LSCF-GDC Cathode Research Articles

Electrochemical Performance of Direct Ammonia Solid Oxide Fuel Cell

Recently, as regulations on greenhouse gas emissions have been strengthened in various industries such as automobiles and ships, many studies on alternative energy are being conducted. Hydrogen is a good energy carrier and an environmentally benign fuel because the reaction product is only water or water vapor when it is burned or is used for fuel cells. However, hydrogen suffer from its low specific volume, so the storage issue is still remained unsolved.Ammonia (NH3) has a higher energy density (12.7 MJ/L) than liquid hydrogen (8.5 MJ/L). Liquid hydrogen has to be stored at cryogenic conditions of –253°C, whereas ammonia can be stored at a much less energy-intensive –33°C. In addition, ammonia is much less flammable than hydrogen although it is hazardous to handle.In this study, anode-supported SOFCs consisting of LSCF-GDC cathode, YSZ electrolyte, and Ni/YSZ cermet anode was fabricated and their electrochemical performances (Impedance spectra, J-V curve and cell voltage degradation with operation time) were investigated at 600, 650, and 700°C under ammonia and hydrogen. Regardless of the operating temperature, the power densities of ammonia-fueled SOFCs were almost similar to those of hydrogen-fueled SOFCs. This suggested that the ammonia cracking reaction effectively occurred in the Ni-YSZ anode support. However, the cell voltage at 0.2 A/cm2 was slightly decreased as a function of operating time, which means that the nickel was degraded under ammonia atmosphere. The change of an anode’s microstructure before and after the degradation test will be discussed. In addition, Ni-YSZ catalysts for ammonia cracking were fabricated and their catalytic activity was measured in terms of operating temperature and space velocity using in-line gas chromatography. The phase evolution and microstructure of catalysts were characterized by XRD, SEM and TEM.

Nanostructured Lscf-GDC Cathodes Via a Sol–Gel Method for High Performance Solid Oxide Fuel Cells

Cobaltite-based perovskite materials such as La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF), Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) which have rapid surface activation and mixed ionic and electronic ionic conductivity (MIEC) usually mixed with ionic conducting electrolyte material to lower cathodic polarization resistance. Moreover, decreasing the particulate size of the cathode at nanoscale can effectively enhance the ORR activity of the cathode by extending the electrochemically active sites due to its high surface area. However, due the high surface energy, the nanoparticles easily aggregate before mixing into homogeneous heterophase nanoparticles, resulting in significant coarsening between adjacent particles during high-temperature sintering. In this study, GDC and LSCF-GDC composite powders were synthesized via advance sol-gel method. The physicochemical properties such as sintering behavior, phase stability, etc. of the synthesized nanopowders were investigated. The optimal sintering conditions of GDC nanopowders were evaluated based on the relative density and total conductivity of the sintered GDC pellets. Furthermore, the electrochemical performances of the LSCF-GDC nanocomposite cathodes were measured by the electrochemical impedance spectroscopy (EIS). The microstructural characteristics of the LSCF-GDC nanocomposite cathode were quantified via a 3D reconstruction techniques and related to electrochemical performance of the Ni-YSZ anode supported SOFC single cell.

Influence of practical operating temperature on the Cr poisoning for LSCF-GDC cathode

The influence of operating temperature on Cr poisoning effect on solid oxide fuel cells (SOFCs) cathode was comprehensive studied. The La0.6Sr0.4Co0.2Fe0.8O3-δ-Ce0.90Gd0.10O1.95 (LSCF-GDC) symmetric cells were investigated at 550, 600, 650, 700, 750 °C respectively in the presence of Crofer 22 APU. After aging in the air for 200 h, the cell operated at 650 °C expressed the most serious degradation. At the operating temperature lower than 650 °C, the degradation rate was accelerated with the increased operating temperature. At the operating temperature higher than 650 °C, the degradation rate slowed down with the increased operating temperature. Combined the experimental results with first principles calculation, the mechanism could be put forward that competition existed between the oxygen reduction reaction (ORR) activity and the formation reaction of SrCrO4. The activity of ORR and the formation of SrCrO4 at different operating temperatures codetermined the fluctuating degradation rate of LSCF-GDC cathode. This result could offer a valuable guidance for the rational application of LSCF-GDC cathode in SOFCs.

