Investigation of LSM-8YSZ cathode within an all ceramic SOFC. Part I: Chemical interactions

This work focuses on a novel, co-sintered, all-ceramic solid oxide fuel cell (SOFC) concept. The objective is the understanding of interaction and degradation mechanisms of the cathode and current collector layers within the design during co-sintering. Half cells consisting of silicate mechanical support, lanthanum strontium manganite (LSM) current collector, LSM mixed with 8 mol% yttria-stabilized zirconia (8YSZ) composite cathode and 8YSZ electrolyte were co-sintered at 1150 °C < T < 1250 °C. Crystallographically stable LSM compositions within the design were identified. However, the cathode and silicate/electrolyte interacted by interdiffusion of Zn (gas diffusion) and Mn (solid diffusion), and by the formation of several reaction phases (between silicate and cathode only). Introducing silicate poisoning decreased the electrochemical performance of the cell by around 40%. This is likely due to the formation of the Zn- and Mn-rich phase in the cathode, but may also be caused by a higher ohmic resistance of the current collector.     

Impedance studies of cathode/electrolyte behaviour in SOFC

This paper reports impedance studies of the cathode/electrolyte behaviour in solid oxide fuel cells (SOFC), based on comparative investigation of half-cells with yttria stabilized zirconia (YSZ) electrolyte and different cathode materials: lanthanum strontium manganite (LSM), and composite LSM/YSZ with low ionic conductivity as well as the electron conducting Ag, Pt and Au. For improved impedance data analysis the technique of the differential impedance analysis is applied. It ensures structural and parametric identification without preliminary assumptions about the working model. It is found that despite the low ionic conductivity of LSM, the cathode reaction of the oxide cathode materials is a two-step process including: (i) charge transfer with activation energy of the resistivity E_a increasing with the temperature and (ii) transport of oxygen ions through the bulk of the electrode (rate-limiting stage) with E_a independent on the temperature. For the metal (electron conducting) electrodes, the reaction behaviour is described with one step process with higher E_a at higher temperatures. The activation energy of the electrolyte conductivity decreases with the increase of the temperature. The observed changes in E_a for the electrolyte and the cathode reaction (the charge transfer step for the LSM-based electrodes) appear in the same temperature interval. This interesting coincidence suggests for correlation between the bulk (electrolyte) and surface conduction properties. Approaches for improvement of both the ionic conductivity and the supply with electrons in LSM should be also searched.     

High temperature electrical conductivity and electrochemical investigation of La2-xSrxCoO4 nanoparticles for IT-SOFC cathode

Single-phase Ruddlesden popper of La_2-xSr_xCoO₄ nanopowders with x?=?0.7, 0.9, 1.1 and 1.3, were successfully synthesized by a modified sol-gel method. Structural stability and morphology of the prepared samples were examined using HT-XRD analysis, FE-SEM and SEM techniques. HT-XRD analysis of the samples, in the range of room temperature to 850?°C, revealed that the structure of all samples was tetragonal. The electrical conductivity measurements, in the range of room temperature to 850?°C, indicated that by increasing the temperature the electrical conductivity mechanism inverts from variable range hopping to the nearest-neighbor hopping of small polarons. In addition, it was found that by increasing Sr concentration the structure of the sintered samples becomes more stable. The electrochemical characterization was carried out using the impedance spectroscopy (EIS) measurements on symmetrical cells at three different temperatures, 650?°C, 750?°C and 850?°C. The area specific polarization resistance (ASR) of La_2-xSr_xCoO₄-CGO-La_2-xSr_xCoO₄ symmetrical cell, in oxygen flow, was obtained about 1.07, 0.35, 0.33 and 0.43 Ωcm² at 850?°C for the samples with x?=?0.7, 0.9, 1.1 and x?=?1.3, respectively. According to our EIS results, the main rate-limiting step for La_2-xSr_xCoO₄ cathode performance is the dissociation process of oxygen at the surface of cathode at 650?°C and the charge transfer limiting in the cathode/electrolyte at 750?°C and 850?°C. Our results showed that the samples with Sr contents of x?=?0.9 and x?=?1.1 can be the promising cathodes for IT-SOFC applications.     

