Study On The Preparation And Electrochemical Properties Of Cobalt-based Oxides For Oxygen Electrode

The ever-increasing demands of sustainable energy promote the extensive research on the development of alternative energy conversion and storage systems with high effciency,low cost and environmental friendness.The oxygen evolution and reduction reactions?OER and ORR?are the keypoints of many renewable-energy technologies such as reversible fuel cells and rechargeable metal?metal hydride?-air batteries.However,the sluggish kinetics of these two reactions severely limits the commercialization of these technologies.Therefore,developing electrocatalysts with high ORR/OER activities is the key goal of these technologies.Currently,Pt and its alloys are the irreplaceable electrocatalysts for oxygen reduction reaction,RuO₂ and IrO₂ are the best electrocatalysts for oxygen evolution reaction.But the scarce resources and high costs of these metals limit their commercial applications.Therefore,it is extremely important to develop inexpensive and robust non-precious metal bifunctional catalysts.In this thesis,after the summarization of the current catalysts for oxygen electrode reactions,and based upon the electrocatalytic principles,the cobalt-based bifunctional oxygen electrode catalysts with high electrocatalytic activity were designed,and their corresponding electrochemical performance were systematically studied.The main research results are summarized in following aspects:PrBaCo₂O₅ was selected as a model material to evaluate the effect of Co⁴⁺to the oxygen electrode performance.A series of novel cobalt-based perovskite bifunctional catalysts with high electrocatalytic activity were synthesized through the Sr²⁺doping and Ba²⁺-deficiency strategy.The results indicate that Sr²⁺doping and Ba²⁺deficiency increase both Co⁴⁺?e_g=1?and oxygen vacancy concentrations,and therefore promotes the electrocatalytic activity towards both OER and ORR.The PrBa_0.25Sr_0.75Co₂O_5.95catalyst,with the highest concentration of Co⁴⁺,exhibits the superior electrocatalytic activity.The onset potential,tafel slope and the overpotential at given current density of10.0 mA·cm^-2 for the OER are 1.52 V,75.8 mV·dec^-1 and 0.42 V,respectively,comparable to the currently reported high performance OER catalysts.At the same time,the onset potential,Tafel slope,limiting current density and electron transfer number of PrBa_0.25Sr_0.75Co₂O_5.95.95 for ORR are 0.753 V,58.3 mV·dec^-1,4.081 mA·cm^-2 and n=3.92,respectively,implying much improved ORR activity.A facile route was designed in this study to realize a novel composite material of reduced graphene oxide-encapsulated porous Co₃O₄ yolk-shell nanocage?rGO-Co₃O₄YSNCs?.Through a rational integration of the high conductivity of r-GO together with the promising catalytic properties of the highly porous Co₃O₄,the shortages of Co₃O₄are compensated,and the advantages of both constituents are synergistically combined,therefore the enhanced OER activity and stability are realized.The Tafel slope,current density at over potential of 450 mV and the over potential at 10.0 mA·cm^-2 for rGO-Co₃O₄ YSNCs composite electrode are 84.9 mV·dec^-1,19.9 mA·cm^-2 and 410 mV,respectively.Moreover,the composite electrode exhibits 2.8%attenuation of current density after 500^th cyclic voltammogram tests.These results are comparable to the currently reported high-performance precious metal OER catalysts,which prove that rGO-Co₃O₄ YSNCs composite material is a potential catalyst for oxygen evolution reaction.In order to obtain cobalt-based bifunctional catalyst,a novel material with core-shell Co@Co₃O₄ nanoparticles embedded in N-doped mesoporous carbon rhombic dodecahedrals?Co@Co₃O₄/CN RDCs?was realized by carbonization and subsequent oxidation strategy of cobalt metal-organic framework?ZIF-67?.Owing to the synergetic catalytic effects of Co@Co₃O₄ nanoparticles and N-doped mesoporous carbon,the composite material is endowed with superior bifunctional catalytic activity and strong durability.The catalytic activity of Co@Co₃O₄/CN composite is closely related to the oxidization degree of Co nanoparticles and the content of N-doped mesoporous carbon.When oxidized at 250? in air for 1 h,the formed Co@Co₃O₄/NC-1 composite catalyst exhibits the highest electrocatalytic active surface area,the voltage difference between ORR and OER is 0.79 V,and the current density is only weakly attenuated after 500^thh cyclic voltammogram tests,which is promising bifunctional catalytic activity and durability that surpass the currently reported high-performance bifunctional catalysts,such as Co@Co₃O₄/NC,Co@Co₃O₄/NCNT,Co₃O₄/NPGC and Co₃O₄/PGC.