Structural electroneutrality in Onsager–Stefan–Maxwell transport with charged species

Abstract

We present a method to embed local electroneutrality within Onsager–Stefan–Maxwell electrolytic-transport models, circumventing their formulation as differential systems with an algebraic constraint. Flux-explicit transport laws are developed for general multicomponent electrolytes, in which the ionic conductivity, component diffusivities, and transference numbers relate to Stefan–Maxwell coefficients through invertible matrix calculations. A construction we call a ‘salt–charge basis’ implements Guggenheim’s transformation of species electrochemical potentials into combinations describing a minimal set of neutral components, leaving a unique combination associated with electricity. Defining conjugate component concentrations and fluxes that preserve the structures of the Gibbs function and energy dissipation retains symmetric Onsager reciprocal relations. The framework reproduces Newman’s constitutive laws for binary electrolytes and the Pollard–Newman laws for molten salts; we also propose laws for salt solutions in two-solvent blends, such as lithium-ion-battery electrolytes. Finally, we simulate a potentiostatic Hull cell containing a non-ideal binary electrolyte with concentration-dependent properties.