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Br− redox reactions in zinc-bromine flow batteries
Electrochimica Acta 318 (2019) 69e75
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
N-doped graphene nanoplatelets as a highly active catalyst for Br2/Br redox reactions in zinc-bromine flow batteries M.C. Wu a, H.R. Jiang a, R.H. Zhang a, L. Wei a, K.Y. Chan b, *, 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 Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 April 2019 Received in revised form 6 June 2019 Accepted 11 June 2019 Available online 13 June 2019
The low power density, due primarily to the sluggish reaction kinetic of Br2/Br, is one of the main barriers that hinder the widespread application of zinc-bromine flow batteries (ZBFBs). Here, N-doped graphene nanoplatelets are synthesized by a facile method and applied as a catalyst for the Br2/Br redox reactions. Electrochemical characterizations reveal that N-doped graphene nanoplatelets exhibit a remarkable catalytic activity toward Br2/Br reactions, thus enabling the ZBFB to achieve an energy efficiency of as high as 84.2% at 80 mA cm2, far surpassing those with the non-doped counterpart and pristine graphite-felt electrodes. More strikingly, even when the current density is raised up to 120 mA cm2, the battery can still maintain an energy efficiency of 78.8%, which represents the highest performance for the ZBFBs reported in the open literature. Additionally, the ZBFB with the N-doped graphene nanoplatelets catalyst shows no degradation after 100 cycles. These superior results demonstrate that N-doped graphene nanoplatelets are an efficient and promising catalyst for high-performance bromine-based flow batteries. © 2019 Elsevier Ltd. All rights reserved.
Keywords: N-doped graphene nanoplatelets Zinc-bromine flow batteries Br2/Br redox reactions Energy storage
1. Introduction During the past several decades, concerns about environmental pollution and shortage of fossil fuels have provoked a rapidly increasing deployment of renewable energies (e.g., wind and solar power). Unfortunately, these renewables are unpredictable and intermittent, making it quite challenging to integrate the generated electricity into the grids [1]. One promising solution to this issue is to adopt large-scale energy storage (LES) devices to smooth out their intermittency and therefore ensure a stable power output [2,3]. Redox flow batteries (RFBs) have drawn tremendous attention for this application because of their flexible design, good scalability, quick response times, and long cycle life [4]. Thus far, a wide variety of RBFs have been developed, including vanadium flow battery (VFB), Fe/Cr flow battery (ICFB), ZBFB and others [5e8]. Although numerous RBFs have been proposed, presently, only VFB, ICFB and ZBFB are on the verge of commercialization. In particular, the VFB has attracted enormous interests and several commercial
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (K.Y. Chan), [email protected] (T.S. Zhao). https://doi.org/10.1016/j.electacta.2019.06.064 0013-4686/© 2019 Elsevier Ltd. All rights reserved.
demonstration pilot plants have been built [9e12]. However, the broader market adoption of this technology is still impeded by several crucial issues. For example, the energy density of VFB is only ~25 Wh L1, and the suitable temperature range for operation this battery is restricted within 10e40 C, resulting from the poor solubility of the vanadium salts in aqueous solution [13]. Moreover, using expensive vanadium species and Nafion membrane inevitably results in a high capital cost that retards the commercial adoption of this technology. Take a VFB system with a 4-h discharge capacity for example, the electrolyte has already accounted for more than 40% of the total capital cost, leading to a much higher total cost of this system (447 $ kWh1) than that of the targeted one ($150 kWh1) for LES applications [14]. Although ICFB employs cheap iron and chromium salts as active materials, this system has almost been abandoned because of the sluggish kinetics of Cr3þ/ Cr2þ reactions, chromium aging effect, severe hydrogen evolution reaction (HER) and low energy density (~10 Wh L1) [8]. In contrast, due to the high energy density, abundant active material resources and low cost, ZBFB is gaining increasing research interests for LES applications [15,16]. The electrochemical reactions in a typical ZBFB are simply summarized as:
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at negative electrode: Zn 2e 4Zn2þ (E0 ¼ 0.76 V vs. SHE) (1a) at positive electrode: Br2 þ 2e 42Br (E0 ¼ 1.08 V vs. SHE)
(1b)
and overall: Br2 þ Zn4ZnBr2 (E ¼ 1.08 V vs. SHE)
(1c)
Zinc is electrochemically deposited at the negative electrode during charge. At the same time, element bromine is produced in the positive side. Reversed reactions occur during the discharge process. It is worthy to note that, although the standard potential of Zn2þ/Zn is much lower than that of SHE, the large overpotential of hydrogen evolution reaction (HER) on zinc electrode enables the deposition/stripping of zinc to achieve a relatively high current efficiency. By virtue of the high ZnBr2 solubility as well as the large potential difference between Zn and Br2, the ZBFB exhibits a high cell voltage and energy density. More importantly, the initial electrolytes for both positive and negative sides in a ZBFB are identical. This is a free-contamination feature of RFBs that enables easy recovery of species crossover induced capacity loss via a simple re-mixing and re-separating process [17]. All these appealing advantages make ZBFB a promising alternative for LES applications. Nevertheless, the low power density, primarily arising from the sluggish Br2/Br reactions, will lead to a larger stack size and thus material consumption, hampering its widespread applications. Hence, developing highly active catalysts for Br2/Br reactions is urgently needed. Over the last few decades, tremendous efforts have been devoted to developing positive materials for bromine based flow batteries. For example, Zhang et al. coated active carbons onto a porous membrane to form a positive electrode for ZBFB and demonstrated an increased energy efficiency (EE) of 75%, which was ascribed to reduced overpotential in the positive electrode and decreased internal resistance [18]. Munaiah et al. investigated the