| With the global energy crisis and increasingly serious environmental pollution,it is urgent to develop clean and sustainable energy to replace fossil fuels.With the development and utilization of renewable energy sources,people put forward higher requirements for rapidly store electricity.Supercapacitors with high power density,short charging time,long cycle life and strong environmental performance received widespread attention in the field of energy storage.Hydrogen with high energy density and environmentally friendly,has become the most promising new e-nergy instead of fossil fuels.Electrocatalytic water splitting to produce hydrogen is one of the most optimal methods at present.Pt is the current electrocatalytic electrode.Its low reserves and high price greatly limit the wide application of electrocatalytic water splitting technology in the future practical production.Therefore,based on the future development prospect of supercapacitor and electrocatalytic water splitting technology,the exploration and design of supercapacitor electrode materials and non-noble metal catalyst with high performance and low cost become the important premise of energy storage and conversion.At present,the development trend of supercapacitor and electrocatalytic water splitting technology is the exploration of electrode materials with abundant reserves,low cost and excellent performance on earth to promote the further development of both.Transition metal nitride has attracted more and more attention in the field of energy storage and conversion due to its unique physical and chemical properties.In transition metal nitrides,nitrogen atoms mixed with metal lattice,cause the shrinkage of the original metal d orbitals,so as to make it close to the electronic structure of precious metal catalyst.Because the nitrogen atom radius is lesser,which can easily embedded metal lattice space position,keep the original structure of close packing.The structure makes transition metal nitride have high conductivity.Nitrogen doping makes it obtain excellent corrosion resistance.These advantages make transition metal nitrides become an important research content of electrode materials in the field of supercapacitor and electrocatalytic water splitting.Based on the problems of current supercapacitor and electrocatalytic water splitting catalysts,this paper selects transition metal nitrides(TiN,VN,MoN,WN)with good physical and chemical properties as the main research object.From the relationship between material structure and performance,morphology,components and defects of transition metal nitrides are regulated by combining the preparation of nanomaterials and the micro-structure construction.The charge storage mechanism of supercapacitors and the thermodynamic process of electrocatalytic decomposition of aquatic hydrogen were studied by theoretical calculation.Based on the interface,defects,electronic structure synergy of composites,optimizing their supercapacitor and electrocatalytic performance to further promote the application of transition metal nitrides in the field of energy storage and conversion.The main research contents and conclusions of this paper are as follows:In the first chapter of paper,we introduced the research background and significance of supercapacitor and electrocatalytic water splitting,and their basic principles and active evaluation indexes.We also made simple introduction of research progress of supercapacitor and electrocatalytic water splitting to produce hydrogen,and then introduced the application of transition metal nitrides in supercapacitor and electrocatalytic water splitting,including the material preparations,properties and modification strategies.Based on the relationship between material structure and performance,the research idea of further improving the performance of transition metal nitrides in supercapacitor and electrocatalytic water splitting through the preparation of nanomaterial and the regulation of microstructure is put forwardIn chapter 2,the effects of morphology regulation and composite construction on the properties of transition metal nitrides as supercapacitor electrode materials are studied.(1)TiN nanostructures with different morphologies were synthesized in situ on conductive substrate Ti foil by controlling the synthesis conditions.With the increase of hydrothermal temperature and time,the morphology of precursor changed from fibrous to nanosheet.Through regulating the temperature and time,we prepared the precursor of nanosheet After the preparation of porous TiN nanosheet perfect keep the same structure as the precursor,which has a larger specific surface area.The hole structure is conducive to make electrolyte contact with materials effectively.The synthesis of active materials in situ on conductive Ti can accelerate the electron transfer.After capacitance performance test,its highest capacitance is 81.63 F g-1,and the electrode shows good stability and can maintain 100%capacitance after 2,500 cycles.After assembling two electrodes with solid electrolyte into a device,which has high capacitance,good energy density and power density.