Solvent-filled polymers are an important class of membrane-electrolytes because the imbibed solvent imbues the material with high conductivity while maintaining mechanical stability. Both these attributes are essential for membrane-electrolytes in numerous energy-storage and conversion devices. However, these membranes’ favorable ion-transport properties pose a challenge when multiple ion types are present. Specifically, there are few quantitative material design guides for polymers to balance the movement of certain species that are desirable, such as current-carrying ions, against transport of contaminants, additives, or redox-active species, which decrease device performance. To address this challenge and provide material design insights, this work describes the theory of multi-ion transport in these polymer-electrolyte membranes.<br/><br/>Specifically, we use Stefan-Maxwell-Onsager theory to rigorously describe concentrated-solution transport in the membrane. To connect performance to the molecular-level structure of the polymer, hydrodynamic theory provides constitutive relations for the involved ion/solvent and ion/ and solvent/polymer friction coefficients, with classical porous-media theories scaling tortuosity. Additionally, electrostatic relaxation dictates friction between ions. To account for the contribution of weak electrolytes, the model incorporates ion speciation into species’ transport coefficients.<br/><br/>The model compares favorably to a number of published measured membrane transport properties (e.g. conductivity, transference numbers, electroosmosis, and water transport coefficient) in dilute and concentrated binary and ternary electrolytes. The model isolates the contribution of each type of molecular-scale interaction between species to the measured properties. Moreover, we show how to encode concentrated-solution theory into dilute-solution transport parameters and explore the limitations of both approaches.

Theory of multi-ion transport in solvent-filled membranes

2019 Spring National Meeting

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