Next Article in Journal
Evaluating Performance of Metal-Organic Complexes as Electrodes in Hydrogen Peroxide Fuel Cells
Previous Article in Journal
Research on Energy Management Strategy for Mining Trucks with Methanol Range-Extender and Hybrid Energy Storage System
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Energies
Volume 18
Issue 10
10.3390/en18102597
energies-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Carbon Dot-Modulated Phase-Change Composites for Wide Temperature Range and High-Density Heat Storage and Release

by
Jingya Liang
1,
Ning Li
1,
Jie Wu
2,
Qing Chang
1,
Jinlong Yang
1,3,4 and
Shengliang Hu
1,*
1
Research Group of New Energy Materials and Devices, State Key Laboratory of Coal and CBM Co-Mining, North University of China, Taiyuan 030051, China
2
School of Applied Science, Taiyuan University of Science and Technology, Taiyuan 030024, China
3
State Key Laboratory of Coal and CBM Co-Mining, Nancun Town, Zezhou County, Jincheng 048012, China
4
State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University, Beijing 100084, China
*
Author to whom correspondence should be addressed.
Energies 2025, 18(10), 2597; https://doi.org/10.3390/en18102597
Submission received: 8 April 2025 / Revised: 30 April 2025 / Accepted: 12 May 2025 / Published: 16 May 2025
(This article belongs to the Section J1: Heat and Mass Transfer)
Browse Figures
Versions Notes

Abstract

:
Organic phase-change materials (PCMs) offer great promise in addressing challenges in thermal energy storage and heat management, but their applications are greatly limited by low energy density and a rigid phase transition temperature. Herein, by introducing carbon dots (CDs) with abundant oxygen-related groups, we develop a novel kind of erythritol (ET)-based composite PCMs (CD-ETs) featuring an enhanced latent heat storage capacity and a reduced degree of supercooling compared to pure ETs. The optimally formulated CD-ETs increase the latent heat storage capacity from 377.3 to 410.2 J·g−1 and the heat release capacity from 209.0 to 240.2 J·g−1 compared to the pristine ETs. Moreover, the subcooled degree of CD-ETs is more than 30 °C lower than that of pristine ETs. By successively encapsulating CD-ETs and CD-containing polyethylene glycol (PEG) with a low melting point in a reduced graphene oxide-modified melamine sponge, the resultant shape-stabilized system not only prevents leakage of molten PCMs but also allows for a wide response temperature window and promotes the heat transfer ability of melted PEG in close contact with solid CD-ETs. Stepped melting and crystallization guarantee phase changes in high-melting-point ETs via solar heating, Joule heating or a combination thereof. Specifically, the melting enthalpy of this system is as high as 306.5 J·g−1, and its cold crystallization enthalpy reaches 196.5 J·g−1, surpassing numerous organic PCMs. This work provides a facile and efficient strategy for the design of ideal thermal energy storage materials to meet the needs of application scenarios in a cost-effective manner.

1. Introduction

Phase-change materials (PCMs) capable of reversibly absorbing and releasing large amounts of latent heat during melting and solidifying processes offer considerable potential in many areas, such as electronic cooling, waste heat recovery, green building, off-peak power storage, smart textiles, infrared stealth and so on [1,2,3,4,5,6]. Currently, solid–liquid PCMs, which are intensively utilized in practice, owing to their high energy storage density and feasibility, can be classified as inorganic or organic [7]. In comparison, inorganic PCMs possess superior heat storage capabilities and thermal conductivity, while organic PCMs can offer lower corrosiveness without severe detriments with respect to matrix materials [8]. Regardless of the kind, PCMs suffer from the burden of volume changes and liquid leakage upon melting [9]. Therefore, solid–liquid PCMs are required to be encapsulated before utilization. Meanwhile, the selection of a PCM for an intended application needs to take into account heat storage density, the operation and transformation temperatures, thermal conductivity, cost effectiveness and cyclability [10]. However, significant challenges in the use of solid–liquid PCMs include their high degree of supercooling, high phase-change temperatures for PCMs with large amounts of latent heat and the limited temperature range for thermal energy regulation [11,12].
Supercooling, which is a unique phenomenon related to the crystallization process, is regarded as an adverse factor in phase-change heat storage systems, since it prevents latent heat release below the normal freezing point of PCMs [13,14]. Apart from the cooling condition, supercooling behavior relies on the nature of PCMs. To date, a small number of organic PCMs, like paraffin, have been found to easily crystallize without a supercooled state once the temperature decreases to the phase-change temperature, whereas most inorganic PCMs, like salt hydrates, and some organic PCMs, like polyalcohols, frequently present a supercooling phenomenon even if the cooling rate is quite low [15]. A common method to address the issues of supercooling is the addition of nucleating agents, which can reduce nucleation barriers [14]. Several effective additives have been reported, including polymers [16], graphene oxides [17] and other inorganic nanoparticles [18]. Among them, low-mass-density nanoparticles exhibit excellent structural stability and homogeneous dispersion in PCMs. This property minimizes their detrimental effects on the interfacial thermal resistance, resulting in an efficient heat transfer pathway. Thus, such nanoparticles play a key role in improving the overall thermal performance of composite systems, especially in optimizing the latent heat storage capacity and reducing the subcooling effect [19]. Unfortunately, there are two intrinsic limitations associated with this approach: the sacrifice of a part of the energy storage density of PCMs and the poor compatibility of these additives with PCMs, which influences the performance of PCMs in terms of cyclic stability. Therefore, there is an urgent need to develop desirable additives.
Erythritol (ET)—a kind of four-carbon-atom polyalcohol—exhibits an extremely high phase transition enthalpy (over twice that of paraffin) [20]. Nevertheless, it also exhibits a high degree of supercooling of more than 90 °C, resulting in a crystallization heat as low as half of its fusion heat [21]. Moreover, its phase-change temperature reaches ~120 °C; thus, it is hardly utilized for the storage of low-grade heat below 100 °C. To expand application scenarios, ET-based PCMs with adjustable thermophysical parameters like phase-change temperature, supercooling, and latent heat release are desired. This demand inspired us to seek an efficient approach to overcome the shortcomings of ET PCMs by simply regulating their intermolecular interactions, which usually dominate molecular mobility, nucleation and crystal growth during phase-change processes for multiple energy conversion applications [22].
Other studies on the subcooling regulation of erythritol have almost always resulted in a loss of enthalpy while reducing the degree of subcooling [23]. In this contribution, we demonstrate that carbon dots (CDs) with abundant surface groups and dimensions of fewer than 10 nm can mediate intermolecular forces of ETs surrounding CDs during the process of ET melting and freezing [24,25]. The integration of CDs with ETs (CD-ETs) not only unprecedentedly enhances the phase transition enthalpy of ETs but also reduces the degree of supercooling over 30 °C, thereby promoting heat storage and release. Previous research has focused on single-stage phase-change materials that are subject to limitations in their ability to adapt to different temperature demands. Therefore, we created a composite consisting of CD-ETs and polyethylene glycol (PEG) in optimum proportions using a rational encapsulating technique. This composite can perform stepped phase changes in different temperature ranges and offers the ability to convert sunlight and electricity into thermal energy, making it compatible with a wide variety of needs for low-grade heat storage and multiple energy conversion utilizations.

