Nano Energy 52 (2018) 292–299
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Redox-targeted catalysis for vanadium redox-flow batteries a
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Feifei Zhang , Songpeng Huang , Xun Wang , Chuankun Jia , Yonghua Du , Qing Wang a b
a,⁎
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Department of Materials Science and Engineering, Faculty of Engineering, National University of Singapore, Singapore 117576, Singapore Institute of Chemical & Engineering Sciences, A⁎STAR, Agency for Science, Technology and Research, Singapore 627833, Singapore
A R T I C LE I N FO
A B S T R A C T
Keywords: Vanadium redox-flow battery Redox targeting Electrocatalyst
Vanadium redox-flow battery (VRB) as a promising electrochemical power source for large-scale energy storage, suffers from various polarization losses despite that it has been extensively studied in the past decades. Among these losses, the sluggish interfacial charge transfer of the vanadium species on the respective electrode renders large overpotentials giving rise to inevitable hydrogen and oxygen evolutions during the charging process. In this study, we report an unprecedented method based on the redox targeting concept to tackle the above issues. Prussian blue (PB) and a Prussian blue analogues (PBA) with identical redox potentials to VO2+/VO2+ and V2+/ V3+ are grafted on cathode and anode, respectively. Upon operation, the reversible proton-coupled redox targeting reactions between PB and VO2+/VO2+ on cathode, PBA and V2+/V3+ on anode facilitate the interfacial charge transfer of the vanadium species and concomitantly inhibit the hydrogen and oxygen evolutions, which improves the selectivity of the redox reactions and considerably enhances the round-trip energy efficiency and cycling performance of VRB in a wide range of current densities. The above redox-assisted catalytic reactions were scrutinized and the mechanisms are unequivocally manifested with various electrochemical and spectroscopic measurements. We anticipate the surface immobilized redox catalysis approach demonstrated here would generically provide a paradigm for improving the sluggish kinetic processes in a variety of electrochemical devices.
1. Introduction Redox flow battery, due to its superior operation flexibility and system scalability, is presently one of the most promising battery technologies for large-scale applications [1–4]. Among various flow battery technologies, vanadium redox-flow batteries (VRBs) have received considerable attention for eliminated cross contamination by employing vanadium of difference valence states in both catholyte and anolyte [5–8]. VRBs store and release electrical energy through the redox reactions of VO2+/VO2+ and V2+/V3+ in acidic electrolytes on the surface of carbon-based electrode [9–12]. The electrode, which conducts electrons and provides the sites for redox reactions, is commonly made of carbon felt with three-dimensional microstructures, high electrical conductivity and chemical stability in concentrated vanadium-based electrolytes [13–16]. However, these materials generally have low electrochemical activity towards vanadium species [17]. For the cathode, the performance is dictated by the redox reaction of VO2+/VO2+ involving the transfer of an oxygen atom from H2O molecule in the electrolyte, which inherently leads to a slow kinetics and induces oxygen evolution at high potentials [10,18]. The performance of anode is constrained by the sluggish heterogeneous electron transfer
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rates of V2+/V3+ and parasitic hydrogen evolution [19,20]. These not only give rise to large overpotential loss impairing the power performance and round-trip energy efficiency of VRBs, the evolutions of H2 and O2 also lead to reduced Coulombic efficiency and imbalanced electrolyte composition compromising the cycling stability. Various surface treatments of the carbon electrode have thus been investigated to address the above issues, such as electrochemical oxidation, acid treatment, thermal activation and modification with metal or metal oxides, etc. [19,21–26] Nevertheless, controversial results are commonly found in different reports due to a lack of consistence of the electrode materials employed in the studies. In addition, a clear understanding differentiating the kinetics of VO2+/VO2+ on the positive side and V2+/V3+ on the negative side is crucial to the development of electrodes with enhanced electrochemical activities. For instance, it is discovered that oxidative surface of the anode favors the reaction of V2+/V3+ while reductive surface of the cathode promotes the reaction of VO2+/VO2+ [9,27]. These surfaces are however liable to changes when the anode undergoes reduction while the cathode undergoes oxidation. As such, robust and effective electrocatalysts with targeted catalytic capability for VO2+/VO2+ on the cathode and V2+/V3+ on the anode are highly desired.
