**4. Conclusions**

1. We successfully synthesized magnesium oxide nanoparticles in situ in the ternary nitrate system through the high-temperature decomposition of the Mg(OH)2 precursor. When the content of MgO nanoparticles does not exceed 3 wt %, the dispersion performance is more appropriate, and there is no agglomeration. The size of the nanoparticles synthesized in situ is concentrated in the range of 50–200 nm.

2. Nano-magnesium oxide can improve the heat storage performance of nitrate. When the content of MgO nanoparticles is 2 wt %, the specific heat capacity of the solid increases by 51.54%, and the specific heat capacity of the liquid increases by 44.50%. The increase in specific heat capacity is related to the interface thermal resistance and the semi-solid layer.

3. MgO nanoparticles synthesized in situ significantly improve the heat transfer performance of ternary nitrate. When the content of MgO nanoparticles is 5 wt %, the thermal diffusion coefficient of the modified salt increased by 39.3%. In the molten state, MgO nanoparticles and ternary nitrate form a heat transfer fluid. Molten salt nanofluid is an excellent heat exchange medium in the field of heat storage, with a high heat exchange capacity.

**Author Contributions:** Conceptualization, X.C. and Z.T.; methodology, Z.T.; software, L.L.; validation, L.L., Y.L. and Q.W.; formal analysis, Z.T.; investigation, Y.L.; resources, Q.W.; data curation, Z.T.; writing—original draft preparation, Z.T.; writing—review and editing, X.C.; visualization, L.L.; supervision, Y.L.; project administration, X.C.; funding acquisition, X.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Technology and Innovation Major Project of Hubei, grant number 2021BGE023 and 2020BED002.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** All the data are available within the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **References**

