Beyond the 3 rd Generation of Planar SOFC: Development of Metal Foam Supported Cells with Thin Film Electrolyte F. Han a, R. Semerad b, G. Constantin c,d, L. Dessemond c,d, R. Costa a a German Aerospace Center (DLR), Pfaffenwaldring 38-40, Stuttgart D-70182, Germany b Ceraco Ceramic Coating GmbH, Rote-Kreuz-Str. 8, Ismaning D-85737, Germany c Université Grenoble Alpes, Laboratoire d Electrochimie et de Physico-Chimie des Matériaux et des Interfaces, F-38000 Grenoble, France d CNRS, Laboratoire d Electrochimie et de Physico-Chimie des Matériaux et des Interfaces, F-38000 Grenoble, France The manufacturing of thin and gas tight electrolyte in solid oxide fuel cells (SOFCs) remains challenging especially when alloys are used for the substrate. In this work, we present a porosity graded multi-layer structure supporting thin film electrolyte, as a further evolution of the metal supported planar cell architecture. The substrate is made of NiCrAl metal foam impregnated with La 0.1 Sr 0.9 TiO 3-α (LST) ceramic. Furthermore, a 20 µm thick composite anode functional layer with LST and Gd 0.1 Ce 0.9 O 2-α (GDC) was deposited onto the substrate. The surface of the anode functional layer is later engineered with a suspension of nanoparticles with 40 nm average particle size of yttria-stabilized zirconia (YSZ). This results in a mesoporous layer with a smooth surface. Finally, a gas-tight GDC electrolyte was deposited by EB- PVD method. The thickness of the thin-film YSZ layer and the gas-tight CGO layer was approximately 1 µm and 2 µm, respectively. Button half cells with size up to 25cm² were successfully produced showing an air leakage rate down to 0.7 Pa m s -1 satisfying the gas-tightness quality control threshold of the state of the art metal supported SOFCs at DLR. Full cells with La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) cathodes showed an OCV of 0.97 V at 750 C and power density of about 80mW/cm² was obtained at 0.7 V. Introduction Solid oxide fuel cells (SOFCs) are highly efficient energy devices directly converting variety of chemical fuels into electrical power and heat. Several cell architectures have been proposed and developed aiming at increased power density and durability. In recent years, as a 3 rd generation of SOFC, metal-supported cells (MSCs) became more popular than ever. In such MSC design, porous metal substrates are applied to replace the conventional ceramic or cermet supports, present in the Electrolyte Supported Cell (ESC) and Anode Supported Cell (ASC) respectively. The well-known advantages of MSCs include cost-effective material options, improved mechanical robustness, thermal shock resistance and fast start-up capability (1, 2), which make MSCs especially attractive for mobile applications such as auxiliary power units (APUs). Cr-containing ferritic stainless
steels with good oxidation resistance are often used as substrates for MSCs for operation in the temperatures range of 600 800 o C (2 7). However, processing issues remain as challenges for their fabrication. Among them, the manufacturing of gas tight YSZ electrolyte remains the main challenges. Indeed, the standard procedure of high temperature sintering in air is incompatible with the nature of the substrate. Hereby, physical vapor deposition (PVD) performed at low temperature (400-800 o C) is extremely attractive for the fabrication of hermitic electrolyte on metal supports (3, 5, 7). The state-of-the-art MSCs often apply Ni/YSZ cermet as anode due to the excellent catalytic activities and electrical conductivity. However, Ni agglomeration, coking, sulfur poisoning and redox cycling instability, remain obstacles for the commercialization of SOFCs based on Ni/YSZ cermet anodes (8-12). Among the promising alternative anode materials, lanthanum or yttrium-substituted SrTiO 3 materials demonstrate high electrical conductivity after thermal treatment in reducing atmosphere, compatible thermal expansion coefficient with neighboring ceramic layers and excellent dimensional stability upon redox cycling (13-17). Porous perovskite based substrates with typical thickness of less 1 mm can hardly provide reliable mechanical stability. In this work, as a further development of MSCs, the processing of a novel type of cell is presented and demonstrated. It implements an innovative substrate, combining NiCrAl metal foam with LST aiming at improving mechanical stability and redox tolerance, with a thin film YSZ-GDC bi-layer electrolyte. Cell Manufacturing Experiments The processing route combines standard procedure from the ceramic processing which is widely used for SOFC manufacturing. An overview of the different process is given in Figure 1. Figure 1. Flow-chart of the cell fabrication.
