Ernesto C. Pereira

Investigation of the electrical properties of SnO2 varistor system using impedance spectroscopy

The electrical properties of tin oxide varistors doped with CoO, Nb2O5, and Cr2O3, were investigated using the impedance spectroscopy technique with the temperature ranging from 25 to 400 °C. The impedance data, represented by means of Nyquist diagrams, show two time constants with different activation energies, one at low frequencies and the other at high frequencies. These activation energies were associated with the adsorption and reaction of O2 species at the grain boundary interface. The Arrhenius plots show two slopes with a turnover at 200 °C for both the higher and lower frequency time constants. This behavior can be related with the decrease of minor charge carrier density. The barrier formation mechanism was associated with the presence of CrSn⋅ at the surface, which promotes the adsorption of the O′ and O″ species which are in turn proposed as being responsible for the barrier formation.

Characterisation of PEO–Al2O3 composite polymer electrolytes

Abstract The behaviour of PEO 8 LiClO 4 with different quantities of α-Al 2 O 3 or γ-Al 2 O 3 was investigated using DSC, AC conductivity and 7 Li NMR experiments. DSC results showed that the presence of the filler does not change the glass transition temperature of the electrolyte but, on the other hand, modifies the quantity of its crystalline phase. From the AC impedance measurements, it was observed that the sample with the highest conductivity at room temperature is PEO 8 LiClO 4 5.3 wt.% α-Al 2 O 3 . The change in the quantity of crystalline phase cannot alone explain the conductivity data, and it is suggested that the space charge contribution in the interphase of the filler particles and the polymeric chains influences the behaviour of the samples. The 7 Li NMR results showed that line width narrowing begins at temperatures close to T g . From the hydrogen decoupling experiments it was possible to estimate the LiH average distances as 2.7 A. The LiLi distance was calculated as being between 2.6 and 3.5 A depending on the number of near neighbours lithium nuclei used in the model.

Application of a distributed impedance model in the analysis of conducting polymer films

The problem analysed is whether the electrochemical transfer in the doping of conducting polymer electrodes must be described as a macroscopic resistance localised at the outer boundaries of the polymer layer, or by a distributed process inside the film region. Impedance measurements of poly(thiophene-3-acetic acid) film in its oxidised state are reported for various values of pH in solution. The analysis shows that the observed response consists on the coupling, throughout the polymer film, of the transfer resistance with other conduction and polarisation processes. This is interpreted as evidence of a distributed charge-transfer resistance in the internal polymer/solution interface.

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

Investigation of polypyrrole degradation using electrochemical impedance spectroscopy.

In this paper, the electrochemical degradation of polypyrrole film was studied by means of overpotential application. The overpotential was 0.58 V versus SCE, and after every 5 min of application of 0.58 V, a cyclic voltammogram was recorded in the range of -0.7 to 0.5 V as well as an electrochemical impedance spectroscopy and electrochemical quartz crystal microbalance (EIS and EQCM). The main characteristic is the huge increase in the charge transfer resistance (r(ct)), which indicates that the insertion process of ions in the polymer matrix is hindered by the electrochemical degradation. Once the process of insertion is damaged, the number of intercalated ions in the matrix should decrease, which is expressed by the low-frequency capacitance, which is proportional to the number of intercalated ions in the polymeric matrix. The decrease of intercalated ions has an influence in the mass variation of the polymer film, which is confirmed by EQCM measurements.

Development of low-cost metal oxide pH electrodes based on the polymeric precursor method.

In this work, the polymeric precursor method was used to prepare low-cost solid-state sensors for pH determination based on iridium oxide as the main pH sensitive material. The iridium content was reduced with addition of TiO(2), forming the binary system IrO(x)-TiO(2), whose electroanalytical properties were evaluated in comparison with a commercial glass pH electrode. The minimum iridium content which gave suitable results was 30mol%, and the electrode presented Nernstian and fast response in the pH range from 1 to 13, with no hysteresis effect observed. Besides, the electrode showed high selectivity in the presence of alkali ions as Li(+), Na(+) or K(+). The amount of iridium in the prepared electrodes was very small (<0.1mg), supporting the efficiency of this method on the simple preparation of functional low-cost pH electrodes.

