LSCF-GDC Composite Cathodes Research Articles

Fabrication of Durable and High-Performance Flat Tubular Anode-Supported Solid Oxide Cells for Stack Application

Solid oxide cell (SOC) technology is highly admissible for distributed power generation since it enables the most efficient and clean conversion of fuels into energy and heat. SOCs are also regarded as a potential technology for their decisive role in strengthening the hydrogen economy concept by storing renewable energy resources such as green hydrogen and synthetic fuels. In this work, high-performance and durable flat tubular anode-supported SOC is developed for SOC stack applications. NiO-8YSZ composite flat tubular anode support is fabricated by employing an extrusion process. Anode functional layer and electrolyte layers are deposited by the dip-coating technique, whereas the GDC diffusion barrier, LSCF-GDC composite cathode, and Ag-glass composite interconnect layers are fabricated by the conventional screen-printing process. Finally, the prepared flat tubular SOCs possess an active cathode area of 45 cm2 and demonstrated a high-power density of ~430 mW cm-2 at operating current density, voltage, and temperature conditions of 501 mA cm-2, 0.87 V, and 700 °C. During a long-term stability test at 700 °C for 1000 h, the SOC showed a voltage improvement of 0.013 % while operating at a fuel efficiency of 65%. The flat tubular anode-supported SOC’s high performance and durable operation showed its potential for commercial applications in distributed power generation and hydrogen production. Together with the research to further enhance the SOC’s performance and stability, the flat tubular SOC stack design and fabrication are also in progress.

Thermo-Electro-Chemo-Mechanical Coupled Modeling of Solid Oxide Fuel Cell with LSCF-GDC Composite Cathode

Intricate relationships between transport phenomena, reaction mechanisms, and mechanical aspects likely affect the durability of solid oxide fuel cell (SOFC) stack. This study presents a modeling framework that combines thermo-electro-chemo models (including the methanol conversion process and the electrochemical reactions of the carbon monoxide as well as the hydrogen) and a contact thermo-mechanical model that considers the effective mechanical properties of composite electrode material. Detailed parametric studies are performed focusing on the inlet fuel species (hydrogen, methanol syngas) and flow arrangements (co-flow, counter-flow) under typical operating conditions (operating voltage 0.7 V), and performance indicators of the cell, such as the high-temperature zone, current density, and maximum thermal stress were discussed for parameter optimization. The simulated results show that the high temperature zone of the hydrogen-fueled SOFC is located at the central part of units 5, 6, and 7, and the maximum value is about 40 K higher than that of methanol syngas-fueled SOFC. The charge transfer reactions can occur throughout the cathode layer. The counter-flow improves the trend of the current density distribution of hydrogen-fueled SOFC, while the effect on the current density distribution of methanol syngas-fueled SOFC is small. The distribution characteristics of the stress field within SOFC are extremely complex, and the inhomogeneity of the stress field distribution can be effectively improved by feeding methanol syngas. The counter-flow improves the stress distribution state of the electrolyte layer of methanol syngas-fueled SOFC, and the maximum tensile stress value is reduced by about 37.7%.

Restrictions of nitric oxide electrocatalytic decomposition over perovskite cathode in presence of oxygen: Oxygen surface exchange and diffusion

Nitric oxide (NO) abatement from engine exhaust is of great significance to alleviate air pollution and haze. Compared with the traditional selective catalytic reduction (SCR) technology, electrocatalytic decomposition of NO simplifies the reductant supply system and therefore avoids secondary pollution. In this study, typical perovskite La0.6Sr0.4CoxFe1-xO3-δ (LSCF) infiltrated by different dosages of nano ceria Ce0.9Gd0.1O1.9 (GDC) was used as composite cathodes, in order to explore the critical factors to restrain NO conversion in excess of O2. The results show that electron as reactive species transfers among NO, ABO3-type cathode and oxygen vacancy. The maximum of NO removal efficiency can reach 96.27 % in absence of O2 and up to 80.55 % in presence of 1% O2 in case of LSCF infiltrated by moderate dosages LSCF-GDC(2), which is superior to those of LSCF, LSCF-GDC(4) and LSM-GDC(nano) composite cathode. Compared to oxygen storage capacity (OSC) caused by the infiltration of nano ceria, higher surface oxygen exchange coefficient (kδ) and chemical diffusion coefficient (Dchem) lead to the significant decrease in polarization resistance (Rp), and consequently to the enhancement of NO removal in presence of O2. No matter what kind of oxygen deriving from oxygen reduction reaction (ORR) and NO reduction reaction (NORR), GDC infiltration into LSCF improves oxygen transport property and however, the property of cathode in ORR is dominant over in NORR in presence of O2. Moderate GDC loading has the highest oxygen transport kinetics, and oxygen surface exchange is faster than chemical diffusion, due to lower activation energy. Over loading of GDC with greater ohmic resistance (Rs) inversely influences the NO removal.