Corresponding author. E-mail address:
[email protected] (Q. Wang).
https://doi.org/10.1016/j.nanoen.2018.07.058 Received 16 June 2018; Received in revised form 20 July 2018; Accepted 25 July 2018 Available online 26 July 2018 2211-2855/ © 2018 Elsevier Ltd. All rights reserved.
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Fig. 1. (a) Schematic illustration of VRB operating upon surface immobilized redox catalysis during charge process. The carbon felt electrodes are grafted with PB and PBA for catalyzed reactions of vanadium species in the respective compartment. (b) Scheme of the targeted redox catalytic reactions of VO2+/ II VO2+ and FeIII 4 [Fe (CN)6·H2O]3 (PB) on the cathode, and that of V2+/V3+ and CrII4[CrIII(CN)6OH]2 (PBA) on the anode during charge process. The above processes reverse upon discharge.
II Herein, Prussian blue (PB, FeIII 4 [Fe (CN)6·H2O]3) and a Cr-based II III Prussian blue analogue (PBA, Cr4 [Cr (CN)6OH]2) material with identical redox potentials to VO2+/VO2+ and V2+/V3+ were judiciously selected and decorated on the cathode and anode (Fig. 1), respectively, to facilitate the reactions of vanadium ions and inhibit the side reactions. The redox-targeted catalytic reactions between VO2+/VO2+ and PB on the cathode and that of V2+/V3+ and PBA on the anode (Fig. 1b) were scrutinized with various electrochemical and spectroscopic techniques. The modified electrodes demonstrated considerably enhanced electrochemical activity towards vanadium species, which substantially improves the power performance and capacity retention of the VRB cell during prolonged cycling test.
vacuum. Carbon felt was then immersed in 10 mL 0.01 M K4Fe(CN)6 solution. The solvent was a mixture of DI water and ethanol with a volume ratio of 1:1. 10 mL 0.02 M FeCl3 solution was dropped into the above K4Fe(CN)6 solution. The carbon felt was collected after reacted for 24 h and washed with DI water and ethanol for several times before finally dried at 60 °C for 24 h under vacuum. 2.2. Characterization The morphology and microstructure of the above materials and electrodes were characterized by a Zeiss Supra 40 field-emission scanning electron microscope (FESEM) at 5 kV. Energy dispersive X-ray spectroscopy (EDX) was recorded at an acceleration voltage of 15 kV. Xray diffraction (XRD) patterns were measured on a Bruker D8 with Cu Kα radiation under an accelerating voltage of 40 kV. Fourier transform infrared (FTIR) spectra were collected with a PerkinElmer Frontier MIR/FIR system by 16 scans with a nominal resolution of 1 cm−1 through an attenuated total reflection (ATR) mode. The X-ray absorption near edge structure (XANES) measurement of PBA was performed at the XAFCA beamline of the Singapore Synchrotron Light Source. The X-ray photoelectron spectroscopy (XPS) analysis was conducted with a Kratos Analytical Axis Ultra DLD spectrometer. Monochromated Al K radiation was used as the radiation source, and all the measurements were taken in vacuum. Carbon, C(1 s), was used as the reference for all the samples. The XPS results were analyzed with XPS Peak software.
