Cathode For Intermediate-temperature Solid Oxide Fuel Cells Research Articles

Prevention of Reaction between (Ba,Sr)(Co,Fe)O3 Cathodes and Yttria-stabilized Zirconica Electrolytes for Intermediate-temperature Solid Oxide Fuel Cells

Perovskite (Ba,Sr)(Co,Fe)O3 (BSCF) cathodes are highly electrochemically active in the oxygen reduction process for intermediate-temperature solid oxide fuel cells (IT-SOFCs). However, the phase of perovskite zirconate is formed between the BSCF cathode and yttria-stabilized zirconia (YSZ) electrolyte, which increased the cathode polarization resistance. In the present work, the (Ba,Sr)ZrO3 formation was not prevented by the insertion of a porous gadolinia-doped ceria (GDC) interlayer between the cathode and the electrolyte, when the BSCF cathode was sintered at 1100°C. Energy dispersive X-ray spectroscopy showed that the product of (Ba,Sr)ZrO3 formed between BSCF and YSZ was thicker than that of SrZrO3 formed between a conventional (La,Sr)(Co,Fe)O3 (LSCF) and YSZ. When a BSCF cathode was sintered at 900°C, no (Ba,Sr)ZrO3 formation was observed between the cathode and the electrolyte. The maximum power density at 650°C was improved from 7.5 to 740mW/cm2 by decreasing the sintering temperature of BSCF cathode from 1100°C to 900°C. Distribution of relaxation times (DRT) analysis showed that the polarization resistance of the oxygen exchange process in the BSCF cathode sintered at 900°C was lower than that in the LSCF cathode. The sintering temperature is important for obtaining high-performance BSCF cathodes for IT-SOFCs.

Electrochemical performance of Ba0.8Sr0.2Co0.7Fe0.2Nb0.1O3−δ–Ce0.85Sm0.15O1.925 composite cathodes for intermediate temperature solid oxide fuel cells

Ba1−xSrxCo0.7Fe0.2Nb0.1O3−δ (x = 0.0–0.2) are proposed as the electrodes for intermediate temperature solid oxide fuel cells (IT-SOFCs) for the first time. The minimum polarization resistance value is achieved at Ba0.8Sr0.2Co0.7Fe0.2Nb0.1O3−δ(BSCFN). In this work, BSCFN-xSDC(x = 0–40wt.%) are investigated as the cathodes for IT-SOFCs with SDC electrolyte. The structures of the cathodes are characterized by X-ray diffraction and SEM. The electrode performance of BSCFN-xSDC is measured at various temperatures and oxygen partial pressures. Comparison of the results demonstrate that introduction of ionic conductive SDC to BSCFN-xSDC composite cathode can make the oxygen reduction reaction processes from the three processes into two processes, the charge transfer process and the adsorption and dissociation of gaseous oxygen process. The charge transfer process is the major rate limiting step when oxygen partial pressure is higher than 0.05 atm. The microstructure of BSCFN-30SDC is characterized by more appropriate grain size and more adequate porosity. The area conductivity of adsorption and dissociation of gaseous oxygen process for BSCFN-30SDC is the highest. Comparison of the power density of single cell results demonstrate that BSCFN-xSDC composite cathode has higher power density than BSCFN cathode. BSCFN-30SDC electrode exhibits the highest cathode performance among BSCFN-xSDC composite cathodes.

Performances of YBaCo1.4Cu0.6O5+δ–Ce0.8Sm0.2O1.9 composite cathodes for intermediate-temperature solid oxide fuel cells

The electrochemical properties of YBaCo1.4Cu0.6O5+δ–xCe0.8Sm0.2O1.9 (YBCC–xSDC, x=20, 30, 40, 50wt%) have been investigated for the potential application in intermediate-temperature solid oxide fuel cells (IT-SOFCs). No chemical reactions between YBCC cathode and SDC electrolyte, and YBCC and La0.9Sr0.1Ga0.8Mg0.2O3−δ (LSGM) occur. The thermal expansion coefficient (TEC) of YBCC cathode decreases with SDC addition. The TEC of YBCC–30SDC cathode is 13.60×10–6K−1 from 30 to 850°C in air and it exhibits the best electrochemical performance among the YBCC–xSDC cathodes. The polarization resistance (Rp) of YBCC–30SDC is 0.027Ωcm2 at 850°C, 0.044Ωcm2 at 800°C and 0.075Ωcm2 at 750°C. The maximum power density value of electrolyte-based cell with YBCC–30SDC cathode is 662, 483 and 319mWcm−2 at 850, 800 and 750°C, respectively. Preliminary results indicate that YBCC–30SDC is especially promising as a cathode for IT-SOFCs.

