Chen Carmen, Salinger Jamie L, Essig Molly E, Walton Ian M, Fulvio Pasquale F, Walton Krista S
School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive NW, Atlanta, Georgia 30332, United States.
ACS Appl Mater Interfaces. 2024 Jul 31;16(30):40275-40285. doi: 10.1021/acsami.4c09456. Epub 2024 Jul 17.
To combat water scarcity in remote areas around the world, adsorption-based atmospheric water harvesting (AWH) has been proposed as a technology that can be used alongside existing water production capabilities. However, commonly used adsorbents either have low water adsorption loadings or are difficult to regenerate. In this work, we developed two novel hierarchical silica-salt composites that both exhibit high water adsorption loadings under dry and humid conditions. The total water vapor loading, kinetics, and heats of water adsorption for both silica-salt composites were investigated. As hierarchical silicas have tunable pores and large pore volumes, these materials serve as effective host matrixes for the hygroscopic salt LiCl. Our results suggest that hierarchical pores play a significant role in water adsorption: micropores and some smaller mesopores act as "storage" sites for hygroscopic salt, whereas larger mesopores and macropores increase the accessibility of water vapor into the silica. Using this mix of pores, we achieved greater than 0.4 g HO/g composite at 10% RH and 27 °C. Additionally, we found that the salt-impregnated silica and bare silica had the same heat of adsorption: 80-90 kJ/mol. The results suggest that the H-bond interactions are similar for both systems and that the primary mechanism at play here is water cluster adsorption/desorption. Despite the similar energies, the LiCl-containing materials exhibited considerably slower kinetics than bare silica materials. Of equal importance to the adsorption capacity and kinetics of these composites is their mechanical stability. To assess their mechanical stability, high-energy ball milling of silica was conducted to create more uniform particle sizes. However, reduced particle sizes came at a cost─the BET surface areas and pore volumes were drastically decreased after more than 1 h of ball milling. Findings from this study suggest that short-term ball milling may be a viable large-scale option to reduce particle size in silica materials without sacrificing significant performance.
为应对全球偏远地区的水资源短缺问题，基于吸附的大气取水（AWH）技术被提出来，可与现有的水生产能力一并使用。然而，常用的吸附剂要么水吸附量低，要么难以再生。在这项工作中，我们开发了两种新型的分级硅盐复合材料，它们在干燥和潮湿条件下均表现出高水吸附量。研究了两种硅盐复合材料的总水蒸气吸附量、吸附动力学和水吸附热。由于分级硅具有可调的孔隙和大孔体积，这些材料可作为吸湿盐LiCl的有效主体基质。我们的结果表明，分级孔隙在水吸附中起重要作用：微孔和一些较小的中孔作为吸湿盐的“储存”位点，而较大的中孔和大孔增加了水蒸气进入二氧化硅的可及性。利用这种孔隙组合，我们在10%相对湿度和27°C下实现了大于0.4 g H₂O/g复合材料的吸附量。此外，我们发现盐浸渍二氧化硅和裸二氧化硅具有相同的吸附热：80 - 90 kJ/mol。结果表明，两种体系中的氢键相互作用相似，此处起主要作用的机制是水簇的吸附/解吸。尽管能量相似，但含LiCl的材料的动力学比裸二氧化硅材料慢得多。对于这些复合材料的吸附容量和动力学同样重要的是它们的机械稳定性。为评估其机械稳定性，对二氧化硅进行了高能球磨以获得更均匀的粒径。然而，粒径减小是有代价的——球磨超过1小时后，BET表面积和孔体积急剧下降。这项研究的结果表明，短期球磨可能是一种可行的大规模选择，可在不牺牲显著性能的情况下减小二氧化硅材料的粒径。
