Electrospun Poly(vinylpyrrolidone)/Calcium Chloride Composite Microfibers as Permeable Sorbents for Low-Temperature Thermochemical Heat Storage

Abstract Salt hydrates are promising materials for low-temperature thermochemical heat storage because of their high energy density, low cost, and reversible interaction with water vapor. However, their direct use is limited by structural instability, agglomeration, deliquescence, and vapor transport limitations during hydration/dehydration cycling. In this work, electrospun poly(vinylpyrrolidone)/calcium chloride composite microfibers were investigated as permeable water-sorption sorbents for low-grade heat utilization. The effect of calcium chloride loading on fiber morphology, salt dispersion, thermal stability, water vapor uptake, sorption/desorption heat, and permeability was evaluated. At moderate salt loadings, the electrospun mats preserved a fibrous morphology without visible macroscopic salt crystals on the fiber surface or between adjacent fibers. X-ray diffraction suggested the presence of finely dispersed calcium chloride-containing crystalline domains within the polymeric matrix. Water vapor sorption at 10 mbar increased from 10.4 wt % for pristine poly(vinylpyrrolidone) microfibers to 14.6 and 17.1 wt % for composite microfibers with moderate calcium chloride contents. Isobaric heat measurements further showed that calcium chloride incorporation enhanced the sorption heat by approximately 25% at 10 mbar, reaching 294.1 kJ kg–1 for PVP20Ca. Water-vapor cycling over 21 consecutive cycles at 23.4 mbar showed reproducible sorption behavior, with average cyclic water-uptake variations of approximately 17.9 wt % for pure PVP and 23.9 wt % for PVP20Ca. Postcycling SEM analysis showed that the fibrous networks remained recognizable, although more pronounced fiber swelling was observed for PVP20Ca. In addition, the fibrous structure retained measurable permeability under low-pressure vapor-flow conditions, supporting vapor transport through the sorbent layer.