Anion exchange membrane design for reserve electrodialysis

Abstract

Reverse electrodialysis (RED) is a clean, sustainable technology for the generation of energy from the mixing of solutions with different salinity. The ion exchange membranes are key elements in RED. Especially the study of anion exchange membranes is crucial since limited research has been done specifically for RED. This PhD thesis investigates the design and development of the RED process, focusing on fabrication, characterization and optimization of anion exchange membranes. Chapter 2 deals with the fabrication of anion exchange membranes in a simple, environmentally friendly manner. Different membrane properties and characterization methods are studied and implemented. The directions towards how to tune membrane properties specifically for RED are defined. For the first time, it was shown that tailor-made anion exchange membranes were installed in a RED stack. Chapter 3 systematically investigates the bulk membrane properties of both a series of commercially available membranes and tailor-made membranes (both anion and cation exchange membranes) and correlates these to experimental RED performance data. The results set directions to decrease the area resistance rather than to improve permselectivity because area resistances of the membranes were found to be the dominant parameter regarding RED performance. In Chapter 4, the practical potential of monovalent ion selective anion exchange membranes is investigated. As RED in natural conditions requires the use of natural seawater and river water, the presence of multivalent ions in the feed has a decreasing effect on the power output in RED. To prevent that, monovalent ion selective membranes were fabricated. Monovalent selectivity comparable to that of their commercially available counterparts membranes was achieved. Chapter 5 is dedicated to the fabrication of micro-structured membranes that eliminate the spacer shadow effect, which occurs when non-conductive spacers are used. They reduce the effective area of the membranes for ionic transport. To eliminate that, structured anion exchange membranes having straight-ridge, wave or pillar structures were fabricated. Pillar-structured membranes were advantageous compared to the other types. The last part, Chapter 6 discusses the future potential of the RED process and the directions to further develop the process.