Figure 6. Density functional theory calculation for OER mechanism of Na₂CoP₂O₇.

(a) Wulff structure of Na₂CoP₂O₇, showing the equilibrium crystal shape based on the relative surface energies and the enlarged (101) surface with active sites. (b) Local environment around the active cobalt atom located on the (101) surface before and after P₂O₇ rotation; the active cobalt atom is depicted in pink. (c) Schematic of the Gibbs free energy changes for the four elementary steps during the OER based on DFT calculations. The local structure and valence change of the cobalt atoms in the active sites are shown at the corners. The inset shows the free-energy landscape on the (101) surface of Na₂CoP₂O₇ compared with an ideal catalyst at pH 7.0. An ideal catalyst, Na₂CoP₂O₇ for U=0 and Na₂CoP₂O₇ for U=0.817 are depicted by the dashed line, blue line and red line, respectively.