Preparation And Research Of Carbon Black Composite NiFe Layered Double Metal Hydroxides For Oxygen Evolution Reaction

With the increasing consumption of fossil fuels,hydrogen energy,as a high-calorie,clean gas,has attracted widespread attention from researchers around the world.Water splitting is one of the most promising ways to exploit hydrogen energy.However,during the splitting of water,the slow four-electron transfer process of OER greatly restricts the reaction kinetics,which leads to an increase in overpotential.Ruthenium-based and iridium-based precious metal catalysts are the most effective electrocatalysts in OER processes.Unfortunately,high prices and scarce storage have severely limited their commercialization.Therefore,it is urgent to develop cost-effective catalysts.In recent years,non-precious metal catalysts such as nickel,cobalt,and iron have attracted wide attention due to their high OER activity and good alkali resistance.Among all the alternative non-precious metal catalysts,Ni Fe layered double hydroxides?Ni Fe LDHs?are considered as a standard oxygen evolution electrocatalyst under alkaline conditions.Compared to a single nickel and iron component,the high effect of Ni Fe LDHs on catalytic performance is attributed to the synergistic interaction between nickel and iron atoms.Nonetheless,the low active site exposure rate and poor conductivity of Ni Fe LDHs still hinder their OER performance.In addition,the characteristics of Ni Fe LDHs materials which are easy to agglomerate and the heterogeneity of the components will cause their instability.Based on the above problems,a series of researches were carried out on the design,preparation and characterization of Ni Fe LDHs,Ni Fe LDHs/CB and Ni Fe LDHs/CB-TC catalysts.The main results are as follows:Using urea as the precipitant and ammonium fluoride as the crystal form regulator,a highly spherical Ni Fe LDHs OER catalyst was successfully prepared by a simple one-step hydrothermal method.Electron microscopy results showed that the flower ball-shaped Ni Fe LDHs material produced by this method had a flower ball diameter of about 6?m,a single-layered plate was hexagonal,and the surface was smooth without holes.In this work,the synthesis conditions of Ni Fe LDHs:metal ion ratio,hydrothermal temperature and solvent self-volatility were precisely controlled.The results of electrochemical experiments show that in 1M KOH electrolyte,the Ni Fe LDHs catalyst has the best OER performance when the molar ratio of nickel to iron is2:1 and the hydrothermal temperature is 120°C.At a current density of 10 m A/cm²,its overpotential is 336 m V.In addition,the self-volatilizing environment of the solvent will also help the OER performance of the Ni Fe LDHs catalyst.Carbon black is used as the conductive substrate,and ammonium fluoride is used as the crystal type regulator.By adding the pretreatment step before the hydrothermal method and regulating the multivariable conditions of metal salt type,precipitating agent type,carbon black?CB?type,the solvent mixing ratio and pretreatment conditions,a highly active Ni Fe LDHs/CB catalyst was successfully screened and successfully prepared.Electron microscopy results showed that after the addition of nitric acid-modified carbon black supplemented by the corrosion of ammonium fluoride,the main laminates of Ni Fe LDHs gradually appeared with circular holes of about 4-7?m,and the lateral size of the single sheet was about 200 nm.The results of electrochemical test showed that the best OER performance of Ni Fe LDHs/CB catalyst was obtained in 1M KOH electrolyte with nitrate as metal salt,urea as precipitant,BP2000 as carbon black and 60°C as pretreatment condition for 10 min plus self volatilization.In addition,at a current density of 10 m A/cm²,its overpotential is 287m V.The introduction of nitric acid-modified carbon black brings high conductivity and rich pores to the composite LDHs material,which accelerates the charge transfer effect,provides a large number of active sites,and significantly improves the OER performance of the catalyst.In addition,the self-volatilizing environment of the solvent will also help the OER performance of the Ni Fe LDHs/CB catalyst.Using carbon black as the conductive substrate,ammonium fluoride as the crystal form modifier,and trisodium citrate?TC?as the surfactant,a highly active OER catalyst with a coated structure was successfully prepared by a modified hydrothermal method after the pretreatment step:Ni Fe LDHs/CB-TC.The electron microscope results show that the randomly arranged nanoplates are completely wrapped in the amorphous CB,which ensures a strong interface connection between the LDHs and the CB,which will facilitate the electron transfer in the OER process.The electrochemical test results show that the overpotential is 236 m V and the Tafel slope is 31 m V/dec in 1M KOH electrolyte?10 m A/cm²?.In addition,the specific surface area of Ni Fe LDHs/CB-TC(627.4 m² g^-1)is approximately 17 times of Ni Fe LDHs and 2 times of Ni Fe LDHs/CB,showing a clear mesoporous structure.Its large specific surface area and suitable pore size not only facilitate the diffusion of electrons and OH-,but also rapidly release O₂during the oxygen release reaction.At the same time,a larger specific surface area can also provide a large number of active sites and more defects,resulting in excellent OER activity.