Study On Electrocatalytic Activities Of Bifunctional Manganese/cobalt-based Materials For Zinc Air Batteries

The rapid consumption of fossil fuels and the severe ecological crisis have led to the increasing demand for sustainable energy conversion and energy storage equipments.As a promising energy device,zinc-air battery has advantages of high energy density,environmental friendliness,safety and low cost;however,its slow reaction kinetics on air cathode restricts the further development of zinc-air battery.Therefore,developing bifunctional catalysts with high catalytic activity toward oxygen reduction reaction?ORR?and oxygen evolution reaction?OER?is very important to improve the performance of zinc-air battery.In this paper,transition metal oxides are selected as research objects,and various strategies are adopted to improve their catalytic performances.The studies in this article are as follows:The MnO₂/LSM composite material?LSM30?with core-shell structure was synthesized by selectively dissolving the cations in a site of La_0.7Sr_0.3MnO₃?LSM?perovskite.For the ORR process,the onset potential of the LSM30 sample is 0.873 V;for the OER process,this sample shows the lowest overpotential of 606 mV at 10 mA cm^-2.The excellent catalytic performances of LSM30 sample are related to the good electronic conductivity,enhanced oxygen adsorption capacity and oxygen adsorption capacity.In addition,when the LSM30 sample was used as a catalyst of air cathode for zinc-air battery,the maximum power density of this battery is 181.4 mW cm^-2.This indicates that LSM30composite material is an excellent bifunctional catalyst,and it also shows that selective dissolution is an effective method for preparing bifunctional catalysts.The Co₃O₄/LaCoO₃ composite material?LCO-1?was prepared by selective dissolution of LaCoO₃?LCO?synthesized by the sol-gel method.The LCO-1 composite material shows the initial potential of 0.640 V,and its mass specific activity at 0.7 V is10.03 mA mg^-1.During OER process,this material presents a small overpotential of 503mV at 10 mA cm^-2.When the LCO-1 composite material is used as a bifunctional catalyst of the air cathode for zinc-air battery,this battery shows the maximum power density of155.7 mW cm^-2,and its potential difference increased by 0.03 V after 100 times of charge-discharge cycles.The enhanced catalytic activities of the LCO-1 material composite are related to the larger surface area,porous structure,more surface defects and weaker Co-O bonds.Metastable Li_1-x-yHyCo O₂ materials?LC-x-E,x=4,8 and 12?and Co₃O₄/Li_1-xCoO_2-?composites?LC-x-H,x=4,8 and 12?were synthesized by treating LiCoO₂?LC?with acid treatment and acid treatment-post annealing methods,respectively.After acid treatment,part of the Li⁺was replaced by H⁺in the bulk LiCoO₂ material,thus metastable Li_1-x-yH_yCoO₂ nanosheets formed.The metastable material transformed into a main phase of spinel Li_1-xCoO_2-?material with more oxygen vacancies after annealing.Electrochemical results show that the LC-12-H composite material has excellent ORR and OER catalytic properties.For the ORR process,the half-wave potential of the LC-12-H material is 0.683V;for the OER process,its overpotential at 10 mA cm^-2 is 513 mV.The zinc-air battery with the LC-12-H sample as a bifunctional catalyst can reach the maximum power density of 141.2 mW cm^-2,and its initial and final potential differences are 0.738 V and 0.834 V.The Mn₂O₃/LiNi_1/3Co_1/3Mn_1/3O₂ material?NCM-2?was successfully prepared by treating Li?NiCoMn?_1/3O₂?NCM?with treatment-post annealing method.The NCM-2material shows excellent ORR and OER electrocatalytic performances and stability in alkaline solution.The potential difference of the NCM-2 composite material between the oxygen reduction potential(ORR,-3 mA cm^-2)and the oxygen evolution potential(OER,10 mA cm^-2)is 1.090 V.In addition,the maximum power density of the zinc-air battery with NCM-2 composite material is 120.0 mW cm^-2.The catalytic activities of the NCM-2material are attributed to the large specific surface area,pore structure and enhanced oxygen adsorption capacity.