The water shortage crisis is a serious global problem which limits and endangers the lives of many people. With growing world population, ongoing climate change and increasing water pollution, the situation is projected to worsen in the near future. In our Review Article, we describe the recent efforts to address the water shortage crisis by utilizing metal–organic frameworks (MOFs) to harvest moisture from air—a research direction pioneered recently by our research group.

MOFs are crystalline, porous materials which are constructed from inorganic metal cluster nodes linked by organic ‘linker’ molecules. A large variety of inorganic and organic constituents can be employed, thus making this class of materials highly tuneable and applicable to a myriad of use cases. One such application is their use as powerful water sorbents; capable of energy efficient uptake and release of large amounts of moisture at low relative humidities and upon mild heating. In comparison, conventional sorbents, such as zeolites and silica gels, require either extreme temperatures for water release or take up only small quantities of water.

In principle, moisture can be condensed by direct cooling without using any sorbents. However, large amounts of air would need to be cooled down to generate small amounts of water in dry and hot climates, thus making this technology extremely inefficient and sometimes even infeasible. In contrast, appropriate sorbents, such as MOFs, allow for moisture concentration in their pores even in arid environments. Then, the captured water can be liberated upon mild heating (e.g., through solar thermal energy) and the released concentrated vapour can be easily condensed (e.g., through natural cooling).

While the design and synthesis of new crystalline, porous materials is the main focus of my PhD adviser Professor Yaghi, in the last couple years, we demonstrated the practical use of MOFs for water capture from air by developing MOF water harvesters and employing them in field experiments (see Figure 1 for a photograph of the water harvesting team in the Mojave Desert, California, along with the latest device developed in our lab). In our Review Article, we focus on both MOF materials design and their integration in devices for atmospheric water harvesting.

As of now, MOF-based water harvesting is in its infancy, and, as we outline in our article, there are many opportunities for improving water output and energy efficiency by developing new MOF sorbents and prudently implementing them in future water harvesters. Importantly, however, this technology has a large potential to provide decentralized, community-managed, off-grid access to safe potable freshwater anywhere and anytime.

If you are interested in learning more about this topic, you can read our article "MOF water harvesters" at https://www.nature.com/articles/s41565-020-0673-x.