Electrolytes, transport properties

Fergus (2007), Effect of Cathode and Electrolyte Transport Properties on Chromium Poisoning in Solid Oxide Fuel Cells , Int.J. Hydrogen Energy, 32, 3664-3671. 

Key words Sealant/Glass-Ceramics/Thermal Expansion/Solid Oxide Electrolyte/Transport Properties... 

II. STATISTICAL MECHANICAL THEORY OF ELECTROLYTE TRANSPORT PROPERTIES... 

Very little work has been done in this area. However, good work on electrolyte transport for electrolyte systems is now available [65, 66]. Gering has developed a model for estimating the full set of electrolyte transport properties [67]. Newman etal. [68] applied molecular dynamics to transport in lithium-ion electrolytes and showed that although lithium ions have different interactions with different solvents, ion transport causes only small changes in solvent composition across the separator. 

Based on the competing needs of these factors, an electrolyte with some diffusivity of reactants is needed. For the PEFC, the diffusivity has been correlated from experimental data for 1100-EW Nafion, the most well-studied polymer electrolyte. Other electrolyte polymers should have similar trends, but actual values change with EW values. In general, low-EW polymers that absorb more water will have higher gas-phase species diffusion coefficients than higher EW electrolytes. Transport properties in the polyflourosulfonic acid (PFSA) based membranes such as Nafion are generally dependent on the water sorption in the membrane. For dry membranes, ionic conductivity and water mobihty are very low. As water sorption is increased, the membrane swells, and particularly at over 60% RH,... 

This database provides thermophysical property data (phase equilibrium data, critical data, transport properties, surface tensions, electrolyte data) for about 21 000 pure compounds and 101 000 mixtures. DETHERM, with its 4.2 million data sets, is produced by Dechema, FIZ Chcmic (Berlin, Germany) and DDBST GmhH (Oldenburg. Germany). Definitions of the more than SOO properties available in the database can be found in NUMERIGUIDE (sec Section 5.18). 

The behavior of ionic liquids as electrolytes is strongly influenced by the transport properties of their ionic constituents. These transport properties relate to the rate of ion movement and to the manner in which the ions move (as individual ions, ion-pairs, or ion aggregates). Conductivity, for example, depends on the number and mobility of charge carriers. If an ionic liquid is dominated by highly mobile but neutral ion-pairs it will have a small number of available charge carriers and thus a low conductivity. The two quantities often used to evaluate the transport properties of electrolytes are the ion-diffusion coefficients and the ion-transport numbers. The diffusion coefficient is a measure of the rate of movement of an ion in a solution, and the transport number is a measure of the fraction of charge carried by that ion in the presence of an electric field. 

Ren, X. Springer, T. E. and Gottesfeld, S. (1998). Direct Methanol Fuel Cell Transport Properties of the Polymer Electrolyte Membrane and Cell Performance. Vol. 98-27. Proc. 2nd International Symposium on Proton Conducting Membrane Euel Cells. Pennington, NJ Electrochemical Society. 

Chapter 15 gives an extensive and detailed review of theoretical and practical aspects of macromolecular transport in nanostructured media. Chapter 16 examines the change in transport properties of electrolytes confmed in nanostructures, such as pores of membranes. The confinment effect is also analyzed by molecular dynamic simulation. 

Finally, it must be recalled that the transport properties of any material are strongly dependent on the molecular or ionic interactions, and that the dynamics of each entity are narrowly correlated with the neighboring particles. This is the main reason why the theoretical treatment of these processes often shows similarities with models used for thermodynamic properties. The most classical example is the treatment of dilute electrolyte solutions by the Debye-Hiickel equation for thermodynamics and by the Debye-Onsager equation for conductivity. 

Because of the inherent technical difficulties, investigations of transport properties in molten salts are much less common than those of aqueous solutions. However, interpretation of the phenomena seems to be even simpler in molten salts where water is not involved. Molten salt systems are considered to be the simplest liquid electrolytes. Data have been compiled largely due to the great efforts of the Janz group." "... 

