The Preparation,Modification And Electrocatalytic Properties Of Cobalt-based Oxides

Oxygen reduction and evolution reactions（ORR and OER）are the key reactions on the air electrode of Zn-air batteries.Currently,Pt-and Ru-based noble metal materials are widely applied as the commercial catalysts,but there are still several problems such as high cost,poor stability,and single catalytic performance.It is urgent to develop alternative high-performance non-precious metal catalysts.Transition metal oxides are considered as the promising alternatives for OER/ORR electrocatalysts due to their low price and good redox capacity.However,this type of catalyst also has the drawbacks including inadequate catalytic activity and poor electrical conductivity.How to enhance and optimize their electrocatalytic performances has become a common concern in the research community.In this work,ZIF-67 organic template was selected as a precursor to prepare spinel-type Co₃O₄catalysts,and the physicochemical properties of Co₃O₄ catalyst can be modified by doping metal ions(Ru³⁺,Mn²⁺)and non-metal atoms（P）to improve its ORR/OER electrocatalytic performances.Moreover,the potential application of this material was evaluated by a self-assembling Zn-air battery.This work mainly includes the following three parts:（1）Ru@Co₃O₄ composite metal oxides with different Ru doping amounts were prepared by modulating the dosage of RuCl₃.Material characterizations results showed that the introduction of Ru can induce the generation of more crystal defects and optimize the microstructure and surface active composition.The Ru@Co₃O₄-1.0 catalyst prepared with the addition of 1 mL RuCl₃ solution exhibited excellent bifunctional electrocatalytic activity.Ru@Co₃O₄-1.0 had a higher half-wave potential（0.77 V）and a larger limiting current density(6.1 mA cm^-2)during ORR;meanwhile it presented a low overpotential of 380 mV at the current density of 10 mA cm^-2 during OER.The assembled Zn-air battery using this catalyst as an air cathode performed higher specific capacity(788.1 mAh g^-1)and power density(101.2 mW cm^-2),and it also exhibited more superior cycling stability than the commercial Pt/C at a current density of 10 mA cm^-2.（2）The same Mn-doped Co₃O₄ metal oxides（Mn/Co-T）were prepared under different calcination temperatures.Material characterizations results showed that Mn/Co-T catalysts exhibited more remarkable lattice defects,more abundant surface metal active species and oxygen vacancies compared with the single Co₃O₄.Particularly,when the calcination temperature was 450?C,the Mn/Co-450 catalyst had the most surface adsorbed oxygen species and vacancies,and thus presented excellent bifunctional electrocatalytic activity.It possessed the most positive onset potential（0.91 V）,half-wave potential（0.78 V）and highest limiting current density(7.04 mA cm^-2)in ORR;meanwhile it exhibited the lowest overpotential（380 mV）at the current density of 10 mA cm^-2 in OER.The assembled Zn-air battery using this catalyst as an air cathode performed high specific capacity(757.2 mAh g^-1)and power density(115.5 mW cm^-2),and it also exhibited excellent cyclic stability at a current density of 5 mA cm^-2.（3）P-doped Co₃O₄ metal oxides were prepared by adjusting the mass ratio of red phosphorus（P）to ZIF-67.Material characterizations results showed that the introduction of P element can optimize the concentration of active metal species and oxygen vacancies on the catalyst surface.Among all,when the P/ZIF-67 ratio was 1/15,P/C-2 catalyst exhibited the best ORR catalytic performance.Compared with the single Co₃O₄,P/Co-2had a more positive half-wave potential（0.70 V）and a larger limiting current density(3.94mA cm^-2),which indicated that the introduction of trace P can significantly improve the catalytic performance of Co₃O₄ catalyst.