Abstract
The supercapacitive properties of manganese oxide (MnO2) thin films electrodeposited on three-dimensionally (3D) aligned inverse-opal nickel nanostructures are investigated. Compared to conventional planar or two-dimensionally (2D) aligned nanostructures, 3D-aligned nanostructures can provide considerably increased and controllable contacts between the electrode and electrolyte. As a result, saturation of the areal capacitance with the electrode thickness and associated decrease of the specific capacitance, Csp, become much slower than those of the planar and 2D-aligned electrode systems. While, for planar MnO2 electrodes, the Csp of a 60-cycle electrodeposited electrode is only the half of the 10-cycle electrodeposited one, the value of the 3D-nanostructured electrode remains unchanged under the same condition. The maximum Csp value of 864 F g−1, and Csp retention of 87.7% after 5000 cycles of galvanostatic charge-discharge are obtained. The voltammetric response is also improved significantly and the Csp measured at 200 mV s−1 retains 71.7% of the value measured at 10 mV s−1. More quantitative analysis on the effect of this 3D-aligned nanostructuring is also performed using a deconvolution of the capacitive elements in the total capacitance of the electrodes.
Highlights
Pseudocapacitors, or supercapacitors that use transition metal oxides and conducting polymers as electrode materials, are attracting growing attention due to their superior specific energy and capacitance compared to the current electrochemical double layer capacitors (EDLCs)[1,2,3]
This uniform deposition of the MnO2 implies that the amount and areal capacitance of the MnO2 electrode can be extended continuously by increasing the number of stacking layers of the 2D-arrayed PS template while preserving the electrode thickness and advantages of the nanostructure. It is very important for the practical use of nanostructured pseudocapacitor electrodes that a new current collector nanostructure is continuously provided and MnO2 is uniformly deposited thereon. This is because, for areal capacitance saturates and the specific capacitance rapidly decreases as the electrode thickness increases, making it difficult to obtain desired specific energy
An increase in the areal capacitance was almost proportional to the thickness of the electrode and the decrease in the specific capacitance, Csp, was retarded significantly
Summary
Pseudocapacitors, or supercapacitors that use transition metal oxides and conducting polymers as electrode materials, are attracting growing attention due to their superior specific energy and capacitance compared to the current electrochemical double layer capacitors (EDLCs)[1,2,3]. The areal capacitance of the electrode is saturated and the Csp value decreases rapidly as the MnO2 film thickness increases. This indicates that conventional two-dimensional nanostructures are not suitable for obtaining sufficient specific energy necessary for practical use because the nanostructuring effect can be exerted only when the extremely small amount of electrode materials is used. The voltammetric response and cycle life of the electrode were much improved
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