One good method of sucking out carbon from atmospheric carbon dioxide (which is necessary to avoid further global warming), apart from growing trees, is ‘direct air capture’, which is being tried in some places.
In this method, air passes through chemicals that absorb carbon dioxide. But the challenge arises when you want to separate carbon dioxide from these chemicals —so that the chemicals can be reused — as that would require intense heat of nearly 800 degrees C. This rubs the economics of the process the wrong way.
Now, Prof Bryan McCloskey of Lawrence Berkeley National Laboratory, California, has come up with a potentially cheaper approach. His method uses electrochemistry to capture carbon dioxide.
Electrochemistry broadly involves atoms giving or receiving electrons; this science is the basis of all batteries and fuel cells. Prof McCloskey’s process gets carbon dioxide to react with hydroxide ions to form bicarbonates. It then uses electrochemical methods to separate carbon dioxide and the hydroxide ions, so that the gas can be put away and the hydroxide reused.
As Prof McCloskey explains the process, you bubble air through an absorber containing a solution of sodium hydroxide. This will result in the formation of sodium bicarbonate. The bicarbonate is fed into a special electrochemical cell, where the reaction regenerates sodium hydroxide.
In the electrochemical cell, two reactions occur at each of its electrodes. At one electrode, bicarbonate is oxidised to form a pressurised stream of carbon dioxide, which can be sequestered. At the other electrode, hydrogen gas is generated, which consumes protons to regenerate the alkaline solution. “The hydrogen production is certainly a bonus of our alkaline regeneration scheme,” says McCloskey. Thus, the process produces a stream of concentrated carbon dioxide and another stream of hydrogen.
McCloskey reckons it would be possible to capture carbon dioxide for $100 a tonne, compared with other methods that cost six times as much.
He cautions that while the science is decided, the systems will have to be engineered for perfection. He points out that the whole system involves innovations at three points.
The first is the electrochemical cell’s stability. The electrodes need to be robust. The cell should also be energy-efficient.
The second area of innovation is the membrane that separates the two electrodes from each other. Otherwise, the hydrogen and carbon dioxide would mix together; they’re more valuable as pure streams, says McCloskey. In the prototype, the researcher has used a special membrane, called Nafion, which is often employed in fuel cells but is expensive. Research is on to develop a cost-effective membrane.
The third innovation is around the development of a suitable catalyst for the bicarbonate-to-carbon dioxide reaction. The catalyst would enhance the reaction.
McCloskey is “very confident” that these aspects will be fixed in the course of time, not in the least because of the expertise available at Berkeley Lab. "We have experts in all these different areas, such as membrane technology, molecular simulation and modelling, and electrocatalysis,” notes McCloskey.