Thermophysical Enhancement of Graphene Oxide-Enhanced Quaternary Nitrate for Concentrated Solar Power Applications

Abstract

With the continuous progress of global renewable energy, the reliability of the performance of heat storage materials is becoming increasingly important. In this study, graphene oxide (GO) was used as an additive to investigate its influence on the heat storage performance of quaternary nitrate molten salt. Quaternary nitrate molten salts doped in different proportions of 0.5, 1.0, 1.5, and 2.0 wt.% were prepared by the high-temperature hot melting method, and their properties were characterized in detail. The results show that the optimal concentration value of graphene oxide nanosheets is 1.0 wt.%, at which point the thermal parameters such as the specific heat capacity and thermal conductivity of the molten salt are optimal. Meanwhile, differential scanning calorimetry and thermogravimetric analysis tests verified the enhanced effect of the thermal performance. Furthermore, transmission electron microscopy and scanning electron microscopy analyses indicated that the insertion and encapsulation of nanosheets in the channel structure between nitrate crystals were effective. The modification methods used in this paper can enhance the thermophysical properties of nitrates. Meanwhile, the methods proposed in this paper can provide new ideas for the practice of heat-requiring systems.

1. Introduction

The global demand for energy is rapidly increasing, driven by industrialization and significant population growth. The current dominance of fossil fuels in energy generation is widely recognized as a key contributor to a range of pressing environmental issues, including climate change, air pollution, and resource depletion [1,2,3,4]. A transition to renewable energy is, therefore, essential in the pursuit of a sustainable future. Concentrated solar power (CSP) technology, which uses solar energy to generate electricity, is considered the best alternative for power generation to fossil fuels [5,6,7].

The selection of molten salts as heat transfer fluids in CSP systems is driven by their exceptional thermal stability, superior heat storage, and minimal vapor pressure properties [8,9,10]. Recent research findings have highlighted the cost-effectiveness and reduced corrosive properties of nitrates, in contrast to the higher corrosivity of halides [11,12,13].

However, the relatively low thermal conductivity of conventional nitrate salts has been shown to limit their heat transfer efficiency, which in turn adversely affects the overall performance and economic returns of CSP plants. This obstacle hinders the efficient extraction and application of solar energy, further weakening the competitive advantage of CSP technology in the market.

In the optimization study of the thermophysical properties of nitrate-based molten salt systems, improving the specific heat capacity and thermal conductivity has become a key research goal. Over the past few decades, researchers have achieved some degree of improvement in the molten salt thermal performance by incorporating nanoparticles such as SiO₂, MgO, and Al₂O₃. The Anagnostopoulos research group [14] conducted a comprehensive and systematic study on the application of nanomaterials in solar molten salts, focusing on analyzing the impact of the regulatory mechanisms of SiO₂ at different concentration gradients on the thermodynamic properties of composite molten salts. The experimental data show that the addition of trace nanoparticles can change the wetting behavior of the nanoparticle surface, such as dynamic contact angle changes, and can also alter rheological properties, such as viscosity control. Additionally, it can effectively delay the solid–liquid phase transition process of the molten salt system. Han et al. [15], by controlling the size distribution of Al₂O₃ nanoparticles, comprehensively evaluated their impact on the thermal properties of molten salts. Based on experimental measurements of the specific heat capacity and thermal diffusivity data, the research team built solid-phase thermal conductivity models under different temperature ranges and revealed the structure–activity relationship between the nanoparticle size and heat transfer efficiency. The experimental data showed that Al₂O₃ nanoparticles with size distributions of 200 nm and 135 nm could increase the average specific heat capacity of the solid and liquid phases by 17.2% and 19.7%, respectively. Li et al. [16] developed composite phase change materials using eutectic nitrate salts as the phase change matrix. By introducing MgO as a structural enhancer and optimizing the thermal conductivity network with graphite, the melting point was stabilized at 89.56 °C, with a decomposition temperature of 628 °C. Over the 50–600 °C temperature range, the material exhibited a high energy storage density exceeding 626 kJ/kg. The optimized CPCM formulation contained a 50% mass ratio of MgO matrix and allowed for more than 10% graphite addition, which increased the thermal conductivity of the composite system to over 1.4 W/(m·K). Nithiyanantham et al. [17] used in situ high-temperature microscopy combined with ζ potential analysis to systematically study the size-dependent effects of SiO₂ nanoparticles on molten nitrate salt systems. The study found that SiO₂ nanoparticles with a size greater than 450 nm exhibited better long-term stability compared to the 27 nm particle system, which could be attributed to the aggregation inhibition effect of larger particles in the molten salt medium. However, traditional nanoparticles generally exhibit lower thermal conductivity. This limitation has prompted researchers to explore two-dimensional nanosheets with high thermal conductivity and a large specific surface area as novel materials for improvement. Such materials include graphene oxide, hexagonal boron nitride, molybdenum disulfide, and expanded graphite. Zhang et al. [18] improved the solid-state thermal management performance by preparing a composite system of hexagonal boron nitride and solar salt with different boron nitride contents. When the mass fraction of nano-BN was 0.8%, the system’s specific heat capacity reached 1.81 J/(g·K), a 16.03% increase compared to the pure solid-state matrix without BN. This performance improvement was attributed to the reconstruction of nitrate salt crystal planes on the nano-BN surface, which increased the overall surface energy of the system, thus broadening the applicable temperature range and increasing the thermal conductivity by 18.52%. Xiao et al. [19] used a step-by-step process to prepare the composite materials, first introducing graphite oxide into HITEC salt or solar salt matrices via ultrasonic dispersion, then impregnating the nickel/copper metal foam in the salt solution, and introducing graphite to construct a nanocomposite structure. The experimental results showed that the thermal diffusivity of the salt/graphite/metal foam composite system was significantly better than the pure salt matrix, with an increase of 110–270% in the solid-state and 150–360% in the liquid state.

