molecules

Article Excellent -Control Based on Reversible Thermochromic Materials for Light-Driven Phase Change Materials System

Caixia Ren 1, Fangfang Liu 1, Malik Muhammad Umair 1 , Xin Jin 2, Shufen Zhang 1 and Bingtao Tang 1,2,*

1 State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, China; [email protected] (C.R.); [email protected] (F.L.); [email protected] (M.M.U.); [email protected] (S.Z.) 2 Eco-chemical Engineering Cooperative Innovation Center of Shandong, Qingdao University of Science and Technology, Qingdao 266042, China; [email protected] * Correspondence: [email protected]; Tel./Fax: +86-411-8498-6464

 Academic Editor: Ana Ines Fernandez Renna  Received: 28 February 2019; Accepted: 23 April 2019; Published: 24 April 2019

Abstract: Light-driven phase change materials (PCMs) have received significant attention due to their capacity to convert visible light into thermal energy, storing it as latent . However, continuous photo-thermal conversion can cause the PCMs to reach high thermal equilibrium after . In our study, a novel light-driven phase change material system with temperature-control properties was constructed using a thermochromic compound. Thermochromic phase change materials (TC-PCMs) were prepared by introducing 2-anilino-6- dibutylamino-3-methylfluoran (ODB-2) and bisphenol A (BPA) into 1-hexadecanol (1-HD) in various proportions. Photo-thermal conversion performance was investigated with solar radiation (low power of 0.09 W/cm2) and a xenon lamp (at a high power of 0.14 W/cm2). The TC-PCMs showed a low equilibrium temperature due to variations in absorbance. Specifically, the temperature of TC-PCM180 (ODB-2, bisphenol A and 1-HD ratio 1:2:180) could stabilize at 54 ◦C approximately. TC-PCMs exhibited reversibility and repeatability after 20 irradiation and cooling cycles.

Keywords: photo-thermal conversion; phase change materials; thermochromic compound; temperature control

1. Introduction As a fossil fuel substitute, solar energy is a renewable and clean alternative energy resource [1,2]. Solar energy utilization—including thermal utilization, photovoltaic conversion and photo-thermal conversion technology—has increasingly advanced [3–7]. However, thermal utilization of solar energy suffers from inferior direct heat usage factors due to inefficient utilization of visible light, which accounts for 44% of solar radiation [8,9]. To solve this problem, investigators have introduced light harvesting materials (such as dye, carbon materials and metal, etc.) into phase change material systems to obtain high photo-thermal conversion and energy storage efficiency [10–14]. On the other hand, continuous photo-thermal conversion can lead to high thermal equilibrium temperatures of phase change materials (PCMs) after phase transition [15–17]. This critical issue can have certain implications on the application of PCM systems based on photo-thermal conversion materials in some fields. Typically, in a photo-thermal conversion system, light-harvesting materials absorb visible light, causing valence electrons to jump into an excited state and then deexcite to the ground state mainly by means of non-radiative transition [18,19]. Vast heat energy is emitted and then stored by PCMs [20].

