Figure 2.

A) ORR cyclic voltammograms of Pt@HGS 1–2 nm in 0.1 M HClO₄ saturated with Ar (black) and with oxygen at (400–2500 rpm, blue) recorded at room temperature with a sweep rate of 0.05 V·s⁻¹. B) Same as in A) after subtraction of capacitive currents. C) Plot of the specific activity as derived from the anodic scan at 0.9 V_RHE versus the electrochemically active surface area of platinum illustrating the particle size dependent changes in activity. D) Tafel plots from a representative measurement of Pt-poly (orange), Pt@HGS 1–2 nm (blue), Pt@HGS 3–4 nm (red) and Pt/Vulcan 3–4 nm (green). The three Tafel plots of the Pt/C catalysts are almost identical, which indicates comparable specific activities of Pt@HGS based catalysts with standard high surface area fuel cell catalysts.