Mixed Ionic And Electronic Conducting Research Articles

La0.6Sr0.4Co0.2Fe0.8O3-δ nanoparticles modified Ni-based anode for direct methane-fueled SOFCs

Direct utilization of methane in solid oxide fuel cells (SOFCs) is highly desirable, due to the merits of cost, simplicity and energy saving. However, the most commonly used Ni/YSZ anode suffers from carbon deposit issue under CH4. Here, a novel composite anode is developed to solve the problem, where La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) perovskite-type mixed ionic and electronic conductor (MIEC) is used to modified the anode by infiltration method. Simultaneously, LSCF/YSZ nano-structure cathode is obtained using infiltration method as well, which remarkably simplified the preparation process and is in favor of the practical applications. High electrochemical activity is demonstrated for this cell with a peak power density of 705 mWcm−2 under H2 and 486 mWcm-2 under CH4 at 800 °C. Durability test suggests the stability of cell in CH4 is dramatically improved, and it operates for 100h with little degradation under 0.3 Acm−2 and 750 °C. Results show that LSCF NPs, which are partially decomposed to (Sr,La)2(Co,Fe)O4 with CoFe alloy phase and disperse on the anode as a very thin layer in fuel atmosphere, dramatically enhance the coking resistance of anode. The work delivers an efficient way for direct CH4 fueled SOFC.

A Nano-Structured SOFC Composite Cathode Prepared via Infiltration of La0.5Ba0.25Sr0.25Co0.8Fe0.2O3-δ into La0.9Sr0.1Ga0.8Mg0.2O3-δ for Extended Triple-Phase Boundary Area

Mixed ionic and electronic conductors (MIECs) show greatly enhanced electrochemical properties as cathode materials for intermediate-temperature solid oxide fuel cells (IT-SOFCs). Among promising MIEC candidates, La0.5Ba0.25Sr0.25Co0.8Fe0.2O3-δ (LBSCF) is chosen as a cathode material in this study due to its high oxygen ion diffusivity and oxygen reduction reaction (ORR) activity. It is well known that the composite configuration with superior oxygen ion conductor, e.g., La0.9Sr0.1Ga0.8Mg0.2O3-δ (LSGM), results in the improvement of cell performance. Here, we successfully fabricate the LBSCF-LSGM composite cathode by infiltration method at a low sintering temperature (< 800°C), and thus nanoparticles are uniformly distributed on the LSGM porous scaffold. A half-cell exhibits an extremely low area-specific resistances (ASRs) of 0.010, 0.020, and 0.049 Ω cm2 at 700, 650, and 600°C, respectively. Further, the peak power density about 0.9 W cm−2 is attained at 700°C, suggesting that infiltrated LBSCF-LSGM composite demonstrates sufficient ORR activities at an intermediate temperature and thus to be a highly promising cathode material for IT-SOFCs.

Preparation and Testing of Metal/Ce0.80Gd0.20O1.90 (Metal: Ni, Pd, Pt, Rh, Ru) Co-Impregnated La0.20Sr0.25Ca0.45TiO3 Anode Microstructures for Solid Oxide Fuel Cells

La0.20Sr0.25Ca0.45TiO3 (LSCTA-) is a novel mixed ionic and electronic conductor (MIEC) material which can act as a potential replacement Solid Oxide Fuel Cell (SOFC) anode ‘backbone’ microstructure, for the current state-of-the-art Ni-based cermet. By impregnating this ‘backbone’ with electrocatalytically active coatings of metal oxides and metallic particles, it is possible to create high performance SOFC anodes which offer improved redox stability and tolerance to non-optimal fuel gases. Here, we present short-term test data for SOFC containing LSCTA- anode ‘backbones’ impregnated with a variety of catalyst systems including: Ni/CGO, Pd/CGO, Pt/CGO, Rh/CGO and Ru/CGO. Electrolyte-supported SOFC containing Ni/CGO impregnated anodes showed large reductions in Area Specific Resistance (ASR), in comparison to previous generation research (0.55 Ω cm2 versus 1.2 Ω cm2, respectively). Exchange of the Ni component, for Pd and Rh, led to much lower ASR of 0.39 Ω cm2 and 0.41 Ω cm2 (in 97% H2:3% H2O, at 900°C and 0.8 V), respectively. Equivalent circuit fitting of AC impedance spectra revealed the absence of an anode charge transfer process for the Rh/CGO catalyst system above 875°C, in comparison to all other systems, identifying this system as a potential replacement for the Ni-based cermet.