Electrochemical performance of double perovskite Pr2NiMnO6 as a potential IT-SOFC cathode

Double perovskite oxide Pr2NiMnO6 is evaluated as a potential cathode for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The reactivity test indicates that no reaction occurs between Pr2NiMnO6 and CGO at 1200 °C in air. The thermal expansion coefficients (TEC) of Pr2NiMnO6 are 10.6 × 10−6 K−1 in air and 11.5 × 10−6 K−1 in nitrogen atmosphere within a temperature range of 100–800 °C. The electrochemical properties of Pr2NiMnO6 cathode are characterized by ac impedance and dc polarization methods, respectively. The lowest area specific resistance (ARS) is 0.38 Ω cm2 at 700 °C in air. The dependence of ASR with oxygen partial pressure indicates that the charge transfer process is the rate-limiting step for oxygen reduction reaction on Pr2NiMnO6 cathode.

Study of Electrode Performance for Nanosized La0.4Sr0.6Co0.8Fe0.2O3-Δ IT-SOFC Cathode

The strontium-lanthanum cobalt-rich perovskites are the most extensively studied mixed conductor SOFC cathode materials[1-3]. Recently, different methods have been developed to improve their microstructure and to make them suitable for use as electrodes in Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFC) [1, 3-5]. It is well known that reducing particle size leads to an increase in surface area, thereby reducing the electrode polarization resistance. However, the elevated temperatures typical of SOFC operation can cause particles coarsening, a dominant contribution to long-term degradation for nanomaterials [6, 7]. Another cause of degradation is Sr-segregation on the surface of these perovskites, which blocks the O-surface exchange sites [8, 9] and decreases the effective surface. In this work we present the study of the cell performance and long-term stability of a nanoscaled La0.4Sr0.6Co0.8Fe0.2O3-δ cathode obtained by the combustion method (LSCF4682) at 800˚C. Details about the synthesis method, the microstructure and the mechanism of oxygen reduction reaction for this cathode has been previously reported in [4]. The unique microstructure achieved by this method, which exhibits a high surface area, good interconnectivity of grains and a high volume fraction of mesopores induces a change of mechanism controlling the O-surface rate. Below 600 °C, the mechanism of reaction is dominated by the O-surface and transport rates, whereas as T increases the mass transfer due to the O2gas diffusion becomes important [4]. SOFC tests were performed on single cells of anode–electrolyte–cathode systems. The samples were constructed from a porous (Sr0.8La0.2TiO3-α) STO-LSGM supported multilayer cell [10]. The anode consists of a Ni infiltrated porous LSGM separated from the cathode side by a thin dense LSGM electrolyte. The cathode was prepared as a bilayer electrode consisting of a thin LSGM porous interlayer and a thicker LSCF4682-LSGM 50% composite. Figure 1a shows post mortemSEM images of the tested cell, and Figures 1 b and c show the cathode and anode microstructures. Figure 2 shows I-V and I-P curves collected between 500 and 650 °C. A maximum power density of 1.23 Wcm-2was achieved at 650 °C suggesting that this is a competitive IT-SOFC assembly. Above 600 °C the gas mass transfer controls the power density at high current density in agreement with the oxygen reduction mechanisms previously proposed [4]. The time-dependent evolution of cathode polarization resistance for this material deposited in symmetrical LSCF/LSGM/LSCF cell was studies by Electrochemical Impedance Spectroscopy (EIS) and compared with that of a commercial La0.6Sr0.4Co0.2Fe0.8O3-δ cathode (Praxair LSCF6428). In order to reduce the time for data acquisition, an accelerated aging test was performed, where ageing would take place at 800 °C, with brief reduction in temperature to 700 °C for EIS collection. Figure 3a shows the initial and 500 h EIS spectra of cells manufactured with either, the commercial LSCF6428 and the LSCF4682 synthetized by us. Despite that the degradation rate of both cathodes are similar, the low LSCF4682 polarization resistance guarantees a competitive IT-SOFC cathode during longer-term operation. Figure 3b indicates that the time evolution of total cathode polarization resistance (Rtot) is due to both contributions by O-surface exchange and O2gas diffusion suggesting a microstructural change with time. Figure Caption Figure 1. a) SEM cross-section images of SOFC full cell b) cathode consisting of LSCF4682 as synthetized powder c) Ni-LSGM infiltrate anode. Figure 2. Voltage and power density versus current density measured between 200 sccm air and 100 sccm humidified H2 as a function of temperature. Figure 3. a) Comparison between EIS spectra taken at 700 °C at the beginning and after 500 h of the 800 ˚C aging test. b) Resistance evolution with time for High (RHF) and Low (RLF) frequency contribution to Total cathode polarization resistance (Rtot) .

