Korean researchers have developed a technology that can convert seawater into drinking water using only sunlight without any power supply. The breakthrough is expected to significantly help address water shortages in developing countries and island regions lacking electrical infrastructure.
A research team led by Professor Jang Ji-hyun at the Department of Energy and Chemical Engineering at Ulsan National Institute of Science and Technology (UNIST) announced on Monday that they have developed a ternary oxide-based evaporator that heats seawater by absorbing sunlight. The device can produce drinkable freshwater without electricity by evaporating seawater and then condensing the vapor.
The evaporator developed by the research team can produce approximately 4.1 liters of drinking water per hour when floated on seawater at a size of one square meter. This is nearly seven times faster than the natural seawater evaporation rate and represents the world's highest evaporation speed among oxide material-based devices reported in academia to date.
The secret to this high performance lies in a new photothermal conversion material. Photothermal conversion materials absorb sunlight and convert it into heat, and are thinly coated on the evaporator surface.
The research team created a ternary oxide photothermal conversion material by substituting some of the manganese in corrosion-resistant manganese oxide with copper and chromium. This "bandgap engineering" technology designs the solar wavelength bands that materials can absorb by adjusting their composition. While typical oxide materials only absorb light up to the visible light wavelength range, the developed material absorbs 97.2 percent of light from ultraviolet through visible light to the near-infrared region.
The absorbed sunlight is also efficiently converted into heat. When chromium or copper occupies manganese sites, the absorbed solar energy is more likely to convert into heat rather than being re-emitted as light.
As a result, the material's surface temperature rises to 80 degrees Celsius. This significantly outperforms conventional manganese oxide, which only reached 63 degrees Celsius under the same conditions, and copper-manganese oxide, which recorded 74 degrees Celsius.
The device structure was also optimized to minimize salt accumulation. It features an inverted U-shaped structure, with cotton material that readily absorbs water applied to the evaporation surface coated with photothermal conversion material, while polyester material is used for the remaining parts. The fiber structure of polyester rapidly draws water upward like a straw, while the hydrophobic properties of polyester itself serve as a channel to wash away salt, preventing it from adhering to the evaporation surface.
"We were able to create a high-performance evaporator by fundamentally improving upon the narrow light absorption bands that limited the efficiency of existing oxide photothermal conversion materials, while also enhancing photothermal conversion characteristics," Professor Jang said. "The material has excellent durability and is easy to scale up, so it could help solve actual drinking water shortage problems."
The research findings were published online on December 16 in the international academic journal Advanced Materials and are awaiting formal publication.
The research was conducted with support from the Engineering Research Center project of the "Micro-plastic Response Chemical and Bio Convergence Process Research Center," the Mid-career Researcher Program, the Brain Pool Program, and the Innocore Project.
