Investigation Of Energy Conversion And Storage Performance Of Cobalt Based N-doped Carbon Composites

Transition metal integrated carbonaceous materials have been widely used as the active materials in energy conversion and storage devices due to their excellent electrochemical performance.So far,cobalt based carbon composites have been significantly studied in fuel cells and lithium-ion batteries.Generally,efficient and controllable methods are favored to readily design,prepare and control the morphologies of the cobalt based carbon composites which play crucial roles in determining their electrochemical performance.In the present thesis,two cobalt based carbon composites are reported.The factors in controlling the morphologies of the resulting products and their electrochemical performance have been systematically investigated.1)Catalytic performance of Co-CoO@NC/NC for oxygen reduction reaction?ORR?:A hierarchical structure catalyst comprising N-doped carbon shell coated uniform sized Co-Co O nanoparticles?Co–Co O@NC/NC?supported on N-doped carbon nanosheets has been produced via calcination of a the dried mixture consisting of Co?NO3?2,glucose,and urea.The obtained Co–Co O@NC/NC is found to be highly active for the ORR.Specifically,the Co–Co O@NC/NC exhibits an ORR onset potential of 0.961 V vs.RHE and a half-wave potential of 0.868 V vs.RHE.Both the onset and half-wave potentials are higher than most of catalysts reported previously and even close to the commercial Pt/C?one of the best ORR catalysts reported to date?.In addition,the Co–Co O@NC/NC also shows high stability towards the ORR.These results indicate that the Co–Co O@NC/NC are suable as a promising electrocatalyst for oxygen reduction.Systematic investigations show that the calcination temperature has a profound effect on the performance of Co–Co O@NC/NC.Particularly,the sample synthesized at the calcination temperature of 800 oC shows impressive electrocatalytic performance for the oxygen reduction reaction.Urea is demonstrated to be necessary for preparing uniform sized Co-Co O nanoparticles and improving the electrocatalytic activity of the Co–Co O@NC/NC.Most importantly,there exists a strong electronic interaction between the Co-Co O nanoparticles and the N-doped carbon,which plays an important role in the high performance of the Co–Co O@NC/NC.2)Lithium ion storage performance of Co?OH?₂ hexagonal nanoplates:Hexagonally shaped Co?OH?₂ nanoplates have been synthesized through a simple hydrothermal method.The hexamethylenetetramine?HMTA?and p H value of the reaction solution are founded to be crucial in controlling the morphology of the product.The obtained Co?OH?₂ nanoplates are then transformed to Co O through the calcination and subsequently coated with the carbon,forming the Co O/C composite with a core/shell structure.When use in the LIBs,the Co O/C exhibits a high reversible specific capacity of 882 m Ah g-1.The high performance of the Co O/C arises its plate structure,which shortens the diffusion length of Li+ and electron and reduces the inner resistance of the Co O/C.The result suggests that the Co O/C is a promising and efficient material for LIBs.