Advances in the Stabilization of Eutectic Salts as Phase Change Materials (PCMs) for Enhanced Thermal Performance: A Critical Review

Abstract

Inorganic phase change materials (PCMs) can be employed in passive thermal regulation systems as building envelopes to decrease energy consumption. Nonetheless, they present a manifold of issues, such as leakage, incongruent melting, crystallization, and supercooling, which limit their performance and durability. A widely explored approach to address these shortcomings is the development of eutectic salts and their stabilization through techniques such as the use of porous substrates and encapsulation, in addition to combining them with the incorporation of carbon derivatives as fillers and nucleating agents to enhance thermal performance and durability during charge and discharge cycles. In this study, a critical review is developed via analysis and discussions of different methods for incorporating inorganic PCMs. The focus is mainly on eutectic salts and the challenges associated with their application, the generation of new eutectic salts, stabilization methods, and use cases where the incorporation of fillers, the use of porous substrates, and the implementation of nucleating agents have contributed to improving thermal performance, reducing the degree of supercooling, and minimizing PCM leakage during phase transitions.

1. Introduction

The temperature increase driven by climate change has intensified reliance on air-conditioning systems to maintain indoor thermal comfort. However, the immediate effect of this increase is a corresponding energy consumption increase and higher greenhouse gas emissions [1,2,3]. One strategy to deal with this issue is the application of exterior coating on buildings, which plays a critical role as thermal envelopes provide comfort. The selection of the materials employed to develop such coatings can either increase or reduce heat transfer into the buildings [4,5].

The use of inorganic phase change materials (PCMs) is regarded as an effective solution to lowering the energy demand for cooling and heating, as they reduce indoor temperature fluctuations and improve thermal comfort through heat storage [6,7]. When incorporated into the building envelope, these materials act as thermal regulators, dampening temperature peaks during phase transitions. By maintaining a stable indoor temperature and reducing peak loads, the performance of heating and cooling systems is optimized, thereby decreasing their operating time to reach the desired temperature [8,9,10].

Nonetheless, one of the main drawbacks of inorganic PCMs is the leakage of material in its liquid phase during phase transitions [11,12,13]. To address this shortcoming, the development of PCMs confined within porous support materials has been proposed. Among these, carbon-based materials such as multilayer graphene oxide have proven effective in retaining significant amounts of PCMs within their structure through capillary action and interfacial interactions, thus preventing leakage [14,15]. Likewise, expanded perlite has been reported to stably contain up to 55% hydrated salt while reducing thermal conductivity by more than 70% [16]. In this arrangement, porous materials not only prevent liquid leakage but also modulate thermal conductivity [12,17]. Current research on inorganic PCMs is therefore focused on their stabilization through the use of supports and encapsulation techniques [18,19,20].

An alternative approach consists of the use of boron nitride foam (BNF) as a substrate and eutectic hydrate salt (EHS), the techniques of differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and thermal conductivity were evaluated using a thermal conductivity instrument, Xiatech TC3000E, which has demonstrated reduced supercooling, higher latent heat, elimination of leakage issues, and a 10-fold improvement in thermal conductivity with merely a 4% PCM incorporation regarding poly (acrylamide-co-acrylic acid) copolymer (PAAAM) [21]. To further enhance the applicability of PCMs in the construction industry, both stabilization and enhanced energy storage capacity are required. In this regard, graphene nanoplatelets emerged as a promising support material. Graphene nanoplatelets effectively prevent PCM leakage while improving thermal conductivity by 114%, with a low degree of supercooling of only 0.1 °C [22].

In this study, an analysis and discussion of different methods for PCMs incorporation is provided with a primary focus on eutectic salts and the challenges associated with their application, the development of new eutectic salts, and the recently explored stabilization strategies. These include incorporation of fillers, the use of porous substrates, and the implementation of nucleating agents. These strategies aim at enhancing thermal performance, reducing the degree of supercooling, and minimizing PCM leakage during phase transition [23,24].

2. Data Collection Methodology

Prior to establishing the database, a search was conducted in Scopus and Web of Science (WoS), using the same keywords and Boolean operators, resulting in a large number of duplicate items, in addition to the fact that the number of articles found in WoS was greater. This literature review covered studies published between 1998 and 2025 from the Web of Science database applying the methodology established in Figure 1.

Table 1. Criteria for evaluating the quality of the database.

Reference	Year	Theoretical Model	Experimental Model	Statistical Analysis	Durability Tests	Quality Assessment
[25]	2024		√			Low
[26]	2024	√	√		√	Medium
[27]	2024		√			Low
[28]	2023	√	√		√	Medium
[29]	2023	√	√		√	Medium
[30]	2023	√	√			Medium
[31]	2023	√	√		√	Medium
[32]	2023		√		√	Medium
[33]	2023		√		√	Medium
[34]	2022		√		√	Medium
[35]	2022		√			Low
[36]	2022		√	√		Medium
[37]	2022		√			Low
[38]	2022		√		√	Medium
[39]	2021		√	√		Medium
[40]	2021		√			Low
[41]	2021		√		√	Medium
[42]	2020		√			Low
[43]	2020		√		√	Medium
[44]	2020		√	√		Medium
[45]	2020	√	√			Medium
[46]	2020	√	√	√		Medium
[17]	2018		√		√	Medium
[47]	2018		√		√	Medium
[48]	2018	√	√	√	√	High
[49]	2017		√			Low
[50]	2017	√	√	√	√	High
[51]	2017		√		√	Medium
[52]	2016		√		√	Medium

establishes the quality control parameters for the 29 items filtered after the trend analysis, comparing 4 aspects outlined in the manuscript: it defines whether the study is purely experimental or uses any theoretical simulation or modeling method, whether it lacks statistical analysis, and finally, whether it includes a durability analysis.

From a total of 319 documents retrieved from the Web of Science (WoS) database and subsequently exported to VOSviewer (version 1.6.20), Figure 2 presents the results of the keyword analysis used by the authors and their thematic correlations. This analysis was conducted with the aim of identifying trends and innovations within the field of study, while also considering the temporal relationship between the keywords and their year of publication.

