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Made available in DSpace on 2020-12-12T01:30:24Z (GMT). No. of bitstreams: 0 Previous issue date: 2020-01-01 Herein, to balance the overall performance of SPEEK membrane, the imidazolium hydrogen sulfate (Im.HSO4), 1-methylimidazolium hydrogen sulfate (MI.HSO4), and 1-butyl-3-methylimidazolium hydrogen sulfate (BMI.HSO4) ionic liquids (ILs) were used as dopants in polymer matrix. A series of SPEEK/ionic liquid composite membranes were successfully fabricated by casting method and evaluated. Comparative studies showed that the membranes doped with 5 wt.% of MI.HSO4 or BMI.HSO4 ILs showed an increase in the proton conductivity of 50% and 30% at 80 °C, respectively, compared with the pristine SPEEK membrane. The proton exchange membrane fuel cell (PEMFC) performance demonstrated that the membrane doped with 5 wt.% of BMI.HSO4 IL (SBMI5) has the highest current density value and the highest power density without loss of performance as temperature increases. This result indicates that this membrane is promising for fuel cells application. Chemistry Department Federal University of São Carlos, P. O. Box 676 Department of Chemistry Universidade Estadual Paulista–Unesp, P.O. Box 473 LRC-Institute of Chemistry Universidade Federal do Rio Grande do Sul, P. O. Box 15003 Department of Chemistry Universidade Estadual Paulista–Unesp, P.O. Box 473

Pt is still the state of the art electrocatalyst for the oxidation of ethanol but it is poisoned by the CO-intermediates resulting in a slow ethanol oxidation reaction kinetic. Here, we show that the electrocatalytic activity of unsupported Pt nanoparticles for the ethanol electro-oxidation in acid medium can be significantly enhanced by the addition of small amounts of CeO2 nanoparticles (≤5 wt%). Comparative studies on bare Pt, Pt-CeO21 wt% and Pt-CeO2—5 wt% catalysts composed of Pt nanoparticles with different sizes (from 10 to >40 nm for bare Pt, and from 6 and 20 nm for the Pt-CeO2 composites) revealed an increase of the microstrain with the decrease of the Pt's average particle size. Besides, there is a strong electronic interaction between the Pt and CeO2 nanoparticles, demonstrated by a systematic shift of the Pt 4f7/2 peak to lower binding energy with the addition of CeO2. Chronoamperometric data showed that the steady state current density normalized to Pt's electrochemical surface area for ethanol electro-oxidation is mainly governed by the microstrain, and it decreases with increasing strain. This trend is explained in terms of a more facile activation of the ethanol molecule at the surface of less strained Pt nanoparticles. Thus, the best performance was obtained with Pt-CeO2 1 wt% formed by Pt nanoparticles with an average size of 18 nm, reaching 1.0 A·m−2 after 900 s at 0.6 V vs SCE.

The behavior of gas bubbles produced during electrochemical processes is of large interest because of the increase in cell overpotential induced by their production, the electrode surface covering by them and, finally, the detachment of these bubbles and their direct impact on the energy consumption during the reaction. In this work, we used coupled experimental and computational approaches to investigate bubble formation under those conditions where there are important convective effects. We have measured both normal and parallel velocity of the solution and built up a computational fluid dynamics (CFD) model. The results were compared to experimental data. Then, having the computational model validated by experimental data, we have simulated different conditions following bubble displacement. An important change of the surface covering is observed during the electrochemical reaction. Considering just the covering change, for example, there is up to 23 % increase comparing to the electrode edge and center.

Two ionic liquids (ILs) with different alkyl chains, 1-butyl-3-methylimidazolium chloride (BMI.Cl) and 1-hexadecyl-3-methylimidazolium chloride (C16MI.Cl), were incorporated into ZnO particles using the microwave-assisted hydrothermal (MAH) method. The morphology and microstructure of ZnO and ZnO/IL composites were characterized along with their photocatalytic effect for dye degradation. While the incorporation of ILs into the ZnO particles did not alter their morphology, it converted the shallow defects into deep defects. These changes improved Rhodamine B (RhB) photodegradation efficiency. The dye degradation reached 62% when ZnO/C1640 was used, whereas it reached 30% when pure ZnO was used, during the same time interval.

We developed Ptpc/Co/Pt nanostructured metallic multilayers (MM) electrodes for methanol and carbon monoxide electro-oxidation. This material uses as an intermediate layer a non-noble metal (cobalt), which is a low cost material. Cyclic voltammetry was used to characterize the electrode electrochemical behaviour for methanol and carbon monoxide. In addition, electrochemical impedance spectroscopy (EIS) was used to study the methanol oxidation on these multilayers. An increase in the peak current density of about 35% for Ptpc/Co10/Pt5 nanostructured metallic multilayer compared to Ptpc electrode was observed for methanol oxidation reaction (MOR). Carbon monoxide electro-oxidation showed large current density for all multilayers electrodes and the onset potentials for this reaction was displaced toward more negative potentials, which indicates weaker adsorption energy for CO over the surface of the materials compared to Ptpc. EIS data indicated a lower charge transfer resistance (Rct) for Ptpc/Co10/Pt5 MM nanostructured during the methanol electro-oxidation, that indicates higher electrocatalytic activity for this electrode compared to Ptpc. Computational modelling results indicate that the presence of a Co-underlayer for Pt/Co/Pt (111) nanostructured system decrease the center of d-band and, consequently, increase the catalytic activity compared to Pt (111) system.