A detailed kinetic model for the reduction of oxygen on LSCF-GDC composite cathodes

A kinetic investigation of the Oxygen Reduction Reaction (ORR) is performed on LSCF-GDC composite cathodes (La0.4Sr0.6Co0.2Fe0.8O3-δ/Ce0.9Gd0.1O2-δ 50/50) spanning a wide range of operating conditions. EIS tests are carried out on symmetric cells between 700 °C and 560 °C at OCV, with O2/N2 mixtures at varying O2 molar fraction (5–21%). A dynamic, one-dimensional, physic model of the LSCF-GDC electrode is applied to rationalize the experimental results. The model simulates the spectra by solving mass and charge conservation equations, including terms for gas diffusion in the porous electrode and solid state transport in both the LSCF and the GDC lattice. A thermodynamically consistent, detailed kinetic scheme is applied to describe the ORR mechanism, which takes into account elementary steps of adsorption and desorption, first and second electronation at the gas/electrode interface, interfacial and lattice ion transfer. A full set of rate parameters (pre-exponential factors and activation energies) is derived by fitting to inhouse-measured impedance data, and validated against a well-established literature dataset. The sensitivity analysis supports the prevailing role of the TPB route over the 2PB route, and highlights that the transfer of a single-charged oxygen adatom from the LSCF surface to the GDC lattice governs the ORR. The model clarifies the origin of distortions in measured impedance arcs, and captures the effect of O2 pressure on the observed electrochemical activity.

Effect of Microstructure on Electrochemical Performance of Nano-Structured La0.8Sr0.2Co0.2Fe0.8O3-δ-Gd0.2Ce0.8O1.9 Composite Cathodes

In this article, the effect of microstructure on the electrochemical performance of LSCF-GDC composite cathode is mainly studied. LSCF-GDC composite cathodes with different morphologies are prepared via electrospinning and infiltration. The LSCF-GDC composite cathodes with nanoparticle structure achieve their best performance at the LSCF/GDC mass ratio of 1:0.18, and the corresponding polarization resistance(Rp) is 0.12 Ω cm2 at 750 °C. Similar, 200nm and 280nm nanofiber-structured LSCF-GDC composite cathodes achieve their optimum performance at LSCF/GDC mass ratios of 1:0.30 and 1:0.76, respectively, the corresponding Rp are 0.07 Ω cm2 and 0.056 Ω cm2 at 750 °C. On this basis, carbon fibers are used as pore-former to further increase the porosity of composite cathodes. After pore formation, the optimum LSCF/GDC mass ratio of 280nm nanofiber-structured composite cathode reaches to 1:1.01, and the corresponding Rp decreases to 0.045 Ω cm2. Arrhenius formula shows that the 280nm nanofiber-structured composite cathodes after pore formation gains the lowest activation energy of 1.27 eV.

Effect of H2O and CO2 on LSCF-GDC Composite Cathodes

Polarization losses associated with the cathode oxygen reduction reaction (ORR) and degradation of cathode materials remain as hurdles for widespread implementation of solid oxide fuel cells (SOFC). Rates of degradation depend significantly on the operating temperature and gas conditions, such as the presence of unwanted oxygen-containing compounds, namely H2O and CO2. In this study, we explore H2O and CO2 degradation mechanisms for a common composite cathode material, La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) - Ce0.90Gd0.10O1.95 (GDC) using electrochemical impedance spectroscopy (EIS) under various temperatures, oxygen partial pressure, and contaminant conditions to determine the effect of H2O and CO2 on LSCF-GDC. Our results suggest that CO2 and H2O compete with the ORR, and a strong correlation between blocking, degradation, and sintering effects caused by CO2 and H2O is directly observed. The interactions of CO2 and H2O with the cathode surface changes as a function of operating temperature, resulting in different short-term or long-term degradation mechanisms.

