Zirconia-based Electrolytes Research Articles

Enhanced densification of screen-printed GDC interlayers for solid oxide fuel cells using nitrate-based precursor in various chelating agents as pore-filling solution

The Gd-doped CeO2 (GDC) diffusion barrier between the zirconia-based electrolyte and the LSCF-based cathode in a solid-oxide fuel cell (SOFC) is essential for preventing the formation of Sc2ZrO3 as a secondary phase. However, the reaction between ceria and zirconia at high temperatures (>1300 °C) impedes the formation of a highly dense GDC layer. Herein, we present a cost-effective approach for fabricating a highly dense GDC diffusion barrier layer at lower sintering temperatures by filling the pores of the GDC skeleton using a gelatin or polyvinylpyrrolidone (PVP) solution as a chelating agent for GDC cations. We investigated the interaction between the cations and the chelating agent molecules and its effect on the diffusion barrier. In addition, the flow behavior of the pore-filling solution was evaluated to determine its penetrability. The proposed method yielded a pore-filled GDC (PF-GDC) interlayer with enhanced density at 1000 °C, a remarkable 250 °C below the conventional sintering temperature for a porous GDC interlayer. The effectiveness of the PF-GDC was investigated by analyzing the performance of electrolyte-supported cells (ESCs) and anode-supported cells (ASCs). On ASCs, the observed peak power density at 800 °C was enhanced 1.5-fold, from 1.91 W⋅cm−2 (porous GDC sintered at 1250 °C) to 2.61 W⋅cm−2 (PF-GDC sintered at 1200 °C). These findings highlight the potential for pore-filling methods to improve the performance of SOFCs.

Synergetic effect of spontaneous Fe diffusion for enabling immediately switchable operations in NiFe-supported solid oxide cells

Solid oxide cells (SOCs) are highly efficient energy conversion systems, offering switchable capabilities for power and fuel production. NiFe metal-supported structures in SOCs show promising advantages in structural stability and scalability. This study explores the synergistic effect of spontaneous Fe diffusion in NiFe-supported SOCs, enhancing alloy catalyst formation in the electrode and improving the sinterability of the zirconia-based electrolyte. This results in increased activity for carbon fuel reactions, resistance to coking, and a remarkably low ohmic resistance of 0.06 Ω cm2 at 800 °C. The NiFe support also enhances structural stability, enabling reversible and multifuel operation. The single cell achieves exceptional maximum power densities of 2.6 and 2.0 W cm−2 in fuel cell mode (3 % H2O/H2 and dry CO) and current densities of − 1.51 and − 1.38 A cm−2 in electrolysis cell mode (1.3 V, 20 % H2O/H2 and 20 % CO2/CO) at 800 °C, respectively.

Boosting the Stability and Performance of SOFCs: A New Approach to Oxygen Storage Capacity Using Lanthanum-Doped Ceria Interlayer Technology

