CO2 and steam electrolysis using a microtubular solid oxide cell

Nickel-yttria stabilized zirconia (Ni-YSZ) supported tubes were fabricated by plastic extrusion molding (PEM). YSZ was used as the electrolyte and LSM-YSZ (lanthanum-strontium doped manganite) as the oxygen electrode. Both layers were deposited by dip coating and were then sintered at 1500 degrees C and 1150 degrees C, respectively. Coelectrolysis experiments were performed in these cells at 850 degrees C, using different fuel gas conditions varying the amount of steam, carbon dioxide, nitrogen and hydrogen. Area specific resistance (ASR) values ranged from 0.47 Omega cm(2), when rich steam and CO2 flows are used, to 1.74 Omega cm(2), when a diluted composition is used. Gas chromatography was used to examine the amount of H-2 and CO in the output gas. The obtained results are consistent with the equilibrium of the water gas shift reaction. For all the different analysed conditions, faradaic efficiency was found to be close to 100%. This experiment confirmed that there is no electronic conduction taking place through the YSZ electrolyte. The threshold for electronic conduction in the diluted feeding conditions (Poor H2O and CO2) for these particular YSZ-based cell was found at voltages of about 1.65 V

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

Reversible operation performance of microtubular solid oxide cells with a nickelate-based oxygen electrode

This paper describes the reversible operation of a highly efficient microtubular solid oxide cell (SOC) with a nickelate-based oxygen electrode. The fuel cell was composed of a microtubular support of nickel and yttria stabilized zirconia (Ni-YSZ), an YSZ dense electrolyte, and a double oxygen electrode formed by a first composite layer of praseodymium nickelate (PNO) and gadolinium-doped ceria (CGO) and a second one of PNO. A good performance of the cell was obtained at temperatures up to 800 °C for both fuel cell (SOFC) and electrolysis (SOEC) operation modes, specially promising in electrolysis mode. The current density in SOEC mode at 800 °C is about -980 mA cm-2 at 1.2V with 50% steam. Current density versus voltage curves (j-V) present a linear behavior in the electrolysis mode, with a specific cell area resistance (ASR) of 0.32 O cm-2. Durability experiments were carried out switching the voltage from 0.7V to 1.2V. No apparent degradation was observed in fuel cell mode and SOEC mode up to a period of about 100 h. However, after this period especially in electrolysis mode there is an accumulated degradation associated to nickel coarsening, as confirmed by SEM and EIS experiments. Those results confirm that nickelate based oxygen electrodes are excellent candidates for reversible SOCs. © 2019 Hydrogen Energy Publications LL

Lanthanide nickelates for their application on Solid Oxide Cells

High-temperature technologies like solid oxide cells (SOC) have been employed to provide power-to-fuel and vice versa for energy conversion and storage. These technologies are a work in progress due to durability and compatibility issues between components at high temperatures. For this reason, the pursuit of optimal physical, mechanical, and chemical properties of SOC materials at lower temperatures has become more diligent. Finding suitable air electrodes has become one of the more notable obstacles to complete implementation in the industry. One of the most recent alternatives is the use of lanthanide nickelates with the Ruddlesden-Popper (RP), Lnn+1NinO3n±1 (Ln = La, Nd or Pr), and perovskite, LnNiO3-δ, structures. These materials present fast ionic and electronic transport, as well as flexible oxygen stoichiometry that makes them compelling for this purpose. As part of an ongoing study on alternative air electrode advanced materials, this review is focused on documenting the relevant findings of RP nickelates over the years, especially focusing on the current status in research and development while comparing the electrochemical performance of nickelate air electrodes

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