An assembly of electrochemical cells for an electrochemical reactor, including a first electrochemical cell, including a first membrane/electrode assembly including a first anode and a first cathode on either side of a proton exchange membrane; first and second flow guides positioned on either side of the first assembly; a second electrochemical cell, including a second membrane/electrode assembly including a second anode and a second cathode on either side of a proton exchange membrane; third and fourth flow guides on either side of the second membrane/electrode assembly; the first and third flow guides have one and the same geometry; the first anode and the second anode have different distributions of surface densities of electrocatalytic material on respective faces of the first and second proton exchange membranes.