Study On The Preparation,catalytic Performance And In-situ Derivations Behavior Of Cobalt-based Oxygen Evolution Catalysts

The depletion of traditional fossil fuels and the increasing energy demand have caused a series of urgent problems to be solved.It is necessary to develop sustainable clean energy systems,such as fuel cells,rechargeable metal-air cells,and water electrolysis devices.Among many energy storage and conversion technologies,electrolyzed water technology is regarded as a sustainable hydrogen production method,which has been widely concerned.Because the oxygen evolution reaction（OER）that is half-reaction of the water splitting process is slow in kinetics,the overall energy conversion efficiency for the production of hydrogen from electrolyzed water is greatly reduced.Currently,commercial OER catalysts mainly include Ir O₂and Ru O₂.However,their high cost and scarcity undoubtedly limit their large-scale application.Therefore,researchers are very interested in the development of high-activity and low-cost OER electrocatalysts.Many transition metal oxides,hydroxides and chalcogenides have been extensively studied.Cobalt-based catalysts are considered to be a promising oxygen evolution catalyst due to their rich content,low cost and high electrocatalytic activity.In this thesis,by selecting cobalt-based materials with abundant reserves,low price and easy-to-control microstructure as the research object,the micro-nano structure of cobalt-based materials is adjusted to explore its electrocatalytic oxygen evolution performance and the phase derivation behavior in the electrocatalytic process.The following research works have been carried out:（1）Using cobalt nitrate as the cobalt source,the Co₃O₄micro-nano cubes were prepared by hydrothermal and high-temperature calcination methods.Then,using thioacetamide as the sulfur source,the prepared Co₃O₄micro-nano cubes were treated with hydrothermal vulcanization.The sulfurization treatment greatly enriches the active sites on the catalyst surface.The interaction between S^2-and O^2-and cations optimizes the electronic structure of the active site.The synergistic interaction between S^2-,O^2-and cation optimizes the active site in the electronic structure.The catalytic activity curve of the catalyst presents a volcanic shape with increasing sulfur content.At a current density of 10 m A·cm^-2,the optimized S-Co₃O₄catalyst has an over-potential of only 314 m V and a Tafel slope of 62.8 m V·dec^-1,which shows excellent OER performance.The samples were collected after the catalytic reaction,and then the substances before and after the OER reaction were characterized and compared.It was found that S-Co₃O₄only acts as a“pre-catalyst”.However,the Co OOH species derived in-situ on the surface and rich in oxygen vacancies are the real active substances for OER catalysis.In addition,the 36 h i-t test shows that the S-Co₃O₄catalyst has good stability.（2）The CoFe-LDH/r GO-X composite was successfully synthesized by solvothermal method.Because CoFe-LDH grows on both sides of r GO in-situ,the catalyst has good conductivity.CoFe-LDH nanosheets have a high specific surface area,and the porous structure can accelerate the mass transfer rate.The synergistic effect and unique structure between Co and Fe significantly improve the electrocatalytic performance.With a current density of 10 m A·cm^-2,the overpotential is only 272 m V,and the Tafel slope is 72.2 m V·dec^-1.The above results prove that it has good electrochemical durability and stability.Through the characterization of the catalyst after the OER reaction,it was found that the surface species of CoFe-LDH undergoes an oxidation reaction.（3）First,α-Co（OH）₂nanosheets were synthesized by a simple co-precipitation method.Secondly,using the silver ammonia solution as the silver source,a simple green light reduction deposition method was used to anchor the precious metal silver nanoparticles on theα-Co（OH）₂nanosheets.The method is time-saving and simple,and does not require any pretreatment of theα-Co（OH）₂nanosheets.In addition,the silver is in close contact with the surface of theα-Co（OH）₂nanosheets,so there is no need to use any adhesive.The catalystα-Co（OH）₂/Ag shows excellent OER catalytic performance.When the current density is 10 m A·cm^-2,the overpotential and Tafel slope are only 278 m V and 78.0 m V·dec^-1,respectively.When silver nanoparticles are loaded under the irradiation of a green laser with a wavelength of 532 nm,the LSV test results of the catalyst show that the local plasmon resonance effect（LSPR）is generated on the surface of the catalyst,which significantly improves the OER performance.When the current density is 10 m A·cm^-2,the overpotential is reduced by45 m V.By comparing the characterization results of the catalyst before and after the catalytic OER reaction,it is found that the phase of the catalyst surface has changed fromα-Co（OH）₂/Ag to Co OOH/Ag during the catalytic process.