Controllable Synthesis And Properties Of Novel Hybrid Nanoelectrocatalysts

To promote the commercial application of regenerative H₂-O₂ fuel cells,it is urgently needed to find proper electrocatalysts to improve the sluggish dynamics of cathodic oxygen reduction（ORR）in fuel cells and reduce the overpotential of cathodic hydrogen evolution reaction（HER）in electrolyzer.In this thesis,many highly monodisperse Pd-based bimetallic or trimetallic nanocrystals（NCs）with the size smaller than 10 nm have been integrated on graphene（G）or reduced graphene oxide（rGO）nanosheets via a versatile electrostatic assembly and hydrogen reduction strategy or step by step assembly or in-situ reduction methods.By optimizing experimental parameters,a series of well-coupled nanohybrids or nanocomposites（NCPs）,such as G-MPd₃（M = Fe,Cu,Ag,Au,Cr,Mo,W）NCPs,rGO-Co-Pd NCPs,rGO-branched NiPd₂ NCs NCPs and G-AuxFeyPdz NCs NCPs,have been successfully obtained.The EDS,XRD,ICP,XPS,TEM,HRTEM,elemental mapping analysis and Raman spectra have been used to characterize the components and microstructures of the obtained NCPs.On the basis of this,the electrocatalytic ORR or HER performances of those NCPs have been systematically investigated.Additionally,the corresponding catalytic mechanisms have been explored.Some interesting primary results have been achieved.The details are given below:（1）A series of well-coupled G and MPd₃（M = Fe,Cu,Ag,Au,Cr,Mo,W）NCs NCPs（G-MPd₃ NCPs）have been synthesized via a versatile electrostatic assembly and hydrogen reduction strategy,i.e.sequential assembly of coordination anions and cations on excess cationic polymer modified graphene oxide（GO）to form composite precursors and then thermal treating them under H₂/Ar gases atmosphere.All the obtained NCPs can be directly used to catalyze ORR in alkaline media.Among them,G-FePd₃ NCPs exhibit the highest catalytic activity.This work not only provides a general strategy for fabricating well-coupled G-MPd₃ NCPs but also paves the way for future designing multicomponent NCPs with multiple interfaces to apply in alkaline fuel cells.（2）The rGO-Co-Pd NCs NCPs have been successfully prepared via a simple"two-step" chemical route.Firstly,highly monodispere Co-Pd alloy NCs were prepared by reduction of Pd（acac）₂/Co（acac）₂ precursors in the presence of tert-butylamine-borane complex（BTBC）,oleic acid（OA）and dodecylamine（DDA）.Secondly,those Co-Pd alloy NCs were integrated on presynthesized rGO nanosheets through two phase disperson and solvent evaporation induced assembly strategy.Electrocatalytic tests demonstrate that the obtained rGO-CoPd NCPs can efficiently catalyze ORR in alkaline media and their onset reduction potential is about-0.06 V（vs.SCE）.Except for catalyzing ORR,the obtained rGO-CoPd NCPs also exhibit superior electrocatalytic activity for HER in acidic media,whose onset potential for evolution of H₂ is about-0.020V（vs,RHE）.This work indicates that the rGO-CoPd NCPs can be served as a promising "bifunctional" catalyst to apply in regenerative H₂-O₂ fule cells.（3）The novel NCPs composed of rGO and branched NiPd₂ NCs（rGO-NiPd₂ NCPs）have been successfully synthesized via a simple one-pot chemical route,i.e.thermal treatment of GO,Ni（acac）₂ and Pd（NO₃）₂ solid precursors in the liquid mixture of N,N-Dimethylformamide（DMF）and dodecylamine（DDA）.In the obtained NCPs,the active metallic components（NiPd₂ NCs）are relatively uniform and highly branched,which are well-anchored on in-situ generated rGO nanosheets and the avoids among them can be clearly seen.Electrocatalytic tests demonstrate that the rGO-NiPd₂ NCPs possess excellent electro-catalytic activity and stability for ORR in both acidic and alkaline media.In addition,the electrocatalytic performance of rGO-NiPd₂ NCPs is also better than that of commercial Pt/C catalyst,showing great promise for substituting conventional Pt-based catalysts to apply in fuel cells.（4）The small-sized monodisperse AuxFeyPdz trimetallic NCs have been successfully integrated on G nanosheets to form well-coupled multicomponent NCPs with multiple interfaces via a developed electrostatic assembly and hydrogen reduction strategy.By controlling the molar ratio of objective metals in the composte precursors and thermal treatment temperature,a series of well-coupled NCPs including G-AuFe2Pd₂,G-AuFePd,G-Au₂FePd and G-Au₄FePd can be easily obtained.Electrocatalytic tests demonstrate that all the obtained G-AuxFeyPdz NCPs can efficiently catalyze ORR in alkaline media.Among them,G-AuFePd NCPs possess the highest electrocatalytic activity and their onset reduction potential is about 0.12V（vs.SCE）,which is much better than that of recently reported some Pd-based,Ag-based,and Au-based bimetallic electrocatalysts as well as commercial Pt/C catalyst.These results demonstrate that the G-AuFePd NCPs are an attractive non-Pt electrocatalyst that can be used to replace conventional Pt-based catalyst for applying in fuel cells.