Lanthanum Strontium Manganite Yttria-stabilized Zirconia Research Articles

Revitalizing Oxygen Reduction Reactivity of Composite Oxide Electrodes via Electrochemically Deposited PrOx Nanocatalysts.

Solid oxide fuel cells that operate at intermediate temperatures require efficient catalysts to enhance the inherently poor electrochemical activity of the composite electrodes. Here, a simple and practical electrochemical deposition method is presented for fabricating a PrOx overlayer on lanthanum strontium manganite-yttria-stabilized zirconia (LSM-YSZ) composite electrodes. The method requires less than four minutes for completion and can be carried out under at ambient temperature and pressure. Crucially, the treatment significantly improves the electrode's performance without requiring heat treatment or other supplementary processes. The PrOx-coated LSM-YSZ electrode exhibits an 89% decrease in polarization resistance at 650°C (compared to an untreated electrode), maintaining a tenfold reduction after ≈400h. Transmission line model analysis using impedance spectra confirms how PrOx coating improved the oxygen reduction reaction activity. Further, tests with anode-supported single cells reveal an outstanding peak power density compared to those of other LSM-YSZ-based cathodes (e.g., 418mWcm-2 at 650°C). Furthermore, it is demonstrated that multicomponent coating, such as (Pr,Ce)Ox, can also be obtained with this method. Overall, the observations offer a promising route for the development of high-performance solid oxide fuel cells.

Electrochemical Deposition of Nanocatalysts on an Oxide Scaffold Enhances the Activity of Oxygen Reduction

Solid oxide fuel cells (SOFCs) are devices that directly convert the chemical energy of hydrogen and oxygen into electrical energy, and are attracting attention for their high efficiency and eco-friendliness. Since the recent research trend is to lower the operating temperature of the device, there is a considerable demand for a way to effectively introduce a catalyst to overcome the poor electrochemical activity of the most commercially available lanthanum strontium manganite–yttria-stabilized zirconia (LSM-YSZ) composite electrode.Praseodymium oxide (PrOx) is an excellent catalyst for the ORR and has also been applied to LSM-YSZ electrodes via infiltration, the most widely used catalyst fabrication method. However, this previously well-established method still experiences time-consuming and energy-intensive limitations; therefore, other catalyst fabrication approaches are required.Cathodic electrochemical deposition (CELD) is chosen as a central strategy to decorate the PrOx catalyst which strongly empowers the exclusive ORR activity of the LSM-YSZ electrode. CELD is an excellent catalyst fabrication method that combines electroplating and chemical precipitation, and is simple, fast, cost-effective, and capable of deposition at room temperature and ambient pressure.Herein, we present an electrochemical deposition method that fabricating a PrOx overlayer significantly improves the catalytic activity of composite electrodes with only a short process of less than 4 min, even in an ambient environment. Moreover, it does not require additional processes such as heat treatment. The PrOx-coated electrode exhibits a decrease in initial polarization resistance compared to the bare, and maintained an oxygen reduction reaction characteristic by more than 10 times even after about 400 hours of operation at 650 °C. Transmission line model analysis with impedance spectra describes how PrOx improves the reactivity of the oxygen reduction reaction of composite electrodes. Finally, we demonstrate that a two-element material, (Pr, Ce)Ox, was electrochemically deposited. Electrochemical deposition considerably improves the catalytic properties of the cathode via a concise and straightforward process.

Electrical Conductivity of LSM—YSZ Oxygen Electrode for Determining Active Electrode Zone in Solid Oxide Cells

