Highly porous carbons derived from MOFs for shape-stabilized phase change materials with high storage capacity and thermal conductivity

Abstract

Highly porous carbons (HPCs) are successfully prepared using a controlled carbonization of metal organic frameworks (MOFs) method. New micropores and mesoporous channels are produced during the migration and aggregation of small ZnO particles in the carbon matrix, while larger nanocavities are created after the evaporation of ZnO particles. The HPCs with high surface area (up to 2551 m² g⁻¹) and large total pore volume (up to 3.05 cm³ g⁻¹) have high adsorption of polyethylene glycol (PEG) (up to 92.5 wt%) for shape-stabilized phase change materials (ssPCMs). In the PEG@HPCs composites, the nanocavities with a large mesopore volume guarantee the high storage of PEG molecules, and the micropores and mesoporous channels induced surface tension and capillary force to ensure the high thermal stability of the composites. With a high content of PEG and good shape-stabilities, the PEG@HPCs show high phase change enthalpy, which is close to or even higher than that of pure PEG. The thermal conductivity of PEG can also be improved by HPCs.

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1. Introduction

Latent thermal energy storage (LTES) with high energy storage capacity and constant phase change temperature is one of the most preferred ways of energy storage.^1–3 Phase change materials (PCMs), as the main energy storage mediums in LTES, are applied to attain energy savings by their phase transitions and reduce the time gap between supply and demand of energy.^4–7 Up to now, organic PCMs, inorganic PCMs, and a hybrid of both the PCMs have been developed.⁸ As a kind of organic PCM, polyethylene glycol (PEG) is especially promising as a solid–liquid PCM for LTES at low temperature, owing to its chemical and thermal stabilities, high phase change enthalpy, and suitable melting temperature. However, the leakage above its melting temperature and the low thermal conductivity still remain as two drawbacks that limit its application.^9–12

Currently, the methods of inserting solid–liquid PCMs into porous carriers (such as porous silica,^13–18 porous carbons^10,19–23 or silicate minerals^24–30) to prepare shape-stabilized PCM composites (ssPCMs) have been developed to resolve the leakage and low thermal conductivity problems.^14,26–28 In the phase change process, the solid–liquid PCMs in ssPCMs work by storing and releasing latent heat, and the porous carriers serve as supporting materials preventing the working substances from leaking out.³¹ For example, the PEG/SiO₂ PCMs were prepared by impregnating PEG into silica gel. Owing to the stable core–shell structures, the composites exhibited high stabilization during the phase transition.¹⁴ Carbon nanofibers (CNF) and carbon nanotubes (CNT) with good thermal conductivity can also be used to improve the thermal performance of soy wax PCMs and the obtained composites show much higher thermal conductivities than that of pure soy wax.³² Graphene/ceramic composites were used as effective fillers to improve the thermal transfer properties of stearic acid, due to the high thermal conductivity of graphene and excellent wettability between graphene and stearic acid.³³ However, low content of working substances in these composite ssPCMs inevitably reduce the energy storage density and cause a lower efficiency in energy storage. So far, developing porous materials with new structures as supporting materials for ssPCMs, which possess high PCM loading, good thermal conductivity and high shape stabilization capability, are still a challenge for practical applications.^9,34

Recently, porous metal organic frameworks (MOFs), which possess one-, two-, and three-dimensional channel structures, have attracted great attention. A large number of MOFs have been synthesized in the past decades.^35–40 With high thermal stability, MOFs can be used as novel templates to prepare porous carbons.^41–43 The MOFs-derived nanoporous carbon materials have three-dimensional (3D) micro-, meso-, and macropores with high surface area and an exceptionally high pore volume.^44–46 With these excellent porous parameters, such kind of porous materials have high potential as candidates to prepare ssPCMs with high energy storage density. However, to the best of our knowledge, the MOF-derived porous carbon materials have not been investigated for LTES yet. Notably, their porosity characteristics of simultaneously high pore volume and surface area are rarely found in the family of porous carbons that are used for ssPCMs.

