“…12 Cobalt oxides (CoO x ) present high catalytic activity for OER in alkaline solution, especially the Co 3 O 4 spinel that has been widely investigated as an attractive anode material with high activity, corrosion resistance in alkaline electrolytes, excellent redox property originating from its Co 2+ /Co 3+ mixed valence nature. 1,3,5,6,[13][14][15] Furthermore, the development of acid-stable OER electrocatalysts is still a challenge. 5 The electrolyte pH has a signicant impact on the choice of the electrocatalyst, for instance, for the application of proton exchange membrane (PEM) electrolyzer for hydrogen production at acid pH, active and resistant catalysts are necessary.…”