Study On The Controllable Synthesis And Performance Of A Novel Cu-based Hybrid Nano Electrocatalyst

As a clean and regenerable energy conversion device,fuel cells have attracted much attention and considered as one of efficient means for resolving current energy crisis problems.Designing and developing low-cost,high-acitivity and long-durability electrocatalyst to improve the sluggish dynamics of cathodic oxygen reduction reaction（ORR）is vital to advance fuel cells technologies and promote its commercial applications.In this thesis,in view of the research status of current ORR electrocatalysts,a series of highly monodispersed Cu-based bimetallic（CuxPty,CuxPdy）or trimetallic（CuxAuyPdz）nanocrystals（NCs）have been synthesized and further integrated on graphene（G）or N-doped graphene（N-G）nanosheets to obtain strongly-coupled nanocomposites（NCPs）,such as G-CuxPdy NCPs and N-doped G-CuxAuyPdz NCPs.The components,microstructures and electron-coupling of those bimetallic and trimetallic NCs as well as related NCPs are characterized by EDS、XPS、ICP、XRD、TEM、HRTEM、Raman and electrochemical impedance spectrum（EIS）.On the basis of those characterizations,their electrocatalytic performances in different media have been systematically evaluated by using ORR or methanol oxidation as the probe reaction.Moreover,the corresponding ORR mechanism has been explored based on electrocatalytic dynamic experiments data.Some preliminary results have been obtained and the details are given below:（1）The monodisperse flower-like CuPt₃ NCs with the average size of about 23.1 nm have been synthesized by thermal treatment of acetylacetone copper（Cu（acac）2）and dehydrate chloroplatinic acid（H2PtCl6·2H2O）solid precursors in the liquid mixture of dodecylamine（DDA）and oleic acid（OA）.Related HRTEM analysis reveals that those flower-like CuPt₃ NCs are assembled by many small-sized CuPt₃ NCs according to a certain means and continue and clear lattice fringes are observed.The lattice spacing is about 0.22 nm,which is between the（111）plane of fcc Cu and fcc Pt.This result confirms that the obtained CuPt₃ NCs are nanoalloy.Further element mapping analysis confirms that the Cu and Pt elements are uniformly distributed in the obtained CuPt₃ NCs.A series of control experiments demonstrate that the adding sequence of metal precursors,reaction temperatue and surfactants affect the formation of flower-like CuPt₃ NCs.Under the optimized experimental conditions,other Cu-Pt NCs with different Cu and Pt molar ratios,including the irregular polyhedron-like CuPt NCs and hollow Cu₃Pt NCs,also can be obtained by simply changing the mole ratio of used metal precursors.Electrocatalytic measurements demonstrate that the obtained flower-like CuPt₃ NCs can efficiently catalyze ORR reaction in acidic media,whose oneset oxygen reduction potential is much better than that of polyhedron-like CuPt NCs and hollow Cu₃Pt NCs as well as commercial Pt/C catalyst.Moreover,the obtained three kinds of Cu-Pt NCs also can catalyze the methanol oxidation（MOR）reaction in acidic media.Among them,CuPt₃ NCs possess the best electrocatalytic MOR performances,whose onset potential and the ratio of forword and backword current densities（Ie/Ib）are better than those of CuPt and Cu₃Pt NCs.This work suggests that the obtained flower-like CuPt₃ NCs can be served as a promising"bifunctional catalyst" for replacing Pt/C catalyst to apply in direct methanol fuel cells.（2）A series of G-CuxPdy（Cu₄Pd,Cu₃Pd,CuPd,CuPd₃,CuPd₄）NCPs have been obtained by assembly of pre-synthesized CuxPdy bimetallic NCs on G nanosheets using a simple "dispersing-mixing-evaporizing solvent" strategy.Among those NCPs,the CuxPdy NCs are synthesized by thermal treatment of Pd（acac）2 and Cu（acac）2 solid precursors in the liquid mixture of oleylamine（OLA）and triethylene glycol,which show component-dependent electronic properties and well anchored on the planar planes,crumples or edges of G sheets to form desired G-CuxPdy NCPs.Electrochemical impedance spectra（EIS）analysis reveal that the interfacial electron transfer dynamics of those G-CuxPdy NCPs are dependent on alloy components and show a strong "double-volcano" type correlation with the Cu or Pd molar ration in CuxPdy NCs components.The related electrocatalytic tests exhibit that the ORR activities of those NCPs in alkaline solution are also dependent on the alloy components and show a "double-volcano" type features between the mass specific activity and the Pd or Cu molar ratio in alloy components,which is consistent with EIS results.Among them,G-Cu₃Pd NCPs possess the highest electrocatalytic activity,which is much better than that of some reported ORR electrocatalysts and commercial Pd/C catalyst,and close to Pt/C catalyst.It should be mentioned that the mass specific activity of G-Cu₃Pd NCPs at-0.4 V（vs SCE）is 0.433 A/mgPd,which nearly reaches that of MOE 2017 target for ORR electrocatalyst（0.44 A/mgpt）.By correlating the Pd 3d binding energies and the sizes of CuxPdy NCs with the mass specific activities of G-CuxPdy NCPs and considering the interfacial electron transfer dynamics,the best catalytic activity of G-Cu₃Pd NCPs may results from the unique electronic structure and the smallest size of Cu₃Pd NCs as well as the strong synergistic effect between G and Cu₃Pd NCs.Moreover,the durability of G-Cu₃Pd NCPs is superior to Pt/C catalyst.This work not only realizes component-controlled synthesis of Cu-based bimetallic NCs but also screens out a low-cost,high-acitivity and long-durability Cu-rich hybrid electrocatalyst for replacing commercial Pt/C to apply in alkaline fuel cells.（3）On the basis of previous work,the monodisperse Cu₃AuPd NCs with the average diameters of～7.27 nm have been synthesized by thermal treatment of Cu（acac）2,Pd（acac）2 and HAuC14·2H2O solid precursors in the liquid mixture of oleylamine（OLA）and 1-octadecene（ODE）.Under the optimal experimental conditions,a series of other CuxAuyPdz（Cu₄AuPd NCs,CuAuPd NCs and CuAuPd2）NCs are also obtained by simply adjusting the mole ratio of used metal precursors.Using a simple"dispersing-mixing-evaporizing solvent" strategy,those monodisperse CuxAuyPdz have been assembled or integrated on pre-synthesized N-doped G nanosheets to generate a series of desired N-doped G-CuxAuyPdz NCPs.The electrocatalytic performances of those NCPs are evaluated in different media by using ORR as the probe reaction.Compared with that of other N-doped G-CuxAuyPd,ones,the N-doped G-Cu₃AuPd NCPs exhibit the best electrocatalytic activity in alkaline media,which is much better than that of reported Pd-based bimetallic and trimetallic NCs electrocatalysts and close to commercial Pt/C catalyst.Corresponding electrocatalytic dynamic experiments demonstrate that the ORR on N-doped G-Cu₃AuPd NCPs is the first-order reaction and the ORR mechanism complies with the direct "4e" pathway.In acidic media,the ORR activities of those NCPs exhibit the similar tendency as that in alkaline media.Among those NCPs,N-doped G-Cu₃AuPd NCPs also possess the highest catalytic activity in acidic media.This work demonstrates that N-doped G-Cu₃AuPd NCPs can be served as a high-efficient cathodic ORR electrocatalyst to apply in regenerative H2-O₂ fuel cells under different media.