Controlled Synthesis Of Pd/Pt Based Nanocrystals And Their Electrocatalytic Properties Toward Oxygen Reduction Reaction

It has been already a wordwide challenge to develop environment-friendlly energy sources and reduce our dependence on fossil fuels.Proton exchange memberane fuel cell is an efficient,non-pollutiong and clean method to transfer chemical energy into electric energy.Pd-and Pt-based nanocrystals are the most frequently-used catalysts for the fuel cell,especially oxygen reduction reaction?ORR?at cathode.Owing to the scarcity,limited supply,high cost,ever increasing demand,however,it has been challenging to commercialize the fuel cell at an industrial scale.Intensive studies have shown that the performence of these nanocatalysts closely depends on their shape,size and composition.As a result,controlled synthesis of the Pd-and Pt-based nanocrystals with specific morphology and size is of great importance to enhance their catalytic properties.Most of the protocols for the controlled synthesis of nanocrystals were based on batch reactors such as flasks and vials,in which the quantity and reproducibility cannot be guaranteed from batch to batch,making it a big challenge to apply the nanocrystals to large-scale applications.Continuous-flow synthesis method offers an attractive strategy for the scalable production of colloidal nanocrystals.In this dissertation,Pd-and Pt-based noble-metal nanocrystal were employed to study the formation mechanism of the shape controlled nanocrystals,thus guiding the exploration of simple and effective synthetic methods.The enhanced catalytic activities of the corresponding products toward ORR have been studied.The main research work of this dissertation was summarized as follows:?1?Pd icosahedral nanocrystals with uniform,controlled morphology were synthesized in continuous-plug reactor separated by air.The reaction channel could be reach a millimeter level?1.6 mm?,Na₂PdCl₄ was used as metal precursor,and diethylene glycol?DEG?was used as solvent and reductant.The average diameters of the Pd icosahedra could be readily controlled in the range of 12-20 nm by manipulating the reaction time or the amount of hydrochloric acid.Compared to droplet reactors involving silicone oil or fluorocarbon liquids as carrier phases,the use of air as a carrier phase could reduce the production cost by avoiding additional procedures for the separation of products from the oil.The reduction kinetics with different carriers was studied,the oxygen contained in the air segments not only contributed to the generation of a reductant from diethylene glycol in situ,but also oxidized elemental Pd back to the ionic form via oxidative etching and thus slowed down the reduction kinetics.Thus,the carrier phase should be carefully controlled in this system.During the preparation of high yield Pd icosahedra,the length ratio between reaction solution and air should be controlled with 1:6 to 1:18.The Pd icosahedra were further employed as seeds for coating two to three layers of Pt atoms via dropwise method,to produce Pd@Pt core-shell icosahedra,which could serve as a catalyst toward ORR with greatly enhanced activity,the specific and mass activity can reach 1.08 mA·cm_Pt^-2 and 1.1 A·mg_Pt^-1 at0.9 V_RHE,which were 3 and 5.7 times than commercial Pt/C catalyst,respectively.Pd core was sacrificed to prevent the corrosion of Pt shell during the durability test,thus enhancing the catalytic durability.?2?Facil and scalable method based on continuous-flow-mode reactor was reported for conformally coating the surfaces of facet-controlled Pd icosahedral seeds with two to five ultrathin shells made of Pt,to form Pd@Pt core-shell icosahedra.The key to the success of such an approach is the identification of a proper polyol?tetraethylene glycol,TTEG?,suitable temperature and concentration of Na₂PtCl₆ to generate the Pt atoms at a relatively slow rate to ensure adequate surface diffusion and thus the formation of uniform shells in a layer-by-layer fashion,avoiding Pt self-nucleation.The reaction conditions and formation of the nanocrystals were discussed based on the Pd@Pt icosahedra?twinned crystal with{111}facet?.The strategy was successfully extended to the synthesis of Pd@Pt nanoscale octahedra?single crystal with{111}facet?and cubes?single crystal with{100}facet?,which were synthesized via Pd octahedral