The electrode was exhaustively rinsed with 1 M KOH solution before being introduced in the electrochemical cell again with fresh electrolyte. A new sequence of 15 voltammograms was recorded, by ensuring that the potential window is above the OCP (Figure ). The OER onset potential remained unchanged, although the Mn(VII)/Mn(VI) redox waves disappeared. We did not observe any evolution of the OER current upon cycling. The electrolyte near the electrode became again purple, but at a lower extent than in the previous experiment. The electrode was then soaked into the electrolyte overnight at OCP. It recovered an intense green color. 50 new cyclic voltammograms were recorded (Figure ) and showed similar activity as above. The appearance of the green color in the electrolyte may be associated with the formation of MnO4 2-by MnO4 - reduction in an alkaline environment, which is a slow reaction. Different manganese oxides, have been studied towards OER electrocatalysis in 1 M KOH by Heese-Gärtein et al. The activity presented herein after activation, hence after evolution of the hydroxyapatite, is similar to the most active material reported by the authors, cryptomelane, and suggests that the hydroxyapatite surface evolves into a MnO2-based phase by solid-stat transformation or by dissolution-redeposition. 32 To avoid decomposition into permanganate, we assessed the possibility of maintaining the electrode at a sufficiently high potential by performing chronopotentiometry (CP) at 5 mA cm -2 for 20 h (Figure ). Some OER activation of material is verified in the first 1.5 h, achieving a value of 2.90 V vs RHE at this time. Then, the electrochemical response retained a good electrochemical stability, only showing a 5.5 % potential increase after 20 h. Noteworthy, no coloring of the electrolyte occurred, neither at the electrode surface nor at the electrolyte, which corroborates material stability. Meanwhile, we observe the evolution of O2 bubbles from the electrode. Nonetheless, reaching such a current density requires a high potential, thus further supporting the interpretation that the OER activity measured upon cyclic voltammetry (Figure ) arises from a MnO2-type electrocatalytic material forming at the surface of the material.