Ion exchange membranes are commonly used as electrolyte separator in all vanadium redox flow battery (VRFB). Due to the high acidity and ionic concentration in battery electrolyte, a substantial amount of electrolyte species can be imbibed into ion exchange membrane. The ionic equilibrium between the ion exchange polymer and solution can significantly influence membrane transport behavior, morphology and mechanical properties. We are investigating the role of polymer properties in controlling this equilibrium. In this initial phase of the work, the sulfuric acid equilibrium, swelling behavior and mechanical strength of Nafion were quantitatively studied to investigate the ionic species equilibrium within sulfuric acid environment. Membrane sulfuric acid and water contents were characterized after equilibration with sulfuric acid solutions. Sulfuric acid can overcome the Donnan exclusion, derived from electrostatic anion (co-ion) repulsion by sulfonic acid groups in the membrane, to enter membrane micropores. The water content in the membrane was substantially reduced at equilibrium. With sulfuric acid present in Nafion, membrane density increases, mainly due to decreased porosity caused by the deswelling effect of sulfuric acid. Strain-stress curves of Nafion samples were measured with dynamic mechanical analysis after sulfuric acid equilibration. Higher sulfuric acid molality in membrane micropores leads to higher stiffness, as represented by elevated Young's modulus. Thermodynamic models of swelling pressure and Donnan potential were utilized with the measurement to illustrate the ionic equilibrium in Nafion membrane. The theoretical analysis found that the mean ionic activity factor of sulfuric acid in the ion exchange polymer was consistently lower than that in bulk solution phase. For less concentrated sulfuric acid, the mean ionic activity factor ratio of sulfuric acid in the membrane phase to that in solution is much higher than that in concentrated solution case.

Electrolyte equilibrium in ion exchange polymer

248th National Meeting (2014)

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