In the Cu catalyst and micro-porous layer region (Figure ), which is directly in contact with the wetted AEM, the Cs + ions move freely when a potential is applied. This figure demonstrates a strong correlation between potential and Cs + content, as well as H2O content, due to migration and electroosmosis as discussed earlier. However, the Cs + levels before reaching 200 mA cm -2 do not match completely the Cs + levels after reaching 200 mA cm -2 for 50 and 100 mA cm -2 , respectively. The Cs + content is continuously increasing, which can account for these differences. Regarding the H2O signal in Figure , when the system returns to OCV, the H2O signal at the Cu catalyst layer suddenly increases. This increase can be attributed to similar phenomena as observed in Figure , namely, the decrease in H2O consumption, the stopping of electroosmosis from anion movement, and an increase in water content within the membrane due to the absence of OER (Supplementary Note 4). These additional factors, that affect the H2O signal, create the mismatch in H2O and Cs + signals. It is worth mentioning that a slightly higher H2O content is observed in the catalyst layer at OCV after electrolysis compared to its initial state, suggesting an irreversible change of wettability due to electrowetting. Figure further presents the changes in Cs + and H2O content in the macro-porous layer. In contrast to Figure , the correlation between cell potential and the content of H2O and Cs + has weakened. As discussed earlier, the movements of Cs + and H2O towards the macro-porous layer are expected to be diffusion. The Cs + and H2O contents increase and reach a maximum at 200 mA cm -2 for this experiment. Afterwards, the H2O content shows a plateau, while the Cs + diffuses back into the anolyte. While the Cs + returns to the anolyte almost immediately due to a concentration gradient, no such gradient is present for the H2O content, with the anode being completely saturated. In Figure , it may appear that the electric potential exerts an influence on the movement of H2O; however, it is important to note that diffusion remains independent of the applied cell potential. Instead, the increase of H2O content across the AEM to the Cu catalyst is primarily driven by electroosmotic drag (Figure ), subsequently influencing the diffusion of H2O towards the macroporous layer. The same phenomena are observed in a comparison of the normalized H2O content in the GDL of 0.01 M and 0.1 M CsHCO3 electrolyte concentrations in Figure . This figure shows that the macro-porous layer tends to be flooded more rapidly with a higher concentration of CsHCO3. The elevated H2O level in the GDL (i.e. flooding) results in mass transport limitations of CO2 to the catalyst and explains one of the biggest issues for zero-gap CO2 electrolyzers, which essentially results in HER. In Figure the differential changes in H2O signal are shown at different operating conditions. This figure shows the difference between the respective scans and the scan prior to the first scan of each figure under the same operating conditions. At OCV before the experiment (Figure ) and after the experiment (Figure ) there is no movement of H2O. Generally, the water transport in the MEA systems across the AEM is attributed to diffusion, hydraulic permeation, and electroosmotic drag. However, since no movement is observed at OCV, the contributions from diffusion and hydraulic permeation are negligible within this timeframe.