recovers as the cations migrate to the neighborhood of the cathodic catalyst layer (CL). It takes around 10 seconds for the cations to accumulate in the cathode CL and over 10 minutes to diffuse back to the bulk of the membrane after shutdown. Using data from quantified neutron images and electrochemical overpotential breakdown analysis, we found that even very small amounts of cations can severely impair cell performance. The fraction of occupied exchange groups (2.5%) does not match the membrane resistance increase (10%). The reason behind this higher than expected impact most likely stems from additional yet unknown mechanisms (e.g. polymer morphology altera-
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tion). Since the membrane resistance is affected only by the cations remaining in the bulk of the membrane, we performed cyclic current density ramp up and down measurements to confirm that the amount of the cations in the membrane will follow the membrane resistance values. We observed a very good agreement between the two and concluded that performing polarization curve measurements cyclically can be a good way to check for ionic presence in the membrane – identified as a hysteresis between the up and down current sweeps. As the next step, we developed a model, which predicts the distribution and movement of these