Long-term stability of the IT-SOFC cathode materials La0.6Sr0.4CoO3 − δ and La2NiO4 + δ against combined chromium and silicon poisoning

Long-term degradation effects of combined Cr- and Si-poisoning for the promising intermediate temperature solid oxide fuel cell (IT-SOFC) cathode materials La0.6Sr0.4CoO3−δ (LSC) and La2NiO4+δ (LNO) were investigated at 700°C in dry and humid atmospheres for subsequent periods of 1000h by using dc-conductivity relaxation measurements. Changes in the chemical composition and morphology of the contaminated sample surfaces were studied by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) with energy and wavelength dispersive X-ray analysis and by atomic force microscopy (AFM). Upon exposure to dry and humid gas flows containing Cr- and Si-species both materials exhibit a strong decrease of the chemical surface exchange coefficient of oxygen by a factor of 110 and 40 for La0.6Sr0.4CoO3−δ and La2NiO4+δ, respectively, which can be attributed to the formation of Cr-containing crystallites on the degraded sample surfaces. AFM-measurements reveal faceted crystallites of lateral extensions of up to 2μm and heights of up to 500nm on the surface of La0.6Sr0.4CoO3−δ. XPS-analyses confirm the presence of high levels of Cr and Sr in these crystals, indicating the decomposition of the perovskite phase through SrCrO4-formation. The significantly smaller crystallites that are found on the surface of La2NiO4+δ also contain Cr, as well as elevated levels of La. For both compounds, silicon is found to spread in small patches across the entire sample surface, as determined by EDXS elemental mapping analysis.

B-Site Metal (Pd, Pt, Ag, Cu, Zn, Ni) Promoted La1−xSrxCo1−yFeyO3–δ Perovskite Oxides as Cathodes for IT-SOFCs

Perovskite oxides La1−xSrxCo1−yFeyO3–δ (LSCF) have been extensively investigated and developed as cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFCs) due to mixed ionic–electronic conductivity and high electrooxygen reduction activity for oxygen reduction. Recent literature investigations show that cathode performances can be improved by metal surface modification or B-site substitution on LSCF. Although the specific reaction mechanism needs to be further investigated, the promoting effect of metal species in enhancing oxygen surface exchange and oxygen bulk diffusion is well recognized. To our knowledge, no previous reviews dealing with the effect of metal promotion on the cathodic performances of LSCF materials have been reported. In the present review, recent progresses on metal (Pd, Pt, Ag, Cu, Zn, Ni) promotion of LSCF are discussed focusing on two main aspects, the different synthesis approaches used (infiltration, deposition, solid state reaction, one pot citrate method) and the effects of metal promotion on structural properties, oxygen vacancies content and cathodic performances. The novelty of the work lies in the fact that the metal promotion at the B-site is discussed in detail, pointing at the effects produced by two different approaches, the LSCF surface modification by the metal or the metal ion substitution at the B-site of the perovskite. Moreover, for the first time in a review article, the importance of the combined effects of oxygen dissociation rate and interfacial oxygen transfer rate between the metal phase and the cathode phase is addressed for metal-promoted LSCF and compared with the un-promoted oxides. Perspectives on new research directions are shortly given in the conclusion.