Research on porous nanocomposite materials has opened up new directions for applications in thermal energy storage. Madathil et al. [20] developed a ternary molten salt system based on MoS₂ and CuO nanoparticles, which effectively improved the thermal conductivity and specific heat capacity of nitrate-based molten salts by adjusting the nanoparticle ratio. Li et al. [21] prepared NaNO₃-LiNO₃-NaCl/expanded graphite composite phase change materials, and experiments showed that compared to pure NaNO₃, the phase change temperature of the ternary molten salt decreased by 45.3 °C, and the phase change latent heat increased to 59.4 J/g. When the amount of expanded graphite reached 20%, the thermal conductivity of the system increased from 1.350 W∙m⁻¹∙K⁻¹ to 5.017 W∙m⁻¹∙K⁻¹, demonstrating its potential for application in medium- and high-temperature energy storage fields. Liu et al. [22] innovatively designed a composite-phase change material block consisting of molten salt, expanded graphite, and graphite paper. By optimizing the component ratio, the thermal conductivity of the CPCM block reached 12.76 W∙m⁻¹∙K⁻¹, which was 2.08 times higher than that of the MgCl₂-KCl/EG block. After 1000 heating–cooling cycles, the thermal stability of the composite system performed excellently, verifying its reliability for long-term energy storage applications.

This study aims to investigate in depth how GO can be modified to better optimize the thermal storage and transport properties of high-temperature molten salts. Using different amounts of modified MXene, a mixture of GO and QN (GO/QN) was successfully fabricated, with the goal of developing an innovative composite material that is stable, safe, has high thermal conductivity, a low melting point, and a high SHC. To this end, the SHC, surface area, melting point, and decomposition temperature were measured. Improvements in these parameters were then analyzed to understand their effects on the thermal storage and heat transfer properties of solar salt. An in-depth study of the microstructure and chemical composition to reveal its microstructure and material composition at 30 °C was also conducted. The objective of this study is twofold: first, to enhance the thermal storage and heat transfer properties of solar salts in TES systems, and second, to further promote the popularization and application of MXenes in the field of high-temperature molten salt technology. In comparison with conventional materials, the material adopted in this work is anticipated to result in the substantial increase in the thermal energy storage efficiency to minimize energy loss and thus improve the efficiency of the system as a whole. While the cost of material currently is relatively high, the production cost of the material could be drastically decreased via optimization in the synthesis and scaling-up production process. In addition, with the CSP system efficiency improving, this material can shorten the energy recovery cycle and enhance the energy output; thus, it can gain more of an economical advantage in long-term production.

