Preparation Of Heteroatom Doped Carbon As Oxygen Reduction Electrocatalysts And Their Electrochemical Performance Of Zinc-air Battery

Zinc-air batteries,as a potential electrochemical energy storage device,have attracted wide attention from researchers because of their low cost,abundant resources,environmental friendliness,and high energy density.Its energy density is 2-5 times higher than that of the most commonly used lithium-ion batteries,but there are also many problems,in which the lack of effective catalysts for oxygen reduction and oxygen evolution reaction on air electrode is the biggest challenge.As a positive electrode catalyst,carbon material has the advantages of low cost and good stability,but its activity is low.In view of the above analysis,this paper promoted the activity of carbon as a catalyst for the air electrode of zinc air batteries by doping with heteroatoms,which focused on using organic salt pyrolysis,electrospinning,and subsequent heat treatment to dope N,F,B,and P elements,and systematically study the mechanism of doping on the catalytic performance.Through the assembly of zinc-air batteries and testing the performance,we built the relationship of carbon catalyst composition-structure-performance.The main contents and innovative points are listed as follows:?1?In Chapter 2,we designed a very simple route for the synthesis of highly nitrogen-doped carbon nanosheets?HNCNSs?,avoiding the complex and costly disadvantages of traditional methods for preparation.We have found that nitrogen-doped carbon nanosheets could be prepared by annealing disodium ethylenediaminetetraacetate dihydrate in an argon atmosphere,and the nitrogen doping amount can reach 18.96 wt%.When being used as electrocatalysts,due to the rich active sites in their structure,the as-prepared HNCNSs exhibited high catalytic activity for ORR through a highly efficient four-electron reaction mechanism.Importantly,the nitrogen-doped carbon nanosheets were much more stable than the commercial Pt/C in the ORR electrocatalytic performance test.Therefore,the high-nitrogen-doped carbon nanosheets had stable catalytic properties in alkaline solutions and could be used as excellent ORR electrocatalysts for zinc-air batteries.?2?In Chapter 3,we developed a simple self-template method for synthesizing N-doped porous carbon materials by directly pyrolysing commercial D-pantothenate calcium?Vitamin B5?in argon.We could adjust the specific surface area and nitrogen doping of N-doped carbon materials by controlling the annealing temperature.When used as an ORR electrocatalyst,the as-prepared 3D porous nitrogen-doped carbon material exhibited good catalytic activity for ORR through an effective 4e^-pathway and had good long-term stability.In the test of primary zinc-air cells,a high power density of 80 mW cm^-2 and excellent stability were achieved under the catalytic action of the catalyst.The unique physical structural features of the 3D porous structure composed of N-doped carbon nanoplatelets promoted excellent electrochemical activity and their application in zinc-air cells.The porous structure of the catalyst provided a suitable electrode/electrolyte interface to promote catalysis.At the same time,in-situ N-doping improved the electron conductivity of the carbon layer and contributed to external defects and a large number of active sites by contributing one or two electrons.In conclusion,the synergistic effect of these factors ultimately lead ed to excellent performance of zinc-air batteries.?3?In Chapter 4,we prepared nitrogen,fluorine and boron ternary doped carbon fiber?TD-CFs?and nitrogen single doped carbon fiber?ND-CFs?materials by simple electrospinning and subsequent heat treatment.The morphologies and stru ctures of TD-CFs were characterized by SEM,TEM,XRD,Raman and XPS test analysis techniques.It was found that the synthesized heteroatom-doped carbon nanofibers had uniform morphology.Moreover,they intersected with each other to form a three-dimensional conductive network structure,which facilitated the transmission of electrons.The doping of heteroatoms could further improve the electronic structure of carbon fibers and form more active sites.Under alkaline conditions,TD-CFs as catalysts exhibited higher electrocatalytic performance than ND-CFs through a highly efficient four-electron transfer mechanism.When metal-free nitrogen,fluorine and boron ternary doped carbon fiber were used as catalysts for rechargeable zinc-air batteries,it showed more excellent catalytic performance than Pt/C-RuO₂ and Vulcan XC-72 carbon black electrocatalysts.After being discharged/charged 130 cycles at a current density of 10 mA cm^-2,no significant voltage decay was observed for rechargeable zinc-air battery with the TD-CFs catalyst,while the zinc-air batteries with Vulcan XC-72 and Pt/C+RuO₂ as catalysts exhibited significant voltage drops only after 100 and 55 cycles.?4?In Chapter 4,we firstly synthesized N,F,and P doped macroporous carbon fibers?NFPC?by dissolving sodium hexafluorophosphate?NaPF₆?and polyacrylonitrile?PAN?in N,N-dimethylformamide?DMF?followed by electrospinning and subsequent heat treatment method.The as-prepared NFPC had a large specific surface area(1230.1 m² g^-1)with more active reaction sites,thereby enhancing its electrocatalytic activity.The NFPC catalyst exhibited excellent ORR and OER performance and could be used as a bifunction catalyst in zinc-air batteries,which delivered a specific capacity of 520 mA h g^-1 at a current density of 10 mA cm^-2.And after 200 cycles,no significant voltage decay was observed in the NPFC-based batteries.These results confirmed that NFPC-based zinc-air batteries had very stable cycling performance.