First-row transition metal oxides and chalcogenides have been found to rival or exceed the performance of precious metal-based catalysts for the interconversion of water and O2, central reactions that underlie renewable electricity storage and utilization. However, the high lability of the first-row transition metal ions leads to surface dynamics under the conditions of catalysis and results in active site structures distinct from those expected by surface termination of the bulk lattice. While these surface transformations have been well-characterized on many metal oxides, the surface dynamics of heavier chalcogenides under electrocatalytic conditions are largely unknown. We recently reported that the heazlewoodite Ni3S2 bulk phase supports efficient ORR catalysis under benign aqueous conditions and exhibits excellent tolerance to electrolyte anions such as phosphate which poison Pt. Herein, we combine electrochemistry, surface spectroscopy and high resolution microscopy to characterize the surface dynamics of Ni3S2 under ORR catalytic conditions. We show that Ni3S2 undergoes self-limiting oxidative surface restructuring to form an approximately 2 nm amorphous surface film conformally coating the Ni3S2 crystallites. The surface film has a nominal NiS stoichiometry and is highly active for ORR catalysis. Using DFT simulations we show that, to a first approximation, the catalytic activity of nickel sulfides is determined by the Ni-S coordination numbers at surface exposed sites through a simple geometric descriptor. In particular, we find that the surface sites formed dynamically on the surface of amorphous NiS during surface restructuring provide an optimal energetic landscape for ORR catalysis. This work provides a systematic framework for characterizing the rich surface chemistry of metal-chalcogenides and provides principles for the development of structure-energy-activity descriptors leading to a broader understanding of electrocatalysis mediated by amorphous materials.The interconversion of water and oxygen is a central chemistry underlying the storage of renewable electricity in energy-dense chemical bonds (Lewis and Nocera, 2006). The oxidation of H2O to O2 is the efficiency limiting half reaction for the splitting of water to generate H2 fuel, whereas the reduction of O2 to H2O is the efficiency limiting cathode reaction in low temperature fuel cells (Katsounaros et al., 2014). Platinum group metals and their corresponding oxides and chalcogenides are well-known catalysts for these reactions(Matsumoto and Sato, 1986) - (Gasteiger et al., 2005), but recent studies have uncovered a diversity of earth abundant first-row transition metal oxides(Kanan and Nocera, 2008) - (Long et al., 2014) and chalcogenides (Gao et al., 2012) , (Gao et al., 2013) that, depending on the reaction conditions, rival the activity of their precious metal analogs.Unlike their precious metal congeners, first-row transition metal ions are labile, Merbach, 1999, 2005) and as a result, the surfaces of these materials are expected to ...