New ZincâVanadium (ZnâV) Hybrid Redox Flow Battery: High-Voltage

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New Zinc-Vanadium (Zn-V) Hybrid Redox Flow Battery: High Voltage and Energy Efficient Advanced Energy Storage System Mani Ulaganathan, Subramanian Suresh, Mariyappan Karuppusamy, Padikassu Periasamy, and Ragupathy Pitchai ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06194 • Publication Date (Web): 18 Feb 2019 Downloaded from http://pubs.acs.org on February 20, 2019

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ACS Sustainable Chemistry & Engineering

New Zinc-Vanadium (Zn-V) Hybrid Redox Flow Battery: High Voltage and Energy Efficient Advanced Energy Storage System Mani Ulaganathana,b,*, Subramanian Suresha,b, Mariyappan Karuppusamy a, Padikkasu Periasamy a,b and Ragupathy Pitchaia,b,* aFlow

Battery Section, Electrochemical Power Sources Division, CSIR-Central Electrochemical Research Institute, Karaikudi, Tamil Nadu, 630 003, India. bAcademy

of Scientific and Innovative Research, CSIR-Campus, New Delhi, India

*Corresponding Author: [email protected] [email protected] (P. Ragupathy). Tel: +91 4565 241109 (off)

(M.

Ulaganathan),

Abstract Herein for the first time, we have reported the performance and characteristics of new high voltage zinc-vanadium (Zn-V) metal hybrid redox flow battery using zinc bromide (ZnBr2) based electrolyte. The Zn-V system showed an open circuit voltage of 1.85 V, which is very close to that of zinc-bromine flow cell. The obtained results exhibited a voltaic, coulombic, and energy efficiency of 88, 82 and 72% at 20 mA cm-2, respectively, in which low-cost microporous membrane was used as a separator. On the other hand, the cell tested using Nafion-117 membrane showed voltaic, coulombic, and energy efficiency of 84, 83 and 71%, respectively at a current density of 20 mA cm-2. Furthermore, the Zn-V cell performance is also compared with the Zn-Br2 flow system to highlight the advancement of the new Zn-V system. The cell also showed honest performance upto 50 cycles.

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Keywords: Zinc-Vanadium; Metal hybrid redox flow battery; High voltage; Energy density; Cell efficiency.

Introduction To develop low-cost, high performance and environmentally benign energy storage devices that can meet the ever increasing energy demand at various levels has become very essential1-6. In recent years, renewable energy resources are extensively dominant because of eco-friendliness, profuse availability, low-cost, and importantly zero emission of CO2 which supports reduced global warming rate. In this regard, redox flow batteries are widely recognized as a potential candidate for storing large-scale electricity due to their flexibility in design, high energy density, good rate ability, efficient round-trip efficiency, long cycle life, low capital cost and low self-discharge2, 7. Further, the redox flow batteries in practice reduce the issues associated with intermittent fluctuation in renewable energy sources and thus cutting down the exploited use of fossil fuels1. Moreover, the decoupling of energy density and power density is not possible with the other existing conventional batteries utilizing solid active materials. Based on the redox couple involved in the cell reactions, RFBs have been named as Fe/Cr8, Fe/V9, allvanadium10-14, Zn/Br2(ZBB)15-20, Zn/polyiodide and Li/polyiodide21, Iron-Chloride22 and Zn/Fe2324.

Among these, all vanadium redox flow battery (VRFB), and Zn-Br2 have been

commercialized successfully; but they still suffer from intrinsic disadvantages such as low energy density, corrosive electrolytes raising safety concerns, and dendrite formation. For instance, all VRFB developed during the 1980s owes to some of the major issues such as low energy density (25 Wh l-1) and high-cost of ion selective membranes25. Similarly, elemental bromine generated during the charging process in Zn-Br2 flow batteries has been of critical concern for its success. Further, bromine corrosion caused to all the key components used in the


