ELECTROCHEMICAL ANALYSIS USING ANGULAR POLARIZATION MODULATED ATIR SPECTROMETRY AND FLUOROMETRY

The absence of suitable in situ methods for obtaining electrical (charge-potential) and chemical information at electrode/electrolyte interfaces has represented a serious experimental incapacity in a broad range of fundamental and applied areas. This paper addresses the use of attenuated total internal reflection spectrometry and total internal fluorometry in conjunction with system modulation, both electrical cell control and spatially selective optical polarization, to probe this experimentally difficult region. Although applications of these techniques to electrolyte/solid electrode interfaces under equilibrium conditions will be described, emphasis will be given to interfacial characteization under current densities that significantly depart from equilibrium. The experimental configurations and difficulties in such measurements will also be addressed.     

A calibration technique for an Ag/AgCl reference electrode utilizing the relationship between the electrical conductivity and the KCl concentration of the internal electrolyte

For ensuring the long-term performance of an Ag/AgCl reference electrode, a calibration technique was developed by using the linear relationship between the electrical conductivity and the KCl concentration of an internal electrolyte. It was observed that the electrical conductivities increased with an increase of the KCl concentration of the internal electrolyte, and the potentials of the Ag/AgCl electrode shifted anodically with a decrease of the KCl concentration. For simulating a long-term exposure of the electrode, we designed a reference electrode with a tiny perforation to accelerate the diffusion of the internal electrolyte. During the acceleration tests, the potentials of the Ag/AgCl electrode were shifted by almost the same amount as the potentials calculated from the change of the electrical conductivity of the internal electrolyte. Consequently, this technique was confirmed to be applicable for the calibration of an Ag/AgCl electrode, especially when exposed to non-electrolytic aqueous solutions.     

全固态锂离子电池的研究进展与挑战

全固态锂离子电池具有安全性高、电化学性能优异等优点,但存在电极与电解质界面相容性差、室温离子电导率低等问题。本文总结了以上问题产生的原因及解决方案。对于正极界面,可复合正极材料与固态电解质、构造三维多孔结构固态电解质或在界面处引入缓冲层。对于负极界面,可设计界面层、原位聚合生成固态电解质、构造固态电解质骨架或使用自愈合和弹性固态电解质。对于固态电解质自身,以聚氧化乙烯（PEO）固态聚合物电解质为例,可添加增塑剂、无机陶瓷填料或构造聚合物共混物与嵌段共聚物。最后,对今后的研究方向提出了建议：应注重优化电极/固态电解质界面层;探索锂离子传输机理;构建具有高离子电导率的固态电解质等。     

Effect of silica on the interfacial stability of the PEO-based polymer electrolytes

Summary In order to evaluate the effect of silica on stabilizing the interface of lithium metal electrode/solid polymer electrolyte, the cyclic behavior for silica-free and silica-containing polymer electrolyte under electrical stress was investigated using cyclic voltammetry. These electrolytes have an ionic conductivity of the order 10^-4 S/cm at above 60°C and most importantly the introduction of hydrophilic silica in PEO-based polymer electrolyte has brought about the enhanced stability of lithium metal electrode/polymer electrolyte interface especially under electrical stress. This in turn supports the suitability of the composite polymer electrolytes with hydrophilic silica for fabrication of enhanced rechargeable solid lithium polymer batteries. Received: 7 May 2002/ Revised version: 10 July 2002/ Accepted: 12 July 2002     

Model interpretation of electrochemical impedance spectroscopy and polarization behavior of H2/CO mixture oxidation in polymer electrolyte fuel cells

A mathematical model is proposed based on electrode kinetics analysis for the oxidation of 2% CO+H₂ mixture in polymer electrolyte fuel cells. Successful simulation of the polarization curve and experimental impedance spectra for Pt/C electrode system confirm the validity of the model which shows that the impedance is strongly dependent on electrode potential. With the increase of potential, an inductive behavior will occur. It is believed that the appearance of this inductive pattern can be used as a criterion for the onset of CO oxidation by the coincidence of the potential at which inductive behavior occurs with the ignition potential for CO oxidation. The effects of change of CO oxidation rate constant and CO adsorption equilibrium constant on impedance pattern, as well as on CO surface coverage and Faradaic current are also delineated with the use of the proposed model.     

