Enhancing power density of reverse electrodialysis by adding non-permeable ions in dilute electrolytes

Abstract

Reverse electrodialysis (RED) generates energy from salinity gradients between water sources such as seawater and river water, separated by ion-exchange membranes. Although RED is a promising technology for sustainable power generation, its performance is limited by low power density. Lowering the ion concentration in the dilute solution enhances the salinity gradient but also increases resistance, creating a fundamental trade-off. Here, we introduce a new strategy to resolve this limitation by adding non-permeable ions to the dilute electrolyte. Confined by monovalent selective membranes, these ions reduce resistance while preserving the salinity gradient, thereby boosting current flux. Using MgSO4 as a model additive, we demonstrate up to 168 % improvement in power density compared with conventional RED systems. The system using only MgSO4 in the dilute electrolyte achieved a maximum power density of 0.74 W/m2 at 15.2 A/m2, maintaining 93.9 % of its initial performance over 144 h. This finding overturns prior assumptions about the detrimental role of multivalent ions and highlights their potential as performance enhancers. Supported by experiments and numerical simulations, our study establishes a new design principle for advancing RED and other membrane-based energy conversion technologies.