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flow battery is inevitable and it strictly affects the overall cell efficiency. So far many efforts have been paid to address these critical challenges. It is noteworthy to mention that quaternary salt based compounds such as N‐ethyl‐N‐methyl‐morpholinium bromide (MEM) and N‐ethyl‐N‐methyl‐pyrrolidinium bromide (MEP) are used as suitable complexing agent to capture the elemental bromine26 but the ionic liquids are expensive in nature; besides, as anticipated the addition of complexing agent does not cause any improvement in the cell performance upon cycling26. In general, the broadening of operating voltage and increasing the concentration of electrolytes are other important strategies for enhancing the energy density of RFBs27. In these aqueous based flow batteries, as solute from the electrolyte is surrounded by large amounts of water molecules forming a solvation complex as large species hinder high concentration resulting in poor energy density. Therefore, it is highly desired to develop new flow battery chemistry with low-cost, high energy density and excellent durability9. Recently, various approaches have been attempted to enable a remarkable innovation in this field which includes many exciting advances, tailored redox species, novel battery structures, low-cost and high-performance membranes28-34. For instance, Zhang et.al35 has reported a low-cost iron-aluminum (Fe-Al) battery which delivers the high energy density of 166 Wh l-1 with reasonable operating cell voltage. However, this system cannot be considered as a viable storage option as it involves various undesired organic solvents. Recently, organic based redox flow batteries are being projected as next-generation storage technology. However, the intrinsic problem of low conductivities and solubility of most organic redox species still remain as challenges3, 28.



Experimental section: Cell assembly and characterizations: A schematic representation of working principle of the Zn-V flow cell used in the present investigation is show in Scheme 1. In a flow cell fabrication, given graphite composite plate (6 mm) is grooved with a depth of 1 mm on the surface in which the carbon felt was placed to provide an active site to the electrochemical reactions. It is crucial that the selection of membrane in a battery system where the plating process is involved; therefore, the dendrite growth will be effectively minimized during the cell operands. To understand the properties, here the cell characteristics were analyzed using both microporous separator (Daramic, 1 mm thickness, Polyethylene battery separator) and Nafion-117 membrane, separately.

Freshly prepared anolyte and catholyte were circulated through the given flow

direction via negative and positive half cells, respectively. A lab-scale cell setup used to study the characteristics of the proposed novel Zn-V RFB system is depicted in the below photograph (figure 1). 80 ml of 3M ZnBr2 +1M ZnCl2 + 1:1 M MEP: MEM electrolyte solution was prepared and used as negative electrolyte (anolyte). On the other hand, the given volume of as prepared 1.7 M of V3.5+ containing 4 M H2SO4 solution was employed as a catholyte in the positive half cell; further, the flow rate of the electrolytes are maintained at 20 ml.min-1 on both sides. The GCD performance was carried out using constant current mode (each charge & discharge at 1 h) in which the charging voltage is not fixed and the cell was discharged to lower voltage limit of 1.0 V. The cell was tested at the different current rate starting from 10 to 50 mA.cm-2. All the cell parameters were evaluated at an ambient temperature. Cyclic Voltammetry (CV) and


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electrochemical impedance spectral (EIS) measurements were carried with help of electrochemical workstation model Solartron 1470E, UK at a half cell configuration. Results and Discussion Inspired by the outstanding electrochemical behavior of zinc and vanadium, here we have demonstrated for the first time an aqueous based metal hybrid Zn-V redox flow battery with high energy density and excellent current density compared to other conventional flow batteries. This new Zn-V system utilizes simple salts and oxides such as ZnBr2, ZnCl2, V2O5, and V2O3 for electrolyte preparation. Further, the redox potential of V4+/V5+ (1.0 vs SHE) occurs merely close to the potential of Br-/Br2 (1.08 vs SHE) redox reaction. This proposed Zn-V system delivered high theoretical energy density (559.68 Wh.kg-1) over other established systems in particular ZnBr2 (438.45 Wh.kg-1)36-37. A proof of concept study of this Zn-V system with a high theoretical energy density (559.68 Wh.kg-1) than the other systems particularly Zn-Br2 is expected to make a significant impact on the large-scale storage of electricity. The working principle of the presented new

Zn-V

flow

battery

is

shown

in scheme

1, in

which

Zn2+/Zn0

(anode)

and

VO2+/VO2+ (cathode) were used as a redox couples. With an aim to understand the redox behavior of zinc and vanadium species, cyclic voltammograms were recorded at a scan rate of 50 mV s-1. Accordingly, figure 2a shows the CV curves of zinc and vanadium redox reactions indicating the characteristic behavior of both metals. From the CV curve, it is clearly showed that the proposed half-cell reaction i.e transition from V4+ to V5+ in the positive half-cell starts at a potential of ~0.97 V vs Ag/AgCl (conversion of V4+ to V5+) whereas the zinc reduction i.e Zn2+ to Zn0 occurred on the negative potential of 0.88 V vs Ag/AgCl38. The CV curves for the graphite electrode used in the present work



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recorded at various sweep rates ranging from 5 to 50 mV.s-1 in 1.7 M V3.5+ and 3M ZnBr2 electrolytes are shown in figure S1 and S2. The cell reaction to the proposed new system is given below: Negative half cell reaction: Zn0↔ Zn2+ + 2e-