2. Materials and Methods

2.1. Materials

The quaternary molten salt matrix employed in this research was sodium nitrate (NaNO₃, ≥99.0%, Sigma-Aldrich, St. Louis, MO, USA), potassium nitrate (KNO₃, ≥99.0%, Merck, Darmstadt, Germany), calcium nitrate (Ca(NO₃)₂·4H₂O, ≥99.0%, Sigma-Aldrich, St. Louis, MO, USA), and sodium nitrite (NaNO₂, ≥99.0%, Alfa Aesar, Ward Hill, MA, USA), added in a given molar percentage. All chemical reagents were commercialized by reputable companies and were not further purified. The quaternary mixture was chosen because it possesses good thermal conductivity and melting temperature is relatively low, which makes this kind of material especially suitable for the application of effective thermal energy storage. As major ions, NaNO₃ and KNO₃ could offer relatively high heat storage capacity, whereas Ca(NO₃)₂·4H₂O and NaNO₂ can lower melting temperature and enhance the thermal conduction efficiency in general. Moreover, the composition of the four-component mixture also considers cost and its possibility in the application. The raw materials for the preparation of graphene oxide sulfuric acid (H₂SO₄, AR, 98%), phosphoric acid (H₃PO₄, AR, 75–85%), potassium permanganate (KMnO₄, AR), hydrogen peroxide (H₂O₂, AR, 35%), and ethanol absolute (C₂H₆O, AR, 99.5%) were acquired from Sinopharm Chemical Reagent Co. (Shanghai, China). Flake graphite with high purity (500 mesh) was purchased from Nanjing XFNANO Materials Tech Co., (Nanjing, China).

2.2. Experimental Procedure

2.2.1. Synthesis of GO Sheets

The preparation of GO followed a modified Hummers method. Graphite powder (3 g, 500 mesh) was placed into a large flask (500 mL) containing a mixture of concentrated H₂SO₄ (360 mL) and H₃PO₄ (40 mL). The flask was preset in a magnetic stirrer containing ice water. Subsequently, KMnO₄ (18 g) was slowly added under constant stirring, maintaining the reaction temperature at no more than 35 °C. Following the dissolution of the KMnO₄, the flask was stirred in a water bath at 50 °C for 12 h. Upon completion of the reaction and subsequent cooling to room temperature, the resulting acid mixture was gradually poured into pre-cooled 3% H₂O₂ (~400 mL). Notably, this process is accompanied by the release of substantial amounts of heat and sulfide gases, necessitating the execution of the reaction under conditions of optimal ventilation and continuous stirring. Subsequent to achieving room temperature, the mixture underwent centrifugation, and the whole process was carried out three times by centrifugation with 500 mL of deionized water, 3% HCl, and anhydrous ethanol in rotation for 30 min at 1000 rpm group for washing. The culmination of this process yielded brownish-yellow graphite oxide powder, subsequently obtained through a vacuum drying procedure at 50 °C for 2 h.

2.2.2. QN Synthesis

The NaNO₃, KNO₃, NaNO₂, and Ca(NO₃)₂ samples were subjected to a drying process at 150 °C in an oven for a period of 48 h, with the objective of removing moisture. Following this, the dried samples were meticulously mixed in a precise proportion, and subsequently ground in a planetary ball mill for 30 min. Thereafter, the samples were transferred to a silicon carbide crucible and placed in a muffle furnace, which increased in temperature from 30 to 400 °C at a rate of 10 °C/min and held at 400 °C for 4 h to ensure complete melting and thorough mixing of the nitrates. This entire process is known as the static melting method. The muffle furnace was then cooled to 280 °C and the molten salt mixture was removed and allowed to cool to room temperature. The mixture was then ground in a grinder for 2 min, after which the nitrates were sealed and stored in a constant temperature drying oven.