Electrical and Electrochemical Performances Evaluation of LaNi0.6Fe0.4O3 Cathode Contact and Current Collecting Layer in SOFCs

In this work, the electrical and electrochemical performances of LaNi0.6Fe0.4O3 (LNF) cathode contact and current collecting layer are investigated. The screen-printed LNF thin film on LSCF-GDC cathode effectively improves the performance of anode supported single cell with maximum power density increase by ∼15% and polarization resistance decrement by ∼24%. However, the LNF layer is found to hinder oxygen diffusion under low cathode oxygen partial pressure below ∼0.07 atm. For the application as thick contact layer, an optimized method is developed in combination with alternate ink deposition and drying processes, to provide decent structural stability and interfacial contact. The area specific resistance (ASR) of thick LNF contact layer shows high long-term stability under current load of 300 mA cm−2. The ASR stabilizes at 0.086 Ω·cm2 for more than 1600 h. Moreover, the LNF contact layer operates stably after 7 thermal cycles. The results indicate that, LNF is promisingly applicable as current contact and collecting material in solid oxide fuel cells.

Boosting solid oxide fuel cell performance via electrolyte thickness reduction and cathode infiltration

The Ni–YSZ-supported SOFC with LSCF–GDC cathode yields the maximum power density of 2.38 W cm−2 at 800 °C via electrolyte thickness reduction and cathode infiltration with PrOx, with a resulting decrease in both the ohmic and polarization resistances.

A highly active and Cr-resistant infiltrated cathode for practical solid oxide fuel cells

An SCT conformally coated LSCF–GDC cathode shows significantly improved ORR activity with a strong Cr-tolerance.

Numerical Estimation of Electrochemical Activity of Triple-phase Boundary in LSCF-GDC Cathode for Solid Oxide Fuel Cells

Numerical Estimation of Electrochemical Activity of Triple-phase Boundary in LSCF-GDC Cathode for Solid Oxide Fuel Cells

In Situ Synthesized La0.6Sr0.4Co0.2Fe0.8O3−δ–Gd0.1Ce0.9O1.95 Nanocomposite Cathodes via a Modified Sol–Gel Process for Intermediate Temperature Solid Oxide Fuel Cells

Composite cathodes comprising nanoscale powders are expected to impart with high specific surface area and triple phase boundary (TPB) density, which will lead to better performance. However, uniformly mixing nanosized heterophase powders remains a challenge due to their high surface energy and thus ease with which they agglomerate into their individual phases during the mixing and sintering processes. In this study, we successfully synthesized La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF)–Gd0.1Ce0.9O1.95 (GDC) composite cathode nanoscale powders via an in situ sol–gel process. High-angle annular dark field scanning transmission electron microscopy analysis of in situ prepared LSCF–GDC composite powders revealed that both the LSCF and GDC phases were uniformly distributed with a particle size of ∼90 nm without cation intermixing. The in situ LSCF–GDC cathode sintered on a GDC electrolyte showed a low polarization resistance of 0.044 Ω cm2 at 750 °C. The active TPB density and the specific two phase (LSCF/pore) boundary...

Electrochemical Properties of YSZ/GDC Bi-Layer Produced by One-Side Dip Coating Method for the Intermediate Temperature Solid Oxide Fuel Cell

The Gadolinium Doped Ceria (GDC) diffusion barrier layer was formed on the Yttrium Stabilized Zirconia (YSZ) electrolyte to apply La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) cathode system for lowering the operating temperature of Solid Oxide Fuel Cell. In this study, YSZ and GDC layers were coated by one-side dip coating method which possibly coat one-side of anode substrate. Cu(NO3)2 was infiltrated into pre-sintered substrate of anode and electrolyte as a sintering aid for lowering the co-firing temperature which was enough to produce dense electrolyte layer. LSCF-GDC cathode was coated on the GDC layer by screen printing method. The cells were operated at 600℃ and 650℃. The microstructures of the surface and cross-section were observed by Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS). The ohmic and polarization resistance were measured by the Electrochemical Impedance Spectroscopy (EIS).

Electrochemical Properties of YSZ/GDC Bi-Layer Produced by One-Side Dip Coating Method for the Intermediate Temperature Solid Oxide Fuel Cell

The Gadolinium Doped Ceria(GDC) diffusion barrier layer was formed on the Yttrium Stabilized Zirconia(YSZ) electrolyte to apply La0.6Sr0.4Co0.2Fe0.8O3(LSCF) cathode system for lowering the operating temperature of Solid Oxide Fuel Cell. In this study, YSZ and GDC layers were coated by dip coating method which possibly coat one-side of anode substrate. Cu(NO3)2 was infiltrated into pre-sintered substrate of anode and electrolyte as a sintering aid for lowering the co-firing temperature which was enough to produce dense electrolyte layer. LSCF-GDC cathode was coated on the GDC layer by screen printing method. The cells were operated at 600℃ and 650℃. The microstructures of the surface and cross-section were observed by Scanning Electron Microscopy (SEM). The ohmic and polarization resistance were measured by the Electrochemical Impedance Spectroscopy (EIS). The cell performance was measured by I-V curve observation.