In Situ Impedance Analysis of Oxygen Exchange on Growing La0.6Sr0.4CoO3-δ Thin Films.

The further development of solid oxide fuel and electrolysis cells (SOFC/SOEC) strongly relies on research activities dealing with electrode materials. Recent studies showed that under operating conditions many perovskite-type oxide electrodes are prone to changes of their surface composition, leading to severe changes of their electrochemical performance. This results in a large scatter of data in literature and complicates comparison of materials. Moreover, little information is available on the potentially excellent properties of surfaces immediately after preparation, that is, before any degradation by exposure to other gas compositions or temperature changes. Here, we introduce in situ impedance spectroscopy during pulsed laser deposition (IPLD) as a new method for electrochemical analysis of mixed ionic and electronic conducting (MIEC) thin films during growth. First, this approach can truly reveal the properties of as-prepared MIEC electrode materials, since it avoids any alterations of their surface between preparation and investigation. Second, the measurements during growth give information on the thickness dependence of film properties. This technique is applied to La0.6Sr0.4CoO3−δ (LSC), one of the most promising SOFC/SOEC oxygen electrode material. From the earliest stages of LSC film deposition on yttria-stabilized zirconia (YSZ) to a fully grown thin film of 100 nm thickness, data are gained on the oxygen exchange kinetics and the defect chemistry of LSC. A remarkable reproducibility is found in repeated film growth experiments, not only for the bulk related chemical capacitance but also for the surface related polarization resistance (±10%). Polarization resistances of as-prepared LSC films are extraordinarily low (2.0 Ω cm2 in 40 μbar O2 at 600 °C). LSC films on YSZ and on La0.95Sr0.05Ga0.95Mg0.05O3−δ (LSGM) single crystals exhibit significantly different electrochemical properties, possibly associated with the tensile strain of LSC on LSGM.

Fabrication and characterization of microtubular solid oxide cell supported with nanostructured mixed conducting perovskite fuel electrode

Mixed ionic and electronic conducting (MIEC) perovskites demonstrate advantages over Ni-cermet as fuel electrode materials for solid oxide cells (SOCs). However, SOCs are primarily electrolyte-supported planar designs in literature when MIEC perovskite fuel electrodes are employed, which are relatively easy to fabricate but usually have high electrolyte ohmic resistance. Perovskite fuel electrode-supported designs are rarely studied particularly for microtubular SOCs. In this research, (La0.3Sr0.7)0.9Ti0.9Ni0.1O3-δ-Sm0.2Ce0.8O1.9 (LSTN-SDC) electrode-supported microtubular cell LSTN-SDC/YSZ/(La0.8Sr0.2)0.95MnO3-δ is fabricated and characterized. The LSTN-SDC microtubular substrate is prepared using an in-house built spinneret extrusion system in combination with modified phase inversion method, featuring radially well-aligned microchannels open at the inner surface. A thin YSZ electrolyte of ~15 μm and (La0.8Sr0.2)0.95MnO3-δ electrode of ~25 μm are then fabricated on the substrate, respectively. Upon reducing treatment, nickel is exsolved from LSTN grains and uniformly decorated onto grain surface as Ni nanoparticles, and therefore on inner surface of microchannels in the substrate. With CO/CO2 gas mixture as the fuel, the complicated electro-/chemical reactions are identified in the substrate electrode. The electrolysis process in combination with surface catalytic process of nanostructured electrode substrate leads to highly efficient CO production from CO2 with conversion efficiencies of well above 100%. The electrolysis also facilitates to regenerate surface catalytic functionality of nanostructured electrode substrate. The redox stability advantages of the cell are demonstrated in both alternative reduction (CO)/oxidation (air) atmospheric conditions and reversible operating mode.

