Nafion/PEG hybrid membrane for CO2 separation: Effect of PEG on membrane micro-structure and performance

In the present work, PEGDME with different molecular weight (Mn∼250 and 500 g/mol) was added into Nafion-based membranes as CO2-philic additive, aiming at improving their CO2 capture performance. The physical, chemical and morphological characteristics of the hybrid membranes were thoroughly investigated using different techniques, including TGA, XRD, SEM and FTIR. The gas transport properties were studied by means of mixed gas permeation tests at different relative humidity conditions. CO2 permeability is greatly enhanced upon the addition of the PEGDME. The addition of 40 wt% PEGDME 250 into the Nafion matrix shows a CO2 permeability of 57.4 Barrer at the dry state, which is 36 folds higher than the pristine Nafion. The presence of water vapor in the gaseous streams further enhances the CO2 permeability and CO2/N2 selectivity, reaching a value of 446 Barrer and 37, respectively, under fully saturated conditions. However, the further increase of the PEGDME content in the Nafion matrix leads to undesirable micro phase separation (defects were observed from the morphological analysis), causing serious loss of the selectivity. Finally, in order to improve the theoretical understanding of the transport mechanism, a modified Maxwell model was successfully applied to describe the separation performances of the resulted Nafion/PEGDME hybrid membrane. The model results suggest that an interconnected CO2-philic structure is obtained upon the addition of PEGMDE and water to the ionomer matrix, forming preferential pathways for gas permeation able to enhance the membrane performance.