2. Material and methods 2.1. Preparation of PB and PBA-modified Carbon Felt All chemicals for this experiment were used directly as received without further purification. For the PBA-modified electrode, the carbon felt was firstly pretreated with 2 M H2SO4 at 80 °C for 10 h, rinsed with plenty of DI water and dried at 60 °C for 24 h under vacuum. A piece of pro-treated carbon felt was then immersed in 10 mL 0.01 M K3Cr(CN)6 solution. The solvent was a mixture of DI water and ethanol with a volume ratio of 1:1. 10 mL 0.02 M CrCl2 solution was dropped into the above K3Cr(CN)6 solution. The reaction was protected by N2. The carbon felt was collected after reacted for 24 h and washed with DI water and ethanol for several times before finally dried in a vacuum oven at room temperature for 24 h. The PBA nanoparticle was synthesized by using the same method without carbon felt. For the synthesis of PB-modified electrode, the carbon felt was only washed with DI water and ethanol and dried at 60 °C for 24 h under
2.3. Electrochemical measurement Cyclic voltammetry (CV) was conducted on an Autolab electrochemical workstation (Metrohm, PSTA30) using a three-electrode system. Saturated calomel electrode (SCE) was used as reference 293
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electrode. A solution containing 0.1 M VO2+ in 2 M H2SO4 was used for tests of positive electrode and 0.1 M V3+ in 2 M H2SO4 was used for the tests of negative electrode. Tafel plot and Rotating disk electrode (RDE) measurement: 12 mg PB powder, 12 mg BG powder, 3 mg carbon black and 3 mg Nafion solution (5 wt.%) were added into 100 mL DI water to make PB paste; 24 mg PBA powder, 3 mg carbon black, and 3 mg Nafion solution (5 wt. %) were added into 100 mL DI water to make PBA paste. A glassy carbon (GC) disk electrode (5 mm in diameter) was polished by using Al2O3 (0.05 µm) paste. 20 µL PB or PBA paste was then dripped onto the GC electrode and repeated for 3 times. The PB/GC and PBA@GC electrodes were dried in an oven at 60 °C for 2 h prior to use. For Tafel measurement, the electrolytes were 100 mM VO2+/VO2+ in 2 M H2SO4 for the positive electrode, 100 mM V2+/V3+ in 2 M H2SO4 for the negative electrode; the scan rate was 0.5 mV s−1. For RDE measurement, the electrolytes were 10 mM VO2+/VO2+ in 2 M H2SO4 or just 2 M H2SO4 for the positive electrode, 10 mM V2+/V3+ in 2 M H2SO4 or just 2 M H2SO4 for the negative electrode. The RDE measurements were conducted by using a Modulated Speed Electrode Rotator (AFMSRCE, PINE). The rotating rate was set at 1600 rpm and the scan rate was 10 mV s−1. All the above electrochemical measurements were performed on an Autolab electrochemical workstation.
electrode (Fig. 2f) shows sluggish reactions with large hysteresis extending to high potentials where the oxygen evolution reaction prevails, and to low potentials amalgamated with the reaction of V3+/ VO2+. In contrast, the PB-modified electrode exhibits nearly reversible wave for both the oxidation of VO2+ and reduction of VO2+. These results along with the above spectroscopic studies suggest that PB has a good electrocatalytic activity towards VO2+/VO2+. The redox catalytic reactions between VO2+/VO2+ and PB/BG during the charge process is illustrated in Fig. 1b and the respective reactions are shown below: III − + II III FeIII 4 [Fe (CN)6⋅H2 O]3 ⇌ Fe4 [Fe (CN) 6 (OH)]3 + 3e + 3H
(1)
III III II 3VO2 + + FeIII 4 [Fe (CN)6 (OH)]3 + 3H2 O ⇌ Fe4 [Fe (CN)6⋅H2 O]3
+ 3VO+2 + 3H+ VO2 + + H2 O ⇌
VO+2 +2H+
+ e−
(2) (3)
Upon charging, the surface-immobilized PB loses electrons and is oxidized to BG which accompanies the release of H+ (Eq. (1)). Given both the PB and VO2+/VO2+ share identical equilibrium potential, the activity changes of various species induce a Nernstian potential difference between the two at different SOC [30], which drives the oxidation of VO2+ by BG when the latter becomes predominant on electrode surface (Eq. (2), see more discussion in SI). The overall reaction is the oxidation of VO2+ to VO2+ (Eq. (3)) while the PB remains unchanged before and after the reaction. Considering PB electrically passivates the electrode surface, such a surface modification is proved to be effective in suppressing the evolution of O2 (see Fig. 2f). Upon discharging, all the above processes reverse and VO2+ is reduced to VO2+ catalyzed by PB.
2.4. VRB single cell test The cells were assembled by sandwiching the different electrodes on the positive and negative sides with a total active area of 13.3 cm2. An untreated Nafion 212 membrane was employed as the separator. Two glass containers placed in both the negative and positive sides were filled with electrolytes containing a mixture of 1.5 M vanadium ions in 2.0 M H2SO4 solution. The volumes of catholyte and anolyte are each 20 mL. Electrolytes were pumped in and out of the cell stack using peristaltic pumps. The charge-discharge performance of the flow cells was investigated in galvanostatic mode with an Arbin battery tester.
