Unraveling Geometrical Site Confinement in Highly Efficient Iron-Doped Electrocatalysts toward Oxygen Evolution Reaction

Introduction of iron in various catalytic systems has served a crucial function to significantly enhance the catalytic activity toward oxygen evolution reaction (OER), but the relationship between material properties and catalysis is still elusive. In this study, by regulating the distinctive geometric sites in spinel, Fe occupies the octahedral sites (Fe³⁺ _(Oh)) and confines Co to the tetrahedral site (Co²⁺ _(Td)), resulting in a strikingly high activity (η_{j = 10 mA cm} ⁻² = 229 mV and η_{j = 100 mA cm} ⁻² = 281 mV). Further enrichment of Fe ions would occupy the tetrahedral sites to decline the amount of Co²⁺ _(Td) and deteriorate the OER activity. It is also found that similar tafel slope and peak frequency in Bode plot of electrochemical impedance spectroscopy indicate that Co²⁺ _(Td) ions are primarily in charge of water oxidation catalytic center. By means of electrochemical techniques and in situ X-ray absorption spectroscopy, it is proposed that Fe³⁺ _(Oh) ions mainly confine cobalt ions to the tetrahedral site to restrain the multipath transfer of cobalt ions during the dynamic structural transformation between spinel and oxyhydroxide, continuously activating the catalytic behavior of Co²⁺ _(Td) ions. This material-related insight provides an indication for the design of highly efficient OER electrocatalysts.