Electrolytes For Solid Oxide Fuel Cells Research Articles

Chromium-Related Degradation of Thin-Film Electrolyte Solid Oxide Fuel Cell Stacks

SOFC short stacks with thin-film electrolyte anode-supported cells were operated for up to ∼3,000 h and post-test characterized. The cells consisted of thin electrolyte films (≤ 3 μm) and LSCF or LSC cathodes. Typical manganese-oxide chromium evaporation protection layers and perovskitic cathode contact layers were used. Operating parameters were: temperatures between 600°C and 750°C, a current density of 500 mA/cm², hydrogen with 20% water vapor as fuel, and a fuel utilization of ∼40%. Microstructural characterization revealed that Cr-containing phases formed at different locations. SEM and TEM analysis of some cells additionally showed microstructural changes within the GDC layer. A possible explanation for the different chromium-containing phase locations based on theoretical assumptions is given.

Microstructure, hardness, and fracture toughness of suspension plasma sprayed yttria-stabilized zirconia electrolytes on stainless steel substrates

Abstract Recent research interest in solid oxide fuel cells (SOFCs) has grown towards effective manufacturing processes that are compatible with low-cost materials such as stainless steel structural supports. Suspension plasma spraying is a promising technique that rapidly produces a fully consolidated, thin electrolyte layer with good microstructure control and without any post-deposition heat treatment. In the present study, suspension plasma spraying was used to deposit yttria-stabilized zirconia electrolyte coatings on porous stainless steel substrates using a range of spray conditions. Mechanical properties of the coatings, such as hardness and fracture toughness, were characterized. These properties can affect the durability of SOFC electrolytes, coupled with the thermal stresses generated during operation. Coating toughness and hardness were observed to be dependent on the torch power and stand-off distance during fabrication, which affect coating microstructural features such as porosity, microcracks, and segmentation cracks. The fracture toughness measured using an indentation technique of the coatings produced with a torch power of 133 kW and a stand-off distance of 90 mm was found to be 1.5 ± 0.15 MPa m1/2.

Enhanced electrochemical properties of Sm0.2Ce0.8O1.9 film for SOFC electrolyte fabricated by pulsed laser deposition

The cubic fluorite structure Sm0.2Ce0.8O1.9 (SDC) film was fabricated by pulsed laser deposition (PLD) to be used as electrolyte for solid oxide fuel cells (SOFCs). The SDC target was prepared by co-precipitation process at a low sintering temperature (1,250 °C) with a relative density of 97 %. By inserting the PLD seed layer with the thickness of 3 μm, robust electrolyte films were successfully fabricated on La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) substrate. The SDC film growing at a substrate temperature of 650 °C with thermal treatment and oxygen gas pressure of 10 Pa has high ionic conductivity of 0.075 S·cm−1 at 800 °C. The results demonstrate that the dense SDC thin films prepared by PLD are suitable for low-temperature SOFC applications.

Failure Mode and Effect Analysis of Composite Cathode on Anode-Supported SOFCs under the Accelerated Durability Testing

