Electrodeposition and soft oxidation of nickel‑iron hydroxides: An efficient two-step approach for the synthesis of highly active and stable iron-rich NiFe-LDHs with controlled Ni/Fe composition for oxygen evolution reaction.

Nickel‑iron layered double hydroxides (NiFe-LDHs) are promising low-cost and active electrocatalysts for the alkaline oxygen evolution reaction (OER), yet conventional synthesis methods face two key limitations: (i) poor control over the Ni/Fe atomic ratio and (ii) difficulty in producing active, stable iron-rich NiFe-LDHs due to the formation of inactive iron hydroxide phases. Given iron's higher abundance and lower cost compared to nickel, developing iron-rich NiFe-LDHs is highly desirable. In this study, we present a two-step synthesis approach to overcome these challenges: first, rapid cathodic electrodeposition (<1 min) of NiFe(OH) onto an electrode, followed by controlled ambient oxidation of Fe to Fe, yielding phase-pure NiFe-LDH. By adjusting the electrodeposition potential, we achieve precise control of the Ni/Fe ratio in the film, matching the precursor solution composition. SEM-EDS analysis confirms this correlation for films electrodeposited at -2.0 V vs. Ag/AgCl (3 M KCl) across a broad Ni/Fe range. The oxidation step integrates iron (as Fe and Fe) into the LDH lattice without forming inactive iron oxide phases. XPS analysis reveals a consistent M/M ratio (∼1/3, M = Ni and Fe), consistent with the hydrotalcite (honessite) structure, indicating Fe oxidation is governed by the LDH structure. Electrochemical testing in 1 M aqueous KOH demonstrates outstanding performance: a NiFe-LDH with 35.1 ± 2.0 at. % Fe exhibits an overpotential of 167 ± 22 mV at 30 mA·cm, while a 76.6 ± 4.0 at. % Fe film requires 249 ± 10 mV. Both catalysts maintain stability over 100 h at 100 mA·cm, surpassing previously reported NiFe-LDHs with comparable Fe content in activity and durability. This work provides a scalable route to iron-rich NiFe-LDHs synthesis with controlled composition and high catalytic performance.