| With the rapid development of the world economy,the demand for energy from various countries is also increasing.However,in recent years,global environmental degradation has increased,pollution has increased,and energy shortages.Faced with these problems,people have realized the importance of developing new energy sources.Therefore,the development of green,efficient,low-cost and sustainable new energy sources has increasingly attracted the attention of researchers.Fuel cell and metal air battery are new energy batteries with high efficiency,which have the advantages of green,zero pollution and no noise.Oxygen reduction reaction?ORR?and Oxygen evolution reaction?OER?,as the two most important semi-reactions in fuel cell and metal air cell,involve four-electron transfer process and the kinetic reaction is very slow.It is imminent to study efficient and durable electrocatalysts.At present,there are still many deficiencies in the research and development of efficient electrocatalysts and the understanding of catalytic mechanism.ORR and OER are mainly based on precious metal catalysts?such as ORR:Pt/C;OER:Ir O2/Ru O2?.However,their disadvantages such as low reserves,high cost and poor stability have hindered their large-scale application.Compared with precious metals,carbon materials,transition metals,and sulfur are abundant,low cost and easy to be popularized,making them ideal for commercial electrocatalysts.As a kind of half-battery and semi-fuel cell,zinc-air battery has attracted people's attention because it has many advantages such as high specific energy?theoretically value:1350 Wh/kg?,large energy density,rich reserves,and smooth discharge curve and so on.In this thesis,carbon-based materials were used as precursors to effectively combine carbon-based materials with transition metal sulfides by sol-gel method,and different carbon-based transition metal sulfides composite aerogel nanomaterials were prepared,which were applied to OER,ORR and zinc-air batteries respectively.The synthesis strategy is simple and easy to generalize.The specific research contents are as follows:?1?Ni3S4 particles were embedded in N,P co-doped honeycomb porous carbon aerogels by hydrogel method.First,the sol-gel method was used to polymerize chitosan,sodium tripolyphosphate,guanidine polymer,and Ni2+precursors,Ni2+is uniformly doped in the gel matrix,after freeze-dried,it was carbonized in an inert atmosphere to obtain Ni3S4/N,P-HPC.This synthesis method solves the difficulty of synthesizing pure Ni3S4 phase due to the poor stability of Ni3S4 at high temperature,and has a good control effect on the porous structure and the co-doping of N and P of the carbon aerogel.The synergistic effect between the structural advantages of N,P co-doped carbon aerogels?such as accessible active sites,high specific surface area and excellent electron transport capacity?and the inherent electrochemical properties of Ni3S4 enables Ni3S4/N,P-HPC to have excellent electrocatalytic performance.The overpotential at 10 m A·cm-2 is only 0.37 V,which is close to commercial Ru O2?0.36V?.The current density at 1.65 V is 23.7 m A·cm-2,which is much larger than the comparative samples Ni/N,P-HPC(2.9 m A·cm-2),N,P-HPC(1.6 m A·cm-2),and commercial Ru O2(14.3 m A·cm-2).The Tafel slope is 96 m V·dec-1,which is basically consistent with Ru O2.It is shown that during the OER reaction,Ni3S4/N,P-HPC is conducive to electron transport,thereby improving electrocatalytic performance.?2?Doping transition metals into carbon aerogels is an effective method to develop highly active and robust bifunctional oxygen electrocatalysts.Based on the previous work,we continue to use a new hybrid hydrogel method to prepare three-dimensional porous carbon aerogels doped with Co9S8.In our strategy,chitosan,sodium tripolyphosphate and polyhexamethylguanidine phosphate were polymerized effectively by sol-gel method.The repeat unit of the polymer contains a binding site?-NH2?with Co2+ions.After pyrolysis,it can ensure the uniform anchoring of Co9S8particles in the carbon aerogel.Due to the significant synergy between Co9S8 and the three-dimensional porous N,P co-doped carbon aerogel,this catalyst has excellent bifunctional electrocatalytic performance for ORR and OER.Electrochemical tests showed that the initial potential and half-wave potential of the catalyst in ORR were Eonset=0.89 V and E1/2=0.78 V,respectively,and the overpotential at 10 m A·cm-2 in OER was only 363 m V.Tafel slope(ORR:90.2 m V·dec-1,OER:94.7 m V·dec-1)proves that the catalyst has faster reaction kinetics.Different from previous studies on Co9S8,this time we also verified that the activity of Co9S8 material on OER is higher than ORR through density functional theory?DFT?calculation.?3?The calcination-hydrothermal method is used to prepare graphene aerogels anchored with Ni3Fe-Co9S8 particle bifunctional electrocatalyst.A bifunctional air cathode catalyst was assembled using the catalytic properties of Ni3Fe to OER and Co9S8 to ORR.The synergistic effect between multiple components was significantly better than that of single component Ni3Fe/r GO and Co9S8/r GO.At the same time,the additional synergy between the porous r GO aerogel and the Ni3Fe-Co9S8 porous composites endows the composites with more exposed active sites,which is conducive to faster electron/ion transmission rate and better structural stability.Thanks to reasonable material selection and structural design,Ni3Fe-Co9S8/r GO not only has excellent ORR activity(Eonset=0.91 V and E1/2=0.80 V),but also has excellent OER activity with an overpotential of 0.39 V at 10 m A·cm-2.After assembled into a rechargeable zinc-air battery,with a high open-circuit voltage(1.45V vs.Zn/Zn2+)and the peak current density up to 125 m W·cm-2,which is significantly better than a zinc-air battery driven by Pt/C and Ru O2(112 m W·cm-2).The charge-discharge cycle test was performed at 10 m A·cm-2,and the zinc-air battery driven by Ni3Fe-Co9S8/r GO operated stably for 4000 minutes.All test results show that the composite catalyst has excellent chargeability,fast dynamic response,and great practicality. |
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