3.2. Structural and electrochemical characterizations of PBA-grafted anode In parallel to the cathode, the same concept by employing a PB analogue material (PBA) was applied to the anode to promote the reactions of V2+/V3+. With similar synthetic method, PBA powder with an average size of ~50 nm (Fig. S4a) was prepared to examine its physical properties. The XRD pattern (Fig. S4b) of the PBA powder is indexed to a face-centered-cubic structure (Fm3m) [31], with sharp characteristic peaks commensurate with CrII4[CrIII(CN)6OH]2 (or CrIIN≡C-CrIII). The CV of PBA has a pair of peaks at −0.50 V vs. Ag/AgCl (Fig. S4c) corresponding to the reversible reaction of CrII-N≡C-CrIII and CrII-N≡C-CrII, nearly the same as that of V2+/V3+. PBA nanoparticles were then deposited on carbon felt pretreated by H2SO4 to explore its catalytic properties towards V2+/V3+. The SEM images in Fig. 3a and b show nanoparticulate PBA (~ 50 nm) is uniformly deposited on the surface of carbon felt, consistent with the EDX mapping of N and Cr elements (Fig. S5). To identify the catalytic effect of PBA, CV measurements of pristine and PBA-modified carbon felt were conducted. As shown in Fig. S4d, the reduction of V3+ on pristine carbon felt is distorted because of hydrogen evolution and shows a poorly developed cathodic peak. In contrast, the modified electrode presents much better electrochemical performance with higher anodic and cathodic current and smaller peak potential separation. FTIR and XANES were employed to examine the reactions between PBA and V2+/V3+. As the FTIR spectra shown in Fig. 3c, a new vibration peak assigned to the stretching of N≡C-CrII appeared at 2065 cm−1 after a reaction with V2+ [32–34], which is in contrast to that of N≡C-CrIII at 2185 cm−1 suggesting the reduction of CrII-N≡CCrIII to CrII-N≡C-CrII. Interestingly, the vibration at 2065 cm−1 vanished after immersing the reduced PBA in V3+ electrolyte, indicating N≡C-CrII has been re-oxidized by V3+. Fig. 3d shows the Cr K-edge XANES spectra of PBA before and after reaction with V2+, which shifts towards a lower binding energy indicating a decrease in the average valence of Cr in PBA. Such a reduction of CrIII is consistent with the FTIR measurement.
3. Results and discussion 3.1. Structural and electrochemical characterizations of PB-grafted cathode PB nanoparticles were synthesized and grafted onto carbon felt to explore its catalytic properties towards VO2+/VO2+. The SEM images (Fig. 2a and b) show the nanoparticles with similar shape and size (~ 50 nm) are distributed uniformly on the surface of carbon felt, which is consistent with the EDX mapping in Fig. S1. The CV curves show that PB has a redox potential of 0.90 V (vs. Ag/AgCl) (Fig. S2) corresponding to the reversible reaction of PB/Berlin green (BG), which is nearly the same as that of VO2+/VO2+. When measured in VO2+/VO2+-containing electrolyte, the PB-modified electrode exhibits significantly improved peak current and reduced peak potential difference compared with the pristine carbon felt (Fig. S3), implying a more reversible reaction of VO2+/VO2+. XPS and FTIR spectroscopy were preformed to probe the redox reaction between PB/BG and VO2+/VO2+. The XPS spectrum of pristine PB (Fig. 2c) shows the FeII 2p3/2 and FeII 2p1/2 peak at 708.4 and 721.5 eV, respectively. After reacting with VO2+, two enhanced peaks appear at 710.1 and 723.1 eV revealing the transformation to FeIII [28]. The FTIR spectra in Fig. 2d show similar reaction — compared with the pristine PB, a new peak appears at 2170 cm−1 after reacting with VO2+, which is assigned to the vibration of N≡CFeIII substantiating the oxidation of PB to BG by VO2+ [29]. To examine the kinetics of the redox catalytic reactions between PB and VO2+/VO2+, polarization curves of the electrodes were measured (Fig. 2e). It is evident from the Tafel plots that PB-modified GC electrode has an exchange current density (3.8 × 10−4 A cm−2) two orders of magnitude as high as the pristine electrode (3.0 × 10−6 A cm−2), implying a considerably enhanced kinetics of the modified electrode. The linear sweep voltammetry (LSV) of VO2+/VO2+ on a GC disk 294
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Fig. 2. (a, b) SEM images of PB-modified carbon felt. (c) Fe 2p XPS and (d) FTIR spectra of PB before and after reacted with 1.5 M VO2+ in 2 M H2SO4. (e) Tafel plots of GC and PB-modified GC electrodes measured in 0.1 M VO2+/VO2+ and 2 M H2SO4 electrolyte. (f) LSV curves of an RDE and PB-modified RDE electrodes measured in 10 mM VO2+/VO2+ and 2 M H2SO4 electrolyte, respectively. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article).
