Roberto M. Torresi

Transport Coefficients, Raman Spectroscopy, and Computer Simulation of Lithium Salt Solutions in an Ionic Liquid

Lithium salt solutions of Li(CF3SO2)2N, LiTFSI, in a room-temperature ionic liquid (RTIL), 1-butyl-2,3-dimethyl-imidazolium cation, BMMI, and the (CF3SO2)2N(-), bis(trifluoromethanesulfonyl)imide anion, [BMMI][TFSI], were prepared in different concentrations. Thermal properties, density, viscosity, ionic conductivity, and self-diffusion coefficients were determined at different temperatures for pure [BMMI][TFSI] and the lithium solutions. Raman spectroscopy measurements and computer simulations were also carried out in order to understand the microscopic origin of the observed changes in transport coefficients. Slopes of Walden plots for conductivity and fluidity, and the ratio between the actual conductivity and the Nernst-Einstein estimate for conductivity, decrease with increasing LiTFSI content. All of these studies indicated the formation of aggregates of different chemical nature, as it is corroborated by the Raman spectra. In addition, molecular dynamics (MD) simulations showed that the coordination of Li+ by oxygen atoms of TFSI anions changes with Li+ concentration producing a remarkable change of the RTIL structure with a concomitant reduction of diffusion coefficients of all species in the solutions.

Calibration of the Electrochemical Quartz Crystal Microbalance

The quartz crystal microbalance seems to be a very usefui tool in electrochemical studies, but, up to now, no attempts have been made to calibrate the microbalance under electrochemical conditions. The aim of this paper is to determine not only the average sensitivity for different active areas but also the differential sensitivity as a function of the radial position of a localized change of mass. In this way, for homogeneous mass perturbation, it is possible to calculate the change of mass (Am) from the frequency shift (AJ) for different active areas by taking into account the average sensitivity value. In the case of localized Am, the differential sensitivity value allows one to calculate hm from hfif the position of the event is known. The calibration procedure was made for an AT-6 MHz quartz crystal. Quartz crystals are important sensing devices in liquid phase. They can be used to monitor changes in electrode mass (1-3) or changes in the liquid properties (4, 5). In an electrochemical process, a shift of the value of the quartz crystal resonance frequency (fo) can be attributed to a change in mass (Am) of the electrode, provided that the true relationship between hf and Am is known. The problem of the sensitivity of the quartz crystal microbalance (QCM) has been already dealt with in the air (6-8). The differential sensitivity of the QCM has been calculated from the local damage produced in a thin polymer film by an ion beam sputtering. In the liquid phase the distribution of the vibration amplitude of the quartz crystal has been investigated by putting a tungsten wire probe in contact with the quartz crystal to provoke a change in frequency (9, 10). The results obtained in both liquid and gas phases are rather similar, in spite of the properties of liquid phase which extend the vibration of the quartz crystal even beyond the active region defined by the metal deposit. These works show the complexity of the system and that the boundary conditions used for the derivation of Sauerbreys equation (11, 12) are not always fulfilled. The problem of the QCM calibration has been extensively studied in air. on the contrary, despite the wide use of the QCM in electrochemistry, the calibration procedure of this technique in electrochemical conditions is not depicted in the literature. The aim of this work is to calibrate the electrochemical quartz crystal microbalance (EQCM), not only in the case where hm is uniformly spread on the active electrode surface, but also for localized mass changes. The last case is very relevant to all electrochemical phenomena which produce a change of mass on a small area, e.g., in localized corrosion or gaseous bubble evolution. The general case where the mass change is continuously but nonuniformly distributed across the surface is not addressed in the paper.

The role of ion exchange in the redox processes of polypyrrole/dodecyl sulfate films as studied by electrogravimetry using a quartz crystal microbalance

Abstract The electrochemical redox mechanism of a dodecyl sulfate-doped polypyrrole was studied in detail by electrogravimetry using a quartz crystal microbalance. In order to distinguish the nature of the inserted/deinserted species, different salts were used as aqueous electrolytes. A quantitative analysis of the results indicated that the surfactant counter anion was trapped in the polymer matrix and that both anions and cations of the electrolyte play an important role in the redox processes. Quantitative calculations showed that the hydration sphere of these ions must be taken into account when dealing with the redox processes of conductive polymers adhered to the surface of electrodes.

