Effect Of Vanadium Doping And Size Regulation Of NiFe Double Hydroxide On Electrocatalytic Oxygen Evolution Reaction

The development of global economy,the increase of population and the acceleration of industrialization have caused the rapid consumption of energy all over the world and brought serious pollution to the global environment.In this context,the development of renewable energy came into being,and researchers gradually shifted their research focus to this field.Hydrogen energy is known as one of the cleanest energy sources in the world since the energy revolution.It has the advantages of rich reserves and high energy supply efficiency.It will become one of the most potential energy sources.Electrolysis is one of the most convenient methods to produce hydrogen,which is the process of electrolyzing hydrogen into water.Among them,the oxygen evolution half reaction needs the transfer of four electrons and protons,which needs to overcome a higher energy barrier than the hydrogen evolution reaction.Therefore,the speed of its reaction efficiency directly determines the rate of water decomposition reaction.It is reported that noble metals and their compounds doped with transition metals are widely used catalysts.Common noble metal catalysts for electrolytic water include iridium dioxide（Ir O₂）and ruthenium dioxide（Ru O₂）.These catalysts have good catalytic activity,but their development potential and availability are still limited due to their low reserves and high cost.In addition,these catalysts can not maintain stability in alkaline solution for a long time（Ru O₂ and Ir O₂are very easy to be oxidized in the electrolysis process,and then lose catalytic activity）.These disadvantages seriously limit the practical industrial application of precious metal catalysts.In order to improve the cost of noble metal catalysts,in recent years,people have shifted the research focus to the transition metal group with rich reserves and low cost.It is found that layered double hydroxides（LDHs）nanosheets composed of transition metals can significantly improve the OER reaction rate,and have the advantages of low cost,flexible composition and material structure.Among the many transition metals that can be selected,NiFe layered bimetallic hydroxides（NiFe-LDHs）show good catalytic activity in the oxygen evolution half reaction of alkaline electrolytic water.Some studies have shown that vanadium doped NiFe-LDHs has better electrocatalytic activity and stability,but the mechanism of vanadium doping in NiFe-LDHs catalyst for improving the stability of the catalyst is not clear.In addition,in previous studies,it was found that the OER performance of LDHs with smaller size is better.Therefore,it is generally considered that the edge site of LDHs is the active site of oxygen evolution reaction,but it has not been verified by detailed experimental data.In view of the above two problems,this paper will study from the following two aspects:1.In this study,three LDHs catalysts were synthesized by double drop method,namely NiFe V-LDHs,VO₃^-intercalated NiFe-LDHs and CO₃^2-intercalated NiFe-LDHs.Then,the stability of the electrolytic system was optimized by adjusting the concentration of metavanadate（VO₃^-）in the alkaline electrolyte,and the concentration parameters with the best performance were explored.Finally,the interaction between V element and NiFe-LDHs was deeply studied to further explain the mechanism.The experimental results show that the overvoltage of NiFe-LDHs intercalated with VO₃^-is the lowest when the electrolyte concentration is 0.5 m M Na VO₃ and 1 M KOH.Under the same electrolyte condition,its stability is better than NiFe V-LDHs and CO₃^2-intercalated NiFe-LDHs.Combined with the XPS after the reaction and the ICP test results of the electrolyte,the experimental conclusions are further explained.It is found that VO₃^-has an inhibitory effect on the dissolution of Ni and Fe active sites,which significantly improves its OER stability.2.In order to study the effect of LDHs size on OER active sites and accurately judge the location of active sites through experiments,we use the density gradient centrifugation to obtain different sizes and undamaged structure NiFe-LDHs,and carried out the OER test to explore catalytic activity.Under the condition of high temperature and high pressure in the reactor,the synthesized NiFe-LDHs had a relatively complete hexagonal sheet structure,and then the size was separated by density gradient centrifugation.and four different sizes of NiFe-LDHs were obtained.Sucrose is the most widely used water-soluble gradient medium at present,which has the advantage of low price and no damage to the activity of LDHs.Therefore,five gradient concentrations of high-purity sucrose solutions were used as the medium for density gradient centrifugation.The sucrose gradient solution is added into the centrifugal tube by the"laying down method",the concentration gradient sucrose solution is added into the centrifugal tube in turn,and finally the catalyst suspension is tiled on the top layer of the gradient concentration solution.When centrifuging,the appropriate centrifugal speed（w）should be selected first,after a certain centrifugation time,the suspension is divided into four layers,namely four different sizes of NiFe-LDHs.After OER electrochemical test and Energy Dispersive X-Ray Spectroscopy（EDS）test of hydrotalcite with different sizes,it is found that the larger size of NiFe-LDHs and the larger Ni/Fe ratio exhibit the worse OER performance;The smaller size of NiFe-LDHs and the smaller Ni/Fe ratio show the better OER performance.This study reveals that the different performance of NiFe-LDHs with different sizes in OER is due to the difference number of Ni-O-Fe active sites.In addition,It is also proved that the active sites are more located on the hydrotalcite laminate.The conclusions of this study will provide guidance for the preparation strategy of synthetic binary hydrotalcite.