Biomass derived carbon aerogel as an ultrastable skeleton of form-stable phase change materials for efficient thermal energy storage

Fresh pumpkin and carrot were bought from local markets. 1,3-Divinyltetramethyldisiloxane (purity 98%) was purchased from acr GmbH (Karlsruhe, Germany). 2,2'-(Ethylenedioxy) diethanethiol (purity 95%) was purchased from Sigma Aldrich Company (Mexico). Dicumyl peroxide (purity 98%) was purchased from Sigma Aldrich Company (Italy). 1,1,1-Trimethylolpropane(3-mercaptopropionat) was purchased from Bruno Bock Chemische Fabrik GmbH & Co. KG (Marschacht, Germany). Ethanol and palmitic acid (PA) were obtained from Aladdin agent corporation. The agents used without further purification.

The scheme of preparation of carbon aerogels is shown in figure 1. Firstly, the rind, seeds, and soft pulp from pumpkin (the rind from carrot) were removed. Then the flesh of the pumpkin (or carrot) was cut into an appropriate shape and dimension, and then placed into a Teflon-lined stainless steel autoclave. The autoclave was heated at 180 °C for 10 h. The pumpkin (carrot) hydrogels were immersed in hot water (around 60 °C) and ethanol for 2 days, respectively, to remove soluble impurities. The pumpkin aerogel (or carrot aerogel) was obtained via lyophilisation process. Finally, the pumpkin aerogels (or carrot aerogel) were fully converted to pumpkin carbon aerogel (or carrot carbon aerogel) through sintering in a tube furnace at 800 °C or 1000 °C for 90 min in argon atmosphere. The products were marked as PCA800, PCA1000, CCA800 and CCA1000 according to the sintering temperature of carbon aerogel.

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Firstly, 10 g 1,3-Divinyltetramethyldisiloxane, 5.872 g 2,2'-(Ethylenedioxy) diethanethiol, and 5.706 g 1,1,1-Trimethylolpropane(3-mercaptopropionat) were added into a beaker, they were treated by ultrasonic at 80 °C till it became a homogeneous solution. Secondly, 21.578 g PA (50 wt%) and 0.647 g dicumyl peroxide were added into the beaker and treated by ultrasonic for another 30 min. The previous mixture was named as PA/TE solution. Then the PA/TE solution was poured into a preheated Teflon mold and cured at 120 °C for 4 h, palmitic acid was sealed in thiol-ene thermosetting resin after curing and this product was labelled as 50PA/TE. Meanwhile, the CCA (or PCA) and PA were put into a beaker and then they were placed in a vacuum oven, and heated to 120 °C for 30 min after holding the pressure at 0.1 bar for 10 min. The samples were marked as PA@CCA800, PA@CCA1000, PA@PCA800, and PA@PCA1000. Finally, the cooled PA/TE solution and CCA (or PCA) were put into a beaker and placed in a vacuum oven, and then heated to 120 °C for 4 h after holding the pressure at 0.1 bar for 10 min. The samples were separately labelled as 50PA/TE@CCA800, 50PA/TE@CCA1000, 50PA/TE@PCA800, and 50PA/TE@PCA1000. The scheme of preparation of 50PA/TE@CA composites is shown in figure 2.

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X-ray diffraction (XRD, Ricoh, D/max2550VB/PC) was used to characterize the crystalline structure of the carbon aerogel (CA). Laser Micro-Raman Spectrometer (Renishaw, Invia reflex) was used to examine the graphitization of the carbon aerogel. The morphologies of the as-synthesized carrot aerogels, pumpkin aerogel and their corresponding carbon aerogels were determined by scanning electron microscopy (SEM, Hitachi, S4800). The specific surface area and pore distribution were measured by N₂ absorption-desorption isotherms (Micromeritics ASAP2020 Analyser). DSC analysis was performed on a differential scanning calorimeter (DSC, TA Instrument, DSC Q-2000). Thermal conductivity (λ) was calculated by equation (1):

$$\begin{eqnarray}&&\lambda =\alpha \,{C}_{P}\rho \end{eqnarray} \tag{ 1 }$$

where α, C_P and ρ are the thermal diffusivity, specific heat capacity and density of specimen, respectively. α of samples (12.7 mm × 12.7 mm × 1 mm) at 25 °C were measured by laser flash method utilizing a NETZSCH LFA 447 Nanoflash instrument. C_P was determined by a TA Instrument Q-2000 differential scanning calorimetry. The infrared camera (Optris, PI 400) was used to measure the temperature curves of the samples.

