Structure Regulation And Electrocatalytic Properties Research Of Manganese Dioxide

Zinc-air batteries have many merits such as low cost of raw material,high specific capacity,mechanical replacement,friendly to environment and so on,and also have the following advantages for zinc-air fuel cells（ZAFC）,such as higher specific energy density,more attractive cost,better adaptability to environment and rapid on-site energy recharge which only needs to provide a standard power supply.In this paper,four kinds of MnO₂ with crystalline structures were synthesized by simple hydrothermal synthesis method on controlling the reaction between manganese-containing oxidants and different reducing agents.The composition,microstructure and surface morphology of the catalysts were qualitatively analyzed and observed by XRD,SEM and TEM.The oxygen reduction and oxygen evolution properties of four MnO₂ under alkaline conditions were evaluated.The experimental results show thatα-MnO₂ has higher oxygen reduction activity,and the half-wave potential ofα-MnO₂ is only 0.032 V different from commercial Pt/C,while the catalytic activity ofα-MnO₂ for oxygen evolution needed to be further improved.The mechanism of the electrocatalysis activity was studied by means of EIS and BET.At the same time,the different catalyst materials were designed and assembled into rechargeable ZAFC.During the long cycle test,the coulombic efficiency ofα-MnO₂are kept above 90%all the time after 100 cycles of charge and discharge,and it has excellent activity and stability even under high working current density,which verified its practical application value.Secondly,theα-MnO₂ was simulated by density functional theory,and the samples containing oxygen vacancies were obtained by three simple methods.The oxygen vacancies play an important role in changing the geometric,electronic structures and the chemical properties of MnO₂.In this study,the functional used GGA+PBE mode to optimize the structure ofα-MnO₂ with different oxygen vacancies.By studyingα-MnO₂（110）,it is found that the generation of oxygen vacancies led to the change of electronic structure（band structure,density of total states and density of partial states,electron density difference and Milliken populations）.These results were compared with the experimental results to study the electrocatalytic performance of the samples.The results show that a large number of Mn-d orbitals overlap and form defect energy bands due to the medium concentration of oxygen vacancies,which leads to the introduction of defect energy levels into the forbidden band and increases the density of states at fermi level,which increases the conductivity ofα-MnO₂（110）.However,the high concentration of oxygen vacancies will impede the catalytic activity of MnO₂ on oxygen evolution.