Suppressed Sr segregation and performance of directly assembled La0.6Sr0.4Co0.2Fe0.8O3-δ oxygen electrode on Y2O3-ZrO2 electrolyte of solid oxide electrolysis cells

Active and stable oxygen electrode is probably the most important in the development of solid oxide electrolysis cells (SOECs) technologies. Herein, we report the successful development of mixed ionic and electronic conducting (MIEC) La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) perovskite oxides directly assembled on barrier-layer-free yttria-stabilized zirconia (YSZ) electrolyte as highly active and stable oxygen electrodes of SOECs. Electrolysis polarization effectively induces the formation of electrode/electrolyte interface, similar to that observed under solid oxide fuel cell (SOFC) operation conditions. However, in contrast to the significant performance decay under SOFC operation conditions, the cell with directly assembled LSCF oxygen electrodes shows excellent stability, tested for 300 h at 0.5 A cm−2 and 750 °C under SOEC operation conditions. Detailed microstructure and phase analysis reveal that Sr segregation is inevitable for LSCF electrode, but anodic polarization substantially suppresses Sr segregation and migration to the electrode/electrolyte interface, leading to the formation of stable and efficient electrode/electrolyte interface for water and CO2 electrolysis under SOECs operation conditions. The present study demonstrates the feasibility of using directly assembled MIEC cobaltite based oxygen electrodes on barrier-layer-free YSZ electrolyte of SOECs.

Crystal structure, thermal expansion and long-term behaviors of SmBaCoO5+δ as cathode for intermediate-temperature solid oxide fuel cells

SmBaCo2O5+δ (SBC) was studied as cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The crystal structure, thermal expansion behavior, and electrochemical performance with long-term operation of SBC were characterized. An orthorhombic layered perovskite structure was observed in SBC cathode by a GSAS program for refinement. The average thermal expansion coefficient (TEC) is 21.6 x 10-6K-1 in the temperature range of 100oC-800oC. For long-term testing, the polarization resistance of SBC cathode increases gradually from 25.77 Ω cm2 for 2 h to 38.77 Ω cm2 for 96 h at 600°C, and an increasing-rate for polarization resistance is around 13,8% h-1. Based on the electrochemical properties, SBC cathode with mixed ionic and electronic conductor (MIEC) behavior is a potential cathode for intermediate temperature solid oxide fuel cells based on a SDC electrolyte.

Structural and chemical stability of high performance Ce0.8Gd0.2O2-δ – FeCo2O4 dual phase oxygen transport membranes

Ceramic oxide membranes are widely being researched for Carbon Capture and Storage/Utilization sector applications. Foreseen applications of these membranes are oxygen generation for oxyfuel combustion in e.g. power plants, glass-, cement- or steel production. Major drawback with Mixed Ionic and Electronic Conducting (MIEC) perovskite structure membranes is their limited long term stability at high temperatures in aggressive atmospheres. Dual phase composite membranes have been reported to excel overcoming this drawback. In addition to performance evaluation, Ce0.8Gd0.2O2-δ – FeCo2O4 (CGO-FCO) membranes were subjected to stability test in flue gas conditions closely mimicking industrial flue gas atmosphere. The dual phase composites are investigated for their phase stability at the operating temperature of 850°C in a gradient of oxygen chemical potential. The composites were also exposed to a series of gas mixtures over a period of time at their operating temperature to test for the chemical stability. CGO-FCO membranes are identified to possess chemical stability in gas mixtures of CO2, SO2 along with oxygen over a period of 200h at 850°C under oxygen partial pressure gradient.

Comparative Study of Metal Impregnated Lanthanum Doped Strontium Titanate Anodes and Conventional Nickel Oxide Anode for Solid Oxide Fuel Cell Application

