Figure 2. ORR on the Fe–N/C catalyst across the pH scale.

(a) Rotating disk electrode measurements of Fe–N/C catalyst at different pH values in O₂ saturated 0.5 M electrolytes, 1,600 r.p.m., 5 mV s⁻¹; loading 270 μg cm⁻² data corrected for solution resistance, capacitive background and different oxygen solubility and diffusivity. (b) Plot of the potential at a current density of 0.1 mA cm⁻² (iR-free) versus pH (bottom of panel) versus saturated calomel electrode; linear fit shows a slope of 57±2 mV/pH in the pH range 0–9 (top of panel) corrected to RHE scale. All values become the same within the error margin in the pH range 0–9 corrected to RHE scale. (c) Tafel plot of mass-transport corrected currents from (a) corrected to the RHE potential scale. All plots collapse into one, especially at high potentials.