2. Experimental Section

Sample preparation of CDs-ETs: Typically, the specific mass of ETs was dissolved in deionized water, and subsequently, a certain volume of the dispersed suspension of CDs was added, which were fabricated by selective oxidation of coal tar pitch as reported previously [26], with the concentration of 16.6, 7.0, 5.0, 4.2, 3.3 g·L−1. A uniform solution was achieved through thorough stirring. Then, the resultant solution was placed in a vacuum drying oven at 140 °C to completely remove the solvent. After cooling the room temperature, the sample of CDs-ETs was finally obtained for further characterizations.
Method toward encapsulating PCMs: Firstly, rGO@MS was obtained by our reported method [27,28]. The sample of CDs-ETs was melted at 140 °C. Let rGO@MS absorb a certain volume of liquid CDs-ETs. The resultant sample was cooled and then impregnated into the solution containing 20 g·L−1 of CDs-PEG at 70 °C. Afterward, it was cooled again to obtain a shape-stabilized composite system. The product was coated using a PDMS solution and was finally dried at 40 °C for 24 h.

3. Results and Discussion

The DSC heating curves of pristine ETs and the CDs-ETs samples with different mass ratios are close (Figure 1a), whereas their corresponding cooling curves are highly dependent on the mass ratios of CDs and ETs (Figure 1b). According to the DSC results [29,30], we extracted the thermal parameters for convenient comparison and summarized them in Figure 1c,d. The onset melting and solidifying points of ETs are 118.2 and 23.2 °C, respectively. The five samples of CDs-ETs have slightly lower melting points than ETs (117 °C for 1:30; 117.2 °C for 1:70; 116.6 °C for 1:100; 116.8 °C for 1:120; and 116.9 °C for 1:150), while exhibiting significantly higher solidifying points than ETs (49.3 °C for 1:30; 45.4 °C for 1:70; 54.6 °C for 1:100; 51.7 °C for 1:120; and 48.1 °C for 1:150). The increase in crystallization temperature demonstrates that CDs serve as nucleating agents by providing nucleation sites. In comparison, CDs-ETs exhibit more interaction energy than pure ETs, requiring larger thermal energy for melting. A series of composite CDs-ETs were prepared by mixing ETs and CDs thoroughly in the solution (Scheme 1) to achieve an optimal energy storage density while minimizing the supercooling degree of ETs. Consequently, the composite CDs-ETs achieved a higher melting enthalpy than pristine ETs. The melting enthalpy for ETs is 377.3 J·g−1. With the addition of CDs, the value does not reduce but instead increases to 401.6, 382.9, 410.2, 405.0 and 388.0 J·g−1 for 1:30, 1:70, 1:100, 1:120 and 1:150 samples, respectively. Specifically, the crystallizing enthalpy of the 1:100 sample is 240.2 J·g−1, markedly larger than that of pure ETs (209.0 J·g−1). Accordingly, it is clear that the incorporation of CDs into ETs can enhance their energy storage density. This is an abnormal phenomenon compared with most, if not all, of the reported composite PCMs (Figure 1e, Table 1) [16,31,32,33,34,35]. Moreover, the addition of CDs enables the supercooled degree of ETs to decrease (Figure 1c and Figure S2). Notably, the minimum supercooled degree of CDs-ETs (1:100 sample) is more than 30 °C lower than pristine ETs under the same cooling rates. We further created the temperature–time curves of ETs and composite CDs-ETs at a fixed cooling rate (Figure S3). It can be observed that the optimal sample of CDs-ETs presents a higher onset freezing point and a lower degree of supercooling compared to ETs (Figure 1f), meaning faster and more energy release.
Optical microscopy was used to observe the crystal evolution of ETs with the addition of CDs. Solid ETs are featured with ordered flaky crystals (Figure 2a). With an increasing additive amount of CDs, the long-range ordered features of ETs gradually disappeared and were transformed into a long-range disordered structure (Figure 2b–f). The X-ray diffraction (XRD) patterns of ETs and CDs-ETs (Figure 2g) display characteristic peaks at 14.7°, 19.6°, 20.2°, 24.5° and 29.6° that can be assigned to the (101), (220), (211), (301) and (202) crystal planes of ETs, respectively. [16] In contrast, CDs-ETs demonstrate significantly lower intensities of the diffraction peaks at 2θ of 14.7°, 20.2° and 29.6°than ETs, whereas no apparent peak shifts were observed. As a result, CDs interrupted the ordinary order of ET molecules but did not influence their crystal unit cells. The Fourier-transform infrared (FTIR) spectra (Figure 2h) of both ETs and CDs-ETs present the stretching vibrations of –CH and are observed at 2970 and 2920 cm−1, while manifesting the stretching and bending vibrations of –OH at 3300 and 1420 cm−1, respectively, confirming the molecular structure of ETs. Moreover, the stretching vibrations of C=O at 1632 cm−1 appear in the FTIR spectrum of CDs-ETs. This C=O signal should derive from the COOH groups on the surface of CDs by comparison with the FTIR spectrum of CDs. No new vibration peaks other than the ones of the components in the FTIR spectrum of CDs-ETs confirm the noncovalent binding of CDs and ETs.
In general, the hydroxyl group on each carbon atom facilitates strong hydrogen bonding between neighboring ET molecules [36]. This hydrogen bonding enables ETs to absorb significant amounts of heat during melting, thereby achieving high heat storage density. After CDs with rich O-related functional groups were introduced into ETs, they interacted with the OH on ETs to form multiple binding forces, incurring changes in the phase-change latent heat of ETs. To confirm this point, the molecular dynamics (MD) simulation was conducted to examine the interaction of ETs and CDs [37,38]. As illustrated in Figure 3a, three kinds of bond lengths were observed from the interaction of surface groups of CDs with OH on ETs. The bond length is shorter, and the interaction force is stronger. The intermolecular hydrogen bond force of ETs is slightly inferior to the interaction force between the O of C=O on the surface of CDs and the H of OH on ETs. But it is much weaker than the interaction force between the H of COOH on the surface of CDs and the O of OH on ETs. Overall, the interaction strength of the COOH groups on the surface of CDs with ETs is higher than that of the intermolecular hydrogen bonds of ETs. Figure 3b shows equilibrated conformations and atom distribution of ETs and CDs-ETs under the temperatures of 298 and 423 K. The intermolecular interaction of ETs and the interaction of CDs with ETs at 298 K are higher than those at 423 K. One can observe the phase separation phenomenon of CDs in ETs at 423 K due to their melting. According to the in situ FTIR spectra of CDs-ETs (Figure S4), the intensity of -OH groups belonging to ETs increased with temperature, reflecting gradual decay in the interaction of ETs and CDs during the melting process of ETs. The calculated interaction energy for ETs and CDs-ETs is negative (Figure 3c), indicating a mutual attraction between them. In comparison, CDs-ETs exhibit more interaction energy than pure ETs, requiring larger thermal energy for melting. Consequently, the composite CDs-ETs achieved a higher melting enthalpy than pristine ETs. Analogously, the attraction force endowed by surface functional groups of CDs still works during the crystallization process upon cooling, enabling faster crystallization of ETs. This not only reduced the degree of supercooling but also facilitated the release of latent heat, consistent with the experimental results (Figure 3d).