Substrate Manufacturing. First of all, LST powder (CerPoTech, Norway) was milled in methyl-ethyl-ketone (MEK)/ethanol solvent for 6 h to form suspensions. Polyvinyl butyral (PVB, Sigma-Aldrich, Germany) and polyethylene glycol (PEG, Sigma-Aldrich, Germany) were added and mixed into to the suspensions. Homogeneous slurries with LST powder were obtained after another 6 h milling process. NiCrAl foam (Alantum GmbH, Germany) was cut into substrates with diameter of 45 mm or 50 mm 50 mm. The substrates were impregnated with LST slurry and dried at 75 o C for 30 min. Then the substrates were uni-axially pressed to increase the LST powder packing density and reduce the substrate thickness. Functional Anode Layer Deposition. LST powder (CerPoTech, Norway) and Gd 0.1 Ce 0.9 O 2-α (GDC) powder (Treibach AG, Austria) were mixed and milled in MEK/ethanol solvent for 24 h to obtain a suspension. PVB (Sigma-Aldrich, Germany) and PEG (Sigma-Aldrich, Germany) were added and mixed into to the suspension. The mixture was milled for an additional 6 hours into homogeneous slurry. The LST-GDC slurry was cast with a doctor blade into a green tape with a thickness of 35 µm. The green tape was laminated onto the substrates under a uni-axial pressure. The substrate and the anode layer were co-fired at 1000 o C with a heating rate of 3 K/min and dwelling time of 1 hour. Electrolyte Layer. The electrolyte layer is a combination of a YSZ nano-porous layer together with a GDC gas tight layer produced by PVD. Thin-film YSZ layer was first deposited by dip-coating of aqueous YSZ nano-suspension. The experimental details were reported elsewhere in literature (18). GDC layers were fabricated by electron beam physical vapor deposition (EB-PVD) method. Cathode Deposition. La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) powder (CerPoTech, Norway) was mixed with α-terpineol and ethyl cellulose and milled into an ink. The cathode layers were screen printed on the half-cells and dried at 75 o C in air before the electrochemical tests. Characterization Over the process, the microstructure of the different components was checked by means of SEM using a Zeiss ULTRA PLUS SEM (Carl Zeiss AG, Germany) with an X- Flash Detector 5010 (Bruker, Germany) for elemental mapping and detect distribution of elements among the multilayer structure. The gas-tightness of the electrolyte was characterized by an air leakage testing module (DLR, Germany). Fuel cell was tested in an open flanges test set-up from Fiaxell Sarl (Switzerland) at 750 C with non-humidified hydrogen and nitrogen gas mixture as fuel and synthetic air as oxidant. The total gas flow for each side was kept stable of 15 ml.min -1.cm -2. Gold grids (Heraeus, 1024 mesh.cm -2 ) were used as current collectors, because gold is inert for the hydrogen activation. A fine platinum grid (Heraeus, 3600 mesh.cm -2 ) was added at the cathode side to improve the current collecting. Electrochemical characterization was carried out at open circuit voltage (OCV) using an Autolab (PGSTAT302) potentiostat/galvanostat frequency response analyzer with frequencies between 0.01 Hz and 10 khz. The Autolab analyzer was coupled with an Autolab Booster (20 A) to record the I-V curves.
Results and Discussion Anodic Substrate It is observed in Figure 2 (a) that the NiCrAl alloy foam has an open pore size of about 450 µm. The pore size of the substrates was reduced to 200 nm (Figure 3) through the impregnation of LST ceramic into the metal foam and a calcination at 1000 o C, as shown in Figure 2 (b). The NiCrAl metal foam as the structural framework ensured the mechanical strength of the substrates. The pore size of the LST-GDC anode is further reduced to less than 100 nm, as seen in Figure 2 (c), which is suitable for the processing of the electrolyte. Figure 2. Top view SEM: (a) NiCrAl foam, (b) NiCrAl foam with impregnated LST, (c) LST-GDC anode functional layer. Cumulative pore volume (mm³/g) 100 80 60 40 20 Cumulative pore volume (mm³/g) dv/dlogr (mm³/g, R) 200 150 100 50 dv/dlogr (mm³/g, R) 0 10 100 1000 10000 100000 Pore radius (nm) Figure 3. Pore size distribution of calcined NiCrAl alloy substrates with impregnated LST fired at 1000 o C for 1 hour. Electrolyte and Hermeticity Aiming at reducing the electrolyte thickness, nano-porous YSZ layers were deposited by dip-coating method in order to further reduce the pore-size and allow deposition of thin dense hermitic GDC layer by means of EB-PVD. It is observed in Figure 4 that the 0