Compared to other-type of fuel cells, solid oxide fuel cells (SOFCs) have many advantages, such as higher efficiency and performance, environmental friendless, fuel flexibility and no use of precious metal catalysts [1]. For making the commercialization of SOFCs, however there is still forced to improve durability of components with high performance. In particular, researches for durability are only in the early stages, compared to the effort of developing materials to reduce operating temperature with enhanced performance.Recently, studies of SOFC durability have extended to understand main degradation mechanisms of components. The degradation phenomena of Ni cermets anode is often observed in low concentrations of hydrogen at high current density. Chen et al.[2] reported that the poor tolerance of Ni-ceria cermets may result in delamination of each layer of SOFC components by variation of anode volume. Other researchers examined that chemical reactions of LSM with YSZ during hot-temperature SOFC operations form insulating phases like SrZrO3 or La2ZrO7 [3, 4]. This phenomena lead to decrease SOFC performance with high polarization loss. The humidified air also occurs degradation of cathode materials because of variable hydration properties or formation of secondary phases [5, 6].In this study, degradation mechanisms of the neodymium doped ceria (NDC) based anode-supported SOFCs with cathodes related to perovskite structure are investigated by accelerated degradation testing methods. As the preparation of experiments, NiO-NDC cermets are used for anode substrates and NDC is synthesized by combustion process [7]. The NDC slurries are deposited on the substrate as an electrolyte. These green pellets are sintered at 1550 °C for 4 h with a 2 °C min-1 ramp rate in air. To compare influences of durability on the cathode materials, Ba0.5Sr0.5Co0.8Fe0.2O3- δ (BSCF) and Nd(Ba0.5Sr0.5)Co1.5Fe0.5O5+ δ (NBSCF) are prepared with NDC. The mixtures are screen-printed on anode-supported cells and sintered at 1150 °C for 2 h. A sintered cell is loaded on a reactor and sealed by glass rings.Durability of anode-supported SOFCs is investigated under various failure modes such as hydrogen and air depletion and highly humidified air. The experiments are carried out periodical turn- on/off of hydrogen (and air) in anode (and cathode) side under constant current density of 180 mA cm-2 at 650 °C. To find main degradation mechanism after failure tests, the impedance spectra and overpotential analysis are performed and the microstructure analysis is also carried out by SEM, TEM and XRD [8, 9]. Those experiments contribute to find design parameters for highly durable SOFC components.Lowering the Temperature of Solid Oxide Fuel Cells, E. D. Wachsman, and K. T. Lee, Science , 334, 935 (2011).Rapid degradation mechanism of Ni-CGO anode in low concentrations of H2 at a high current density, G. Chen, G. Guan, S. Abliz, Y. Kasai, A. Abudula, Int. J. Hydrogen energy,36, 8461 (2011).Microstructural studies on degradation of interface between LSM–YSZ cathode and YSZ electrolyte in SOFCs, Y. L. Liu, A. Hagen, R. Barfod, M. Chen, H. J. Wang, F. W. Poulsen, P.V. Hendriksen, Solid state ionics, 180, 1298 (2009).Effect of cathode gas humidification on performance and durability of solid oxide fuel cells, J. Nielsen, A. Hagen, Y. L. Liu, Solid state ionics, 181, 517 (2010).Fundamental mechanisms limiting solid oxide fuel cell durability, H. Yokokawa, H. Tu, B. Iwanschitz, A. Mai, J. Power sources, 182, 400 (2008).Hydration properties and rate determining steps of the oxygen reduction reaction of perovskite-related oxides as H+-SOFC cathodes, A. Grimauda, F. Mauvya, J. M. Bassata, S. Fourcadea, L. Rocherona, M. Marronyb, J. C. Grenier, J. the electrochemical society, 159, B683 (2012).Various synthesis methods of aliovalent-doped ceria and their electrical properties for intermediate temperature solid oxide electrolytes, G. Kim, N. Lee, K. -B. Kim, B. -K. Kim, H. Chang, S. J. Song, J. -Y. Park, Int. J. Hydrogen energy, 38, 1571 (2013).Polarization measurement of anode-supported solid oxide fuel cells studied by incorporation of a reference electrode, H. Xiao, T. L. Reitz, M. A. Rottmayer, J. Power sources, 183, 49 (2008).Reliability and accuracy of measured overpotential in a three-electrode fuel cell system, S. H. Chan, X. J. Chen, K. A. Khor, J. Applied electrochemistry, 31, 1163 (2001). * Corresponding authors: jyoung@sejong.ac.kr (J.-Y. Park).

