A layered compound composed of crystalline ruthenic acid sheets interleaved with layers of water can be exfoliated (delaminated) to yield colloidal nanosheets. This material is a mixed conductor where the crystalline nanosheets contribute to the electron conductivity and the hydrous interlayer supplies proton transport (see diagram). A large active surface area and a high specific capacitance is promising for electrochemical supercapacitor applications.

Electrochemical impedance spectroscopy was conducted on a series of hydrous ruthenium oxides, RuO(2).xH(2)O, (x = 0.5, 0.3, 0) and a layered ruthenic acid hydrate (H(0.2)RuO(2.1).nH(2)O) in order to evaluate their protonic and electronic conduction. The capacitor response frequency was observed at lower frequency for RuO(2).xH(2)O with higher water content, which was suggested to be due to electrolyte exhaustion within the film and/or utilization of hydrated interparticle micropores that have high ionic resistance. Analysis of the impedance data indicated that the charge-transfer resistance through the film is not significantly affected by the water content in RuO(2).xH(2)O, and the capacitor frequency response is dominated by the protonic conduction. The capacitor response frequency of layered H(0.2)RuO(2.1).nH(2)O was comparable to RuO(2).0.5H(2)O. The high specific capacitance at low frequency for layered H(0.2)RuO(2.1).nH(2)O is attributed to the utilization of the expandable hydrous interlayer, which accounts for the ionic conduction. The present results demonstrate the importance of hydrous regions (either interparticle or interlayer) to allow appreciable protonic conduction for high energy and high power electrochemical capacitors.

Abstract

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The charge storage mechanism of nanostructured anhydrous and hydrous ruthenium-based oxides was evaluated by various electrochemical techniques (cyclic voltammety, hydrodynamic voltammetry, chronoamperometry, and electrochemical impedance spectroscopy). The effects of various factors, such as particle size, hydrous state, and structure, on the pseudocapacitive property were characterized. The electric double layer capacitance (C dl), adsorption related charge (C ad), and the irreversible redox related charge (C irr) per unit mass and surface area of electrode material has been estimated and the role of structural water within the material either in micropores or interlayer are discussed.

Unilamellar crystallites of conductive ruthenium oxide having a thickness of about 1 nm were obtained via elemental exfoliation of a protonic layered ruthenate, H(0.2)RuO(2).0.5H(2)O, with an alpha-NaFeO(2)-related crystal structure. The obtained RuO(2) nanosheets possessed a well-defined crystalline structure with a hexagonal symmetry, reflecting the crystal structure of the parent material. The restacked RuO(2) nanosheets exhibited a high pseudocapacitance of approximately 700 F g(-1) in an acidic electrolyte, which is almost double the value of the nonexfoliated layered protonated ruthenate.

Over the past decade, interest in electrochemical capacitors as an energy-storage technology has increased enormously, spurring the development and evaluation of a large number of new materials and device configurations. This perspective article aims to propose guidelines by which new materials and devices should be evaluated, and how resulting data should be reported with respect to critical metrics such as capacitance, energy and power. In recent years the number of publications dedicated to electrochemical capacitors (ECs) has increased enormously, while drawing from a diverse community of researchers in the fields of electrochemistry, material science, and engineering.1,2 Contributions from such different perspectives have been invaluable to the advancement of EC science and technology, but has also resulted in some confusion in the corresponding scientific literature because of the wide range of materials (active and inactive), device configurations, and electrochemical testing protocols that are used. Current StatusUnfortunately, a comparison of the electrochemical properties of materials for EC is often challenging because key experimental parameters and procedures are not fully described in literature reports, and in some cases data from single electrode experiments are improperly extrapolated to projected device performance. Inconsistency in reporting key performance metrics hinders the comparison of data from different laboratories on otherwise related materials and devices. 1,2The present perspective article identifies some useful guidelines for correctly evaluating the electrochemical performance of EC materials and devices and reporting on the resulting findings. Future Needs and ProspectsMaterials for ECs (electrode and electrolyte).-When a new (active) material is proposed, its synthesis procedure should be described in detail sufficient to allow for replication in another laboratory; basic material properties should be also reported (particle/crystallite/film morphology, crystal structure, etc). Any electrode preparation using the resulting materials must be clearly described, including information on the respective ratios (or weight %) between active material, conductive agent (if present) and binder(s) used in electrode fabrication. Furthermore, one should report the active material mass loading per electrode area, for example in units of mg cm −2 . This information is essential for readers to clearly evaluate the electrochemical performance of both the active material and full device, particularly when comparing against results from other laboratories or from commercial products. Authors should consider that the mass loading of commercialized activated carbon electrodes is ∼10 mg cm −2 . * Electrochemical Society Member. z E-mail: andrea.balducci@uni-jena.de When a new electrolyte component (solvent or salt) is proposed, the composition of the investigated electrolyte should be clearly described, for example with concentration of the salts noted in units of mol L −1 . The viscosity and ionic c...

A tetrabutylammonium-H 2 Ti 4 O 9 ?xH 2 O intercalation compound was obtained by a guest exchange reaction between tetrabutylammonium hydroxide and an ethylammonium-H 2 Ti 4 O 9 ?xH 2 O intercalation compound, and its dispersion state in aqueous and non-aqueous solutions were studied. Spontaneous exfoliation of H 2 Ti 4 O 9 ?xH 2 O into colloidal nanosheets occurred when the tetrabutylammonium-H 2 Ti 4 O 9 ?xH 2 O intercalation compound was dispersed in water, methyl alcohol, isopropyl alcohol, acetonitrile, N,N-dimethylformamide, dimethyl sulfoxide, and propylene carbonate, while exfoliation did not occur in tetrahydrofuran. A tetrabutylammonium-H 2 Ti 4 O 9 ?xH 2 O film was obtained by a reassembly process by casting the colloidal suspension containing exfoliated nanosheets, while a H 2 Ti 4 O 9 ?xH 2 O film was directly obtained by electrophoretic deposition. Thermal treatment of the electrophoretically deposited film led to an oriented TiO 2 (B) film with the (0k0) planes lying perpendicular to the substrate.