In the figure, the yellow color represents publications after 2022; the size of the spheres indicates the frequency of occurrence of each keyword in the literature, and the larger the sphere, the higher the number of repetitions.

The network analysis reveals a significant trend in the use of the term “composite”, which generally refers to the combination of techniques involving a substrate, a modified salt, and a coating method. Likewise, a relevant connection is observed with the keyword “eutectic hydrated salt”, allowing the identification of an emerging research line over the past five years related to these concepts.

In addition, other prominent keywords such as “phase separation”, “nucleation”, “graphite”, and “optimization” were also considered in the information search and development for this manuscript. These terms are used in the literature both to describe specific challenges and to propose solutions in studies involving eutectic salts as part of the synthesis of composite materials.

3. Eutectic PCM Stabilization

PCMs are classified according to their chemical composition and divided into three sub-groups: organic PCMs (these include paraffin waxes and fatty acids), inorganic PCMs (including hydrated salts and metallic compounds), and a mixture derived from the previous ones called eutectic PCMs (organic–organic, inorganic–inorganic, and organic–inorganic). Inorganic PCMs offer several advantages over organic PCMs, including lower costs, suitable phase change temperatures, and greater durability. However, they also present drawbacks such as salt leakage during phase transitions, phase separation, and supercooling [40]. These limitations highlight an opportunity for the development and application of eutectic salts, which can be produced from different types of PCMs. The possible combinations fall into three main categories: organic–organic, inorganic–inorganic, and organic–inorganic.

Eutectic salts, obtained by mixing two or more hydrated salts, are particularly notable for their lower melting points, higher energy density, and enhanced thermal properties compared to their precursor salts [20,40,51]. As illustrated in Figure 3, the present study focuses on analyzing the advances, advantages, disadvantages, and production methods of eutectic salts, emphasizing stabilization strategies for hydrated salts. These include the use of porous support materials, the addition of graphite derivatives as fillers, the use of polymers for encapsulation, and even the combination of two or more of these stabilization techniques.

During the selection process of PCMs for a specific application, several factors must be considered, including cost, availability, and thermal properties such as phase change temperature, specific heat, and thermal conductivity. However, when the properties of individual PCMs do not meet the desired requirements, they can be combined to produce eutectic PCMs.

The choice of salts to be combined in a eutectic mixture depends on the intended application. For instance, in the construction industry, where the thermal regulation range typically lies between 20 and 45 °C, low phase change temperature hydrated salts such as Na₂SO₄·10H₂O, MgSO₄·7H₂O, and CaCl₂·6H₂O are commonly used. Conversely, when temperature control is required at higher ΔT values, salts with higher melting points, such as Al₂(SO₄)_3·18H₂O, MgCl₂∙6H₂O, and Ba(OH)₂∙8H₂O are recommended, as they exhibit fusion temperatures above 35 °C.

Equally important is the determination of the eutectic point, at which the mixture melts simultaneously, ensuring uniformity and preventing segregation during the phase change process. For this purpose, Schrader’s equation (Equation (1)) can be applied, as it provides the relationship between the thermophysical properties of eutectic components at different compositions, as shown in Equation (1) [53].

Subsequently, the determination of the eutectic point is paramount to developing a binary or pseudo-binary eutectic mixture. For this purpose, the Schroder-van Laar (SvL) equation (Equation (1)) is commonly applied, which bridges the behavior between the thermophysical properties of the components and their different compositions.

$$X_i=exp\left[{\displaystyle \frac{∆H_{mi}}{R}}\left({\displaystyle \frac{1}{T_{mi}}}-{\displaystyle \frac{1}{T_0}}\right)\right]$$

(1)

where X_i is the molar fraction of each component, $\sum X_i=1$; ∆H_mi enthalpies of phase transition (kJ·kmol⁻¹), R is the universal gas constant (8.314 J·kmol⁻¹·K⁻¹), and T_mi and T₀ are the melting temperature of each component and the melting temperature of eutectic mixture (K).

For a binary system, the melting point of the eutectic mixture can be determined by simplifying the equations from Equation (1) as follows:

$$T_0={\left[{\displaystyle \frac{1}{T_{0,A}}}-R{\displaystyle \frac{{lnX}_A}{{∆H}_{m,A}}}\right]}^{-1}$$

(2)

$$T_0={\left[{\displaystyle \frac{1}{T_{0,B}}}-R{\displaystyle \frac{{lnX}_B}{{∆H}_{m,B}}}\right]}^{-1}$$

(3)

The SvL equation derives from the van’t Hoff relation and works under several assumptions [54]: ideal liquid behavior, negligible heat-capacity difference between phases, and absence of impurities or intermediate compounds. The SvL equation is adequate for many inorganic and organic eutectic systems and provides a convenient first-order prediction of the eutectic point, as has been amply discussed in several studies [53,55]. Nonetheless, as has been mentioned before, the SvL equation has its limitations. Experimental studies have shown systematic departures from SvL predictions, especially for hydrated salts and deep eutectic mixtures where bound water, solvation, and strong ionic interactions might produce metastable phases or non-ideal activity coefficients. Palmelund et al. [56], for example, reported deviations in ternary hydrated salts, mainly attributable to water-of-crystallization effects. Similarly, Valenti et al. [57] reported experimental DSC results that fall below SvL predictions in several eutectic mixtures.

From the SvL equation, the theoretical ideal eutectic mass ratio and eutectic temperature are determined by designing a series of eutectic mixtures which are later evaluated experimentally using a characterization technique, such as DSC. For typical binary or ternary eutectic mixtures, it is expected that only one endothermic peak is generated during the phase transition process. However, non-ideal conditions that often arise in real systems (such as water loss, strong ionic associations, and a myriad of kinetic hindrances) might lead to altered activity coefficients and, thus, render the SvL predictions inaccurate. Consequently, SvL models provide a useful first-order thermodynamic descriptor, but not a complete representation of real phase equilibria, where the interplay between metastable transitions, water loss, and non-ideal kinetics governs the effective eutectic point.

