Can we separate oxygen from carbon dioxide?
Alternatively, you could heat CO2 over a catalyst of iron doped zeolite and hydrogen to produce water and ethylene. A nonthermal plasma applied to ethylene will generate carbon soot and recover the hydrogen. Electrolysis of water gives back the extra hydrogen and produces oxygen. (Hey! I didn't say it was efficient.) It might be useful to someone on Mars who has endless power in the form of a nuclear reactor and plenty of CO2 but not so much oxygen.