Being carbon neutral is one of the world's most important objectives. More than 65 percent of the countries that emit harmful greenhouse gases have committed to achieving net-zero emissions by the middle of the century.[1] There are numerous essential ways to gather energy for worldwide missions, including fuel cells, solar power, wind power, etc. The growing marketplace of fuel cells is following the trend of green-energy demand. Solid oxide fuel cells (SOFCs) are gaining popularity in the next-generation energy industry due to their high energy conversion efficiency, fuel flexibility, and low emissions.[2]The main challenges for the commercialization of SOCs that need to address are high performance, long-term durability, and effective cost. Stabilized zirconia such as Yttria-stabilized zirconia (YSZ) and Scandia-stabilized zirconia (ScSZ) is commonly used as a self-standing electrolyte support. The inability of zirconia-based electrolytes to react with Sr-rich cathode materials like LSC, LSCF, BSCF, etc. Therefore, the electrodes and the zirconia-based electrolyte must have the best-interlayered buffer structure. Optimizing interlayer microstructures to make them more effective at high-performance ESCs is of great interest. Several Rare-earth doped ceria (RDC) materials, including (Gd, Ce)O2, (Sm, Ce)O2, and (La, Ce)O2, have been used as efficient interlayers to lessen reactivity and preserve ionic conductivity in the interface. However, the difference in the thermal expansion coefficient (TEC) between the GDC and the YSZ poses a significant problem for starting and halting SOCs. In the present work, we focus on the commercial lanthanum-doped ceria (Ce0.6La0.4O1.8 - LDC) as a buffer layer between the YSZ electrolyte and the LSCF cathode because of its higher oxygen storage capacitance. The reactivity of the YSZ/GDC composite was higher than that of the YSZ/LDC composite. And the total conductivity of the LDC/YSZ composite is higher than GDC/YSZ composite even though LDC has lower conductivity than GDC. Moreover, the mismatching of TEC was reduced by replacing the GDC with the LDC bilayer.Consequently, the maximum power density for the electrolyte-supported cell SOFC with LDC interlayer was ~0.55 W.cm−2 at 1073 K. A variety of additives (Co, Mn, and Cu) was added to the LDC buffer layer to examine the sintering properties. The addition of copper not only obtained the highest microstructural density but also improve mechanical stability under cost-effective processes.

Lowering the Temperature of Solid Oxide Electrochemical Cells Using Triple-Doped Bismuth Oxides.

Despite the great potential of solid oxide electrochemical cells (SOCs) as highly efficient energy conversion devices, the undesirable high operating temperature limits their wider applicability. Herein, a novel approach to developing high-performance low-temperature SOCs (LT-SOCs) is presented through the use of an Er, Y, and Zr triple-doped bismuth oxide (EYZB). This study demonstrates that EYZB exhibits > 147 times higher ionic conductivity of 0.44 S cm-1 at 600 °C compared to commercial Y-stabilized zirconia electrolyte with excellent stability over 1000h. By rationally incorporating EYZB in composite electrodes and bilayer electrolytes, the zirconia-based electrolyte LT-SOC achieves the unprecedentedly high performance of 3.45 and 2.02W cm-2 in the fuel cell mode and 2.08 and 0.95 A cm-2 in the electrolysis cell mode at 700 °C and 600 °C, respectively. Further, a distinctive microstructural feature of EYZB that largely extends triple phase boundary at the interface is revealed through digital twinning. This work provides insights for developing high-performance LT-SOCs.

Thickness effect of thin-film barrier layers for enhanced long-term operation of solid oxide fuel cells

Highly efficient solid oxide cells are one of the most promising technologies for a sustainable future based on renewable hydrogen. The diffusion barrier layer employed between zirconia-based electrolytes and state-of-the-art oxygen electrodes aims to limit the formation of electrically insulating secondary phases that dramatically reduce the cells’ performance. Conventional barrier layers manufactured by screen-printing technology lead to porous microstructures that enable the formation of insulating SrZrO3, partially blocking the active area of the cells. Opposite, homogeneous and dense barrier layers have proven to be the ultimate solution to limit interdiffusion, substantially improving the cells’ performance. Despite the relevance of this solution, the impact of the barrier layer thickness on the final performance of the cells is still unknown. In this work, gadolinia-doped ceria barrier layers with thicknesses between 200 and 800 nm made by pulsed laser deposition were studied in button cells. Excellent electrochemical performance was obtained for all the cells, improving 45% of the power output of the reference counterparts. Moreover, durability tests performed on the cell with the thinnest layer (200 nm) did not show any measurable degradation for 3500 h of continuous operation under high current densities of 0.77 A cm−2 (∼0.87 V) at 750 °C. Post-mortem characterization by synchrotron nano-x-ray fluorescence of a pristine cell and the aged cell allowed us to observe that some spots of SrZrO3 were present at the cathode/electrolyte interface since the cell manufacturing step without increasing during long-term operation. Indeed, the discontinuity of this insulating phase seems not to be critical for cell operation.

