Assembly of electrochemical cells for an electrochemical reactor, comprising: - a first electrochemical cell, including: - a first membrane/electrode assembly comprising a first proton exchange membrane, a first anode and a first cathode positioned on either side of the proton exchange membrane and each including an electrocatalytic material; - first and second flow guides positioned on either side of the first membrane/electrode assembly; - a second electrochemical cell, including: - a second membrane/electrode assembly comprising a second proton exchange membrane, a second anode and a second cathode positioned on either side of the proton exchange membrane and each including an electrocatalytic material; - third and fourth flow guides positioned on either side of the second membrane/electrode assembly; - characterized in that: - the first and third flow guides have one and the same geometry; - the second and fourth 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, or the first cathode and the second cathode have different distributions of surface densities of electrocatalytic material on respective faces of the first and second proton exchange membranes (143, 113), the first proton exchange membrane comprising a central portion completely covered by the first anode and by the first cathode, the second proton exchange membrane comprising a central portion having a zone not covered by the second anode and by the second cathode. - a first electrochemical cell, including: - a first membrane/electrode assembly comprising a first proton exchange membrane, a first anode and a first cathode positioned on either side of the proton exchange membrane and each including an electrocatalytic material; - first and second flow guides positioned on either side of the first membrane/electrode assembly; - a second electrochemical cell, including: - a second membrane/electrode assembly comprising a second proton exchange membrane, a second anode and a second cathode positioned on either side of the proton exchange membrane and each including an electrocatalytic material; - third and fourth flow guides positioned on either side of the second membrane/electrode assembly; - characterized in that: - the first and third flow guides have one and the same geometry; - the second and fourth 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, or the first cathode and the second cathode have different distributions of surface densities of electrocatalytic material on respective faces of the first and second proton exchange membranes (143, 113), the first proton exchange membrane comprising a central portion completely covered by the first anode and by the first cathode, the second proton exchange membrane comprising a central portion having a zone not covered by the second anode and by the second cathode. - the first and third flow guides have one and the same geometry; - the second and fourth 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, or the first cathode and the second cathode have different distributions of surface densities of electrocatalytic material on respective faces of the first and second proton exchange membranes (143, 113), the first proton exchange membrane comprising a central portion completely covered by the first anode and by the first cathode, the second proton exchange membrane comprising a central portion having a zone not covered by the second anode and by the second cathode.