Semiconductor oxides are a class of materials used in many different applications, such as catalysis, solar cells and magnetic devices. In particular, composites with different semiconducting oxides follow synergetic effects with changes of electrical conductivity or magnetic properties. In this case, the synthesis conditions are very important for controlling their properties due to structural defects and non-stable phases at room temperature. In this paper, we analyzed the influence of the synthesis temperature and different concentrations of cerium in the formation of the orthorhombic SnO2 phase. The structural and morphological characterization was performed using experimental techniques such as X-ray diffraction (XRD), allied with Rietveld refinement, field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS). The reflections in the XRD pattern and clear lattice fringes observed by HRTEM confirm the formation of the orthorhombic SnO2 phase in the (Sn1−xCex)O2 with x = 0, 0.05, 0.1 and 0.3 synthesized at 750 °C. The reducing atmosphere generated by the heat treatment process of the samples and the strain originated due to the difference of ionic radii between the tin and cerium ion can play a crucial role in the formation of the orthorhombic phase of SnO2.

The high sensitivity of ATR-SEIRAS allowed simultaneous recording of time-resolved infrared spectra of the Pt-electrolyte interface and of the current flowing during CO-charge displacement experiments. These experiments revealed the dynamics of CO adsorption on Pt, as well as of the rearrangement of the interfacial water layer. Our experiments show that COL sites are populated first, COB appearing only at slightly higher coverages. At potentials sufficiently separated from the potential of zero total charge, the charge displaced upon CO adsorption on Pt and the integrated intensity of the COL band run approximately parallel, as also do the current transient and the time derivative of the integrated intensity of the COL band. This suggests that, for low to medium CO coverage, the current flowing at a given instant during the CO-charge displacement experiment corresponds to the instantaneous rate of CO adsorption, while the charge displaced when a CO submonolayer is formed is a good estimate of the corresponding relative coverage. Although the structure of the interfacial water layer is affected at low coverages, the band corresponding to interfacial water on top of a CO-covered Pt surface only appears at a relatively high CO coverage, suggesting that this kind of interfacial water is linked to the presence of compact islands of adsorbed CO.

Anatase titanium dioxide nanoparticles were synthesized via fast microwave-hydrothermal route and they were characterized by BET, SEM, XRD and EIS. The synthesis process took barely 20 s, which means a low sintering time which provides a decrease in the cost and energy required in the synthesis process. Pure anatase colloidal TiO2 particles were obtained under these synthesis conditions. Particulate TiO2 films were deposited onto high porous graphite substrates and their catalytic activity toward the oxygen reduction reaction in alkaline media was qualitatively demonstrated by cyclic voltammetry and linear sweep voltammetry. The ORR mechanism showed to follow the two-electron pathway via hydrogen peroxide production and further reduction to water.

A novel microwave-electrochemical process is shown to allow Fe-Co alloy to be formed at a stainless steel electrode under fast mass transport − high temperature deposition conditions. In contrast to the Fe-Co alloy formed under ambient conditions, the microwave Fe-Co alloy is produced with a Fe:Co ratio close to unity (mass transport controlled deposition). Unusual properties of the new alloy material include good electrocatalytic performance for the hydrogen evolution reaction (HER), showing low onset overpotential (145 mV), high exchange current density (20 mA cm−2) and Tafel parameters close to those for Pt on stainless steel. It is suggested that in future the microwave-electrochemical deposition method can help providing novel sustainable alloy catalyst materials for a wider range of applications.

In this work, one-step electrochemical synthesis of a supercapacitor with high performance and stability based on carbon nitride and polypyrrole is described. The new composite was easily obtained through the electrodeposition of polypyrrole on the surface of carbon nitride using an aqueous suspension. This material was fully characterized using both theoretical studies and experimental techniques. The sample showed a capacitance value of 810 F g−1 (at 0.2 A g−1), retaining 92% of its initial value after 6000 charge-discharge cycles at 6.0 A g−1. Furthermore, the composite material has a capacitance value of 610 F g−1 at 9.0 A g−1. To explain these results, it was used impedance spectroscopy and DFT calculations. The computational calculations showed the formation of hydrogen bonds between the N H of the polypyrrole and the π-system of the carbon nitride which stabilized the structures. Besides, a planarization effect could also be observed over the polypyrrole chains. Both results are supported by the experimental data collected from the impedance experiments.

Different in situ coupled techniques were used to characterize the naphthenic corrosion of API 5L X70 steel at room temperature. In situ optical microscopy coupled with an electrochemical noise technique indicated a general corrosion process. In situ AFM images showed the early formation of the corrosion product with a thickness of 600 nm after 90 min of immersion. The main regions observed in the temporal evolution of FTIR spectra are evidence of the formation of an iron naphthenate film even at room temperature.

