I've actually looked into this in the past (synthesizing, for example, methane from the carbon dioxide in the atmosphere) but I haven't found anything that seemed achieveable from a DIY perspective. Do you know anywhere that I can get more information? It seems like this might be a good, carbon-neutral way to solve the problem of wind/solar fluctuation and (probably) would not require massive infrastructure changes (since you'd be synthesizing the stuff that cars and trucks are already designed to run on).