Fig. 5: X-ray fluorescence time traces during CV cycles.

Data were collected for the 20s-deposition/air-dried sample in CO₂-saturated 0.1 M KHCO₃ (pH 6.8) during two CV cycles (CV start at OCP; forward scan down to −0.7 V, backward scan to +0.8 V, 1 mV s⁻¹). Detection of the CV current was combined with recording of the X-ray fluorescence at four excitation energies; here the normalized 8982 eV X-ray fluorescence intensity is shown. Top panel: potential (yellow line) and current (blue line) time traces. Bottom panel: normalized X-ray fluorescence time-trace (red line). The Cu₂O content is taken from the linear combination of XANES spectra in Supplementary Fig. 5c. ΔF1 and ΔF2 represent the fluorescence changes during oxidation process for cycle 1 and cycle 2, respectively. The normalized process for fluorescence intensity and the calculation of integrated charge for oxidation and reduction are described in section 2.3 of the Supplementary Information (SI). The integration ranges were chosen such that outside the integration range the X-ray signal was constant, thereby verifying the absence of redox-state changes of the Cu material outside the indicated integration ranges.