Creating and collecting and concentrating CO2 is a much more involved processes then merely separating nitrogen off from its oxygen component! It's just air! CO2 is measured in parts per million. Besides,. that defeats the whole point: if I give you pure CO2 to breathe, you'd be in distress, suffering inhaling a toxic compound. You are breathing 78% nitrogen right now. Raise it to 80% and you will likely black out and die while still feeling like you never stopped breathing regular air.
Why do you care? Got someone in mind you'd like to gas?