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In the process, as light radiates unremittingly, the temperature of PCM systems rises due to energy [20]. In the process, as light radiates unremittingly, the temperature of PCM systems rises due to conversion until thermal equilibrium is attained. In a sense, the thermal equilibrium temperature energy conversion until thermal equilibrium is attained. In a sense, the thermal equilibrium depends on the balance between the photo-thermal conversion rate and heat conduction rate. temperature depends on the balance between the photo-thermal conversion rate and heat conduction Fortunately, thermochromic materials, which are typically composed of former, developer and rate. Fortunately, thermochromic materials, which are typically composed of color former, developer solvent, show superior performances in temperature control [21,22]. The tendency of thermochromic and solvent, show superior performances in temperature control [21,22]. The tendency of materials to become colorless and lose light absorption capacity with increases in temperature enables thermochromic materials to become colorless and lose light absorption capacity with increases in them to achieve quick thermal equilibrium at a low temperature. At present, thermochromism-induced temperature enables them to achieve quick thermal equilibrium at a low temperature. At present, methods of PCM-based systems have been studied widely to achieve energy storage or temperature thermochromism-induced methods of PCM-based systems have been studied widely to achieve control [23–26]. Peng Tang et al. designed thermochromism-induced temperature self-regulation and energy storage or temperature control [23–26]. Peng Tang et al. designed thermochromism-induced alternatingtemperature photothermal self-regulation nanohelix and alternating clusters, photot and appliedhermal these nanohelix to synergistic clusters,chemo and applied/phototherma these to therapysynergistic for tumours chemo/phototherm [26]. a therapy for tumours [26]. InIn the the present present study, study, 2-anilino-6-dibutylamino-3-methylfluoran 2-anilino-6-dibutylamino-3-methylfluoran (ODB-2) (ODB-2) was was selected selected as as a a color color former.former. 1-hexadecanol 1-hexadecanol (1-HD) (1-HD) and and bisphenol bisphenol A A (BPA) (BPA) served served as as solvent solvent andand developer,developer, respectively.respectively. TheThe thermochromic thermochromic materials materials were were black black at at room room temperature, temperature, and and when when 1-hexadecanol 1-hexadecanol melted melted into into anan amorphous amorphous state,state, the color color faded. faded. This This process process caused caused notable notable light light absorption absorption changes changes due to due the tocolor the colorvariation variation of ODB-2, of ODB-2, and the and thermal the thermal equilibrium equilibrium under under irradiation irradiation was achieved was achieved at a lower at a lowertemperature, temperature, which which was beneficial was beneficial for increasing for increasing the photostability the photostability of the light-harvesting of the light-harvesting molecules moleculesunder continuous under continuous irradiation. irradiation. The mechanism The mechanism is as is follows: as follows: ODB-2, ODB-2, like like many many otherother similarsimilar chromophores,chromophores, presents presents two two di differentfferent isomers: isomers: the the leuco leuco lactone lactone form, form, which which is is colorless, colorless, and and the the openopen carboxylate carboxylate state, state, which which is colored.is colored. When When 1-HD 1-HD starts starts to crystalize to crystalize as temperature as temperature decreases, decreases, the equilibriumthe equilibrium between between both isomersboth isomers of ODB-2 of ODB-2 is displaced is displaced by interactions by interactions with bisphenol with bisphenol A through A hydrogenthrough hydrogen bonding, whichbonding, favors which ring-opening favors ring-ope of thening color of former the color (Figure former1)[ (Figure27–30]. 1) [27–30]. InIn this this work, work, thermochromic thermochromic phase phase change change materials materials (TC-PCMs) (TC-PCMs) were were obtained obtained via via combining combining ODB-2ODB-2 and and BPA BPA with with 1-HD 1-HD in in various various ratios. ratios. Then, Then, the the photo-thermal photo-thermal conversion conversion property property was was 2 evaluatedevaluated under under di differentfferent light light sources: sources: solar solar radiation radiation (low (low power power approximately approximately 0.09 0.09 W W/cm/cm )2) and and 2 xenonxenon lamp lamp (high (high power power approximately approximately 0.14 W 0.14/cm ).W/cm A similar2). A trendsimilar was trend observed was inobserved both conditions: in both temperatureconditions: cantemperature be controlled can be at acontrolled certain range. at a certain After 20 range. irradiation After 20 and irradiation cooling cycles, and cooling temperature cycles, controltemperature properties control of TC-PCMs properties was of TC-PCMs found to be was still found achievable. to be still achievable.

FigureFigure 1. 1.Diagram Diagram of of the the thermochromic thermochromic process. process.

