L.O.S. Bulhões

Impedance of constant phase element (CPE)-blocked diffusion in film electrodes

Abstract We construct a model of ac impedance response to blocked linear diffusion that has a sloped low frequency region in the impedance plot. The approach is based on the transmission line analogy to linear diffusion, and it is equivalent to solving Fick’s law with a boundary condition that allows us to set an arbitrary impedance response at low frequencies. We argue that roughness at the blocking interface gives rise to constant phase element (CPE) response at low frequencies, and we give an impedance model function that can fit data along the whole frequency range when such a CPE is found. This is tested in an experiment of Li + insertion in Nb 2 O 5 . The model should be of significance for metal oxide thin film electrodes and modified polymer electrodes.

Dip-coated TiO2CeO2 films as transparent counter-electrode for transmissive electrochromic devices

Abstract The dip-coating process is an attractive way for the preparation of thin films used in the field of electrochromism. The scope of the present paper is focused on the TiO 2 CeO 2 compounds since they exhibit a reversible electrochemical insertion of lithium ions maintaining a high optical transmissivity. These films can be used as transparent counter-electrode in an all solid state electrochromic transmissive device with, for example, WO 3 as electrochromic material and a lithium conductive polymer as electrolyte.

Electrochemical oxidation of o-aminophenol in aqueous acidic medium : formation of film and soluble products

The electrochemical oxidation of o-aminophenol (OAP) has been studied by cyclic voltammetry on platinum (Pt) and glassy carbon (GC) electrodes. Films formed on Pt exhibited a voltammetric response that is dependent on the final potential applied during the electropolymerization. Although a less electroactive film has been obtained on Pt electrodes cycled at higher potentials, modified GC electrodes showed a well-defined redox response independently of the potential range used. This result is attributed to an activation process of the GC electrode rather than to an electroactive film deposited on the electrode. The deposition process of thin films was also investigated with quartz crystal microbalance experiments. Poly-o-aminophenol (POAP) films were insoluble with an electrochromic response from colorless to brown, and a ladder structure confirmed by infrared spectra. Exhaustive electrolysis of OAP produced a reasonable amount of soluble product, which after solvent extraction, was characterized as 2-aminophenoxazin-3-one (APZ), a product also prepared by chemical oxidation of OAP. A route of electrooxidation of OAP has been proposed based on electrochemical and spectroscopic results.

Preparation and Characterization of a Dip‐Coated SnO2 Film for Transparent Electrodes for Transmissive Electrochromic Devices

A new method for the synthesis of SnO 2 is proposed and thin films are prepared by a dip-coating method. In the present paper we report that these SnO 2 films exhibit a reversible electrochemical insertion of lithium ions while maintaining high optical transmissivity. These films can be used as transparent counterelectrodes in electrochromic transmissive devices and in gas sensors

Structural and electrochemical properties of LiCoO2 prepared by combustion synthesis

LiCoO2 powders were prepared by combustion synthesis, using metallic nitrates as the oxidant and metal sources and urea as fuel. A small amount of the LiCoO2 phase was obtained directly from the combustion reaction, however, a heat treatment was necessary for the phase crystallization. The heat treatment was performed at the temperature range from 400 up to 700 °C for 12 h. The powders were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and specific surface area values were obtained by BET isotherms. Composite electrodes were prepared using a mixture of LiCoO2, carbon black and poly(vinylidene fluoride) (PVDF) in the 85:10:5% w/w ratio. The electrochemical behavior of these composites was evaluated in ethylene carbonate/dimethylcarbonate solution, using lithium perchlorate as supporting electrolyte. Cyclic voltammograms showed one reversible redox process at 4.0/3.85 V and one irreversible redox process at 3.3 V for the LiCoO2 obtained after a post-heat treatment at 400 and 500 °C. Raman spectroscopy showed the possible presence of LiCoO2 with cubic structure for the material obtained at 400 and 500 °C. This result is in agreement with X-ray data with structural refinement for the LiCoO2 powders obtained at different temperatures using the Rietveld method. Data from this method showed the coexistence of cubic LiCoO2 (spinel) and rhombohedral (layered) structures when LiCoO2 was obtained at lower temperatures (400 and 500 °C). The single rhombohedral structure for LiCoO2 was obtained after post-heat treatment at 600 °C. The maximum energy capacity in the first discharge was 136 mA g−1 for the composite electrode based on LiCoO2 obtained after heat treatment at 700 °C.