Durability and performance of CGO barriers and LSCF cathode deposited by spray-pyrolysis

Ce_0.9Gd_0.1O_1.95 (CGO) protective layers are prepared by two different methods to prevent the reaction between the Zr_0.84Y_0.16O_1.92 (YSZ) electrolyte and the La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) cathode. In the first method, the CGO layers are deposited by an airbrushing technique from an ink containing CGO particles without and with cobalt as sintering aids. The second strategy consists in preparing both a dense CGO barrier layer and a porous LSCF cathode by spray-pyrolysis deposition, in order to further reduce the fabrication temperature and minimize the reaction between the cell components. The samples prepared by spray-pyrolysis exhibit better performance and durability than those obtained by conventional sintering methods. The results suggest that the interfacial reactivity between YSZ and LSCF as well as the Sr-enrichment at the cathode surface can be avoided by using low-temperature fabrication methods and by operating at temperatures lower than 650?°C.     

Development of LSM/YSZ composite cathode for anode-supported solid oxide fuel cells

This paper presents the effect of (La,Sr)MnO₃ (LSM) stoichiometry on the polarization behaviour of LSM/Y₂O₃-ZrO₂ (YSZ) composite cathodes. The composite cathode made of A-site deficient (La_0.85Sr_0.15)_0.9MnO₃ (LSM-B) showed much lower electrode interfacial resistance and overpotential losses than that made of stoichiometric (La_0.85Sr_0.15)_1.0MnO₃ (LSM-A). The much poorer performance of the latter is believed to be due to the formation of resistive substances such as La₂Zr₂O₇/SrZrO₃ between LSM and YSZ phases in the composite electrode. A slight A-site deficiency (∼0.1) was effective in inhibiting the formation of these resistive substances. A power density of ∼1 W cm⁻² at 800 °C was achieved with an anode-supported cell using an LSM-B/YSZ composite cathode. In addition, the effects of cathodic current treatment and electrolyte surface grinding on the performance of composite cathodes were also studied.     

Novel CO2-tolerant Co-based double perovskite cathode for intermediate temperature solid oxide fuel cells

For the commercial application of solid oxide fuel cells (SOFCs), CO₂-tolerant cathode materials with high electrochemical activity are required. Here, we discuss the performance of double perovskite Pr_0.2Sr_1.8CoTiO_6?δ (P02STC) as a potential cathode material for SOFCs. P02STC has a cubic structure and keeps lattice structure stable in the CO₂ atmosphere. The average thermal expansion coefficient is 17.8 × 10^–6 K^–1 at 30–900 °C in air. The P02STC cathode exhibits good electrochemical performance with a low polarization resistance of 0.080 Ωcm² at 700 °C. The P02STC cathode shows good structure stability, electrochemical performance stability, and excellent tolerance to CO₂ poisoning for the symmetrical cells based on the 350 h stability test in air and the 150 h stability test in O₂ containing 5%CO₂ at 700 °C. The electrolyte-supported single cell with a P02STC cathode shows a maximum power density of 677 mW cm^? 2 at 800 °C. The single cell operates stably for 250 h at a constant current of 0.3 A/cm² without obvious degrading performance. According to all of the experimental results, the P02STC sample might be a promising candidate cathode for SOFCs.     

Morphology and electrochemical activity of SOFC composite cathodes: II. Mathematical modelling

This paper presents a mathematical model of mass and charge transport and electrochemical reaction in porous composite cathodes for application in solid oxide fuel cells. The model describes a porous composite cathode as a continuum, and characterises charge and mass transfer and electrochemical kinetics using effective parameters (i.e. conductivity, diffusivity, exchange current) related to morphology and material properties by percolation theory. The model accounts for the distribution of morphological properties (i.e. porosity, tortuosity, density of contacts among particles) along cathode thickness, as experimentally observed on scanning electron microscope images of LSM/YSZ cathodes of varying thickness. This feature allows the model to reproduce the dependence of polarisation resistance on thickness, as determined by impedance spectroscopy on LSM/YSZ cathodes of varying thickness. Polarisation resistance in these cathodes is almost constant for thin cathodes (up to 10 μm thickness), it sharply decreases for intermediate thickness, to reach a minimum value for about 50 μm thickness, then it slightly increases in thicker cathodes.     

