A new type of electrode strips 80 to 90 percent of the dissolved carbon out of seawater and locks it away as limestone. It was developed by a team at the Korea Advanced Institute of Science and Technology (KAIST) together with a group at the Massachusetts Institute of Technology. The work appeared in the journal "Advanced Energy Materials".
The ocean is the planet's largest carbon store. It absorbs around 30 percent of the carbon dioxide released by human activity. Take dissolved carbon out of the water, and it can take up fresh gas from the air. That is the aim of the process the researchers call electrochemical dissolved ocean carbon removal, or e-DOC. An electric current shifts the pH of the seawater towards the alkaline. The dissolved carbon then precipitates as calcium carbonate. That mineral does not return to the atmosphere.
Until now such systems have been defeated by their own product. Freshly formed scale settled on the electrodes and blocked the reaction. The equipment had to be cleaned or replaced often. That cost energy and money.
Hydrogen bubbles as a brush
The team's answer is a hollow fibre. Its outer shell is made of porous stainless steel and serves as the working electrode. Seawater flows past platinum and iridium wires inside it. The hydrogen produced there bubbles outwards through the porous shell. Those bubbles act like a brush and keep the surface clear. The scale therefore forms beside the electrode rather than on it. Every two to three hours the researchers also raise the water pressure for a tenth of a second. The pulse flushes deposits away.
The system was tested first in the laboratory with artificial seawater, then off the Korean island of Jeju. There it ran for more than 120 hours without a marked drop in performance, according to the group. In a 100-hour run the fibre treated 40 litres of seawater at a current density of around 20 milliamps per square centimetre. Electricity consumption was up to 54 percent below that of comparable methods. Pure hydrogen and magnesium hydroxide, an industrial raw material, are produced as by-products.
"Electrochemical direct ocean capture opens a viable route to decarbonisation at the gigatonne scale," write lead author Inhwan Park and his colleagues. Dong-Yeun Koh of KAIST, who led the study, expects the technique to speed up the commercialisation of marine carbon removal.
The gap between the laboratory and the gigatonne remains wide. Forty litres of treated seawater and 120 operating hours are a proof of concept, not a plant. The fibres can be bundled and installed on ships or offshore platforms, KAIST says. How the resulting mineral would be used or stored at scale is not addressed by the study.