electro-catalytic activities of the single-walled and multi-walled carbon nanotubes for ZBFBs. It is found that SWCNT presents a higher electrocatalytic effect for Br2/Br reactions compared with its multi-walled counterpart. As a result, higher energy efficiency and superior rate were achieved in ZBFBs by employing CNT based electrodes [19,20]. Recently, Wang et al. reported a porous nanosheet carbon (PNSC) to boost the performance of ZBFBs. It is demonstrated that at 80 mA cm2, a high EE of 82% was attained by using the high-surface-area PNSC [21]. These previous works clearly prove that it is effective to improve the performance of ZBFBs by developing highly catalytic materials and more works are still needed to further improve their performances. Graphene, which possesses excellent electrical conductivity, superb structural stability and high specific surface area, has attracted numerous interests for various applications [22,23]. More attractively, its electronic properties can be well modulated via heteroatom doping (e.g., N, B). Very recently, Venkatesan et al. reported boron-doped reduced graphene oxide (B-rGO) as a catalyst for ZBFBs [24]. Although improved electrochemical performance was achieved, the operating current density remained as low as 30 mA cm2, and degradation was also observed upon cycling. On the contrary, by doping nitrogen, which possesses a higher electronegativity, is an effective approach to achieve electron modulation that offers favorable electronic structures for various electrocatalytic processes [25e27]. Moreover, replacing rGO with graphene will offer much enhanced durability due to its higher graphitization. Thus, it is believed that N-doped graphene will be a promising positive material for Br2/Br redox reactions. Herein, we synthesized N-doped graphene nanoplatelets (NGnP) by a simple pyrolysis method and, applied as a catalyst for Br2/ Br reactions. Intensive material and electrochemical characterizations were conducted to confirm the successful doping of
nitrogen as well as its electrocatalytic activity. The practical application of N-GnP was further evaluated by real flow battery tests. All results indicate that N-GnP presents an exceptionally high activity and stability toward Br2/Br reactions, thus allowing the ZBFB to operate at 120 mA cm2 maintaining an EE of 78.8%. Stable cycling performance is also demonstrated, indicating that N-GnP is an efficient and promising catalyst for bromine based FBs. 2. Experimental 2.1. N-GnP synthesis Graphene nanoplatelets (GnP) were purchased from Sigma Aldrich. The N-doped graphene nanoplatelets (N-GnP) were fabricated by a facile pyrolysis process in the presence of urea. Specifically, 100 mg GnP and 500 mg urea were placed in a porcelain boat separately, where urea was put near the inlet side. A porcelain plate was positioned on the top of the boat to avoid the fast escape of the generated ammonia gas. The sample was heated at 800 C in a tube furnace for 1 h under argon flow. The collected sample was denoted as N-GnP. 2.2. Material characterizations The morphologies of GnP and N-GnP were observed by a scanning electron microscope (SEM, JEOL-6700F). The surface properties of the samples were studied by Raman spectra using a RM 3000 (Renishaw) micro-Raman spectrometer at 532 nm. X-ray photoelectron spectroscopy (XPS) tests were performed on a Physical Electronics PHI 5600 multi-technique system. The Al monochromatic X-ray power was set at 350 W. 2.3. Electrode fabrication N-GnP and Nafion emulsion (5 wt%, DuPont®) were dispersed in absolute ethanol, and subjected to ultrasonication for 50 min to form a suspension. The weight ratio of N-GnP to Nafion was controlled to be 9: 1, and the N-GnP concentration in the resultant suspension was 3 mg mL1. This suspension was then uniformly dropped cast on the graphite felt (GF, SGL carbon group, Germany) with a loading of about 3 mg cm2. Original GF (6 mm) was evenly cut into two pieces with a thickness of 3 mm for use. The resultant electrode was denoted as N-GnP/GF. The same procedure was carried out by using GnP as the control group and the electrode was denoted as GnP/GF. 2.4. Electrochemical characterizations A typical three-electrode system was used for Cyclic voltammetry (CV) test. The reference and counter electrodes were saturated calomel electrode (SCE) with a salt bridge and platinum mesh electrode, respectively. The working electrode was prepared as followed. The prepared catalyst (10 mg) was dispersed into 190 mL absolute ethanol, which was then ultrasonically stirred for 40 min to achieve a uniform suspension. Then, 100 mL Nafion emulsion (5 wt%, DuPont®) was added and the suspension was subjected to strong ultrasonication for another 20 min. At last, 10 mL catalyst ink was dropped onto the glassy carbon (GC, d ¼ 5 mm) electrode surface and naturally dried. The electrolyte for CV test consisted of 0.1 M ZnBr2 and 1 M HCl. The electrochemical impedance spectroscopy (EIS) measurements were conducted by means of a twoelectrode connection, where negative Zn electrode was used as both the counter and reference electrode. The impedance was obtained at the open circuit voltage (OCV) by sweeping the frequencies from 10 kHz to 100 mHz with a wave amplitude of 10 mV
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after the ZBFB was charged at 20 mA cm2 for 1 h. Full battery tests were evaluated by using our lad-made flow cell. Briefly, a piece of GF was used as the negative electrode, and a non-conductive porous polyacrylonitrile (PAN) felt was laid on its surface to reserve space for zinc deposition, which was clamped together with the positive electrode separated by a Nafion membrane (Nafion 211, DuPont®). Nafion membrane is used because of its excellent stability and effectiveness in suppressing bromine crossover [15,28]. The compression ratio of the electrodes (effective area of 2 cm 2.5 cm) is about 17%. Solutions (20 mL) containing 2 M ZnBr2 and 4 M NH4Cl were employed as both positive and negative electrolytes, and circulated through the electrodes with a flow rate of 46 mL min1 via a 2-channel peristaltic pump (Longer pump, WT600-2J). The charge-discharge profiles were recorded by using Arbin Instrument under a constant-current mode. The batteries were charged at a constant areal capacity (20 mAh cm2) and then completely discharged to a cut-off voltage of 0.5 V.
