I agree with Sean Condon that the machines produce oxygen. Passing electricity through water splits it into oxygen and hydrogen. Pretty simple stuff. The process is called electrolysis.
So one simple way Zion could be set up is to use an underground river to provide both fresh drinking water, and a source of hydroelectric power. The electricity could be used to split some of the water into hydrogen and oxygen. The hydrogen could be ejected to the Earth's surface, along with excess carbon dioxide.
I suppose hydrogen could be burned as a fuel. But that depletes oxygen, which is needed for breathing, so I'd vote against that. They already have their hydroelectric power source.