W. Sitte

Fast CO2-selective potentiometric sensor with open reference electrode

Abstract Performance, long-term stability and selectivity of fast potentiometric CO2-sensors constructed by combining a Na2CO3 measuring electrode, a Na-β″-alumina electrolyte and a reference electrode consisting of Na 2 Ti 6 O 13 TiO 2 or Na 2 Ti 3 O 7 Na 2 Ti 6 O 13 two phase mixtures are described. The materials of the reference electrode are chemically inert against CO2, so that the whole cell, without sealing the reference electrode, can be exposed to the ambient gas atmosphere. The sodium activity of the reference electrodes is fixed by the presence of the two phase mixture and the contact with oxygen in the gas phase. Thus thermodynamically defined, the sensor signal is essentially drift-free. Moreover, the influence of the oxygen partial pressure fixing the activity of the elements on both sides, cancels in the overall cell reaction. In this paper special emphasis is laid on demonstrating the fast Nernstian response with respect to CO2 and the independence of the signal from the oxygen partial pressure at temperatures above 500 °C.

Potentiometric detection of complex gases: Application to CO2

Abstract After a review of most common sensor principles for the detection of CO 2 with emphasis on solid state potentiometric sensors, the thermodynamic principles of recently developed potentiometric CO 2 sensors with open reference electrodes are briefly discussed. These sensors are based on acid-base interactions and allow sealing-free cell arrangements with signals being independent of the oxygen partial pressure. Results concerning the CO 2 and O 2 partial pressure as well as temperature dependence are presented. At least one of the reference electrodes could be identified to show a very fast response to varying CO 2 partial pressures. Coulometric titrations were performed in order to check the phase stability of the reference electrodes. In this way the TiO 2 -rich part of the quasi-binary phase diagram TiO 2 -Na 2 O is elucidated.

Fast potentiometric CO2 sensors with open reference electrodes

Abstract It is shown how a stable, simple and fast potentiometric EMF sensor for CO 2 can be constructed based on an early idea by one of us. Whilst the gas sensitive electrode consists of Na 2 CO 3 , the reference electrode uses mixtures of SnO 2 and Na 2 SnO 3 or TiO 2 and Na 2 Ti 6 O 13 . The activity of the elements is simply fixed by the ambient oxygen partial pressure. In this way the oxygen partial pressure cancels in the overall electrochemical reaction and the EMF measured does not depend on P O2 . Since the reaction free enthalpies of the systems chosen are such that CO 2 does not interfere with Na 2 SnO 3 or Na 2 Ti 6 O 13 , the reference side need not be sealed at all. Detailed results on the P CO2 - and T-dependence of the cell EMF and also results concerning the response time (∼ 1 s or even better) are presented.

Transport properties of La0.4Sr0.6CoO3−δ

La 0.4 Sr 0.6 CoO 3-δ was prepared by the glycine nitrate process and characterized in respect of purity and phase composition by powder X-ray diffraction and analytical transmission electron microscopy. The oxygen-nonstoichiometry, as a function of oxygen partial pressure (10 -4 bar <P O2 < 10 2 bar) and temperature (300°C < T < 900°C) as well as the chemical diffusion coefficient (725°C and 825°C), were determined by means of oxygen exchange measurements. Electronic conductivity as measured with the van der Pauw-technique is given as a function of temperature (between 300°C and 900°C) and oxygen partial pressure. With the obtained values for the oxygen-nonstoichiometry of La 0.4 Sr 0.6 CoO 3-δ , it is possible to take into account the change of the oxygen content of the sample with temperature and to calculate isostoichiometric (δ = constant) electronic conductivities. Additionally, chemical diffusion and surface exchange coefficients could be obtained from oxygen exchange measurements using simplified and exact fitting procedures.

Investigation of the binary system AgTe in the temperature range between 25 and 200°C using solid silver electrolytes

Abstract A thermodynamic study of the silver-tellerium system was made using suitable solid state galvanic cells with RbAg 4 I 5 and AgI as solid electrolytes in the temperature range between 25 and 200°C. The composition of the system was continously varied by use of the galvanostatic intermittent titration technique (GITT). The standard Gibbs energies of formation of Ag 5 Te 3 , Ag 1.9 Te and Ag 2 Te are −37.7, −44.0 and −45.3 kJ/mole, respectively, at 200°C and ideal stoichiometry. The homogeneity ranges of the intermediate phases as well as the activities of silver and tellerium for the two-phase regions Te(Ag)Ag 5 Te 3 , Ag 5 Te 3 Ag 1.9 Te and Ag 1.9 TeAg 2 Te are given.

