Michele Molinelli’s research while affiliated with Swiss Federal Institute of Technology in Lausanne and other places

In order to characterize a new SOFC stack design, to investigate degradation processes and to validate models, a diagnostic test station was designed and realized. It allows to characterize repeat-elements locally by measuring the local potential, current density and temperature over the active area. The active area is segmented in 18 small electrically insulated measurement points (segments), in addition to a main segment. The profile of current density was investigated up to an average of 0.6A/cm2 and 67% of fuel utilization, showing as expected a different response depending on the position along the flowpath. In addition, the local Nernst potentials were measured by temporarily disabling the polarization of the concerned segments. The temperature profile was also investigated as a function of the output current, showing a large heat transfer with the test furnace in this single-element configuration. In addition, local degradation behavior was studied over 1900 hours. In particular, it was found that the repeat-element showed a large sensitivity to fuel composition, with larger degradation rates under pure hydrogen than under diluted fuel mixture. Impedance spectroscopy results showed large differences in degradation behavior, depending on the location in the repeat-element and on the polarization history. In particular, segments that had not been polarized during the test showed lower ohmic resistances than the polarized ones, and zones located near the gas inlets degraded more than those near the outlet. With this experiment, it was clearly demonstrated that degradation processes depend on local operating conditions.

Planar SOFC stack technology based on a unique concept (SOFConnex™) uses structured gas distribution layers between unprofiled metal sheet interconnects and thin Ni-YSZ anode supported electrolyte cells. The layers are flexible both in material and designand allow to implement new configurations relatively simply; manifolding can be internal, external, or combined. Together with thin stack components, independent of the supplier, the SOFConnex™ stacking approach allows compact planar assembly with low cost potential and adequate power density. Different cell and flow designs have been realized. With a basic flow configuration, short stacks (50 cm2 cell active area) were assembled and tested, power density at 800°C reaching 0.5 W/cm2 at 0.7 V average cell voltage (1.5 kWe /L, 0.36 cm2 area specific resistance), for 65% fuel utilization and 35% lower heating value electrical efficiency. Short stacks were thermally cycled and operated with both hydrogen and syngas. Degradation was essentially Ohmic(confirmed from impedance spectroscopy on stacks) and at first mainly due to the cathode-electrolyte interfacial reaction, performance loss was subsequently strongly reduced after cathode replacement. Using multiple voltage probes with additional interconnects allowed to separately monitor current collection losses during polarization. With an improved design in terms of sealing, postcombustion control and flow field, stacks up to 1 kWe have been operated.

A planar solid oxide fuel cell repeating unit, 50 cm2 in total active electrode size, consisting of an anode supported electrolyte cell bearing two 7mm holes for fuel and air injection, and contacted to two dense metal current collector plates via gas distribution layers, was constructed with the aim of measuring local current densities rather than the integral current over the full area. The cathode side was entirely segmented (i.e. cathode layer, gas distribution layer, metal current collector plate) into eight galvanically separated parts of ca. 6.5 cm2 each, with own current and potential leads. The element was characterised at 750–800 ◦C and different H2 fuel flows, by total and local current–voltage recording as well as by local electrochemical impedance measurement. The segment that incorporates the fuel injection hole for the whole cell always outperforms all other segments, the corner segments furthest away from the fuel injection perform least. Differences in local potential can be higher than 200mV. Polarizing one segment individually and recording the change in potential of the other segments reveals the different contributions of convection and diffusion on the flow field. Contrarily to small ideal single cells, total performance of such larger sized, stackable cells is decisively governed by the distribution fields and their weakest zones.

Our planar SOFC stacking technology uses unprofiled metallic interconnects (MIC) and thin cells of tape cast anode supported YSZ. The key element is the gas diffusion layer (GDL) between cell and MIC, which consists of so-called SOFConnex™. Using square cells with internal manifolds, 0.5 W/cm2 stack power density (800°C) can be obtained on short stacks. However, this open design configuration limits the assembly of large stacks and the durability of operation, owing to postcombustion and redox cycling occurring at unprotected cell edges. A new design, inspired from modeling work and the adaptability of the SOFConnex™ GDL, led to oblong-shaped cells, assembled in a closed stack casing with external air manifolding and fuel recovery manifolding, avoiding postcombustion. While stack power density in both designs remains similar, the operation at increased fuel utilization has become more stable in the 2nd design. Furthermore, a correlation of performance homogeneity during stack testing was drawn to assembly quality control. A 36-cell stack in dilute H2 at 800°C achieved 625 Wel (28% LHV efficiency, 0.35 W/cm2) under continuous polarisation, with all 6 clusters of 6 cells showing coincident i-V-output.