The composite of CDs-ETs as a kind of PCM still maintains a high melting point, which is unfavorable for the storage and utilization of low-temperature heat energy. The reduced-graphene-oxide-modified melamine sponges (rGO@MS) were employed to encapsulate ETs and CDs-ETs for solar energy harvesting and storage (Figure S5). The resultant shape-stabilized composite PCMs are named S1 and S2, respectively. Using a home-made setup as depicted in Figure S6, we provided their temperature−time plots under a simulated sunlight radiation of 400 mW·cm−2. It is clear that the temperature of S1 and S2 increases slowly and cannot reach the melting temperature of ETs (Figure 4a). Thermal images of S1 and S2 are shown in Figure S7. The absence of the exact phase-change platforms certifies that both S1 and S2 fail to store solar energy with the help of only rGO as the light absorber. To fulfill solar energy storage under the same light intensity, a step heating strategy is proposed. First of all, the proper mass of CDs was mixed with polyethylene glycol (PEG) to prepare composite CDs-PEG with a lower melting point than ETs. As illustrated in Figure 4b, CDs-PEG was filled into the holes left by ETs to gain the sample of S3. From Figure 4c and Figures S3 and S7, unlike S1 and S2, enables its temperature to reach the melting point of ETs or CDs-ETs under the solar irradiation condition. When the simulated sunlight is turned off, its temperatures decrease until the temperature peak appears, reflecting a buffering effect upon cooling owing to the heat release. Hence, this confirms that S3 can execute solar energy storage and utilization through the phase transition of ETs under the light intensity of 400 mW·cm−2.
To further shed light on the influence of the incorporation of CDs-PEG into CDs-ETs, we tuned the mass ratio of CDs-PEG and CDs-ETs while encapsulating with rGO@MSs to determine the melting temperature of composites and the amount of thermal energy discharged. Figure 4d contains five samples with the mass ratios of 1:9, 3:7, 5:5, 7:3 and 9:1 for CDs-PEG and CDs-ETs, which are labeled as S3-1, S3-2, S3-3, S3-4 and S3-5, respectively. During heating, S3-3 exhibited the fastest rate in temperature increase and first reached the phase transition temperature of CDs-ETs compared with the other four samples under an irradiation of 400 mW·cm−2. During the cooling process, the samples with higher contents of CDs-PEG manifested two buffering effects on temperature because of the differences in solidification points for PEG and ETs. Figure 4e presents the DSC curves of five samples during the heating and cooling process. Each sample exhibits two endothermic peaks and one exothermic peak. Specifically, the exothermic peak of S3-3 is significantly broad. According to their relevant thermal enthalpies provided in Figure 4f and Table S2, S3-3 is endowed with a quite high melting enthalpy of 306.5 J·g−1 and an optimal crystallization enthalpy of 196.5 J·g−1 compared to the other samples. At present, many common organic solid–liquid PCMs [31,39,40,41,42,43,44,45] are inferior to S3-3 in phase-change storage performance (Figure S8 and Table 2).
An important role of CDs in CDs-PEG serves as photothermal agents to promote solar harvesting and conversion. It is necessary to design the proper proportion of CDs in CDs-PEG. As shown in Figure 5a,b, the rate of temperature increase for S3 and the time required to reach the melting temperature are highly dependent on the proportion of CDs in CDs-PEG upon irradiation. Notably, the excessive proportion of CDs lessens the temperature of S3 under the same irradiation time and requires a longer irradiation time to realize melting instead. The main reason is that the high content of CDs minimizes the specific heat capacity of CDs-PEG and impairs the accumulation of solar-converted thermal energy for heating ETs. PEG and the corresponding composite CDs-PEG feature a low melting point and easily undergo solid–liquid transformation under solar irradiation. The melting PEG has a larger volume and induces pressure in the small pores of rGO@MS, resulting in close contact of liquid PEG with solid CDs-ETs. According to the contact melting experiments and theoretical simulations as reported in the literature [46], the heat transfer efficiency of the contact melting region is beyond an order of magnitude higher than that of the non-contact melting region. Considering that the thermal conduction is the primary mode of heat transfer in the contact melting region, we explored the thermal conduction of S3 and found that the value at around 80 °C is higher than that at the room temperature (Figure S9). This upgrades the heat transfer rate and reduces the melting time of PCMs. Therefore, the melting PEG can accumulate heat energy for the phase change in CDs-ETs, benefiting from the melting and crystallization of ETs.
Thanks to the conductive property of rGO@MS, we compare two different ways of energy storage and release through S3-3 by introducing electric heating. One is that S3 is heated to completely melt under a simulated sunlight radiation of 100 mW·cm−2 and is followed by coupling electric heating (applying a voltage of 15 V). The other is one-step heating of S3 by exerting a light of 100 mW·cm−2 and a voltage of 15 V simultaneously. As shown in Figure 5c, both heating methods result in S3 attaining different temperatures. It can be found that the melting PEG plays the role of a booster in heat storage charging (melting) processes for CDs-ETs. The melting PEG enhances the melting efficiency of CDs-ETs by minimizing the thermal resistance between solid ETs and photothermal materials mentioned above. It is worth noting that only applying a light of 100 mW·cm−2 or a voltage of 15 V cannot fulfill energy charging and storage of S3 (Figure S10). Figure 5d shows the time-dependent temperature profiles at controlled voltages accompanying a light irradiation of 100 mW·cm−2. A higher voltage applied in S3 results in a higher saturated temperature. The saturated temperature can reach 62.4 °C (5 V), 105.6 °C (10 V), 120.0 °C (12 V) and 140.0 °C (15 V). The integration of solar heating and electric heating provides multiple driving forces and significantly extends the range of applications and service duration.
Moreover, CDs-ETs and CDs-PEG can be effectively retained within rGO@MS. The SEM images indicated that rGO sheets were coated on the skeleton of the porous sponge, forming hierarchical pores (Figure 6a–c). The rGO@MS was impregnated by liquid CDs-ETs and CDs-PEG in turn to achieve a shape-stabilized sample, i.e., S3 mentioned above. According to SEM observation, the vast majority of pores in rGO@MS were filled by PCMs (Figure 6d–f). Although the capillary action induced by the hierarchical pores enabled the stability of PCMs within the confined hierarchical pores, the leakage of melted PCMs was not completely prevented during the solid–liquid phase transition (Figure S11a). To solve this problem, polydimethylsiloxane (PDMS) was used to cover the whole surface of S3, and the corresponding SEM images are displayed in Figure 6g–i. We observed that the pores were filled more fully and the surface became very smooth after PDMS modification. FTIR spectra were analyzed to give insight into the interaction of PDMS with S3. After PDMS coating, the stretching vibration peak of –OH at 3200 cm−1, ascribed to the ETs and PEG, and the bending vibration peak of C-NH2 at 860 cm−1, derived from MS, almost disappeared in the FTIR spectrum (Figure S11b), suggesting that the surface of S3 was fully covered by PDMS. We further measured the water contact angles of S3 before and after coating with PDMS. The water contact angle was enhanced to 101.74° from 65.17° after coating with PDMS (Figure 7a). Consequently, PDMS-modified S3 was endowed with a unique hydrophobic property. Subjecting PDMS-modified S3 to 11 heating–cooling cycles demonstrated no marked change in the temperature–time curves (Figure 7b) and morphology profiles (Figure 7c). This indicates that the heat management capability is significantly stable, and no liquid leakage occurs.

