Imagine a small device that can be placed in the mouth and converts CO2 into oxygen, using batteries. This brief analysis looks at how realistic this is.
A human consumes 550 liters of oxygen per day (https://health.howstuffworks.com/human-body/systems/respiratory/question98.htm), and the CO2 produced can be converted back to O2 easily with potassium superoxide. But it’s not practical, because potassium superoxide is explosive when in contact with water.
550 liters is 24.5 mols per day, or 1 mole per hour. According to this paper (https://bmcplantbiol.biomedcentral.com/articles/10.1186/1471-2229-14-111), photosynthesis occurs at 20 micromole per meter squared per second, and per hour that’s 3600*20 = 72000 umol = 72 mmol = 0.072 mol. So 14 m2 would be needed to supply one person.
The efficiency of photosynthesis is up to 26% (https://www.britannica.com/science/photosynthesis/Energy-efficiency-of-photosynthesis), so the 14m2 would have to receive sizeable energy in the form of light.
Perhaps the potassium superoxide method is most realistic after all. But keeping plants in a nuclear sub could work well, too.
This is already well answered here: https://www.quora.com/How-much-oxygen-does-a-typical-household-plant-produce?share=1