Researchers tap into world's largest sink of CO2 for carbon capture

The process is complex and costly as the membranes needed for the process are expensive and also need chemicals to drive the reactions at the electrodes. A research team led by Alan Hatton, a professor of chemical engineering at MIT, worked to simplify the process and make it cost-effective.

Their newly developed method does not use membranes and instead uses a reversible reaction at the electrodes to capture carbon. Reactive electrodes release protons into seawater, thereby acidifying seawater and converting dissolved bicarbonates into molecular carbon dioxide, which is then captured under a vacuum.

The water is then fed to a second set of cells where the voltage is reversed, and protons are recovered to turn the water alkaline before it is released back into the sea. The press release said that when one set of electrodes is depleted of protons, the roles of the two cells can be reversed.

MIT researchers are looking at the reinjection of alkaline water as a localized measure to reduce the acidification of oceans. This is expected to help to address the threat to coral reefs and shellfish and could be done offshore to ensure that alkalinity spikes do not disrupt ecosystems.

To reduce the capital expenditure associated with setting up such facilities, the researchers suggest co-locating the technology with existing infrastructure such as desalination plants, offshore drilling platforms, and aquaculture farms.

Carbon, thus removed from the oceans, could be converted into compounds like ethanol or specialty chemicals. Alternatively, it could be buried in geological formation under the sea floor, the researchers added.

The research findings were reported in the journal Energy and Environmental Science.

Abstract

In recent years, the ocean has come to be recognized as a global-scale reservoir for atmospheric CO2. The removal of CO2 from ocean water is thus considered a compelling approach to reduce ambient CO2 concentrations, and potentially achieve net-negative emissions. As an effective means of oceanic CO2 capture, we report an asymmetric electrochemical system employing bismuth and silver electrodes that can capture and release chloride ions by Faradaic reactions upon application of appropriate cell voltages. The difference in reaction stoichiometry between the two electrodes enables an electrochemical system architecture for a chloride-mediated electrochemical pH swing, which can be leveraged for effective removal of CO2 from ocean water without costly bipolar membranes. With two silver-bismuth systems operating in tandem in a cyclic process, one acidifying the ocean water, and the other regenerating the electrodes through alkalization of the treated stream, CO2 can be continuously removed from simulated oceanwater with a relatively low energy consumption of 122 kJ/mol, and high electron efficiency.