Solid oxide cells (SOCs) show high potential in applications related to sustainable society. The substantial contribution of the SOCs is their high efficiency in terms of the electric energy utilization due to the operating temperature up to 800 °C. The effect of high operation temperature is favorable in two ways. Firstly, it accelerates the kinetics of electrochemical reactions preventing the need of the use of Pt-based catalysts. Secondly, the high temperature offers favorable thermodynamic conditions decreasing the equilibrium potential for water splitting. This makes the SOCs technology a direct competitor to low-temperature technologies, such as alkaline or PEM electrolysis.The SOCs are composed of electrolyte, fuel electrode and oxygen electrode. The electrolyte allows transport of oxygen ions between the electrodes due to its high ionic conductivity. Yttria-stabilized zirconia (YSZ) represents a typical example of such a material. The oxygen electrode is where the oxygen reduction reaction or the oxygen evolution reaction takes place according to the operation mode of the SOC. Lanthanum strontium manganite (LSM) has been the state-of-the-art oxygen electrode material for almost 25 years and is considered as a pure electron conductor. Lack of ionic conductivity of the LSM leads to limitation of the overall reaction rate to the amount of triple phase boundary (TPB) where gaseous phase, electron-conducting phase and ion-conducting are in simultaneous contact. According to the literature it is optimal for the electrode material to be mixed in the 50:50 ratio with YSZ to increase the amount of the TPB. Thus, the active zone is extended further from the electrolyte surface into the volume of the electrode body. Even though the mixed electrode material exhibits great performance, information on the optimal thickness of the electrode is scarce in the literature. The electrode thickness is a crucial parameter since not all of the electrode volume is electrochemically active and represents just surplus resistance for the electric current impairing the overall electrode performance. The goal of our research is to determine of the extent of the active zone that we can estimate the optimal thickness of the LSM—YSZ electrode for various operation conditions.A macrohomogenous 1D model was developed to simulate the active electrode zone extent. Initial parametric study has shown that the extent of the active region of the electrode is influenced by many factors including the kinetics of the occurring reactions, the ionic and electronic conductivity of the respective domains and working conditions, such as temperature and operating current density. Even though the literature offers data on the conductivity of bulk the LSM and YSZ, the data of the respective conductivities in the LSM—YSZ are not applicable due to the electrode structure. The conductivity of the structure consisting of packed particles with a small contact area is limited by the contact resistance. This study targets to determine the electronic conductivity of the LSM phase and the ionic conductivity of the YSZ phase in the LSM—YSZ electrode framework.The LSM conductivity in the electrode framework was determined by resistance measurement of the casted layers of the LSM—YSZ using electrochemical impedance spectroscopy (EIS). The results show an exponential decrease of the LSM conductivity with decreasing LSM content. Furthermore, the 50:50 LSM—YSZ exhibits conductivity comparable to that of the bulk YSZ electrolyte. This result proves that the energy losses due to the electrical resistance of the LSM—YSZ electrode cannot be neglected in the terms of SOC performance.The determination of the ionic conductivity of YSZ in the electrode framework required the substitution of LSM with an inert material. The material of choice was CaSO4 due to its availability and its melting point being relatively close to that of the LSM. Similarly to the LSM, YSZ exhibited an exponential decrease of ionic conductivity with a decrease in YSZ volume fraction in the samples. Furthermore, the trend was compared to the Koh-Fortini relationship, showing that grain boundaries have a strong influence on the total conductivity of the material.The results of the experiments helped us to determine the extent of the active electrode zone leading to the estimation of optimal thickness of the LSM—YSZ oxygen electrode thickness.The authors acknowledge the financial support of the Czech Science Foundation (GACR), contract No: 19-142-44-J and by the Technology Agency of the Czech Republic under project no. TK04030143.

Lowering the Impedance of Lanthanum Strontium Manganite-Based Electrodes with Lanthanum Oxychloride and Lanthanum Scavenging Chloride Salts

The impact of infiltrating chloride salts on the electrochemical behavior of lanthanum strontium manganite-yttria stabilized zirconia (LSM-YSZ) cathodes was investigated under solid oxide fuel cell operation. Infiltrating a lanthanum chloride solution resulted in the formation of a lanthanum oxychloride (LaOCl) phase. A LaOCl phase also formed by infiltrating an ammonium chloride solution; however, lanthanum was scavenged from the LSM phase to form LaOCl. The third infiltrating solution, a combination of zirconium chloride and yttrium nitrate, formed LaOCl by scavenging lanthanum from LSM and produced YSZ nanoparticles. Electrochemical impedance spectroscopy results suggest that LaOCl improves oxygen adsorption kinetics compared to a baseline LSM-YSZ cathode, reducing the low frequency impedance by 30%. In addition, scavenging lanthanum from LSM improved oxygen ion diffusion polarization as indicated by the observed 40% reduction in high frequency impedance and improved serial ohmic resistance by 19%. Finally, YSZ nanoparticles further reduced the high frequency impedance and ohmic resistance by 45% and 23%, respectively. The findings reveal new strategies for lowering the impedance of LSM-YSZ cathodes.

