Understanding the relationship between gas diffusion electrode (GDE) structures and the performance of electrochemical CO2 reduction reaction (CO2RR) is crucial to developing industrial-scale technologies to convert CO2 to valuable products. We studied how the microporous layer (MPL) on GDE's coated with silver nanoparticle catalysts affects the electrochemical CO2 conversion to CO in a flow cell electrolyser. We demonstrate a convenient method to measure the rate of catholyte seepage through a GDE during CO2RR experiments and used this method to show how the MPL thickness affects flooding of the GDE. We found the GDE with the thickest MPL (39BB) had the best selectivity for CO and stability at current densities above 100 mA cm−2 as the thick MPL minimized flooding. However, at low current densities the 39BB electrode achieved a lower CO selectivity than the GDE with thinner MPL. These results suggest opportunities to improve CO2 electrolyser performances at high current by optimisation of the MPL structure and wettability.@en