This work presents the development and characterization of a ring-shaped sensor built around a steel sample (AISI 1020) to determine the near-surface solution pH (NSSpH) during pitting corrosion. The pH sensor is constituted by a ring of IrOx, a material that is sensitive to pH changes, synthesized using the Pechini method on a glass tube with a gap of 1 mm between the pH sensor (ring) and the metal sample (disc). The evolution of the corrosion process was also followed using a temporal series of micrographs with an optical microscope. At the same time, the sensor followed the NSSpH during the passivation of the metal, during the breakdown of the passive film, and during the pit evolution in an aqueous basic solution containing HCO3− and Cl− species. When the pit potential was reached, the NSSpH was reduced by one pH unit.

In the development of supercapacitors and rechargeable batteries based on conducting polymers, the stability of the material is an important concern. The objective of this work is to investigate the electrochemical aging of layer-by-layer (LBL) films of two polymers, namely poly(o-methoxyaniline) and poly(3-thiophene acetic acid), which present a self-doping effect, and compare them with poly(o-methoxyaniline) casting film electrochemical properties. Cyclic voltammetry and impedance spectroscopy are used to analyze the behavior of the films. The results show that POMA/PTAA LBL films have a higher electrochemical stability than POMA casting films. This last one presents a continuous degradation as the number of ageing cycles are increased resulting in an 80% decrease from the initial specific capacitance value after 3000 cycles. For LBL films, on the other hand, film stability has a different behavior, and after 3000 cycles, only 1% of decrease in the specific capacitance is observed. Impedance data shows a continuous increase in both polymer resistance and ionic charge transfer resistance in the case of the POMA casting film whereas no changes is observed for the LBL one indicating the film degradation is possible related to the counter ion intercalation/deintercalation which occurs mainly in POMA samples.

This work investigates the possibility of interfacial pH changes during a hydrogen evolution reaction using a finite elements simulation approach. This reaction is a common side step observed in many electrochemical systems, such as electrodeposition or corrosion. To develop a general approach, different mechanisms, i.e., Volmer/Tafel and Volmer/Heyrovský, were investigated. It is observed that for V-H mechanism the interfacial pH change increases 4.2 pH units for those cases where the bulk pH is 5.0. Therefore, in this case, the instant pH at the interface become alkaline, although the bulk is still acidic, which could justify parallel reactions such as metal hydroxide formation in metal electrodeposition. Besides, the interfacial pH changes and the corresponding pH profiles were calculated under several experimental conditions including the composition of the metallic electrode, the bulk solution pH and the total buffer concentration ([HA] + [A − ]).

In this paper, the electrochemical ageing of poly(3-hexilthiophene) was studied using two different films electrodeposited under different experimental conditions. The first one has been was prepared in acetonitrile with 0.1 mol L −1 of monomer + 0.1 mol L −1 of LiClO 4 (Film I). The second one has been synthesized in the same solvent with 0.2 mol L −1 of monomer + 0.2 mol L −1 of LiClO 4 , plus the addition of 10 mmol L −1 of pure water (Film II). The electrochemical ageing has been carried out by the application of an overoxidation potential. At every 20 minutes, a cyclic voltammogram and electrochemical impedance experiments were measured. Each film has exhibited different pathway for the ageing process. Film II showed a smooth degradation of the electrochemical properties, mainly at the interface polymer chains and solution in the pores. The other one, Film I, has presented a two steps degradation process, been the first, up to 120 min, characterized by smooth changes in all the electrochemical parameters necessary to describe the polymer. After 120 min, the second degradation process occurs characterized by an abrupt change in in all the parameters, indicating an important change not only in the electrochemical degradation of the polymer but also in its morphology, measured by SEM micrographs. Furthermore, two electrode impedance electronic experiments confirmed an important degradation of the bulk polymer for Film I.

The study of metallic multilayers (MM) has presented interesting highlights to understand electrocatalytic properties for organic molecules oxidation. This work reports the preparation and characterization of MM using rhodium or ruthenium as an intermediate layer, Ptpc/Rh/Pt and Ptpc/Ru/Pt, aiming to evaluate the electrocatalytic activity toward methanol and ethanol oxidation in acid medium. For methanol electro-oxidation, the evaluated MM presented higher current density than Ptpc, especially Ptpc/Rh2/Pt1 and Ptpc/Ru5/Pt2, where the increase in peak current density was 283% and 422% compared to Ptpc. By in situ infrared spectroscopy (FTIR in situ), it was observed that in MM the CO2 formation from methanol oxidation occurred in lower potentials compared to Ptpc. In the ethanol oxidation, Ptpc/Rh2/Pt1 and Ptpc/Ru5/Pt2 increase in peak current density 32% and 67% compared to Ptpc, respectively. The bands referring to the acetaldehyde, acetic acid and CO2 were observed first to MM than for Ptpc. The better electroactivity of the metallic multilayers in the electro-oxidation of alcohols could be explained by the changes in the electronic interaction between the exposed platinum surface and the rhodium or ruthenium intermediate layer, which facilitated the oxidation of intermediates at lower potentials, and thus, increasing of the electrocatalytic activity of these materials.