High-Performance Solid Oxide Electrochemical Cells with Ultrathin and Dense Double-Doped Ceria Interlayers

To mitigate the drastic global climate change, which mainly originated from the indiscriminate use of fossil fuels, various alternative eco-friendly energy conversion technologies have been studied. Among them, solid oxide electrochemical cells (SOCs) are one of the most promising devices with superior power and energy densities and low environmental impact. The current main objective of the SOCs research field is lowering the operating temperature to secure thermal cycling stability and minimize system costs. To achieve a feasible performance for commercial usage at reduced temperatures, it is inevitable to use state-of-the-art cobalt-based oxygen electrode materials. However, cobalt-based materials inherently react with conventional zirconia-based electrolytes at high temperatures forming insulating phases at the interface. Thus, the ceria interlayer is essentially required to prevent unwanted chemical reactions. In this study, we newly developed thin and dense doubly doped ceria interlayers (~250 nm) using a non-vacuum solution deposition process with gelatin. The SOCs with the thin-film ceria interlayers achieved superior electrochemical performance both in the fuel cell (FC) and the electrolysis cell (EC) mode (~3.5W/cm2 and ~2.1A/cm2 at 750°C for the FC and EC modes, respectively) compared to the SOC with a conventional porous and thick interlayer. The mechanistic analyses showed that the thin-film interlayer provides a much shorter route for oxygen ionic conduction and more active sites for oxygen reduction reaction (ORR) and oxygen evolution reaction (ORR) at the interface compared to the porous interlayer. Our results suggest that the thin-film doubly-doped ceria interlayer fabricated using the gelatin-assisted process has a high potential for development of high-performance SOCs.

Sintering Aid Strategy for Promoting Oxygen Reduction Reaction on High-Performance Double-Layer LaNi0.6Fe0.4O3-δ Composite Electrode for Devices Based on Solid-State Membranes.

Strontium and cobalt-free LaNi0.6Fe0.4O3-δ is considered one of the most promising electrodes for solid-state electrochemical devices. LaNi0.6Fe0.4O3-δ has high electrical conductivity, a suitable thermal expansion coefficient, satisfactory tolerance to chromium poisoning, and chemical compatibility with zirconia-based electrolytes. The disadvantage of LaNi0.6Fe0.4O3-δ is its low oxygen-ion conductivity. In order to increase the oxygen-ion conductivity, a complex oxide based on a doped ceria is added to the LaNi0.6Fe0.4O3-δ. However, this leads to a decrease in the conductivity of the electrode. In this case, a two-layer electrode with a functional composite layer and a collector layer with the addition of sintering additives should be used. In this study, the effect of sintering additives (Bi0.75Y0.25O2-δ and CuO) in the collector layer on the performance of LaNi0.6Fe0.4O3-δ-based highly active electrodes in contact with the most common solid-state membranes (Zr0.84Sc0.16O2-δ, Ce0.8Sm0.2O2-δ, La0.85Sr0.15Ga0.85Mg0.15O3-δ, La10(SiO4)6O3-δ, and BaCe0.89Gd0.1Cu0.01O3-δ) was investigated. It was shown that LaNi0.6Fe0.4O3-δ has good chemical compatibility with the abovementioned membranes. The best electrochemical activity (polarization resistance about 0.02 Ohm cm2 at 800 °C) was obtained for the electrode with 5 wt.% Bi0.75Y0.25O1.5 and 2 wt.% CuO in the collector layer.