(-0.76 V vs SHE)

------- (1)

(1.00 V vs SHE)

------- (2)

Positive half cell reaction: VO2++2H+ + e-↔VO2+ + H2O Over all cell reaction: 2VO2++4H+ +Zn0 ↔ 2VO2+ + Zn2+ + 2H2O

(1.76 V)

------- (3)

The CV curves recorded in a catholyte solution indicate the excellent reversibility and clearly revealing the existence of the controlled diffusion process figure S1. At the same time, in the case of anolyte (Figure S2), the CV curves show sharp peaks which show the surface controlled process of Zn plating. This behavior is well correlated with the reported results on Zn plating [17]. From the CV curves, it is observed that the largest potential difference of 1.85 V is established between the anode and cathode half-cell reactions. Very interestingly, this value is almost equal to the open circuit potential of Zn-Br2. Further, the obtained cell voltage is higher than that of all VRFB systems (1.26 V). From the above CV curve analysis, due to the high cell operating voltage, it is expected that the energy density of the cell will be higher when compared with the all-vanadium redox flow battery system. To further understand the insights into the performance characteristic of the system, EIS spectra of the electrodes were recorded and the results are shown in figure S3. Impedance results clearly indicate the diffusion controlled process of both positive and negative half cells. The plot showed the solution resistance and charge transfer resistance of the graphite electrode materials


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used in the present investigation. For both ZnBr2 and vanadium electrolytes, the graphite electrode shows charge transfer resistance value less than 5 Ω. Moreover, the presence of smaller range of semicircle in the vanadium electrolyte indicates the faster electron transfer reaction of the electrode materials. The performances characteristics of the present Zn-V flow cell fabricated using Nafion117 were evaluated at different current density ranging from 10 to 50 mA cm-2. Thus, the GCD profile (second cycle of each current density) recorded for the Zn-V cell is shown in figure 2b. The GCD plateau of cell shows an excellent performance in terms of voltage efficiency of about 91%. As expected, the voltage drop increased linearly with increase in the current density. Here, the observed voltage drop is mainly due to the resistance of flow of electrons through the electrode materials. Further, the voltage drop is increased from 0.099 V (10 mA.cm-2) to 0.85 V at 50 mA.cm-2. For better understanding, the performance characteristics of the new Zn-V metal hybrid system have also been compared with the Zn-Br2 RFB. It was noted that the Zn-Br2 cell shows a huge voltage drop of 191 mV when compared to the Zn-V system (96 mV) tested at the same current density of 10 mA cm-2 (Figure 2c). Further, the GCD performance tested at different current densities for the Zn-Br2 flow battery system is shown in figure S4. The voltage drop measured at different current densities for both Zn-V and Zn-Br2 were compared and the plot is shown in figure S5. It is worth to mention that Zn-V system shows better performance than that of Zn-Br2 even at high current density. The voltage drop difference is very high for the Zn-Br2 system than the Zn-V at all tested current density. For instance, Zn-V flow cell shows a potential drop of 270 mV, which is threefold lower than the voltage drop of 693 mV (Figure S5) obtained for the Zn-Br2 system at 40 mA.cm-2. It clearly reveals the excellent reversibility and good conductivity of the electrolyte of the Zn-V system. This increased voltage drop of Zn-



Br2 also indicates the poor electrode kinetics of the electrode materials against the redox couples which is also directly related to the conductivity of the electrolyte. Importantly, the cell voltage of Zn-V (1.85 V) is well comparable and almost closer to the potential of Zn-Br2 (1.84 V) cell. More importantly, the cell potential (1.85 V) of the newly designed Zn-V flow cell is higher than that of Fe/V[4]11 (1.02 V) and all vanadium[5e]

16 (1.26

V) systems. Since the energy density

mainly depends on the ion concentration and the cell potential of the system, it is expected that the present system will deliver high energy density than the other conventional systems likely, Zn-Br2 (60-70 Wh.l-1) all vanadium (10-25 Wh.l-1) and Fe/V[21]. Hence, it is clearly shown that the studied new Zn-V system will be a potential replacement for the exiting Zn-Br2 and allvanadium redox flow battery systems. Furthermore, the GCD profile showed huge mass transport loss in both cases which is a usual phenomenon in flow battery because during the GCD process, there is a huge change in the electrolyte concentration. During the Zn deposition, the ion concentration in the electrolyte becomes lower and there are not enough charge carriers available at the electrode surfaces; in addition, electrolyte conductivity will be reduced; thus, it leads to huge mass transport. It can be minimized by adding excess of electrolyte and or by modifying the electrolyte medium. For example, in a typical Zn-Br2 battery electrolyte, ZnCl2, NaCl have been added as an additive to balance the conductivity and for tuning the pH of the electrolyte16. It was also observed that the Ohmic region of the discharge curve has improved with an increase in the rated current. The ZnV cell showed better Ohmic region at 40 mA cm-2 compared to the cell performance at 10 mA cm-2. This enhanced performance is mainly attributed to the increase of state-of-charge of the electrolyte and may be due to the depletion of the residual Zn at this high current rate. Further, the cells were subjected to longtime charge-discharge cycling (2 h charge and 2 h discharge).