In this paper, highly porous carbons (HPCs) derived from MOFs are applied as supporting materials to prepare PEG based ssPCMs. The HPCs show high PEG adsorption (up to 92.5 wt%), and the obtained PEG@HPCs composite PCMs exhibit high phase change enthalpy, which is close to or even higher than that of pure PEG, and the thermal conductivity of PEG can also be improved by HPCs. The experiment results suggest the excellent performance of the composite PCMs is highly related to the highly porous structure of HPCs, which is formed by controlling the nucleation, aggregation and evaporation of ZnO particles in the carbon matrix.

2. Results and discussion

Characterization of carbon materials

The MOF-5 was synthesized via the “direct mixing” synthesis strategy as reported in existing literature.^47,48 XRD results (Fig. S1†) confirm the as-prepared samples have the crystalline structure of MOF-5.⁴⁹ The HPCs were obtained through a controlled carbonization of MOF-5 under N₂ atmosphere. TGA results of the carbonization of MOF-5 show three phases of the weight loss (Fig. 1): the mass loss (15 wt%) below 250 °C is attributed to the evaporation of solvent molecules inside pores, and XRD results demonstrate the thermal stability of MOF-5 during the solvent evaporation process. When the temperature is increased to 500 °C, the peaks of ZnO appear, and the peaks that belong to MOF-5 become weak, which indicates the mass-loss between 400 °C and 750 °C (30 wt%) is attributed to the decomposition of MOF-5 framework and Zn²⁺ → ZnO. The mass-loss between 750 °C and 1000 °C (44 wt%) is due to the evaporation of CO₂, CO and Zn via the reduction of ZnO by carbon, and about 10 wt% porous carbon material is obtained. As showed in Fig. 1, the XRD peaks corresponding to ZnO disappeared, and no sharp peaks could be found after carbonization, which indicates ZnO evaporated and the resultant porous carbon is in amorphous state.⁴⁹ Interestingly, the weak and broad XRD peaks at ∼24° and ∼44°, which assigned to carbon (002) and (101) diffractions, indicate the formation of disordered tiny graphitic structure in HPC-1000 carbon (Fig. S3†). SEM images of HPC-1000 show the HPCs possess similar shapes to their precursors (Fig. S4a and b†).

Fig. 1 TGA curves of MOF-5.

To understand the effect of the formation of ZnO on the pore structure in the carbon matrix, the HPCs were synthesized under 450 °C by controlling the carbonization time. The samples were studied by XRD and HRTEM. As shown in Fig. 2a-1–a-3, after 5 minutes of carbonization, XRD results show that the peaks assigned to ZnO emerged, which indicates MOF-5 structure begins to decompose and ZnO clusters start to appear, but no ZnO could be observed from the HRTEM image, due to the too small size of the ZnO clusters. After 20 minutes of carbonization at 450 °C, by the comparison of the full width at half maximum (FWHM) of XRD, it is found that the particle size of ZnO has increased, which can also be confirmed by HRTEM. As shown in Fig. 2b-2, the size of ZnO nano particles are 1–3 nm, which is mainly attributed to the migration and aggregation of ZnO nanoclusters into ZnO nano particles in the carbon matrix (Fig. 2b-3). When the carbonization time is increased to 80 min, the peaks belonging to MOF-5 disappeared, which indicates the structure of MOF-5 has decomposed completely, and only the peaks of ZnO could be observed (Fig. 2c-1). As shown in Fig. 2c-2, some of ZnO particles are aggregated into larger particles, and its size increased to 5–10 nm. After 320 min reaction, all ZnO are transformed into ∼10 nm ZnO particles. The migration of ZnO particles generates a large number of micropores and mesoporous channels, and the evaporation of these ZnO particles at higher temperature generates a large number of nanocavities in the carbon matrix (Fig. 2e-1–e-3). The porous structures in HPCs are formed during the process of nucleation, migration, aggregation and evaporation of ZnO particles.

Fig. 2 XRD patterns (a-1–e-1) and HRTEM images (a-2–e-2) for the obtained HPCs, and schematic representation (a-3 to e-3) of highly porous structure formation for carbons (micropores, mesoporous channels and nanocavities (white); ZnO (red)). The HPCs in (a–d) were obtained from MOF-5 carbonized at 450 °C for different times (5 min, 20 min, 80 min, and 320 min) and the sample in (e) was got by the direct carbonization of MOF-5 at a heating rate of 5 °C min⁻¹ under N₂ flow to 1000 °C and maintained for 6 h.

