Study Of The Electrocatalytic Ethanol Oxidation In Transition Metal Oxides Modified Palladium-based Nanocomposites

In recent decades,direct alcohol fuel cells（DAFCs）have become potential candidates as a highly efficient clean energy for the next generation due to their simple structure,friendly environment,high energy density and easy storage and transportation.As a basic criterion of DAFCs,the catalytic activity and stability of anodic electrocatalytic materials play a crucial role in the practical applications.Therefore,it is particularly important to explore high-performance electrocatalytic materials toward alcohol oxidation.Compared to the traditional platinum-based electrocatalytic materials,the palladium（Pd）based materials recently have aroused extensive concern in the field of DAFCs because of their rich reserves,especially the excellent alcohol-oxidation activity and long-term stability in alkaline media.However,the toxic intermediates like the carbon monoxide(CO_ads)adsorbed on the Pd surface seriously hinder the further catalytic reaction for alcohol oxidation,which leads to the slow alcohol oxidation kinetics,low-efficiency utilization of reaction active sites and poor durability.It has been found that the adsorption energy of reaction intermediates can be effectively reduced by regulating the position of the d-band orbit center of Pd.Thus,the poisoning species on the Pd surface can be removed to further improve the electrocatalytic performance.To date,it has been demonstrated that adding the carrier materials can serve as an effective strategy to improve the electrocatalytic activity.On one hand,when supporting Pd nanoparticles on these materials,the dispersion and utilization efficiency of Pd can be significantly improved due to the larger specific surface area of the carrier material.On the other hand,strong metal-support interactions such as chemical bonding or electron transfer between the Pd nanoparticles and carrier material can tailor the electronic structure of Pd.Based on the above analyses,transition metal oxides can be regarded as the potential cocatalysts to promote the catalytic performance by the strong chemical interactions with Pd owing to their high conductivity and rich oxidation state.Therefore,in this thesis,we mainly studied the electrocatalytic ethanol oxidation in transition metal oxides modified Pd-based nanocomposites.Particularly,the reaction dynamics of ethanol oxidation are considerably promoted by manipulating the electronic state of Pd to reduce the adsorption energy of toxic intermediates.The research contents and results in details are as follows:1.Tungsten oxide（WO₃）modified Pd/C composite catalysts（Pd-WO₃/C）were successfully fabricated by a microwave irradiation method to investigate the electrocatalytic ethanol oxidation in alkaline medium.A series of the tests,such as the surface morphology,micro-structure and crystallization of the samples,reveal that adjusting the electronic structure of Pd can promote the catalytic activity and stability due to the strong electronic interaction between crystalline WO₃and Pd nanoparticles.In particular,the mass specific activity can reach to 2236.38 m A mg_Pd^-1for Pd-WO₃/C which is around 2 times larger than that for the commercial Pd/C（10%）.Our findings provide a design ideas for synthesizing high-performance composite catalytic materials.2.Molybdenum oxide（Mo O₃）decorated Pd/C nanocomposites（Pd/Mo O₃-C）were successfully synthesized by a facile hydrothermal method to explore the electrocatalytic activity and stability toward ethanol oxidation in alkaline medium.The electrochemical tests show that the mass specific activity of the Pd/Mo O₃-C composite is 2909.04 m A mg_Pd^-1,which is about 2.6 times larger than that for the commercial Pd/C（10%）.Meanwhile,the Pd/Mo O₃-C composite has the stronger anti-poisoning ability for CO,which effectively improves the stability of the catalytic reaction as well.These excellent catalytic properties mainly benefit from the strong electron coupling interaction between amorphous Mo O₃and Pd nanoparticles,which can weaken the adsorption of toxic intermediates on the Pd surface and thus provide more active sites for the electrocatalytic oxidation reaction.3.Using the nickel molybdate（Ni Mo O₄）nanorods as the support material,the composite catalyst（Pd/Ni Mo O₄-C）,composed of polycrystalline Ni Mo O₄-C nanorods supported Pd nanoparticles was successfully synthesized to realize the highly efficient and stable electro-oxidation reaction of ethanol.The mass specific activity can reach to 3318.84m A mg_Pd^-1for Pd/Ni Mo O₄-C,which is around 2.82 times larger than that for the commercial Pd/C（10%）.The superior catalytic performance is mainly derived from:1)the strong electron interaction between Ni Mo O₄and Pd can induce the rapid electron transfer kinetics on the Pd surface;2)the Ni Mo O₄nanorods provide a good carrier platform for Pd nanoparticles,which not only increases the dispersion of Pd nanoparticles and refines the particle size（the average particle diameter is about 3.33 nm）,but also exposes more active centers by expanding the contact area.Furthermore,we also demonstrate that the highly conductive C with the stronger anchoring effect in the chemically synthesized Pd/Ni Mo O₄-C composite plays an auxiliary role in further boosting the electrocatalytic performance.4.Nickel cobalt oxide（Ni Co₂O₄）decorated Pd/C nanocomposites（Pd/Ni Co₂O₄/C）were prepared by a facile hydrothermal approach.Using the strong electronic interaction between amorphous Ni Co₂O₄and Pd nanoparticles effectively alters the electron state of Pd and weakens the adsorption capacity of toxic intermediates on the Pd surface so that the higher electrocatalytic efficiency toward ethanol oxidation and better anti-CO poisoning ability and stability are obtained.In particular,the mass specific activity can reach up to 3541.22 m A mg_Pd^-1for Pd/Ni Co₂O₄/C,which is approximately 3.19 times larger than that for the commercial Pd/C（10%）.It significantly enhances the electrocatalytic oxidation capacity,making the Pd/Ni Co₂O₄/C nanocomposite a potential alternative electrode material for the practical applications of DAFCs.