Preparation And Electrochemical Performance Of Nickel-and Cobalt-based Nanocatalysts For Water Splitting

Electrochemical water splitting is considered to be one of the clean and green hydrogen production methods,which has the advantages of abundant raw materials and high product purity.However,the high energy barriers of the two half reactions that make up the electrolyzed water reaction,the hydrogen evolution reaction（HER）and the oxygen evolution reaction（OER）,limit and affect the energy conversion efficiency of the electrolysis process and the development of this technology.Therefore,it is necessary to develop highly active catalysts to reduce the energy barrier of reaction.To date,the state-of-the-art electrocatalysts for OER and HER are precious-metal-based compounds.However,their large-scale application is greatly confined by the scarcity and high cost.In addition,these catalysts usually display high activity for one half-reaction while poor activity for another half-reaction.Therefore,from the aspects of cost reduction and process simplification,the developments of bifunctional non-noble metal compound catalysts with high HER and OER activities are very important to achieve an efficient overall water splitting.Based on this,this thesis constructed two transition metal-based materials with the design of nanostructures and the regulation of chemical components,achieving a synthetical improvement of HER and OER performances.The research contents and results were summarized as follows:（1）Preparation of Ni_0.7Co_0.3P nanoflower and the study of its performances for electrochemical water splittingA series of Ni Co P catalysts with different Co doping contents were synthesized by low temperature phosphating of hydroxides with different Ni:Co molar ratios.The effect of Co content on the electrocatalytic activities was investigated.When the molar ratio of Ni:Co is 7:3,the as-prepared Ni_0.7Co_0.3P exhibits the best catalytic activities for both HER and OER.In particular,Ni_0.7Co_0.3P has a unique nanoflower structure and a large number of hierarchical defective pores,which can expose more active sites and provide more electron/ion diffusion paths,thereby accelerating the electrochemical reaction process.Under alkaline conditions,the overpotentials required for hydrogen evolution and oxygen evolution at the current density of 10 m A cm^-2 are 105 and 230m V,respectively.The Tafel slopes are 58 and 78 m V dec^-1,respectively.Additionally,it can operate stably for 25 h under the constant current density of 10 m A cm^-2.（2）Preparation of Co/CoO@NC@CC and the study of its performances for electrochemical water splittingCo and CoO embedded in N-doped carbon layer coated on carbon cloth（Co/CoO@NC@CC）was synthesized by a facile self-assembly-carbonization method.During the carbonization process,due to the collapse and shrinkage of the zeolite imidazolate skeleton,a large number of irregular defective pores were generated on the surface of carbon fiber,which effectively expanded the accessible area of the electrolyte,and better exposed and utilized the active sites.In addition,by changing the types of active materials in the composite,the effects of the synergistic effects of two active components（Co and CoO）on the adsorption capacity of H atoms and the reaction intermediates,as well as the reaction kinetics of HER and OER were explored.The obtained composites show excellent electrochemical performances:under alkaline conditions,the overpotentials required for hydrogen evolution and oxygen evolution at10 m A cm^-2 are 152 and 284 m V,as well as the corresponding Tafel slopes are 80 and76 m V dec^-1,respectively.In addition,employing it as both cathode and anode simultaneously,the device only needs a cell voltage as low as 1.66 V at 10 m A cm^-2,along with good stability in continuous 25 h of testing.In summary,in this paper,two transition metal-based catalysts,Ni_0.7Co_0.3P nanoflowers and Co/CoO@NC@CC,are designed and synthesized.The electrocatalytic activities for hydrogen evolution and oxygen evolution of these two materials under alkaline conditions are examined.The experimental results demonstrated that both catalysts exhibited excellent HER/OER activities and excellent stability.In addition,the corresponding structure-activity relationships and HER/OER mechanisms of two materials were proposed,which provided new ideas and methods for the design and preparation of bifunctional electrolytic water catalysts.