Creep behaviour of porous SOFC electrodes: Measurement and application to Ni-8YSZ cermets

A methodology is proposed in this study to investigate the creep properties of porous Ni-8YSZ cermet. Creep experiments have been conducted under reducing atmosphere at the typical SOFC operating temperatures. Specimens have been loaded in a four-point bending test bench. A special attention has been paid in this work to the analytical and numerical modelling of the mechanical test. It has been highlighted that Ni-8YSZ exhibits substantial creep strain rates even at relative low temperatures (700 °C < T < 850 °C). The creep exponent has been found to be just slightly higher than unity (1 < n < 2) while the activation energy has been determined equal to Q = 115 kJ mol⁻¹.High-temperature plastic strains of both Ni and 8YSZ phases have been estimated through the local stress acting on the cermet particles. This analysis indicates that creep behaviour of the Ni-8YSZ composite is not influenced by the metallic phase, but is controlled by the deformation of the 8YSZ matrix. It is also proposed that cermet creep mechanism involves Zr⁴⁺cations diffusion at the surface rather than in the bulk of the 8YSZ material.Impact of the Ni-8YSZ cermet creep on the internal stresses distribution in SOFC is discussed considering the anode supported cell (ASC) design. It is shown that cermet creep strain can induce a substantial stress decrease in the thin electrolyte.     

Influence of doctor blade gap on the properties of tape cast NiO/YSZ anode supports for solid oxide fuel cells

In this study, various tape cast NiO/YSZ anode support layers with similar geometric properties are fabricated by varying the doctor blade from 100?µm to 200?µm with an increment of 25?µm. The mechanical properties of the anode support layers are investigated by three point bending tests of 30 samples for each doctor blade gap. The reliability curves of the flexural strength data are also obtained via two-parameter Weibull distribution method. The effects of the doctor blade gap on the microstructure and the electrochemical performance of the anode support layers are determined via SEM investigations and single cell performance-impedance tests, respectively. The apparent porosities of the samples are also measured by Archimedes’ principle. The results indicate that the doctor blade gap or the resultant tape thickness influences the microstructure of tape cast NiO/YSZ anode supports significantly, yielding different mechanical and electrochemical characteristics. At a reliability level of 70%, the highest flexural strength of 110.20?MPa is obtained from the anode support layer with a doctor blade gap of 175?µm and the 16?cm² active area cell with this anode support layer also exhibits the highest peak performance of 0.483?W/cm² at an operating temperature of 800?°C. Thus, a doctor blade gap of 175?µm is found to have such a microstructure that provides not only better mechanical strength but also higher electrochemical performance.     

Influence of electrode thickness on the performance of composite electrodes for SOFC

Measurements on half-cells consisting of yttria-stabilized zirconia (YSZ) electrolyte pellets and slurry-coated cathodes of different thickness were performed in order to determine the active area for oxygen reduction in composite cathodes of lanthanum strontium manganite (LSM) and YSZ. Electrochemical impedance spectroscopy was used to evaluate the main electrochemical parameters of the cathodic process. The temperature range was between 500 and 900 °C. The experimental results show a remarkable effect of the electrode thickness on the overall reaction rate in all the temperature range. At 750 °C a change in the controlling regime of the oxygen reduction is detectable and has been ascribed to the transition of the rate-determining step from a charge transfer to a mass transfer of the ionic species. A simplified theoretical model of the cathode that accounts for charge transfer and ionic conduction was developed to give insight into the experimental results. The model simulations compared satisfactorily with the experimental data confirming that the behaviour experimentally observed could be approached with the proposed model.     

