Solid Oxide Cell Electrodes Research Articles

Sr Substituted La2−xSrxNi0.8Co0.2O4+δ (0 ≤ x ≤ 0.8): Impact on Oxygen Stoichiometry and Electrochemical Properties

Lanthanide nickelate Ln2NiO4+δ (Ln = La, Pr, or Nd) based mixed ionic and electronic conducting (MIEC) materials have drawn significant attention as an alternative oxygen electrode for solid oxide cells (SOCs). These nickelates show very high oxygen diffusion coefficient (D*) and surface exchange coefficient (k*) values and hence exhibit good electrocatalytic activity. Earlier reported results show that the partial substitution of Co2+ at B-site in La2Ni1−xCoxO4+δ (LNCO) leads to an enhancement in the transport and electrochemical properties of the material. Herein, we perform the substitution at A-site with Sr, i.e., La2−xSrxNi0.8Co0.2O4+δ, in order to further investigate the structural, physicochemical, and electrochemical properties. The structural characterization of the synthesized powders reveals a decrease in the lattice parameters as well as lattice volume with increasing Sr content. Furthermore, a decrease in the oxygen over stoichiometry is also observed with Sr substitution. The electrochemical measurements are performed with the symmetrical half-cells using impedance spectroscopy in the 700–900 °C temperature range. The total polarization resistance of the cell is increased with Sr substitution. The electrode reaction mechanism is also studied by recording the impedance spectra under different oxygen partial pressures. Finally, the kinetic parameters are investigated by analyzing the impedance spectra under polarization. A decrease in exchange current density (i0) is observed with increasing Sr content.

Reaction Mechanisms of La2NiO4+δ Oxygen Electrodes Operated in Electrolysis and Fuel Cell Mode

The reaction mechanisms governing the electrochemical behavior of La2NiO4+δ (LNO) oxygen electrodes for Solid Oxide Cells have been investigated through a coupled experimental and modeling approach. In this frame, a set of experiments was performed on a symmetrical cell using a three-electrode setup. A micro-scale electrode model considering two reaction pathways, i.e. bulk and surface paths, has been developed to describe the experimental results. The microstructural parameters of the electrode were obtained by FIB-SEM tomography. The model was calibrated using the experimental polarization curves measured at different temperatures, and it was validated using electrochemical impedance diagrams recorded at open circuit potential (OCP) and under polarization for different oxygen partial pressures. It has been evidenced that the LNO reaction mechanism depends on both the temperature and the polarization. At OCP, the reaction mechanism is controlled by the bulk path at 650 °C and by the surface path at higher temperatures. A transition from the bulk path towards the surface path was observed under cathodic polarizations. These results have been interpreted by considering the evolution of the LNO over stoichiometry with the electrode polarization. The evolution of the electrode polarization resistance with the oxygen partial pressure has been also investigated.

A high-performance oxygen electrode for solid oxide cells: Compositional optimisation of barium cobaltite-based composites

Solid oxide electrochemical cells are promising devices for energy conversion and chemical synthesis. In this respect, one of the main challenges to improving cell performance is the decrease in the polarisation resistance (Rp) of the oxygen electrodes. Here, the electrochemical properties of composite electrodes of Ba2Co9O14/Ce0.8Gd0.2O2-δ (BCO/CGO) are investigated by electrochemical impedance spectroscopy at 600–800 °C. The composite with 50 vol% CGO exhibits the lowest polarisation resistance, 0.3 Ω cm² at 600 °C in O2. The comparatively superior performance results from an increased triple-phase boundary (TPB) length due to the increased percolation of both phases. This successful result presents a high-performing oxygen electrode for solid oxide electrochemical systems.

