| Hydrogen energy as a kind of renewable an environmentally friendy new energy source.Due to the characteristic of high efficiency and pollution free,it is considered to be the ideal energy.Electrolysis technology is based on the mechanism of hydrogen evolution by an electrochemical process and is proposed as one of to obtain sustainable hydrogen production due to its advantages of simplicity and high efficiency.However,a large amount of noble metal e.g.Pt has to be used for conventional electrodes during the electrolysis in which high cost gradually becomes the bottleneck problem for large scale production of hydrogen in industry.Therefore,development of non-noble electrocatalyst for water electrolysis is necessary.Recent research has indicated that nanostructured transition metal oxides exhibit superior room temperature hydrogen evolution characteristics featuring high activity,low cost and simple facility.On the other hand,transition metal caribide and metal nitride have similar electronic structure and catalytic properties to noble metals such as Pt and Pd,and been proposed as the potential non-noble hydrogen evolution catalyst to take the place of noble metals,however,they are difficult to prepare due to high melting point.In this thesis,3D molybdenum oxide nanostructures such as nanobelts and nanowires have been obtained on different substrates by a facile hydrothermal method.In addition,3D molybdenum carbide and nitride nanostructures have been obtained by carbidization and nitrition of nanostructured molybdenum oxide.Effect of substrate,reaction temperature,growth time,precursor concentration on the morphology,size,phase structure,crystallinity,surface chemistry and hydrogen evolution in acid conditions has been systematically investigated.Based on the optimized parameters,structure and electrocatalytic properties of the nanostructured molybdenum oxide,molybdenum carbide and molybdenum nitride have been investigated and compared,and the enhanced electrocatalysisi mechanism has been discussed.Main rsults of the research can be summarized as follows:(1)3D molybdenum oxide nanobelts and nanorods have been obtained on different substrates by one step hydrothermal method.Experimental results indicate that,metastable h-MoO3 nanostructures were generated under lower temperature or shorter growth time,and the metalstable phase gradually transformed into stableα-MoO3 phase with increasing reaction temperature or time.The optimized paramters for the molybdenum oxide nanobelts growth on the carbon paper were temperature of 170℃,growth time of 15 h,precursor concentration of 10wt%,while those for nanorods growth on molybdenum mesh were temperature of 190℃,growth time of 10 h.While single crystalline molybdenum oxide nanobelts were obtained on the carbon paper,amorphous molybdenum oxide nanorods were generated on the molybdenum oxide nanorods.Electrochemical measurement results demonstrated that 3D molybdenum oxide nanostructures on the two substrates exhibited better hydrogen evolution performance than freestanding molybdenum oxide nanobelts.(2)β-Mo2 C nanorods with hcp structure were obtained by carbridizing molybdenum oxide nanobelts and nanorods using CH4 as the carbon source in a chemical vapor method.While molybdenum oxide nanobelts were almost totally converted to molybdenum carbide on the carbon paper,only part of molybdenum oxide nanorods on the molybdenum mesh transformed to molybdenum carbide and XRD characterization indicated the mixture of molybdenum carbide and molybdenum oxide of the product.Electrochemical measurement results showed that onset potential of hydrogen evolution for β-Mo2 C nanorods shifted positively around 0.21 V compared to molybdenum oxide nanobelts,and the cathode current density at-1.0V(vs.Ag/AgCl)of β-Mo2 C nanorods increased by almost 3 times compared to molybdenum nanobelts,showing obvious enhanced hydrogen evolution properties.(3)Porous γ-Mo2 N and MoN nanotbelts were obtained by nitridizing molybdenum oxide nanobelts and nanorods using NH3 as the nitrogen source in a chemical vapor method.While molybdenum oxide nanobelts were almost totally converted to molybdenum nitride on the carbon paper,the products on the molybdenum mesh were composed of the mixture of γ-Mo2 N,which might be related to the different crystallinity characteristics of the molybdenum oxide nanostructures and the difference in the regional reaction environment on the two substrates.Electrochemical measurement results showed that onset potential of hydrogen evolution for the porous 3D molybdenum nitride nanobelts shifted positively around 0.19 V compared to α-MoO3 nanobelts,and the cathode current density at-1.0V(vs.Ag/AgCl)of the molybdenum nitride nanobelts increased by around 2.5 times compared to molybdenum oxide nanobelts,showing enhanced hydrogen evolution properties.Our research suggest that 3D molybdenum carbide and nitride nanostructures may have superior hydrogen evolution performance in acid conditions and can be promising non-noble catalysts for hydrogen evolution. |
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