Highly promoted performance of triple-conducting cathode for YSZ-based SOFC via fluorine anion doping

One of the most important factors to commercialized yttria-stabilized zirconia (YSZ)-based solid oxide fuel cell (SOFC) is a highly active mixed-conducting cathode at reduced temperatures. Herein, we propose a new strategy of fluorine anion (F⁻) doping to enhance electrochemical performance of the H⁺/O²⁻/e triple-conducting BaCo_0.4Fe_0.4Zr_0.1Y_0.1O_3-δ (BCFZY) perovskite cathode for YSZ-based SOFCs. The F⁻-doped BCFZY as BaCo_0.4Fe_0.4Zr_0.1Y_0.1O_2.95-δF_0.05 (BCFZYF) retained the cubic structure with better symmetry. The doping of minor fluorine in oxygen-site was found to increase the oxygen exchange capability and thus oxygen reduction reaction (ORR) catalytical activity, as reflected by lower area specific resistance (ASR) of cathode on symmetrical cells. Maximum power density of 786 mWcm⁻² was achieved at 800 °C for anode-supported single cell with BCFZYF cathode, being 1.7 times higher than that with BCFZY cathode. Furthermore, the single cell with BCFZYF cathode demonstrated excellent stability without any degradation of current density over 200 h at 700 °C. The present work clarifies that the fluorine doping strategy is highly effective to promote the ORR activity of the triple-conducting BCFZY cathode for state-of-the-art oxygen-conducting SOFC.     

Morphology and electrochemical activity of SOFC composite cathodes: I. experimental analysis

This paper describes the first part of an experimental and theoretical study performed on composite Lanthanum Strontium Manganite (LSM) and Yttria-stabilized Zirconia (YSZ) electrodes. Cathode electrocatalytic activity was investigated using different cell configurations and carrying out potentiodynamic polarisation and electrochemical impedance spectroscopy measurements (EIS). Measurements were carried out at different oxygen partial pressures, overpotentials, temperatures and electrode geometries. In order to identify the main steps involved in cathodic oxygen reduction, the NLLS-Fit procedure was used. The results for different cell geometries agree with each other, suggesting a transition in the overall reaction mechanism, from charge transfer to mass transfer control, at a critical temperature of about 750 °C. The experimental results also show a remarkable effect of electrode thickness on the overall reaction rate, throughout the temperature range tested. A grey level gradient along the thickness of the thicker electrodes were detected by analyzing microscopic images of the cells. These results, together with electrochemical measurements on cathodes with different thickness, confirm that morphology plays a key role in determining the performance of Solid Oxide Fuel Cells (SOFC) composite cathodes.     

3D imaging and quantification of interfaces in SOFC anodes

Solid oxide fuel cells (SOFCs) are functional electrochemical conversion devices whose performance is strongly dependent on electrode microstructure. Both the performance and lifetime of these electrochemical devices can be considerably enhanced by the ability to design better electrodes. Data acquired from high resolution 3D imaging techniques were used in the quantification of two electrode structures of different compositions. The quantified nickel-based anode data through the analysis of particle sizes with their metal–metal and ceramic–ceramic neck sizes, metal–ceramic interface sizes, volume fractions, and triple-phase boundary densities, demonstrate it is possible to understand how microstructure contributes to differences observed in electrochemical and mechanical performance; facilitating optimisation of electrode micro/nano structure towards improved performance. In doing so, new insights are gained that could be used to develop better electrodes.     

Layered YSZ/SCSZ/YSZ Electrolytes for Intermediate Temperature SOFC Part I: Design and Manufacturing

(Sc₂O₃)_0.1(CeO₂)_0.01(ZrO₂)_0.89 (SCSZ) ceramic electrolyte has superior ionic conductivity in the intermediate temperature range (700–800 °C), but it does not exhibit good phase and chemical stability in comparison with 8 mol% Y₂O₃–ZrO₂ (YSZ). To maintain high ionic conductivity and improve the stability in the whole electrolyte, layered structures with YSZ outer layers and SCSZ inner layers were designed. Because of a mismatch of coefficients of thermal expansion and Young's moduli of SCSZ and YSZ phases, upon cooling of the electrolytes after sintering, thermal residual stresses will arise, leading to a possible strengthening of the layered composite and, therefore, an increase in the reliability of the electrolyte. Laminated electrolytes with three, four, and six layers design were manufactured using tape‐casting, lamination, and sintering techniques. After sintering, while the thickness of YSZ outer layers remained constant at ∼30 μm, the thickness of the SCSZ inner layer varied from ∼30 μm for a Y–SC–Y three‐layered electrolyte, ∼60 μm for a Y–2SC–Y four‐layered electrolyte, and ∼120 μm for a Y–4SC–Y six‐layered electrolyte. The microstructure, crystal structure, impurities present, and the density of the sintered electrolytes were characterized by scanning and transmission electron microscopy, X‐ray and neutron diffraction, secondary ion mass spectroscopy, and water immersion techniques.     