NiO-SDC Mixed Ionic Electronic Conducting Anode for Low Temperature Thin Film SOFCs

Solid oxide fuel cells (SOFCs) are promising candidates for next generation energy conversion devices due to its fuel flexibility, high efficiency, high energy density, and no CO poisoning. However, high operation temperature which is required due to poor ionic conductivity of oxide electrolyte of typical SOFCs disturbs the commercialization.Various thin film techniques for fabrication of electrodes and electrolyte are applied to lower the operation temperature of SOFCs. Pulsed laser deposition (PLD), sputter, chemical vapor deposition (CVD), and atomic layer deposition (ALD) are used to prepare thin film electrodes or electrolyte.Typical low temperature SOFCs use platinum (Pt) as an electrode material due to high electrical conductivity and superior catalytic activity of Pt for both hydrogen oxidation reactions and oxygen reduction reactions. Despite of these strong points, high cost, thermal degradation due to agglomeration, mismatch of thermal expansion coefficient and limited triple phase boundaries (TPB) are still remained. Mixed ionic electronic conducting (MIEC) anode materials such as NiO-YSZ, NiO-GDC, and NiO-SDC are usually used at typcial SOFCs due to high density of triple phase boundaries. These MIEC materials can relieve the thermal degradation and thermal stress. Cost is also reduced compared with Pt. Therefore, we applied the MIEC anode for low temperature thin film SOFCs.In this study, we prepared anodic alumina oxide (AAO) based low temperature thin film SOFCs. NiO-SDC anode was fabricated via radio frequency sputtering method using NiO-SDC oxide target. The reactive RF sputtering process is conducted to prepare yttria-stabilized zirconia (YSZ) electrolyte on a NiO-SDC anode by using Y-Zr alloy target and Ar-O2 mixing gas. After electrolyte deposition, porous Pt cathode was deposited on the electrolyte by direct current sputtering method. Open circuit voltage (OCV), performance, and electrochemical impedance spectroscopy (EIS) were measured at 500C.

Molecular Dynamics Simulation on Oxide Ion Conduction of La-Based Perovskite Oxides for SOFCs Electrolyte

La-based perovskite oxides have various physical and electrical properties by substitution of various elements at A and/or B-sites. Especially, doping of low-valent cation generates oxygen vacancies to maintain charge neutrality, leading to high oxide ion conductivity. Also, B-site is comprised of transition metals such as Co, Mn or Fe, the La-based perovskite oxides become a mixed ionic-electronic conducting material. So, these La-based perovskite oxides can be applied to electrode, electrolyte or interconnect materials of solid oxide fuel cells (SOFCs). Recently, the SOFCs operating temperature is getting lower, much attention has been paid to the La-based perovskite oxides to alternate a conventional electrolyte material, yttria-stabilized zirconia (YSZ). To obtain an optimal La-based perovskite oxide with high oxide ion conductivity, it is very important to understand a correlation between experimental variables such as dopant type or concentration and various factors affecting the electrical conductivity. Therefore, we microscopically analyzed the effects of A-site dopant types and B-site cation types in LaInO3 perovskite oxide with orthorhombic structure on the oxide ion conductivity using molecular dynamics simulation. La0.9A0.1InO2.95 (A=Ca, Sr and Ba) and La0.9Sr0.1MO2.95 (In, Ga and Sc) compositions were modeled. The pair potential used in this study has two parts. One part represents long-range interaction described by Coulombic force. The other part represents short-range interaction by Born-Mayer-Huggins pair potential. The simulation box consisted of a total of 3960 atoms with periodic boundary conditions. Main calculations were carried out for 1.6ns with time step of 1fs after relaxation under NPT ensemble. From the radial distribution function results, it was found that the interionic distance between oxide ion and A-site dopant is important factor determining the ionic conductivity.

