Behavior of  S  x 2 −  Species in Molten Sodium Polysulfide

Abstract

An electrolytic cell having four electrodes, each separated by a/~-alum/na plate or a porous ceramic separator and impregnated with molten sodium polysu~de was constructed. The electrolysis was carried out between two of these two sets of electrodes, and the composition change of the melt in the test electrode was observed from the OCV change during the electrolysis. The results of this study strongly suggested that S22-ions migrated preferentially as a carrier on the electrolysis and that they would exist in the equilibrium with $42-or $52-ions even in a melt with the composition of Na~$4.66. The $42-or $52-ions also seem to contribute to the charge transport as the depletion of $22-ions in the melt, whereas Na + is considered not to take part in the charge transport. These phenomena characterize the determination of the distribution of sodium polysulfide in the porous current collector of the sodium sulfur battery.Since sodium sulfur batteries are expected to have a high energy density of more than 150 W-hr/kg, a considerable number of research and development programs are exploring their applications for load leveling and transportation power source. The behavior of molten sodium polysulfide, Na2Sx, in a charge-discharge reaction affects the performance of this type of battery, and much effort has been devoted to clarify the properties of the compound, the phase diagram of Na2S~ (1,2), and properties such as electrical conductivity (3), density (4), and sulfur activities (5). However, there seem to be few experimental studies on the behavior of molten sodium polysulfides as battery electrode.The composition of molten sodium polysulfide is known to change (Na2S5 --> Na2S4 ~ Na2S3) progressively during the discharge process. The termination of the reaction is ascribed to the formation of solid sodium polysulfide in the electrode.The reaction distribution in a porous current collector of a sodium sulfur battery has been estimated (6-9) by applying a network model. However, the reaction distribution would not necessarily correspond with the resultant distribution of the composition, when Sx 2-ions formed during the reaction migrate through the current collector. Since the sulfur utilization would be determined by the resultant distribution of the composition, the effect of migration of Sx 2-species seems essential.Cleaver and Davies (5) derived the Washburn number instead of the transference number to observe the activity of sulfur in molten sodium polysulfide. This number only defines the number of moles of sulfur per unit charge passing through a reference frame in the melt, where the frame is chosen so that sodium ion does not pass through it. However, the mutual migrations of Na + and Sx 2-species should be considered for a better understanding of the resultant distribution of the composition in the porous current collector.

Studying the spatially resolved immobilisation of enzymes on a capacitive field‐effect structure by means of nano‐spotting

“…Figure 4 depicts a typical dynamic ConCap response (a) and a calibration curve (b) of four EIS sensors recorded in Titrisol buffer with different pH values from pH 4 to 9. In this experiment, the capacitance of EIS structures has been kept at a fixed value (within the depletion region at~60% of the maximum capacitance [20][21][22]) using a feedback-control circuit, and the pH-dependent signal changes due to the protonation/deprotonation of the surface amphoteric hydroxyl groups were directly measured. As can be seen, all four sensors show clear, pH-dependent potential steps with a nearly Nernstian pH-sensitivity of 57-59 mV/ pH and a small hysteresis (~3 mV at pH 7).…”

Low‐cost Zinc‐Iron Flow Batteries for Long‐Term and Large‐Scale Energy Storage

“…As shown in Figure 1b, the research of ZIFBs was started in 1981, [19] driven by the energy crisis in 1975 and the demand for automobile power [20] . However, it is very difficult to stably control the morphology of zinc deposition.…”

Transference Number Calculations for Sodium Polysulfides

“…Measurements of melt conductivity, viscosity, and surface tension are presented in their work, but data for the transference number and the diffusion coefficient are not included. Some recent work by Yoshida and Nakajima (2) gives some values for transference number obtained from experimental measurements of the discharge potential of the sodium sulfur cell. However, the authors have incorrectly determined the transference number by not considering diffusion through the cell.…”