Synthesis Of Noble Metal-Based Catalysts Under The Control Of1-Naphthol And Their Electrocatalytic Performance

With the massive consumption of fossil fuels and the excessive emission of carbon dioxide,the energy crisis and environmental pollution have become increasingly serious.Electrocatalysis is considered to be one of the most effective ways to alleviate energy problems and produce fuel and chemical value-added products.However,most of the electrocatalytic performance is still at a moderate state,far from reaching the requirements of practical applications.Currently,the most used commercial catalysts are still the noble metals,but the cost is too high and the reserves are limited.Therefore,there is an urgent need to explore low-cost,high-activity noble metal electrocatalysts.The synthesis of noble metal electrocatalysts with specific structure and composition can effectively improve the atomic utilization and electrocatalytic activity of noble metals.According to the functional requirements,1-naphthol was chosen as the key structure-directing agent and reducing agent to fabricate ultrathin Au nanowires in this dissertation.By using Au nanowires as the template,the bifunctional oxygen electrocatalyst Au wire/Ni₆Mn O₈ for zinc-air battery and the trimetallic Au@Pd Pb nanowires are further prepared.In addition,the 1-naphthol reaction system is extended to the preparation of Pt₃Ag nanocorals and Pd_xAg_y nanotrees with two-level nanostructure.The growth mechanism and bifunctional electrocatalytic performance of the two electrocatalysts were discussed in detail.The main results are summarized as follows:(1)Ultrathin Au nanowires are synthesised via a simple mixing of HAu Cl₄aqueous solution with 1-naphthol ethanol solution and aging for a few minutes,in which 1-naphthol were used as the structure-directing agent and reducing agent.Physical characterizations show that the resultant Au nanowires possess a Boerdijk-Coxeter helix-structured morphology dominated by(111)facets.The growth mechanism investigation revwal that the rapid coordination-induced formation of Au nanowire-arrayed microspheres and their gradual dissociation account for the formation of ultrathin Au nanowires.The ORR measurements demonstrate that the half-wave potential of ultrathin Au nanowires(0.88 V)is more positive than that of commercial Pt black catalyst(0.84 V).After the stability test of 40,000 s,the Au nanowires show a negligible degradation with only 3 m V negative shift of E_1/2,which is remarkably lower than that(17 m V)observed on commercial Pt black.This might be attributed to the one-dimensional anisotropy,abundant surface atoms and structural defects of ultrathin Au nanowires.(2)The bifunctional Au-NWs/Ni₆Mn O₈ oxygen electrocatalysts are fabricated via the in-situ growth of ultrathin Au nanowires on the porous rigid Ni₆Mn O₈ microspheres.Rigid Ni₆Mn O₈ microspheres not only offer the three-dimensional porous structure,but also serves as a highly-active component for the OER;while ultrathin Au-NWs contributes to the excellent ORR activity and acts as a flexible conductive electronic network.By forming a metal-metal oxide interface,the electronic structure of the electrocatalysts can be effectively regulated.The electrochemical measurements demonstrate that Au-NWs/Ni₆Mn O₈ shows significantly improved ORR activity in terms of its more positive walf-potential(E_1/2=0.90 V)as compared with Au-NWs(0.88 V)and Ni₆Mn O₈(0.62 V).Meanwhile,Au-NWs/Ni₆Mn O₈ also has very good OER activity.At the current density of 10 m A cm^-2,Au-NWs/Ni₆Mn O₈ has a very low overpotential(E_J10)of 0.36 V,outperforming Ru O₂ catalyst(0.39 V).Moreover,a Zn-air battery adopting Au-NWs/Ni₆Mn O₈ as air-cathode exhibits a larger peak power density(118 m W cm^-2),a higher specific capacity(768 m Ah g _Zn^-1)and longer cycle life than that of a mixed Pt/C+Ru O₂ air-cathode.