Figure 4. OH⁻ adsorbed surfaces for SFO and CCFO and corresponding OER mechanism.

(a) OH⁻ adsorbates on Fe-terminated (100) plane of SFO for Fe-mediated route (ER type). The interatomic distance between the nearest neighbouring OH⁻ adsorbates is ∼3.9 Å. (b) OH⁻ adsorbate on (Ca,Cu)O-terminated (100) planes of CCFO for Cu-mediated route (ER type). (c) OH⁻ adsorbates on FeO₂-terminated (100) planes of CCFO for Fe-mediated route (LH type). The interatomic distance between the nearest neighbouring OH adsorbates is ∼2.6 A. In all cases, the Cu²⁺/Cu³⁺ or Fe⁴⁺/Fe⁵⁺ redox couple acts as the reaction mediator under the assumption that the adsorbed OH⁻ ions occupy the original oxygen sites in the crystal structure.