Understanding the Specific Heat Enhancement in Metal-Containing Nanofluids for Thermal Energy Storage: Experimental and Ab Initio Evidence for a Strong Interfacial Layering Effect

Nanofluids have emerged as an addition for thermal management and energy conversion applications. The dispersion of a small amount of solid nanoparticles occasionally leads to an unexpected enhancement in the specific heat of the dispersant fluid. This effect has technical, economic, and social significance, and for that, it has received a lot of attention from applied research, but the associated physical and chemical phenomena explaining this phenomenon are yet to be described. We report here a combined experimental and theoretical investigation of nanofluids consisting of palladium nanoplates in a typical heat transfer oil used for concentrating solar power. Their specific heat per unit volume is found to be maximally enhanced at intermediate nanoparticle concentrations, at all temperatures. This is consistent with the phenomenological description provided by the mesolayer model. Density functional theory calculations of adsorption energies and diffusion/desorption activation barriers reveal a strong interaction between the base fluid molecules and palladium surfaces, leading to a nanofluid model where the metal particles are decorated by a static layer of organic molecules. Such layering is potentially responsible for the anomalous enhancement on the thermal properties of the nanofluid, such as the specific heat. Our contribution with this work is a first step toward a complete understanding on the structure and properties of nanofluids using ab initio molecular simulation techniques rather than phenomenological descriptions only.