In conventional SOFC, composite anode made up of nickel oxide (NiO) and yttria stabilized zirconia (YSZ) are used [1]. NiO is an excellent anode for SOFC application because of its high electrochemical activity towards H2 oxidation. But the amount of NiO used is around 30- 40 wt% to reach a percolation threshold [2]. Agglomeration of nickel particles due to sintering is a serious concern during SOFC operation at elevated temperatures (800 ˚C). In order to address the above mentioned issue, we have developed doped strontium titanate (SrTiO3) based perovskite anodes. Perovskite anodes are mixed ionic and electronic conductors (MIEC) providing enhanced triple phase boundary (TPB) which lowers the over-potential losses compared to pure conductors [3]. To increase the electrical conductivity, lanthanum (La) has been doped at A-site of SrTiO3 using solid state route (SSR), to form La0.3Sr0.7TiO3+δ (LST). Physical characterizations like field emission scanning electron microscope (FESEM), x-ray diffraction (XRD) and image analysis (ImageJ) were carried out before and after reduction of anodes in H2 environment to analyze their microstructure, phase stability and particle sizes as shown in Figure 1 and 2. Conductivity measurements were carried out using two probe conductivity measurement system to determine the activation energy (Ea) required in reducing the anodes in H2 environment. Along with physical characterizations, electro-chemical characterizations were carried out as described. The halfcell (LST/ YSZ) gave a high polarization resistance (Rp) of 62 Ω cm2, low current density of 20 mA cm-2 and power density of 5 mW cm-2 at 800 ˚C. We impregnate the porous anode with varying NiO content using wet impregnation technique and observed that LST impregnated with 4 wt% NiO forms a percolated network and provided enhanced performance. A decrease in Rp to 4 Ω cm2 and increase in current density to 156 mA cm-2 in 4 wt% NiO impregnated LST halfcells is seen at 800 ˚C. The performance increased sharply upto 900 mA cm-2 in case of halfcell with LST anode when impregnated with both nickel(4%) and ceria(6%) with low Rp of 0.5 Ω cm2, giving a maximum power density of 200 mW cm-2 at 800 ˚C as shown in Figure 3(a, b). The above obtained power density is better in performance compared to the one given by half cells of NiO-YSZ anodes [2]. Thus, metal impregnated A-site doped perovskite anodes can be potential substitute for NiO-YSZ anodes for SOFC application. Key words:A-site doped perovskites, Mixed ionic and electronic conductor, Activation energy and half-cell study.

Iron Exsolution Phenomena in Lanthanum Strontium Ferrite SOFC Anodes

As a mixed ionic and electronic conductor (MIEC), lanthanum strontium ferrite (La0.6Sr0.4FeO3-δ, LSF) is frequently investigated as a new material for SOFCs to lower the operating temperatures. Due to the duality of their conductivities, MIECs are suited for use as electrode as well as electrolyte materials. LSF has been proposed as a possible anode material and a number of investigations have analyzed its electrocatalytic activities. However, it has been shown that iron exsolution occurs in reductive environments at higher temperatures, leading to the formation of Fe rods on the surface.[1] Other experiments revealed that this segregated metallic Fe also appears when applying cathodic biases, causing a pronounced enhancement of the electrochemical water-splitting kinetics.[2] However, this structural instability would render this perovskite unsuitable for the use in SOFCs. It is therefore necessary to investigate the details of the exsolution in order to find ways to eventually suppress it. Utilizing an array of complementary in situ and ex situ techniques such as XPS, in situ XRD, scanning EXAFS, in situ TGA/DSC, and in situ TEM, we characterize this phenomenon regarding its thermodynamic and kinetic properties. According to in situ XRD, there are two distinct processes taking place: in the lower temperature regime, the perovskite exhibits a non-linear shift of the diffraction peaks to larger unit cell volumes, while, at temperatures above 773 K, Fe starts to crystallize. The shift allows the hypothesis to be made that the Fe atoms are relocated from their lattice positions to interstitial sites, thereby possibly forming diffusion channels, before they segregate to the surface and crystallize as metallic iron agglomerates. With XPS, the iron oxidation state in the surface-near region can be observed as a function of the reduction treatment in dry, flowing hydrogen. Starting from a state with only Fe(III) present, the perovskite is continuously reduced at temperatures above 673 K, resulting in the presence of only Fe(II) after reduction at 773 K. However, upon raising the temperature further, the observed Fe(II) concentration drops again: at 873 K, only 57% of the sampled iron is in the form of Fe(II), whereas the amount is zero after reduction at 973 K. This increase in the Fe(III) content is readily explained by TEM investigations: due to the transport of the samples in air, the exsolved iron particles are oxidized at their surface, forming a 2 nm thick oxide overlayer.[1] This shell prevents the surface-sensitive XPS measurements from sampling the metallic core. However, since the surface-near region of the perovskite itself only consisted of Fe(II) at 773 K, any Fe(III) in the spectrum originates from the oxide hull of segregated metallic Fe particles. Since the observed Fe(III) concentration, and thus the amount of segregated iron, increases above 773 K, the XPS results agree well with the in situ XRD measurements. Scanning EXAFS measurements of single LSF particles show that there is a difference in the electronic structure between the surface and bulk regions, corroborating the findings from the XPS analyses. [1] Thalinger, R.; Gocyla, M.; Heggen, M.; Klötzer, B.; Penner, S. J. Phys Chem. C 2015, 119, 22050–22056 [2] Opitz, A. K.; Nenning, A.; Rameshan, C.; Rameshan, R.; Blume, R.; Hävecker, M. Knop-Gericke, A.; Rupprechter, G.; Fleig, J.; Klötzer, B. Angew. Chem. Int. Ed. 2015, 54, 2628­–2632 Figure 1: Temperature-programmed i n situ XRD measurements in hydrogen/argon. Arrows mark the peaks resulting from segregated metallic Fe. Figure 1