Preparation and characterisation of Ti/RuO2 anodes obtained by sol–gel and conventional routes

The preparation and characterisation of Ti/RuO2 anodes and RuO2 powders obtained by the thermal decomposition of RuCl3 using isopropanol solution method (ISM) and the polymeric precursor method (PPM) is described. The materials were investigated by SEM and XRD attempting to correlate its morphological and microstructural properties with electrochemical features of the electrodes. SEM micrographies show that the electrode coatings obtained by PPM are rougher than those prepared by ISM. XRD analyses show that the crystallite sizes of the materials obtained by PPM are smaller than those from ISM. It was also observed that RuO2 powders obtained by ISM show an expansion in the unit cell. The electrochemical properties were investigated by cyclic voltammetry and accelerated stability tests for the oxygen evolving reaction in acid medium. Higher voltammetric charge and stability were observed for the electrodes prepared by PPM. Microstructural, morphological and electrochemical features were correlated to the methods of preparation. D 2002 Elsevier Science B.V. All rights reserved.

Nuclear magnetic resonance study of PEO-based composite polymer electrolytes

Abstract The effect of carbon black and ceramic (a-Al 2 O 3 and g-Al 2 O 3 ) particles on the cationic and polymer chain mobilityhave been investigated in poly(ethylene oxide)–lithium salt (LiClO 4 and LiBF 4 ) solid composite polymer electrolytesby using nuclear magnetic resonance (NMR) techniques. Proton ( 1 H) and lithium ( 7 Li) lineshapes and spin–latticerelaxation times were measured as a function of temperature. The 7 Li data indicates that the lineshape and relaxationis affected by the coupling of its quadrupolar moment to the surrounding electric field gradients. Activation energiesextracted from the 7 Li relaxation data are in the range 0.20–0.25 eV. Results show that the addition of a-Al 2 O 3 influenced both, the relative cation mobility and the polymer chain motion. Calculation shows that this result is inquantitative agreement with the conductivity data.

Preparation and characterization of Ti/RuO2-Nb2O5 electrodes obtained by polymeric precursor method

Abstract A new dimensionally stable electrode based on the RuO 2 –Nb 2 O 5 system prepared by a polymeric precursor method is described. The effects of the layer composition and preparation temperature on the electrochemical, morphological and microstructural characteristics and durability were investigated. An increase in the voltammetric anodic charge was observed, which has a maximum value for the nominal composition RuO 2 –Nb 2 O 5 70:30 mol%. The voltammetric charge of the RuO 2 –Nb 2 O 5 70:30 mol% electrodes was maximum for those prepared at 300°C. A decrease in the voltammetric charge was detected as the preparation temperature increased, which could be related to the crystallization process and reduction of the surface area of the layer. X-ray diffraction patterns show the main peaks of RuO 2 in rutile phase for nominal compositions higher than 30 mol% of RuO 2 . On the other hand, Nb 2 O 5 orthorhombic crystalline phase was observed only for electrodes prepared at 600°C. The highest value of the accelerated stability test (AST) for the oxygen evolution reaction (OER) was 22.0 h per mg of RuO 2 for the Ti/RuO 2 –Nb 2 O 5 70:30 mol% composition.

Fractional factorial design applied to investigate properties of Ti/IrO2–Nb2O5 electrodes

The effects of the preparation variables on the electrochemical and morphological properties of Ti/IrO2–Nb2O5 (60:40 mol%) electrodes prepared by the polymeric precursor method were investigated. In order to reduce the number of experiments, the study was carried out using a two-level fractional factorial design with resolution III. It was observed that the charge depends mainly on the temperature of calcination and on the composition of the precursor solution. An average decrease of 26.9 (mC cm−2 mg−1) was observed by changing the calcination temperature from 400 to 500°C. The average anodic charge decreased by 8.93 (mC cm−2 mg−1) when the molar ratio CA/EG (citric acid/ethylene glycol) was changed from 1:6 to 1:12. A similar behaviour was observed when the molar ratio between CA/PS (citric acid/precursor salt) was changed from 6:1 to 12:1, a decrease of 10.3 (mC cm−2 mg−1) being observed. The anodic charge decrease also occurred by increasing the time of calcination as well as controlling the heating rate. Once the temperature was the main variable affecting the anodic charge it was widely investigated in a range between 250 and 700°C. A maximum anodic charge was observed at 300°C while the electrode calcinated at 600°C showed the maximum value in the accelerated stability test (AST) for the oxygen evolving reaction in acid medium. The effects observed on the electrochemical properties were correlated with the morphological and microstructural changes occurring in the samples.