A review on recent advances and trends in symmetrical electrodes for solid oxide cells

Symmetrical solid oxide cells (SSOCs) with identical air and fuel electrodes have gained significant scientific interest in the last decade because they offer several advantages over conventional cell configurations. Among other features, simpler fabrication, better chemical and thermo-mechanical compatibility between cell layers, and electrode reversibility, make them attractive for electricity generation, H 2 production and CO 2 electroreduction. This review offers an overview of the most recent advances in the field of SSOCs, paying special attention to the relationship between electrode composition, crystal structure and properties. With that aim, symmetrical electrodes are classified in four groups according to their redox stability, i.e. single phases, composites, electrodes with exsolved metal particles and those that suffer a drastic phase transformation under reducing conditions, known in the literature as quasi-symmetrical electrodes. Furthermore, an outlook of other cell configurations with increased scientific interest are also discussed, i.e. symmetrical protonic fuel cells (H–SSOCs) and solid oxide electrolyzers for CO 2 electroreduction. With this overview in mind, the authors would like to highlight the challenge ahead of finding electrode materials that optimally work under both oxidizing and reducing conditions in terms of redox stability and electrochemical properties, and further conclude on the future development of SSOCs. • Symmetrical solid oxide fuel cells with identical air and fuel electrodes are reviewed. • The state-of-the-art and future perspectives of SSOFCs and SSOECs are provided. • Doping strategies, structure, microstructure and electrochemical properties of electrodes are discussed. • This review will be helpful for the development of new symmetrical electrodes.

Highly promoted electrocatalytic activity of spinel CoFe2O4 by combining with Er0.4Bi1.6O3 as a bifunctional oxygen electrode for reversible solid oxide cells

The CoFe2O4–Er0.4Bi1.6O3 as a novel bifunctional oxygen electrode exhibits efficient ORR and OER bifunctionality for reversible SOCs at intermediate temperatures.

A critical review of the nano-structured electrodes of solid oxide cells.

Renewable energies from solar and wind power are playing an ever increasing role in meeting the tremendous global energy demand with substantially reduced carbon emissions; however, their intermittent nature poses a critical challenge for sustainability and practical applications. The capability of solid oxide cells (SOCs) to operate in both fuel cell and electrolysis modes makes them one of the most important candidates for efficient storage and regeneration of renewable energies. Here we present a critical review and prospects on the development of key fabrication techniques, e.g. wet infiltration, exsolution and direct assembly, for nanostructured electrodes, one of the most critical issues affecting the performance and stability of SOC technologies. The traits and challenges of these techniques are thoroughly discussed, with an aim to provide a critical guide for future design and development of more refined, high-performing nanostructured electrodes for SOCs.

Development of intertwined nanostructured multi-phase air electrodes for efficient and durable reversible solid oxide cells

The development of nanostructured electrodes of solid oxide cells is largely hindered by the high temperature sintering process and limited in the loading and choices of catalytic phases. Here, a nanostructured multi-phase air electrode is fabricated via facile combination of Gd0.2Ce0.8O1.9 (GDC) decorated PrBa0.8Ca0.2Co2O5+δ (PBCC) and direct assembly approach. The highly flexible decoration process and the in situ formation of multi-phases lead to the formation of intertwined core-shell type nanostructures with intimate electrode/electrolyte interface. A cell with a 40 wt% GDC decorated PBCC electrode achieves a peak power density of 1.74 W cm−2 at 750 °C and an electrolysis current density of 1.77 A cm−2 at 1.3 V with excellent durability for 200 h. The combined decoration and direct assembly approach provides a unique and general pathway to develop a new class of nanostructured air electrodes for efficient and durable solid oxide cells.

Smart utilization of in-situ exsolution occurring in Ag doped PrBa0.5Sr0.5Co2O5+δ nanofiber air electrode of intermediate temperature reversible solid oxide cells

Low-temperature operation of reversible solid oxide cells can effectively improve their stable and economical application. However, insufficient catalytic activity of the air electrode is the limiting factor for reversible solid oxide cells. Here, we fabricate a novel Pr0.9Ag0.1Ba0.5Sr0.5Co2O5+δ nanofibers with enhanced electrocatalytic activities via an electrospinning technique and in-situ exsolution. The study confirms the advantage of morphology engineering in enlarging the catalytic interface and reactive sites, and the cells with Pr0.9Ag0.1Ba0.5Sr0.5Co2O5+δ nanofibers air electrode exhibit obviously decreased polarization resistance (0.06 Ω cm2), increased electrolysis current density of 0.65 A cm-2 (50 vol% absolute humidity and 1.5 V), and adequate power density (~ 0.5 W cm-2) at 700 °C. Also, the cell exhibits exceptional reversibility and stability during the long-term test. The enhancement may be assigned to the in-situ exsolution of Ag nanoparticles at Pr0.9Ag0.1Ba0.5Sr0.5Co2O5+δ surface and better interface combination between the air electrode and electrolyte. This work provides an exemplificative study on the favorable nanofibers air electrode for high performance intermediate temperature reversible solid oxide cells.