Oxygen nonstoichiometry and ionic transport properties of La0.4Sr0.6CoO3-δ ☆

Homogeneous samples of La0.4Sr0.6CoO3-delta were obtained by the glycine nitrate process. The oxygen nonstoichiometry was determined from oxygen exchange measurements as a function of oxygen partial pressure (10(-4) bar < PO2 < 10(-2) bar) and temperature (300 degrees C < T< 900 degrees C). The chemical diffusion coefficient D was obtained from oxygen exchange measurements applying a stepwise variation of the oxygen partial pressure of the ambient atmosphere of a disk-shaped sample. The amount of oxygen absorbed or desorbed by the perovskite was analyzed as a function of time. Chemical diffusion data were evaluated using simplified and exact fitting procedures taking into account the surface exchange coefficient. Alternatively, galvanostatic polarization measurements were performed in a PO2-range between 10(-4) and 10(-2) bar to yield D and the ionic conductivity sigma(i) from the long time solution of the diffusion equation. Values for D from polarization measurements at T= 775 degrees C and from oxygen exchange measurements at T= 725 degrees C are in good agreement with each other. D and sigma(1) increase with increasing PO2 (10(-4) to 10(-2) bar). The ionic conductivity shows a maximum at 3-delta approximately 2.82 and decreases with decreasing oxygen content indicating the possible formation of vacancy ordered structures.

Chemical diffusion in mixed conductors: α'-Ag2Te and β-Ag2Se

Abstract The silver chalcogenides α′-Ag 2 + δ Te and β-Ag 2 + δ Se represent two contrary examples regarding the composition dependence of the chemical diffusion coefficient. Whereas in the vicinity of the stoichiometric composition the coulometric titration curve of β-Ag 2 Se is determined by the chemical potential of the silver ions (due to its cationic Frenkel disorder), the coulometric titration curve of α-Ag 2 Te is determined by the chemical potentials of the electrons (due to its structural disorder). The experimentally determined maximum of the chemical diffusion coefficient, in the case of α-Ag 2 + δ Te, near the stoichiometric point as well as the monotonie decrease of the chemical diffusion coefficient with decreasing silver activity in the case of β-Ag 2 + δ Se, can be explained in detail.

Conductivity relaxation experiments on Ni1−δO

The chemical diffusion coefficient D of polycrystalline nickel oxide was studied as a function of oxygen activity (10 5 atm < p O2 < 1 atm) and temperature (700°C < T < 1200°C) employing electronic conductivity relaxation experiments in the van der Pauw configuration. With this technique the sample is equilibrated under a given oxygen pressure. The oxygen content of the atmosphere surrounding the sample tablet is then changed stepwise and re-equilibration of the sample is followed by monitoring the conductivity relaxation. The chemical diffusion coefficient is obtained from the time-dependent variation of the electronic conductivity. The temperature dependence of D between 700 and 1200°C can be described by D(cm 2 s -1 ) = 2.40 × 10 -3 exp(-0.70±0.05 eV/kT). The chemical diffusion coefficient is found to be almost independent of the oxygen partial pressure and thus of oxide composition. The electronic conductivity at 1000°C shows a 1/6 dependence on the oxygen partial pressure, indicating the presence of doubly ionized nickel vacancies and electron holes as predominant defects. The D values are compared with literature results on single crystals.

Electrochemical cell for composition dependent measurements of electronic and ionic conductivities of mixed conductors and application to silver telluride

Abstract An electrochemical cell is described which allows measurements of ionic and electronic conductivities of mixed conductors as a function of composition. On one side of the disc-shaped sample four peripheral, roughly equidistant electronic contacts are mounted; on the other side four ionic contacts are positioned in the same manner. As the cell is based on the van der Pauw geometry, only the thickness of the sample and not the spacings between the electronic and ionic contacts needs to be determined. The composition of the sample can thus be altered by means of coulometric titration. For conductivity measurements on mixed conductors the composition gradient of the sample, arising from the flow of electrons and ions in the mixed conductor, is considered in detail. To demonstrate the high stoichiometric resolution the cell has been applied to study the composition dependence of ionic and electronic conductivities of α-Ag 2 Te at 300 °C within the homogeneity range and with high stoichiometric resolution. Experimental details as well as an interpretation of the composition dependence of the electronic conductivity are given.

Composition dependence of chemical diffusion coefficient and ionic conductivity of α′- and α-Ag2Te

Abstract The chemical diffusion coefficient and the ionic conductivity of Ag2Te have been measured simultaneously with high stoichiometric resolution between 160 and 300°C as a function of composition by a recently developed polarization technique. The composition of the system was varied by coulometric titration employing AgI as solid ionic conductor. The results indicate the existence of two structurally cationic-disordered, mixed conducting phases of α-Ag2Te with narrow homogeneity ranges and high ionic conductivities. Between 160 and 220°C the chemical diffusion coefficient of α′-Ag2Te shows a distinct maximum at the inflexion point of the coulometric titration curve with values between 0.02 and 0.35 cm2s−1. At temperatures between 260 and 350°C a monotonically decreasing function of the chemical diffusion coefficient could be observed for α-Ag2Te. For both phases, the ionic conductivity was found to be almost independent of the silver activity showing rather high values between 1.0 and 1.6 S cm−1.