Based on the eutectic melting temperature, a phase diagram can be constructed in which the intersection point of the components defines the eutectic melting point, while the composition indicates the eutectic mixture ratio. Figure 4 illustrates a binary system where the liquid phase (L) is in equilibrium above the eutectic temperature (E), while two solid phases (A and B) constitute the equilibrium state below that temperature. The three phases can be in equilibrium at the eutectic temperature, but only at the eutectic composition [28,53].

In particular, the application of thermodynamic equations, numerical simulations, and phase prediction methods such as the Schrader equation and phase equilibrium-based models has made it possible to design eutectic compositions with high precision in their transition temperature and fusion enthalpy, significantly reducing the need for empirical experimentation [58].

In the study by Kalidasan et al. [30], a numerical modeling approach based on the Schrader equation was employed to determine the optimal proportions of three salts: sodium sulfate decahydrate (SSD), sodium phosphate dibasic dodecahydrate (SPDD), and sodium carbonate decahydrate (SCD) in a ternary eutectic system. This model allowed the prediction of a theoretical melting point of 21.5 °C and an enthalpy of 207 J·g⁻¹, values later confirmed experimentally (21.3 °C and 202.2 J·g⁻¹). The correlation between theoretical and experimental data demonstrates the reliability of the thermodynamic model for the design of multicomponent PCMs, eliminating the need for extensive trial-and-error testing.

Similarly, in another work by Kalidasan et al. [26], the Schrader equation was again used to predict the optimal binary composition of the SSD/SPDD system (62:38 wt%) with a transition temperature of 27.6 °C and an enthalpy of 216 J·g⁻¹. This theoretical approach established the foundation for the experimental development of the MXene@SSD/SPDD nanocomposite, where the model accurately predicted the thermal operating window.

Finally, the integration of theoretical–computational approaches with advanced experimental methodologies (DSC, FTIR, UV–Vis, CFD simulations) reveals an interdisciplinary synergy essential for the development of new high-performance hybrid eutectic materials. These studies consolidate a predictive design paradigm in the field of PCMs, the combination of thermodynamic modeling, heat-transfer simulation, and experimental characterization capable of guiding sustainable innovation toward more efficient and resilient energy systems.

Table 2 summarizes various hydrated salts used in the production of eutectic mixtures, including the proportions of each salt, the resulting phase change temperature, and the enthalpy of fusion. A notable trend is the frequent use of Na₂SO₄·10H₂O as a precursor in eutectic systems. This salt is considered a promising medium for thermal energy storage due to its physical properties: a specific heat capacity of solid of 1.760 kJ·kg⁻¹·°K⁻¹, thermal conductivity of 0.6 W·m⁻¹·°C⁻¹ in the solid state, and an incongruent melting point will start to form crystalline decahydrate at about 32.4 °C and will remain at that temperature until either the water or the anhydrous phase is exhausted [59,60,61,62]. Although its incongruent melting behavior is a limitation, studies have shown that when combined with Na₂HPO₄·12H₂O in proportions greater than 60%, the phase change temperature decreases by at least 4 °C, the enthalpy of fusion increases to above 205 J·g⁻¹, and supercooling is reduced by up to 40% or more when a porous support substrate is employed [26,28,30,36]. Moreover, the addition of borax, polypropylene, and sodium alginate has been reported to further mitigate phase separation [36].

Among the affordable options and commercially available salts, potassium-based compounds such as KCl and KNO₃ stand out. Sharing the same K⁺ cation enables the formation of a new structure with the original ionic group of barium hydroxide octahydrate, which promotes ion hydration and thereby reduces the phase-transition temperature [39].

Conversely, the combination of Al₂(SO₄)₃·18H₂O and FeSO₄·7H₂O is suitable for achieving higher melting points, making them attractive for applications in solar energy storage and industrial processes [14]. However, ensuring the stability of eutectic salts during charge–discharge cycles remains essential. Durability testing typically involves subjecting eutectic salts to 100–200 phase change cycles. The incorporation of carbon derivatives—particularly carbon nanotubes—has been shown to enhance the stability of the matrix by forming a porous structure that limits salt expansion. Additionally, carbon nanotubes act as nucleating agents, accelerating the phase change process and reducing the degree of supercooling [39,63,64].

Recent studies continue the trend of developing composites based on the generation of a eutectic salt, with the aim of tailoring them to specific application requirements. Subsequently, a porous material is used as a supporting substrate (such as melamine sponge, expanded perlite, carbon aerogel, diatomite, or expanded graphite) [65,66,67,68,69], all of which share a low-density structure capable of storing PCMs. Finally, to minimize PCM leakage, the encapsulation method is employed, with polymers being the most commonly used due to their low cost and ease of acquisition, such as epoxy resin and polyurethane resin, among others.

Table 2. Summary of the most commonly used PCMs for the preparation of eutectic salts and stabilization materials.

