Preparation And Performance Of Atomically Fe-based Nitrogen Doped Carbon Catalysts For Oxygen Reduction

The slow cathode oxygen reduction reaction（ORR）is one of the major limiting factors for the large-scale promotion of Zn-air batteries.The current best-performing Pt-based noble metal catalysts have the disadvantages of low reserves,high cost,susceptibility to poisoning,and poor stability.Therefore,it is crucial to develop high-activity and low-cost non-precious metal ORR catalysts.In view of the problems of low activity and insufficient stability for non-precious metal catalysts,in this paper,atomically dispersed iron-based nitrogen-doped carbon non-precious metal（Fe-N-C）catalysts with different dimensions（1D,2D and 3D）were prepared.The ORR catalytic performance of these catalysts was improved by optimizing the specific surface area,pore volume and the density of atomic Fe-N_x active sites through structural regulation.The 1D hierarchically porous carbon nanorods（Fe/N-CNRs）catalyst embedded with atomic Fe-N₄ active sites was prepared by in-situ polymerization combined with pyrolysis using Fe₂O₃ nanorods as template.The pyrolysis temperature was optimized（800℃was the best）,and the effects of Fe₂O₃nanorods on morphology,structure,composition and electrochemical properties of the catalysts were investigated.It is demonstrated that Fe₂O₃ nanorods template can be used as the precursor of polymerization initiator(Fe³⁺)and the template to generate 1D structure.Moreover,the residual Fe₂O₃ template after polymerization can prevent the collapse of porous structure and promote the uniform doping of Fe in the carbonization process.In addition,the residual Fe₂O₃ template after pyrolysis can be removed by pickling as the pore-forming agent to improve the porosity of the materials,which can provide unique 1D structure,high specific surface area,hierarchical porous structure and uniformly dispersed atomic Fe-N₄active sites for Fe/N-CNRs catalyst.The above merits lead to excellent ORR activity and stability for Fe/N-CNRs catalyst,including half-wave potential of0.90 V and current retention rate of 89.62%after chronoamperometric responses test for 10000 s.In addition,the Zn-air battery assembled with Fe/N-CNRs catalyst showed excellent discharge performance(Peak power density:181.8 m W cm^-2).The 2D hierarchically porous carbon nanosheets（SAs-Fe/N-CNSs）embedded with atomic Fe-N_x active sites were prepared by simple pyrolysis of D-glucosamine/Fe Zn/g-C₃N₄ complex using Zn and g-C₃N₄ as dual templates.The effects of pickling and Fe doping on the structure,composition and electrochemical performance of the catalysts were studied.It was demonstrated that pickling and Fe doping can produce higher specific surface area and pore volume,more defect sites and Fe-N_x active sites while maintaining the 2D structure of the catalyst,which is beneficial to the improvement of ORR performance.In addition,the mechanism of the Zn/g-C₃N₄ dual templates was studied,and it was demonstrated that g-C₃N₄ can be used as a structure-oriented2D template,and provided rich sites for anchoring Fe atoms.Zn Cl₂ can be used as a self-sacrificing template to produce a hierarchical porous structure through its volatilization and to inhibit the agglomeration of Fe single atoms through steric hindrance effect during pyrolysis.Finally,the Zn/Fe molar ratio in the precursor was optimized,when it was 2/1,the prepared SAs-Fe/N-CNSs catalyst exhibited the highest specific surface area,pore volume and atomic-scale Fe-N_x active site density.Furthermore,it exhibited the best ORR catalytic performance:the half-wave potential was 0.91 V,and the current retention rate after 10000 s chronoamperometry was 92.10%.The discharge performance of the Zn-air battery assembled with SAs-Fe/N-CNSs catalyst is superior to that of the commercial Pt/C catalyst.Using ZIF-8 as self-sacrificing soft template,Nitrogen-doped carbon support with hollow structure（N-CHSs）was prepared by glucosamine-assisted hydrothermal combined with pyrolysis.Subsequently,3D atomically dispersed Fe and N co-doped hierarchically porous carbon（Fe/N-CHSs）catalyst with high Fe loading was prepared by Fe Pc adsorption at room temperature.The effects of glucosamine dosage on the morphology,structure,composition and electrochemical properties of N-CHSs-x carrier were studied.It was demonstrated that when the amount of glucosamine was too low,it was not enough to completely react with the decomposition products of ZIF-8,and the final carrier showed random block morphology.Too high glucosamine dosage will lead to too thick shell and too large particle size for N-CHSs-x carriers.Compared with the XC-72carrier,the Fe/N-CHSs catalyst prepared on the optimized N-CHSs carrier has a unique 3D hollow structure,higher specific surface area and pore volume,and higher density of Fe-N_x active site,showing better ORR activity and stability（half wave potential up to 0.926 V;the electric current retained 91.87%of the initial value after 10000 s timing current test）.After CV tests for 5000 cycles,there was no obvious attenuation of half-wave potential.Furthermore,the Zn-air battery with Fe/N-CHSs possesses better discharge performance,exceeding that of the commercial Pt/C catalyst.