Aromatic Polymers Functionalized with Anionic and Cationic Groups for Ion Exchange Membranes

Abstract: Fuel cells, redox flow batteries and some water desalination technologies such as reverse osmosis and electrodialysis require efficient ion exchange membranes to perform in an optimum way. Thus, by enhancing the properties of these membranes there is a possibility to improve this type of applications and technologies. Depending on the specific application, anion or proton exchange membranes with good thermal, chemical and mechanical stability combined with an efficient ion transport are sought for. In the current study, the effect of local ion concentration on properties such as water uptake and ion conductivity was investigated. This was done by systematically controlling the local density of ionic groups. When four instead of two sulfonic acid groups were located on neighbouring aromatic rings on a polysulfone backbone, the proton conductivity under reduced relative humidity was significantly increased. A correlation between distinctive ion clustering, reduced water uptake and an increase in ion conductivity was observed when the stiffness of the polymer backbone was increased by the replacement of the flexible ether bridges in the polymer structure by stiffer sulfone bridges. Anion exchange materials are less prone to form ion clusters. To study this, polymers with a well-controlled local ion density were synthesized. The ion concentration was varied from being random to a placement of the ions in pairs, triplets and quartets. This allowed for a systematic study of the effect of ion distribution and a high local ion concentration was observed to promote ionic clustering. However, a reduction in ion conductivity was also seen as a result of high local ion concentration, probably due to the formation of ion condensates. In order to be able to use anion exchange membranes in fuel cells they need to withstand a highly alkaline environment. This was not the case for the polysulfone-based materials, thus a material with poly(2,6-dimethyl-1,4-phenylene oxide) as polymer backbone was produced. In this case, the cationic groups were distanced from the polymer backbone by a flexible aliphatic spacer. This did not only allow for an enhancement of the ion conductivity, but also a significant increase in the stability under alkaline conditions, thus making it an interesting material for further analysis and development.