Synthesis And Electrocatalytic Activity Of Co₃O₄ Hollow Polyhedrons

With the increasing of energy demands and fossil fuels depletion,it is important to develop energy storage and conversion technologies.Oxygen reduction reaction?ORR?and oxygen evolution reaction?OER?play important roles in energy transform fields,but traditional electrocatalysts are precious metals and their metal oxides.Exorbitant price of these electrocatalysts limit the use of ORR and OER in practical applications,so it is import to develop highly efficient and inexpensive electrocatalysts.Metal-organic frameworks material is a class of inorganic-organic hybrid materials with porous crystalline characteristic.This material has the advantages of large surface areas,high porosity and tunable pore sizes.MOFs are extended crystal structure built with metal ions and electron-donating organic ligands.Utilizing MOFs as templates,MOFs-based inorganic materials have been synthetized by thermal decomposition.These materials have the advantages of large surface areas and high porosity as MOFs.By this method,we can transform MOFs into metal oxides and metal/Carbon hybrid nanomaterials.These materials have been widely used in chemical sensors,lithium ion battery and electrocatalysis fields.The thesis mainly contains two parts:1.Preparation and characterization of the Co₃O₄ hollow polyhedrons.Using methanol as solvent,by mixing Co?NO₃?₂·6H₂O and 2-methylimidazole,Co-based MOFs?ZIF-67?with rhombic dodecahedral structures were synthesized.After annealing ZIF-67 in air,Co₃O₄ hollow polyhedrons were prepared.Co₃O₄ hollow polyhedrons exhibit high porosity?28.53 nm?and large specific surface areas(49.27 m² g^-1).Also,this kind of materials shows small electrochemical impedance,suggesting fast electron conduction and diffusion ability.Because of the diversity of MOFs,this method by annealing MOFs can present an idea to synthesize other metal oxides with porous hollow structures.2.ORR and OER performances of the Co₃O₄ hollow polyhedrons.Compare with particulate Co₃O₄,Co₃O₄ hollow polyhedrons show earlier onset potential and higher current density(the peak current density for ORR is 0.61 mA cm^-2,the onset overpotential for OER is 436 mV and the current density is 10 mA cm^-22 at the overpotential of 536 mV).Moreover,the methanol-tolerance and durability of Co₃O₄hollow polyhedrons are better than the commercial Pt/C?Co₃O₄ hollow polyhedrons maintain 82.6%of the original activity after 25 000 s?.With higher porosity and larger specific surface areas,the Co₃O₄ hollow polyhedrons can provide more electrocatalytic active sites to catalyze ORR and OER.Also,this kind of porous hollow polyhedral structures can increase more accessible channels to improve ORR and OER activities by four-electron pathway to promote electron diffusion and conduction.