Symmetrical Solid Oxide Electrolyzer Cells (SOECs) with La0.6Sr0.4Co0.2Fe0.8O3 (LSCF)-Gadolinium Doped Ceria (GDC) Composite Electrodes

Scandia (Sc2O3)-stabilized zirconia (ScSZ) electrolyte-supported symmetrical solid oxide electrolyzer cells (SOECs), in which lanthanum strontium cobalt ferrite (LSCF)-gadolinia (Gd2O3)-doped ceria (GDC) composite materials are used as both the cathode and anode, were fabricated and their high temperature steam electrolysis (HTSE) performance was investigated. Current density-voltage curves were obtained for cells operated in 10% H2O/90% Ar at 750, 800, and 850ºC. It was possible to determine the ohmic, cathodic, and anodic contributions to the total overpotential using the three-electrode technique. The HTSE performance was significantly improved in the symmetrical cell with LSCF-GDC electrodes compared to the cell consisting of an Ni-YSZ cathode and LSCF-GDC anode. It was found that the overpotential due to the LSCF-GDC cathode largely decreased and, at a given current density, the total cell voltage decreased, which resulted in the enhanced hydrogen production rate in the symmetrical cell. Key words: HTSE, SOECs, Symmetrical cell, LSCF-GDC, Hydrogen production rate

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.

Precursor solution additives improve desiccated La0.6Sr0.4Co0.8Fe0.2O3–x infiltrated solid oxide fuel cell cathode performance

Here, the addition of the surfactant Triton X-100 or the chelating agent citric acid to Solid Oxide Fuel Cell (SOFC) La0.6Sr0.4Co0.8Fe0.2O3–x (LSCF) precursor nitrate solutions is shown via scanning electron microscopy (SEM) and X-ray diffraction (XRD) to reduce average infiltrate nano-particle size and improve infiltrate phase purity. In addition, the desiccation of LSCF precursor solutions containing the aforementioned organic solution additives further reduces the average LSCF infiltrate nano-particle size and improves the low-temperature infiltrate phase purity. In particular, CaCl2-desiccation reduces the average size of Triton X-100 derived (TXD) LSCF particles fired at 700 °C from 48 to 22 nm, and reduces the average size of citric acid derived LSCF particles fired at 700 °C from 50 to 41 nm. Modeling and electrochemical impedance spectroscopy (EIS) tests indicate that particle size reductions alone are responsible for desiccation-induced cathode performance improvements such as CaCl2-desiccated TXD La0.6Sr0.4Co0.8Fe0.2O3-x – Ce0.9Gd0.1O1.95 (LSCF-GDC) cathodes reaching a polarization resistance of 0.17 Ωcm2 at 540 °C, compared to 600 °C for undesiccated TXD LSCF-GDC cathodes. This excellent low-temperature performance, combined with a low open-circuit 540 °C degradation rate, suggests that the desiccation of organic-additive-containing infiltrate precursor solutions may be useful for the development of durable, high-power, low-temperature SOFCs.

Nano-ceria pre-infiltration improves La0.6Sr0.4Co0.8Fe0.2O3−x infiltrated Solid Oxide Fuel Cell cathode performance

Here, scanning electron microscopy, X-ray diffraction, and thermo-gravimetric analysis experiments show that the pre-infiltration of Ce0.9Gd0.1O1.95 (GDC) nano-particles reduces the average size of La0.6Sr0.4Co0.8Fe0.2O3−x (LSCF) produced from the subsequent infiltration of precursor nitrate solutions containing the surfactant Triton X-100 or the chelating agent citric acid. In contrast, GDC pre-infiltration has no effect on the average size of LSCF particles produced from precursor solutions containing only lanthanum, strontium, cobalt, and iron nitrate. Consistent with the observed particle size trends, electrochemical impedance spectroscopy measurements show that GDC pre-infiltration improves the performance of Triton X-100 Derived (TXD) LSCF–GDC cathodes and Citric Acid Derived (CAD) LSCF–GDC cathodes, but has no effect on the performance of Pure Nitrate Derived (PND) LSCF–GDC cathodes. In particular, TXD LSCF–GDC cathodes with more than ∼5 vol% of GDC pre-infiltration display average LSCF particle sizes of 21 nm and open-circuit polarization resistance values of 0.10 Ωcm2 at 540 °C, compared to 48 nm and 640 °C without GDC pre-infiltration. Results suggest that this 100 °C reduction in cathode operating temperature is caused solely by LSCF particle size reductions. 7.4 vol% GDC pre-infiltrated TXD LSCF–GDC cathodes also display lower 540 °C degradation rates than conventionally infiltrated PND LSCF–GDC cathodes.

