Surface Modification of Few-Layered Graphene Nanoplatelets for Enhanced Energy Transportation in Nanofluids

Low-dimensional carbon materials, such as graphene, have superior thermal conductivity (3,000-5,000 W m^-1 K^-1) properties and are due to their lightweight materials of choice as advanced heat transfer media, especially for the transport of heat from sustainable power stations like concentrated solar power plants. Excellent thermal transport properties of the transport media can be achieved when the low-dimensional carbon nanomaterial is dispersed into conventional host fluids like water (0.613 W m^-1 K^-1) or ethylene glycol (EG; 0.250 W m^-1 K^-1). These so-called nanofluids can significantly improve the heat transfer characteristics. The combination of two-dimensional structure and high surface area as well as its cost-efficiency, make graphene nanoplatelets (GNPs) suitable for large-scale application in colloidal thermal conductive fluids. For an efficient dispersion of GNPs in base fluids, intrinsically hydrophobic GNPs were treated with a mixture of nitric and sulfuric acid for a surface modification to obtain highly concentrated (4 wt.%) graphene-based nanofluids with high thermal conductivity and low viscosity. Investigations on different reaction parameters and different GNP sizes showed significant influences of the surface area on the resulting thermal conductivity values of the nanofluids regarding the pristine carrier fluid. After 14h of measurement in a dormant system, the most efficient particles reached a thermal conductivity of 0.586 W m^-1 K^-1 (base fluid: 0.391 W m^-1 K^-1) and low viscosity of 6.39 cP resulting in an overall higher efficiency of 77 %, when compared to the base fluid.