Synthesis And Electrocatalytic Performance Of Silver-Based Single-Atom Catalysts

Energy shortage and huge carbon emission caused by the extensive use of fossil energy are two key problems facing the sustainable development of human civilization.Fuel cells and carbon dioxide electroreduction are feasible means to deal with these problems.The former can effectively alleviate the fossil energy crisis from energy consumption,and the latter will achieve global carbon balance from environmental emissions.The oxygen reduction（ORR）and carbon dioxide electroreduction（CO₂RR）are the core half-reactions involved in these two conversion technologies respectively.At present,ORR still relies on Pt-based materials,but the rapid development of fuel cell devices has been seriously hindered by the rarity and preciousness of noble metal Pt.Therefore,the development of high efficiency Pt-substituting catalysts for novel fuel cell devices has become the focus of attention today.On the other hand,the sluggish reaction kinetics of CO₂RR due to the inherent inertness of CO₂ molecules,which requires high overpotentials to achieve the improvement of product selectivity.The reports showed that Ag is a potential candidate to catalyze these two half-reactions.In a range of precious metals,Ag is cheap and abundant in the earth,and it is an excellent material to replace other precious metal catalysts.However,Ag has weak adsorption capacity for O₂ and CO₂,resulting in its unsatisfactory catalytic activity for ORR and CO₂RR.The size effect can bring differentiated catalytic behavior to the catalyst.When the size of the catalytic material reduces from nano-level to the sub-nanometer level and then to the single-atom level,their electronic structure is obvious regulated,and thus achieving better catalytic performance.Single-atom catalysts（SACs）have attracted extensive attention in the field of energy catalysis because of their special geometric environment and strong metal-support interactions to enhance the intrinsic activity of isolated metal centers.On the other hand,the catalytic material at the single-atom scale can expose active sites as much as possible,improve the utilization efficiency of metal atoms,and effectively reduce the cost of catalysis.SACs have been in the forefront of research due to their unique structural properties,which provide an opportunity to develop efficient Ag-based single-atom catalytic materials.However,the preparation of highly efficient Ag SACs is still a formidable challenge due to the easy migration and accumulation of metal Ag.This thesis focuses on the controlled synthesis,coordination environment design,and co-catalytic site construction of Ag-based single-atom catalysts,and then performs ORR,CO₂RR performance evaluation and related battery device assembly.A series of highly efficient Ag-based SACs were synthesized via the morphology control and functional design of metal-organic framework（MOF）.Meanwhile,the structure-activity relationship between the coordination microenvironment and catalytic reaction was investigated to reveal their catalytic behavior,and further enrich the science of single-atom catalysis.The research works are as follows:1.The Ag-N_x SACs were designed and regulated to obtain high ORR and CO₂RR catalytic performances.Through ligand-assisted pyrolysis strategy,the coordination structure of Ag center on MOF-derived N-doped carbon substrates were regulated to achieve the effective preparation of novel Ag-N_x SACs.The porous concave structure of carbon matrix was constructed by coating-etching-pyrolysis steps to reduce the cover of carbon layer,and further expose the isolated Ag single-atom sites as much as possible.The atomic dispersion of Ag species and the evolution of Ag–N coordination number were investigated by spherical aberration electron microscopy and X-ray absorption spectroscopy.The synthesized Ag-N_xSACs exhibited excellent catalytic activity for both ORR and CO₂RR.In particular,the ORR half-wave potential reached 0.881 V on the Ag–N₃single-atom site under alkaline conditions,which greatly exceeded that of Ag nanomaterials.Similarly,the CO current density of Ag–N₃ single-atom site reached 7.6 m A cm^-2 at-0.55 V,and the CO Faradaic efficiency achieved 96%at the ultralow overpotential of 260 m V.2.By designing the coordination environment of the metal center of SACs,a heteroatom coordinated Ag-based SAC was synthesized,which improved the catalytic performance of ORR,and further assembled the device to obtain a high-performance zinc-air battery.The heteroatom functionalization of carbon matrix was achieved by ligand polymerization-coating imidazole zeolite framework.And the functionalized heteroatoms（N,P,Cl,S）were precisely used to modulate the coordination environment of Ag single-atoms,thereby optimizing its ORR catalytic activity.The heteroatom coordinated Ag-based SAC showed excellent ORR performance in alkaline medium,with the half-wave potential of 0.896 V,the mass activity of 21.1 A mg _Ag^-1 and TOF of 5.9 s^-1 at 0.85 V,respectively.And it also performed excellence in the assembly test of the zinc-air battery,with a peak power density（PPD）of 270 m W cm^-2 at 460 m A cm^-2.Theoretical simulations revealed that the heteroatom coordination environment reduces the energy barrier of Ag SACs for the generation of*OOH intermediates,thereby improving the ORR catalytic performance.3.By constructing a SAC with multiple metal center sites to control activity,an Ag Fe dual-site SAC was designed and synthesized,which exhibited excellent ORR catalytic performance,and further applied in alkaline membrane fuel cell.Ag Fe dual-site SAC with high activity and stability was synthesized by metal co-doping method.X-ray absorption spectroscopy combined with X-ray photoelectron spectroscopy analyses found that there was electron transfer between Ag and Fe single-atomic sites,which regulated their adsorption capacity for ORR intermediates,resulting in improved ORR activity and excellent stability.The half-wave potential of the Ag Fe dual-site SAC was up to 0.917 V,and only decreased12 m V after 5000 cyclic voltammetry cycles.Using Ag Fe dual-site SAC as cathode material for alkaline membrane fuel cell assembly,its PPD value was as high as 1.26 W cm^-2,which significantly exceeded that of commercial Ag/C catalysts.Theoretical calculations explained the catalytic mechanism of the Ag Fe bimetallic atomic site and the evolution of the electronic density of states at the bimetallic center,indicating that the ORR preferentially occurred at the Ag site on the Ag Fe dual-site SAC,and the introduction of Fe site reduced the ORR overpotential,thus promoting ORR catalytic performance.