Understanding the basic properties of pristine carbon nitride electrodes is of great importance for their further applications as supercapacitor materials. To this end, a comparative study of unmodified carbon nitride is crucial to understand the difference between the bare material and its composite counterparts described in the literature. Therefore, the aim of this paper is to explore the electrochemical behaviour of casting-produced C3N4 electrodes using cyclic voltammetry, charge/discharge curves and impedance spectroscopy. The results from this study show a capacitance value of 113.7 F g−1 at 0.2 A g−1 with an impressive retention of 89.2% after 5000 charge and discharge cycles at 3.0 A g−1. In addition, this material shows a large amount of specific energy (76.5 W h kg−1) at an operation power of 11.9 W kg−1, decreasing only 10.7% due to the electrochemical aging process. Hence, C3N4 constitutes a long-life pristine material with a large amount of energy and a moderate operation power with better performance than other C3N4-based composites found in the literature. These results are important to gain a better understanding of the inherent properties of carbon nitride – to further design composites with higher specific capacitance, longer lifetime, and specific energy.

In this work, the corrosion process of AISI 1020 in crude oil, to which different amounts of sea water (0.5 and 4.0% v/v), naphthenic acid (500 and 3000 ppm), and H 2 S (50 and 1000 ppm) were added, was investigated using electrochemical noise at open circuit potential. The different oil samples were prepared using a 2 3 factorial design involving the three species described above. To analyze the obtained data, wavelet transform and energy distribution plot were used. Using this technique it was possible to separate the contributions of two types of corrosion, generalized and localized, in oil samples with different compositions and to analyze the effect of those variables on the changes that occurred during 55 days of immersion in the experiment. The results showed an increase in pitting formation in those steel samples studied in oil containing H 2 S at 50 ppm and 4.0% sea water. The contribution of generalized corrosion on the metallic surface is higher in oil containing 3000 ppm naphthenic acid than in the other experimental conditions. The results were confirmed by morphological analysis of the corroded samples.

In the present work, it is described the fabrication and characterization of TiO 2 nantotubes prepared potentiostatically using 1.0 or 5.0 vol.% ionic liquid electrolyte medium at 10 or 20 °C. These experimental conditions led to TiO 2 nanotubes with 63-77 nm pore diameter, crystallite sizes between 27-33 nm, and band-gap values in the 3.1-3.2 eV range. Although the morphology and band-gap values were similar, the water splitting reaction showed differences in terms of photocurrent, up to 300%. Besides, we report also important changes in those parameters related with electronic defects, such as carrier density ( N D ), width of depletion layer ( w ) and Debye length (λ D ). Then, the enhancement on photoelectrochemical properties could be related to a decrease in the rate of surface recombination processes. Finally, these proposition are in agreement with the open-circuit voltage-decay, where was observed a higher electron lifetime for TiO 2 NT obtained at higher vol.% of ionic liquid.

Multivariate linear regression aided by a successive projections algorithm (SPA-MLR) was applied in the evaluation of anodic stripping voltammetry data obtained in the simultaneous determination of metals under conditions where there were significant complications due to interference processes such as the formation of intermetallic compounds and overlapping peaks. Using simulated data, modeled from complex interactions experimentally observed in samples containing Cu and Zn, as well as Co and Zn, it was demonstrated that SPA-MLR selected variables that allow chemical interpretation. This feature was used to make inferences about the underlying electrochemical processes during the simultaneous determination of four metals (Cu, Pb, Cd, and Co) in a concentration range where all responses were complicated by interference processes (10-100 ng mL −1 ). Additionally, the analytical performances of MLR models for quantitative predictions were excellent despite the complexity of the system under study.

Pitting corrosion is difficult to detect, predict and design against. Modeling and simulation can help to increase the knowledge on this phenomenon as well as to make predictions on the initiation and progression of it. A cellular automaton based model describing pitting corrosion is developed based on the main mechanisms behind this phenomenon. Further, a sensitivity analysis is performed in order to get a better insight in the model, after which the information gained from this analysis is employed to estimate the model parameters by means of experimental time series for a metal electrode in contact with different chloride concentrations.

This work presents a method to study corrosion reactions in real time by the analysis of a temporal series of micrographs using in situ optical microscopy coupled to electrochemical techniques. The approach was applied to study the pitting corrosion of AISI 1040 steel during chronoamperometric measurements, in carbonate and chloride solutions. Using this coupled approach, it was possible to measure different pit parameters as well as to estimate the mean pit depth. The number of pits and the total pit area presented different behaviors as the chloride concentration increased and active and passive pits were detected.

Electrochemical noise coupled to in situ optical micrographs was used to study corrosion processes in AISI 1020 steel immersed in seawater. The noise data was analysed using wavelet transform. The results were represented in energy diagrams and showed that considerable changes occurred in the corrosion mechanism during the initial step of the experiment (until 7200 s). During this initial step, it was observed that the corrosion was predominantly localized by means of pitting formation and intergranular corrosion. After approximately 50,000 s (14 h) of immersion, general corrosion processes become dominant. The conclusions obtained from the analysis of the current noise signal using the wavelet transform were confirmed by optical micrograph which suggests that the coupling of electrochemical noise and optical microscopy techniques can be valuable in studying corrosion processes in this system. Another advantage of this procedure is the elucidation and separation of corrosion processes, such as, for example, pitting and intergranular ones, that are not commonly identified by other techniques.

