# An Assessment of Anion Exchange Membranes for CO2 Capture Processes: A Focus on Fumasep® and Sustainion®

## TL;DR

This study evaluates two anion exchange membranes for their ability to capture CO2 under dry conditions, comparing their performance in terms of absorption, permeability, and selectivity.

## Contribution

The study provides new data on CO2 absorption and permeation in Fumasep® and Sustainion® membranes under dry conditions, which were previously unreported.

## Key findings

- Fumasep® and Sustainion® show similar CO2 absorption levels, but Fumasep® has higher CO2 permeability and CO2/CH4 solubility selectivity.
- CO2 permeability in both membranes reaches a minimum at around 2–3 bar pressure and increases with CO2 concentration due to matrix swelling.
- The CO2/CH4 perm-selectivity of both membranes is comparable to fluorinated ion exchange membranes like Nafion®.

## Abstract

Anion exchange membranes are utilised in cutting-edge energy technologies including electrolysers and fuel cells. Recently, these membranes have also emerged as a promising tool in CO2 capture techniques, such as moisture-driven direct air capture and the separation of CO2 from other gases, leveraging the moisture-induced sorption/desorption and diffusion of CO2 in its ionic forms. In this study, we examine the absorption and permeation of CO2 and CH4 in two commercially available anion exchange membranes, Fumasep® and Sustainion®, under dry conditions. With the exception of CO2 sorption in Fumasep®, these measurements have not been previously reported. These new data points are crucial for evaluating the fundamental separation capabilities of these materials and for devising innovative CO2 capture strategies, as well as for the simulation of novel combined processes. In a dry state, both materials demonstrate similar CO2 absorption levels, with a higher value for Sustainion®. The CO2 solubility coefficient decreases with pressure, as is typical for glassy polymers. Fumasep® exhibits higher CO2/CH4 ideal solubility selectivity, equal to ~10 at sub-ambient pressures, and higher diffusivity. The CO2 diffusion coefficient increases with the CO2 concentration in both membranes due to swelling of the matrix, varying between 0.7 and 2.2 × 10−8 cm2/s for Fumasep® and between 1.6 and 9.0 × 10−9 cm2/s for Sustainion®. CO2 permeability exhibits a minimum at a pressure of approximately 2–3 bar. The CO2 permeability in the dry state is higher in Fumasep® than in Sustainion®: 3.43 and 0.72 Barrer at a 2-bar transmembrane pressure, respectively. The estimated perm-selectivity was found to reach values of up to 40 at sub-ambient pressures. The CO2 permeability and CO2/CH4 estimated perm-selectivity in both polymers are of a similar order of magnitude to those measured in fluorinated ion exchange membranes such as Nafion®.