Highly active cobalt-free perovskites with Bi doping as bifunctional oxygen electrodes for solid oxide cells

Cobalt-containing perovskite materials have been intensively investigated as bifunctional oxygen electrodes for solid oxide cells because of their mixed ionic-electronic conducting nature. However, the direct application of these materials to the most widely used zirconia-based electrolytes has been limited because of their thermo-chemical incompatibility. Herein, we introduce a cobalt-free manganite-perovskite with significantly improved electrocatalytic activity by doping with Bi which possesses stereochemically active lone pair electrons. Replacing Bi with Pr in Pr0.8Sr0.2MnO3−δ increases the oxygen vacancy concentration and promotes oxygen ion incorporation and charge transfer processes at the electrode. Consequently, we apply Pr0.5Bi0.3Sr0.2MnO3−δ, to the interlayer-free yttria-stabilized zirconia electrolyte-based cells, which exhibits record high performance in the fuel cell (1.91 W∙cm−2) as well as electrolysis (1.90 A∙cm−2) modes at 700 °C along with remarkable long-term stability (450 h). Thus, our findings demonstrate that Pr0.5Bi0.3Sr0.2MnO3−δ is a highly promising bifunctional catalyst for solid-state energy storage and conversion applications.

Zirconia- and ceria-based electrolytes for fuel cell applications: critical advancements toward sustainable and clean energy production.

Solid oxide fuel cells (SOFCs) are emerging as energy conversion devices for large-scale electrical power generation because of their high energy conversion efficiency, excellent ability to minimize air pollution, and high fuel flexibility. In this context, this critical review has focussed on the recent advancements in developing a suitable electrolyte for SOFCs which has been required for the commercialization of SOFC technology after emphasizing the literature from the prior studies. In particular, the significant developments in the field of solid oxide electrolytes for SOFCs, particularly zirconia- and ceria-based electrolytes, have been highlighted as important advancements toward the production of sustainable and clean energy. It has been reported that among various electrolyte materials, zirconia-based electrolytes have the potential to be utilized as the electrolyte in SOFC because of their high thermal stability, non-reducing nature, and high mechanical strength, along with acceptable oxygen ion conductivity. However, some studies have proved that the zirconia-based electrolytes are not suitable for low and intermediate-temperature working conditions because of their poor ionic conductivity to below 850°C. On the other hand, ceria-based electrolytes are being investigated at a rapid pace as electrolytes for intermediate and low-temperature SOFCs due to their higher oxygen ion conductivity with good electrode compatibility, especially at lower temperatures than stabilized zirconia. In addition, the most emerging advancements in electrolyte materials have demonstrated that the intermediate temperature SOFCs as next-generation energy conversion technology have great potential for innumerable prospective applications.

The synthesis of nanoscale structure of calcia-stabilized zirconia electrolytes by lamellar liquid crystal template method

Calcia-stabilized zirconia (CSZ) is a potential zirconia-based electrolyte for intermediate-temperature solid-oxide fuel cells. The grain size and crystal form of zirconia ceramics have a significant effect on the electrical properties of CSZ, particularly on ionic conductivity. In this work, nanosized particles of calcia-stabilized cubic zirconia were synthesized using citrate-assisted lamellar liquid crystal template method. The morphology and particles size of CSZ could be controlled by simply optimizing the amount of three components in the template. A cubic CSZ could be established by controlling the amount of calcia doped at 800 °C sintered in air. In addition, a band gap variation of zirconia crystal occurred after Ca doping. This work provides a feasible strategy to develop intermediate-temperature solid-oxide fuel cells through Ca doping and template regulation.

α-PbO2-type niobate as efficient cathode materials for steam and CO2 electrolysis

