Thermodynamic and Physical Properties of Molten Sodium Polysulfides from Open-Circuit Voltage Measurements

Abstract

The open-circuit voltage of the cell Na/~-alumina/Na2Sx/pyrolytic graphite has been measured as a function of temperature and sodium-polysulfide composition. From these data, differential free energy, entropy, and enthalpy values for the potential-determining reaction and the differential free energy of formation for various polysulfide compositions were calculated. The partial differential enthalpy (H1) of adding Na to the polysulfide melt is --47.6 kcal/ tool independent of melt composition and temperature. The solubility of sulfur in sodium pentasulfide was determined between 280~176 and the Na2S-S phase diagram of Pearson and Robinson was revised in the Na2S2-S range and extended to show the Na-rich border of the miscibility gap. " Electrochemical Society Active Member. Key words: free energy of formation, phase diagram. ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 128.210.126.199 Downloaded on 2015-05-29 to IP Vol. I19, No. 8 ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 128.210.126.199 Downloaded on 2015-05-29 to IP

The electrical conductivity of sodium polysulfide melts

“…Ky(VrVi) is the drag force as a result of the relative motion of the species. Newton's third's j law requires that (2)(3)(4) 'Dii is not defined. Therefore, the number of transport parameters 'Dds, defined by equation , is N(N-1 )f2, where N is the number of species.…”

“…Consequently; the number of independent equations (2-1) is one less than the number of species. 2F Xe (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19) Do denotes the activity of sulfur in the melt. and Xe denotes the mole fraction of sodium sulfide in phase ~.…”

“…Xe dlnlle+Xodlnao =0, and equation (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20), the term of In lle in equation can be expressed in terms of dVI· On the other hand, let us apply the definition of the electrolyte molality, and the chain rule to the term of din m on the right side of equation . Consequently, equation can be rewritten as…”

“…The three equations for the transport properties, equations (2-13), (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14), and (2-18), can be inverted into expressions explicit in t])ij 's, 23…”

“…The simplest type of dependence of the current density on the surface overpotential and the composition adjacent to the electrode surface is given by the Butler-Volmer equation: (3)(4)(5) The exchange current io depends on the composition of the solution adjacent to the electrode, as well as the temperature and the nature of the electrode surface. For large values of io, a given current density can be obtained with small surface overpotentials.…”