Hans-Dieter Wiemhöfer

Oxide ion conducting solid electrolytes based on Bi2O3

The high oxide ion conductivity of solid solutions of bismuth oxide was initially discovered by Takahashi and coworkers. The bismuth oxide based compounds are much better solid electrolytes than the well-known stabilized zirconia. The only difficulty which has prevented their use in high temperature fuel cells and gas sensors up to now is their instability against reduction at low oxygen partial pressures. In this article, we review the structural properties, thermal expansion, electrical conductivity, thermodynamic stability, and surface properties of bismuth oxide and solid solutions of bismuth oxide with face centred cubic, rhombohedral, tetragonal or layer structures.

Electronic conductivity of Gd-doped ceria with additional Pr-doping

Abstract The electronic conductivity of ceria doped with 20− x mole% Gd and x mole% Pr according to the composition Ce 0.8 Gd 0.2− x Pr x O 1.9 (with 0.01≤ x ≤0.03) was measured as a function of oxygen activity in the range from a O 2 ≈10 2 to a O 2 ≈10 −15 ( a O 2 =1 corresponding to a gas with oxygen partial pressure p O 2 =1.013 bar) for temperatures between 600°C and 750°C and compared with results for Pr-free Ce 0.8 Gd 0.2 O 1.8 . The Hebb–Wagner polarization technique was used with an encapsulated Pt-microcontact in an N 2 atmosphere and a Cu 2 O/CuO reference electrode. Addition of 1–3 mol% Pr leads to a slightly decreased electron conductivity in the n-type range, but to a large increase in the p-type range. Moreover, the minimum of the electronic conductivity is shifted to lower oxygen activities by doping with Pr (e.g. at 700°C: from an oxygen activity of 5×10 −4 for Ce 0.8 Gd 0.2 O 1.9 to 5×10 −6 for Ce 0.8 Gd 0.17 Pr 0.03 O 1.9 ). The activity dependence of the electronic conductivity in the n-type and p-type range slightly deviates from the theoretically expected values ( σ n , p ∝ a ±1/ m O 2 with 4 m ≤5). The effect of Pr-substitution on the p-type conductivity is explained by a model which assumes low lying 4f levels of Pr that cause an excess p-type conductivity due to a beginning valence change from Pr 3+ to Pr 4+ under oxidizing conditions. The activation energy as determined from the temperature dependence of the electronic conductivity was E A,e =2.5 eV for electrons in the n-type range, independent of the Pr concentration. In the p-type range, the activation energy decreases with Pr concentration: e.g. E A,h =1.16 eV for Ce 0.8 Gd 0.2 O 1.9 to E A,h =0.7 eV for Ce 0.8 Gd 0.17 Pr 0.03 O 1.9 . Measurement of the total impedance yielded an increase in the overall oxygen ion conductivity with Pr addition due to a lower grain boundary resistivity.

Active Reaction Sites for Oxygen Reduction in La0.9Sr0.1MnO3 / YSZ Electrodes

Funding for this work was provided by the R&D Management Center for Energy and Resources (Korea). S. M. Oh gratefully acknowledges the financial support from the Alexander yon Humboldt Foundation.

Active Reaction Sites for Oxygen Reduction in La0.9Sr0.1,MnO3/YSZ Electrodes

Funding for this work was provided by the R&D Management Center for Energy and Resources (Korea). S. M. Oh gratefully acknowledges the financial support from the Alexander yon Humboldt Foundation.

Li ion diffusion in the anode material Li12Si7: ultrafast quasi-1D diffusion and two distinct fast 3D jump processes separately revealed by 7Li NMR relaxometry.

The intermetallic compounds Li(x)Si(y) have attracted considerable interest because of their potential use as anode materials in Li ion batteries. In addition, the crystalline phases in the Li-Si phase diagram turn out to be outstanding model systems for the measurement of fast Li ion diffusion in solids with complex structures. In the present work, the Li self-diffusivity in crystalline Li(12)Si(7) was thoroughly probed by (7)Li NMR spin-lattice relaxation (SLR) measurements. Variable-temperature and -frequency NMR measurements performed in both the laboratory and rotating frames of reference revealed three distinct diffusion processes in Li(12)Si(7). The diffusion process characterized by the highest Li diffusivity seems to be confined to one dimension. It is one of the fastest motions of Li ions in a solid at low temperatures reported to date. The Li jump rates of this hopping process followed Arrhenius behavior; the jump rate was ~10(5) s(-1) at 150 K and reached 10(9) s(-1) at 425 K, indicating an activation energy as low as 0.18 eV.

