Effect of Interlayer Anions in Ni–Fe Layered Double Hydroxides for Electrochemical Seawater Splitting

Nickel–iron layered double hydroxides (Ni–Fe LDHs) are among the active earth abundant electrocatalysts for the oxygen evolution reaction (OER) in alkaline media. The electrocatalytic performance is strongly influenced by both catalyst composition and the intercalated ions. However, there are lack of systematic comparison of the effect of intercalated anions which are abundantly found in sea water towards electrochemical water splitting. Here, Ni–Fe LDHs intercalated with chloride, nitrate, sulfate, and carbonate anions were synthesized via a hydrothermal route and systematically evaluated the role of interlayer anions under idealized and realistic electrolytes. Structural and compositional analyses confirm the formation of well-defined LDH phases with comparable Ni:Fe ratios with different interlayer compositions based on the precursor used. In 0.1 M KOH, divalent oxyanion intercalated LDHs (SO₄²⁻ and CO₃²⁻) exhibit enhanced OER activity, achieving lower onset potentials (1.26–1.30 V vs Ag/AgCl/sat. KCl) than monovalent anion systems (Cl⁻ and NO₃⁻). In contrast, when natural seawater is used as the electrolyte, all samples display nearly identical onset potentials (~1.60 V vs Ag/AgCl/sat. KCl), indicating that the intrinsic benefits of interlayer anion engineering are largely suppressed by the complex ionic environment. These results demonstrate that interlayer anion chemistry provides an effective approach for tuning Ni–Fe LDH activity in alkaline media but becomes secondary under seawater conditions.