On the conductivity degradation and phase stability of solid oxide fuel cell (SOFC) zirconia electrolytes analysed via XRD

Abstract The degradation of ionic conductivity in 6ScSZ and 3YSZ SOFC electrolytes was analysed via X-ray diffraction (XRD), 4-point conductivity and electrochemical impedance spectroscopy (EIS) and the results present a consistent story: in 6ScSZ subtle but reproducible changes to the phase assembly in the form of a shrinking “cubic shoulder” are obtained with ageing at 900 °C, which closely corresponds to the decrease in ionic conductivity. This appears to be the gradual transformation of the residual high-temperature c or t″ phase to t′. Both the shoulders and the conductivity return after “healing” at temperatures above the ageing temperature but below the sintering temperature, bringing the samples back to the as-sintered state. Evidence of a type of short-range ordering (SRO) acting in parallel with the phase changes was seen in the EIS. Distortions on the XRD peaks such as humps on the left shoulder were almost never seen and appear to be a polishing artefact unrelated to changes in the conductivity.

Development of improved solid oxide fuel cell electrolyte sheet by microimprinting for layered material

At present, the solid oxide fuel cell (SOFC) is attracting much attention because it possesses the highest power generation efficiency among many types of fuel cell, and SOFC emits only water that does not harm the environment. Recently, the electrolyte structure for SOFC has been processed into various forms to increase the efficiency of SOFC. In this work, we tried to improve the performance of SOFC by changing the mesostructure on the interfaces between the electrolyte and electrodes. This hundred-micrometer-scaled mesostructure has been proposed to increase the reaction on the interfaces. However, there has been no effective method of fabricating the cells with the microstructure along the interfaces. We have already proposed the micro powder imprint (µPI) method to create fine patterns on ceramic samples. In this study, the µPI method for layered material was newly developed to fabricate the micropatterns on both sides of an electrolyte sheet. The optimization of process parameters, such as compounding ratio, debinding temperature, and sintering temperature, was examined to obtain desired wavy patterns on both surfaces of the electrolyte sheet.

Effect of internal and external constraints on sintering behavior of thin film electrolytes for solid oxide fuel cells (SOFCs)

The effect of internal and external constraints on the sintering behavior of yttria-stabilized zirconia (YSZ) thin film electrolytes is investigated for intermediate-temperature solid oxide fuel cells (SOFCs). The external constraints imposed by rigid substrates result in major process flaws, such as vertical and delamination cracks during film densification, in a chemical solution deposition (CSD) process. In this study, YSZ nanoparticles are incorporated into the precursor solution to impose internal constraints and control the shrinkage rate of the sol–gel matrix. Because the structural stability of the thin film is determined by the grain size distribution, it is critical to optimize the content and dispersion structure of the nanoparticles and the sintering temperature to obtain a gas-tight thin film electrolyte for SOFCs. A clustering model is proposed to obtain a comprehensive understanding of the sintering behavior of the YSZ thin-film electrolyte containing the nanoparticles in the CSD process.

Hydrothermal Synthesis, Structure Investigation, and Oxide Ion Conductivity of Mixed Si/Ge-Based Apatite-Type Phases

Apatite-type oxides ([A(I)4][A(II)6][(BO4)6]O2), particularly those of the rare-earth silicate and germanate systems, are among the more promising materials being considered as alternative solid oxide fuel cell electrolytes. Nonstoichiometric lanthanum silicate and germanate apatites display pure ionic conductivities exceeding those of yttria-stabilized zirconia at moderate temperatures (500-700 °C). In this study, mixed Si/Ge-based apatites were prepared by hydrothermal synthesis under mild conditions rather than the conventional solid-state method at high temperatures. Single-phase and highly crystalline nanosized apatite powders were obtained with the morphology changing across the series from spheres for the Si-based end member to hexagonal rods for the Ge-based end member. Powder X-ray and neutron analysis found all of these apatites to be hexagonal (P63/m). Quantitative X-ray microanalysis established the partial (<15 at%) substitution of La(3+) by Na(+) (introduced from the NaOH hydrothermal reagent), which showed a slight preference to enter the A(I) 4f framework position over the A(II) 6h tunnel site. Moreover, retention of hydroxide (OH(-)) was confirmed by IR spectroscopy and thermogravimetric analysis, and these apatites are best described as oxyhydroxyapatites. To prepare dense pellets for conductivity measurements, both conventional heat treatment and spark plasma sintering methods were compared, with the peculiar features of hydrothermally synthesized apatites and the influence of sodium on the ionic conductivity considered.