To further illustrate the formation of the porous structure in HPCs, N₂ sorption isotherms were performed to analyze the pore volume values, pore-size distribution and BET of HPC-1000 (Fig. 3, Table 1). After the carbonization, the pore size distribution of HPC-1000 is distributed from 1 to 20 nm (Fig. 3b), and the predominant pore smaller than 2 nm in HPC-1000 originates from the porous MOF-5 during the carbonization; the pores ranged in size from ∼2 to 7 nm are attributed to the aggregation and migration of small ZnO particles into large particles in the carbon matrix. The pore size in width >7 nm belongs to the nanocavities, which are retained after the large ZnO nano particles evaporated. The HPC-900 and HPC-1100 samples further confirm the porous characteristic in HPCs, as shown in Fig. 3b, the pore size distribution of HPC-900 and HPC-1100 are similar to that of HPC-1000. The N₂ sorption isotherms for all samples could be classified as type-IV isotherms (Fig. 3a), which also suggest the presence of micro-, meso- and macropores in porous carbon. The BET surface area and mesopore volume of MOF-5 calculated by BET and t-plot methods are 687 m² g⁻¹ and 0.5 cm³ g⁻¹, respectively. After the carbonization of MOF-5, the BET surface area of 1060 m² g⁻¹ and mesopore volume of 0.86 cm³ g⁻¹ are obtained for HPC-900, which is attributed to the micropores and mesoporous channels generated by the migration of ZnO particles in the carbon matrix. The increased mesopore volume of HPC-1000 to 2.11 cm³ g⁻¹ is due to the reservation of the nanocavities after the release of ZnO particles by the evaporation of CO₂, CO and Zn through the reduction of ZnO by carbon (the boiling point of Zn metal is about 908 °C) (Fig. 1, S3 and S4b–d†).⁴⁹ In PEG@HPCs composites, the nanocavities with high mesopore volume of HPCs can guarantee higher storage of PEG molecules, and the micropores and mesoporous channels having induced the surface tension and capillary force can stabilize PEG in the carbon matrix.^10,14

Fig. 3 (a) N₂ adsorption–desorption isotherms and (b) pore size distributions of the as-prepared samples (MOF-5, HPC-900, HPC-1000, and HPC-1100).

Table 1 The surface area and total pore volume of the obtained samples (MOF-5, HPC-900, HPC-1000, and HPC-1100)

Samples	Mesopore area (m^2 g^−1)	Mesopore volume (cm^3 g^−1)	Micropore area (m^2 g^−1)	Micropore volume (cm^3 g^−1)	Total pore volume (cm^3 g^−1)	BET special surface (m^2 g^−1)
MOF-5	110.86	0.27	589.9	0.23	0.5	687.5
HPC-900	427.86	0.86	237.4	0.11	1.24	1060
HPC-1000	980.80	2.11	481.76	0.23	3.00	2491
HPC-1100	957.16	1.92	547.02	0.31	3.05	2551

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Morphology and crystallization properties of the shape stabilized composite PCMs

Fig. 4A shows SEM images of the PEG@HPC-1000 composite PCMs with different PEG weight percentages. When the weight percentage of PEG is below 90 wt%, the HPC-1000 particles in the PEG@HPC-1000 PCMs maintain their particular shapes (Fig. 4Aa and b), which means the PEG is absorbed into the pores of HPC-1000. When the weight percentage of PEG is increased to 95 wt%, most of the HPC-1000 particles are embedded in the PEG blocks (Fig. 4Ad), suggesting that the excessive PEG cannot be absorbed into the pores of HPC-1000. In order to confirm that the 92.5 wt% PEG-4000 composite sample can keep its shape-stabilization, leakage tests were carried out by keeping the composite PCMs pellets in an oven at 80 °C (above the melting point of PEG) for 30 min. As shown in Fig. 4B, no leakage of PEG from the surfaces of the composite PCM can be found. After 50 cycles of cooling–heating between 25 °C and 80 °C, no leakage of the molten PEG is observed from the composite PCM, which indicates the high shape-stabilization of the composite PCMs.