Investigation of the electrochemical reactions at a limited-contact La1−Sr MnO3/Y-doped ZrO2 interface with a rod-type ionic-probe

Abstract A more quantitatively controllable triple phase boundary (TPB) of a lanthanum strontium manganite/yttria-stabilized zirconia (LSM/YSZ) interface was constructed by using an YSZ ionic probe with well-defined dimensions. A bar-shaped, dense YSZ sintered body was employed as an ionic probe and embedded in the pellet-shaped, dense LSM bulk. The TPB length of the LSM/YSZ interface can be simply determined from the circumference of the YSZ bars. To identify the reaction mechanism of the oxygen reduction reaction (ORR) at the TPB of the LSM/YSZ interface, limited-contact AC impedance spectroscopy was used; this distinguished the LSM/YSZ interface related polarization from other polarizations present in the Pt/LSM/YSZ/Pt electrochemical cell. By analyzing the electrode-related polarizations in the electrochemical cell with the geometrically quantified YSZ ionic probe, the rate determining step of the ORR was the diffusion of adsorbed oxygen along the LSM bulk surface. In this paper, emphasis is placed on the experimental versatility and the limitations of our designed electrochemical analysis with bar-shaped ionic probes.

A Sintering Kinetics Model for Ceramic Dual‐Phase Composite

An analytical model is developed for the sintering kinetics of ceramic dual‐phase composites of the cathodes in solid oxide fuel cells (SOCFs). The model simulates the isothermal and pressureless sintering processes and formulates volumetric three‐phase boundary (TPB) length and porosity as a function of sintering time, surface/interface energies, grain‐boundary diffusivities, particle sizes, and dual‐phase composition. Lanthanum strontium manganite (LSM)–yttria‐stabilized zirconia (YSZ) composite is used as an example to develop and validate the model. LSM–YSZ composites are sintered at 1100°C for various sintering time, and the TPB length and porosity are estimated from SEM images by using stereological analysis to validate the model. Parametric studies are performed at various conditions, illustrating novel insights into the sintering kinetics. This analytical model is generic and applicable to the sintering kinetics of ceramic dual‐phase composites for use in solid‐state electrochemical devices, such as SOCFs, electrolyzers, and gas separation membranes. This analytical model can also be easily extended to the sintering processes of other ceramic dual‐phase and triple‐phase composites.

Tortuosity characterization of 3D microstructure at nano-scale for energy storage and conversion materials

A distance propagation method is presented for calculating tortuosity with relatively low computation time from three-dimensional (3D) tomographic data. Moreover, a novel concept of tortuosity distribution is developed to provide a more comprehensive picture of inhomogeneous microstructures where tortuosity depends on the actual 3D paths. Instead of using one single tortuosity value, the tortuosity distribution both as spatial distribution map and also statistic histogram can provide a more complete description. The method, which can be applied to any porous medium, is tested against a diffusion-based tortuosity calculation on two 3D microstructures: a LiCoO2 cathode electrode of lithium ion battery measured by x-ray nano-tomography and a lanthanum strontium manganite–yttria-stabilized zirconia, solid oxide fuel cells cathode measured using focused ion beam-scanning electron microscopy serial sectioning. The present method is shown to provide good-agreement with the effective diffusion-based tortuosity values.

