NOVELTY - An electrolyzer (100) comprises pressure vessel (115) having shell and opposite closed ends; electrolysis cell stack (101) inside pressure vessel, having bipolar electrolysis cells stacked between first terminal end plate (107a) and second terminal end plate (108a); fluid connections for feeding electrolyte to cell stack and for collecting product(s) of the electrolysis from cell stack, and electric connections having anode and cathode connection. The stationary head is equipped with fluid connections (122) and electric anode and cathode connections (120, 121) with the cell stack.    USE - Electrolyzer used for production of hydrogen from electrolysis of water.    ADVANTAGE - The electrolyzer is safe and reliable to carry out process of electrolysis at pressure much greater than in the prior art, allows to make available gas product at a pressure above 30 bar which are substantially the limit of the prior art; produces hydrogen from electrolysis of water, directly at a pressure of hundreds of bar, as required for hydrogen-fueled vehicles; and starts up electrolysis process at low pressure, up to the final operating pressure, gas pressurization of the electrolyzer pressure vessel grows proportionally, following the building up of the electrolysis process pressure.    DETAILED DESCRIPTION - An electrolyzer (100) comprises pressure vessel (115) having shell and opposite closed ends; electrolysis cell stack (101) inside pressure vessel, having bipolar electrolysis cells stacked between first terminal end plate (107a) and second terminal end plate (108a); fluid connections for feeding electrolyte to cell stack and for collecting product(s) of the electrolysis from cell stack, and electric connections having anode and cathode connection. The first terminal end plate of cell stack is integral with one of closed ends of the pressure vessel, and forming stationary head (107) of the cell stack; stationary head equipped with fluid connections (122) and electric anode and cathode connections (120, 121) with the cell stack; second terminal end plate (108a) of the cell stack inside the pressure vessel (115) and free to move in longitudinal direction relative to first terminal end plate and to the vessel, in response to thermal expansion or contraction, and forming a floating head (108) of the cell stack. An INDEPENDENT CLAIM is included for equipment for electrolysis.    DESCRIPTION OF DRAWING(S) - The drawing shows a cross-sectional view of the electrolyzer.    Electrolyzer (100)    Electrolysis cell stack (101)    Stationary head (107)    First terminal end plate (107a)    Floating head (108)    Second terminal end plate (108a)    Pressure vessel (115)    Cathode connections (120, 121)    Fluid connections (122)