Effects of strontium doping on the structure, oxygen nonstoichiometry and electrochemical performance of Pr2−xSrxNi0.6Cu0.4O4+δ (0.1 ≤ x ≤ 0.5) cathode materials

Pr2−xSrxNi0.6Cu0.4O4+δ (PSNCO, x = 0.1, 0.3, 0.5) oxides with K2NiF4-type structure are synthesized by a glycine–nitrate process as potential cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFCs). The influence of strontium substitution on the structure, oxygen nonstoichiometry, electrical property and electrochemical performance is investigated. Iodometric titration method is employed to study the oxygen nonstoichiometry and the formal valence of nickel. The maximum electrical conductivity reaches 109 S cm−1 at 450 °C in air for Pr1.7Sr0.3Ni0.6Cu0.4O4+δ material. Area specific resistance (ASR) values of all compositions are below 0.11 Ω cm2 at 800 °C, and the Pr1.7Sr0.3Ni0.6Cu0.4O4+δ composition has the lowest ASR value of 0.063 Ω cm2 at 800 °C. Open circuit voltage (OCV) and maximum power density of the single cell (NiO–SDC/SDC/PSNCO) are 0.767 V and 353 mW cm−2 at 800 °C, respectively. The preliminary results indicate that Pr2−xSrxNi0.6Cu0.4O4+δ materials could be good candidates for IT-SOFCs cathode materials.

Superior electrochemical performance and oxygen reduction kinetics of layered perovskite PrBaxCo2O5+δ (x = 0.90–1.0) oxides as cathode materials for intermediate-temperature solid oxide fuel cells

The layered perovskite PrBaxCo2O5+δ (PBxCO, x = 0.90–1.0) oxides have been synthesized by a solid-state reaction technique, and evaluated as the potential cathode materials for intermediate-temperature solid oxide fuel cells (IT-SOFCs). Room temperature X-ray diffraction patterns show the orthorhombic structures which double the lattice parameters from the perovskite cell parameter as a ≈ ap, b ≈ ap and c ≈ 2ap (ap is the cell parameter of the primitive perovskite) in the Pmmm space group. There is a good chemical compatibility between the PBxCO cathode and the Ce0.9Gd0.1O1.95 (CGO) electrolyte at 1000 °C. The electrical conductivity and thermal expansion coefficient of PBxCO are improved due to the increased amount of electronic holes originated from the Ba-deficiency. The results demonstrate the high electrochemical performance of PBxCO cathodes, as evidenced by the super low polarization resistances (Rp) over the intermediate temperature range. The lowest Rp value, 0.042 Ω cm2, and the cathodic overpotential, −15 mV at a current density of −25 mA cm−2, are obtained in the PrBa0.94Co2O5+δ cathode at 600 °C in air, which thus allow to be used as a highly promising cathode for IT-SOFCs. A CGO electrolyte fuel cell with the PrBa0.94Co2O5+δ cathode presents the attractive peak power density of ∼1.0 W cm−2 at 700 °C. Furthermore, the oxygen reduction kinetics of the PrBa0.94Co2O5+δ cathode is also studied, and the rate-limiting steps for oxygen reduction reaction are determined at different temperatures.

Electrochemical properties of B-site Ni doped layered perovskite cathodes for IT-SOFCs

Offering fast oxygen reduction kinetics and high electrical conductivities relative to those of ABO3-type perovskite oxides, layered perovskite cathode materials have received considerable attention for intermediate temperature solid oxide fuel cells (IT-SOFCs). In this study, we have investigated the effects of Ni substitution for the Co site in SmBa0.5Sr0.5Co2O5+δ in terms of structural characteristics, electrical properties, and electrochemical performance of SmBa0.5Sr0.5Co2−xNixO5+δ (x = 0, 0.1, and 0.2) in relation to its application as an IT-SOFC cathode material. At the given temperature, the electrical conductivities of SmBa0.5Sr0.5Co2−xNixO5+δ (x = 0, 0.1, and 0.2) decrease with increasing Ni content. For x = 0.1, the lowest ASR value, 0.125 Ω cm2, and the highest single cell performance, 1.78 W cm−2 at 600 °C, are obtained.

The characteristic of strontium-site deficient perovskites SrxFe1.5Mo0.5O6−δ (x = 1.9–2.0) as intermediate-temperature solid oxide fuel cell cathodes

As the cathodes for intermediate-temperature solid oxide fuel cells (IT-SOFCs), A-site deficient SrxFe1.5Mo0.5O6−δ (x = 1.9–2.0) (SxFM) materials have been successfully synthesized using the sol–gel combustion method. In the perovskite structure of these oxides, the unit cell varies from pseudocubic to cubic with increasing deficiency. Thermal expansion coefficient of SxFM has also been measured and compared with that of Scandium-stabilized zirconium (ScSZ) electrolyte. X-ray photoelectron spectroscopy (XPS) results indicate that the Sr-deficiency has changed the proportion of Fe2+/Fe3+ and Mo6+/Mo5+ ratios, which directly influences the conductivity of SxFM materials. S1.950FM possesses the largest electrical conductivity and the lowest polarization resistance (Rp) among all the samples. The maximum power densities of a single cell with the S1.950FM cathode reaches 1083 mW cm−2, and the area specific resistance value is 0.17 Ω cm2 at 800 °C. These results indicate that the A-site deficiency could promote the electrochemical performance of SFM materials as cathodes for IT-SOFCs.

