(666b) Water Sorption and Ion Transport in Ion-Exchange Membranes

Membrane-based separation technologies are attractive desalination methods due to their energy- and cost-efficiency and minimized emission and chemical waste. Perfluorosulfonic acids (PFSA) ion-exchange membranes (IEM) such as Nafion®, Aquivion® and 3M™ ionomers are used for industrial water treatment. Despite excellent chemical stability and ion-exchange performance, high manufacturing cost, HF generation, and toxicity of the material remain issues with PFSA membranes. Key technical challenges in the development of non-fluorinated separator membranes are the improvement of ionic conductivity, permeability and selectivity while achieving good chemical stability. We designed a non-fluorinated sulfonated precision polymer for cation-exchange membranes with various counterions (Li⁺, Na⁺, K⁺, Mg²⁺, and Ca²⁺). The water solubility of the polymer was regulated by precisely adjusting the degree of ionization. The ionic interactions between the functional groups and cations in the membranes with different forms were investigated using Fourier transform infrared spectroscopy (FTIR). After each membrane equilibrated with a salt solution matching the membrane's cation, the water absorption, and ion transport properties were measured. These properties showed significant differences depending on the type of cation. Higher water content, ionic conductivity and ion permeability were observed in the membranes with smaller cations. The membranes with multivalent ions exhibited excellent dimensional stability in dilute solution, implying that divalent cations function as ionic crosslinking agents. The results from this study suggested the possibility of the ion selectivity of the cation-exchange membranes. The results from this study revealed the potential for the selective ion separation with the newly developed IEMs. Future study will be conducted to investigate the ion separation performance to demonstrate the effectiveness and feasibility of the IEMs under practical operational conditions.