Preparation Of Bifunctional Oxygen Catalyst MOF-derived Materials And Their Application In Zinc-air Batteries

Fuel cells and metal-air batteries are becoming effective and most promising green electrochemical energy conversion and storage due to their high energy conversion efficiency,noiselessness,environmental friendliness and low cost.These sustainable technologies include two important half-reactions,namely oxygen evolution reaction（OER）and oxygen reduction reaction（ORR）.However,the reduction mechanism of O₂ is more complicated,the cathode reaction kinetics is slow,and the energy conversion efficiency is low,which limits their application.Therefore,it is particularly important to design and prepare cheap bifunctional electrocatalysts with high activity and high stability.Noble metals such as ruthenium oxide（RuO₂）and Pt/C are regarded as benchmarks for OER and ORR electrocatalysts due to their excellent electrocatalytic performance.Unfortunately,their practical applications are affected by issues such as high prices,scarcity,and poor stability.In recent years,metal-organic framework（MOF）carbon-based ORR catalysts have attracted much attention due to their simple synthesis method,high porosity,large specific surface area,and orderly internal structure,which provide non-noble metal catalysts designed for ORR new opportunities.The specific research content is as follows:（1）Through the pre-implantation of poly 3,4-ethylenedioxythiophene（PEDOT）nanowires,a catalyst（Co/C@NS NWs）co-doped carbon nanowires loaded with Co nanoparticles was designed and synthesized.XRD,SEM,TEM,and XPS characterizations proved that ZIF-67’s structural collapse and Co atom agglomeration were suppressed during the pyrolysis process,and N and S co-doping was realized at the same time.The Co/C@NS NWs catalyst exhibits excellent ORR performance,with an initial potential of 0.91 V（vs RHE）and a limiting current density of-4.8 mA cm^-2,which is even better than commercial Pt/C.Co/C@NS NWs-catalyzed ZAB has an open circuit voltage of 1.45 V and a maximum power density of 129.3 mW cm^-2at a discharge current density of 180 mA cm^-2,and a specific capacity of 800 mA h g _Zn^-1 at a discharge current density of 10 mA cm^-1.During the 35-hour continuous discharge/charge cycle test,the voltage did not change significantly.This work provides new inspiration for the design and development of high-performance non-noble metal carbon-based bifunctional oxygen electrocatalysts.（2）In order to improve the catalytic performance of Ce-MOF materials in redox reactions,this research has developed a unique Ce-N-C catalyst modification strategy.The synthesis includes mixed-valence bundled Ce-MOF₉₀₀(Ce³⁺/Ce⁴⁺)carbon nanorods.The template was prepared by using ethanolamine intercalation to adsorb cobalt ions and anchored on carbon nanorods,and finally low-temperature hydrothermal reduction to prepare Co₃O₄-CeO₂@NC catalyst.Co₃O₄ nanoparticles are highly dispersed on the CeO₂carbon frame,avoiding the agglomeration of metal ions,and improving the conductivity of the Ce-based electrocatalyst.At the same time,the Co₃O₄ adsorbed on the surface and CeO₂work together to promote ORR/oxygen storage performance.The initial potential of the optimized Co₃O₄-CeO₂@NC catalyst relative to the reversible hydrogen electrode（RHE）is E₀=0.80 V,the limiting current density is j _L=6.0 mA cm^-2 and the low Tafel slope of 81 mV dec^-1.Finally,the zinc-air battery assembled with Co₃O₄-CeO₂@NC as the oxygen electrode has a high specific capacity of 780 mAh g^-1 and a stable discharge/charge potential at a constant current discharge current density of 10 mA cm^-2.And excellent cycle stability.（3）Use Ce（NO₃）₂·6H₂O and Co（NO₃）₂·6H₂O as raw materials to polymerize aniline monomer at room temperature to prepare CeO₂C₂/NC non-noble metal catalyst and systematically study its promotion of oxygen reduction reaction effect.The results show that the CeO₂C₂/NC catalyst is in the shape of nanorods,with a large amount of irregular main phase CeO₂C₂ attached to it.Electrochemical tests and battery tests show that the prepared catalyst has good catalytic activity and battery performance.The CeO₂C₂/NC-catalyzed zinc-air obtained an open circuit voltage of 1.44 V and a maximum power density of 90.2mW cm^-2 at a discharge current density of 130 mA cm^-2.The constant current discharge test shows that the zinc-air battery based on CeO₂C₂/NC has good catalytic stability to ORR in actual ZAB.