Eutectic Salt	Mass Ratio	Substrate	Core–Shell	Filler	Phase Transition Temperature (°C)	Melting Enthalpy (J·g−1)	Reference
Na2HPO4·12H2O/Na2S2O3·5H2O	80:20	melamine sponge	PU light-curing resin	-	26.1	134.54	[65]
CH3COONa·3H2O/CH4N2O	6:4	-	silica/nanosilica shell	-	21.31	286.8	[70]
Na2HPO4·12H2O/Na2CO3·10H2O	6:4	Expanded perlite	Epoxy resin and fly ash	-	23.73	92.87	[66]
MgCl2·6H2O/NH4Al(SO4)2·12H2O	30:70	Carbon aerogels	-	-	64.7	156.93	[67]
CH3COONa·3H2O/C2H5NO2/Na2HPO4·12H2O	85.15:13.86:0.99	Expanded graphite	-	-	48.36	128.1	[71]
BaCl2/KCl/NaCl	53:28:19	Geopolymer half shells	Al2O3	-	-	215	[72]
NaAc∙3H2O/NH2CH2COOH	88:12	Expanded graphite	-	-	48.62	258.5	[73]
Na2CO3·10H2O/MgSO4·7H2O	70:30	polycrystalline silicon	-	-	33.2	230.5	[25]
Na2CO3·10H2O/Na2HPO4·12H2O	-	expanded graphite	-	-	23.5	196.2	[40]
Al2(SO4)3·18H2O/FeSO4·7H2O	2:1	carboxymethyl cellulose	-	carbon nanotubes	80–150	420	[64]
MgCl2∙6H2O/MgSO4∙7H2O	92.15:4.85	Activated carbon	-	-	90.21	156.14	[63]
Ba(OH)2∙8H2O/KCl	90:10	expanded graphite	-	-	66.25	206.4	[39]
Ba(OH)2∙8H2O/KNO3	88:12				67.71	231.5
CH3COONa·3H2O/Na2S2O3·5H2O	28:72	Melamine sponge	-	Polyurethane	41.45	186.6	[20]
Na2SO4·10H2O/Na2HPO4·12H2O	80:20	-	-	-	33.4	253.5	[36]
Na2CO3·10H2O/Na2SO4·10H2O/Na2HPO4·12H2O	0.4:1.0:0.7	Graphene Nanoplatelets	-	-	21.5	207	[30]
Na2SO4·10H2O/Na2HPO4·12H2O	62:38	Coconut shell biochar	-	-	27.8	218.1	[28]
Na2SO4·10H2O/Na2HPO4·12H2O	62:38	MAX Phase material (Ti3AlC2)	-	-	27.6	216	[26]
NaH2PO4·2H2O/Na2S2O3·5H2O/K2HPO4·3H2O	6:6:6	-	-	Nanoactivatedcarbon/Sodiumtetraborate/Sodiumfluoride	11.9 10.6 14.8	127.2 118.6 82.56	[48]
Na2SO4·10H2O/Na2HPO4·12H2O	75:25	-	-	nano-α-Al2O3	31.2	280.1	[50]

Table 2. Summary of the most commonly used PCMs for the preparation of eutectic salts and stabilization materials.

Eutectic Salt	Mass Ratio	Substrate	Core–Shell	Filler	Phase Transition Temperature (°C)	Melting Enthalpy (J·g−1)	Reference
Na2HPO4·12H2O/Na2S2O3·5H2O	80:20	melamine sponge	PU light-curing resin	-	26.1	134.54	[65]
CH3COONa·3H2O/CH4N2O	6:4	-	silica/nanosilica shell	-	21.31	286.8	[70]
Na2HPO4·12H2O/Na2CO3·10H2O	6:4	Expanded perlite	Epoxy resin and fly ash	-	23.73	92.87	[66]
MgCl2·6H2O/NH4Al(SO4)2·12H2O	30:70	Carbon aerogels	-	-	64.7	156.93	[67]
CH3COONa·3H2O/C2H5NO2/Na2HPO4·12H2O	85.15:13.86:0.99	Expanded graphite	-	-	48.36	128.1	[71]
BaCl2/KCl/NaCl	53:28:19	Geopolymer half shells	Al2O3	-	-	215	[72]
NaAc∙3H2O/NH2CH2COOH	88:12	Expanded graphite	-	-	48.62	258.5	[73]
Na2CO3·10H2O/MgSO4·7H2O	70:30	polycrystalline silicon	-	-	33.2	230.5	[25]
Na2CO3·10H2O/Na2HPO4·12H2O	-	expanded graphite	-	-	23.5	196.2	[40]
Al2(SO4)3·18H2O/FeSO4·7H2O	2:1	carboxymethyl cellulose	-	carbon nanotubes	80–150	420	[64]
MgCl2∙6H2O/MgSO4∙7H2O	92.15:4.85	Activated carbon	-	-	90.21	156.14	[63]
Ba(OH)2∙8H2O/KCl	90:10	expanded graphite	-	-	66.25	206.4	[39]
Ba(OH)2∙8H2O/KNO3	88:12				67.71	231.5
CH3COONa·3H2O/Na2S2O3·5H2O	28:72	Melamine sponge	-	Polyurethane	41.45	186.6	[20]
Na2SO4·10H2O/Na2HPO4·12H2O	80:20	-	-	-	33.4	253.5	[36]
Na2CO3·10H2O/Na2SO4·10H2O/Na2HPO4·12H2O	0.4:1.0:0.7	Graphene Nanoplatelets	-	-	21.5	207	[30]
Na2SO4·10H2O/Na2HPO4·12H2O	62:38	Coconut shell biochar	-	-	27.8	218.1	[28]
Na2SO4·10H2O/Na2HPO4·12H2O	62:38	MAX Phase material (Ti3AlC2)	-	-	27.6	216	[26]
NaH2PO4·2H2O/Na2S2O3·5H2O/K2HPO4·3H2O	6:6:6	-	-	Nanoactivatedcarbon/Sodiumtetraborate/Sodiumfluoride	11.9 10.6 14.8	127.2 118.6 82.56	[48]
Na2SO4·10H2O/Na2HPO4·12H2O	75:25	-	-	nano-α-Al2O3	31.2	280.1	[50]

4. Eutectic Salts Stabilization via Porous Substrates

One of the main challenges associated with the use of inorganic PCMs is liquid leakage during the phase change process. To mitigate this issue, the development of composite PCMs using porous support materials has been proposed, where liquids are stabilized through capillary interactions. Commonly used support materials include carbon [14,15], polymers [16], and expanded graphite [74,75]. These materials not only prevent liquid leakage but also act as regulators of thermal conductivity [12,17].

Figure 5 illustrates the most commonly employed method: vacuum impregnation. In this process, the substrate is pre-dried to maximize PCM absorption during impregnation.

