Study On Hydrogen Production Performance And Mechanism Of Co/Mo Based Self-supporting Electrocatalysts

Energy is an essential and decisive factor in human social progress and economic growth.Since the first industrial revolution,traditional energy based on fossil fuels play an important role in social and economic development.However,this process is also accompanied by a great number of greenhouse gas emissions,acid rain and other serious environmental problems.With the further advancement of industrialization and urbanization,the traditional energy crisis and environmental pollution problems will certainly grow.Hence,the exploration and utilization of environmental energy is extremely urgent and challenging.Hydrogen（H₂）,with the merits of environmental friendliness,rich sources and high energy density,has become the hotspot in the field of energy research.Electrochemical water splitting to yield H₂ through oxygen evolution reaction（OER）and hydrogen evolution reaction（HER）is undoubtedly one of the most promising H₂ production technologies.Therefore,it is vitally important to develop efficient electrocatalysts for H₂ production by triggering water splitting.Materials based on noble metals,such as Pt,Ir O₂ and Ru O₂,are recognized as the most effective HER and OER electrocatalysts to date,yet,scarcity and high cost seriously hinder their large-scale application.Transition metals,with the virtues of rich reserves,strong activity,and easy access,are important contributors to the development of catalytic materials.It has been found that most transition metal-based materials exhibit excellent H₂ evolution properties in acidic,neutral,or alkaline electrolytes over recent years.Some of these catalysts have similar or even superior properties to noble metal-based catalysts.Most notably,transition metal compounds,including sulfides,phosphides,nitrides,carbides,oxides,etc.,show excellent catalytic performance,while their design principles and mechanisms are different.Nano-engineering provides an unprecedented opportunity for the development of rational design,synthesis,and characterization of these metal catalysts.Meanwhile,it also provides a new idea for designing and fabricating efficient electrocatalysts.The purpose of this paper is to design a series of transition metal-loaded self-supporting catalysts with low cost and excellent performance for H₂ production by electrochemical water splitting.Surface and interface control strategies have been developed,such as heteroatom doping,interface regulating,defects engineering,local coordination environment modulation,and so on.A series of high efficiency electrocatalysts are prepared,and various electrochemical measurement technologies are implemented to explore their catalytic performance.Meanwhile,the effects of regulation methods on the geometric and electronic structures of materials are investigated by various fine structure characterization techniques.The relationship between the electronic structure of the surface and interface and catalytic activity is further analyzed by combining with theoretical calculation to delve into the catalytic mechanism,thus providing a train of feasible ideas for the design of high efficiency electrocatalysts.Combined with the concept of“carbon peak and neutrality”,it is put forward that the present social development has an increasing demand for new energy.The key role of H₂production by electrochemical water splitting in the development of new energy is highlighted,and the mechanism of water electrolysis and the current bottleneck problems in different environmental media are described in detail.The important role of transition metal-based materials in the development of electrocatalysts and the corresponding design strategies are further proposed.Starting with HER,one of the half reactions of water splitting,molybdenum disulfide（Mo S₂）nanoflower structures are successfully grown on graphite felt（GF）（Mo S₂/GF）by a simple hydrothermal method,and a feasible and effective electrochemical activation technique is proposed to improve the HER activity of the composites.Electrochemical activation can induce H proton intercalation between Mo S₂ and GF and act as electron shuttles to increase Mo S₂ interlayer spacing and enhance the synergistic effect between GF and Mo S₂,thus improving the electron transfer efficiency between the S sites in Mo S₂ and the oxygen-containing functional groups in GF.Electrochemical measurement results display that the overpotential of the modified composites for HER is only 82 m V at a current density of 10 m A cm^-2,and the Tafel slope is 48 m V dec^-1 with excellent long-term operation stability under acidic conditions.This simple electrochemical activation method obtains favorable kinetics and proliferation of highly active catalytic sites in the Mo S₂/GF nanostructure and endows the material with superior and stable catalytic