Abstract
This paper describes the wet-chemistry synthesis of highly crystalline hexagonal flakes of Ni-Fe layered double hydroxide (LDH) produced at temperature as low as 100 °C. The flakes with diameter in the range of 0.5–1.5 μm and the thickness between 15 and 20 nm were obtained by homogeneous precipitation method with the use of triethanolamine (TEA) and urea. By analyzing the intermediate products, it is suggested that, differently from previous reports, a thermodynamically metastable iron oxyhydroxide and Ni-TEA complex are firstly formed at room temperature. Subsequently, when the mixture is heated to 100 °C and the pH increases due to the thermal decomposition of urea, Ni2+ and Fe3+ are slowly released and then recombine, thus leading to formation of pure, highly-crystalline Ni-Fe LDH flakes. This material showed promising results as an electrocatalyst in oxygen evolution reaction (OER) providing an overpotential value of 0.36 V.
Highlights
Ni-Fe LDH layered double hydroxide (LDH) have become the focus of an extensive scientific research, mainly due to the high electrocatalytic activity of this material in oxygen evolution reaction (OER), oxygen reduction reaction (ORR) and electrode material for supercapacitor[1,2,3,4,5]
The morphological and structural characterization of the as-prepared LDH was carried by Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), Atomic force microscopy (AFM), Fourier-transform infrared (FT-IR) and Thermogravimetric analysis (TGA) analysis
The homogenous contrast observed in the TEM micrographs (Fig. 1b) suggests that the flakes have a uniform thickness and the selected area electron diffraction (SAED) pattern (Fig. 1c) shows a hexagonal symmetry (a = 3.08 Å, c = 23.55 Å), which is confirmed by X-ray diffraction (XRD) patterns (Fig. 1d)
Summary
Ni-Fe LDH layered double hydroxide (LDH) have become the focus of an extensive scientific research, mainly due to the high electrocatalytic activity of this material in oxygen evolution reaction (OER), oxygen reduction reaction (ORR) and electrode material for supercapacitor[1,2,3,4,5]. The synthesis of Fe3+-containing LDH with high crystallinity and a well-defined shape is challenging because usually gel-like, water-insoluble Fe(OH)[3] precipitates at pH above 2, which impedes the further incorporation of Ni2+ within its structure[10,11,12] Several techniques, such a as reversed co-precipitation[13], ball milling[14], topochemical routes[10,15] or co-precipitation with the use of long-chain organic acid[16], were implemented in order to synthesize high-quality Ni-Fe LDH. The topochemical route relies on an initial precipitation of Fe2+ and its subsequent oxidation to Fe3+ using iodine[10] or anthraquinone-2-sulfonate[10,15] This additional synthesis step led to the formation of regular hexagonal platelets[15], Fe2+ ions and oxidizing agents are possible sources of contamination. Preliminary electrochemical measurements suggest that this material might be potentially used as a catalyst for OER
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