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It was noted that the cell showed a huge over potential drop (195 mV) (Figure 3a) when compared with the cell tested for 1 h charge and discharge (93 mV). This indicates the ion crossover from positive to negative side which caused an imbalance of the electrolyte concentration. It is well-known that ion exchange membrane (IEM) holds 50% of the total cost of the system.

In

the

present

study,

Nafion-117 is

used as IEM,

although

it

shows appreciable performance, it is always desirable to find replacement for Nafion based membrane for feasible cost reduction of the system. Here, we have also used the thick Daramic microporous separator to study the performance of the Zn-V system which will help to minimize the effect of dendrites growth. Figure 3b shows the GCD profile of the Zn-V flow system by employing the Daramic microporous separator and Nafion-117 IEM. The cell tested using microporous separator shows enhanced Ohmic region than the Nafion-117 membrane employed cell, tested at a current density of 10 mA cm-2. The cell configured using microporous separator delivered a high capacity of 85 mAh cm-2 than the Nafion117 based system (71 mAh cm-2) at 10 mA cm-2. Thus, the improved performance of the microporous separator will support for further cost reduction of new Zn-V flow battery. However, the microporous based cell shows capacity fading in successive cycles. This may be due to the ion crossover from positive to negative compartments through the pores of the microporous separator, in the present study a noticeable V4+ crossover is observed at ppm level. It is noteworthy here that the crossover of vanadium (V4+) through the microporous separator to the anode side does not influence the cell potential since the anodic deposition of the zinc is well above the vanadium transition potential (V4+/V3+). Thus, the crossed V4+ will convert to V3+ and undergoes V3+ to V2+ conversion in the successive GCD cycles. As a result, the self-discharge of



the flow system getting faster; hence, the ion crossover should be minimized by placing a suitable membrane to get better performance. It was observed that elemental Br2 (during the charge) is diffused to the positive side while using the microporous separator in the flow cell. It is believed that various chemical reactions are anticipated when elemental bromine reacts with vanadium metal on the cathode side and causes self-discharge of the cell. The possible side reactions during the cell operation are given below: Br2+H2O → HBr + HBrO

---------------- (4)

Br2+ V4+→ VBr2

---------------- (5)

Thus, the by-product formations likely VBr2, HBr will cause severe corrosive issues which will strictly affect the cell components and the overall performance. Furthermore, the cross diffusion ions will lead to severe self-discharge of the cell. These limitations will be minimized by using pore-filled membranes; Recently, Kim et al39., successfully demonstrated the effect of ultra-thin Nafion filled membrane for Zn-Br2 RFBs; The pore-filled membrane will effectively block the Br2 ion crossover, hence the efficiency of the system can increased to 77.7%. Since the major cost of the flow system depends on the nature of membrane, the use of microporous membrane not only effectively reduces the crossover of electrolyte, but also has a significant impact on the total cost of the flow system. In order to confirm the crossover from the negative to positive side, the cycled electrolyte (after 20 cycles) have been subjected for CV studies and the obtained CV curves are shown in figure S7. It is clearly, noted that there were small traces of Br-/Br2 reaction were observed while using microporous separator, which indicates the presence of bromine reaction. However, further investigation is required for supporting this result.


