Fig. 2. Electrochemical characterizations of Co₃O₄ and Co₃O₄/CeO₂ (prepared with 10 at% Ce) catalysts on FTO electrodes in 0.5 M H₂SO₄ solution.

a iR-corrected CV curves of both catalysts, the inset shows the overpotential (with error bar) required for each catalyst to reach a geometric catalytic current density of 10 mA cm^–2 based on the averages of three individual electrodes. b Magnified CV curve of the Co₃O₄ catalyst that highlights the three pre-OER redox features and the corresponding C1, C2, and C3 cathodic peaks. c The corresponding Tafel plots of both catalysts. d ECSA-normalized CV curves of both catalysts, the inset shows the ECSA-normalized catalytic current density (J_{ECSA-normalized}) of each catalyst at the overpotential of 450 mV.