Solid Electrochemical Cells Research Articles

Potentiometric NO 2 gas sensor using LiRESiO 4 (RE=Nd and Sm)

The properties of two kinds of lithium ionic conductors, LiNdSiO 4 and LiSmSiO 4 as solid electrolytes were investigated by designing the solid electrochemical cells such as (−) air, Pt|lithium ionic conductor|Au, LiNO 3, NO 2, O 2 (+), for the NO 2 gas sensor. The electromotive force, EMF, of these sensors increased linearly with an increase in the logarithmic value of nitrogen dioxide partial pressure, in accordance with Nernst's law. It was suggested from the slope of Nernst's equation that the one-electron reaction associated with the nitrogen dioxide molecule takes place at the detection electrode around 150°C. The 90% response time of EMF for an increase in NO 2 concentration were within 12 min at 150°C.

Applying polarized electrochemical vapor deposition (PEVD) to solid state ionic devices

A new vapor deposition process, polarized electrochemical vapor deposition (PEVD), has been developed recently. The significance of PEVD is that it applies solid state ionic techniques to modify the widely accepted CVD technique under well-controlled conditions in a solid electrochemical cell. This provides PEVD with easy process control and unique electrocrystallization behavior. Consequently, it holds promise for a wide range of potential applications for solid state ionic devices. Investigations show that PEVD is the most suitable technique to improve solid electrolyte/electrode contact and, subsequently, the performance of solid state ionic devices. In this paper, current applications of PEVD for potentiometric gaseous oxide sensors and solid oxide fuel cells is discussed.

Electrochemical Study of a Polarized Electrochemical Vapor Deposition Process

The interfacing of modern vapor deposition technology and solid‐state ionic technology has led to the recent development of polarized electrochemical vapor deposition (PEVD). Due to the unique electrocrystallization behavior of its products and easy process control through a solid electrochemical cell, PEVD holds promise for a wide range of potential applications. However, the migration of charged ionic and electronic carriers during PEVD is a kinetic process. The task related to studying PEVD working electrode kinetics in this investigation is to explain the sequence of partial reactions constituting the overall PEVD electrochemical reaction for product formation at the working electrode. The PEVD process for auxiliary phase deposition at the working electrode of a potentiometric sensor was selected for the current electrochemical studies. The dependence of current density, or of reaction rate, upon various working electrode overpotentials and temperatures (500–550°C) was studied by a steady‐state potentiostatic method. The PEVD reaction rate‐limiting steps are solved for samples undergoing second‐stage growth. The results from this investigation help in understanding the kinetics of the PEVD reaction and subsequent product formation in a PEVD system, improve knowledge of PEVD kinetics, and elucidate the possibility of further process control in PEVD. © 2000 The Electrochemical Society. All rights reserved.

Potentiometric CO2 Gas Sensor Prepared from Lithium Ionic Conductors.

The properties of five kinds of lithium ionic conductors, LiLaSiO4, LiNdSiO4, LiSmSiO4, Li2Zr(PO4)2 and LiAlSi2O6, as solid electrolytes were investigated by designing the solid electrochemical cells such as (-) air, Pt|lithium ionic conductor| Au, Li2CO3, CO2, O2 (+), for the CO2 gas sensor. The electromotive force, EMF, of these sensors increased linearly with an increase in the logarithmic value of carbon dioxide partial pressure, in accordance with Nernst's law. It was suggested from the slope of Nernst's equation that the two-electron reaction associated with carbon dioxide molecule takes place at the detection electrode above 400°C. The 90% response times of EMF for all these sensors for an increase in CO2 concentration were within 2min at 450°C.

Electrical properties of a non-stoichiometric copper sulfide/solid electrolyte interface

A solid electrochemical cell copper-metal/electrolyte/mineral was studied by impedance spectroscopy between 10 5 and 2×10 −3 Hz in the temperature range 120–165°C. The mineral was a mixture of hexagonal chalcocite and high digenite of natural origin with a composition Cu 1.88S. The electrolyte was RbCu 4Cl 5. The impedance spectra at equilibrium and low cathodic over-voltages showed two distinct regions. At high frequencies, the general shape of the diagram was a depleted arc of a circle. At low frequencies, a diffusion branch was observed but not when the working electrode was replaced by copper metal. The results were interpreted in terms of copper insertion into the sulfide. The real contact area was determined and the impedance parameters were calculated and analyzed. At 145°C the chemical diffusion coefficient of copper vacancies calculated from impedance data is 2.0×10 −7 cm 2 s −1.