Solid Oxide Cell Electrode Nanocomposites Fabricated by Inkjet Printing Infiltration of Ceria Scaffolds.

The enhancement of solid oxide cell (SOC) oxygen electrode performance through the generation of nanocomposite electrodes via infiltration using wet-chemistry processes has been widely studied in recent years. An efficient oxygen electrode consists of a porous backbone and an active catalyst, which should provide ionic conductivity, high catalytic activity and electronic conductivity. Inkjet printing is a versatile additive manufacturing technique, which can be used for reliable and homogeneous functionalization of SOC electrodes via infiltration for either small- or large-area devices. In this study, we implemented the utilization of an inkjet printer for the automatic functionalization of different gadolinium-doped ceria scaffolds, via infiltration with ethanol:water-based La1−xSrxCo1−yFeyO3−δ (LSCF) ink. Scaffolds based on commercial and mesoporous Gd-doped ceria (CGO) powders were used to demonstrate the versatility of inkjet printing as an infiltration technique. Using yttrium-stabilized zirconia (YSZ) commercial electrolytes, symmetrical LSCF/LSCF–CGO/YSZ/LSCF–CGO/LSCF cells were fabricated via infiltration and characterized by SEM-EDX, XRD and EIS. Microstructural analysis demonstrated the feasibility and reproducibility of the process. Electrochemical characterization lead to an ASR value of ≈1.2 Ω cm2 at 750 °C, in the case of nanosized rare earth-doped ceria scaffolds, with the electrode contributing ≈0.18 Ω cm2. These results demonstrate the feasibility of inkjet printing as an infiltration technique for SOC fabrication.

Characterization of La0.65Sr0.35MnO3-based double-layered oxygen electrode for reversible solid oxide cells

Characterization of La0.65Sr0.35MnO3-based double-layered oxygen electrode for reversible solid oxide cells

Addressing planar solid oxide cell degradation mechanisms: A critical review of selected components

AbstractIn this review paper, a critical assessment of the main degradation processes in three key components of solid oxide fuel cells and electrolysers (negative and positive electrodes and the interconnect) is undertaken, attempting prioritization of respective degradation effects and recommendation of the best approaches in their experimental ascertainment and numerical modeling. Besides different approaches to quantifying the degradation rate of an operating solid oxide cell (SOC), the latest advancements in microstructural representation (3D imaging and reconstruction) of SOC electrodes are reviewed, applied to the quantification of triple‐phase boundary (TPB) lengths and morphology evolution over time. The intrinsic degradation processes in the negative (fuel) electrode and the positive (oxygen) electrode are discussed, covering first the composition and governing mechanisms of the respective electrodes, followed by a comprehensive evaluation of the most important factors of degradation during operation. By this systematic identification of dominant degradation processes, measurement techniques, and modeling approaches, the foundations are laid for the definition of meaningful accelerated stress testing of SOC cells and stacks, which will help the technology achieve the constantly more demanding durability targets in market applications.

Electrochemical performance of La0.65Sr0.35MnO3 oxygen electrode with alternately infiltrated Sm0.5Sr0.5CoO3-δ and Sm0.2Ce0.8O1.9 nanoparticles for reversible solid oxide cells

La1-xSrxMnO3 is a well-known oxygen electrode for reversible solid oxide cells (RSOCs). However, its poor ionic conductivity limits its performance in redox reaction. In this study, we selected Sm0.5Sr0.5CoO3-δ (SSC) as catalyst and Sm0.2Ce0.8O1.9 (SDC) as ionic conductor and sintering inhibitor to co-modify the La0.65Sr0.35MnO3 (LSM) oxygen electrode through an alternate infiltration method. The infiltration sequence of SSC and SDC showed an influence on the morphology and performance of LSM oxygen electrode, and the influence was gradually weakened with the increasing infiltration time. The polarization resistance of the alternately infiltrated LSM-SSC/SDC electrode was 0.08 Ω cm2 at 800 °C in air, which was 3.36% of the LSM electrode (2.38 Ω cm2). The Ni-YSZ/YSZ/LSM-SSC/SDC single cell attained a maximum power density of 1205 mW cm−2 in SOFC mode at 800 °C, which was 8.73 times more than the cell with LSM electrode. The current density achieved 1620 mA .cm−2 under 1.5 V at 800 °C in SOEC mode and the H2 generation rate was 3.47 times of the LSM oxygen electrode.