Direct In Situ Probe of Electrochemical Processes in Operating Fuel Cells

The function of systems and devices in many technologically important applications depends on dynamic processes in complex environments not accessible by structure and property characterization tools. Fuel cells represent an example in which interactions occur under extreme conditions: high pressure, high temperature, in reactive gas environments. Here, scanning surface potential microscopy is used to quantify local potential at electrode/electrolyte interfaces in operating solid oxide fuel cells at 600 °C. Two types of fuel cells are compared to demonstrate two mechanisms of ionic transport at interfaces. Lanthanum strontium ferrite-yttria-stabilized zirconia (LSF-YSZ) and lanthanum strontium manganite-yttria-stabilized zirconia (LSM-YSZ) cross-sectional electrode assemblies were measured to compare mixed ionic electronic conducting and electronic conducting mechanisms. Direct observation of the active zones in these devices yields characteristic length scales and estimates of activation barrier changes.

Design and Optimization of Composite Electrodes in Solid Oxide Cells

AbstractThe performance of solid oxide cells (SOCs) heavily relies on the population of three‐phase boundaries (TPBs) in the composite electrodes. In this study, SOC composite electrodes are described by percolating binary particle aggregates that are constructed from random loose packing models and classical sintering theories. Summed perimeters of the sintering necks represent the total TPB lengths. A case study has been carried out on lanthanum strontium manganite (LSM)–yttria‐stabilized zirconia (YSZ) composite electrodes. By employing three‐dimensional data that are converted from relevant two‐dimensional data, the TPB length of baseline LSM–YSZ electrodes investigated in this study is 35.4 μm μm–3. The parametric and sensitivity analyses show the changes of TPB lengths in functions of the weight fraction of powders, particle size and particle size ratio of powders, void fraction of electrodes, and density of materials. In the case of baseline LSM–YSZ electrodes, proper electrode design and optimization would result in 2–3 times of the enlargement of TPBs. Technical guidelines on the design and optimization of SOC composite electrodes are proposed.

Simulation of sintering kinetics and microstructure evolution of composite solid oxide fuel cells electrodes

A three-dimension (3D) kinetic Monte Carlo (kMC) model is developed to study the sintering kinetics and microstructure evolution of solid oxide fuel cell (SOFC) composite electrodes during the co-sintering processes. The model employs Lanthanum Strontium Manganite (LSM) – Yttria-stabilized Zirconia (YSZ) composites as the example electrodes but can be applied to other materials. The sintering mechanisms include surface diffusion, grain boundary migration, vacancy creation, and annihilation. A morphological dilation method is used to generate the initial LSM–YSZ compacts as the input structures for the kMC simulation. The three-phase boundary (TPB) length, porosity, and tortuosity factor of the composite cathodes are calculated during kMC sintering. Simulation results are compared with literature data and good agreement is found. Parametric study is conducted to investigate the effects of particle size, size distribution, and sintering temperature on sintering kinetics as well as the evolution of electrode microstructures. The kMC model is capable of simulating the initial and a part of intermediate sintering stages of SOFC electrodes by considering various sintering mechanisms simultaneously. It can serve as a useful tool to design and optimize the sintering processes for composite SOFC electrodes.

The Effect of Humidity and Oxygen Partial Pressure on LSM–YSZ Cathode

AbstractTwo series of anode supported solid oxide fuel cells (SOFC) were prepared, one with a composite cathode layer of lanthanum strontium manganite (LSM) and yttria stabilized zirconia (YSZ) on top and the other further has a LSM current collector layer on top. The fuel cells were heat treated at 1,000 °C in air or nitrogen choosing dry or humid (70% steam) conditions. XRD, SEM, and XPS investigations were performed on the various samples. The most severe modifications were observed in humid nitrogen atmosphere at low oxygen partial pressure. LSM surface change goes along with a decrease of manganese concentration and strontium enrichment on the surface of the materials. Formation of monoclinic zirconia and zirconate phases was also observed. These results give a closer insight into possible degradation mechanisms of SOFC composite cathode materials in dependence of humidity and oxygen partial pressure.

