Importantly, the observations from the model system discussed above are consistent with ORR activities determined in the MEA configuration, i.e., MEA with Aquivion/Pt/C exhibiting a higher rate than Nafion/Pt/C . Thus, the molecular level insights into the ionomer/Pt interface could help understand device level performance and guide the design of ionomers. The limiting currents of the ORR on all single and polycrytalline Pt surfaces employed in this work with and without Nafion are similar (Figure , Figures ), indicating the lack of mass transport of O2 to the interface. Atomic force microscopy images show that the Nafion film on Pt(111) is ~5 nm in thickness and exhibits a porous structure (Figure ), which could be a key factor contributing to the facile O2 transport to the Pt surface. To investigate whether the polymeric overlayer of perfluoro-alkyl backbone leads to the suppression of the ORR, the PTFE-covered Pt(111) with the same loading was employed to measure the ORR activity. Since PTFE is composed of similar perfluoro-alkyl backbone but without sulfonate groups, the impact of OHNafion caused by sulfonate adsorption and polymeric backbone on the measured ORR activity can be isolated. As shown in Figure , the ORR activity on PTFE-covered Pt(111) electrode is nearly identical to that on the bare Pt(111) when normalized to the geometric area, suggesting that the polymeric overlayer is unlikely to be the main cause in suppressing the ORR on the Nafion-covered Pt(111). It should be noted that ionomers play multiple roles under operating condition of fuel cells other than affecting the kinetic properties of surface bound intermediates, such as maintaining sufficient ion conductivity and allowing for adequate mass transport through the interface . Thus, the Pt/ionomer ratio and interfacial morphology are key aspects in the optimization of the fuel cells' performance . Leveraging the fundamental insights gained from this work to guide interfacial engineering, e.g., by suppressing the formation of kinetically hindered OHNafion species on Pt, could help improve the performance of fuel cells. The possibility of the Nafion overlayer impacting the ORR activity via altering the local oxygen concentration under the conditions employed in this work is excluded based on control experiments discussed in the Supplementary Note 5.