Controllable Synthesis Of Self-supported Cobalt-based Compound Heterogeneous Nanostructured Catalysts For Electrocatalytic Water Splitting

The rapid development of modern society has led to an increasing demand for energy.However,traditional fossil energy resources are not only limited in reserves but also cause increasingly serious environmental pollution.As an emerging and promising energy carrier,hydrogen（H₂）has the advantages of high energy density(about 140 MJ kg^-1),pollution-free and renewable,having the potential to become a substitute for fossil energy.Electrocatalytic water splitting technology is an extremely attractive new hydrogen energy sustainable conversion technology.It has the advantages of high hydrogen production efficiency,mature technology,simple equipment and operation,high purity of H₂ produced and no pollution during hydrogen production.In a typical water electrolysis system,the ideal potential between the cathode hydrogen evolution reaction（HER）and the anode oxygen evolution reaction（OER）is 1.23 V.However,due to the high overpotentials of both the anode and the cathode,actual water electrolysis usually requires a higher battery voltage,resulting in greater energy consumption for the reaction.In order to reduce the power consumption of the reaction process and accelerate the reaction kinetics,it is critical to develop green and efficient catalysts.At present,commercial electrocatalytic water splitting catalysts are noble metal-based（Pt,Ir,and Ru,etc.）catalysts,but noble metals have disadvantages such as high prices and low earth reserves,which limit their development.Therefore,it is of great significance for the industrial application of water electrolysis to find a non-noble metal hydrogen evolution/oxygen evolution reaction（HER/OER）electrocatalyst with abundant reserves on the earth to replace noble metal-based catalysts.In addition,considering both cost reduction and process simplification,the development of highly active self-supporting non-noble metal electrocatalysts is essential to achieve an efficient water electrosis.Based on this,this thesis constructed two non-noble metal cobalt-based nanomaterials with the design of self-supported heterostructure electrodes and the regulation of chemical composition,which achieved the comprehensive improvement of HER and OER performance.The research contents and results are summarized as follows:（1）Preparation of self-supported Co₃O₄@Co WP/CC composite material and its catalytic performance in water electrolysisThrough a simple hydrothermal-phosphatization procedure,the surface of Co WP nanowire which in situ grown on carbon cloth（CC）is compounded with an ultra-thin layer composed of Co₃O₄ nanoparticles to synthesize self-supported Co₃O₄@Co WP/CC composite electrode.Compared with Co₃O₄/CC and Co WP/CC,Co₃O₄@Co WP/CC displays excellent electrocatalytic activity and improved kinetics of OER and HER.In an alkaline electrolyte,the Co₃O₄@Co WP/CC composite electrode has low overpotentials of 269 m V（OER）and 118 m V（HER）to achieve the current density of 10 m A cm^-2,respectively.On the basis of the bifunctional property of the electrode material,Co₃O₄@Co WP/CC is applied as both anode and cathode in the overall water splitting electrolyzer.The assembled electrolyzer requires a 1.61 V cell voltage to reach 10 m A cm^-2,which is superior to most non-noble metal electrocatalysts that have been reported.In addition,the chronopotentiometry under constant current density shows that the potential has no change after 24 hours of reaction,indicating that the Co₃O₄@Co WP/CC composite has excellent stability.The open three-dimensional nanowire array structure and the synergetic effect between heterogeneous components not only enlarges the effective contact area between the electrolyte and the catalyst,shortens the substance/electron transfer path during the reaction,but also optimizes the free energy of adsorption/desorption between catalyst and reaction intermediates,making the composite electrode to achieve the best electrolysis performance.（2）Preparation of self-supported Co Se₂@MoSe₂/CC composite material and its hydrogen evolution performance in a wide p H rangePreparing high activity non-noble metal-based HER electrocatalysts in a wide p H range is the core of the large-scale production of hydrogen and water-alkali industries.Through a unique dissolution-regeneration strategy,a three-dimensional core-shell heterostructure of hollow Co Se₂ nanotubes supported with ultra-thin MoSe₂ nanosheets on the carbon cloth substrate（Co Se₂@MoSe₂/CC）is successfully synthesized.The prepared heterostructure composite electrode exhibits excellent performance of HER in a wide p H range,which is attributed to its unique hollow core-shell heterostructure,maximizing exposed edge active sites and increased electron/ion transmission speed.Under optimal conditions,the composite electrode only needs an overpotential(η₁₀)of108 m V to achieve a current density of 10 m A cm^-2 in 1.0 M KOH,much lower than most non-noble metal-based electrocatalysts currently had reported.In addition,the Co Se₂@MoSe₂/CC composite electrode also displays excellent performance under acidic and neutral conditions,withη₁₀ of 128 and 148 m V,respectively.Moreover,after48 hours of the HER test in the full p H range,the overpotential hardly decayed,indicating its feasibility in practical applications.Herein,we designed and prepared two self-supported non-noble metal-cobalt-based catalysts:Co₃O₄@Co WP/CC and Co Se₂@MoSe₂/CC composite electrodes,and studied the electrocatalytic hydrogen and oxygen evolution behaviors of these two self-supported materials.The experimental results showed that both catalysts had excellent electrocatalytic activity and stability.In addition,the corresponding synthesis and electrocatalytic mechanisms of the two electrode materials were discussed separately,which provided new ideas and methods for the design and synthesis of self-supported non-noble metal water electrolysis catalysts.