Composites of Ce0.8Gd0.2O1.9 and Gd0.7Ca0.3CoO3−δ as oxygen permeable membranes for exhaust gas sensors

The transport and catalytic properties of mixed conducting composites prepared from Gd0.7Ca0.3CoO3-δ (GCC) and Ce0.8Gd0.2O1.9 (CGO) were determined. In total, three compositions with 43, 60 and 75 vol.% CGO have been prepared. The composites have potential applications as oxygen permeable membrane in an amperometric sensor for NOx detection in exhaust gases. At all compositions, three different phases were found in the annealed composite as a result of solid state reactions during annealing. The gadolinium content of the CGO phase was increased. The GCC was transformed into a phase with K2NiF4-structure and an additional CoO phase appeared. Electron conductivities were determined in the range 100 to 750°C. The oxygen ion conductivities were measured by a microelectrode technique between 650 and 750°C. Permeation measurements were carried out between 850 and 1000°C. Effective ionic conductivities were calculated from the permeation results. The latter agreed well with the microelectrode data extrapolated to higher temperatures. The composition dependence of the conductivities was explainable by a statistical percolation model. The investigated composites exhibit good oxygen ion conductivities. Typical ionic conductivity values for a composite with 75 vol.% CGO increased from 4x10-4 (Ω cm)-1 at 650°C to 4.2x10-2(Ω cm)-1 at 1000°C. The heterogeneous catalysis of surface gas reactions was tested by exposing powdered samples to various gaseous mixtures with O2, N2, CO, CO2, H2O, SO2 and propene at temperatures up to 800°C. No conversion of NO to oxygen and nitrogen as well as no extensive formation of N2O was detected. It is concluded that the composite membranes are suitable for the intended applications.

Reversible switching between p- and n-type conduction in the semiconductor Ag10Te4Br3

Semiconductors are key materials in modern electronics and are widely used to build, for instance, transistors in integrated circuits as well as thermoelectric materials for energy conversion, and there is a tremendous interest in the development and improvement of novel materials and technologies to increase the performance of electronic devices and thermoelectrics. Tetramorphic Ag(10)Te(4)Br(3) is a semiconductor capable of switching its electrical properties by a simple change of temperature. The combination of high silver mobility, a small non-stoichiometry range and an internal redox process in the tellurium substructure causes a thermopower drop of 1,400 microV K(-1), in addition to a thermal diffusivity in the range of organic polymers. The capability to reversibly switch semiconducting properties from ionic to electronic conduction in one single compound simply by virtue of temperature enables novel electronic devices such as semiconductor switches.

NMR relaxometry as a versatile tool to study Li ion dynamics in potential battery materials.

NMR spin relaxometry is known to be a powerful tool for the investigation of Li(+) dynamics in (non-paramagnetic) crystalline and amorphous solids. As long as significant structural changes are absent in a relatively wide temperature range, with NMR spin-lattice (as well as spin-spin) relaxation measurements information on Li self-diffusion parameters such as jump rates and activation energies are accessible. Diffusion-induced NMR relaxation rates are governed by a motional correlation function describing the ion dynamics present. Besides the mean correlation rate of the dynamic process, the motional correlation function (i) reflects deviations from random motion (so-called correlation effects) and (ii) gives insights into the dimensionality of the hopping process. In favorable cases, i.e., when temperature- and frequency-dependent NMR relaxation rates are available over a large dynamic range, NMR spin relaxometry is able to provide a comprehensive picture of the relevant Li dynamic processes. In the present contribution, we exemplarily present two recent variable-temperature (7)Li NMR spin-lattice relaxation studies focussing on Li(+) dynamics in crystalline ion conductors which are of relevance for battery applications, viz. Li(7) La(3)Zr(2)O(12) and Li(12)Si(7).

Leaching of dimethylferrocene, a redox mediator in amperometric enzyme electrodes

Abstract Glucose electrodes were prepared by electrochemical oxidation of graphite and covalent binding of glucose oxidase to the functionalized surface; some of the electrodes were additionally modified by adsorption of the artificial electron acceptor 1,1′-dimethylferrocene. The long-term responses of two kinds of electrodes were compared in a flow-injection system. While the unmodified electrode (current generation by H 2 O 2 oxidation at 600 mV versus SCE) remained stable for more than 3 days, the dimethylferrocene-mediated system (100 mV versus SCE) rapidly lost its activity. However, response could in part be restored by adsorption of new mediator. The loss of 1,1′-dimethyl-ferrocene from graphite electrodes could be shown by cyclovoltammetry and steady-state current—voltage curves. In rotating ring-disk electrode experiments, current generation at the concentric platinum electrode could only be observed when the central graphite disk electrode, modified with 1,1′-dimethylferrocene, was poised to a potential higher than the oxidation potential of ferrocene. Consequently, the 1,1′-dimethylfericinium cation must be the leaching agent. The results are discussed with respect to electrode stability and intoxication problems arising in the case of in vivo application.

Investigation of the influence of zirconium substitution on the properties of neodymium-doped barium cerates

Abstract Preparation, electrical conductivity and stability are discussed for Nd-doped barium cerate with partial substitution of zirconium for cerium. BaCe 0.9− x Zr x Nd 0.1 O 2.95 with 0.1 ≤ x ≤ 0.9 was prepared by direct reaction of mixtures of barium carbonate and the corresponding metal oxides. Structural properties of BaCe 0.9− x Zr x Nd 0.1 O 2.95 were investigated by X-ray diffraction and raman scattering. Single phases were obtained over the entire range of x -values, if reaction temperatures above 1500 °C were applied. The total conductivity was measured as a function of temperature and for different conditions of p ( H 2 O ), p ( O 2 ) in N 2 . Only a slight decrease in the total conductivity was found up to 60%-zirconium substitution. In addition, the oxygen ion conductivity was determined for 600 °C–800 °C using an ion conducting microelectrode made from stabilized zirconia. The oxygen ion conductivity was lower than the total conductivity by about two orders of magnitude. The observed total conductivity is therefore attributed primarily to proton conductivity.