Improved Thermophysical Properties of Developed Ternary Nitrate-Based Phase Change Material Incorporated with MXene as Novel Nanocomposites

In this study, nanocomposite of ternary nitrate molten salt induced with MXene is developed. LiNO3-NaNO3KNO3 with wt% of 35:12:53 and 35:10:55 are produced and doped with MXene in the wt% of 0.2, 0.5, 1.0, and 1.5. FTIR result indicates the composites had no chemical reaction occurred during the preparation. UV-VIS analysis shows the absorption enhancement with respect to the concentration of MXene. Thermogravimetric analysis (TGA) was used to measure the thermal stability of the LiNO3-NaNO3-KNO3 induced with MXene. The ternary molten salts were stable at temperature range of 600–700°C. Thermal stability increases with the addition of MXene. 1.5 wt% of MXene doped with LiNO3-NaNO3-KNO3 with wt% 35:10:55 and 35:12:53, increases the thermal stability from 652.13°C to 731.49°C and from 679.82°C to 684.57°C, respectively. Using thermophysically enhanced molten salt will increase the efficiency of CSP.

A major drawback in harvesting solar energy is, it is only available when there is sunlight. This can be overcome by using HTF as Thermal Energy Storage (TES). TES addresses the interval between the availability of solar power and the electricity needs, as well as reducing higher costs of electricity produced by CSP. Sensible TES was installed in industrial CSP plants in a two-tank molten salt facility. Latent TES with PCM is considered a cost-efficient option, since during transition process it provides higher energy storage density and insulating behaviour. It has been thoroughly studied in recent years and currently is transitioning from research by pilot-scale demonstration to commercial deployment [3][4][5].
Water is a good low-temperature sensible heat storage medium because it is inexpensive, has high specific heat and readily available. Its usage temperature is around 25-90°C. For 60°C temperature transition, waters store 250 kJ/kg. As transport medium and as energy storage, water, for instance, can be used for solar energy systems. However, it needs to be expensive isolation and stress tolerant because of its high vapour pressure for high temperature applications [6].
Because of the high temperature operating CSP systems, inorganic salts and salt eutectics with melting points above 300°C and metals and metal alloys are considered attractive candidates for PCMs [3,7,8]. In order to produce effective and stable PCM-TES systems the PCM should have the necessary high latent fusion heat, phase change temperature, reasonable chemical and thermal stability and reasonable corrosion in the containment material at high pressure and after repeated thermal [9][10][11].
Due to their massive latent melting heater, flexibility over a relative range of temperatures, an outstanding heat power and a huge variation of melting temperatures from one salt to another, the molten salts tend to be quite fascinating [12][13][14]. Salt mixtures with greater thermal stability can certainly provide better performance by extending the range of working temperatures [15,16]. Molten salts consisting of the sodium nitrate and potassium nitrate were used successfully in the big-scaled, SCR (Solar Control Power Panel) at temperatures up to 565°C [17] as a thermal energy collection and storage fluid. However, salt mixtures with lower melting points will increase performance by expanding the working temperature and by simplifying the start-up process [18,19]. The NaNO 3 , KNO 3 , and NaNO 2 ternary mixtures (7%-53%-40%) with the melting point of 142°C, will also typically be used in solar thermal power stations for heat transfer storage material. The molten salt is higher at the same pressure and the vapour pressure is below, the thermal conductivity doubles the volume of most organic thermal fluids. It also has the viscosity of high temperature water, making it an effective heat transfer and heat transfer storage material for thermal solar power [20]. Ternary molten salt is also environmental friendly and non-toxic [21]. Tab. 1 represents the recent studies on the ternary nitrate-based molten salts.
However, its relative poor energy storage capacity is the biggest downside of molten salt acting as a heat carrier [23]. One of the main parameters influencing device size and output price is the basic heat capacity that specifically exposes the responsive heat that a material will store by unit mass or volume. Technical breakthroughs are important to enhance device performance and ultimately reduce the unit electricity price. Molten salt-based, special nature nanofluids will provide a potential alternative for reducing CSP  (20) 130 600 N/A Li 2 CO 3 (32.1)-Na 2 CO 3 (33.4)-K 2 CO 3 (34.5) 400 800-850 N/A device sizes and costs. Shin et al. [24] observed that with the addition to carbonite eutectics of a limited number of nanoparticles of silica (not more than 1%) their particular heat potential could increase up to 24%. This rise was also detected by the dispersal of nanoparticles in various salts: CuO [25], Al 2 O 3 [26] and TiO 2 [27].
The special physical and chemical properties of two dimensional (2D) materials have gained great interest after graphene was first rendered in a single layer in 2004 through mechanical exfoliation [28]. Recently, the latest class of 2D metal carbide/nitride intermediate materials known as MXene has received considerable attention from both theoretical and experimental physicists and chemists [29][30][31][32].
MXenes have excellent property such as high chemical stability, high electrical conductivity and ecofriendly characteristics due to their special structures. Up to now, the materials MXenes are promising in semi-conducting, hydrogen storage, supercapacitors, and lithium-ion batteries [33,34]. MXenes' hydrophilic nature and its large functions on its surface have rendered it successful in many molecular and ionic speed adsorbents MXenes.
In this research work, a novel nanocomposite with nitrate-based phase change materials is developed and induced with MXene to optimize the thermophysical properties. To the best of authors' knowledge, this is the first study of developing ternary nitrate-based nanocomposites incorporating promising MXene nanoparticles. This study has investigated various ratio of nitrate-based molten salts to develop two different types of ternary nitrate-based molten salts as host materials in order to develop novel nanocomposites. In this study, four samples are prepared with different concentrations of MXene nanoparticles.

