Controllable Preparation And Properties Of Non-Precious Metal Electrocatalysts

Energy is an important resource on which economic development is highly dependent.At present,fossil energy still occupies a dominant position in energy consumption,and problems such as energy shortage and environmental pollution are becoming more and more serious.In order to break the situation of excessive dependence on fossil energy and seek the sustainable development of human society,the development and application of new clean energy and advanced energy devices must be accelerated.Among them,fuel cells and zinc-air batteries（ZABs）have the superiorities of outstanding energy utilization,cleanness and low pollution,and are considered as bright energy storage and conversion devices.The oxygen reduction reaction（ORR）is a central reaction in fuel cell devices and zinc-air battery devices,but sluggish reaction kinetics severely limit the efficiency of these clean devices.The currently commercialized ORR catalysts are platinum-based catalysts.Platinum-based catalysts show excellent catalytic efficiency in the ORR process,But platinum reserves are scarce and expensive,which limit the widespread application of fuel cell systems.Therefore,the development of inexpensive,highly active and durable non-precious metal electrocatalysts is the key to large-scale commercialization of fuel cells and zinc-air batteries.In this dissertation,aiming at designing and preparing high-efficiency,durable and low-cost carbon-based non-precious metal electrocatalysts,a series of composite materials were developed。Focusing on the strategies of size effect,alloy effect and core-shell structure,etc.,the structural composition and catalytic performance of composite catalysts were studied,and methods to improve the catalytic activity and stability of non-precious metal catalysts were discussed.The main research contents are as follows:（1）Preparation and properties of covalent organic polymer-derived oxygen reduction catalyst Fe₃O₄/CCOP_HM-F127.A novel two-dimensional covalent organic polymer with high nitrogen content(named COP_HM)was developed by polymerizing hexaketocyclohexane（C₆O₆·8H₂O）with melamine（C₃H₆N₆）under acetic acid conditions.The ORR catalyst Fe₃O₄/CCOP_HM-F127 with Fe₃O₄ as the active site was prepared by pyrolysis of the mixture of COP_HM,Fe salt and surfactant F127.The TEM images demonstrated that Fe₃O₄ nanoparticles in the pyrolyzed catalyst showed smaller particle size in the presence of surfactant F127.The COP_HM derivative catalyst Fe₃O₄/CCOP_HM-F127,prepared with the assistance of anionic surfactant F127,exhibited excellent performance in electrocatalytic oxygen reduction with a half-wave potential of 0.87 V（vs.RHE）,which is better than Pt/C.Moreover,Fe₃O₄/CCOP_HM-F127 exhibited excellent stability and methanol tolerance during testing.In this work,novel nitrogen-rich covalent organic polymers(COP_HM)were synthesized,and the effect of surfactant F127in preventing metal particle agglomeration and improving particle distribution uniformity was investigated.This work provides a facile and efficient strategy for optimizing the structure and activity of non-noble metal electrocatalysts.（2）Preparation and properties of covalent organic polymer-derived high-performance bifunctional oxygen catalyst Fe Ni@NC.Based on previous studies on covalent organic polymers(COP_HM),a novel covalent organic polymer with high nitrogen content(named COP_HM-2)was prepared by the polymerization of hexaketocyclohexane（C₆O₆·8H₂O）and melem(C₆H₁₀N₁₀).In the presence of surfactant F127,Fe and Ni salts were added,and then Fe Ni alloy-supported hierarchical porous carbon composites（Fe Ni@NC）were prepared by further pyrolysis,in which the Fe Ni alloy was coated with several layers of graphitic carbon.Due to the numerous Fe Ni nanoparticles in Fe Ni@NC and the structure of carbon shell-coated Fe Ni alloy particles,the Fe Ni@NC catalyst exhibits excellent activity and durability for both ORR and OER under alkaline conditions.The Zn-air battery with Fe Ni@NC catalyst as the air electrode exhibited higher discharge power density than Pt/C+Ru O₂,and also has excellent charge-discharge performance and high long-term stability.These results demonstrate that Fe Ni@NC are excellent bifunctional electrocatalysts and can be used for practical applications in rechargeable zinc-air batteries.（3）A Dual MOFs Strategy for Synthesis of Fe Co@NC Bifunctional Oxygen Catalysts.Using a dual MOFs（Metal-Organic Frameworks）strategy.By pyrolyzing the mixture of two MOFs（Zn Co-ZIF and MIL-101（Fe））,a bifunctional oxygen catalyst Fe Co@NC was synthesized with the structure of a porous carbon-supported Fe Co alloy.Among them,Zn Co-ZIF acts as the structural support and provides the cobalt/nitrogen source,while the iron-based MIL-101 provides the iron source.During pyrolysis,Zn metal evaporation in Zn Co-ZIF facilitates the formation of porous channels and the exposure of Fe Co alloys in carbon supports.Therefore,the as-synthesized Fe Co@NC catalyst exhibited good catalytic activity for both ORR and OER.The Zn-air battery with Fe Co@NC as the air electrode shows a maximum power density of 139.6 m W cm^-2 and excellent cycle life.The excellent bifunctional properties and stability exhibited by Fe Co@NC in alkaline media are mainly attributed to the obtained Fe_0.26Co_0.74 crystal phase and the smaller particle size and hierarchical pore structure resulting from the dual MOF synthesis strategy.（4）Fe doping for enhance the oxygen reduction performance of carbon-based chromium nitride catalysts under acidic conditions.The ORR catalyst materials loaded with Cr N and Cr₂O₃ nanoparticles were prepared based on ZIF-derived carbon,and Fe doping was adopted to enhance the ORR catalytic activity of catalysts.A series of characterizations proved that Fe had been successfully doped into Cr N and Cr₂O₃ lattices.The results of the performance test prove that Fe doping can significantly improve the catalytic performance of carbon-supported Cr N and Cr₂O₃ materials.Among them,Fe-Cr N@NC exhibited excellent catalytic activity in acidic solution with a half-wave potential of 0.742 V（vs.RHE）.In addition,Fe-Cr N@NC also exhibited superior stability and methanol tolerance over commercial Pt/C.In actual PEMFC tests,the Fe-Cr N@NC catalyst also showed excellent performance,with the highest power density reaching 708 m W cm^-2.The results show the important application value of Cr-based ORR catalyst materials and Fe-doped strategies.