Gas-sensitive characteristics of metal/semiconductor polymer Schottky device

Abstract Metal/polymer Schottky barrier diodes have been fabricated using electrochemically prepared films of polyaniline as the semiconductor and aluminium as the metal. Polyaniline was doped with HCl at room temperature to form a p-type semiconductor. On a sandwich-type device, the junction at Al/polyaniline showed rectifying behavior, and was used as a sensor for the detection of methane gas. The current–voltage ( I – V ) and capacitance–voltage ( C – V ) characteristics have been studied at room temperature, and the shifts in the I – V and C – V dependence of the diodes were measured with different concentrations of gas for different time intervals. The data have been analysed and interpreted on the basis of the thermionic emission mechanism.

Pt/HClO4 interface CPE: influence of surface roughness and electrolyte concentration☆

Abstract The double-layer region impedance of polycrystalline Pt/perchloric acid solutions interfaces was measured to determine the influence of electrode surface roughness and solution electrolyte concentration on the fractional power n of the constant phase element cpe associated to frequency dispersion in the impedance behaviour of this system. The surface roughness factor was obtained through the adsorption charge of hydrogen from cyclic voltammograms. The solution electrolyte concentration was found to have a significant effect on the cpe only when smaller than 0.5 moll −1 . On the other hand, the frequency dispersion was found to depend directly on the surface roughness. An empirical linear relationship was found between the exponent n and the inverse if the roughness factor, leading to a minimum value of 76° for the phase angle; this value contrasts to the theoretically predicted value of 45°. From a general logarithmic relationship between roughness factor δ and the fractal dimension d of the electrode surface, derived from known equations, and the linear relationship between n and 1/δ, an empirical logarithmic relationship between n and d is obtained. This empirical relationship contrasts with linear relationships previously derived by other authors.

The potentiometric response of chemically modified electrodes

Abstract The electrochemical polymerization of ortho-methoxyaniline and ortho-methylaniline was studied on platinum electrodes. The electrochemical response of electropolymerized films exhibits three well-defined redox processes similar to those observed for polyaniline. The potentiometric response of these electrodes was analyzed, and the polymeric film prepared with ortho-methylaniline showed a better response as a pH sensor than that prepared with ortho-methoxyaniline. The electrode prepared with poly(ortho-methylaniline) showed an apparent Nernstian response in a 1–12 pH range with a slope of 63.8 mV/ decade, while the linear range of the electrode prepared by poly(ortho-methoxyaniline) exhibited a linear relationship in the 2–11 pH range with a slope of 62.0 mV/decade.

Nanostructured Systems Containing Rutin: In Vitro Antioxidant Activity and Photostability Studies

The improvement of the rutin photostability and its prolonged in vitro antioxidant activity were studied by means of its association with nanostructured aqueous dispersions. Rutin-loaded nanocapsules and rutin-loaded nanoemulsion showed mean particle size of 124.30 ± 2.06 and 124.17 ± 1.79, respectively, polydispersity index below 0.20, negative zeta potential, and encapsulation efficiency close to 100%. The in vitro antioxidant activity was evaluated by the formation of free radical ·OH after the exposure of hydrogen peroxide to a UV irradiation system. Rutin-loaded nanostructures showed lower rutin decay rates [(6.1 ± 0.6) 10−3 and (5.1 ± 0.4) 10−3 for nanocapsules and nanoemulsion, respectively] compared to the ethanolic solution [(35.0 ± 3.7) 10−3 min−1] and exposed solution [(40.1 ± 1.7) 10−3 min−1] as well as compared to exposed nanostructured dispersions [(19.5 ± 0.5) 10−3 and (26.6 ± 2.6) 10−3, for nanocapsules and nanoemulsion, respectively]. The presence of the polymeric layer in nanocapsules was fundamental to obtain a prolonged antioxidant activity, even if the mathematical modeling of the in vitro release profiles showed high adsorption of rutin to the particle/droplet surface for both formulations. Rutin-loaded nanostructures represent alternatives to the development of innovative nanomedicines.

Surface Characterization of Thermally Prepared, Ti‐Supported, Ir‐Based Electrocatalysts Containing Ti and Sn

Surface characterization of Ir-based Ti- and Sn-containing electrodes of nominal composition, Ir 0.3 Ti (0.7-x) Sn x O 2 (0 ≤ x ≤ 0.7), was performed ex situ by scanning electron microscopy and energy-dispersive x-ray and in situ by open-circuit potential measurements and cyclic voltammetry. Despite the use of SnCl 2 as precursor, energy-dispersive x-ray results showed the real composition to be very distinct from nominal due to SnCl 4 volatilization during the calcination step in the electrode preparation procedure. SnCl 4 formation in the precursor mixture was confirmed by visible spectrophotometric measurements. The substitution of TiO 2 by SnO 2 results in a significant increase in electrochemically active surface area, as supported by scanning electron microscopy, anodic voltammetric charge, q s , and the double-layer capacity, C dl, as a function of composition. Roughness factors between 3600 and 5100 were obtained. A linear C dl vs. q s graph with an angular coefficient close to one was obtained.