The distance of Fe-Nioutside path changes dramatically, from 3.98 Å in the dry sample to 3.80 Å at no bias and  = 0.22 V to 3.34 Å at  = 0.27 V and further to 3.21 Å at  = 0.37 V. This result indicates strong structural changes at the interface of the -FeOOH clusters and the -NiOOH support. From the dry sample to the catalyst at the beginning of OER ( = 0.27 V), the structural change can be accounted for by considering a 52 o tilt of the FeO6 octahedron relative to the NiO6 octahedron (Fig 3c ). A further 3.6 o tilt of the FeO6 octahedron can account for the structural change from  = 0.27 V to  = 0.37 V. While the more than 50 o tilt is significant, it seems feasible since the FeO6 octahedrons in pure -FeOOH are tilted at about 23 o degree one over the other (Supplementary Fig. ). Notably, this significant structural change only occurs on NF-AC-NiOx-Fe catalyst. The NiFe LDH reference sample, which is considered as the active form of NiFeOx, exhibits no such structural changes according to operando XAS (Supplementary Fig. ). This comparison further underscores the unique nature of the present NF-AC-NiOx-Fe catalyst relative to NiFeOx and the peculiar role of the interaction between the NiOOH support and the discrete FeOOH clusters. The DFT computations shown in Figs. and assist in understanding how NF-AC-NiOx-Fe can significantly outperform the state of the art NiFeOx catalyst. Here, a generic isolated -FeOOH was constructed. This model contains the key features of the active catalyst, i.e., the brucite type structure and the octahedral coordination by six oxygen ligands (Fig. ). Computationally, this model has the advantage of representing potentially active Fe sites while avoiding complications resulting from interfacing the cluster with the -NiOOH support. As shown in Fig. , -FeOOH binds O too strongly and is placed on the strong binding side of the Sabatier volcano (Fig. ). The formation of Fe=O has an equilibrium potential of only about 1.2 V. The potential determining step is the hydroxide attack on M=O, which has a potential of 1.7 V and results in a theoretical overpotential of 0.5 V (Fig. ). Thus, -FeOOH alone is only a modest catalyst. However, if the O-O bond forming step on -FeOOH can proceed through an alternative pathway, such as a bifunctional mechanism that avoids the potential limiting formation of M-OOH, the overpotential can be reduced . The bifunctional mechanism assumes the direct formation of O2 through a nucleophilic attack of OH -coupled with a concerted H transfer to an adjacent acceptor site, A :