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The Zn-V cell delivered a maximum discharge capacity of 390 mAh cm-2 at a current density of 50 mA cm-2 (GCD profile obtained at 50 mA cm-2, Figure S6) and the corresponding estimated energy density is 16 Wh.l-1. This indicates the effective utilization of Zn during the charge-discharge process even at a high current rate. Rated power for the Zn-V flow system at different current rate has been calculated and the corresponding plot of rated power vs current density is shown in figure S8. The cycle life of the Zn-V cell employing Nafion-117 membrane is evaluated at a current density of 20 mA cm-2 for 30 mins each charge and discharge; first five cycles of the cell tested at this current density is shown in figure 3c. The cell exhibits steady performance with increased efficiency. Figure 3d shows the characteristics plot of cycle number vs efficiency of the cells. It is noted that the cell voltage efficiency has increased from 84 to 85.9% within the studied cycle range, whereas, the coulombic efficiency has decreased from 83.46% to 78.5%. It also depicts a promising energy efficiency value which is merely 71% at the initial cycle, these obtained preliminary values are well comparable with the recently reported other aqueous hybrid redox flow battery system15-16, 40 (Table S1). The increasing trend in the voltage efficiency indicates the high reversibility of Zn and the increased state of charge for the electrolyte system. Overall the cell exhibits good voltaic efficiency over first 50 cycles which will favor in improving the performance of the new Zn-V system even at stack level, in particular, this will improve the power density of the stack. In order to check the reproducibility of the present system, the cell was fabricated with same configuration and their cell characteristics were evaluated at 20 mA cm-2. The standard deviation is calculated for each of the values and the obtained results are shown in figure S9. Further, the cell fabricated using the microporous separator is subjected to ensure improved cycle life of the cell. The cell is cycled upto 25 cycles and the obtained results



are shown in figure S10. The cell has vanadium ion crossover, from positive to negative side and was observed visually by color change. Thus, the cycle life has been studied only up to 25 cycles. It is well documented that voltage drops in the electrochemical cell mainly depends on various factors such as electrode polarization which is mainly due to the electrode kinetics, Ohmic loss, and mass transfer loss associated with bulk reagent delivery to the electrode41. In this study, polarization curves were recorded at charging the cell for about 1 h by applying a steady current of 20 mA cm-2. The cell was discharged for different current starting from 20 mA to 850 mA by stepping 20 mA (2 mA cm-2) for a constant time interval of 30 sec. The polarization behavior has also been recorded for different drain current rate likely 5 (by step 50 mA) and 10 mA cm-2 (by step 100 mA). The recorded polarization plot is shown in figure 4a-c. It was noted that the voltage drop is linearly varied with the current density in the Ohmic region, this indicates that transport process is mainly due to the ionic transport; it was noted that the kinetic polarization was decreased by increasing the drain current rate. Further, the power density was increased while increasing the drain current rate. The improved performance in the Ohmic region was entailing the better contact of the electrode materials. In figure 4a, a huge starvation was observed due to the mass transfer above 650 mA. This is mainly due to the ion transport between the active layers. These mass transfers were improved when drained at high current (50 mA steps for 30 s and 100 mA for 30 s) form the cell. On the other hand, the cell delivered the power density of 810, 826 and 840 mW cm-2 at 2, 5, 10 mA.cm-2 which indicates the good rate capability of the present flow cell (Fig. 4a-c). The obtained power density value is two times higher than the power density achieved in the recent alkaline flow battery systems41-42. Thus, the achieved high power density is considered as one of the prime key factors for high


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performance in the reported new Zn-V system. The obtained results are also well correlated with the power density calculated from the GCD studies. Conclusion In conclusion, new Zn-V metal hybrid redox flow battery was successfully demonstrated using Zn2+/Zn0 and V5+/V4+ redox couples. This system exhibits high voltage (1.85 V) which is merely close to the voltage window (1.84 V) of Zn-Br2 flow cell. Further, the energy density of Zn-V flow battery is comparable with the Zn-Br2 and all-vanadium flow battery. It is an added advantage for Zn-V system that the vanadium-based electrolyte is highly stable compared to the bromine counterpart as in the case of the Zn-Br2 flow battery. The Zn-V system shows better performance in terms of the over potential drop which is two times lower than that of the Zn-Br2. This new system delivered as high as power density of 840 mW cm-2. Thus, high voltage, high power density, high energy density, eco-friendliness, low electrode polarization, and high-efficiency of the present Zn-V system enables the development of a viable futuristic off-grid energy storage technology.

Acknowledgement Dr. M. U and K. M. thanks the financial support from Science & Engineering Research Board (SERB), a statutory body of the Department of Science & Technology, Govt. of India through Ramanujan Fellowship (SB/S2/RJN-082/2016).



