V. A. C. Haanappel

Performance improvement of (La,Sr)MnO3 and (La,Sr) ×(Co, Fe)O3-type anode-supported SOFCs

Targets in the development of anode-supported or planar solid oxide fuel cells (SOFCs) are low operation temperatures, high durability, high reliability, high power density, and low production costs. During the past ten years steps have already been taken at Forschungszentrum Jiilich to lower the operating temperatures while maintaining the power output. This was achieved by optimizing processing and microstructural parameters of the electrodes. This paper presents the latest results concerning performance improvement through variations of the processing route and the microstructure of La 0.65 Sr 0.3 MnO 3 (LSM) and La 0.58 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF)-type SOFCs. In the case of the LSM-type single cells, the following aspects relating to the electrochemical performance were investigated in more detail: (I) production of the anode substrate by tape casting versus warm pressing; (2) deposition of the anode functional layer (AFL) and electrolyte by screen printing versus vacuum slip casting; (3) use of noncalcined and non-ground YSZ for applying the cathode functional layer (CFL); and (4) sintering temperature of the CFL and cathode current collector layer (CCCL). In the case of LSCF-type cells, a systematic approach was initiated for optimizing the Ce 0.8 Gd 0.2 O 2-δ (CGO) diffusion barrier layer: (1) deposition techniques of the CGO layer and (2) sintering temperature of the screen-printed CGO layer. Results have shown that certain modifications of the processing route led to a slightly lower electrochemical performance, whereas others did not affect the performance at all. Regarding LSCF-type SOFCs, a slight improvement of the performance was achieved by optimizing the sintering temperature of the CGO layer.

Sintering Behavior of (La,Sr)MnO3 Type Cathodes for Planar Anode-Supported SOFCs

One of the main targets in the development of anode-supported solid oxide fuel cell (SOFCs) is to improve the electrochemical performance. This can be achieved by optimizing processing and microstructural parameters of the SOFCs. Variations of the thickness of the cathode functional layer and the cathode current collector layer, the grain size of the powders used for applying these layers, and the sintering temperature, can influence the electrochemical performance as such that lower operation temperatures become possible without detrimentally affecting the power output to a great extent. In this study the effect of variations of the sintering temperature of the cathode on (I) the microstructure, (2) the gas diffusivity and permeability in the cathode, and (3) electrochemical performance of FZJ-type anode-supported single cells, was investigated. The FZ-Julich cell design is based on anode-supported type cells, which are characterized by a relatively thick anode (thickness: 1.0-1.5 mm) consisting of a NiO/8YSZ cermet, a thin 8YSZ electrolyte, and a bi-layered cathode. The cathode distinguished two separated layers: first a cathode functional layer consisting of La 0.65 Sr 0.3 MnO 3 (LSM)/Y 2 O 3 -stabilized ZrO 2 (8YSZ) and a cathode current collector layer of pure La 0.65 Sr 0.3 MnO 3 (LSM). This study can be considered as a follow-up of that (Journal of Power Sources 141 (2005) 216-226) describing the improvement of the cell performance by a systematic variation of the microstructure. The experiments described in this paper and the corresponding results are part of a more extensive study to investigate in more detail the effect of the sintering temperature on the electrochemical performance of LSM-type SOFCs. Since research is still going on, conclusions, drawn in this contribution, are yet not definitive.

The Electrochemical Performance of Anode-Supported SOFCs with LSM-Type Cathodes Produced by Alternative Processing Routes

The electrochemical performance of La 0.65 Sr 0.3 MnO 3 -type (LSM) anode-supported single cells, produced by alternative production processes, has been investigated at intermediate temperatures. In particular, three different variations of the production route were investigated in more detail: (I) the use of nonground LSM powder for the cathode current collector layer, (2) the use of noncalcined and nonground YSZ powder for the cathode functional layer, and (3) the use of tape casting versus warm pressing as the production process for anode substrates. Results from electrochemical measurements performed between 700 and 900 °C with H 2 (3 vol % H 2 O) as fuel gas and air as the oxidant showed that performance increased with increasing grain size of the outer cathode current collector layer: the highest performance was achieved with nonground LSM powder. Furthermore, performance was not adversely influenced by the use of noncalcined and nonground YSZ for the cathode functional layer. Also the use of anode substrates with a thickness of about 0.7 mm and produced by tape casting, instead of those with a thickness of about 1.5 mm and applied by warm pressing, did not detrimentally affect the electrochemical performance of this type of SOFC.

Advances in Research, Development, and Testing of Single Cells at Forschungszentrum Jülich

This paper presents an overview of the main advances in solid oxide fuel cells (SOFCs) research and development (R&D), measurement standardization, and quality assurance in SOFC testing at the Forschungszentrum Julich. These activities have resulted in both a significant improvement of the electrochemical performance and a better understanding of the electrochemical behavior of SOFCs. Research and development of SOFCs was mainly focused on two types of anode-supported cells, namely, those employing either La 0.65 Sr 0.3 MnO 3 (LSM) or La 0.58 Sr 0.4 Co 0.2 fe 0.8 O 3-δ (LSCF) cathode materials. In both cases the optimization of processing and microstructural parameters resulted in satisfactory power output and long-term stability at reduced operation temperatures. Standardization and quality assurance in SOFC testing was also addressed with the goal of producing consistent and reliable tests and measurement results. At present, under optimized experimental conditions, SOFCs with LSM or LSCF cathodes can deliver a power output of about 1.0 W/cm 2 and 1.9 W/cm 2 at 800°C (700 mV), respectively.

