Studies On The Strategies For The Improved Activities And Stabilities Of Cobalt Sulfide Based Bifunctional Electrocatalysts

The sluggish kinetics of oxygen reduction reaction and oxygen evolution reaction are the key factors that affect the performance of metal-air batteries.Therefore,it is necessary to develop bifunctional oxygen electrocatalysts with excellent catalytic activities and stabilities.Cobalt sulfide is proposed as one of the most promising bifunctional oxygen electrocatalysts.However,the poor stability of cobalt sulfide in the alkaline environment severely deteriorates the electrochemical properties.In this thesis,we first figured out the failure mechanism of cobalt sulfide catalysts during the catalytic process,and then proposed two strategies（carbon coating and anion doping）to improve the catalytic stabilities and activities.The detailed research is stated below:（1）Polypyrrole-coated cobalt sulfide hollow porous nanosheets（Co₉S₈/PPy HPNS）catalyst for the improved electrocatalytic stability.This work has investigated the deactivation mechanisms of cobalt sulfide catalysts and proposed a surface carbon coating method,which not only greatly improved the long-term stability but also introduced Fe-N-C active sites to enhance the catalytic activity.We first demonstrated that cobalt sulfide is evolved into Co OOH during the catalytic process towards the ORR,in accompany with the collapse of the hollow porous structure,which deteriorates the catalytic activity.To address this issue,we proposed an in-situ polymerization and subsequent pyrolysis method to coat～8 nm polypyrrole（PPy）on the surface of hollow Co₉S₈ to form core-shell Co₉S₈/PPy HPNS catalyst.It is well demonstrated that the surface carbon coating can effectively avoid the structure collapse and the composition change,and thus greatly enhance the catalytic stability.Moreover,the carbon layer can increase the electron conductivity and introduce the Fe-N-C active sites to synergistically improve the catalytic activity.Electrochemical characterizations revealed that the E_1/2 of the Co₉S₈/PPy HPNS catalyst positively shifted by 174 m V,in comparison with pristine Co₉S₈ catalyst.More importantly,the performance of the Co₉S₈/PPy HPNS catalyst is well retained after10000 CV cycles,while the Co₉S₈ HPNS catalyst almost lost the catalytic activity after4 CV cycles.Also,the OER overpotential at 10 m A cm^-2 is 370 m V for the Co₉S₈/PPy HPNS catalyst and showed negligible change after 3000 CV cycles.（2）Cobalt sulfide selenide/nitrogen,sulfur co-doped graphene nanofoam（Co S_xSe_y/N,S-GNF）catalysts for the enhanced bifunctional electrocatalytic activity.Although the carbon coating method can significantly improve the long-term stability of cobalt sulfide-based oxygen electrocatalysts,the catalytic activity,especially OER catalytic activity,was still unsatisfactory.To address this issue,we designed a nanofoam structure that can fully expose the cobalt sulfide as well as Co-N-C active sites for improving ORR catalytic activity.Besides,Se atoms are doped into the cobalt sulfide lattice via the chemical vapor deposition（CVD）method to form metallic Co-Se bonds,which exhibited the strong adsorption of*OOH species and significantly improved the intrinsic OER activity.The Co S_xSe_y/N,S-GNF catalyst showed excellent bifunctional catalytic performance and good stability in alkaline environment.Its ORR half-wave potential is 0.880 V vs.RHE,which is 25 m V more positive than that of Co₉S₈/PPy HPNS catalyst.The OER overpotential at 10 m A cm^-2is 347 m V for the Co S_xSe_y/N,S-GNF catalyst,which is 23 m V smaller than that of Co₉S₈/PPy HPNS catalyst.