Construction And Catalytic Performance Of Transition Metal(Fe/Co/Ni) Based Catalyst For Electrochemical Water Splitting

With the rapid development of science and technology,energy crisis and environmental problems are becoming more and more serious.For sustainable development in energy resources,hydrogen is a secondary energy source with high energy density,wide sources,clean and carbon free,flexible and efficient,and rich application scenarios,which regarded as an ideal energy storage medium to replace traditional fossil fuels.Among many hydrogen production methods,the traditional fossil energy reforming hydrogen production and industrial by-product hydrogen production methods are both at the cost of consuming fossil energy or generating carbon emissions,and prepared is the"grey hydrogen",which is obviously contrary to the proposed goal of"carbon neutralization".The"green hydrogen"prepared by electrolytic water splitting is considered to be the most convenient and clean way of hydrogen production.The theoretical working voltage of electrolytic water is only 1.23V.However,the bipolar half reaction of electrolytic water(anodic oxygen evolution reaction（OER）;cathodic hydrogen evolution reaction（HER）is thermodynamically non-spontaneous,so an additional overpotential is required to reduce the energy barrier in the reaction process.At present,the working voltage of industrial electrolytic cell is usually 1.8-2.0 V,so it is necessary to use efficient and stable catalyst to improve its reaction kinetics.So far,the materials with the highest catalytic activity for water splitting are still precious metal based electrocatalysts,but their wide application is limited by low reserves and high price.Therefore,it is of great significance to develop catalyst materials with low price,easy preparation,excellent catalytic activity and stability.On the basis of summarizing the research results of electrolytic water splitting in recent years,this paper is based on the synthesis of cheap transition metal based catalysts with both catalytic activity and stability.By selecting appropriate supports,optimizing target catalysts,doping and introducing impurities,enriching highly active catalytic sites,and constructing porous self-supporting catalytic electrodes to maximize the exposure of catalytic active sites.This paper looks for ideal electrolytic water catalysts and catalytic performance and mechanism were deeply explored.The main research contents are as follows:1.Construction of Ni（OH）₂ Supported RhCo Alloy Nanoparticles and Exploration of Overall Water SplittingThe interaction between support and supported materials is very important for the preparation of high-performance and stable catalysts.Guided by the strong metal support interaction（SMSI）,ultra-thin flake Ni（OH）2 synthesized by a simple method was selected as the carrier to load trace RhCo alloy（Ni（OH）2@RhCO）as a highly active total hydrolysis catalyst in alkaline medium.In order to explore the catalytic mechanism,we used in-situ Raman spectroscopy to test the cathodic and anoodic reaction process of water splitting.The test results showed that in the catalytic process on the anode side（OER）,the catalyst underwent electrochemical reconstruction,and Ni（OH）2 underwent in-situ phase transformation to NiOOH,but this transformation did not occur on the cathode side（HER）.Considering the oxygen enriched and highly oxidized environment of the anode,we speculate that this is conducive to the electrochemical conversion to the corresponding hydroxyl oxide.XPS data further proved the occurrence of this transformation.More worthy of our attention is that when we use the reconstructed material as the cathode for total hydrolysis test,the activity and durability of the material have been greatly improved compared with that before reconstruction.In order to further verify the effect of this transition on performance,we made Ni（OH）2@RhCo reconstituted to NiOOH@RhCo and OER and HER test were conducted,which further showed that the electrochemical reconstruction on the anode caused the improvement in performance.In situ Raman measurement under reduction conditions shows that the reconstruction process from Ni（OH）2 to NiOOH is irreversible,which also provides evidence for the excellent stability of the material.In general,we synthesized the pre catalyst Ni（OH）2@RhCo then in situ reconstituted into a real catalyst（NiOOH@RhCo）during electrochemical tests,an excellent total hydrolysis catalyst is constructed by setting the anode and cathode,which also provides a new strategy for the selection of efficient supported catalyst support.2.Regulation of CdP₂-CDs-CoP Hierarchical Structure and Robust Electrocatalyst for oxygen evolutionThe powder catalyst material obtained in the previous work needs to add organic polymer binder in the use process,which inevitably buries the active site of the catalyst and increases the contact resistance,which has an adverse impact on the electron transfer in the catalytic reaction.In this work,we are inspired by Lego building toys.The carbon quantum dots（CDs）with rich negative charge functional groups are joined by electrostatic interaction with the positively charged Co²⁺/Cd²⁺.At the same time,a graded structure CdP2-CDs-CoP nanometer array self-supporting electrode on the surface of nickel foam is constructed due to the difference of solubility product（Ksp）.Data analysis shows that the combination of negatively charged CDs and positively charged metal ions Co²⁺/Cd²⁺can effectively prevent the agglomeration of CoP particles and help to expose more active sites.At the same time,due to the low evaporation temperature of Cd and the difference of Ksp of precursors,trace CdP2preferentially grows on the surface of nano-array to realize the growth of hierarchical structure.In the phosphidation process,part of Cd will evaporate with the increase of temperature to produce structural defects and accelerate the desorption of reaction products.The synergistic effect of CDs and CdP2 enriches the surface defects of the catalyst,constructs more active sites,speeds up the reaction kinetics on the catalytic interface and realizes efficient catalytic reaction.The combination strategy of positive and negative charges inspired by building block toys and the construction of hierarchical structure provide a simple and effective method for the construction of transition metal based OER catalysts,through which more efficient catalysts can be constructed.3.Construction of FeNi-OH-MOF Electrode and Catalytic Mechanism:Self-Supported Electrodes for Efficient Oxygen EvolutionThe self-supporting nano-array materials obtained in the previous work show excellent oxygen evolution catalytic activity,while it’s stability under high current density is not satisfactory.Therefore,we consider whether we can prepare an integrated self-supporting catalyst without external metal source to improve this problem.Based on this,we choose the metal organic frameworks（MOF）material as the research object by the self dissociation strategy.We choose Ni/Fe alloy foam as the self-supported electrode and also the metal source.During the process of synthesis,in the system of organic-water coexistence,all the benzoic acid anhydride will hydrolyze and make the liquid phase system acidic.The metal ions on the surface of the Ni/Fe alloy foam will be dissolved into the system to provide metal ions,and trace metal sources will be added to form the seeds to induce nucleation.At the same time,during the synthesis process,Ni/Fe alloy foam and liquid interface will cause local PH rise due to the consumption of H⁺,and a layer of hydroxide（Ni/Fe-OHx）will be grown on the surface of the alloy foam.The etching effect of metal ions will make the surface of alloy foam rough and make the material grow more firmly.The experimental results show that the prepared self-supporting FeNi-OH-MOF composite electrode has excellent oxygen evolution performance.At the current density of 10 m A cm^-2,only needs 213 m V overpotential,which is far lower than the commercial Ru O2（265 m V）under the same test conditions,and is better than most reported oxygen evolution catalysts.After 70 h stability test at 90 m A cm^-2,there is no obvious decrease,which proves that it also has excellent stability.