Electrochemical reactor, comprising: - a flow guide; - a membrane/electrode assembly; - a peripheral seal arranged between the flow guide and the membrane/electrode assembly; characterized in that it further comprises: - an intermediate seal arranged between the flow guide and the membrane/electrode assembly, the intermediate seal being encircled by the peripheral seal and encircling a reaction zone of the electrochemical reactor; - at least one first flow circuit for cooling fluid arranged between the peripheral seal and the intermediate seal, the flow circuit comprising: - a first flow channel extending along the peripheral seal between a first end connected to a first manifold for cooling fluid and a second end opposite to the first; - a second flow channel extending along the intermediate seal between a first end connected to a second manifold for cooling fluid and a second end opposite to the first; and - either a third flow channel connecting the respective second ends (547, 549) of the first and second flow channels (541, 545), so that a fluid introduced at the level of the first manifold must pass through the third flow channel to return to the second manifold; - or fourth flow channels distributed along the first flow channel and each connecting the first flow channel to the second flow channel, so that a fluid introduced at the level of the first manifold must pass through a fourth flow channel to return to the second manifold, the first end of the first flow channel being positioned close to the second end of the second flow channel, said fourth flow channels being configured to induce pressure losses in a fluid flow between the first and second flow channels. - a flow guide; - a membrane/electrode assembly; - a peripheral seal arranged between the flow guide and the membrane/electrode assembly; - an intermediate seal arranged between the flow guide and the membrane/electrode assembly, the intermediate seal being encircled by the peripheral seal and encircling a reaction zone of the electrochemical reactor; - at least one first flow circuit for cooling fluid arranged between the peripheral seal and the intermediate seal, the flow circuit comprising: - a first flow channel extending along the peripheral seal between a first end connected to a first manifold for cooling fluid and a second end opposite to the first; - a second flow channel extending along the intermediate seal between a first end connected to a second manifold for cooling fluid and a second end opposite to the first; and - either a third flow channel connecting the respective second ends (547, 549) of the first and second flow channels (541, 545), so that a fluid introduced at the level of the first manifold must pass through the third flow channel to return to the second manifold; - or fourth flow channels distributed along the first flow channel and each connecting the first flow channel to the second flow channel, so that a fluid introduced at the level of the first manifold must pass through a fourth flow channel to return to the second manifold, the first end of the first flow channel being positioned close to the second end of the second flow channel, said fourth flow channels being configured to induce pressure losses in a fluid flow between the first and second flow channels. - a first flow channel extending along the peripheral seal between a first end connected to a first manifold for cooling fluid and a second end opposite to the first; - a second flow channel extending along the intermediate seal between a first end connected to a second manifold for cooling fluid and a second end opposite to the first; and - either a third flow channel connecting the respective second ends (547, 549) of the first and second flow channels (541, 545), so that a fluid introduced at the level of the first manifold must pass through the third flow channel to return to the second manifold; - or fourth flow channels distributed along the first flow channel and each connecting the first flow channel to the second flow channel, so that a fluid introduced at the level of the first manifold must pass through a fourth flow channel to return to the second manifold, the first end of the first flow channel being positioned close to the second end of the second flow channel, said fourth flow channels being configured to induce pressure losses in a fluid flow between the first and second flow channels.