LSM-YSZ Composite Cathode Research Articles

Electrochemical Reaction Mechanism of LSM-YSZ Composite Cathode Based on 3D Simulation of Oxygen Diffusion and Oxygen Labeling Experiment

Electrochemical reaction mechanism of an LSM-YSZ composite cathode is investigated using a combined measurement of oxygen labeling experiment and 3D microstructural reconstruction. First, oxygen isotope gas 18O2 was exposed to the LSM-YSZ cathode at 750 °C and a current density of 0.052 A/cm2. After 2 minutes of 18O2 supply, the gas was switched to water-cooled He to quench the cell and fix the incorporated isotope at the operating condition. After the oxygen labeling experiment, the cell piece was filled with an epoxy resin and polished to prepare the sample. Three-dimensional microstructure was obtained using a focused ion beam-scanning electron microscopy (FIB-SEM). Furthermore, between the measurement of the FIB-SEM, secondary ion mass spectroscopy (SIMS) analysis was conducted at the middle plane of the observed volume to correlate 18O distribution with the 3D microstructure. Based on percolation analysis of the microstructure, several isolated LSMs showed quite low 18O concentration, whereas some isolated LSMs showed 18O incorporation. For a further understanding of the correlation between the 18O distribution and microstructure, 3D numerical simulation using the reconstructed microstructure was carried out. First, potential distribution was obtained by solving gas diffusion and charge transportations which are coupled with the reaction at the reaction sites using the lattice Boltzmann method. Then, 18O distribution was predicted by solving oxygen diffusion along the calculated potential field. The electrochemical reaction mechanism was discussed by comparing the results of numerical simulation and experiment.

Electrochemical Reaction Mechanism of LSM-YSZ Composite Cathode Based on 3D Simulation of Oxygen Diffusion and Oxygen Labeling Experiment

Electrochemical reaction mechanism of an LSM-YSZ composite cathode is investigated using a combined measurement of oxygen labeling experiment and 3D numerical simulation using a reconstructed microstructure. First, an oxygen labeling experiment was conducted using 18O2 at 750°C under polarized condition. Then, a three-dimensional microstructure was obtained using a focused ion beam-scanning electron microscopy (FIB-SEM). Secondary ion mass spectroscopy (SIMS) analysis was conducted at the middle plane of the measured volume to correlate 18O distribution with the 3D microstructure. Using the 3D microstructure, electrochemical simulation was carried out considering the oxygen potential dependence of LSM ionic conductivity. Then, 18O transport was solved considering the balance between diffusion by concentration gradient, ionic current transport, and production by reaction current. The electrochemical reaction mechanism was discussed by comparing the numerically solved 18O concentration with that obtained from oxygen labeling experiment.

Determination of the rate-determining step of the oxygen reduction reaction of La0.8Sr0.2MnO3(LSM)- 8mol% yttria-stabilized zirconia(YSZ): Composition and microstructure

In this study, LSM-YSZ composite cathodes were analyzed by changing the firing temperature, composition, and operating temperature to determine the contribution of each step of the oxygen reduction reaction (ORR). The overall resistance of the cathode reaction was characterized by fitting the AC impedance spectra with an equivalent circuit model. It was found that initial reactions of ORR (dissociative adsorption) are the main rate-determining step (RDS) regardless of operating or sintering temperature, while reactions on LSM surface become the main RDS when the ratio of LSM catalysts has a relatively small proportion. The [LSM-YSZ]5:5 cathode fired at 1100 °C showed the best microstructure and lowest resistance in the ORR at an operating temperature of 700 °C (RHF: 0.18 Ωcm2, RMF: 0.20 Ω cm2, RLF: 0.25 Ωcm2, Rcathode: 1.14 Ωcm2). This demonstrates the potential use of LSM-YSZ cathodes for IT/LT-SOFC without the use of expensive materials, such as LSCF and BSCF.

Interface and grain boundary degradation in LSM-YSZ composite Solid Oxide Fuel Cell cathodes operated in humidified air

The interfaces between the different phases and the associated triple phase boundaries (TPBs) in solid oxide fuel cell (SOFC) cathodes are critical for the oxygen reduction reaction, and their degradation impacts SOFC performance and durability. This work examines nanostructure degradation of composite LSM/YSZ cathodes induced by electrochemical operation in humidified air. Three commercial button cells operated in humidified air for various durations at 800 °C exhibited more severe performance degradation than the cell operated in dry air. Microscopy imaging reveals nanostructure degradation within the cathode active layer, especially in the regions nearest to the electrolyte. Newly formed Mn-enriched nano-precipitates accompanied by nano-voids initiate at the original TPBs and propagate along the LSM/YSZ interface. The abundance of the nano-precipitates at LSM/YSZ interfaces increases with operation time, and prolonged operation in humidified air further promotes the formation of the Mn-enriched precipitates along YSZ/YSZ grain boundaries. The formation mechanism of those precipitates and their influence on the cathode performance are discussed.

