A Cost-Effective and High-Performance Core-Shell-Nanorod-Based ZnO/α-Fe2O3//ZnO/C Asymmetric Supercapacitor

A novel core-shell design for nano-structured electrode materials is introduced for realizing cost-effective and high-performance supercapacitors. In the proposed core-shell design, thin shell-layers of highly pseudo-capacitive materials provide the platform for surface or near-surface-based faradaic and non-faradaic reactions together with shortened ion-diffusion path facilitating fast-ion intercalation and deintercalation processes. The highly-conducting core serves as highway for fast electron transfer toward current collectors, improving both energy and power performance characteristics of the core-shell structure in relation to pristine component materials. Furthermore, use of carbon (C)-based materials as a shell layer in either electrode not only enhances capacitive performance through double-layer formation but also provides enough mechanical strength to sustain volume changes in the core material during long-cycling of the supercapacitor improving its cycle life. In order to enhance electrochemical performance in terms of specific capacitance and rate capability via core-shell architecture and nano-structuring, an asymmetric supercapacitor (ASC) is assembled using ZnO/α-Fe2O3 and ZnO/C core-shell nanorods as respective negative and positive electrodes. The ASC exhibits a specific capacitance of ∼115 F/g at a scan rate of 10 mV/s in a potential window as large as 1.8 V with a response time as short as ∼39 ms and retains more than 80% of its initial capacitance after 4000 cycles. Interestingly, the ASC can deliver an energy density of ∼41 Wh/kg and a power density of ∼7 kW/kg that are significantly higher than those reported hitherto for iron-oxide-based ASCs.