Developing Redox Stability of Ceramic Anode Supported Intermediate Temperature Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) are eco-friendly and promising electrochemical devices which can convert chemical energy of fuel to electrical energy. They have many advantages including high conversion efficiency (45~65%) and fuel flexibility [1]. However they have to stand under harsh condition such as frequent load cycles, operational temperature fluctuations, and pump malfunctions. They should also operate over 40,000h on combined head and power (CHP) plants for commercial uses [2]. Mixed ionic and electronic conductors (MIEC) normally (particularly, Ni-YSZ cermets) use for the anode material of SOFCs, because of their good electrical conductivity and high catalytic activity for hydrogen oxidation. However Ni is likely to agglomerate at high temperature, thus resulting in reducing electrical conductivity with the significant loss of Ni-Ni contacts [3]. In addition, when SOFCs are exposed on the fuel starvation condition in the anode side, it can occur mechanical failure of anode substrates with the significant chemical expansion due to oxidation of Ni to NiO [4]. Furthermore Ni is not albe to use hydrocarbon fuel because of carbon deposition and has low tolerance to Sulphur.[4] In order to solve those issues, ceramic anode materials, La0.75Sr0.25Cr0.5Mn0.5O3- δ(LSCM), have been considered as a candidate material for the SOFC anode due to high chemical stability and good electrical conductivity. Furthermore, LSCM anode materials are fabricated with the highly conductive Nd0.1Ce0.9O2(NDC) electrolytes to investigate the practicability of SOFCs at the intermediate temperature (IT), since decreasing the operating temperature of SOFCs is one of possible solutions to enhance operational stability and long-term durability. In this work, LSCM anode-supoorted cells are fabricate by the using uniaxial pressing and drop-coating methods. For the electrical characterizations of LSCM-based cells electrochemical impedance spectra (EIS) and I-V (current-voltage) polarization curves are measured by using the Potentiostat/Galvanostatntequipped with impedance spectroscopy (Biologic) field emission scanning electron microscopy (FE-SEM), and X-ray diffraction (XRD) spectroscopy are used for the physicochemical analysis of LSCM anode-cells. [1] Steele, Brian CH, and Angelika Heinzel. "Materials for fuel-cell technologies." Nature 414.6861 (2001): 345-352. [2] Wachsman, Eric D., and Kang Taek Lee. "Lowering the temperature of solid oxide fuel cells." Science 334.6058 (2011): 935-939. [3] Simwonis, D., F. Tietz, and D. Stöver. "Nickel coarsening in annealed Ni/8YSZ anode substrates for solid oxide fuel cells." Solid State Ionics 132.3 (2000): 241-251. [4] Sarantaridis, D., and A. Atkinson. "Redox Cycling of Ni‐Based Solid Oxide Fuel Cell Anodes: A Review." Fuel cells 7.3 (2007): 246-258. [5] Yokokawa, Harumi, et al. "Fundamental mechanisms limiting solid oxide fuel cell durability." Journal of Power Sources 182.2 (2008): 400-412. [6] Tao, Shanwen, and John TS Irvine. "A redox-stable efficient anode for solid-oxide fuel cells." Nature materials 2.5 (2003): 320-323. * Corresponding author: jyoung@sejong.ac.kr (J.-Y. Park). Keywords : solid oxide fuel cells, ceramic anode, La0.75Sr0.25Cr0.5Mn0.5O3- δ, electrochemical performances, durability.