2. Results and Discussion

2.1. FT-IR Analysis Figure 2 shows the Fourier Transform (FT-IR) spectra of 1-HD and TC-PCMs. The characteristic peak of COO− at 1607 cm−1 attributed to the addition of thermochromic compound [31], appeared in all TC-PCMs’ curves. It confirmed the ring-opening structure of the lactone ring in ODB-

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−1 −1 2. ResultsFurthermore, and Discussion the peaks at 3303 cm (stretching vibration of O–H), 2956 cm (C–H stretching vibration of –CH3), 2850 cm−1 (C–H stretching vibrations of –CH2) and 1062 cm−1 (C–O stretching vibration)2.1. FT-IR Analysisare typical of 1-HD Figure2 shows the Fourier Transform Infrared (FT-IR) spectra of 1-HD and TC-PCMs. 1 The characteristic peak of COO− at 1607 cm− attributed to the addition of thermochromic compoundMolecules 2019 [, 3124,], x appeared in all TC-PCMs’ curves. It confirmed the ring-opening structure of3 of the 9 lactone ring in ODB-2. Furthermore, the peaks at 3303 cm 1 (stretching vibration of O–H), 2956 cm 1 −1 − −1 − 2. Furthermore, the peaks at 3303 cm (stretching1 vibration of O–H), 2956 cm (C–H stretching 1 (C–H stretching vibration of –CH3), 2850 cm− (C–H stretching vibrations of –CH2) and 1062 cm− vibration of –CH3), 2850 cm−1 (C–H stretching vibrations of –CH2) and 1062 cm−1 (C–O stretching (C–O stretching vibration) are typical of 1-HD. vibration) are typical of 1-HD

Figure 2. FT-IR spectra of 1-hexadedocanol (1-HD), thermochromic phase change material TC-PCM180,

TC-PCM130, TC-PCM90 and TC-PCM50.

2.2. Thermal Property of TC-PCMs Phase transition temperature and enthalpy play important roles in measuring the energy- storage performances of PCMs, which were characterized by differential scanning calorimetry (DSC). Figure 2. FT-IR spectra of 1-hexadedocanol (1-HD), thermochromic phase change material TC-PCM , Figure 3 and Table 1 show the DSC curves and specific data on TC-PCMs, respectively. The180 1-HD TC-PCMFigure 2. FT-IR, TC-PCM spectra ofand 1-hexadedocanol TC-PCM (1-HD), thermochromic phase change material TC-PCM180, curve featured130 two exothermic90 peaks, which50. were caused by solid–solid transition at 42 °C (forming TC-PCM130, TC-PCM90 and TC-PCM50. a2.2. rotation Thermal phase) Property [32] of and TC-PCMs liquid–solid transition at 48 °C, and an endothermic peak overlapped by two2.2. transitionsThermal Property (as mentioned of TC-PCMs above) [33]. The melting point (Tm) of 1-HD was 48.76 °C, and the Phase transition temperature and enthalpy play important roles in measuring the energy-storage cooling points including liquid-solid transition (Tc) and solid–solid transition (Tt) were 48 °C and performancesPhase transition of PCMs, temperature which were characterizedand enthalpy byplay diff erentialimportant scanning roles in calorimetry measuring (DSC). the energy- Figure3 41.87 °C, respectively. andstorage Table performances1 show the of DSC PCMs, curves which and were specific characteri datazed on TC-PCMs,by differential respectively. scanning calorimetry The 1-HD (DSC). curve The phase transition temperatures (Tc and Tm) of TC-PCMs decreased subtly with increasing featuredFigure 3 twoand exothermicTable 1 show peaks, the DSC which curves were and caused specific by data solid–solid on TC-PCMs, transition respectively. at 42 C The (forming 1-HD a doping amounts (mass fraction of ODB-2 and BPA). Contrary to this, enthalpy values◦ increase to rotationcurve featured phase) two [32] exothermic and liquid–solid peaks, transitionwhich were at caused 48 C, by and solid–solid an endothermic transition peak at 42 overlapped °C (forming by varying degrees. This result can be attributed to the addition◦ of a thermochromic compound, which twoa rotation transitions phase) (as [32] mentioned and liquid–solid above) [transition33]. The meltingat 48 °C, point and an (T endothermic) of 1-HD was peak 48.76 overlappedC, and by the promoted the formation of the rotator phase and crystalline structurem of 1-HD [28].◦ Therefore, coolingtwo transitions points including (as mentioned liquid-solid above) transition[33]. The melting (T ) and point solid–solid (Tm) of transition 1-HD was (T 48.76) were °C, 48 andC the and appropriate addition of ODB-2 and BPA had a negligiblec effect on the phase transitiont temperature◦ 41.87coolingC, points respectively. including liquid-solid transition (Tc) and solid–solid transition (Tt) were 48 °C and and enhanced◦ the enthalpy values of TC-PCMs. 41.87 °C, respectively. The phase transition temperatures (Tc and Tm) of TC-PCMs decreased subtly with increasing doping amounts (mass fraction of ODB-2 and BPA). Contrary to this, enthalpy values increase to varying degrees. This result can be attributed to the addition of a thermochromic compound, which promoted the formation of the rotator phase and crystalline structure of 1-HD [28]. Therefore, appropriate addition of ODB-2 and BPA had a negligible effect on the phase transition temperature and enhanced the enthalpy values of TC-PCMs.