Boosting the performance of conventional air electrodes for solid oxide cells by in-situ loading of nano praseodymium oxide

State‐of‐the‐art air electrodes such as (La,Sr)MnO3 (LSM) and La0.6Sr0.4Co0.2Fe0.8O3+δ (LSCF) for solid oxide cells (SOCs), suffer from sluggish oxygen exchange kinetics and high activation energy at reduced temperatures, thus dramatically decreasing their performance. Herein, we demonstrate a simple strategy to boost the activities of these conventional air electrodes. Praseodymium oxide nano-catalysts were in-situ homogeneously incorporated into LSM, LSCF, and La0.6Sr0.4CoO3 (LSC) air electrodes during the SOC startup via a facile one-step urea-based infiltration method. The polarization resistance of LSM can be reduced to<1/10 of its original value, while LSCF and LSC can be reduced to about 1/2 – 1/4 of their original resistances at 600–800 °C. This approach dramatically enhances the cell performances, typically increases the SOFC power outputs and electrolysis current densities (at 600 °C) by>5 times for LSM, more than two times for LSCF, and approximately two times for LSC. Furthermore, the infiltrated LSC electrode show good durability of 132 h at 700 °C under electrolysis mode. This work demonstrates a facile and cost-effective way to boost the performance of conventional air electrodes for SOC and is also applicable to the studies involving similar oxygen exchange reactions for various energy conversion and storage systems.

Examining Operando Generated Ni-Based Alloy Nanomaterials as Fuel Electrodes in Solid Oxide Cells

Constructing nanostructure through exsolution has been demonstrated as an effective approach to producing electrode materials with superior performance and stability in the application of solid oxide cells (SOCs), primarily due to the excellent catalytic properties and the significantly enhanced stability offered by the well-embedded nanoparticles that are exsolved from a supporting oxide. Here, we focus on investigating the titanate perovskites with exsolved Ni-Co alloy nanocatalysts as fuel electrodes for SOCs, particularly on operando generating these nanomaterials by applying a potential bias in CO2 electrolysis operating conditions. Three compositions of titanate perovskite were examined, La0.43Ca0.37Ti0.94Ni0.06O3−δ (LCT-Ni6), La0.43Ca0.37Ti0.94Ni0.03Co0.03O3−δ (LCT-Ni3Co3), and La0.43Ca0.37Ti0.90Ni0.05Co0.05O3−δ (LCT-Ni5Co5). Various techniques, including X-ray diffraction, thermogravimetric analysis, and DC conductivity measurement, were applied to study the crystal structure, reduction behavior, conductivity property, and microstructure of these materials. SOCs with these titanate fuel electrodes were fabricated and evaluated, with emphasis placed on operando generation of active nanomaterials through electrochemical switching in pure CO2 atmosphere, and on understanding the materials properties linking to their microstructure and performance toward CO2 electrolysis and H2 fuel cell.

A highly active and stable hybrid oxygen electrode for reversible solid oxide cells

Reversible solid oxide cells (RSOCs) have attracted increasing attention due to the potential realizing the deep coupling between hydrogen and electricity. An efficient and stable oxygen electrode is needed for developing RSOCs. Herein, we report a nanostructured hybrid with a nominal composition of BaZr0.2Co0.8O3-δ as oxygen electrode. The chemical composition, crystal structure and physicochemical properties of the BaZr0.2Co0.8O3-δ hybrid have been characterized. The results show that the sintered BaCo0.8Zr0.2O3-δ has grown into a hybrid, consisting of cubic perovskite BaZr0.82Co0.18O3-δ, hexagonal perovskite BaCo0.96Zr0.04O2.6+δ and a small quantity of hexagonal perovskite BaCoO3. The cell with BaZr0.2Co0.8O3-δ hybrid oxygen electrode delivers a current density of 2.09 A cm−2 @0.7 V in SOFC mode, and gives −1.43 A cm−2 @1.3 V in SOEC mode for steam electrolysis. The BaCo0.8Zr0.2O3-δ cell shows a smooth transition from SOEC to SOFC, and gives no obvious degradation after 100 times SOEC↔SOFC cycle operation.

