Preparation,structural Regulation And Catalytic Performance Of Nickel-based Catalysts For Urea Electro-oxidation

With the continuous consumption of fossil fuels such as coal and oil,the energy crisis has become one of the major challenges to the rapid development of human society.Therefore,the continuous exploration of new energy sources and the development of efficient energy conversion technologies to get rid of the dependence on fossil fuels is the current direction of efforts.Due to the advantages of urea molecule with high energy density,non-toxic and abundant sources,energy conversion technologies based on urea such as urea electrolysis/photolysis to hydrogen and urea fuel cell have attracted the attention of researchers in recent years.In addition,a large amount of urea-containing wastewater is produced daily in production and life,and the use of electrolysis to remove urea from wastewater can effectively avoid environmental pollution caused by ammonia release from urea decomposition.The development of an efficient urea oxidation reaction（UOR）electrocatalytic system is the basis for the realization of all-electric/photolytic hydrogen production from urea,urea fuel cells and treatment of urea-containing wastewater,which has the dual relevance of producing clean energy and purifying wastewater.In the research of UOR catalysts,non-precious metal catalysts have attracted more and more attentions from researchers due to their abundant sources and low prices.Among them,nickel-based catalysts have shown excellent catalytic performance,which has led to the flourishing of related research.In this thesis,based on nickel-based metal-organic framework materials（Ni MOFs）,we have developed various novel nickel-based catalysts for UOR electrocatalysis by regulating their structures and properties through metal node engineering,defect engineering,hybridization,and calcination,and have investigated the reasons for their enhanced catalytic performance.The details are as follow:1.Nickel/cobalt bimetallic organic framework（Ni Co MOF-Fc）modulated by ferrocene carboxylic acid ligands were successfully synthesized by a simple one-step solvothermal preparation method.Compared with the pristine Ni MOF,ferrocene carboxylic acid（Fc-COOH）ligand modulation introduced the Fc group with good electrical activity into the MOF,which improved the electrical conductivity of the material and produced linker deficiencies,which created defects in the MOF structure and changed the coordination environment of the Ni active center.While the introduction of Co optimized the electronic structure of the Ni active site.The synergistic effect makes the Ni active sites of Ni Co MOF-Fc show higher electrocatalysis performance.Electrochemical test results showed that the optimized Ni Co MOF-Fc loaded nickel foam electrode（Ni Co MOF-Fc/NF）was able to achieve a current density of 140 m A cm^-2 at a low potential of 1.299 V and 782 m A cm^-2 at 1.5 V.The Tafel slope was 46.2 m V dec^-1and exhibited high catalytic stability.This study provides a new idea for the development of efficient UOR electrocatalysts and promotes the application of MOFs materials in electrocatalysis.2.Ni MOF-Fc hybrid materials with MWCNT conductive network（MWCNT-Ni MOF-Fc）were prepared by introducing the one-dimensional nanomaterial with high electrical conductivity,multi-walled carbon nanotubes（MWCNT）,into a nickel-based metal organic framework modulated by ferrocene carboxylic acid ligands（Ni MOF-Fc）.The hybrid material exhibited nanoflower-like morphology with high electrical conductivity and large accessible surface area.The electrochemical test results show that the hybrid material has excellent UOR electrocatalytic properties,with a current density of 50 m A cm^-2at 1.437 V.In addition,the material shows good oxygen evolution reaction（OER）catalytic activity,requiring an overpotential of only 274 m V to reach a current density of 10 m A cm^-2 in 1 M KOH solution.The excellent MWCNT-Ni MOF-Fc catalytic activity may be attributed to the fact that the MWCNT which embedded in the Ni MOF sheet structure can act as a conductive network and improve the electrical conductivity of the material and the unique flower-like morphology of MWCNT-Ni MOF-Fc increases the electroactive surface area and provides more accessible active sites.In addition,the modulation of Fc-COOH introduces defect sites in the Ni MOF,which changes the coordination environment of the Ni active center.The combination of the above factors resulted in MWCNT-Ni MOF-Fc exhibiting high electrocatalytic performance.This study provides a new approach for the development of high-performance UOR-OER bifunctional electrocatalysts.3.UOR is the basic reaction for urea based-energy conversion technologies such as urea electrolysis,photoelectrochemical urea splitting and urea fuel cell.It is of great practical significance to develop efficient UOR catalysts.In this paper,N-doped carbon-encapsulated nickel oxide and nickel molybdate composite（C-N@Ni O-Ni Mo O₄）was synthesized by stepwise hydrothermal as well as annealing methods,and was loaded on nickel foam（NF）for UOR electrocatalysis.The N-doped carbon-encapsulated structure and the synergistic effect of C-N@Ni O and Ni Mo O₄ can enhance the conductivity,charge transfer ability,poisoning resistance and increase the electroactive surface area,which significantly improve the electrocatalytic activity and long-term stability of the C-N@Ni O-Ni Mo O₄.It was found that the current density of C-N@Ni O-Ni Mo O₄/NF at 1.8V is 1.7 times higher than that of Ni Mo O₄/NF and 3.6 times higher than that of C-N@Ni O/NF.There was no significant change in the catalytic performance after 57 h of reaction.The results showed that the prepared C-N@Ni O-Ni Mo O₄/NF has a good practical application prospect.