Preparation And Performance Research Of NiFe Hydroxide And Co Oxide Electrocatalyst

With the increasing depletion of petrochemical resources,the contradiction between energy demand and environmental pollution has become more acute,and it is particularly important to find renewable and environmentally friendly new energy sources.Hydrogen energy has high energy density,no carbon emissions,and does not have problems such as intermittent power generation and installation environment limitations,which compared with other new energies such as solar energy,wind energy,tidal energy,and nuclear energy.It is regarded as one of the most development potential new energy.At present,in commercial hydrogen production pathways,hydrogen production by electrochemical water splitting not only obtain high-purity hydrogen,but also could be recycled,which is a simple and reliable hydrogen production technologies.In order to further improve hydrogen production efficiency and reduce the price,non-noble metal transition catalysts have been preferred by researchers.However,there is still a certain gap in its catalytic performance compared with precious metal catalysts.Partly due to its poor electrical conductivity,sluggish kinetics and low intrinsic activity,how to effectively solve the above problems is the keyto attain efficient transition metal catalyst.The doping of atoms and the regulation of the microstructure are one of the methods to solve the problem.Heteroatoms replace the original atoms or enter the transition metal latti ce to change the electronic structure environment,which can effectively improve the intrinsic activity and conductivity while improving the adsorption energy of the intermediate.This article focuses on the controllable preparation of transition metal catalysts,the introduction of heteroatoms and the regulation of nanostructure morphology obtain a series ofhigh-efficiency electrocatalytic materials,and we analyze their formation mechanism and performance.It provides a reference for further analyzing the mechanism of heteroatoms to improve the performance of transition metal catalysts.The main contents of this article are as follows:NiFe layered hydroxide（Ni Fe-LDH）nanosheets with composition gradient characteristics are prepared in situ on the Ni foam conductive substrate by using trisodium citrate as a chelating agent.The array of three-dimensional nanostructures has a large electrochemically active surface area,which can expose more active sites to participate the redox reactions.At the same time,a chelating reaction is used to make Ni and Fe atoms build a concentration gradient on the longitudinal direction of the nanosheet,which provides a transfer channel for efficient carrier transport which reduces the resistance of the material.In addition,the introduction of Fe ions changes the electronic environment of Ni OOH,improves the intrinsic activity and adjusts the adsorption energy of the intermediates.Facilitate the adsorption of reactive components to tntermediate particles.Compared with Ni Fe-LDH without composition gradient,the composition gradient Ni Fe-LDH nanosheets have better oxygen evolution catalytic performance in alkaline electrolyte.The overpotential is only 270 m V at a current density of 50 m A·cm^-2,and the Tafel slope is 63.6 m V·dec^-1,which has excellent chemical and structural stability in alkaline electrolyte solutions.Using three-dimensional porous nickel foam as the growth substrate,the Co（OH）F precursor with a urchin-like morphology is grown on the substrate through a hydrothermal reaction,Co₃O₄ with different characteristics can be obtained by sintering under different heat treatment conditions（air,nitrogen,nitrogen and phosphorus source）:Co₃O₄ with a normal amount,Co₃O₄-Ov with a large number of oxygen vacancies and phosphorus-doped Co₃O₄-P with phosphorus atoms filling part of oxygen vacancies.The unique urchin-like morphology provides a large specific surface area and more reactive sites during the electrochemical water splitting process.The introduction of vacancies and phosphorus atoms not only improves the conductivity of Co₃O₄,but also adjust the electronic environment to enhance the intrinsic activity of the material and at the same time as an active site to participate in catalysis,thereby obtaining a high-efficiency electrocatalytic performance.In addition,the vacancy will destroy the integrity of the crystal,the introduction of P atoms will transform the Co₃O₄ crystal into amorphous state,which has higher catalytic activity than the crystalline state.Co₃O₄-Ov exhibits more excellent catalytic activity in the oxygen evolution reaction.The current density of 20 m A·cm^-2 corresponds to an overpotential of 279 m V,while Co₃O₄-P only needs an overpotential of 110 m V in the hydrogen evolution reaction when the current density is 20 m A·cm^-2.Therefore,an electrolytic cell composed of Co₃O₄-Ov and Co₃O₄-P as anode and cathode can achieve a current density of 20 m A·cm^-2 with only 1.58 V during the overall water splitting.Co₃O₄-P with good conductivity and excellent HER performance is used as the current collector,and amorphous Ni Fe-LDH layered hydroxide is deposited on nanoneedle surface.The dendritic structure formed by combining the nanosheets with the urchin-like morphology not only increases the catalytic activity area,but also facilitates the rapid transfer of electrons required for the redox reaction from the electrode to the reaction area.Because of its special composite structure and morphology,the electrocatalytic reaction has the following characteristics:Ni Fe-LDH will be the main reactant during the oxygen evolution reaction,while Co₃O₄-P is mainly involved in the reaction during the hydrogen evolution reaction.This composite structure has excellent performance of oxygen evolution and hydrogen evolution,and only needs an overpotential of 269.9 m V and-168.5 m V to achieve a current density of plus or minus 50 m A·cm^-2.At the same time,as a bifunctional catalyst,the overpotential is only 1.51 V when the current density is 50 m A·cm^-2 during theoverall water splitting.