Non-blocking Electrodes Research Articles

Impedance studies of the cell Ag/Ag beta″ alumina/Ag

Measurements of the cell impedance were carried out at constant temperature between 25 and 450°C at frequencies between 10 −2 and 10 4 Hz. The surfaces of the solid electrolyte, which was obtained by exchanging the sodium ions of the starting material, Na beta″ alumina from Ceramatec Inc., with silver ions in molten AgNO 3, were either polished or cut with a saw. Silver electrodes were attached by silver paint after applying the initial silver layer by sputtering. The silver electrodes display teh behaviour of a blocking electrode in a wide frequency range at temperatures below about 200°C. This behaviour is more clearly pronounced on the polished surfaces. The saw-cut surfaces show an intermediate behaviour between blocking and non-blocking electrodes. A transition from blocking to non-blocking electrodes occurs between 150 and about 200°C for both types of surface. The impedance data are analysed in the lower temperature range in terms of an analogue network.

Depolarization thermocurrents in ice Ih at low temperature depending on the electrode material. Polarization mechanism

Using the depolarization thermocurrent (DTC) method, we studied the low-temperature DTC peak in polycrystalline hexagonal ice Ih with nonblocking, totally blocking and partially blocking electrodes. We found the activation energyW=(0.24±0.01) eV to be independent of the electrode material. The peak temperature was found to beTM=(126±1) K with nonblocking electrodes,TM=(113±2) K with totally blocking electrodes and between them with partially blocking electrodes. The heating rate was 3.5 K/min. The value of the dielectric constant eβ−e∞ was found to be equal to 174±14 at the temperatureT=Tp=130 K with nonblocking electrodes and lower with blocking electrodes. Our results are consistent with a model in which the low-temperature DTC peak in ice is due to the reorientation of water molecules, realized by motion of Bjerrum defects. In the case of blocking electrodes additional Bjerrum and ionic defects (d.c. conductivity of ice) accumulate at the surfaces of the sample. This leads to the lowering of the polarization (smaller “dielectric constant» eβ−e∞) and to faster relaxation times (lowerTM).

Proton Diffusion in Tungsten Trioxide Thin Films

A small signal a‐c impedance method has been applied to the cell surface area carbon, in which the carbon electrode acts as a nonpolarizable electrode. Tungsten trioxide films were prepared by vacuum evaporation onto tin oxide covered glass substrate. The films, which were initially amorphous, were heated in air at temperatures from 300° to 400°C. The a‐c impedance measurements were carried out at several temperatures as a function of frequency from 10−2 to 105 Hz. The Randles circuit was fitted to the data by nonlinear chi‐squared minimization. The amorphous films have higher diffusion coefficients, i.e., about at 20°C, than crystalline samples, i.e., at 20°C. The activation energy for the amorphous sample is about 0.25 eV as compared to about 0.4 eV for crystalline films. Such a difference might be explained by a difference in composition and density between the amorphous and crystalline films or by different transport mechanisms such as hopping of H+ and diffusion of oxygen ion vacancies. The diffusion coefficient does not change significantly with composition in the range . The data suggest that may act as a nonblocking electrode with an exchange current density as high as 0.02 A · cm−2.

Ion transport in peo-alkali salt complex polymeric electrolytes

We have prepared and characterized association complexes of polyethylene oxide with LiCF3COO, NaSCN and KSCN. The readily hydrated films show a markedly higher electrical conductivity than the dry films. The samples are only useful at temperatures below the melting transition of 155°C for (PEO)5LiCF3COO, 12OC for (PEO)4KSCN and 195°C for (PEO)4NaSCN. The comductivity of (PEO)5LiTFA is 3×10-8 (ohm cm)−1 at 25C and 1.0×10−5 at 100C. Solid state NMR data and conductivity using blocking and non-blocking electrodes suggests that Li ions are the predominant conducting species.

