Suppression of the ORR activity on Aquivion-covered Pt(111) is much less pronounced than that on Nafioncovered Pt(111) (Figure ), which is consistent with the hypothesis of OHNafion retarding the ORR. This correlation also holds on Pt(554), a stepped facet with a 110 step every 10-atom wide 111 terrace. OHNafion peak is clearly visible in CV on Nafion-covered Pt(554) (Figure ), with its size varying in sync with that of the sulfonate adsorption peak. The ORR activities decreases with the size of the peak attributed to OHNafion on Nafion-covered Pt(554) (Figure ) . Single crystal Pt surface/Nafion interfaces employed above are idealized model systems to reduce the complexity of electrode/polymer electrolyte interfaces.