ANCILLARY INFORMATION Supporting Information CV and EIS curve of the graphite electrode in vanadium and ZnBr2 electrolytes, GCD profile of the Zn-Br single flow cell, Voltage drop comparison of ZnBr2 and Zn-V, GCD profile of Zn-V at 50 mA/cm2, Rated power vs current rate plot are shown in figure S(1-8). Standard deviation Coulombic, Voltage and Energy efficiency, Cycle life, GCD curve obtained at different cycles of the Zn-V cell having micro porous separator are shown in S(9and 10). Performance comparisons of the other flow battery with the Zn-V system (Table S1). References: 1. Wang, W.; Sprenkle, V., Redox flow batteries go organic. Nature Chemistry 2016, 8, 204, doi 10.1038/nchem.2466. 2. Park, M.; Ryu, J.; Wang, W.; Cho, J., Material design and engineering of next-generation flow-battery technologies. Nature Reviews Materials 2016, 2, 16080, doi 10.1038/natrevmats.2016.80. 3. Luo, J.; Hu, B.; Debruler, C.; Liu Tianbiao, L., A π-Conjugation Extended Viologen as a Two-Electron Storage Anolyte for Total Organic Aqueous Redox Flow Batteries. Angewandte Chemie International Edition 2017, 57 (1), 231-235, doi 10.1002/anie.201710517. 4. Li, W.; Fu, H.-C.; Li, L.; Cabán-Acevedo, M.; He, J.-H.; Jin, S., Integrated Photoelectrochemical Solar Energy Conversion and Organic Redox Flow Battery Devices. Angewandte Chemie International Edition 2016, 55 (42), 13104-13108, doi 10.1002/anie.201606986. 5. Wedege, K.; Azevedo, J.; Khataee, A.; Bentien, A.; Mendes, A., Direct Solar Charging of an Organic–Inorganic, Stable, and Aqueous Alkaline Redox Flow Battery with a Hematite Photoanode. Angewandte Chemie International Edition 2016, 55 (25), 7142-7147, doi 10.1002/anie.201602451. 6. Perry, M. L., Expanding the chemical space for redox flow batteries. Science 2015, 349 (6255), 1452, doi 10.1126/science.aad0698. 7. Gong, K.; Xu, F.; Grunewald, J. B.; Ma, X.; Zhao, Y.; Gu, S.; Yan, Y., All-Soluble AllIron Aqueous Redox-Flow Battery. ACS Energy Letters 2016, 1 (1), 89-93, doi 10.1021/acsenergylett.6b00049. 8. Lopez-Atalaya, M.; Codina, G.; Perez, J. R.; Vazquez, J. L.; Aldaz, A., Optimization studies on a Fe/Cr redox flow battery. Journal of Power Sources 1992, 39 (2), 147-154, doi 10.1016/0378-7753(92)80133-V. 9. Wang, W.; Kim, S.; Chen, B.; Nie, Z.; Zhang, J.; Xia, G.-G.; Li, L.; Yang, Z., A new redox flow battery using Fe/V redox couples in chloride supporting electrolyte. Energy & Environmental Science 2011, 4 (10), 4068-4073, doi 10.1039/C0EE00765J. 10. Maharjan, M.; Bhattarai, A.; Ulaganathan, M.; Wai, N.; Oo, M. O.; Wang, J.-Y.; Lim, T. M., High surface area bio-waste based carbon as a superior electrode for vanadium redox flow battery. Journal of Power Sources 2017, 362, 50-56, doi 10.1016/j.jpowsour.2017.07.020.