Figure 3. Curves of TC-PCM180, TC-PCM130, TC-PCM90, TC-PCM50, and pure 1-HD. Figure 3. Curves of TC-PCM180, TC-PCM130, TC-PCM90, TC-PCM50, and pure 1-HD.

Figure 3. Curves of TC-PCM180, TC-PCM130, TC-PCM90, TC-PCM50, and pure 1-HD.

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The phase transition temperatures (T and T ) of TC-PCMs decreased subtly with increasing Molecules 2019, 24, x c m 4 of 9 doping amounts (mass fraction of ODB-2 and BPA). Contrary to this, enthalpy values increase to varying degrees. This result can beTable attributed 1. Thermal to the date addition of TC-PCMs of a thermochromic and pure 1-HD. compound, which promoted the formation of the rotator phase and crystalline structure of 1-HD [28]. Therefore, appropriate additionSample of ODB-2 andTC (% BPA wt) had aT negligiblec (°C) effTectt (°C) on the phaseΔHc (J/g) transition T temperaturem (°C) Δ andHm (J/g) enhanced the enthalpy values of TC-PCMs. 1-HD 0.00 48.00 41.87 229.5 48.76 233.8 Table 1. Thermal date of TC-PCMs and pure 1-HD. TC-PCM30 1.89 46.23 41.12 242.9 46.04 244.5

Sample TC (% wt) Tc (◦C) Tt (◦C) ∆Hc (J/g) Tm (◦C) ∆Hm (J/g) TC-PCM90 1.08 47.01 40.81 244.9 46.95 240.3 1-HD 0.00 48.00 41.87 229.5 48.76 233.8 TC-PCMTC-PCM130 30 0.75 1.8947.47 46.23 41.85 41.12 248.9 242.9 46.0447.03 244.5247.2 TC-PCM90 1.08 47.01 40.81 244.9 46.95 240.3 TC-PCMTC-PCM180 130 0.55 0.7547.45 47.47 41.81 41.85 236.3 248.9 47.0347.13 247.2238.7 TC-PCM180 0.55 47.45 41.81 236.3 47.13 238.7

2.3. X-Ray Diffraction (XRD) Analysis 2.3. X-ray Diffraction (XRD) Analysis The crystallization property was characterized by X-Ray Diffraction (XRD). As shown in curve The crystallization property was characterized by X-ray Diffraction (XRD). As shown in curve a of Figure 4, typical diffraction peaks of 1-HD were observed at angles of 2θ = 20.6, 21.4, 21.8, a of Figure4, typical di ffraction peaks of 1-HD were observed at angles of 2θ = 20.6 , 21.4 , 21.8 , 22.1, 24.2, 24.7, which corresponded to the TC-PCMs. In curves c–e, the peaks at 21.4◦  and◦ 24.2◦ 22.1 , 24.2 , 24.7 , which corresponded to the TC-PCMs. In curves c–e, the peaks at 21.4 and 24.2 weakened—probably◦ ◦ ◦ due to the doping of dyes. In curve b, the diffraction peaks were◦ nearly◦ weakened—probably due to the doping of dyes. In curve b, the diffraction peaks were nearly unchanged, unchanged, indicating that the doping dose of TC-PCM180 had no significant influence on the indicating that the doping dose of TC-PCM had no significant influence on the crystallinity of 1-HD. crystallinity of 1-HD. 180

