To evaluate the opportunities of using the Mn(V) hydroxyapatite for oxidizing water, we focused on the nanostructured hydroxyapatite to maximize the surface area exposed to the electrolyte. The open-circuit potential (OCP) is 1.42 V vs RHE in 1 M KOH; which is already above the thermodynamic potential for OER (1.23 V vs RHE), thus suggesting that Mn(V) is not stable in such conditions. Indeed, cyclic voltammograms recorded in the same electrolyte at 20 mV s -1 (Figure ) show an OER onset potential around 1.60 V vs RHE, an activation of the material over the first 5 cycles, and the concomitant growth of a reversible reduction wave, which keeps increasing upon cycling. The redox wave with oxidation and reduction peaks at 1.49 and 1.28 V vs RHE, respectively, indicates a Mn(VII)/Mn(VI) couple. Accordingly, the electrolyte became purple. This indicates the release of permanganate. Mayhofer et al. reported a similar behavior for manganese-based oxides that degrade and produce permanganate at OER potentials. This is also in agreement with the work of Rabe et al. who reported the dissolution of Mn as permanganate when a manganese oxide electrocatalyst was submitted to an OER potential in a 0.1 M NaOH electrolyte. In addition, a simultaneous deposition of MnO2 during manganese dissolution during OER has been previously proposed. This mechanism is likely related to the activation of OER (Figure ).