Polyethylene glycol based self-luminous phase change materials for both thermal and light energy storage

Journal Pre-proof Polyethylene glycol based self-luminous phase change materials for both thermal and light energy storage Liang Jiang, Yuan Lei, Qinfeng Liu, Jingxin Lei PII:

S0360-5442(19)32497-1

DOI:

https://doi.org/10.1016/j.energy.2019.116802

Reference:

EGY 116802

To appear in:

Energy

Received Date: 7 August 2019 Revised Date:

11 December 2019

Accepted Date: 17 December 2019

Please cite this article as: Jiang L, Lei Y, Liu Q, Lei J, Polyethylene glycol based self-luminous phase change materials for both thermal and light energy storage, Energy (2020), doi: https://doi.org/10.1016/ j.energy.2019.116802. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Polyethylene Glycol Based Self-luminous Phase Change Materials for Both Thermal and Light Energy Storage Liang Jiang, Yuan Lei, Qinfeng Liu, Jingxin Lei* State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu 610065, China *Corresponding authors. E-mail addresses: [email protected] (J. Lei).

Abstract Except for the improvement enthalpy value and thermal conductivity of conventional solid-solid phase change materials (SSPCMs), expansion of additional functions other than thermal energy storage function of that has been particularly attractive. In this work, a novel self-luminous SSPCMs based polyethylene glycol have been successfully synthesized via incorporation of long afterglow luminescence (LAL) particles into SSPCMs in the absence of any isocyanates and solvents. The prepared self-luminous SSPCMs have high melting latent heats with a maximum value at 120.2 J g-1, maximum encapsulation ratio of 80.6%, and a suitable phase change temperature around 28 °C. Importantly, the prepared self-luminous SSPCMs with different concentrations of LAL particles can absorb and store visible light sources in the daylight but can slowly release blue light in the dark over a long time. Furthermore, the prepared self-luminous SSPCMs after thermal cycling tests and storing-releasing light energy cycling tests have preeminent thermal reliability, luminescence repeatability and chemical structure reliability for a long time practical application. Keywords: Self-luminous solid-solid phase change materials, Thermal and light energy storage, Long afterglow luminescence particles, Polyethylene glycol.

1. Introduction The ever-increasing energy shortages and global warming is one of the widely discussed hot issues due to the ceaselessly increase in the depletion of fossil fuels and greenhouse gas emissions[1, 2]. To resolve the inherent trade-off between energy supply and demand, improving energy utilization efficiency has been acted as one of the most effective strategies in terms of energy conservation and environmental protection[3, 4]. The consideration of utilization of thermal energy storage (TES) has been paid attention to phase change materials (PCMs) that have the intrinsic capacity of absorbing 1

and releasing abundant heat during their phase transition process. PCMs can be classified into organic PCMs, inorganic PCMs and eutectic mixtures[5, 6]. Especially organic PCMs including paraffin[7, 8], fatty acids[9, 10], alcohols[11, 12] and polyethylene glycol(PEG)[13, 14], have great advantages of high latent heat, non-toxicity and low cost. Therefore, organic PCMs are being extensively applied in solar energy utilization, buildings materials, coatings of smart materials, cement and smart textiles[15-19]. However, a leakage problem limits its practical application during the solid-liquid phase change process. Many approaches have been used to solve the leakage problem, such as microencapsulation of PCMs with protective shell[20], the mixture of PCMs into porous supporting materials[6, 10, 21] and fabrication of polymeric PCMs via chemical bonding (grafting, blocking or crosslinking polymerization)[22, 23]. In comparison, fabrication of polymeric PCMs has attracted many researchers’ attention owing to the stable nature of the chemical bond, which consequently imparts them with excellent performances of no liquid leakage, homogeneous dispersion and outstanding long-term utilization performance[24, 25]. Thus, tremendous interest has been focused in PEG based organic PCMs due to its remarkable merits of relative high phase enthalpy and suitable phase change temperature, good chemical stability, nontoxicity, and good biocompatibility[26-30]. Commonly, the terminal hydroxyl groups of PEG could react with multifunctional isocyanate to prepare the PEG based solid-solid PCMs (SSPCMs) for the unique performance of cross-linked polyurethane like easy fabrication, no gas or liquid generation and the availability of commodity raw materials[31, 32]. Recently, researches focused their great interest on the improvement enthalpy value and thermal conductivity of conventional SSPCMs, but these SSPCMs have almost no additional functions other than TES function. Luminescent materials that can emit light after absorption of energy from an excitation source have received considerable interest[33]. If the excitation source is initiated by photoexcitation (excitation by photons), the light emission is called photoluminescence. Some studies have integrated the luminescent nanoparticles into various matrixes to prepare the self-luminous composites and expand the application of luminescent materials in the optical fields[34-37]. Liu et al. [34] reported a novel luminescent and transparent wood composite by impregnating luminescent nanoparticles to the wood template. However, few studies combined the luminescent materials with PCMs to extend the functional application of SSPCMs. Wang et al. [35] proposed a facile, low-cost and controllable strategy to fabricate highly graphitized 3D network carbon for shape-stabilized composite PCMs 2