Direct Current Distribution Measurement of an Electrolyte-Supported Planar Solid Oxide Fuel Cell under the Rib and Channel by Segmented Electrodes

Solid oxide fuel cells have been of practical concern as a high efficiency power generation device. Problems of the SOFCs include current distribution that decays the total cell performance and efficiency, and causes electrode degradation chemically and thermo-mechanically. In the case of the planar SOFCs, the fuel/oxidant distributions and current collecting resistance cause current and temperature distributions over the electrodes under the separator ribs and flow channels. Optimized design of the separator is hence required to improve the power generation characteristics and durability of practical fuel cell stacks. Although there have been a number of numerical analyses, very few experimental investigations confirming in-situ current distributions to reveal the influence of the separator structure have been reported. The aim of the present study is thus to measure in-plane spatial current variations of an electrolyte-supported planar SOFC with segmented cathodes under the rib and the flow channel. We used the planar cell having three segmented cathodes (Fig. 1) assembled with segmented cathode separators for electrical insulation. The segmented cathodes were placed opposing to a rib and a set of parallel flow channels of the anode separator. The cell was composed of NiO-10Sc1CeSZ anode, 10Sc1CeSZ electrolyte, GDC interlayer, and LSCF-GDC cathode. The electrode area was 1.4 cm2(2.8 x 0.5 cm) each. The anode and cathode separator made of SUS430 had the flow channels with a width of 3 mm, a depth of 1 mm, and a length of 2.8 cm. Silver mesh was employed for the current collection of both sides. Current voltage (I-V) measurements were carried out under voltage control using three electric loads to reproduce the electrode potential of a single cell[1]. The anode and cathode were electrically connected with the four-terminal method. The anode NiO was reduced to Ni by feeding H2/N2 mixture gas for 2 hours prior to measurements. During measurements, anode and cathode were fed upward with mixtures of H2/N2and dried air at constant flow rates, respectively. The cell was maintained at 800 °C by an electric furnace at open circuit voltage (OCV) We prepared two types of the cathode separators. One of them had the same flow channel configuration as the anode separator where the rib and flow channels faces each other. The other had additional flow channels opposing to the anode rib. With the former cathode separator, the overpotential at the electrodes under the anode rib was significantly large compared with the flow channel part, whereas the latter case exhibited smaller overpotentials under the rib. This indicates that oxygen transport becomes poor under the wider rib in the cathode side. The latter case also indicated that the electrode under the rib exhibited smaller overpotential than the flow channel part when fuel is sufficient, owing to the smaller contact resistance under the rib by the compression. We also calculated current and hydrogen/oxygen partial pressure distributions by finite element simulation (COMSOL Multiphysics). The current distributions can be calculated so that they agree with the in-situ current distributions derived by the segmented cathodes from setting the exchange current densities, electrode porosities, electrolyte ion conductivities, and electrode ion/electron conductivities. Thereby oxygen starvation under the wider cathode rib and larger electron conduction resistance for the flow channel part were numerically evaluated to explain the in-situ currents for the segmented cathodes. We will present the results for the case of H2 starvation. [1] Ö. Aydin, T. Koshiyama, H. Nakajima, T. Kitahara, In-situ Diagnosis and Assessment of Longitudinal Current Variation by Electrode-Segmentation Method in Anode-Supported Microtubular Solid Oxide Fuel Cells, J. Power Sources, Vol. 279, 218–223 (2015).Fig. 1 Segmented cathodes on the planar SOFC. Figure 1

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.

