As a remedy to the increasing concentration of greenhouse gases and depleting fossil resources, the electrochemical CO2 reduction closes the carbon cycle and provides an alternative carbon feedstock to the chemical and energy industry. While most contemporary research focuses on the catalyst activity, we emphasize the importance of the reactor design for an energetic efficient (EE) conversion. A design strategy for an electrochemical membrane reactor reducing CO2 to hydrogen, carbon monoxide (CO) and ethylene (C2H4) is developed. We present the stepwise development from an H-cell like setup using full-metal electrodes to a cell with gas diffusion electrodes (GDE) towards high current efficiencies (CE) at high current densities (CD). At 300 mA.cm(-2) a CO-CE of 56% for a Ag GDE and a C2H4-CE of 94% for a Cu GDE are measured. The incorporation of the developed GDEs into a zero-gap assembly eliminates ohmic losses and maximizes EE, however the acidic environment of the ion exchange membrane inhibits CO2 reduction. As a compromise a thin liquid buffer layer between cathode and membrane is a prerequisite for a highly active conversion. We demonstrate that industrial relevant CDs with high CEs and EEs can only be achieved by moving beyond today's research form catalyst development only to an integrated reactor design, which allows to exploit the viable potential of electrochemical CO2 reduction catalysts.