with superior thermal energy harvesting. Novel self-luminous wood composite based on PCMs with superior thermal energy storage and long afterglow luminescence (LAL) materials with excellent light energy storage is reported[37]. To our best knowledge, integration of LAL particle into PCMs to synthesize PEG based self-luminous SSPCMs for both thermal and light energy storage, have not been reported. In this work, we prepared a novel self-luminous SSPCMs via incorporating the LAL particle into PEG based SSPCMs for both thermal and light energy storage. The self-luminous SSPCMs were synthesized by two steps: the first step was that PEG was modified by phthalic anhydride to prepare the intermediate, and the second step was that the intermediate was cross-linked by tri-functional aziridine as a specific crosslinking agent. Especially, the synthesized SSPCMs can be synthesized in the absence of any isocyanates and solvents, meanwhile can possess high latent heats without leakage. Incorporation of LAL particles into SSPCMs for absorbing and releasing light energy can achieve both thermal and light energy storage. Importantly, first incorporation of LAL particles into the PEG based SSPCMs can boost the flourishment of a new era of PCMs with additional functions other than TES function. The synthesized self-luminous SSPCMs have great potential application in the self-luminous wallboard for buildings or smart highways by absorbing sunlight in the day and then slowly releasing it in the form of light over a long time.

2. Experimental 2.1. Materials Polyethylene glycol (PEG, analytical grade, PEG4K, Mn=4000 g mol-1 from Chengdu Kelong Chemical Reagent Co. Ltd., China) were dried under vacuum at 102 °C for 2 h to remove water before use. Phthalic anhydride (PA, analytical grade) was supplied by Aladdin Reagent Co., Ltd. Trimethylolpropane-tris [3-(2-methylazopropyl)] propionate (HD-100, analytical grade) was supplied from Shanghai Kanglejia Material Co., Ltd. (Yancheng, Chain). Long-afterglow luminescent particles (LAL, SB-8C, technical grade) were supported by Luming Technology Group Co., Ltd. 2.2. Preparation of self-luminous SSPCMs As shown in Fig.1, the intermediate (PEG4K-PA) was firstly synthesized by the reaction between PEG4K and PA with the molar ratio of 1:2 at 100 °C for 2 h, followed by adding various weight percentages of long-afterglow luminescent (LAL) particles under ultrasound for 30 min to ensure a 3

uniform dispersion. After this, a stoichiometric amount of HD-100 slowly added and stirred to adequately react between PEG4K-PA and HD-100 for 20 min at room temperature. Finally, the synthesized PEG based self-luminous SSPCMs were poured into a PTFE mold and then at 60 °C for 5 h under vacuum. In Table 1, the obtained self-luminous SSPCMs were named as PCM4K-x (where x represents the weight percentage of LAL particles in the whole SSPCMs weight).

Fig. 1 Synthetic routes of self-luminous SSPCMs Table 1 Formula of self-luminous SSPCMs with different concentrations of LAL particles LAL particle Samples PEG4K (g) PA (g) HD-100 (g) LAL particle(g) contents (wt%) PEG4K-PA 10 0.74 0 0 0 PCM4K-0 10 0.74 0.78 0 0 PCM4K-1 10 0.74 0.78 1 8.10 PCM4K-2 10 0.74 0.78 2 14.79 PCM4K-3 10 0.74 0.78 4 25.77 2.3. Characterization Fourier transform infrared (FTIR) spectra was obtained using infrared spectrophotometer (Nicolet-560, Nicolet Co., USA) in the wavenumber range of 4000-400 cm-1 with a resolution setting of 4 cm-1. X-ray diffraction (XRD) patterns were conducted by an X-ray diffractometer (X’PertPro 4

MPD, Netherlands) under Cu Ka at 35 kV and 30 mA with 2θ ranges of scans from 5 to 90° at ascanning rate of 4 min-1. Thermal properties were measured via a differential scanning calorimeter (DSC 204 F1, German) from -20 to 100 °C with a heating rate of 10 °C min-1 and from 100 to -20 °C with a cooling rate of 10 °C min-1 under nitrogen atmosphere. The thermal stability was conducted using a thermal gravimetric analyzer (SDTQ600, USA) from 30 to 600 °C with a heating rate of 10 °C min-1 under nitrogen atmosphere. The morphologies were observed using a scanning electronic microscope (SEM, JEOLJSM-5900LV, Japan) under an accelerated voltage of 20 kV. The crystalline morphology was observed a polarizing optical microscope (POM, XPR-500D, Shanghai, China) at room temperature. Accelerated thermal cycling tests were performed in a high-low temperature chamber for 100 consecutive heating and cooling cycles from 20 to 90 °C at a heating or cooling rate of 10 °C min-1. Afterward, DSC and FTIR analyses were employed to investigate the variations of thermal performances and chemical structures. The photoluminescence spectra were obtained by Hitachi F-7000 fluorescence spectrophotometer (Japan). All samples were laid in the dark for 24 h before measurement and irradiated under filament lamp for 1 h, and then the decay measurement was conducted by using digital photos in the dark.

