Re: The CO2 problem interests me
Here's a starter description of the ISS's life support system:
Here's the NASA overview of the ISS life support:
Basically, there's multiple systems at work. The US has contributed a fancy CO2 scrubber and air monitor; both the US and Russia have contributed water-cracking systems prone to failure; and there are back-up oxygen candles and chemical CO2 scrubbers.
As for the purpose of bringing up pure oxygen and air instead of just oxygen, that's a side effect of the life support and leakage on the station. The station is filled with an oxygen-nitrogen atmosphere like on Earth. The main life support systems attempt to recycle oxygen, but there are inevitable inefficiencies because not all astronaut-consumed oxygen is exhaled as CO2 and water. Some oxygen also ends up in the urine (which is recycled) after the body gets done using combining oxygen with food throughout the body, but some oxygen gets bound up elsewhere: feces, flesh, hair, nails, etc. that aren't recycled. So, because of those oxygen recycling losses, the composition of the ISS's atmosphere would tend to get nitrogen-rich without adding some pure oxygen to make up the losses. Hence, the delivery of pure oxygen.
But there are also nitrogen losses through leaks and airlock operation. The life support system could keep pressure at its set point, but this would result in the atmosphere getting oxygen-rich. Or the life support system could keep the oxygen percentage steady, but this would result in a pressure drop. Or you could bring up some canned air to make up the nitrogen losses.
You could also use pure nitrogen, but since you're going to also need to replace lost oxygen, you might as well bring up canned air.