In the present work, an innovative methodology is used to contribute in the elucidation of methanol electro-oxidation reaction on polycrystalline Pt surface. We coupled infrared spectroscopy to linear voltammetry and evaluate the data using parallel factor analysis (PARAFAC) to extract hidden information concerning the oxidation reaction. Five adsorbed species were simultaneously extracted and quantified at different applied potentials corresponding to: formate, sulfate, water, total CO and COL shift. It was observed that the CO adsorbed content starts at maximum on low potentials and gradually decreases up to 1.0 V. Meanwhile, the interfacial water content on the surface of the CO-free Pt increases, which causes the shift on the COL band, resulting in a further decrease of the interfacial water content. The water content increases again until up to 0.8 V, which it reaches a constant value, due to formate production and subsequent adsorption of sulfate. Using this technique, it was possible to determine the potential window where each chemical species is adsorbed on the surface, as well as the effect of methanol concentration and scan rate on the mechanism.

This work presents a study of the initial instants in the pitting corrosion of AISI 1040 steel, analyzed by temporal series micrographs coupled to an open circuit potential (Eoc) and polarization curves. During the Eoc measurement, the pit induction time and the initial pit growth in MnS inclusions was detected in alkaline sulfide solution. The pit area behavior has two distinct rate of area changes in specific regions directly associated to current slope changes. Finally, it was possible to create a three-dimensional model of the pit depth evolution on the metal, using Faraday’s law and the bullet-shaped geometry.

Multilayered Pt/Ir/Pt films were electrodeposited over titanium substrates, controlling the film thickness by the deposition charge. Two probe molecules were used to perform the evaluation of catalytic activity, CH3OH and CO. Using a 22 factorial design with a central point, optimized intrinsic catalytic activity was obtained over the Ti/Pt25mC/Ir6mC/Pt electrode. An enhancement of the current density of up to 2.76 times was observed for the methanol oxidation reaction (MOR) compared with the reference sample (Ti/Pt electrodes), even though both samples had the same electroactive area. In addition to the methanol oxidation voltammetry, CO stripping voltammetry suggests that MM-like systems are less susceptible to the catalyst-poisoning phenomenon compared to the Ti/Pt ones. Impedance spectroscopy was mainly used to monitor the charge transference resistance (Rct), whose results showed a clear Rct reduction for the MOR process over MMs compared with Ti/Pt electrodes, corroborating the previous observations regarding the general catalytic improvement.

We report the microwave-assisted hydrothermal synthesis of bimetallic Pt-Au nanoparticles with different Pt/Au mole ratio, and investigate their performance towards the electro-oxidation of formic acid. The as-synthesized Pt-Au sol was dispersed on a graphite electrode, without any binding agents, which allowed us to control the mass of alloy deposited. Pt-Au alloys showed better activity than bulk Pt and/or Pt nanoparticles towards the oxidation of formic acid, as evidenced by the decrease in the onset potential and the higher currents in the corresponding cyclic voltammograms. The higher activity is due both to atomic-ensemble effects, which lead the reaction through the so-called direct pathway with insignificant CO poisoning, and to electronic effects, which optimised the interaction between the catalyst surface and the reactive intermediate in the direct path. Further insight into the individual contributions of atomic-ensemble and electronic effects and their effect on the catalytic activity was provided by the analysis of galvanostatic potential oscillations.

Oxidative stability and cold-filter plugging points (CFPP) of blends of biodiesel from palm, sacha-inchi, jatropha and castor oils were evaluated. Blends were made as a strategy to obtain a biodiesel with a better performance. These properties of biodiesel depend on the type of methyl-ester constituents and they are generally opposed, i.e., a biodiesel with good oxidative stability exhibits bad CFPP. Biodiesel was produced through KOH-catalyzed methanolysis of the oils. Binary blends of biodiesel from castor-jatropha, palm-castor and palm-sacha inchi were made, in proportions of 25:75, 50:50 and 75:25. The oxidative stability was evaluated following the standard EN 14112. CFPP of pure biodiesels and binary blends were evaluated according to ASTM D6371. An induction time greater than 6 hours and a CFPP below 0 degrees C were set as quality criteria. Among the pure biodiesels, only castor oil biodiesel achieved this quality because its induction time and CFPP were 31 h and -7 degrees C, respectively. The best biodiesel blend was made of 75% jatropha and 25% castor. This blend achieved an induction time of 7.56 h and a CFPP of -12 degrees C. However, this blend has a viscosity higher than the required by international standards. The oxidative stability (induction time) and the CFPP were correlated with the structural indices APE (allylic position equivalent), BAPE (bis-allylic position equivalent), SME (saturated methyl esters content), MUME (mono-unsaturated methyl esters content) and PUME (poly-unsaturated methyl esters content); it was found that BAPE and PUME correlate with IT, while CFPP does not correlate with any of these indices. (C) 2012 Elsevier B.V. All rights reserved.

