Fig. 4.

Cation selective layer generation during activation in salty electrolyte. (A) Three-electrode OER constant current activation of Ni³ in 1 M KOH + 0.5 M NaCl, resistance 1.4 ± 0.05 ohm, electrode area 0.5 cm². The decrease in voltage that occurred between 3 and 4 h was due to the etching-passivation process. Note that Ni³ was first activated in 1 M KOH under 400 mA/cm² for 12 h before activation in 1 M KOH + 0.5 M NaCl. (B) OER R_FE plots for O₂ production taken during A. Decrease in voltage at 3 to 4 h corresponds to a small decrease in R_FE. Error bars were obtained by three parallel tests. (C) Raman spectra of Ni³ and NiS_x/Ni after 12-h activation in 1 M KOH + 0.5 M NaCl, suggesting polyatomic anion intercalated LDH phase and formation of sulfate species at the LDH/NiS_x interface. (D) TOF-SIMS mapping of SO_x^2-/1- and CO₃^2-/1- fragments from a Ni³ and NiS_x/Ni electrode surface after activation in 1 M KOH + 0.5 M NaCl at 400 mA/cm². Negative TOF-SIMS counts were collected from m/z = 96/48/80/40 (SO₄⁻/SO₄^2-/SO₃⁻/SO₃^2-) and 60/30 (CO₃⁻/CO₃^2-) after Ar plasma milling for 5 to 15 min to clean the surface adsorbed electrolytes. (Scale bars: 10 μm.)