2.2.3. Synthesis of GO/QN CPCM

The GO/QN composite nanofluidic materials were prepared using a physical mixing method. Initially, the QN was ground in a planetary ball mill for 15 min. Subsequently, GO with a mass fraction of 0.5, 1.0, 1.5, and 2.0 wt.% per 100 g of QN was added and ground for a further 30 min. The resultant mixture was transferred to a crucible and placed in a muffle furnace held for 30 min. The resultant mixture was transferred to a crucible and placed in a muffle furnace held at 400 °C for 60 min to allow the GO to disperse perfectly in the QN via the Brownian motion of the nanofluidic. The resultant GO/QN nanofluidic material was then obtained. The samples added with 0.5, 1.0, 1.5, and 2.0 wt.% GO are hereafter denoted by QN0.5GO, QN1.0GO, QN1.5GO, and QN2.0GO, respectively.

2.3. Material Characterization

For microscopic morphological characterization of the experimental materials, field emission scanning electron microscopy (FE-SEM) was used, equipped with an energy-dispersive X-ray spectrometer (EDX) for elemental distribution and relative content analysis. Transmission electron microscopy (TEM) was used to obtain transmission images of materials and reveal their nanostructural features. Atomic force microscopy (AFM) was used to characterize the morphological dimensions, particle shapes, and surface roughness of the materials. Crystal structure analysis was performed using an X-ray diffractometer (XRD) with a scanning angle range of 5° to 90°. The chemical composition, elemental valence states, and chemical states of the material surfaces were determined using X-ray photoelectron spectroscopy (XPS).

The experimental thermal performance tests were carried out using a differential scanning calorimeter (DSC, DSC3/700, Mettler Toledo, Zurich, Switzerland). The test conditions included a temperature range of 50–450 °C, a heating rate of 10 °C/min, and a nitrogen (N₂) protective atmosphere with a flow rate of 50 mL/min. The thermal conductivity measurements were performed using a hot-plate thermal constant analyzer (CPTS-500, Foreda Co., Tianjin, China). The sample was pressed into a cylindrical shape with a diameter of 34 mm and a height of 10 mm at 15 MPa pressure using a powder press. During testing, the probe was clamped between two samples of the same size to obtain heat transfer data. The high-temperature thermal stability of the materials was assessed using a combined thermogravimetric-differential scanning calorimeter (TGA/DSC3+/1100, Mettler Toledo, Zurich, Switzerland). The test parameters included a temperature range of 50–800 °C, a heating rate of 10 °C min⁻¹, and a nitrogen/air mixed protective atmosphere with a flow rate of 20 mL min⁻¹.

All the data in the experiments were measured more than 1 time and average was calculated to validate the data and guarantee it. Every experiment was carried out at least three times and averaging data from independent attempts were completed to avoid random errors. These experiments have confirmed the repeatability of data and reliability of experiment.

2.4. Material Preparation

GO/QN composites were prepared by means of the impregnation melting method. GO and QN were prepared in the first instance. The resultant GO was subsequently amalgamated with QN to yield the final CPCM (see Figure 1).

3. Results and Discussion

3.1. Characteristics of the Composite Material

SEM measurements were utilized to analyze the morphology of the powder sample and the dispersion uniformity of the GO nanosheets in the tetramine nitrate. Ternary nitrate, GO, and their mixture were separately dispersed in an ethanol solution. After 20 min of ultrasonic oscillation in a water bath, a few drops of the dispersed liquid were taken and added dropwise to the ultra-thin copper grid. Following the drying process, gold was sprayed for 45 s using a Quorum SC7620 sputtering coater (Quorum, EastSussex, UK) at 5 milliamps. SEM analysis of the QN displayed distinct eutectic characteristics, exhibiting a smooth surface (Figure 2a). SEM analysis of the GO showed a flake-like structure, with some areas exhibiting fine wrinkles and folds due to its large monomers, indicating the successful preparation of the monolayer (few-layer) GO (Figure 2b). After the amalgamation of GO and QN, as depicted in Figure 2c, the nitrate molten salt is coated with GO on the interior surface, with GO functioning as the external contact surface. This configuration serves to reduce the interfacial thermal resistance between GO and QN and significantly enhance the thermal conductivity.