Fig. 4 (A) SEM images of PEG-4000@HPC-1000 PCMs with different PEG percentages: (a) 87.5 wt%, (b) 90 wt%, (c) 92.5 wt%, and (d) 95 wt%, (B) the form-stable effect photos of pure PEG-4000, 92.5 wt% PEG-4000@HPC-1000 PCM before and after 50 times cycling, (C) XRD patterns of the PEG@HPC-1000 PCMs with various PEG-4000 weight percentages, (D) powder XRD pattern of PEG@HPC-1000 PCMs with 92.5 wt% PEG-4000 collected at various temperatures.

To study the effect of porous structure on the shape-stabilized PCM composites, XRD patterns of the PEG-4000@HPC-1000 PCMs with various PEG weight percentages were tested and the results are shown in Fig. 4C. The typical diffraction peaks of pure PEG-4000 are observed at 2θ = 19.2°, 23.4°. Compared to pure PEG, the 2θ positions of the peaks corresponding to PEG in composite PCMs are maintained, which implies PEG still can maintain its crystallization after the introduction of HPC-1000. In order to evaluate the stability of the composite PCMs, after 50 repeated cycles of heating–cooling, the PEG-4000@HPC-1000 PCM with 92.5 wt% PEG was also tested by XRD. As shown in Fig. 4C the relative intensity of the peaks of PEG in the composite PCM indicates the good crystallization behaviors of PEG in the composite.⁵⁰ To study the phase transformation behavior of the composite PCMs, XRD investigations were carried out on the as-synthesized PEG-4000@HPC-1000 PCM with 92.5 wt% PEG in a temperature cycle 25–70–25 °C with the 2θ range from 10° to 25° (Fig. 4D). As the temperature increases, a clear phase transformation around 60 °C is observed and the diffraction peaks belonging to PEG-4000 disappeared, which indicates the PEG is in the molten state. When the temperature decreased to 40 °C, the melted PEG recrystallized, and the typical diffraction peaks of PEG appeared again, indicating another phase transformation of PEG-4000 from amorphous to crystalline.

Thermal stability of PEG@HPCs composite PCMs

Thermal stability is a critical parameter in thermal energy storage applications. The thermal stability tests of PEG@HPC-1000 PCMs are shown in Fig. S5.† It is observed that pure PEG and PEG@HPC-1000 PCMs exhibit a one-step loss of weight (Fig. S5a†), and the weight loss process occurs between 250 °C and 430 °C, which is attributed to the thermal degradation of pure PEG. No obvious weight loss is found below 250 °C, which indicates that the composite PCMs have excellent thermal stability. The residual weight is about 2% for pure PEG.¹⁴

M_T (wt%) = M_R × 2% + (1 − M_R)

where M_T represents the theoretical residual mass; M_R means the weight percentage of PEG absorbed in composite PCMs.

The residual mass (M_R) of the composite PCMs with 80%, 85%, 87.5%, 90% and 92.5% (added weight) of PEG-4000 is about 23%, 16%, 13%, 12% and 10%, respectively, which is in agreement with the theoretical value (M_T) of 21.8%, 16.7%, 14.3%, 11.8%, 9.4%, respectively. Furthermore, the composite PCMs with different molecular weights were also tested by TGA, no obvious differences in the weight lost processes are observed from Fig. S5b.† The results indicate the actual PEG content in PEG@HPCs composite PCMs is in agreement with that of theoretical value.

Thermal properties of PEG@HPCs composite PCMs

The phase change temperatures and enthalpies of the PEG@HPC-1000 PCMs with various PEG weight percentages were investigated by DSC analyses (Fig. 5a and Table S1†). In the melting process, the solid to liquid phase transition of PEG@HPC-1000 PCMs occurs between 45 °C and 65 °C. While in the cooling process, the liquid to solid phase transition occurs between 25 °C and 43 °C. The melting temperature decreases with the increase of HPC-1000 content in the PEG@HPC-1000 PCMs, which should be attributed to the surface tension and capillary force of HPC-1000.¹⁴ The motion of the PEG chain is restrained by the micropores and mesoporous channels in porous carbon, and the restricted PEG chain acting as a heterogeneous nucleating seeds makes the crystalline regions of PEG smaller and leads to the slight decline of the melting point.^51,52

