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identified through the open circuit potential (OCP) measurements under different pH conditions. Through impedance analysis, the single zeolite membrane resistance with different electrolyte pHs (acid-acid, acid-base, and base-base) was analyzed and compared with that of a Nafion IEM membrane. Cyclic voltammetry (CV) was performed using different electrolyte combinations to understand the redox potential shift of V4+/V5+ and S42−/2S22− better. Separate electrolysis was carried out with higher current density in different combinations of electrolytes (acid-base) using spectral analysis to identify the crossover of ions through a zeolite membrane. Charging- discharging analysis was carried out with suitable electrodes and current density by a single zeolite membrane and optimized. Along with the coulombic, voltage, and energy efficiencies identification, the possible minimization of ion crossover and future applications of the single membrane usage were discussed. Experimental Materials.—1 M Na2S4 was prepared from 1 M Na2S by dissol- ving additional sulfur powder upon heating at the required mole ratio (∼3 mole) using the literature procedure.36 A ceramic tube (∼31 vol % porosity, diameter (Inner diameter (ID): 2.54 cm, Outer diameter (OD): 3.2 cm), thickness 0.24 cm, length 14.5 cm, ∼0.11 μm nominal pore size) was supplied by Chosun Refractories Co. Ltd., Korea. A zeolite was coated on the ceramic substrate, which was confirmed by X-ray diffraction (XRD) and scanning electron microscopy—energy-dispersive X-ray spectroscopy (SEM-EDS) using the published procedures.37,38 V4+ (VOSO4) (from Sigma- Aldrich), H2SO4, KOH, NaOH and KMnO4 were obtained from Sam Chun Chemicals, Korea, and ferrous sulfate (Fe(II)SO4.7H2O) was purchased from Junsei Chemical Co., Ltd., Japan. Unless stated otherwise, all chemicals were used as received. All aqueous solutions were prepared in water that had been purified by reverse osmosis (Human Power III plus, Korea). Electrochemical characterizations.—CV and OCP measure- ments were carried out in a tailor-made glass cell with an electrolyte volume of 25 ml furnished with two compartments separated by a piece of a zeolite-coated ceramic membrane or a Nafion324 (From DuPont, U S A) membrane for the working and counter electrodes. A graphite or nickel foam electrode served as the working electrodes. A platinum mesh and Ag/AgCl were used as the counter and reference electrodes, respectively. The measurements were taken using a PARC VersaSTAT 3 instrument. The scan rate was 20 mVs−1. Figure 1A presents the prototype electrochemical cell with a zeolite-coated ceramic membrane-divided cell. The impe- dance measurements were performed using the same PARC VersaSTAT 3 instrument to examine the effect of the acid-base electrolyte on the zeolite-coated ceramic membrane and the Nafion membrane. Galvanostatic mode with an applied current of 1 mA (vs OCP) using a two-electrode configuration was applied with Pt mesh as the counter and working electrodes (0.5cm distance) with frequencies, ranging from 200 kHz to 0.01 Hz. Charge-discharge cycling.—The charge-discharge tests were conducted using the following: a prototype plate and frame (made using Teflon, Viton, and SS end plate materials), zeolite-coated ceramic piece, or Nafion324 membrane-divided RFB system with 1M V4+ in 1M H2SO4 and 1M Na2S4 in 1M NaOH solution, as shown in Fig. 1B. Two electrolyte tanks attached to each side of the prototype RFB system were filled with 200 ml of electrolyte. The graphite felt and Ni foam electrode with a geometrical working area of 4 cm2 at a 0.5 cm distance for the Nafion 324 membrane and 1.0 cm for the zeolite-coated ceramic membrane were mounted and separated. Peristaltic pumps (Masterflex) were used to circulate the electrolyte through the flow cell at a rate of 50 ml min−1. All electrodes were cleaned using an electrochemical cycling method between 0 V to −1.4 V in 0.1 M KNO3 solution and washed with ethanol, and finally with RO water. Analysis.