| In recent years,many scientists have devoted to developing the new energy storage devices with high energy density and high power density.The electrostatic capacitor with conventional configurations exhibits high power density,but it shows low energy density.In this work,the electrostatic nano-capacitors were fabricated using anodization and nanotechnology,aiming to increasing their specific capacitance on the condition that the characteristic of high power density was reserved.Firstly,the highly ordered porous anodic alumina(PAA)templates were fabricated,and then aluminum nanorod arrays were obtained by direct nanoimprinting at high temperature in vacuum.Finally,anodic alumina films were formed on the surface of the nanorod arrays using anodizing method,aiming to construct electrostatic nano-capacitors.The main research contents are as follows:Firstly,the barrier layer thickness of PAA films was measured by using re-anodizing and electrochemical impedance spectroscopy methods in order to detennine the electric field strength during anodization.The results show the high-field anodization of aluminum can be realized by improving the applied voltage in the highly concentrated(0.75 M)oxalic acid solution,but cannot in the dilute(0.1 M)oxalic acid solution.In addition,the ordering qualities of PAA films increase with increasing field strength instead of the applied voltage.Secondly,anodizing conditions of high ordering PAA films were optimized by adjusting the key anodizing factors,including aluminum purity,grain orientation,electrolyte concentration,temperature,voltage,and time.Thus,PAA films were formed in high temperature(30?)in highly concentrated(0.75 M)oxalic acid electrolyte at 50 V for 50 min,and then the barrier layer side of PAA films was corroded in dilute phosphoric acid for 3 h,aiming to fabricating high ordering PAA templates.Then,some key anodizing factors,such as the electrolyte,temperature and voltage,were investigated in order to form alumina dielectric films with excellent capacitive performance.It was demonstrated that the alumina dielectric films formed in 15wt%ammonium adipate solution at 60? exhibited high specific capacitance,low leakage current and dielectric dissipation factor.Besides,the ratio of film thickness to applied voltage was 1.48 nm V-1.Finally,the aluminum nanorod electrodes were fabricated by direct nanoimprinting at high temperature in vacuum,and then alumina films were formed on surface of electrodes to investigate the effect of film thickness on its capacitive performance by varying anodizing voltage.According to developed theoretical derivation,the specific capacitance of nano-capacitors can be improved by increasing the length and diameter of nanorods or decreasing film thickness and center distance of nanorods.In addition,the energy density of nano-capacitors can be improved by increasing the length and diameter of nanorods or decreasing substrate thickness and center distance of nanorods.As well,it exhibits maximum energy density when the films thickness is a quarter of nanorod diameter.Experimental results demonstrated that the nanorod arrays with different length and diameter can be fabricated successfully by simultaneously imprinting and increasing the temperature in the range of 500?650?.For nanorod electrode fabricated using 300 nm pore-diameter PAA templates,the alumina films formed at 20V on the surface of 16.5 ?m-length nanorods show the better capacitance of 11.60 ?F cm-2,which is 38.8 times greater than the capacitance of planer structure films formed at 20 V.Besides,the nano-capacitors constructed using the films formed at 50 V showed maximum energy density(2.37 mJ cm-2)in accordance with developed theoretical derivation.For nanorod electrodes fabricated using 75 nm pore-diameter PAA templates,the alumina films formed at 10V on the surface of 6.3 ?m-length nanorods show the capacitance of 20.10 ?F cm-2,which is 33.5 times greater than the capacitance of planer structure films formed at same voltage(10 V). |
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