Fabrication of vertical graphene-loaded nickel-iron nanoparticles and their oxygen evolution reaction performance

Abstract

<p indent="0mm">In recent years, the rapid consumption of traditional fossil fuels such as coal, oil and natural gas has caused inevitable harm to the environment. Hydrogen and other clean energy have attracted increasing attention. Electrolysis of water is one of the most promising technologies for hydrogen production compared with other techniques. However, due to the anode oxygen evolution reaction (OER) with slow dynamics, the catalysts such as RuO₂ and IrO₂ are not only expensive but also limited in reserve. Therefore, it is significant and urgent to develop OER catalysts with low cost and excellent catalytic activity and stability. As a representative transition metal, Ni is abundant on the earth and has excellent corrosion resistance. It is usually combined with Fe to prepare Ni-Fe oxides, hydroxides, sulfides, phosphating compounds and Ni-Fe alloys that can catalyze OER efficiently. From the view of catalytic kinetics, it is beneficial to construct the catalyst structure and reasonably design the specific morphology of nanomaterials. In other words, the activity of the electrocatalyst can be improved by increasing the number of active sites on a given electrode reasonably. In order to improve the catalytic activity of Ni-Fe catalytic system, a variety of processes have been developed to regulate the morphology and increase the number of active sites. Most of these processes focus on hydrothermal or calcination methods, which take a long time. Therefore, a simple and efficient method for preparing Ni-Fe catalysts with high catalytic activity for OER remains to be developed. Unlike the other three states (gas, liquid, and solid), plasma is composed of a mixture of electrons, ions, radicals and neutral particles. Compared with traditional methods, plasma-assisted modification (e.g., plasma etching, doping, and other surface treatments) and plasma-assisted synthesis (e.g., deposition, conversion, and decomposition) are becoming powerful tools for producing catalyst materials. Herein, vertical graphene nanosheets (VG) were prepared by plasma-enhanced chemical vapor deposition (PECVD). Then, Ni-Fe alloy nanoparticles were prepared by electrodeposition on the surface of VG as the active materials. The morphology and composition of the Ni-Fe electrocatalyst were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectrometry (EDS), Raman and X-ray photoelectron spectroscopy (XPS). Electrochemical workstation was used to characterize its electrocatalytic performance as OER catalyst. The overpotential of the catalyst for OER was obtained by linear sweep voltammetry (LSV), and the electrochemical active surface area (ECAS) was assessed by cyclic voltammetry (CV). The stability of the catalyst was also tested. The results show that the prepared nanocatalyst has excellent properties. In the solution of <sc>1 mol L^–1</sc> KOH, the overpotential is only 242 mV at <sc>10 mA cm^–2,</sc> the slope of Tafel is 43.13 mV dec^–1, and the stability is pretty good. This is mainly due to the open array structure of graphene nanosheets with small size of Ni-Fe nanoparticles, which is beneficial to mass transfer and gas diffusion. The synergistic structure has a large specific surface, so the catalyst exposes more active sites. The efficient method of preparation will provide an important technical basis for the preparation of transition metal-based OER nanocatalysts.