Synthesis And Electrocatalytic Performance Of Co/Co₃O₄ Encapsulated In Nitrogen-doped Porous Carbon Catalyst

With the rapid growth demand for clean renewable energy,energy conversion and storage devices have gained considerable interest.The oxygen reduction reactions(ORR)and oxygen evolution reactions(OER)are at the heart of many new energy systems including metal-air batteries,fuel cells and water-splitting devices.Preparation of bi-functional catalyst with high catalytic activity has become the focus of improving the performance of new energy systems.However,the ORR catalyst(e.g.Pt)and OER catalyst(e.g.Ir and Ru)have disadvantages of high cost,small reserve,and difficulty in large scale application.Moreover,precious metal catalyst has a single catalytic activity,which cannot satisfy the bi-functional catalytic activity of oxygen reduction and oxygen evolution.Co₃O₄ with bi-functional catalytic activity and nitrogen-doped porous carbon with hierarchical porous structure were compounded to prepare bi-functional catalysts.The research contents as follows:(1)A novel bi-functional catalyst was prepared by encapsulating Co/Co₃O₄nanoparticles into N-doped macroporous/mesoporous carbon(Co/Co₃O₄/NPC)with dual-template method.The catalyst possesses hierarchically porous structure consisting of 3D ordered macropores and mesopores,large specific surface areas(982.4 m²·g^-1),high pore volumes(2.88 cm³·g^-1)and uniform Co/Co₃O₄ nanoparticles enclosed in the carbon walls.After electrochemical tests,the catalyst has excellent ORR catalytic performance,which possesses an ORR half wave potential of 0.84 V,an ORR Tafel slope of 59.7 m V·dec^-1 and a transferred electrons number of ca.4.Co/Co₃O₄/NPC also shows excellent OER catalytic performance,which has a potential of 1.62 V at 10 m A·cm^-2 and an OER Tafel slope of 114.3m V·dec^-1.The overpotential of Co/Co₃O₄/NPC is calculated to be 0.78 V,showing excellent bifunctional performance.In addition,the catalyst exhibits better durability in alkaline solution for ORR and OER compared with commercial Pt/C and Ir/C.(2)A bi-functional catalyst of Co/Co₃O₄ nanoparticles encapsulated in 3D ordered N-doped mesoporous carbon sphere array was synthesized by in situ synthesis strategy from dual-templates method(Co/Co₃O₄/NMCS).The work also explored the effect of different nanoparticle contents on the morphology and electrocatalytic performance of the catalyst.As the content of nanoparticle increases,the catalytic activity increases at the begining and then decreases.The results show Co/Co₃O₄/NMCS-2(nanoparticle content is ca.24%)has hierarchically porous structure with a combination of macropores(the voids between ordered carbon spheres)and mesopores,large specific surface areas(363.5 m²·g^-1),large pore volumes(0.92 cm³·g^-1),and uniform Co/Co₃O₄ nanoparticles encapsulated in carbon layer.After electrochemical tests,Co/Co₃O₄/NMCS-2 has excellent ORR catalytic performance with an ORR half-wave potential of 0.84 V,an ORR Tafel slope of 47.3 m V·dec^-1 and a transferred electrons number of 4.The catalyst also shows excellent OER catalytic performance which has a potential of 1.64 V at 10 m A·cm^-2 and an OER Tafel slope of 76.1m V·dec^-1.The overpotential of Co/Co₃O₄/NMCS-2 is calculated to be 0.80 V,showing superior bifunctional catalytic performance.In addition,Co/Co₃O₄/NMCS-2 exhibits better durability in alkaline solution for ORR and OER compared with commercial Pt/C and Ir/C.In our work,Co/Co₃O₄ nanoparticles encapsulated in the nitrogen-doped carbon with different structures were synthesized by an in-situ synthesis strategy from dual-templates method.The type of catalyst has excellent bifunctional catalytic performance,which could be ascribed to the synergistic effects between encapsulated Co/Co₃O₄nanoparticles and the unique hierarchically porous carbon structure.The former can provide catalytic active sites and the latter can effectively shorten the molecule/ion diffusion length.Moreover,N doping could further furnish active sites.Tightly packed mesoporous carbon as support instead of macroporous/mesoporous carbon could futher improve high mechanical stability and tap density to support more nanoparticles.Catalytic activity and durability were futher improved.