Investigation of LSM-YSZ Composite Cathode Performance Degradation with a Multistep Charge Transfer Model

The long-term performance degradation of LSM/YSZ composite cathodes is investigated via calibrated multiphysics simulation with a multistep oxygen reduction reaction (ORR) model and structural coarsening data from a phase field study. Multiphysics simulations, including a multistep ORR mechanism, are first developed for better understanding of the limiting processes in solid oxide fuel cells. The multistep ORR mechanism includes two parallel pathways, surface pathway and bulk pathway, which consist of elementary reaction steps. The multiphysics model is simultaneously calibrated with experimental polarization curves and impedance behavior for various air/fuel supply conditions. To our knowledge, this is the first time that a multistep ORR model is calibrated with such datasets. Next, the calibrated simulations are utilized to simulate a 2D half-cell constructed with measured microstructural data and random heterogeneity. Finally, the long-term performance degradation of the half-cell is predicted by the calibrated multiphysics model coupled with structural coarsening trends simulated using a phase field-based coarsening model. Degradation of both polarization curves and impedance behavior is investigated. Thorough analyses, including changes of contributions from different pathways, the resistance components, and overall reaction order, are performed to provide more insights into cathode performance degradation due to grain coarsening phenomena.

Tailoring the pore structure of cathode supports for improving the electrochemical performance of solid oxide fuel cells

Two types of lanthanum doped strontium manganite (LSM)-yttria-stabilized zirconia (YSZ) composite cathodes were prepared, one with the finger-like straight open pores by the phase inversion tape casting, and the other with the randomly distributed tortuous pores by the conventional tape casting. A gas permeation flux of 42.5 × 105 Lm−2 h−1 was measured under a trans-membrane pressure of 0.6 bar for the former while only 10.6 × 105 Lm−2 h−1 for the latter. Fuel cells supported on the as-prepared LSM-YSZ composite cathodes were fabricated, comprising a 15 μm thick YSZ electrolyte layer and a 20 μm thick NiO-YSZ anode. The electrochemical performance of the fuel cells was measured using H2 as fuels and air as oxidants. The cell supported on the phase-inversion derived cathode showed a maximum power density of 362 mWcm−2 at 850 °C, while only 149 mWcm−2 for the cell supported on the cathode formed by the conventional method. The difference in the electrochemical performance between the two cells can be attributed to the pore structure of the cathode supports. It is concluded that the phase inversion tape casting provides a simple and effective approach for tailoring the pore structure of the cathode support and thus enhancing the electrochemical performance.

Elementary kinetics of the oxygen reduction reaction on LSM-YSZ composite cathodes

A multi-physics based transient, continuum model of a solid oxide half-cell comprising of a porous LSM-YSZ composite cathode sintered to a dense YSZ electrolyte was developed to investigate the oxygen reduction reaction kinetics. The model coupled species, electron and ion transport through the porous cathode to surface and electro-chemistry. The electrochemical reduction of O2 was modeled using three candidate elementary kinetic mechanisms. Each mechanism included parallel surface and bulk pathways for O2 reduction and was driven by three different electric phase potentials. The mechanisms were compared against three sets of electrochemical impedance spectra and polarization curves measured by Barbucci et al. (2009), Cronin et al. (2012) and Nielsen and Hjelm (2014) over a wide range of operating temperatures (873–1173K), inlet O2 concentrations (5–100%) and overpotentials (−1V to +1V). Two of the three mechanisms were able to quantitatively reproduce the three sets of experiments by only tweaking the microstructural parameters for each individual set. Yet on analyzing their kinetic and thermodynamic parameters, the mechanism postulating the chemisorption of gas-phase O2 on LSM to form the superoxo-like adsorbate O2- was determined to be the most realistic. A model-based sensitivity analysis revealed that ionic transport in the YSZ phase, O2 dissociation in conjunction with surface to bulk charge transfer in the LSM phase and charge transfer at the three phase boundary were the rate limiting steps throughout the operating space. Additionally, the bulk pathway was found to be insignificant.

