| The global energy crisis and environmental pollution have made the development,storage,and conversion of renewable new energy sources a hot research topic.Hydrogen has the advantages of high energy density,clean and pollution-free,while ammonia has higher energy density than hydrogen and easy liquefaction feature,thus both of hydrogen and ammonia are ideal secondary energy sources.Overall water splitting and renewable H2-O2 fuel cell technology can achieve the efficient production,storage,and conversion of hydrogen and electrical energy,while electrocatalytic nitrogen reduction can achieve the green production of ammonia with low energy consumption,thus all of these three technologies have good development prospects.In addition to engineering issues,the construction of high-performance,inexpensive electrocatalysts has been shown to reduce the overpotential of hydrogen evolution reaction?HER,the cathode reaction of overall water splitting?,accelerate the kinetics of oxygen evolution reaction?OER,the anode reaction of overall water splitting?and oxygen reduction reaction?ORR,the positive reaction of fuel cell?,improve the yield and selectivity of nitrogen reduction reaction?NRR?.Although noble metal catalysts such as Pt,Ru,Ru O2,and Ir O2 can significantly increase the activities of these reactions,the high cost and limited stability limit their widespread applications.By integrating constituent components with different intrinsic properties,hybrid nanomaterials can not only reduce the dosage of precious metals,but also inherit the advantages of these components,and generate abundant defects or special electronic interactions at the interface,finally contributing to optimized electrocatalytic properties.In view of such research status,this thesis aims to improve the electrocatalytic performances of different catalysts and reduce their cost.By hybridizing different metals or compounds with different intrinsic properties,a series of hybrid nanoelectrocatalysts with low-precious metal content and even precious metal-free,have been developed,including noble metal@precious metal alloy nanocrystal,metal@metal phosphide-precious metal nanorod,three-dimensional nanostructure of noble metal/rare earth hydroxide,transition metal oxide/rare earth oxide hybrid nanostructure,and multiple heterogeneous metal/metal carbides nanoparticles.Moreover,we have systematically studied their growth mechanism and electrocatalytic performances,and analyzed the sources of the electrocatalytic activity,as well as the relationship between the hybrid nanostructure and their electrocatalytic performance based on series of optical,electrical characterization and theoretical calculations.The main contents and some preliminary results are summaried as follows.?1?Controllable synthesis of concave octahedral Pd@Pd Pt nanocrystals and their electrocatalytic oxygen reduction and hydrogen evolution performances.The development of core@shell nanostructure with inexpensive metal as the core and noble metal as the shell can not only reduces the usage of noble metal,but also induces electronic interactions between the core and shell,thus contributing to improved electrocatalytic activity.In addition,compared with planar nanostructures,concave polyhedral nanocrystals?NCs?usually have more lattice steps,kinks,and low coordination surface atoms.These defects can serve as active sites to accelerate electrocatalytic process.Based on related research status,through controlling the reduction kinetics of Pd?NO3?2 and K2Pt Cl6 in ethylene glycol system,this work synthesized a novel Pd@Pd Pt nanaocrystal by a one-pot method,which integrates three structural characteristics of concave,octahedron and core-shell.Series of microstructure analyses reveal that a large number of lattice defects are formed on the surface of the Pd@Pd Pt NCs,and there is electron transfer between the Pd core and Pd Pt alloy shell,as well as the Pd Pt alloy shell.This leads to abundant electrocatalytic active sites and the redistribution of electrons.Therefore,when applied to the field of electrochemical,the Pd@Pd Pt NCs show superior electrocatalytic bifunctionality,comparable and even better than that of the commercial 20%Pt/C.For ORR,the Pd@Pd Pt catalyst shows a preeminent half-wave potential up to 0.91 V?vs.RHE?,mass specific activity of 0.95 A mgand decent cyclic stability?1000 cycles?.For HER,the catalyst displays a small onset potential of?5 m V?vs.RHE?,Tafel slope of 38 m V dec-1,overpotential(?10)of 39 m V and long-term stability?4000 cycles?.By designing and synthesizing nanostructures with special morphologies and structures,this work realized the dual control of surface defects and electronic structures,as well as the improvement of electrocatalytic performance,thus providing a new idea for the development of new and efficient hybrid nanoelectrocatalysts.?2?Synthesis of multiple heterogeneous Ni@Ni2P-Ru nanorods and their excellent electrocatalytic hydrogen evolution performance.Ni2P is an attractive HER electrocatalyst with high activity and low price,but its HER catalytic performance still inferior to that of the 20%Pt/C;Ru has similar hydrogen bonding energy as Pt,and lower price?1/25?.Based on these research status,this work firstly adopted theoretical calculation to predict the electrocatalytic HER activity of Ru-Ni2P catalyst,and found that Ru-Ni2P hybrid nanostructure has significantly optimized hydrogen adsorption free energy??GH?than individual Ru and Ni2P catalysts.This means that Ru-Ni2P nanohybrid may have remarkable HER catalytic performance.Under the guidance of such calculations,we prepared a multiple heterogeneous Ni@Ni2P-Ru nanostructure by a one-pot method,in which Ru nanoclusters are uniformly anchored on the surface of Ni@Ni2P nanorod.XANES and EXAFS analyses indicate that due to the Ru-Ni coordination,the firstly formed metallic Ni cores are not easily phosphated,thus leading to multiple heterogeneous metal@phosphoride-metal nanostructure.Spherical aberration HRTEM and series of electrochemical