Hydrogen straight from sea water
New electrolyser splits seawater without complex pre-treatment or desalination
The next step towards hydrogen production on the high seas: researchers have developed an electrolysis system that also works efficiently with seawater – without the saltwater having to be desalinated and pretreated beforehand. This is made possible by a specially coated catalyst that protects the electrodes from corrosion and deposits. It intercepts the reactive ions formed during electrolysis before they can cause any damage, as the scientists report in “Nature Energy”.
The production of green hydrogen on the high seas or on the sea coast would have several advantages: offshore wind farms can supply climate-friendly electricity for electrolysis. For these, the hydrogen in turn serves as chemical energy storage, which can be used to buffer excess electricity. In addition, the most important raw material for hydrogen production is abundantly available on and in the sea – one would think so.
corrosion and deposits
Unfortunately, it is not quite that simple: Common electrolysis systems do not tolerate seawater. The dissolved salts in seawater are released during water splitting and react with the material of the electrodes. The result is rapid corrosion and blocking deposits, rendering the electrolysers unusable. So far, seawater has had to be desalinated first. Alternatively, strong acids or bases are used to bind the released ions.
In addition: “The activity of catalysts for oxygen production at the anode and hydrogen production at the cathode is very low in neutral seawater,” explain Jiaxin Guo from Tianjin University in China and his colleagues. “Therefore, a much higher voltage is required. However, this triggers even more harmful corrosion and oxidation by chloride ions.”
Combination of chromium oxide and cobalt oxide as a catalyst
As a solution to this problem, Guo and his colleagues have now developed a new type of catalyst that can work efficiently even with seawater and at the same time protect against corrosion and deposits. The catalyst consists of thin carbon threads with a coating of cobalt oxide (CoO x ). This material is a good catalyst for water splitting, but quickly loses its function in seawater.
To prevent this, the scientists additionally coated these catalyst filaments with an extremely thin layer of chromium(III) oxide (Cr 2 O 3 ). “This compound is electrochemically stable over the entire working range of seawater electrolysis,” explain Guo and his team. More importantly, chromium(III) oxide is a strong Lewis acid – it can bind electrons and negatively charged ions, keeping them away from the electrodes.
Just as powerful as a pure water electrolyser
In initial practical tests, the researchers compared the performance and durability of their electrolysis system with a standard pure water system that used platinum and ruthenium dioxide as a catalyst. For seawater splitting, they constructed a flow-through electrolyzer in which a titanium mesh served as the electrodes, Cr 2 O 3 – CoO x as the catalyst, and a semipermeable Nafion membrane as the separator. In the test, 60 milliliters of water per minute flowed through both electrolyzers – seawater in one case and distilled water in the other.
The result: The seawater electrolyser worked just as efficiently and stably as the conventional pure water system. He achieved Faraday efficiencies of 93 and 02 percent for hydrogen and oxygen. “This indicates that almost no chloride and hydroxide by-products were formed,” report Guo and his colleagues. This is one of the reasons why the system was still working stably and without significant loss of performance even after more than 100 hours at 500 milliamps per centimeter.
Promising approach
“We have used this to split normal seawater into oxygen and hydrogen with almost 100 percent efficiency – without the need for pre-treatment of the seawater,” says co-author Shizhang Qiao from the University of Adelaide. With the untreated seawater at 1.87 volts and 60 degrees, the electrolyser achieved a power density of one ampere per square centimeter – this corresponds to industrial standards, according to the researchers. In addition, the catalyst does not consist of expensive platinum metals, but of a non-precious, comparatively inexpensive material.
According to the scientists, this technology opens up new possibilities for generating hydrogen directly and comparatively cheaply from seawater – for example on offshore plants. Guo and his colleagues are already constructing their system on a larger scale so that it can be integrated into commercial applications. (Nature Energy, 2023; doi: 10.1038/s41560-023-01195-x )
Source: University of Adelaide