Performance of double-perovskite YBa0.5Sr0.5Co1.4Cu0.6O5+δ as cathode material for intermediate-temperature solid oxide fuel cells

Double-perovskite YBa0.5Sr0.5Co1.4Cu0.6O5+δ (YBSCC) is synthesized by the EDTA–citrate complexing method, and investigated as a novel cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs). There are no serious reactions between YBSCC and La0.9Sr0.1Ga0.8Mg0.2O3−δ (LSGM) except a slight peak shift. The electrical conductivities of the YBSCC sample occurs a semiconductor–metal transition at about 350°C. The thermal expansion coefficient (TEC) of the YBSCC sample is 16.4 ×10−6K−1 between 30°C and 850°C in air. The lowest polarization resistance of 0.036Ωcm2 is achieved at 850°C. The maximum power densities of a single cell with the YBSCC cathode on a 300μm-thick LSGM electrolyte are 398mWcm−2 and 272mWcm−2 at 850°C and 800°C, respectively. This study suggests that the double-perovskite YBSCC can be potential candidate for utilization as IT-SOFCs cathode.

A new family of barium-doped Sr2Fe1.5Mo0.5O6−δ perovskites for application in intermediate temperature solid oxide fuel cells

Sr2−xBaxFe1.5Mo0.5O6−δ (x = 0, 0.2, 0.4, 0.6, 0.8, 1) perovskite materials have been prepared as potential cathodes for intermediate temperature solid oxide fuel cells (IT-SOFCs) via a sol–gel combustion method. X-ray diffraction (XRD) confirms that the expansion of the unit cell derived from the substitution of the large radius Ba atom, with the impurity phase structure only being detected when x = 0.8 and 1. Concurrently, thermal expansion coefficients (TECs) increase with increasing Ba doping. X-ray photoelectron spectrometry (XPS) measurements show that A-site doped Ba in the perovskite has an insignificant impact on the existence and ratio of Fe2+/Fe3+ and Mo6+/Mo5+ species within the Sr2−xBaxFe1.5Mo0.5O6−δ (x = 0.2, 0.4, 0.6) (SBFM) samples. Compared with Sr2Fe1.5Mo0.5O6−δ, the SBFM cathodes exhibited better electrochemical performance, which has been confirmed by the electrochemical impedance spectra (EIS). In particular, Sr1.8Ba0.2Fe1.5Mo0.5O6−δ (SB0.2FM) shows the lowest interface polarization (Rp). Furthermore the peak power densities of single cells based on the SB0.2FM cathode are 0.87 and 1.30 W cm−2 at 700 and 750 °C, respectively. All these results suggest that SBFM materials are promising cathodes for IT-SOFCs.

Effect of Cu doping on YBaCo2O5+δ as cathode for intermediate-temperature solid oxide fuel cells

Layered-perovskites YBaCo2-xCuxO5+δ (YBCCO, x=0.0, 0.2, 0.4, 0.6, 0.8) are synthesized by the sol-gel method and are assessed as potential cathode materials for utilization in intermediate-temperature solid oxide fuel cells (IT-SOFCs) on the La0.9Sr0.1Ga0.8Mg0.2O3-δ (LSGM) electrolyte. With increasing x from 0.0 to 0.8, the structure does not change but the lattice parameters and unit-cell volumes increase. From X-ray photoelectron spectra, it can be seen that both Cu and Co exist in a mixed oxidation state. Cu ions are prone to low oxidation state (Cu+ and Cu2+), while Co ions basically exhibit high oxidation state (Co3+ and Co4+). The conductivity of the YBCCO samples decreases with increasing Cu content, and the maximum conductivity of YBCCO is 143 S cm−1 at 300°C for the x=0.0 sample. The thermal expansion coefficient of samples significantly decreases from 17.8×10−6K−1 for x=0.0 to 13.4×10−6K−1 for x=0.8 in the temperature range of 30–850°C. With increasing Cu substitution content for Co, the electrode polarization resistance (Rp) increases. The Rp values of the YBCCO (x =0.6) cathode on the LSGM electrolyte are 0.125Ω cm2 and 0.076Ω cm2 at 700°C and 750°C, respectively. The maximum power densities of the single cells fabricated with the LSGM electrolyte, Ce0.8Gd0.2O1.9 (GDC) interlayer, NiO/GDC anode and YBCCO (x=0.6) cathode reach 815, 642 and 479 mW cm−2 at 850, 800 and 750°C, respectively. This study suggests that the layered-perovskites YBCCO can be potential candidates for utilization as IT-SOFC cathodes.