Support materials can be of different types, but they generally share the characteristics of low density and high porosity, such as expanded perlite [16], expanded graphite [76,77], diatomite [12], and expanded vermiculite [6]. It has been reported that when using diatomite as a porous substrate for eutectic salt preparation, the operating parameters were heated in a water bath at 55 °C with constant stirring for 25 min. Subsequently, the eutectic salt was added drop by drop to the diatomite at 55 °C, with constant stirring for one hour in mass proportions of 50, 55, 60, and 65% by weight [12]. On the other hand, the eutectic salt hydrate/expanded vermiculite is obtained by vacuum impregnation. The PCM was added completely melted in proportions of 75, 70, 65, and 60% by weight; it was added drop by drop at a vacuum pressure of 0.1 MPa, with constant stirring for 30 min, and subsequently, the compounds were kept under vacuum for 3 h until completely absorbed [6].

Expanded graphite oxide, a porous material produced through microwave irradiation of expandable graphite, has been employed as a porous substrate to improve the thermal conductivity of PCMs. Durability tests over 200 phase change cycles have shown only an 8.1% loss in latent heat. According to the literature, accelerated thermal cycling tests were conducted on phase change composites to evaluate their long-term thermal stability and reliability. The samples (10 mg) were subjected to up to 450 heating and cooling cycles between 10 and 50 °C in a nitrogen atmosphere, using differential scanning calorimetry. Specific cycles (1st, 150th, 300th, 450th) were run at 2 °C/min, while the others used 20 °C/min. A key factor in these experiments was addressing PCM leakage. Without proper cleaning, leaked material would remain in the container, masking actual latent heat loss. The samples were carefully cleaned with absorbent paper, and left for 24 h to eliminate any exuded PCM [17,78].

Another porous material employed as a support is hectorite aerogel, which exhibited fusion and solidification enthalpies of 215.3 and 186.9 J·g⁻¹, respectively, in addition to offering tunable phase change temperatures and potential to mitigate supercooling and phase separation [79]. Similarly, porous silica materials include zeolites, mesoporous silica nanoparticles, porous glass, aerogels, and xerogels. Zeolites have an ordered structure with micropores smaller than 2 nm, which limits their suitability for PCM nanocomposites despite pore control. Silica nanoparticles, though nonporous, can be synthesized as uniform spheres (≈20 nm) whose packing creates interparticle porosity [80]. These features make them suitable as supporting materials.

The porous mesh design increases the material’s surface area and provides active sites that enhance the adsorption of more PCMs, thereby improving heat conductivity. The microporous cross-linked carbon tubes hold the PCM securely, preventing leakage and ensuring that the Glauber’s salt composite maintains high heat storage capacity and stable thermal cycling performance [17,81]. Furthermore, the combination of eutectic salts with porous substrates has been investigated to assess the substrate’s effectiveness in preventing PCM leakage and to evaluate the overall thermal performance of the composite system [42,51].

5. Eutectic Salts Stabilization via Encapsulation

Encapsulation methods for PCMs help mitigate issues such as salt leakage during the solid–liquid phase transition, control of volume changes, reduction in corrosion, and improvement of thermal stability, while also protecting PCMs throughout multiple charge–discharge cycles [82,83]. The main challenge with salt hydrates is their corrosive effect on metals, which leads to rust formation. This rust contaminates the storage medium, altering its thermal performance and reducing durability. Applying corrosion protection enhances the practicality and cost-effectiveness of metals, making salt hydrate-based PCMs more viable for energy storage [26,84].

In the literature, the proposal to use MXene@eutectic salt hydrate composites was found as an alternative to mitigate the leakage of PCMs and reduce supercooling. The results of FT-IR tests allow us to infer that by maintaining a spectral patter similar to the target sample, the chemical stability of the eutectic PCM is suggested. On the other hand, they offer better resistance samples, which is why more studies are required to determine the parameters of affectation with other metals, commonly used in construction.

The encapsulation process involves coating particles with a uniform shell to produce capsules, which can be classified by size into macroencapsulation (1 mm to 1 cm) [18,85,86], microencapsulation (1 μm to 1 mm) [87,88], and nanoencapsulation (less than 1 μm) [89,90]. The thermal degradation of particles depends on factors such as particle size and surface roughness, and salt content samples with higher salt content degrade more slowly. Chemical compatibility between PCMs and shell materials limits the usable temperature range. Additionally, stirring rate influences particle size; if particles are too small, encapsulation efficiency decreases. Microencapsulation stands out among the nanoencapsulation and macroencapsulation techniques; this is because it requires techniques that are not as specialized as in nanoencapsulation, which allows greater control in the encapsulation efficiency, and its size allows it to be used in combination with other elements in the creation of cement-based matrices. However, in the case of macroencapsulation processes of elements in the range of less than one cm, the macroencapsulation method allows encapsulating even the porous substrate impregnated with PCMs, but in the case of larger capsules in the cm range, they even require the modification of the construction process and generate an additional cost [86,91,92].

Table 3. Comparative table of advantages and disadvantages of the type of encapsulation [18,78,83,85,90,93,94,95,96,97].