Inkjet Printing of Solid Oxide Fuel Cells and Proton Ceramic Fuel Cells

This paper reports successful synthesis of high performance components for solid oxide fuel cells (SOFCs) and proton ceramic fuel cells (PCFCs) by inkjet printing. The tested materials include La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) and Gd0.1Ce0.9O2-δ (GDC) for SOFCs and the PrBa0.5Sr0.5Co1.5Fe0.5O5+δ (PBSCF) for PCFCs. The ink sources are synthesized by solving the raw particles in an ethanol-water solution satisfying the fluidic conditions desired for use in a commercial HP inkjet printer. The inkjet-layers are adopted in anode-support type ceramic fuel cells. As a result, SOFCs with the inkjet-printed LSCF cathode has exhibited power of about 270 mW/cm2 at 650ºC while the power is enhanced to 330 mW/cm2 with the inkjet-printed LSCF-GDC composite cathode. The PCFCs with the inkjet-printed PBSCF cathode have demonstrated power of about 720mW/cm2 at 650ºC.

Numerical simulation of La0.6Sr0.4Co0.2Fe0.8O3- Gd0.1Ce0.9O1.95 composite cathodes with micro pillars

In the current study, electrochemical performances of La0.6Sr0.4Co0.2Fe0.8O3-Gd0.1Ce0.9O1.95 (LSCF-GDC) composite cathode microstructures are numerically simulated in order to clarify the effects of GDC pillar which is introduced to enhance the effective ionic conductivity of the solid oxide fuel cell (SOFC) cathode. The numerical simulation is carried out based on the three-dimensional microstructure reconstructed with focused ion beam scanning electron microscopy (FIB-SEM). In order to further investigate the characteristics of the GDC pillars, we examine the electrochemical effects of GDC pillars inside the pure LSCF and LSCF-GDC composite cathode microstructures with different particle sizes. According to the simulation results, the GDC pillars effectively improve the performance of the pure LSCF cathode, and the improvements by the GDC pillars are more pronounced in the microstructures with small particle sizes.

Fabrication of Lanthanum Strontium Cobalt Ferrite-Gadolinium-Doped Ceria Composite Cathodes Using a Low-Price Inkjet Printer.

In this work, we have successfully fabricated lanthanum strontium cobalt ferrite (LSCF)-gadolinium-doped ceria (GDC) composite cathodes by inkjet printing and demonstrated their functioning in solid oxide fuel cells (SOFCs). The cathodes are printed using a low-cost HP inkjet printer, and the LSCF and GDC source inks are synthesized with fluidic properties optimum for inkjet printing. The composition and microstructure of the LSCF and GDC layers are successfully controlled by controlling the color level in the printed images and the number of printing cycles, respectively. Anode-support type SOFCs with optimized LSCF-GDC composite cathodes synthesized by our inkjet printing method have achieved a power output of over 570 mW cm-2 at 650 °C, which is comparable to the performance of a commercial SOFC stack. Electrochemical impedance analysis is carried out to establish a relationship between the cell performance and the compositional and structural characteristics of the printed LSCF-GDC composite cathodes.

Numerical Simulation of LSCF-GDC Composite Cathodes with Various Microstructures

In the present study, the electrochemical performances of different LSCF-GDC microstructures are numerically calculated in order to investigate the effects of GDC particles and GDC pillars on LSCF cathode microstructures with various particle sizes. Lattice Boltzmann method (LBM) is applied for solving the governing equations. It is found from the numerical results of pure GDC and LSCF-GDC composite cathodes with various particle sizes that GDC addition improves LSCF cathode electrochemical performance. This performance improvement is more pronounced for cathodes with smaller particle sizes and when operated at lower temperatures. In addition, it is shown that LSCF cathode with GDC pillars can enhance the performance of the cathode. The GDC pillars are even more effective when the reactive region of the cathode becomes thin by using small size powders.

Investigation of Electrochemical Reaction Mechanism with LSC-GDC Composite Cathode Based on 3D Reconstructed Microstructures

In the present study, electrochemical reaction mechanisms of La0.6Sr0.4CoO3 (LSC)-Gd0.1Ce0.9O1.95 (GDC) composite cathodes were investigated by a Lattice Boltzmann Method (LBM) based on 3D reconstructed cathode microstructures with different volume ratios. In the simulation, the exchange current density for the surface reaction was fitted using an experimental data of pure LSC cathode. On the other hand, the exchange current density for the TPB reaction was fitted using composite cathodes. Overpotentials and potential distributions were quantified. Then, the results were compared with those of LSCF-GDC composites reported in our previous work (Y.T. Kim, Z. Jiao, N. Shikazono, J. Power Sources, 342, pp. 787-795, 2017). LSC surface area density was 1.4 μm2/μm3, which was smaller than the LSCF cathode. Compared with the LSCF-GDC composite cathode, it is considered that TPB reaction is more contributed for the LSC-GDC composite cathodes.

