Figure 4 . (a) Limiting potential of the selected candidates (Mn2@site1, Ru2@site1, Co2@site2, W2@site2, Os2@site3, V2@site4, Cr2@site4, Ni2@site4, W2@site4, and Re2@site4, respectively. (b)-(e) Gibbs free energy changes diagrams for Co2@site2, V2@site4, and Cr2@site4, respectively.