Synthesis And Electrocatalytic Properties Of Binary And Ternary Pd-based Nanoalloys Or Corresponding Nanohybrids

As one class of clean energy conversion device,regenerative H2-O2 fuel cells(RHOFCs)are mainly composed of two sections:one is the H2-O2 fuel cells(HOFCs)section and the other is the water electrolyzer cell(WECs)section.Due to the sluggish kinetics of cathodic oxygen reduction(ORR)in HOFCs as well as anodic oxygen evolution reaction(OER)and cathodic hydrogen evolution reaction(HER)in WECs,it is ugently needed to develope cost-effective,highly active and robust electrocatalysts to promote those three electrode reactions and reduce their overpotentials.In view of current research status on ORR and HER electrocatalysts,this thesis is focused on desin and synthesis of Pd-based binary and ternary alloy nanostructures or related nanohybrids,and exploration of their applications in those two electrode reactions.By developing new solvent systems,serveral kinds of newly Pd-based nanoalloys and related nanohybrids,such as a series of strongly coupled MnxPdy/C nanohydrids,ultrathin 2D MnPd3 nanosheets based 3D flower-like porous nanoarchitectures,and network-like ternary Pd2Ni2-xMox(0<x<2)nanoalloys,have been successfully fabricated by using solvothermal-assisted in-situ growth and controllable annealing technique.The electrocatalytic ORR or HER properties of those materials have been systematically studied.Moreover,by combining a series of spectroscoptic and electrochemical analyses,the origins of the catalytic activities of those materials have also been suggested.Some primary results have been achieved,and the details are given below:(1)A series of strongly coupled MnxPdy(MnPd3,MnPd-Pd,Mn2Pd3,Mn2Pd3Mn11Pd21)/C nanohydrids(NHs),have been synthesized by solvothermal treatment of Mn2(CO)10 and Pd(acac)2 precursors with proper molar ratios in the presence of the carboxylic-functionalized Vulcan XC-72 C and citronellol to in-situ integrate MnxPdy nanocrystals on Vulcan C,and then annealing under Ar/H2 atmosphere.In the obtained NHs,MnPd3C ones exhibit unique "half-embedded and half-encapsulated"interfacial structure,i.e.one half of MnPd3 nanocrystals are embedded into C support and the other half are encapsulated by thin mesoporous C shell layer,which not only facilitates to improve mass transport process and trigger the occurrence of the reaction but also be helpful to enhance its electrocatalytic stability.Electrocatalytic tests demonstrate that all the MnxPdy/C NHs can efficiently catalyze ORR in alkaline media.Amone them,MnPd3/C NHs exhibit the highest catalytic activity with the onset reduction potential of 0.953 V(vs.RHE)and half-wave potential of 0.80 V(vs.RHE),which are close to commercial Pt/C,and superior to recently reported some ORR catalysts,such as Ag-Co alloy,PdCo/N-doped C NHs,PdNiCu/PdNiSn,and so on.Moreover,the durability and methanol-tolerant capability of MnPd3/C NHs are better than Pt/C catalyst yet.As revealed by spectroscopic and electrochemical analyses,the excellent catalytic performance of MnPd3/C NHs results from the proper component ratio of Mn and Pd and the strong interplay of their constituents,which not only facilitate to optimize d-band center or electronic structure of Pd but also induce the phase transformation of MnPd3 active components and enhance their conductivity or interfacial electron transfer dynamics.This work demonstrates that MnPd3/C NHs are a promising methanol-tolerant cathode electrocatalyst that may be employed in fuel cells or other renewable energy option.(2)The ultra thin MnPd3 nanosheets(NSs)based 3D flower-like porous nanoarchitectures(NSNAs)have been successfully fabricated for the first time through a facile solvothermal method by using DMF and citronellol as the mixed solvent,Mn2(CO)10 and Pd(acac)2 as the metallic precursors,and polyoxyethylene ether(Brij C10)as the capping or structure-directing reagent.The related N2 adsorption-desorption tests reveal that the obtained MnPd3 NSNAs possess the mesoporous structure with the average pore size of?5.7 nm,and and their BET surface area is about 49.03 m2 g-1.Electrocatalytic tests demonstrate that the 3D flower-like porous MnPd3 NSNAs can efficiently catalyze ORR in alkaline media with the onset reduction potential of 1.0 V(vs.RHE)and half-wave potential of 0.86 V(vs.RHE),which are slightly higher than Pt/C,and much higher than MnPd3 nanoparticles as well as recently reported some Pd-based alloy catalysts or related nanohybrids.Moreover,the durability of such MnPd3 NSNAs are superior to Pt/C catalyst yet.In relative to quasi-spherical MnPd3 nanoparticles,the excellent electrocatalytic performance of MnPd3 NSNAs results from their ultrathin 2D nanosheet building blocks and unquie 3D porous motifs,which not only can increase surface specific area and expose more available active sites but also offer 3D elenctron conducting nework or parthway and facilitate to improve the interacial electron transfer kinetics of the NSNAs.This work not only offers a facile avenue for synthesis of novel ultrathin Pd-based alloy nanosheets based 3D flower-like porous nanoarchitectures but also screens out a highly active and robust ORR electrocatalysts that outperforms Pt/C,showing great potential in RHOFCs or other renewable energy technologies fields.(3)Three kinds of network-like(NL)Pd2Ni2-xMox(0<x<2)nanoalloys(NAs),which are abbreviated as Pd2Ni2-xMox NLNAs,have been successfully synthesized through solvothermal-assisted reduction of Pd,Ni,and Mo salt precursors by using BTBC as the reductant,the liquid mixture of DMF and citronellol as the solvent,and the combination of oleylamine and oleic acid as the capping or structure-directing reagents.Those ternary NLNAs are made up of many small Pd2Ni2-xMox nanocrystals building blocks that spontaneously organize according to a certain means,as evidenced by micro structural analyses.Related N2 adsorption-desorption tests reveal that the obtained Pd2Ni2-xMox NLNAs possess the mesoporous structure with the average pore size of?5.8 nm,and the BET surface area of about 94.82 m2 g-1.Compared with pure binary Pd-Ni and Pd-Mo alloy nanostructures,those ternary Pd2Ni2-xMox(0<x<2)NLNAs show greatly enhanced electrocatalytic activity toward HER in acidic electrolyte.Among them,Pd2NiMo NLNAs exhibit the highest catalytic activity with the small overpotential 34 mV to reach the current density at 10 mA cm-2,which are superior to recently reported some HER electrocatalysts(e.g.PdNi/CNFs,CuPdPt/C,PtNiCu etc.).Moreover,Pd2NiMo NLNAs also exhibit better stability,which can continuously work for 15,000 cycles(90 h)that nearly without loss of their catalytic activity.As revealed by a series of spectroscopic and electrochemical analyses,the excellent electrocatalytic performance of Pd2NiMo NLNAs mainly originates from the proper component ratio,and their efficient electronic coupling or synergistic effect,which not only facilitate to increase the available catalytic active sites but also modulate or optimize their surface electronic structure and improve their interfacial electron transfer dynamics that can balance the adsorption-desorption between H+ ions and H2 molecules.This work not only synthesizes three kinds of ternary Pd2Ni2-xMox(0<x<2)NLNAs and enriches current Pd-based nanoalloys family but also screens out a highly active and robust ternary nanoalloy electrocatalyst toward HER,showing great potential in the WEC sections of RHOFCs or produce clean H2 fuels.