Henricus J.M. Bouwmeester

Dense ceramic membranes for methane conversion

Dense ceramic membranes made from mixed oxygen-ionic and electronic conducting perovskite-related oxides allow separation of oxygen from an air supply at elevated temperatures (>700 °C). By combining air separation and catalytic partial oxidation of methane to syngas into a ceramic membrane reactor, this technology is expected to significantly reduce the capital costs of conversion of natural gas to liquid added-value products. The present survey is mainly concerned with the material properties that govern the performance of the mixed-conducting membranes in real operating conditions and highlights significant developments in the field.

Electrode properties of Sr-doped LaMnO3 on yttria-stabilized zirconia. II. Electrode kinetics.

A series of six cathodes Sr0.15La0.85MnO3 (SLM) on yttria-stabilized zirconia with different morphology of the electrode/electrolyte interface were characterized by ac impedance and dc polarization measurements. It is found that the electrode kinetics at elevated temperature (945°C) are governed by two serial processes. An activation process can be identified to occur at high cathodic overpotential, whereas a transport process competes with charge-transfer at comparatively low overpotential. Attention is drawn to the profound change in the electrocatalytic properties of Sr0.15La0.85MnO3 upon current passage and its influence in elucidation of the interfacial kinetics.

Electrode Properties of Sr‐Doped LaMnO3 on Yttria‐Stabilized Zirconia I. Three‐Phase Boundary Area

The interface microstructure of the state-of-the-art cathode material for solid oxide fuel cells, SrxLa1–xMnO3 (SLM), was investigated with respect to its electrochemical performance. The interface microstructure was characterized by grain size and coverage of SLM on the electrolyte surface. Variation of the grain size was obtained by using three different sintering temperatures, whereas variation of the coverage was obtained by using two powders with a different morphology. This resulted in a set of six cathode/electrolyte samples with different combinations of grain size and SLM coverage at the interface. The cathode overpotential, as a measure for the electrochemical performance, could not be related to the length of the three-phase boundary. Based on the constriction resistance occurring in the electrolyte a model was developed which provides an estimate for the width of the active three-phase boundary zone.This zone is most likely to extend outside the cathode particle across the zirconia surface. The width calculated in this way was found to vary in the range of 0.03 to 0.07 µm for the different electrode microstructures. It is argued that the actual values may be smaller by one or two orders of magnitude.

Ion and mixed conducting oxides as catalysts

This paper gives a survey of the catalytic properties of solid oxides which display oxygen ion or mixed (i.e. ionic + electronic) conductivity. Particular consideration is given to the oxidation-reduction reactions of gas phase components, but attention is also devoted to oxygen exchange between gas and oxide. An attempt has been made to relate and explain the observed phenomena such as catalytic activity and selectivity in terms of the electrical conducting properties of the oxides, which depend on their crystal and defect structures. In a number of cases possible applications of these materials in (electro)catalytic reactors, sensors, fuel cells, oxygen pumps, etc. are indicated.

Oxygen permeation of La0.3Sr0.7CoO3-δ

The oxygen permeability of dense La0.3Sr0.7CoO3−δ membranes has been measured in the range 750–1100°C under various oxygen partial pressure gradients. A sweep gas method was employed. Results indicate that in the range of thickness 0.057–0.215 cm used in the present study, the oxygen flux is predominantly controlled by bulk diffusion across the membrane. The measured activation energy is 60 kJ mol−1. By fitting the permeation data for various thicknesses to the transport equation obtained upon assuming linear kinetics for the surface exchange reactions and bulk ionic transport, we could derive the oxygen ionic conductivity and the characteristic membrane thickness. The latter quantity determines the transition from predominant control by diffusion to that by surface exchange. The ionic conductivity is about 0.5 S cm−1 at 1000°C. The characteristic thickness is extrapolated at a value of about 80 μm.

Oxygen permeation modelling of perovskites

A point defect model was used to describe the oxygen nonstoichiometry of the perovskites La0.75Sr0.25CrO3, La0.9Sr0.1FeO3, La0.9Sr0.1CoO3 and La0.8Sr0.2MnO3 as a function of the oxygen partial pressure. Form the oxygen vacancy concentration predicte by the point defect model, the ionic conductivity was calculated assuming a vacancy diffusion mechanism. The ionic conductivity was combined with the Wagner model for the oxidation of metals to yield an analytical expression for the oxygen permeation current density as a function of the oxygen partial pressure gradient. A linear boundary condition was used to show the effect of a limiting oxygen exchange rate at the surface.

Interpretation of the Gerischer impedance in solid state ionics

The Gerischer impedance in its most elementary form, ZG(ω)=Z0(k+jω)−1/2, has been observed in the frequency response of mixed conducting solid electrolyte systems. This simple transfer function can be derived directly from Ficks second law by including a reaction term. The Gerischer impedance was observed in the electrode dispersion of heavily Tb-doped yttria-stabilised zirconia (YSZ) ceramics. The mixed conducting samples were provided with ionically blocking gold electrodes. The occurrence of a Gerischer impedance in the Tb–YSZ system is tentatively modelled with the formation of immobile complexes of oxygen vacancies and trivalent cations. The presented results indicate that for mixed conducting materials it is not allowed to represent the electronic and ionic conduction path as independent components in an equivalent circuit.

Reactor Flush Time Correction in Relaxation Experiments

The present paper deals with the analysis of experimental data from conductivity relaxation experiments. It is shown that evaluation of the chemical diffusion and surface transfer coefficients for oxygen by use of this technique is possible only if accurate data for the conductivity transient can be measured at short times, i.e., immediately after the step change in the surrounding oxygen partial pressure. The flushing behavior of the reactor volume may, however, significantly influence the early stage of the relaxation process. Large errors in the transport parameters are obtained from fitting the relaxation data to the theoretical equations if this phenomenon is not properly recognized. Equations are presented which describe the transient conductivity taking into account the finite flush time of the reactor. The regimes of surface- and diffusion-controlled kinetics are discussed quantitatively

Influence of diffusion plane orientation on electrochemical properties of thin film LiCoO2 electrodes

Submicrometer LiCoO2 films have been prepared on silicon substrates with RF sputtering and pulsed laser deposition (PLD). The electrochemical activity of both types of thin film electrodes is compared using scanning cyclic voltammetry, galvanostatic and potentiostatic intermittent titration, and electrochemical impedance spectroscopy. The RF films exhibit a axis orientation and have an accessible diffusion plane alignment, unlike the c axis oriented PLD films. The preferential orientation of the host crystal lattice toward the electrolyte solution is critical for the intercalation rate and cycling efficiency. The RF films show superior electrochemical performance and faster relaxation characteristics than the PLD films. Based on the analysis of the time and frequency domain measurements a model for the electrode response is proposed. Apparently, the intercalation rate of the RF films is not diffusion-limited, but hindered by the large charge-transfer resistance, the phase boundary movement, and the hindrance by the solid electrolyte interface.

Chemical diffusion and oxygen surface transfer of

The chemical diffusion coefficient and oxygen-transfer coefficients of selected compositions in the series