Preparation Of Manganese Dioxide With Optimized Electronic Structure For Supercapacitors

The gradual depletion of fossil fuels and the increase of carbon dioxide emissions have been stimulated the development of renewable energy sources.Correspondingly,energy storage technologies with long-life,high energy/power density have been extensively investigated.In recent years,supercapacitors(SCs),as a new type of green energy storage technology with high power density and long cycle life,have been attracted lots of attentions.Electrode materials play an important role in determining the SCs performance.Manganese dioxide(MnO2)is a promising SCs electrode material due to its low manufacturing cost,high availability,high safety and low toxicity.However,manganese dioxide has low electronic conductivity and poor cationic diffusivity,thus leading to low capacitance and poor rate performance.Therefore,it remains a challenge to design and prepare manganese dioxide electrode materials to achieve excellent capacitive performance.Herein,we regulated the electronic structure of MnO2 through introducing oxygen vacancy and optimizing coordination environment to improve its SCs performance.(1)MnO2 with rich bulk oxygen vacancies(MnO2-C)was successfully prepared through complex induced chemical precipitation.Electron paramagnetic resonance(EPR)results show that MnO2-C exhibits a remarkably increased(5-fold)oxygen vacancies concentration compared to the control sample of MnO2.Density-functional theory calculations verify that oxygen vacancies can distinctly induce reduced band gap and electron delocalization,then promoting fast electron transfer.Bader charge analysis and charge density differences results demonstrate that more electron transfer and higher electron delocalization degree can be achieved with increased oxygen vacancies concentration.Moreover,a formed robust local electric-field in bulk MnO2-C can accelerate ion transfer kinetics,thus enhancing the capacitance and improving the rate capability.The simulations on Na+adsorption and diffusion barrier confirm that rich bulk oxygen vacancies can accelerate Na+ion diffusion.Owing to enhanced electrical conductivity,robust local electric field and low diffusion energy barrier,the as-prepared MnO2-C has a remarkable specific capacitance of 286.9 F-g’1.An as-fabricated ASC by using MnO2-C as cathode and MoO3-x as anode,can deliver a maximum energy density of 54.2 Wh-kg-1 and power density of 3279.6 W-kg-1.(2)MnO2-TEA nanosheets with optimized coordination environment were successfully prepared through a solution-based chemical deposition process using triethanolamine(TEA)as complexing agent.Scanning transmission electron microscopy,X-ray photoelectron spectroscopy and X-ray absorption spectroscopy results confirm that the coordination environment of MnO2-TEA has been successfully optimized.To reveal the effect of optimized coordination environment on the capacitive performance,we carried out DFT calculations and analysis of Bader charge.It suggests that optimized coordination environment(more oxygen vacancy and more corner-shared Mn-Mn interaction)changes the electronic structure of MnO2-TEA by forming a robust local electric-field.As a result,it remarkably enhances the electrical conductivity and ion diffusion ability of MnO2.The as-prepared MnO2-TEA exhibits a remarkable specific capacitance of 417.5 F·g-1 at 1 A·g-1 and the fabricated asymmetric supercapacitor delivers a maximum energy density of 228.3 Wh·kg1 at a power density of 2055.0 W·kg-1.In summary,MnO2-C and MnO2-TEA electrode materials were prepared as high-performance electrode materials for SCs.Combining with the experimental data and simulation results,we unravel the energy storage mechanism of MnO2 electrode with optimized electronic structure based on clarifying the relationship of its structure-performance.This strategy of optimizing electronic structure can be extended to prepare other metal oxides for a variety of energy storage applications.