Two-thirds of the global population is experiencing water scarcity at various levels. While numerous research efforts have been made to develop desalination and water purification technologies, the moisture in the atmosphere, which is estimated to be more than ten thousand cubic kilometres, is another sustainable source of freshwater that can be harvested to mitigate the current water shortage. Compared to conventional water purification technologies that rely on the existence of a waterbody, the extraction of water from air represents a decentralized approach regardless of geographical or hydrologic conditions. The key steps of atmospheric water harvesting (AWH) involve moisture capture and water release, followed by a simple filtration or purification process (Fig. 1a). The earlier approaches, such as fog capturing and dew condensation, require the presence of high relative humidity (RH) (>90%RH), which is not a viable solution considering more than one-third of the global terrestrial area has an average annual humidity less than 40% (Fig. 1b).

Porous sorbent materials have been exploited for AWH over a wide range of humidity, especially under low RH (≤ 30%). However, current moisture-harvesting materials suffer from one or more limitations, including low water uptake at low RH, slow sorption/desorption kinetics, high energy consumption for water release, and high cost. Polymeric gels have emerged as another promising platform to enable high water uptake at low RH due to their high water retention, tuneable structures, and tailorable polymer-water interactions.

In our recent work, we developed super hygroscopic polymer films (SHPFs) to extract water vapor from arid climates (≤ 30% RH) with exceptional kinetics. The SHPF consists of earth-abundant biomasses, konjac glucomannan (KGM) and hydroxypropyl cellulose (HPC) as the hybrid polymer matrix to hold uniformly dispersed LiCl solution (Fig. 1c), achieving 0.64 g g^-1 and 0.96 g g^-1 water uptake at 15% RH and 30% RH, respectively. Specifically, SHPFs have hierarchically porous structures facilitated by KGM, which provide enlarged sorbent-air interfaces and rapid water vapor transport pathways. Thermoresponsive HPC permits controllable interactions between polymer chains and water molecules, realizing the release of water within 10 min to achieve 14 sorption-desorption cycles at 15% RH and 24 cycles at 30% RH per day with the assistance of electric heating. In addition, the aggregation of salt particles in SHPFs is effectively suppressed, which warrants the stable water sorption performance during cycles, offering a daily water yield of up to 5.8 - 13.3 L kg^-1. Synthesized via a simple casting method using sustainable raw materials, SHPFs highlight the potential for low-cost and scalable atmospheric water harvesting technology to mitigate the global water crisis (Fig. 1d).

For more details of this work, please see our recent publication in Nature Communications:

Scalable super hygroscopic polymer films for sustainable moisture harvesting in arid environments, https://www.nature.com/articles/s41467-022-30505-2