seeds and cubic seeds with sizes of 15 and 18 nm,respectively.These core-shell nanocrystals showed great enhancement in catalytic activity toward ORR.Our results suggest that seed-mediated growth could be combined with continuous-flow reactor to achieve scalable production of bimetallic and even trimetallic core-shell structures with controlled sizes,shapes,and compositions.?3?Pd@Pt core-shell octahedra and icosahedra with controlled size and shape were synthesized by co-reduction in one-pot method.Quantitative understanding of the reduction kinetics responsible for the synthesis of Pd-Pt bimetallic nanocrystals with two distinctive structures was studied.When ethylene glycol?EG?was used as the solvent,in the absence of KBr,the ratio between the initial reduction rates of Na₂PdCl₄and K₂PtCl₄ was greater than 5,leading to the formation of Pd@Pt core-shell octahedra with average diameter of 6.3 nm.The Br^–involved in KBr could impact the redox potentials of metal precursor ions by ligand exchange,in the presence of a certain amout of KBr,PdBr₄^2-and PtBr₄^2-were formed,thus manipulating the reaction kinetics.In this case,the ratio between the initial reduction rates of the two precursors decreased to less than 4,the products became Pd-Pt alloy nanocubes with average diameter of 6.1nm.When EG was replaced by TTEG,Pt?acac?₂ and Na₂PdCl₄ were used as metal precursors,the reaction kinetics was following slowed down,in addition,the solubility of Pt?acac?₂ and complexing action of acetylacetone provide enough time for Pd atoms to form the icosahedral core,the corresponding product became uniform Pd@Pt core-shell icosahedra with size of 10 nm.Quantitative understanding of the reduction kinetics offers a theoretical direction for the fabrication of noble metal nanocrystals with core-shell structure.The Pd@Pt core-shell octahedra showed enhanced catalytic activity and durability toward ORR,Specifically,the specific and mass activity can reach 1.51 mA·cm_Pt^-2 and 1.05 A·mg_Pt^-1 at 0.9 V_RHE,which were 4.2 and 5.5 times than commercial Pt/C catalyst,respectively.Even after 20,000 cycles of accelerated durability test,the core-shell octahedra still exhibited a mass activity of 0.68 A·mg_Pt^-1.The Pd core of Pd@Pt icosahedra can be selectively removed away by oxidative etching,thus Pt nanocages were formed.Enhanced Pt atom utilization can be achieved by this howllow structure,the specific and mass activity can reach 0.91 mA·cm_Pt^-2 and 0.48A·mg_Pt^-1 at 0.9 V_RHE,the Pt shells were relatively thicker and the number of layers was different on different facets of a single particle,the co-reduction synthesis of Pd@Pt icosahedra should be further optimized to improve their catalytic activities.?4?Cubic nanoframes comprised of twelve ultrathin Pt ridges?only 1.7 nm?with open faces and a hollow interior,which offer a greatly large surface area,were synthesized as an advanced catalyst toward ORR.The Pt cubic nanoframes were fabricated by site-selected deposition of Pt atoms on Pd nanocubes?templates?in EG,Na₂PtCl₆ was used as metal precursor,and then remove the Pd cores by oxidative-etching.During the deposition of Pt atoms,the kinetics of the deposition and the diffusion of Pt adatoms on the sufaces of the templates were carefully adjusted by manipulating the injection rate of Na₂PtCl₆ solution and the reaction temperature,respectively,which was the key to fabricate Pt nanoframes.After selective removing Pd cores from the Pd-Pt core-frame nanocubes,the Pt cubic nanoframes with open and hollow structure were obtained.The Pt frames/C catalyst showed an enhanced initial mass activity,meanwhile,exhibited substantially improved durability compared with a commercial Pt/C catalyst at 0.9 V_RHE,the mass and specific activity were 0.45 A·mg_Pt^-1and 0.53 mA·cm_Pt^-2,respectively.Due to the increase in ridge thickness of the Pt nanoframes during repeated potential cycling,the proportion of low-coordinated surface atoms was decreased,thus enhancing the overall specific activity.As a result,the mass activity of the Pt frames/C increased with cycling and became 1.4 times of the initial value after cycling 30,000 potential sweeps.The greatly enlarged contact area between the nanoframes and the carbon support prevented aggregation as well as leaching of the Pt from the catalyst,there was only¹0%of Pt lost from the Pt frames/C catalyst.