3. Results and discussion The microstructures of the GnP and N-GnP are shown in Fig. 1. It is seen that the morphology of N-GnP resembles that of GnP, indicating the nitrogen doping does not alter the microstructure of GnP. Both GnP and N-GnP are crumpled and abundant pores are observed, which is able to offer more reactive sites for the reaction of Br2/Br. Raman measurement was carried out to study the properties of the GnP and N-GNP, and the spectra are presented in Fig. 2a. Three peaks are detected within the Raman shift range from 200 to 3200 cm1, i.e., D, G, and 2D band, as marked in the figure. The intensity ratio of D band to G band (ID/IG) is generally used as an indicator to evaluate the graphitization degree. From Fig. 2a, we can see that the N-GnP presents a higher value of ID/IG than GnP, suggesting more defects are generated in the carbon framework after nitrogen doping [29]. The 2D band located at about 2650 cm1 arises from a two-phonon double resonance and is a characteristic feature of few-layer graphene. The similar intensity of this peak in
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GnP and N-GnP indicates the layer number remains unchanged after nitrogen doping [30]. XPS was also conducted to further investigate the surface properties of the samples. XPS survey reveals that the nitrogen content in N-GnP is 0.42%. Fig. 2b and c show the high-resolution O1s XPS spectra. Three different peaks located at about 531.3, 532.4 and 533.6 eV are deconvoluted, referred as O1, O2 and O3, respectively. Here, O1 is ascribed to C]O of carbonyl or ketone groups, O2 to carbonyl oxygen of esters, anhydrides, amides, and oxygen atom of phenol, alcohol or ether groups, and O3 to oxygen of carboxylic groups [31]. Results show that the dominant type in GnP is O1 while its content reduces after nitrogen doping with the increase of O3 species. The high resolution N1s XPS spectra are displayed in Fig. 2d, which were deconvoluted into three peaks, namely the pyridinic-, pyrrolic- and graphitic-N. An illustration of different N species is presented in Fig. S1 [32]. It is found that the pyridinic-N presents the highest content, which is believed to favor the Br2/Br redox reactions because this type of nitrogen species is widely regarded as the active site for electrochemical reactions [33,34]. To evaluate the catalytic activity of the N-GnP toward Br2/Br reactions, CV test was first conducted. As displayed in Fig. 3a, N-GnP exhibits a larger anodic and cathodic peak current density compared with GnP, indicating that the electrochemical activity of N-GnP is much higher than its non-doped counterpart, primarily ascribed to the incorporated N-doped active sites. In addition, a reduced anodic and cathodic peak potential separation is observed for N-GnP, which suggests the reversibility of Br2/Br is enhanced on N-GnP. The Br2/Br redox reaction is reported to involve two consecutive electrochemical steps in the anodic and cathodic processes [35]:
Br2 þ e 4Brads þ Br
(3a)
Brads þ e 4Br
(3b)
where reactions (3a) and (3b) are the rate-determining steps in
Fig. 1. SEM pictures of (a&b) GnP; (c&d) N-GnP.
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Fig. 2. Material characterizations of GnP and N-GnP. (a) Raman spectra; (b) high-resolution XPS O1s spectra of GnP; (c) high-resolution XPS O1s spectra of N-GnP; (d) highresolution XPS N1s spectra of N-GnP.
cathodic and anodic processes, respectively. By incorporating nitrogen into the carbon frameworks, the electronic properties of the nearby carbon atoms are modulated because of their higher electronegativity. Accordingly, the adsorption of negatively charged bromide species (e.g., Br, Br 3 ) is improved and the charge transfer between the adsorption ions and the electrode is enhanced, thus boosting the kinetics of the Br2/Br redox reactions [21,36]. To better understand the reaction of Br2/Br, CV profiles were recorded using different scan rates and results are displayed in Fig. 3b and c. It is revealed that the anodic and cathodic peaks of both GnP and N-GnP undergo a positive and negative shift, in response to the increase of scan rate, respectively, suggesting the quasi-reversibility of the Br2/Br reaction [37]. Furthermore, the Br anodic peak of GnP becomes not noticeable when the scan rate reaches 40 mV s1, while it can still be well recognized even at 60 mV s1 for the N-GnP electrode, further implying the enhanced activity of N-GnP toward Br2/Br redox reactions. The relation of anodic and cathodic peak current densities (Ip) of N-GnP with the square root of scan rates (v1/2) is illustrated in Fig. S2. It is seen that both anodic and cathodic Ip are linearly proportionally to v1/2, indicating that Ip is controlled by the diffusion process. The stability of N-GnP was then evaluated by repetitively scanning CVs (100 cycles at 50 mV s1). No change of the CV curves is observed after 100 cycles as compared with the first one (See Fig. 3d), indicative of the favorable durability of N-
GnP. EIS was also applied to further study the catalytic activity of NGnP. Fig. 4 depicts the Nyquist plots of N-GnP/GF, GnP/GF and pristine GF electrodes. The diameters of semicircles at high frequencies decrease in an order of pristine GF > GnP/GF > N-GnP/GF, indicating that the N-GnP/GF presents the lowest charge transfer resistance. This confirms that N-GnP possesses faster kinetics than GnP and pristine GF, consistent with the CV results. The practical application of N-GnP was verified in ZBFBs. Fig. S3 shows that GnP and N-GnP are successfully anchored onto the carbon surface, offering much more active sites for Br2/Br redox reactions. Although the electrodes are hydrophobic, as evidenced by the contact angles in Fig. S4, the electrolyte is forced to flow through their porous structure and the Br2/Br redox reactions can take place on the carbon surface. Charge-discharge profiles of ZBFBs assembled with N-GnP/GF, GnP/GF and pristine GF electrodes at 80 mA cm2 are compared in Fig. 5a. The charge and discharge voltage plateaus for the ZBFB with a pristine GF electrode are 2.02 and 1.48 V, respectively, revealing the large polarization in this battery, which possibly results from the poor electrochemical activity of pristine GF toward Br2/Br. In contrast, the charge/ discharge voltage plateaus are substantially reduced/enhanced after decorating the GF electrode with GnP and N-GnP catalysts. Notably, the use of N-GnP catalyst allows the ZBFB to be charged at a voltage plateau of as low as 1.93 V and discharged at a plateau of
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Fig. 3. CV results of GnP and N-GnP. (a) CVs of GnP and N-GnP at a scan rate of 20 mV s1; (b) CV profiles of GnP at different scan rates; (c) CV curves of N-GnP at different scan rates; (d) repeating sweeping measurement of N-GnP at 50 mV s1.
Fig. 4. EIS results of pristine GF, GnP/GF and N-GnP/GF electrodes by ranging the frequency from 100 mHz to 10 kHz.
