Modification of ion exchange membrane with polyacrylic acid to reduce the effect of polyvalent anion transfer during reverse electrodialysis

Abstract

This study investigated the generation of electrical energy by reverse electrodialysis using a laboratory device with internal manifolds equipped with Fujifilm Type X membranes. The device was fed with a more concentrated solution in which a total electrolyte concentration was 170.9 mmol-eq/L and a more dilute solution in which a total electrolyte concentration was 1.71 mmol-eq/L. The study aimed to test the performance of the laboratory reverse electrodialysis device using three variants of pairs of model solutions differing in the presence of polyvalent anions, to determine the maximum potential drop between the endplate electrodes and the power generated in the external circuit, to confirm the observation of a decrease in these characteristics in the variants where polyvalent anions were present in the solutions, and to test the possibility of reducing the negative impact of polyvalent anions by modifying the anion exchange membranes with polyacrylic acid. It was shown that in all cases the device generated a potential drop exceeding that expected according to the Nernst equation; the maximum generated power in the absence of polyvalent ions was 0.32 W/m² of membranes (1.29 W/m² of electrode), which is comparable to that achieved by pilot-scale units. Replacing 10% of monovalent anions with polyvalent ones in the concentrated solution channel reduced the maximum potential drop by 5%, and the generated power decreased to 0.19 W/m² of membranes; an analogous replacement of monovalent anions with polyvalent ones in the dilute solution channel resulted in a lesser effect. Modification of the anion exchange membrane with polyacrylic acid made it possible to eliminate the decrease in the maximum potential. At this stage it is impossible to conclude if the modification eliminates the decrease in the maximum power due to high experimental dispersion. It is expected that the effect will become clearer when the number of cell pairs would be increased. The obtained results can be used in the field of both reverse and traditional electrodialysis. Of interest for further research is the determination of the causes of experimental potential drop between the endplates exceeding the theoretical value, the possible beneficial use of this effect, as well as the creation of laboratory-scale reverse electrodialysis devices with a large number of cell pairs.