Preparation Of ZnCo-ZIF-derived N-doped Carbon-based Catalysts Via Salt-assisted Strategy And Their Oxygen Reduction Reaction Performance

Zinc-air batteries（ZABs）,as one of the emerging devices in current energy storage and conversion systems,have attracted extensive attention from researchers in recent years due to their advantages such as high energy density,high stability,low price,and superior safety and reliability.However,the slow cathode reaction kinetics and the high cost of precious metal catalysts have greatly hindered large-scale application of zinc-air batteries.Therefore,one of the effective ways toward their large-scale application is to explore non-noble metal electrocatalysts with high catalytic activity.Among the non-noble metal electrocatalysts,MOFs（Metal-Organic Frameworks）derived transition metallic（Fe,Co,Ni）nitrogen-doped carbon-based catalysts have been extensively investigated for their advantages such as tunability of components and structures,high pore size and abundance of active sites.Nevertheless,common pyrolysis at high temperatures can cause deformation and collapsing or self-aggregation of precursors,resulting in a significant decrease in utilization of active sites.Therefore,in this thesis,we use a"salt-assisted"pyrolysis strategy to modulate the surface electronic structure of the catalysts in terms of both morphological control and defect design to obtain non-noble metallic carbon-based catalytic materials with excellent performance.The specific work and conclusions of this thesis are as follows:1.We have proposed a"dimensionality reduction"synthesis method.The original three-dimensional Zn Co-ZIF was"thermally exfoliated and etched"into a Zn Co-MS-NC catalyst by using a"salt-assisted"high-temperature pyrolysis strategy.The Zn Co-MS-NC carbon nanosheet catalysts with ultra-thin two-dimensional morphology,high porosity and abundant catalytic activity sites have been synthesized to afford excellent oxygen reduction（ORR）catalytic activity in alkaline electrolytes.Compared to conventional pyrolysis,the Zn Co-MS-NC nanosheets have a higher half-wave potential（0.82 V）,comparable to that of commercial Pt/C.The Zn Co-MS-NC assembled Zn-Air cells exhibit an open circuit voltage of 1.5 V,a specific capacity of 786.0 m Ah g^-1and a peak power density of 146.61 m W cm^-2,which is superior to the that of commercial Pt/C.2.We obtained transition metal nitrogen doped porous carbon catalysts with high density defect by using NH₄Cl"salt assisted"pyrolysis strategy.The NH₃and HCl gases produced during NH₄Cl pyrolysis"etch"carbon structure.As a result,a highly defective Zn Co-NC-AC catalyst was obtained.The catalyst showed excellent ORR activity in alkaline environments,with an onset potential of 0.947 V,a half-wave potential of 0.844 V and a low Tafel slope（53.02 m V/dec）.The zinc-air battery assembled using Zn Co-NC-AC catalyst as cathode catalysts have a power density of145.4 m W cm^-2and a specific capacity of 807.6 m Ah g^-1.This synthetic strategy provides a new avenue for the preparation of non-noble metal nitrogen-doped carbon catalysts derived from MOFs.