YSZ layer and GDC layer could be deposited all over the surface LST-CGO anode layer. The thickness of the calcined LST-GDC anode layer is about 20 µm. The total thickness of the crack-free YSZ and GDC double layer is approximately 3 µm. Undesired diffusion of elements among the layers could not be detected through elemental mapping (Figure 4c). Figure 4. SEM of half cells with dip-coated YSZ layer and EB-PVD GDC layer: (a) overview cross section, (b) cross section at LST-GDC/YSZ/GDC interfaces, (c) elemental mapping at LST-GDC/YSZ/GDC interfaces. The electrolyte fabrication process described here is proved being a reliable and reproducible way for manufacturing of half cells with a gas tightness as low as 0.7 Pa m s -1 despite the extremely thin electrolyte (seen in Figure 4), which satisfy the quality control threshold of an air leakage rate below 5 Pa m s -1 (Figure 5). 1000 Leak rate (Pa*m 3 * s -1 ) 100 10 1 quality control threshold 0,1 TD 01 TD 02 TD 03 TD 04 TD 05 TD 06 TD 08 TD 09 Sample Figure 5. Leak rate results on half cells with bi-layered YSZ/GDC thin film electrolyte. Electrochemical Testing The electrochemical characteristics of the cells are shown on Figure 6 and Figure 7. The open circuit voltage (OCV) was measured at 0.97 V, which is lower than theoretical Nernst Potential at 750 C. This may originate in the open atmosphere testing set up causing water formation at the anode side and or current leak through the thin film
electrolyte. Considering the low air leak rate of the cell, H 2 permeation through the membrane may not be the primary origin for this low OCV. Figure 6. I-V and P-I characteristics at different times recorded on cell at 750 C using 100 ml/min air at the cathode and a mixture of H 2 and N 2 as a fuel, with flows of 80 ml/min and 20 ml/min respectively. Cell was run at OCV between each record. A continuous increase of performance was observed over time especially during the first 30 hours of operation. Only small performance improvement was observed in the following 65 hours of testing indicating that a plateau was reached. The frequency distribution of the polarization resistance does not show major change and remains the same over the period of testing (Figure 7), indicating no significant change in the electrode reaction processes. Nonetheless, the ohmic area specific resistance (ASR) reduces from 2.1 Ω.cm 2, down to 1 Ω.cm 2. This decrease of the ohmic resistance explains the improvement of the total cell performance and is attributed to the in-situ sintering of the green LSCF cathode during operation, which by necking and bounding particles between each other improves the current collecting at the cathode side. Figure 7. Nyquist plot of EIS spectra recorded at OCV and at 750 C using 100 ml/min air at the cathode and a mixture of H 2 and N 2 as a fuel, with flows of 80 ml/min and 20 ml/min respectively. After 95 hours of operation, a power density of 80 mw/cm² at 0.7 V was obtained while peak power density reached nearly 100 mw/cm². Performance might be improved by decreasing the ohmic resistance of the substrate. Reducing the LST only at 750 C may not be suitable to reach high electronic conductivity. Therefore reducing the thickness of
the substrate may lead to significant improvement of performance. Similarly, the low frequency contribution (< 1 Hz) seems to be predominant to the cell polarization. Though additional experiment are required to determine the nature of the limiting physical process, it might be linked to a gas diffusion in porous phase and most likely in the substrate since this was measured at merely about 200 nm. Therefore, tuning the pore size and the thickness of the NiCrAl-LST substrate may lead to significant improvement of performance. Conclusion Innovative cell architecture combining metal support tolerant to air atmosphere at high temperature with nickel free perovskite based electrocatalyst anode is presented. A gas tight bi-layer thin film electrolyte combining nano-porous YSZ layer with gas tight EB-PVD GDC is demonstrated. The cell reached a power density of 80 mw/cm² at 0.7 V and 750 C. Performance limiting factors are identified as being the low electrical conductivity of the substrate and fine porosity of the substrate which may hinder the gas diffusion and the fuel feeding in the anode. Further development work is required to address these issues and increase performance to a higher level. Acknowledgments The research leading to these results has received funding from the European Union s Seventh Framework Program (FP7/2007-2013) for the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreement No. 303429 (Project EVOLVE). The authors also thank Ina Plock, Gudrun Steinhilber, Davide Vattuone and Tang Rui for all experimental supports and helpful discussions. References 1. M. C. Tucker, G. Y. Lau, C. P. Jacobson, L. C. DeJonghe, S. J. Visco, J. Power Sources, 171, 477-482 (2007). 2. M. C. Tucker, J. Power Sources, 195, 4570-4582 (2010). 3. R. Vaßen, D. Hathiramani, J. Mertens, V.A.C. Haanappel, I.C. Vinke, Surface & Coatings Technology, 202, 499 508 (2007). 4. T. Franco, R. Henne, M. Lang, G. Schiller, P. Szabo, F. Meyer, L. Otterpohl, 5th European Solid Oxide Fuel Cell Forum, Luzern (2002). 5. M. Lang, T. Franco, G. Schiller, N. Wagner, J. Applied Electrochemistry, 32, 871-874 (2002). 6. M. Lang, P. Szabo, Z. Ilhan, S. Cinque,T. Franco, G. Schiller, J. Fuel Cell Science and Technology, 4, 384-391 (2007). 7. M. Haydn, K. Ortner, T. Franco, S. Uhlenbruck, N. H. Menzler, D. Stöver, G. Bräuer, A. Venskutonis, L. S. Sigl, H. P. Buchkremer, R. Vaßen, J. Power Sources, 256, 52-60 (2014). 8. H. Tu, U. Stimming, Advances, J. Power Sources, 127, 284-293 (2004). 9. S. McIntosh, R.J. Gorte, Chem. Rev., 104, 4845-4866 (2004). 10. S. Zha, Z. Cheng, M. Liu, J. Electrochemical Soc., 154, B201-206 (2007).
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