Carsten Korte

Electrochemical blackening of yttria-stabilized zirconia – morphological instability of the moving reaction front

Electrochemical reduction was performed on both polycrystals of calcia-stabilized zirconia (CSY) and single crystals of yttria-stabilized zirconia (YSZ) at 500°C. A glass-encapsulation was used to prevent access of molecular oxygen from the gas phase and to realize a virtually two-dimensional geometry for the reacting crystal. The resulting blackening process which occurs by the advancement of a morphologically unstable reaction front was observed in-situ in a heating-stage by the use of an optical microscope.

Results of a 20 000 h lifetime test of a 7 kW direct methanol fuel cell (DMFC) hybrid system – degradation of the DMFC stack and the energy storage

With a proven life of 20 000 operation hours in a lifetime test with a realistic dynamic load profile, the direct methanol fuel cell (DMFC) system V3.3-2 represents a milestone for the commercialization of DMFC systems. The hybrid DMFC system V3.3-2 comprises an in active serial connected 1.0 kW DMFC system and a 45 A h lithium-ion high-power battery pack. This hybrid system replaces the battery tray of a class 3 forklift truck and can supply a peak load of 7 kW. The advantages of this energy-supply module compared to conventional lead-acid batteries are its higher range (24 h use with a 20 L methanol canister instead of 8 h with battery recharging) and its higher availability (a few minutes are required to exchange methanol canisters instead of hours to recharge the battery). However, in order to ensure that use of the DMFC system V3.3-2 is economic, the DMFC stack must have a durability of at least 10 000 h. This publication describes the degradation behavior of the DMFC stack and of the energy storage system during a lifetime test of the DMFC system V3.3-2 with a dynamic load profile of a material handling vehicle. In the first-ever test worldwide lasting 25 600 h, the hybrid system is successfully operated for 20 000 h. Operation for 20 000 hours is equivalent to the life cycle of a vehicle in the material handling sector. The development and validation of the DMFC system V3.3-2 shows that this system is suitable for use in a forklift truck and that it not only meets the economic system requirements for commercialization but goes well beyond them.

18O-tracer diffusion along nanoscaled Sc2O3/yttria stabilized zirconia (YSZ) multilayers: on the influence of strain

Abstract The oxygen tracer diffusion coefficient describing transport along nano-/microscaled YSZ/Sc2O3 multilayers as a function of the thickness of the ion-conducting YSZ layers has been measured by isotope exchange depth profiling (IEDP), using secondary ion mass spectrometry (SIMS). The multilayer samples were prepared by pulsed laser deposition (PLD) on (0001) Al2O3 single crystalline substrates. The values for the oxygen tracer diffusion coefficient were analyzed as a combination of contributions from bulk and interface contributions and compared with results from YSZ/Y2O3-multilayers with similar microstructure. Using the Nernst–Einstein equation as the relation between diffusivity and electrical conductivity we find very good agreement between conductivity and diffusion data, and we exclude substantial electronic conductivity in the multilayers. The effect of hetero-interface transport can be well explained by a simple interface strain model. As the multilayer samples consist of columnar film crystallites with a defined interface structure and texture, we also discuss the influence of this particular microstructure on the interfacial strain.

The magnetoresistance of homogeneous and heterogeneous silver-rich silver selenide

The magnetoresistance (MR) effect of the low-temperature phase of silver selenide (α-Ag2+δSe) is measured as a function of composition. Very small composition variations in the order of Δδ=10−6 are achieved by coulometric titration and can be performed simultaneously during the MR measurement. A homogeneous Ag2+δSe shows an ordinary magnetoresistance (OMR) effect, which can be well described by the two-band model. For silver selenide with a heterogenous silver excess, we found quite a different MR behavior. Up to a minor silver excess of 1×10−4 10−2) shows again an OMR effect.

Study of the Soret effect in mixed conductors by the measurement of ionic and electronic thermopower

Abstract The redistribution of a mobile component in a mixed conducting compound A1 + δXv in a temperature gradient can be studied by means of time-dependent measurements of either the ionic or the electronic thermovoltage. The reduced heat of transport of the mobile component ( Q A ∗ or Q x ∗ ) results directly from the difference of the partial thermovoltages at t → 0 and t → ∞: For materials with predominant ionic conductivity, the electronic thermopower has to be measured; for materials with predominant electronic conductivity, the ionic thermopower has to be measured. Experimental examples are presented.

