- Until now, the extraction of hydrogen from salty seawater was only possible with low efficiency because the chloride ions accumulated on the electrode
- An optimized electrolyser with a proton exchange membrane (PEM) can significantly increase efficiency
- In the future, coastal desert regions with high levels of solar radiation could use the process to produce green hydrogen
An optimized electrolysis can produce hydrogen directly from salty sea water with high efficiency. In the future, the process could be used to produce hydrogen in coastal desert regions.
Tianjin (China). In the future, green hydrogen should replace fossil fuels as fuel and in many industrial processes. Because production requires large amounts of renewable energy, countries like Germany are already considering importing hydrogen from countries with high levels of solar radiation. A study by the German Aerospace Center (DLR), the Wuppertal Institute and the Institute for Future Energy and Material Flow Systems (IZES) recently shows that in the Mena region (Middle East and North Africa) around 400,000 terawatt hours of renewable energy are generated annually Energies could be produced with which green hydrogen is then produced.
The problem here, however, is that the electrochemical splitting (electrolysis) of water into hydrogen and oxygen requires not only electricity but also water that is as clean as possible, i.e. a resource that is rarely found in desert regions , even near the sea . Although electrolysis from salty seawater is possible in principle, it has only a low level of efficiency.
Greater efficiency in electrolysis from salty seawater
Scientists from Tianjin University have now presented an optimized electrolysis method in the journal Nature Energy that significantly increases the efficiency of extracting hydrogen from salty seawater.
In seawater electrolysis, the high proportion of chloride ions previously reduced the hydrogen yield because the chloride ions accumulated on the positively charged electrodes and thus prevented the accumulation of hydroxyl groups (OH) in the water. To stop this process, large amounts of potassium hydroxide could be added to the seawater. However, this significantly increased the effort and costs of hydrogen production.
Electrolysis with a proton exchange membrane (PEM)
The scientists working with Tao Ling therefore designed an electrolyser with a proton exchange membrane (PEM), which ensures that the efficiency remains high even with a high chloride content in the water. The system’s electrode consists of a nanostructured cobalt oxide with a thin layer of chromium oxide on the surface. The chromium oxide forms a Lewis acid to which electron pairs attach preferentially.
This ensures that within the electrolytic cell, hydroxyl groups reach the electrode more easily and frequently than the problematic chloride ions. The high salinity of the sea water only minimally influences the splitting process of the water molecules.
Laboratory prototype with high efficiency
A laboratory prototype of the optimized PEM electrolyser achieved an efficiency of up to 40 liters of hydrogen gas per hour. The experiment was carried out with normal seawater, which was only freed from coarse impurities with a filter.
This proves that on a laboratory scale, Lewis acid on the electrodes can be used to extract hydrogen from salty seawater with almost the same efficiency as from purified seawater and low-salinity drinking water. The researchers now want to scale their development to larger systems and increase long-term stability.
Nature Energy, doi: 10.1038/s41560-023-01195-x