4. Conclusions

We have demonstrated a facile and efficient strategy to modulate the phase transition enthalpy and supercooling degree of ETs by leveraging the surface functional groups of CDs, which have ultrafine sizes and large specific surface areas. The proportion of CDs in ETs was investigated to achieve optimal thermal energy storage performance and a minimal degree of supercooling. The optimally formulated CD-ET composites showed a significant improvement in thermal storage capacity compared to the pristine ETs. Specifically, the latent heat storage capacity of the optimally formulated CDs-ETs was increased from 377.3 to 410.2 J·g−1, and the exothermic capacity increased from 209.0 to 240.2 J·g−1, a significant improvement of about 14.9%. In terms of supercooling performance, the subcooled degree of CDs-ETs was lower than beyond 30 °C than the pristine ETs. Theoretical calculation revealed that the intermolecular interactions of ETs were regulated by evenly dispersed CDs and impacted their melting and course of crystallization, resulting in higher energy storage density and latent heat release than pure ETs. The inherent high melting temperature of ETs and CDs-ETs limits their utilization in low-temperature heat energy. In this regard, we proposed a reliable solution that CDs-ETs and CDs-PEG with a low melting point were encapsulated into rGO@MS in turn and then coated with PDMS. The resultant shape-stabilized composite not only prevents leakage of the molten PCMs but also allows for a wide storage temperature window and increases heat transfer capability by putting melted PEG in close contact with solid CDs-ETs. Together, these characteristics improve the reliability and adaptability of thermal energy storage systems, enabling them to efficiently capture, store and release heat under varying operating conditions. This is critical for advancing applications such as solar thermal harvesting, industrial waste heat recovery and grid-scale energy storage, where a wide temperature range and consistent thermal performance are essential. Stepped melting and crystallization guarantee the phase change in CD-ETs and ETs by solar heating, Joule heating or their coupling. In addition, the melting enthalpy of this composite is as high as 306.5 J·g−1, and the cold crystallization enthalpy reaches 196.5 J·g−1, surpassing many organic solid–liquid PCMs. By integrating material property tailoring with engineering-oriented encapsulation, this approach offers a scalable and adaptable solution for translating lab-based thermal energy storage innovations into real-world sustainable energy systems.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/en18102597/s1, Figure S1: Physical drawing of DSC test instrument, Figure S2: The dependence of degree of supercooling upon the mass ratio of ETs and CDs, Figure S3: (a)Temperature-time curves of composite CDs-ETs with different contents of CDs resulted from isothermal testing; (b) Enlarged view of the crystallization process, Figure S4: In situ FTIR spectra of CDs-ETs during heating process, Figure S5: Schematic illustration for preparing samples including the reduced- graphene-oxide-modified melamine sponges (rGO@MS), S1 and S2, Figure S6: The schematic of solar heating and temperature monitoring, Figure S7: Thermograms from 400 to 800 s for S1, S2 and S3, Figure S8: Heat storage performance comparisons of the S3 with those of other PCMs reported in literatures, Figure S9: The thermal conductivity of the S2 and S3 at 25 ℃ and 80 ℃, Figure S10: The heating and cooling process of S3 under one sun irradiation (100 mW·cm−2) or applied a voltage of 15 V, Figure S11: (a) The photograph of the leakage of S3 without coating PDMS; (b) FTIR spectra of the S3 before and after coating PDMS, Table S1: Phase change temperature and enthalpy of ETs and its composites, Table S2: Thermophysical parameters of our presented samples.