Figure 4. X-ray Diffraction (XRD) patterns of (a) 1-HD, (b) TC-PCM180, (c) TC-PCM130, (d) TC-PCM90, Figure 4. X-Ray Diffraction (XRD) patterns of (a) 1-HD, (b) TC-PCM180, (c) TC-PCM130, (d) TC-PCM90, (e) TC-PCM50. (e) TC-PCM50. 2.4. Photo-Thermal Conversion Analysis 2.4. Photo-Thermal Conversion Analysis To study the temperature-control property of thermochromic materials, photo-thermal conversion wasTo tested study (Figure the5 ).temperature-control Figure5a,b shows the property photo-thermal of thermochromic conversion curves materials, of TC-PCMs photo-thermal and CB 180 . conversionIn Figure5a, was under tested sunlight (Figure irradiation 5). Figure (0.09 5a,b W / cmsh2ows), the the thermal photo-thermal equilibrium conversion temperature curves of TC-PCMs of TC- PCMswas evidently and CB180 lower. In Figure than that 5a,of under CB180 sunlight. This result irradiation was due (0.09 to the W/cm structural2), the transformationthermal equilibrium of the temperaturethermochromic of TC-PCMs compound was when evidently the temperature lower than was that higher of CB than180. This the result phase was transition due to temperature the structural of transformation1-HD (Figure5d) of [ 22the,33 thermochromic]. Furthermore, compound the thermal wh equilibriumen the temperature temperature was of TC-PCMhigher than180 stabilized the phase at transition56 ◦C in contrast temperature with that of of 1-HD CB180 , whose(Figure temperature 5d) [22,33]. exceeded Furthermore, 64 ◦C. Figure the 5thermalb shows theequilibrium curves of 2 temperatureTC-PCMs under of TC-PCM the higher180 powerstabilized of a simulativeat 56 °C in light contrast source with (IR removal, that of CB approximately180, whose temperature 0.14 W/cm ). exceededThe same 64 tendency °C. Figure was 5b shows exhibited: the curves TC-PCMs of TC achieved-PCMs under lower the thermal higher power equilibrium of a simulative temperatures. light sourceTherefore, (IR asremoval, the lowest approximately TC compound 0.14 ratio W/cm (mass2). The fraction same of tendency TC compound, was exhibited: 1.63%), TC-PCMTC-PCMs180 achieved lower thermal equilibrium temperatures. Therefore, as the lowest TC compound ratio (mass fraction of TC compound, 1.63%), TC-PCM180 can attain lower thermal equilibrium temperatures under high or low irradiating power. This result indicates that TC-PCM180 exhibits good temperature- control property for photo-thermal conversion.

Molecules 2019, 24, 1623 5 of 9 can attain lower thermal equilibrium temperatures under high or low irradiating power. This result Molecules 2019, 24, x 5 of 9 indicates that TC-PCM180 exhibits good temperature-control property for photo-thermal conversion. In order to further investigate the temperature-control property of different PCMs with In order to further investigate the temperature-control property of different PCMs with thermochromic compound, we selected 1-tetradecanol as the PCM, at an optimum ratio of 1:2:180 thermochromic compound, we selected 1-tetradecanol as the PCM, at an optimum ratio of 1:2:180 (ODB-2: BPA: 1-tetradecanol) under solar irradiation. The results are shown in Figure5c. The curves (ODB-2: BPA: 1-tetradecanol) under solar irradiation. The results are shown in Figure 5c. The curves of 1-tetradecanol and 1-HD reached different thermal temperatures of 51.5 ◦C and 43.4 ◦C respectively, of 1-tetradecanol and 1-HD reached different thermal temperatures of 51.5 °C and 43.4 °C which indicated that the system temperature could be controlled under irradiation. respectively, which indicated that the system temperature could be controlled under irradiation.

