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Chinese scientists have developed a new method to transform seawater into hydrogen without desalinating it. They have incorporated a waterproof, breathable membrane and a self-damping electrolyte (SDE) into the electrolyser, so that water migrates from seawater through the membrane to the SDE, without additional energy consumption.
Researchers from Shenzhen University and Nanjing University of Technology (China) have developed a seawater electrolysis (SES) system for the direct electrolysis of seawater, without side reactions or corrosion.
The new method works by self-propelled water migration to avoid the need for a separate desalination process. The power consumption is supposedly comparable to that of industrial alkaline electrolysis with pure water.
To develop the electrolyzer on a laboratory scale, the scientists separated the two electrode layers with a diaphragm layer and then immersed them in a concentrated hydroxide (OH-) solution – SDE. They placed a polytetrafluoroethylene (PTFE) membrane between the seawater and the SDE. They state that this design allows biased diffusion of water vapor, but completely prevents the penetration of liquid seawater and impurity ions such as magnesium, chloride, and sulfate.
“During operation, the water vapor pressure difference between seawater and SDE across the membrane provides a driving force for the spontaneous gasification of seawater (evaporation) on the seawater side and the diffusion of water vapor through the short path of gas inside the membrane to the SDE side, where it is re-liquidated by SDE absorption”, explains the scientist.
The consumption of water in the SDE during electrolysis maintains the pressure difference across the membrane, ensuring the continuous input of fresh water, without extra energy consumption.
When the water migration rate is equal to the electrolysis rate, a new thermodynamic equilibrium is established between seawater and SDE, and a continuous and stable water migration occurs through a “liquid-gas- liquid” to provide fresh water for electrolysis,” the researchers explain.
The laboratory-scale electrolyzer produced hydrogen from seawater for more than 72 hours, with average voltages of approximately 1.95 V and 2.3 V at current densities of 250 mA cm-2 and 400 mA cm-2 , respectively. After 72 hours, the concentrations of chloride, sulfate and magnesium impurities were only 0.008%, 0.052% and 0.089% of the original.
“Calculations show that the electrical cost of [hydrogen] production is approximately 4.6 kWh Nm-3H2 and 5.3 kWh Nm-3H2 at current densities of 250 mA cm-2 and 400 mA cm-2, respectively, which is comparable to that of industrial alkaline electrolysis with pure water,” the scientists stated.
To prove the concept, they built a scaled SES measuring 82 cm x 62 cm x 70.5 cm. It reportedly showed stable performance for more than 3,200 hours and power consumption of about 5.0 kWh Nm-3H2, with no increase in impurity ions detected.
The scientists said their method can be further developed using other electrolytes capable of absorbing water vapor and conducting ions, or high-performance electrocatalysts capable of operating in the SDE environment. They added that the strategy could be applied to other non-volatile liquids such as highly concentrated solutions of acids, alkalis and salts for use in industrial wastewater treatment.
Furthermore, it could be used to produce hydrogen and recover useful resources such as lithium from water. Further research is also needed so that the compatibility of the electrolyser with renewable energy sources is equal to that of conventional electrolysers.
The scientists have reported their findings in “ A membrane-based seawater electrolyser for hydrogen generation ,” recently published in Nature. |