Electrochemical impedance spectroscopy of the alkaline manganese dioxide electrode

Two-probe electrochemical impedance spectroscopy measurements were carried out on the electrolytic manganese dioxide electrode in concentrated KOH electrolytes under a variety of experimental conditions. These included varying the electrode thickness and compaction pressure, electrolyte content and concentration, degree of manganese dioxide reduction and the presence of TiO₂ (anatase) as an additive. The overall electrode impedance was found to decrease when thin electrodes, prepared under high compaction pressures, with an excess of electrolyte, were used. The impedance of the EMD/electrolyte interface was also minimized when 5.0 M KOH was used as the electrolyte. This correlates with a maximum in electrolyte conductivity. The electrode impedance also increased as the degree of EMD reduction was increased, as was expected. Under these experimental conditions the electrode impedance increased in the presence of TiO₂ (anatase), which has negative implications for its commercial use. This conclusion was reached despite the differences in experimental conditions between this work and in commercial applications. An equivalent circuit was also derived and used as an aid in interpreting the impedance data.     

Anisotropy of the Optical Properties of Electrode Surfaces

Differential reflection spectroscopy of electrode/electrolyte interfaces gives a quantity which in analogy to thermodynamic properties of the surface region shall be called “dielectric surface excess”. The dielectric surface excess has two complex components representing the anisotropy between the dielectric properties tangential and normal to the surface. Methods are described for the experimental determination of the two components. Results for oxide layers on platinum, bromide adsorbed on gold, rhodamin B adsorbed on silver and monomolecular films of lead on silver are discussed.     

The evaluation of the polarization resistance in a tubular electrode and its application to the hydrogen electrode reaction

An alternative method for the determination of the kinetic parameters involved in the elementary steps of the reaction mechanism of the hydrogen electrode reaction is proposed. It is based on the determination of the variation of the polarization resistance in a tubular platinum electrode with a laminar flow of electrolyte as a function of the activity of protons of the electrolyte solution. A theoretical expression that relates the experimental variables and the equilibrium polarization resistance is developed, which takes into account the current distribution along the electrode surface. The results are compared with others obtained previously, contributing to the verification of the kinetic mechanism through a completely different experimental procedure.     

Energy Conversion Via Pigmented Bilayer Lipid Membranes

Abstract

One approach to the problem of understanding photosynthesis quantum conversion is to investigate the manner in which the thy-lakoid membrane of the chloroplast achieves many of its physio-chemical processes. Ideally, a great deal of information may be obtained from electrical measurements in the dark and in the light. This procedure entails the-placing of electrodes across the membrane, much as has been done for the nerve membrane of axons. However, such an approach is not yet feasible for most photoactive membranes owing to their tiny size. An alternative approach is to investigate experimental pigmented bilayer lipid membranes (BLM), which are very suited for studying light transduction and light-initiated redox reactions.

This paper will discuss certain photoeffects observed in pigmented BLM in terms similar to those that occur at semiconductor/ metal interfaces (Schottky barriers). The pigmented BLM separating two aqueous solutions behaves as a bipolar electrode, into and from which electrons and holes can be transferred. Also owing to its ultra-thinness (< 100 Å), a very large field strength can be developed across the BLM under a small electrical bias. General principles for electron transfer processes across the BLM are discussed. The conversion of light into electrical and/or chemical energy by pigmented BLM is demonstrated, and proposed schemes for the understanding of green plant photosynthesis via the thy-lakoid membrane are described.     

Montages coaxiaux pour l'etude des phenomenes rapides de relaxation aux interfaces metal-electrolyte aqueux

For the study of the relaxation phenomena at the metal-aqueous electrolyte interfaces in the 10^?9 s or 10⁸ Hz range, the experimental set-ups must be entirely coaxial with properly selected electrode diameters. Set-ups using a coaxial line (transmission and reflexion) are proposed after having been tested in the time domain.     

