Trade-off between selectivity and permeability in organic-solution-based ionic osmotic energy conversion

Organic waste liquids generated in industrial processes contain organic solvents and high ion concentrations, representing a substantial source of osmotic energy. Ionic osmotic energy conversion is a promising technology for recovering this energy, but its performance is constrained by the trade-off between ion selectivity and permeability. The effects of organic solvents on this balance remain unclear. In this study, aqueous solution was employed as the reference system, and the area-specific resistance, the open-circuit voltage, the short-circuit current, and the power density were measured in methanol, ethanol, and isopropanol systems. The results showed that area-specific resistance increased with solvent molecular size, whereas the power density was highest in methanol and lowest in isopropanol. Notably, the methanol system achieved a power density of 0.71 W·m⁻², surpassing the 0.50 W·m⁻² obtained in the aqueous system. Molecular dynamics simulations revealed that permeability decreased as solvent molecular size increased. Compared with water, methanol formed larger ion solvation structures and induced stronger electrostatic exclusion, thereby increasing the energy barrier for anions entering the nanochannels. Consequently, the cation transference number increased from 0.533 to 0.583, indicating enhanced ion selectivity. Although ion permeability was reduced, the improvement in selectivity predominated in the methanol system, resulting in superior energy conversion performance. These findings indicate that in water and methanol systems, selectivity plays a dominant role, whereas in ethanol and isopropanol systems, insufficient permeability becomes the primary factor limiting energy conversion despite stronger anion rejection. This work provides theoretical guidance for optimizing osmotic energy recovery from organic liquids.