• Amir Muhaisen

Carbon Capture : Artifial Leaf

in this article, Amir illustrates the artificial leaf process compared to conventional atmospheric direct air capture to fight climate change.

While humanity has many options for reducing emissions none but carbon dioxide removal enables significant reduction in the overall climate risk because risks grow with the accumulation of atmospheric carbon dioxide, which cannot be reduced without carbon-negative technologies such direct air capture.

Direct Air Capture

The removal of carbon dioxide directly from the ambient air is called direct air capture (DAC) as opposed to carbon capture and storage (CCS) which captures carbon dioxide from industrial point sources such as power plant flue gas line using common technologies including ethanolamines absorption or temperature swing adsorption.

The concept of direct air capture is based on capturing atmospheric carbon dioxide through contacting with dilute caustic or caustic potash thus converting carbon dioxide into aqueous carbonate anions (blue). Furthermore, carbonate anions are precipitated by reaction with calcium cations forming insoluble calcium carbonates while calcium cations are replenished by dissolution of calcium hydroxide (red).

Finally, calcium carbonates are regenerated to liberate carbon dioxide -preferably inside isolated greenhouses- producing calcium oxide which is hydrated to provide calcium hydroxide (black).

Artificial Leaf Process

Recently, engineers from the University of Illinois Chicago have built an electrolytic cell that can capture carbon dioxide at higher rates compared to conventional direct air capture as this is the developed form of direct air capture, the lab-scale prototype is called the artificial leaf.

The electrolytic cell is composed of dissolved caustic in ethylene glycol, in which atmospheric carbon dioxide is captured upon contacting with the organic solvent forming bicarbonate anions that is pulled through anion exchange membrane toward the aqueous side to further convert bicarbonates into released carbon dioxide to be utilized for industrial operations. hydroxide ions are regenerated in the organic side using porous cathode that pulls hydroxide ions from an adjacent humidified inert nitrogen stream thus producing hydrogen byproduct at the cathode besides oxygen produced at the anode.

The electrolytic cell is used to speed up the transfer of bicarbonate anions through membrane from cathode to anode, thus higher voltage source -preferably photovoltaics- means higher capture rate.

Utilization Challenge

Despite that direct air capture (DAC) is simple technology and artificial leaf process is promising regarding the potential for scaling up. The challenge is not in capturing atmospheric carbon dioxide, it is in utilizing the captured into higher value products. Similarly, captured carbon dioxide from industrial point sources (CCS) is being utilized in renewable methanol and in upstream enhanced oil recovery.

Chemiprobe started “probing” for technical insights and continuously networking with professionals in carbon capture. Therefore, atmospheric capture requires higher operational expenses compared to industrial sources capture due to low atmospheric concentrations (400 ppm) compared to industrial point sources.