Jan Van herle

Ammonia as a fuel in solid oxide fuel cells

The use of ammonia as a source of hydrogen for fuel cells has received little attention until now. Ammonia offers several advantages over hydrogen as a fuel and is produced commercially in massive quantities and as a biogas. This paper describes the results of a solid oxide fuel cell-based system running on ammonia and compares the performance with respect to hydrogen. A novel catalyst concept has been devised and employed with success. Results indicate that the ammonia performance, using the catalyst is comparable to hydrogen suggesting ammonia can be treated as an attractive alternative fuel.

Energy balance model of a SOFC cogenerator operated with biogas

A small cogeneration system based on a Solid Oxide Fuel Cell (SOFC) fed on the renewable energy source biogas is presented. An existing farm biogas production site (35 m3 per day), currently equipped with a SOFC demonstration stack, is taken for reference. A process flow diagram was defined in a software package allowing to vary system operating parameters like the fuel inlet composition, reforming technology, stack temperature and stack current (or fuel conversion). For system reforming simplicity, a base case parameter set was defined as the fuel inlet of 60% CH4:40% CO2 mixed with air in a 1:1 ratio, together with 800 8C operating temperature and 80% fuel conversion. A model stack, consisting of 100 series elements of anode supported electrolyte cells of 100 cm2 each, was calculated to deliver 3.1 kWel and 5.16 kWth from an input of 1.5 N m3/h of biogas (8.95 kW LHV), corresponding to 33.8 and 57.6% electrical and thermal efficiencies (Lower Heating Values (LHVs)), respectively. The incidence on the efficiencies of the model system was examined by the variation of a number of parameters such as the CO2 content in the biogas, the amount of air addition to the biogas stream, the addition of steam to the fuel inlet, the air excess ratio l and the stack operating temperature, and the results discussed.

A Review of RedOx Cycling of Solid Oxide Fuel Cells Anode

Solid oxide fuel cells are able to convert fuels, including hydrocarbons, to electricity with an unbeatable efficiency even for small systems. One of the main limitations for long-term utilization is the reduction-oxidation cycling (RedOx cycles) of the nickel-based anodes. This paper will review the effects and parameters influencing RedOx cycles of the Ni-ceramic anode. Second, solutions for RedOx instability are reviewed in the patent and open scientific literature. The solutions are described from the point of view of the system, stack design, cell design, new materials and microstructure optimization. Finally, a brief synthesis on RedOx cycling of Ni-based anode supports for standard and optimized microstructures is depicted.

Sintering behaviour and ionic conductivity of yttria-doped ceria

Abstract Highly sinterable yttria-doped ceria powder (YO1.5)x (CeO2)1 − x (x = 0.1–0.33) was fabricated by an optimized coprecipitation route. Compacted bodies could be sintered to impermeability at 1200 °C and near full density at 1300 °C, among the lowest temperatures reported for doped ceria densification. Ceria diffusion is important above 1400 °C. Excellent conduction properties were observed: high ionic conductivity (6.5 S m−1 at 750 °C for Ce0.8Y0.2O1.9), low activation energy (0.7 eV) and vanishing grain-boundary resistance.

Conductivity of Mn and Ni-doped stabilized zirconia electrolyte

We develop SOFC based on a NiO–YSZ anode support, a thin YSZ layer (5–10 micron) and a LaSrMnO3 cathode. Mn and Ni can dissolve into thin zirconia during high temperature fabrication. On cells, we consistently measured higher ohmic losses than expected from the YSZ resistivity. To investigate whether ionic conductivity of YSZ is affected, dense tape cast YSZ doped with 1 at.% Mn or 2 mol% NiO was fabricated and the conductivities in O2 and H2 measured between 300 and 850

Oxygen transport through dense La0.6Sr0.4Fe0.8Co0.2O3-δ perovskite-type permeation membranes

In this study, we examine the parameters that govern the overall oxygen flux through a permeation membrane. The chemical diffusion (D) and the surface exchange (k) coefficients for oxygen in La0.6Sr0.4Fe0.8Co0.2O3-d were determined as a function of temperature using a specially designed electrochemical cell combined with impedance spectroscopy. Typically, D=1.7 10-5 cm2/s and k=3.6 10-4 cm/s in air at 900°C. These values were compared with literature 18O/16O isotope exchange data. Oxygen permeation measurements were also performed on the same material in an air/Ar gradient, in the temperature range of 800 to 1000°C. At 900°C, the oxygen flux across a 1.53 mm thick membrane was 8.0 10-8 mol/(cm2s). The measured fluxes were compared with fluxes calculated on the basis of the D and k values using expressions derived from Fick’s law. Comparison showed that the flux is controlled by both bulk diffusion and surface exchange, even for such thick membranes, and that the apparent k? varies significantly from one experiment to another.

