Preparation And Catalytic Performances Of High-Efficient Pt-Based Composite Catalysts For Methanol Electrooxidation

Fuel cell is a new open system that can truly realize the efficient and continuous conversion from chemical energy to electric energy and act as an important kind of future energy sources.Its development and wide application,however,suffer still from the cost and performances of catalysts.Especially,the catalysts have been the development bottleneck of the low-temperature fuel cells with small organic molecules.Despite the home and abroad researches on the preparation of non-noble metal catalysts and their catalytic behaviors,at present from the perspective of catalytic activity and stability,the Pt-based materials are still the most effective fuel cell catalysts for the electrochemical oxidation of small alcohols.Therefore,the studies on the preparation and the performances of Pt-based catalysts are not only of great significance to basic researches but also of much value for practical applications.For many years,great research efforts have focused on two aspects:one is improving the utilization rate of precious metal atoms to reduce the costs;another is adding in other elements or compounds to improve the resistance of Pt active sites to intermediates?such as COads?through bifunctional mechanisms,electronic effects or spatial effects.From the viewpoint of catalyst particle sizes,nano-preparation has become an effective way to improve the atomic utilization rates and catalytic activities of precious metals.Additionally,the amount of platinum can be reduced and the resistance of Pt to CO poisoning can be improved by combining platinum with other metal elements to regulate the surface adsorption free energy of CO and OH.Platinum-based polymetallic catalysts mainly include the platinum-based bulk alloys or intermetallic compounds and the surface or near-surface alloys that can form by covering the platinum surface with other metal atomic layers.Although platinum-based nano-alloy catalysts have been explored and prepared either by solution chemical reduction or by physical methods such as film sputtering,the problem of how to prepare platinum-based nano-alloy catalysts more easily,greenly and controllably is still important and challenging in catalyst researches.It is worth noting that the preparation and performances of platinum-based nano-alloy catalysts supported on transitional metal hydroxides or oxides have been becoming a hot point of the fuel cell catalyst study.Based on the investigation and analysis of the related home and abroad researches,the work in this thesis aimed to obtain the Pt-based nano-composite catalysts supported on porous hydroxides.The catalysts were prepared by hydrothermal,electrochemical and photochemical methods,and then their catalytic performances were tested and given a mechanism explain.This study included four parts:?1?hydrothermal preparation and electrodeposition of porous hydroxide supports;?2?preparation of Pt,Ag and PtAg nanoparticles by electrodeposition and liquid phase photochemical reduction;?3?characterization of the microstructure,phase analysis and element composition of the as-prepared products;?4?electrochemical study of the catalytic performances of the as-prepared catalysts towards the electrochemical oxidation of small alcohol molecules.The main experimental results are described as follows:?1?The experimental results showed that,the porous nickel hydroxide thin films could be prepared by electrodeposition on copper foils or nickel foam.This method,however,is unsuitable for large-scale preparation and limited to some supports.A better method is hydrothermal reaction,by which nano-sized nickel hydroxide with different morphologies was prepared with different base sources.The honeycomb-like?-phase nickel hydroxide was prepared with hexamethylenetetramine as the base source.Its specific surface area is about 248m²·g^-1,which is comparable to the most widely used fuel cell catalyst support VULCAN XC-72R(250 m²·g^-1).The sea urchin-like nickel hydroxide was prepared with urea and almost presented as?-Ni?OH?₂.In addition,the experiment of adding in cobalt element showed that the honeycomb-like morphology of pure nickel hydroxide changed gradually with the content of Co element.?2?Nano-Pt/Ni?OH?₂ composite catalyst was successfully prepared by the liquid-phase photochemical reduction in absolute ethanol solutions,which adopted common xenon lamps as irradiation light source at room temperature.When the prepared Pt-Ni?OH?₂ composite catalyst was applied to modify the working electrode,Nafion solutions were added in different modes.The cyclic voltametric experiments of methanol electrooxidation exhibited that the electrodes modified with the prepared catalyst and then Nafion solution resulted in obviously better performances than those modified with the mixture of Nafion solution and the catalyst.It is worthwhile to point out that,the bad electric conductivity of Ni?OH?₂ can make the Pt-Ni?OH?₂composite catalyst inferior to the Pt catalyst prepared by the same process in the aspect of catalyzing the methanol electrooxidation.?3?Pt/Ag/Ni?OH?₂ composite catalyst was prepared by the same photochemical reduction process as that for the Pt/Ni?OH?₂ composite catalyst,except for the addition of silver nitrate solution to the H2PtCl6 solution.The catalytic performances of the prepared catalyst towards methanol electrooxidation were studied by cyclic voltammetry,chronoampermetry?i^t?and Tafel curve techniques.All electrochemical experiments were carried out with a standard three-electrode system,in which the glassy carbon electrode modified with the prepared catalyst served as the working electrode.Cyclic voltametric experiments verified that the addition of Ag could improve the catalytic activity of Pt.The peak current density obtained with Pt/Ag?1:1?as the catalyst was more than 1 times higher than that obtained with pure Pt as the catalyst.When the Pt/Ag/Ni?OH?₂ composite catalyst was used,the peak current density corresponding to methanol electrooxidation is about 3 times that for the Pt catalyst.The i^t tests?3 600 s?showed that the current density for Pt/Ag/Ni?OH?₂composite catalyst was significantly higher than that for pure Pt catalyst.The results from Tafel curve experiments further demonstrated that the Pt/Ag/Ni?OH?₂ composite catalyst could offer the best catalytic performances from the viewpoints of both the Tafel slope and the current density at a selected overpotential.Through the analysis of a series of experimental results,this study arrived at three conclusions.?1?Although electrodeposition has the advantage of easy operation,the hydrothermal method will be preferable if the large-scaled production and support availability are considered.The hydrothermal synthesis with HMT as the OH^-source and template can easily and effectively produce the ordered porous hydroxide with large specific surface area.?2?Photochemical reduction in alcohol/water solution is a green and convenient approach to preparing platinum-group metal nanoparticles,whose mild experimental conditions are beneficial to preserving the morphology and structure of catalyst support.?3?The prepared Pt/Ag/Ni?OH?₂ composite catalyst has excellent catalytic activity towards the methanol electrooxidation.In the catalyst,Ni?OH?₂ can promote the dissociation of water molecules to produce the adsorbed OH species.The OH radical coming from the surface of nickel hydroxide can react with the CO molecules adsorbed on the Pt surface,and the Ag atoms can improve the electric conductivity of catalyst system and impose an electronic effect on Pt atoms.This study has made three points of innovation:?1?the introduction of new photochemical methods into the controllable preparation of platinum-based fuel cell catalysts,?2?the construction of the highly efficient Pt-based composite catalysts combined with honeycomb-like porous hydroxide,and?3?a new viewpoint that the OH spillover from nickel hydroxide surface to Pt active sites is an important step in the oxidative removal of CO_ads.This study is an active exploration for the development of fuel cell catalysts.The obtained experimental results are not only of great significance to basic researches but also of much value for practical applications,advancing the research of small-organic-molecule fuel cells.