Author Contributions

Conceptualization, J.L. and S.H.; Methodology, N.L.; Software, J.W.; Resources, Q.C.; Writing—original draft, J.L. and S.H.; Supervision, J.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Science and Technology Innovation Teams of Shanxi Province (202304051001014), Research Project Supported by Shanxi Scholarship Council of China (2022-136) and Shanxi Provincial Postgraduate Scientific Research Innovation Project (2023SJ237).

Data Availability Statement

The original contributions presented in the study are included in the article/Supplementary Materials, further inquiries can be directed to the corresponding authors.

Acknowledgments

This work is jointly supported by the special fund for Science and Technology Innovation Teams of Shanxi Province (202304051001014), Research Project Supported by Shanxi Scholarship Council of China (2022-136) and Shanxi Provincial Postgraduate Scientific Research Innovation Project (2023SJ237). We thank the reviewers for their valuable comments and the editors for their careful guidance, and the publisher for their great support!

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

References

  1. Xiong, Q.A.; Alshehri, H.M.; Monfaredi, R.; Tayebi, T.; Majdoub, F.; Hajjar, A.; Delpisheh, M.; Izadi, M. Application of phase change material in improving trombe wall efficiency: An up-to-date and comprehensive overview. Energy Build. 2022, 258, 111824. [Google Scholar] [CrossRef]
  2. Junaid, M.F.; Rehman, Z.U.; Ijaz, N.; Cekon, M.; Curpek, J.; Elhag, A.B. Biobased phase change materials from a perspective of recycling, resources conservation and green buildings. Energy Build. 2022, 270, 112280. [Google Scholar] [CrossRef]
  3. Ge, H.S.; Li, H.Y.; Mei, S.F.; Liu, J. Low melting point liquid metal as a new class of phase change material: An emerging frontier in energy area. Renew. Sust. Energy Rev. 2013, 21, 331–346. [Google Scholar] [CrossRef]
  4. Zhang, B.P.; Chen, Y.; Mu, Y.; Yang, B.Z.; Li, X.; Ke, S.R.; Min, X.; Mi, R.Y.; Wu, X.W.; Liu, Y.G.; et al. A novel cobalt-reinforced graphene aerogel composite phase change material with excellent energy storage capacity for low-temperature industrial waste heat recovery. J. Energy Storage 2025, 107, 114920. [Google Scholar] [CrossRef]
  5. Lu, Y.; Xiao, X.D.; Fu, J.; Huan, C.M.; Qi, S.; Zhan, Y.J.; Zhu, Y.Q.; Xu, G. Novel smart textile with phase change materials encapsulated core-sheath structure fabricated by coaxial electrospinning. Chem. Eng. J. 2019, 355, 532–539. [Google Scholar] [CrossRef]
  6. Zhang, H.J.; Zhang, X.G.; Pan, D.; Ai, Y.H.; Chen, Y.S. Preparation and application of high-temperature composite phase change materials. J. Energy Storage 2023, 68, 107669. [Google Scholar] [CrossRef]
  7. Li, X.; Liu, Y.; Xu, Y.; Tao, P.; Deng, T. Solid–Liquid Phase Change Composite Materials for Direct Solar–Thermal Energy Harvesting and Storage. Acc. Mater. Res. 2023, 4, 484–495. [Google Scholar] [CrossRef]
  8. Tao, J.L.; Luan, J.D.; Liu, Y.; Qu, D.Y.; Yan, Z.; Ke, X. Technology development and application prospects of organic-based phase change materials: An overview. Renew. Sust. Energ. Rev. 2022, 159, 112175. [Google Scholar] [CrossRef]
  9. Wang, P.; Diao, X.M.; Chen, X. Intelligent phase change materials for long-duration thermal energy storage. Mater 2024, 7, 2716–2718. [Google Scholar] [CrossRef]
  10. Vitorino, N.; Abrantes, J.C.C.; Frade, J.R. Quality criteria for phase change materials selection. Energy Convers. Manag. 2016, 124, 598–606. [Google Scholar] [CrossRef]
  11. Qiao, Y.J.; Gu, X.; Gao, Z.Y.; Bai, C.Y.; Zhang, L.L.; Wang, X.D.; Zheng, T. Preparation and characterization of eicosane-paraffin/expanded graphite/polyurethane as form-stable composite phase change materials with wide phase change temperature range. Mater. Lett. 2024, 366, 136518. [Google Scholar] [CrossRef]
  12. Shamseddine, I.; Pennec, F.; Biwole, P.; Fardoun, F. Supercooling of phase change materials: A review. Renew. Sust. Energ. Rev. 2022, 158, 112172. [Google Scholar] [CrossRef]
  13. Safari, A.; Saidur, R.; Sulaiman, F.A.; Xu, Y.; Dong, J. A review on supercooling of Phase Change Materials in thermal energy storage systems. Renew. Sust. Energ. Rev. 2017, 70, 905–919. [Google Scholar] [CrossRef]
  14. Zahir, M.H.; Mohamed, S.A.; Saidur, R.; Al-Sulaiman, F.A. Supercooling of phase-change materials and the techniques used to mitigate the phenomenon. Appl. Energy 2019, 240, 793–817. [Google Scholar] [CrossRef]
  15. Shi, J.M.; Qin, M.L.; Aftab, W.; Zou, R.Q. Flexible phase change materials for thermal energy storage. Energy Storage Mater. 2021, 41, 321–342. [Google Scholar] [CrossRef]
  16. Shao, X.-F.; Yang, S.; Lin, J.-C.; Teng, H.-R.; Fan, L.-W.; Chiu, J.N.; Yuan, Y.-P.; Martin, V. Polyvinylpyrrolidone (PVP)-enabled significant suppression of supercooling of erythritol for medium-temperature thermal energy storage. J. Energy Storage 2022, 46, 103915. [Google Scholar] [CrossRef]
