Fig. 2.

In situ XANES spectra of an as-deposited δ-MnO₂ film subject to multipotential step activation. XANES spectra were collected after applying a potential between 1.1 and −0.4 V 25 times with a cycle terminating at the anodic or cathodic potential. One cycle (designated c) = 25 steps. The cycle number is designated numerically, and A and C denote cycles terminating at anodic or cathodic potential, respectively. Inset shows the energy position of the XANES edge determined from the inflection point as deduced from the maximum of the first derivative. The edge positions in Inset are color-coded to match the XANES spectra for cycles A0 (teal), c1C (purple), c2A (orange), c3C (red), c4A (green), and c5C (blue). The normalized absorption was determined from the full spectra shown in SI Appendix, Fig. S5.