Two experiments with anolyte concentrations of 0.01 M (left, Figure ) and 0.1 M CsHCO3 (right, Figure ) and the spatial changes of H2O and Cs content was monitored as a function of time and applied current density. The customized zero-gap electrolyzer was operated by using a cathodic gas stream of humidified CO2, a sputtered Cu cathode, a commercial AEM (Sustainion X37-50), and a commercial IrOx anode. Further experimental details are given in the section of Experimental Procedures. During the experiment, a systematic electrochemical analysis was performed, starting from open circuit voltage (OCV), gradually increasing the current density from 50 to 200 mA/cm 2 in galvanostatic mode, and subsequently descending back to OCV (Figure ). XRF and WAXS spectra were obtained during the electrolyzer operation, monitoring the Cs and H2O signals as a function of time and position in MEA (Figure ). The individual MEA layers identified by WAXS (Figure and Supplementary Note 1) are represented by the bars on the right-hand side of Figure . The origin of the small distortions in cell potential at 1.2 hours for 0.01 M CsHCO3 and at 1.5 hours for 0.1 M CsHCO3 are unknown, and correlate to a drop in WAXS and XRF signal. However, in all cases, the signal then returns to the normal level.