Why is a tiny startup Prometheus able to succeed where Google has failed?
Prometheus is not the first startup to take on the problem, but they seem to be the first to to make fuel-from-air cost effective.
They take water and carbon, and using a chemical reaction convert this mixture to alcohol. Then, and this is their secret sauce, instead of using a traditional thermal process which requires a lot of energy, Prometheus is converting this alcohol at room temperature, using electricity and a carbon nanotube membrane, yielding fuel at a much cheaper cost.
Note: This technical explanation to why Prometheus is able to do what Google has not comes from the Prometheus CEO's recent post on Hacker News