Mechanism and Kinetics of Oxygen Reduction in Porous Nd2NiO4+ δ Electrodes

Solid Oxide Fuel Cells (SOFC) cathodes must exhibit high catalytic activity for oxygen reduction, as well as good thermal and chemical compatibility with solid electrolytes in order to minimize resistance and promote stabile operation [1]. The Ln2NiO4+δ (Ln = La, Nd, Pr, for example) nickelates, belonging to the Ruddlesden-Popper series, have been proposed as cathode materials. These oxides have the aptitude to accommodate oxygen ions into interstitial sites in the LnOx layer, which gives rise to a high ionic conductivity by interstitial oxygen diffusion [2]. In addition, the Ln2NiO4+δ materials present Thermal Expansion Coefficients (TEC) similar to the most widely used SOFC or IT-SOFC electrolytes: 8mol% Y2O3 stabilized ZrO2 (YSZ), Ce0.9Gd0.1O1.96 (CGO) and La0.9Sr0.1Ga0.8Mg0.2O2.85 (LSGM) [3-4].In this work, we present a study of the reaction mechanism and the kinetic parameters of porous Nd2NiO4+δelectrodes. Three different microstructures have been obtained using different preparation methods. The conventional Solid State Reaction (SSR) and Pechini methods, and a soft chemical route based on a sol-gel method. Details of these syntheses are described in [5]. These materials have been deposited by spin coating on dense LSGM electrolytes and were fired at 1000°C. The resulting microstructures were quantified by 3D focused ion beam-scanning electron microscope (FIB-SEM) tomography. Figure 1 shows example 2D images from the 3D FIB-SEM data sets showing markedly different microstructures.Electrochemical Impedance Spectroscopy (EIS) was used to measure the cells for temperatures ranging from 600 to 700°C and oxygen partial pressures (pO2) between 1 atm and 6∙10-4 atm in order to identify possible rate-limiting steps of the oxygen reduction reaction. The EIS spectra, which showed a low and a high frequency response, were fit using an electrical equivalent circuit model composed of two elements – the high frequency contribution was fit using a Gerischer-type element, whereas a parallel resistor/capacitor (RC) element was used for the low frequency arc. The low frequency contribution was only detected at low pO2 and temperatures, with its size increasing with increasing particle size. The Gerischer element suggests a co-limitation by surface exchange kinetics and oxygen diffusion. The variation of the resistance (RG) and relaxation time (τG) of the Gerischer-type contribution are plotted in Figure 2.The 3D FIB-SEM data (porosity, specific surface area, tortuosity) was used in the ALS model [6] to find the characteristic kinetic parameters, oxygen surface exchange (k) and oxygen chemical diffusion coefficient (Dchem) that correctly predict the EIS-measured RG and τG values. The results will be compared with reported values for Nd2NiO4+δ. Note that the main change in microstructure, the changes in particle size and specific surface area (Fig. 1), agree with the measured RG values according to the ALS model. That is, high surface area (smaller particle size) led to lower RG. The microstructure plays an important role in determining the mechanism of the reaction. Thus, the lower pO2 dependence for RG and τG and the high polarization resistance suggest that the kinetics of the oxygen reduction reaction is mainly limited by a bulk transport path for the SSR sample.

