High-Performance Pseudocapacitor Electrodes Based on α-Fe₂O₃/MnO₂Core–Shell Nanowire Heterostructure Arrays

Abstract

A simple wet chemical technique has been employed to fabricate MnO2 nanolayer-coated α-Fe2O3/MnO2 core–shell nanowire heterostructure arrays to prepare unique pseudocapacitor electrodes. The coating of MnO2 on α-Fe2O3 nanowires is triggered by the reduction of KMnO4 solutions by the metallic (Au) film on which the polycrystalline α-Fe2O3 nanowires have been grown electrochemically. This metallic film also acts as the current collector by making direct contact with the arrays of the 1D nanoheterostructures. The as-prepared α-Fe2O3/MnO2 nanoheterostructures are found to exhibit excellent specific capacitance, high energy density, high power density, and long-term cyclic stability as compared with the bare α-Fe2O3 nanowire electrodes. The unique geometry of the 1D nanoheterostructures with high effective surface area to allow faster redox reaction kinetics, the incorporation of two highly redox active materials in the same structure, and the porous surface structures of the heterostructure to allow facile electrolyte diffusion help in the superior electrochemical performance of the α-Fe2O3/MnO2 nanoheterostructures. The maximum specific capacitance of 838 F g–1 (based on pristine MnO2) has been achieved by cyclic voltammetry at a scan rate of 2 mV s–1 in 1 M KOH aqueous solution. The hybrid α-Fe2O3/MnO2 nanocomposite electrodes also exhibit good rate capability with excellent specific energy density of 17 Wh kg–1 and specific power density of 30.6 kW kg–1 at a current density of 50 A g–1 and good long-term cycling stability (only 1.5% loss of its initial specific capacitance after 1000 cycles). These studies indicate that the α-Fe2O3/MnO2 nanoheterostructure architecture is very promising for next-generation high-performance pseudocapacitors.

Influence of different materials on the microstructure and optical band gap of α-Fe2O3 nanoparticles

“…α-Fe 2 O 3 composites with different materials have been considered as potential candidates for pseudocapacitors, photocatalysis, high-performance anode material for lithium ion batteries, electromagnetic wave absorption materials etc. Sarkar et al [5] have reported that α-Fe 2 O 3 /MnO 2 nanoheterostructures exhibit excellent specific capacitance, high energy density, high power density, and long-term cyclic stability as compared with the bare α-Fe 2 O 3 nanowire electrodes. Their studies indicated that the α-Fe 2 O 3 /MnO 2 nanoheterostructure architecture is very promising for the next-generation * E-mail: pravanjan phy@yahoo.co.in high-performance pseudocapacitors.…”

α-Fe2O3-Based Core-Shell-Nanorod–Structured Positiveand Negative Electrodes for a High-Performance α-Fe2O3/C//α-Fe2O3/MnOxAsymmetric Supercapacitor

“…[5][6][7] However, their energy densities still remain limited and there is a definite need to increase it to cope with the global energy demand for electric traction and nextgeneration portable electronic devices. 8 The energy density of supercapacitors can be enhanced by increasing their capacitance as well as their operating-potential window, which can be achieved more easily with asymmetric supercapacitors (ASCs) than with the symmetric supercapacitors (SSCs) since, with suitable combination of positive and negative electrode materials, the operational potential window as high as 2 V has been achieved even with aqueous electrolytes. 6,[9][10][11][12][13][14] In recent years, efforts have been expended to explore several ASCs with different electrode materials based on redox-active transition-metal oxides, like RuO 2 , 15 SnO 2 , 23 etc., which are not commercially viable owing to their high cost and environmental incompatibility.…”

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

“…7,8 Hematite (α-Fe 2 O 3 ) is a promising negative electrode material for electrochemical capacitors (ECs) due to its high electrochem- * Electrochemical Society Member.…”

“…6,7 In a core-shell nanostructure, nanometer-thin shell layer of electroactive materials significantly shortens ion-diffusion pathlength for effective ion-intercalation and deintercalation in conjunction with fast electron transfer to the core, while the highly conducting core, which serves as scaffold for electroactive shell, rapidly transfers the electrons to the current collector. 7,8 Hematite (α-Fe 2 O 3 ) is a promising negative electrode material for electrochemical capacitors (ECs) due to its high electrochem-ical activity, high theoretical-specific-capacitance, suitable negativepotential-window because of high hydrogen over-potential in aqueous electrolytes, mostly environment friendly and abundance in nature. [8][9][10][11] However, poor electrical conductivity (∼10 −14 S/cm) and short iondiffusion length limit its electrochemical performance significantly that can be overcome by creating oxygen vacancies within α-Fe 2 O 3 thin films.…”

“…7,8 Hematite (α-Fe 2 O 3 ) is a promising negative electrode material for electrochemical capacitors (ECs) due to its high electrochem-ical activity, high theoretical-specific-capacitance, suitable negativepotential-window because of high hydrogen over-potential in aqueous electrolytes, mostly environment friendly and abundance in nature. [8][9][10][11] However, poor electrical conductivity (∼10 −14 S/cm) and short iondiffusion length limit its electrochemical performance significantly that can be overcome by creating oxygen vacancies within α-Fe 2 O 3 thin films. 9,11,12 Likewise, zinc oxide (ZnO) is an attractive material for solar cells owing to its higher electron mobility as compared to TiO 2 .…”