3. Results and discussion 3.1. Preparation and characterization The self-luminous SSPCMs were synthesized via a two-step method according to Fig. 1. The intermediate of PEG4K-PA was first synthesized via a ring-opening reaction between PEG4K and PA, whose chemical structure was confirmed by FTIR and 1H NMR (Fig. S1 and S2 in Supporting Information). After this, various weight percentages of LAL particles and a stoichiometric amount of HD-100 as a crosslinking agent were added into that to prepare self-luminous SSPCMs. To confirm the chemical structures of raw materials and final products, FTIR spectra of PEG4K, PEG4K-PA, PCM4K-0, PCM4K-1, PCM4K-2 and PCM4K-3 are shown in Fig. 2. As can be seen in Fig. 2, in the FTIR spectra of pure PEG4K, the characteristic peak at 3431 cm-1 and 1114 cm-1 were respectively ascribed to the O-H stretching vibration and the C-O-C symmetric stretching vibration, and the characteristic peaks at 2882 cm-1, 1467 cm-1, 1340 cm-1, 962 cm-1 and 842 cm-1 were attributed to the C-H vibration. As for PEG4K-PA, the wide and loose absorption bands around 3000 cm-1 corresponding to the O-H stretching vibration of carboxyl groups are detected, and the characteristic peak of the C=O stretching vibration appears at 1723 cm-1. The results showed that the intermediate 5

of PEG4K-PA was successfully synthesized based on the FTIR spectra and 1H NMR analysis. The characteristic peaks corresponding to the stretching vibration of -COOH groups totally disappeared; however, the new characteristic peaks of PCM4K-0 at 1631 cm-1 corresponding to the -N-H bending vibration appeared[25, 38]. The results of FTIR spectra showed that the self-luminous SSPCMs were successfully synthesized. Moreover, the FTIR spectra of PCM4K-1, PCM4K-2 and PCM4K-3 were almost identical to that of PCM4K-0 without new characteristic peaks appeared, indicating that there was no chemical reaction but only physical interactions between pristine SSPCMs and the LAL particles.

Fig. 2 FTIR spectra of the pure PEG4K, PEG4K-PA and self-luminous SSPCMs

3.2. Shape-stability performance The shape-stability performance of self-luminous SSPCMs was conducted by the leakage test and the results of the leakage test were displayed using digital photos. As shown in Fig. 3, the solid pure 6

PEG4K completely melted into liquid state once the temperature increased from 30 to 90 °C for 20 min, but PCM4K-0, PCM4K-1, PCM4K-2 and PCM4K-3 can remain the original shape without PEG4K leakage. It can confirm that the self-luminous SSPCMs have excellent shape-stability performance due to the network structure via the stability of chemical bonds.

Fig. 3 Leakage test of pure PEG4K and self-luminous SSPCMs

3.3. Phase change performances Phase change performances are of vital importance for applications from the perspective of applicable phase change temperature and high latent heat. The DSC curves of the pure PEG4K and the self-luminous SSPCMs with different content of LAL particles were presented in Fig. 4, and the detailed melting and freezing temperatures, latent heats and encapsulation ration are included in Table 2. As shown in Fig. 4, DSC curves of PEG4K have an asymmetric exothermic and endothermic peak (even double peak) during crystallization and melting process, respectively. These result from the coexistence of amorphous/crystalline phase, contributing to the crystalline grains of different size with the help of crystal transformation, thermal history including heating/cooling rate or impurities working as nucleating agent[39, 40]. The DSC curves of the self-luminous SSPCMs with different content of LAL particles exhibited similar curves with that of the pure PEG4K, providing evidence that the PEG was mainly acted the role of the latent thermal energy storage during the phase change process. In Table 2, compared with the melting point and freezing point of the pure PEG4K at 58.4 °C and 27.9 °C, that of the self-luminous SSPCMs different content of LAL particles experienced few change around 52 °C and 28 °C. But, contrast with pure PEG4K, the latent heat of synthesized self-luminous SSPCMs obviously decreased. The freezing latent heats of the self-luminous SSPCMs obviously decreased from 118.1 to 70.7 J mol-1 and the melting latent heats 7

of that decreased from decreased from 120.2 to 72.4 J mol-1 with addition of LAL particles contents from 0 to 4 wt%. Meanwhile, the encapsulation ratio of the self-luminous SSPCMs gradually decreased from 80.6 to 48.4% with the increase of LAL particles contents. This result was contributed to the chemical restriction in free movement of PEG chains to align and the LAL particles acted as an impurity leading to the lower degree of crystallinity for the self-luminous SSPCMs[41, 42].

