The first step is acquiring a carbon source. One possibility is to remove carbon dioxide directly from the atmosphere, by passing air through systems that use either filters or chemicals to extract the CO2. That has the benefit of being truly carbon-neutral, in that no more carbon dioxide is released into the atmosphere than has already been removed from it. Unfortunately, because CO2 constitutes a mere 0.04 percent of all atmospheric gases, removing and concentrating it on an industrial scale is an immense challenge. “The problem with pulling it out of air is, it’s hard to be efficient, because it’s very dilute,”

The irony of that paragraph

Hydrogen and regular gas fuel cells would fall into that reason as well.