The importance of the iron valence state in NiCoFe nanosheet array catalysts for the oxygen evolution reaction

Abstract

The development of low-cost, highly active and stable electrocatalysts for the oxygen evolution reaction (OER) is desirable and challenging. In this work, we report nickel foam (NF) supported low-crystalline NiCoFe nanosheet array electrocatalysts derived from NiCo layered double hydroxide via spontaneous cation exchange reaction. Driven by the redox properties of NiCo layered double hydroxide and iron salt precursors, the entire cation exchange reaction process is completed without extra energy input. The chemical composition and catalytic properties of the as-prepared NiCoFe/NF catalysts are strongly dependent on the valence state of the Fe cation in the iron salt precursors. The NiCoFe/NF–FeSO₄ catalyst with FeSO₄ as the iron salt precursor exhibits satisfactory OER activity with a much lower overpotential of 293 mV to reach a current density of 100 mA cm⁻² as compared with NiCoFe/NF–Fe₂(SO₄)₃ (η₁₀₀ = 333 mV), NiCo/NF (η₁₀₀ = 425 mV) and commercial RuO₂ (η₁₀₀ = 399 mV) catalysts. Moreover, NiCoFe/NF–FeSO₄ also exhibits impressive long-term durability for the OER. The current density remains nearly unchanged during the 95 h of continuous OER test, demonstrating a promising application in actual industrial water electrolysis. These findings indicate that the iron ion valence states in the iron salt precursors have a significant impact on the composition of the catalysts and thus affect their catalytic performances. This work is instructive for the rational design and preparation of highly efficient Earth-abundant electrocatalysts for the OER.