Chromium poisoning in (La,Sr)MnO3 cathode: Three-dimensional simulation of a solid oxide fuel cell

Abstract A three-dimensional numerical model of a single solid oxide fuel cell (SOFC) considering chromium poisoning on the cathode side has been developed to investigate the evolution of the SOFC performance over long-term operation. The degradation model applied in the simulation describes the loss of the cathode electrochemical activity as a decrease in the active triple-phase boundary (TPB) length. The calculations are conducted for two types of cell: lanthanum strontium manganite (LSM)/yttria-stabilized zirconia (YSZ)/Ni-YSZ and LSM-YSZ/YSZ/Ni-YSZ. Their electrode microstructures are acquired by imaging with a focused ion beam scanning-electron microscope (FIB-SEM). The simulation results qualitatively reproduce the trends of chromium poisoning reported in the literature. It has been revealed that the performance degradation by chromium is primarily due to an increase in the cathode activation overpotential. In addition, in the LSM-YSZ composite cathode, TPBs in the vicinity of the cathode–electrolyte interface preferentially deteriorate, shifting the active reaction site towards the cathode surface. This also results in an increase in the cathode ohmic loss associated with oxide ion conduction through the YSZ phase. The effects of the cell temperature, the partial pressure of steam at the chromium source, the cathode microstructure, and the cathode thickness on chromium poisoning are also discussed.

Effect of GDC addition method on the properties of LSM–YSZ composite cathode support for solid oxide fuel cells

Equal amounts of Gd0.1Ce0.9O2−δ (GDC) were added to La0.65Sr0.3MnO3−δ/(Y2O3)0.08(ZrO2)0.92 (LSM/YSZ) powder either by physical mixing or by sol–gel process, to produce a porous cathode support for solid oxide fuel cells (SOFCs). The effect of the GDC mixing method was analyzed in view of sinterability, thermal expansion coefficient, microstructure, porosity, and electrical conductivity of the LSM/YSZ composite. GDC infiltrated LSM/YSZ (G-LY) composite showed a highly porous microstructure when compared with mechanically mixed LSM/YSZ (LY) and LSM/YSZ/GDC (LYG) composites. The cathode support composites were used to fabricate the button SOFCs by slurry coating of YSZ electrolyte and a nickel/YSZ anode functional layer, followed by co-firing at 1250°C. The G-LY composite cathode-supported SOFC showed maximum power densities of 215, 316, and 396mWcm−2 at 750, 800, and 850°C, respectively, using dry hydrogen as fuel. Results showed that the GDC deposition by sol–gel process on LSM/YSZ powder before sintering is a promising technique for producing porous cathode support for the SOFCs.

Microtubular solid oxide fuel cells with lanthanum strontium manganite infiltrated cathodes

Microtubular solid oxide fuel cells (mT-SOFCs) with infiltrated cathodes were fabricated and their electrochemical performance were compared with standard cells. For this purpose, NiO-YSZ (yttria stabilized zirconia) microtubular supports were fabricated by cold isostatic pressing (CIP) of NiO, YSZ and pore former powders, followed by spray coating of the YSZ electrolyte and co-sintering at 1400 °C. The LSM (La0.8Sr0.2MnO3−δ)-YSZ oxygen electrode is deposited by infiltration of LSM (into a thin porous YSZ layer). One of the advantages of this fabrication method is an increase of TPB (triple-phase boundary) length compared with the standard LSM-YSZ composite due to the much finer dispersed LSM particles having a higher active surface area towards oxygen reduction.The effect of the infiltrated amount in cell performance was studied. Two cells with identical anode support and thin layer electrolyte and 22 vol% (cell A) and 35 vol% (cell B) infiltrated LSM were prepared. The infiltrated cells showed an increase of up to 50% in terms of power density compared to the standard cell (550 mW cm−2 at 0.7 V and 850 °C for a standard cell having 50 vol% LSM and 720 and 805 mW cm−2 at 0.7 V and 850 °C for infiltrated cells A and B, respectively). The results indicate that the infiltrated cathode with fine distributed LSM particles improve the fuel cell performance using a lower LSM content compared with standard LSM-YSZ composite cathodes.

