Impact of Cycling on the Structure of Crystal Nanocellulose-Salt Composite Materials for Thermal Energy Storage

Thermochemical energy storage represents a promising technology due to its high energy density with long-term storage efficiency. One thermochemical reaction of interest for energy storage in buildings is the hydration-dehydration of hygroscopic salts. However, these salts have challenges including particle agglomeration, deliquescence, and structural degradation over the course of hydration-dehydration cycling. Various composite materials have been investigated to address the cycle stability issue. One promising composite utilizes crystalline nanocellulose (CNC) as a stabilizing additive. In this study, various salt-CNC composite materials, as well as controls of pure salts, are investigated. The composites include SrCl2 and an 80:20 SrCl2:CaCl2 blend with CNC, both at salt-to-CNC weight ratios of 90:10 and 80:20. The cycle behavior was assessed by measuring the hydration water uptake. The structural changes were determined through visual inspection at the macroscale, while the microstructural changes were assessed via scanning electron microscopy. The dehydration enthalpy of the samples was measured. The samples have relatively stable water uptake, with a slight decrease for the salt-CNC composites throughout cycling. CNC helps in preventing particle agglomeration and deliquescence. Also, for SrCl2 samples, the addition of a small amount of CNC retained the dehydration enthalpy over multiple cycles. The results are useful for guiding future material development, particularly salt-CNC composites, for thermochemical storage.