As shown in figures 3(a) and (d), the SEM images of carrot aerogel and pumpkin aerogel present an interconneted 3D networks that are mainly composes of a typical layer structure. The interlayer distance is estimated to be in the range of 5–10 μm, the oriented hierarchical 3D structure was formed by the growth of ice crystals from the bottom to the top, and the pumpkin flesh and carrot flesh were discharged to the boundary between the ice crystals during the freezing-casting process. Besides, aerogels shows a dark brown colour (figure 1), indicating that low-temperature hydrothermal treatment cannot fully convert the tissue of pumpkin or carrot to carbon. Further sintering treatment of pumpkin aerogel and carrot aerogel at 800 °C or 1000 °C under an argon atmosphere leads to the evaporation of volatile organic species and the formation of PCA and CCA (figure 1). Figures 3(b), (d), (e) and (f) show the micro-structure of CCA800, CCA1000, PCA800 and PCA1000, respectively. Compared with carrot aerogel and pumpkin aerogel, the carbon aerogels show similar interconnected structure. However, a large number of pores appeared on the carbon nanosheets and formed a porous structure, and the higher sintering temperature further causes damages on the porous structure, which leads to lower densities of CCA1000 and PCA1000 than that of CCA800 and PCA800, respectively.

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As shown in figure 4, specific surface area and pore distribution of the as-prepared carbon aerogels are characterized by N₂ absorption-desorption isotherms. The Brunauer–Emmett–Teller (BET) specific surface area of CCA800, CCA1000, PCA800, and PCA1000 are 33.80, 12.59, 12.77, and 8.74 m² g⁻¹, respectively. The specific surface area of carbon aerogels with sintering temperature of 800 °C higher than that of carbon aerogels with sintering temperature of 1000 °C because the high temperature destroyed the porous structure of carbon aerogels. As shown in figure 4(b), the carbon aerogels with a low sintering temperature show more micropore and mesopore than the carbon aerogels with a high sintering temperature. The specific surface area of CA also influenced by the micro-structure of raw materials, and the specific surface areas of CA derived from different raw materials were listed in table 1.

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The thermogravimetric curves of aerogel and carbon aerogels are shown in figure 5. The aerogel decomposed from about 200 to 700 °C, and the weight loss of carrot aerogel was 64%. The TGA curves illustrated that the flesh of pumpkin and carrot were fully converted to carbon aerogel after sintering treatment. All of the carbon aerogels show a small mass loss during the TGA analysis due to the decomposition when the tmeperature higher than 650 °C and the adsorption of water in the micropores and mesopores of carbon aerogels.

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XRD patterns of the carrot-derived and pumpkin-derived carbon aerogels are shown in figure 6(a). In the case of carbon aerogel, two broad peaks located at 24° and 44° were observed, suggesting a poor ordering of graphene nanosheets along their stacking direction. Meanwhile, it is observed from the XRD patterns that with the increase of sintering temperature, the obtained carbon aerogels present enhanced graphitization, as indicated by the higher intensity of the spectra of CCA1000 and PCA1000 than that of CCA800 and PCA800, respectively. Raman spectra of these four samples were also acquired to further confirm the graphitization of the samples, as shown in figure 6(b). The peaks of D-band and G-band located at 1330 and 1590 cm⁻¹, respectively. Here D-band is related to the A_1g vibration mode of the disordered carbon, while G-band represents the E_2g vibration mode of the ordered graphitic carbon [25]. The defect extent of carbon materials can be directly reflected in the intensity ratio of D band and G band (I_D/I_G). The lower I_D/I_G values of CCA1000 and PCA1000 indicate that the higher sintering temperature resulted in the higher extent of graphitization of the carbon aerogels.

