Personal anecdote: I do freediving, so CO2 tolerance training is common, and I've done it on and off. Basically you do breath holds to train your body to get used to high leves of CO2.

I've found that brief high C02 levels are very good for activation, and to get out of a lethargic state. I don't know if a mix of cortisol and vasoconstriction and dive reflex triggered by the CO2, but I feel like it's a tool I have on my toolbox whenever I need to so something difficult or that requires a lot of will power.

It's not for everyone tho, because many people can't get past the initial urge to breathe, and would probably freak out with the first involuntary contraction.