Ceria-Zirconia Composite Electrolyte for Solid Oxide Fuel Cells

Ceria-zirconia-ceria sandwich structured composite filmelectrolytes were designed in order to offer high ionic and low electronicconductivity electrolyte films. Calculation of oxygen potential profile inthe composite film electrolyte indicates that a very thin zirconia filmkills the electronic current of ceria without affecting the ionicconductivity. The composite films were successfully prepared by a co-fireprocess. The main problem was the fact that the ceria and zirconia greenfilms had different shrinkage behaviors. Successful co-firing was achievedby controlling the temperature program and amount of binder. De-laminationbetween yttria stabilized zirconia(YSZ) and gadolinia doped ceria(GDC)layers was overcome by the formation of a solid solution phase at theinterface of the two films. The resultant composite films, however, showedpoor electrical conductivity compared with theoretical values. The formationof a solid solution phase can have a negative effect on compositeelectrolytes. Also, the composite film is unsatisfactory in terms ofmechanical strength. This could be due to the lattice expansion in reducingatmospheres, thermal expansion coefficient mismatch, or the intrinsicweakness of the ceria texture.

Co-casting and co-sintering of porous MgO support plates with thin dense perovskite layers of LaSrFeCoO3

A tape casting co-sintering route is described in which thin dense layers of LaSrFeCoO3 (LSFC) have been formed on planar, porous MgO substrates 100- 200 micron thick. SEM analysis of the sintered structure showed that it was possible to eliminate most of the residual porosity in the LSFC layer, but maintain a porosity between 25 and 45% in the MgO support layer. The LSFC layer dit not reveal many cracks. The overall shrinkage of the co-sintered structure was about 25%. The LSFC layer topography was smooth and uniform with a metallic-like lustre. A good correlation was obtained between the observed microstructure and the gas permeability measurements made at room temperature.

Sulfur as pollutant species on the cathode side of a SOFC system

Sulfur poisoning in a strontium doped lanthanum cobaltite (LSC) cathode current collection layer was revealed in a solid oxide fuel cell (SOFC) tested in repeat-element and stack configuration. Sources of sulfur contamination, other than trace SOx in air, were identified. Strontium sulfate (SrSO4) and strontium chromate (SrCrO4) enriched with sulfur were found at the interface between LSC and air channels. Understanding degradation mechanisms is a major issue in the development of SOFCs. In particular, in repeat-element and stack configuration, an important coupling of different degradation processes exists, with internal sources at the stack level and exogenous sources coming from system components. A specific diagnostic test station was developed to allow locally-resolved measurements of electrochemical performance and degradation in a repeat-element. Large differences in local degradation behavior were observed, affecting different electrochemical processes. Post-mortem analysis, mainly done by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX), allowed to identify the pollutant species responsible for degradation. Complete results are published elsewhere [1-2]. In this study, an anode-supported cell, with an active strontium doped lanthanum manganite (LSM) - yttria stabilized zirconia (YSZ) composite cathode and a LSC current collection layer, was tested over 1900 h around 1073 K. Chromium and sulfur were found as major pollutant species on the cathode side; both are dependent on upstream conditions and components. While chromium poisoning is well known for SOFC cathodes, sulfur contamination of the cathode has received more attention recently. Sulfur poisoning is a major limitation for wide use of perovskite catalysts for treatment of auto exhaust gas [3]. Sulfur pollution from trace SOx in air and involving sulfate formation was reported by Xu et al. [4], where perovskite-type ceramics were used for oxygen permeation membranes. Yokokawa et al. identified sulfur as impurity after long-term operation of SOFC stacks in [5], and Xiong et al. reported and studied sulfur poisoning of different cathode materials just recently [6]. In the present experiment, a commercial vulcanized polymer tube for air inlet and an insulating high temperature sealing paste were identified as potential sulfur sources. Figure 1 shows the LSC microstructure before and after exposure to sulfur containing air. Strontium sulfate growth on porous LSC surface leads to an almost dense sulfate layer. Strontium sulfate was found over the whole LSC thickness without affecting the LSM/YSZ cathode. The more pronounced strontium sulfate formation in LSC compared to LSM is explained by the higher activity of strontium oxide (SrO) in LSC [7]. Chromium on the LSC surface was only found at the air-inlet, indicating that Cr was principally evaporated from upstream system components. SEM/EDX analysis allowed to identify sulfur-rich strontium chromate with a composition close to Sr(Cr0.85S0.15)O4. The chromate structure is changing along the airflow direction, from an almost dense structure to isolated particles, while maintaining the same composition. Further analyses are ongoing to confirm sulfate and chromate crystalline structures.

In situ Reduction and Oxidation of Nickel from Solid Oxide Fuel Cells in a Transmission Electron Microscope

Environmental transmission electron microscopy was used to characterize in situ the reduction and oxidation of nickel from a Ni/YSZ solid oxide fuel cell anode support between 300-500°C. The reduction is done under low hydrogen pressure. The reduction initiates at the NiO/YSZ interface, then moves to the center of the NiO grain. At higher temperature the reduction occurs also at the free NiO surface and the NiO/NiO grain boundaries. The growth of Ni is epitaxial on its oxide. Due to high volume decrease, nanopores are formed during reduction. During oxidation, oxide nanocrystallites are formed on the nickel surface. The crystallites fill up the nickel porosity and create an inhomogeneous structure with remaining voids. This change in structure causes the nickel oxide to expand during a RedOx cycle.