Figure shows Cs + distribution across the MEA within the first three scans (3.2 minutes each) at an OCV of approximately 0.3 V. The consistency observed in different scans suggests the absence of a Cs + net flux. Intriguingly, there is a slight accumulation of Cs + content (~5 arb.u.) near the AEM (yellow shaded region), surpassing the content present in the outer anode GDL region (~2 arb.u.). Since diffusion is driven by concentration gradients, this accumulation contradicts the diffusion phenomenon, suggesting the involvement of other factors. While the origin of the Cs + accumulation remains unidentified, some hypotheses are presented in Supplementary Note 2.