  17. Cui, H.Z.; Wang, P.Z.; Yang, H.B.; Xu, T. Design and preparation of salt hydrate/graphene oxide@SiO2/SiC composites for efficient solar thermal utilization. Sol. Energy Mater. Sol. Cells 2022, 236, 111524. [Google Scholar] [CrossRef]
  18. Tang, J.; Wang, Y.; Liu, H.; Belfiore, L.A. Effects of organic nucleating agents and zinc oxide nanoparticles on isotactic polypropylene crystallization. Polymer 2004, 45, 2081–2091. [Google Scholar] [CrossRef]
  19. Nazari, M.A.; Maleki, A.; Assad, M.E.H.; Rosen, M.A.; Haghighi, A.; Sharabaty, H.; Chen, L. A review of nanomaterial incorporated phase change materials for solar thermal energy storage. Sol. Energy 2021, 228, 725–743. [Google Scholar] [CrossRef]
  20. Del Barrio, E.P.; Cadoret, R.; Daranlot, J.; Achchaq, F. New sugar alcohols mixtures for long-term thermal energy storage applications at temperatures between 70 °C and 100 °C. Sol. Energy Mater. Sol. Cells 2016, 155, 454–468. [Google Scholar] [CrossRef]
  21. Shao, X.F.; Wang, C.; Yang, Y.J.; Feng, B.; Zhu, Z.Q.; Wang, W.J.; Zeng, Y.; Fan, L.W. Screening of sugar alcohols and their binary eutectic mixtures as phase change materials for low-to-medium temperature latent heat storage. (I): Non-isothermal melting and crystallization behaviors. Energy 2018, 160, 1078–1090. [Google Scholar] [CrossRef]
  22. Shao, X.F.; Yang, S.; Fan, L.W.; Yuan, Y.P. Sugar alcohol phase change materials for low-to-medium temperature thermal energy storage: A comprehensive review. J. Energy Storage 2023, 68, 107848. [Google Scholar] [CrossRef]
  23. Liu, C.; Cheng, Q.; Li, B.; Liu, X.; Rao, Z. Recent advances of sugar alcohols phase change materials for thermal energy storage. Renew. Sustain. Energy Rev. 2023, 188, 113805. [Google Scholar] [CrossRef]
  24. Rosso, C.; Filippini, G.; Prato, M. Carbon Dots as Nano-Organocatalysts for Synthetic Applications. ACS Catal. 2020, 10, 8090–8105. [Google Scholar] [CrossRef]
  25. Wu, H.; Lu, S.; Yang, B. Carbon-Dot-Enhanced Electrocatalytic Hydrogen Evolution. Acc. Mater. Res. 2022, 3, 319–330. [Google Scholar] [CrossRef]
  26. Meng, X.; Chang, Q.; Xue, C.; Yang, J.; Hu, S. Full-colour carbon dots: From energy-efficient synthesis to concentration-dependent photoluminescence properties. Chem. Commun. 2017, 53, 3074–3077. [Google Scholar] [CrossRef]
  27. Li, L.; Xue, C.; Chang, Q.; Ren, X.; Li, N.; Yang, J.; Hu, S.; Xu, H. Polyelectrolyte Hydrogel-Functionalized Photothermal Sponge Enables Simultaneously Continuous Solar Desalination and Electricity Generation Without Salt Accumulation. Adv. Mater. 2024, 36, 2401171. [Google Scholar] [CrossRef]
  28. Su, J.; Chang, Q.; Xue, C.; Yang, J.; Hu, S. Electrochemical oxidation reconstructs graphene oxides on sponge for unprecedentedly high solar water evaporation. Carbon 2022, 194, 267–273. [Google Scholar] [CrossRef]
  29. Moffat, R.J. Describing the uncertainties in experimental results. Exp. Therm. Fluid Sci. 1988, 1, 3–17. [Google Scholar] [CrossRef]
  30. Chen, H.-C.; Wu, P.-C.; Huang, J.-Y.; Chen, L.-A. Uncertainty analysis for measurement of measurand. Measurement 2010, 43, 1250–1254. [Google Scholar] [CrossRef]
  31. Chen, Y.-H.; Jiang, L.-M.; Fang, Y.; Shu, L.; Zhang, Y.-X.; Xie, T.; Li, K.-Y.; Tan, N.; Zhu, L.; Cao, Z.; et al. Preparation and thermal energy storage properties of erythritol/polyaniline form-stable phase change material. Sol. Energy Mater. Sol. Cells 2019, 200, 109989. [Google Scholar] [CrossRef]
  32. Li, Q.; Feng, Y.; Li, Y.-T.; Ning, Y.-H.; Hu, P.; Lin, C.-H.; Zhang, Y.-H.; Zhou, M.; Yu, L.-P.; Li, C.-C.; et al. Erythritol confined in multiwalled carbon nanotubes reinforced silica aerogel as novel form-stable phase change materials. J. Mol. Liq. 2022, 360, 119589. [Google Scholar] [CrossRef]
  33. Ma, C.; Wang, Y.; Xie, S.; Wang, J.; Ji, Z. Phase transition characteristics and supercooling suppression of erythritol with organic salts as nucleating agents. J. Energy Storage 2023, 73, 108889. [Google Scholar] [CrossRef]
  34. Shin, H.K.; Rhee, K.-Y.; Park, S.-J. Effects of exfoliated graphite on the thermal properties of erythritol-based composites used as phase-change materials. Compos. B Eng. 2016, 96, 350–353. [Google Scholar] [CrossRef]
  35. Zhang, Q.; Luo, Z.; Guo, Q.; Wu, G. Preparation and thermal properties of short carbon fibers/erythritol phase change materials. Energy Convers. Manage. 2017, 136, 220–228. [Google Scholar] [CrossRef]
  36. Liu, Y.; Deng, Y.; Zheng, J.; Wu, F.; Lu, J.; Sun, S.; Wu, D.; Wu, T. Role and influence of hydrogen bonds in composite phase change materials: A critical review. Sol. Energy Mater. Sol. Cells 2022, 248, 112031. [Google Scholar] [CrossRef]