Key technological elements in dye-sensitized solar cells (DSC)

The best cell efficiency of lab scale dye sensitized solar cell (DSC) exceeds 11%, but there are still many technological problems to overcome for commercialization. This review describes key technological elements in DSC, including working electrodes with dye/TiO₂/electrolyte interfaces, quasi solid state electrolyte with ion diffusion, and counter electrodes with electrolyte-catalytic electrode interfaces. Their operating principles, equivalent electric circuits and measurement techniques are described.     

Electrocatalytic reforming of carbon dioxide by methane in SOFC system

The reaction of carbon dioxide catalytic reforming with methane is an attractive route because these greenhouse gases can be converted into variable feedstocks. However this reaction is a highly energy consuming and coke forming process. These problems were improved by the electrocatalytic reforming of CO₂ with CH₄ in a solid oxide fuel cell (SOFC) membrane reactor system, which generates high electrical power and synthesis gases. The single cell consists of catalyst electrode (NiO–MgO), counter electrode ((La,Sr)MnO₃) and Y₂O₃ stabilized ZrO₂ (YSZ) electrolyte. The reaction rates of CO₂ and CH₄, and the electrochemical properties were investigated by an on-line GC and impedance-analyzer under open- and closed-circuit conditions, respectively. It was found that reaction rates of CO₂ and CH₄ under the closed-circuit condition were more stable than those of the open-circuit. The results were interpreted that the stability of catalyst anode was maintained by the reaction of oxygen ion transferred from the cathode with the surface carbon formed in the internal CO₂ reforming by CH₄ in SOFC system.     

In situ wetting of aluminium during the growth of porous alumina by anodic oxidation

The growth of nanoporous alumina at an aluminium surface which is not completely covered by an electrolyte has been investigated. At one point the electrolyte forms three interfaces: oxide/electrolyte, metal/electrolyte, electrolyte/air. At this three-interfaces-point a self-wetting induced oxide growth occurs. The wetting increases up to 7 mm from the electrolyte contact edge. At this final stage of wetting an equilibrium between the wetting induced flow (bottom-up) and the gravity-driven flow (top-down) is reached. The thickness of the nonporous layer decreases with the distance from the starting point.     

Large-area TiO2 nanotube dye-sensitized solar cells using thermal-sprayed Ti layers on stainless steel

This work presents large-scale dye-sensitized solar cells and methods for their manufacture. A dye-sensitized solar cell device contains a photosensitive dye adsorbed on a large surface of the anode, and a transparent conductive cathode disposed opposite the anode, wherein platinum nano-catalytic particles adhere to its surface, and an electrolytic solution is sealed between the anode and the transparent conductive cathode. A titania nanotube film was fabricated by thermo-spraying titanium film on 304 stainless-steel substrate. The photo-current conversion efficiency was tested under an AM 1.5 solar simulator. The dye-sensitized solar cell device has a short current density of 8.22 mA cm^–2, open voltage of 0.71 V, fill factor of 0.59, and conversion efficiency of 3.4%. The internal impedance of the dye-sensitized solar cell was detected and simulated using an electrical impedance spectra technique with inductance, resistance, and capacitance characteristics. The stainless-steel/titania, titania/electrolyte, electrolyte, and electrolyte/(platinum/indium tin oxide) interfaces were simulated using an resistor–capacitor parallel circuit, and bulk materials such as stainless steel, tin doped indium oxide, and conducting wire were simulated by using a series of resistors and inductance.     

Curvature effects on electric-double-layer capacitance

Understanding the microscopic structure and thermodynamic properties of electrode/electrolyte interfaces is central to the rational design of electric-double-layer capacitors (EDLCs). Whereas practical applications often entail electrodes with complicated pore structures, theoretical studies are mostly restricted to EDLCs of simple geometry such as planar or slit pores ignoring the curvature effects of the electrode surface. Significant gaps exist regarding the EDLC performance and the interfacial structure. Herein the classical density functional theory (CDFT) is used to study the capacitance and interfacial behavior of spherical electric double layers within a coarse-grained model. The capacitive performance is associated with electrode curvature, surface potential, and electrolyte concentration and can be correlated with a regression-tree (RT) model. The combination of CDFT with machine-learning methods provides a promising quantitative framework useful for the computational screening of porous electrodes and novel electrolytes.     