Fig. 4 DSC curves of the pure PEG4K and self-luminous SSPCMs Table 2 Latent heat storage properties of PEG4K and self-luminous SSPCMs Freezing process Melting process PCM -1 Tf/°C ∆Hf/J g Tm/°C ∆Hm/J g-1 PEG4K 27.9 -145.7 58.4 -149.9 PCM4K-0 27.8 -118.1 52.7 -120.2 PCM4K-1 28.3 -99.9 50.6 -104.1 PCM4K-2 28.7 -79.1 52.1 -80.1 PCM4K-3 27.1 -70.7 52.3 -72.4 Tf and Tm represented the melting and freezing point, respectively. ∆Hf and ∆Hm represented the melting and freezing latent heats, respectively. 8

Encapsulation ratio/% 100 80.6 69.0 53.9 48.4

To investigate the crystalline property of the system, the XRD patterns of the pure PEG4K and self-luminous SSPCMs were displayed in Fig. 5. The strong diffraction peaks of the pure PEG4K appeared at 2θ = 19.2° and 23.4° correspond to the lattice plane of (120) and (112), respectively[43]. The typical diffraction peaks of self-luminous SSPCMs were similar to that of pure PEG4K, showing the same sharp and position within the measurement range, indicating the existence of crystals. However, the intensity of diffraction peaks of self-luminous SSPCMs apparently decreased with the increase of LAL particles contents and the corresponding decrease of PEG4K contents, suggesting that the crystallinity of PEG4K in SSPCMs is much lower than that of pure PEG4K. This result was contributed to the high restriction of chemical effects, leading the rearrangement and orientation of PEG chains in self-luminous SSPCMs are hindered by the introduction of covalent bonds with amorphous units, which reduced crystallite size and crystallization ability of PEG4K.

Fig. 5 XRD patterns of the pure PEG4K and self-luminous SSPCMs

To further verify the crystalline properties, POM micrographs of the pure PEG4K and self-luminous SSPCMs were presented in Fig. 6. The obvious cross extinction patterns were observed in pure PEG4K and self-luminous SSPCMs, illustrating that all of them had the same 9

crystalline morphology in the presence of crystal spherulites at room temperature. However, the spherulites of self-luminous SSPCMs were smaller in size than that of pure PEG4K because of the restriction in chain movement of PEG4K posed by the chemical bonds, leading to hinder the growth of crystal spherulites. The crystallization reduces in self-luminous SSPCMs were consistent with XRD and DSC results. Meanwhile, compared with PCM4K-0, the spherulites of PCM4K-1, PCM4K-2 and PCM4K-3 gradually decreased due to the added fillers of LAL particles acted as a nucleating agent to promote more crystal spherulites. It can conclude that self-luminous SSPCMs have comparable crystallization capacity with pure PEG4K, and their crystallinity can be affected to some extent by chemical bonds and the filler of LAL particles.

Fig. 6 POM images of the pure PEG4K and self-luminous SSPCMs SEM images of cryo-fractured surface of the self-luminous SSPCMs containing various LAL particles contents were shown in Fig. 7. It can be seen from the SEM images that the cryo-fractured surface of self-luminous SSPCMs was rough, which can indicate the growth of crystal spherulites. Reactive to the cryo-fractured surface of PCM4K-0, that of PCM4K-1, and PCM4K-2 and PCM4K-3 slightly became smooth, implying the more crystal spherulites possessed by addition of LAL particles. This observation indicated that the filler of LAL particles as a nucleating agent was homogeneously dispersed in the SSPCMs matrix, promoting a relatively smooth cryo-fractured surface.

10

Fig. 7 SEM images of self-luminous SSPCMs

3.4. Luminescence performances Fig. 8 displayed the corresponding photoluminescence spectra of self-luminous SSPCMs with different concentrations of LAL particles. As shown in Fig. 8a for the emission spectra, all samples showed similar broadband shape and position with a peak at 467 nm in the blue wavelength range of 400~480 nm[33, 44]. Therefore, the self-luminous PCMs can absorb visible light and then emit the blue light due to the 4f65d1→ 8S7/2(4f7) transition of Eu2+, which is consistent with the blue light in Fig. 8b[45-47]. As can be seen in Fig. 8b, the decay measurement of self-luminous SSPCMs was conducted by using digital photos in the dark. The macroscale change of luminance intensity of PCM4k-1, PCM4K-2 and PCM4K-3 gradually get less over time due to a principle to the quenching of the luminescence[48]. Additionally, a comparison among the prepared self-luminous SSPCMs with different concentrations of LAL particles suggested that PCM4K-3 has the strongest emission intensity and the longest decay time, resulting from higher LAL particles concentrations in SSPCMs that more energy transfer took place between europium ions, which was contributing to the improvement of the emission intensity and decay time[33]. Unfortunately, the increase of LAL particles contents diminished latent heats of the self-luminous SSPCMs, thus the trade-off between 11

the high latent heats and decay time will be considered for the practical application.

Fig. 8 (a) Photoluminescence emission spectra of self-luminous SSPCMs (excitation wavelength = 370 nm). (b) Digital photographs of PCM4K-1, PCM4K-2 and PCM4K-3.