Electrolytic Conduction in Transition-Metal-Free Oxides with K2NiF4 Structure

La2NiO4 and related materials with the K2NiF4 structure are known to support high electronic conductivity (about 100 S•cm-1) and oxygen ionic conductivity (comparable to or exceeding that of yttria-stabilized zirconia).1, 2 Unlike other well known oxygen ion conductors, these materials conduct via an interstitial rather than a vacancy mechanism. While the K2NiF4 crystal structure is robust across a fairly wide solid solution compositional space, nearly all known compositions exhibit mixed ionic electronic conduction due to one or more transition-metals on the B-site. One exception, LaSrAlO4,3 was previously found to be unstable to the introduction of ionic defects. For these reasons, despite high oxygen ion conductivity, oxides with the K2NiF4 structure have been limited in application as electrode and not electrolyte materials in solid oxide fuel cells (SOFC). In this work, we aimed to develop a new type of SOFC electrolyte material from transition-metal-free oxides with the K2NiF4 structure. The first successful composition, La1.6Sr0.4Al0.4Mg0.6O4,4 was created by solid-state reaction. Oxygen ion defects were able to be created in the lattice by adjusting the La/Sr ratio. We hypothesize that defects are stabilized in this composition but not LaSrAlO4 due to higher charge separation between the rocksalt and perovskite layers in the crystal lattice. Similar compositions substituting Al, Mg or Sr with Ga, Zn or Ca, respectively, were also synthesized and were confirmed to maintain this crystal structure. X-ray diffraction revealed that the crystal structure was consistently more stable to higher concentrations of vacancies relative to interstitial defects. Four-electrode conductivity measurements indicated that the B-site composition had a larger effect on total electrical conductivity than the A-site composition. Specifically, replacing Al or Mg with Ga or Zn, respectively, improved the conductivity by 1 to 2.5 orders of magnitude. Unfortunately, further electrochemical studies suggested significant hole conduction in these samples. Substituting Sr with Ca in the baseline composition had no effect on the total conductivity value, and La1.6+xCa0.4-xAl0.4Mg0.6O4 series samples were all found to be pure ionic conductors. Nevertheless, the oxygen ion conductivity values achieved in the compositions to date remain less than technologically desirable. Results from experiments on newer compositions will show that Li can be placed on the B-site. Such compositions may have utility in both Li-ion and Li-air batteries.

Tunability and Electrical Properties of Yttria-doped-ceria Films Fabricated via Atomic Layer Deposition

Current Solid Oxide Fuel Cell (SOFC) designs have operating temperatures between 800-1000 °C. Reducing this temperature to 500-600 °C would allow efficient utilization in varied applications. Yttria Doped Ceria (YDC) films show high ionic conductivity when used as fuel cell electrolytes in lower temperature ranges. This investigation shows that stoichiometric tunability of YDC film composition can be achieved through Atomic Layer Deposition (ALD). Film resistivity at room temperature is measured using a custom four-point probe sensor Film resistance analysis provides a method for evaluating the effect of doping and thickness of the film on desirable SOFC electrolyte characteristics such as high ionic conductivity.

Investigations on structural and electrical properties of calcium substituted LSGM electrolyte materials for IT-SOFC

Strontium and magnesium doped lanthanum gallate, La0.9Sr0.1Ga0.8Mg0.2O2.85 (LSGM) and a few compositions of calcia (CaO) incorporated LSGM, La(0.9−x)CaxSr0.1Ga0.8Mg0.2O2.85; x≤0.10, were prepared by conventional solid state ceramic route. The X-ray diffraction patterns showed orthorhombic LSGM phase along with a few impure secondary phases. SEM micrograph depicted fairly dense grain morphology of the system. The textural properties were also found to be quite suitable for solid oxide fuel cell electrolyte applications. The electrical conductivity of the systems was measured in the frequency range 20Hz to 1MHz and temperature range 300–800°C. The total conductivity of the system was found to first increase up to x=0.05 and thereafter it decreased for higher concentrations of the dopant. A small amount of calcia (upto x=0.05) scavenges the intergranular secondary impure phases resulting in a significant increase in grainboundary conduction. The presence of extra secondary phases in the higher compositions has been attributed to decrease in electrical conductivity.