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24. Xie, C.; Duan, Y.; Xu, W.; Zhang, H.; Li, X., A Low-Cost Neutral Zinc–Iron Flow Battery with High Energy Density for Stationary Energy Storage. Angewandte Chemie International Edition 2017, 56 (47), 14953-14957, doi 10.1002/anie.201708664. 25. Skyllas‐Kazacos, M.; Rychcik, M.; Robins, R. G.; Fane, A. G.; Green, M. A., New All‐Vanadium Redox Flow Cell. Journal of The Electrochemical Society 1986, 133 (5), 10571058, doi 10.1149/1.2108706. 26. Winardi, S.; Poon, G.; Ulaganathan, M.; Parasuraman, A.; Yan, Q.; Wai, N.; Lim, T. M.; Skyllas–Kazacos, M., Effect of Bromine Complexing Agents on the Performance of Cation Exchange Membranes in Second-Generation Vanadium Bromide Battery. ChemPlusChem 2014, 80 (2), 376-381, doi 10.1002/cplu.201402260. 27. Li, B.; Nie, Z.; Vijayakumar, M.; Li, G.; Liu, J.; Sprenkle, V.; Wang, W., Ambipolar zinc-polyiodide electrolyte for a high-energy density aqueous redox flow battery. Nature Communications 2015, 6, 6303, doi 10.1038/ncomms7303. 28. Hu, B.; DeBruler, C.; Rhodes, Z.; Liu, T. L., Long-Cycling Aqueous Organic Redox Flow Battery (AORFB) toward Sustainable and Safe Energy Storage. Journal of the American Chemical Society 2017, 139 (3), 1207-1214, doi 10.1021/jacs.6b10984. 29. Tang, C.; Zhou, D., Methanesulfonic acid solution as supporting electrolyte for zincvanadium redox battery. Electrochimica Acta 2012, 65, 179-184, doi 10.1016/j.electacta.2012.01.036. 30. Oh, K.; Ketpang, K.; Kim, H.; Shanmugam, S., Synthesis of sulfonated poly(arylene ether ketone) block copolymers for proton exchange membrane fuel cells. Journal of Membrane Science 2016, 507, 135-142, doi 10.1016/j.memsci.2016.02.027. 31. Aziz, M. A.; Shanmugam, S., Zirconium oxide nanotube–Nafion composite as high performance membrane for all vanadium redox flow battery. Journal of Power Sources 2017, 337, 36-44, doi 10.1016/j.jpowsour.2016.10.113. 32. Park, J.-H.; Ramasamy, P.; Kim, S.; Kim, Y. K.; Ahilan, V.; Shanmugam, S.; Lee, J.-S., Hybrid metal–Cu2S nanostructures as efficient co-catalysts for photocatalytic hydrogen generation. Chemical Communications 2017, 53 (22), 3277-3280, doi 10.1039/C7CC00071E. 33. Jiang, B.; Wu, L.; Yu, L.; Qiu, X.; Xi, J., A comparative study of Nafion series membranes for vanadium redox flow batteries. Journal of Membrane Science 2016, 510, 18-26, doi 10.1016/j.memsci.2016.03.007. 34. Jiang, B.; Yu, L.; Wu, L.; Mu, D.; Liu, L.; Xi, J.; Qiu, X., Insights into the Impact of the Nafion Membrane Pretreatment Process on Vanadium Flow Battery Performance. ACS Applied Materials & Interfaces 2016, 8 (19), 12228-12238, doi 10.1021/acsami.6b03529. 35. Zhang, L.; Zhang, C.; Ding, Y.; Ramirez-Meyers, K.; Yu, G., A Low-Cost and HighEnergy Hybrid Iron-Aluminum Liquid Battery Achieved by Deep Eutectic Solvents. Joule 2017, 1 (3), 623-633, doi 10.1016/j.joule.2017.08.013. 36. Wu, M. C.; Zhao, T. S.; Zhang, R. H.; Wei, L.; Jiang, H. R., Carbonized tubular polypyrrole with a high activity for the Br2/Br− redox reaction in zinc-bromine flow batteries. Electrochimica Acta 2018, 284, 569-576, doi 10.1016/j.electacta.2018.07.192. 37. Wu, M.; Zhao, T.; Zhang, R.; Jiang, H.; Wei, L., A Zinc–Bromine Flow Battery with Improved Design of Cell Structure and Electrodes. Energy Technology 2017, 6 (2), 333-339, doi 10.1002/ente.201700481, . 38. Gallaway, J. W.; Gaikwad, A. M.; Hertzberg, B.; Erdonmez, C. K.; Chen-Wiegart, Y.-c. K.; Sviridov, L. A.; Evans-Lutterodt, K.; Wang, J.; Banerjee, S.; Steingart, D. A., An In Situ


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Page 17 of 22

Zn2+

Discharge

Zno Zn2+ Charge

Charge

V4+

V5+

Current Collector

Synchrotron Study of Zinc Anode Planarization by a Bismuth Additive. Journal of The Electrochemical Society 2014, 161 (3), A275-A284, doi 10.1149/2.037403jes. 39. Kim, R.; Kim, H. G.; Doo, G.; Choi, C.; Kim, S.; Lee, J.-H.; Heo, J.; Jung, H.-Y.; Kim, H.-T., Ultrathin Nafion-filled porous membrane for zinc/bromine redox flow batteries. Scientific Reports 2017, 7 (1), 10503, doi 10.1038/s41598-017-10850-9. 40. Zhao, Q.; Huang, W.; Luo, Z.; Liu, L.; Lu, Y.; Li, Y.; Li, L.; Hu, J.; Ma, H.; Chen, J., High-capacity aqueous zinc batteries using sustainable quinone electrodes. Science Advances 2018, 4 (3), doi 10.1126/sciadv.aao1761. 41. Aaron, D.; Tang, Z.; Papandrew, A. B.; Zawodzinski, T. A., Polarization curve analysis of all-vanadium redox flow batteries. Journal of Applied Electrochemistry 2011, 41 (10), 1175, doi 10.1007/s10800-011-0335-7. 42. Yuan, Z.; Duan, Y.; Liu, T.; Zhang, H.; Li, X., Toward a Low-Cost Alkaline Zinc-Iron Flow Battery with a Polybenzimidazole Custom Membrane for Stationary Energy Storage. iScience 2018, 3, 40-49, doi 10.1016/j.isci.2018.04.006.

