Development of high efficacy super-porous hydrogel composites-based polymer desiccants to capture water vapors from moist air

Abstract

Deliquescent salts are well known for their high-water vapor adsorption capacity, but they form crystalline hydrates and dissolve in adsorbed water; therefore, they cannot be used in most water vapor adsorption applications. To counter this issue, we fabricated solid polymer desiccants comprising polyacrylic acid (PAA) and sodium chloride (NaCl), which were able to capture water vapors from humidity in large quantities and avoid the dissolution of NaCl by keeping it intact inside the polymer matrix. Polymer desiccants, i.e., super-porous hydrogels (SPHs), were synthesized using gas-blowing and foaming techniques to create a porous structure. Due to dense capillary channels, the polymer matrix alone (i.e., without NaCl salt) could capture a high amount of water vapors (0.82 g_w/g_ads). Introducing NaCl salt in the polymer matrix drastically improved desiccant performance (3.1 g_w/g_ads). Further, the polymer matrix avoided salt dissolution in the adsorbed water and kept it intact within the polymer matrix. Adsorption isotherm was found to be type-III isotherm and best explained using GAB and FHH isotherm models, suggesting that the high desiccant performance of synthesized solid polymeric adsorbents was due to the presence of dense capillary channels in the polymer structure and the presence of NaCl salt within the polymer matrix. The adsorption kinetics followed the linear driving force (LDF) model and the case-II type diffusion mechanism. The desorption performance and kinetics of water release from fully hydrated desiccant samples after capturing water vapors were studied at different temperatures, suggesting that the water release rate depends highly on desorption temperature. Furthermore, the synthesized desiccants exhibited good cyclic performance for six adsorption cycles with a little loss in the desiccant performance.