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Researchers from the University of Adelaide and their international partners have succeeded in using raw seawater to produce green hydrogen. For this they introduced an acid layer on the catalysts in situ.
Researchers from the University of Adelaide have led an international team that has discovered a method that allows water directly from the ocean to be used in a commercial electrolyser to efficiently produce green hydrogen.
“We use seawater as a raw material without the need for any pre-treatment processes, such as reverse osmosis desolation, purification or alkalinisation,” explains Associate Professor Yao Zheng, from the University of Adelaides School of Chemical Engineering.
The team limited itself to filtering seawater, coming from the Huanghai Sea (China), to remove solids and microorganisms.
“The performance of a commercial electrolyzer with our catalysts in seawater is similar to that of platinum/iridium catalysts in highly purified deionized water,” Zheng says.
The discovery responds to concerns about water scarcity that have been present in the debates about green hydrogen. Researchers have described the ocean as a “nearly infinite resource”, accounting for 96.5% of Earths water reserves, but this has proven difficult due to the complexities of the water profile. The teams solution boils down to adjusting the local reaction environment of the catalyst.
The team is made up of researchers from the Chinese universities of Tianjin and Nankai and from the American Kent State. They published their work this week in Nature Energy, after noting that the method was more practical in regions with long coastlines and abundant sunlight, such as Australia.
The team will now work on expanding the system using a larger electrolyser so that it can be used in commercial processes such as hydrogen generation for fuel cells and ammonia synthesis.
salt in the wound
Given the vastness of the ocean, it is considered a natural electrolyte. As the researchers point out, "direct electrolysis of seawater without a purification process or chemical additives is very attractive and has been researched for about 40 years."
The problem: Using seawater for electrolysis tends to cause reactions and corrosion on the electrodes, reducing the efficiency and stability of the electrolysis system.
This is because seawater has high concentrations of harmful chlorine ions, as well as positively charged unwanted ions such as magnesium and calcium ions. Since the pH value of seawater near the cathode increases remarkably during electrolysis, these magnesium and calcium ions can form massive precipitates such as magnesium hydroxide, becoming insoluble solids that can block the electrode.
To solve these problems, the team introduced a "hard Lewis acid shell" on the catalysts surface to split the water molecules and capture many of the negatively charged ions that surround the catalyst. They also found that their method created a strongly alkaline environment, pH 14, which inhibited chlorine production in the catalyst, reducing the formation of these electrode-blocking solids.
The team introduced this acid layer to a number of common catalysts to manipulate the local reaction microenvironment, they said, – noting that this was a “general strategy that can be applied to different catalysts without the need for specifically designed catalysts and electrolyser design”.
“With this local alkaline environment generated in situ on a series of Cr2O3-modified catalysts, a substantial improvement in activity was achieved while avoiding harmful chlorine chemistry and precipitate formation,” they noted.
“The seawater flow electrolyzer with Cr2O3 modified catalysts offered good stability up to 100 h at 500 mA cm-2 and had an industrial current density of 1.0 A cm-2 at 1.87 V and 60 °C ”, the team concluded. |