as high as 1.64 V, demonstrating its excellent electrochemical activity. It should be mentioned that as all other battery conditions are kept unchanged during tests, the decrease/increase of charge/
discharge voltage are attributable to reduced overpotentials in the positive electrodes, where GnP and N-GnP provide more electrochemical reactive sites for Br2/Br, especially the N-GnP. The resultant coulombic efficiencies (CEs), voltage efficiencies (VEs) and EEs of these ZBFBs are summarized in Fig. 5b. It is found that the ZBFBs exhibits an EE of up to 84.2%, whereas those with GnP/GF and pristine GF only show an EE of 80% and 71.8%, respectively. This result represents the highest value ever reported for ZBFBs at this current density [21]. Moreover, the CEs in all tested ZBFBs are near 99%, confirming that Nafion membrane is effective in preventing bromine crossover, primarily due to the Donnan effect as well as the small ionic cluster size of the untreated Nafion membrane, as discussed in our previous work [36]. The rate performances of ZBFBs were evaluated by gradually increasing the current densities from 20 to 120 mA cm2. The resulting charge-discharge curves of the ZBFB equipped with the N-GnP/GF electrode are presented in Fig. 5c. It is shown that increasing the current densities leads to the increase/decrease of the charge/discharge voltage as a result of the increased electrochemical and ohmic polarizations. Similar trends are observed for the other two ZBFBs with GnP/GF and pristine GF electrodes. Due to the reduced reaction time by increasing the current density and thus less crossover of bromine, the CEs of the ZBFBs increase slightly (See Fig. S5). Because the CEs are similar under each current density, the EE (Fig. 5d) follows the same tendency as VE. Under all tested current densities, ZBFBs with both GnP and N-GnP catalysts present much higher EEs compared with that with the pristine GF electrode. Furthermore, it is found that the
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Fig. 5. Full battery performances. (a) Charge-discharge curves of ZBFBs with pristine GF, GnP/GF and N-GnP/GF electrodes at 80 mA cm2; (b) CE, VE and EE comparison of different ZBFBs at 80 mA cm2; (c) Charge-discharge curves of ZBFB with the N-GnP/GF electrode at various current densities; (d) EE comparison of ZBFBs with different electrodes at different current densities.
N-GnP magnifies its catalytic effect under larger current densities by maintaining a slower drop in EE. Exceptionally, even with at 120 mA cm2, which is the highest operating current density for
ZBFBs, the ZBFB with N-GnP/GF electrode is still capable of exhibiting an EE of as high as 78.8%, demonstrating an excellent rate capability. In comparison, the EEs of the ZBFB with the GnP/GF and
Fig. 6. Cycling performance of a ZBFB with the N-GnP/GF electrode. (a) voltage versus time plot; (b) CE, VE, EE and discharge capacity during the 100 charge-discharge cycles.
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pristine electrode are only 73.5% and 65.7%, respectively. These superior results again confirm the extraordinary electrochemical activity of N-GnP toward the Br2/Br redox reactions. Additionally, the cycling stability is a crucial factor for LES applications. To reveal the stability of the N-GnP catalyst in ZBFB, the battery with an NGnP/GF electrode is repetitively charged and discharged at 80 mA cm2. The resulting curves in Fig. 6a reveal that both charge and discharge voltage plateaus remain the same throughout the cycle test. The plots of CE, VE, EE and discharge capacity versus cycle number are also presented in Fig. 6b. No decay is observed during the cycling test, suggesting the outstanding durability of the N-GnP catalyst in ZBFB applications. 4. Conclusions In summary, N-GnP was synthesized via a simple pyrolysis process and applied as a catalyst to boost the kinetic of Br2/Br reactions in ZBFBs. It is found that N-GnP exhibits an outstanding electrochemical activity toward Br2/Br, which enables the ZBFB to achieve a high EE of 84.2% at 80 mA cm2. In contrast, the ZBFBs with its non-doped counterpart and the pristine electrode can only output an EE of 80.0% and 71.8%, respectively. More remarkably, even when the current density is raised to 120 mA cm2, the ZBFB with the N-GnP catalyst is still capable of maintaining an EE of 78.8%, which represents the highest performance in ZBFBs. In addition, no decay is observed over 100 cycles for the battery assembled with this N-GnP catalyst. These results demonstrate that N-GnP is an efficient and promising catalyst for high-performance bromine-based flow batteries. Acknowledgement The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. T23-601/17-R). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2019.06.064. References [1] Z. Yang, J. Zhang, M.C.W. Kintner-Meyer, X. Lu, D. Choi, J.P. Lemmon, J. Liu, Electrochemical energy storage for green grid, Chem. Rev. 111 (2011) 3577e3613. [2] C. Zhang, L. Zhang, Y. Ding, S. Peng, X. Guo, Y. Zhao, G. He, G. Yu, Progress and prospects of next-generation redox flow batteries, Energy Storage Mater 15 (2018) 324e350. [3] Y. Li, Y. Feng, X. Sun, Insight into interface behaviors to build phase-boundarymatched Na-ion direct liquid fuel cells, ACS Sustain. Chem. Eng. 6 (2018) 12827e12834. [4] G.L. Soloveichik, Flow batteries: current status and trends, Chem. Rev. 115 (2015) 11533e11558. [5] Y. Zeng, F. Li, F. Lu, X. Zhou, Y. Yuan, X. Cao, B. Xiang, A hierarchical interdigitated flow field design for scale-up of high-performance redox flow batteries, Appl. Energy 238 (2019) 435e441. [6] Q. Wu, X. Zhang, Y. Lv, L. Lin, Y. Liu, X. Zhou, Bio-inspired multiscale-porenetwork structured carbon felt with enhanced mass transfer and activity for vanadium redox flow batteries, J. Mater. Chem. A 6 (2018) 20347. [7] X. Zhou, L. Lin, Y. Lv, X. Zhang, Q. Wu, A Sn-Fe flow battery with excellent rate and cycle performance, J. Power Sources 404 (2018) 89e95. n, L. Berlouis, C.T.J. Low, F.C. Walsh, Progress in redox [8] P. Leung, X. Li, C.P. de Leo flow batteries, remaining challenges and their applications in energy storage, RSC Adv. 2 (2012) 10125e10156. [9] W. Wang, Q. Luo, B. Li, X. Wei, L. Li, Z. Yang, Recent progress in redox flow battery research and development, Adv. Funct. Mater. 23 (2013) 970e986. [10] M. Skyllas-Kazacos, M.H. Chakrabarti, S. a. Hajimolana, F.S. Mjalli, M. Saleem, Progress in flow battery research and development, J. Electrochem. Soc. 158 (2011) R55eR79.