α-PbO2-type Mn0.5Ti0.5NbO4-based oxides are studied as cathode materials in solid oxide electrolysis cell (SOEC) for the direct electrolysis of steam or CO2 at 800 °C. Comparison between Mn0.5Ti0.5NbO4 and the Fe-doped ones, Fe0.3Mn0.35Ti0.35NbO4 and Fe0.6Mn0.2Ti0.2NbO4, indicates that the Fe3+ doping increases the electric conductivity in air but decreases the stability of the α-PbO2 structure in reducing atmosphere as in Fe0.6Mn0.2Ti0.2NbO4. The electric conductivity in reducing atmosphere is found to be ascribed to Ti4+-O-Ti3+ and Fe3+-O-Fe2+ in Mn0.5Ti0.5NbO4 and Fe doped ones, respectively. The steam or CO2 electrolsysis indicates that 30% Fe3+ substitution for Mn/Ti decreases the polarization resistance at low bias, but 60% Fe doping increases the polarization resistance dramatically than Mn0.5Ti0.5NbO4. The excessive Fe3+ doping is found to induce the phase transformation and delamination on the cathode/electrolyte interface under a cathodic current. The electrolyser with Fe0.3Mn0.35Ti0.35NbO4 cathode on zirconia-based electrolyte imparts a stable current density of 2.32 Acm−2 at −1.6 V if Ar-88% H2O is used as feeding gas. Analysis of the gas product indicates that H2 is produced and the Faradaic efficiency at −1.4 V and −1.6 V is found to be 100% and 93%, respectively.

Phase transformation and electrical properties of bismuth oxide doped scandium cerium and gadolinium stabilized zirconia (0.5Gd0.5Ce10ScSZ) for solid oxide electrolysis cell

Scandium cerium and gadolinium stabilized zirconia (SCGZ) is among the zirconia-based electrolytes which exhibits high ionic conductivity. However, stabilization of high conducting cubic phase is needed to maintain the ionic conductivity during prolonged operation. In this study, 1–2 mol% of Bi2O3 is added in the SCGZ to enhance the stability of the cubic phase. The phase characterization, sintering behavior, and electrochemical performance of the electrolyte-supported cells are studied. Bi2O3 is found to act as both phase stabilizer and sintering aid. Doping of Bi2O3 results in a partial decrease of unwanted rhombohedral phase (Sc2Zr7O17) which is a low conducting phase. Increasing Bi2O3 content also significantly increases electrolyte densification. The unwanted rhombohedral phase is found to form at high sintering temperature. Thus, the lowest sintering temperature which is able to provide sufficiently dense electrolyte is required. In the present work, adding 2 mol% of Bi2O3 in the SCGZ helps reduce sintering temperature to 1350 °C with sufficiently high relative density (>96.5%). The ionic conductivity of the electrolyte is also improved with adding Bi2O3.

HfO2-based electrolyte potentiometric oxygen sensors for liquid sodium

The use of potentiometric sensors for the on-line monitoring of oxygen in molten sodium in fast breeder reactors has been studied since the 1970s. However, their lifetime and the reproducibility of the results are inadequate for commercial development. In this study, the performance of the sensor using yttria-doped hafnia is studied and compared with results obtained with thoria- and zirconia-based electrolytes. The yttria-doped hafnia sensor tested, in the form of a tube, exhibits a very reproducible signal and remarkable stability under very low oxygen levels (ca. 0.02 ppm) and low temperatures (200–300 °C), with a lifetime of ca. two months. Consequently, this electrolyte can be viewed as a promising material for sensing oxygen in molten sodium. The main sources of errors observed in the application of these sensors are reviewed under severe conditions at low temperatures. Discrepancies between the calculated electromotive force (emf) of the cell and the slope of the theoretical Nernst law and the experimental values are analyzed and hypotheses are discussed in order to explain the deviations from the expected values. For the zirconia-based sensor, an original interpretation of the sensor’s response to a change in oxygen concentration, obtained at low temperatures, is proposed, involving peroxide ions.