believed to arise from the reversible redox reactions between PBA and V2+/V3+ as illustrated in Fig. 1b and stated below:
As shown in Fig. 3e, the polarization curves of the electrodes were measured to quantitatively assess the kinetics of the redox catalytic reactions between PBA and V2+/V3+. The PBA-modified GC electrode presents a 4-fold higher exchange current density (4.6 × 10−6 A cm−2) than the pristine electrode (9.1 × 10−7 A cm−2). While well dwarfed by the cathodic side, it still represents a substantial enhancement of the reaction. The LSV of vanadium electrolyte was tested by using an RDE at 1600 rpm. V2+/V3+ presents fairly sluggish reactions on the GC disk electrode with large voltage hysteresis (Fig. 3f). In contrast, the modified electrode exhibits augmented current with nearly no hysteresis, implying reversible V2+ oxidation during the positive scan and V3+ reduction during the negative scan. The enhanced catalytic current is
Cr II4 [Cr III (CN)6⋅OH]2 + 2H+ + 2e− ⇌ Cr II4 [Cr II (CN)6⋅H2 O]2
(4)
Cr II4 [Cr II (CN)6⋅H2 O]2 + 2V 3 + ⇌ Cr II4 [Cr III (CN)6⋅OH]2 + 2H+ + 2V 2 + (5)
V 3 + + e− ⇌ V 2 +
(6)
Upon charging, the surface grafted CrII-N≡C-CrIII receives electrons from the electrode and is reduced to CrII-N≡C-CrII in the presence of H+ (Eq. (4)), which follows the reduction of V3+ by CrII-N≡C-CrII 295
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Fig. 3. (a, b) SEM images of PBA-modified carbon felt. (c) FTIR spectra of PBA powder at different stages of reactions with V2+ and V3+. (d) XANES spectra of PBA before and after reacted with 0.1 M V2+ in 2 M H2SO4. (e) Tafel plots of GC and PBA-modified GC electrodes measured in 0.1 M V2+/ V3+ and 2 M H2SO4 electrolyte. (f) LSV curves of an RDE and PBA-modified RDE electrodes measured in 10 mM V2+/V3+ and 2 M H2SO4 electrolyte, respectively.
releasing H+ (Eq. (5)). As those on the cathodic side, the Nernstian potential difference between the PBA and V2+/V3+ induced by activity changes relies on the predominance of reduced PBA on the surface. This is especially so at the latter stage of charging. The overall reaction is the reduction of V3+ to V2+ (Eq. (6)) during which the PBA stays unchanged. Another significant effect of PBA is the effective suppression of parasitic H2 evolution upon charging. As shown in Fig. 3f, the PBAmodified electrode reveals considerably enhanced overpotentials for H+ reduction, which deemed to be crucial for the electrolyte stability. Upon discharging, all the above processes reverse and V2+ is oxidized to V3+ catalyzed by PBA.