Barthel, J., Transport properties of electrolytes from infinite dilution to saturation, Pure Appl. Chem.y 57, 355 (1985). 

The term limiting-current density is used to describe the maximum rate at 100% current efficiency, at which a particular electrode reaction can proceed in the steady state. This rate is determined by the composition and transport properties of the electrolytic solution and by the hydrodynamic condition at the electrode surface. 

The use of excess inert electrolyte so as to reduce differences in transport properties of the solution at the electrode surface and in the bulk. In such a solution, the ionic diffusivity of the reacting ion, for example, Cu2 + or Fe(CN)g, should be employed in the interpretation of results, and not the molecular diffusivities of the compounds, for example, CuS04 or K3Fe(CN)6. 

The transport behavior of Li+ across membranes has been the focus of numerous studies, the bulk of which have concentrated upon the human erythrocyte for which the Li+ transport pathways have been elucidated and are summarized below. The movement of Li+ across cell membranes is mediated by transport systems which normally transport other ions, therefore the normal intracellular and subcellular electrolyte balance is likely to be disturbed by this extra cation. Additionally, Li+ has been shown to increase membrane phospholipid unsaturation in rat brain, leading to enhanced fluidity in the membrane, which could have repercussions for membrane-associated proteins and for membrane transport properties. 

Since the early days of Faraday and Arrhenius, electrolytic solutions have provided a most challenging field for both the experimental and the theoretical physico-chemist. In particular, the long range of the Coulomb forces between the electric charges located on the ions gives rise to highly non-trivial effects on the equilibrium and transport properties of electrolytes. 

In order to gain some familiarity with the more elaborate treatment we shall need in considering the transport properties of electrolytes, we shall however follow here a less traditional approach, due to Balescu and Taylor,2 which is based on the dynamical formulation we developed in Section II. 

We now turn to transport properties of electrolytes. As mentioned above this problem is much more complicated than the equilibrium one because the solvent plays a crucial role in the description of dissipative phenomena. As a matter of fact, no general solution exists to this problem, because the statistical description of the liquid state is still at a very primitive stage. 

Results of the experimental and theoretical investigations on bridging electrolyte-water systems as to thermodynamic and transport properties of aqueous and organic systems. Revised version of chapter four in Number 35. 

A compilation of various thermodynamic and transport properties, including aqueous electrolyte and non-electrolyte solutions... 

Stokes, R. H. Mills, R. "Viscosity of Electrolytes and Related Properties" in "The International Encyclopedia of Physical Chemistry and Chemical Physics Topic 16, Transport Properties of Electrolytes" Vol. 3, Stokes, R. H., Ed., Pergamon Press, Oxford, 1965. 

Here Tq is — C2 and is a prefactor proportional to which is determined by the transport coefficient (in this case at the given reference temperature. The constant B has the dimensions of energy but is not related to any simple activation process (Ratner, 1987). Eqn (6.6) holds for many transport properties and, by making the assumption of a fully dissociated electrolyte, it can be related to the diffusion coefficient through the Stokes-Einstein equation giving the form to which the conductivity, <7, in polymer electrolytes is often fitted,... 

This sharp decline in cell output at subzero temperatures is the combined consequence of the decreased capacity utilization and depressed cell potential at a given drain rate, and the possible causes have been attributed so far, under various conditions, to the retarded ion transport in bulk electrolyte solutions, ° ° - ° ° the increased resistance of the surface films at either the cathode/electrolyte inter-face506,507 Qj. anode/electrolyte interface, the resistance associated with charge-transfer processes at both cathode and anode interfaces, and the retarded diffusion coefficients of lithium ion in lithiated graphite anodes. - The efforts by different research teams have targeted those individual electrolyte-related properties to widen the temperature range of service for lithium ion cells. 

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