(2)We prepared VN and C composite on nickel foam(NF).We regulate the surface morphology of the samples by controlling the amount of chitosan solution.The electrode with the morphology of flower lamellar structure showed excellent performance of capacitance,the high capacity is 386.6 g F-1 at the scanning rate of 2 mV s-1.The electrode could still maintain good performance under high current density.The self-supporting electrode prepared by loading the active material on the conductive substrate can be directly applied to capacitor devices,which improves the effective contact between the active material and the substrate,improves the efficiency of electronic transmission,and saves the complex electrode preparation work.In addition,the capacitance of transition metal nitride was improved by improving its morphology and conductivity.In chapter 3,the improvement performance of transition metal nitrides by complex construction on electrocatalytic decomposition water to produce hydrogen is studied.The porous nanosheet Mo2N and CeO2 composite material were synthesized on nickel foam by two-step hydrothermal method and subsequent nitridation.CeO2 has abundant reserves and moderate price and is widely used in industrial catalysis.The valence state of Ce in CeO2 can be flexibly converted between+3 and+4,which enables the stable existence of oxygen vacancy,which facilitates the adsorption and dissociation of water.The characteristic provides the possibility for electronic interaction to establish the interface with other materials,improves the dispersion of Mo2N,and thus improves the electrocatalytic hydrogen production activity of Mo2N/CeO2 material system.XPS analysis showed that in Mo2N/CeO2@NF composites,there is a strong electronic interaction between Mo2N and CeO2.The calculated results of DFT further indicate that the composite system of Mo2N and CeO2 can significantly reduce the energy barrier of the intermediate and the product,thus promoting the catalytic reaction.By comparing the electrocatalysis test results,it shows that the catalytic activity of the composite material system is higher than the single phase Mo2N or CeO2.The overpotential is only 26 mV(10 mA cm-2),which measured in the alkaline medium.Mo2N/CeO2@NF could maintain good stability in many-current stability test(25 h),moreover the electrode also maintained stable current density after 100 h.The experimental results show that the performance of transition metal nitride for HER can be improved by constructing a reasonable composite material system,and the advantages of the composite material system can be brought into full play.In chapter 4,the effects of morphology regulation on the HER performance of transition metal nitride were studied.The different sources of molybdenum(phosphomolybdic acid,molybdic acid and ammonium molybdate)can affect the morphology of MoNx,which loaded on CeO2/NF.The morphology of MoNx is porous nanosheet prepared by using ammonium molybdate.After calculation,the HER performance of MoNx prepared by using ammonium molybdate shows that the porous nanosheet with large specific surface area and enough space,can be effectively exposed active sites on the surface of the material.XPS analysis shows that the surface elements of the sample prepared by ammonium molybdate containing high levels of Mo3+.The HER tests showed that the MoNx prepared by using ammonium molybdate has the optimal production activity,as well as the smallest Tafel slope(32 mV dec-1),which is below the Tafel slope of Pt/C(35 mV dec-1).The long-time HER test proved that the sample has good HER stability.Therefore,by adjusting the surface morphology of the electrocatalyst and the active sites of the material can effectively promote the catalytic reaction.In chapter 5,we mainly introduce the method of complex construction to improve the HER performance of other transition metal nitrides.WN/CeO2/NF and VN/CeO2/NF were prepared on nickel foam by hydrothermal method and high-temperature nitridation.The flake structure composed of particles increased the roughness of the material surface and provided more reactive sites for the reaction.By comparing the HER performance of the samples prepared in the experiment,the catalytic performance of the composite material system with CeO2 can be improved,indicating that the method is universal for improving the electrocatalytic performance of transition metal nitrides.The HER performances of WN/CeO2/NF and VN/CeO2/NF can be comparable to those of Pt/C(20 wt%).Their overpotential were 26 and 24 mV respectively,at the current density of 10 mA cm-2,Tafel slope were 31.4 and 30 mV dec-1.Moreover,two samples showed good HER stability,after 24 h hydrogen production experiments,the current density has no apparent change.By comparing the HER performance of the samples obtained by reduction in hydrogen atmosphere,it is shown that the excellent electrocatalytic performance of the composite material is due to the synergistic effect of two phases,which makes the whole composite material system show excellent electrocatalytic activity and long-time stability.In chapter 6,first,we summarized the research contents and conclusions of this paper,then the innovation points of this paper are summarized,and the shortcomings of this paper are discussed at last,and the future work is also prospected.In summary,the researches in this paper prove that transition metal nitrides have excellent performance and wide application prospect in energy storage and conversion,especially in supercapacitors and electrocatalytic water splitting.Based on the relationship between structure and properties of the material,morphology regulation and construction of composite material system are effective measures to improve performance of transition metal nitrides in supercapacitors and electrocatalytic water splitting.Exploring the generality of performance adjustment methods to expand the transition metal nitrides in practical application of energy storage and transformation has strategic significance. |
PDF Full Text Request