Design,Synthesis And Electrocatalytic Performance Of Porous Iron/Cobalt-Nitrogen-Carbon Composites

The transitional consumption of traditional fossil energy has brought about energy crisis and many environmental pollution problems.The development of alternative new energy storage devices is an important strategy to solve this problem.The zinc-air batteries are considered to be one of the most promising electrochemical energy storage devices due to their advantages of high energy density,environmental friendliness,and low cost.However,the sluggish electron transfer process at the air electrode severely restricts the further development of zinc-air batteries.Noble metal platinum-based catalysts and iridium dioxide/ruthenium dioxide are excellent oxygen reduction reaction（ORR）and oxygen evolution reaction（OER）catalysts,but their high price,scarce reserves and short service life are not conducive to the commercial application of zinc-air batteries.The transition metal-nitrogen-carbon materials and metal sulfides have well catalytic performance on ORR and OER,respectively.And their low cost and abundant reserves make them have the potential to replace noble metals as air electrode catalysts for zinc-air batteries.Therefore,this paper prepared porous transition metal-nitrogen-carbon materials as well as metal sulfide/transition metal-nitrogen-carbon composites using high-temperature pyrolysis methods,and studied their electrocatalytic properties.The main contents and conclusions are as follows:（1）Graphene-like mesoporous iron and nitrogen co-doped carbon materials（Fe₃C/Fe-N-C）were prepared by high-temperature pyrolysis using polyaniline and FeCl₃ as raw materials.The results show that FeCl₃ is very important for forming two-dimensional graphene-like morphology,and graphene-like morphology cannot be formed without adding FeCl₃.Fe₃C/Fe-N-C exhibited excellent ORR activity under alkaline conditions.Its initial potential,half-wave potential and limiting current density were 0.95 V,0.80 V and 6.0 m A cm^-2,respectively,far better than N-C catalyst.Its excellent electrocatalytic performance is due to the doping of Fe improves the surface defects of carbon materials and forms a highly catalytically active Fe-N_xmoieties,which promotes the ORR catalytic process.（2）Porous Fe-N-C nanospheres were prepared by using carbon spheres prepared from glucose as the carbon source,melamine as the nitrogen source,FeCl₂ as the iron source,and KOH as the activating agent.The experimental results found that the doping of Fe is essential to maintain the original morphology of the carbon nanospheres,which can prevent the carbon sphere precursor from sintering during high temperature,and is beneficial to increase the specific surface area of the carbon material.KOH treatment can not only promote the formation of micropore structure and increase the specific surface area of carbon materials,but also increase the surface defects,which is conducive to improving the catalytic activity of ORR.The Fe-N-C porous nanospheres with the highest ORR activity can be obtained by adjusting the doping amount of Fe.The Fe-N-C porous nanospheres with the optimum Fe doping have better ORR catalytic performance than commercial Pt/C.The initial potential,half-wave potential,and limiting current density were 1.0 V,0.88 V,and 5.6 m A cm^-2,respectively,and the electron transfer number was 3.88,the yield of hydrogen peroxide was5.8%,very close to the Pt/C catalyst.In addition,the kinetic reaction rate and catalytic stability of Fe-N-C porous nanospheres are better than commercial Pt/C.Various characterization and test results indicate that the Fe-N_x moieties are the catalytic active site of Fe-N-C materials.（3）Using carbon spheres as the carbon source,CoCl₂ and FeCl₂ as the metal sources,melamine as the nitrogen source,Na₂S as the sulfur source,and KOH as the activating agent,Co_9-xFe_xS₈ nanoparticles supported on Co,Fe-N-C porous nanospheres(Co_9-xFe_xS₈@Co,Fe-N-C)were synthesized by one-step high-temperature solid-phase method.The experimental results show that Co_9-xFe_xS₈@Co,Fe-N-C formed by high temperature has very good crystallinity.And after being treated by KOH,it has a rich microporous structure and a large specific surface area,which provides efficient mass transfer channels during OER and ORR catalytic processes.By adjusting the doping amount of Fe,a bifunctional catalyst with the highest performance can be obtained.0.2-Co_9-xFe_xS₈@Co,Fe-N-C has the best bifunctional activity,and its OER overpotential at 10 m A cm^-2 is 343 m V,better than the noble metal Ir O₂,the half-wave potential of ORR is 0.81 V,slightly worse than the commercial Pt/C,and the difference of OER and ORR metrics is 0.76 V.Through electrocatalytic performance tests,it was found that Co_9-xFe_xS₈ is the catalytic active site of OER,and Co/Fe-N is the main catalytic active site of ORR.The synergistic catalytic effect between Co_9-xFe_xS₈ and Co,Fe-N-C Produce better catalytic effect.