An electrolyser (100) comprising an electrolysis cell stack (101) inside a pressure vessel (115), wherein the first terminal end plate (107a) of the cell stack is integral with one a closed ends of the pressure vessel, thus forming a stationary head (107) of the cell stack equipped with the fluid and electric connections, and the second terminal end plate (108a) of the cell stack is inside the vessel and is free to move in a longitudinal direction in response to thermal expansion or contraction, thus forming a floating head (108) of the stack. The pressure vessel (115) is preferably pressurized using a gaseous product obtained in the process of electrolysis.