As the Fe ions in NF-AC-NiOx-Fe are located on the edges of the -NiOOH support, we explored the H transfer to various nearby sites on -NiOOH as potential hydrogen acceptors. It was found that an Ni3O site at a terrace plane of -NiOOH was a suitable hydrogen acceptor, with a potential of 1.3 V for Ni3O + H + + e -to Ni3OH (Fig. ). Such a site is abundantly present at the proximity of the Fe center (Fig. ). Incorporation of the Ni3O hydrogen acceptor completely alters the energy landscape of the OER on -FeOOH (Fig. and). The O-O bond forming step now only has a potential of about 1.3 V, resulting in a theoretical overpotential of only 0.1 V. This bifunctional catalysis avoids the high-energy OOH intermediate and introduces two new slopes for a revisited Volcano plot (green curves in Fig. ; see also Supplementary Fig. ). The left slope represents a region where the O-O bond formation through a bifunctional mechanism determines the overpotential. A narrow plateau is found at the top of this Volcano plot, where the formation energy of M=O ranges from 1.1 to 1.3 eV. At this plateau the overpotential is determined by the recovery of the H acceptor site and the theoretical overpotential is at its minimum value of 0.1 V. The -FeOOH-Ni3O bifunctional catalyst, which is proposed to be the active component of NF-AC-NiOx-Fe, sits exactly at the top of the "bifunctional Volcano" and represents a new benchmark for metal oxide OER catalysts.