Hydrogen evolution reaction on anodic titanium oxide films

Abstract Oxide titanium films were prepared using linear potential sweeps up to different limiting potential values between 0 and 60 V in 0.5 M H 2 SO 4 . The electronic characteristics of these films were investigated through impedance measurements during their formation. The hydrogen evolution reaction (HER) in either HClO 4 or H 2 SO 4 solutions in the pH range between 1 and 4 was studied on the different oxide films. The obtained experimental results could be explained assuming that the recombination of adsorbed hydrogen atoms is the rate determining step (rds), and that the Temkin type isotherm applies to the adsorbed intermediates. The log i vs E plot after correction for diffusion control in the bulk gives a slope of 2.3 RT/F . The influence of the physico-chemical properties of the oxide films on the HER has also been studied from the changes in the current—potential profiles. The oxide film thickness changes the HER overpotential, but the slope of the log i vs E relationship remains independent of it.

Polyaniline Based Acrylic Blends for Iron Corrosion Protection

Instituto de Qui´mica, Universidade de Sa˜o Paulo, 05513-970 Sa˜o Paulo, BrazilCoating strategies for corrosion protection considering intrinsically electronic conducting polymers have become important mainlybecause of restrictions on the use of heavy metals due to their environmental problems. This work presents the electrochemicalbehavior of an acrylic blend formed by sulfonate-doped polyaniline and poly~methyl methacrylate! used for corrosion protectionof iron in chloride solutions. The results would indicate that these blends have a dual protection mechanism; forming a passivatingcomplex with the dopant anion ~camphorsulfonate! and simultaneously acting as a physical barrier to avoid chloride anionpenetration.© 2001 The Electrochemical Society. @DOI: 10.1149/1.1381288# All rights reserved.Manuscript submitted October 28, 2000; revised manuscript received April 20, 2001. Available electronically June 15, 2001.

Electrochromism of WO3 in acid solutions: An electrochemical, optical and electrogravimetric study

Abstract The electrochromic reaction of a tungsten trioxide electrode prepared by electrochemical deposition was studied. Chronopotentiometric experiments associated with transmittance measurements were carried out for several film thicknesses. The dependence of electrochromic efficiency on the coloration/bleaching current density, the wavelength of the light source and the film thickness were analysed. The mass and structural changes due to the electrochromic reaction were followed using a quartz crystal microbalance and stress experiments respectively. Film formation and dissolution processes for WO 3 in acidic media are also discussed.

Cathodes for lithium ion batteries: the benefits of using nanostructured materials

Commercially available lithium ion cells, which are the most advanced among rechargeable batteries available so far, employ microcrystalline transition metal oxides as cathodes, which function as Li insertion hosts. In search for better electrochemical performance the use of nanomaterials in place of these conventional ones has emerged as excellent alternative. In this review we present a brief introduction about the motivations to use nanostructured materials as cathodes in lithium ion batteries. To illustrate such advantages we present some examples of research directed toward preparations and electrochemical data of the most used cathodes in nanoscale, such as LiCoO2, LiMn2O4, LiMnO2, LiV2O5 e LiFePO4.