The aim of this paper is to investigate changes in morphology and microstructure of TiO2 films, prepared by sparking anodization of Ti in a H3PO4 solution, by applying different formation charges. We show that although films obtained by this technique are rarely used in photocatalytic applications, the morphological and microstructural changes during sparking anodization produce TiO2 films that can be used as photocatalysts. In contrast to qualitative analysis commonly found in the literature, we used quantitative methods of analysis to quantify average pore diameter and pore density from the morphology and structural parameters from X-ray diffraction (XRD) patterns using the Rietveld refinement. The results indicated that changes in both the morphology and crystalline structure have a strong influence on the photoactivity of the films. From this investigation, we concluded that, for films prepared in early stages of anodization, the morphology had the biggest influence on photoactivity, and after applying 72C of charge, crystalline properties dominated the photocatalytic characteristics of the films.

This work describes CO and methanol electrooxidation over Bi/Pt 1.2:1.2 or 10:10 ML electrodeposited on a bulk platinum substrate. It could be observed in a blank solution that the same features for bulk Pt and Pt/Bi/Pt MM and the hydrogen region were overlaid. The electroactive areas, calculated using the hydrogen desorption method, are the same for both bulk Pt and Pt/Bi/Pt MM electrodes. This is in agreement with AFM images and RMS values, which were the same for bulk Pt and Pt/Bi/Pt MM electrodes. CO stripping voltammograms showed a shift in the anodic peak potential towards the negative direction of 138 and 197 mV for Pt/Bi/Pt 10:10 and 1.2:1.2 ML, respectively, in comparison to bulk Pt. Moreover, for methanol electrooxidation the Pt/Bi/Pt 1.2:1.2 ML electrodes presented an enhancement of 315% and 22 times in the peak current density compared to bulk Pt using cyclic voltammetry and chronoamperometry techniques respectively. Using electrochemical impedance spectroscopy, it was possible to observe the lowest resistance charge transfer for Pt/Bi/Pt 1.2:1.2 ML compared to Pt/Bi/Pt 10:10 ML and bulk Pt respectively. We designate the Pt/Bi/Pt MM electrodes as a giant multilayer electrocatalytic (GME) system, due to their enhanced electrocatalytical properties.

The aim of this paper is to evaluate the application of valve metal anodization to water heating. The solution heating occurs during the oxide breakdown region, a potential oscillation region where the main process is the electronic current associated with the water oxidation reaction over the oxide surface. It was observed that the heating rate is influenced by the current density since the temperature rise is governed by the spark events during the galvanostatic growth. The heating rate of the galvanostatic heater was compared with some commercial heaters, and found to be 14% higher than a commercial resistance-based heater and 29% higher than a shower element.

This work describes the CO and methanol electrooxidation over an Ir/Pt bilayer electrodeposited on a platinum polycrystalline substrate. In the blank acidic solution it was observed that the electrochemical behavior of both the polycrystalline Pt and Pt/Ir/Pt nanostructured electrodes were very similar. The electroactive area, calculated using the hydrogen desorption method, are the same for both materials. In order to investigate the effect of the thickness change of Ir interlayer, two different samples were prepared. One with 1 Ir monolayer and the second with 3 monolayers thick. CO stripping voltammograms showed a shift in the anodic peak potential towards the negative direction of 160 and 180 mV for Pt/Ir/Pt 3:1 and 1:1 ML, respectively, compared to polycrystalline Pt. Besides, for methanol electrooxidation, the Pt/Ir/Pt electrodes presented an increase of 170% in the peak current density compared to polycrystalline Pt. These results are in agreement with the calculated activation energies which were 31.5, 39.0 and 43.5 kJ mol −1 for Pt/Ir/Pt 1:1, 3:1 ML and polycrystalline Pt electrodes, respectively. Using the electrochemical impedance spectroscopy, surprisingly, the Pt/Ir/Pt electrodes, did not exhibit the inductive arc which means that the poisoning of the electrode surface is not important in this case.

Electrochemical decolourisation of Reactive Orange 16 was carried out in an electrochemical flow-cell, using as working electrodes a Pt thin film deposited on a Ti substrate (Pt/Ti) prepared by the Pechini method and a pure platinum (Pt) foil. Using the Pt/Ti electrodes better results for dye decolourisation were obtained under milder conditions than those used for pure Pt. For the Pt electrode, colour removal of 93 % (λ = 493 nm) was obtained after 60 min, at 2.2 V vs. RHE, using 0.017 mol L−1 NaCl + 0.5 mol L−1 H2SO4 solution. For the Pt/Ti electrode there was better colour removal, 98%, than for the Pt electrode. Moreover, we used 0.017 mol L−1 NaCl solution and the applied potential was 1.8 V. Under this condition after 15 min of electrolysis, more than 80% of colour was removed. The rate reaction constant, assuming a first order reaction, was 0.024 min−1 and 0.069 min−1, for Pt and Pt/Ti electrodes, respectively.

