Metal-organic-framework (MOF) Derived Carbon-based Materials For ORR Applications

To ensure the sustainable development of our planet,it is necessary to develop cost-effective and feasible alternative energy conversion and storage devices.Zn-air batteries（ZAB）have emerged as a promising technology due to their high energy density,low cost,safety,and environmental friendliness.However,the development of cost-efficient cathode catalysts is crucial to the advancement of ZAB because oxygen reduction reaction（ORR）on the cathode is impeded by a considerable energy barrier and constrained by expensive commercial Pt/C catalysts.Metal-organic frameworks（MOF）are excellent precursors for the synthesis of carbon nanomaterials due to their simple synthesis,rich structure and composition.MOF-derived carbon nanomaterials retain the morphology of their precursors at low dimensional scales,endowing themselves with abundant edge defects,larger specific surface area,and excellent electron transport paths.Additionally,MOF’rich composition enables the carbon nanomaterials derived from them to exhibit various physicochemical properties,including stronger electron gaining ability,oxygen affinity,and a higher degree of graphitization,leading to excellent ORR activity.Therefore,rational design of the structure and composition of MOF precursors and precise control of their pyrolysis parameters can be used to develop more efficient ORR catalysts.This paper aims to investigate the ORR properties of MOF-derived carbon-based materials in-depth,with the goal of developing more efficient and cost-effective cathode catalysts for ZAB.（1）Porous carbon nanofibers with encapsulated Fe-ZIF-8 and Zn O particles were synthesized by electrospinning and pyrolysis,etching,and activation treatments（Fe-N-C@CNFs-act）.The Fe-N-C@CNFs-act exhibited dispersed Fe-N-C active sites and were used as cathode catalysts for ZAB.Etching treatments increased the porosity of the fibers,enabling better gas exchange and electrolyte penetration,and exposed more Fe-N-C active sites,reducing the reaction barrier.Activation with NH₃ converted the carbon in the catalyst into more conductive graphite carbon,facilitating electron transfer.The Fe-N-C@CNFs-act catalyst showed comparable ORR performance to commercial Pt/C catalyst(E_onset:0.978 V,E_1/2:0.838 V,j:5.76 m A cm^-2),significantly better than Fe-N-C@CNFs catalysts without etching and activation treatments(E_onset=0.913 V,E_1/2=0.779 V,j:3.95 m A cm^-2).The Tafel slope of the Fe-N-C@CNFs-act catalyst was 79.66 m V dec^-1,lower than that of Pt/C catalysts(96.67 m V dec^-1),indicating better ORR kinetic performance.The Fe-N-C@CNFs-act cathode catalyst achieved a peak power density of 153.55 m W cm^-2 in a ZAB.Further etching and activation treatments of carbon fibers can be an effective way to enhance the electrochemical activity of the catalyst based on the excellent performance of the synthesized Fe-N-C@CNFs-act catalyst in ORR.（2）A MOF-derived Fe Co alloy nanoparticles loaded on one dimensional carbon nanofibers matrix catalyst（Fe Co/N-CNFs）is prepared by means of electrospinning and pyrolysis.By careful designing,the carbon nanofibers produced by electrospinning provide a supporting template for the precursor,which can effectively prevent the aggregation of encapsulated Fe nanoparticles derived from Prussian Blue（PB）and surface-bound Co nanoparticles derived from ZIF-67.In this case,it is guaranteed that each encapsulated PB within the electrospun nanofibers is geographically located within the neighborhood of certain ZIF-67s.Therefore,the ZIF-67 anchored along the surface of the nanofibers can gain easy access to neighboring encapsulated PBs within the nanofibers during the pyrolysis,therefore forming well dispersed Fe Co alloy nanoparticles.The Fe Co/N-CNFs catalyst demonstrated exceptional ORR performance in electrochemical tests,with an Eonset of 0.997 V and an E_1/2 of 0.88 V,surpassing that of Pt/C(E_onset=0.992 V,E_1/2=0.85 V).Moreover,the ZAB assembled with Fe Co/N-CNFs catalyst exhibited impressive device performance,with better discharge current density(529.90 m A cm^-2),peak power density(356.23 m W cm^-2),and longer cycle stability compared to Pt/C-based ZAB.The successful encapsulation of one metal within the nanofibers and surface-bound growth of the other metal on the nanofibers demonstrated by this novel and promising synthetic strategy for bimetallic M-N_X/CNFs has resulted in extraordinary electrocatalytic performance.