There is a significant drop in the Cs + content at the cathode catalyst membrane interface, where the cations are relatively mobile in contrast to those in the macro-porous layer that must transfer across the micro-porous layer. Initially the concentration gradient is also expected to be larger across the membrane, which results in a higher diffusion of cations near the interface. The last two scans show a significant drop in Cs + content within the AEM, which can be attributed to the Donnan exclusion effect.