Material and Method
Potassium nitrate is bought from Fisher Scientific chemicals with a molecular weight of 101.103 g/mol, the melting point of 337°C, the density of 2.1 g/m 3 and CAS-No 7757−79-1 Sodium nitrate (NaNo 3 ) is obtained from R&M chemicals Co with a molecular weight of 84.99 g/mol, melting point of 308°C, the density of 2.3 g/m 3, and CAS-No 7631-99-4. Lithium nitrate (LiNO 3 ) with a molecular weight of 68.95 g/mol, melting point of 255°C, the density of 2.38 g/m 3 and CAS-No 7790-69-4 is purchased from Acros Organics chemical company. Fig. 1 shows the overall methodology flow chart.

Preparation of Ternary Molten Salt
LiNO 3 -NaNO 3 -KNO 3 salt was prepared at 35:12:53 wt%. 3.501 g of LiNO 3 was weighed using microbalance (TX323L, UNIBLOC) and added into 50 ml borosilicate beaker. 30 g of ultrapure deionized (DI) water was injected into the beaker. The mixture is stirred at 50°C on hotplate stirrer at 500 rpm for 30 min to dissolve LiNO 3 in DI water. In the beaker, 1.200 g NaNO 3 was added and stirred for further 30 min. 5.301 g KNO 3 has been added into beaker and stirred for another 1 h. Then the mixture is evaporated for 2 h at 140°C on hot plate. Finally, the salt is put in a dry oven at 140°C for 2 h to eliminate the chemically bonded water molecules. After 2 h, the samples were kept in a 20 ml disposable scintillation vial. The same process was repeated to produce LiNO 3 -NaNO 3 -KNO 3 salt with 35:10:55 wt%. Fig. 2 demonstrates the adopted methodology for the preparation of ternary nitrate-based molten salts.

Preparation of MXene (Ti 3 C 2 T x )
MXene (Ti 3 C 2 T x ) synthesis process was conducted by the following materials as received without any further purification: MAX Phase material (Ti 3 AlC 2 ) from Y-Carbon Inc., Ltd., USA, Ammonium hydrogen difluoride (reagent grade 95%, Sigma Aldrich) and sodium hydroxide (97% purity, pellets, Sigma Aldrich) were obtained. The wet-chemistry method was adopted for the synthesis of MXene nanomaterial. The  The prepared 20 ml solution of ammonium hydrogen difluoride was placed on hot plate magnet stirrer (RCT BASIC, IKA), followed by stirring for one h at 300 rpm and room temperature. Afterward, 1 g of Ti 3 AlC 2 was weighed using a microbalance (Explorer series, EX224, Ohaus), followed by adding to the uniform wellprepared NH 4 HF 2 solution. Due to the exothermic reaction, the addition of MAX Phase (Ti 3 AlC 2 ) to the NH 4 HF 2 solution was performed slowly. In order to perform the etching process, the MAX Phase suspension in the NH 4 HF 2 was stirred using magnetic stirrer at 300 rpm for 48 h and room temperature continuously. A dilute solution of NaOH was added slowly after completion of etching process until the pH of the settlement reached 6, and was filtered and rinsed several times with deionized water. The washing process took place in a high speed centrifuge (Sorvall LYNX 6000, Thermo Scientific) for 4 washes was performed (each wash of 10 min) at 3500 rpm. The obtained Ti 3 C 2 T x multi-layered solution was then sonicated using an ultrasonic probe sonicator (FS-1200 N) for 1 h to obtain delaminated MXene (d-Ti 3 C 2 T x ) in the setting of a power of 60% and on/off time of 7/3 s. By using vacuum oven (VO 500, MEMMERT Germany) the synthesized delaminated flakes of MXene nanomaterial were dried for overnight. The required characterizations for the synthesized MXene are available in our previous research study [19].

