Design,Preparation,and Application Of Carbon-rich Single-atom Oxygen Electrocatalysts

The extensive use of fossil fuels has caused a series of problems such as environmental pollution and energy crisis.Therefore,it is urgent to develop the energy allocation by increasing the uses of clean and sustainable energies.Zinc-air battery and proton exchange membrane fuel cell（PEMFC）are clean energy devices which can continuously convert chemical energy into electricity.They have many advantages,such as high energy conversion efficiency,stable discharge voltage,and environmental protection.However,their oxygen electrode reaction suffers from high overpotential and slow kinetics,hamping their practice applications.At present,platinum group metals are considered to be the most efficient catalysts,but their high cost,poor stability and toxicity limit their applications in the real world.Therefore,developing efficient and stable non-precious metal catalysts is very imporant but has remained challenging.Single-atom catalysts（SACs）with their unique features of structure such as maximized atomic utilization,strong metal-support interaction,and unsaturated metal coordination environment,enable opprotunities for advanced nanotechnologies of non-precious metal catalysts.This paper aims to design,and synthesize carbon-rich single-atom oxygen electrocatalysts for energy conversion and storage.The effects of nature of metal precursors,heteroatom doping,and porous structures of carbon supports on the distribution and atomic structure of active site have been demonstrated.The correlation of structures and performance has been explored.Finnally,the applications of these catalysts in Zn-air battery and PEMFC have been examined.The main research work is as follows:（1）Using zeolite imidazolate frameworks（ZIF-8）derived carbon as support,cobalt（Co）SAC was prepared by an impregnation method.The influence of cobalt precursors on the structure and performance of the catalyst was investigated.The results of atomic absorption spectroscopy（AAS）and inductively coupled plasma emission spectrometer（ICP-OES）tests exhibted that the four cobalt precursors have different adsorption capacities on carbon support,which have certain effects on the morphology and pore structure of the prepared catalyst material.The Co SAC labeled as Co-N-C/EDTA prepared by using sodium cobalt ethylenediamine tetraacetate（Co-EDTA）.Revelaed that the Co-EDTA precursor can be converted into Co N_x active sites during thermal treatment process.This material demonstrated a better catalytic activity than other catalysts prepared by using other metal precursors.In alkaline condition,the Co-N-C/EDTA catalyst has the highest oxygen reduction reaction（ORR）activity with a half wave potential(E_1/2)of 0.85 V,outperforming the commercial Pt/C(E_1/2=0.84 V).Additionally,the Co-N-C/EDTA exhibited excellent activity of oxygen evolution reaction（OER）with a overpotential of 403 m V at 10 m A cm^-2.Therefore,the Co-N-C/EDTA has potential of application in rechargeable Zn-air batteries.（2）Phosphorus（P）-doped cobalt-nickel dual-atom oxygen electrocatalyst was prepared by the impregnation method using ZIF-8 derived carbon as support,and was denoted as P/Co Ni-NC/Na Cl.The X-ray photoelectron spectrometer（XPS）indicated that P was doped into carbon structure to form C-P.In 0.1 M KOH electrolyte solution,E_1/2 of ORR catalyzed by P/Co Ni-NC/Na Cl was 0.88 V,which was higher than that of Co Ni-NC/Na Cl(E_1/2=0.82 V).Moreover,it enhanced OER catalytic activity in 1 M KOH with a low overpotential of 352 m V at 10 m A cm^-2.The results demonstrated that P doping enhanced the ORR-OER bifunctional activity of the catalyst.When P/Co Ni-NC/Na Cl was used as an air electrode,the Zn-air battery delievered a high peak power density of 211m W cm^-2.This battery also showed a good stability by the charging and discharging measurement for 800 cycles（285 h）.（3）The porous structures of carbon supports were regulated by using molten Na Cl and KCl-assisted pyrolysis of ZIF-8.Then,the carbon supports were further used for host single Fe atoms,obtaining Fe-N-C SAC electrocatalyst using the impregnation method.By adjusting the mass ratio of Na Cl,KCl and ZIF-8,the pore size distributions in the carbon carrier were regulated.When Na Cl:KCl was 1:4,the mesopores with pore sizes of 2-4 and 7-10 nm are dominated in the catalyst.The E_1/2 of ORR catalyzed by Fe-NC/0.25KNa in 0.5 M H₂SO₄was as high as 0.80 V,which was 50 m V higher than that of Fe-NC(E_1/2=0.75 V)prepared by carbon support without using molten.In additioan,it showed a better stability with slight activity loss(E_1/2 only drops by 14 m V)after 10000 CV cycles.Therefore,this work shows that the atomic structure of Fe-NC can be optimized and the performance of acid ORR can be improved by adjusting the pore structure of carbon supports.The resulting catalysts have potential of application in PEMFCs.