Numerical Simulation of LSCF-GDC Composite Cathodes with Various Microstructures

In the present study, electrochemical performances of different LSCF-GDC microstructures are numerically simulated in order to investigate the effects of GDC particle sizes and GDC pillars on LSCF cathode performance. Lattice Boltzmann method (LBM) is applied for solving the governing equations. It is found from the numerical results of pure GDC and LSCF-GDC composite cathodes with various particle sizes that the performance improvement by GDC addition is more pronounced for cathodes with smaller particle sizes. In addition, it is shown that LSCF cathode with GDC pillars can enhance the performance of the cathode especially when the reactive region of the cathode becomes thin by using small size powders.

Effects of impregnating palladium on catalytic performance of LSCF-GDC composite cathodes for intermediate temperature solid oxide fuel cells

The effects of palladium introduction on the catalytic activity of LSCF composite cathodes are investigated by comparing the performance of LSCF-GDC and Pd-LSCF-GDC cathodes. Pd-LSCF-GDC cathodes were prepared by impregnation of Pd and LSCF precursor into GDC scaffold separately in two steps. The electrochemical results show that the introduction of Pd promotes the catalytic activity of LSCF-GDC cathodes under the condition of both heating and cooling processes. The promotion is related to the existing state of Pd with LSCF under different conditions. According to in-situ high temperature XRD and XPS results, the PdO phase is stable at temperature below 700 °C, and decomposed to Pd metal at 700–800 °C during heating process. Combination XRD with TG-DSC analysis confirms that some Pd changes into PdO phase at 837 °C. Correspondingly, the phase composition changed little and the coexistence of Pd and tiny PdO is stable in the cooling process from 850 to 25 °C.

Long-Term Cr Poisoning Effect on LSCF-GDC Composite Cathodes Sintered at Different Temperatures

The impact of sintering temperature on Cr-poisoning of solid oxide fuel cell (SOFC) cathodes was systematically studied. La0.6Sr0.4Fe0.8Co0.2O3-δ - Ce0.9Gd0.1O2-δ symmetric cells were aged at 750°C in synthetic air with the presence of Crofer 22 APU, a common high temperature interconnect, over 200 hours and electrochemical impedance spectroscopy (EIS) was used to determine the degradation process. Both the ohmic resistance (RΩ) and polarization resistance (RP) of LSCF-GDC cells, extracted from EIS spectra, for different sintering temperatures increase as a function of aging time. Furthermore, the Cr-related degradation rate increases with decreased cathode sintering temperature. The polarization resistance of cathode sintered at lower temperature (950°C) increases dramatically while aging with the presence of Cr and also significantly decreases the oxygen partial pressure dependence after aging. The degradation rate shows a positive correlation to the concentration of Cr. The results indicate that decreased sintering temperature increases the total surface area, leading to more available sites for Sr-Cr-O nucleation and thus greater Cr degradation.

Microstructure and Polarization characteristics of LSCF-GDC Composite Cathode with Different Volume Fractions

In the present study, La0.6Sr0.4Co0.2Fe0.8O3(LSCF)-Gd0.1Ce0.9O1.95(GDC) composite cathodes with different volume fractions were fabricated by the screen printing method, and their polarization characteristic and microstructure parameters are evaluated. Cathode overpotentials were evaluated at 700 oC and 100% O2. Microstructures were reconstructed by focused ion beam-scanning electron microscope (FIB-SEM). In addition, current and potential distributions inside the reconstructed microstructures were calculated by a Lattice Boltzmann method. The relationship between polarization characteristics and microstructure is quantitatively investigated.

Microstructure and Polarization characteristics of LSCF-GDC Composite Cathode with Different Volume Fractions

In the present study, La0.6Sr0.4Co0.2Fe0.8O3 (LSCF)-Gd0.1Ce0.9O1.95 (GDC) composite cathodes with different volume fractions were fabricated by the screen printing method, and their polarization characteristics and microstructure parameters are evaluated. Cathode overpotentials were measured at the measurement condition of 700 °C and 100 % O2. Microstructures were reconstructed by focused ion beam-scanning electron microscope (FIB-SEM). The relationship between polarization characteristics and microstructure is quantitatively investigated.

