Role of conditioning on water uptake and hydraulic permeability of Nafion® membranes

The performance of ion-exchange membranes under pressure differential conditions is important for a number of separations and for fuel cell applications. Fluid transport properties under pressure are also key to understanding electrochemically-driven pumping and pressurization systems. The physicochemical properties and performance of perfluorosulfonic acid membranes varies considerably with conditioning treatment. In this study, hydraulic permeability and water uptake are measured as a function of membrane treatment for Nafion® membranes that are contacted with aqueous electrolyte solutions. Several conditioning/contacting solution treatments are evaluated including exchanged cation, manner of cation exchange, anion effect, and heat treatment. For Nafion® 117 membranes, these treatments resulted in water content ranging from 3.8 to 20.2 water molecules/fixed sulfonate site and hydraulic permeability ranging from 3.78 × 10−16 to 1.79 × 10−13 m/s Pa. Similar to water uptake, a systematic decrease in hydraulic permeability is observed with the alkali series Li+ > Na+ > K+. Interestingly, the as-received H+ form and the Li+ form have nearly identical water content but the permeability of the H+ form is a factor of 6–7 higher than the Li+ form, regardless of heat treatment. Evaluation based on an equivalent pore model and Poiseuille flow shows good agreement for pore radii ranging from 1.0 to 2.0 nm.