Fig. 2. Evaluation of Pt for oxygen and hydrogen electrocatalysis in HClO₄, HNO₃, and H₂SO₄.

a Representative third-cycle-averaged N₂-subtracted oxygen reduction reaction (ORR) rotating disk electrode (RDE) cyclic voltammogram (CV) of the Pt disk in pH 1 HClO₄, HNO₃, and H₂SO₄. b Representative second-cycle-averaged oxygen evolution reaction (OER) RDE CV of the Pt disk in pH 1 HClO₄, HNO₃, and H₂SO₄ (OER trend is consistent as a function of cycle, Supplementary Fig. 4). c Average OER (top) and ORR (bottom) onset potential (bars, left y-axis) $\pm$0.1 mA ${\mathrm{cm}}_{\mathrm{geo}}^{-2}$ for all three acids and average Tafel slope (circles, right y-axis) in the low current density region. d Representative third-cycle-averaged hydrogen evolution rection (HER) RDE CV of the Pt disk in pH 1 HClO₄, HNO₃, and H₂SO₄. e Representative third-cycle-averaged hydrogen oxidation reaction (HOR) RDE CV of the Pt disk in pH 1 HClO₄, HNO₃, and H₂SO₄. f Average HOR (top) and HER (bottom) onset potential (bars, left y-axis)$\pm$0.1 mA ${\mathrm{cm}}_{\mathrm{geo}}^{-2}$ for all three acids and average Tafel slope (circles, right y-axis) in the low current density region. Error bars represent the standard deviation of separate triplicate measurements. All CVs collected at 1600 rpm, 20 mV s^–1, and iR_u corrected. Color code: HClO₄ (blue), HNO₃ (red), and H₂SO₄ (olive).