3.3. VRB cell performance To assess the influence of the surface-immobilized redox catalysts on battery performance, VRB single cells with the modified electrodes were assembled. These cells were tested at a cutoff voltage of 0.70–1.75 V, which is slightly larger than the commonly used voltage range in order to investigate the inhibiting effects of the modified electrodes towards the deteriorating oxygen and hydrogen evolutions upon overcharging. Fig. 4a presents the typical voltage profiles of these cells measured at 50 mA cm−2. The full cell with both electrodes grafted with PB and PBA shows the lowest overpotential and consequently the highest
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Fig. 4. Cell performance of VRBs assembled with modified electrodes. (a) Voltage profiles of the various VRBs at a current density of 50 mA cm−2. (b) Discharge capacity, Coulombic (CE) and energy efficiencies (EE) of VRBs at different current densities. (c) Areal cumulative discharge capacity (ACDC) of VRBs upon prolonged cycling at a current density of 100 mA cm−2. (d) Power performance vs. current density plots of the various VRB cells measured at 100% SOC. 20 mL 1.50 M V2+/ V3+ and 1.50 M VO2+/VO2+ in 2.0 M H2SO4 were used as the anolyte and catholyte in the above cells, respectively. (b-d use the same legends).
capacity loss during prolonged cycling. This could be understood from the above electrochemical studies that the modified electrodes effectively eliminate the parasitic H2 and O2 evolution during the charge process, alleviating the deteriorative electrolyte imbalance problem. This is corroborated by the considerably enhanced EE of the modified cell (Fig. S8). In addition, polarization curves were measured to examine the power performance of the modified VRB cells. As shown in Fig. 4d, the full cell demonstrates a maximum power of ~ 310 mW cm−2 at 373 mA cm−2, noticeably higher than that of the pristine cell which is ~ 178 mW cm−2 at 243 mA cm−2. Such a superior power performance has also shown in Fig. 4b, where the modified cells deliver markedly higher discharge capacities at a range of current. Due to the significant increase in the electrode overpotentials, the pristine cell has actually failed to operate in the galvanostatic tests when the current density is larger than 250 mA cm−2. We notice despite the considerably faster reaction kinetics as determined by the Tafel plots on GC electrodes (Figs. 2e and 3e), the areal power of VRB cell with PB-modified cathode is notably lower than the cell with PBA-modified anode. The discrepancy may arise from the inhomogeneous deposition of PB on the carbon felt. The thick PB layer impedes charge transport and induces larger electrode resistance. We envisage an optimized cathode would further enhance the cell performance.
charge and discharge capacities. The impact of surface modification is also manifested by the subtle differences in the voltage profile at the latter stage of charging. Compared with the slanted rise of cell voltage (above 1.60 V) in the pristine cell, which is largely a result of the parasitic H2 or O2 evolutions retarding the charging, the cell with modified electrodes all present much steeper voltage profile. Such an effect is also verified by the Coulombic efficiency (CE) shown in Fig. 4b. The average CE for pristine cell is 93.5% at 50 mA cm−2, which increases to 94.2%, 95.0% and 95.0% for PB-, PBA-modified and the full cell, respectively. The CE for PB-modified cell is a little lower than that of the PBA-modified one, implying H2 evolution on the anode is presumably a more dictating factor compared with O2 evolution on the cathode [35]. The CE for all the cells increases at higher current density due to reduced crossover. Hence, as a result of reduced overpotential and parasitic reactions, the energy efficiency (EE) of cells with modified electrodes is evidently superior to the pristine cell. The durability of the modified electrodes was evaluated by galvanostatic measurement in VRB cells at a current density of 100 mA cm−2. Areal cumulative discharge capacity (ACDC), regardless of the quantity of electrolytes, is employed to assess the cycling stability of the electrodes (Fig. 4c). The full cell with modified both cathode and anode shows the least deviation from the theoretical line (with zero decay based on the first discharge capacity of the full cell), implying a reduced
