NOVELTY - The bipolar plate comprises a central proton exchange membranes with unit cells that are electrically connected in series and are chemically distinct from each other, a first central region arranged on one side of its edge portion, a second central region arranged on other side of its edge portion, and a shaped body (2) made of plastic. Flow fields of open cross-flow channels (8) in the shaped body are formed in the central regions. First supply channels (7) are formed in the edge region of the bipolar plate. An inside of the shaped body is present in communicate with cross flow channels.    USE - The bipolar plate is useful for a polymer electrolyte membrane stack reactor (claimed), which is useful as a fuel cell and a hydrolyzer for hydrolytic production of hydrogen from water.    ADVANTAGE - The bipolar plate can be economically manufactured with high efficiency, excellent mechanical property, long term stability and increased electric conductivity, and improves the functionality of the polymer electrolyte membrane stack reactor.    DETAILED DESCRIPTION - The bipolar plate comprises a central proton exchange membranes with unit cells that are electrically connected in series and are chemically distinct from each other, a first central region arranged on one side of its edge portion, a second central region arranged on other side of its edge portion, and a shaped body (2) made of plastic. Flow fields of open cross-flow channels (8) in the shaped body are formed in the two central regions. First supply channels (7) are formed in the edge region of the bipolar plate. An inside of the shaped body is present in communicate with cross flow channels in the first central region and in the second central region. A U-shaped electrode sheet (3) is arranged between two legs of a longitudinal section through a polymer electrolyte membrane stack reactor, and includes openings in the two central regions. Ventilation ducts communicate with the cross-flow channels extending transversely in the edge region of the shaped body on the other side of the bipolar plate and on outer periphery of the bipolar plate. Second supply channels are formed in the edge region of the bipolar plate. A peripheral seal is formed between an electrode sheet and shaped body, seals the edge portion to the second central region, and is an elastomer seal. The elastomer seal includes openings extending through and in the edge region in the electrode plate. The elastomer seal is injection molded on the electrode sheet. The electrode sheet is provided with the openings. The electrode sheet includes a core of titanium metal, and is coated with platinum, preferably iridium-titanium mixed oxide. An associated connector electrode is electrically connected to the electrode sheet, and projects on one of the two sides of the bipolar plate. The shaped body has knob-like projections between which the transverse flow channels crossing each other in directions extend. An INDEPENDENT CLAIM is included for a polymer electrolyte membrane stack reactor.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic cross sectional view of a bipolar plate along a first cutting plane.    Shaped body (2)    Electrode sheet (3)    First supply channels (7)    Cross-flow channels (8)    First and second central regions. (11)