High-Performance and Durable Reversible Fuel Cells Based on Proton Conductors

A reversible fuel cell (RFC) can operate both in a fuel cell mode (to convert chemical fuels to electricity) and in an electrolysis mode (to produce hydrogen and carbon-containing fuels from electrolysis of water and carbon dioxide, respectively). RFCs have potential to be one of the most efficient and low-cost options for co-located energy storage and power generation. To date, oxide ion conductors (such as zirconia-based electrolytes) have been widely studied for RFCs. In comparison, RFCs based on proton conductors have several advantages, including operation at intermediate temperatures (400–600 °C), simplified system design, reduced cost, and enhanced durability due to the generation ‘dry’ hydrogen in the negative electrode compartment. In this presentation, we will report our recent progress in the development of high-performance and durable RFCs based on proton conductors. When operated in the fuel cell mode, the RFC achieves peak power densities of about 1.0 and 1.5 W cm-2 at 600 and 650 °C, respectively. When operated in both the fuel cell mode and the electrolysis mode, the RFC demonstrates a roundtrip efficiency of greater than 70% at 1 A cm-2 and 650 °C while maintain excellent stability (without obvious degradation in performance for continuous operation of more than 400 h).

Annealing of Gadolinium-Doped Ceria (GDC) Films Produced by the Aerosol Deposition Method

Solid oxide fuel cells need a diffusion barrier layer to protect the zirconia-based electrolyte if a cobalt-containing cathode material like lanthanum strontium cobalt ferrite (LSCF) is used. This protective layer must prevent the direct contact and interdiffusion of both components while still retaining the oxygen ion transport. Gadolinium-doped ceria (GDC) meets these requirements. However, for a favorable cell performance, oxide ion conducting films that are thin yet dense are required. Films with a thickness in the sub-micrometer to micrometer range were produced by the dry room temperature spray-coating technique, aerosol deposition. Since commercially available GDC powders are usually optimized for the sintering of screen printed films or pressed bulk samples, their particle morphology is nanocrystalline with a high surface area that is not suitable for aerosol deposition. Therefore, different thermal and mechanical powder pretreatment procedures were investigated and linked to the morphology and integrity of the sprayed films. Only if a suitable pretreatment was conducted, dense and well-adhering GDC films were deposited. Otherwise, low-strength films were formed. The ionic conductivity of the resulting dense films was characterized by impedance spectroscopy between 300 °C and 1000 °C upon heating and cooling. A mild annealing occurred up to 900 °C during first heating that slightly increased the electric conductivity of GDC films formed by aerosol deposition.

Development of High Performance Metal-Supported Solid Oxide Electrolysis Cells

Hydrogen as an energy carrier has the highest energy/weight ratio, and therefore it has been advocated as the desired fuel for automotive power, auxiliary power, stationary power generation, and also as an energy storage medium. Among various hydrogen generating methods, high temperature electrolysis (HTE) of water using solid oxide electrolysis cells (SOECs) has attracted substantial attention over the last decade due to its high efficiency of conversion and low energy consumption. To date, extensive effort has been devoted to the development of SOECs and promising results have been shown.1,2 However, while most of SOECs are based on an anode-supported cell (ASC) structure, only limited research on metal-supported SOECs is reported.3,4 Compared with conventional ASC type SOECs, metal-supported solid oxide electrolysis cells (MS-SOECs) have unique advantages due to low cost of fabrication, high mechanical strength, good redox stability, and excellent thermal cycling resistance.5,6 Because of these characteristics, MS-SOECs are considered as a promising solution for intermittent and mobile electrolysis applications. In this work, symmetric-structure MS-SOECs with stabilized zirconia electrolyte are developed at LBNL. Nanostructured cathode and anode are fabricated by infiltrating catalysts on both steam and oxygen electrodes. The cells are tested in the temperature range of 650 to 800 ◦C, with steam content between 3-75 vol%. Our MS-SOECs show the highest performance for the cells with zirconia-based electrolyte (Figure 1), to the best of our knowledge. At 1.3 V (thermoneutral voltage) and 50 vol% steam content, the measured current densities were 4.8 A/cm2 at 800 ◦C, 3.1 A/cm2 at 750 ◦C, 2.0 A/cm2 at 700 ◦C, and 1.2 A/cm2 at 650 ◦C. This performance was achieved by optimizing the catalyst chemistry, catalyst arrangement, and infiltration process. In order to study the effects of the cell fabrication process, the electrochemical performance at various steam contents and overpotentials is analyzed, and the corresponding microstructures are examined. The high cell performance shows a great potential of MS-SOEC technology, and the insight from these experiments will provide guidance for further research and development. 1S.D. Ebbesen, S. H. Jensen, A. Hauch, and M. B. Mogensen, Chem. Rev., 114(21), 10697-10734 (2014). 2M. A. Laguna-Bercero, J. Power Sources, 203, 4-16 (2012). 3T. Chen, Y. Zhou, M. Liu, C. Yuan, X. Ye, Z. Zhan, and S. Wang, Electrochem. Commun., 54, 23-27 (2015). 4G. Schiller, A. Ansar, M. Lang, and O. Patz, J. Appl. Electrochem., 39(2), 293-301 (2009). 5M. C. Tucker, J. Power Sources, 195(15), 4570-4582 (2010). 6M. C. Tucker, Energy Technol., 5(12), 2175-2181 (2017). Figure 1. Performance of a metal-supported solid oxide electrolysis cell (a) at 700 ◦C with 3%, 25%, 50% and 75% steam contents in the cathode, and (b) with 50% steam content in the cathode at 650, 700, 750 and 800 ◦C. Figure 1

