Fig. 2: In situ Co L₃-edge XAS analysis of [Co-Fe-Pb]O_x.

a, TFY spectra collected for the [Co–Fe–Pb]O_x-functionalized Au-Si₃N₄ membrane in flowed (10 μl min⁻¹) 0.1 M H₂SO₄ + 5 mM Co²⁺ at 1.75 (sky blue), 1.80 (blue), 1.85 (green), 1.90 (yellow), 2.00 (orange), 2.05 (violet) and 2.10 V vs RHE (wine). b, Ex situ TFY (grey) and in situ TFY at open circuit (black) for the as-deposited catalyst, compared to an ex situ TEY spectrum of the CoOOH reference (purple). c, First-order derivative of the data shown in a (inset shows a magnified plot of the region around derivative = 0). Arrows in a and c show changes induced by the application of progressively positive potentials; vertical dashed line in panels a–c shows position of the TFY peak maximum at 1.75 V vs RHE. d,e, Effect of potential on the Co L₃-edge main peak intensity (black) and the integral intensity across the Co L₃-edge (blue) (d) and Co L₃-edge main peak position (e); data collected for the as-deposited catalyst ex situ, in situ at OCP, and for the reference materials are shown as horizontal dash-dotted lines in e (see labels in the figure). Symbols in d and e show experimental data (mean ± standard deviation from n ≥ 5 measurements; Methods), whereas dashed lines show linear fits. y-axis break in e is introduced to facilitate comparisons of the data. [Co-Fe-Pb]O_x was deposited by 15 min galvanostatic (1 mA cm⁻²_geom.) oxidation of quiescent 0.1 M H₂SO₄ solutions containing 5 mM Co²⁺, 1 mM Fe³⁺ and 0.5 mM Pb²⁺ at 23 ± 1 °C.

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