How do astronauts get a supply of fresh air?

The astronauts on a space mission usually have just the air they take with them (there have been some experiments in storing liquid gases to mix). The carbon dioxide is removed from the air, either by chemicals such as lithium hydroxide, or by using hydrogen that is electrolyzed from fuel cell water to recapture oxygen (Sabatier reaction). Oxygen is added from the fuel cell water and reserve tanks. All of this means that a long-term mission is limited by the supplies of hydrogen and oxygen and water they bring along, or are delivered by resupply.Space vehicles can harness substantial energy from small reactors, fuel cells, and solar panels, but they cannot obtain any air or water in space.