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Yu, Synthesis of N-doped graphene by chemical vapor deposition and its electrical properties, Nano Lett. 9 (2009) 1752e1758. [30] K.S. Rao, J. Senthilnathan, Y.F. Liu, M. Yoshimura, Role of peroxide ions in formation of graphene nanosheets by electrochemical exfoliation of graphite, Sci. Rep. 4 (2014). [31] K. Laszlo, E. Tombacz, K. Josepovits, Effect of activation on the surface chemistry of carbons from polymer precursors, Carbon N. Y. 39 (2001) 1217. [32] L. Shi, S. Liu, Z. He, J. Shen, Nitrogen-Doped Graphene:Effects of nitrogen species on the properties of the vanadium redox flow battery, Electrochim. Acta 138 (2014) 93e100. [33] D. Guo, R. Shibuya, C. Akiba, S. Saji, T. Kondo, J. Nakamura, Active sites of nitrogen-doped carbon materials for oxygen reduction reaction clarified using model catalysts, Science 351 (80-) (2016) 361e365. [34] T. Xing, Y. Zheng, L.H. Li, B.C.C. Cowie, D. Gunzelmann, S.Z. Qiao, S. Huang, Y. 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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
N-doped graphene nanoplatelets as a highly active catalyst for Br2/Br redox reactions in zinc-bromine flow batteries M.C. Wu a, H.R. Jiang a, R.H. Zhang a, L. Wei a, K.Y. Chan b, *, 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 Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 April 2019 Received in revised form 6 June 2019 Accepted 11 June 2019 Available online 13 June 2019
The low power density, due primarily to the sluggish reaction kinetic of Br2/Br, is one of the main barriers that hinder the widespread application of zinc-bromine flow batteries (ZBFBs). Here, N-doped graphene nanoplatelets are synthesized by a facile method and applied as a catalyst for the Br2/Br redox reactions. Electrochemical characterizations reveal that N-doped graphene nanoplatelets exhibit a remarkable catalytic activity toward Br2/Br reactions, thus enabling the ZBFB to achieve an energy efficiency of as high as 84.2% at 80 mA cm2, far surpassing those with the non-doped counterpart and pristine graphite-felt electrodes. More strikingly, even when the current density is raised up to 120 mA cm2, the battery can still maintain an energy efficiency of 78.8%, which represents the highest performance for the ZBFBs reported in the open literature. Additionally, the ZBFB with the N-doped graphene nanoplatelets catalyst shows no degradation after 100 cycles. These superior results demonstrate that N-doped graphene nanoplatelets are an efficient and promising catalyst for high-performance bromine-based flow batteries. © 2019 Elsevier Ltd. All rights reserved.
Keywords: N-doped graphene nanoplatelets Zinc-bromine flow batteries Br2/Br redox reactions Energy storage
1. Introduction During the past several decades, concerns about environmental pollution and shortage of fossil fuels have provoked a rapidly increasing deployment of renewable energies (e.g., wind and solar power). Unfortunately, these renewables are unpredictable and intermittent, making it quite challenging to integrate the generated electricity into the grids [1]. One promising solution to this issue is to adopt large-scale energy storage (LES) devices to smooth out their intermittency and therefore ensure a stable power output [2,3]. Redox flow batteries (RFBs) have drawn tremendous attention for this application because of their flexible design, good scalability, quick response times, and long cycle life [4]. Thus far, a wide variety of RBFs have been developed, including vanadium flow battery (VFB), Fe/Cr flow battery (ICFB), ZBFB and others [5e8]. Although numerous RBFs have been proposed, presently, only VFB, ICFB and ZBFB are on the verge of commercialization. In particular, the VFB has attracted enormous interests and several commercial
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (K.Y. Chan), [email protected] (T.S. Zhao). https://doi.org/10.1016/j.electacta.2019.06.064 0013-4686/© 2019 Elsevier Ltd. All rights reserved.
demonstration pilot plants have been built [9e12]. However, the broader market adoption of this technology is still impeded by several crucial issues. For example, the energy density of VFB is only ~25 Wh L1, and the suitable temperature range for operation this battery is restricted within 10e40 C, resulting from the poor solubility of the vanadium salts in aqueous solution [13]. Moreover, using expensive vanadium species and Nafion membrane inevitably results in a high capital cost that retards the commercial adoption of this technology. Take a VFB system with a 4-h discharge capacity for example, the electrolyte has already accounted for more than 40% of the total capital cost, leading to a much higher total cost of this system (447 $ kWh1) than that of the targeted one ($150 kWh1) for LES applications [14]. Although ICFB employs cheap iron and chromium salts as active materials, this system has almost been abandoned because of the sluggish kinetics of Cr3þ/ Cr2þ reactions, chromium aging effect, severe hydrogen evolution reaction (HER) and low energy density (~10 Wh L1) [8]. In contrast, due to the high energy density, abundant active material resources and low cost, ZBFB is gaining increasing research interests for LES applications [15,16]. The electrochemical reactions in a typical ZBFB are simply summarized as:
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at negative electrode: Zn 2e 4Zn2þ (E0 ¼ 0.76 V vs. SHE) (1a) at positive electrode: Br2 þ 2e 42Br (E0 ¼ 1.08 V vs. SHE)
(1b)
and overall: Br2 þ Zn4ZnBr2 (E ¼ 1.08 V vs. SHE)
(1c)
Zinc is electrochemically deposited at the negative electrode during charge. At the same time, element bromine is produced in the positive side. Reversed reactions occur during the discharge process. It is worthy to note that, although the standard potential of Zn2þ/Zn is much lower than that of SHE, the large overpotential of hydrogen evolution reaction (HER) on zinc electrode enables the deposition/stripping of zinc to achieve a relatively high current efficiency. By virtue of the high ZnBr2 solubility as well as the large potential difference between Zn and Br2, the ZBFB exhibits a high cell voltage and energy density. More importantly, the initial electrolytes for both positive and negative sides in a ZBFB are identical. This is a free-contamination feature of RFBs that enables easy recovery of species crossover induced capacity loss via a simple re-mixing and re-separating process [17]. All these appealing advantages make ZBFB a promising alternative for LES applications. Nevertheless, the low power density, primarily arising from the sluggish Br2/Br reactions, will lead to a larger stack size and thus material consumption, hampering its widespread applications. Hence, developing highly active catalysts for Br2/Br reactions is urgently