The influence of reduction conditions on a Ni-YSZ SOFC anode microstructure and evolution

Ni-YSZ cermets are the most widespread material used as SOFC anodes. These materials are generally fabricated through the reduction of a NiO-YSZ composite, but the reduction conditions have a great effect in the final microstructure of the electrode. In the present work several conditions were explored to reduce microtubular anode supports produced via extrusion in order to find a suitable microstructure for SOFC anodes. Samples were reduced in pure and diluted H2, either dry or humidified at temperatures ranging from 400 to 800°C while their DC conductivity was being monitored. The highest value of peak conductivity was measured in the sample reduced in humidified pure hydrogen at 800°C, but it experienced more severe conductivity degradation that other samples. The best value for the duration of the test was obtained for the sample reduced in dry diluted hydrogen at 800°C.Authors would like to thank the project MAT2012-30763, financed by the Spanish Government (Ministerio de Economía y Competitividad) and the Feder program of the European Union.Peer Reviewe

Highly stable microtubular cells for portable solid oxide fuel cell applications

In this work, extruded support tubes based on Nickel Oxide-YSZ (yttria stabilized-zirconia) were manufactured by Powder Extrusion Moulding (PEM). An YSZ layer is then deposited by dip coating as the electrolyte and subsequently, standard La0.8Sr0.2MnO3-δ (LSM)/YSZ composites were deposited by dip coating as oxygen electrodes. Microstructure of the anode support was optimized in order to achieve the maximum fuel utilization and as a consequence, a high performance of the cells. Experiments as a function of the fuel composition showed power densities above 500 mWcm−2 at 800 °C at 0.7 V, with high fuel utilization (∼75%). Long-term durability studies were also performed for a period above 1000 hours. The experiment was conducted at 800 °C using pure humidified hydrogen at a fixed voltage of 0.8 V. It was observed that the current density of the cell is significantly evolving during the initial period of about 100 hours, as a consequence of reconditioning of nickel particles at the anode support. Once the system is stabilized, no degradation was observed up to 1000 hours under operating conditions, obtaining current densities in the range of 400 mAcm−2 at 0.8 V and 800 °C.The project MAT2015-68078-R, financed by the Spanish Government (Ministerio de Economía y Competitividad) and the Feder program of the European Union, is also acknowledged.Peer Reviewe

The influence of the reducing conditions on the final microstructure and performance of nickel-yttria stabilized zirconia cermets

Ni-YSZ (yttria stabilized zirconia) cermets are the most widespread composite materials to be used as SOFC fuel electrodes. These materials are generally fabricated by the reduction of NiO to Ni in a NiO-YSZ composite, where the reducing conditions have a great effect in the final microstructure of the electrode. In the present work, several reducing conditions were explored in order to find the most suitable microstructure for anode-supported microtubular solid oxide fuel cells (SOFCs). Samples were firstly reduced in either pure or diluted H (dry or humidified), at temperatures ranging from 400 to 800 °C while their DC conductivity was monitored. The highest conductivity value was measured for the sample reduced in pure humidified hydrogen at 800 °C. However, this sample experienced conductivity degradation in comparison with samples reduced under dry conditions. For the studied temperature range, nucleation of nano-porous nickel particles is firstly formed during reduction. However, from our experiments it was concluded that those nanoparticles are not stable with time, at least at temperatures between 600 °C and 800 °C. Electrochemical characterization of complete microtubular cells under real wet conditions was also performed under current load, confirming that the microstructure of the Ni-YSZ cermet is still evolving during operation.Authors would like to thank the project MAT2015-68078-R, financed by the Spanish Government (Ministerio de Economía y Competitividad) and the Feder program of the European Union.Peer Reviewe

Editorial

Víctor Orera was our first teacher and scientific father in the laboratory. With him we also learned that research, like any human activity, must be honest and contribute to improving the world around us. He was very passionate about science, dedicated and naturally optimistic. When we were having a bad day at the lab, he had a knack for finding the silver lining in an apparently unsuccessful outcome. Until retirement, he was the leader of our Research Group, Processing and Characterization of Structural and Functional Ceramics, PROCACEF, at the Institute of Materials Science of Aragon, ICMA, an institute that he contributed to found and develop. ICMA has recently become part of the Aragon Institute of Nanoscience and Materials (INMA), from where we write these words..

Direct-methane anode-supported solid oxide fuel cells fabricated by aqueous gel-casting

Direct methane Solid Oxide Fuel Cells (SOFCs) operated under catalytic partial oxidation (CPOX) conditions are investigated, focusing on the processing of the anode support and the anode deactivation caused by carbon deposition. Anode-supported SOFCs based on gadolinium-doped ceria (GDC) electrolyte, and NiO-GDC anode support were fabricated by the gel-casting method. Suitable aqueous slurries formulations of NiO–GDC were prepared, starting NiO-GDC nanocomposite powders, agarose as gelling agent and rice starch as pore former. Electrochemical and mechanical tests evidenced that the support of 550 ± 50 µm thickness and 10 wt% pore former is a good candidate for direct-methane SOFCs. The cells operating under stoichiometric conditions of CPOX reached a performance of 0.64 W·cm−2 at 650 ºC, a very close value to that measured under humidified hydrogen (0.71 W·cm−2). The best electrochemical stability of the cell is achieved at a CH4/O2 ratio of 2.5, showing no evidence of carbon deposition and reducing nickel re-oxidation significantly