Ether-Bond-Containing Ionic Liquids and the Relevance of the Ether Bond Position to Transport Properties

The ionic liquids (ILs) 1-ethoxyethyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide, [EtO(CH(2))(2)MMI][Tf(2)N], and N-(ethoxyethyl)-N-methylmorpholinium bis(trifluoromethanesulfonyl)imide, [EtO(CH(2))(2)MMor][Tf(2)N] were synthesized, and relevant properties, such as thermal stability, density, viscosity, electrochemical behavior, ionic conductivity, and self-diffusion coefficients for both ionic species, were measured and compared with those of their alkyl counterparts, 1-n-butyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide, [BMMI][Tf(2)N], and N-n-butyl-N-methylpiperidinium bis(trifluoromethanesulfonyl)imide, [BMP][Tf(2)N] and N-n-butyl-N-methylmorpholinium bis(trifluoromethanesulfonyl)imide [BMMor][Tf(2)N]. This comparison was done to evaluate the effects caused by the presence of the ether bond in either the side chain or in the organic cation ring. The salt, LiTf(2)N, was added to the systems to estimate IL behavior with regard to lithium cation transport. Pure [EtO(CH(2))(2)MMI][Tf(2)N] and their LiTf(2)N solutions showed low viscosity and the highest conductivity among the ILs studied. The H(R) (AC conductivity/NMR calculated conductivity ratio) values showed that, after addition of LiTf(2)N, ILs containing the ether bond seemed to have a greater number of charged species. Structural reasons could explain these high observed H(R) values for [EtO(CH(2))(2)MMor][Tf(2)N].

Ionic exchanges in dodecylbenzenesulfonate doped polypyrrole Part 1. Optical beam deflection studies

Abstract Cyclic voltammetry and optical beam deflection (OBD) techniques in different electrolytes, KCl, KClO 4 , CsCl, NaCl, LiClO 4 and LiCl, were used to study the ionic exchange processes in polypyrrole (PPy) doped with an anionic surfactant, dodecylbenzenesulfonate (DBS − ). Our results show the role played by the cations during the first redox process of the polymer. To compare the OBD curves in each electrolyte, they were normalized with respect to their dependence on the chemical nature of the electrolyte. The use of temporal convolution, proposed by Vieil et al. ( J. Electroanal. Chem., 368 (1994) 183), to relate electrical current and angular optical beam deflection, allows the modeling of the OBD curves and the calculation of apparent diffusion coefficients for each electrolyte and the diffusion coefficient of each ionic species.

Transport properties of V2O5/polypyrrole nanocomposite prepared by a sol-gel alkoxide route

Nanocomposites of V2O5 xerogel and polypyrrole (Ppy) were prepared from a vanadyl tris(isopropoxide) precursor and pyrrole monomer by in situ oxidative polymerization of the pyrrole in the sol stage followed by gelation. Unlike previous sol-gel nanocomposite synthetic routes, the gel forms a stable solution from which thin homogeneous films can be cast. The nanocomposite was characterized by XRD, FTIR and electrochemical measurements. Three different thin film types with different spatial arrangements were used to characterize the compensation of charge. X-ray diffraction reveals that the Ppy chains are intercalated within the interlayer region of the V2O5, leading to an increase in the d-spacing from 11.85 Afor V2O5 to 13.8 Afor the nanocomposite. Electrogravimetric results showed that electroneutrality during charging and discharging is achieved predominantly by Litransport. EQCM studies of a bilayer film arrangement with the nanocomposite cast over an electropolymerized film of Ppy revealed that the nanocomposite is an effective barrier for anion transport. The nanocomposite showed a higher specific capacity (283 A h kg � 1 ) than that of V2O5 (236 A h kg � 1 ). This result was attributed to differences in structure between the two materials. The galvanostatic intermittent titration technique (GITT) was used to obtain the diffusion coefficients of Liin the nanocomposite and the V2O5 xerogel. These values range from 2� /10 � 11 to 3 � /10 � 13 cm 2 s � 1 , decreasing as the amount of intercalated Liis increased. For V2O5 the Lidiffusion coefficient ranged from 1� /10 � 12 to 8 � /10 � 14 cm 2 s � 1 , also decreasing as the amount of intercalated Liincreased. In situ resistance measurements revealed a very high conductivity over the potential range of 0.4 to � /0.7 V( vs. Ag/AgNO3) for the nanocomposite, more than a factor of five larger than that for V2O5. # 2002 Elsevier Science B.V. All rights reserved.