Solar steam generation (SSG) is recognized as a sustainable technology for seawater desalination, but its practical applications have been hampered by salt fouling, which compromises the evaporation performance and lifespan of evaporators. Addressing this issue, a research team from City University of Hong Kong (CityUHK) has developed a groundbreaking solution - a hierarchical salt-rejection (HSR) strategy that prevents salt precipitation during long-term evaporation, even in high-salinity brine. This innovative research is a significant step forward in advancing various applications such as resource recovery and offshore farming, thereby paving the way for a more sustainable future.

This innovative approach resulted in an impressive evaporation rate of 2.84 kg m^-2 h^-1 and stable evaporation for 7 days in 20 wt% NaCl solution. Furthermore, the freshwater production rate improved by a staggering 54% compared to the control group. Given the low cost of raw materials and the ability for mass production, the HSR evaporator can provide affordable drinking water to regions experiencing water stress and lacking infrastructure.

"We offer a generic solution for long-term, stable, and highly efficient solar-driven seawater desalination. This method solely relies on solar energy without the need for additional equipment, and can potentially deliver low-cost clean water to off-grid regions, enabling agricultural applications and helping to alleviate the freshwater-food crisis in underprivileged areas.", said Prof Lu Jian, Dean of College of Engineering at CityUHK, who led the study together with his research team.

The team's work presents a simple yet effective strategy to a longstanding challenge in solar-driven desalination, achieving long-term salt-rejection while maintaining a rapid evaporation rate in high salinity brine. This breakthrough was achieved by creating a novel salt-resistant solar evaporator, the HSR evaporator. The HSR strategy breaks down the salt diffusion into three aspects: molecular insulation, micron branching diffusion, and macro arterial transport, enhancing salt rejection efficiency without compromising the water evaporation rate.

"The most challenging part of this research was preparing the hierarchical structure in the evaporator." Prof Lu added. However, by leveraging the capabilities of 3D printing, the team successfully fabricated a hierarchical structure comprising a molecular insulation layer, a microporous skeleton, and interconnected channels.

This invention opens up a wide range of application scenarios, including desalination, wastewater treatment, resource recovery, and agricultural irrigation. Looking ahead, the team plans to achieve large-sample preparation and explore applications in high-concentration brine treatment, sea salt collection, and offshore farming.

The findings were published in scientific journal Nano Materials Science, titled “A hierarchical salt-rejection strategy for sustainable and high-efficiency solar-driven desalination”.

For inquiry, please contact Dr. Mao Zhengyi from the Department of Mechanical Engineering at CityUHK, by email at zymao3@cityu.edu.hk.