A study of photocatalytic, chemical, and electrocatalytic water oxidation on ACo2O4 (A = Ni, Cu, Zn) samples through doping different metal ions

• The catalytic activity of ACo2O4 samples follows the catalytic activity order NiCo2O4 > CuCo2O4 > ZnCo2O4.
• The catalytic activity was investigated under photochemical, chemical, and electrochemical conditions.
• The oxidation peak of the NiCo2O4 sample was detected, which may be assigned to oxidation from Ni2+ to Ni3+.
• For the activity order, the doping metal ions of ACo2O4 oxides take part in the reaction.
• Different charge and electron transport rates lead to different catalytic activity.

ACo2O4 samples were synthesized as water oxidation catalysts. The samples were characterized by multiple experiments to confirm their uniform morphology, which belongs to the spinel structure (Fd3m space group). The catalytic activity of ACo2O4 samples was investigated under photochemical, chemical, and electrochemical conditions; it follows the catalytic activity order NiCo2O4 > CuCo2O4 > ZnCo2O4. This activity sequence results from different doping metal ions in {Co4O4} motifs, which act as the active sites of water oxidation. Electrochemical impedance spectroscopy and other experimental data demonstrate that ACo2O4 samples doped with different metal ions may influence the rate of charge and electron transport. Therefore, the catalytic activity of water oxidation followed the above order.

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