  37. Feng, B.; Fan, L.-W.; Zeng, Y.; Ding, J.-Y.; Shao, X.-F. Atomistic insights into the effects of hydrogen bonds on the melting process and heat conduction of erythritol as a promising latent heat storage material. Int. J. Therm. Sci. 2019, 146, 106103. [Google Scholar] [CrossRef]
  38. Sun, H.; Ren, P.; Fried, J.R. The COMPASS force field: Parameterization and validation for phosphazenes. Polymer 1998, 8, 229–246. [Google Scholar] [CrossRef]
  39. Wei, Z.; Fu, X.; Wu, B.; Jiang, L.; Wang, Y.; Yuan, A.; Xu, H.; Lei, J. Paraffin/methyl stearate/multi-walled carbon nanotubes composite phase change materials with wide service temperature and high latent heat. Int. J. Energy Res. 2021, 45, 13636–13645. [Google Scholar] [CrossRef]
  40. Li, Y.; Lu, Q.; Yang, J.; He, W. Delignified wood for thermal energy storage with high efficient photo-thermal conversion efficiency. J. Energy Storage 2024, 80, 110235. [Google Scholar] [CrossRef]
  41. Zhang, J.-L.; Wu, N.; Wu, X.-W.; Chen, Y.; Zhao, Y.; Liang, J.-F.; Bai, X.-B. High latent heat stearic acid impregnated in expanded graphite. Thermochim. Acta 2018, 663, 118–124. [Google Scholar] [CrossRef]
  42. Yan, K.; Feng, Y.; Qiu, L. Thermal and photo/electro-thermal conversion characteristics of high energy storage density expanded graphite/polyethylene glycol shaped composite phase change materials. Sol. Energy 2024, 272, 112477. [Google Scholar] [CrossRef]
  43. Emeema, J.; Murali, G.; Reddi, B.V.; Mangesh, V.L. Investigations on paraffin wax/CQD composite phase change material—Improved latent heat and thermal stability. J. Energy Storage 2024, 85, 111056. [Google Scholar] [CrossRef]
  44. Wei, C.; Li, Y.; Song, J.; Cheng, J.; Tang, Z. Supercooling suppression and thermal conductivity enhancement of erythritol using graphite foam with ultrahigh thermal conductivity for thermal energy storage. Int. Commun. Heat. Mass. Transf. 2024, 153, 107392. [Google Scholar] [CrossRef]
  45. Dong, K.; Sheng, N.; Zou, D.; Wang, C.; Shimono, K.; Akiyama, T.; Nomura, T. A high-thermal-conductivity, high-durability phase-change composite using a carbon fibre sheet as a supporting matrix. Appl. Energy 2020, 264, 114685. [Google Scholar] [CrossRef]
  46. Hu, N.; Li, Z.-R.; Xu, Z.-W.; Fan, L.-W. Rapid charging for latent heat thermal energy storage: A state-of-the-art review of close-contact melting. Renew. Sust. Energ. Rev. 2022, 155, 111918. [Google Scholar] [CrossRef]
Energies 18 02597 sch001
Scheme 1. Schematic diagram of CDs-ETs preparation.
Scheme 1. Schematic diagram of CDs-ETs preparation.
Energies 18 02597 sch001
Energies 18 02597 g001
Figure 1. Thermal behavior of CDs-ETs. (a,b) DSC traces of heating and cooling (heating/cooling rate 10 °C min) cycles of ETs and composites with different ratios of CDs and ETs; (c) the relationship of melting and freezing temperature with the mass ratio of ETs and CDs; (d) melting enthalpies and crystallization enthalpies of pristine ETs and CDs-ETs with different mass ratios; (e) comparison of melting enthalpies for different ET-based phase-change composites (The star represents the carbon dots composites we studied and the dotted lines represent their enthalpies of melting); (f) temperature–time curves of ETs and composite CDs-ETs resulted from isothermal testing.
Figure 1. Thermal behavior of CDs-ETs. (a,b) DSC traces of heating and cooling (heating/cooling rate 10 °C min) cycles of ETs and composites with different ratios of CDs and ETs; (c) the relationship of melting and freezing temperature with the mass ratio of ETs and CDs; (d) melting enthalpies and crystallization enthalpies of pristine ETs and CDs-ETs with different mass ratios; (e) comparison of melting enthalpies for different ET-based phase-change composites (The star represents the carbon dots composites we studied and the dotted lines represent their enthalpies of melting); (f) temperature–time curves of ETs and composite CDs-ETs resulted from isothermal testing.
Energies 18 02597 g001
Energies 18 02597 g002
Figure 2. Characterizations of CDs-ETs. (a–f) Optical images of pristine ETs and CDs-ETs with different mass ratios; (g) XRD patterns of ETs and CDs-ETs; (h) FTIR spectra of ETs, CDs and CDs-ETs.
Figure 2. Characterizations of CDs-ETs. (a–f) Optical images of pristine ETs and CDs-ETs with different mass ratios; (g) XRD patterns of ETs and CDs-ETs; (h) FTIR spectra of ETs, CDs and CDs-ETs.
Energies 18 02597 g002
Energies 18 02597 g003
Figure 3. MD simulation results. (a) Comparison of bond lengths in ETs and CDs-ETs; (b) equilibrated conformations of ETs and CDs-ETs at 298 and 432 K; (c) simulated interaction energy of ETs and CDs-ETs at 298 and 432 K; (d) mechanism illustration of phase-change process.
Figure 3. MD simulation results. (a) Comparison of bond lengths in ETs and CDs-ETs; (b) equilibrated conformations of ETs and CDs-ETs at 298 and 432 K; (c) simulated interaction energy of ETs and CDs-ETs at 298 and 432 K; (d) mechanism illustration of phase-change process.
