The potential determining step is either the formation of M=O (step 2) or the step forming the O-O bond by hydroxide attack of the M=O species (step 3) depending on their respective potentials (black slopes in Fig. ). The adsorption energies of OH, O, and OOH are correlated. In particular, the difference in the surface adsorption energy of OH and OOH is, according to Density Functional Theory (DFT) computations, independent of the catalyst and approximately 3.2 eV . Because of this scaling relationship, the minimal theoretical overpotential () for an oxide catalyst is about 0.4 V. According to computations , on catalysts like NiO, -NiOOH, and RhO2, O binds too weakly which causes the formation of M=O to become potential determining and the overpotential increases beyond 0.4 V. For catalysts like Mn3O4, -MnO2, -MnO2, IrO2, and -CoOOH, O is too strongly bound and the formation of M=O occurs below 1.6 V . The potential determining step is the O-O bond formation, which requires more than 1.6 V. The theoretical overpotential is again larger than 0.4 V. For NiFeOx, both steps 2 and 3 occur at about 1.6 V, putting it near the top of a Sabatiertype Volcano-plot (Fig. ) . Thus, NiFeOx is predicted to be among the "most active" OER catalysts, far superior to NiOOH, CoOx, and MnOx. This prediction agrees well with experimental results . To date, the scaling relationship has proven to govern the majority of metal oxide catalysts, including all the known "most active" mixed oxide catalysts such as NiFeOx, CoFeOx, and FeCoW oxyhydroxide .