(3)The trimetallic Au@Pd Pb core-shell electrocatalyst is fabricated via the grown of ultrathin Pd Pb alloy nanolayers on the surface of ultrathin Au nanowires as the hard template.The core-shell nanostructure of trimetallic Au@Pd Pb and bimetallic Au@Pd nanowires are investigated by physical characterization in detail.The Au@Pd Pb NWs have the merits of anisotropic one-dimensional nanostructure,high utilization efficiency of Pd atoms and doping of Pb atoms.Electrochemical tests show that Au@Pd Pb NWs exhibit remarkably enhanced ORR activity with a high ha If-wave potential(0.827 V),much better than those of commercial Pd black(0.788 V)and bimetallic Au@Pd NWs(0.803 V).After the long-term durability test,the E_1/2 of Au@Pd Pb NWs only negative shifted by 16 m V,which is more stable than Au@Pd NWs(22 m V)and commercial Pd black(28 m V).XPS results show that the introduction of Au and Pb as the core can effectively modify the electronic structure of the Pd,thus decreasing adsorption strength of oxygen species on Au@Pd Pb NWs surface to enhance the ORR activity.(4)Dendritic Pt₃Ag nanocorals are synthesised via a simple mixing of Pt,Ag precursors with 1-naphthol and a following one-pot hydrothermal reaction.The alloy composition and growth mechanism of Pt₃Ag nanocorals were investigated by control experiments and physical characterization in detail.Further electrochemical measurements demonstrate that the half-wave potential(0.85 V)of Pt₃Ag NCs are obviously more positive than that of Pt black(0.82 V).After the stability test,the current density of Pt₃Ag NCs towards ORR retain about 72.6%,whereas Pt black retain 64.1%.Meanwhile,Pt₃Ag NCs exhibit higher mass activity of 217.56 m A mg^-1,which is 3.46 times as large as that of Pt black(62.87 m A mg^-1),better stability and CO-tolerance ability towards FAOR.(5)Pd_xAg_y nanotrees with two-level nanostructures were synthesised via a simple mixing of Pd,Ag precursors and 1-naphthol and aging in a water-bath under a specific temperature.The growth mechanism investigation reveal that the obtained Pd_xAg_y NTs feature two-level structure of tiny Pd₁Ag₁ alloy nanodendrites and Pd₁Ag₂ alloy nanobranches,which can effectively prevent the dissolution and agglomeration of the catalyst,improve the structural stability and provide sufficient electrocatalytic active sites.The electrochemical measurements demonstrate that the Pd₃Ag₁ nanotrees(0.84V)perform more positive half-wave potential towards ORR than the commercial Pd black(0.80 V).After 40000s stability test,the residual of the ORR specific current density for Pd₃Ag₁ NTs is about 85.01%,much higher than Pd black(53.14%).Meanwhile,Pd₃Ag₁ NTs display the uppermost activities normalized by the Pd mass of991.70 m A mg^-1,which is 3.59 times higher than that of commercial Pd black(276.33m A mg^-1).Our researches demonstrate:Ultrathin Au nanowires are prepared on the basis of the 1-naphthol reaction system,and used as electrocatalysts towards oxygen reduction reaction in alkaline conditions.Then,Au/Ni₆Mn O₈ is obtained by combining the Au nanowires with three-dimensional porous carrier Ni₆Mn O₈ with commendable OER performance,and applied as the bifunctional oxygen electrocatalysts in the air cathode of the zinc-air battery.Based on the ultrathin Au nanowires,we further synthesized the Au@Pd Pb core-shell nanowires to greatly improve the ORR performance of nanowires under acidic conditions.Besides,we also design the fabrication of Pt and Pd-based nanocatalysts via the 1-naphthol reaction system to acquire Pt₃Ag nanocorals and Pd_xAg_y nanotrees to effectively improve the bifunctional cathodic ORR and anodic FAOR performances in the formic acid fuel cells.In this dissertation,we aim to solve the high cost and complicated preparation process of the electrocatlysts applied in fuel cells and zinc-air batteries to realize the high efficiency and high performance application of precious metal.