Random-packing model for solid oxide fuel cell electrodes with particle size distributions

A percolation theory based model considering particle size and its distribution is proposed for composite electrodes of solid oxide fuel cells (SOFCs). The model calculation agrees excellently with 3D numerical reconstruction results, suggesting great validity of prediction. Moreover, it is also consistent well with experiment for real LSM (lanthanum strontium manganite)–YSZ (yttria-stabilized zirconia) electrodes with different composition, especially in range from 40:60 to 60:40 wt.% LSM:YSZ. The model can explicitly capture the effects of particle size, distribution, and electrode composition on several basic microstructure features and electrochemical properties of composite electrodes, such as coordination numbers and percolation probability, total and active three-phase boundary length, and interfacial polarization resistance. The model is further used to estimate LSM–YSZ electrode performance with the particle size and distribution of the source materials. The estimation generally coincides with the experiment, showing great potential in predicting power density based on the particle parameters of source materials for SOFCs.

Geometrical Optimization of Double Layer LSM/LSM-YSZ Cathodes by Electrochemical Impedance Spectroscopy

The present-day high-temperature solid oxide fuel cells (SOFCs), based on yttria-stabilized zirconia (YSZ) electrolyte, a lanthanum-strontium manganite (LSM) cathode and a nickel-YSZ cermet anode, operate at 800–1000°C. Cathode materials are restricted to doped lanthanum manganites due to their stability in oxidizing atmosphere, sufficient electrical conductivity, and thermal expansion match to the YSZ electrolyte. Reduction in the operating temperature of SOFCs is desirable to lower the costs and to overcome the technological disadvantages associated with elevated temperatures. However, as the operating temperature is reduced, the decrease in the LSM conductivity and increase in interfacial polarization resistances between the LSM cathode and YSZ electrolyte become critical. Therefore, different approaches have been proposed to improve interfacial quality and electrochemical performance of the LSM/YSZ cathode. The length of the triple-phase boundary (TPB) correlates well with the interfacial resistances to electrochemical oxidation of hydrogen at the anode and reduction in oxygen at the cathode. The extension of the TPB or the number of active reaction sites becomes, therefore, a determining factor in improving electrode performance. This can be achieved by developing electrode materials of higher ambipolar conductivity and by optimizing the microstructure of the electrodes. In order to improve SOFC performance, both composition and structure of the LSM/YSZ interface and of the cathode should be optimized. Recently, functional grade materials (FGMs) were introduced for SOFC technology. However, all studies reported in the literature so far, were focused on cathodes with only compositional gradient. On the other hand, intuitionally the best structure for a functional SOFC should be characterized by both compositional and porosity gradients. Fine grains (and high surface area) close to the electrode/electrolyte surface and large grains (and thus large pore size) at the air/oxygen side are expected to be of advantage. In the present study, “symmetrical” cathode-electrolyte-cathode SOFC single cells were fabricated. The cells consisted of the functional grade LSM cathode with YSZ/LSM cathode functional layer and LSM contact layer. The effects of various geometrical and microstructural parameters of cathode/functional layers on the overall cell performance were systematically investigated. The parameters investigated were the (1) cathode functional layer thickness and grain size and (2) the LSM contact layer thickness. Cathode performances were tested by means of electrochemical impedance spectroscopy (EIS) over a temperature range of 650–950°C, using air as oxidant. The dependence of cell performance on various parameters was rationalized by a comprehensive microscale model. A cathode polarization corresponding to 0.14–0.4 Ω cm2 at 750°C was achieved in this manner.

Oxygen Reduction at LSM–YSZ Cathodes Deposited on Anode-Supported Microtubular Solid Oxide Fuel Cells

Three-electrode methods were employed to evaluate the electrochemical activity of the oxygen reduction reaction (ORR) at lanthanum strontium manganite (LSM)–yttria stabilized zirconia (YSZ) composite cathodes deposited on the outer surface of microtubular, anode-supported, solid oxide fuel cells . The performance of these cathodes, deposited on the highly curved, electrophoretically deposited, thin YSZ electrolyte layers in the microtubular configuration, was compared against LSM-YSZ cathodes, deposited similarly and having the same microstructure, on thick YSZ layers in standard SOFC planar cells. While the EIS response of the high and low frequency arcs for LSM-YSZ in the two-cell geometries differed somewhat, the total polarization resistance and the ORR activation energy were similar. It was also demonstrated that LSM-YSZ cathode performance in the system can be further enhanced when the cell is sintered at 1150 vs .