		Advantages	Disadvantages
Encapsulation	nano (1–1000 nm)	Controlled thermal energy release.	Increase in thermal conductivity.
		Prevention of material exchange with environment.	Nanoparticles are susceptible to instability and agglomeration.
		Protection against degradation during heat uptake/release cycles.	Difficult to scale-up.
		The possibility to use the capsules in powder or paste form as additives to convenient materials (concrete, foam, paint, etc.).	It is complicated and costly.
		Smaller capsules greatly increase the surface area to volume ratio of the material, which improves heat transfer.
		Encapsulating PCMs in capsules of 1 mm in size would increase the surface area by 300 m2 m−3 when compared with the bulk PCM Reducing their diameter to the nanometer range would vastly enhance this effect.
		Prevention of both leakage and reactions with the external environment.
		Corrosion protection for container materials
		It is also possible to form a composite polymer/inorganic shell combining advantages of each.
		Increment in the reliability (charging-discharging cycle life).
		It is one of the most promising solutions to increase the efficiency of PCMs, both organic and inorganic.
	micro (1–1000 µm)	Microcapsules with impermeable walls are used in products where isolation of active substances is needed.	The degree of supercooling is strongly dependent on the morphology and component of shell material.
		PCMs with a melting point ranging from −10 to 80 °C can be microencapsulated.	The leaching of the functional core PCM in microencapsulation.
		The effects achieved with impermeable microcapsules include separation of reactive components, protection of sensitive substances against environmental effects, reduced volatility of highly volatile substances, and conversion of liquid ingredients into a solid state and toxicity reduction.	The high surface polarities of these substances, edge alignment effects, and their tendency to alter their water content.
		Control the changes in the volume as phase change occurs.	Difficult to scale up.
		Controlled release of thermal energy.	It is complicated and costly.
	macro (>1000 µm)	Macroencapsulations are easier to include and therefore cheaper to produce and should be better in case of natural convection with a smaller heat transfer surface compared to the PCM mass.	A lower thickness improves the crystallization time but simultaneously increases the melting time.
		Macroencapsulated salt hydrates are up to three to four times cheaper than microencapsulated paraffins.	Cuboid shapes are easy to produce but have a low heat transfer owing to a thick PCM layer.
		A spherical shape shows the fastest melting time because of an even distribution combined with a large heat transfer surface.	Macroencapsulation produces a much simpler structure but uses hard shells (such as metals) to avoid shape deforming once the solid melts into a liquid
		It requires the modification of the traditional construction process.	The heavy shells lower the energy storage density and increase the costs.
			Metal capsules have corrosion problems.
Porous substrate		The micropore can absorb PCMs with capillary force, thus keeping the composite free of liquid leakage, reducing the sub-cooling degree of PCMs, and alleviating the phase separation.	There is still one problem that microimpregnation with porous matrices cannot correct dehydration. Hydrated salts dehydrate once phase change begins. If the water leaves from the hydrates as a vapor, the composition of the hydrated salts changes, which also changes the phase change characteristics.
		Applying vacuum impregnation, the process effectively removed air from the porous, allowing for deep and uniform infiltration of PCM	Composites prepared from conventional porous materials such as bentonite, zeolites, diatomaceous earth and expanded graphite, might present an initially appealing latent heat, they suffer a considerable amount of PCM leakage caused by exudation from the macropores and the inter-granular space during thermal cycling.
		The microporous structure provides a large specific surface area.	The exudation problems of in use conditions.
		Optimization of pore structure may enhance thermal storage and mechanical properties
		The hydrophilicity of the porous matrix has a considerable impact on the adsorption capacity of hydrates.
		It can be complemented with some form of encapsulation technique.
		Low cost, ease of fabrication and widespread application.

presents a comparative summary of the advantages and disadvantages of the two different types of eutectic salt stabilizations: encapsulation and the use of porous substrates. In the case of encapsulation, it is divided into three sections: nano, micro, and macro.

Based on structural type, encapsulation can be mononuclear, polynuclear, matrix-type, or multilayered, depending largely on the method employed. In general, encapsulation techniques are divided into three major categories: physical, physicochemical, and chemical methods.

Table 4. Main encapsulation strategies used for PCMs [83,89,93,98,99].

Physical Methods	Physic-Chemical Methods	Chemical Methods
air-suspension coating centrifugal extrusion pan coating spray drying vibrational nozzle	coacervation ionic gelation sol-gel solvent evaporation	chemical reduction emulsion polymerization interfacial polymerization in situ polymerization suspension polymerization

summarizes the principal encapsulation strategies used for PCMs.

The choice of encapsulation method largely depends on the intended application and the type of PCM employed. Importantly, encapsulation can also be extended to porous support materials, hence, combining both stabilization strategies and further enhancing PCM retention.

There are about 50 different known polymers that can be used as synthetic as well as natural capsules [86]. An example is the use of an interior coating based on ethylene–vinyl acetate copolymers incorporating nanocapsules of Na₂HPO₄·12H₂O encapsulated with polymethyl methacrylate. This system was evaluated by comparison with the reference chamber coated with conventional coatings which demonstrated high thermal stability after 200 charge–discharge cycles, in addition to imparting flame-retardant properties and providing effective thermal comfort control (24–28 °C) for 141 min, compared with only 34 min for conventional coatings under dynamic heat transfer testing [100].

A wide variety of materials have been investigated for encapsulation, each with specific advantages and limitations. Metals and alloys such as Al–Si, Al–Si–Cu, Al–Si–Mg, Cr–Ni, and Cu–Cr–Ni exhibit excellent thermal conductivity and high melting points, but their practical use is constrained by corrosive effects and potential chemical reactions with salts. Analogously, polymer-based encapsulation avoids corrosion issues, though it is limited to PCMs with relatively low phase-transition temperatures. Commonly studied polymers include polystyrene [101], ethylene-vinyl acetate [100], polyvinyl acetate [100], and epoxy resins [18].

Among the problems that arise in the encapsulation process is mainly the efficiency of the encapsulation process, which refers to the percentage of PCM that is correctly encapsulated. It also addresses the issue of core–shell morphology, which depends on the ratio of coating material to the amount of PCM. Excess polymeric material, for example, promotes agglomeration and surface area. Conversely, a low amount of polymer for encapsulation generates low efficiency in the production of the core–shell, allowing PCMs to escape, which, in the case of applications in the construction industry, affects the reinforcing steel when in contact [92,102,103].

On the other hand, the choice of the correct polymer influences the durability of the core–shell, as it is a material that is exposed to constant cycles of phase change and volumetric changes derived from the same. There are losses in the coating and again the effect of leakage of PCMs occurs, which ends up generating corrosion in the reinforcing steel [86,104].

6. Application of Carbon-Based Fillers in PCM Composites

One of the major challenges in phase change materials (PCMs) lies in enhancing the thermal conductivity and stability of the composites. To address this issue, various carbon-derived fillers have been investigated, particularly graphite-based materials such as graphene, graphite oxide, and graphite nanoparticles, which act as reinforcement agents [63,105].

