In one aspect, reverse electrodialysis systems are described herein having constructions operable to reduce membrane stack resistance, thereby requiring significantly less membrane surface area for meaningful electrical power generation. A reverse electrodialysis system described herein comprises an anode and cathode adjacent to a membrane stack, the membrane stack comprising alternating anion and cation exchange membranes defining diluate and concentrate ionic solution compartments, wherein an ion exchange medium is positioned in a diluate compartment.