3.5. Thermal reliability and luminescence repeatability analysis It is indispensable to estimate the thermal reliability and reusability of self-luminous SSPCMs for a long time practical application via the accelerated thermal cycling process. Fig. 9 a~d showed DSC curves of self-luminous SSPCMs with different concentrations of LAL particles before and after 100 consecutive heating and cooling cycles. The detail the changes of latent heats and the phase change temperature after 100 consecutive heating and cooling cycles were presented in Fig. 9 e and f. DSC curves of self-luminous SSPCMs before and after thermal cycling experienced hardly change. Meanwhile, the latent heat and the phase change temperature of self-luminous SSPCMs after 100 thermal cycling tests was similar to those of original one, manifesting that no obvious leakage occurred heating-cooling process due to the reliable cross-linked structure. It can be concluded that the self-luminous SSPCMs have preeminent thermal reliability for the truly application.

12

Fig. 9 DSC curves of self-luminous SSPCMs before and after thermal cycling: (a) PCM4K-0, (b) PCM4K-1, (c) PCM4K-2, (d) PCM4K-3. The detail the changes of latent heats and phase change temperature before and after thermal cycling: (e) Melting latent heat and freezing latent heat, (f) Melting point and freezing point. Luminescence repeatability of self-luminous SSPCMs is also a critical factor because of the consecutive alternation of day and night for the practical utilization. Thus, the luminescence repeatability of these self-luminous SSPCMs was conducted via storing-releasing light energy test for 50 cycles. As shown in Fig. 10, for PCM4K-3, compared with the time interval from 0 to 40 min, there was a significant change in luminescence intensity before and after 50 cycles under the same 13

conditions. It demonstrated that the self-luminous SSPCMs have outstanding luminescence repeatability, which is meant for practical application.

Fig. 10 Digital photographs of PCM4K-3 before and after storing-releasing light energy test for 50 cycles. To further study the chemical structure reliability, the FTIR spectra of self-luminous SSPCMs with different concentrations of LAL particles after 100 thermal cycling tests and 50 storing-releasing light energy cycling tests were presented in Fig. 11. As can be seen in Fig. 11, absorption peaks and shapes of FTIR spectra after thermal cycling were in agreement with that of the original one, and no 14

additional new absorption peaks occurred after thermal cycling and light energy cycling, indicating that chemical structure of the synthesized self-luminous SSPCMs is not affected and any chemical degradation in the synthesized self-luminous SSPCMs will not occur after thermal cycling. These result proved good long-period chemical structure reliability of the self-luminous SSPCMs.

Fig. 11 FTIR spectra of self-luminous SSPCMs before and after thermal cycling.

3.6. Thermal stability The thermal stability is crucial for self-luminous SSPCMs that need work a long time yet the outer temperature fluctuates frequently during usage. For practical consideration, the self-luminous SSPCMs should be satisfactorily durable without thermal decomposition or degradation. Thus, the thermal stability was investigated by TGA method that is the commonly adopted method to evaluate thermal stability. TGA curves of the PEG4K and self-luminous SSPCMs with different concentrations of LAL particles were presented in Fig. 12 and the results derived from the curves are 15

given in Table 3. In Fig. 12, all samples displayed only one decomposition step. The initial decomposition temperature of the pure PEG4K was 347.6 °C and temperature at the maximum decomposition rate appeared at 373.9 °C. However, the initial decomposition temperature of self-luminous SSPCMs obviously decreased around 250 °C due to the degradation of ester groups under base catalysis of secondary amide[24, 49]. Compared with the temperature at the maximum decomposition rate of pure PEG4K, that of self-luminous SSPCMs slightly increased because of the addition of the LAL inorganic particles. Additionally, the final residual mass of PEG4K and PCM4K-0 without the LAL particles was all quite close to 0. In contrast, the final residual mass of PCM4K-1, PCM4K-2 and PCM4K-3 is about 9.88%, 15.84% and 28.67%, respectively, and the residual mass is in agreement with the mass ratio of LAL particle contents in SSPCMs. The TGA results demonstrate that the self-luminous SSPCMs have good thermal stability in a broad phase transition temperature range of outdoor practical applications.

Fig. 12 TGA curves of the pure PEG4K and self-luminous SSPCMs.

16

Table 3 TGA results of the pure PEG4K and self-luminous SSPCMs Initial Temperature at the Final residual Samples decomposition maximum decomposition mass (%) temperature (°C) rate (°C) PEG4K 347.6 373.9 0.24 PCM4K-0 247.2 393.8 0.11 PCM4K-1 248.8 389.2 9.88 PCM4K-2 253.7 388.3 15.84 PCM4K-3 256.0 386.8 28.67 3.7. The mechanism of thermal and light energy storage Consideration of the thermal and luminescence performance, the suggested mechanism of thermal and light energy storage was shown in Fig. 13. As shown in Fig. 13a, the self-luminous SSPCMs can absorb a lot of thermal energy when the ambient temperature increased above the melting point while the phase changed from the crystalline state to the amorphous state. However, when the ambient temperature gradually decreased below freezing point, the self-luminous SSPCMs can release large amounts of thermal energy while the reversed phase changed from the amorphous state to the crystalline state. Thus, the self-luminous SSPCMs can be achieved to store and release in the reversible phase change process. In Fig. 13b, incorporation of LAL particles into the self-luminous SSPCMs can endow the self-luminous function, absorbing and storing visible light sources in the daylight but slowly releasing blue light in the dark over a long time. Therefore, the self-luminous SSPCMs can achieve thermal and light energy storage, which can be used for light and thermal energy efficient application in emergency signs and wallboard for buildings.

