SCIENTISTS have developed a system that can produce green hydrogen directly from seawater without the need for any pre-treatment processes like desalination. The team behind the development, which involves the introduction of a Lewis acid layer on a transition metal oxide catalyst, say the method shows high potential for commercial application.
Over 97% of the water on Earth’s surface is saline water in the oceans, 2% is stored as fresh water in ice caps, glaciers and snow-capped mountain ranges, and just 1% is available for our daily water supply needs.
Saline water can be made into potable water through a process called desalination, a technique that some areas around the world rely on to produce fresh water for human consumption and for domestic and industrial use. But desalination is an energy-demanding process, and worse still it is often powered by energy sources which are unsustainable.
Splitting water into its constituent parts is also well understood. The process – known as electrolysis – uses a direct current between two electrodes immersed in an electrolyte to split water into hydrogen and oxygen. Hydrogen is formed at the cathode, or negative electrode, and oxygen at the positive electrode, or anode.
Because a mix of the gases can explode, most electrolysers separate the anode and cathode with a thick, porous plastic sheet, and metal catalysts such as nickel and iron are used to speed up reactions.
Putting both of these processes together, namely desalinating seawater, and then splitting it to create hydrogen has long been hailed as one of the best solutions to provide clean and affordable fuel for energy, that in turn could power everything from a city’s electricity, to making steel, producing fertiliser, and even as fuel for airplanes – the list of potential uses is a long one.
However, one of the reasons we’re not already using hydrogen fuel to fly around the world, is that saltwater and other impurities corrode electrodes, shortening their life. As those components are typically made of rare metals such as platinum, it costs too much to keep replacing them. Chloride ions in seawater are also a problem and chlorine electro-oxidation reactions (ClOR) compete with oxygen evolution reaction (OER) on the anode during electrolysis. This reaction results in the release of toxic and corrosive chlorine species such as hypochlorite. Hypochlorite is relatively unstable, it can release toxic chlorine gas when mixed with ammonia or acid and it can also eat away at stainless steel.
To get around this, the seawater could be desalinated and purified before processing it, but this is not always financially viable either. Another option is to coat the electrodes with polyanions to suppress corrosion, but this too can be costly.
Lewis acid layer on a metal oxide catalyst
Scientists working on the problem have now found another solution using cheaper materials. Instead of using catalysts made of rare precious metals to dynamically split water molecules and capture hydroxyl anions, the team introduced a Lewis acid layer (chromium oxide), on a transition metal oxide catalyst, which promotes water splitting to H and OH. “This is a general strategy that can be applied to different catalysts without the need for specifically engineered catalysts and electrolyser design,” write the authors in their research paper.
Captured hydroxyl anions can then be oxidised into oxygen molecules with anodic potential, and as the process produces large amounts of OH−, it reduces the amount of Cl− that is formed. “These results demonstrate that the harmful Cl− chemistry in direct seawater electrolysis can indeed be avoided by preferentially enriching OH− on the electrode surface,” the authors write.
Additionally, the team only filtered the seawater to remove solids and microorganisms, but did not purify it beforehand, which is a step that is normally needed when using conventional electrolysers.
“The performance of a commercial electrolyser with our catalysts running in seawater is close to the performance of platinum/iridium catalysts running in a feedstock of highly purified deionised water,” explains co-author Yao Zheng at the University of Adelaide, adding that the experiment is nearly 100% efficient when producing green hydrogen from splitting seawater. As such is could help negate the need to use freshwater – a commodity that is already scarce.
The team hopes to scale up their project for commercial production in ammonia synthesis, and to generate hydrogen fuel cells.
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