These fillers exhibit even greater performance when incorporated into porous substrates. Graphene and expanded graphite possess inherently high thermal conductivity due to their two-dimensional carbon atom arrangement, which also helps mitigate phase separation and degradation of PCMs. However, their use is limited by high production costs, low availability, and, most importantly, the difficulty of achieving homogeneous dispersion. Figure 6 illustrates the most widely applied method for dispersing nanoparticles in aqueous media, in which the suspension is kept under constant agitation, followed by immersion of the porous substrate. PCMs are then introduced to ensure homogeneous distribution. Notably, the processing sequence in the preparation of PCM composites plays a decisive role in determining their thermal properties [14,64,106].

The composite of eutectic salt (barium hydroxide octahydrate-KCl) and expanded graphite [39], according to the results of Scanning Electron Microscopy (SEM), exhibitsa parallel laminar structure characteristic of reinforcement morphology of microspores that generate spaces capable of storing the PCM. This storage occurs thanks to physical phenomena such as capillarity and surface tension, typical of real porous confinement. On the other hand, thermal conductivity tests indicate that the increase in the addition of expanded graphite increases the thermal conductivity, although it reduces the latent heat, which indicates improved nucleation behavior during the phase change process, this is considered positive when evaluating the composite alone [14,15], but comparative data for cement-based composites are insufficient in the current literature; on the other hand, thermal conductivity tests and SEM do not directly measure the capacity of the substrate to retain the PCM nor its efficiency as a nucleator.

Despite the aforementioned benefits, the application of carbon-based fillers to PCMs also faces a manifold of challenges in practice. The improvement in thermal conductivity provided by carbon-based fillers is largely controlled by the efficiency of heat transfer at the filler–matrix interface and by the formation of continuous conductive paths within the PCM. Despite the high intrinsic conductivity of carbon materials, imperfect contact and vibrational mismatch at the interface create thermal boundary resistance that limits phonon transport [107]. Analogously, the amount of filler material incorporated plays a crucial role in the stability of the substrate. Excessive dosages may lead to agglomeration in specific regions, thereby affecting the hydration products in cement-based matrices or compromising the mechanical properties [23]. In this context, various carbon derivatives such as carbon spheres, graphene nanoplatelets, carbon nanotubes, mesoporous carbon, nanographite, and graphite oxide have been reported [108], among others. For instance, the incorporation of 1.5 wt% graphite increased the thermal conductivity by 10% in a barium carbonate–poly(hydroxy)urethane blend. While the addition of barium carbonate alone tends to reduce thermal conductivity, the inclusion of graphite effectively counterbalances this drawback [3].

Another important feature is the percolation threshold, which is the critical filler content above which a connected thermal network forms. Below this point, conductivity increases slowly; once percolation is reached, conductivity rises sharply [109]. The percolation threshold depends on filler geometry and dispersion quality, and exceeding it without controlling dispersion often introduces new trade-offs.

In summary, pursuing higher conductivity by increasing filler content introduces trade-offs that directly affect PCM performance. Added filler mass displaces active phase change material, reducing latent-heat capacity per unit mass and increasing density, while agglomeration at high loading raises viscosity and impairs melt infiltration and heat exchange efficiency [110]. Optimal design requires balancing interfacial coupling, percolation continuity, and material rheology rather than merely maximizing filler content.

On the other hand, the amount of filler material incorporated plays a crucial role in the stability of the substrate. Excessive dosages may lead to agglomeration in specific regions, thereby affecting the hydration products in cement-based matrices or compromising the mechanical properties [23]. In this context, various carbon derivatives such as carbon spheres, graphene nanoplatelets, carbon nanotubes, mesoporous carbon, nanographite, and graphite oxide have been reported [108], among others. For instance, the incorporation of 1.5 wt% graphite increased the thermal conductivity by 10% in a barium carbonate–poly(hydroxy)urethane blend. While the addition of barium carbonate alone tends to reduce thermal conductivity, the inclusion of graphite effectively counterbalances this drawback. [3].

7. Mitigation of Supercooling and Stability Enhancement

Supercooling is the phenomenon in which a PCM does not crystallize and remains liquid below its melting temperature, limiting the practical application of PCMs [28]. The temperature difference between the freezing and melting points is defined as the degree of supercooling [111] and can be evaluated using a hysteresis equation (Equation (4)) [112]:

$${∆T}_s=T_m-T_f$$

(4)

where ∆T_s indicates the degree of supercooling, T_m is the melting temperature, and T_f is the freezing temperature.

PCMs adjust their temperature by storing and releasing large amounts of latent heat during the phase change process. For a complete phase transition, the operating temperature must encompass at least the melting and solidification temperature ranges. By establishing the operating temperature range, the latent heat of fusion, as well as the specific heats of both the solid and liquid phases of the PCMs, are estimated [112]. Supercooling can lead to delayed solidification, reduced thermal storage efficiency, and influence incongruent melting behavior.

The general formula for salt hydrates is AB·nH₂O, where AB represents the salt and n the number of water molecules [113]. During phase transitions, these compounds lose water (dehydrate), forming either a hydrate with fewer water molecules or an anhydrous salt [114]. This process can lead to incongruent melting, where the released water is insufficient to dissolve the remaining salt, causing density differences, phase separation, and salt sedimentation. Phase separation is a common issue in salt hydrates but can be minimized by incorporating thickening agents such as cellulose, polyethylene glycol, or titanium oxide nanoparticles. Other techniques include material mixing, encapsulation, or adding excess water, though the latter increases the system’s mass and volume [115].

The use of nucleating agents and thickeners can mitigate this effect by generating initiation points for crystallization. Key factors affecting the degree of supercooling include heterogeneous nucleation, homogeneous nucleation, and the cooling rate [116].

The use of nucleating agents can reduce the degree of supercooling by facilitating crystal formation [80]. As shown in

Table 5. Nucleating agents and the impact of supercooling temperatures.