Fig. 5 (a) The DSC curves, (b) thermal conductivity and phase change enthalpies of pure PEG and the PEG@HPC-1000 PCMs with various PEG-4000 weight percentages, (c) the phase change enthalpies of PEG@HPC-900, PEG@HPC-1000 and PEG@HPC-1100 with the maximum content of PEG-4000, respectively, (d) the phase change enthalpies and thermal conductivity of 92.5 wt% PEG@HPC-1000 composite form-stable PCMs with different PEG molecular weight, respectively.

The phase change enthalpy is used to evaluate the thermal energy storage capacity of the composite PCMs. As shown in Fig. 5b and Table S1,† the melting latent heat of the composite PCMs increases along with the content of PEG increased. When the PEG weight percentage is increased to 92.5 wt%, the melting latent heat (162 J g⁻¹) of the composite PCMs is close to that of pure PEG-4000 (164.9 J g⁻¹). In the PEG@HPCs composites, both the high mesopore volume nanocavities and the nano channels that prevent the leakage of the PEG guarantee the high storage of PEG, which determines the high melting latent heat of the composite PCMs.⁵³ In addition, the thermal conductivity of PEG@HPC-1000 PCMs with various weight percentages of PEG was tested at room temperature. The results are shown in Fig. 5b and Table S1.† The thermal conductivity of the PEG-4000@HPC-1000 PCM with 92.5 wt% PEG is 0.42 W m⁻¹ K⁻¹, which is more than 50%-fold higher than that of pure PEG-4000 (0.27 W m⁻¹ K⁻¹). The disordered tiny graphitic porous carbon with very high specific surface area may benefit to the thermal conductivity (Fig. 2 and S3†).

The influence of the structure of HPCs on thermal properties of the composite PCMs was also investigated, and the results are shown in Fig. 5c, S6 and Table S2.† As demonstrated, the highest stabilized PEG content is 80 wt%, 92.5 wt% and 87.5 wt% for HPC-900, HPC-1000 and HPC-1100, respectively. Compared to HPC-900, the ZnO particles in HPC-1000 are removed and generate a large number of nanocavities (Table 1), which promise a higher adsorption of PEG in PEG@HPC-1000 composite PCM. Although the special surface and the total pore volume of HPC-1100 are 2551 m² g⁻¹ and 3.05 cm³ g⁻¹, respectively, which is higher than that of HPC-1000, the highest stabilized PEG content in HPC-1100 was lower than that in HPC-1000. This is probably due to the mesopore volume (about 1.92 cm³ g⁻¹) in HPC-1100 being smaller than that in HPC-1000 (about 2.11 cm³ g⁻¹) (Fig. 3b and Table 1).¹⁴ It indicates that controlling the pore size distribution of HPCs by the carbonization temperature can improve the adsorption capacity of HPCs.⁵²

The thermal characteristics of PEG@HPC-1000 (92.5@7.5 wt%) composite PCMs with different PEG molecular weights were present in Fig. 5d, S7–S11 and Table S3.† The composite PCMs exhibit a large heat storage capacity (160.4–181.9 J g⁻¹). After 50 cycles of the composite PCMs, no significant change in enthalpy is observed, which indicates the good stability of thermal energy storage. It is worth mentioning that the phase change enthalpies of the composite PCMs with PEG-2000, PEG-4000 and PEG-6000, respectively, are close to or even higher than that of pure PEG. Compared with that of pure PEG system, the interactions between PEG and surfaces of HPCs in the composite PCMs such as the surface tension and capillary force result in larger absorbed energy that transformed into intermolecular potential energy.^5,53,54 However, the phase change enthalpies of the composite PCMs with PEG-8000 and PEG-10 000 are lower than that of pure PEG, respectively, which is probably due to the longer polymer chains of PEG-8000 and PEG-10 000. The movement of some PEG chains is confined by the surface tension and capillary force in micropores and mesoporous channels.⁵² The thermal conductivity of the PCMs is about 50% fold higher than that of pure PEG (Fig. 5d and Table S4†). The disordered porous carbon with tiny graphitic structure and high specific surface area can improve the thermal conductivity of PEG effectively.

