Isotonic separation enabled efficient low-grade heat conversion with thermal-responsive ionic liquids

The osmotic heat engine (OHE) stands out as a promising approach for converting low-grade heat into electricity owing to its scalability. However, challenges arise from the substantial latent heat consumption or the restricted concentration difference during the regeneration stage, constraining improvements in energy efficiency and power density. Here, we introduce an isotonic separation process to a thermal-responsive ionic liquid (IL)-based OHE system, where the water-rich phase of the IL after phase transition is further filtrated into pure water and another part of the IL-rich phase with constant osmotic pressure. This isotonic process amplifies the net energy generated in the mixing process. Ultimately, a state-of-the-art Carnot-relative efficiency of 48% is achieved within a temperature difference of 25–40 °C, where the hot side is maintained at 60 °C. Additionally, a forward-osmosis device yields an exemplary peak power density of 10.9 W/m 2 .