The present work aims to study the oxygen evolution reaction (OER) activity or catalytic properties of the Co₃O₄ facets, by applying density functional theory with an on-site Hubbard U term.

The effect of Fe/Ni doping at Co₃O₄ (001) on the OER activity was explored. The relative stability of the terminations of (001) surface was assessed by the surface phase diagram and the Poubaix diagram. By modelling, the termination with octahedral Co (B-layer) has a lower overpotential η (0.46 V) than the A-/0.5A-layer with two/one tetrahedral Co. The η of the former is decreased from 0.46 to 0.34 V by Ni doping. Likewise, the η of the A-layer is lowered from 0.63 V to 0.37 V/0.41 V due to Fe/Ni doping. Therein, the octahedral Co remains the active site. The inclusion of implicit solvation model does not alter the tendency of reduction in η by doping. The scaling relationship between the binding energies of ∗OOH and ∗OH is reproduced. Plotting -η as a function of (∆G^b_∗O -∆G^b_∗OH) contributes to a volcano, where the Ni-doped B-layer and Fe-doped A-layer are located near its peak. Furthermore, the OER activity trend was correlated to the electronic and magnetic properties of the Co active site during OER.

The oxidized/reduced Co₃O₄(001) surfaces were characterized and O₂ adsorption on both surfaces were explored. Firstly, the formation of O vacancies on the surface were identified for different vacancy concentration: (1) 1/4 ML: the vacancy at O bonding to three octahedral Co cations (O_3O) is easier to form than the one bonding to two octahedral and one tetrahedral Co (O_2O); (2) 1/2 ML: the double O vacancy V(O¹_3O , O²_2O) has the lowest formation energy. Secondly, we performed STM simulation for the surfaces with and without O vacancies using the Tersoff-Hamann model. Simulation of O₂ adsorption on the oxidized/reduced surfaces indicated that the experimental data are in the range of the theoretical binding energies. By analysis of the O₂ bond lengths, charge density difference and
projected density of states before and after O₂ adsorption, it is possible for the formation of superoxide at the reduced surface, consistent with the experiments. In addition, the vibrational spectrum were also simulated.

Finally, the OER performance between the Co₃O₄(001) and (111) surfaces was compared. For the (111) surface, terminations by tetrahedral Co (0.25-layer), both octahedral and tetrahedral Co (0.5-layer) and the cases precovered by ∗O and ∗OH groups were considered. Among them, the 0.25-layer has the lowest η (0.56 V). This is still higher than the B-layer (0.46 V) of the (001) surface. Higher overpotentials are observed at the ∗O-precovered cases for the two reaction sites: tetrahedral (0.58 V) and octahedral Co (0.80 V); at the ∗OH-precovered cases: tetrahedral (0.70 V) and octahedral Co (0.97 V), compared to the (001) for ∗O-precovered systems: B-layer (0.50 V) and A-layer (0.40 V). In conclusion, the (001)
surface exhibited better OER catalytic activity than the (111) surface.

Hopefully, this work could provide some inspiration to improve OER efficiency of Co₃O₄.