Study Of Mo₂C As Bifunctional Catalysts For Li-Air Batteries

Lithium-air batteries?LABs?have already attracted much attention as one of highly efficient electrochemical energy storage systems due to high specific energy density(5200Wh kg^-1,including oxygen)and environment friendly.However,because of sluggish dynamics of oxygen reduction reaction?ORR?in discharging process and oxygen evolution reaction?OER?in charging process,there are some drawbacks of LABs,such as low discharge/charge specific capacities,rate capability and bad cycling stability.According to these bottleneck problems,we aimed to develop novel ORR/OER bifunctional catalysts of transition metal carbides?Mo₂C?,and systematically study its ORR/OER catalytic activities in 0.1 M KOH solutions,then explore fabricating method to prepare new binder-free air electrodes and discuss their influence on the performance of LABs.In chapter 3,we have in-situ prepared Mo₂C-C hybrids microspheres with carbon microspheres as template and reactant through a simple hydrothermal process.Through controlling the content of MoCl₅,Mo₂C-C hybrids microspheres with different morphologies could be obtained,such as core-shell structure,yolk-shell structure and hollow structure etc.Rotating-ring-disk electrode measurements show that the obtained Mo₂C-C hybrids exhibit excellent ORR/OER bifunctional activities and stabilities,especially,yolk-shell structured Mo₂C-C hybrids microspheres have the best catalytic performance in all Mo₂C-C hybrids microspheres,which better ORR/OER catalytic stabilities than that of commercial Pt/C and RuO₂ catalysts.The better catalytic activities and stabilities of the yolk-shell structured Mo₂C-C hybrids microsphere should be attributed to appropriate Mo₂C content to provide enough strength;the unique yolk-shell structure and high special surface area to provide enough active sites for the reactions;the non-crystalline amorphous MoO_x nano-film to enhance conductivity.In chapter 4,we have reported our findings in exploration of a binder-free electrode based on Mo₂C nanoparticles grown on carbon cloth?Mo₂C@CC?,derived from a facile infiltration and high-temperature etching process.When tested in a Li-O₂ battery?LOB?,the Mo₂C@CC electrode(with a Mo₂C loading of 0.3 mg cm^-2)demonstrates excellent rate capability(achieving discharge capacity of 7,646,9,751,and 11,853 mAh g^-1 at a rate of 200,500,and 1000 mA g^-1,respectively)while maintaining good cycle stability(for over 700 h at a rate of 500 mA g^-1 with a cut-off capacity of 500 mAh g^-1).This new electrode architecture opens a promising avenue for the development of high-performances LOBs through optimizing the electrode microstructure.The high performances of the Mo₂C@CC electrode are ascribed to the unique porous structures of the carbon fibers in the CC,which is critical to trapping Mo₂C nanoparticles in the pores on CFs surface to avoid aggregation and exfoliation during extensive charge/discharge cycling.Further,the large pores within the CC allow fast Li⁺ion transport and oxygen diffusion;it can also accommodate more insoluble discharge products,increasing the rate capability and capacity.This new cathode architecture opens a promising avenue for the development of high-performance LOBs electrode through optimization of electrode microstructure.In chapter 5,in order to further explore and validate the feasibility of binder-free electrode,we have prepared MoP@CC air electrode with carbon cloth?CC?as current collector,?NH₄?₆Mo₇O₂₄·4H₂O and NaH₂PO₂·H₂O as reactants through facile hydrothermal and high-temperature phosphating process.LOBs based on MoP@CC air electrode show better discharge/charge specific capacities,lower overpotential and higher coulombic efficiency than that based pure CC electrode.Especially,MoP@CC air electrode based LOBs demonstrate an actual discharge capacity as high as 2.448 mAh at an actual current of 100?A.Moreover,the battery shows good discharge/charge stability?for over 400 cycles at a rate of 100?A with a cut-off capacity of 0.25 mAh?with only a 9.0%dropping of discharged voltage.All results indicate that binder-free MoP@CC air electrode could be used in efficient LOBs.