On twin substantially improved thermal properties and stability of pyrrolidinium-based ionanofluids with long multi-walled carbon nanotubes

The quest for advanced heat transfer fluids with superior thermal properties has led to the exploration of ionanofluids (INFs). Such systems are composed of ionic liquids (ILs) and thermoactive dispersed nanoparticles, and offer promising opportunities for the enhanced heat transfer. In this work, we selected long multi-walled carbon nanotubes (MWCNTs) as well as 1-propyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([C3C1pyr] [NTf2]) and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([C4C1pyr][NTf2]) as base liquids. The chosen ILs have the same thermal conductivity and similar viscosity which results in the 'twin' thermal conductivity of related INFs in the whole range of MWCNTs loading as well as similar rheological performance. We find the remarkable synergistic effects of incorporating 5.0 wt% of long MWCNTs into [C3C1pyr][NTf2]-based INF, resulting in a sky-rocketing 560% increase in thermal conductivity of INF compared to the base IL. The [C4C1pyr][NTf2]-based INFs exhibited excellent thermal and sedimental long-term stability (4 years), characterized by change in thermal conductivity and in density lower than declared uncertainties. We proved that remarkably enhanced thermal conductivity was mainly influenced by the high aspect ratio of MWCNTs enabling formation of long-range, 3D nanotube networks additionally stabilized by nanolayers of IL on the MWCNTs surface.