A hydrogel-based system could offer a long-term solution for harvesting potable water from ambient air, according to a study.

Hydrogels have been researched for many years as a means to produce water at low cost almost anywhere. The materials, which are made of salt and polymers, soak up large amounts of moisture from the air, even in desert-like conditions. The water is then condensed back into liquid water and collected for drinking.

The challenge is that these solar-powered harvesting systems only last for around eight months or 30 cycles of filling up and releasing water before they degrade. A team at Stanford Doerr School of Sustainability explored why this happened and what they could do to create a longer-lasting system that could be scaled. 

The hydrogel system they designed uses a highly absorbent salt called lithium chloride and an equally absorbent polymer commonly used in nappies called polyacrylamide. This hydrogel is placed within a 30cm² metal frame. Painted black, this aluminium sheet absorbs heat from the sun during the day, warming the hydrogel and causing it to release water as vapour. During cooler conditions at night, the vapour condenses back into liquid water. 

Having previously tested their solar-powered system in the parched environment of Chile’s Atacama Desert, they knew it could absorb water from the air very well, but they also wanted to figure out why it degraded so quickly. 

In the lab, they investigated how the hydrogel breaks down and found that problems arise from the gel’s contact with the metal surface as it releases ions that form radicals in the hydrogel and attack the polymer’s long chains. Essentially, the gel disintegrates as bits of polymer leach into the water, destroying the potability of the water.

The researchers tested interventions to block the metal ions. When they applied an anti-corrosion coating to the metal, the hydrogel’s lifespan dramatically extended. 

According to Carlos Diaz-Marin, an assistant professor of energy science and engineering in the Stanford Doerr School of Sustainability and co-lead author of the study, in one of the tests the hydrogel remained stable for more than eight months at 167°F (75°C), a temperature used to stress-test the material under extreme conditions. They also found the hydrogel on coated metal remained stable for more than 190 water-harvesting cycles.

Diaz-Marin said: “These improvements could let us get to a point where we produce water at maybe one cent per litre. We see a path to this technology to perhaps even being competitive with tap water.”

While the researchers foresee its use in bringing potable water to rural communities facing water shortages, the system could also find a use in other applications. “There are also very water-intensive industries like semiconductor manufacturing and data centres that are putting even more pressure on water systems. We believe this could potentially be a way to provide additional water resources,” said Diaz-Marin.

Before they reach that point, the team are now working on further improving the system’s efficiency and cost. The goal is to increase its output to five litres daily, which they believe is achievable. 

Their study – Long-term stability of moisture-capturing hydrogels by preventing metal-mediated degradation – has been published in the journal Nature Communications.