Contribution of Triple-Phase Boundary Reaction in Cathodic Reaction of Solid Oxide Fuel Cell

In a conventional SOFC cathode using mixed ionic and electronic conductor (MIEC), the electrochemical reaction proceeds not only at triple-phase boundaries (TPB: gas/electrode/electrolyte) but also on double-phase boundaries (DPB: gas/electrode). In the MIEC cathode, electrochemical reaction generally occurs within a limited area from electrode/electrolyte interface because the resistance for oxide ion conductivity becomes significant compared to that for surface reaction as the diffusion length of oxide ion becomes longer. Our research group has experimentally clarified the reaction area in a porous MIEC cathode by operando micro X-ray absorption spectroscopy (XAS) measurements (1). In a porous La0.6Sr0.4CoO3-δ electrode under a constant cathodic overpotential of 140 mV at 873 K in 10-2 bar of oxygen partial pressure P(O2), it was found the electrochemical reaction proceeded within 2 µm from the electrode/electrolyte interface. In the higher P(O2), the reaction area would become even shorter. It is in general believed that the total electrochemical reaction in a porous MIEC cathode is dominated by the reaction on DPB and the contribution of the reaction at TPB is insignificant. However, when the electrochemical reaction proceeds only in the vicinity of the electrode/electrolyte interface, as mentioned above, it is considered that the electrochemical reaction at TPB cannot be ignored. In this study, for investigating the contribution of the TPB reaction, patterned thin film electrodes with/without TPB, which were kinds of columnar electrodes, were fabricated. The distributions of electrochemical reaction in the model electrodes were evaluated by operando micro XAS measurements. In a La0.6Sr0.4CoO3-δ patterned electrode without TPB, the electrochemical reaction proceeded within 60 µm from the electrode/electrolyte interface under a constant cathodic overpotential of 190 mV at 973 K in 10-1 bar of P(O2). On the other hand, in the patterned electrode with TPB, the electrochemical reaction proceeded within 40 µm from the interface under the similar experimental conditions. The distribution of electrochemical reaction was decreased by the introduction of TPB, which demonstrated the contribution of the TPB reaction even in the MIEC cathode. The distribution of electrochemical reaction was also evaluated by isotopic exchange experiments and following secondary ion mass spectrometry (SIMS) measurements (2). 18O/16O isotopic exchanges were performed with/without cathodic overpotential, and the isotope exchange profiles were analyzed by SIMS measurements. The electrochemically incorporated 18O was evaluated as the difference between the isotope exchange profiles with/without cathodic overpotential. In the presentation, the contribution of the TPB reaction in the MIEC cathode will be discussed based on the results obtained by isotopic exchange measurements. 1. Y. Fujimaki, H. Watanabe, Y. Terada, T. Nakamura, K. Yashiro, S. Hashimoto, T. Kawada and K. Amezawa, ECS Trans., 2013, 57 (1), 1925-1932. 2. Y. Fujimaki, T. Nakamura, K. Develos-Bagarinao, K. Yamaji, K. Yashiro, T. Kawada, F. Iguchi, H. Yugami and K. Amezawa, ECS Trans., 2015, 68 (1), 623-630.

Structure and electrical properties of Y, Fe-based perovskite mixed conducting composites fabricated by a modified polymer precursor method

In this work, samples of Y0.07Sr0.93Ti1-xFexO3-δ with 20, 40, 60 and 80 mol% of iron amount were prepared by a low-temperature polymer precursor method. The SEM-EDS analysis proved that analyzed Y0.07Sr0.93Ti1-xFexO3-δ samples were composites of two Ti- and Fe-rich perovskite samples. This kind of composite consists of two phases in which one has a good ionic and the other electronic conductivity, which makes such a composite a potential mixed ionic and electronic conductors (MIECs) material. The total electrical conductivities of analyzed samples were measured in air atmosphere (cathode conditions in Solid Oxide Fuel Cell). The values changed from ∼10−3 to 10−1 S cm−1 and depended on the ratio between two observed perovskite phases. The 0.12 S cm−1 conductivity value at 800 °C for sample with the highest amount of Fe-rich perovskite in the structure makes this composite material a candidate for air electrode in electrochemical devices.