In the present paper we developed and simulated a two-dimensional model for the electrodeposition inside the porous anodic alumina template, which deals with one only potentiostatic pulse applied to one pore. The potential distribution within the oxide is described by Laplace equation and the diffusion equation takes into account the transport of the metal ions in the solution. These equations are coupled by time-dependent boundary conditions at the deposition interface. The charge transfer process is described by a Butler–Volmer relationship. Two limit situations corresponding, respectively, to a completely insulating template and to a metallic cavity are described by the model. The first case corresponds to the ideal condition for a truly one-dimensional deposition because there is no current on the pore wall. In the second limit situation, the reacting interface is treated as an equipotential and the deposition kinetic is limited by the charge transfer process. Furthermore, in this case one has a bi-dimensional electrodeposit growth. Between these limit situations, the charge transfer process and the voltage drop inside the oxide are equally significative. In this case, the model captures the dynamic interaction between the concentration gradients and the potential distribution inside the oxide which allows us to explain several experimental observations. © 2008 Elsevier Ltd. All rights reserved.

In this present paper the influence of viscosity on the ionic dynamics of polymer gel electrolytes prepared by the Pechini polymeric precursor method is investigated by impedance spectroscopy, differential scanning calorimetric (DSC) and NMR techniques. Polymer gel electrolytes are formed by ethylene glycol (EG) and citric acid (CA) and lithium perchlorate. Room temperature conductivity of the order of 2.3 × 10 −4 S/cm was obtained for the sample of EG/CA:LiClO 4 with lower viscosity ( η = 197 cP). The results show that the ionic conductivity of the electrolytes increases for decreasing viscosity. Proton ( 1 H) and Lithium ( 7 Li) NMR lineshapes and spin–lattice relaxation times were measured as a function of temperature and viscosity (197–868 cP). The 7 Li relaxation process was found to be dominated by quadrupolar couplings. The activation energy extracted from the 1 H and 7 Li relaxation data (∼0.23 eV) was found to be independent of the viscosity of the gel electrolyte. The 7 Li NMR relaxation results indicate an increase of the lithium ion mobility with decreasing viscosity.

The electrodeposition of cobalt from sulphate solutions containing boric acid was investigated using EQCM technique coupled with potentiostatic measurements. The boric acid was added to electrolyte as a buffer to avoid the local pH rise caused by parallel hydrogen evolution reaction (HER). The results showed that the buffer contribution of boric acid is effective in the cobalt electrodeposition at 25 ◦ C; however, cobalt hydroxide is formed simultaneously with cobalt deposition at 48 ◦ C. The M/z values calculated using the Sauerbrey equation and the Faraday Law showed that in the initial stages of deposition at 48 ◦ C, only cobalt deposits were detected, but after 2 s, an important amount of Co(OH)2 started to be formed.

Electrodeposition of cobalt from sulfate solutions at different pH values was investigated using the EQCM technique coupled with cyclic voltammetry. The results show that cobalt hydroxide is formed simultaneously with cobalt deposition during the early stages of reduction due to the pH variation near the electrode surface caused by the parallel hydrogen evolution reaction (HER). This result was confirmed using M/z values calculated using the Sauerbrey equation and Faraday's law, which showed the presence of cobalt hydroxide in the electrodeposits. A flux model was developed and it assumes a direct reduction of cobalt, simultaneous HER and the formation of cobalt hydroxide during the early stages of deposition at pH 4.10. When the solution pH is decreased to 3.33 only the direct cobalt reduction is observed without any hydroxylated species formation.

Polypyrrole films modified with Ni Particles were eletrodeposited using a constant potential on a Pt/quartz crystal electrode with the aim to investigate the changes in the voltammetric and mass profiles compared with pure polypyrrole. The electrochemical behavior of the polypyrrole with Ni showed a new cathodic peak at −0.73 V associated to Ni2+ reduction. Besides, it was characterized that there is a redox-pair of the polymer and Ni and a model was proposed to explain the phenomenon considering the eletroneutrality principle. The transport of the ClO 4 - ion into (or outside) the polymer is opposite to the expected direction. This occurs due to the redox-pair between the PPy chain and Ni species which can be explained by a loss of anions during Ni oxidation and the gain of anions during the Ni reduction.

This paper presents the preparation of a completely new Pt/Rh/Pt multilayer. This material was characterized by means of X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry (where the change in the electroactive area in the material during the deposition of the bilayer was discarded). The material was further tested for the oxidation of small organic molecules (methanol, ethanol, formaldehyde and formic acid) with the aid of cyclic voltammetry and chronoamperometry. In this study, increases of 295%, 266%, 140% and 380% in the voltammetric peak current density for methanol, ethanol, formaldehyde, and formic acid oxidation at the Pt/Rh/Pt bilayer, respectively, are presented.