Ionic Conductivity of MAlSi<sub>2</sub>O<sub>6</sub>(M=Li, Na, K, Rb and Cs) and Its Application as a Potentiometric CO<sub>2</sub> Gas Sensors

Five kinds of alkali-metal aluminosilicate ceramics, MAlSi2O6(M=Li, Na, K, Rb or Cs), were prepared by sintering a mixture of M2CO3, Al2O3 and SiO2, and their properties as a solid electrolyte were investigated by designing a series of solid electrochemical cells such as (-)CO2, O2, Pt|MAlSi2O6|Au, M2CO3, CO2, O2(+), for the CO2 gas sensor, where MAlSi2O6 functions as an alkali-metal ionic conductor. Electromotive force, EMF, increased linearly with an increase in the logarithmic value of carbon dioxide partial pressure, in accordance with the Nernst's law and the slope of the Nernst's equation suggests two electron reaction of carbon dioxide molecule at the detection electrode around 400°C. At this temperature, the 90% response times were about 1min for all of the sensors. The sensor prepared KAlSi2O6 as the solid electrolyte was found to work at the lowest temperature and over the widest temperature range.

On the unique properties of heterogeneous solid electrochemical cells

Heterogeneous solid electrochemical cells that contain two mixed ionic-electronic conductors (MIECs) exhibit quite unexpected I-V relations when the two MIECs conduct ionic defects having different absolute values of their effective charge | z|. For example, the larger voltage drop need not occur on the MIEC with the larger resistance to electronic (electron/hole) current. When the ionic transference number of one of the two MIECs is close to unity and its resistance to electronic current is large enough, then a simple relation is imposed on the ionic and electronic currents in the other MIEC. This is an ambipolar motion of ionic and electronic charge carriers with a non zero total current. The effect on the I-V relations of adding a solid electrolyte in series with a MIEC, conducting defects with different | z |, is discussed in detail, including the implications on ionic conductivity measurements.

Electrical properties of silver vanadate electrochemical cells

The silver iodide based ternary system xAgI- yAg 2 O- zV 2O 5 ( x = 10, 20, 30, …, 90; y/ z = 2) was prepared by rapid quenching of the melt at liquid nitrogen temperature. X-ray diffraction confirmed the glassy or polycrystalline nature of the powdered phases. The 70AgI-20Ag 2O-10V 2O 5 phase has the highest room temperature (300 K) electrical conductivity of 0.011 S m −1 at 1 kHz. Scanning electron microscopy showed that the surface of the 70AgI-20Ag 2O-10V 2O 5 as-quenched phase contains separate agglomerates. IR spectroscopy revealed bands at approximately 960, 920, 890, 850, 820 and 700 wavenumbers, indicating the possible existence of [VO 4] 3− clusters. Solid electrochemical cells fabricated from the phase with the highest electrical conductivity showed that the transference number is almost unity and that the phase is an ionic conductor. The internal resistance of this battery is approximately 460 ω. When discharged at a load current of 30 μA, the current density is 0.04 mA cm −2, the discharge capacity is 3.78 C, the power density is 0.012 W kg −1 and the energy density is 1.512 J g −1 for a circular cell of mass 1.55 g and a surface area of 1.3 cm 2.

Electrochemical properties of complexes of iodine with triphenyl-amine, -phosphine, -arsine and stibine

The iodine complexes of triphenylamine, triphenylphosphine, triphenylarsine and triphenylstibine were prepared in the solid state. The complexes are stable over wide ranges of iodine concentration. Triphenylstibine forms a liquid complex. Free energies of complex formation were determined using solid electrochemical cells with an AgI electrolyte. The thermodynamic data and results of electrical conductivity measurements are discussed.

O2 chemical potential of nonstoichiometric ceria, CeO2−x, determined by a solid electrochemical method

The chemical potential of oxygen in nonstoichiometric ceria, CeO2−x, was measured versus composition x (10−4 ≤x ≤10−2) and temperature (823≤T≤1073 K) using a novel solid electrochemical cell, utilizing yttrium stabilized zirconia. The composition of CeO2−x was varied by coulometric titration. The O2 chemical potential was determined from the electromotive force of the cell with air used as the reference gas. With this new experimental setup, it is possible to carry out long coulometric titrations at very low oxygen partial pressures (limited by 10−30 atm at 823 K or by 10−13 atm at 1473 K). For x between 10−4 and 10−2, the O2 partial pressure in equilibrium with CeO2−x was found to change from 10−8 to 10−18 atm at 1073 K and from 10−16 to 10−30 atm at 823 K. The oxygen vacancies in CeO2−x were determined to be V″0 in this (x,T) range. The charge on aliovalent cations impurities was found to dominate the negative (effective) charge concentration for x≲10−3. The oxygen partial molar enthalpy ΔH̄(O2) is 870 kJ/mol (9 eV) for x≳3×10−3 decreasing for smaller x to a value of 670 kJ/mol (7 eV) for x=10−4.

