Controlled Synthesis And Property Study Of Pd-Based Nano-composite For Anode Catalyst Of Direct Fuel Cell

Noble metals have gained the same value recognition among different civilizations in the world since ancient times,mainly because of their scarcity and their unique physicochemical properties.In recent thirty years,depending on the rapid development of TEM,SEM and other characterization methods,the study of noble metal catalysts has made great improvements.The concept of controllable synthesis has been widely accepted by academics.As one of the noble metals,the wide applications of palladium in the field of modern catalysis has greatly improved human's daily life.Great efforts are being made to further enhance the controllable synthesis way to fabricate palladium-based nanoparticles.In this dissertation,we designed and synthesized a series of palladium-based nanocatalysts anchored on activated carbon,carbon nanotube,graphene nanoribbon,as the anodic catalysts for anion exchange membrane fuel cells.The aim is to obtain the desired high-performance nanocatalysts by a novel and simple synthesis method.At the same time,the morphology,structure,and composition of the obtained catalysts were studied,and the structure-activity relationship between the catalyst and electrochemical performance was investigated in detail.The main research contents include the following aspects:1.Controlled synthesis and property study of carbon black supported palladium nanocomposite catalystControllable synthesis of palladium nanoparticles and particle size control is a very challenging work so that it's become a hot topic of research.Small-sized palladium nanoparticles have excellent surface electronegativity due to their extremely high surface atomic ratio and a large number of low-coordination high-energy atoms located at the edge of the crystal.We successfully prepared palladium nanoparticles with an average particle size of 2.7nm and uniform loading on the surface of commercial carbon black supports(Pd/CB)by our novel synthesis system in which ethylene glycol as the solvent,hydrazine hydrate as the reducing agent and palladium nitrate solution as the metal precursors.Through a series of comparative experiments,it is confirmed that the excellent morphology of the catalyst mainly depends on the following two points:(1)the rapid consumption of the palladium precursor in the early stage inhibits the growth of the palladium nanoparticles after the nucleation,and the short reaction time suppresses the Ostwald ripening phenomenon;(2)The strong adsorption capacity of the support to the palladium nanoparticles and the good dispersibility in the reaction system effectively prevent the agglomeration of the palladium nanoparticles in the preparation process.This novel Pd/CB catalyst exhibit the extremely high electrochemical activity of electrochemical specific surface area,almost three times that of conventional commercial palladium/carbon catalysts.At the same time,the Pd/CB catalyst has excellent electrocatalytic activities and good electrochemical stabilities towards the electro-oxidation of small molecule alcohols such as methanol and ethanol has good application prospect in the future anion exchange membrane fuel cell.2.Controlled synthesis and property study of carbon black supported palladium nanoflower and palladium nanowire catalystsBy expanding above controllable synthesis strategy,we successfully prepared two kinds of composite catalysts:carbon black supported palladium nanoflower and palladium nanowire in the reaction system of ethylene glycol/hydrazine hydrate:(1)ammonia was introduced into the reaction system to adjust the reduction activity of hydrazine hydrate to change the formation rate of palladium atoms,a unique palladium nano-flower structure was prepared.Importantly,carbon black supported palladium nanoflower nanocomposite could achieve higher palladium loading(50 wt%),thereby effectively reducing the catalytic layer resistance,improving the electrocatalytic properties at the same time,the unique structure of palladium nano-flower promotes the rapid diffusion of the electrolyte in the catalytic process,thereby enhancing the catalytic reaction efficiency;(2)the usage of agent including Cl-in the reaction system can control the aggregation rate of palladium nanoparticles in the process of preparation by steric effect.We obtained a loose nanostructured palladium nanowire with an average diameter of only 2-3 nm.After adsorbed by carbon black,the as-prepared nanocomposite shows higher electrocatalytic activity and excellent anti-toxic stability towards methanol and ethanol electrooxidation under alkaline conditions.3.Controlled synthesis and property study of activated carbon supported palladium-bismuth,palladium-nickel,palladium-cobalt,and palladium-copper alloy composite catalysts ·In order to further expand our controllable synthesis system,palladium-bismuth alloy nanopart:icles of different atomic ratios loaded on carbon black with an average particle size of2.6-3.3 nm was successfully synthesized.The as-prepared nanocomposites were first taken the cyclic voltammetry(CV)measurement in glycerol and sodium hydroxide solutions which make the palladium-bismuth alloy nanoparticles in nanocomposites achieved surface de-alloying by introducing Pd-rich shell structure.After the surface de-alloying,the electrocatalytic activities and cycle stabilities of the nanocomposites increased greatly towards small molecule alcohols electrooxidation such as ethanol.After that,we successfully prepared a series of palladium-based alloy catalysts,such as palladium-nickel,palladium-cobalt and palladium-copper alloyed nanoparticles supported on carbon black.Their microstructures and electrocatalytic properties were detailed studied and analyzed,the new alloy catalysts showed excellent catalytic performance for the electrooxidation of small molecule alcohols such as ethanol,which were significantly more competitive than the carbon supported palladium nanocomposite and commercial palladium-carbon catalyst.Controlled synthesis and property study of graphene nanoribbons and low-defect carbon nanotube-supported palladium nanocomposite catalystsThe catalyst support is also one of the key factors,which greatly affects the electrocatalytic performance of the noble metal catalyst.We extend the activated carbon support system to the current popular types of carbon nanomaterials,such as carbon nanotubes and graphene nanoribbons.First,with the aid of NMP,we could obtain low-defect carbon nanotubes.Meanwhile,by enhancing the ultrasonic power,we could obtain highly dispersed graphene nanoribbons.Using our controllable synthesis method,we successfully synthesized palladium nanoparticles with average particle sizes of 2.9and3.5 nm supported on graphene nanoribbons or low-defect carbon nanotubes.Due to the huge surface area of graphene ribbon,the metal loading on graphene nanoribbons can be significantly increased up to 80wt%,thereby effectively reducing the thick:ness of the anode catalyst layer thus reducing the mass transfer resistance of the electrolyte and improving the catalytic efficiency of the catalytic system.The results of electrochemical measurements show that the palladium nanoparticles supported on graphene ribbon have excellent catalytic activity and stability for methanol,ethanol electro-oxidation.In addition,the palladium nanoparticles supported on low-defect carbon nanotubes also shows better catalytic performance for formic acid electrooxidation,which is significantly superior to conventional acid-treated carbon nanotubes,chemically reduced graphene,activated carbon supported palladium catalysts.