Construction Of Nickel/Cobalt-Based Electrode Materials And Its Research In Electrolysis Of Water For Hydrogen Evolution Coupled With Oxidation Of Organic Compounds

As a clean,non-polluting renewable energy source,hydrogen can well solve the environmental pollution problem caused by excessive use of fossil energy and contribute to the achievement of the dual carbon goal.Among the many hydrogen production methods,hydrogen prepared by electrocatalytic water splitting is called green hydrogen,which has a better prospect compared with gray hydrogen and blue hydrogen.However,the anodic oxygen evolution reaction（OER）has inherent slow kinetics and high thermodynamic barrier,which leads to high energy consumption and high cost of electrocatalysis and restricts the large-scale industrial application.Therefore,how to solve the electrocatalytic water splitting anodic oxidation reaction and at the same time can be cathodic hydrogen production is of great importance.Organic small molecules are widely available and most of them possess low theoretical oxidation potential,which can perfectly replace OER and couple with hydrogen evolution reaction（HER）to reduce the overall voltage of hydrogen production by electrocatalytic water splitting.In terms of anode products,the O₂ produced by conventional OER is not only of low economic value but also an explosion hazard when mixed with H₂,while the anode from organic oxidation is not only free of hazardous products but may also produce chemicals with high added value.However,whether it is cathodic hydrogen precipitation reaction or anodic organic oxidation reaction,it is crucial to design the corresponding reaction system and prepare cheap and efficient electrocatalysts.Traditional noble metal electrodes have little content and high cost,so non-precious metal materials are chosen to replace noble metals.Among many non-precious metals,transition metals Ni and Co have a broad development prospect in electrocatalysis due to their abundant content,environmental friendliness,excellent catalytic performance,excellent stability,low cost and unique d electronic structure.The nickel/cobalt-based electrodes are tuned by suitable preparation strategies to change their crystal and electronic structures and further optimize the catalytic performance.Therefore,in this thesis,the anode is selected from the reaction pathway to replace the OER with high thermodynamic potential by a variety of organic oxidation reactions under the premise that the cathode electrocatalytic hydrogen precipitation reaction remains unchanged.Taking nickel/cobalt-based electrocatalysts as the main research object,we use a variety of nanostructure modulation means to construct them,optimize their catalytic performance by adjusting the crystal structure and electronic structure of nickel/cobalt-based materials,and combine with theoretical calculations to deeply investigate their conformational relationships to achieve efficient electrocatalytic generation of green hydrogen and anodic OER substitution.The main research of this thesis is as follows.In Chapter 1,we first briefly introduce the necessity of hydrogen energy and the advantages of green hydrogen,and focus on the basic principle of electrocatalytic water splitting,and evaluation index.Subsequently,several alternative organic oxidation reactions to anodic oxygen precipitation reactions are introduced,including the thermal/kinetic advantages,basic principles,and current research status.And then,we describe the research progress of several nickel-based materials and cobalt-based materials and provide a systematic introduction to the common optimization strategies of electrocatalysts.To address the problem of high energy consumption for electrocatalytic hydrogen production,we propose the use of organic oxidation reactions instead of OER and the preparation of mono/bifunctional electrocatalysts by means of suitable material modulation.In Chapter 2,to prepare high-performance HER electrocatalysts,we synthesized NiMoO₄ electrode materials by simple hydrothermal synthesis.However,in order to improve the poor conductivity of the oxide,then the nitrogen in NH₃ was doped into the oxide by high temperature annealing,which not only solved the conductivity problem of the original substrate material NiMoO₄,but also increased the electrochemical active area.To further enhance the HER performance,Ni₃N,as a recognized HER material,was selected to construct the NNiMoO₄/Ni₃N heterostructure by electrodeposition and high-temperature annealing,which can promote the adsorption and desorption of water and hydrogen intermediates.Meanwhile,it was verified that the electrocatalyst still has good performance and stability under high concentration of KOH,which is beneficial for industrial applications.The experimental and theoretical results demonstrate that the electrodes prepared by both strategies have better HER performance due to the synergistic effect compared to the single strategy.In Chapter 3,we use the thermodynamic advantage of small urea molecules to replace OER given the high thermodynamic potential of OER.By controlling the time and temperature in doping,non-metallic ions can be doped into it without destroying the original structure,and different kinds of anion doping have specific preferences for the electrocatalytic reaction poles.The N-CoMoO₄ electrode doped with nitrogen is more inclined to the HER,while the PCoMoO₄ electrode doped with phosphorus has a better urea oxidation reaction.DFT calculations reveal that the band gap of the oxide narrowed whenever doping occurred,indicating an increasing material conductivity,which is favorable for carrier transport.Moreover,the doped material has a significant increase in active sites and catalytic rate.In addition,the N-CoMoO₄ and P-CoMoO₄ electrodes were used as the cathode and anode of the urea-assisted electrolytic water splitting to hydrogen solution cell,respectively,and the voltage driving the whole system is lower than that of the overall water splitting to hydrogen and oxygen production.In Chapter 4,the nitrogen-containing small molecule N₂H₄,which has a lower theoretical oxidation potential and faster kinetics,was chosen to replace the OER for its inherently slow kinetics.We transformed the nanoneedle electrode into a nanosheet electrode in situ by secondary hydrothermal heating,which increased the contact area of the electrode with the electrolyte.Further calcination at high temperature transformed the electrode into a porous structure and caused the production of metallic Co,which increased the conductivity and active sites of the electrode.Theoretical calculations reveal that the redistribution of electrons at the heterogeneous interface of the MoO₂/Co heterostructure compared to the pure metal contributes to the adsorption of water and the adsorption desorption of hydrogen intermediates,and for N₂H₄ oxidation,the heterostructure not only improves N₂H₄ adsorption but also makes*NHNH dehydrogenation to*N₂H a rate-determining step and becomes a spontaneous reaction.Coupling the anode hydrazine hydrate oxidation reaction to the HER,the total reaction can still be driven stably for a long time at very low cell voltage using energy sources such as used batteries and solar cells.In Chapter 5,in response to the increasingly serious plastic pollution problem,PET is dissolved with strong alkali as an oxidation reaction instead of OER.Using the needle-like structure of NiMoO₄ in Chapter 2,Ni-MoO₂ nano-needle heterojunction bifunctional electrode is built on the basis of this structure by high temperature annealing.The good conductivity of metallic Ni supports the carrier transport in the electrode,while MoO₂ can effectively regulate the Ni electronic structure of the electrode.In addition to the low overpotential and stability in HER,the electrode also shows high activity and selectivity in PET electro-reforming,starting the current at 1.33 V and finally obtaining the high value-added product formic acid at 85%Faraday efficiency.Moreover,the voltage required to drive the coupling of PET electrooxidation and hydrogen evolution has a significant advantage over the total hydrolysis reaction,which is significant for energy-saving hydrogen production and treatment of pollutants.In Chapter 6,the main research contents of this thesis are comprehensively summarized,the innovation points of this paper are summarized,and the problems and shortcomings of the current work are pointed out,and finally the outlook of the future work is proposed.