A team in China has developed a combined desalination-electrolysis system that can produce green hydrogen directly from seawater. This integrated process uses an energy-efficient method to purify seawater, making it one of the first viable approaches to using saltwater as a hydrogen source. The purification step uses phase transitions to remove impurities and could have additional applications in wastewater treatment and resource recovery.
Splitting water with electricity has been experimented with for over 200 years and the reactions involved are well understood: at the cathode, H+ ions gain electrons to form hydrogen gas, while OH- loses electrons at the anode to form oxygen. But despite the simplicity of the underlying chemistry, effective electrolysis is a very complicated process. Water splitting is thermodynamically unfavorable and requires both specifically designed catalytic electrodes and a significant amount of energy to drive the reaction. Even trace amounts of impurities can damage the delicate structure of the cell, clogging membrane pores, corroding expensive electrodes and forming unwanted by-products.
Chloride ions in seawater are a particular problem and undergo competitive oxidation at the anode to produce chlorine. Not only does this side reaction reduce the cell’s electrochemical efficiency, but chlorine is an extremely corrosive gas that quickly corrodes the electrodes and inactivates the cell. “Approaches to suppress corrosion by coating catalysts have met with modest success,” explains Heping Xie, an energy chemist at Shenzhen University in China. ‘But the composition of seawater is changing [with] place, season [and] human behaviour, so electrolysers cannot be universally compatible.’ With an average salt concentration of around 3.5%, the chloride content of seawater makes direct electrolysis unfeasible.
“Desalination of seawater before electrolysis can prevent problems,” said Zongping Shao, an electrocatalytic chemist at Nanjing Tech University in China. ‘But [it] requires extra energy and space [so it’s] economically and practically less attractive.’ Currently, the energy cost of desalination outweighs the value of hydrogen generated by electrolysis. However, the abundance of seawater combined with the urgent need for green fuels motivates researchers to find innovative solutions to these problems.
Split sea water
Using the purifying power of evaporation, Xie and Shao have developed the first practical and scalable seawater electrolysis system. Their in-situ purification system uses a liquid-gas-liquid phase transition to generate pure water from seawater directly in the electrochemical cell, a process driven by subsequent electrolysis.
A PTFE-based porous membrane separates seawater from the inside of the cell, with the high density of fluorine atoms forming a hydrophobic barrier impermeable to water and its impurities, but permeable to water vapour. On the other hand, a concentrated potassium hydroxide solution surrounds the electrodes and provides the driving force for water vapor migration. ‘The potassium hydroxide electrolyte has a higher concentration than the salt concentration in the seawater’, explains Alexander Kuwan, a renewable fuels researcher at the University of Liverpool, UK. ‘The resulting difference in water vapor pressure (due to the salinity gradient) causes the water to pass from the seawater side through the membrane to the potassium hydroxide solution.
Left in isolation, this system would eventually reach equilibrium and water migration would stop as concentrations on each side of the membrane became equal. However, the consumption of purified water by the electrolysis reaction provides a continuous driving force and maintains the concentration gradient across the membrane. By changing the rate of water migration or electrolysis, the system effectively regulates itself and ensures that the pure water is used as quickly as it is produced. “It can actually be thought of as a dynamic balance system,” explains Xie. ‘If the initial electrolysis rate is higher than the water migration rate, the electrolyte concentration increases, leading to an increase in the water vapor pressure difference and, as a result, the water migration rate.’
After successful lab trials, the team was eager to demonstrate the utility of this approach on a large scale and installed a demo device in Shenzhen Bay. The compact unit ran for an initial test period of 133 days and produced more than one million liters of hydrogen without any apparent corrosion of the catalyst or build-up of impurities. “It’s a nice demonstration of the technical feasibility of running direct seawater electrolysis over extended periods of time without apparent loss of activity,” says Cowan. “A challenge will be to develop a device that can achieve a significant reduction in operating potential to make them comparable to more conventional membrane electrolysers.”
“It solves a long-standing technical bottleneck in this area,” notes Xuping Sun, an electrocatalysis researcher at Shandong Normal University in China. ‘But [it still] needs further development. To make seawater electrolysis systems more relevant for industrial applications, higher current densities are needed.’ Xie and Shao are eager to develop the device for industrial use and are currently exploring ways to reduce energy consumption and improve the performance of the catalysts. “This technology has great potential,” says Shao. ‘[We hope that] humans can use this liquid-gas-liquid phase transition mechanism in other fields of fuel production and resource recovery.”