17

Fig. 13 Suggested mechanism of thermal and light energy storage.

4. Conclusion In this work, a series of self-luminous SSPCMs with different concentrations of LAL particles have been fabricated in the absence of any isocyanates and solvents by using a facile synthesis method. The prepared self-luminous SSPCMs have excellent shape-stability performance due to the chemical bonds cross-linkage. DSC, XRD and POM results revealed that the self-luminous SSPCMs have high freezing latent heats with a maximum value at 120.2 J g-1, maximum encapsulation ratio of 80.6%, and suitable phase change temperature around 28 °C. The intrinsic crystalline structure in self-luminous SSPCMs was not influenced by the crosslinking reaction and the LAL particles but crystallite size and crystallization ability in that were hindered. Importantly, the self-luminous SSPCMs can absorb and store visible light sources in the daylight but can slowly release blue light in 18

the dark over a long time, which can be applied in light energy storage. Furthermore, the self-luminous SSPCMs after thermal cycling tests and storing-releasing light energy cycling tests have preeminent thermal reliability, luminescence repeatability and chemical structure reliability for a long time practical application. Therefore, the self-luminous SSPCMs with high latent heat, suitable phase change temperature, effective storage of thermal energy and light energy, and outstanding stability and reliability, have a new way to expand the additional functions other than TES function in self-luminous emergency signs and wallboard for buildings.

Conflicts of interest There are no conflicts to declare.

Acknowledgements The authors acknowledge Hui Wang from the Analytical & Testing Center of Sichuan University for her help with SEM characterization.

Reference [1] Sarbu I, Dorca A. Review on heat transfer analysis in thermal energy storage using latent heat storage systems and phase change materials. Int J Energy Res. 2019;43(1):29-64. [2] Nazir H, Batool M, Bolivar Osorio FJ, Isaza-Ruiz M, Xu X, Vignarooban K, et al. Recent developments in phase change materials for energy storage applications: A review. Int J Heat Mass Transfer. 2019;129:491-523. [3] Yang J, Tang L-S, Bao R-Y, Bai L, Liu Z-Y, Xie B-H, et al. Hybrid network structure of boron nitride and graphene oxide in shape-stabilized composite phase change materials with enhanced thermal conductivity and light-to-electric energy conversion capability. Sol Energy Mater Sol Cells. 2018;174:56-64. [4] Liang J, Zhimeng L, Ye Y, Yanjun W, Jingxin L, Changlin Z. Fabrication and characterization of fatty acid/wood-flour composites as novel form-stable phase change materials for thermal energy storage. Energy Build. 2018;171:88-99. [5] Umair MM, Zhang Y, Iqbal K, Zhang S, Tang B. Novel strategies and supporting materials applied to shape-stabilize organic phase change materials for thermal energy storage-A review. Appl Energy. 2019;235:846-73. [6] Huang X, Chen X, Li A, Atinafu D, Gao H, Dong W, et al. Shape-stabilized phase change materials based on porous supports for thermal energy storage applications. Chem Eng J. 2019;356:641-61. [7] Li M, Liu J, Shi J. Synthesis and properties of phase change microcapsule with SiO2-TiO2 hybrid shell. Sol Energy. 2018;167:158-64. [8] Jiang Z, Ouyang T, Yang Y, Chen L, Fan X, Chen Y, et al. Thermal conductivity enhancement of phase change materials with form-stable carbon bonded carbon fiber network. Mater Des. 19

