The Controllable Design Of Porous Carbon-based Single-atom Catalysts And Study Of Electrocatalytic Performance

Clean-energy technologies such as proton exchange membrane fuel cells and metal-air batteries rely heavily on the oxygen reduction reaction（ORR）performance of cathode catalysts.Single-atom catalysts（SACs）have attracted much attention because of their maximized atomic utilization efficiency,adjustable electronic structure and excellent ORR performance.However,the low intrinsic activity,low porosity,insufficient exposure of the active sites and low mass transfer efficiency seriously hinder the practical application process.It is urgent to develop M-N-C single atom catalysts with high activity,high conductivity and rich porosity.In this paper,a series of novel porous carbon-based single atom catalysts were designed,and the influences of porous structure,heterogeneous element doping,strain environment on the single atom center were investigated.By coupling metal particles,the catalyst can simultaneously realize efficient ORR and oxygen evolution reaction（OER）.The microstructure and composition of the porous materials were investigated by means of aberration-corrected high angle angular dark field-scanning transmission electron microscopy（AC HAADF-STEM）,synchrotron radiation and X-ray photoelectron spectroscopy（XPS）.The main contents are as follows:（1）Fe atoms supported on hierarchical porous carbon was fabricated,and the effects of porous structure,pyrolysis temperature and Fe content on the ORR performance were studied.Firstly,ordered macro-microporous single crystal ZIF-8doped with Fe was synthesized by polystyrene sphere（PS）template and heterogeneous nucleation induced by solvents.Subsequently,the synthesized Fe-containing material was carbonized to obtain ordered porous carbon-supported Fe-N_xSACs（HP-Fe NC）.The obtained HP-Fe NC shows a three-dimensional ordered and interconnected porous carbon skeleton,and the AC HAADF-STEM test shows that Fe atoms uniformly dispersed on the carbon substrate.The interconnected porous structure promotes the construction of triple-phase boundary,shortens the mass transfer distance in the catalyst,and exposes a large number of active sites.The hierarchical porous HP-Fe NC catalyst with moderate Fe doping and pyrolysis temperature（1000°C）exhibited the best ORR performance(E_1/2=0.86V vs.RHE)under alkaline conditions compared with Pt/C（20 wt%）and Fe NC catalyst without hierarchical porous structure.The Zinc-air batteries assembled by HP-Fe NC showed an open-circuit voltage of 1.45 V and a maximum discharge power density of 154m W cm^-2,which was higher than that of the Pt/C+Ru O₂.This design method can solve the problems of low utilization rate of active sites and poor mass transfer efficiency in the catalyst.The findings will be helpful for further exploration of structure design of the ORR catalysts.（2）The above work indicates that the porous structure and Fe sites can significantly improve the ORR performance,but the overall performance still needs to be further improved.We doped B and F elements to further adjust the electronic structure of Fe sites while designing the porous structure.We also developed an efficient strategy for fabricating integrated porous electrode.The designed integrated electrode consists of porous carbon cages and reticular fibers.Fe-N₄sites were supported on a high density B,F co-doped carbon substrate（Fe-SA-FPCS）.Fe-SA-FPCS showed higher ORR half-wave potential（0.89 V vs.RHE）.Fe-SA-FPCS exhibit a high discharge power density of 168.4 m W cm^-2and excellent durability when used as electrodes for liquid Zinc-air batteries.The assembled solid state Zinc-air battery shows a stable charge/discharge process under the flat/bent state.DFT calculations revealed that Fe-N₄sites regulated by B and F elements promoted the desorption of*OH.This work offers a novel design concept to construct the integrated electrode with adjustable porous structure and electronic structure for advanced electrochemical energy systems and catalysis.（3）To further regulate the ORR activity of Fe-N₄sites in porous carbon cages,we designed a double-template method together with electrostatic spraying technique to prepare the high-curvature carbon supported Fe single atom catalyst（T-Fe SAC）.The microstructure,morphology and formation mechanism of the carbon materials were investigated.EXAFS measurements showed that the Fe-N bond was elongated and the valence of Fe atoms was enhanced on the curvature carbon.T-Fe SAC exhibited excellent ORR activity with half-wave potential(E_1/2)of0.91 V vs.RHE,and the peak power density of 199 m W cm^-2when used as air electrodes in Zinc-air battery.In situ ATR-SEIRAS tests indicate that stress-regulated Fe-N₄sites promote O-O bond breaking and accelerate the evolution of*OOH intermediate to*O intermediate.While,the DFT calculation results show that the electron density around Fe atom decreases under the tensile stress,which promotes the desorption of*OH intermediate.The rapid breaking of the O-O bond in*OOH with the formation of*O and the rapid desorption of*OH reduce the energy barrier of the 4e^-reaction pathway,achieving high ORR activity and selectivity.The present work provides new insights into understanding and designing the microstructure of single atom catalysts.（4）To further enhance the bifunctional catalytic activity of the catalyst,highly active Co-Co F₂particles was introduced on the basis of the SACs.Porous carbon supported Co single atoms and Co-Co F₂nanoparticles were synthesized using PTFE spheres as templates.Co single atoms were uniformly dispersed on the porous carbon surface and the Co-Co F₂particles were evenly distributed on the surface of the hole and in the carbon nanotubes（CNTs）.The interconnected porous structure and CNTs increase the exposure of active sites,while improving the conductivity and mass transport efficiency in the electrocatalyst.The abundant Co-N_xsites and Co-Co F₂heterojunctions promoted the electrocatalytic activity of ORR and OER.The ORR half-wave potential of PCF@CCFCNT is 0.852 V vs.RHE,and the onset potential and overpotential of OER(η_j=10 m A cm^-2)were 1.47 V and 300 m V,respectively.The assembled Zinc-air battery shows a peak power density of 184 m W cm^-2and long-term charge-discharge cycle of more than 1000 h.This work provides a new strategy for the design of advanced bifunctional oxygen electrocatalysts and provides guidance for the practical application of rechargeable zinc-air batteries.