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High latent heat is a critical property for a PCM, and the thermal energy storage capacity of the PCMs depends on the latent heat. In this work, the latent heat of the PCMs were examined by using DSC instrument. Figure 7 shows the DSC curves of pure PA and composite PCMs. There are several kinds of shapes of DSC curves, but all of the DSC curves of PCMs are broader than that of pure PA except PA@PCA800 and PA@PCA1000, which indicates the introduction of CCA and thiol-ene broaden the temperature range of the phase transition process. The corresponding thermal performances of pure PA and composite PCMs are listed in table 2. For PA, the melting point and melting enthalpies are 62.8 °C and 197.78 J g⁻¹, respectively. The melting temperature of PA@CA and 50PA/TE@CA are located in the range of 62.8 ± 2 °C, indicating that the thiol-ene resin and carbon aerogel have a slight influence on phase change temperature.

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The thermal degradation of PA, thiol-ene resin, 50PA/TE, and 50PA/TE@CA are shown in figure 8. The temperature of 5% mass loss of pristine PA and thiol-ene resin are located at 213.5 and 313.8 °C, respectively. Pristine PA shows a typical one-stage decomposition process and thiol-ene resin shows a two-stage decomposition process. The introduction of carbon aerogels exhibited no influence on the decomposition process, and the residue of 50PA/TE@CCA800, 50PA/TE@CCA1000, 50PA/TE@PCA800, and 50PA/TE@PCA1000 after decomposition process are 4.3%, 4.2%, 8.9%, and 5.6%, respectively. According to the results of TGA and DSC, the latent heat of composite PCMs with melting enthalpy is mainly determined by the mass ratio of pure PA in the composite PCMs.

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In a thermal energy storage system, the shape stability is highly important. As shown in figure 9, the carbon aerogels prevented the PCMs from leaking even the temperature over the melting point of the PCM, which resulted from the porous structure of carbon aerogels, suggesting that the involvement of carbon aerogels led to outstanding improvement in shape stability of the pristine PA and 50PA/TE composite.

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The fast storage/release of heat energy by the PCMs is highly significant for a thermal energy storage system. The thermal performance of composite PCMs can be characterized by measuring the thermal conductivity and temperature-time curves of the PCMs. The dominant method of heat transfer in PCMs is heat conduction and it is necessary to enhance the thermal conductivity of PA/TE composite for fast storing and releasing thermal energy. The thermal conductivities of PA and composite PCMs are shown in figures 10(a) and (b). The thermal conductivity of PA is 0.029 W·m⁻¹·K⁻¹, which is too low to apply in a thermal energy storage system. As shown in figures 10(a) and (b), the introduction of CA improved the thermal conductivities of PA and PA/TE significantly. The influence factors of thermal conductivities of PCM composites include the degree of graphitization of CA and the mass ratio of pristine PA encapsulated in the composite PCMs. The thermal conductivities of PA@CCA1000 and PA@PCA1000 are almost 10 times higher than that of pristine PA, the corresponding thermal conductivity are 0.245 W·m⁻¹·K⁻¹ and 0.253 W·m⁻¹·K⁻¹. Moreover, the thermal conductivity of 50PA/TE@PCA1000 (0.149 W·m⁻¹·K⁻¹) is 60.5% higher than that of 50PA/TE (0.093 W·m⁻¹·K⁻¹). The low thermal conductivity of 50PA/TE@CCA1000 (0.106 W·m⁻¹·K⁻¹) mainly attributes to the high content of PA/TE in the composite PCM, and the thermal conductivities of PA/TE@CA lower than that of PA@CA owing to higher interface thermal resistance within the composites.

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The thermal energy storage efficiency could be characterized by temperature-time curves during the charging process. As shown in figures 10(c) to (h), the samples with higher thermal conductivities showed higher temperature raising rates. In conclusion, a high thermal conductivity is more efficient for charging and discharging of thermal energy from the PCMs.