This paper describes the study of the preparation of Ti/Ti1-xCexO2 electrodes using the Pechini method and their application for the electroreduction of nitrobenzene in aqueous acid medium. The electrodes were studied using X-ray diffraction, and scanning electron microscopy and it was found that the electrocatalytic properties are influenced by the microstructure and morphology of the electrodes. All doped electrodes presented an enhanced performance for nitrobenzene electroreduction when compared to pure TiO2. The Ti0.995Ce0.005O2 electrode presented the best performance, with an increase of 58% in the aniline yield under galvanostatic electrolysis conditions.

This paper describes Rh electrodeposition on Pt electrodes in HClO4 solutions. Using cyclic voltammetry (CV), coupled with electrochemical quartz crystal microbalance (EQCM) studies, we determined the main characteristics related to oxide formation on rhodium deposited on platinum. During the deposition, the results demonstrated that Rh0 ad-atoms are stable on Pt in the potential range investigated at different saturation coverages. Data obtained using EQCM and CV show that the number of electrons transferred in the oxidation of Rhads on Pt is 3 and a Rh(OH)3 species was proposed as the adsorbed product. At potentials higher than 1.0 V, Rh(OH)3 is oxidized to RhO(OH)3. This species is reduced to Rh(OH)3 and then to Rh0 during the negative potential sweep.

The influence of the space charge created by the presence of TiO2 nanoparticles on the lithium and polymer chain mobility have been investigated in solid composite polymer electrolytes (CPE), poly(ethylene oxide) (PEO) LiClO4, by using complex impedance spectroscopy and nuclear magnetic resonance (NMR). Special care was taken with the synthesis and the characterization of the TiO2 particles and with the composite preparation. The conductivity and NMR measurements were undertaken in composite samples nanoparticles having constant total surface area. Proton (1H) and lithium (7Li) lineshapes and spin-lattice relaxation times were measured as a function of temperature. Activation energies extracted from the 7Li relaxation data are in the range 0.20–0.22 eV. The NMR decoupling experiment suggests that the Li–Li interactions are stronger in the composites when compared with those of the ceramic free electrolytes.

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.

The small signal ac response of IrO2-based electrodes was investigated using a general impedance model for porous electrodes. This model consists in a distributed equivalent circuit that describes the transport and charge storage of electroactive species coupled with the reaction throughout the internal surface. The evolution of the impedance spectra with the variation in the dc-applied voltage was predicted, and the measurements obtained showed a very good agreement with the theory. The model allows the evolution of parameters related to transport, charge storage, and reaction, to be followed under widely varying conditions, from the capacitive domain to the region of heavy current flow by intense surface reaction. The active surface sites play a dominant role both in the capacitive charging process and in the transport component of the impedance. © 2001 Elsevier Science B.V. All rights reserved.

The article is concerned with the improvement of electrochemical impedance models for analysing conducting polymer electrodes. The basic framework is that of distributed impedance models (transmission lines) which have proved quite effective in the electrical study of porous systems. Impedance measurements of poly(thiophene-3-acetic acid) films are reported at different stages during oxidation, varying the stationary applied potential. The main issues addressed are: (a) the direct observation of anomalous transport features in the polymeric matrix which provides a solid explanation for the otherwise intriguing high-frequency wing of the spectra; (b) the sharp variation of the dc polymer resistance at a threshold potential Eth that clearly marks the insulator to conductor transition between the polymer oxidation states; and (c) the observation of two different inner polarisation impedances, namely, one parallel and the other normal to the outer macroscopic interfaces. This analytical tool allows us to extract relevant and accurate information about the electrical characteristics of the polymer film electrodes. In addition, some relations between electrical and morphological properties are suggested from the dependences of the fitted parameters with the applied potential.

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. © 2001 Elsevier Science Ltd. All rights reserved. Keywords :Polymer electrolytes; NMR; Composite; PEO; Carbon blackwww.elsevier.nl:locate:electacta

TiO2 doped Nb2O5 thin films of concentration 0.1–10 mol% were prepared by the Pechini method and their electrochemical and electrochromic properties were characterized by cyclic voltammetry, chronoamperometry, ‘in situ’ UV–Vis spectrophotometry and scanning electron microscopy. The films obtained have a thickness between 440 and 550 nm. The cyclic voltammograms showed an increase in the cathodic current densities for the doped films. The highest values of the insertion charge density were observed for films containing TiO2 between 0.05 and 0.5 mol%. The undoped Nb2O5 films showed a transmittance variation (ΔT at 620 nm) of 56.8% and a coloration efficiency (CE) of 14.9 cm2 C-−1. An increase in ΔT and CE was observed with the increase of the TiO2 concentration up to 1.0 mol%, which presented ΔT=84% and CE=26.9 cm2 C−1. Above this concentration, the ΔT and CE values decreased. These results are discussed considering the electronic and morphological properties of the samples.

The effects of three synthesis variables on the electrochemical properties of TiO2 prepared by the Pechini method are described. To minimize the number of experiments, a factorial design was used. It was shown that, in the presence of nitrobenzene, the temperature and the number of layers were the most important variables. The effects of these variables on the cathodic charges measured by cyclic voltammetry were −0.75 and 2.76 mC cm−2, respectively. Explanations of these findings are based on the effect of the conditions of preparation on the morphology and microstructure of the 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.