Numerical analysis of electrical response: Biased small-signal a.c. response for systems with one or two blocking electrodes

The small-signal a.c. response of systems with two blocking electrodes, or one blocking and one ohmic electrode, subject to an external steady voltage bias is examined. The electrolyte is unsupported and may contain one or two mobile charge carrier species. The small-signal response displays features which are readily associated with the redistribution of charge within the system caused by the external bias and the injection of charge into a system with one non-blocking electrode. A procedure is given for approximating the small-signal response of a system containing many Debye lengths from the small-signal response of a thinner system. In a addition to describing the impedance and admittance of an initially flat-band system subject to an external bias, the results obtained are also applicable to a limiting case of an unbiased system with intrinsic space charge (Frenkel) layers. An equivalent circuit is found which usually provides an adequate approximate representation of the system response, and the physical interpretation of the circuit parameters is discussed.

Impedance measurements on Li 3 N single crystals

Lithium nitride has been well known for about thirty years; but because of its tendency to react with water and oxygen, together with its low free enthalpy of formation it did not attract the interest of electrochemists though early work suggested Li-ion conduction in this binary Li compound. In recent months the rapidly accelerating interest in Li 3N is due to the possibility of synthesizing poly- and monocrystalline Li 3N in a way that it is stable in the atmosphere and has a very high Li-ionic conductivity perpendicular to the c-axis at room temperature. In addition the simple hexagonal structure of Li 3N together with the clear evidence of N 3− ions in its structural arrangement may explain the extraordinary kinetic hindrance to its electrolytic decomposition. In this paper impedance measurements on Li 3N monocrystals with both ionically blocking and non-blocking electrodes are reported. It is shown that Li 3N is a rather interesting and promising Li-ionic conductor.

Ohmic contacts and dielectric relaxation in transition metal phosphate glasses

Criteria used to establish that electrical measurements on semiconducting phosphate glasses are bulk limited and can be described in terms of a Debye type polarization are reviewed. The practical and theoretical considerations needed to prepare non-blocking electrodes are outlined and it is concluded that any non-reproducibility of results arises from absorbed surface layers rather than the fundamental properties of the contact.

Hydronium beta″ alumina: A fast proton conductor

Single crystals of sodium beta″ alumina (0.84Na 2O·0.84MgO·5 Al 2O 3) undergo rapid ion exchange in concentrated sulfuric acid to produce “hydronium” beta alumina (0.84) H 2O·0.84MgO·5Al 2O 3·2.8H 2O). Hydronium beta″ alumina undergoes a partial, reversible dehydration between 250–300°C and irreversibly decomposes into alpha alumina and water above 700°C. The conductivity of hydronium beta″ alumina has been measured with blocking and non-blocking electrodes and is 5 × 10 −3 (ohm cm) −1 at 25°C. The high conductivity is interpreted on the basis of a two dimensional liquid model.

Space charge in silver halides—Theory of surface charge and electrode charge

It is suggested that the recent space-charge theory of Fatuzzo and Coppo omits consideration of several effects important in general space-charge situations and probably also important in these authors' experimental studies of silver halides. These effects are discussed in some depth and may explain, at least in part, why Fatuzzo and Coppo's theoretical analysis does not apply well to their experimental results. A detailed discussion is given of the diffusion potential and of surface and electrode charge in blocking and nonblocking electrode situations. Although a careful distinction between electrode and surface (absorbed) charge is usually made in the ideally polarized electrode (completely blocking) electrolyte case, the need for this necessary separation has apparently not been well recognized heretofore for situations where a solid material of interest is assumed to be contained between electrodes which are completely blocking for some or all types of mobile charge carriers in the solid. When this distinction is made for the problem considered by Fatuzzo and Coppo, quite different results than theirs are obtained. Three important improvements over the Fatuzzo-Coppo work are the present use of more appropriate general expressions for static and differential dielectric constants (or capacitances), the avoidance of their implicit assumption that surface charge is independent of total applied potential difference, and the avoidance of their assumption that the applied potential difference divides equally between the left and right crystal-electrode regions. The new analysis, appropriate for intrinsic materials with completely blocking electrodes, leads to expressions for the differential capacitance of such a system for a specific situation where surface charge may depend on the applied potential difference.

Space charge limited ac conduction with non-blocking electrodes

A model calculation has been performed for space charge limited ac transport at a reversible interface. The continuity equation for current carriers is solved by a numerical method. The calculated frequency dependence of the conductance and the capacitance agrees with the experimental results on solution-silver chloride-solution couples.