The Preparation And Performance Of Atomically Dispersed Ruthenium-based Electrocatalytic Materials

Last few years,with the continuous in-depth research on heterogeneous catalytic materials,the new type single-atom catalytic materials have become an intense focus of research.Due to the theoretical 100%atomic use efficiency and extremely high catalytic specific activity,single-atom catalytic materials possess the integrated characteristics of both homogeneous catalysts and heterogeneous catalysts.These features also make it possible to use noble metal based heterogeneous catalytic materials in large scale.However,the current researches on single-atom or atomically dispersed catalytic materials are still in its infancy.Therefore,the development of atomically dispersed Ru-based electrocatalytic materials with high catalytic activity and stability is very significant for their low-cost and wide applications.In this thesis,we parepred a series of ruthenium-based electrocatalysts with different scales of atomically dispersed active sites through the synergistic effect of the carrier and the ligand,the interaction between the ligand and the active site,and the selection of the metal ruthenium precursor.We also successfully modulated the composition and structure of atomic-level dispersed active sites to improve the catalytic intrinsic activity of the active sites and the selectivity to different electrocatalytic reactions.Meanwhile,we also designed and optimized the carrier and the corresponding porous structure to further increase the density of active sites and enhanced the mass transfer effect of the reaction,which is significant for the application of the actual energy conversion device.This paper provides new methods and design for the preparation of other atomic-level dispersed catalytic materials.The main innovations achieved are as follows:(1)Boron-doped ordered mesoporous carbon microspheres were prepared by in-situ doped and soft template methods,and were adopted as supports.Meanwhile,urea was used as nitrogen-containing ligand,and ruthenium trichloride was used as the metal precursor.Above-mentioned raw materials were dispersed and mixed in the solution.Afterwards,by virtue of the interaction between the supports,ligands and metal atoms,the single-atom ruthenium electrocatalyst with the Ru-N-B-C active site structure was successfully synthesized through the method of ultraviolet photochemical activation.By adjusting the composition of the supports and the selection of ligands,the active sites with different structures(such as Ru-C,Ru-N-C,RuC-B,etc.,)were constructed and explored the effect of different active sites on electrocatalytic activity.The study found that compared with the constructed active sites such as Ru-C,Ru-N-C,Ru-C-B,the Ru-N-B-C active site utilized the electronegativity difference of each element to form the ion characteristic sites with alternating positive and negative charges,which can enhance the interaction between ruthenium and nitrogen and shorten the coordination bond length of ruthenium and nitrogen(1.38 ?).Thereby Ru-NB-C active site exhibited the favorable free energy of adsorbed hydrogen(0.16 eV),improved the electrocatalytic hydrogen evolution activity(the overpotential in 1 M KOH is:51 mV@10 mA cm-2;in 0.5 M H2SO4:79 mV@10 mA cm-2)and enhanced the catalytic stability at high working current.(2)The integrated hydrogen evolution electrode with low-contained Ru species was prepared by photochemical activation method.Especially,we focused on the design of electrode porous structure and made it be suitable for industrial high-current electrolysis of water to produce hydrogen.The dualtemplate method was adopted to construct the electrode structure with threedimensional(3D)ordered hierarchical porous structure.The soft template of polystyrene microspheres was used to construct ordered macropores with the inverse opal structure,and the soft template of Pluronic F127 was used to construct ordered mesopores with the hexagonal arrangement.Based on this ordered hierarchical porous carbon substrate,low-contented Ru species(0.12 mgRu cm-2)were loaded by the UV photochemical activation method,and an efficient hydrogen evolution electrode with the characteristics of rapid mass transfer and electrolyte diffusion was prepared.Through the selective usage of dual templates,microporous structure electrodes,mesoporous structure electrodes,and macroporous structure electrodes were also synthesized.The obtained 3D ordered hierarchical porous electrodes exhibited lowest charge transfer resistance(0.436 Ω)and diffusion resistance(1.560),and the overpotential of hydrogen evolution at high current was very low(-0.084 V@50 mA cm-2),which is beneficial for actual industrial production.(3)Nitrogen-doped hierarchical porous carbon derived from biomass cattle bone was adopted as the carbon support.By virtue of the abundant microporous and mesoporous structure,the corresponding ruthenium precursors can be adsorbed and anchored.Meanwhile,through the selection of different kinds of precursors and utilization of nitrogen-containing ligands,atomically dispersed ruthenium active sites with different structures were constructed.For the first time,the dimer of p-cymene ruthenium dichloride was used as a dinuclear ruthenium precursor,and phenanthroline was used as the nitrogen source to construct diatomic ruthenium active sites.Compared with the single-atom ruthenium active site derived from traditional ruthenium trichloride precursor,dual-atom ruthenium active site not only had good performance for hydrogen evolution,but also had better catalytic activity for oxygen reduction reaction.Especially,dual-atom ruthenium active site could still maintain good catalytic activity in acidic electrolytes(E1/2=0.77 V).The structure of the active site was characterized and analyzed by XAS and HADDF-STEM.The study found that the active site of the dual-metal center was more falvorable to form bridgingadsorption constructure and promote the O-O bond cleavage than the singlemetal center.Thererfore,dual-metal center promoted oxygen reduction reaction via four-electron pathway.(4)On the basis of the previous part of the work,we further explored the specific effect of the nuclearity in the atomically dispersed active site on the oxygen reduction catalytic activity.In order to precisely control the nuclearity of the active site and avoid the metal aggregation effect when metal atoms increased,a series of organic ruthenium sandwich/half-sandwich compound precursors(bis(1,2,3,4,5-pentamethylcyclopentadienyl)ruthenium(Ⅱ),[RuCp*2];dichloro(p-cymene)ruthenium(Ⅱ)dimer,[Ru(p-cymene)Cl2]2;chloro(1,2,3,4,5-pentamethylcyclopentadienyl)ruthenium(Ⅱ)tetramer,([RuCp*Cl]4)were adopted as precursors.Meanwhile,porous commercial carbon black BP2000 was adopted as the carbon support,with melamine as the source of nitrogen.After the pyrolysis,we obtained a series of atomically dispersed RuxNy/C clusters with different numbers of Ru atoms(Ru4Ny/C,Ru2Ny/C,RulNy/C).The structure of the active sites were characterized and analyzed by XAS and HADDF-STEM,and the structure of tetranuclear,dualatom,and single-atom metal sites were further determined.Through the electrochemical oxygen reduction reaction tests and characterization,the active site of tetranuclear Ru4Ny was found(E1/2=0.80 V)had the comparable activity with commercial Pt/C(E1/2=0.81 V),and its mass activity(100.16 mA mgRu-1@0.90 V)was also much higher than the dual-atom Ru2Ny site(52.99 mA mgRU1),the single-atom Ru1Ny site(5.28 mA mgRu-1)and the ruthenium nanoparticle site(5.99 mA mgRu-1)in the acid electrolyte.The partial density of states(PDOS)and d-orbital band centers of different active sites were calculated by density functional theory(DFT).The study found that the Ru4Ny active site possessed the lowest d-band center compared with other active sites with different nuclearity.Such a result was mainly attributed to the shortened Ru-Ru bond length in the active site of Ru4Ny and the relatively large metal coordination number,which could enhance the interaction between the d-orbital of ruthenium and widen the d-orbital band.Due to the too strong oxygen adsorption energy of ruthenium,widening the d-orbital band and lowering d-band center could further adjust and reduce its adsorption energy for transition state O*.Therefore,Ru4Ny cluster would be the most flavorable active site for oxygen reduction reactions according to the Sabatier principle.