The comparison of thermal properties of PEG@HPC-1000 with other composite PCMs was also investigated. It shows that the energy storage capacity of PEG@HPC-1000 PCMs is relatively higher than that in the literatures (Table 2). PEG@HPC-1000 composite PCMs show good heat storage capacity, and are suitable for thermal energy storage application. In the PEG@HPCs composites, the nanocavities with large mesopore volume created in HPCs after the evaporation of ZnO particles guarantee high storage of PEG molecules, and the mesoporous channels that produced during the migration and aggregation of small ZnO particles induce the surface tension and capillary force to ensure the high thermal stability of the composites. Both the high mesopore volume nanocavities and the nano channels that prevent the leakage of the PEG guarantee the high storage of PEG, which determines the high melting latent heat of the composite PCMs. Furthermore, the HPCs with tiny graphitic structure and high specific surface area is benefit to the thermal conductivity of PEG, despite the porous carbons have lower degree of graphitization, compared to other carbon materials such as graphene oxide (GO), expanded graphite (EG), carbon nanotube (CNT). Using MOFs to prepare highly graphitic porous carbon will be the next challenge for the thermal energy storage.

Table 2 The comparison of thermal properties of PEG@HPC-1000 composite PCMs with that of composite PCMs in literature^a

Composite PCMs	Tm (°C)	ΔHm (J g^−1)	ΔHm/ΔHp (%)	Reference
a Tm, ΔHm and ΔHp represent melting temperature, melting latent heat of composite PCMs and pure PEG in the melting process, respectively.
PEG-1000(90 wt%)/EG	61.46	161.2	86.1	55
PEG-6000(90 wt%)/EG	63	155	93	52
PEG-6000(70 wt%)/AC	63	90	68	52
PEG-6000(96 wt%)/GO	64	142.8	90.4	9
PEG-6000(90 wt%)/CMK-5	63	150	89	32
PEG-2000(80 wt%)/SiO2	57.4	133.9	71	14
PEG-6000(80 wt%)/SiO2	55.57	71.8	78.5	16
PEG-4000(60 wt%)/cellulose	58.51	84.6	40.6	56
PEG-4000(70 wt%)/agarose	57.73	110.9	53.2	56
PEG-4000(80 wt%)/chitosan	57.18	152.2	73	56
PEG-2000(92.5 wt%)@HPC-1000	53.12	160.4	104.1	Present study
PEG-4000(92.5 wt%)@HPC-1000	60.03	162	98.2	Present study
PEG-6000(92.5 wt%)@HPC-1000	60.45	176	101.7	Present study
PEG-8000(92.5 wt%)@HPC-1000	61.78	170.8	85.8	Present study
PEG-10 000(92.5 wt%)@HPC-1000	64.11	181.9	89	Present study

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3. Conclusion

In summary, HPCs possessing high surface area (up to 2551 m² g⁻¹) and large total pore volume (up to 3.05 cm³ g⁻¹) were prepared by direct carbonization of MOFs method. The nucleation and aggregation of ZnO particles into large particles generate a large number of micropores and mesoporous channels, while the evaporation of ZnO particles further forms nanocavities in the carbon matrix during the carbonization process. The nanocavities with high volume guarantee HPCs have higher adsorption of PEG, and the micropores and mesoporous channels that induced the surface tension and capillary force ensure their high shape-stabilities. The HPCs exhibit excellent adsorption capacity for PEG (up to 92.5 wt%). After 50 repeated cycles of heating–cooling, no significant change in enthalpy confirms the prepared composite ssPCMs have high thermal stability for thermal energy storage. The phase change enthalpies of the ssPCMs are close to or even higher than that of pure PEG, due to the interaction between PEG and the surfaces of HPCs. In addition, the thermal conductivity of the composite PCMs with 92.5 wt% PEG is about 50%-fold higher than that of pure PEG. The MOF-based porous carbon materials possess great potential for efficient thermal energy storage.