Development and Testing of Impregnated La0.20Sr0.25Ca0.45TiO3 Anode Microstructures for Solid Oxide Fuel Cells

The A-site deficient perovskite: La0.20Sr0.25Ca0.45TiO3 (LSCTA-) is a mixed ionic and electronic conductor (MIEC) which shows promising performance as a Solid Oxide Fuel Cell (SOFC) anode ‘backbone’ material, when impregnated with metallic and oxide-ion conducting electrocatalysts. Here, we present data on the complete ceramic processing and optimisation of the LSCTA- ‘backbone’ microstructure, in order to improve current distribution throughout the anode. Through control of ink rheology, screen printing parameters and sintering protocol an advantageous LSCTA- microstructural architecture was developed, exhibiting an ‘effective’ conductivity of 21 S cm-1. Incorporation of this LSCTA- anode microstructure into SOFC and impregnation with Ce0.80Gd0.20O1.9 and either Ni, Ru, Rh, Pt or Pd resulted in promising initial performances during fuel cell testing in a fuel stream of 97% H2:3% H2O. Area Specific Resistances of 0.41 Ω cm2 and 0.39 Ω cm2 were achieved with anodes containing Rh/CGO and Pd/CGO, respectively.

Improved performance of molten carbonate fuel cells with (Li/Na)2CO3 electrolytes by using BYS coated cathode at low operating temperatures

In order to improve the stack life time of MCFCs, it is necessary to reduce the operating temperature of MCFCs below 600 °C, because reduced operating temperature minimizes electrolyte loss due to evaporation and corrosion. However, at the low operating temperature below 600 °C, the cell performance of MCFCs with (Li/Na)2CO3 electrolyte is too low to operate the fuel cell stack and system. In this study, we have performed wettability control of the liquid molten carbonate electrolyte by coating NiO cathodes with poor wetting property of the mixed ionic and electronic conductor (MIEC) such as BYS (Bi1.5Y0.3Sm0.3O3-δ). From experiments with symmetrical cells, each polarization component with various temperatures and gas conditions were studied. To investigate effects of the BYS coated cathode on the performance of MCFCs, a 100 cm2 single cell of MCFCs was employed. The performance of a 100 cm2 single cell with BYS coated cathode was better than that with conventional cathode by a factor of 1.84, because BYS coated cathode reduces activation polarization and mass transfer resistance greatly.

(Invited) Triple Phase Boundary Reaction in a Mixed-Conducting SOFC Cathode

The contribution of the triple phase boundary reaction in a mixed ionic and electronic conducting (MIEC) cathode in solid oxide fuel cells (SOFCs) was investigated. For this purpose, patterned thin film electrodes with or without triple phase boundaries, which simplified the microstructure of a practical porous electrode, were proposed and fabricated. In this work, an La0.6Sr0.4CoO3-δ (LSC) electrode on a Ce0.9Gd0.1O1.95 electrolyte was chosen as a model SOFC MIEC cathode. Effective reaction area was evaluated by means of operando micro X-ray absorption spectroscopy with the model electrodes under 10-2 bar of p(O2) at 873 K. It was found that the introduction of triple phase boundaries shortened the effective reaction area. The results may indicate the non-negligible contribution of the triple phase boundary reaction even in SOFC MIEC cathodes.

Generalized electrical conductivity relaxation approach to determine electrochemical kinetic properties for MIECs

Electrical conductivity relaxation (ECR) technique has been widely used to determine kinetic properties of mixed ionic and electronic conductors (MIECs). However, this technique is only applicable for reliable determination of kinetic parameters within a confined range of chemical Biot numbers. Since the kinetic properties of the materials are not known a priori, this imposes great difficulties on how to verify the obtained kinetic parameters from obtained ECR data. Further due to the ill-posed nature of ECR problem, the measurement noise could lead to significant uncertainties in the determined kinetic parameters. Herein a generalized ECR (g-ECR) approach is developed by coherently incorporating multiple ECR measurements simultaneously into an inverse algorithm. This new approach is able to improve the accuracy of calculated kinetic parameters and attenuate uncertainties induced by measurement noise over a wider range of chemical Biot numbers than the ECR approach with a single measurement. The capability of both reducing uncertainties and increasing the range of chemical Biot numbers for accurate parameter determination can be achieved by increasing the number of employed ECR measurement responses. A case study of ECR measurement for PrBa(Co0.75Fe0.25)2O5+δ is performed in a mixture gas of N2/Air at 600°C. The kinetic parameters are determined and evaluated using this g-ECR approach. The rationality of using g-ECR approach instead of ECR approach is verified.