Transport processes in temperature gradients thermal diffusion and soret effect in crystalline solids

Abstract The redistribution of a mobile component in a mixed conductor of the type A 1+δ X ν in a temperature gradient is governed by the ratio of the heat of transport of the mobile component and the thermodynamic factor. Both quantities exhibit extrema at or close to the stoichiometric composition. As a consequence, the resulting thermal segregation factor (dδ/d T ) j =0 shows a strong composition dependence. We present a formal description which allows the prediction of (dδ/d T ) j =0 and discuss experimental results.

Carbon NMR investigation of the polybenzimidazole–dimethylacetamide interactions in membranes for fuel cells

Polybenzimidazole dissolved in dimethylacetamide has been studied by 1H and 13C one and two dimensional NMR methods to investigate the polymer–solvent interactions. The resonance signals have been fully assigned, giving a complete picture of the present species and allowing a study of the involved dynamic processes. The results have been correlated with the formation of H-bonds, and with the presence of tautomeric processes in the polymer.

Cross Effect between Heat and Matter Fluxes in Mixed Conducting Solids — Definition of the Heats of Transport

cause heat fluxes but also induce matter fluxes. In the framework of irreversible thermodynamics, this coupling is expressed by the definition of so-called heats of transport of the mobile components. In mixed conducting solids, the measurable fluxes of ions and electrons are in general composed of contributions of various mobile point defects (structure elements). Thus, the measurable transport coefficients and heats of transport are necessarily a combination of several independent defect properties. For the case of mixed conducting solids with Frenkel and electronic disorder we present a formal analysis of the coupling on the basis of a distinction between structure elements and building units of the crystal. The relations between the measurable transport properties and the a priori not measurable point defect properties are derived and discussed. Experimental results for two different systems (low-temperature phases a-Ag2+,¡S, a-Ag2+liSe) are analysed on the basis of our formal results.

Phosphoric Acid and its Interactions with Polybenzimidazole-Type Polymers

In high-temperature polymer electrolyte membranes, phosphoric acid is used as dopant for polybenzimidazole-type membranes to provide the protonic conductivity. In addition, phosphoric acid also serves as proton conductor in the porous electrodes in order to establish the three-phase boundary. In the first part of this chapter a short overview is given on the physico-chemical properties of (aqueous) phosphoric acid. In the second part the focus is on the adsorption of phosphoric acid as a protic electrolyte on polybenzimidazole-type polymers. Although polybenzimidazole-type membranes are routinely doped with phosphoric acid, few studies on the exact nature of the acid inside the membrane have been published. Experimental data from our institute and data compiled from literature indicate that the polymer chain is protonated by the acid and that the anions are bound by coulomb interactions. Additional electrolyte molecules can interact with the polymer chain by formation of H bonds or via H bonds with other H3PO4 molecules. It is demonstrated that the uptake of phosphoric acid can be described by a modified BET isotherm, assuming multilayer-like adsorption. The assumption of free phosphoric acid in the membrane at high doping levels is supported by Raman spectroscopy.

Influence of texture and grain misorientation on the ionic conduction in multilayered solid electrolytes – interface strain effects in competition with blocking grain boundaries

Interface strain and its influence on the ionic transport along hetero-interfaces has gained a lot of attention over the last decade and is controversially discussed. We investigate the relaxation of mismatch induced interfacial strain as a function of the degree of orientation/texture of the columnar crystallites and assess the impact on the oxygen ion conductivity in Er2O3/YSZ multilayer systems. Results from X-ray diffraction clearly show, that the width of the strained hetero-interface region increases with an increasing degree of orientation of the crystallites. The combined impact of film texture and strain at the hetero-interfaces of the film on the ionic conductivity however is not easily deduced from these measurements. The samples with the highest degree of orientation, i.e. with only one azimuthal variant, show strong anisotropic electrical properties. In samples with a lower degree of orientation, i.e. samples with a fiber texture, anisotropic properties cannot be detected, possibly due to a geometrical averaging of the electrical properties. The expected strain induced monotonic increase of the ionic conductivity with decreasing layer thickness and thus increasing interfacial influence could only be detected for samples with a fiber texture and a considerable degree of crystallite misorientation. This leads to the important conclusion that the texture and therefore the nature of the grain boundaries and their network influence the ionic conductivity of the multilayer thin films in the same order of magnitude as the misfit induced interface strain. Thus, the potential design of strain-controlled ionic conductors requires additionally the control of the microstructure in terms of grain orientation.