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Journal of The Electrochemical Society, 2021 168 020531 Combination of Acid-Base Electrolyte at Each Half-Cell with a Single Zeolite Membrane for Crossover Free and Possible Increased Energy Density in an All Aqueous Redox Flow Battery G. Muthuraman,1,= P. Silambarasan,1,= K. Bae,1,2 and I. S. Moon1,*,z 1Department of Chemical Engineering, Sunchon National University, Suncheon, 57922 Jeollanam-do, Korea 2Top Ecoenergy Co, Seongnam-si, Gyeonggi-do, Republic of Korea Instead of an organic medium, a simple change in pH could lead to a high energy density redox flow battery (RFB). Besides, ion crossover and membrane optimization are problems that limit its commercialization. In this investigation, a zeolite-coated ceramic single membrane is adopted in an acid-base pH electrolyte combination for the vanadium (V4+/V3+)/sulfur (S42−/2S22−) (V/S) redox couple as a model system. First, the potential widening with a change in pH is explained by difference in OCP (open circuit potential) between the acid-acid and acid-base electrolyte combination that differs by 0.8 V. A 300 mV decrease in the V4+/V5+ redox peak potential and the 10 mV increase in the negative direction in the S42−/2S22− redox peak potential between acid-acid and acid-base electrolyte combination show the pH effect predominant in anodic half-cell than the cathodic half-cell. UV-visible analysis for the migration of vanadium and sulfur ions demonstrates no migration of vanadium and sulfur ions to each other half- cell via zeolite coated ceramic membrane. The current efficiency of 94%, voltage and energy efficiencies of 45%–50% are achieved under the given current density of 5 mA cm−2. In addition, the acid-base combination of V/S RFB system shows an energy density of 233.2 Wh l−1 © 2021 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/ by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/ 1945-7111/abe39f] Manuscript submitted November 10, 2020; revised manuscript received December 22, 2020. Published February 18, 2021. The redox flow battery (RFB) has attracted considerable attention because of its potential to enhance power and energy storage.1,2 RFBs store energy in chemical form at external reservoirs,3–5 which are withdrawn instantly or deposited by redox reactions that occur on the active surfaces of the electrode when a liquid electrolyte is flowing into the cell. Therefore, the RFB system can be scaled up to the megawatt-hour (MW h) range by simply expanding the volume of the reservoir.1,2 Despite the potential, there has been continuing research interest in the development of the RFB because of the many issues, such as low energy density,6 problems with the membrane for ions crossover,7 inefficient electrodes,8 and capacitance fading.9 One way to increase the energy density in an aqueous medium is to increase the potential window, which has been achieved by adopting different combinations of metal ions because each metal ion has its unique oxidation/reduction potential. By adopting this approach, the following combinations of metal ions have been developed for high energy density battery production: Ce/V,10 Zn/Br2,11 Zn/polyiodide and V-MH (metal hydride),12 iron chloride,13 and Zn/Fe.14 On the other hand, the potential window can be widened by selecting a suitable electrolyte medium, such as a non-aqueous medium that is devoid of H2O.15 Different types of redox-active materials are involved in a non-aqueous RFB, such as inorganic active materials (such as sulfur and iodine species),16,17 redox-active polymers,18 and organo- metallic compounds.19,20 Nevertheless, these nonaqueous lithium- RFBs have drawbacks. For example, because of the use of lithium metal, they are quite sensitive to moisture and gases, such as O2, N2, and CO2.20,21 In addition, the ionic mobility in non-aqueous solutions is much slower than in their aqueous counterparts.22 Another possibility for achieving a broader potential window is to change the pH to widen the potential window in an aqueous medium using either highly acidic or highly alkaline solutions to favor the generation of an electrocatalyst (redox-active material) in an electrochemical cell because of the gain in potential window by postponing the oxygen evolution reaction (OER) or hydrogen evolution reaction (HER).23 Using this approach, the potential window of cathodic half-cell can be extended using a 10 M KOH medium and used to develop low valent =These authors contributed equally to this work. *Electrochemical Society Member. zE-mail: ismoon@sunchon.ac.kr metal complexes for the mediated catalytic reduction (MER) of many air pollutants in the electro-scrubbing process for the first time.24–26 The true acid-base electrolyte combination for a widened potential window was tested in the generation of two electron mediators (Co1+ and Co3+) by adopting an acid for Co3+ and base for Co1+ generation.27 Recently, the potential widening by pH variation was introduced by adopting the Zn/I3− redox couple in KOH and neutral medium for the RFB field.28 Despite the potential widening by a change in pH, ion crossover through the membrane causes capacitance fading in the RFB system.29 The zincate ion permeability coefficients through Nafion 117 were 6.74 × 10−7 cm2 min−1 and 11.62 × 10−7 cm2 min−1 in 2 M and 6 M KOH electrolytes, respectively.28 A double ion exchange membrane (IEM) was adopted to minimize the crossover without30 and with gap12 between membrane and electrode, but the capacity fading was equal to that of a single IEM. A triple or double IEM membrane-divided electrochemical cell was introduced to produce a high energy density flow battery for the Ce/Zn system with a widened potential window,31 but the polymeric IEM membrane allows ions and capacity fading equal to the single IEM. Recently, different types of polymeric membranes have been devised, particularly polybenzylimidazole, and shown to have much less Fe2+ and V4+ permeability than the Nafion212 membrane.32,33 Instead of an IEM, size-selective membranes favored excellent ion crossover minimization with high proton transfer in an all-vanadium redox flow battery,34,35 where both half-cells adopted an acid electrolyte. Different electrolyte pH media, i.e., acid-base electro- lytes at each half-cell, were never tested in the size-selective membranes in the RFB system. In addition, the size-selective zeolite ceramic membrane was adopted in the generation of two mediators, Co1+ in 10 M KOH and Co3+ in 5 M H2SO4, where a complete absence of ion crossover was observed.27 In the present investigation, a single zeolite coated ceramic membrane was used as a separator with a model redox pair of the V/S RFB system. The V4+/V5+ redox pair with a potential of 1.175 V in the acid medium and the S42−/2S22− redox pair with a potential of −0.45V in the base medium,31 giving a total cell potential of 1.625 V, which is slightly higher but more suitable for the preliminary analysis of a single membrane application in the acid-base electrolyte system. The investigation started with unpre- cedented power density gain by dual electrolytes pH change thatPDF Image | Single Zeolite Membrane for Crossover
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