Fig. 4. OER mechanism on the γ-phase of MFe LDHs.

a Structures of different surface phases and OER intermediates; adsorbates of surface phases are highlighted by blue circles on the sides views, and OER intermediates are differentiated by colors (yellow instead of white for hydrogen and rose instead of red for oxygen, respectively). A dashed rose circle indicates the formation of a surface O vacancy. The reaction centers are highlighted by large white circles. The magnetic moments of Ni and Fe during OER are also given on the top views. b Surface phase diagram of of γ-NiOOH, γ-NiFe LDH, and γ-CoFe LDH. The representative surface phases are given in a. c Reaction free-energy diagrams for OER on γ-NiOOH, γ-NiFe LDH, and γ-CoFe LDH; the potential limiting steps and the overpotentials are also given. d Volcano plot of the OER overpotential as a function of Gibbs free energies of the reaction intermediates.