PCM	Nucleating Agent	Addition wt%	Melting Point °C	Enthalpy J·g−1	Supercooling °C	Reference
Na2HPO4·12H2O/Na2S2O3·5H2O	Na2SiO3·9H2O	3.0	26.1	134.5	2.3	[65]
NaAc∙3H2O/ NH2CH2COOH	Borax	0.5	48.62	258.5	1.49	[73]
CH3COONa·3H2O/Na2S2O3·5H2O	SrCl2·6H2O	2.0	41.45	186.6	0.462	[20]
Na2 SO4·10H2O/Na2HPO4·12H2O	Nanometer AlN, Borax	1.8		180.2	3.1	[36]
		1.5
Na2SO4·10H2O/Na2HPO4·12H2O	Borax	1	42.2	149.3	7.7	[28]
Na2SO4·10H2O/Na2HPO4·12H2O	Borax	0.3		209.5	3.4	[26]
		0.6		209.0	5
		0.9		215.5	4.5
		1.2		207.5	4.2
MgCl2·6H2O/ Mg(NO3)2·6H2O	Sr(OH)2·8H2O	0.5	60	139.5	1	[118]
NaH2PO4·2H2O/Na2S2O3·5H2O/K2HPO4·3H2O	Nanoactivatedcarbon/Sodiumtetraborate/Sodiumfluoride	0	11.9	127.2	1.5	[48]
		3	10.6	118.6
		5	14.8	82.56
Na2SO4·10H2O/Na2HPO4·12H2O	Nano-α-Al2O3/Borax	3	31.2	280.1	1	[50]
		1
Na2HPO4·12H2O	Na2SiO3·9H2O		30.4	163.4	1.3	[5]
Na2HPO4·12H2O	Na2SiO3·9H2O	2	28.9	153.1	3.7	[4]
		3			2.0
		4			2.7
		5			0.8
		6			2.4
		7			1.8

, borax (Na₂B₄O₇·10H₂O) has been studied as a nucleating agent in eutectic salt-based PCMs, particularly in hydrates such as sodium sulfate decahydrate. When used at concentrations of 0.3–1.2 wt%, borax can reduce supercooling by approximately 3–15 °C, thereby improving the solidification process. Nonetheless, its use may present disadvantages if it is incompatible with the PCM structure, and concentrations below 2% are not recommended. Similarly, Na₂HPO₄·12H₂O has been reported to reduce the degree of supercooling in sodium acetate trihydrate by up to 4 °C [117].

8. Discussion on the Holistic Design of PCMs

A key shortcoming in much of the PCM literature is the isolated treatment of thermal performance (latent heat, melting point, and conductivity), mechanical durability (encapsulation integrity and volume change fatigue), and chemical stability (corrosion, hydrolysis, and leakage). In practice, these properties are deeply interrelated, and optimal PCM design requires simultaneous consideration of all.

It is insufficient to treat thermal, mechanical, and chemical stability of PCM systems as independent virtues, while in practice they form an interdependent triad, and design decisions in one domain invariably impact the others. For example, increasing the encapsulation shell thickness improves leakage resistance (mechanical/chemical stability) but adds thermal resistance, reducing effective heat transfer rate (thermal trade-off) [119]. Similarly, adding conductive fillers enhances thermal performance but often increases system density or reduces latent heat (thermal–mechanical trade-off), may increase brittleness of the composite (mechanical risk), and in some cases promote corrosion or chemical degradation by providing pathways for ingress or galvanic coupling (chemical risk).

This review emphasizes that future work should adopt multi-objective optimization (response surface methodology, for instance) rather than single-parameter targets. Instead of only maximizing conductivity, authors should report the optimum filler loading where conductivity gain, latent heat loss, density increase, mechanical fatigue, and chemical degradation are jointly minimized. Additionally, this critical review highlights the need for standardized lifecycle testing protocols (e.g., 1000 standard thermal cycles, leakage quantification, and corrosion tests) for PCM systems ahead of real-world deployment.

9. Conclusions

The present review highlights that the stabilization of eutectic salts as PCMs is fundamentally governed by the interplay among thermodynamic tuning, microstructural confinement, and interfacial transport mechanisms. The combination of the SvL equations with experimental validation enables not only convenient prediction of eutectic composition and transition temperature but also provides a framework for an exploratory screening for rational PCM design guided by thermodynamic principles rather than pure empirical optimization. Nonetheless, its limitations should acknowledged. It is useful and convenient as a first-approximation method.

A comparative evaluation of stabilization techniques reveals that porous substrates primarily enhance physical confinement and capillary retention, while encapsulation introduces chemical and mechanical stabilization at the expense of high thermal resistance. Analogously, carbon-based fillers act as multifunctional additives, coupling thermal conduction enhancement with heterogeneous nucleation to reduce supercooling and accelerate phase reversibility. This review also highlights the interplay and unavoidable trade-offs of enhancing siloed properties. Moreover, this emerging design paradigm shifts from isolated enhancement toward multi-objective optimization, integrating thermodynamic modeling, materials selection and properties, and durability and lifecycle testing to enable predictable, durable, and eventually scalable PCM systems.

Based on the reviewed literature, the collective evidence establishes a design map for eutectic PCM composites:

• In terms of PCM content, 60–70 wt% within porous matrices ensures leakage-free operation;
• Regarding filler loading, ≤3 wt% carbon-based additives provide a conductivity gain of 100–200% with <10% latent heat penalty;
• Cycle durability of encapsulated or supported PCMs can retain >90% enthalpy after 200–300 cycles when proper pore confinement and interfacial coupling are achieved.

10. Future Perspectives

Expanding upon the reviewed literature, the field must advance toward multi-objective design frameworks that integrate several properties to achieve durable, scalable, and eco-efficient PCM systems. Simultaneously, future research should prioritize the development of standardized testing protocols encompassing thermal cycling, leakage resistance, and corrosion behavior, as well as the mechanistic coupling of modeling and experiment to understand phase behavior under real conditions. Moreover, efforts should emphasize eco-design and scalability, leveraging bio-derived encapsulants, recyclable carbon supports, and low-temperature synthesis routes to minimize environmental impact. Finally, the incorporation of stabilized eutectic PCMs into building envelopes, cementitious composites, and HVAC systems represents a promising direction for practical deployment, linking material innovation to tangible energy efficiency gains in the built environment along with continuous monitoring for assessment.