New material provides clean water and electricity using nothing but the Sun

Our atmosphere holds six times more water than you’ll find in all the rivers on Earth. The dew drops you see on grass and water droplets on a cold juice bottle are evidence of this natural reservoir of water. Despite its ubiquity, 2 billion people on Earth still don’t have access to clean drinking water.

A technique called atmospheric water harvesting (AWH) can allow us to extract some of this freshwater out of the air. But there are various challenges that have prevented us from implementing AWH on a large scale. In order to create an effective and continuous AWH system, scientists need to ensure two things. The first is that the water absorption from the air is fully reversible so that the water can be retrieved for use.

The second is efficient waste heat management. When an AWH system captures water from the air, the condensation of water releases heat. If this excess heat is not processed carefully, it can interfere with the entire process. However, it seems that we are now closer to a solution. Inspired by the structure of plant leaves, a team of researchers in China has created a core-shell structural cellulose nanofibre-based aerogel (called Core-Shell@CNF for short) that promises to overcome these challenges.

Not only does it operate using only sunlight, but it produces electricity as well.

Producing fresh water out of thin air

The Core-Shell@CNF comes with a hydrophilic (attracts water molecules) core and a hydrophobic (tends to repel water) shell. The former comprises LiCl particles, which are great at absorbing water and work as a sorbent. The latter contains carbon black particles and has a water-resistant Polydimethylsiloxane, or PDMS, coating. This layered design takes its inspiration from plant leaves that also “display well-designed core-shell structures where the cuticle of the leaf protects the interior mesophyll tissue from dehydration and oxidation, while the stomata allow the free transport of gaseous water molecules,” the study authors note.

The AWH process begins with water absorption in the aerogel at night. When air, along with water vapor, passes through the material, its big pores allow water molecules to reach the LiCl particles inside, which absorb them. As more and more water goes in, the hydrated salts turn into a liquid film and then a salty solution. Meanwhile, the external hydrophobic shell prevents liquid water from leaking outside. This combination helps the Core-Shell@CNF continue gathering water.

During the day, the carbon black particles absorb sunlight and heat up quickly. As a result, the temperature inside the aerogel increases, and the salty solution starts releasing water vapor, re-forming the original salt. Because of carbon’s ability to quickly absorb sunlight and turn it into heat energy, the aerogel is able to release water quickly. Carbon’s porous structure also aids in transferring heat and water molecules, making the desorption process effective.

The researchers performed multiple absorption-desorption cycles to test the limits of their AWH material and achieve maximum efficiency. This led to several improvements in their material’s design; for instance, the final aerogel structure has fewer pores and 10 times thicker external walls than the original. Thanks to these changes, “Even under [pressure], the CB-PDMS@CNF could prevent the water from penetrating, showing good hydrophobicity and mechanical strength,” the researchers note.

The strong hydrophobic shell is also great at keeping the aerogel clean, as it separates any dust particles or contaminants that come along with water molecules. When the researchers tested the aerogel in an outdoor environment for 24 hours, each kilogram of material was able to collect a bit under a gram of freshwater. While the researchers hope to boost the efficiency, the water itself was ready to drink. “The results of the inductively coupled plasma mass spectrometry (ICP-MS) test indicated that the collected water met the drinking water requirements of the World Health Organization (WHO) and the US Environmental Protection Agency (EPA),” they added.

Turning the excess heat into electricity

The aerogel has produced freshwater, but what about the extra heat generated by the carbon black particles during desorption? Overheating could potentially harm the entire Core-Shell@CNF material; to prevent that from happening, the researchers fed the heat into a thermoelectric module, a device that can generate electricity by utilizing temperature differences.

The researchers connect the Core-Shell@CNF to one side of the thermoelectric device while maintaining a small gap between the two so that enough heat is retained to power the ongoing AWH process. When excess heat is produced, the Core-Shell@CNF creates a big temperature difference between the hot and cold sides of the thermoelectric module, resulting in electricity generation.

When the researchers tested this system in an outdoor environment under different sunlight conditions, they achieved a maximum power production of 12 W per square meter—about 10 percent of what you'd get from a traditional solar panel. This peak of production took place after the system had dried out. “This promising strategy combines AWH and solar thermal conversion while simultaneously producing fresh water using natural light as the sole energy input, promoting the commercialization of the next generation of advanced AWH,” the study authors note.

They argue that Core-Shell@CNF would be very useful for travelers and researchers who explore harsh environments and continuously struggle to meet their power and water needs. The device also has the potential to provide clean water and energy access to billions of people living in poor parts of the world. However, the current design will need to go through a lot of additional improvements to make this possible.

“We envision that such a scalable, ultralight, and super-hygroscopic TE generator system with continuous water production ability will provide new opportunities for AWH materials in outdoor equipment for efficient solar thermoelectricity-freshwater cogeneration,” the authors added.

Matter, 2023. DOI: 10.1016/j.matt.2023.07.015 (About DOIs)

Rupendra Brahambhatt is a journalist and filmmaker who covers science and culture news.