Atomic-Scale Imaging of Built-In Electric Field Reveals Fe-O_v Synergistic Enhancement for Oxygen Evolution Reaction

Imaging difficulties of the built-in electric field within water-splitting catalysts have hampered the acquisition of visual evidence for establishing the direct relationship between electronic structure and catalytic behavior. Herein, modeling on a cobalt-nickel bimetallic oxide comodified by iron doping and oxygen vacancies (O_v), we establish an explicit and comprehensive picture of the enhanced built-in electric field arising from the synergy effect of Fe and O_v at the atomic-level by utilizing differential phase-contrast scanning transmission electron microscopy. Heterogeneous Fe ions are revealed to introduce additional electronic conduction pathways for enhancing the overall electric field intensity. While O_v acted as absorption stations for attracting electrons and leading to a localized accumulation of electron density, Fe and O_v synergistically induce a strong and uniform built-in electric field, thereby enhancing charge migration. In situ spectroscopies further reveal that Fe doping promotes OH* intermediate adsorption by enhancing electric fields, and oxygen vacancies induce localized electric fields to facilitate the lattice oxygen oxidation mechanism. NiCo_2-xFe_xO_δ fully exploits the Fe-O_v synergistic effect to achieve long-term stability and high catalytic activity in the anodic oxygen evolution reaction (OER). We anticipate this work can elucidate the interplay between cation-anion coregulation and the built-in electric field, offering a rational design strategy for high-performance OER electrocatalysts.