Although the Cs signal at the catalyst-membrane interface increases slightly, a drastic increase is observed in the cathode GDL. The ohmic resistance across the GDL is expected to be < 13 mΩ 30 , which would result in a rather small migration driving force for such a significant accumulation. Furthermore, the Cs + distributions at the cathodic compartment are divided into two peaks. To explain the Cs + accumulation in the GDL and the Cs + drop at a depth near 0.12 mm, the composition of the GDL and its correlation to the H2O transport need to be considered. Typically, a GDL consists of two layers: a micro-porous layer to load the catalyst (<1 µm pore diameter) and a macroporous layer consisting of carbon fibers with high porosity to provide high gas transport capacity (25 -200 µm pore diameter). To optimize the water management in the GDL, the micro-porous layer normally consists of carbon powder and polytetrafluoroethylene (PTFE) as a hydrophobic binder (e.g., 5 wt% of PTFE in Sigracet 39BB). The H2O content will be lower in the micro-porous layer due the small pores and correspondingly lower free volume, in addition to the hydrophobic binder. Figure -B provides a more in-depth insight into the H2O and Cs + content in the two layers of the GDL and their correlation. As expected, the H2O content displays a significant drop in the region of the micro-porous layer (0.1~0.17 mm) and an increase in the region of the macro-porous layer (0.03-0.1 mm). Up to this point, our analysis has focused solely on Cs + content. However, it is crucial to account for the amount of H2O, as Cs + ions move through aqueous environments, given that concentration (measured in moles per unit volume) can also be the driving force for diffusion. At the Cu catalyst layer, where the electrochemical CO2 reduction reaction takes place, that potential drop is high and results in an accumulation due to the migration of Cs + ions. The micro-porous layer near the Cu catalyst-membrane interface is mostly carbon with a limited free volume for H2O, and consequently results in a higher Cs + concentration within this region than that of the macro-porous layer region, as shown in Figure (further information in Supplementary Note 5). This high concentration provides a driving force for the Cs + to diffuse towards the macro-porous layer. Nevertheless, due to the dry conditions (Figure ) and slow diffusion of H2O, the movement of Cs + will be limited to that of H2O. The correlation between the H2O and Cs + movement is further illustrated in Figure , spanning six distinct locations within the macroporous layer, which are based on data from the five scans depicted in Figure . Figure displays a prominent linear trend, suggesting that Cs + diffusion is constrained by the diffusion of H2O. This linear trend also shows that the concentration of Cs + remains consistent throughout the entire macro-porous layer, with variations in Cs + content primarily attributed to the changes in H2O content. At the end of the experiment, the potential was reverted from 2.75 V (at 50 mA cm -2 ) to OCV, which was approximately 1 V. This higher OCV, as compared to the initial OCV of approximately 0.3 V, is attributed to the presence of residual active reaction intermediates and charged species at the cathode catalyst surface. Figure shows the Cs + content change after returning to OCV. The shape and size of the peak around the anodic region is very similar to the peak at OCV before operation, which suggests a rapid return to steady-state configuration at the anode. Despite the changes in Cs + content that have been observed throughout the experiment, the Cs + signal at the anolyte at a 0.5 mm scan height remains constant at 2 arb.u. The change in Cs + content is negligible in contrast to the content in the bulk electrolyte because the anolyte is continuously flowing and has a large reservoir. Even though there is a small driving force from the electric field caused by the potential difference of approximately 1 V, the Cs + content is decreasing across the whole cathode compartment. Consequently, the Cs + removal is expected to be dictated by diffusion due to the concentration gradient.