Preparation And Catalytic Properties Of Ni（OH）₂ Nanosheets Modified With Noble Metal Atoms

Fuel cell is a kind of clean and efficient new energy,among which direct methanol fuel cell is very promising and has attracted the attention and research of many researchers.However,the large-scale application of direct methanol fuel cells still faces great challenges.Pt-based catalysts have a good catalytic effect on methanol,but the development of direct methanol fuel cells is limited by the disadvantages of easy"poisoning"and high price of Pt catalysts.Depositing Pt onto a carrier with a large specific surface area,or introducing a second element into a single element of Pt catalyst,can effectively alleviate the CO"poisoning"problem of Pt electrocatalysts and reduce the cost.In this thesis,the Pt/Ni（OH）₂ composite catalyst and Pt Ag/Ni（OH）₂ composite catalyst were prepared by depositing Pt and Ag atoms on the nickel base hydroxide as the support,and the pure Pt catalyst and Pt Ag catalyst were prepared as the control.The catalytic mechanism of Pt Ag/Ni（OH）₂ composite catalysis on the electrooxidation of methanol was investigated.The main research results are as follows:（1）Ni（OH）₂ carriers with different morphologies were prepared by solution chemical method.Ni（OH）₂ carriers prepared by liquid crystal template method were dozens to hundreds of nanometers in size and showed porous structure,fragmentation and overlap.The Ni（OH）₂ prepared by hydrolysis method is nanosheet with the size of about 50 nm.The maximum size of Ni（OH）₂ prepared by hydrothermal method is about 400 nm,which shows rod-like morphology and serious agglomeration.The results of cyclic voltammetry of methanol with three kinds of Ni（OH）₂ showed that the initial oxidation potential of Ni（OH）₂ prepared by hydrolysis method was 322.8m V and the peak oxidation current density was 2.49 m A·cm^-2,which had the best catalytic effect on the electrooxidation of methanol.（2）Pt nanoparticles were prepared by liquid phase photoreduction method and deposited on Ni（OH）₂ support to prepare Pt/Ni（OH）₂ composite catalyst.The introduction of Ni（OH）₂ makes Pt particles more dispersive and improves the utilization rate of Pt atoms.Cyclic voltammetry results show that the oxidation peak current density of Ni（OH）₂ supported composite catalyst prepared by hydrolysis method is 527.2 m A·mg^-1,which is higher than that of Ni（OH）₂ supported composite catalyst prepared by liquid crystal template method and hydrothermal method,but slightly lower than that of pure Pt catalyst,because of the poor conductivity of Ni（OH）₂.Blocking the transfer of electrons between the catalyst and the electrode.The results of i～t curve show that the composite catalyst prepared by hydrolysis method with Ni（OH）₂ as the support has the best stability.（3）Pt Ag catalyst and Pt Ag/Ni（OH）₂ composite catalyst were prepared by liquid phase photoreduction method with Ag on the basis of Pt/Ni（OH）₂ composite catalyst to improve the conductivity.The cyclic voltammetry results show that the current density of Pt Ag catalyst is 1.99 times that of Pt catalyst.The Pt Ag/Ni（OH）₂composite catalysts with different proportions were prepared by changing the amount of Ni（OH）₂.With the increase of the amount of Ni（OH）₂,the current density first increased and then decreased.The current density of the composite catalyst is 2.65times that of the pure Pt catalyst at a specific ratio.Under this ratio,the slope of Tafel curve corresponding to the composite catalyst is the lowest,and the current density of the i～t curve is also higher than that of other catalysts when the curve is stable.The results showed that it was necessary to optimize the ratio between Ni（OH）₂ carrier and Pt load.（4）In this thesis,the catalytic mechanism of Pt Ag/Ni（OH）₂ composite catalysis on the electrooxidation of methanol was analyzed and discussed.Its excellent catalytic performance can be attributed to the combined action of Pt,Ag and Ni（OH）₂.Pt is the main component of the electrooxidation of methanol,and CO_ads produced will be adsorbed on the surface of Pt.Ni（OH）₂ as a carrier can improve the dispersion of Pt particles and promote the dissociation and adsorption of surface water molecules.The resulting OH_ads react with CO_ads on the surface of Pt to produce the final reaction products CO₂ and H₂O.The incorporation of Ag makes Pt particles more dispersed and further improves the utilization of Pt atoms.As an excellent electronic conductor,it improves the electrical conductivity of Pt Ag/Ni（OH）₂ composite catalyst.Ag can also improve the electronic structure of Pt and enhance its anti-poisoning ability.The innovation of this thesis is to explore the simple preparation of Ni（OH）₂nanosheets and the photodeposition of the noble metal alloy nanoparticles,and to analyze the catalytic mechanism of the electrooxidation of methanol.