Development and characterization of salt hydrate and nanocellulose composites for thermal energy storage

Abstract

Salt hydrates have demonstrated thermal energy storage capabilities via reversible bonding of water molecules. Materials exhibit an energy density of 400-870 kWh/m 3, operate at low temperatures (<150°C), are generally low-cost, but are prone to degradation. Previous efforts to improve stability have primarily focused on impregnating a porous host matrix with salt. However, salt expansion during hydration leads to degradation of the host matrix and salt leakage. Cellulose nanocrystals (CNCs) have shown promise in strengthening the structural frameworks of composites across numerous applications. CNCs have generated significant interest due to their high mechanical strength, high aspect ratio, high surface area, liquid-crystalline nature, and hydrophilicity, which support interaction between salt and water. CaCl 2, MgSO 4, and SrCl 2 were employed in the study, as well as several blends of species (MgSO 4:SrCl 2; SrCl2:CaCl 2; MgSO 4:CaCl 2). Salts and CNCs are combined to produce composites with varying mass ratios (60:40, 80:20, 90:10). Material performance is evaluated using simultaneous thermal analysis. SrCl 2:CNC and SrCl 2:CaCl 2:CNC (SrCl 2:CaCl 2-90:10) are the most promising materials that were developed based on energy density and uniformity. SrCl 2-based formulations possess high energy storage capabilities exceeding 600 J/g and demonstrate unique interactions with CNC through water molecules. CNCs were produced from waste sugar beet pulp (SBP) to develop a more sustainable and cost-effective process. SBPCNC-containing formulations exhibit lower energy density than those using control CNC, attributed to reduced purity, zeta potential magnitude, crystallinity, and a larger aspect ratio. FTIR results indicate that the salt:CNC chemical interaction is mediated by electrostatic forces between CNC and the water molecules of the salt hydrate. The interpretations are supported by Raman spectroscopy, which also indicates unique salt:CNC lattice vibrations and/or CNC effects on water vibrations--the hydrophilic properties of CNC increase drying resistance by binding free water and water molecules in salt hydrates. The composite materials form through mechanical and electrostatic interactions, with strong chemical affinity between the components, which supports material stability.