Synthesis and characterization of SmSrCo2−xMnxO5+δ (x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0) cathode materials for intermediate-temperature solid-oxide fuel cells

Double perovskite SmSrCo2−xMnxO5+δ (SSCM, x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0) oxides are designed and evaluated as cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFCs). Effect of Mn/Co molar ratio on structural, conductivity, thermal and electrochemical properties for SSCM is systematically investigated. With increasing x from 0.0 to 1.0, the structure does not change. At the same time, the SSCM material exhibits good chemical compatibility with Ce0.9Gd0.1O1.95 (GDC) electrolytes at 950°C for 6h. The thermal expansion coefficient of SSCM decreases with increasing x, which is helpful to thermally match with the electrolyte. The electrical conductivity decreases with x and the SSCM system exhibits a metal to semiconductor transition around x=0.4 and 0.6. With increasing Mn substitution content for Co, the area specific resistance (ASR) increases from 0.073Ωcm2 (x=0.0) to 0.140Ωcm2 (x=0.8) at 700°C. Preliminary results indicate that SSCM is especially promising as a cathode for IT-SOFCs.

Electrochemical characterization of La0.6Ca0.4Fe0.8Ni0.2O3 cathode on Ce0.8Gd0.2O1.9 electrolyte for IT-SOFC

For Solid Oxide Fuel Cells (SOFCs) to become an economically attractive energy conversion technology, suitable materials and structures which enable operation at lower temperatures, while retaining high cell performance, must be developed. Recently, the perovskite-type La0.6Ca0.4Fe0.8Ni0.2O3 oxide has shown potential as an intermediate temperature SOFC cathode. An equivalent circuit describing the cathode polarization resistances was constructed from analyzing impedance spectra recorded at different temperatures in oxygen. A competitive electrode polarization resistance is reported for this oxygen electrode using a Ce0.8Gd0.2O1.9 electrolyte, determined by impedance spectroscopy studies of symmetrical cells sintered at 800 °C and 1000 °C. Scanning electron microscopy (SEM) studies of the symmetrical cells revealed the absence of any reaction layer between cathode and electrolyte, and a porous electrode microstructure even when sintered at a temperature of only 800 °C. The performance of this cathode shows favorable oxygen reduction reaction (ORR) properties potentially making it an excellent choice for IT-SOFC application.

A collaborative study of sintering and composite effects for a PrBa0.5Sr0.5Co1.5Fe0.5O5+δIT-SOFC cathode

Recently, a novel cathode material PrBa0.5Sr0.5Co1.5Fe0.5O5+δ (PBSCF) has been proposed as a solution to overcome the drawbacks of a conventional cathode for intermediate-temperature solid oxide fuel cells (IT-SOFCs). Here we report systematic procedures to optimize the sintering temperature and the composite for PBSCF as an IT-SOFC cathode. For optimization of the heat treatment conditions for a PBSCF composite cathode, the effects of sintering temperature on the microstructure and electrical transport properties of the material are examined. We also suggest the optimization processes to effectively expand the electrochemical reaction zone based on a combination of a mixed ionic and electronic conductor (MIEC) electrode and an ionically conducting phase (PBSCF-Ce0.9Gd0.1O1.95 (GDC)x, x = 0, 20, 40, 50, and 60 wt%). The optimal intersection point between these two processing systems is revealed to be 50 wt% of GDC containing a composite cathode sintered at 950 °C for 4 h. The area specific resistance (ASR) of PBSCF-GDC50 sintered at 950 °C for 4 h reaches a minimum value of 0.052 Ω cm2 at 600 °C, which is consistent with the electrochemical performance results representing peak power density of ∼2.0 W cm−2 at 600 °C.

