Controllable Preparation and Electrocatalytic Mechanism Study of MOF-derived Carbon-based Oxygen Reduction Electrocatalysts

Author:Lai Qing Xue

Supervisor:he jian ping


Degree Year:2018





Oxygen reduction reaction(ORR)electrocatalysts are extensively applied in new energy technologies,including fuel cell,metal-air battery,and chlor-alkali industry,to overcome the slugguish kinetics of ORR and elevate the devices’overall performance.However,the significant disadvantages for tranditional precious metal-based ORR electrocatalysts,including high cost,low stability and bad resistance to small molecules,hinder the large-scale applications of above technologies.Therefore,it’s much necessary to develop cheap and efficient non-precious metal ORR electrocatalysts to replace precious metal materials for practical application in above technologies.The focus of this dissertation is to develop metal-organic-framework(MOF)-derived nanocarbon-based ORR electrocatalysts.The microstructures of carbon matrix and configurations of active sites were carefully regulatedin order to investigate their influence on electrocatalytic performance and then the electrocatalytic mechanism of ORR on the surface of this kind of catalysts was discussed.The main points in this dissertation are summarized as follows:1.The interface interaction forces between zeolite imidazolate framework-8(ZIF-8)and polyvinylpyrrolidone(PVP)during pyrolysis process was employed to in situ transform microporous structure to micro-mesoporous hierarchical one for ZIF-8 derived nitrogen-enriched carbon,achieving a three-dimensional nitrogen-enriched micro-mesoporous hierarchical carbon(3D NEMC).The specific surface area ratio of mesopores to micropores was improved to 0.57 from 0.43 for nitrogen-enriched carbon nanoparticles(NECNP)prepared by direct carbonization of ZIF-8.Extensive investigation of the influence on products’nanostructures from the sizes of ZIF-8,weight ratio of ZIF-8 to PVP,and pyrolysis temperature revealed a step-by-step carbonization process for PVP and ZIF-8,inspiring us to propose an interface-interaction-dominated in situ confinement pyrolysis transformation mechanism.2.One-dimensional nitrogen-enriched micro-mesoporous carbon nanofiber(1D NEMCF)and two-dimensional nitrogen-enriched micro-mesoporous carbon/graphene nanosheet(2D NEMC/G)were successfully fabricated by the in-situ confinement pyrolysis transformation strategy combining with electrospinning technology and graphene oxide-based structure-directing method,respectively.Especially for 2D NEMC/G,it not only has a high content of nitrogen(8.92 at.%)and specific surface area(655 m2/g),but also pocessess a high specific surface area ratio of mesopores to micropores(1.17).Therefore,such an improved synthesis strategy can achive efficient regulation over porous structures without obviously sacrificing the specific surface area and nitrogen content,which significantly enhanced the utilization of nitrogen-doped sites and mass transfer kinetics,and then promoted the ORR electrocatalytic performance of as-prepared catalysts.3.Highly active Cu-N sites were successfully incorporated into the surface of ZIF-8-derived nitrogen-enriched carbon(N/C)by an ex-situ method.The formed strong synergistic effect between Cu-N and nitrogen doped sites promised a ten times higher kinetic current density of 43.7 mA/cm2 for Cu-N/C material than N/C(4.1 mA/cm2);meanwhile,the electron transfer number for Cu-N/C increased from 3.5 for N/C to 3.75,closer to a highly efficient four-electron reaction path.Afterward,the potential application of prepared Cu-N/C in zinc-air battery was discussed.4.ZIF-8-assisted in-situ metal-doping-inducing synthesis strategy was proposed to fabricate a series of advanced Cu-N/C catalysts by in-situ doping Cu2+into ZIF-8 as the single precursor for Cu-N/C.The dopping of Cu2+not only promotes the formation of highly active Cu-N sites with high density on ZIF-8 derived carbon matrix,but also transforms microporous structure to micro-mesoporous one synchronously.As a result,the prepared Cu-N/C catalyst not only had a high nitrogen content of 11.04at.%and specific surface area of 1182 m2/g,but also had a high specific surface area ratio of mesopores to micropores of 1.84,resulting to excellent electrocatalytic performance both in half-cell and zinc-air battery.Furthermore,a novel hybrid coordination configuration of Cu(0)-Cu(II)-N was revealed to be responsible for excellent electrocatalytic performance by electrochemical molecular probe technique combining with physical characterization.5.Novel Fe-N/C catalysts with high performance in acid electrolyte were successfully fabricated by a ZIF-8-based host-guest chemistry strategy.Fe-mIm nanoclusters(NCs)with well-defined Fe-N sites as guests were introduced into ZIF-8 hosts.The host-guest relationship between ZIF-8 and Fe-mIm NCs lead the in-situ confinement pyrolysis of Fe-mIm in ZIF-8 hosts,suppressing the formation of inactive inorganic Fe-containing nanoparticles and promoting controllable transformation of Fe-mIm NCs to highly active and atomically dispersed Fe-Nx sites.As a result,the prepared 5%Fe-N/C delivered comparable ORR electrocatalytic activity to commercial 30%Pt/C in acid electrolyte.Synchrotron radiation characterizations and density functional theory(DFT)calculations revealed that five-coodinated Fe-Nx configuration was the best electrocatalytic sites for ORR in acid condition,which provided a novel approach to design and optimize Fe-based non-precious metal ORR electrocatalysts.