NaBr and NaI–silica gel composites as novel sorbents for adsorption desalination and thermochemical energy storage driven by ultra-low-grade heat

Composite sorbent materials based on sodium bromide (NaBr) and sodium iodide (NaI) embedded in mesoporous silica gel are proposed as novel working pairs for adsorption heat transformation applications targeting ultra-low-grade thermal sources. Unlike widely investigated salt hydrates such as CaCl₂ and LiCl, alkali halides remain largely underexplored despite their favorable thermodynamic properties for low-temperature operation. Three salt loadings (20, 25 and 30 wt%) were synthesized and characterized, with the 30 wt% composites showing the best performance in terms of water uptake and cycling stability over six consecutive hydration–dehydration cycles below 80 °C. Equilibrium data, fitted through the Dubinin–Astakhov model, were used to assess thermodynamic performance under application-relevant boundary conditions. For adsorption desalination, the 30NaBrSG composite exhibited stable water vapor exchange Δw of 0.38–0.40 kg/kg. Crucially, while LiCl-based composites outperform 30NaBrSG at desorption temperatures above 60 °C, their equilibrium curves predict a significant reduction in water vapor exchange below 50 °C — a range where 30NaBrSG retains stable performance, making it uniquely attractive for ultra-low-grade waste heat valorization. For thermochemical energy storage, the 30NaISG composite achieved energy storage densities of 318–428 kJ/kg at desorption temperatures of 50–60 °C, competitive with chloride-based materials at low charging temperatures while offering reduced corrosion risk. These results establish NaBr/SG and NaI/SG as a new class of composite sorbents for renewable and waste heat-driven applications in a temperature range currently underserved by existing materials.