Contribution of properties of composite cathode and cathode/electrolyte interface to cell performance in a planar solid oxide fuel cell stack

Solid oxide fuel cells (SOFCs) with distinct cathode materials usually differ in output performance. In this study, 2 μm-thick Pt voltage probes are embedded into the cathode/electrolyte interface. The effects of the electrical properties and cathode/electrolyte interfaces of LSCF-GDC and LSM-YSZ composite cathodes on cell performance are investigated in situ for anode-supported planar SOFCs. Results show that the voltage and maximum output power density measured by the probes on both sides of the LSCF-GDC and LSM-YSZ composite cathodes are 7% and 4%, respectively, of those of the corresponding cell during instantaneous current–voltage testing. The enhanced LSCF cell performance is mainly attributed to the rough GDC/LSCF-GDC interface that is responsible for the three-dimensional contact between the GDC layer and LSCF-GDC cathode particles and increases the triple-phase boundary (TPB) length. The LSM-YSZ cathode performance degradation is attributed to the variation in polarization resistance caused by cathode particle growth. However, the primary factor for the degradation of LSCF-GDC cathode performance is structural instability, such as inner cracks.

Micro scale dynamic modeling of LSM/YSZ composite cathodes

Abstract In this study, a detailed reaction mechanism for oxygen reduction on LSM/YSZ is applied to dynamic one-dimensional micro-scale modeling of LSM–YSZ composite cathodes. The model incorporates the effects of cathode microstructural properties on the local transport phenomena and electrochemistry inside the cathode. The reaction mechanism used in the model features two parallel routes for bulk gas oxygen conversion to electrolyte lattice oxygen, namely two-phase boundary (2 PB) and three-phase boundary (3 PB) pathways. The model predicts field distributions of local thermodynamic values, over-potential, Faradaic current and other parameters relevant to cathode performance. Electrochemical impedance simulations are performed in the time domain to analyze the contribution of specific mechanistic steps to the overall impedance. A parametric study was performed to assess the sensitivity of the model predictions to selected model parameters and cathode micro-structural properties. Results reveal that the relative contribution from 3 PB and 2 PB pathways to the total charge transfer is dependent upon operating conditions. Parametric sensitivities observed in the impedance results are compared with polarization simulations to assess the physical plausibility of the proposed reaction and transport model. Simulation results are further compared to experimental findings reported in literature.

Performance studies of electrolyte-supported solid oxide fuel cell with Ni–YSZ and Ni–TiO2–YSZ as anodes

Redox cycling of Ni-based anode induces cell degradation which limits the cell's lifetime during solid oxide fuel cell operation. In the present study, the redox testing of electrolyte-supported cells has been investigated with TiO2-added NiO–YSZ anode matrix. Button cells were fabricated by die-pressing YSZ powder as electrolyte, and onto which NiO–YSZ or NiO–TiO2–YSZ anode and LSM–YSZ composite cathode were painted. The electrochemical performance and stability have been evaluated by measuring current–voltage characteristics followed by impedance spectroscopy after each redox cycling. Anode matrices before and after cell operation have been characterized by X-ray diffraction (XRD), elemental dispersive X-ray (EDX), and scanning electron microscopy (SEM). During cell operation the peak power density decreases from 111 mW cm−2 (239 mA cm−2) to 84 mW cm−2 (188 mA cm−2) between 23 and 128 h with five redox cycles for cell having NiO–YSZ (40:60) anode. But for cell with NiO–TiO2–YSZ (30:10:60), the anode peak power density was constant and stable around 85 mW cm−2 (194 mA cm−2) throughout the cell run of 130 h and five redox cycles. No loss in the open circuit voltage was observed. SEM and XRD studies of NiO–TiO2–YSZ (30:10:60) anodes revealed formation of ZrTiO4, which may be responsible for inhibition of Ni coarsening leading to stable cell performance.

Analysis of impact of sintering time on microstructure of LSM-YSZ composite cathodes by X-ray nanotomography

Analysis of impact of sintering time on microstructure of LSM-YSZ composite cathodes by X-ray nanotomography

Feasibility of using ash-free coal in a solid-oxide-electrolyte direct carbon fuel cell

The feasibility of using ash-free coal as the fuel in a direct carbon fuel cell (DCFC) was investigated. Tubular-electrolyte-supported single cells were operated at high temperatures with continuous fuel feeding equipment. A single cell consisted of an yttria-stabilized zirconia (YSZ) electrolyte tube support, a NiO-YSZ cermet anode, and an LSM-YSZ composite cathode. The solvent extraction method was used to remove mineral matter and ash from raw coal. From thermal gravimetric analysis, the ash content of the ash-free coal was found to be nearly zero, but its volatile matter content was 38.34 wt.%. Scanning electron microscopy (SEM) observations and visualization tests were carried out for characterization of the interface of the ash-free coal; the results showed that the ash-free coal was in a liquid-like phase at high temperatures because the volatile matter in the coal produced by solvent extraction could help maintain carbon in the liquid-like state on the anode at high temperatures. The cell performance was affected by this liquid-like state at high temperatures.