BaZr0.2Ce0.8−xYxO3−δ solid oxide fuel cell electrolyte synthesized by sol–gel combined with composition-exchange method

This study reports the synthesis of proton-conducting BaZr0.2Ce0.8−xYxO3−δ (x = 0–0.4) oxides by using a combination of citrate-EDTA complexing sol–gel process and composition-exchange method. Compared to those oxides prepared from conventional sol–gel powders, the sintered BaZr0.2Ce0.8−xYxO3−δ pellets synthesized by sol–gel combined with composition-exchange method are found to exhibit improved sinterability, a higher relative density, higher conduction, and excellent thermodynamic stability against CO2. Moreover, the Pt/electrolyte/Pt single cell using such a BaZr0.2Ce0.6Y0.2O3−δ electrolyte shows an obviously higher maximum powder density in the hydrogen-air fuel cell experiments. Based on the experimental results, we discuss the improvement mechanism in terms of calcined particle characteristics. This work demonstrates that the BaZr0.2Ce0.8−xYxO3−δ oxides synthesized by sol–gel combined with composition-exchange method would be a promising electrolyte for the use in H+-SOFC applications. More importantly, this new fabrication approach could be applied to other similar ABO3-perovskite material systems.

Reduction in ohmic contact resistance at interface between Gd-doped CeO2 interlayer and Sc2O3-stabilized ZrO2 electrolyte in SOFCs to improve performance

Abstract Influence of the ohmic contact resistance at the interface between Gd-doped CeO2 (GDC) interlayer and scandia-stabilized zirconia (ScSZ) electrolyte on the performance of solid oxide fuel cells (SOFCs) was investigated. When GDC interlayer was used, the ohmic resistance of SOFCs was increased due to insufficient contact conditions between the GDC interlayer and ScSZ electrolyte. To improve the contact conditions, the effect of sintering temperature and sintering aids (Mn, Fe, Bi) on the ohmic contact resistance was evaluated. As a result, using an optimized Mn-modified GDC interlayer (with added 0.3 mol% Mn and sintered at 1300 °C) to decrease the ohmic contact resistance, an anode-supported cell with Sr-doped LaFeO3 cathode and ScSZ electrolyte achieved a power density of 1.5 W cm− 2 at 800 °C.

Doped Germanate‐Based Apatites as Electrolyte for Use in Solid Oxide Fuel Cells

AbstractApatite ceramics, known for their good electrical conductivities, have garnered substantial attention as an alternative electrolyte for solid oxide fuel cells (SOFCs). However, studies focusing on the electrochemical performances of SOFCs with apatities as electrolytes remain rare, partly due to their high sintering temperature. In this study, the effects of Mg2+, Al3+, Ga3+, and Sn4+ dopants on the characteristics of La9.5Ge6O26 ± δ are examined and their potential for use as SOFC electrolytes evaluated. The results indicate that La9.5Ge5.5Al0.5O26 is stabilized into a hexagonal structure, while the La9.5Ge5.5Sn0.5O26.25, La9.5Ge5.5Ga0.5O26, and La9.5Ge5.5Mg0.5O25.75 ceramics reveal triclinic cells accompanied with the second phase La2Sn2O7 or La2GeO5. The study further demonstrates that a high sintering temperature is needed for both the La9.5Ge5.5Mg0.5O25.75 and the La9.5Ge5.5Sn0.5O26.25 ceramics, and the worst electrical conductivity among the examined systems appears in the La9.5Ge5.5Ga0.5O26 ceramic. The La9.5Ge5.5Al0.5O26 ceramic is accordingly selected for cell evaluation due to its ability to reach densification at 1,350 °C, its good electrical conductivity of 0.026 S cm–1 at 800 °C, and its acceptable thermal expansion coefficient of 10.1 × 10–6 K–1. The maximum power densities of the NiO‐SDC/La9.5Ge5.5Al0.5O26/LSCF‐SDC single cell are found to be respectively 0.22, 0.16, 0.11, and 0.07 W cm–2 at 950, 900, 850, and 800 °C.

