Impact of humidity on gas transport in polybenzimidazole membranes

Polybenzimidazoles (PBIs) are promising materials for high temperature H2/CO2 separation in applications such as steam reforming and pre-combustion carbon capture where significant amounts of water are often present. However, PBIs are hydrophilic, and the impact of humidity on PBI gas separation properties is relatively unexplored. Furthermore, opportunity exists to elucidate the interplay between plasticization, free volume, and gas transport in glassy polymer membranes such as PBIs. This study investigates the effect of humidity on H2, O2, and CO2 permeabilities at 35 °C in a commercial PBI and two sulfone-containing PBIs. Water uptake significantly reduces PBI gas permeabilities at low humidities due to competitive sorption and antiplasticization. At high humidities, plasticization increases the permeabilities of larger gases in more hydrophilic PBIs. Effective fractional free volumes evaluated from gas permeation data and previously reported water sorption and dilation data suggest water plasticizes PBIs by increasing accessible free volume via enhanced molecular dynamics rather than by creating new free volume cavities. [Display omitted] •Humidity causes loss of H2/CO2 selectivity in three PBIs at 35 °C by up to 90%.•At low relative humidities, water uptake reduces PBI gas permeabilities at 35 °C.•At high relative humidities, plasticization can increase CO2 permeabilities.•Low amounts of sorbed water fill and reduce PBI free volume, slowing gas permeation.•Plasticization may increase effective free volume by enhancing chain mobility.