Our planar SOFC stacking technology uses unprofiled metallic interconnects (MIC) and thin cells of tape cast anode supported YSZ. The key element is the gas diffusion layer (GDL) between cell and MIC, which consists of so-called SOFConnex™. Using square cells with internal manifolds, 0.5 W/cm2 stack power density (800°C) can be obtained on short stacks. However, this open design configuration limits the assembly of large stacks and the durability of operation, owing to postcombustion and redox cycling occurring at unprotected cell edges. A new design, inspired from modeling work and the adaptability of the SOFConnex™ GDL, led to oblong-shaped cells, assembled in a closed stack casing with external air manifolding and fuel recovery manifolding, avoiding postcombustion. While stack power density in both designs remains similar, the operation at increased fuel utilization has become more stable in the 2nd design. Furthermore, a correlation of performance homogeneity during stack testing was drawn to assembly quality control. A 36-cell stack in dilute H2 at 800°C achieved 625 Wel (28% LHV efficiency, 0.35 W/cm2) under continuous polarisation, with all 6 clusters of 6 cells showing coincident i-V-output.

Electrical output behaviour obtained on solid oxide fuel cell stacks, based on planar anode supported cells (50 or 100 cm2 active area) and metallic interconnects, is reported. Stacks (1–12 cells) have been operated with cathode air and anode hydrogen flows between 750 and 800 °C operating temperature. At first polarisation, an activation phase (increase in power density) is typically observed, ascribed to the cathode but not clarified. Activation may extend over days or weeks. The materials are fairly resistant to thermal cycling. A 1-cell stack cycled five times in 4 days at heating/cooling rates of 100–300 K h−1, showed no accelerated degradation. In a 5-cell stack, open circuit voltage (OCV) of all cells remained constant after three full cycles (800–25 °C). Power output is little affected by air flow but markedly influenced by small fuel flow variation. Fuel utilisation reached 88% in one 5-cell stack test. Performance homogeneity between cells lay at ±4–8% for three different 5- or 6-cell stacks, but was poor for a 12-cell stack with respect to the border cells. Degradation of a 1-cell stack operated for 5500 h showed clear dependence on operating conditions (cell voltage, fuel conversion), believed to be related to anode reoxidation (Ni). A 6-cell stack (50 cm2 cells) delivering 100 Wel at 790 °C (1 kWel L−1 or 0.34 W cm−2) went through a fuel supply interruption and a thermal cycle, with one out of the six cells slightly underperforming after these events. This cell was eventually responsible (hot spot) for stack failure.

Results on solid oxide fuel cell stacks tested at 800° C with H2 fuel and using planar Ni-zirconia anode supported cells, 80x80x0.2 mm in size, are presented. Modeling and numerical simulation is used to interpret observed results and develop improved designs. Where neccessary, the models are calibrated with additional experimental data. Emphasis is placed on the critical issue of nickel anode reoxidation, related to the fuel flow field. Consideration is also given to gradients of temperature and current density developing over the cells and predicted by the models; local current density could be validated by measurement. Flow distribution within stacks is also illustrated by both experimental and modeling results.

Our SOFC stack development technology is based on the unique SOFConnexTM concept, using flexible gas distribution layers between metal sheet interconnect and thin ASE cells. Flexibility is given both in material and design. This ensures proper electrical contact over the whole cell (50 cm2 active), without necessitating restrictive cell fabrication tolerances, and allows easy adaptation and evolution of the flowfields. With the presently used configuration, several multiple cell stacks were assembled and tested. Reproducible stack power density (H2 fuel, λ = 1.5-2, 800°C maximum local temperature) is 0.5 W/cm2 at 0.7 V average cell voltage (1.5 kWe/L), for 67% fuel utilisation (35% LHV electrical efficiency). Performance with simulated POX-syngas (H2/CO/N2) was close to that with H2. Degradation is the focus of attention now that adequate power density and efficiency using the SOFConnex™ approach have been established and reproduced.

Solid Oxide Fuel Cell (SOFC) application development is very well represented in Switzerland by two companies. Sulzer Hexis AG is one of the world leaders in the commercialization of SOFC systems for single family houses. A smaller company, HTceramix, is active in novel processing routes for cells and in innovative stack designs. This article first presents the benefits of implementing SOFC in selected applications and markets. Then the current state-of-the-art in stacking is described for both Swiss stack designs, looking at power density, and electrical efficiency. It is remarkable that both stacks currently exhibit a unique characteristic in SOFC design: the absence of side sealing, which permits to significantly simplify the stack assembly and thus improve its reliability. Finally, the two generations of SOFC systems produced by Sulzer Hexis are presented. The HXS 1000 Premiere preseries system is evaluated on the basis of the extended demonstration program currently underway where 110 systems are in operation in single family houses and public buildings. The near-series system is then introduced with respect to the identified needs in reduction of investment and operating costs as well as size and weight.

Progress on anode supported cell stacks (SOFCONNEX design, 50 m2 per cell) is presented. A 6-cell stack and a 8 cell-stack were mounted and tested with hydrogen fuel at 800 degre C, yielding 100 W el and 140 W el, corresponding to a power density of 1kW el/L (0.34 W/cm2). Fuel utilisation was 50% and electrical efficiency 25%. A one-cell stack delivered 0.4 W/cm 2 at 70% fuel utilisation and 33 % electrical efficiency, and showed a performance increase over its 450 h test period. Another one-cell stack was monitored and variable conditions (20-50 % fuel utilisation, 0.2-0.5 A/cm 2) for 5500 h including several thermal cycles, with -5%/1000 h degradation