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Journal of The Electrochemical Society, 2021 168 020531 Figure 6. Deep charging-discharging profile of the V/S RFB system performed using a single zeolite membrane in an acid-base electrolyte combination. Current density = 5 mA cm−2; Electrode = Carbon felt and Ni-foam (4 cm2); Electrolyte (200 ml) = 1 M V4+ in 1 M H2SO4 and 1 M Na2S4 in 1 M NaOH; Solution flow rate = 50 ml min−1. double membrane divided acid-base electrolyte combination31), as shown in Fig. 7A. The charging and discharging potentials of 2.15 V and 0.81 V, respectively, were maintained until 50 cycles (displayed only 50 cycles to avoid clumsiness) with a potential difference of 1.34 V. The CE (based on the SOC calculation) was maintained at 94% in all 200 cycles (Fig. 7B curve a). In the case of VE (Fig. 7B curve b) and EE (Fig. 7B curve c), there was a slight increase from 43.3% to 47.6% (Fig. 7B curve b) and 40.8% to 44.6% (Fig. 7B curve c) after 200 cycles. As there was no ion migration via the zeolite membrane, capacity fading had been completely minimized after 200 cycles (Fig. 7B curve d). The specific capacity of the cell under the given conditions (2 M concentration and number of electrons transferred (3)) was 160.8 Ah l−1, which is slightly lower than that of the alkaline–neutral redox couple, Zn/I3− (202.2 Ah l−1).28 The volumetric energy density for the acid-base electrolyte pH combination of the V4+/S42− redox couple under given conditions was 233.2 Wh l−1, which is competitive with the highest volumetric energy density achieved.28 Finally, the acidity and alkalinity of each half-cell were cross-checked with suitable indicators. The lack of change in the acid and alkaline concentration confirmed that the single zeolite membrane effectively allows the transfer of water molecules. Conclusions This study evaluated an acid-base pH combination of electrolytes at each half-cell using a single zeolite membrane towards redox flow battery application. When the acid-base pH electrolyte combination was combined, the OCP was 0.8 V higher than that of the only acid- acid electrolyte combination, which supported the widening of the potential window in the extreme pH variation. The 300 mV difference in the V4+ redox peak potential between the acid-acid and acid-base electrolyte combination, confirmed that the redox potential is affected by pH. The relative lack of variation in the internal resistance with different combinations of electrolyte pH highlighted the size-selective nature of the zeolite membrane. In addition, no migration of vanadium and sulfur ion to the other half-cells confirmed that the zeolite membrane effectively restricted the ion crossover. Despite the greater than 94% current efficiency, the voltage and energy efficiencies were 45%–50% because of the acid-base electrolyte OCP potential. At the same time, no fading in capacitance supported the zeolite membrane divided with the acid-base pH electrolyte combination is effective in redox flow battery applications. Moreover, the energy density of the −1 acid-base combination in the V/S RFB system was 233.2 Wh l , Figure 7. (A) Long term cell cycling performance of V/S RFB system with a single zeolite membrane in acid-base electrolyte combination and (B) corresponding coulombic (curve a), voltage (curve b), energy (curve c) efficiencies, and capacitance (curve d) variation. The other conditions are the same as in the figure legend of Fig. 6. which is much higher than the V/V RFB in acid-acid electrolyte combinations. These preliminary studies can be improved further to achieve high voltage and energy efficiencies. Acknowledgments This work was supported by the Korea Sanhak Foundation (KSF) in 2019. 1. 2. 3. 4. 5. 6. 7. 8. ORCID https://orcid.org/0000-0002-8191-9662 References A. Orita, M. G. Verde, M. Sakai, and Y. S. Meng, Nature Commun., 7, 13230 (2016). E.S.Beh,D.DePorcellinis,R.L.Gracia,K.T.Xia,R.G.Gordon,andM.J.Aziz, ACS Energy Lett., 2, 639 (2017). T. Janoschka, N. Martin, M. D. Hager, and U. S. Schubert, Angew. Chem. Int. Edi., 55, 14427 (2016). K. Lin, R. Gómez-Bombarelli, E. S. Beh, L. Tong, Q. Chen, A. Valle, A. Aspuru- Guzik, M. J. Aziz, and R. G. Gordon, Nat. Energy, 1, 16102 (2016). C. S. Sevov, D. P. Hickey, M. E. Cook, S. G. Robinson, S. Barnett, S. D. Minteer, M. S. Sigman, and M. S. Sanford, J. Am. Chem. So., 139, 2924 (2017). B. Li, Z. Nie, M. Vijayakumar, G. Li, J. Liu, V. Sprenkle, and W. Wang, Nat. Commun., 6, 6303 (2015). M. Skyllas-Kazacos and L. Goh, J. Membr. Sci., 399–400, 43 (2012). S.-C. Kim, H. Lim, H. Kim, J. S. Yi, and D. Lee, Electrochim. Acta, 348, 136286 (2020). I. S. MoonPDF Image | Acid-Base Electrolyte at Each Half-Cell with a Single Zeolite Membrane
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