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The source of the electrode overpotentials is mostly kinetic. Because of •4*-^ = the high Zn and Cl ionic concentrations, mass transfer overpotential is small during the charging process. Mass transfer overpotential can become significant only during the discharging process at the Cl^ electrode. The solubility of Cl^ in the solution is on the order of 0.01 M; however, the use of a flow-through porous electrode eliminates the significance of mass transfer overpotential. Kim and Jorne [3] measured the mass transfer and the kinetics of the cathodic reduction of dissolved Cl and showed that the kinetics of the reaction are 22 moderately fast (i = 1.1 mA /cm ). Therefore, using a flow-through porous elec trode ensures that mass transfer is not the rate determining step during the dis charging process. A complete cycling of the single cell is shown in Figure 4, where the po tentials of the Zn and Cl^ electrodes were measured vs. SCE. The average charging voltage was 2.3 V and the average discharging voltage was 1.8 V. The voltaic efficiency is given by 1.8/2.3 = 78%. The coulombic efficiency was measured from the total charge passed during the charging and discharging, 6,000 and 5,040 coul, respectively. Hence, the coulombic efficiency is 5,040/6,000 = 84%. The energy efficiency is given by energy efficiency = (voltaic efficiency) x (coulombic efficiency) = 0.78 x 0.84 = 66% The overpotentials at the Zn electrode are -50 mV and +40 mV for the charging and discharging, respectively. The overpotentials at the Cl^ electrode are +120 mV and -200 mV for the charging and discharging, respectively. At the end of the discharging process, all of the zinc is removed from the graphite substrate and Cl^ is evolved and the recorded potential corresponds to that of the anode during the preceding charging process (see Figure 4). Tafel's Kinetic Parameters* 2 The Tafel polarization data for Cl^/Cl on a dense graphite electrode is shown in Figure 5. The data was corrected for ohmic drop using Newman's method 2 [4], The apparent exchange current density, i = 1.1 mA/cm , is obtained from the intercept of the cathodic and anodic polarization branches. The anodic and cathodic Tafel slopes are b = 124 and b = -120 mV/decade, respectively. The cl C present parameters are in agreement with the results of Hine [5] for NaCl solution A.-8PDF Image | Development of the Zinc-Chlorine Battery for Utility
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