## 2 Nanofluid Synthesis and Specific Heat Measurements
### Specific Heat Measurements  
The specific heat was measured using a DSC (TA Instruments, New Castle, DE, USA, Q20) up to 560 \({}^{\circ}\)C with a ramping rate of 20 \({}^{\circ}\)C/min. According to the measured heat-flow data, the specific heat of the nanofluids was determined via the standard test method ASTM E1269 [41]. For every measurement, DSC runs of an empty pan (denoted as \"baseline run\") and sapphire (denoted as \"sapphire run\") were conducted prior to DSC runs of the nanofluid (denoted as \"sample run\") and were employed to calculate the specific heat of each nanofluid. The specific heat capacity in the liquid phase was averaged within the temperature range of 525 to 555 \({}^{\circ}\)C. Avoiding inaccurate measurements  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Mole Fraction of Each Salt** & **Specific Heat in Liquid Phase [J/g-K]** \\ \hline
**Sample Name** & **Li\({}_{2}\)CO\({}_{3}\) [mol \%]** & **K\({}_{2}\)CO\({}_{3}\) [mol \%]** & **Specific Heat in Liquid Phase [J/g-K]** \\ \hline Sample 1 & 90 & 10 & 2.582 \\ Sample 2 & 80 & 20 & 2.503 \\ Sample 3 & 62 & 38 & 1.646 \\ Sample 4 & 50 & 50 & 1.543 \\ Sample 5 & 40 & 60 & 1.325 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Chemical compositions and specific heat of carbonate salt mixtures for Group 1 samples.  
Figure 2: Synthesis procedures for molten salt nanofluids.  
of the heat flow supplied to the aluminum pans due to the warming of the heaters in the calorimeter, we executed a heating cycle with the same ramping rate in the DSC and maintained an isothermal state of 560 \({}^{\circ}\)C for more than 5 min prior to recording the heat flows. Hermetically sealed aluminum pans were employed for the sample runs to prevent mass loss during the thermal cycles. The same thermal cycle from 150 to 560 \({}^{\circ}\)C was repeated five times for the sample runs to check for variations and deteriorations in the specific heat of the nanofluids. Finally, the enhancements were calculated as follows.  
\[Enhancement[\%]=\frac{\mathcal{C}_{p,Nanofluid}-c_{p,pure}}{c_{p,pure}}\times 100 \tag{1}\]  
The specific heat of pure carbonate salt eutectic was measured in this study to check the sample preparation and measurement protocol by comparing the measurement to a reference value. On the other hand, the specific heat values of the pure salt mixtures were obtained from a previous study [31] in which the specific heat was measured via the same protocol employed in this study. Using the specific heat values of the pure salt mixtures, the specific heat enhancement was determined for each nanofluid. According to our experimental measurements, it was observed in this study that the GA cannot increase the specific heat of the carbonate salt mixtures. Moreover, it is well known that the GA is thermally decomposed between 250 and 350 \({}^{\circ}\)C [42]. Therefore, the GA is decomposed during the first DSC ramping process. For these reasons, the effect of the GA on the specific heat increase of the molten salt nanofluids is negligible or even unfavorable. Hence, the increase of the specific heat is solely attributed to the nanoparticles dispersed into the salt mixture. Scanning electron microscopy (SEM, JSM-7500F, JEOL, Japan) was used to capture images of the nanoparticles in the salt mixtures and examine the dispersion homogeneity of the nanoparticles.