A good method of sucking carbon from atmospheric carbon dioxide (which is necessary to avoid further global warming), apart from tree growth, is “direct air capture”, which is being tested in some places .
In this method, the air passes through chemicals that absorb carbon dioxide. But the challenge arises when you want to separate the carbon dioxide from these chemicals – so that the chemicals can be reused – because that would require intense heat of almost 800 degrees C. This rubs the economy of the process in the wrong sense.
Now Professor Bryan McCloskey of Lawrence Berkeley National Laboratory in California has come up with a potentially cheaper approach. His method uses electrochemistry to capture carbon dioxide.
Electrochemistry generally involves atoms donating or receiving electrons; this science is the basis of all batteries and fuel cells. Professor McCloskey’s process reacts carbon dioxide with hydroxide ions to form bicarbonates. It then uses electrochemical methods to separate the carbon dioxide and hydroxide ions, so the gas can be stored and the hydroxide reused.
As Professor McCloskey explains the process, you bubble air through an absorber containing a solution of sodium hydroxide. This will cause the formation of sodium bicarbonate. Bicarbonate is introduced into a special electrochemical cell, where the reaction regenerates sodium hydroxide.
In the electrochemical cell, two reactions occur at each of its electrodes. At an electrode, the bicarbonate is oxidized to form a pressurized stream of carbon dioxide, which can be sequestered. At the other electrode, hydrogen gas is generated, which consumes protons to regenerate the alkaline solution. “Hydrogen production is definitely a bonus of our alkaline regeneration system,” McCloskey says. Thus, the process produces a stream of concentrated carbon dioxide and another stream of hydrogen.
McCloskey estimates it would be possible to capture carbon dioxide for $100 a ton, compared to other methods that cost six times as much.
He warns that while the science is decided, systems will have to be designed for perfection. He points out that the whole system involves innovations in three points.
The first is the stability of the electrochemical cell. The electrodes must be sturdy. The cell must also be energy efficient.
The second area of innovation is the membrane that separates the two electrodes from each other. Otherwise, hydrogen and carbon dioxide would mix; they are more valuable as pure flows, says McCloskey. In the prototype, the researcher used a special membrane, called Nafion, often used in fuel cells but expensive. Research is ongoing to develop a cost effective membrane.
The third innovation concerns the development of a suitable catalyst for the bicarbonate-carbon dioxide reaction. The catalyst would enhance the reaction.
McCloskey is “very confident” that these aspects will be corrected over time, in particular thanks to the expertise available at the Berkeley Lab. “We have experts in all of these different areas, such as membrane technology, molecular simulation and modeling, and electrocatalysis,” notes McCloskey.
May 08, 2022