The TEM image reveals that high-energy electron beam exposure leads to the formation of a honeycomb structure within the QN, causing the molten nitrate to undergo a phase transition to a gaseous state (Figure S1a). The TEM observation of the QN/GO CPCM (Figure S1b) demonstrates that GO is coated on the surface of the molten nitrate, thereby validating the extent of the mixture as determined by the SEM. This coating can optimize the interfacial thermal resistance between GO and QN, thereby enhancing the thermal storage performance through a continuous heat transfer path [23]. According to AFM measurements, the vertical height of QN is 102 nm (Figure 2d,g). Moreover, QN also exhibits high surface roughness (Ra = 34 nm) (Figure S2), attributed to its excellent crystal properties with abundant grain boundaries (as shown in the XRD pattern). GO exhibits a typical two-dimensional sheet structure with a thickness of 473 pm (Figure 2e,h), indicating that effective exfoliation played a key role in maximizing the surface area and enhancing the interaction with QN. The surface of GO is relatively smooth (Ra = 0.258 nm, Figure S2) and has very few defects. The thickness of the QN/GO composite (Figure 2f,i) is 34 nm, a fraction of the thickness of the QN. Furthermore, the Ra of the QN/GO composite is considerably lower than that of QN (Ra = 5.67 nm, Figure S2), suggesting that GO has a uniform coating on QN. According to the AFM images and TEM images, the GO lamellae are uniformly and stably mixed with the molten salt matrix. The structure and thickness of the GO sheet is crucial for enhancing the thermal conductivity. Thinner GO sheets are more easily dispersed, evenly in a molten salt matrix, forming a better thermal-conduction network structure, resulting in higher thermal conductivity. Nevertheless, thicker or agglomerated GO sheets will lead to an interrupted thermal–conductive path of the material, thus obstructing the enhancement of the thermal conductivity. The influence of the surface functional groups of GO’s (e.g., carboxyl and hydroxyl groups) interaction with molten salt also led to a certain interface of thermal resistance being formed, which further hinders its thermal performance.

As shown in Figure 3, the elemental mapping of the salt composite reveals that the elements Na, Ca, K, O, C, and N are uniformly dispersed throughout the entire observation area, indicating a close binding and uniform distribution of the QN salt and GO in the composite. This phenomenon can be attributed to the fact that in the QN/GO composite, the microscopic movement of the liquid salt causes the GO to migrate and rearrange. This redistribution of nanosheets leads to improved uniformity within the microstructure, facilitating the formation of a dense and homogeneous composite.

QN demonstrates remarkable crystallinity, evidenced by the presence of multiple sharp diffraction peaks that can be indexed to KNO₃ (PDF#71-1558), NaNO₃ (PDF#079-2056), Ca(NO₃)₂ (PDF#07-0204), and NaNO₂ (PDF#06-0392). The absence of secondary phases is indicative of the presence of high-quality eutectics. The XRD patterns of QN and QN2.0GO (see Figure 4a–e) demonstrate a general trend of a decreasing peak intensity with an increasing GO content. This phenomenon can be attributed to the coverage of GO sheets, which hinder the interaction of the X-ray beam with the crystal structure, thereby reducing the peak intensity. Concurrently, the prominent peaks exhibit a slight shift towards lower angles with an increasing GO content, suggesting that the lattice of the GO intercalated material leads to an increase in layer spacing. GO materials possess a larger sheet structure, and their incorporation distorts the crystal structure or strains the material. The calculation of the grain size indicates that with the increase in the GO content, the grain size of the material changes from approximately 300 nm to 200 nm, which suggests that the GO content increases the lattice spacing.

To further elucidate the mechanism of changes in thermal conductivity and SHC, the surface area of the different materials was calculated using the Brunauer–Emmett–Teller (BET) method from the N₂ adsorption–desorption curve. As demonstrated in Figure 4f, the specific surface areas of GO, QN, QN0.5GO, QN1.0GO, QN1.5GO, and QN2.0GO are 136, 65, 71, 83, 72, and 66 m²/g, respectively. The interaction between the GO nanosheets and the molten salt fluid is predominantly influenced by the specific surface area, with an increase in the surface area corresponding to an increase in the contact area. The incorporation of GO nanosheets into QN has been shown to enhance the specific surface area, thereby increasing the number of heat transfer channels and, consequently, enhancing the thermal conductivity and SHC [24].

