<p>The electrocatalytic CO₂ reduction reaction (CO₂RR) presents a promising approach to achieving carbon neutrality. Currently, anion exchange membranes (AEMs) are widely used in CO₂RR electrolyzers. However, the widespread industrial application of CO₂RR electrolyzers faces significant challenges due to the poor alkali resistance, high mass transfer resistance, limited electrode surface area, and high costs associated with conventional AEMs. This paper describes the amplification and stable operation of a horizontal electrolyzer achieved via combining a diaphragm, a double-layer cathode catalyst, and a redesigned flow channel. Furthermore, a new pH measurement device has been proposed to detect the ion concentration on the cathode surface, elucidating the relationship between electrolytic cell components, ion concentration, and Faraday efficiency (FE). The results reveal that the electrolyzer resistance and cell voltage are reduced, and the hydrogen evolution reaction (HER) is effectively reduced due to the high alkali environment and rapid mass transfer enabled by the diaphragm. Furthermore, the uniform flow of electrolytes and rapid desorption of O₂ bubbles on the anode surface have been achieved through the redesigned flow channel using the computational fluid dynamics (CFD) simulation. Diaphragm-based electrolyzer with a double-layer cathode catalyst structure has been adopted, and the catalytic area has been expanded to 100 cm². After running for 100 h at a current density of 200 mA/cm² in a 1 M KOH solution, the FE_CO still exceeds 90%, with a cell voltage of approximately 3 V.</p>