Preparation Of Cobalt-based Heterostructured Nanocatalysts And Their Electrocatalytic Performanc

To achieve"carbon peak"and"carbon neutral"has become an important strategic goal and development trend of China.Replacing fossil energy with sustainable clean energy such as hydrogen,wind power and nuclear power,reforming the supply side of the traditional energy industry and improving the new energy industry chain will not only reduce carbon emissions,but also reduce dependence on traditional energy,and improve energy independence and national competitiveness.Hydrogen energy is a green and low-carbon secondary energy with broad development prospects,and has gradually become one of the important carriers of energy transformation and upgrading.Electrochemical water decomposition is a safe and efficient method to produce hydrogen energy,including water reduction reaction to produce hydrogen（HER）at the cathode and water oxidation reaction to produce oxygen（OER）at the anode.Theoretically,the decomposition voltage of water in the electrolytic cell is 1.23 V（vs.RHE）,but the actual electrochemical water decomposition process involves complex electrochemical reaction,resulting in slow kinetics and low efficiency.It is necessary to overcome resistance such as electrode polarization and solution resistance,and the actual voltage far greater than 1.23 V is often applied to make the reaction happen smoothly.Therefore,catalysts are often used to reduce overpotential during electrochemical water decomposition to reduce energy consumption and improve efficiency.At present,the commonly used effective catalysts mainly include Pt based and Ir/Ru based noble metal materials,which have outstanding electrocatalytic performance during HER and OER processes,respectively.However,their scarcity and high cost limit their application in large-scale practical applications.Therefore,it is urgent to develop non-noble metal catalysts with abundant reserves,high efficiency,stability and low cost to replace noble metal catalysts to achieve efficient hydrogen production from water electrolysis.Cobalt（Co）based nanostructured catalysts（oxides,hydroxides,sulfides,nitride, phosphide,etc.）show obvious advantages in the field of electrochemical water decomposition due to their large specific surface area,stable array structure and controllable morphology and composition.In recent years,researchers have synthesized many cobalt-based electrocatalysts through various methods,which not only effectively reduce the overpotential in the catalytic process,reduce the consumption of electric energy,but also significantly enhance the stability of the catalyst.However,the electrocatalytic performance of most reported cobalt-based electrocatalysts is not as good as that of noble metal catalysts.In this thesis,two kinds of transition metal cobalt-based electrocatalysts were constructed.Through the design of nano-structure and construction of heterogeneous interface,the electronic state,electronic structure and energy band distribution of the catalyst were controllable adjusted,thus enhancing the performance of electrochemical decomposition of water based on transition metal cobalt-based catalysts.Co O/Co₃O₄ rod-like nanoarray electrocatalyst（Co O/Co₃O₄ NA/Ti）with heterogeneous structure was prepared on titanium mesh substrate by hydrothermal method and high temperature hydrogen reduction to achieve the improvement of electrocatalytic hydrogen evolution performance in alkaline medium.The synergistic effect of heterostructure can effectively promote electron transfer rate and expose more active sites.In addition,the rod-like nanoarrays also have significant structural stability and are completely covered on the titanium mesh substrate,which can effectively prevent the corrosion of the substrate during the catalytic process,thus prolonging the service life of the catalyst.Impressively,cobalt-based sulfide composites are also potential hot spots for electrochemical water decomposition.Therefore,the hierarchical heterogeneous Co₃S₄/Ce O₂ rod-like nanoarray electrocatalyst（Co₃S₄/Ce O₂-CF）was successfully prepared on cobalt foam（CF）substrate by one-step hydrothermal method to achieve the improvement of the performance of overall water hydrolysis in alkaline medium.Co foam substrates not only provide cobalt source in the preparation of catalysts,but also provide rich self-supporting interfaces and skeleton structures.Co₃S₄/Ce O₂-CF catalyst exhibits excellent electrocatalytic performance,among which,the lamellar Co₃S₄and rod-like Ce O₂ compound to form a unique microscopic heterostructure,which greatly promoted the rapid transfer of electrons and exposed a large number of active sites.More importantly,the rod-like nanoarray structure has a large specific surface area,which can effectively promote the full contact between catalyst and electrolyte solution and ensure the active site to play a full role,thus improving the catalytic activity of catalyst.It is proved by density functional theory（DFT）calculation that the addition of Ce O₂ can not only reduce the hydrogen adsorption energy in the process of hydrogen evolution,but also reduce the Gibbs free energy of the intermediate products in the process of oxygen evolution,thus promoting the overall catalytic performance of the catalyst.