Figure 5. (a) Photo-thermal conversion curves under solar radiation (b) Photo-thermal conversion curves underFigure simulative 5. (a) Photo-thermal light (c) Photo-thermal conversion conversion curves under curves solar of 1-hexadecanolradiation (b) Photo-thermal (I) and 1-tetradecanol conversion (II) ascurves PCMs under (d) visible simulative light absorption light (c) ofPhoto-thermal TC-PCMs under conversion liquid and curves solid state.of 1-hexadecanol (І) and 1- tetradecanol (Ⅱ) as PCMs (d) visible light absorption of TC-PCMs under liquid and solid state. 2.5. Cycle Performance 2.5. TheCycle light Performance reversibility and durability of TC-PCMs is a key factor to determine practical application in thermalThe storage light systems.reversibility TC-PCMs and underwentdurability 20of irradiationTC-PCMs andis a cooling key factor cycles, to and determine the photo-thermal practical conversionapplication performance in thermal storage before systems. and after TC-PCMs cycling test underwent is shown 20 in Figureirradiation6. The and curves cooling of TC-PCMscycles, and beforethe photo-thermal and after 20 irradiationconversion cycles performance were essentially before an consistent.d after cycling The resultstest is indicateshown in that Figure TC-PCMs 6. The keptcurves good of photo-thermalTC-PCMs before conversion and after performance20 irradiation after cycles 20 timeswere essentially cycles. And consistent. temperature The canresults be controlledindicate that in a TC-PCMs certain scope. kept good photo-thermal conversion performance after 20 times cycles. And temperature can be controlled in a certain scope.

MoleculesMolecules2019 2019, 24, 24, 1623, x 66 of of 9 9

Figure 6. Photo-thermal conversion before and after 20 times irradiation and cooling cycles of

(aFigure) TC-PCM 6. Photo-thermal50 (b) TC-PCM conversion90 (c) TC-PCM before130 and(d) TC-PCMafter 20 times180. irradiation and cooling cycles of (a) TC- PCM50 (b) TC-PCM90 (c) TC-PCM130 (d) TC-PCM180. 3. Materials and Methods 3. Materials and Methods 3.1. Materials 3.1. 1-HexadecanolMaterials (HD), 1-tertradecanol and bisphenol A (BPA, 2,2-bis(4-hydroxyphenyl)propane) were supplied by Tianjin Damao chemical reagent factory. 1-tertradecanol, 2-Anilino-6- 1-Hexadecanol (HD), 1-tertradecanol and bisphenol A (BPA, 2,2-bis(4-hydroxyphenyl)propane) dibutylamino-3-methylfluoran (ODB-2) was supplied by Shenzhen Duao science and technology Ltd. were supplied by Tianjin Damao chemical reagent factory. 1-tertradecanol, 2-Anilino-6- (Shenzhen, China). Carbon black purchased from Cabot Corporation (Boston, MA, USA) was modified dibutylamino-3-methylfluoran (ODB-2) was supplied by Shenzhen Duao science and technology Ltd. with diazonium salt to disperse in the oil phase. (Shenzhen, China). Carbon black purchased from Cabot Corporation (Boston, MA, USA) was 3.2.modified Preparation with ofdiazonium TC-PCMs salt to disperse in the oil phase.

3.2. DiPreparationfferent ratios of TC-PCMs of TC-PCMs were obtained by mixing 1-HD, BPA and ODB-2 at weight ratios of 1:2:30, 1:2:60, 1:2:120 and 1:2:180 (TC-PCM30, TC-PCM60, TC- PCM120 and TC-PCM180, respectively). Different ratios of TC-PCMs were obtained by mixing 1-HD, BPA and ODB-2 at weight ratios of 1-HD was melted at 80 ◦C for 1 h via oil bath, then BPA and ODB-2 were dissolved in 1-HD with 1:2:30, 1:2:60, 1:2:120 and 1:2:180 (TC-PCM30, TC-PCM60, TC- PCM120 and TC-PCM180, respectively). 1- varying percentages in a 185 ◦C thermostatic oil bath for 2 min. CB180 was obtained by replacing HD was melted at 80 °C for 1 h via oil bath, then BPA and ODB-2 were dissolved in 1-HD with ODB-2 with CB (carbon black) in TC-PCM180. 1-tertradecanol (replacing 1-hexadecanol at the ratio of varying percentages in a 185 °C thermostatic oil bath for 2 min. CB180 was obtained by replacing ODB- 1:2:180) systems were obtained identically. 2 with CB (carbon black) in TC-PCM180. 1-tertradecanol (replacing 1-hexadecanol at the ratio of 1:2:180) 3.3.systems Characterization were obtained of TC-PCMs identically.

