Transport mechanism of carbon dioxide through perfluorosulfonate ionomer membranes containing an amine carrier

A new facilitated transport model for CO2 through ion-exchange membranes containing a diamine complexing agent was developed. The diamine ion behaves as a mobile carrier for CO2. Although the morphology of the ion-exchange membrane affects carrier transport, the effect of morphology on ionic carrier transport is not clear. The Nernst-Planck equation and the penetration model were employed in this modeling study. The electrical double layer effect and friction effect in the ion-exchange membrane was also considered. In the membrane, there are two kinds of counter ions (NH3+-R-NH2 and NH3+-R-NH3+), CO2 and NH3+-R-NHCOO- (carbamate ion). The carbamate ion can be treated as a neutral molecule because it has both plus and minus charge. Commercial Nafion117 (N117) and heat treated Nafion117 (HN117) were used as ion-exchange membranes. The water content of N117 and HN117 was 16 and 45%, respectively. Nafion has cluster channels which were filled with water, and HN117 has a larger cluster channel size than N117. Monoprotonated ethylenediamine was used as a carrier. Mobile counter ion diffusivities were measured by membrane conductivity. Carbon dioxide diffusivity was determined from transport measurements in a nonreactive Nafion membrane. The diffusivity ratio of carbamate ion to CO2 was estimated by the group contribution method which is effective in aqueous solutions. We estimated a friction effect for the carbamate ion which reduces the carbamate ion diffusivity ratio in the cluster channel. For the HN117 membrane case, experimental results and simulations were in good agreement when we used the diffusivity ratio which was estimated from the group contribution method The counter ion diffusivities, which are restricted by electrical forces, are the rate limiting step for CO2 transport through large clusters. For the N117 case, we must consider the friction effect, and when we use a small carbamate diffusivity ratio, simulations and experimental results agreed well. The diffusivity of the carbamate ion, which is the largest molecule in the membrane, is the rate limiting step for transport through small cluster channels. This model can explain the permeate-side CO2 pressure effect as the permeate-side CO2 pressure seriously reduces the facilitation effect. Copyright (C) 1996 Elsevier Science Ltd