Ni migration in solid oxide cell electrodes: Review and revised hypothesis

AbstractSevere degradation of Ni‐YSZ (yttria stabilized zirconia) electrodes of solid oxide cells (SOCs) due to Ni migration is well known, but the literature contains apparent contradictions. The mechanisms are still under debate. Fine structured Ni‐YSZ composite electrodes often degrade at operation temperature (700–950°C), because Ni particles lose electrical contact with each other as larger Ni‐particles grow on the expense of smaller ones. Another type of Ni migration, which may be very damaging, is the relocation of Ni in the most active part of the Ni‐YSZ cermet electrode next to the dense YSZ electrolyte. Emphasis is put on the migration of Ni away from the YSZ electrolyte in solid oxide electrolysis cells (SOECs). This is seen as an important obstacle to the commercialization of SOC systems.Apart from temperature, degradation of Ni‐YSZ electrodes in SOCs is related to Ni‐YSZ electrode overpotential and the local redox potential of the gas mixture inside the porous Ni‐YSZ electrode. A unifying Ni migration mechanism is proposed, and methods of alleviating the electrode degradation are discussed. The hypothesis is that Ni migrates via surface diffusion of Ni(OH)x species below ca. 800°C and via Ni(OH)x species in gas phase above ca. 900°C.

A highly efficient and durable air electrode for intermediate-temperature reversible solid oxide cells

Solid oxide cells (SOCs) are considered the most efficient system for reversible conversion between chemical and electrical energy, thus having potential to be an attractive technology for a sustainable energy future. To achieve high round-trip efficiency, highly efficient and durable air electrode materials are needed to minimize energy loss associated with oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Here we report a bi-functional air electrode material, PrBa0.9Co1.96Nb0.04O5+δ, demonstrating outstanding electrochemical performance (e.g., achieving peak power densities of over 1.5 and 1 W cm−2, respectively, for Gd0.1Ce0.9O1.95 and BaZr0.1Ce0.7Y0.1Yb0.1O3-δ based fuel cells at 600 °C) while maintaining excellent stability (e.g., having a degradation rate of 40 mV per 1,000 h for H2O electrolysis cells). The excellent property of the new electrode is attributed to the improved stability from Nb doping and the enhanced electrocatalytic activity from tuning Ba deficiency, as confirmed by experimental results and computational analysis.

Efficient and Economically Favorable Co-Free Air Electrodes for Solid Oxide Cells