Tailoring cathode composite boosts the performance of proton-conducting SOFCs fabricated by a one-step co-firing method

A strategy of tailoring the ceramic cathode composite is presented to improve the performance of proton-conducting solid oxide fuel cells (SOFCs) prepared by a one-step co-firing process. Comparing to the conventional way of using BaCe_0.7Zr_0.1Y_0.2O_3-δ (BCZY) in the composite cathode for BCZY-electrolyte based cells, the replacement of BCZY by BaZr_0.8Y_0.2O_3-δ (BZY) mitigates the reaction between the two ceramic phases in the composite cathode during the co-firing process and also keeps the cathode with sufficient porosity for ample gas diffusion which could assist in adequate cathode reactions. As a result, the BCZY-electrolyte based cell with Sm_0.5Sr_0.5CoO_3-δ (SSC)-BZY composite cathode shows a power output of 300?mW?cm^?2 at 600?°C, which is the largest ever reported for proton-conducting SOFCs prepared by a one-step co-firing process. The strategy of tailoring the composite cathode offers both small ohmic resistance and polarization resistance, providing a promising way to develop single-step co-fired proton-conducting SOFCs.     

Boosting the electrocatalytic performance of Fe-based perovskite cathode electrocatalyst for solid oxide fuel cells

The development of cathode materials with excellent electrocatalytic activity and CO₂ tolerance is an important direction for the wide application of solid oxide fuel cells. Herein, the cobalt-free perovskite oxides Bi_0.5Sr_0.5Fe_1-xV_xO_3-δ (BSFVx, x = 0.025, 0.05 and 0.075) are developed as the efficient cathode electrocatalysts for SOFCs. The V-doping strategy is beneficial to improve the thermal stability, CO₂ tolerance and electrochemical performance of undoped Bi_0.5Sr_0.5FeO_3-δ. Among all samples, Bi_0.5Si_0.5Fe_0.95V_0.05O_3-δ (BSFV0.05) cathode presents excellent oxygen reduction reaction activity, achieving a lower polarization resistance of 0.076 Ω cm² and the peak power density of the single cell with the BSFV0.05 cathode reaches to 1.16 W cm⁻² at 700 °C, which can be comparable to those of the representative cobalt-based cathodes. Furthermore, the improved CO₂ tolerance of the BSFV0.05 cathode can be ascribed to the high acidity of the V⁵⁺ and the larger average bonding energy in the oxide.     

Fabrication and electrochemical modelling of 8YSZ and GDC10 freeze tape cast scaffolds for solid oxide cells (SOCs)

The morphology of electrodes in Solid Oxide Cells (SOCs) has a great impact on their mechanical stability during operation as well as transport properties and kinetics, which in turn affect electrode and cell performance. This study proposes a new experimental procedure based on the freeze tape casting technique for the manufacturing of graded porous electrodes for SOCs. The use of water-based freeze tape casting has enabled the effective fabrication of hierarchical porous ionic backbones featuring the typical porosity of functional and supporting electrodes in a single tape. The porous samples are morphologically characterized and subsequently, for the first time according to the authors knowledge, a Computational Fluid Dynamic (CFD) model has been developed to compare the gas transport properties of conventional spongy-like and graded porous electrodes of planar SOCs. The results presented strongly suggest that hierarchical porous electrodes enable higher performance by decreasing the voltage concentration losses.     

Residual Stress Assessment for Thin 8YSZ Electrolytes Using Focused Ion Beam Milling and Digital Image Correlation

With respect to solid oxide fuel cell power density it has been verified that 1–2 μm thick 8YSZ electrolytes have significant advantages, offering the potential to operate stacks at temperatures of 600 °C. However, reliability of the component depends on integrity and hence residual stress state. In this work, an advanced method is used to determine the electrolyte residual stress locally using stress relaxation tests by a combination of focused ion beam milling and digital image correlation.