Scale-Down and Sr-Doping Effects on La4Ni3O10-δ–YSZ Nanocomposite Cathodes for IT-SOFCs

Among the Ruddlesden-Popper series, Lan+1NinO3n+1 (n = 1, 2, and 3) has attracted tremendous attention as an intermediate temperature solid oxide fuel cell (IT-SOFC) cathode because of its favorable properties for realizing high performance. Inter alia, La4Ni3O10-δ (n = 3) has in particular gained recognition as a promising cathode material owing to its high electrical conductivity and cell performance in IT-SOFC applications. The fabrication of nano-sized La4Ni3O10-δ composites requires a low sintering temperature process because high sintering temperature gives rise to solid state reactions between the constituents, which leads to the formation of an undesired electrical insulator and the formation of a low surface area thin-film on the surface. The infiltration method is therefore chosen to fabricate nano-sized La4Ni3O10-δ composites because it can provide reduced particle size of La4Ni3O10-δ by separating the sintering process of the cathode from the high temperature sintering process of the electrolyte. In this work, we investigated the electrochemical properties of La4Ni3O10-δ composites prepared by an infiltration method in terms of application as an IT-SOFC cathode material as well as the effect of strontium doping at La sites.

Effect of Bi2O3 on the electrochemical performance of LaBaCo2O5+δ cathode for intermediate-temperature solid oxide fuel cells

The LaBaCo2O5+δ−x wt.% Bi2O3 (LBCO-xBi2O3, x=10, 20, 30, and 40) were prepared as composite cathodes for intermediate-temperature solid oxide fuel cells (IT-SOFCs) via the conventional mechanical mixing method. The effect of Bi2O3 on polarization resistance, overpotential, and long-term stability of the LBCO cathode was investigated. An effective sintering aid for LBCO cathode, Bi2O3 not only lowers its sintering temperature by ~200°C, but also improves the electrochemical performance within the intermediate temperature range of 600–800°C. Electrochemical impedance spectroscopy measurements showed that the addition of 20wt% Bi2O3 to LBCO exhibited the lowest area-specific resistance of 0.020Ωcm2 at 800°C in air, which was about a seventh of that of the LBCO cathode at the same condition. At a current density of 0.2Acm−2, the cathodic overpotential of LBCO-20Bi2O3 was about 12.6mV at 700°C, while the corresponding value for LBCO was 51.0mV. Compared to B2O3–Bi2O3–PbO frit, the addition of Bi2O3 significantly improved the long-term stability of cathode. Therefore, LBCO-20Bi2O3 can be a promising cathode for IT-SOFCs.

Internal Activation Strain and Oxygen Mobility in a Thermally Stable Layered Fe3+ Oxide

The behavior of the layered Fe3+ perovskite superstructure with composition Ba1.7Ca2.4Y0.9Fe5O13 (BCYF) as an intermediate temperature solid oxide fuel cell (IT-SOFC) cathode is investigated by an integrated modeling, secondary ion mass spectroscopy (SIMS), processing, and electrochemistry approach. Electronic structure calculations identify the defect chemistry and energetics of the multiple diffusion pathways possible in the superstructure. This reveals a wide distribution of activation energies with the favorable pathways corresponding to transport along the layers. In contrast, interlayer oxygen transport paths require higher activation barriers, whose limiting step corresponds to incorporation of oxide ion vacancies in the Ca2Fe2O5 sub-block of the structure with tetrahedrally coordinated Fe3+ ions, with a calculated activation barrier of 1.46 eV. The latter is similar to the diffusion barrier of 1.41 eV evaluated by direct SIMS measurement on polycrystalline BCYF samples. The experimentally determined oxygen tracer diffusion coefficients (D*) vary by a factor of 4 depending on selected location, confirming the anisotropic nature of the oxygen ion migration. The random orientations of the grains in the polycrystal also result in highly anisotropic oxygen surface exchange properties. and the surface exchange coefficient, k*, shows a 4-fold difference from 1.2 × 10–8 to 2.9 × 10–9 cm s–1 at 587 °C, depending on the grain-sized region selected. Enhanced ceramic processing lowers the area specific resistance (ASR) to 0.10 Ω cm2 at 750 °C, with a wide electrochemically active zone of 30–60 μm. The low calculated activation energies for in-plane oxygen transport are explained by the presence of structural strains between the component units of the layer superstructure that increase the equilibrium energy of the mobile ions, which we define as internal activation strain. The observation of good surface activity and bulk diffusion performance, limited by microstructure in the current ceramic samples, indicates that BCYF is a potential candidate for IT-SOFC cathode applications, open to property optimization by textural control.