A combined desalination and electrolysis system capable of producing green hydrogen directly from seawater has been developed by a team in China. This integrated process uses a low-energy method to purify seawater, making it one of the first viable approaches to using saltwater as a source of hydrogen. The purification step uses phase transitions to remove impurities and could have additional applications in wastewater treatment and resource recovery.
The separation of 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, efficient electrolysis is a particularly complicated process. Water separation is thermodynamically unfavorable and requires both specially designed catalytic electrodes and a large energy input to drive the reaction. Even traces of impurities can damage the delicate structure of the cell, leading to clogging of membrane pores, corrosion of expensive electrodes, and formation of unwanted by-products.
Chloride ions in seawater are a particular problem and undergo concurrent oxidation at the anode to produce chlorine. Not only does this side reaction reduce the electrochemical efficiency of the cell, but chlorine is an extremely corrosive gas that rapidly degrades the electrodes and inactivates the cell. “Approaches to suppress corrosion by coating catalysts have had modest success,” says Heping Xie, an energy chemist at Shenzhen University in China. “But the composition of seawater changes [with] location, season [and] human behavior, so electrolysers cannot be universally compatible. With an average salt concentration of around 3.5%, the chloride content of seawater makes direct electrolysis impossible.
“Desalination of seawater before electrolysis can eliminate problems,” says Zongping Shao, an electrocatalytic chemist at Nanjing Technical University in China. ‘But [it] requires additional energy and space [so it’s] less attractive economically and practically. Currently, the energy cost of desalination exceeds the value of the hydrogen generated by electrolysis. However, the abundance of seawater, coupled with the urgent need for green fuels, motivates researchers to find innovative solutions to these problems.
Fractionation of sea water
By harnessing 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 porous PTFE-based membrane separates seawater from inside the cell, the high density of fluorine atoms creating a hydrophobic barrier impermeable to water and its impurities but permeable to water vapor . On the other side, a concentrated solution of potassium hydroxide surrounds the electrodes and provides the driving force for water vapor migration. “Potassium hydroxide electrolyte is at a higher concentration than the concentration of salt in seawater,” says Alexander Cowan, a sustainable fuels researcher at the University of Liverpool, UK. “The resulting difference in water vapor pressure (due to the salt gradient) drives water from the seawater side through the membrane and into the potassium hydroxide solution.
Left isolated, this system would eventually reach equilibrium and water migration would stop when the concentrations on either side of the membrane became equal. However, the consumption of water purified 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 self-regulates and ensures that pure water is used as quickly as it is produced. “It can actually be considered a dynamic equilibrium system,” Xie explains. “If the initial electrolysis rate is greater than the migration rate of water, the electrolyte concentration increases, leading to an increase in the water vapor pressure difference and, therefore, the migration rate of the water increases.”
After successful lab trials, the team was keen to demonstrate the practicality of this large-scale approach and installed a demonstration device in Shenzhen Bay. The compact unit operated for an initial test period of 133 days, producing over one million liters of hydrogen without any obvious corrosion of the catalyst or increase in impurities. “It’s a great demonstration of the technical feasibility of direct electrolysis of seawater for extended periods without any apparent loss of activity,” says Cowan. “A challenge will be to develop a device capable of achieving a significant decrease in operating potentials to make them comparable to more conventional membrane electrolysers.”
“It solves a long-standing technical bottleneck in this field,” observes Xuping Sun, an electrocatalysis researcher at Shandong Normal University in China. ‘But [it still] needs more development. To make seawater electrolysis systems more suitable for industrial applications, higher current densities are required. Xie and Shao are eager to develop the device for industrial use and are currently investigating ways to reduce power consumption and improve catalyst performance. “This technology has great potential,” says Shao. ‘[We hope that] people can use this liquid-gas-liquid phase transition mechanism in other areas of fuel production and resource recovery.
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