric electrodes and pristine CF at different current densities. Current density/mA cm2
50 60 70 80 100
Asymmetric electrodes
Pristine CF
CE/%
VE/%
EE/%
CE/%
VE/%
EE/%
94.9 95.7 96.3 96.8 98.0
91.8 90.2 88.5 86.6 83.6
87.2 86.3 85.2 83.9 82.0
93.0 94.4 95.2 95.8 96.5
91.1 89.0 87.0 85.2 82.5
84.7 84.0 82.8 81.6 79.7
Fig. 7. Discharge capacity of the VRFB single cell with different electrodes at different current densities.
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4. Conclusions In the paper, the CNTs and Bi-based compound have been embedded in the ECNFs to make composite electrode for VRFB, respectively. Among the three kinds of electrodes (pristine ECNFs, CNTs/ECNFs and Bi/ECNFs), the CNTs/ECNFs shows the best electrochemical activity toward VO2þ/VO2 þ redox couple, and the Bi/ ECNFs presents the best electrochemical activity toward V2þ/V3þ redox couple. Because of the low overpotential of hydrogen evolution on CNTs, the side-reaction on the CNTs/ECNFs is quite serious. On the contrary, the Bi/ECNFs can restrain the hydrogen evolution greatly. Due to the performance difference between the two composite electrodes, the CNTs/ECNFs and Bi/ECNFs are used as positive and negative electrode for VRFB, respectively. Compared with the VRFB single cell just with pristine CF, performance of the cell with asymmetric electrodes is improved greatly. Acknowledgments This work is funded by National Basic Research Program of China (No. 2010CB227203). References [1] M. Skyllas-Kazacos, M. Rychcik, R.G. Robins, et al., J. Electrochem. Soc. (1986)
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