Study On Preparation Of Palladium Based Nanocomposites And Their Electrocatalytic Properties

In the rapid development of modern society,energy is playing a vital role.Due to the apparent drawbacks of traditional fossil fuels,the development of green and sustainable energy resources is imminent.As a device that can directly convert the chemical energy of the fuel into available electric energy,the fuel cell posesses the advantages of mild reaction process,environmental friendliness,high specific energy,and high energy conversion efficiency,and it is one of the most promising new energy technologies.However,in the commercial application of fuel cells,there are some problems such as higher cost and shorter life that remained as bottlenecks.To solve these problems,the key is to design and develop electrocatalysts with low-cost,high-activity and robust long-term stability.In this thesis,a simple,safe,and efficient experimental protocol has been developed to prepare a series of palladium-based nanoelectrocatalysts with low-cost,high-activity,and robust long-term stability.The relationships between the structure and function were studied by means of the characterization tools,and electrochemical tests.The main content of this thesis includes the following three parts:?1?Firstly,we used the metal-organic framework MOF-5 as precursor and obtained porous carbon nanosheets through the high-temperature calcinations process.Then PdCl₂ was employed as a Pd source to obtain different doping ratios of porous carbon confined ultrasmall Pd nanoclusters by in-situ reduction method.Through transmission electron microscopic?TEM?analysis,the average particle size of Pd nanoparticles was between 1 and 2 nm.The electrochemical tests showed that,compared with commercial palladium black,Pd/CNs with a mass ratio of 20%had better oxygen reduction catalytic activity and cyclic stability.The superior oxygen reduction catalytic performance of this material is mainly derived from:?i?The carbon sheet has a porous layered structure with a large specific surface area of 1517.3 m² g^-1;?ii?The in-situ synthesized Pd nanoparticles are smaller in size,evenly distributed,without agglomeration,hence maximal active site fully exposed.?2?Secondly,we utilized the alloying effect between the bimetals to synthesize different ratios of PdAu alloy nanoclusters with a average particle size of 1 to 2 nm via thiol ligand-reduced glutathione,chloroauric acid,potassium chloropalladate,and sodium borohydride.The PdAu alloy was doped into the porous carbon nanosheets by mechanical stirring.Through electrochemical tests,the PdAu ratio shows the best oxygen reduction catalytic performance with a Pd-to-Au atomic ratio of 1:2.Compared with commercial platinum carbon,it had a larger current density and electron transfer number and long-term cyclic stability.This is mainly due to the alloying effect caused by the electron transfer between PdAu,the larger specific surface area as well as the more active sites can be exposed by the ultra-small particle size.?3?Finally,we used PVP as a ligand,ascorbic acid as a reducing agent to synthesize a palladium cube structure,and then partially encapsulated it with molybdenum to form a Pd@Mo core-shell structure,and combined Pd@Mo core-shell nanoparticles with carbon black XC-72 by mechanically stirring.By this way,a Pd@Mo/C electrocatalytic material was obtained.Through TEM,Line-scanning and elemental mapping characterization,it was affirmed that Pd@Mo core-shell structure was acquired.This material was tested for electrochemical oxygen reduction and hydrogen evolution reactions,and it exhibited excellent oxygen reduction catalytic performance,considerable electrochemical hydrogen evolution performance,and prolonged electrochemical cyclic stability.The excellent electrocatalytic activity of Pd@Mo/C is mainly due to the structure effect and alloying effect of Pd and Mo of Pd@Mo core-shell nanoparticles,and the Pd?100?crystal plane,which possesses a particular catalytic enhancement effect on oxygen reduction.