Superior oxygen evolution reaction activities of highly crystalline Ni1−xFex-LDH (0.20 ≤ x ≤ 0.51) synthesized using soft chemistry

Abstract

Developing oxygen evolution reaction (OER) catalysts is an urgent issue for the practical application of electrochemical devices such as water electrolysers and rechargeable batteries. NiFe-based layered double hydroxides (NiFe-LDH) are known as some of the most active OER catalysts. However, LDH is a metastable phase, making it difficult to synthesize it with high crystallinity and controlled composition and morphology. In this study, NiFe-LDHs are synthesized by soft chemistry using NaNi_1−xFe_xO₂ as the starting material. The soft chemistry samples have higher crystallinity with more uniform composition and morphology than the conventional coprecipitation samples. In addition, the substitution of Ni for Fe is successfully achieved at 51%, exceeding the conventional solid solution limit. The soft chemistry samples show superior OER specific activity with an overpotential of less than 0.20 V at 1.0 mA cm_BET⁻² in 1.0 mol dm⁻³ KOH, exceeding the activity of the conventional NiFe-LDH, indicating that the crystalline site has higher OER activity than the amorphous site. The soft chemistry samples exhibit the lowest Tafel slope (24.7–27.0 mV dec⁻¹) among the reported NiFe-LDH (>27.8 mV dec⁻¹). It is also found that the Fe substitution ratio for maximum OER activity exists in the range of 20–51 at%.