| Zn-air battery is a very competitive power source because of its high energy density,ecofriendliness,low cost,and high safety.However,the sluggish kinetics of oxygen reduction reaction(ORR)during discharge on the cathode limits the application.The commonly used Ptbased catalyst has high cost,so it is necessary to develop efficient and cheap non-noble metal catalysts for ORR.Hierarchical pores are important for battery performances such as power density and specific energy because mass transfer processes require optimized pore size distribution with macropores for fast mass transfer,and micropores for sufficient utilization of materials.Hence,reasonable regulation of pore size distribution is of great significance to energy storage and conversion equipment.Nitrogen-doped graphene aerogel(NGA)has hierarchical pores and large specific surface area to maximize mass transfer and reaction efficiency.The bimetallic alloy catalysts exhibit superior activity compared with their individual hybrids,and the synergistic effect of different metals can promote catalytic reactions.NGA can anchor NiCo alloy nanocrystals through the N-sites and load NiCo alloy with high ORR catalytic activity.It can not only improve the intrinsic activity,but also maintain the porous structure and large specific surface area,due to the support of nanocrystals.In this process,changing the metal content is expected to optimize the pore size distribution and improve the catalytic efficiency of ORR by regulating the size and number of alloy nanocrystals without complex templates and additives.Therefore,NiCo alloy nanocrystals@NGA(NiCo@NGA)coassemblies with different metal content were constructed in this thesis.By separately adjusing the total metal content while keeping the Ni/Co ratio,and adjusing the Ni/Co ratio while keeping the total metal amount,the pore size distribution,electronic structure of the coassemblies were optimized,and the adsorption of O2 and reaction intermediates on NiCo@NGA was optimized.The coassemblies were studied by comparing the composition,morphology and structure with ORR catalytic activities including kinetic current density,and Zn-air battery performances including power density.To further understand the main active sites in the ORR process,in situ XRD and in situ Raman spectroscopy were performed,and the possible reaction mechanism was proposed.In chapter 3,with GO as the precursor and dopamine(DA)as the N source and crosslinking agent,the NiCo@NGA coassemblies were prepared by loading NiCo alloy nanocrystals uniformly on NGA by a hydrothermal method and subsequent thermal process.With the increase of metal salt added in the preparation process,the diameter of the nanocrystals formed gradually increases from 20 nm to 50 nm,and the number of alloys increases continuously.NiCo alloy nanocrystals and NGA play synergistic roles for the performances.The NiCo@NGA 5 is the best among all NiCo@NGA coassemblies due to(1)the pore size distribution of NiCo@NGA 5 is the optimized,and the number of micropores and mesopores is increased,which facilitates the transport of reactants(such as H2O and O2)in the electrochemical process,and accelerates the mass transfer and reaction rate;(2)NiCo alloy can improve the electronic conductivity of coassembly and facilitate the rapid electron transfer;(3)the doping of N in NGA reduces the adsorption energy of O2;(4)both NiCo alloy and NGA have ORR activity,which provides abundant active sites for ORR process;(5)the chemical bonds between NiCo alloy and NGA are beneficial to maintain the hierarchically porous structure and improve the stability of structure and properties.So the initial potential(Eonset),half-wave potential(E1/2)and diffusion-limited current density(jd)of NiCo@NGA 5 are 0.97 V,0.80 V,and 5.90 mA cm-2,which are close to commercial Pt/C.Tafel slope(53 mV dec-1)and long-term stability(90.8%current retention after 50000 s test)are better than Pt/C(55 mV dec-1,67.8%current retention after 50000 s test)and other NiCo@NGA with different metal content.The intensity of NiCo alloy diffraction peaks was observed by in situ XRD,which proves that alloys are the main active sites of ORR process.The characteristic peaks at 451,691 and 730 cm-1 were observed by in situ electrochemical Raman spectroscopy,corresponding to CoⅡ-O,CoⅢ-O and OOH intermediates,respectively.The possible reaction mechanism was proposed by monitoring the changes of the peaks.Zn-air battery with NiCo@NGA 5 as air cathode has high power density(149.5 mW cm-2),good rate performance(higher discharge voltage at the same current density),high specific capacity(804.6 mAh gZn-1)and specific energy(995.3 Wh kgZn-1),and long-term stability for up to 800 charge and discharge cycles.In chapter 4,NixCoy@NGA coassemblies with optimized electronic structure,large specific surface area and hierarchical pores distribution were prepared by a one-pot method.The appropriate ratio of Ni/Co and the strong interaction between alloy and NGA make NiCo alloy exhibit appropriate binding energy and give good electronic conductivity to the coassembly.The series resistance(Rs)and charge transfer resistance(Rct)of Ni1Co3@NGA are 1.9 and 1.6 Ω,respectively.The Eonset,E1/2 and jd of Ni1Co3@NGA for ORR are 0.99 V,0.81 V,and 6.22 mA cm-2,which are close to Pt/C(0.98 V,0.82 V and 6.41 mA cm-2).Tafel slope(50 mV dec-1)and stability(91.2%current retention after 50000 s test)are better than Pt/C.The liquid phase Zn-air battery with Ni1Co3@NGA as air cathode has a power density of 161.2 mW cm-2,a specific capacity of 806.1 mAh gZn-1,and a specific energy of 1023.6 Wh kgZn-1.After 1000 charge and discharge cycles,the voltage gap does not change significantly,which is better than the liquid phase Zn-air batteries with Pt/C+IrO2 and transition metal/graphene composites reported as air cathodes.For solid-state Zn-air batteries,Ni1Co3@NGA also performs better than Pt/C+IrO2,proving its feasibility in portable electronic devices.This work provides a new idea for the structural design of transition metal/carbon-based ORR catalysts and guides application in energy storage and conversion devices such as Zn-air battery. |
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