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4. Conclusion
[23] D.M. Kabtamu, J.Y. Chen, Y.C. Chang, C.H. Wang, J. Mater. Chem. A 4 (2016) 11472–11480. [24] B.S. a.M. Skyllas-Kanzacos, Electrochim. Acta 37 (1992) 2459–2465. [25] W. Zhang, J. Xi, Z. Li, H. Zhou, L. Liu, Z. Wu, X. Qiu, Electrochim. Acta 89 (2013) 429–435. [26] X. Wu, H. Xu, Y. Shen, P. Xu, L. Lu, J. Fu, H. Zhao, Electrochim. Acta 138 (2014) 264–269. [27] S.M. Taylor, A. Pătru, D. Perego, E. Fabbri, T.J. Schmidt, ACS Appl. Energy Mater. 1 (2018) 1166–1174. [28] L. Shen, Z. Wang, L. Chen, Chemistry 20 (2014) 12559–12562. [29] Z. Liu, G. Pulletikurthi, F. Endres, ACS Appl. Mater. Interfaces 8 (2016) 12158–12164. [30] M. Zhou, Q. Huang, T.N. Pham Truong, J. Ghilane, Y.G. Zhu, C. Jia, R. Yan, L. Fan, H. Randriamahazaka, Q. Wang, Chem 3 (2017) 1036–1049. [31] R. Chen, Y. Huang, M. Xie, Q. Zhang, X. Zhang, L. Li, F. Wu, A.C.S. Appl, Mater. Interf. 8 (2016) 16078–16086. [32] D. Su, A. McDonagh, S.Z. Qiao, G. Wang, Adv. Mater. 29 (2017) 1604007. [33] E.C. Helena Prima-Garcia, Juan P. Prieto-Ruiz, Francisco M. Romero, Nanoscale Res. Lett. 7 (2012) 232–236. [34] E. Coronado, M. Makarewicz, J.P. Prieto-Ruiz, H. Prima-Garcia, F.M. Romero, Adv. Mater. 23 (2011) 4323–4326. [35] D. Aaron, C.-N. Sun, M. Bright, A.B. Papandrew, M.M. Mench, T.A. Zawodzinski, ECS Electrochem. Lett. 3 (2013) A29–A31.
In conclusion, we have demonstrated a pair of low-cost redox catalysts, PB and PBA, grafted on carbon felt electrodes, and examined their respective electrocatalytic activity towards VO2+/VO2+ and V2+/ V3+ for vanadium redox-flow batteries. Spectroscopic and electrochemical studies have unambiguously elucidated that the considerably promoted electrode reactions of vanadium ions stem from the reversible proton-coupled redox targeting reactions between the surface-immobilized PB or PBA and the respective vanadium ions, which lead to an enhancement of the reaction kinetics by many folds. These redoxmediated catalytic reactions have good selectivity to the parasitic H2 and O2 evolution reactions upon charging and stabilize the electrolyte compositions upon prolonged cycling. As a result, the VRB cells assembled with the modified electrodes exhibit substantially boosted performance in terms of energy efficiency, capacity retention and power. To further enhance the performance, the deposition of the PB or PBA material on carbon felt should be better controlled with improved uniformity and reduced thickness, so that it won’t impede charge transport. We anticipate the redox catalysis disclosed in this study provide a credible approach to the performance enhancement of vanadium redox-flow batteries, and more broadly to other redox-flow battery chemistries.
Dr. Feifei Zhang obtained her Ph.D. degree from Changchun Institute of Applied Chemistry, Chinese Academy of Sciences in 2016. She is currently a research fellow in the Department of Materials Science and Engineering, National University of Singapore. Her research interest is in the development of redox flow batteries and lithium-sulfur batteries.
Acknowledgements This research was supported by the Energy Market Authority, Singapore under its Energy Innovation Research Program – Energy Storage (NRF2015EWT-EIRP002). Appendix A. Supporting information
Mr. Songpeng Huang received his B.S. degree from National University of Singapore. He is now a research engineer in the Department of Materials Science and Engineering at National University of Singapore under supervision of Associate Prof. Wang Qing. His research is focused on optimization of operation conditions for vanadium flow battery.