Cobalt-based oxides are considered as state-of-the-art air electrode materials for solid oxide cells (SOC). It is due to their extraordinary catalytic activity and high mixed ionic-electronic conductivity. However, geo-politically unstable regions which supply cobalt (approx. 70% global mining production in 2018 took place in the Democratic Republic of the Congo), depleting reserves and the forecasted continuously growing demand for the element in next decades (related to the Li-ion batteries market) may result in risks related to the security of supply. Due to mentioned considerations, in 2011 cobalt was classified by EU as a critical raw material [1]. For that reason, European Commission provide incentives for the development of Co-free air electrodes which will eliminate the aforementioned risks and also address the toxicity of cobalt. Moreover, special attention is paid to the socio-economic aspects related to its mining which often does not meet global standards related to the ethical supply of the element.Among proposed in literature air electrodes dedicated both solid oxide fuel cells (SOFC) and solid oxide electrolyzers (SOE) the La(Ni,Cu)O3-δ-based materials seem to be a promising and economically viable alternative. Alike cobalt, nickel and copper are of great economic importance, however they are not considered as critical raw materials and, what is worth to emphasize, can be effectively recycled without compromising the quality. Parent LaNiO3 and LaCuO3 perovskites are characterized by a set of beneficial physicochemical properties, including high electric conductivity, which in the case of LaCuO3 reaches 106 S cm-1 in an oxidizing atmosphere, large range of oxygen nonstoichiometry (up to 0.5 mol per mol of oxide) as well as high catalytic activity [2,3]. As we presented in [4] mixed Ni-Cu perovskites are stable in air while exposed to the temperature up to 900 °C, and only in the case of Cu-rich composition, 0.75 mol Cu per mol of La(Ni,Cu)O3-δ, phase-transition from P4/m to P4/mbm symmetry was recorded. Comparing to the Co-based materials La(Ni,Cu)O3-δ oxides demonstrate moderate TEC in the range from 10 to 14·10-6 K-1. This feature should provide a good thermomechanical compatibility with CGD buffer layer deposited on the 8YSZ electrolyte. LaNi0.5Cu0.5O3-δ and LaNi0.25Cu0.75O3-δ oxides are characterized by a large variation of oxygen nonstoichiometry (up to 0.15 mol of oxygen per oxide at 850 °C) and relatively high electrical conductivity (order of several dozen). The electrodes based on these oxides show low polarization resistance in air at 800 °C equal to 0.056 Ω cm2 and 0.054 Ω cm2 for LaNi0.5Cu0.5O3-δ and LaNi0.25Cu0.75O3-δ, respectively. Preliminary studies of the LSGM-supported full cell with LaNi0.5Cu0.5O3-δ as air electrode (0.87 W cm-2 in SOFC mode and ca. 1 A cm-1 at 1.3 V in SOE mode at 900 °C) revealed potential of (Ni,Cu)-based perovskite in application as an electrode for reversible solid oxide cells (rSOC).In this work the assessment of (Ni-Cu)-based perovskites in terms of the availability of critical raw materials, sustainability and economic aspect in relation to the Co-based oxides is presented. Moreover, the electrochemical studies of fuel electrode-supported cells with selected La(Ni,Cu)O3-δ materials making the air electrodes are presented. LaNi0.5Cu0.5O3-δ and LaNi0.25Cu0.75O3-δ made using sol-gel method were used to fabricate air electrodes layers sintered at relatively low temperature (below 1,000 °C) and characterized by fine microstructure allowing sufficient gas diffusion. The electrochemical measurements included current density-voltage curves, registered in SOFC and SOE modes, and EIS studies performed at different condition (temperature in the 700-800 °C range and various inlet gases composition).

Investigation of La2-XSrxNi0.8Co0.2O4+d (0.0 ≤ x ≤ 0.2) Materials as Oxygen Electrodes for Solid Oxide Cells

Lanthanide Nickelate Ln2NiO4+δ (Ln = La, Pr or Nd) has drawn significant attention as an oxygen electrode for solid oxide cells (SOCs) due to its mixed ionic and electronic conductivity (MIEC) property. They exhibit very good electro-catalytic activity as well as high oxygen diffusion coefficient (D*) and surface exchange coefficient (k*) values [1]. Earlier reported results [2, 3] show that the partial substitution of Co2+ at B-site in La2Ni1−xCoxO4+δ (LNCO) lead to an enhancement in the transport and electrochemical properties of the material. The present study aims to investigate the effect of Sr2+ substitution at A-site in LNCO i.e. La2-xSrxNi0.8Co0.2O4+δ (LSNCO) in order to further investigate the structural, physico-chemical and electrochemical properties of the materials. Three different compositions i.e. LSNCO5, LSNCO10 and LSNCO20 were studied with x = 0.05, 0.10 and 0.20 respectively. First the materials were characterized by XRD, TGA and SEM. Then, the electrochemical characterizations were carried out by fabricating the symmetrical half-cells (8YSZ//GDC//LSNCO) as well as single cells (NiO-YSZ//8YSZ//GDC//LSNCO) using Electrochemical Impedance Spectroscopy (EIS) in 700 – 900 °C temperature range.All these materials exist with orthorhombic structure and a decrease in the cell volume as well as oxygen content (4+δ) is observed with Sr-substitution. The TGA analyses as a function of temperature reveals that these materials show reversible oxygen exchange and are always over-stoichiometric in 25-1000 °C temperature range. However, the loss of oxygen with increasing temperature is decreased with increasing Sr-content. The EIS data analysis of the symmetrical half-cells shows an increase in polarization resistance (Rp) with increasing the Sr-content. For instance, the Rp values of 0.047 and 0.88 Ω.cm2 are observed for LSNCO5 and LSNC20 respectively at 800 °C under air at OCV conditions. The EIS measurements under different partial pressure of oxygen reveals that four electrode processes (as fitted by 4R//CPE along with a series resistance Rs) are involved in the oxygen reduction/evolution mechanism. The observed electrode processes are; ionic transfer of oxide ion through electrode electrolyte interface (R1), charge transfer process (R2), adsorption/desorption of atomic species (R3) and molecular gas diffusion process (R4). The impedance data analysis under polarization (-0.2 to +0.2 V) reveals a decrease in the exchange current density with the increase of Sr-content. For instance, the io value is 0.040 A.cm-2 for LSNCO5 which is decreased to 0.003 A.cm-2 for LSNCO20 at 800 °C. Current-Voltage characteristics obtained from the single cell measurements show a decrease in the current density with increasing Sr-content.The detailed structural, physico-chemical and electrochemical properties of these materials will be presented and discuss in detail.

