The charge storage mechanism in MnO2 electrode, used in aqueous electrolyte, was investigated by cyclic voltammetry and X-ray photoelectron spectroscopy. Thin MnO2 films deposited on a platinum substrate and thick MnO2 composite electrodes were used. First, the cyclic voltammetry data established that only a thin layer of MnO2 is involved in the redox process and electrochemically active. Second, the X-ray photoelectron spectroscopy data revealed that the manganese oxidation state was varying from III to IV for the reduced and oxidized forms of thin film electrodes, respectively, during the charge/discharge process. The X-ray photoelectron spectroscopy data also show that Na+ cations from the electrolyte were involved in the charge storage process of MnO2 thin film electrodes. However, the Na/Mn ratio for the reduced electrode was much lower than what was anticipated for charge compensation dominated by Na+, thus suggesting the involvement of protons in the pseudofaradaic mechanism. An important finding o...

Charge Storage Mechanism of MnO2 Electrode Used in Aqueous Electrochemical Capacitor

Electrochemical supercapacitors (ECs), characteristic of high power and reasonably high energy densities, have become a versatile solution to various emerging energy applications. This critical review describes some materials science aspects on manganese oxide-based materials for these applications, primarily including the strategic design and fabrication of these electrode materials. Nanostructurization, chemical modification and incorporation with high surface area, conductive nanoarchitectures are the three major strategies in the development of high-performance manganese oxide-based electrodes for EC applications. Numerous works reviewed herein have shown enhanced electrochemical performance in the manganese oxide-based electrode materials. However, many fundamental questions remain unanswered, particularly with respect to characterization and understanding of electron transfer and atomic transport of the electrochemical interface processes within the manganese oxide-based electrodes. In order to fully exploit the potential of manganese oxide-based electrode materials, an unambiguous appreciation of these basic questions and optimization of synthesis parameters and material properties are critical for the further development of EC devices (233 references).

Manganese oxide-based materials as electrochemical supercapacitor electrodes

Carbon materials for electrochemical capacitors

The development of more efficient electrical storage is a pressing requirement to meet future societal and environmental needs. This demand for more sustainable, efficient energy storage has provoked a renewed scientific and commercial interest in advanced capacitor designs in which the suite of experimental techniques and ideas that comprise nanotechnology are playing a critical role. Capacitors can be charged and discharged quickly and are one of the primary building blocks of many types of electrical circuit, from microprocessors to large-sale power supplies, but usually have relatively low energy storage capability when compared with batteries. The application of nanostructured materials with bespoke morphologies and properties to electrochemical supercapacitors is being intensively studied in order to provide enhanced energy density without comprising their inherent high power density and excellent cyclability. In particular, electrode materials that exploit physical adsorption or redox reactions of electrolyte ions are foreseen to bridge the performance disparity between batteries with high energy density and capacitors with high power density. In this review, we present some of the novel nanomaterial systems applied for electrochemical supercapacitors and show how material morphology, chemistry and physical properties are being tailored to provide enhanced electrochemical supercapacitor performance.

The role of nanomaterials in redox-based supercapacitors for next generation energy storage devices

Metal oxide thin film based supercapacitors

Electrochemical capacitor of magnetite in aqueous electrolytes

Iron oxide film directly grown on iron, steel, or other substrates by chemical or electrochemical oxidation is a promising material for energy storage through surface adsorption of static charges. As the electrode materials of energy-storage devices, the hydrated iron compound has a chemical composition of FexOyHz, where 1.0≦x≦3.0, 0.0≦y≦4.0, and 0.0≦z≦1.0. An aqueous or organic solution of a metallic salt including sulfates, sulfites, hydroxides, chlorides, phosphates and nitrates is used as electrolyte for the electrochemical devices. Cyclic voltammetry indicates that the iron-oxide electrodes in the electrolytes can store charges as high as 0.5 F/cm2 or 320 F/g of the electrode materials. Electrochemical capacitors using the iron oxide as the electrode material is an economical and viable power source for use in portable electronics, power tools, and electrical vehicles.

Lih-Ren Shiue

Papers

Electrochemical capacitor of magnetite in aqueous electrolytes

Electrochemical capacitor with electrode material for energy storage