Zeolitic Imidazolate Frameworks Derived Transition Metal Manganese And Nitrogen Co-doped Carbon Catalysts For Oxygen Reduction

The kinetics of cathode oxygen reduction reaction(ORR)in fuel cells and metal-air cells is sluggish,so it is necessary to use efficient catalyst to accelerate this reaction.Currently,the high cost and low reserve of the widely-used commercial platinum and its alloys limit the large-scale application.It is of great significance to develop non-noble metal catalysts for ORR with low cost and excellent catalytic performance.In this thesis,the manganese(Mn)and zinc(Zn)bimetallic zeolite imidazolate metal-organic framework materials(ZIFs)with high nitrogen content,large surface area and adjustable pore size were used as precursors,and a series of Mn single atom electrocatalysts were then synthesized through the metal ion adsorption,carbonization and ammonia activation strategies.The effect of catalyst composition and structure on electrochemical performance was revealed.First,Mn and Zn bimetallic ZIFs were prepared by solvothermal method using dimethylimidazole as ligand and zinc nitrate and manganese nitrate as metal sources.The as-prepared bimetallic ZIFs show regular dodecahedral morphology and possess a specific surface area of 1,904 m2 g-1,a micropore percentage of 95.9%,and a nitrogen content of 19 at.%.Taking advantage of the isolate effect of Zn on Mn in the bimetallic ZIFs,the manganese and nitrogen co-doped carbon(Mn-N-C)catalyst with the atomically dispersed Mn sites was prepared through one-step pyrolysis.Results show that the ORR performance was achieved at the Mn/Zn molar ratio of 1:15 in ZIFs precursor and the pyrolysis temperature of 1100℃,showing the half-wave potential of 0.815 V in the alkaline electrolyte.To further increase the Mn doping amount in the catalyst,the Mn-N-C catalyst with a high Mn-doped content was prepared by adsorbing Mn ions in the above bimetallic ZIFs precursor.Inductively coupled plasma emission spectroscopy combined with X-ray photoelectron spectroscopy analyses show that the doping amount of Mn in the catalyst was effectively increased through the Mn ion-assisted adsorption approach.Compared with the catalyst without adsorption,the Mn doping amount in the catalyst was increased by 3.5 times.The adsorption strategy is beneficial to increase the number of active sites of Mn-Nx in the catalyst,and thus improve the ORR catalytic activity.Electrochemical tests show that the half-wave potential of the catalyst is 0.872 V in alkaline electrolyte.Finally,a high performance atomically dispersed Mn-N-C catalyst was prepared through the ammonia activation strategy,and the effect of activation conditions(time and temperature)on the catalyst composition,structure and ORR performance was clarified.Results show that the specific surface area of the catalyst can be effectively increased by ammonia activation.When the activation temperature reaches 1000℃,the specific surface area of the catalyst is as high as 1557 m2 g-1,which is 1.3 times that of the un-activated catalyst.Meantime,ammonia can react with carbon and be doped into the carbon matrix in the form of graphite-N(content:59.58%),thereby changing the chemical state of N on the surface of the catalyst.Owing the increase of specific surface area and the content of graphite-N in the catalyst,the ORR catalytic activity was significantly improved,showing a higher half-wave potential(0.885 V)than commercial Pt/C and an outstanding electrochemical stability,for the optimized catalyst that prepared with the activation time of 1 h and the activation temperature of 1000℃.