Seawater could be a source of clean hydrogen fuel
Tamara Kulikova / Alamy
For the first time, electrodes that can make hydrogen from seawater without generating corrosive and toxic chlorine gas will be produced at commercial scales.
“Traditional electrolysis has only been possible with pure water, an increasingly scarce global resource,” Douglas Wicks at the US Department of Energy’s Advanced Research Projects Agency–Energy (ARPA-E) said in a press release. “[These electrodes] eliminate the process’s dependence on pure water and it taps into the world’s most abundant water resource instead: the ocean.”
The process uses a negatively-charged cathode and a positively-charged anode to split seawater into four “streams” – useful oxygen and hydrogen, and harmless acidic and alkaline streams that can be easily recycled back into the ocean. Equatic, the California-based startup that designed the technology with support from ARPA-E, plans to sell the hydrogen and oxygen created in the process to offset their costs. The alkaline stream reacts with CO2 in the atmosphere to form stable minerals that can be poured back into the sea, while the acidic stream can be returned to the ocean once it is restored to a higher pH after flowing over silica-rich rocks.
Like standard techniques that split water to produce hydrogen, this process takes place in an electrolyser, a machine that uses stacks of electrodes to separate water molecules with electricity. But existing devices have trouble working with seawater because it destroys them: it is full of dissolved salt, other minerals, metals and microorganisms that degrade components and gum up the works. Also, the electrical charge that attracts oxygen to the anode separates the salt in seawater, generating toxic chlorine gas that rapidly corrodes the machine.
To avoid this problem, Chen and his colleagues designed an anode that can selectively split oxygen from the water molecules without splitting the salt. They used a chlorine-blocking layer to allow water to flow through the catalyst while stopping the salt. Based on laboratory tests, Chen says they expect the anodes will work for at least three years before they need to be removed and recoated.
Pau Farras at the University of Galway in Ireland, who is not involved with the company, says three years would be a strong performance, and these oxygen-selective anodes are a promising approach to using seawater to make hydrogen fuel. But he says they haven’t yet shown they can work in the wild. “What we need to do is see the real performance in a real environment,” he says.
The company will now begin producing anodes at a factory in California capable of making 4000 of them a year. They will be used in a demonstration plant being built in Singapore, which the company says will be able to remove 10 tonnes of CO2 and produce 300 kilograms of hydrogen per day.
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