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.

Stability of nanorod-structured La0.8Sr0.2Co0.2Fe0.8O3−δ–Gd0.2Ce0.8O1.9 composite cathodes for intermediate temperature solid oxide fuel cells

In this paper, the stability of the nanorod-structured La0.8Sr0.2Co0.2Fe0.8O3−δ (LSCF)–Gd0.2Ce0.8O1.9 (GDC) composites prepared with combined methods of electrospinning and infiltration has been investigated. For comparison, conventional nanoparticle-structured LSCF–GDC composite cathodes were also developed. Electrochemical impedance spectra of these LSCF–GDC composite cathodes at open circuit potentials were measured using an electrolyte-supported LSCF–GDC/GDC/LSCF–GDC symmetric cell configuration for stability evaluation. The results show that the conventional LSCF–GDC composite cathode has poor stability, with its polarization resistance increasing from 0.80 to 1.37Ωcm2 after being polarized under a current density of 100mAcm−2 for 120h at 650°C. This degradation is attributed to the agglomeration of cathode materials and to poor contact between the cathode and the electrolyte. In contrast, the nanorod-structured LSCF–GDC composite cathode exhibits an excellent performance stability, since its polarization resistance decreases from 0.097 to 0.077Ωcm2 after polarization for 144h under a current density as high as 300mAcm−2. In addition, a nearly constant polarization resistance is maintained during a 20-cycle thermo-cyclic test. The outstanding stability of the nanorod-structured LSCF–GDC composite cathode is ascribed to the stability of its microstructure with continuous electronic and ionic conduction pathways and high porosity, this microstructure is realized through the suppression of LSCF agglomeration by the compact GDC layers. These results demonstrate that the nanorod-structured LSCF–GDC composite is a reliable material for use as intermediate temperature solid oxide fuel cell cathode with a high performance.

Effects of cathode materials on the characteristics of electrolyte supported micro-tubular solid oxide fuel cells

The effects of the GDC–LSCF (Ce0.8Gd0.2O0−δ–La0.6Sr0.4Co0.2Fe0.8O3−δ) cathode layer and the GDC interlayer on the electrochemical performance of the ScSZ (Zr0.8Sc0.2O2−δ) electrolyte supported (≈270 μm) micro-tubular SOFC cells are investigated in this study. Material formulation and sintering profile for fabricating the micro-tubular SOFC cells are developed to avoid physical defects caused by the large sintering shrinkage mismatch among the layers. Cell B (with the LSCF–GDC composite cathode layer and the GDC interlayer) reports an ohmic resistance slightly higher than that of Cell A (with the GDC–La0.8Sr0.2MnO3−δ, i.e. LSM, composite cathode), while its polarization resistance emerges to be significantly smaller than that of Cell A. In terms of cell performance, Cell B demonstrates an OCV value (>1.07 V) similar to that of Cell A and a maximum power density (0.26 W cm−2) 44.4% greater than that of Cell A (0.17 W cm−2) at 850 °C. It can thus be concluded that using the LSCF–GDC composite-cathode layer and inserting the GDC interlayer help reduce the total cell impedance, thereby improving the power density of the tubular cells.

A stability study of impregnated LSCF–GDC composite cathodes of solid oxide fuel cells

Abstract The performance degradation of composite cathodes of La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3− δ and Gd-doped ceria (LSCF–GDC), prepared by impregnating the porous GDC scaffold with a nitrate solution containing La, Sr, Co and Fe in desired composition, is investigated at 750 °C and open circuit in air for 500 h. The performance of the impregnated LSCF–GDC composite cathodes deteriorates after testing at 750 °C for 500 h; the electrode polarization resistance ( R p ) increases from 0.38 to 0.83 Ω cm 2 , and the electrode ohmic resistance ( R o ) increases from 1.79 to 2.14 Ω cm 2 . The grain growth and coarsening of impregnated LSCF nanoparticles are responsible for the performance degradation of the cathodes. XPS analysis shows the enrichment of cobalt on the surface of the infiltrated LSCF–GDC cathodes and such surface segregation could also contribute to the degradation of the electrocatalytic activity of the cathodes. Introducing MgO and LaNi 0.6 Fe 0.4 O 3 phases can effectively suppress the coarsening of LSCF nanoparticles and enhance the stability of the cathodes. However, the enhancing effect is related to the conductivity and electrocatalytic activity of the introduced phases.