Metal-organic Framework Derivative Bifunctional Catalysts And Rechargeable Zn-air Batteries

With the introduction of―carbon peaking and carbon neutrality‖goal,there is a growing focus on environmental protection.In the face of the environmental pollution caused by fossil fuels as a source of energy,there is a growing awareness of the need to reduce the amount of fossil fuels employed and to gradually replace them with environmentally friendly,renewable energy sources.Currently,the use of green energy sources such as wind,water and tidal energy can effectively reduce the use of fossil fuels.Due to geographical differences or other factors,this has the disadvantage of an unstable energy supply.Therefore,the development of energy storage and conversion technologies to make energy sustainable is the focus of research.Zn-air batteries（ZABs）are a promising new energy source due to their high energy density,safety factor and cost effectiveness.In the study of Zn-air batteries,a catalyst is essential to accelerate the chemical reaction at the air electrode and to stabilise the current input and output.For Zn-air batteries,the development of superior air cathode materials is a technical challenge and one of the priorities for alleviating environmental energy problems.As air cathode materials,carbon-based materials are the most widely used.They have been developed by many researchers due to their high electrical conductivity,controlled specific surface area and low preparation cost.In this thesis,metal-organic framework-derived composites were designed and synthesised for application as bifunctional catalysts in rechargeable zinc-air batteries.In the design of the metal-organic framework materials,both material composition and structure were considered.Different types of MOFs are formed by selecting elements from the transition metals and rare earth metals as metal centres,which are subjected to calcination and other processes to enable the derivation of different types of carbon-based composites.The research for this thesis is as follows.（1）Carbon-based materials derived from bimetallic MOF（Co/Y-BTC）oxides compounded with graphene were synthesised by a hydrothermal method.On this basis,the materials were nitrogen-doped by high-temperature pyrolysis to form nitrogen-containing carbon-based materials,which were applied as bifunctional catalysts for the oxygen reduction reaction（ORR）and oxygen precipitation（Oxygen evolution reaction OER）reactions.In performance tests,the ORR half-wave potential E_1/2 of CoY/C-N reached 0.840 V and the overpotentialηof OER was 420 mV(10mA·cm^-2).The performance was tested after assembly into a liquid Zn-Air battery and showed that the specific capacity reached 873.4 mAh·g^-1;the power density was139.27 m W·cm^-2 and the charge/discharge cycle time was able to reach 4586 minutes.（2）Nickel-doped rare earth based MOF（Ni/Y-BTC）was synthesised by hydrothermal method and compounded with ZIF-67 to form precursors.The MOF was derived from the metal-oxide-carbon matrix composite by stepwise pyrolysis.Novel metal/rare earth metal oxide coupled carbon based bifunctional electrocatalysts were prepared.The structure and active sites were adjusted by the ratio of rare earth elements to transition metal elements,and the rare earth-based bimetallic MOF was pyrolysed to give the corresponding oxide（Y₂O₃）,allowing the promotion of electron transfer.Doped nickel elements are also able to improve the performance of OER.In addition,the ZIF-67 ligand concentration was explored to modulate the number of Co-N active sites,providing an auxiliary effect on the overall catalytic performance.Physical characterisation such as scanning electron microscopy,transmission electron microscopy and powder X-ray diffractometry were used to confirm the analysis of the active components of the material.A half-wave potential of 0.830 V and an overpotential of 392 mV@10 mA·cm^-2 demonstrated the bifunctionality of the catalyst.After applying it to zinc-air batteries,it showed a superior specific capacity of 899.6mAh·g^-1.（3）Two-dimensional nanosheets of MoS₂ anchored with large amounts of ZIF-67were grown on hollow nanospheres Ni-BTC by hydrothermal,annealing and room temperature stirring and pyrolysed to carbon based materials.A three-dimensional hierarchical structure was constructed from the nanospheres and nanosheets as well as surface-loaded orthododecahedral ZIF-67,resulting in an enhanced specific surface area.The properties of each layer of the structure were examined to show the advantages of the structure.In alkaline environments,HM@MoS₂/Co-NC exhibited excellent half-wave potentials for ORR（0.875 V）and OER overpotentials close to RuO₂(η=370@20 mA·cm^-2).The assembly into Zn-air batteries exhibited excellent cycling stability,cycling for 140 h at constant current charge and discharge.The results show that the materials synthesised in this thesis,CoY/C-N,NiY/C@Co/C and HM@MoS₂/Co-NC,are able to maintain a good morphological structure,providing a larger specific surface area as well as better stability for the electrocatalytic process.In addition,they are able to improve the slow kinetics of the air cathode of Zn-air batteries,which is a good reference in metal-organic framework materials as air cathode catalysts.