Preparation of LiNO 3 -NaNO 3 -KNO 3 /MXene
Mixed with LiNO 3 -NaNO 3 -KNO 3 (35:12:53 wt%), are four distinct levels of MXene (0.2, 0.5, 1.0, and 1.5 wt%). Preparation of 0.2 wt% of MXene of LiNO 3 -NaNO 3 -KNO 3 /MXene: 0,998 g molten salt was weighed in microbalance to a 100 ml of borosilicate beaker. The beaker added then 0.002 g MXene. The beaker holds 40 ml of DI water, with the mixture being extracted at 60°C and 700 rpm for 30 min. This is then accompanied by a 30 min stirring in ultrasonic probe sonicator (FS1200N) using 7-second duration on time and 3-second off time setting. Sonicator capacity is held at 55%. The mixture is then moved to a glass petri dish, placed on a hot plate for evaporation at temperature 120°C. After that, the petri dish is put in an oven (VO 500, MEMMERT GERMANY) with 60°C, 100 millibar of pressure for 1 h. The resulting nanocomposite is then placed in a vial (50 ml) until the thermophysical properties are calculated. This process is repeated to prepare 1 g each of LiNO 3 -NaNO 3

Characterisation
In this study, the specific heat capacity (c p ) and melting temperature measurements of the pure binary molten salt and alkali binary molten salt-based hBN nanocomposites are performed using a differential scanning calorimetry (DSC). DSC-8000 (Perkin Elmer, USA) is a double-furnace, power compensation DSC with greater sensitivity and accuracy instrument. The colorimetric and temperature accuracy of the above-mentioned DSC is considered as <±0.2% and ±0.05°C, respectively. The measurements are conducted using aluminum crucibles of 40 μl. The temperature range for measurements is between 25-500°C with the heating rate of 10°C/min. The c p values and melting temperature analysis was through Pyris software.

FTIR
PerkinElmer Spectrum Two-UATR spectra with integrated detector of MIR TGS (15000-370 cm -1 ) is used to detect the peak and the functional group of pure LiNO 3 -NaNO 3 -KNO 3

UV-Vis
PerkinElmer Lambda 750 is used to conduct Ultraviolet-visible spectroscopy (UV-Vis) to obtain optical absorbance. Absorption data is obtained at room temperature with the wavelength range from 800 to 200 nm. The scan speed adjusted is 266.75 nm/min with the 860 nm monochromatic.

Thermogravimetric Analysis
The thermal stability of the molten salts is obtained using thermogravimetric analysis (TGA). Based on the outcome, the mixture of pure LiNO 3 -NaNO 3 [35]. No other new peaks were found within the FT-IR spectrum of the composites. This result show that the composites were physical combined, no chemical reaction occurred during the preparation and the composites have good thermal stability [36,37].

Conclusion
In this paper, thermal stability, FTIR and UVVIS analysis has been conducted on ternary nitrate salt of pure LiNO 3 -NaNO 3 -KNO 3 with the ratio of (35:10:55) and LiNO 3 -NaNO 3 -KNO 3 for the ratio of (35:12:53) are investigated by adding MXene as the nanoparticles with loading concentrations of 0.2, 0.5, 1.0, and 1.5 wt%. Thermal stability of the samples assessed using TGA. From the results obtained, ternary  . With the addition of MXene the thermal stability increases. FTIR shows that composites were physical combined and no chemical reaction occurred during the preparation. UVVIS shows that the absorption increases with the concentration of MXene. Enhancing thermal stability would improve the thermal energy that can be retained before being injected to the power block for the production of electricity. This is critical as a higher temperature could be required to work in a turbine centered on the conventional Rankine cycle. Future studies could focus on the investigation of other important parameters, such as enthalpy, melting point, viscosity, specific heat and thermal conductivity. Conflicts of Interest: The authors declare that they have no conflicts of interest to report regarding the present study.