NOVELTY - Membrane-separated, bipolar multicell electrochemical reactor has bipolar plate electrodes and membrane separators alternately stackable in horizontal position with other face of frames carrying O-ring gaskets facing upward. A retention counterflange is mounted on perimetral portion of the electrode or separator on the inner flange of the frame opposite the other face.    USE - For implementing a redox flow battery system for use on nonpolluting vehicles.    ADVANTAGE - The electrochemical reactor has an architecture that makes it more easy to assemble by allowing to stack fully preassembled elements horizontally, one on top of the other, and suitable to be operated in the same horizontal orientation of the bipolar elements.    DETAILED DESCRIPTION - Membrane-separated, bipolar multicell electrochemical reactor for half-cell reduction and oxidation reactions in respective positive and negative liquid electrolytes, without gas evolution, includes alternately disposed bipolar plate electrodes and ion-exchange membrane separators (2). A positive electrolyte flow chamber is defined on one side of each membrane and a negative electrolyte flow chamber is defined on the opposite side. The separators are sealingly assembled together in a filter-press arrangement between two end electrodes electrically coupled into an electric circuit. The electric circuit includes an electrical source forcing a current through the electrochemical reactor or an electrical load absorbing a current from the electrochemical reactor. The electrodes and the separators include a frame of an electrically nonconductive and chemically resistant material cooperating with sealing gasket and having through holes and recesses in coordinated locations. Upon assembly, ducts for the separate circulation of a negative electrolyte and of a positive electrolyte are formed cascadedly in all negative and positive electrolyte flow chambers. All the frames (1E) have an inner flange recessed from a first planar face of the frame on the opposite side of the other face of the frame having grooves for accommodating O-ring gaskets (8) around pass-through electrolyte-ducting holes and around an outer seal perimeter. A perimetral portion of the respective electrode or separator is also accommodated. Retention pins project out of the surface of the flange and pass through holes of the perimetral portion of the accommodated electrode or separator. A retention counterflange (4, 15) of an electrically nonconductive and chemically resistant material having holes (5, 17) is coordinated with the positions of the retention pins. The counterflange is functionally mounted over the perimetral portion of the electrode or separator on the recessed flange portion of the frame and is permanently fixed by hot-flattened heads of retention pins protruding out of the coordinated holes of the counterflange. Pre-assembled bipolar plate electrodes and membrane separators are alternately stackable in horizontal position with the other face of the frames carrying the O-ring gaskets facing upward.    DESCRIPTION OF DRAWING(S) - The figure shows an exploded view of a membrane separator and a bipolar electrode.    Frame (1E)    Membrane separator (2)    Retention counterflange (4, 15)    Holes (5, 17)    O-ring gasket (8)    Keying pins (10)