Fig. 4: Potentiodynamic in situ XAS and eQCM analysis of [Co-Fe-Pb]O_x.

a, Cyclic voltammograms (v = 0.005 V s⁻¹) recorded during the hard XAS (solid red and dashed red), soft XAS (sky blue) and eQCM (orange) experiments in flowed (1 ml min⁻¹ for Co K-edge and eQCM, 10 μl min⁻¹ for Co L₃-edge) 0.1 M H₂SO₄ (dashed) and 0.1 M H₂SO₄ + 5 mM Co²⁺ (solid). b,c, Positions of energies used for FEXRAV on typical Co K- (b) and L₃-edge spectra (c). d–k, Corresponding potentiodynamic (v = 0.005 V s⁻¹) evolution of the TFY signals at 7,720 (red) and 8,000 eV (grey; multiplied by a factor of 10) (d,e,h,i) in 0.1 M H₂SO₄ (d,h) and 0.1 M H₂SO₄ + 5 mM Co²⁺ (e,i), TFY signal at 780.75 eV (f,j), and mass change (g); arrows show the sweep direction. k, Effect of the scan rate (values in the figure) on the potentiodynamic evolution of the TFY signal at 7,720 eV in 0.1 M H₂SO₄ + 5 mM Co²⁺. h–k show time-resolved data on a potential scale with vertical dashed lines showing the potential switching points. [Co-Fe-Pb]O_x was deposited onto CFP (Co K-edge), Au-Si₃N₄ membrane (Co L₃-edge) and Au-coated quartz crystal (eQCM) by galvanostatic oxidation at 10, 1 and 5 mA cm⁻², respectively, of quiescent 0.1 M H₂SO₄ + 5 Mm Co²⁺, 1 mM Fe³⁺ and 0.5 mM Pb²⁺ at 23 ± 1 °C.

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