Preparation Of Co/Co₃O₄-Nanocarbon Bifunctional Catalysts For Oxygen Reduction/Evolution Reaction

The sluggish kinetics of air cathodes hinders commercial applications of rechargeable metal-air batteries,thus,the researchers are dedicated to seeking the efficient bifunctional catalysts.Transition metal/metal oxide-nanocarbon hybrid materials exhibit fantastic ORR/OER catalytic performance,becoming the most promising bifunctional catalysts.However,the development of bifunctional catalysts also suffers from some problems:(1)the synethetic methods are complex,enhancing the production cost;(2)the active sites of catalysts cann't be exposed sufficiently,it is necessary to rationally construct the structure of the materials;(3)the interface interaction between active component and active component/support is weak,decreasing the reactivity and stability of the catalysts.To address these problems,the main contents are listed as follows:(1)First,we synthesized Co3O4/NG catalysts via in situ method,g-C3N4 was chosen as two-dimensional(2D)template and cobalt phthalocyanine(CoPc)as the single precursor.However,roating disk electrode(RDE)measurements found that Co3O4/NG didn't show catalytic activity towards both ORR and OER.Thus,we adopted an approach allowing both the Co3O4 and Co/NG to be tailored individually prior to integration,optimizing their structure and electrocatalytic performance.The prepared Co3O4@NG catalysts exhibited comparable ORR activity and superior OER activity compared to 30 wt% Pt/C,giving a current density of 10 mA/cm2 at a potential of 0.76 V vs.SCE.(2)Core-shell Co3O4@NC bifunctional catalysts was synthesized via self-sacrificing template method from metal salts and 2-methylimidazole.Starting with ZnCoZIF as sacrificial template,followed by carbonization,to prepare NC,which serve as the cores and Co-LDH as the shells.Core-shell Co3O4@NC catalysts give electron transfer number of 3.78 and current density of 10 mA/cm2 at a potential of 0.72 V vs.SCE,are outstanding bifunctional catalysts.At the same time,we analyze the bifunctional activity of the catalysts by combining with the structure-activity relationship.(3)Strong coupled and core-shell Co/NC bifunctional catalysts was synthesized via epitaxial growth method,employing metal salts and 2-methylimidazole as sources and ZnCoZIF as template.Introducting the core-shell structure and epitaxial growth method can facilitate mass/charge transport and strengthen interface interaction,promoting the electrochemical activity and stability of the catalysts.The core-shell Co/NC-900-900 shows a diffusion-limited current density of-4.75 mA/cm2 for ORR,and a potential of 0.72 V vs.SCE at the current density of 10 mA/cm2 for OER,which is lower than the 30 wt% Pt/C(-5.58 mA/cm2 and 0.91 V vs.SCE).