## 3 Results and Discussion  
To probe structural and chemical-state changes of the MnO\({}_{2}\) electrocatalyst under current loads characteristic of battery operation, operando XAS investigation at the Mn K-edge was carried out. The architecture and composition of the electrode, as well as the configuration of the electrochemical setup and the imposed operating conditions, were those of a real battery, some aspects of which were designed to achieve optimal edge-jumps and signal-to-noise ratio in the absorption spectra for the Mn K-edge, with no impact on the current densities applied during testing and on electrode performance, as detailed in the Experimental Section. The operando XAS experiments, devised to follow discharge/charge cycles, consisting of a sequence of cathodic (ORR) and anodic (OER) current density intervals of j = 5 mA\(\cdot\)cm\({}^{-2}\), were carried out in ambient air without a CO\({}_{2}\) trap. It should be emphasized that the current densities used in this study are one to two orders of magnitude higher than those reported for other operando XAS studies performed on other GDE design concepts. We thus consider that our results provide information on structural, chemical, and electronic modifications occurring when electrocatalyst material is subjected to application-relevant operating conditions. The operando XAS experiments were performed on the GDE at three different stages: before (GDE before O\({}_{2}\) starvation), after partial (GDE--partial O\({}_{2}\) starvation, Figure 2 shows the corresponding voltammogram), and after complete O\({}_{2}\) starvation condition (GDE--complete O\({}_{2}\) starvation). OER experiments after complete O\({}_{2}\) starvation were not performed because the interest of the experimental protocol was to assess the chemical state change of Mn resulting from extensive O\({}_{2}\) starving, rather than checking the anodic performance after the GDE has been exposed to such stress conditions. Instead, we believed that it would have been interesting to assess the residual anodic electrocatalytic activity after exposure to milder and more realistic stress conditions. Figure 3a shows the operando X-ray absorption near edge structure (XANES) spectra of the three GDEs at the Mn K-edge. The reference spectra of Mn\({}_{3}\)O\({}_{4}\) and pyrolusic MnO\({}_{2}\) powders are also included in the plot. The electrode examined in pristine condition corresponds to the GDE before O\({}_{2}\) starvation (see solid blue and red lines for ORR and OER condition, respectively). It can be seen that the two XANES spectra nearly overlap, with only a minor negative shift (to lower energies) of the rising portion for the spectrum measured at ORR (see blue solid line). The XAS experiment for GDE--partial O\({}_{2}\) starvation was carried out after imposing a short reduction step on the electrode. This was achieved by sealing the GDE with a thin Kapton foil under imposed cathodic polarization at a rate typical of ORR operation (j = \(-\)10 mA\(\cdot\)cm\({}^{-2}\)) for about 25 min. These severe operating conditions result in a gradual decrease in electrode potential over time, as shown in Figure 2.  
The duration of the reduction process, however, was insufficient to complete the electrochemical reduction of \(\alpha\)-MnO\({}_{2}\). When compared with the GDE before O\({}_{2}\) starvation, the equivalent XANES spectra collected at ORR and OER (see blue and red dotted lines) following such partial electrochemical reduction are now clearly moved to lower energies. In the case of bulk metal oxides, a shift in the edge position usually entails a change in the oxidation state of the element, where a shift to lower energies correlates to a reduction of the Mn oxidation state, respectively [3]. The reduction stage was continued for the GDE --complete O\({}_{2}\) starvation until the associated XANES spectrum remained unaltered (no additional shift to lower energy); see black solid line in Figure 3a.