This mainly suggests three things: (i) Comparing Pt-alloy cathodes at both wet as well as dry conditions can reveal performance differences related to the presence of the less noble metal ions in the MEA. In particular, the present data also suggests that measuring only at wet conditions could even be misleading and mask transport issues related to the presence of less noble metal ions. Thus, measuring at lower (dryer) RH as well as in air rather than in O2 is critical for adequate evaluation of Pt-alloys, especially when considering and understanding the effects of the transition metal cations on the performance. (ii) 'Tolerance' for Ni ions in single-cells far exceeds that of Cu ions. When comparing both A-activated Pt-alloy catalysts at hot-wet conditions (Figures and), d-Pt-Ni/C-NIC-A exhibits significantly better BoL performance than d-Pt-Cu/C-NIC-A despite having even a small amount of pure-Ni phase (Figure ) that most likely, in addition to leaching of Ni from Pt-Ni NPs, additionally contributed towards additional contamination with Ni ions. However, transition metal ions can also be introduced in the MEA upon Pt-M electrocatalyst degradation. Thus, the second part of this work focuses on the effect of the LVL on the dissolution of the less noble metal from the Pt-M electrocatalysts. (iii) As predicted by the prior observations in TF-RDE and EFC-ICP-MS (Figure ) S-activated Pt-M NIC analogues significantly outperformed the Aactivated ones in the 50 cm 2 single cells, namely at the HCDs. This not only supports our arguments on the CO-electrooxidation shifts, but also suggests that the presence of the transition metal cations in the CCM is one of the most critical parameters governing the performance of Pt-alloys in PEMFCs. However, as already suggested, even if the Pt-alloy is 'adequately' activated and no significant amount of the less noble metal ions are introduced in the CCM at the BoL, due to aging of the Pt-M NPs, the operation of the PEMFC will overtime introduce fresh transition metal ions. Thus, in the second part of this study, the focus will be in understanding a highly underestimated parameter in PEMFC operation, the lower voltage limit (LVL). ).