Waste heat recovery using thermally responsive ionic liquids through TiO2 nanopore and macroscopic membranes

Waste heat is the untapped heat produced by a thermodynamic process. It is usually released into the surrounding environment without any valorization. But recently, industrial waste heat has been identified as a promising energy source and many techniques have been proposed for its recovery. In this paper we present a method to convert low temperature waste heat (T < 100 °C) into salinity gradients, from which osmotic power is harvested across a nanoporous membrane. The heat is used to trigger the phase separation of a lower critical solution temperature (LCST) water–ionic liquid mixture. The two phases, of different salt concentrations, are fed in two reservoirs separated by a membrane with nanopores ≈30–100 nm in diameter. An osmotic electric current is measured across the membrane for various concentration ratios and pHs, the origin of which is shown to be a diffusio-osmotic (DO) process occurring at the TiO2 pore surface. The power density across the nanoporous TiO2 membrane is found to reach 7 W m−2. This opens up new avenues for the harvesting of waste-heat using nanoporous membranes and ionic liquids in a closed-loop configuration.