Pore structure modified diatomite-supported PEG composites for thermal energy storage

A series of novel composite phase change materials (PCMs) were tailored by blending PEG and five kinds of diatomite via a vacuum impregnation method. To enlarge its pore size and specific surface area, different modification approaches including calcination, acid treatment, alkali leaching and nano-silica decoration on the microstructure of diatomite were outlined. Among them, 8 min of 5 wt% NaOH dissolution at 70 °C has been proven to be the most effective and facile. While PEG melted during phase transformation, the maximum load of PEG could reach 70 wt.%, which was 46% higher than that of the raw diatomite. The apparent activation energy of PEG in the composite was 1031.85 kJ·mol−1, which was twice higher than that of the pristine PEG. Moreover, using the nano-silica decorated diatomite as carrier, the maximum PEG load was 66 wt%. The composite PCM was stable in terms of thermal and chemical manners even after 200 cycles of melting and freezing. All results indicated that the obtained composite PCMs were promising candidate materials for building applications due to its large latent heat, suitable phase change temperature, excellent chemical compatibility, improved supercooling extent, high thermal stability and long-term reliability.


Acid treatment
Acid treatment was used to remove the fine impurities and to have well-opened pores on the porous structure of the diatomite. In a typical acid leaching step, 40 g RD-1 powders and 160 mL 20 wt.% sulfuric acid were added into a 3-mouth flask in the heated thermostatic water bath, which was connected to a mechanical agitator equipped with twin-bladed impeller. The mixture was heated to 80 °C (±0.5 °C) for 2 h. Then, the slurry was filtered and the residue was washed with distilled water for several times until the pH value reached 7. The acid treated diatomite was dried at 105 °C for 24 h, ground into to powder, and then kept in desiccators (denoted as RD-2).

Alkali leaching
Diatomite was treated with sodium hydroxide to enhance its performance as a carrier. The RD-2 samples were immersed in sufficient amount of 5% (w/w) sodium hydroxide solution at 70 °C for 8 min. The digested diatomite was washed several times by deionized water, filtered, dried at 105°C, sieved and stored in closed containers for further tests, which was denoted as RD-3.

Decorate by nano-silica particles
The temperature-assisted sol-gel method was employed to prepare the nano-SiO 2 decorated diatomite [2]. In a typical experimental operation, 5 g RD-2 powder was dissolved in 50 mL of Na 2 SiO 3 solution (0.03 mol· L −1 ) under stirring for overnight and the temperature was controlled at 60 o C in a constant temperature bath. Then acetic acid solution (10 wt.%) was gradually added into the solution under continuous stirring to form silica gel until the pH value reached to 4. After aging for 2 h, the composite powder was washed with distilled water for three times and then collected.
The products were dried in a vacuum oven at 80 o C for 48 h. Finally, the nano-SiO 2 decorated diatomite was obtained and denoted as RD-4.

Preparation of PEG/diatomite ss-PCMs
20 g of diatomite sample was placed inside a filtering flask, which was connected to a vacuum pump apparatus to evacuate air from its porous surface. Then, the valve between the flask and a container filled with liquid PEG was opened to let liquid PEG flow into the flask to cover the diatomite sample. After a period of time, air was allowed to enter the flask again to force the liquid PEG to penetrate into the pore space of diatomite. The porous diatomite materials filled with the PEG were taken out from liquid PCM and then for removing liquid PCM captured by the surface of composites or not supported in pore. They were kept in the furnace which was keeping at 80 °C. Finally, the products were taken out from the furnace and dried.

Analysis methods
The specific surface area and pore volume of diatomite were determined by a N 2 adsorption analyzer (Quantachrome Instruments, US). Transmission electron microscope (TEM, JEM-2100HR, Japan) and scanning electronic microscope (SEM, Model HITACHI S-4800) was adopted to observe the microstructures of the modified diatomite and the prepared ss-PCMs. The chemical compatibility of ss-PCMs was obtained via Fourier transform infrared spectroscopy (FT-IR, Model Frontier) and X-ray diffraction (XRD, Model XD-3) method. Besides, thermal property and stability of the ss-PCMs were explored through differential scanning calorimeter (DSC, Q2000) and thermo-gravimetric analysis (TGA, Q50), respectively.

Exudation stability of the prepared PEG/diatomite ss-PCMs
The prepared PEG/diatomite ss-PCMs were heated to 80 o C for 2-10 h to investigate their exudation stability. The macroscopic photographs of different modified diatomite , ss-PCMs, and pristine PEG after heating are presented in Fig.   S3(a). No liquid PEG was observed on the surface of PEG/diatomite composite. In addition, from Fig. S3(b), the mass loss during the melting process can be neglected.
Based on the results of the above two methods, PEG/diatomite composite prepared in this study is quite stable.