Etude electrochimique a l'etat solide du couple anthraquinone—9.10—anthraquinol,en tant que matiere active d'electrode de generateur secondaire

The—9.10 anthraquinone—anthraquinol couple has the proper characteristics to be used, in the solid state, in negative electrode reactions of secondary electrochemical generators in the presence of an aqueous electrolyte.Our research work covered the conditions of electrochemical formation of anthraquinol, of little stability in the presence of oxygen, within the electrode and starting with anthraquinone. The electrochemical characteristics of such electrodes were then measured.The equilibrium potential of the electrochemical system was about 80 mV/enh in a normal sulphuric medium and the actual capacities recorded were measured with respect to different operation conditions of the electrodes. The variation of these characteristics, according to electrode composition, medium acidity and temperature, were studied.This research led to the development of an experimental battery supplying a voltage of approx 0,5 V and having a mass energy ratio of about 25 Wh/k of electrode.     

Current distribution at electrode surfaces as simulated by finite element method

In this article, an approach of finite element method has been introduced to predict the distribution of potential and current on electrode surfaces. In this method, the bulk of electrolyte solution and the main body of solid electrodes are modeled to be normal conductive media; while the surface of the electrode, whose current-potential (I-V) curve satisfies experimental data, is modeled to be special nonlinear boundary conditions. This modeling approach for electrochemistry system is successfully verified by experimental data, and would contribute to a better understanding of the distribution of electrical potential and current at electrode surfaces in practical systems with high voltage coupling effect.     

Potential and current density distributions along a bipolar electrode

Potential and current density distributions were modelled and measured for an electrochemical cell with a bipolar electrode. The dimension of the bipolar electrode in the direction of current flow was extended, to enable experimental determination of the electrode potential and the local current densities at various positions inside the electrolyte and in the electrode body. The experimental results showed that the most active regions of the bipolar electrode are located at the ends of the bipolar electrode facing the terminal electrodes. The equations corresponding to the mathematical model of the experimental cell were solved using the finite volume method and gave very good qualitative agreement with the experimental data. However, some discrepancies between model predictions and experimental data were evident in the active parts of the bipolar electrode and in the variation of the terminal voltage with the total current. This was explained in terms of the active electrolyte cross-section and the electrode surface area being diminished due to the presence of gas bubbles in the system.     

Bruit electrique dans les electrolytes 1.1 aqueux - 2. Cas des fortes concentrations - bruit d'interface

The conductivity of concentrated aqueous solution of KCl (10^?2 to 2 molar) has been measured in terms of electrical noise voltages. Under conditions of thermodynamic equilibrium, the liquid produces pure white noise; while there may arise a potential difference between the two electrodes while measuring noise voltages, there is no net current flow since there is no electrochemical reaction to supply the energy required for a non-zero mean current. A noise spectrograph has been developed which allows the measurement of very low values of equivalent noise resistance using a cross correlation method. The evaluation of the results is automated by means of a multiprogrammer coupled to a computer. Noise produced at the electrode interfaces is independent of the electrolyte concentration. Deducting the interfacial noise component from the total noise product by the cell allowed the evaluation of the equivalent noise conductivity of the electrolyte. Up to 1 molar concentration, the resulting values agree well with tabulated values within the errors of the measurements.     

L'electrode poreuse percolante (EPP)—III.: Difference de potentiel metal—solution au sein de l'electrode en regime diffusionnel

An analytical expression is given for the local metal—solution potential difference in a Percolating Porous Electrode working under limiting current conditions. The solution is obtained for the case where the general electrolyte flow direction is parallel to the current lines, for a given location of the current feeder and for an equipotential dispersed phase.This solution can be useful in using a PPE for determining mass transfer coefficients between a liquid and the particles of a fixed bed, as well as for the recuperation of metals from dilute solutions in potential conditions close to those of the diffusional regime.Metal-solution potential distributions have been determined experimentally as a function of the height in a percolating porous electrode made of high conducting grains which are fixed or fluidized. The results are discussed with respect to their variation with electrolyte flow rate.When the grains are fixed and for limiting current conditions in the whole bed, the experimental distributions are in excellent agreement with the calculated distributions.It is shown that a fluidized electrode should have a very low efficiency and that the problem of electrical conductivity in two-phase system is important in the application of solid—liquid fluidized beds in electrochemistry.