Role of Site-Specific Iron in Fe-Doped Nickel Hydroxide Toward Water Oxidation Revealed by Spatially Resolved Imaging at the Single-Particle Level

Water electrolysis driven by renewable electricity is limited by the slow-kinetic oxygen evolution reaction (OER). NiFe-based hydroxides are considered promising non-noble electrocatalysts toward the OER but require profound insight into the role of site-specific iron incorporation. Herein, we determined the critical role of edge sites on single-crystalline NiFe-based hydroxide toward the OER using spatially resolved in situ single-particle imaging techniques. The potential-driven incorporation of Fe into the specific edge or plane sites was achieved on two-dimensional (2D) Ni layer double hydroxide (LDH) single crystals. The spatially resolved scanning electrochemical cell microscopy imaging illustrated that Fe-doped edge sites dominated the activity of the OER rather than Fe-doped plane sites. In situ Raman spectroscopy imaging of single particles was used to monitor the evolution of edge and plane sites, revealing that the incorporation of Fe impeded the oxidation of Ni. Moreover, spatially resolved ¹⁸O-isotope-labeling experiments demonstrated that Fe doping hindered the oxygen exchange between Ni LDH and the electrolyte, inducing the switch of partial active sites from Ni to Fe. Combined with theoretical calculations, the Fe-O_bridge-Ni sites contributed to the enhanced OER activity on Ni LDH with Fe doping at the edge, whereas the O_hollow (NiNiFe) sites induced by the infiltration of Fe into the plane were detrimental to the OER performance. This work provides direct spectroscopic evidence for understanding the specific sites at the single-particle level and guides the rational design of optimal electrocatalysts.