Use carbon dioxide as a method for capital punishment

I am against capital punishment's in general.
But here is the idea, instead of keeping people in death row for years for some reason, and then get the convict go through all the possible pain and fear. A carbon dioxide should be used when inmate sleeps, this way the death would be painless and also could happen without exposing inmate to all the fear related to procedure and so on. I think this could be the most humanitarian way of capital punishement.
Oh, its also possibly super cheap :)