Erasing carbon’s footprint with sunshine

Hank Hogan, Contributing Editor, hank.hogan@photonics.com

The key to rolling back climate change could be the sun, according to George Washington University researchers who have demonstrated a carbon capture process that promises to use solar power to remove carbon dioxide from the air. As a bonus, the technique could generate a profitable product.

“Because the process simultaneously uses both solar thermal and solar visible, it captures and uses more solar energy than solar cells, and it removes and converts carbon dioxide into a useful form,” said Stuart Licht, a chemistry professor at the university’s Columbian College of Arts and Sciences in Ashburn, Va.

Licht was lead author on a Journal of Physical Chemistry Letters paper published online on Aug. 18, 2009, describing the process, which the researchers dubbed solar thermal electrochemical photo, or STEP, carbon capture. It has a carbon dioxide capture efficiency of up to 50 percent; i.e., as much as half of the energy in sunlight can be used to remove CO₂ from the air.

The end product is solid carbon or carbon monoxide. The latter could be used as feedstock for synthetic jet, kerosene and diesel fuels as well as for plastics and medicine. The revenue derived from selling such feedstock or the solid carbon could change the economics of combating rising CO₂ levels, making mitigation much more attractive.

In a demonstration of the new carbon capture technique, the investigators bubbled CO₂ through an electrolysis cell filled with molten lithium carbonate. They used concentrated solar thermal, or infrared, energy to heat the cell to a temperature above the 723 °C melting point of the material, forcing the temperature as high as 950 °C. They used photovoltaics to convert the visible part of sunlight into electricity that drove the electrochemical reaction.

Sunshine powers an electrolysis cell (right) that turns atmospheric CO₂ into solid carbon (left) or carbon monoxide. Courtesy of Stuart Licht, George Washington University. Reprinted from the Journal of Physical Chemistry Letters.

The high temperature decreased the energy required for that reaction. What’s more, the output of that reaction depended upon the temperature.

In their demonstration, the researchers plated out solid carbon on the cathode of the electrolysis cell at temperatures as low as 750 °C. At higher temperatures, the amount of solid carbon decreased and the production of gaseous carbon monoxide increased. At 950 °C, only the gas was produced.

Because the technique is highly immune to poisoning effects, it could be used to directly treat the waste stream coming out of a smokestack, something that Licht noted was not possible with other proposed carbon capture methods.

Calculations showed that it is feasible with STEP carbon capture to reduce CO₂ in the atmosphere to preindustrial levels within 10 years. There is enough annual lithium carbonate production to do this, although it would require a significant fraction of the world’s output. Roughly 700 km² of photovoltaics would be needed. How long it would take to remove the CO₂, Licht said, depends on the commitment to capturing carbon.

The technique also could use potassium carbonate, of which there is a much larger potential supply than there is of lithium carbonate. The downside is that using potassium would require higher electrolysis potentials and would therefore be not as attractive as the lithium alternative.

The STEP method also could reduce atmospheric CO₂ in other ways. Licht and his team showed in a Chemical Communications paper published online on Aug. 23, 2010, that a variation of the STEP technique can produce pure iron from two common ores, hematite and magnetite, without emitting carbon dioxide. This approach could significantly reduce the estimated 6.8 billion tons of CO₂ emitted each year by the iron industry.

Licht said that the group welcomes commercial or governmental inquiries about the STEP technique. He added that the method can be used for environmentally benign generation of fuels, a process that requires teaming it up with solar-energy-produced hydrogen.

Noting that the extraction of CO₂ from the air is the culmination of efforts stretching over 20 years, he said, “Until now, it has been a challenge to convert the stable molecule carbon dioxide into a useful product and remove it from the atmosphere. It is exciting to watch carbon dioxide be bubbled into the STEP process and be easily converted into solid carbon.