| With the development of modern society,people's demand for portable energy is overgrowing,and lithium-air batteries are receiving more and more attention because of the highest energy density among all battery systems.Cathode catalyst materials,as the core component,are being explored by more and more researchers in recent years.CeOx has become a promising candidate because its unique oxygen defect structure can anchor the lithium-oxygen electrochemical reaction.However,the large particle size of bulk CeOx significantly reduces the proportion of oxygen defects on the surface,thereby weakening its catalytic performance.Furthermore,its poor electrical conductivity urges it to modify with nanotechnology and compositing with conductive materials such as mesoporous carbon(MC).This dissertation focuses on the electrochemical reaction path,proposes a reasonable composition and structure consisting of ultra-small and rich-oxygen-defect CeOx and mesoporous carbon,and evaluates their electrochemical performance comprehensively.The main work includes the following two parts.(1)Ultra-small and rich-oxygen-defect CeOx nanocrystals uniformly disperse on the MC nanosheets via the chemical adsorption of Ce3+ions on the corn stover surface,followed by one-step sintering.The CeOx nanocrystals,with an average particle size of 1.98 nm and concurrently high loading of 43.8%,evenly distribute on the MC surface.The CeOx is full of oxygen defects,where x is 1.813 calculated by fitting the XPS peak area.The CeOx/MC exhibits an ultra-high discharge specific capacity of 12753 mAh·g-1 in the ultimate charge/discharge test at the current density of 100 mA·g-1 and the voltage window of 2.5-4.5 V.The cells run stably for 55 cycles under the capacity limitation of 1000 mAh·g-1 at 200 mA·g-1.The excellent catalytic capability of CeOx is due to the complex characteristics of ultrasmall particle sizes,high mass loading,and especially abundant oxygen defects.The oxygen defects provide massive active sites for oxygen redox reactions and adjust the nucleation of insoluble product Li2O2 to form a uniform and excellent distribution.CeOx/MC-X samples,obtained at different calcination temperatures(X=600,700,800,and 900),confirm the effect of temperatures on the morphology,chemical component,and catalytic performance.The increasing temperature increases the CeOx size,enlarges the redox overpotential,and deteriorates the cycling performance.Compared with commercial CeO2,the CeOx/MC-600 has much richer oxygen defects and better catalytic performance.(2)The CeOx/MC-600 composite is modified further by decorating sub-10 nm RuO2 and removing excess carbon substrate.The CeOx/RuO2/C-1:1,obtained at the Ru/Ce molar ratio of 1:1,consists of sub-10-nm CeOx/RuO2 with(>98%)and little carbon material(<2%).The carbon material crosslinks with the oxide nanoparticles to form a self-supporting nanosheet.The rich pore structure facilitates the transmission of reactants and the storage of discharge products,thus contributing to a high specific discharge capacity of 9700 mAh·g-1 in the ultimate charge/discharge test.The cells run stably in the initial 75 cycles under the capacity limitation of 1000 mAh·g-1.The performance improvement is due to that sub-10-nm RuO2 dramatically reduces the charge overpotential.The charge/discharge potential difference is only 1.45 V,lower than that(1.65 V)of the CeOx/MC-600.The effect of the Ce/Ru molar ratio on the final performance is studied.Even at the Ce/Ru molar ratio of 50:1,the Ru utilization is very high,and the electrochemical activity of RuO2 is robust.The electrochemical performance of CeOx/RuO2/C-50:1 is very close to that of CeOx/RuO2/C-1:1.This dissertation proposed a simple preparation method of sub-10-nm Ce-based catalysts and revealed the synergistic effect between CeOx,RuO2,and carbon in the electrochemical environment of lithium-air batteries.These works enrich the catalyst system of lithium-air cells and broaden the design ideas of advanced catalysts. |
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