Synthesis Of Iron-based Oxide And Hydroxide And Their Electrocatalytic Performance Of Oxygen Evolution

Hydrogen energy is a secondary energy source which is clean,non-polluting,green and low-carbon.Electrocatalytic water splitting is a currently commercial way to produce hydrogen powered by renewable energy.However,the proportion of hydrogen production from water electrolysis in China is only 1%due to the high operating load,high energy consumption and high cost.The oxygen evolution reaction（OER）in water splitting involves four-electron transfer,which has a slower kinetics and more complex reaction mechanism than the hydrogen evolution reaction（HER）,and is the bottleneck limiting the rate of the overall electrolytic water splitting reaction.Introducing the suitable catalysts can significantly lower the anode overpotential and thus reducing energy consumption.It is of great significance to develop catalysts with abundant reserves,low prices,stable properties and excellent catalytic performances.Iron-based elements（Fe,Co,Ni）are easy to lose their outer electrons to form variable oxidation states due to their unfilled d electrons.Iron-based compounds as electrocatalysts for OER have been widely explored,but their catalytic activities and stabilities still fail to meet the requirements of commercial applications.In addition,the exploration of the underlying mechanisms of doping,construction of heterojunctions and other modification strategies for enhancing catalytic activity is still in progress.In this paper,a series of iron-based catalysts for OER were synthesized by heteroatom doping,creation of defective sites and construction of self-supporting hierarchical heterojunctions.The effects of the modification strategies on the microscopic morphology and electronic structure of the catalysts and the underlying mechanism of catalytic activity enhancement were also investigated by combining experimental characterization and density functional thoery（DFT）calculations.The main studies are as follows:Ni,Co,Yb-FeOOH nanorod arrays grown on carbon cloth（CC）by a simple one-step hydrothermal method.X-ray photoelectron spectroscopy（XPS）,electron paramagnetic resonance spectroscopy and high-resolution transmission electron microscopy reveal that the introduction of Ni²⁺and Co²⁺can occupy Fe²⁺and Fe³⁺sites in FeOOH,and the doped Yb³⁺can enter the lattice or interstitial sites,leading to an increase in the concentration of oxygen vacancy and the density of edge dislocation in Ni,Co,Yb-FeOOH/CC,which enrich the active sites in FeOOH.DFT calculations confirm that the doped Ni²⁺,Co²⁺and Yb³⁺drive the d-band center of the main active site Fecloser to the Fermi level and reduce the Gibbs free energy change of the rate-determining step in the OER.The most efficient Ni,Co,Yb-FeOOH/CC can be obtained by adjusting the feeding ratio of metal salts to regulate the elemental composition and defect concentration of the catalyst,which requiring a overpotential of 230.9 m V at a current density of 10 m A cm^-2 and the Tafel slope is 22.7 m V dec^-1.NiCo₂O₄/Cu_xO（x=1 or 2）heterojunctions are grown on Cufoam using chemical etching,hydrothermal and calcination methods.The obtained catalyst has a unique three-dimensional self-supporting structure.X-ray diffraction,Raman spectroscopy,and XPS combined with DFT calculations confirm that the charges of Cuatoms in Cu₂O transfer to the O atoms in NiCo₂O₄ through the Cu-O bond formed at the interface,resulting in more active Cu²⁺sites in the heterojunction.The conductivity obtained by the four-probe method increases after constructing the heterojunction,indicating faster electronic transmission in the heterojunction.The heterogeneous interface in contact with the electrolyte in NiCo₂O₄/Cu_xO/Cuincreases and then decreases with increasing the hydrothermal time,and the catalytic activity of NiCo₂O₄/Cu_xO/Cushows the same trend.The most efficient NiCo₂O₄/Cu_xO/Cuis obtained when the hydrothermal time is 6 h:only 219 m V and 93 m V overpotential are required for OER and HER at a current density of 10 m A cm^-2.The three-dimensional self-supporting structure has abundant channels,which are conducive to the diffusion of electrolyte into the catalyst and the escape of gas products.The bonding strength between NiCo₂O₄,Cu_xO and Cufoam is strong,which gives NiCo₂O₄/Cu_xO/Cuexcellent mechanical stability:at a current density of 10 m A cm^-2,the OER and HER can remian stable for 125 h and 150 h,respectively.Self-supported p-n heterojunction FeCoNiLDH/CuO arrays grown on Cufoam by chemical etching,calcination and cathodic electrodeposition.Scanning electron microscopy and atomic force microscopy show that the size and thickness of FeCoNiLDH in p-n junctions are significantly reduced compared to FeCoNiLDH deposited directly on Cufoam,exposing more active sites on the edges.UV photoelectron spectroscopy,UV-vis diffuse reflectance spectroscopy,and XPS combined with DFT calculations confirm the existence of a strong built-in electric field of 1.1 e V at the p-n heterogeneous interface.The electrons flow from the n-type semiconductor FeCoNiLDH to the p-type semiconductor CuO,making FeCoNiLDH more positively charged near the interface and therefore more favorable for the adsorption of the initial reactant OH^-.In addition,the construction of p-n heterojunctions modulates the electronic structures of the main active sites Fe,Co,and Ni,making their d-band centers closer to the Fermi level and enhancing the interactions with the reactant molecules.The intrinsic OER activity of FeCoNiLDH is higher than that of CuO,and the construction of p-n heterojunction exposes more edge active sites and sites with positive charge at the interface in FeCoNiLDH,so the more FeCoNiLDH ultrathin nanosheet in contact with the electrolyte is more beneficial to the performance enhancement.The amount of FeCoNiLDH nanosheets in contact with the electrolyte in FeCoNiLDH/CuO/Cuis regulated by changing the electrodeposition time to obtain the most efficiennt catalyst.The FeCoNiLDH/CuO/Cuobtained with an electrodeposition time of 600 s requires only 243.1m V overpotential to obtain a current density of 50 m A cm^-2,and the Tafel slope is63.8 m V dec^-1.In addition,it remains stable for 26 h（continuous reaction for 104 h）at current densities of 10,25,50 and 90 m A cm^-2 respectivily.