Porous Cellulose/Zn-Containing Composite Aerogel with Zinc Nitrate Hexahydrate as a Hydrated-Salt Phase Change Material: Synthesis Optimization and Thermal Performance

Abstract

Salt hydrates are attractive phase change materials for low-temperature thermal energy storage, but supercooling, phase segregation and leakage limit their use. In this study, cotton cellulose fibers were mixed with aqueous zinc nitrate hexahydrate at 75 or 85°C, at cotton-to-salt mass ratios of 0.5–1%, and freeze-dried. The composites were screened by infrared spectroscopy and X-ray diffraction, and one composite was further examined by scanning electron microscopy, energy-dispersive X-ray spectroscopy, thermogravimetry and differential scanning calorimetry. The infrared spectra were dominated by nitrate and water bands, and the surface particles had a composition (Zn 35.5, N 12.8 and O 51.7 wt%) close to that of zinc nitrate. The diffraction patterns showed narrow reflections of crystalline Zn-containing phases, including reflections compatible with layered zinc hydroxide nitrate, whereas zinc oxide was not a major phase. The selected composite melted with an onset of 34.9°C and a peak at 40.4°C, and its enthalpy of fusion (157.1 J/g) was close to that reported for the pure salt hydrate. Dehydration began at about 63°C, which sets the upper limit of the working temperature. Mercury intrusion porosimetry of P 2 T 2 M 2 showed a dominant peak at 82.73 µm equivalent diameter, a cumulative intrusion volume of 273.86 mm³/g and 13.61% porosity, consistent with accessible pore space after freeze-drying. A small fraction of cotton cellulose can therefore host the salt hydrate without loss of its latent heat, while crystallization behavior, cycling stability and leakage resistance remain to be evaluated before practical use.

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