| Advanced clean energy techniques are the key to achieve a sustainable development of human society.Not only do urea-based energy conversion technologies improve the efficiency of energy utilization,but also they have the capability of mitigating current environmental issues,benefitting from the high efficiency and environmental friendliness.Urea oxidation reaction（UOR）is the key reaction that determines the efficiency of modern urea-based energy conversion technologies.Advanced catalysts are still the key to improve the performance of UOR.The researchers have made outstanding contributions to the design and synthesis of non-expensive transition metal-based catalysts.Among them,Ni-based catalysts are the most attractive candidates for urea-based electrocatalytic reactions owing to their promising transformation into active species under alkaline conditions.However,the drawbacks of lack of active sites and poor stability are the major reactions for their limited roles in urea catalytic reactions.In this paper,simple synthesis routes were designed to synthesize the complex NiC2O4 and Ni（OH）2@NiC2O4 in view of the problems existing in UOR of Ni-based catalysts.A series of derived materials with controllable morphology were obtained by annealing,phosphating and compounding ways.The growth mechanism of precursors NiC2O4,Ni（OH）2@NiC2O4 and the impact of annealing temperature,phosphating conditions on the morphology and microstructure of the derived materials were studied.The NiC2O4 and Ni（OH）2@NiC2O4 derived materials were modified by means of metal/nonmetallic element doping,electronic structure regulation and hierarchical structure construction.The performance of these materials for UOR was investigated.The research content includes the following five parts:1.The uniform rod-like NiC2O4 nanomaterial was prepared by means of precipitation reaction at room temperature with Na2C2O4 and Ni（NO3）2·6H2O in the solution of glycol containing a certain amount of HMT.Meanwhile,the impact of the solvent composition on the morphology of NiC2O4 nanorods was studied.Therefore,the rod-like NiO/Ni materials were prepared by thermal annealing under different temperatures in N2.The impact of the thermal annealing temperature on the morphology,specific surface area and pore size distribution of NiO/Ni nanorods was also studied.The rod-like NiO/Ni-2 catalyst which was prepared at 340℃ exhibited outstanding activity for UOR,compared with other samples.The enhanced UOR activity can be mainly attributed to the component and structure.A large number of metallic Ni nanoparticles enriched the active sites of the catalyst and improved the electronic conductivity.Moreover,the uniform nanorods assembled by nanoparticles and uniform distribution of mesoporous characteristics were benefcial for the rapid charge transmission.Additionally,the possible E-C mechanism of mesoporous NiO/Ni nanorods was discussed by the microstructure and surface chemical states analysis after UOR.2.The mesoporous rod-like NiO/Ni nanomaterials were demonstrated to have outstanding activity for UOR.However,due to the agglomeration effect of excessive elemental Ni nanoparticles,the formation and release of gas-phase catalytic products were limited.The phosphorization process can not only regulate the electronic structure to improve the activity of electrocatalytic urea,but also reduce the content of Ni to improve the stability.Hence,the rod-like Ni2P/Ni nanomaterials were prepared by means of a one-step phosphating reaction with NaH2PO2 and rod-like NiO/Ni nanomaterials.The impact of the phosphating conditions（phosphating amount,temperature and time）on the morphology,degree of phosphating,specific surface area and pore size distribution was studied.The Ni2P/Ni-2 was prepared by introducing 100 mg NaH2PO2 into the reaction system at the temperature of 330℃ for 1h.The results of the characterization analysis indicated that Ni2P/Ni-2 had all the advantages of the factors discussed above.DFT and DOS calculations suggested that the phosphating sample can effectively reduce the energy of adsorption OH-and increase the electronic conductivity.The Ni2P/Ni-2 catalyst exhibited enhanced activity and stability toward UOR in the three-electrode and two-electrode configurations,compared with other samples.The possible mechanism of E-C was discussed by the characterization of SEM,XRD and XPS after UOR.3.As mentioned in the second part,phosphating is an effective strategy to reduce the nickel content,but the agglomeration of metal nanoparticles is inevitable.The Ni（OH）2@NiC2O4 with hierarchical structure was synthesized by a one-step hydrothermal reaction（90℃）based on the method of synthesizing rod-like precursor NiC2O4 nanomaterials,and the synthesis route was adjusted benefitting from the hydrolysis characteristics of HMT.Meanwhile,the impact of the hydrolysis time on the morphology of Ni（OH）2@NiC2O4 hierarchical structure was studied.The results suggested that HMT played a dual role:regulating the growth of NiC2O4 nanorods;controlling the growth of Ni（OH）2 nanosheets.Therefore,the C/N doped NiO hierarchical structure samples were prepared by different temperatures thermal annealing in O2.The impact of the thermal annealing temperature on the morphology,specific surface area and pore size distribution of N-NiO/C hierarchical structure was studied.Experimental results suggested that the catalyst of N-NiO/C-2 can greatly enhance the electrocatalytic activity and stability toward the UOR.Clearly the enhanced UOR performence can be mainly attributed to the C/N doping of NiO hierarchical structure.The prepared N-NiO/C-2 hierarchical structure,can not only enhance the stability but also expose abundant active sites.Moreover,the nanosheets exposed to the hierarchical structure were beneficial for capturing urea molecule and OH-,shortting mass transport distance,and thus promoting the formation and release of gas-phase catalytic products.Additionally,doping C/N elements in catalysts could enhance the activity of UOR.The possible UOR mechanism of mesoporous N-NiO/C hierarchical structure was verified by electrochemical behavior,the microstructure and surface chemical states analysis.4.Based on the above research conclusions,materials based on the phosphating and hierarchical structure were of great importance for efficient electrochemical UOR systems.By introducing 100 mg NaH2PO2 into the reaction system at the temperature of 330℃ for 1h,the phosphating reaction induced the transformation of N-NiO/C to N-Ni2P/C with hierarchical structure.It was found that the performance of UOR was largely ascribed to the phosphating degree,morphology,specific surface area,and pore size distribution.Owing to all merits,the N-Ni2P/C-2 with hierarchical structure exhibited superior UOR catalytic activity and stability,which were much better than that of other samples.To further investigate the possible mechanism of mesoporous N-Ni2P/C-2 hierarchical structure in UOR,a series of characterization tests were conducted to characterize electrochemical behavior,the microstructure,surface chemical states.5.Benefitting from the advantages of hierarchical structure in UOR and the outstanding merits of layered MOF including large surface areas,acceptable electron transfer channels and abundant active sites,a series of Ni-MOF composed of nickel oxalate derivatives hierarchical structures were designed via a one-step solvothermal method.The synthesis of Ni-MOF@NiO/Ni and the performence of UOR were discussed.Regulating the growth of ultra-thin nanosheets was the key to design Ni-MOF@NiO/Ni hierarchical structure.The ultrathin nanosheets Ni-MOF@nanorod NiO/Ni hierarchical structure delivered outstanding UOR performances in three-electrode system and two-electrode system.The active centers,active species and possible UOR mechanism were analyzed by XRD（after solvent immersion）,XPS,SEM,TEM and the distribution after cycling test.Ni-MOF@Ni2P/Ni,Ni-MOF@N-NiO/C and Ni-MOF@N-Ni2P/C hierarchical structures were designed via a similar way and their performence of UOR were studied. |
