In step one, industrial fans draw in air and blow it through a mix of water and other compounds, which McGinnis says could include sodium carbonate. That then readily reacts with carbon dioxide molecules in the air, shifting much of the carbonate to bicarbonate.
The resulting solution then moves into a battery-like cell with a membrane in the middle and electrodes on either end, which uses electricity to spark a series of chemical reactions that produce complex alcohols. It’s equipped with a catalyst based on technology licensed from Oak Ridge National Laboratory. In an earlier description, that lab said it had developed a catalyst made from tiny carbon spikes embedded with copper nanoparticles. When a voltage was applied, it converted carbon dioxide dissolved in water into ethanol “with a yield of 63%.”
Prometheus’s carbon nanotube membranes come into play in step three, separating the alcohols from the water.
And in a final step, different catalysts are used to combine the alcohols and convert them into synthetic gasoline, diesel, or jet fuel. In 2020, Prometheus licensed separate technology from the Oak Ridge lab that can be used to produce jet fuel from ethanol, through a multistep process that relies on a novel though unspecified catalyst.
The overall process is substantially different from the one other companies converting captured carbon into fuels are taking. As McGinnis explained in a Joule commentary, the Prometheus systems can operate under standard atmospheric pressure and at room temperature. The technology also avoids the heat energy needed to produce concentrated carbon dioxide as well as the capital costs of an electrolyzer dedicated to producing hydrogen. Instead, the company claims, it can synthesize alcohols straight from carbon dioxide dissolved in water, and then convert those into standard fuels.
If they’ve indeed figured out how to do this, it “could lead to significant energy and cost savings,” says Evan David Sherwin, a postdoctoral researcher at Stanford who produced the Environmental Science & Technology study.
One of the last slides in the investor materials shows a Prometheus-branded fuel station, with a red neon “Zero Net Carbon” sign advertising gas prices of $3.50 a gallon and diesel at $3.75, well below current average US prices.
‘Out over their skis’
McGinnis later pushed back his target date for delivering fuels, saying in that March 2020 Joule piece: “We project that by putting all of these advances together, it will be possible to offer renewable gasoline from [direct air capture] CO2-to-fuels within the next two years that is price competitive with fossil gasoline.”