Study On Preparation And Electrocatalytic Performance Of Carbon-Based Single-Atom Iron Catalysts For Oxygen Reduction

Fuel cell is a clean and efficient energy conversion technology.The kinetic rate of cathode oxygen reduction reaction is the key factor determining the efficiency of energy conversion.Although traditional Pt-based catalysts can achieve high current output,the low Pt storage and poor durability seriously impede the large-scale application of various fuel cells.Therefore,the development of non-noble metal electrocatalysts with high efficiency,durability and low cost has great practical significance.In recent years,carbon-based single-atom catalysts have been regarded as appealing alternatives to Pt-based catalysts due to the excellent catalytic performance,unique structure and high atomic utilization.However,the surface free energy of the metal increases at the atomic scale,leading to the occurrence of agglomeration,and vast metal sites located in micropore or deep of carbon material are difficult to be effectively utilized.Therefore,it is necessary to reasonably design the preparation conditions for increasing the metal loading,and optimize the configuration of carbon materials for improving the utilization of atomic sites,thereby improving the overall catalytic performance of materials.In this paper,a series of carbon-based single-atom iron catalysts（Fe-NC）with superior ORR activity were prepared by developing the simple and efficient synthesis method to improve the metal dispersion and mass transfer,as well as optimize the coordination environment of atomic sites.The detail contents of these works are as follows:（1）Petal-like porous Fe-NC nanosheet with improved mass transport for oxygen reduction reactionPetal-like porous carbon nanosheets（Fe NC-D）with highly accessible Fe-N4 sites are prepared by a reliable hard-template strategy combining the space confinement and charge induction.Using mesoporous silica nanoparticles（MSN）as template,the morphology,pore structure and surface characteristics of the catalysts were improved by adjusting the ratio of carbon/nitrogen sources（melamine and diethylenetriaminepentaacetic acid）.When the ratio is1:1,the resulted Fe NC-D0.5 catalyst had high mesoporosity balanced hydrophobicity/hydrophilicity,which could promote the transfer of reactants/products and greatly increase the exposure of Fe-N4 moieties,resulting in efficient utilization of active sites.Therefore,Fe NC-D0.5 exhibited superior ORR performance with a half-wave potential（E1/2）of 0.866 V,exceeding the commercial Pt/C in alkaline solution.when utilized as the cathode of microbial fuel cell（MFC）and Zn-air battery（ZAB）,the Fe NC-D0.5 displayed high power densities(1041.3 m W m^-2 for MFC,356 m W cm^-2 for ZAB)and possessed remarkable stability,far surpassing the commercial Pt/C catalyst.（2）Hydrangea-Like carbon with highly accessible Fe-N4 active sites for efficient oxygen reduction reactionIn this chapter,a silica-templated strategy was employed to prepare hydrangea-like 3D porous material with open space and large external surface area.By introducing the MSN template,Fe-doped ZIF-8 precursors（2-methylimidazole and zinc nitrate）were assembled into flower-like carbon spheres,which possess porous nanosheets stretching from inside out that interweaves to form an open and interconnected architecture.This unique structure could fully release the dense Fe-N4 sites inside the carbon substrate and accelerate the mass transfer of the catalyst layer,thereby resulting in high active-site utilization.In addition,the morphology and porosity of Fe NC-Fn could be significantly adjusted by changing the diameter of MSN.When the diameter of MSN is 500 nm,the optimized Fe NC-F3 material exhibited high catalytic activity（E1/2=0.905 V in alkaline medium）,and achieves high power densities in MFC(3200m W m^-2)and ZAB(288 m W cm^-2).（3）Controllably incorporating sulfur functionality to boost oxygen reduction of Fe-N4 sitesIn this chapter,the innovative strategy of combining a Mg（OH）2,KCl template with KOH activation was designed to prepare high activity Fe-NC catalyst by constructing agarose and thiosemicarbazide composite gel.Due to confinement and catalysis effects,the strategy not only could optimize the pore structure,metal loading,and sulfur functional group content,but also controls the types of sulfur functional groups and preferentially generates oxidized sulfur.Theoretical calculations showed that the adjacent sulfur functional groups could affect the electronic structure of the Fe-N4 site,thereby optimizing the adsorption energy of oxygen-containing species and greatly accelerating the ORR kinetics of the active center.The obtained order in terms of reaction kinetics is as follows:oxidized S doping>thiophene-like S doping>pristine Fe-N4.In addition,the effect of the template content and functional groups,and the formation of atomically dispersed Fe-N4 moieties were systematically studied.When using thiosemicarbamide as source,the optimized Fe NC-SN-2 catalyst had the enhanced activity and high Fe loading,and exhibited excellent ORR activity with positive E1/2 of 0.890 in alkaline solution.When Fe NC-SN-2 was employed as air electrode,both corresponding MFC and ZAB demonstrate excellent catalytic efficiency and stability compared with commercial Pt/C,with high power density of 1785 m W m^-2 and 260 m W cm^-2,respectively.（4）Engineering metal sites for enhanced oxygen reduction activityIn this chapter,a template and activation combination strategy was designed to prepare Fe-NC nanoparticles with high catalytic performance by constructing chitosan and thiosemicarbazide composite gel.The effect of Mg（OH）2,KCl template and KOH activation was systematically studied.Combining a multi-step pickling and pyrolysis process,the Fe species was transformed from Prussian blue to Fe S,and finally to stable monodispersed iron sites.The optimized catalyst（Fe NSC-2Fe）with high porosity and abundant sulfur functional groups were synthesized by adjusting the content of metal（0.84 g）and type of template.Theoretical calculations showed that the adjacent sulfur atom and OH could affect the electronic structure of the metal center,thereby optimizing the adsorption energy of oxygen-containing species on Fe sites and greatly accelerating the ORR kinetics of the active center.Owing to the enhanced activity and fast mass transfer,the catalyst exhibited ultra-high catalytic activity,with a E1/2 of 0.913 V in an alkaline electrolyte.When used as the cathode of MFC,it exhibited high power density(2485 m W m^-2)and enhanced stability（voltage retention rate up to 86%after 12cycles）.When used as the cathode of ZAB,the catalyst also showed excellent catalytic performance(power density of 306 m W cm^-2;retention rate of 98%after 3 cycles).