Water trapping inside anion exchange membranes during practical reverse electrodialysis applications

The power output of reverse electrodialysis (RED), an important renewable energy technology, can be improved using high-salinity feed solutions. Herein, a RED stack of ultrathin ion exchange membranes was operated continuously for 10 days using reverse osmosis brine (~0.9 M NaCl_equivalent) and underground water (~0.01 M NaCl_equivalent). The net power and net energy efficiency were initially 1.8 W m⁻²_{cell pair} and 40.8%, respectively, and then decreased gradually, as did the generated current and stack resistance. This deterioration was caused not by conventional membrane fouling but by trapped water inside the polymer matrix of the anion exchange membrane, especially near the cathode. The high salinity gradient and ultrathin membranes caused a flux imbalance between co-ion transport and osmotic water permeation. Further, bulk mass transfer was enhanced inside the RED stack to maintain electroneutrality. Therefore, combinations of membranes with high water permeability and permselectivity may be required to achieve stable RED operation.