XPS is a technique used to analyze the composition and chemical state of materials. The high-resolution N 1s, K 2p, and C 1s XPS spectra of QN and QN/GO are shown in Figure 5a–c. In QN, N 1s shows a peak at 405.9 eV, corresponding to the coordination environment of NO₃⁻ (Figure 5a). Concurrently, the K 2p XPS spectrum of QN displays two peaks at 294.5 and 291.6 eV, corresponding to 2p 1/2 and 2p 3/2, respectively (see Figure 5b). The high-resolution C 1s XPS spectrum of the composite material (Figure 5c) reveals three C 1s peaks at 284.8, 291.6, and 294.4 eV, which can be attributed to C-C and satellite peaks in the test, respectively. Following the introduction of GO, no substantial alterations were observed in the N 1s, K 2p, and C 1s peaks of QN/GO.

As illustrated in Figure 5d, the XPS batch map of Na in the QN and QN/GO CPCMs reveals a peak at 1070 eV in QN. However, with the incorporation of GO, the Na 1s peak undergoes a gradual shift towards lower energies, which may be attributed to the electrostatic interaction between oxygen from GO and sodium cations, leading to a reduction in the binding energy of Na. Figure 5e presents the high-resolution XPS spectra of Ca 2p. The QN spectrum displays two double peaks at 349.7 and 346.1 eV, corresponding to 2p 1/2 and 2p 3/2, respectively. With the incorporation of GO, the Ca 2p peak undergoes a gradual shift towards higher energy, suggesting that the oxygen functional groups present in GO, such as the hydroxyl, carboxyl, and epoxy groups, interact with calcium. These oxygen functional groups possess the capacity to attract the electron density from calcium, thereby increasing the effective nuclear charge of calcium and causing the calcium 2p peak to shift to a higher binding energy. This effect can be attributed to the reduction in the electron density around calcium caused by the negative charge of the oxygen atoms. Figure 5f presents the high-resolution O 1s XPS spectrum. In QN, a single O 1s peak is observed at 531.5 eV, corresponding to the oxygen element in the NO₃⁻ ion. Following the introduction of GO, the peak intensity of O 1s increases, but the peak begins to shift to lower energy. When GO is mixed with QN, it exhibits highly functionalized oxygen groups. In comparison with the initial state (QN devoid of GO), the incorporation of GO into QN may result in a greater number of oxygen atoms becoming exposed on the surface. This is due to the fact that GO, when present in a thin layer or dispersed state within the QN system, leads to the exposure of a greater number of oxygen-containing functional groups on the surface, as measured by XPS. Concurrently, metal cations and nitrate (NO₃⁻) groups are capable of donating the electron density to the oxygen atoms in GO. The donation of electrons by these groups to oxygen atoms increases the electron density surrounding oxygen atoms, thereby reducing their electronic defects and, consequently, their binding energy. The introduction of GO leads to an increase in interaction sites, which facilitates enhanced mixing between QN and GO. The enhancement of intermolecular forces is likely to result in an improvement in the thermal stability of the CPCM.

3.2. Thermophysical Properties of the QN/GO Composite

The latent heat of the material was tested by differential scanning calorimetry (DSC), and the experimental data indicate that the latent heat value of the proposed material can be high in the material phase transition process. The material’s operating temperature range for phase transition is related to the working temperature range of the CSP system and it indicates a good heat storage performance. And, its latent heat of the material is 112 J/g under the temperature range of 69–173 °C. DSC testing demonstrated that the melting point of QN is approximately 76 °C (Figure 6a). The incorporation of GO nanosheets into the CPCMs shifted their melting points by +6.92, −6.32, −3.32, and −5.84 °C, respectively, relative to QN (the error is less than 1%). This result indicates a substantial reduction in the melting point of QN, accompanied by a broadening of the operational temperature range. Additionally, observations of the crystallization curve revealed that the solidification temperatures of the CPCMs increased by 2.8, 10.2, 13.9, and 15.5 °C, respectively, significantly reducing the supercooling rate of QN molten salt (Figure S3).

