Solute-volume effects in electrolyte transport

Abstract

A model elucidates two transport mechanisms associated with the volumes dissolved electrolytes occupy: the ‘excluded-volume effect’, which arises when concentration polarization induces solution-density gradients that drive volume redistribution; and ‘Faradaic convection’, which occurs when interfacial electrochemical reactions induce bulk flow. The excluded-volume effect can be accounted for in Newman's concentrated-solution theory by incorporating a thermodynamic state equation that describes the solution's local molar volume. Faradaic convection is introduced through boundary conditions that include volume-average velocity, which is distributed throughout a solution by a volume-balance governing equation. Two dimensionless parameters quantify the importances of these phenomena, which prove relevant when modeling nonaqueous electrolytes. Analytical formulas are derived to describe concentration polarization and diffusion potentials in parallel-electrode cells undergoing symmetric ion-deposition/stripping half-reactions. In moderately concentrated nonaqueous electrolytes, Faradaic convection elevates limiting currents by as much as ten percent above those predicted by a theory neglecting it. The excluded-volume effect similarly impacts diffusion potentials.