Performance and Stability of Solid Oxide Fuel Cell LSM-YSZ Composite Cathodes

not Available.

Fabrication of Ni-YSZ anode supported tubular SOFC through iso-pressing and co-firing route

In the present work, a co-pressing and co-firing process has been developed to fabricate thin YSZ electrolyte film over tubular NiO-YSZ anode support for solid oxide fuel cell (SOFC) application. The one-end-closed tubular anode support was fabricated by cold isostatic pressing (CIP) and electrolyte coating was made by dip coating a terpineol based YSZ slurry over the anode support. The anode support along with coated electrolyte layer was co-pressed in CIP and co-fired at 1350 °C to get impervious electrolyte layer. LSM-YSZ composite cathode layer and LSM cathode current collector layer was formed by dip coating terpineol based slurries followed by sintering at 1150 °C. Symmetric cells in planar configuration were fabricated and tested by electrochemical impedance spectroscopy (EIS) in order to determine area-specific resistance (ASR) for electrode polarization. An ASR value of 0.79 Ω cm2 was obtained for Ni-YSZ|YSZ interface at 900 °C in humidified H2. The ASR value was 0.55 Ω cm2 for LSM-YSZ|YSZ interface at 900 °C in air. Tubular single cell having ∼1 cm2 cathode current collecting area was generated and the cell was found to give a maximum of 62 mW cm−2 power density when tested at 900 °C. The interfacial microstructures and electrochemical test results illustrate the feasibility of fabrication of tubular anode supported SOFC by co-pressing and co-firing.

A Micro-Scale Model for Oxygen Reduction on LSM-YSZ Cathode

In this study, a micro-scale model is developed to simulate the oxygen reduction on LSM-YSZ composite cathode. The model incorporates the effects of cathode microstructural properties on the local transport phenomena and electrochemistry inside the cathode. A detailed reaction mechanism is used in the model which has two parallel routes for oxygen conversion into oxide ions, namely two-phase boundary and three-phase boundary pathways. The model predicts field distributions of local thermodynamic values, over-potential, Faradaic current and other parameters relevant to cathode performance. Electrochemical impedance simulations are performed using the current model to analyze the contribution of various processes to the overall impedance.

Sintering behavioral and electrochemical performances of LSM-BSB composite cathode for IT-SOFCs

In this paper, La0.8Sr0.2MnO3-Ba0.1Bi0.9O1.5-δ (LSM-BSB) composite cathode was prepared and characterized for intermediate temperature solid oxide fuel cells (IT-SOFCs). XRD results show that no reaction occurred between LSM and BSB at 900°C. SEM results show that the LSM-BSB composite cathode formed good contact with YSZ electrolyte after sintered at 900°C for 2 h, which significantly reduced the sintering temperature of cathode. Compared with the LSM-YSZ electrode sintered at 1200°C for 2 h, LSM-BSB electrode exhibits better electrochemical performance. At 800°C, the area specific resistance (ASR) of the LSM-BSB30 electrode is about 0.168 Ωcm2, which is nearly 1.5 times lower than that of LSM-YSZ composite cathode.

Low temperature fabrication of nano-structured porous LSM–YSZ composite cathode film by aerosol deposition

A low temperature (≤500 °C) process for fabricating nano structured porous LSM–YSZ composite films, ∼35 μm in thickness, for use as a cathode in the low temperature solid oxide fuel cell (LT-SOFC) was developed using a ceramic–polymer composite powder and an aerosol deposition (AD) process. Polyvinylidene fluoride (PVDF) polymer powder was mixed with LSM and YSZ ceramic powders, as a pore former to prepare the composite powder for AD. The deposited LSM–YSZ composite ceramic film maintained good adhesion with the YSZ electrolyte and showed >40% porosity with a particle and pore size of 10–100 and 100–200 nm, respectively. The peak power density of the YSZ electrolyte-based, anode-supported-type cell with the nano-structured LSM–YSZ cathode produced by AD was 140 mW/cm 2 at 600 °C.