Various Lanthanum Ferrite-Based Cathode Materials With Ni and Cu Substitution for Anode-Supported Solid Oxide Fuel Cells

The electrochemical performance of solid oxide fuel cells with cathodes made of La 0.58 Sr 0.4 Fe 0.8 Ni 0.2 O 3-δ , La 0.58 Sr 0.4 Fe 0.8 Cu 0.2 O 3-δ , La 0.58 Sr 0.4 F e 0.6 Cu 0.2 Co 0.2 O 3-δ , La 0.58 Sr 0.4 Fe 0.7 Cu 0.1 Co 0.2 O 3-δ , and La 2 Ni 0.6 Cu 0.4 O 4 has been investigated. As reference, electrochemical data from cells with La 0.58 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ cathodes were taken into account. The cathode layers were sintered at various temperatures. After testing, cross-sectional analyses were made in order to investigate microstructural changes in the various layers. Electrochemical tests have shown that only cells with a non-sintered Cu-containing cathode or with a similar cathode treated with relatively low sintering temperatures can be considered for SOFC applications. However, it was clear that the tested cells with cathodes including Cu and/or Ni showed electrochemical performance which was always lower than that of reference cells with La 0.58 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ cathode. No electrochemical measurements were possible with cathodes sintered at or above 1000°C. Cross-sectional analyses revealed that in all these cases the presence of Cu exhibited severe chemical interaction with the electrolyte. In addition, several undesired phases were formed in the cathode as well as in the diffusion barrier layer. The extent of these phases and the interaction with the electrolyte layer increased with increasing sintering temperature.

Characterization of Anode-Supported Solid Oxide Fuel Cells With Nd2NiO4 Cathodes

A systematic study was initiated of anode-supported single cells with Pr 0.58 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (PSCF) cathode. These solid oxide fuel cells (SOFCs) were characterized by electrochemical and diffusion and permeation measurements. In particular, the influence of various sintering temperatures of the cathode and various types of Ce 0.8 Gd 0.2 O 2-δ (CGO) interlayer was investigated in more detail. Results from electrochemical measurements performed between 650°C and 800°C showed that the performance of anode-supported SOFCs with screen-printed porous CGO interlayer and a PSCF cathode was excellent. Even at 650 ° C, the area-specific resistance was lower than 0.5 fl cm 2 . The microstructure of the cathode and the performance of this type of SOFC were not obviously affected by variations in the sintering temperature of the cathode. Higher electrochemical performance, in particular, in the temperature range 650- 750°C, was achieved by applying a thin and dense CGO interlayer using reactive sputtering or electron beam physical vapor deposition.

Demonstration of a 4-Cells SOFC Stack Under Different Experimental Conditions

Forschungszentrum Juelich (FZJ) has developed over the past years a well-known SOFC stack technology for a planar cell configuration. 1.5 mm thick anode supported largearea cells based on a NiO/YSZ (anode) and a YSZ (electrolyte) system are integrated in a stack using bipolar plates made of a special ferritic stainless steel acting as current collector and gas distributor Materials, processes, and surface treatments have been selected and optimized to reduce mismatch between different components and to achieve high power densities. The recent SOFC stacks of F-design, fuelled with humidified hydrogen and air has demonstrated a power density as high as 0.6 W/cm 2 at 800°C and a stable behavior up to 8000 h of operation. These tests are usually performed in standard conditions, using a low value of fuel utilization and a precise control of gas composition, fuel, and airflow rates and temperature distribution. It seems quite difficult to maintain the same strict control of these parameters when the stack operates in an actual device. Therefore, it is of interest to investigate the effect of different experimental conditions on the electrochemical performance that can simulate stack behavior as integrated in a real system. For example, an actual fuel cell system is designed to operate with fuel utilization in the range between 70% and 85% due to efficiency considerations. In addition, variations of the electric load and of the corresponding flow rates must be expected that give rise to temperature changes and gradients within the stack. Part of these effects might be detrimental for the stack service life, and can be limited by a well-designed balance of plant of the system. In this contribution, results of a series of tests on a 4-cells SOFC stack of F design, manufactured by FZJ and conducted in a new SOFC test bench by Eurocoating, are presented. This facility allows the investigation of the performance of stacks up to 1 kW as a function of several experimental parameters, including the amplitude of compressive loading, inlet gas temperature and pressure, flow rate, and fuel utilization.

A Comparative Study Between Resistance Measurements in Model Experiments and Solid Oxide Fuel Cell Stack Performance Tests

Several combinations of glass-ceramic and steel compositions with excellent chemical and physical properties have been tested in the past in solid oxide fuel cell (SOFC) stacks, but there have also been some combinations exhibiting pronounced chemical interactions causing severe stack degradation. Parallel to the examination of these degradation and short-circuiting phenomena in stack tests, recently less complex model experiments have been developed to study the interaction of glass-ceramic sealants and interconnect steels. The sealants and steels were tested in the model experiments at operation temperature using a dual air/hydrogen atmosphere similar to stack conditions. The present work compares electrochemical performance under constant current load of SOFC stack tests with the resistance changes in model experiments. In addition, microstructural results of post-operation inspection of various sealant-steel combinations are presented. The model experiments have shown that under the chosen experimental conditions, distinct changes of the specific resistance of the specimens correlate well with the changes of the electrochemical performance of SOFC stacks, indicating that this method can be considered as an excellent comparative method to provide useful information on the physical and chemical interactions between glass-ceramic sealants and ferritic steels.