Interface Regulation Of Transition Metal Matrix Composites And The Hydrogen Evolution Reaction For Electrocatalysis Water Splitting

With the development of economy and the demand for energy is increasing.The non-renewability of traditional fossil forces human begins to demand efficient,clean and renewable energy.As a future energy with high energy density,clean,pollution-free and recyclable,hydrogen（H₂）production and preparation technology has become a research hotspot.Electrocatalysis HER has become an important way because of its simple equipment,effective conversion efficiency of electric energy and chemical energy,and its large-scale popularization.However,the water splitting will suffer thermodynamic non-spontaneous process.Therefore,efficient and inexpensive catalytic materials are needed to accelerate the reaction rate.The catalytic activities of Pt and its alloys often are unparalleled,but their reserves in the earth’s crust are scarce,making their large-scale used in industrial production become unrealistic.In contrast,transition metal-based compounds,such as Fe,Co and Ni,which are abundant and cheap,have become the hotspot of relevant researchers.Over the past decades,people have greatly improved their understanding of the catalytic activities of transition metal-based alloys,oxides,carbides,nitrides and chalcogenides.Among most transition metal-based compounds,transition metal phosphides exhibit good catalytic activity.Nickel phosphide（Ni₂P）,as a catalyst with high catalytic activity in acidic electrolyte for HER,will be weaken when used in more mature alkaline electrolytic water equipment in the current market.Therefore,it is imperative to improve the HER catalytic activity in alkaline conditions.At present,the modification methods of catalytic activator for transition metal matrix composites mainly focus on two aspects.One is to increase the number of active sites per unit area.Nevertheless,with the development of nano-scale materials,the modification of catalytic activity by this method has reached the limit.Another aspect is to modify the intrinsic catalytic activity by regulating the electronic structure of the catalyst.Constructing multi-component nano-heterostructures and introducing heterogeneous elements can effectively regulate the electronic distribution and interaction among the elements in the main catalyst,thus realizing the regulation of the intrinsic catalytic activity.Based on this,a nitrogen-doped carbon-coated hollow cerium oxide/nickel phosphide(H-CeO_2-x/Ni₂P@NC)heterojunction was successfully fabricated using Si O₂ nanospheres as template by wet chemical method and CVD method.And the hollow carbon sphere-supported sulfur-doped nickel phosphide nanosheet（NC@S-Ni₂P）composites were prepared by hydrothermal and two-step CVD method,respectively.To achieve high efficiency HER application under alkaline conditions.The main research contents are as follows:（1）H-CeO_2-x was synthesized by solvothermal and chemical etching using SiO₂as templates.Then,the multi-stage structure of Nickel Phosphide Supported on the surface of nitrogen-doped carbon-coated cerium oxide was synthesized by wet chemical method and CVD method.The correlation between the relative content of cerium oxide/nickel phosphide and the catalytic activity was investigated.By using density functional theory（DFT）,the internal mechanism of the heterojunction interface structure improving the catalytic activity of the catalyst was systematically studied.Experiments and theoretical show that the existence of hollow nano-CeO_2-xnot only effectively enhance the specific surface area of the material,but also expose more active sites.CeO_2-x can also be used as an activation site of water molecule to produce protons needed for subsequent reaction.In addition,due to the variability of valence electrons of CeO_2-x,the electrons of Ni₂P at the CeO_2-x/Ni₂P interface redistribute,which weakens the proton adsorption(（?）G_H*),and finally accelerates the HER process in alkaline.The overpotential of H-CeO_2-x/Ni₂P@NC catalyst is only 123 mV at the current density of10 m A·cm^-2,which is due to the carbon layer structure coated on the surface of heterojunction.As a cathodic catalyst for hydrogen production in electrolytic cell,the material can work steadily for more than 27 hours.（2）Ni（OH）₂ nanosheets loaded on hollow carbon nanospheres were prepared by wet chemical method,CVD and hydrothermal treatment using Si O₂ template.S-doped Ni₂P nanosheets were obtained by CVD treatment,and NC@S-Ni₂P catalysts with different sulfur-doped content were obtained.The effect of S element doping on the catalytic activity of Ni₂P was study.The catalysis mechanism of S-Ni₂P was revealed by DTF.The results show that the introduction of S can weaken the adsorption of hydrogen protons by the vacancies in Ni₃ of Ni₂P exposed surface,that is,the poisoning effect of H*is weakened.The DOS result further confirms that the addition of S can cause the d-band center shift of Ni element,and then increase the anti-bonding component near Fermi level,resulting in the weakening of H*adsorption(（?）G_H*).Compared with the undoped Ni₂P,the introduction of S can further regulate the adsorption of Ni₂P on water molecules,accelerate the capture and subsequent activation of molecules,and then accelerate the reaction rate of electrocatalytic HER.The experimental results show that the overpotential required for NC@S-Ni₂P to obtain 10mA.cm^-2 current density is only 110 mV,and it has good cyclic stability.