Preparation Of Metal-Organic Framework-Derived Porous Cathode Materials And Their Application In Rechargeable Zn-air Battery

The rechargeable Zn-air battery has been paid attention due to the advantages of high energy density,stable discharge voltage,low cost,and sustainable development.The battery is regarded as a kind of new type of energy storage system which is valuable for research and potential application.However,there are still challenges hindering their commercial applications,including sluggish kinetics,low coulombic efficiency and a short cycle life.In this paper,Co/N-C-800 and Ni S-Co S₂-Co/N-C catalysts were prepared,respectively.The microstructures of the catalysts were characterized by XRD,SEM,XPS,and BET etc.A Zn-air battery system was constructed by using nickel foam coated with catalyst as a cathode,copper mesh as an anode and KOH solution with adding Zn O as the electrolyte.The ORR/OER catalytic performance,polarization performance,charge-discharge cycle performance and rate performance of the catalyst were systematically studied by electrochemical tests.The Co/N-C catalyst was prepared by the pyrolysis of ZIF-67 precursor under the protection of N₂.The characterization results show that Co was uniformly distributed on the mesoporous sheet carbon in elemental form.The nitrogen is doped into the catalyst in the forms of pyridine N,graphite N and Co-N_x.Different pyrolysis temperatures have different effects on the catalyst.The catalyst Co/N-C-800 obtained by pyrolysis at 800?has excellent ORR/OER bifunctional catalytic activity and charge-discharge cycling performance.The catalyst stably cycles over 1000 cycles.The coulombic efficiency is between 89%and 95%,and the voltage efficiency is about 92%.The discharge voltage remains at 1.71 V,and the average charge-discharge voltage gap is about 0.16 V.Co/NC-800 has good charge-discharge rate performance.The discharge voltage can be maintained above 1.60 V when the discharge rate is increased from 1 C to 20 C.The discharge voltage can still reach 1.41 V,and the coulombic efficiency has also remained above 90%when the charge rate is increased from 3C to 40 C.The Ni S-Co S₂-Co/N-C catalyst was prepared using Co/N-C as the matrix with adding thiourea and nickel nitrate by a hydrothermal method.The characterization results show that the catalyst contains not only metallic Co,but also has spiculate Ni S and cluster Co S₂supported on mesoporous carbon.The nitrogen is still doped into the catalyst in the forms of pyridine N,graphite N and Co-Nx.Ni S-Co S₂-Co/N-C has more superior ORR/OER bifunctional catalytic activity,which can effectively reduce the charging voltage,prolong the discharge plateau,and significantly improve the charge-discharge cycle performance of the battery.It can stably cycle 1200 cycles with the coulombic efficiency of above 96%.The coulombic efficiency can reach up to 100%.The voltage efficiency stabilizes around 92%.The discharge voltage remains at 1.66 V,and the average charge-discharge voltage gap is about 0.13 V.Ni S-Co S₂-Co/N-C has excellent charge-discharge rate performance.The discharge voltage remains above 1.55 V when the discharge rate is increased from 1 C to 40 C.The discharge voltage can be maintained above 1.59 V,and the coulombic efficiency has been higher than 92%when the charge rate is increased from 3 C to 40 C.