Study On Controllable Preparation Mechanism And Electrochemical Performance Of One-Dimensional MnO₂ Nanotubes

The development of society requires not only research and development of new clean energy sources that replace traditional fuels,but also research and development of efficient new energy storage devices.As a new type of energy storage device,supercapacitors have the advantages of high power density,good cycle performance,wide working range,fast charge and discharge,and environmental friendliness,but their energy density has not reached the current industrial requirements.MnO₂ has many advantages such as rich minerals,high theoretical specific capacitance,diverse crystal structure,wide potential window,and multivalent state.It is one of the current research hotspots for supercapacitor cathode materials,but the conductivity of MnO₂is the main factor hindering its electrochemical performance.Most current articles obtain high specific capacitance by compounding MnO₂ with other conductive substrates,but the conductivity of pure MnO₂ has not been significantly improved,so how to improve the conductivity of MnO₂ simply and effectively is the direction we need to explore.This paper mainly uses polycarbonate membrane（PC membrane）as a template to prepare MnO₂nanotubes.The effects of its crystal structure and micromorphology on its electrochemical performance are studied first,and then its conductivity is improved by different methods.First,the p H of the reaction solution was adjusted using dilute sulfuric acid and potassium hydroxide to obtainδ-MnO₂ nanotubes（MnO₂-7,MnO₂-12）,δ-MnO₂ nanorods（MnO₂-3）,andδ-MnO₂@α-MnO₂nanorods（MnO₂-1.98）andα-MnO₂ nanorods（MnO₂-1）.Specifically,the differences in electrochemical performance caused by different crystal forms and micro-morphologies have been studied.Among them,MnO₂-12 porous nanotubes have obtained the best capacitance performance due to their high specific surface area and special two-dimensional layered structure,which provide more favorable channels for the transmission of ions and electrons.When the current density is 0.5 A g^-1,the specific capacitance of MnO₂-12can reach 364.1 F g^-1,and it has excellent rate performance and cycle performance compared to other MnO₂ nanotubes/nanorods(current density increased to 5 A g^-1,the specific capacitance remains 62.6%,and after 2000 cycles,the specific capacitance remains 90.2%).In this paper,heteroatom doping and introducing oxygen vacancies are used to improve the conductivity of MnO₂.Firstly,cobalt nitrate hexahydrate was used as the source of Co²⁺,and the molar ratio of Co/Mn was controlled to study the effect of Co²⁺on the electrochemical performance of MnO₂ nanotubes.A proper amount of Co²⁺can widen the interlayer distance ofδ-MnO₂ and increase the ion transmission rate,which is conducive to the improvement of electrochemical performance.DFT calculations also confirmed that the proper amount of Co²⁺can narrow the band gap and increase the conductivity of MnO₂.When the Co/Mn molar ratio is 5%,the Co-MnO₂-5 nanotubes have the best specific capacitance,reaching 406.4 F g^-1 at 1 Ag^-1,and 3000 cycles under a current density of 10 A g^-1 can still maintain 91.1%of the initial capacitance.At the same time,Mn-Fe OOH nanotubes prepared using Co-MnO₂-5 nanotubes as templates are used as anode materials.Supercapacitors composed of Co-MnO₂-5 nanotubes and Mn-Fe OOH nanotubes also have excellent capacitance performance at a power density of 999.9 W kg^-1,the energy density can reach 42.1 W h kg^-1.In addition,calcining MnO₂ nanotubes in N₂ atmosphere effectively generated oxygen vacancies and improved electrochemical performance.In this paper,MnO₂ was calcined at different temperatures to study the relationship between the morphology and capacitance of MnO₂ and the calcination temperature.In a N₂ atmosphere,The ov-MnO₂-300 nanotubes prepared at the calcination temperature is 300°C generate a large amount of surface adsorbed oxygen,which increases its surface activity,which is more conducive to the redox reaction,and improves the pseudocapacitive performance of MnO₂.However,when the temperature is too high,MnO₂ is transformed into Mn₃O₄.Although the redox activity is increased,its dense morphology reduces the area in contact with the electrolyte,so the specific capacitance is reduced.ov-MnO₂-300nanotubes have excellent electrochemical performance,which can reach 459.0 F g-1 at1 A g^-1.The rate performance and cycle performance are also the best,which is attributed to the introduction of oxygen vacancies improved the electrochemical activity of MnO₂ and shortens the band gap of MnO₂.This article starts with the research on the crystal structure and microstructure of MnO₂.The conductivity of MnO₂ nanotubes with the best properties is modified,which effectively improves the electrochemical performance of MnO₂ nanotubes.By matching with suitable anode materials.Improved energy density of asymmetric supercapacitors.The study of the capacitance contribution of pure MnO₂ in this paper provides new ideas and promotes the application of pure MnO₂ nanomaterials in supercapacitors.