Lanthanum Nickelate Cathode Materials for Low and Intermediate Temperature for Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) are a clean electrochemical conversion devices from chemical energy of fuel to electrical energy. SOFCs are considered as future green energy devices because of their diverse advantages including high energy conversion efficiency, various fuel acceptance, and negligible gas pollutants [1]. Now, the performance of SOFCs is highly affected by the oxygen reduction reaction (ORR) in the cathode of SOFCs. Until now, the major materials of cathode are cobalt-contained perovskite oxides because they show a low over-potential for ORR with the high SOFC performance [2]. However, cobalt-contained perovskite cathode materials also have several technical issues, such as expensive price, low chemical stability, and big difference of thermal expansion coefficient (TEC) to the zirconia-based electrolyte. The TEC of cobalt-contained perovskite oxides is 20-25×10-6K-1, whereas the yttria-stabilized zirconia and lanthanide-doped ceria electrolytes are 11-13×10-6K-1. This can results in the delamination of cell components during high temperature operation [3]. On the other hand, the TEC of lanthanum nickelates (Lan+1NinO3n+1) is 11-12×10-6K-1, which is similar to that of electrolyte materials and thus high thermal stability is expected for the SOFC[4]. Hence. in this work, the characteristics of lanthanum nickelates in terms of performance and durability are investigated.

Stabilized Zirconia-Based Nanostructured Powders for Solid-Oxide Fuel Cells

High-purity uniform powders of zirconia-based solid electrolytes stabilized with yttria (4 and 8 mol %) are synthesized by co-precipitation with subsequent annealing at different temperatures. The obtained powders were studied using X-ray analysis and transmission electron microscopy; the specific surface area was measured by nitrogen adsorption. The stabilized zirconia powder sintering was studied over temperature range from 1000 to 1600°C. The ionic conductivity of samples containing 8 mol % of yttria was 0.06–0.07 S/сm, that is comparable with that obtained with commercial solid electrolytes.

