Seawater holds enormous potential for green hydrogen production, but realizing that potential remains far from straightforward. Direct seawater electrolysis can damage electrodes, whereas conventional freshwater electrolysis loses much of its energy as low-grade waste heat.

Researchers have now found a way to harness this otherwise wasted heat, using it to desalinate seawater and produce fresh water alongside hydrogen.

In a study published in Nature Energy on September 15, a team led by Prof. DENG Dehui and Associate Prof. LIU Yanting from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences (CAS) proposed a "seawater to hydrogen and fresh water (STHW)" route that couples alkaline water electrolysis (AWE) with low-temperature vacuum distillation desalination.

The STHW process uses waste heat generated during AWE to drive low-temperature seawater desalination, producing fresh water for both electrolysis and external use. The resulting concentrated brine can also be used for resource recovery, including salt, uranium, and bromine.

The research team built a 20-kW industrial pilot STHW system that achieved a hydrogen productivity of 3.8 Nm³ per hour and operated stably for 100 days while co-producing 1.2 kg of fresh water per hour.

When scaled up to 250 kW, the system achieved a hydrogen productivity of 48 Nm³ per hour, producing both high-purity hydrogen and 31.6 kg of fresh water per hour. The system also improved electrical efficiency by 14.4% compared with conventional alkaline electrolysis using fresh water alone.

"This study addresses two key challenges in seawater-based hydrogen production: using low-grade waste heat from water electrolysis and overcoming the stability and energy-efficiency limitations of direct seawater electrolysis," said DENG. "It offers a potential pathway for more efficient hydrogen production from seawater while enabling the co-production of fresh water."

A 250 kW system and its process for co-production of hydrogen and fresh water from seawater. (Image by JIANG Shang)