2018;143:177-84. [9] Wen R, Zhang X, Huang Z, Fang M, Liu Y, Wu X, et al. Preparation and thermal properties of fatty acid/diatomite form-stable composite phase change material for thermal energy storage. Sol Energy Mater Sol Cells. 2018;178:273-9. [10] Lin Y, Zhu C, Alva G, Fang G. Palmitic acid/polyvinyl butyral/expanded graphite composites as form-stable phase change materials for solar thermal energy storage. Appl Energy. 2018;228:1801-9. [11] Geng X, Li W, Wang Y, Lu J, Wang J, Wang N, et al. Reversible thermochromic microencapsulated phase change materials for thermal energy storage application in thermal protective clothing. Appl Energy. 2018;217:281-94. [12] Yang H, Wang Y, Yu Q, Cao G, Yang R, Ke J, et al. Composite phase change materials with good reversible thermochromic ability in delignified wood substrate for thermal energy storage. Appl Energy. 2018;212:455-64. [13] Sun K, Kou Y, Zheng H, Liu X, Tan Z, Shi Q. Using silicagel industrial wastes to synthesize polyethylene glycol/silica-hydroxyl form-stable phase change materials for thermal energy storage applications. Sol Energy Mater Sol Cells. 2018;178:139-45. [14] Yang J, Tang LS, Bao RY, Bai L, Liu ZY, Xie BH, et al. Hybrid network structure of boron nitride and graphene oxide in shape-stabilized composite phase change materials with enhanced thermal conductivity and light-to-electric energy conversion capability. Sol Energy Mater Sol Cells. 2018;174:56-64. [15] Perez G, Allegro VR, Corroto M, Pons A, Guerrero A. Smart reversible thermochromic mortar for improvement of energy efficiency in buildings. Constr Build Mater. 2018;186:884-91. [16] Ke Y, Zhou C, Zhou Y, Wang S, Chan SH, Long Y. Emerging Thermal-Responsive Materials and Integrated Techniques Targeting the Energy-Efficient Smart Window Application. Adv Funct Mater. 2018;28(22):1800113. [17] Zhang L, Zhou K, Wei Q, Ma L, Ye W, Li H, et al. Thermal conductivity enhancement of phase change materials with 3D porous diamond foam for thermal energy storage. Appl Energy. 2019;233-234:208-19. [18] Sheng N, Zhu R, Dong K, Nomura T, Zhu C, Aoki Y, et al. Vertically aligned carbon fibers as supporting scaffolds for phase change composites with anisotropic thermal conductivity and good shape stability. J Mater Chem A. 2019;7(9):4934-40. [19] Weinstock L, Sanguramath RA, Silverstein MS. Encapsulating an organic phase change material within emulsion-templated poly(urethane urea)s. Polym Chem. 2019;10(12):1498-507. [20] Yu Q, Tchuenbou-Magaia F, Al-Duri B, Zhang Z, Ding Y, Li Y. Thermo-mechanical analysis of microcapsules containing phase change materials for cold storage. Appl Energy. 2018;211:1190-202. [21] Atinafu DG, Dong W, Huang X, Gao H, Wang J, Yang M, et al. One-pot synthesis of light-driven polymeric composite phase change materials based on N-doped porous carbon for enhanced latent heat storage capacity and thermal conductivity. Sol Energy Mater Sol Cells. 2018;179:392-400. [22] Liu Z, Wu B, Fu X, Yan P, Yuan Y, Zhou C, et al. Two components based polyethylene glycol/thermosetting solid-solid phase change material composites as novel form stable phase change materials for flexible thermal energy storage application. Sol Energy Mater Sol Cells. 2017;170:197-204. [23] Jiang Y, Yan P, Wang Y, Zhou C, Lei J. Form-stable phase change materials with enhanced thermal stability and fire resistance via the incorporation of phosphorus and silicon. Mater Des. 20

2018;160:763-71. [24] Zhou Y, Liu X, Sheng D, Lin C, Ji F, Dong L, et al. Polyurethane-based solid-solid phase change materials with in situ reduced graphene oxide for light-thermal energy conversion and storage. Chem Eng J. 2018;338:117-25. [25] Wu B, Liu Z, Xiao Y, Wang Y, Zhou C, Zhang X, et al. Polyester-based phase change materials with flexible poly(ethylene glycol) chains for thermal energy storage. J Appl Polym Sci. 2019;136(9):47108. [26] Pielichowska K, Pielichowski K. Phase change materials for thermal energy storage. Prog Mater Sci. 2014;65:67-123. [27] Zhang Y, Wang J, Qiu J, Jin X, Umair MM, Lu R, et al. Ag-graphene/PEG composite phase change materials for enhancing solar-thermal energy conversion and storage capacity. Appl Energy. 2019;237:83-90. [28] Wang C, Chen K, Huang J, Cai Z, Hu Z, Wang T. Thermal behavior of polyethylene glycol based phase change materials for thermal energy storage with multiwall carbon nanotubes additives. Energy. 2019;180:873-80. [29] Fan X, Liu L, Jin X, Wang W, Zhang S, Tang B. MXene Ti3C2Tx for phase change composite with superior photothermal storage capability. J Mater Chem A. 2019;7(23):14319-27. [30] Serrano A, Martín del Campo J, Peco N, Rodriguez JF, Carmona M. Influence of gelation step for preparing PEG-SiO2 shape-stabilized phase change materials by sol-gel method. J Sol-Gel Sci Technol. 2019;89(3):731-42. [31] Zhang Y, Wang L, Tang B, Lu R, Zhang S. Form-stable phase change materials with high phase change enthalpy from the composite of paraffin and cross-linking phase change structure. Appl Energy. 2016;184:241-6. [32] Fu X, Xiao Y, Hu K, Wang J, Lei J, Zhou C. Thermosetting solid–solid phase change materials composed of poly(ethylene glycol)-based two components: Flexible application for thermal energy storage. Chem Eng J. 2016;291:138-48. [33] Rojas-Hernandez RE, Rubio-Marcos F, Rodriguez MÁ, Fernandez JF. Long lasting phosphors: SrAl2O4: Eu, Dy as the most studied material. Renew Sust Energ Rev. 2018;81:2759-70. [34] Gan W, Xiao S, Gao L, Gao R, Li J, Zhan X. Luminescent and Transparent Wood Composites Fabricated by Poly(methyl methacrylate) and γ-Fe2O3@YVO4:Eu3+ Nanoparticle Impregnation. ACS Sustain Chem Eng. 2017;5(5):3855-62. [35] Chen X, Gao H, Yang M, Dong W, Huang X, Li A, et al. Highly graphitized 3D network carbon for shape-stabilized composite PCMs with superior thermal energy harvesting. Nano Energy. 2018;49:86-94. [36] Chen X, Gao H, Yang M, Xing L, Dong W, Li A, et al. Smart integration of carbon quantum dots in metal-organic frameworks for fluorescence-functionalized phase change materials. Energy Storage Mater. 2019;18:349-55. [37] Yang H, Chao W, Wang S, Yu Q, Cao G, Yang T, et al. Self-luminous wood composite for both thermal and light energy storage. Energy Storage Mater. 2019;18:15-22. [38] Wang S-C, Chen P-C, Hwang J-Z, Huang C-Y, Yeh J-T, Chen K-N. A new tri-functional azetidine compound for self-curing aqueous-based PU system. J Appl Polym Sci. 2012;124(1):175-81. [39] Chen C, Liu K, Wang H, Liu W, Zhang H. Morphology and performances of electrospun polyethylene glycol/poly (dl-lactide) phase change ultrafine fibers for thermal energy storage. Sol 21

