Micro-structured membranes for electricity generation by reverse electrodialysis

Reverse electrodialysis (RED) is a technology for extracting salinity gradient power by contacting waters with different salinity, i.e. seawater and river water, through ion exchange membranes. Conventionally, non-conductive spacers are used to separate these ion exchange membranes from each other in RED. The power output is hampered by these non-conductive elements which increase electrical resistance in the RED stack. To eliminate the use of these spacers, structured anion exchange membranes with a structure height of 100 µm were prepared by casting a polymer solution on stainless steel molds followed by solvent evaporation. These self-standing membranes with straight-ridge, wave and pillar structures as well as similarly prepared flat membranes were installed on the river water side in a RED stack (where electrical resistance is the highest). 38% higher gross power density and 20% higher net power density were achieved with the pillar-structured membranes when compared to that of flat membranes with spacers. Further optimization of the structure geometry in combination with the possibility to cast membranes of different chemistries offer a huge potential for further development of homogeneous membranes with the desired electrochemical and physical properties, which could provide high power densities in RED.