Scientists in Shanghai have developed a unique material that combines the best features of a hydrogen and a sponge. This unique ‘hydrosponge’ can extract water from air in a far more energy-efficient way than existing methods.
While hydrogels are known for their ability to soak up and hold large amounts of water, they often hold the water too rigidly. This means it takes a lot of energy (usually heat) to get the water back out.
On the other hand, a sponge has a light, porous structure which makes it easier to release moisture. However, it tends to dry out too quickly as water can pass through it easily.
The hydrosponge is soft and absorbent like a hydrogel and also features a sponge-like airy structure that makes it easier to release water using just gentle heat, like sunlight. It offers the best of both worlds—high water uptake with low energy use.
Creating the perfect hydrosponge
The hydrosponge is mostly made of eco-friendly ingredients, including chitosan (obtained from shellfish waste), γ-polyglutamic acid (a biopolymer), and polyvinylpyrrolidone. To help the material absorb sunlight and turn that into heat effectively, the researchers also added a compound called polypyrrole.
Next, they employed a mix of physical and chemical-based foaming methods to design the material such that 70% of the hydrosponge is empty space comprising interconnected channels. This porous design allows water vapor to flow through it easily and stick to its surface. To further enhance its performance, the researchers added lithium chloride—a salt that’s naturally good at pulling in moisture from the air.
All these ingredients resulted in the formation of CPPY@LiCl, a hydrosponge capable of trapping and releasing water without using any external power supply.
What sets this material apart is how it stores water. The researchers found that water inside it appears in three forms: tightly attached to the material, loosely attached, and freely moving through the pores. CPPY@LiCl contains a lot of loose and free-moving water, which can be removed with far less energy than materials where water binds tightly.
For instance, the amount of energy needed to evaporate water from CPPY@LiCl is about 40% lower than regular water. Moreover, while conventional water harvesting material typically releases water at 80°C (176°F), water starts flowing out from the hydrosponge at just 50°C (122°F), which means that the heat from sunlight is enough to make it work.
Testing the water retention capacity
The team conducted experiments under different humidity conditions to check CPPY@LiCl’s water absorption rate. The scientists found that the material could absorb 1.64 grams of water per gram of material at 30% humidity, 2.65 grams at 60%, and 4.21 grams at 80%.
Next, they conducted tests in outdoor settings, during which the material was left overnight, and then water was collected from it during the day. At the end of the test, the material had harvested 6.29 liters of water per square meter. Plus, it maintained 90% of its water-capturing ability even after exposure to strong UV light.
Finally, they checked whether this water was clean and drinkable. The harvested water turned out to be safe for human consumption as per the water safety guidelines set by the World Health Organization (WHO). These results highlight the incredible potential of CPPY@LiCl in solving water scarcity-related challenges in many regions of the world.
The material is mostly biodegradable, gives out a good yield, and only needs sunlight, making it an ideal solution for rural and remote areas where people don’t have access to clean drinking water. However, further research is required to improve its performance and bring down the cost of the hydrosponge.
The study is published in the journal Advanced Functional Materials.