Abstract
In this study, novel asymmetric integral cation exchange membranes were prepared by the wet phase inversion of sulfonated polysulfone (SPSf) solutions. SPSf with different degrees of sulfonation (DS) was synthesized by variation in the amount of chlorosulfonic acid utilized as a sulfonating agent. The characterization of SPSf samples was performed using FTIR and 1H-NMR techniques. SPSf with a DS of 0.31 (0.67 meq/g corresponding ion exchange capacity) was chosen to prepare the membranes, as polymers with a higher DS resulted in poor mechanical properties and excessive swelling in water. By a systematic study, the opportunity to tune the properties of SPSf membranes by acting on the composition of the polymeric solution was demonstrated. The effect of two different phase inversion parameters, solvent type and co-solvent ratio, were investigated by morphological and electrochemical characterization. The best properties (permselectivity of 0.86 and electrical resistance of 6.3 Ω∙cm2) were obtained for the membrane prepared with 2-propanol (IPA):1-Methyl-2-pyrrolidinone (NMP) in a 20:80 ratio. This membrane was further characterized in different solution concentrations to estimate its performance in a Reverse Electrodialysis (RED) operation. Although the estimated generated power was less than that of the commercial CMX (Neosepta) membrane, used as a benchmark, the tailor-made membrane can be considered as a cost-effective alternative, as one of the main limitations to the commercialization of RED is the high membrane price.
Highlights
As a result of a parametric study carried out at different sulfonation degree, sulfonated polysulfone with 0.31 degrees of sulfonation (DS) was selected considering the mechanical stability of membranes prepared via wet phase inversion
Aiming at the optimization of the electrochemical properties of cation exchange sulfonated polysulfone (SPSf) membranes as a function of their morphology, the experimental work was complemented by the preparation and characterization of membranes using two different main solvents (NMP, DMF) and a co-solvent (IPA) with the aim of appropriately modulating the exchange rate between the non-solvent (IPA) and the solvent mixture
The most promising membrane—i.e., the one resulting from the SPSf dissolved in an 80:20 NMP/IPA solvent mixture was further characterized for different NaCl solution pairs to estimate the theoretical power generation potential under the Reverse Electrodialysis (RED) operation
Summary
Having fixed charged moieties in their polymeric matrix, IEMs are able to repel ions with the same sign (co-ions) in an electrolytic environment, while oppositely charged ions (counter-ions) are attracted. The exclusion property against a specific charge is referred to as permselectivity, considered as one of the two main performance indicators together with ionic resistance. Swelling degree (SD), a parameter strongly affecting the mechanical stability of IEM, increases with increasing ion exchange capacity (IEC), since the elevated hydrophilic fixed charge content of a membrane drastically enhances the interactions of the polymeric chains with water molecules. Resistance and permselectivity decrease with a higher IEC because water channels form across the membrane, favoring non-selective ion transport. Low membrane cost is decisive for affordable technological solutions
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