The electrolyser is a station facility that splits Water into Hydrogen and Oxygen. Every run makes both gases. Only a refinery works can hold one.
It is one step of the hydrogen fuel chain: a Smelter melts ice into water, the electrolyser splits it, and a Refinery combines the hydrogen and oxygen into Fuel. The refinery takes the two gases in the same proportion the electrolyser makes them. A smelter also uses oxygen for steel plate and ceramic.