Current Collector

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Discharge V4+/V5+

Electrode

Membrane Pump

Scheme 1. Schematic the metal hybrid Zn-V

representation of flow battery.



Figure 1: Photograph of the Zn-V flow cell prototype used in the present study.

0.2

VO2++2H+ + e-VO2+ + H2O

Current density / A.cm

-2

Zn0  Zn2+ + 2e-

0.1

0.88 V

0.0

0.97 V

 E= 1.85 V

-0.1

-0.2

-1.2

-0.8

-0.4

0.0

0.4

0.8

1.2

1.6

Potential / V vs Ag/AgCl 2.2

2.0

(b)

2.0 1.8 1.6 10 mA.cm-2 20 mA.cm-2

1.4

0

20

40

96 mV 191 mV

1.6 1.4 1.2

30 mA.cm-2 40 mA.cm-2

1.2 1.0

(c)

1.8

Cell voltage / V

Cell voltage / V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 22

Zn-V Zn-Br

60

Time / mins

80

ACS Paragon Plus 1.0 Environment 100

120

0

20

40

60

80

Time / mins

100

120

Page 19 of 22

(a) Figure 2: (a) Cyclic voltammetry of the positive and negative half cell used in this study at a scan rate of 50 mV s-1 in 1.7 M V3.5+ in 4 M H2SO4 and 3 M ZnBr2 electrolyte, respectively; (b) GCD profile of the Zn-V –Nafion -117 based flow cell tested at different current density; (c) GCD profile of both Zn-V and Zn-Br2 compared at 10 mA.cm-2.

Charge-Discharge (each 1h) Charge-Discharge (each 2h)

1.4

1.6 1.4

1.2

1.2

1.0

1.0 50

100 150 Time / mins

Daramic Separator Nafion 117

1.8

195 mV

1.6

0

(b)

200

250

20

40

60

80

100

120

Time / mins

100

(c)

2.0

0

Ohmic Polarization

Concentration Polarization

93 mV

Cell voltage / V

Cell voltage / V

1.8

2.0

(c)

Active Polarization

2.0

80

Efficiency / %

1.8

Cell Voltage / V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.6 1.4 1.2

60 40 CE EE VE

20

1.0 0

50

100

150

200

250

300

0

0

10

Time / mins

20

30

Number of cycles


40

50


Figure 3: (a) GCD profile of the Zn-V flow cell at various charge –discharge time; (b) GCD profile comparison of Zn-V flow cell fabricated using Daramic microporous separator and Nafion 117 membrane; (c) first five cycle of the GCD profile tested at 20 mA.cm-2; (d) Cycle life of the Zn-V metal hybrid system employing Nafion-117 membrane.

(a) 0.810 W.cm

0.50 0.8 0.25

0.2

0.4

0.6

-2 Current Density / A.cm

0.00 1.0

0.8

0.75

1.2 0.50 0.8 0.25

0.4 0.0 0.0

0.2

0.4

0.6

0.8


(c)

1.00

-2

0.840 W.cm

0.75

1.2 0.50 0.8 0.25

0.4

0.2

0.4

0.6

0.8



0.00 1.0

Power Density / W.cm-2

1.6

0.0 0.0

1.00

-2 0.825 W.cm

1.6

2.0

Cell Voltage / V

Cell Voltage / V

1.2

0.0 0.0

(b)

-2

0.75

0.4

2.0

1.0

0.00 1.2


1.6

1.00

Cell Voltage / V

2.0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 22

Page 21 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. Polarisation curve of the flow cell tested at different conditions: a) discharge current steps 20 mA b) discharge current steps 50 mA c) discharge current steps 100 mA data were collected for every 30 seconds.

Graphical Abstract


Zn2+

Discharge

Zno Zn2+ Charge

Charge

V4+

V5+

Page 22 of 22

Current Collector

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Current Collector


Discharge V4+/V5+

Electrode

Membrane Pump

Synopsis Aqueous metal hybrid redox flow system is highly suitable for storing the renewable energies such as wind and solar.


New ZincâVanadium (ZnâV) Hybrid Redox Flow Battery: High-Voltage

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