Plasma-Sprayed Y2O3-Stabilized ZrO2 Electrolyte With Improved Interlamellar Bonding for Direct Application to Solid Oxide Fuel Cells

Atmospheric plasma spraying was employed to prepare anode, cathode, and Y2O3-stabilized ZrO2 (YSZ) electrolyte to aim at reducing manufacturing cost. YSZ electrolytes were deposited on the anode at different deposition temperatures of 200 °C, 400 °C and 600 °C to optimize the gas tightness of plasma-sprayed YSZ electrolyte. The influences of the deposition temperature on the microstructure and gas-tightness of plasma-sprayed YSZ electrolyte were investigated. The effect of microstructure and the gas-tightness of YSZ electrolyte on the open circuit voltage and the output performance of solid oxide fuel cells (SOFCs) were examined. The results showed with the increase of deposition temperature, the porosity of YSZ electrolytes almost decreased by about 80% and the microstructure of YSZ electrolytes changed from the typical lamellar structure to the continuous columnar crystal structure. At a deposition temperature of 600 °C the gas permeability decreased to 1.5 × 10−7 cm4gf−1s−1, and the highest open circuit voltage can reach 1.026 V, indicating the applicability of the as-sprayed YSZ directly to the SOFC electrolyte.

Prospects of oxide ionic conductivity bismuth vanadate-based solid electrolytes

AbstractThe benefits of lowering the operation temperature of solid oxide fuel cells (SOFCs) have attracted great attention worldwide. There has been an enormous effort in the literature for the improvement of the electrolyte materials working at lower temperatures. The family of BiMeVOx (Bi–bismuth, Me–dopant metal, V–vanadium, Ox–oxide) materials has been found to have specific properties as oxide ion conductor (electrolyte) for low operation temperature. These materials exhibit uniquely structural features rather different from those of other solid electrolytes. It is possible to use these as the electrolyte in SOFCs. This article attempts to review the main structural and electrochemical characteristics of BiMeVOx materials in order to show a guideline for designing novel BiMeVOx-based electrolyte for low-temperature SOFCs. The prospects and challenges for the application of these materials as the electrolyte are discussed.

Evolution of electrochemical interfaces in solid oxide fuel cells (SOFC): a Ni and Zr resonant anomalous ultra-small-angle X-ray scattering study with elemental and spatial resolution across the cell assembly

Electrochemical interfaces are key to the direct conversion of fuels to electrical energy and lend energy converters like solid oxide fuel cells (SOFC) their functionality. Over extended operation at high temperatures, the microstructure of the underlying component materials in the cathodes, anodes and electrolytes evolve to an extent that these interfaces become affected and ultimately impaired, giving rise to performance degradation. We present anomalous ultra-small-angle X-ray scattering (anomalous USAXS) measurements to quantify the component phase interfacial surface areas as a function of position within the electrodes and electrolyte of a SOFC assembly. Using USAXS at a 3rd generation X-ray synchrotron facility, the primary microstructural parameters obtained are the mean feature size, size distribution and surface area, determined over a contiguous length scale from nanometers to micrometers in a single measurement at a given position. Here, a spatial resolution of <20 μm has been achieved perpendicular to the SOFC electrode and electrolyte layers. Anomalous USAXS measurements at X-ray energies just below the Ni and Zr K-absorption edges have enabled the electrochemically-active solid components and their associated void morphologies to be distinguished from each other close to the electrode/electrolyte interfaces. The anomalous variation of the X-ray scattering contrast with X-ray energy has been exploited to distinguish Ni-rich or Zr-rich component microstructures from adjacent phases, and to determine the interfacial surface areas both between specific solid phases and between each phase and the void network. Such information provides improved insights for relating the morphology of the SOFC triple phase boundary (TPB, where the reactant gas, electron-conducting and ion-conducting phases coincide) to the various component interfaces in the adjacent microstructure. We demonstrate how such measurements determine the electrochemically-active interface response to SOFC service life, especially in the anode with and without sulfur present in the fuel. Our approach can be generalized to address degradation issues at a quantitative level in other electrochemical systems such as batteries and photo-electrochemical cells.