In the field of thermodynamics, SHC is a pivotal physical parameter, as it quantifies the amount of heat required to elevate the temperature of a substance. In the context of solar power generation systems, where the operational temperature range of QN is 200–400 °C, SHC plays a crucial role. The SHC of the initial QN salt and the QN/GO CPCM with varying GO mass fractions was calculated. The incorporation of GO was observed to influence the SHC in both solid and liquid phases, as depicted in Figure 6b. The SHC continued to increase as the CPCM recrystallizes approached 80 °C, which corresponds to the phase transition temperature. Upon reaching its operating temperature, the CPCM achieved a state of essentially stable SHC. The SHC of QN was determined to be 1.61 J∙g⁻¹∙K⁻¹ in the range of 250–400 °C. As the mass fraction of GO was increased from 0.5 wt.% to 2.0 wt.%, the corresponding SHCs were 1.46, 1.74, 1.82, and 1.59 J∙g⁻¹∙K⁻¹. However, the SHC remained high only briefly after the addition of 0.5 wt.% GO. A substantial enhancement in SHC was observed upon the incorporation of 1.0 and 1.5 wt.% GO.

The dependence of material properties on the concentration of graphene oxide (GO) may not necessarily be linear. Certain points at a specific concentration of GO may have a more obvious performance enhancement than higher or lower concentrations. For example, in the cement-based composites, the optimal content of GO has been reported to not be the higher the better and an optimum exists [25,26]. Too high a concentration of GO would cause the agglomeration of GO instead of the reinforcing effect and material performance reduction, while too low a concentration still cannot effectively exert the GO-reinforcing effect [27]. This could be associated with GO dispersion in the matrix, an interfacial bonding effect with the matrix material and the matrix microstructure.

To gain a more detailed understanding of the thermal stability of the CPCM with the addition of GO, a series of thermogravimetric tests were carried out. As shown in Figure 6c, the decomposition temperature of the CPCM can reach up to 701 °C with the addition of GO, indicating good thermal stability. From TGA measurement results, the material thermal resistance ability of a high temperature has been effectively improved. We assume that the improvement is mainly reflected in the fact that the graphene oxide structure of GO can construct a high strength network structure in the material. This increases the strength of the material at a high temperature and reduces the material thermal degrading situation. The oxidative functional groups (such as carboxyl and hydroxyl groups) of GO lamella could interact with the ions of the molten salt matrix and thus have the role to improve the thermal stability of the material as well as lower the decomposition temperature. Second, the choice of the quaternary mixture of the molten salt matrix, particularly the ratio of NaNO₃ and KNO₃, has also the key role to improve the thermal stability. These elements can mitigate the high-temperature thermal degradation reaction and facilitate the material thermal tolerance. In the 300–600 °C temperature range, after a period of exposure, its corrosion rate is maintained at a low level so that it has good corrosion resistance under high-temperature conditions. We consider that the addition of GO contributes to improving the corrosion resistance of the material because it can reduce the corrosion reaction on the surface of the material through interactions between its oxidation functional groups and the matrix molten salt. Nevertheless, more long-term corrosion tests are still essential to further test the real application performance of the CSP system. A comparison of QN with QN/GO reveals that the decomposition temperature of the CPCM remained relatively stable, and both materials exhibited standard thermal stability characteristics.

