Fig. 4: XAS experiments on fast oxide dynamics during potential cycling.

Data were collected for the 20s-deposition/air-dried sample at OCP and during application of an electric potential in CO₂-saturated 0.1 M KHCO₃ (pH 6.8). a Time series of XANES spectra; b time courses of normalized X-ray fluorescence for excitation at 9013 eV (left y-axis) and corresponding current density (right y-axis). A total of 17 spectra were collected (for a plot of the stacked spectra, see Supplementary Fig. 15). Spectrum 1 was collected without applied potential (OCP). Spectra 2 to 17 were sequentially collected at −0.7 V with a time interval between spectra of 227 s. Spectra 2-17 are indistinguishable, implying that Cu foam reduction is completed within less than 4 min. In (c) and (d), raw (not normalized) X-ray fluorescence intensities for excitation at 9013 eV at millisecond time resolution (averaging over 100 ms in the plotted data) are shown. In (c), X-ray fluorescence (left y-axis) and current density (right y-axis) time-traces for four +0.6 V (OCP)/−0.7 V potential cycles with non-abrupt transitions between applied potentials facilitated by LSV scans with 100 mV s⁻¹ are shown. In (d), data as in (c), but here shown with an expanded time scale for the four reductive transitions (the time traces of the X-ray signal are labeled by the Jump 1 to Jump 4; the time traces of the corresponding current densities are labeled from 1 to 4). We note that the X-ray data were not corrected for an increase in the incident X-ray intensity during the 1.3 h experiment of Fig. 4c and d, so that the overall increase in the Cu oxidation state may be over-estimated. The redox charges for each reductive and oxidative transition were calculated by integration of the current densities and are shown in Supplementary Fig. 20.