activity,making it highly competitive in HER and other potential reactions.Exploring another half reaction-OER.Based on the mastery of the preparation and properties of Mo S₂ in Chapter 2,the size of Mo S₂ is further adjusted to the level of quantum dots,and an electrocatalyst structure based on three-dimensional porous conductive nickel foam（NF）has been developed.Molybdenum carbide（Mo₂C）nanostructures are successfully grown on NF（Mo₂C@NF）by thermal impregnation,and the modification of ultra-thin Mo S₂ quantum dots on Mo₂C@NF（Mo S₂ QDs@Mo₂C@NF）is successfully realized by the hydrothermal method.The combination of active material and strong conductivity NF significantly improves the mechanical strength and electron transfer rate of the overall catalyst.The Mo S₂QDs@Mo₂C@NF catalyst obtained by collaborative nano-assembly presents excellent electrocatalytic OER behavior under alkaline electrolyte.The overpotential is 110 m V at a current density of 10 m A cm^-2,and the Tafel slope is 57 m V dec^-1.Besides,the electrode holds significant long-term operational stability.On the basis of Chapter 2 and Chapter 3,from local to whole,the hydrogen evolution and oxygen evolution are integrated,and a F doped Co S₂@NF composite without binder（F-Co S₂@NF）is successfully prepared by one-step hydrothermal method to realize the overall water splitting for H₂ production.The F dopant can activate the Co and S sites of Co S₂ in composites so as to improve its inherent conductivity,significantly promoting water splitting activity.Meanwhile,the improved surface wettability in F-Co S₂@NF ensures the rapid penetration of electrolyte,accelerating the migration of hydroxyl and the release of oxygen in the process of water splitting.Electrochemical measurement results show that F-Co S₂@NF composites possess excellent HER and OER properties and long-term operation stability in alkaline medium.Moreover,only 1544 m V of potential is needed to achieve a current density of 10 m A cm^-2 during the overall water splitting process.This work proposes a practicable tactic for enhancing the electrochemical activity of Co-based catalysts,and it also provides a solid theoretical basis for the structure and performance design of other metal or non-metal catalysts.Combining the Mo and Co materials studied in Chapter 2～4,an amorphous bimetallic Co-Mo oxide supported by GF（CoMoO/GF）is prepared by simple electrodeposition,and a simple auxiliary method of anodic treatment is proposed to further introduce defects and non-metallic element doping into the catalyst.The experimental results and theoretical calculations show that the CoMoOF/GF composites have a relatively high electrochemically active surface area（ECSA）from their amorphous structure,a great number of defects caused by electrochemical etching,and synergistic catalysis of Co and Mo components after anodic treatment.Therefore,they show excellent OER catalytic behaviors in both acidic and alkaline media.The overpotential under alkaline conditions is only 79 m V at a current density of 10 m A cm^-2,and the Tafel slope is 43.3 m V dec^-1.Meanwhile,the overpotential under acidic conditions is 94 m V,and the Tafel slope is 60.2 m V dec^-1.In addition,the CoMoOF/GF catalyst can operate stably under alkaline and acidic conditions for a long time.This work provides a feasible way to design multifunctional bimetallic catalysts with a unique structure and excellent performance for energy and environmental-related technical fields.On the basis of Chapter 5,electrocatalytic hydrogen production is combined with urea wastewater purification in water,and a bimetallic Co-Mo oxide nanoflowers are obtained by electrodeposition,which originates from the interfacial assembly of Co-Mo precursors on metallic Co nanosheets vertically grown on a three-dimensional GF surface（CoMoO@Co/GF）.The effects of this self-supporting and bifunctional electrode on H₂ production by simultaneous water splitting and urea oxidation reaction（UOR）in alkaline medium are improved by optimizing the electrodeposition time.The presence of metallic Co enables Co-Mo oxides to grow from an amorphous to a crystalline state.In addition to a great number of accessible and highly electroactive Co-Mo oxide nanoflower structures,the coupling interface between metallic Co and Co-Mo oxides is established to enhance the electron transport and mass transport capabilities of HER and OER/UOR.Electrochemical measurement results show that the CoMoO@Co/GF-based electrolyzer only needs a 1.5 V dry battery to achieve effective H₂production and urea removal simultaneously under alkaline conditions.Furthermore,the electrode exhibits significant long-term operation stability.This chapter constructs a catalytic system combining energy output and wastewater purification,which provides a feasible technical scheme for the broader removal of pollutants in water combined with new energy production.