4. Experimental

Materials

PEGs (2000, 4000, 6000, 8000, 10 000) were purchased from Xilong Chemical Co., Ltd. Zinc nitrate hexahydrate, CHCl₃ and DMF were provided by Sinopharm Chemical Reagent Beijing Co., Ltd, and p-phthalic acid was obtained from Alfa Aesar.

The synthesized procedure of PEG@HPC composite PCMs

The synthesized procedure of PEG@HPC composite PCMs is illustrated in Fig. 6. Firstly, MOF-5 was synthesized according to the literature,⁴⁷ and the HPCs were obtained by carbonization of MOF-5 at a heating rate of 5 °C min⁻¹ to some certain temperature X under N₂ flow and maintained for 6 h, finally PEG@HPC composite PCMs was prepared after the PEG was absorbed into the HPCs via physical blending and impregnation method.

Fig. 6 Schematic illustration of the prepared procedure of PEG@HPC composite PCMs.

Preparation of MOF-5

MOF-5 was synthesized according to the literature.⁴⁷ A solution containing p-phthalic acid (3.06 g), zinc nitrate tetrahydrate (10.89 g) and DMF (360 mL) was stirred in a flask until the reactants were dissolved, then triethylamine (14.4 g) was dropped slowly. After 30 minutes stirring, 2.7 mL of 30 wt% H₂O₂ aqueous solution was added, the solution was stirred for another 30 min. Finally, the obtained solid was filtered off and washed with DMF 3 times, then washed with methanol 3 times and dried at 80 °C for 24 h.

Preparation of HPCs

The HPCs were obtained by the direct carbonization of MOF-5. MOF-5 powder filled in an alumina boat was placed into a tube furnace, then heated at a rate of 5 °C min⁻¹ under N₂ flow to some certain temperature X (900 °C, 1000 °C or 1100 °C) and maintained for 6 h, and then cooled to room temperature. The resultant carbon material was denoted as HPC-X.

Preparation of PEG@HPC-1000 composite PCMs

PEG@HPC-1000 composite PCMs were prepared by physical blending and impregnation method. 1 g of PEG was melted and dissolved in 30 mL ethanol at 80 °C to form a homogeneous solution. Then, HPC-1000 was added into the solution and the mixture was stirred vigorously for 4 h. Finally, the mixture was dried in an oven at 80 °C (above the melting point of PEG) to evaporate the ethanol solvent. The PEG content in PEG@HPC-1000 composite PCMs was increased from 80 to 95 wt% (80, 85, 87.5, 90, 92.5 and 95 wt%) to seek the optimum loading amount of PEG. When the PEG mass fraction reached 95 wt%, leakage of PEG from the carbon matrix occurred. Therefore only the composites PCMs with 80–92.5 wt% of PEG are discussed in this paper.

Characterization

The phase composition of the sample was tested by powder X-ray diffraction (XRD, M21X, Cu Kα radiation, λ = 0.154178 nm). The morphology and structures of the HPCs were characterized on scanning electron microscopy (SEM, ZEISS SUPRA55) and high-resolution transmission electron microscopy (HRTEM, JEM-2010). Thermogravimetric analysis was conducted with an instrument (TGA, Netzsch STA449F) at a heating rate of 10 °C min⁻¹ under N₂ atmosphere. Nitrogen sorption–desorption isotherms and the specific surface area of the samples were tested with a Micromeritics ASAP 2420 adsorption analyzer. The pore size distributions were derived from the adsorption branches of isotherms by using the Barrett–Joyner–Halenda (BJH) model. Fourier-transform infrared (FTIR) was acquired on a Nicolet 6700 using the KBr pellet technique. The PEG@HPC-1000 composite PCMs were characterized in terms of thermal properties by differential scanning calorimetry (DSC) using a NETZSCH STA449F3 (Germany) at 5 °C min⁻¹ heating rate and 5 °C min⁻¹ cooling rate. Samples (5 ± 1 mg) were put into Al₂O₃ pans with covers. Thermal conductivity was measured by a transient plane source technique, using a hot disk thermal constants analyzer (Hot Disk TPS 2500 S, Hot Disk AB Company, Gothenburg, Sweden).

Acknowledgements

The work was supported by National Natural Science Foundation of China (No. 51436001), and National Program on Key Basic Research Project (973 Program, No. 2012CB720404).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra04059d

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