3D-Printed Ion-Conductive Spacers for Enhanced Reverse Electrodialysis in Salinity Gradient Energy Systems

Sammanfattning

Reverse electrodialysis is a membrane-based technology that utilizes the salinity gradient between two solutions (e.g. river water and seawater) to generate sustainable energy. A critical component of RED systems is the spacer, which ensures effective flow distribution and ion exchange through the membranes. Conventional spacers suffer from the shadow effect, which limits ion transport and reduces power output. Moreover, the low ion conductivity in the solution with lower salt concentration leads to higher resistance, influencing power density. This project explores the modeling, functionalization and electrochemical performance of 3D-printed ion-conductive spacers to overcome these limitations. Spacers with different open areas were fabricated using fused deposition modeling, a 3D-printing technique. Ion-conductive spacers were prepared by sulfonation of 3D-printed spacers. Characterization techniques including ion exchange capacity, energy-dispersive X-ray spectroscopy and swelling measurement were employed to understand the degree of sulfonation. An optimal sulfonation time of 15 minutes resulted in a sulfonation degree of 1.22 meq/g while maintaining mechanical stability. Electrochemical testing was conducted on 2 cell and 5 cell RED stacks using artificial feed solutions. The effects of varying river water concentration (concentration gradient), and spacer type on RED performance parameters were analyzed. Increasing the open area of 3D-printed spacer from 58 to 85% improved the power density by 38%. The best performing condition, using seawater (0.5 M) and river water (0.0051 M) with sulfonated spacer, achieved a maximum power density of 0.78 W/m2, 2.7-fold higher than that of commercial spacers. Using 5-cell, it was possible to achieve the same enhancement factor, with only slight variation in power density. Additionally, functionalized spacers contributed to 27% improvement in power density compared to the non-functionalized spacers with identical geometry. Further testing with synthetic saline solution with multivalent ions, simulating natural waters condition, showed a 17% reduction in power density due to increased membrane resistance and lowered open-circuit voltage. These results indicate that a custom-made 3D-printed ion-conductive spacer tailored for RED application could enhance the system performance. Future research should investigate the performance of these spacers with natural waters and evaluate their long-term stability under continuous operations.