Surface exchange model for ITM membrane in transient stage

Mixed Ionic and Electronic Conductors (MIECs) are promising membrane materials for oxygen separation from air at high temperature. The transient stage is critical due to the stress induced by the chemical strain, combined with the oxygen activity field through the membrane. In the literature, permeation models are able to predict the oxygen flux in steady state or to identify the slowest mechanism. In this paper, the proposed model aims to predict the oxygen activity fields through the membrane both in transient stage and steady state. A macroscopic approach is proposed, the balance of a temporary species is introduced to take into account separately the dissociative adsorption, associative desorption and interface reaction between surface and bulk. New densities of probability are proposed to model dissociative adsorption and associative desorption in order to take into account the difference between “classical” catalysis and the MIECs case. This model is implementing in the finite element software COMSOL Multiphysics© thanks to the combination of the modules “coefficient form PDE” and “coefficient form boundary PDE”. The results of the model are in good agreement with steady state and transient stage experimental data. The different kinetics of oxygen release and oxygen intake are well reproduced.

Electrochemical fields within 3D reconstructed microstructures of mixed ionic and electronic conducting devices

Abstract The performance and stability of the mixed ionic and electronic conducting (MIEC) membrane devices, such as solid oxide cells (SOCs) and oxygen separation membranes (OSMs) interplay tightly with the transport properties and the three-dimensional (3D) microstructure of the membrane. However, development of the MIEC devices is hindered by the limited knowledge about the distribution of electrochemical fields within the 3D local microstructures, especially at surface and interface. In this work, a generic model conforming to local thermodynamic equilibrium is developed to calculate the electrochemical fields, such as electric potential and oxygen chemical potential, within the 3D microstructure of the MIEC membrane. Stability of the MIEC membrane is evaluated by the distribution of oxygen partial pressure. The cell-level performance such as polarization resistance and voltage vs. current curve can be further calculated. Case studies are performed to demonstrate the capability of the framework by using X-ray computed tomography reconstructed 3D microstructures of a SOC and an OSM. The calculation method demonstrates high computational efficiency for large size 3D tomographic microstructures, and permits parallel calculation. The framework can serve as a powerful tool for correlating the transport properties and the 3D microstructure to the performance and the stability of MIEC devices.

Hodrogen Oxidation Reaction Mechanisms in Mixed Ionic and Electronic Conducting Oxide Anode for Solid Oxide Fuel Cells

Ceramics anode offers certain advantages in terms of impurity tolerance and other properties, as compared to conventional Ni/YSZ anode for solid oxide fuel cells (SOFC). However, ceramic anodes typically have much lower electrochemical performance, and there is very few studies on the hydrogen oxidation reaction (HOR) mechanisms and kinetics in ceramic anodes. In this research, using Doped YCrO3 as baseline material, we systematically investigated the HOR mechanisms in mixed ionic and electronic conducting (MIEC) ceramic anodes, with the focus on the effect of electronic conductivity, electro-catalytic activity, triple-phase-boundary (3PB), and electrolyte, on the HOR kinetics. It is found that both electrical conductivity and electrode performance of yttrium chromites have been enhanced after Co and Ni doping. Electrochemical impedance spectroscopy (EIS) results indicate that charge transfer process at high frequency and surface adsorption/diffusion processes at low frequency domain can be the dominant anode reaction steps. Ni doping accelerates the surface processes by reducing the related activation energy from 1.2 to 0.5 eV. It also substantially improves the charge transfer process probably by increasing the amount of adsorbed H on electrode surface. The resistance of high frequency is found to be dependent on H2 content. The observed reaction order is 1/4 for Co doped and 1/3 ~ 1/2 for Ni doped yttrium chromites. A model of H2oxidation reaction is proposed, revealing this dependence stems from the reaction between adsorbed H and the lattice oxygen. The influence of electrolyte to electrode performance was investigated by replacing YSZ electrolyte with scandium stabilized zirconium (SSZ). Smaller polarization resistances are observed on each electrode in both wet H2 and air atmospheres. Replacement of electrolyte can alter not only the rate of charge transfer process but also in some cases other surface processes not related to the electrolyte directly. It is proposed that the impact of the electrolyte on each electrode process is passed down as in a chain and the charge transfer step functions as the first ring in the chain.