To get a better understanding of the dynamic evolution, high time-resolution transient state changes of Cs + and H2O were investigated. By using a static cell operation mode in which the X-ray beam is focused at a single fixed position, the time resolution can be significantly improved, decreasing the time from 192 seconds for a 5 mm scan in previous experiments to just 0.8 seconds at one fixed position. Figure presents the Cs + and H2O movement behavior during three different current steps of 0-50, 50-100, and 100-200 mA cm -2 (Figure ). The beam was focused on the catalyst and micro-porous layer interface (depth of 0.18 mm). To minimize Cs + diffusion across the AEM before operation, a relatively low concentration of 0.01 M CsHCO3 was used. Figure illustrates the instantaneous increase in Cs + content after ramping the current density. For all three current density ramps, the Cs + signal exhibits a linear region with varying slopes (indicated by dashed lines). In the first current step, the intensity does not reach a steady state within the first 6 minutes, indicating relatively slow Cs + movement under 2.7 V, consistent with the observation in Figure . However, for the other two current steps, the Cs + content quickly increases and then reaches a steady state, where the Cs + content exhibits only a small increase due to the balance between migration and the competing concentration gradient, as described by the Nernst-Planck equation (Equation ). Within the linear region for each current density step, 63%, 88%, and 72% of the maximum number of cations after ~30 min of operation appears within 6, 1, and 2 minutes for the 0-50, 50-100, and 100-200 mA cm -2 steps, respectively. From this data the calculated slopes are 0.3, 2.4, and 3.0 arb.u./min for the corresponding ramping steps.