[Co（NH₃）₆]²⁺-derived Zero And Two-dimensional Cobalt Sulfide Electrocatalysts For Oxygen Reduction And Evolution Reactions

One of the main challenges of in the field of rechargeable metal-air battery is developing highly efficient and stabile bifunctional oxygen electrocatalysts.Transition metal sulfides-based catalysts were considered as one of the most promising bifunctional catalysts to replace noble metal catalysts（Pt,Ru and Ir）for catalyzing the oxygen reduction and oxygen evolution reactions.In this thesis,we have reported the synthesis of zero-and two-dimensional cobalt sulfide-based catalysts towards the oxygen reduction and evolution reactions via the[Co（NH₃）₆]²⁺-induced polymerization of dopamine precursors and subsequent sulfidation processes.During the polymerization process,dopamine molecules are spontaneously polymerized into polydopamine in the presence of[Co（NH₃）₆]²⁺,and cobalt species are simultaneously attached on polydopamine via strong coordination interaction.This polymerization process can increase the metal loading in the catalysts and inhibit the self-aggregation of metal sulfides during the subsequent sulfidation process.We have investigated the influence of the acid etched time,cation doping and solvents on the structures and compositions of the catalysts,and further studied the electrochemical performance.The details are described as follows:（1）Preparation of Ni-doped Co₉S₈/nitrogen,sulfur co-doped carbon nano-hydrangea for catalyzing the oxygen reduction and evolution reactionsNickel-doped Co₉S₈/nitrogen,sulfur co-doped carbon nano-hydrangea（Ni-Co₉S₈/N,S-CNH）with the controllable metal loading and Ni/Co ratio was successfully synthesized.It’s demonstrated that the main active sites for catalyzing the oxygen reduction reaction（ORR）and oxygen evolution reaction（OER）are Co₉S₈ nanocrystals,and Ni doping can enhance the intrinsic ORR and OER activities.The ORR half-wave potential of oxygen reduction was optimized to 0.872 V（vs.RHE）,and corresponding mass activity was achieved to 7.16 A mg^-1_Co/Niat 0.90 V.The OER overpotential at 10 m A cm^-2 was decreased to 318 m V by optimizing the pore volume and Ni doping content.The improved ORR and OER performances are mainly ascribed to the following factors:1)Doping Ni atoms in Co₉S₈nanocrystals increased the electron density around Co and changes the adsorption and desorption energies of oxygen-containing intermediate;2)More and more defects were formed with introducing Ni atoms into Co₉S₈ crystal lattices,which promotes the intrinsic activities;3)The hierarchical structure with large specific surface area and high pore volume is in favor of exposing the catalytic active sites and ion transport.In addition,the most optimized Ni_0.14-Co₉S₈/N,S-CNH-24h as the air cathode for Zn-Air battery delivered the discharge power density and specific capacity of 96.3m W cm^-2 and 783.0 m Ah g^-1,respectively.The charge-discharge voltage gap slightly increased from 0.89 V to 0.98 V even after 220 h continuous operation at 5 m A cm^-2.（2）Preparation of ultra-thin two-dimensional sheets assembled by cobalt sulfide/nitrogen,sulfur co-doped carbon nanospheres for catalyzing the oxygen reduction reactionIn this chapter,we have reported the solvent engineering towards the synthesis of cobalt sulfide/nitrogen,sulfur co-doped carbon catalysts using[Co（NH₃）₆]²⁺and dopamine precursors via the above mechanism.It was found that ethanol can inhibit the polymerization of dopamine,leading to the formation of cobalt sulfide/nitrogen,sulfur co-doped carbon nanospheres(Co_1-xS/N,S-CNB)in water and ethanol mixed solvents.[Co（NH₃）₆]²⁺and dopamine molecules are polymerized at the water-carbon tetrachloride interface,thus forming cobalt sulfide/nitrogen,sulfur co-doped carbon nanosheets(Co_1-xS/N,S-CNS).Ultra-thin two-dimensional sheet assembled by cobalt sulfide/nitrogen,sulfur co-doped carbon nanospheres are formed in the mixed water,ethanol and carbon tetrachloride,since ethanol inhibits the polymerization of dopamine at the water-carbon tetrachloride interface.U-SA-Co_1-xS/N,S-CNS showed the specific surface area of 235.82 m² g^-1and the pore volume of 0.176 cm³ g^-1,much larger than the other two structures,and have high cobalt sulfide loading,resulting in exhibit excellent ORR performance(E_1/2=0.868 V vs.RHE).Moreover,this method is successfully explored for synthesizing honeycomb cobalt sulfide/nitrogen,sulfur co-doped carbon structures and cobalt sulfide/nitrogen,sulfur co-doped carbon nano-hollow hemispheres.