CFD simulation of a reversible solid oxide microtubular cell

Trabajo presentado al 10th European Solid Oxide Fuell Cell Forum celebrado en Lucerna (Suiza) del 26 al 29 de Junio de 2012.In this work, the authors introduce a comprehensive model and the corresponding 3D numerical tool for the simulation of reversible micro-tubular solid oxide fuel cells. It is based on a previous in-house model for SOFC [1], to which some new features has been added to extend its applicability to SOEC. The model considers the following physical phenomena: (i) fluid flow through channels and porous media; (ii) multicomponent mass transfer within channels and electrodes; (iii) heat transfer due to conduction, convection and radiation; (iv) charge motion; and (v) electrochemical reaction. The numerical algorithm to solve this mathematical model is implemented in OpenFOAM, an open source CFD toolbox based on the finite-volume method.Peer reviewe

Investigation of Graded La2NiO4+ Cathodes to Improve SOFC Electrochemical Performance

Mixed ionic and electronic conducting MIEC oxides are promising materials for use as cathodes in solid oxide fuel cells SOFCs due to their enhanced electrocatalytic activity compared with electronic conducting oxides. In this paper, the MIEC oxide La2NiO4+ was prepared by the sol-gel route. Graded cathodes were deposited onto yttria-stabilized zirconia YSZ pellets by dip-coating, and electrochemical impedance spectroscopy studies were performed to characterize the symmetrical cell performance. By adapting the slurries, cathode layers with different porosities and thicknesses were obtained. A ceria gadolinium oxide CGO barrier layer was introduced, avoiding insulating La2Zr2O7 phase formation and thus reducing resistance polarization of the cathode. A systematic correlation between microstructure, composition, and electrochemical performance of these cathodes has been performed. An improvement of the electrochemical performance has been demonstrated, and a reduction in the area specific resistance ASR by a factor of 4.5 has been achieved with a compact interlayer of La2NiO4+ between the dense electrolyte and the porous La2NiO4+ cathode layer. The lowest observed ASR of 0.11 cm2 at 800°C was obtained from a symmetrical cell composed of a YSZ electrolyte, a CGO interlayer, an intermediate compact La2NiO4+ layer, a porous La2NiO4+ electrode layer, and a current collection layer of platinum paste

Anode supported microtubular solid oxide fuel cells running on methane

Trabajo presentado al "III Iberian Symposium on Hydrogen, Fuel Cells and Advanced Batteries" celebrado en Zaragoza (España) del 27 al 30 de Junio de 2011.We would like to thank grants MAT2009-14324-C0.2-01 and CIT-120000-2007-50 financed by the Spanish Government and Feder program of the European Community for funding project. M. A. Laguna-Bercero would also like to thank the JAEprogram (CSIC) for financial support.Peer Reviewe

Tailoring the microstructure of a solid oxide fuel cell anode support by calcination and milling of YSZ

In this study, the effects of calcination and milling of 8YSZ (8 mol% yttria stabilized zirconia) used in the nickel-YSZ anode on the performance of anode supported tubular fuel cells were investigated. For this purpose, two different types of cells were prepared based on a Ni-YSZ/YSZ/Nd2NiO4+d-YSZ configuration. For the anode preparation, a suspension was prepared by mixing NiO and YSZ in a ratio of 65:35 wt% (Ni:YSZ 50:50 vol.%) with 30 vol.% graphite as the pore former. As received Tosoh YSZ or its calcined form (heated at 1500 °C for 3 hours) was used in the anode support as the YSZ source. Electrochemical results showed that optimization of the fuel electrode microstructure is essential for the optimal distribution of gas within the support of the cell, especially under electrolysis operation where the performance for an optimized cell (calcined YSZ) was enhanced by a factor of two. In comparison with a standard cell (containing as received YSZ), at 1.5 V and 800 °C the measured current density was -1380 mA cm-2 and -690 mA cm-2 for the cells containing calcined and as received YSZ, respectively. The present study suggests that the anode porosity for improved cell performance under SOEC is more critical than SOFC mode due to more complex gas diffusion under electrolysis mode where large amount of steam needs to be transfered into the cell

Electrochemical performance of Nd1.95NiO4+δ cathode supported microtubular solid oxide fuel cells

Nd1.95NiO4+δ (NNO) cathode supported microtubular cells were fabricated and characterized. This material presents superior oxygen transport properties in comparison with other commonly used cathode materials. The supporting tubes were fabricated by cold isostatic pressing (CIP) using NNO powders and corn starch as pore former. The electrolyte (GDC, gadolinia doped ceria based) was deposited by wet powder spraying (WPS) on top of pre-sintered tubes and then co-sintered. Finally, a NiO/GDC suspension was dip-coated and sintered as the anode. Optimization of the cell fabrication process is shown. Power densities at 750°C of ~40 mWcm-2 at 0.5V were achieved. These results are the first electrochemical measurements reported using NNO cathode-supported microtubular cells. Further developments of the fabrication process are needed for this type of cells in order to compete with the standard microtubular solid oxide fuel cells (SOFC).The authors thank grant MAT2009-14324-C02-01 and MAT2012-30763, financed by the Spanish Government (Ministerio de Ciencia e Innovación) and Feder program of the European Community, for funding the project.Peer Reviewe