Energies 18 02597 g003
Energies 18 02597 g004
Figure 4. Heat storage performances of encapsulated composites. (a) Temperature–time curves of S1 and S2 under a solar radiation of 400 mW·cm−2; (b) schematic illustration of S3 preparation; (c) temperature–time curves of S3 under a solar radiation of 400 mW·cm−2, the inset showing an infrared photograph; (d) temperature–time curves of S3 with different mass ratios of CDs-PEG and CDs-ETs under a solar radiation of 400 mW·cm−2; (e,f) DSC curves and the obtained phase-change enthalpies of S3 with different mass ratios of CDs-PEG and CDs-ETs.
Figure 4. Heat storage performances of encapsulated composites. (a) Temperature–time curves of S1 and S2 under a solar radiation of 400 mW·cm−2; (b) schematic illustration of S3 preparation; (c) temperature–time curves of S3 under a solar radiation of 400 mW·cm−2, the inset showing an infrared photograph; (d) temperature–time curves of S3 with different mass ratios of CDs-PEG and CDs-ETs under a solar radiation of 400 mW·cm−2; (e,f) DSC curves and the obtained phase-change enthalpies of S3 with different mass ratios of CDs-PEG and CDs-ETs.
Energies 18 02597 g004
Energies 18 02597 g005
Figure 5. Thermal performance analysis. (a) Effect of CDs contents in CDs-PEG on temperature variation curves of S3-3; (b) temperature–time curves of S3-3 containing different CDs contents in CDs-PEG resulted from isothermal testing; (c) comparison of two different ways of energy storage and release via S3-3 (The shaded area represents the photocoupling time period); (d) time-dependent temperature profiles at given voltages accompanying with solar irradiation of 100 mW·cm−2.
Figure 5. Thermal performance analysis. (a) Effect of CDs contents in CDs-PEG on temperature variation curves of S3-3; (b) temperature–time curves of S3-3 containing different CDs contents in CDs-PEG resulted from isothermal testing; (c) comparison of two different ways of energy storage and release via S3-3 (The shaded area represents the photocoupling time period); (d) time-dependent temperature profiles at given voltages accompanying with solar irradiation of 100 mW·cm−2.
Energies 18 02597 g005
Energies 18 02597 g006
Figure 6. SEM images of encapsulated composites. (a–c) SEM images of rGO@MS; (d–f) SEM images of S3; (g–i) SEM images of S3 coated with PDMS.
Figure 6. SEM images of encapsulated composites. (a–c) SEM images of rGO@MS; (d–f) SEM images of S3; (g–i) SEM images of S3 coated with PDMS.
Energies 18 02597 g006
Energies 18 02597 g007
Figure 7. Performance of PDMS-coated S3. (a) Water contact angles of S3 before and after coating PDMS; (b) cycle stability of PDMS-coated S3; (c) photos of PDMS-coated S3 vs. time at 160 °C.
Figure 7. Performance of PDMS-coated S3. (a) Water contact angles of S3 before and after coating PDMS; (b) cycle stability of PDMS-coated S3; (c) photos of PDMS-coated S3 vs. time at 160 °C.
Energies 18 02597 g007
Table 1. Enthalpy comparison of our presented CDs-ETs with ET-based PCMs reported in the literature.
Table 1. Enthalpy comparison of our presented CDs-ETs with ET-based PCMs reported in the literature.
NameMaterialsΔHm (J g−1)Reference
M1ET-PANI284.2[31]
M2ET-SIO2294.3[32]
M3ET-PVP301[16]
M4ET-10 wt% SCF340.4[35]
M5ET-0.25 wt% EG342[34]
M6ET-1% CAPI344.9[33]
This workET-CDs410.2/
Table 2. Enthalpy comparisons of S3 with other PCMs
Table 2. Enthalpy comparisons of S3 with other PCMs
NamePhase-Change MaterialsΔHm (J g−1)Reference
FSPSM-1Paraffin/Methyl stearate129.69[39]
1%GQ-BN-PCESPolyethylene glycol133.7[40]
EG-SA99Stearic acid 165.44[41]
4% EG-PEGPolyethylene glycol174.38[42]
PWCQD-1Paraffin240[43]
ET-GFErythritol245.3[44]
ET@CFSErythritol282.3[45]
PANI@ETErythritol284.12[31]
S3Erythritol306.5This work
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Liang, J.; Li, N.; Wu, J.; Chang, Q.; Yang, J.; Hu, S. Carbon Dot-Modulated Phase-Change Composites for Wide Temperature Range and High-Density Heat Storage and Release. Energies 2025, 18, 2597. https://doi.org/10.3390/en18102597

AMA Style

Liang J, Li N, Wu J, Chang Q, Yang J, Hu S. Carbon Dot-Modulated Phase-Change Composites for Wide Temperature Range and High-Density Heat Storage and Release. Energies. 2025; 18(10):2597. https://doi.org/10.3390/en18102597

Chicago/Turabian Style

Liang, Jingya, Ning Li, Jie Wu, Qing Chang, Jinlong Yang, and Shengliang Hu. 2025. "Carbon Dot-Modulated Phase-Change Composites for Wide Temperature Range and High-Density Heat Storage and Release" Energies 18, no. 10: 2597. https://doi.org/10.3390/en18102597

APA Style

Liang, J., Li, N., Wu, J., Chang, Q., Yang, J., & Hu, S. (2025). Carbon Dot-Modulated Phase-Change Composites for Wide Temperature Range and High-Density Heat Storage and Release. Energies, 18(10), 2597. https://doi.org/10.3390/en18102597

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop