Development of Solar-Thermal-Electric Energy in Beijing Chemical Industry： Graphene Aerogel and Its Thermal Conductive Phase Change Composites

　　Brief introduction of achievements

　　Organic phase change material (PCM) with high enthalpy is an ideal material for heat storage and release, which is expected to promote the utilization of heat energy and alleviate the energy shortage. However, the inherent shortcomings of ordinary organic phase change materials, such as poor light absorption, poor thermal conductivity and weak shape stability, seriously restrict the absorption, transformation and utilization of solar energy. In this article, Professor Li Xiaofeng from beijing university of chemical technology, Professor Yu Zhongzhen's team published in ACS Sustainable Chem. Eng. the journal titled "High-quality Anisotropic Graphene Aerogels and Their Thermally Conductive Phase Change Compositions for Efficiency". Temporary solar–thermal–electrical energy conversion ",through unidirectional freezing, freeze-drying, carbonization and graphitization at 2800 °C, a high-quality anisotropic graphene aerogel made of preoxidized polyacrylonitrile (OPAN)/ graphene oxide (GO) was designed for the first time.

　　GO component can effectively induce the orientation and graphitization of OPAN component, and transform it into graphite carbon in the graphitization process. After vacuum-assisted impregnation with paraffin wax, an optimal thermal conductivity phase change composite (PCC) was obtained. At the low level of graphene content of 1.07 Vol%, the overall thermal conductivity of PCC increased to 4.36Wm-1K-1, the shape stability was improved, and the latent heat retention rate was as high as 99.7%. Thanks to its excellent light absorption and solar-thermal conversion ability, PCC is very efficient in solar-thermal-electrical energy conversion applications, and the output voltage is as high as 1181mV under the simulated sunlight of 5kWm-2. By releasing the heat energy stored in PCC, it can continue to supply power to LED lamps even after the sunlight stops. This work provides a feasible and effective method for manufacturing thermal conductivity PCC with high latent heat retention rate, which is used for efficient solar-thermal-electric energy conversion.

　　Graphic reading guide

　　Fig. 1. (a) Manufacturing schematic diagram of PG aerogel and its paraffin phase change composite. (b, c) SEM images of the side view of PG4 and (d, e) the top view. (f) Digital photos of PG4.

　　Fig. 2. (a) Apparent density of PG aerogels prepared in OPAN/GO suspension with different initial GO ratios. The illustration shows the sizes of different PG aerogels. (b) XRD pattern of PG aerogel. (c) (002) diffraction angle and FWHM diagram of PG aerogel. (d) Raman images of PG1, (e) PG2, (f) PG3, (g) PG4 and (h) PG5 after graphitization at 2800 C. (i) Average ID/IG value and crystal size of PG aerogels.

　　Fig. 3. Raman spectra of (a) non-annealed PG4 and (b) 1000 C annealed PG4. (c) XPS pattern, and (d) average ID/IG values and C/O atomic ratio of non-annealed PG4, PG4-1000℃ and PG4-2800℃.

　　Fig. 4. (a) Longitudinal and transverse thermal conductivity of PPG composites. (b) Thermal conductivity of PIPG composites. (c) Thermogravimetric curves of paraffin wax and PPG phase change composites. (d) the mass percentage of residue in PPG phase change composites, and (e) the filler content and thermal conductivity improve the efficiency. (f) Comparison of thermal conductivity and latent heat retention of PPG4 with PCC reported. Infrared images show the thermal reaction of paraffin and PPG4 during synchronous (g) heating and (h) cooling.

　　Fig. 5: DSC curves of (a) heating and (b) cooling of paraffin and PPG complex and (c) enthalpy of phase transition. (d) DSC curve of 100 cycles of PPG4. (e) TMA curves of paraffin wax and PPG4 and (f) UV-Vis-NIR absorption spectra. (g) Temperature-time curves of paraffin, (h) PiPG4 and (i) PPG4 under simulated sunlight of 2 kW m-2.

　　Fig. 6. (a) Output voltage of blank, paraffin and thermoelectric semiconductor covering PPG4 when radiator is in air. When the radiator is in water, the thermoelectric semiconductor covering the PPG4 outputs (b) voltage and (c) current. (d) stable output power and (e) power density of PPG4 coated thermoelectric power. (f) Under the irradiation of 10 kW m-2 sunlight, the output voltage/current of the thermoelectric semiconductor is wrapped by PPG4. (g) Schematic diagram of solar-thermal-electric energy conversion experimental device. (h) The brightness change of the LED lamp after the solar simulator stops.

　　summarize

　　In order to solve the problems of poor thermal conductivity, weak solar absorption and poor shape stability of PCM, this paper constructs a high-quality anisotropic graphene aerogel to contain paraffin wax and endow it with better thermal conductivity, stable shape and efficient solar-thermal conversion ability. Through unidirectional freezing, freeze-drying, carbonization at 1000 °C and graphitization at 2800 °C, the thermally conductive graphene skeleton extracted from OPAN/GO was designed for the first time. By adjusting the amount of initial GO, the excessive volume shrinkage of graphene aerogels obtained during carbonization and graphitization can be effectively suppressed, and the orientation and transformation of OPAN to graphite carbon can be induced, thus greatly improving the graphene quality of the obtained graphene aerogels. After vacuum-assisted impregnation with molten paraffin, PPG4 showed a high latent heat retention of 99.7% and the best thermal conductivity of 4.36W m-1 K-1, and the graphene content was low, only 1.07vol%. Porous graphene skeleton can also effectively inhibit the leakage of molten paraffin and give it satisfactory shape stability.

　　In addition, the enhanced solar absorption capacity also gives PPG4 excellent performance in solar-thermal-electric energy conversion applications. Under the simulated sunlight of 5kWm-2, the thermoelectric materials covered with PPG4 can obtain competitive high output voltage of 1181 mV and current of 83.5ma.. This study shows that solar energy-thermal energy phase change composites have broad potential in collecting and utilizing solar energy, which is conducive to alleviating the shortage of fossil energy and the mismatch between supply and demand of solar energy.

　　Literature:

　　https://doi.org/10.1021/acssuschemeng.3c02154