Diffusion-Related SOFC Stack Degradation

Solid Oxide Fuel Cell systems for stationary applications should exhibit low degradation (<< 1%/1,000h voltage loss) to ensure long-term operation (> 40,000h). The rather high operation temperature of SOFCs (≥ 700°C) is on the one hand beneficial for oxygen conduction through the ceramic-based electrolyte, but on the other hand all diffusion-related, energetically-driven mechanisms are also increased. This counts less for gas diffusion processes but is more pronounced for solid-state diffusion (surface, bulk, grain boundary). For gas diffusion processes other parameters like gas stream, individual partial pressures and partial pressure gradients are dominating. Within two different stack tests with four-plane short stacks and their intensive post-test characterization two varying diffusion-related degradation mechanisms have been investigated. One is a short-term (~1250h) test with two different chromium evaporation protection layers on the air side metallic interconnect and frame and the other one is a long-term endurance test (~ 35,000h). For the first stack two planes were coated with a manganese oxide layer applied by wet powder spraying (WPS) while the other two planes were coated by atmospheric plasma spraying (APS) with a manganese-cobalt-iron-spinel layer. Thus the protection layers do not only differ by coating technology but also by material. The voltage loss in the planes with WPS-coated interconnect was markedly higher than in those coated by APS. Finally it turned out that the layers microstructure plays the key role in Cr evaporation minimization. In this stack, gas-phase diffusion is dominating degradation. In the long-term operated stack, the influence of a diffusing element from the cathodic contact layer towards and through the electrolyte has been characterized. Manganese, presumably originating from the contact layer (but it’s additionally also part of the protection layer), diffuses via solid-state diffusion and reacts while reaching reducing conditions in the fuel compartment with the zirconia-based electrolyte. This interaction forms a secondary sponge-like porous phase at the boundary electrolyte-anode. This secondary phase causes stresses and leads, after growing to big islands of some ten micrometers in diameter, to electrolyte-anode delamination and finally to complete cell failure. For the Mn-related degradation solid state diffusion has been supposed, as the electrolyte is coated by a dense thin-film diffusion barrier layer which suppresses gas diffusion. Both results indicate that volatile elements (which could either evaporate via gas phase or diffuse via solid state diffusion) should be avoided as they can lead to severe cell, stack and system failure even after thousands of hours of high-temperature operation.

Quantitative Study of LSCF and LSM-YSZ Cathode Microstructure by FIB/SEM Tomography

The mixed ionic-electronic conducting cathode (La, Sr)(Co, Fe)O3-δ (LSCF) is utilized worldwide for solid oxide fuel cells (SOFCs) operated at intermediate temperatures [1] because of its excellent oxygen reduction reaction rate. The challenge of using LSCF as cathode material is the complex nature of the interface to Zirconia-based electrolytes, which may counteract this advantage [2, 3]. The composite cathode (La, Sr)MnO3 (LSM) – Y2O3 doped ZrO2(YSZ) is an adequate alternative for higher temperatures only. LSM-YSZ offers fewer regions of oxygen reduction (triple phase boundaries) due to spatially separated ionically (YSZ) and electronically (LSM) conductive phases. If electrochemical and microstructural parameters of both types of electrodes are quantified by appropriate techniques, the performance can be modeled and compared for different operating conditions. Advanced imaging techniques such as focused ion beam/scanning electron microscopy (FIB/SEM) or X-ray tomography are promising techniques for microstructure quantification enabling 3D reconstructions of µm- and sub-µm-scaled multiphase electrodes [4, 5, 6]. Reconstructing the composite cathode, in particular, comprises the challenge of overcoming the weak material contrast between LSM and YSZ using scanning electron microscopy. This contribution will show how FIB/SEM tomography can help to identify different microstructural parameters such as porosity, particle size, surface area and tortuosity. We will compare LSCF and LSM-YSZ cathodes and enlighten new challenges in image reconstruction, parameter acquisition and parameter interpretation for the different material sets. [1] H. Yokokawa, et al., J. Power Sources, 182, p. 400 (2008) [2] J. Szasz, et al., ECS Trans., 66(2), p. 79 (2015) [3] Yokokawa, et al., Solid State Ionics, 262, p. 454 (2014) [4] J.R. Wilson, et al., Nature Materials 5(17), p. 541 (2006) [5] J. Joos, et al., Electrochim. Acta 82, p. 268 (2012) [6] Y. C. K. Chen-Wiegart, et al., J. Power Sources, 218, p. 348 (2012) Figure 1