Performance and Stability of Nickelates Based Oxygen Electrodes for Solid Oxide Cells

The K2NiF4-type nickelates i.e. Ln2NiO4+δ (Ln = La, Pr, Nd) and La1.5Pr0.5NiO4+δ have shown promising behaviour as oxygen electrodes for solid oxide cells [1]. These compounds show high anionic bulk diffusion (D*) as well as surface exchange coefficients (k*), combined with good electrical conductivity and thermal expansion properties matching with those of other components (electrolyte, interconnect etc.) of the cell. In order to further enhance the physico-chemical properties, electrochemical performance of these nickelates as oxygen electrode, we performed the substitution of nickel by other transition element such as cobalt. Two compositions of Ln2Ni1-xCoxO4+ δ (Ln = La, Pr, Nd) and La1.5Pr0.5Ni1-xCoxO4+ δ (x=0.1 and 0.2) were considered (as higher cobalt containing nickelates are difficult to synthesize under air atmosphere) and completely characterized using several methods. Finally, the symmetrical half-cells (8YSZ/GDC/electrode) as well as single cells (Ni-YSZ/8YSZ/GDC/electrode) were prepared and electrochemically characterized using DC- and AC-techniques in the temperature range 700 - 900 °C.The current-voltage characteristics of the single cells show an increase in the performance with increasing cobalt content in both SOFC and SOEC operation for Ln2Ni1-xCoxO4+ δ (Ln = La, Pr, Nd) and La1.5Pr0.5Ni1-xCoxO4+ δ electrodes. The best cell performance is obtained with 20% cobalt substitution for all four nickelates. However, the performance of Pr2Ni0.8Co0.2O4+ δ (PNCO20), La1.5Pr0.5Ni0.8Co0.2O4+ δ (LPNCO20) and Nd2Ni0.8Co0.2O4+ δ (NNCO20) are similar and far better than La2Ni0.8Co0.2O4+ δ (LNCO20) [2,3]. For instance under SOFC operation at 900 °C, the maximum power densities obtained at 0.6 V are 1.8, 1.9 and 1.91 W.cm-2 for NNCO20, LPNCO20 and PNCO20 single cells, respectively. Under SOEC conditions, the cell current densities obtained at 900 °C with an applied voltage of 1.5 V are 3.0, 2.84 and 2.8 A.cm-2, respectively for PNCO20, LPNCO20 and NNCO20 single cells with 50% H2O and 50% H2 feed gas mixture. The performance of these electrodes are better than the commercial single cell containing state of art (La,Sr)(Co,Fe)O3 oxygen electrodes. Moreover a large improvement in durability is also observed for the single cells containing cobalt substituted nickelates.Results including physico-chemical and electrochemical properties of these materials, electrochemical performance as oxygen electrode, long term stability under electrolysis conditions and post-test analysis (using SEM-EDX, XRD) will be presented and discussed in detail.