Fig. 4: Operando tracking of O−1 and their decay to molecular oxygen during OCP.
a, Potential profile of OCP decay measurement between 0.6 and 0.8 VRHE, 1.12 and 1.3 VRHE, and 1.3 and 1.5 VRHE in 0.1-M HClO4 electrolyte. b, Change in fluorescence intensity of Ir L edge (11,222 eV). c, Change in fluorescence intensity of O K edge during OCP decay measurement between 0.62 and 0.82 VRHE and 1.1 and 1.27 VRHE at 529 eV, and 1.27 and 1.52 VRHE at 528.7 eV. d, Normalized optical signal decay. The optical signals are taken at the maximum absorption wavelength for each redox transition, with redox transition forming Ir4+ at 600 nm, Ir5+ at 800 nm and O−1 at 500 nm (Fig. 2a and Supplementary Note 3 show the spectra). e, Normalized optical signal decay at 500 nm at different applied potentials. The potentials are iR corrected and in the RHE scale. f, Schematic showing the formation of O−1 on removing an electron and proton from a protonated IrO6 structure by raising the potentials and its corresponding decay during OCP to form molecular oxygen. g, O2 (m/z = 32) detected in EC-MS for pulsed potential measurements from 1.40 to 1.44 VRHE and for a subsequent OCP decay measurement with the same pulsed potential steps and holding time. h, Quantitative comparison between the charge associated with O−1 formation and the detected molecular oxygen during OCP decay. The amount of O−1 formed is quantified by integrating the cathodic current peak during pulsed potential measurements, corresponding to the reduction of accumulated O−1 back to O−2 at 1.415 VRHE, assuming a one-to-one correspondence between O−1 formation and the number of electrons transferred. Above this potential, the charge is dominated by oxygen redox with negligible contribution from iridium (Fig. 2d), and double-layer charging/discharging is minor (Supplementary Note 7 and Supplementary Fig. 28). The amount of O2 released during OCP decay is directly measured by EC-MS, with contributions from potential-jumping controls subtracted (Supplementary Figs. 24 and 27). The change in optical absorption relative to 1.415 VRHE at 500 nm (right axis) is co-plotted to correlate O−1 accumulation with the corresponding optical signal changes resulting from redox transitions.