Energy Mater Sol Cells. 2013;117:372-81. [40] Chen C, Liu W, Wang Z, Peng K, Pan W, Xie Q. Novel form stable phase change materials based on the composites of polyethylene glycol/polymeric solid-solid phase change material. Sol Energy Mater Sol Cells. 2015;134:80-8. [41] Liu Z, Zhang Y, Hu K, Xiao Y, Wang J, Zhou C, et al. Preparation and properties of polyethylene glycol based semi-interpenetrating polymer network as novel form-stable phase change materials for thermal energy storage. Energy Build. 2016;127:327-36. [42] Zhang Y, Gurzadyan GG, Umair MM, Wang W, Lu R, Zhang S, et al. Ultrafast and efficient photothermal conversion for sunlight-driven thermal-electric system. Chem Eng J. 2018;344:402-9. [43] Sundararajan S, Samui AB, Kulkarni PS. Synthesis and characterization of poly(ethylene glycol) acrylate (PEGA) copolymers for application as polymeric phase change materials (PCMs). React Funct Polym. 2018;130:43-50. [44] Li Y, Gecevicius M, Qiu J. Long persistent phosphors-from fundamentals to applications. Chem Soc Rev. 2016;45(8):2090-136. [45] Hai O, Jiang H, Zhang Q, Ren Q, Wu X, Hu J. Effect of cooling rate on the microstructure and luminescence properties of Sr2MgSi2O7:Eu2+,Dy3+ materials. Luminescence. 2017;32(8):1442-7. [46] Xie W, Zou C, Xie M. Effect of H3BO3 on phases, micromorphology and persistent luminescence properties of SrAl2O4: Eu2+, Dy3+ phosphors. J Mater Sci Mater Electron. 2017;28(8):6328-34. [47] Rojas-Hernandez RE, Rubio-Marcos F, Rodriguez MÁ, Fernandez JF. Long lasting phosphors: SrAl2O4:Eu, Dy as the most studied material. Renew Sust Energ Rev. 2018;81:2759-70. [48] Duée N, Ambard C, Pereira F, Portehault D, Viana B, Vallé K, et al. New Synthesis Strategies for Luminescent YVO4:Eu and EuVO4 Nanoparticles with H2O2 Selective Sensing Properties. Chem Mater. 2015;27(15):5198-205. [49] Sundararajan S, Kumar A, Chakraborty BC, Samui AB, Kulkarni PS. Poly(ethylene glycol) (PEG)-modified epoxy phase-change polymer with dual properties of thermal storage and vibration damping. Substain Energy Fuels. 2018;2(3):688-97.

22

Incorporation of long afterglow luminescence particles into SSPCMs for the achievement in thermal and light energy storage.

23

Highlights: 1. The solid-solid phase change materials (SSPCMs) have self-luminous performance. 2. The self-luminous SSPCMs can be synthesized without any isocyanates and solvents. 3. The self-luminous SSPCMs can achieve both thermal and light energy storage. 4. The self-luminous SSPCMs have preeminent thermal and luminescence reliability.

Conflict of interest statement: We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled, “Polyethylene Glycol Based Self-luminous Phase Change Materials for Both Thermal and Light Energy Storage”.