needed. Over the last few decades, tremendous efforts have been devoted to developing positive materials for bromine based flow batteries. For example, Zhang et al. coated active carbons onto a porous membrane to form a positive electrode for ZBFB and demonstrated an increased energy efficiency (EE) of 75%, which was ascribed to reduced overpotential in the positive electrode and decreased internal resistance [18]. Munaiah et al. investigated the electro-catalytic activities of the single-walled and multi-walled carbon nanotubes for ZBFBs. It is found that SWCNT presents a higher electrocatalytic effect for Br2/Br reactions compared with its multi-walled counterpart. As a result, higher energy efficiency and superior rate were achieved in ZBFBs by employing CNT based electrodes [19,20]. Recently, Wang et al. reported a porous nanosheet carbon (PNSC) to boost the performance of ZBFBs. It is demonstrated that at 80 mA cm2, a high EE of 82% was attained by using the high-surface-area PNSC [21]. These previous works clearly prove that it is effective to improve the performance of ZBFBs by developing highly catalytic materials and more works are still needed to further improve their performances. Graphene, which possesses excellent electrical conductivity, superb structural stability and high specific surface area, has attracted numerous interests for various applications [22,23]. More attractively, its electronic properties can be well modulated via heteroatom doping (e.g., N, B). Very recently, Venkatesan et al. reported boron-doped reduced graphene oxide (B-rGO) as a catalyst for ZBFBs [24]. Although improved electrochemical performance was achieved, the operating current density remained as low as 30 mA cm2, and degradation was also observed upon cycling. On the contrary, by doping nitrogen, which possesses a higher electronegativity, is an effective approach to achieve electron modulation that offers favorable electronic structures for various electrocatalytic processes [25e27]. Moreover, replacing rGO with graphene will offer much enhanced durability due to its higher graphitization. Thus, it is believed that N-doped graphene will be a promising positive material for Br2/Br redox reactions. Herein, we synthesized N-doped graphene nanoplatelets (NGnP) by a simple pyrolysis method and, applied as a catalyst for Br2/ Br reactions. Intensive material and electrochemical characterizations were conducted to confirm the successful doping of
nitrogen as well as its electrocatalytic activity. The practical application of N-GnP was further evaluated by real flow battery tests. All results indicate that N-GnP presents an exceptionally high activity and stability toward Br2/Br reactions, thus allowing the ZBFB to operate at 120 mA cm2 maintaining an EE of 78.8%. Stable cycling performance is also demonstrated, indicating that N-GnP is an efficient and promising catalyst for bromine based FBs. 2. Experimental 2.1. N-GnP synthesis Graphene nanoplatelets (GnP) were purchased from Sigma Aldrich. The N-doped graphene nanoplatelets (N-GnP) were fabricated by a facile pyrolysis process in the presence of urea. Specifically, 100 mg GnP and 500 mg urea were placed in a porcelain boat separately, where urea was put near the inlet side. A porcelain plate was positioned on the top of the boat to avoid the fast escape of the generated ammonia gas. The sample was heated at 800 C in a tube furnace for 1 h under argon flow. The collected sample was denoted as N-GnP. 2.2. Material characterizations The morphologies of GnP and N-GnP were observed by a scanning electron microscope (SEM, JEOL-6700F). The surface properties of the samples were studied by Raman spectra using a RM 3000 (Renishaw) micro-Raman spectrometer at 532 nm. X-ray photoelectron spectroscopy (XPS) tests were performed on a Physical Electronics PHI 5600 multi-technique system. The Al monochromatic X-ray power was set at 350 W. 2.3. Electrode fabrication N-GnP and Nafion emulsion (5 wt%, DuPont®) were dispersed in absolute ethanol, and subjected to ultrasonication for 50 min to form a suspension. The weight ratio of N-GnP to Nafion was controlled to be 9: 1, and the N-GnP concentration in the resultant suspension was 3 mg mL1. This suspension was then uniformly dropped cast on the graphite felt (GF, SGL carbon group, Germany) with a loading of about 3 mg cm2. Original GF (6 mm) was evenly cut into two pieces with a thickness of 3 mm for use. The resultant electrode was denoted as N-GnP/GF. The same procedure was carried out by using GnP as the control group and the electrode was denoted as GnP/GF. 2.4. Electrochemical characterizations A typical three-electrode system was used for Cyclic voltammetry (CV) test. The reference and counter electrodes were saturated calomel electrode (SCE) with a salt bridge and platinum mesh electrode, respectively. The working electrode was prepared as followed. The prepared catalyst (10 mg) was dispersed into 190 mL absolute ethanol, which was then ultrasonically stirred for 40 min to achieve a uniform suspension. Then, 100 mL Nafion emulsion (5 wt%, DuPont®) was added and the suspension was subjected to strong ultrasonication for another 20 min. At last, 10 mL catalyst ink was dropped onto the glassy carbon (GC, d ¼ 5 mm) electrode surface and naturally dried. The electrolyte for CV test consisted of 0.1 M ZnBr2 and 1 M HCl. The electrochemical impedance spectroscopy (EIS) measurements were conducted by means of a twoelectrode connection, where negative Zn electrode was used as both the counter and reference electrode. The impedance was obtained at the open circuit voltage (OCV) by sweeping the frequencies from 10 kHz to 100 mHz with a wave amplitude of 10 mV
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after the ZBFB was charged at 20 mA cm2 for 1 h. Full battery tests were evaluated by using our lad-made flow cell. Briefly, a piece of GF was used as the negative electrode, and a non-conductive porous polyacrylonitrile (PAN) felt was laid on its surface to reserve space for zinc deposition, which was clamped together with the positive electrode separated by a Nafion membrane (Nafion 211, DuPont®). Nafion membrane is used because of its excellent stability and effectiveness in suppressing bromine crossover [15,28]. The compression ratio of the electrodes (effective area of 2 cm 2.5 cm) is about 17%. Solutions (20 mL) containing 2 M ZnBr2 and 4 M NH4Cl were employed as both positive and negative electrolytes, and circulated through the electrodes with a flow rate of 46 mL min1 via a 2-channel peristaltic pump (Longer pump, WT600-2J). The charge-discharge profiles were recorded by using Arbin Instrument under a constant-current mode. The batteries were charged at a constant areal capacity (20 mAh cm2) and then completely discharged to a cut-off voltage of 0.5 V.
