Reverse emulsion-templated nanoencapsulation of NaNO3 with crystalline TiO2 shell for high-temperature heat energy storage and thermal corrosion resistance

NaNO 3 is a typical molten salt usually acting as a latent heat-storage medium for use in solar power plants; however, it has some critical drawbacks such as high subcooling, easy leakage in the molten state, and high temperature corrosivity. To address these problems in their applicability for solar power plants, a reverse emulsion-templated in-situ polycondensation technology was successfully developed to encapsulate NaNO 3 with a crystalline TiO 2 shell. The effects of the mass ratio of core to shell and the types of oil phase and emulsifier on the morphology and phase-change performance of the as-synthesized TiO 2 @NaNO 3 nanocapsules were extensively investigated, and the corresponding synthesis mechanism was intensively studied. A higher mass ratio of core to shell enables the formed TiO 2 @NaNO 3 nanocapsules to gain a higher latent heat capacity and smaller particle size. Meanwhile, the use of the oil phase with a lower polarity and emulsifier with a higher lipophilicity can ensure the establishment of a highly stable reverse emulsion templating system for self-assembly of titanic precursors surrounding the NaNO 3 micelles and follow-up in-situ polycondensation. The TiO 2 @NaNO 3 nanocapsules exhibit a minimized nanometric particle size along with a well-defined core-shell structure when synthesized in cyclohexane as an oil phase using poly(ethylene oxide- co -propylene oxide- co -ethylene oxide) as an emulsifier at a core/shell mass ratio of 1/0.5. Under such an optimal condition, the resultant TiO 2 @NaNO 3 nanocapsules not only obtain a maximum latent heat capacity with a Δ H C of 106.5 J g −1 and a Δ H M of 134.2 J g −1 , but also present outstanding excellent high-temperature heat energy-storage perform and good thermal cycling stability. More importantly, the TiO 2 @NaNO 3 nanocapsules exhibit good resistance to thermal metallic corrosion due to the encapsulation of NaNO 3 with an inorganic shell, which effectively protects against the oxidation of iron by contacting molten salt at high temperature. The TiO 2 @NaNO 3 nanocapsules developed in this study show good applicability as a high-temperature heat energy-storage medium for solar power plants. • NaNO 3 was encapsulated with a crystalline TiO 2 shell via reverse emulsion-templating polycondensation. • Optimal synthetic conditions were determined for TiO 2 @NaNO 3 nanocapsules to gain excellent performance. • TiO 2 @NaNO 3 nanocapsules exhibit a high latent heat capacity for high-temperature heat energy storage. • TiO 2 @NaNO 3 nanocapsules present an effective resistance to thermal metallic corrosion. • TiO 2 @NaNO 3 nanocapsules show good applicability as heat energy-storage medium in solar power plants.