Novel nanomaterial extracts drinking water from air

Researchers have developed a novel nanomaterial to efficiently harvest clean drinking water from water vapour in the air. Described in the journal PNAS, the nanomaterial can hold more than three times its weight in water and can achieve this far quicker than existing commercial technologies, suggesting its potential use in direct applications for producing potable water from the air.

The work results from a global collaboration between research groups from Australia, China, Japan, Singapore and India, led by Associate Professor Rakesh Joshi and Nobel Laureate Professor Sir Kostya Novoselov from the Australian Research Council Centre of Excellence for Carbon Science and Innovation (COE-CSI). Joshi is based at UNSW Sydney, while Novoselov is based at the National University of Singapore (NUS).

According to a 2022 United Nations report, 2.2 billion people lack safely managed drinking water — yet there are about 13 million gigalitres of water suspended in Earth’s atmosphere (for comparison, Sydney Harbour holds 500 gigalitres). While this is only a fraction of the total water on Earth, it still amounts to a substantial source of fresh water — a source which the new nanomaterial could harvest in any region where there is sufficient humidity but limited availability of clean potable water, according to Joshi.

The novel nanomaterial is based on graphene oxide, which is an atom-thick carbon lattice functionalised with oxygen-containing groups. Graphene oxide has good water adsorption properties, which are properties that enable water to bond to the surface of a material.

Calcium also has good water adsorption properties. With this in mind, the research team decided to see what happened if you intercalate (insert) calcium ions (Ca²⁺) into the graphene oxide. What happened was unexpected.

An important characteristic of materials that effectively adsorb water is strong hydrogen bonds between the water and the material it adsorbs onto, something that graphene oxide and calcium each have. The stronger the hydrogen bond, the more a material can adsorb water.

But when you intercalate calcium to the oxygen in the graphene oxide, there is a synergy between calcium and oxygen that facilitates the extraordinary adsorption of water. The research team discovered that the way the calcium coordinates with the oxygen in the graphene changes the strength of the hydrogen bonds between the water and the calcium to make those bonds even stronger.

As explained by first author Xiaojun (Carlos) Ren, from UNSW, “We measured the amount of water adsorbed onto graphene oxide by itself and we measured X. We measured the amount of water adsorbed onto calcium itself and we got Y. When we measured the amount of water adsorbed onto the calcium-intercalated graphene oxide we got much more than X+Y. Or it is like 1+1 equals a number larger than 2.

“This stronger-than-expected hydrogen bonding is one of the reasons for the material’s extreme ability to adsorb water.”

There was one more design tweak the team did to enhance the material’s water adsorbing ability — they made the calcium-intercalated graphene oxide in the form of an aerogel, one of the lightest solid materials known.

Aerogels are riddled with micro- to nanometre-sized pores, giving them a massive surface area, which helps this aerogel form adsorb water far quicker than the standard graphene oxide. The aerogel also gives the material sponge-like properties that make the desorption process, or release of the water from the membrane, easier.

“The only energy this system requires is the small amount needed to heat the system to about 50 degrees to release the water from the aerogel,” said study co-author Prof Daria Andreeva, from NUS.

The research is based on experimental and theoretical work, led by Professor Amir Karton from the University of New England, which relied on the National Computational Infrastructure (NCI) supercomputer in Canberra. According to Karton, “The modelled simulations done on the supercomputer explained the complex synergistic interactions at the molecular level, and these insights now help to design even better systems for atmospheric water generation, offering a sustainable solution to the growing challenge of fresh water availability in regional Australia and in water-stressed regions across the globe.”

While the research requires further development, industry have already collaborated on the project to help scale up the technology and develop a prototype for testing. Study co-author Professor Liming Dai, who serves as Director of COE-CSI, concluded, “What we have done is uncover the fundamental science behind the moisture adsorption process and the role of hydrogen bonding. This knowledge will help provide clean drinking water to a large proportion of those 2.2 billion people that lack access to it, demonstrating the societal impact by collaborative research from our Centre.”

Image caption: First author Dr Xiaojun Ren examines the graphene oxide aerogel.