I am surprised no one has mentioned replacing the adsorption CO2 scrubbers on space craft[1] with something that is powered by electricity. The article claims 130 grams of CO2 removed from the air per kilowatt-hour. Astronauts might expel 40 - 50 grams of CO2 per hour into the air, so 500wHrs of power keeps the air breathable forever? That is a good deal. For reference the ISS has a crew of 6 and has 84 - 120kW of power capacity [2].
[1] Study problem on CO2 removal from NASA -- https://www.nasa.gov/pdf/519338main_AP_ED_Chem_CO2Removal_St...
[2] https://www.nasa.gov/mission_pages/station/structure/element...