3. Results and discussion The microstructures of the GnP and N-GnP are shown in Fig. 1. It is seen that the morphology of N-GnP resembles that of GnP, indicating the nitrogen doping does not alter the microstructure of GnP. Both GnP and N-GnP are crumpled and abundant pores are observed, which is able to offer more reactive sites for the reaction of Br2/Br. Raman measurement was carried out to study the properties of the GnP and N-GNP, and the spectra are presented in Fig. 2a. Three peaks are detected within the Raman shift range from 200 to 3200 cm1, i.e., D, G, and 2D band, as marked in the figure. The intensity ratio of D band to G band (ID/IG) is generally used as an indicator to evaluate the graphitization degree. From Fig. 2a, we can see that the N-GnP presents a higher value of ID/IG than GnP, suggesting more defects are generated in the carbon framework after nitrogen doping [29]. The 2D band located at about 2650 cm1 arises from a two-phonon double resonance and is a characteristic feature of few-layer graphene. The similar intensity of this peak in
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GnP and N-GnP indicates the layer number remains unchanged after nitrogen doping [30]. XPS was also conducted to further investigate the surface properties of the samples. XPS survey reveals that the nitrogen content in N-GnP is 0.42%. Fig. 2b and c show the high-resolution O1s XPS spectra. Three different peaks located at about 531.3, 532.4 and 533.6 eV are deconvoluted, referred as O1, O2 and O3, respectively. Here, O1 is ascribed to C]O of carbonyl or ketone groups, O2 to carbonyl oxygen of esters, anhydrides, amides, and oxygen atom of phenol, alcohol or ether groups, and O3 to oxygen of carboxylic groups [31]. Results show that the dominant type in GnP is O1 while its content reduces after nitrogen doping with the increase of O3 species. The high resolution N1s XPS spectra are displayed in Fig. 2d, which were deconvoluted into three peaks, namely the pyridinic-, pyrrolic- and graphitic-N. An illustration of different N species is presented in Fig. S1 [32]. It is found that the pyridinic-N presents the highest content, which is believed to favor the Br2/Br redox reactions because this type of nitrogen species is widely regarded as the active site for electrochemical reactions [33,34]. To evaluate the catalytic activity of the N-GnP toward Br2/Br reactions, CV test was first conducted. As displayed in Fig. 3a, N-GnP exhibits a larger anodic and cathodic peak current density compared with GnP, indicating that the electrochemical activity of N-GnP is much higher than its non-doped counterpart, primarily ascribed to the incorporated N-doped active sites. In addition, a reduced anodic and cathodic peak potential separation is observed for N-GnP, which suggests the reversibility of Br2/Br is enhanced on N-GnP. The Br2/Br redox reaction is reported to involve two consecutive electrochemical steps in the anodic and cathodic processes [35]:
Br2 þ e 4Brads þ Br
(3a)
Brads þ e 4Br
(3b)
where reactions (3a) and (3b) are the rate-determining steps in
Fig. 1. SEM pictures of (a&b) GnP; (c&d) N-GnP.
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Fig. 2. Material characterizations of GnP and N-GnP. (a) Raman spectra; (b) high-resolution XPS O1s spectra of GnP; (c) high-resolution XPS O1s spectra of N-GnP; (d) highresolution XPS N1s spectra of N-GnP.
cathodic and anodic processes, respectively. By incorporating nitrogen into the carbon frameworks, the electronic properties of the nearby carbon atoms are modulated because of their higher electronegativity. Accordingly, the adsorption of negatively charged bromide species (e.g., Br, Br 3 ) is improved and the charge transfer between the adsorption ions and the electrode is enhanced, thus boosting the kinetics of the Br2/Br redox reactions [21,36]. To better understand the reaction of Br2/Br, CV profiles were recorded using different scan rates and results are displayed in Fig. 3b and c. It is revealed that the anodic and cathodic peaks of both GnP and N-GnP undergo a positive and negative shift, in response to the increase of scan rate, respectively, suggesting the quasi-reversibility of the Br2/Br reaction [37]. Furthermore, the Br anodic peak of GnP becomes not noticeable when the scan rate reaches 40 mV s1, while it can still be well recognized even at 60 mV s1 for the N-GnP electrode, further implying the enhanced activity of N-GnP toward Br2/Br redox reactions. The relation of anodic and cathodic peak current densities (Ip) of N-GnP with the square root of scan rates (v1/2) is illustrated in Fig. S2. It is seen that both anodic and cathodic Ip are linearly proportionally to v1/2, indicating that Ip is controlled by the diffusion process. The stability of N-GnP was then evaluated by repetitively scanning CVs (100 cycles at 50 mV s1). No change of the CV curves is observed after 100 cycles as compared with the first one (See Fig. 3d), indicative of the favorable durability of N-
GnP. EIS was also applied to further study the catalytic activity of NGnP. Fig. 4 depicts the Nyquist plots of N-GnP/GF, GnP/GF and pristine GF electrodes. The diameters of semicircles at high frequencies decrease in an order of pristine GF > GnP/GF > N-GnP/GF, indicating that the N-GnP/GF presents the lowest charge transfer resistance. This confirms that N-GnP possesses faster kinetics than GnP and pristine GF, consistent with the CV results. The practical application of N-GnP was verified in ZBFBs. Fig. S3 shows that GnP and N-GnP are successfully anchored onto the carbon surface, offering much more active sites for Br2/Br redox reactions. Although the electrodes are hydrophobic, as evidenced by the contact angles in Fig. S4, the electrolyte is forced to flow through their porous structure and the Br2/Br redox reactions can take place on the carbon surface. Charge-discharge profiles of ZBFBs assembled with N-GnP/GF, GnP/GF and pristine GF electrodes at 80 mA cm2 are compared in Fig. 5a. The charge and discharge voltage plateaus for the ZBFB with a pristine GF electrode are 2.02 and 1.48 V, respectively, revealing the large polarization in this battery, which possibly results from the poor electrochemical activity of pristine GF toward Br2/Br. In contrast, the charge/ discharge voltage plateaus are substantially reduced/enhanced after decorating the GF electrode with GnP and N-GnP catalysts. Notably, the use of N-GnP catalyst allows the ZBFB to be charged at a voltage plateau of as low as 1.93 V and discharged at a plateau of
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Fig. 3. CV results of GnP and N-GnP. (a) CVs of GnP and N-GnP at a scan rate of 20 mV s1; (b) CV profiles of GnP at different scan rates; (c) CV curves of N-GnP at different scan rates; (d) repeating sweeping measurement of N-GnP at 50 mV s1.
