Thermally Conductive Molten Salt for Thermal Energy Storage: Synergistic Effect of a Hybrid Graphite-Graphene Nanoplatelet Filler.

Renewable energy technologies depend, to a large extent, on the efficiency of thermal energy storage (TES) devices. In such storage applications, molten salts constitute an attractive platform due to their thermal and environmentally friendly properties. However, the low thermal conductivity (TC) of these salts (<1 W m-1 K-1) downgrades the storage kinetics. A commonly used method to enhance TC is the addition of highly conductive carbon-based fillers that form a composite material with molten salt. However, even that enhancement is rather limited (<9 W m-1 K-1). In this study, the partial exfoliation of graphite to graphene nanoplatelets (GnP) in a molten salt matrix is explored as a means to address this problem. A novel approach of hybrid filler formation directly in the molten salt is used to produce graphite-GnP-salt hybrid composite material. The good dispersion quality of the fillers in the salt matrix facilitates bridging between large graphite particles by the smaller GnP particles, resulting in the formation of a thermally conductive network. The thermal conductivity of the hybrid composite (up to 44 W m-1 K-1) is thus enhanced by two orders of magnitude versus that of the pristine salt (0.64 W m-1 K-1).

Total exfoliation of graphite in molten salts.

The exfoliation of graphite to graphene nanoplatelets (GnP) in a molten salt medium is investigated in this study. It is shown that this mechanical force-free process yielded a large-sized GnP product (>15 microns) with a low defect density. The effect of the surface tension of the molten salt on graphite exfoliation efficiency was investigated for a series of alkali chloride salts (CsCl, KCl, NaCl and eutectic NaCl-KCl) at 850 °C. It was demonstrated that the produced GnP could be completely and easily separated from the salt. Molten salt with the lowest value of surface tension (CsCl) displayed the highest wettability of the graphitic layers and hence facilitated total exfoliation of the graphite to GnP. The exfoliation of graphite in molten salts is applicable in the thermal energy storage field, as well as in exfoliation of other layered materials. Herein, it is demonstrated that the thermal conductivity of the GnP-CsCl composite is enhanced by ∼300% compared to the neat salt.