Solid electrochemical modification of semiconductors

It is shown that the properties of semiconductors can be modified by solid electrochemical cells. These cells can be used as dopant sources for introduction of impurities into the semiconductor lattice. The solid electrolyte is so chosen that the ionic conducting species are the desired dopants. The controllability of the doping operation is very sensitive and either a constant concentration or a specified concentration profile can be achieved. These cells can also be used to modify the semiconductor surfaces such that only limited penetration of impurities is achieved.

Materials aspects of photo-electrochemical systems

Photo-electrochemical systems are midway between solid state photovoltaic devices and electrochemical cells. On the one hand this creates new material problems, such as corrosion of the photoactive material, while on the other hand the demand for lattice match is replaced by that of rapid kinetics of the electrochemical reactions occuring in the cell. Photocorrosion is dependent on the rate of reaction between light-generated charge carriers and solution species, as well as the rate of removal of reacted solution species from the electrode surface. The occurrence of more than one stable valence state in the semiconductor of the photoelectrode can prevent photocorrosion. Exchange of surface ions with the solution may create a dynamic equilibrium which can lead to effective stabilization of certain photoelectrodes. The substrate material of the photoelectrode should act as a poor electrocatalytic material for the reverse of the photogenerated reaction and its contact with the semiconductor material should be ohmic and physically coherent. The counter-electrode must be kinetically rapid for the reverse of the photogenerated reaction, which may cause complications if even small quantities of electrocatalyst reach the photoelectrode from the counter-electrode. If a storage system is added to the photo-electrochemical system its electrochemical as well as chemical properties have to be carefully matched to that of the photoconverting system. When chemicals are produced instead of electrical energy, the energy difference between the bandgap of the semiconductor and the photopotential obtained can be utilized to overcome overpotentials. This is of special importance for photodiodes, where appreciable overpotentials for dark reactions can prevent side-reactions and recombination, while the photoreaction has sufficient energy available for overcoming these overpotentials. Photodiodes and photocatalysts are conceptually close to each other, their difference being defined by the energetic requirements of the chemical reaction. While the combination of energy production, energy storage and the production of chemicals into one device creates many technical problems, it may turn out to be more economical in the end as has been shown to be true for parallel cases in chemical technology.

Free energies of formation of thorium sulphides from solid-state E.M.F. measurements

Thermodynamic information in the thorium-sulphur system has been obtained from measurements on solid electrochemical cells at temperatures of 825–925°C. The cells employed were of the following types: (I) Th, ThF 4 | CaF 2 | ThF 4, ThS, Th α; (II) Th, ThF 4 | CaF 2 | ThF 4, Th 2S 3, ThS; (III) Th, ThF 4 | CaF 2 | ThF 4, Th 7 S 12, Th 2S 3; (IV) Th, ThF 4 | CaF 2 | ThF 4, ThS 2, Th 7S 12. Free energies, entropies and enthalpies of formation of the sulphides were calculated from the data. In addition, lattice parameter measurements were made on thorium monosulphide to investigate the possibility of nonstoicheiometry in this phase. The results of these measurements are discussed.

Thermodynamic properties of non-stoichiometric thorium monocarbide

Thermodynamic information on nonstoichiometric thorium monocarbide was obtained from measurements on solid electrochemical cells. The cells were of the type Th, ThF 4 | CaF 2 | ThF 4, ThC 1−x. Measurements were made at temperatures of 800–950°C. Partial molar free energies of thorium and carbon were calculated from the data. The monocarbide phase was found to be stable from a composition of ThC 0·66 to ThC 0·96 from measurements of the variation of lattice parameter with composition. The results are discussed in connection with theories of chemical bonding which have been proposed for transition metal and actinide metal carbides. A qualitative picture of the bonding in nonstoichiometric thorium carbide is presented in which it is assumed that thorium and carbon form covalent bonds which increase in strength when some of the carbon is removed from the lattice.