How does processing in supercritical carbon dioxide influence the Nafion film properties?

We address the effect of exposure to sc CO2 on the properties of Nafion membranes. A simple model is proposed to describe the pressure-induced reorganization of the phase-separated matrix. The model predicts that the cylindrical ionic pores in Nafion restructure under pressure applied: wider pores shrink in diameter to a larger degree than narrower ones. As a result, the pores ensemble becomes more monodisperse and narrower as a whole, which should determine detectable changes in the transport properties. The consistency of the model predictions with the main experimental observations was checked. In particular, the membranes were exposed to sc CO2 at 45 °C and pressures ranging from 20 to 750 bar. The modified samples demonstrate the lower water uptake, the higher the pressure during the treatment. This observation correlates well with the cylindrical channels becoming more monodisperse and narrower. Moreover, both proton conductivity and relative crystallinity increase with increasing pressure from 96 to 105 mS/cm and from 20 to 22%, respectively. Methanol permeability decreases from 16 × 10–7 cm2/s for pure Nafion to 10 × 10–7 cm2/s for the treated membrane. DSC measurements reveal larger fraction of bound water for the modified films. Apparently, the bound water is sufficient to ensure high proton conductivity. The improved selectivity is explained by the narrower channels.