Hierarchical Pores Modulation Of FeCo Alloy@Nitrogen-Doped Graphene Aerogel For Catalyzing Oxygen Reduction Reaction

Zn-air battery is an efficient energy storage device,which has the strength of low cost,high safety and environmental friendliness,and the performance of which depends on the activity of oxygen reduction reaction(ORR)of air cathode.In recent years,non-noble transition metal(M)-N/C materials have been widely studied as ORR catalysts to replace noble metal Pt which is scarce in the earth.Fe-N/C materials can effectively catalyze ORR compared with Pt/C.Compared with single transition metal Fe-N/C catalysts,the interaction between FeCo alloy and carbon in FeCo alloy-N/C catalysts is beneficial to improve the electronic conductivity and ORR catalytic activity.Among all carbon materials,N-doped graphene aerogel(NGA)can be used to fix and homogeneously disperse FeCo alloy due to its large specific surface area,hierarchical pores and N sites which can anchor metals.In addition,the macro-,meso-and micropores in proper proportion can maximize mass transfer rate and reaction efficiency.Therefore,if the FeCo alloy can be fixed and homogeneously dispersed on NGA,the highly conductive co-assembly FeCo@NGA with stable structure can be obtained,and the catalytic activity of ORR will be expected to be further improved by modulating the hierarchically porous structure of FeCo@NGA.In this thesis,FeCo@NGA was prepared by controlling reaction conditions.Two methods were proposed to modulate pore size of FeCo@NGA by separately adjusting the lateral size and degree of pore defects of graphene oxide(GO),the precursor of NGA.The relationships between the optimization of pore structure and improvement of the catalytic activity of ORR as well as the performance of Zn-air battery were studied.The catalytic mechanism of FeCo@NGA was studied by in-situ XRD.This study proposes new ideas for the pore size modulation of graphene-based non-noble metal ORR catalysts with hierarchical pores and paves a new way in design and assembly of electrocatalysts for energy storage devices.In chapter 3,the co-assembly of homogeneously dispersed FeCo alloy on NGA was prepared by controlling the pyrolysis temperature,and using pure H2 to shorten the reaction time.FeCo alloy was fixed on NGA by chemical bonding with N,which can improve the electronic conductivity(Rs=3.31Ω,Rct=0.80 Ω)and also conducive to structural stability and performance stability in long-term electrochemical process.The modulation of hierarchically porous structure of FeCo@NGA(GO-P-x)(x=0,4,8)by adjusting the lateral size of GO-P-x(x=0,4,8)(GO obtained with H3PO4 in the preparation process)was studied.The size and the oxidation degree of GO-P-x(x=0,4,8)were controlled by sonication.With the increase of sonication time,the size of GO-P-x(x=0,4,8)decreased and the oxidation degree improved.The GO-P-8 obtained with sonication time of 8 h had the smallest size of 0.08-1.20 μm.With the decrease of the size of GO-P-x(x=0,4,8)and the increase of the oxidation degree,the specific surface area and the number of micropores of FeCo@NGA(GO-P-x)(x=0,4,8)increased,the average pore diameter and pore volume decreased,and the total N content,especially the pyridine N and the content of Fe/Co-N active sites increased.The optimized pore structure can facilitate the rapid transportation of reactant O2 to the fully exposed active sites during ORR.Therefore,the FeCo@NGA(GO-P-8)with the largest specific surface area and optimized pore structure had better ORR catalytic activity.The onset potential(Eonset)was 0.88 V and the diffusion-limited current density(jd)was 5.39 mA cm-2.The Zn-air battery with FeCo@NGA(GO-P-8)as air cathode also exhibited high power density,specific capacity,energy density and good cycling stability(the charge-discharge voltage gap increased by only 0.06 V after 1690 cycles at a current density of 10 mA cm-2).In chapter 4,the effect of the degree of pore defects of GO on the pore structure of FeCo@NGA was studied.GO with different pore defects was obtained with or without the graphene skeleton protector H3PO4 in the GO preparation process.At the same sonication time,GO-8 prepared without H3PO4 had more pore defects than GO-P-8 with H3PO4,which is beneficial to the incorporation of more pyridine N in the subsequent reaction,thus more Fe and Co were fixed to obtain more Fe/Co-N ORR active sites.Due to more pore defects in GO-8,FeCo@NGA(GO-8)had larger specific surface area(444.3 m2 g-1)and more micropores,which is beneficial to the full exposure of active sites.Compared with FeCo@NGA(GO-P-8),FeCo@NGA(GO-8)of larger specific surface area with more pyridine N and Fe/Co-N had excellent ORR activities with the Eonset of 0.90 V and the jd of 5.58 mA cm-2 as well as high performance of Zn-air battery with 818.1 mAh gZn-1 of specific capacity and 1040.1 Wh kgZn-1 of energy density.