The Preparation Of Cobalt-Iron-based Layered Double Hydroxides And Their Application In Electrocatalytic Water Splitting

As an ideal energy carrier with high calorific value of combustion,hydrogen is widely regarded as one of the most promising chemical energy sources that can replace fossil fuels.Among the many hydrogen production schemes,electrocatalytic water splitting is an effective pathway with advantages of better economic benefits and relatively pure products,this technology has become a hot research direction.The electrocatalytic water splitting reaction includes two half reactions of hydrogen evolution reaction（HER） and oxygen evolution reaction（OER）.Among them,the oxygen evolution reaction is a four-electron kinetic slow process,which limits the efficiency of electrocatalytic water splitting.The development of high-efficiency OER catalysts have been extensively studied.Commercial precious metal catalysts such as IrO₂ and RuO₂ have good OER catalytic activity,but precious metals are scarce in resources,expensive,and poor in stability,thus limiting their large-scale application in actual production.In summary,seeking and developing cheap and efficient transition metal catalysts for electrocatalytic water splitting has become the focus of research.In this thesis,CoFe-based layered double hydroxide（LDH） is used as the main material of the study,and its electronic structure is optimized through the third metal doping and interface engineering design strategies,thus improving its OER catalytic activity and stability.Moreover,through the phase and structure characterization before and after the OER reaction,the reaction mechanism of the catalyst was discussed.The specific research content is as follows:1.Preparation of V-doped CoFe LDH grown on nickel foam for superior water oxidation performanceUsing nickel foam（NF） as the conductive framework,the CoFe V LDH nanosheet array was prepared by a one-step hydrothermal method.By adjusting the doping ratio of the V element,the influence of the third metal doping on the catalytic performance of the LDH material was explored and the sample with the best catalytic activity was obtained.In alkaline solution,CoFe V LDH has significantly better OER catalytic activity than CoFe LDH and Co V LDH.The overpotentials of 242 and 330 mV are needed to achieve current densities of 10 and 100 mA cm^-2,while the Tafel slope is only57 mV decade^-1,and the Faradic efficiency is 98.7%.Through structural characterization and mechanism analysis after OER,the results show that the electrophilic properties of vanadium ions in a high valence state can maintain the active site in a high valence state,thereby promoting the OER reaction,making the electrocatalyst have a lower overpotential and higher active.The design strategy of this material provides new ideas for the preparation of non-noble metal OER catalysts with high catalytic activity.2.Preparation of Zr-doped CoFe LDH grown on nickel foam for highly efficient alkaline seawater electrolysisA high-efficiency Zr-doped CoFe LDH electrocatalyst grown on NF was designed through a simple electrodeposition process.The reduction potential method was used to directly electroplate NF in a metal salt solution,and the CoFe Zr LDH/NF electrocatalyst was successfully prepared.The growth of LDH is controlled by adjusting the deposition time to optimize the OER and HER activity.Studies have shown that doping of Zr⁴⁺ can adjust the electronic structure of CoFe LDH and contribute to the exposure of a large number of active sites to enhance electrocatalytic activity.The prepared electrocatalyst has excellent bifunctional catalytic activity,with lower OER and HER overpotentials of 233 and 159 m V at current density of 10 m A cm^-2,respectively.Moreover,it has similar high activity even in alkaline simulated seawater electrolytes.On this basis,when the material is used as an anode and a cathode for electrocatalytic splitting of seawater,a current density of 10 mA cm^-2 can be achieved with only 1.649 V.This work has guiding significance for the design of high-efficiency catalysts in the electrolysis of seawater.3.Preparation of CeO_2-x modified CoFe LDH grown on nickel foam for highly efficient water oxidationCoFe LDH modified with CeO_2-x nanoparticles was grown on nickel foam(CeO_2-x@CoFe LDH/NF)by a one-step hydrothermal method.This chapter investigates the effect of the synergy produced by the heterostructure of CeO_2-x and CoFe LDH on the catalytic activity of OER.In alkaline solution,CeO_2-x@CoFe LDH/NF exhibits excellent electrocatalytic performance and stability,reaching a current density of 100mA cm^-2 at an overpotential of 204 m V.Besides there is almost no attenuation under this current density for 30 hours of continuous work.In addition,when tested in simulated alkaline seawater（1.0 M KOH+0.5 M NaCl）,the material is not affected by the corrosive effect of chloride ions,and still maintains high activity and durability for OER.Through the investigation of its electrocatalytic mechanism,the results show that CeO_2-x nanoparticles containing oxygen vacancies and the interface between CoFe LDH and CeO_2-x have a strong electronic interaction,which significantly improves the electrocatalytic performance.This work provides an important idea for the design and optimization of efficient transition metal-based OER electrocatalysts.