NOVELTY - The electrolyser comprises a stack (1) of a first and a second electrolytic cells. Each electrolytic cell (10) comprises an anode, a cathode, and an ion exchange membrane positioned between the anode and the cathode. The ion exchange membrane of the first and second electrolytic cells is separated by a bipolar electrode (4) constituting the anode of the first electrolytic cell on the one hand and the cathode of the second electrolytic cell on the other hand. The bipolar electrode comprises a bipolar plate, which is associated with a grid and a porous plate.    USE - The electrolyser is useful in an electrolytic ensemble for production of chemical substance such as dihydrogen, dioxygen, chlorine, hypochlorous acid or sodium hydroxide by electrolysis of pure water or water containing salt, base and/or acid such as sodium chloride, hydrogen sulfate, potassium hydroxide or sodium hydroxide (claimed).    ADVANTAGE - The electrolyser is capable of economically and efficiently producing the chemical substance with high purity, and exhibits improved lifespan.    DETAILED DESCRIPTION - The electrolyser comprises a stack (1) of a first and a second electrolytic cells. Each electrolytic cell (10) comprises an anode, a cathode, and an ion exchange membrane positioned between the anode and the cathode. The ion exchange membrane of the first and second electrolytic cells is separated by a bipolar electrode (4) constituting the anode of the first electrolytic cell on the one hand and the cathode of the second electrolytic cell on the other hand. The bipolar electrode comprises a bipolar plate, which is associated with a grid and a porous plate. The grid and the porous plate are integrated with each other before their mounting in the electrolyser. The ion exchange membrane has a thickness of 100-500 mu m. An INDEPENDENT CLAIM is included for an electrolytic ensemble.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a stack of electrolytic cells.    Stack of electrolytic cells (1)    Bipolar electrode (4)    Elastic spacer (5)    Sleeve (8)    Electrolytic cell. (10)