Morphology engineering of ZnO nanostructures for high performance supercapacitors: enhanced electrochemistry of ZnO nanocones compared to ZnO nanowires

Abstract

In this work, the morphology of ZnO nanostructures is engineered to demonstrate enhanced supercapacitor characteristics of ZnO nanocones (NCs) compared to ZnO nanowires (NWs). ZnO NCs are obtained by chemically etching ZnO NWs. Electrochemical characteristics of ZnO NCs and NWs are extensively investigated to demonstrate morphology dependent capacitive performance of one dimensional ZnO nanostructures. Cyclic voltammetry measurements on these two kinds of electrodes in a three-electrode cell confirms that ZnO NCs exhibit a high specific capacitance of 378.5 F g at a scan rate of 20 mV s, which is almost twice that of ZnO NWs (191.5 F g). The charge-discharge and electrochemical impedance spectroscopy measurements also clearly result in enhanced capacitive performance of NCs as evidenced by higher specific capacitances and lower internal resistance. Asymmetric supercapacitors are fabricated using activated carbon (AC) as the negative electrode and ZnO NWs and NCs as positive electrodes. The ZnO NC⫽AC can deliver a maximum specific capacitance of 126 F g at a current density of 1.33 A g with an energy density of 25.2 W h kg at the power density of 896.44 W kg. In contrast, ZnO NW⫽AC displays 63% of the capacitance obtained from the ZnO NC⫽AC supercapacitor. The enhanced performance of NCs is attributed to the higher surface area of ZnO nanostructures after the morphology is altered from NWs to NCs.

High Performance Hydroxide Zinc Carbonate Composite for Supercapacitors

“…The electrochemical richness of HZC composite electrode has been extensively studied using cyclic voltammetry, galvanostatic charge-discharge studies and electrochemical impedance techniques. The CV traces (Figure 4) of HZC shows a pair of redox peaks in the range of 0 -0.6 V (vs Hg/HgO) of HZC composite electrode in three electrode system, which indicates that HZC composite electrode exhibits typical energy storage mode of Faraday pseudocapacitance [3]. It is generally believed that the pseudocapacitance energy storage mechanism of Zn(II)-based oxide or hydroxide electrode in alkaline electrolyte solution is de- Figure 4.…”

“…The specific capacitance of HZC composite electrode can be estimated from the discharge current using the following equation [3]:…”

“…Electrochemical capacitors (EC) or supercapacitors are a well-established class of energy storage devices and have been the subject of intense interest due to their high specific power density and fast recharge capabilities. Depending upon the charge transfer mechanisms, supercapacitors are divided into electrical doublelayer capacitors (EDLCs) and pseudocapacitors [1] [2] [3]. In EDLCs, the charge storage mechanism is non-faradic and the charges are accumulated in the interface between the electrode and electrolytic solution, while faradic redox reactions take place at the electrode surface in pseudo-capacitors [2].…”

“…However, the high cost and toxicity of the compound have placed huge obstacles for its large scale production. Therefore, many researchers are looking for various inexpensive transition metal oxides and hydroxides with high energy density as alternative electrode materials, like ZnO, TiO 2 , and MnO 2 as electrode materials of pseudocapacitors [3] [4] [5]. Among them, Zn-based materials have drawn great attention because of the low cost and excellent electrochemical properties [6] [7].…”

Capacitive Performance and Tortuosity of Activated Carbon Electrodes with Macroscopic Pores

“…9,16,17,a The orientation of solid particles comprising porous electrodes for energy storage has also been used as a strategy to control tortuosity experimentally, and thereby enhance cycling performance relative to conventionally fabricated electrodes. 18,19 In the present work we investigate ion transport in capacitive electrodes that use two disparate length scales of tortuosity. We show through experiments on eight different electrode samples having four different thicknesses, with and without such bi-tortuous structures, that the addition of macroscopic, low-tortuosity pores aligned with the thru-plane direction improves electrode capacitance at high sweep rates.…”