Fig. 4. EIS results of pristine GF, GnP/GF and N-GnP/GF electrodes by ranging the frequency from 100 mHz to 10 kHz.
as high as 1.64 V, demonstrating its excellent electrochemical activity. It should be mentioned that as all other battery conditions are kept unchanged during tests, the decrease/increase of charge/
discharge voltage are attributable to reduced overpotentials in the positive electrodes, where GnP and N-GnP provide more electrochemical reactive sites for Br2/Br, especially the N-GnP. The resultant coulombic efficiencies (CEs), voltage efficiencies (VEs) and EEs of these ZBFBs are summarized in Fig. 5b. It is found that the ZBFBs exhibits an EE of up to 84.2%, whereas those with GnP/GF and pristine GF only show an EE of 80% and 71.8%, respectively. This result represents the highest value ever reported for ZBFBs at this current density [21]. Moreover, the CEs in all tested ZBFBs are near 99%, confirming that Nafion membrane is effective in preventing bromine crossover, primarily due to the Donnan effect as well as the small ionic cluster size of the untreated Nafion membrane, as discussed in our previous work [36]. The rate performances of ZBFBs were evaluated by gradually increasing the current densities from 20 to 120 mA cm2. The resulting charge-discharge curves of the ZBFB equipped with the N-GnP/GF electrode are presented in Fig. 5c. It is shown that increasing the current densities leads to the increase/decrease of the charge/discharge voltage as a result of the increased electrochemical and ohmic polarizations. Similar trends are observed for the other two ZBFBs with GnP/GF and pristine GF electrodes. Due to the reduced reaction time by increasing the current density and thus less crossover of bromine, the CEs of the ZBFBs increase slightly (See Fig. S5). Because the CEs are similar under each current density, the EE (Fig. 5d) follows the same tendency as VE. Under all tested current densities, ZBFBs with both GnP and N-GnP catalysts present much higher EEs compared with that with the pristine GF electrode. Furthermore, it is found that the
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Fig. 5. Full battery performances. (a) Charge-discharge curves of ZBFBs with pristine GF, GnP/GF and N-GnP/GF electrodes at 80 mA cm2; (b) CE, VE and EE comparison of different ZBFBs at 80 mA cm2; (c) Charge-discharge curves of ZBFB with the N-GnP/GF electrode at various current densities; (d) EE comparison of ZBFBs with different electrodes at different current densities.
N-GnP magnifies its catalytic effect under larger current densities by maintaining a slower drop in EE. Exceptionally, even with at 120 mA cm2, which is the highest operating current density for
ZBFBs, the ZBFB with N-GnP/GF electrode is still capable of exhibiting an EE of as high as 78.8%, demonstrating an excellent rate capability. In comparison, the EEs of the ZBFB with the GnP/GF and
Fig. 6. Cycling performance of a ZBFB with the N-GnP/GF electrode. (a) voltage versus time plot; (b) CE, VE, EE and discharge capacity during the 100 charge-discharge cycles.
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pristine electrode are only 73.5% and 65.7%, respectively. These superior results again confirm the extraordinary electrochemical activity of N-GnP toward the Br2/Br redox reactions. Additionally, the cycling stability is a crucial factor for LES applications. To reveal the stability of the N-GnP catalyst in ZBFB, the battery with an NGnP/GF electrode is repetitively charged and discharged at 80 mA cm2. The resulting curves in Fig. 6a reveal that both charge and discharge voltage plateaus remain the same throughout the cycle test. The plots of CE, VE, EE and discharge capacity versus cycle number are also presented in Fig. 6b. No decay is observed during the cycling test, suggesting the outstanding durability of the N-GnP catalyst in ZBFB applications. 4. Conclusions In summary, N-GnP was synthesized via a simple pyrolysis process and applied as a catalyst to boost the kinetic of Br2/Br reactions in ZBFBs. It is found that N-GnP exhibits an outstanding electrochemical activity toward Br2/Br, which enables the ZBFB to achieve a high EE of 84.2% at 80 mA cm2. In contrast, the ZBFBs with its non-doped counterpart and the pristine electrode can only output an EE of 80.0% and 71.8%, respectively. More remarkably, even when the current density is raised to 120 mA cm2, the ZBFB with the N-GnP catalyst is still capable of maintaining an EE of 78.8%, which represents the highest performance in ZBFBs. In addition, no decay is observed over 100 cycles for the battery assembled with this N-GnP catalyst. These results demonstrate that N-GnP is an efficient and promising catalyst for high-performance bromine-based flow batteries. Acknowledgement The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. T23-601/17-R). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2019.06.064. References [1] Z. Yang, J. Zhang, M.C.W. Kintner-Meyer, X. Lu, D. Choi, J.P. Lemmon, J. Liu, Electrochemical energy storage for green grid, Chem. Rev. 111 (2011) 3577e3613. [2] C. Zhang, L. Zhang, Y. Ding, S. Peng, X. Guo, Y. Zhao, G. He, G. Yu, Progress and prospects of next-generation redox flow batteries, Energy Storage Mater 15 (2018) 324e350. [3] Y. Li, Y. Feng, X. Sun, Insight into interface behaviors to build phase-boundarymatched Na-ion direct liquid fuel cells, ACS Sustain. Chem. Eng. 6 (2018) 12827e12834. [4] G.L. Soloveichik, Flow batteries: current status and trends, Chem. Rev. 115 (2015) 11533e11558. [5] Y. Zeng, F. Li, F. Lu, X. Zhou, Y. Yuan, X. Cao, B. Xiang, A hierarchical interdigitated flow field design for scale-up of high-performance redox flow batteries, Appl. Energy 238 (2019) 435e441. [6] Q. Wu, X. Zhang, Y. Lv, L. Lin, Y. Liu, X. Zhou, Bio-inspired multiscale-porenetwork structured carbon felt with enhanced mass transfer and activity for vanadium redox flow batteries, J. Mater. Chem. A 6 (2018) 20347. [7] X. Zhou, L. Lin, Y. Lv, X. Zhang, Q. Wu, A Sn-Fe flow battery with excellent rate and cycle performance, J. Power Sources 404 (2018) 89e95. n, L. Berlouis, C.T.J. Low, F.C. Walsh, Progress in redox [8] P. Leung, X. Li, C.P. de Leo flow batteries, remaining challenges and their applications in energy storage, RSC Adv. 2 (2012) 10125e10156. [9] W. Wang, Q. Luo, B. Li, X. Wei, L. Li, Z. Yang, Recent progress in redox flow battery research and development, Adv. Funct. Mater. 23 (2013) 970e986. [10] M. Skyllas-Kazacos, M.H. Chakrabarti, S. a. Hajimolana, F.S. Mjalli, M. Saleem, Progress in flow battery research and development, J. Electrochem. Soc. 158 (2011) R55eR79.
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