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. $1.5mm$ thick anode supported large-area 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 $0.6W∕cm2$ at $800°C$ and a stable behavior up to $8000h$ 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 $1kW$ as a function of several experimental parameters, including the amplitude of compressive loading, inlet gas temperature and pressure, flow rate, and fuel utilization.