Supplementary data associated with this article can be found in the online version at doi:10.1016/j.nanoen.2018.07.058. References [1] B. Dunn, H. Kamath, J.-M. Tarascon, Science 334 (2011) 928–935. [2] F. Pan, Q. Wang, Molecules 20 (2015) 20499–20517. [3] K. Lin, Q. Chen, M.R. Gerhardt, L. Tong, S.B. Kim, L. Eisenach, A.W. Valle, D. Hardee, R.G. Gordon, J.A. Michael, M.P. Marshak, Science 349 (2015) 1529–1532. [4] Y. Zhao, H. Zhang, C. Xiao, L. Qiao, Q. Fu, X. Li, Nano Energy 48 (2018) 353–360. [5] A. Cunha, J. Martins, N. Rodrigues, F.P. Brito, Int. J. Energy Res 39 (2015) 889–918. [6] S. Kim, J. Choi, C. Choi, J. Heo, D.W. Kim, J.Y. Lee, Y.T. Hong, H.T. Jung, H.T. Kim, Nano Lett. (2018), https://doi.org/10.1021/acs.nanolett.8b01429. [7] M. Park, I.-Y. Jeon, J. Ryu, H. Jang, J.-B. Back, J. Cho, Nano Energy 26 (2016) 233–240. [8] C. Busacca, O.D. Blasi, N. Briguglio, M. Ferraro, V. Antonucci, A.D. Blasi, Electrochim. Acta 230 (2017) 174–180. [9] M. Park, J. Ryu, J. Cho, Chem. Asian J. 10 (2015) 2096–2110. [10] M. Park, I.-Y. Jeon, J. Ryu, J.-B. Baek, J. Cho, Adv. Energy Mater. 5 (2015) 1401550–1401557. [11] Q. Deng, P. Huang, W.-X. Zhou, Q. Ma, N. Zhou, H. Xie, W. Ling, C.-J. Zhou, Y.X. Yin, X.-W. Wu, X.-Y. Lu, Y.-G. Guo, Adv. Energy Mater. 7 (2017) 1700461. [12] Z. González, S. Vizireanu, G. Dinescu, C. Blanco, R. Santamaría, Nano Energy 1 (2012) 833–839. [13] T. Liu, X. Li, H. Nie, C. Xu, H. Zhang, J. Power Sources 286 (2015) 73–81. [14] D.J. Suarez, Z. Gonzalez, C. Blanco, M. Granda, R. Menendez, R. Santamaria, ChemSusChem 7 (2014) 914–918. [15] Y. Liu, Y. Shen, L. Yu, L. Liu, F. Liang, X. Qiu, J. Xi, Nano Energy 43 (2018) 55–62. [16] L. Wu, Y. Shen, L. Yu, J. Xi, X. Qiu, Nano Energy 28 (2016) 19–28. [17] Y. Ji, J.L. Li, S.F.Y. Li, J. Mater. Chem. A 5 (2017) 15154–15166. [18] C. Ding, X. Ni, X. Li, X. Xi, X. Han, X. Bao, H. Zhang, Electrochim. Acta 164 (2015) 307–314. [19] H. Kabir, I.O. Gyan, I. Francis Cheng, J. Power Sources 342 (2017) 31–37. [20] M. Zago, A. Casalegno, Electrochim. Acta 248 (2017) 505–517. [21] B. Li, M. Gu, Z. Nie, Y. Shao, Q. Luo, X. Wei, X. Li, J. Xiao, C. Wang, V. Sprenkle, W. Wang, Nano Lett. 13 (2013) 1330–1335. [22] B. Li, M. Gu, Z. Nie, X. Wei, C. Wang, V. Sprenkle, W. Wang, Nano Lett. 14 (2014) 158–165.
Mr. Xun Wang obtained his Bachelor of Engineering degree from the National University of Singapore. He is now pursuing Ph.D. degree in Materials Science and Engineering at the National University of Singapore under the supervision of Prof. Wang Qing. His work involves designing solid condensed phase materials for the aqueous redox flow batteries.
Chuankun Jia is a professor and the leader of "the 100 Talented Team of Hunan Province". His research focuses on designing materials for the applications of energy conversion and storage. He has 11 years’ expertise in flow battery systems and he has designed a series of high energy density redox flow batteries, with the related works published in Science Advances, Nature communications, Chem and other journals. He has published 2 book chapters and filed more than 10 patents.
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Qing Wang is an associate professor at National University of Singapore. His research focuses on the fundamental understanding of charge transport/transfer in mesoscopic electrochemical systems and their applications for advanced energy conversion and storage. He currently leads a team working on redox targeting-based flow battery for large-scale energy storage.
Dr. Yonghua Du is the Principal Beamline Scientist of Institute of Chemical and Engineering Sciences (ICES), Agency for Science, Technology and Research (A*STAR), Singapore. He received the Ph.D. in 2007 from the Institute of High Energy Physics at Chinese Academy of Sciences, China and worked in same place as postdoctoral researcher from 2007 to 2009. His research focuses on XAFS techniques and applications.
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