pH-dependent thermodynamic stability. Although the polaronic competition in perovskites determines the OER activity, the bulk stability region of CaMO 3 in an aqueous environment is still elusive. To explore the pH-dependent stability of CaMO 3 perovskites, we have calculated the Pourbaix diagrams with respect to their ground-state perovskite structures as shown in Fig. . All graphs represent the equilibrium conditions of CaMO 3 with the concentration of charged solvated ions 10 -8 mol/L. The area filled by the green color is the stable bulk region, in which the formation of CaMO 3 from the solvated ions is spontaneous. We found that the stabilities significantly depend on both pH and potential in an aqueous environment. Non of compounds are stable in an aqueous 13 environment at pH = 0, while the formation of perovskite oxides is pronounced in an aqueous environment at a high pH value of 14. The stability region of CaTiO 3 perovskite and aqueous ions was found to be in a broad range of the electrode potential and pH environments. However, the phase stability of other 3d metals is more evident at pH = 14 and at a portion of the potential, which shifts up by adding electrons in the 3d orbitals. The findings align with those of prior experimental study conducted under conditions where U RHE was maintained at 1.6 V and the pH was set to 13. The results of these studies demonstrate that the concentration of aqueous species approaches approximately 10 -8 M. Therefore, we propose that elevated pH levels, along with the precise regulation of potentials in aqueous en-