3.3. FT-IRCharacterization spectrophotometry of TC-PCMs (Nicolet 6700, Waltham, MA, USA) was used to evaluate the structure of TC-PCMs and 1-HD (using KBr pearls). The thermal properties of TC-PCMs and 1-HD were measured FT-IR spectrophotometry (Nicolet 6700, Waltham, MA, USA) was used to evaluate the structure by DSC (TA Q20, New Castle, Delaware, USA), with a heating rate of 5◦/min in an N2 atmosphere. Theof TC-PCMs crystallinity and of the1-HD samples (using was KBr estimated pearls). throughThe thermal XRD (Dproperties/Max 2400, of Rigaku,TC-PCMs Tokyo, and Japan) 1-HD fromwere measured by DSC (TA Q20, New Castle, Delaware, USA), with a heating rate of 5°/min in an N2 10◦ to 80◦. The UV-visible absorption spectra of solid and liquid TC-PCMs were obtained using a atmosphere. The crystallinity of the samples was estimated through XRD (D/Max 2400, Rigaku, Tokyo, Japan) from 10° to 80°. The UV-visible absorption spectra of solid and liquid TC-PCMs were

Molecules 2019, 24, 1623 7 of 9

Hitachi U-4100 (Tokyo, Japan) spectrophotometer and an Agilent HP 8453 (Santa Clara, CA, USA) spectrophotometer, respectively. In the photo-thermal conversion property test: both solar radiation and a simulated light (CHF-XM35–500W xenon lamp with a parallel light source system, PerfectLight, Beijing, China) were used as photo sources. The xenon lamp produced light with continuous wavelength, ranging from 200 nm to 2000 nm, and its energy distribution in the visible light area is similar to those of the solar spectrum. In contrast, near-infrared light (700 nm to 1100 nm), which heightens energy distribution vastly, was removed by an IR-CUT filter. The light intensity of sunlight was approximately 0.09 W/cm2. 0.14 W/cm2 of simulated light (approaching the strongest amounts of solar radiation) was chosen to study photo-thermal conversion performances under high-intensity light. Meanwhile, the power was measured by an optical power meter (Perfect light PL-MW2000, Beijing, China). The temperature values were automatically recorded using a numerical control thermometer every 4 s.

4. Conclusions In this research, a light-driven PCM system with temperature-control properties was constructed by preparing TC-PCMs. DSC analysis showed a slight shift in phase transition temperature of approximately 1 ◦C, and enthalpy increased to 247 J/kg (in TC-PCM130). The photo-thermal conversion test demonstrated that the ultimate temperature could be stabilized at specific values as the TC compound turned from black to a tinted shade during the phase change of 1-HD, which caused a drastic decrease in absorbance. With the lowest doping dose and minimum absorbance, TC-PCM180 reached 54 ◦C under a certain power range and exhibited the best temperature-control property. Furthermore, 1-tetradecanol systems were explored at an optimal ratio of 1:2:180, in which temperature can be controlled at the same range. In addition, the reversibility and durability of TC-PCMs remained unchanged after 20 irradiation and cooling cycles. Thus, the temperature-control performance of light-driven PCM systems was explored, and, based on thermochromic theory, excellent energy storage properties were achieved.

Author Contributions: C.R. and B.T. conceived and designed the experiments in the manuscript. C.R. wrote the manuscript; F.L. and M.M.U. were involved in the writing and modification of the manuscript. X.J. and S.Z. were involved in editing the manuscript. Funding: This research was funded by National Natural Science Foundation of China (21576039, 21878043, 21536002, 21421005 and U1608223), Program for Innovative Research Team in University (IRT_13R06), the Fundamental Research Funds for the Central Universities (DUT18ZD218). Conflicts of Interest: The authors declare no conflict of interest.

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Sample Availability: Samples of the compounds are not available from the authors.

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