Synthesis Of Ni,Co And Mo-based Nanostructures For Water Splitting

As a promising "secondary energy source",hydrogen is regarded as an ideal "green energy".Among various available hydrogen production technologies,electrochemical water splitting has attracted extensive attention due to merits of high product purity,facile operation,no pollution and superior recyclability.However,the current industrial water electrolysis cell has a voltage of 1.8².0 V and thus large-scale electric energy consumption becomes a major issue that restricts the development of water electrolysis technology.The use of electrocatalysts is an effective way to lower overpotentials and reduce power consumption.Platinum has long been a favored electrocatalyst for hydrogen evolution reaction,while its low abundance in the Earth’s crust and high price cannot support large-scale application.In this sense,developing efficient non-noble metal electrocatalysts composed of earth abundant elements is quite appealing to provide cost-competitive hydrogen.Although plenty of non-precious metal electrocatalysts have been reported,their performance is not so satisfactory compared with platinum.Based on this,aiming at designing and developing high-efficiency non-precious metal catalysts,this paper discussed how to improve the efficiency of water electrolysis via designing a monolithic electrode,utilizing urea to promote hydrogen evolution reaction（designing a Schottky catalyst for urea oxidation）and regulating the electronic structure of catalyst.And the main contents can be summarized as follows:（1）Preparation of the monolithic Ni/NiO electrode and studying of its performance.For hydrogen evolution reaction（HER）,under situations where large amount of hydrogen evolves,how to maintain strong adhesion between catalysts and conductive substrates has been recognized as another general challenge to enable the stable function of the electrodes.From this viewpoint,a Ni/NiO integrated electrode based on Ni foam substrate was prepared by acid activation.The integrated electrode assures intimate contact between the catalyst and the current collector as compared to conventional powder catalysts and catalysts in-situ grown on the substrate,thereby increasing the life span of the catalyst.In addition,after acid activation,its rough and lamellar surface structure endows the electrode a large specific surface area and more active sites.The study of its HER,oxygen evolution reaction（OER）and full water-splitting performance showed that the electrode exhibited excellent catalytic activities.（2）Preparation of CoS₂-MoS₂ Schottky catalyst and studying of its performance for HER and urea oxidation reaction（UOR）.The oxygen evolution reaction（1.23 V）has always been the bottleneck for high-efficiency water splitting.In addition to highly efficient OER catalysts,replacing the sluggish OER with thermodynamically favorable reactions（also known as the chemically assisted hydrogen evolution process（CAHER））is also an effective way to improve the efficiency of hydrogen production.So in the third chapter,we achieved replacing the slow OER process with urea oxidation reaction（UOR）,and designed a Schottky catalyst of CoS₂-MoS₂ based on the feature of urea molecules.The UOR and HER catalytic activities were studied.The mechanism for urea oxidation with this Schottky catalyst was investigated in detail by DFT calculations and experiments.Finally,we studied the influence on full water splitting efficiency when OER is replaced by UOR.（3）Preparation of bimetallic phosphide and studying of its HER performance.Transition metal phosphides have aroused widespread attention due to its high chemical stability,high activity,good electrical conductivity as well as the presence of proton and hydride accepting centers.However,there is still a large gap between its activity and that of platinum.According to the HER mechanism under acidic conditions,when transition metal which having a d⁵（Mo）and d⁸（Ni）electronic structure is recombined,more excellent HER activities can be obtained due to electronic interaction between the two parts.Therefore,a Ni-Mo-P bimetallic phosphide was prepared via electrodeposition and the comparison of catalytic activity with Ni-P was studied.Electrochemical tests exhibited that the introduction of Mo greatly improved the electrocatalytic activity of Ni-P.Finally,on the basis of experiments,a mechanism was proposed to explain why Ni-Mo-P has a higher catalytic activity than Ni-P.