tests showed that the introduction of Ru not only optimizes the?GH of the Ni@Ni2P-Ru catalyst,but also improves its electron transport efficiency?on account of the superior conductivity of the metallic Ni core and Ru nanoclusters?.Therefore,when applied to HER,the Ni@Ni2P-Ru catalyst exhibits obviously enhanced electrocatalytic performances both in acidic and alkaline electrolytes than the individual Ni@Ni2P and Ru catalysts,and even comparable to the commercial 20%Pt/C.In addition,thanks to the dimensional advantages of Ni@Ni2P-Ru nanorods,this catalyst also displays long-term cycling stability.This work shows that component engineering and heterogeneous nanostructure strategy are of great promotion effects to electrocatalytic HER behaviors,thus have promising applied prospects to renewable H2 and O2 fuel cells ae well as many other energy fields.?3?Synthesis of Ru/Y?OH?3 nanohybrid and its robust electrocatalytic hydrogen evolution stability in alkaline media.The rate determining step of HER in alkaline media is the dissociation of water.Ru can promote the dissociation of water,so has high alkaline HER catalytic ability.However,during long-term stability test,Ru nanoparticles are easily agglomerated and deformed,resulting in declining activity.Y?OH?3 is a rare earth hydroxide with excellent structural stability and corrosion resistance,but has rarely been used in the field of electrocatalysis.In this work,novel Ru/Y?OH?3 nanohybrids?NHs?were prepared by an in-situ controlled kinetics reduction method,in which ultra-small Ru nanoparticles?about 2.9 nm?were uniformly anchored in flocculent amorphous Y?OH?3 scaffold.Using HER as a probe reaction,the effect of Y?OH?3 on the electrocatalytic performance of Ru/Y?OH?3 NHs was investigated.It was found that under basic condition,the Ru/Y?OH?3 NHs have significantly improved HER catalytic activity(exchange current density of 0.7 m A cm-2,overpotential(?10)of 100 m V)and cycling stability?20000 cycles?than individual Ru,Y?OH?3 and physically mixed Ru+Y?OH?3 catalyst.Such long-term stability is better than that of most reported HER electrocatalysts in basic media.Series of electrochemical tests and spectral analyses show that Y?OH?3 not only promotes the ability to dissociate water for the Ru/Y?OH?3 to improve its HER catalytic activity,but also produces electronic interactions with Ru nanoparticles to strengthen its cycling stability.This work develops a novel rare earth hydroxide substrate that can significantly improve the electrocatalytic HER stability under alkaline condition,providing a new option for the construction of advanced hybrid nanostructures.?4?Two-dimensional electron gas and oxygen vacancies in Co3O4/Ce O2nanohybrids promoted excellent electrocatalytic oxygen evolution performance.Co3O4 has excellent electrocatalytic OER intrinsic activity,but is poor in conductivity;the rare earth oxide of Ce O2 has reversible surface oxygen ion exchange and easily coupled feature with other materials,and can disturb the electronic structure of Co species,thus is likely to produce some special electronic interaction with cobalt oxides through hybridization.Based on these research statues,this work synthesized well-designed a Co3O4/Ce O2 NHs by a two-step solvothermal method,in which Ce O2nanoparticles are anchored on the surface of Co3O4 nanosheets.Series of spectroscopic,electron microscope analyses,theoretical calculations and electrochemical tests demonstrate that the introduction of Ce O2 not only increases the oxygen vacancies concentration of Co3O4/Ce O2 NHs,but also leads to two-dimensional electron gas?2DEG?at the Co3O4/Ce O2 interface.The carrier concentration of 2DEG is about 3.8×1014 cm-2,which can serve as electron transport channel to promote the electron transfer and conductivity of the catalyst.Therefore,the Co3O4/Ce O2 NHs show superior electrocatalutic OER performance with an overpotential of 270 m V?vs.RHE?and conversion frequency of 0.25 s-1,significantly better than that of the individual Co3O4,Ce O2,and commercial Ir O2 catalysts.This design strategy of hybrid interface induced2DEG is not only applicable to Ce O2 and Co3O4,but also can be extended to other similar transition metal oxides,opening a new direction for the design and development of new advanced hybrid nanostructures.?5?Design and synthesis of Mo/Mo2C/Mo C heterointerface and its enhanced electrocatalytic nitrogen reduction performance.As a potential NRR electrocatalyst,Mo2C has good N2 adsorption and activation free energy,but its activation energy for the*NH2?*NH3 reaction step is high,hindering the formation of NH3.In this work,various kinds of molybdenum-based heterogeneous nanoparticles were selectively synthesized,through precisely controlling the annealing conditions?time,temperature,and atmosphere?of polypyrrole precursor on RGO substrate.When applied to NRR,the heterostructured Mo C/Mo2C?labeled as MoxC?and Mo/Mo C/Mo2C?labeled as Mo/MoxC?catalysts show decent NH3 yield(20.4?g h-1 mg-1)and FE?18.9%?,better than the individual Mo and Mo2C.Spherical aberration HRTEM and electrochemical tests reveal that compared to the Mo and Mo2C,MoxC and Mo/MoxC catalysts own a large amount of heterogeneous interfaces and the resulting lattice distortions.These defects can act as active sites to accelerate the NRR process.Moreover,theoretical calculations indicate that compared with MoxC,the introduction of metallic Mo changes the catalytic path for the Mo/MoxC catalyst,reducing the desorption energy of the adsorbed NH3,promoting the rapid release of NH3 from the catalyst surface,thus finally contributing to superior NRR catalytic performance.By establishing a Mo/Mo2C/Mo C model catalyst,this work reports the modulation effect of heterogeneous interfaces and different molybdenum-based nanomaterials on the electrocatalytic NRR performance,thus is of great reference significance for the design and development of new heterogeneous nanostructures and their electrochemical applications. |
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