Improving La0.6Sr0.4Co0.8Fe0.2O3−δ infiltrated solid oxide fuel cell cathode performance through precursor solution desiccation

Here, for the first time, the average size of solid oxide fuel cell (SOFC) electrode nano-particles was reduced through the chemical desiccation of infiltrated precursor nitrate solutions. Specifically, after firing at 700 °C, CaCl2-desiccated La0.6Sr0.4Co0.8Fe0.2O3−δ (LSCF) – Ce0.9Gd0.1O1.95 (GDC) cathodes contained LSCF infiltrate particles with an average size of 22 nm. This is in contrast to comparable, undesiccated LSCF-GDC cathodes which contained LSCF infiltrate particles with an average size of 48 nm. X-ray diffraction, scanning electron microscopy, and controlled atmosphere electrochemical impedance spectroscopy revealed that desiccation reduced the average infiltrate particle size without altering the infiltrate phase purity, the cathode concentration polarization resistance, or the cathode electronic resistance. Compared to undesiccated LSCF-GDC cathodes achieving polarization resistances of 0.10 Ωcm2 at 640 °C, comparable CaCl2-dessicated LSCF-GDC cathodes achieved 0.10 Ωcm2 at 575 °C. Mathematical modeling suggested that these performance improvements resulted solely from average infiltrate particle size reductions.

Performance stability and degradation mechanism of La0.6Sr0.4Co0.2Fe0.8O3−δ cathodes under solid oxide fuel cells operation conditions

The performance stability and degradation mechanism of La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) cathodes and LSCF impregnated Gd0.1Ce0.9O2−δ (LSCF-GDC) cathodes are investigated under solid oxide fuel cell operation conditions. LSCF and LSCF-GDC cathodes show initially performance improvement but degrade under cathodic polarization treatment at 750 °C for 120 h. The results confirm the grain growth and agglomeration of LSCF and in particular GDC-LSCF cathodes as well as the formation of SrCoOx particles on the surface of LSCF under cathodic polarization conditions. The direct observation of SrCoOx formation has been made possible on the surface of dense LSCF electrode plate on GDC electrolyte. The formation of SrCoOx is most likely due to the interaction between the segregated Sr and Co from LSCF lattice under polarization conditions. The formation of SrCoOx would contribute to the deterioration of the electrocatalytic activity of the LSCF-based electrodes for the O2 reduction in addition to the agglomeration and microstructure coarsening.

Electrochemical characteristics of electrospun La0.6Sr0.4Co0.2Fe0.8O3−δ-Gd0.1Ce0.9O1.95 cathode

The poor activity of cathode materials for electrochemical reduction of oxygen in intermediate and low temperature regime (<700°C) is a key obstacle to reduced-temperature operation of solid oxide fuel cells (SOFCs). In our previous work, the direct methane fuel cell exhibits approximately 1Wcm−2 at 650°C in hydrogen atmosphere without any functional layers when the electrospun LSCF–GDC cathode was applied into the La2Sn2O7–Ni–GDC anode-supported cell, which is approximately two times higher performance than 0.45Wcm−2 of the cell with the conventional LSCF–GDC cathode. For detailed analysis of the fibrous cathode, the symmetrical cells with the electrospun and conventional LSCF–GDC cathode are fabricated, and then their electrochemical characteristics are measured by using electrochemical impedance spectroscopy (EIS). Each resistance contribution is determined by equivalent circuit consisting of a series resistance (Rs) and three arcs to describe the polarization resistance of the cathode. Total polarization resistance of the electrospun LSCF–GDC cathode is approximately two times lower than that of the conventional LSCF–GDC cathode at 650°C, which is attributed to fibrous microstructures and large amount of pores in 100–200nm. The results correspond to the difference in the cell performances obtained from our previous work.

Atomic Layer Deposition Functionalized Composite SOFC Cathode La0.6Sr0.4Fe0.8Co0.2O3-δ -Gd0.2Ce0.8O1.9: Enhanced Long-Term Stability

We report that the long-term stability of a conventional mixed oxide-ion and electron conducting solid oxide fuel cell cathode, La0.6Sr0.4Fe0.8Co0.2O3−δ-Gd0.2Ce0.8O1.9 (LSCF-GDC) composite, can be markedly improved by functionalizing its surfaces with a conformal layer of nanoscale ZrO2 films through atomic layer deposition (ALD). Over a >1100 h testing at 800 °C, the overcoated LSCF-GDC cathode exhibited respective polarization and ohmic area-specific-resistances 3 and 1.5 times lower than the pristine sample, whereas the pristine LSCF-GDC cathode degraded at a rate 4 times faster than the overcoated one. The multifunctionality of porosity, mixed conductivity, and suppressed Sr-enrichment enabled by the nanoscaled ALD-ZrO2 overcoats are attributed to the performance retention observed for the overcoated cathode.