Synthesis And Electrocatalytic Performance Study Of Co-based Catalysts Based On Defect Engineering Control

In recent years,energy consumption and environmental pollution have become the two focal issues all over the world,so developing renewable and clean energy sources is getting more and more attention.Hydrogen energy is considered to be the most potential substitute for fossil energy in the present stage by virtue of its attractive advantages of high energy density and no carbon emission.Electrocatalytic water splitting is considered as a promising and green strategy to produce hydrogen,in which no other toxic by-products are produced.And only water forms after hydrogen combustion,thus create a carbon-free and environment-friendly economy.However,high energy barrier and sluggish kinetics in the hydrogen evolution reaction(HER)and oxygen evolution reaction(OER)during electrocatalytic water splitting result in an inefficient electricity conversion.Therefore,highly efficient electrocatalysts are required to accelerate the reaction rate,reduce reaction energy barrier and thus improve the energy conversion efficiency.Currently,noble metal-based catalysts such as platinum(Pt),iridium(Ir),ruthenium(Ru)are the most effective catalysts for HER and OER.Unfortunately,these catalysts has the disadvantages of low abundance,high cost and poor stability,which greatly limited their large scale application in practical production.Therefore,it is very essential to design and develop non-noble catalysts with high electrocatalytic activity and excellent stability for HER and OER.Recently,transition metal-based materials(for example,iron,cobalt,nickel,etc.)have been proved with considerable electrolytic catalytic activity for water splitting,and on the other hand,it is possible to enhance the catalytic activity through different strategies,such as constructing heterostructures,adjusting catalyst morphology,and defect engineering.In this work,defect engineering including anion vacancies and heteroatomic doping,is used to control the interaction between cobalt(Co)compounds and other substrates,which can adjust electron density,increase active sites,and induce oxygen vacancies and thus improve the electrochemical performance of Co-based catalysts and replace noble metal-based catalysts.The main research contents are included as follows:1.Co-doped Mo O₂ nanorods was successfully prepared through a facile wet chemistry method followed by annealing treatment process.Co doping can not only produce more oxygen vacancies in Mo O₂,but also improve the electrical conductivity of catalyst during HER process.Oxygen vacancies can activate the adjacent oxygen atoms as active sites,thereby increasing the number of active sites on the catalyst surface,and enhancing charge transfer,and greatly improving its electrocatalytic hydrogen production efficiency.Therefore,the optimized Co-Mo O₂-0.01 nanorods(NRs)exhibited an extraordinary electrochemical activity with a very low overpotential of 26 m V at 10 m A cm^-2 and a small Tafel slope of 30.9m V dec^-1,which are much better than that of pure Mo O₂ NRs and even superior to that of the commercial Pt/C catalyst.In addition,the Co-Mo O₂-0.01 NRs also display excellent long-terms stability and durability without obvious catalytic activity degradation after 3000cycles.This work provides a new promising synthetic strategy for developing earth-abundant and highly efficient HER electrocatalysts through rational defect-engineering design.2.Two-dimensional cobalt oxy-hydrate sulfide(Co O(OH)_1-xS_x)nanosheets were synthesized via a facile electrochemical deposition method followed by an anion-exchange process.The obtained Co O(OH)_0.75S_0.25 nanosheets exhibited a low overpotential of 166 m V and a small Tafel slope of 93.4 m V dec^-1 in HER process,while the Co O(OH)_0.75S_0.25 catalyst delivers an extremely small overpotential of 378 m V(at 30 m A cm^-2)and a suppressed Tafel slope of 106.3 m V dec^-1 for OER.It denotes that the specific Co O(OH)_1-xS_x nanosheets can be a promising catalyst for overall water splitting.Benefiting from the modified t_2g orbitals of octahedral Co-O_x,enhanced electrical conductivity and abundant active sites,Co O(OH)_1-xS_xdelivered high catalytic activity toward overall water splitting in alkaline electrolyte.X-ray absorption fine structure analysis and first-principles calculation denotes that The substitution of sulfur heteroatoms in Co OOH could result in the unoccupied t_2g orbit state and hence contribute to enhanced electrical conductivity and reduce the reaction barrier,all of which favor electrocatalytic functions in water splitting.This work provides a facile route to synthesize new transition-metal oxide sulfides for electrocatalytic water splitting.3.Through simple hydrothermal reaction and annealing treatment,employing nickel nitrate as nickel source,a series of Ni_xCo_1-x(OH)₂ nanosheets were firstly prepared by doping with different ratios of cations,then using sodium hypophosphite as phosphorus source,hexagonal Ni_xCo_1-xP bimetallic phosphide nanoplates catalyst was successfully synthesized after annealing and phosphating.The overpotential of the prepared Ni_xCo_1-xP nanosheets was320 m V at 10 m A cm^-2 for OER,Tafel slope was 110.0 m V dec^-1.Meanwhile,the catalytic activity of Ni_xCo_1-xP catalyst almost did not decrease after 24 hours testing.It can be found that the phosphating annealing treatment makes the catalyst form a porous structure,which endows the Ni_xCo_1-xP catalyst more exposed catalytic active sites and increases the contact area between the catalyst and the electrolyte.In addition,nickel doping significantly reduces the resistance of Ni_xCo_1-xP catalyst and accelerates charge transfer during the electrochemical reaction,thereby greatly improving the catalytic performance of the catalyst for OER.This work provides new research ideas for the development of other high-efficiency double transition metal-based oxygen evolution electrocatalysts.