Structure Design And Performance Of The Electrocatalysts For Highly Active Hydrogen Evolution Reaction In A Wide Range Of PH Values

With the rapid development of the social economy,the consumption of fossil fuels and the environmental pollution are becoming more and more serious.Therefore,establishing a global sustainable energy system is the key to solve the problem when we protect the environment.Hydrogen is the most promising clean energy.The use of electrolyzed water is an effective way to produce hydrogen at present.The commonly used electrolytic water equipments are: the alkaline electrolyzer,and proton exchange membrane electrolyzer.However,the requirements of electrode materials for different electrolytic cells are different.The electrolyte used in the alkaline electrolytic cell is a strong alkaline solution and needs an alkali-resistant catalyst.While the proton exchange membrane electrolytic cell generally requires an acid-resistant catalyst under acidic conditions.Therefore,it is necessary to develop highly efficient water splitting catalyst with wide p H values.Due to its unique structure,nanocarbon materials can resist acid and alkali corrosion and can be used as water splitting catalysts with wide p H ranges.In this paper,we focuse on the nanocarbon composite materials and design the highly active electrocatalytic hydrogen evolution catalyst in a wide range of p H values.And we also clarify the relationship between these materials’ structure and their catalytic performance.The main contents of this thesis include the following parts:1.Co,N co-doped carbon nanotubes（U-CNTs）were prepared by using low-cost urea and cobalt chloride hexahydrate as raw materials and then calcined under high temperature in an inert atmosphere.The resulting U-CNTs have excellent catalytic activity over a wide p H range（0-14）,especially under neutral conditions（p H 7）with an overpotential of 240 m V at a current density of 10 m A cm-2.In addition,the seawater resources are of abundance and do not need to be further processed,which can greatly save costs.Therefore,we also study the electrocatalytic activity of this material under buffered seawater conditions.The results show that U-CNTs still maintain excellent electrocatalytic hydrogen evolution properties in buffered seawater.When the current density reaches 10 m A cm-2,the U-CNTs need an overpotential of 250 m V and have good stability.It is also shown that the interactions between the carbon nanotubes and the reactants are promoted due to the co-doping of Co and N increasing the charge density of the carbon nanotubes,and thus improves the electrocatalytic activity.This work opens up a new way for preparing low-cost nanocarbon materials.2.A series of carbon-coated metal phosphides were prepared by urea synthetic route.Among them,urea plays a crucial role in the reduction of metal salts,and in situ carbonization to form nitrogen-doped carbon-coated phosphide composites.The resulting carbon-coated metal phosphides exhibit electrocatalytic hydrogen evolution over a wide p H range（0-14）which Mo P@NC exhibits the excellent performance with an overpotential of 135 m V at a current density of 10 m A cm-2,and can be stable for 20 h.The results show that the carbon coating strategy can effectively disperse phosphide nanoparticles,and reduce the aggregation of nanoparticles,and thus expose more active sites and improve the performance of electrocatalytic hydrogen evolution.3.We use one-step thermal reduction route to prepared a surface clean and pure bimetallic carbide Co₃Mo₃ C using carbon nanotubes,cobalt nitrate and ammonium molybdate as raw materials.In this process,carbon nanotubes not only act as a carbon source,but also as a reducing agent to obtain a pure phase of Co₃Mo₃ C.The intrinsic activity of Co₃Mo₃ C at different p H was studied.Among them,the best activity was found at p H 14 and the overpotential was 167 m V at a current density of 10 m A cm-2.Moreover,it is pointed out that the synergistic effect between the two metals regulates the electronic structure of the material itself and enhances the intrinsic activity of the material.4.A series of multi-metallic carbides were prepared by calcining in the inert atmosphere at 950 oC with carbon nanotubes as raw materials.In particular,this method synthesized trimetal carbides for the first time.The synthesis method is universal,which provides a new route for the synthesis of multi-metal carbides.Among them,tungsten-based carbides generate a low carbon structure T₆W₆C（T stands for Co,Ni or Fe）while molybdenum-based carbides generate a high carbon structure T₃Mo₃ C.The hydrogen evolution properties of the multi-metallic carbides were also studied.After systematically studying the hydrogen evolution properties of the multi-metallic carbides,we found that: the Co₃Mo₃ C has the best performance.And the order of the catalytic activities of the multi-metallic carbides is Co₃Mo₃C?Co₆W₆C?Co_xFe_3-xMo₃C≈Ni₆W₆C?Co_xFe_6-xW₆C?Fe₃Mo₃C?Fe₆W₆C.Moreover,the research shows that the reaction mechanism of molybdenum-based carbides and tungsten-base carbides are Volmer-Heyrovsky mechanism and Volmer mechanism,respectively.