—The migration of the V4+ from acid to base and S42− from base to acid via the zeolite or Nafion324 membrane was measured in a separate electrolysis experiment with an applied current density of 10 mA cm−2. For V4+ migration, samples (5 ml) of the opposite half-cell of the V4+ tank were collected at each timing interval (in 1 M NaOH solution), and UV-visible spectral analysis was performed. In the case of S42− ion migration via the zeolite membrane, samples (5 ml) of the opposite half-cell of the S42− tank were collected at similar time intervals (in a 1 M H2SO4 solution), which was basified with 1M NaOH, and UV-visible spectral analysis was performed to determine its migration using a Scinco s-3100 spectrophotometer. To calculate the SOC (state of charge) via spectral analysis or titration with KMnO4, a 5 ml sample was withdrawn from the cathodic (charging) or anodic (discharging) compartment at the desired time interval. The samples were analyzed by UV-visible spectroscopy to identify the concentration of vanadium or sulfur ions that had migrated using a plot of standard concentrations. Results and Discussions Electrochemical analysis.—Through the OCP, the advantage of the pH change could be realized, as shown in Fig. 2. The OCP for the neutral and its counterpart acid pHs (Fig. 2 curve a) and neutral and base pH (Fig. 2A curve b) was −0.04 V and 0.25 V for the neutral to acid and neutral to base pHs, respectively, which deviated from the neutral and neutral pH (Fig. 2 curve c) OCP value (−0.16 V) (0.12 V and 0.41 V). According to the pH and electro- chemical potential relationship, the Nernst equation predicted a 59 mV shift with a change in one pH value, which was confirmed by Fabbri et al. 39 When the acid and base pHs were combined, the OCP value widened to −1.2 V (Fig. 2 curve f) when divided by the zeolite coated ceramic membrane. In addition, the OCP increased to 0.8 V for the acid-base pH combination when compared to the acid-acid electrolyte combination (Fig. 2 curve d). Morale-Guio et al. suggested that the use of either highly acidic or highly alkaline solutions can be used to favor the generation of an electrocatalyst in an electrochemical cell because of the potential window gain for the OER or HER.23 Apart from the potential widening by pH, the cell resistance may increase due to a change in acid-base pH that had been derived experimentally, as shown in Fig. 3. In the case of Nafion324 membrane divided 5 M H2SO4 and 10 M KOH contained electrolyte, the Nyquist plot showed an internal resistance of 4.72 Ω (1.18 Ω cm2) (Fig. 3A curve b), which is comparatively high compared to the only 10M KOH in each half-cell (4.61Ω (1.15 Ω cm2)) (Fig. 3A curve c). At the same time, the internal resistance was 2.42 Ω (0.62 Ω cm2) (Fig. 3A curve a) for only acid at each half-cell, which is lower compared to the other two-electrolyte combinations. On the other hand, the zeolite-coated ceramic membrane showed 11.43 Ω (2.86 Ω cm2) in the acid-base electrolyte combination (Fig. 3B curve b), which is 2.4-times higher than the resistance from Nafion324 (Fig. 3A curve b). In the case of the only acid electrolyte at each half-cell (Fig. 3B curve a) and only base electrolyte at each half-cell (Fig. 3B curve c), the internal resistance was 17.28 Ω (4.32 Ω cm2) and 32.31 Ω (8.08 Ω cm2), respectively. This is opposite to that observed on the Nafion324 membrane, where the acid-acid electrolyte combination showed less resistance (Fig. 3A curve a). The Nafion resistance in the acid-base was barely observed. In particular, with the acid-base electrolyte combination using the double ion-exchange membranes (Nafion117 and Fumasep FAA) in the acid-base electrolyte combination, the internal resis- tance was 20 Ω cm2,31 which is 4.34 times higher than that of the single Nafion324 membrane (Fig. 3A curve b) in the acid-base electrolyte combination. At the same time, single zeolite coated ceramic membrane in the acid-base electrolyte combination showed an internal resistance of 2.86 Ω cm2 (Fig. 3B curve b), which is approximately seven times lower than the double IEM combination.31 The high internal resistance in Nafion324, especially acid-base and base-base electrolyte combinations, could be due to less or no proton Journal of The Electrochemical Society, 2021 168 020531PDF Image | Acid-Base Electrolyte at Each Half-Cell with a Single Zeolite Membrane
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