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Figure 8. Coulombic, voltage, and energy efficiency for the Nafion 211 and 212 membranes at 320 mA/cm2 and flow rates of a) 800 cc/min/cell and b) 1200 cc/min/cell. Figure 8. Coulombic, voltage, and energy efficiency for the Nafion 211 and 212 membranes at 400 mA/cm2 and flow rates of a) 800 cc/min/cell and b) 1200 cc/min/cell. In all cases, the stack energy efficiencies were greater for the Nafion211 membrane; in general a 2-3% increase was observed with greater increases at the highest current density. The coulombic efficiencies are dependent on the Nafion membrane, the current density, and slightly dependent on flow rate. The decrease in coulombic efficiency with the thinnest Nafion 211 membrane suggests slightly more cross over occurs with the thinner membrane. However, as the current density is increase the coulombic efficiency increases for the Nafion 211 membrane most likely due to the decrease cycle time; the electrolyte spends less time in the stack and in contact with the membrane thereby reducing the amount of crossover at the highest current density. A slight decrease in coulombic efficiency is also observed with increasing flowrate; this decrease is very small and may be within experimental error of the measurements (~0.5-1%). The ohmic losses attributed to the thicker Nafion 212 membrane decrease the voltage efficiency while an increase in flow rate tends to increase the voltage efficiency; these values dictate the overall stack energy efficiency. The stack energy efficiency for the cell fabricated with a Nafion 211 membrane was ~73-74% when operated at 400 mA/cm2 with flow rates of 800 cc/min/cell and 1200 cc/min/cell, respectively. 12PDF Image | High Current Density Redox Flow Batteries
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