To this end, a thermal constant analyzer was used to measure the thermal conductivity of the CPCM at room temperature (25 °C, Figure 6d). The thermal conductivity of pure QN was measured to be 1.05 W∙m⁻¹∙K⁻¹. The incorporation of GO, a material known for its high thermal conductivity, resulted in the following changes: the thermal conductivities of QN0.5GO, QN1.0GO, QN1.5GO, and QN2.0GO were 0.81, 1.13, 1.09, and 0.93 W∙m⁻¹∙K⁻¹, respectively. The thermal conductivity increased significantly at 1.0 and 1.5 wt.% GO; however, due to the stacking effect and content, the thermal conductivity of QN0.5GO and QN2.0GO did not increase proportionally with the GO content. To further explain the potential application of the prepared materials in CSP systems, their high-temperature thermal conductivity was predicted, and the average value of their thermal conductivity in the temperature range from 300 °C to 600 °C can be around 0.7–0.8 W∙m⁻¹∙K^−1. In the temperature range from 300 °C to 600 °C, the thermal conductivity of the material decreases slightly with the increasing temperature. However, there are no sudden changes or large fluctuations in the thermal conductivity. We observed the drop in the material performance at 2.0 wt.% GO. With an increased GO content, lamellar layers of GO in the material might aggregate together, resulting in the lower dispersion of the material, which will influence the thermal conductivity and uniformity of molten salt. When GO particles aggregate, the interfacial area of GO particles will be reduced and the heat conduction path will be irregular, therefore, reducing the heat conduction properties. Second, the interface between GO and the molten salt matrix is saturated when the GO content is at a 2.0 wt.% level. With the increasing quantity of GO, there will be no considerable enhancement of the interface contact, which may cause some irregularities in the structure of the material. It will not effectively change the whole performance of the material. Thus, the optimum GO addition amount ought to be in the range of 1.5–2.0 wt.% to avoid these undesirable effects while enhancing the thermal properties of the material. Compared with the reduced graphene oxide (RGO) and graphene quantum dots (GQDs) used in the study by Hamdy et al. [28], this study directly adopted GO and significantly improved the specific heat capacity, thermal conductivity, and thermal stability of the molten salt at the optimal concentration of 1.0 wt.%, which has the advantage of simple synthesis. The optimization of this proportion was verified by differential scanning calorimetry and thermogravimetric analysis, indicating that the addition of GO effectively enhanced the heat storage capacity and thermal stability. Transmission electron microscopy and scanning electron microscopy analyses have revealed the insertion and encapsulation mechanisms of GO nanosheets in the nitrate crystal channel structure, which provides a new perspective for improving the thermophysical properties of nitrates and offers potential technical support for the application of thermal storage systems in photovoltaic power stations. Ahmed, S. F et al. [29] demonstrated that graphene doping leads to a 5–13% increase in the heat capacity. In this study, a better thermal performance improvement was achieved at a specific GO concentration.

A comparison of the thermophysical properties of the material before and after the addition of GO revealed a significant improvement in the performance of QN during operation. This approach contributes to a reduction in the thermal resistance at the interface between QN and GO and enhances the efficiency of the heat transfer between different components, thereby improving the overall thermal conductivity of the material. This enhancement can increase the heat storage capacity of the material, enabling it to absorb and retain more heat over an extended period. This, in turn, results in the improved overall operating efficiency of the solar thermal power system.

QN molten salt with the addition of GO demonstrates excellent thermal conductivity, thermal stability, heat storage, etc., which has obvious advantages; however, there are still some disadvantages. For example, it is very difficult to synthesize and disperse GO and there are also some manufacturing processes involved, which are expensive and need to be carefully adjusted, and will inevitably increase the additional industrial cost. In addition, while the GO could positively influence the performance at a certain concentration, too high a concentration of GO might result in a loss of the uniform dispersion of the material, and then exert a detrimental influence on the overall performance. We still need to perfect the process of synthesis and resolve the potential problems of dispersion and stability.

Though the synthetic approach of this paper has been proved to be effective in the laboratory, regarding industrial production, the reaction conditions and equipment in large-scale production must be optimized, such as the reaction time and temperature control. Additionally, in the integration process of the QN matrix and GO, there should be special attention paid to the uniform dispersion of GO, which may need more sophisticated stirring and high-temperature treatment technology. Despite these problems, we hold that with the further maturity of technology and with difficulties of cost reduction and high production efficiency, industrial production can be realized, e.g., in centralized solar power generation and thermal energy storage systems.

4. Conclusions

A new type of four-component nitrate/graphene oxide (QN/GO) composite phase change material was prepared via a high-temperature mixing process. In the target operating temperature range, when the graphene oxide (GO) mass fraction was 1.5% (QN1.5GO), the system’s specific heat capacity (SHC) reached 1.82 J∙g⁻¹∙K⁻¹, which was a 13.1% increase compared to the pure QN matrix. Further testing showed that the thermal conductivity of QN1.0GO increased to 1.13 W∙m⁻¹∙K⁻¹, 8% higher than that of the QN matrix. The performance improvement was mainly attributed to the high specific surface area of graphene oxide, whose layered structure enhanced the interface interaction with the QN matrix, increasing the overall surface energy of the system. This optimized the thermal energy storage capacity. Furthermore, the coating of graphene oxide on the QN matrix surface significantly reduced the interfacial thermal resistance, creating more heat transfer pathways and further improving the thermal conductivity. The current key challenge in this research lies in achieving the uniform dispersion of two-dimensional nanomaterials in molten salt systems and optimizing their corrosion resistance, which is crucial for promoting their large-scale application in concentrated solar thermal power systems.