Functionally-graded composite cathodes for durable and high performance solid oxide fuel cells

A double-layer dual-composite cathode is fabricated and has an ideal cathode microstructure with large electrochemical active sites and enhanced the durability in solid oxide fuel cells (SOFCs). The insertion of a yttria-stabilized zirconia (YSZ)-rich functional layer between the electrolyte and the electrode allows for a graded transition of the YSZ phase, which enhances ionic percolation and minimizes the thermal expansion coefficient mismatch. Electrochemical measurements reveal that the double-layer composite cathode exhibits improved cathodic performance and long-term stability compared with a single-layer composite cathode. A cell with a well-controlled cathode maintains nearly constant interfacial polarization resistance during an 80 h accelerated lifetime test.     

Stable glass-ceramic sealants for solid oxide fuel cells: Influence of Bi2O3 doping

Diopside (CaMgSi₂O₆) based glass-ceramics in the system SrO–CaO–MgO–Al₂O₃–B₂O₃–La₂O₃–Bi₂O₃–SiO₂ have been synthesized for sealing applications in solid oxide fuel cells (SOFC). The parent glass composition in the primary crystallization field of diopside has been doped with different amounts of Bi₂O₃ (1, 3, 5 wt.%). The sintering behavior by hot-stage microscopy (HSM) reveals that all the investigated glass compositions exhibit a two-stage shrinkage behavior. The crystallization kinetics of the glasses has been studied by differential thermal analysis (DTA) while X-ray diffraction adjoined with Rietveld-R.I.R. analysis have been employed to quantify the amount of crystalline and amorphous phases in the glass-ceramics. Diopside and augite crystallized as the primary crystalline phases in all the glass-ceramics. The coefficient of thermal expansion (CTE) of the investigated glass-ceramics varied between (9.06–10.14) × 10⁻⁶ K⁻¹ after heat treatment at SOFC operating temperature for a duration varying between 1 h and 200 h. Further, low electrical conductivity, good joining behavior and negligible reactivity with metallic interconnects (Crofer22 APU and Sanergy HT) in air indicate that the investigated glass-ceramics are suitable candidates for further experimentation as sealants in SOFC.     

Performance and durability of nanostructured (La0.85Sr0.15)0.98MnO3/yttria-stabilized zirconia cathodes for intermediate-temperature solid oxide fuel cells

In this communication we report the fabrication of nanostructured (La_0.85Sr_0.15)_0.98MnO₃ (LSM)/yttria-stabilized zirconia (YSZ) composite cathodes consisting of homogeneously distributed and connected LSM and YSZ grains approximately 100 nm large. We also investigate for the first time the role of the cathode nanostructure on the performance and the durability of intermediate-temperature solid oxide fuel cells. The cathodes were fabricated using homogenous LSM/YSZ nanocomposite particles synthesized by co-precipitation, using YSZ nanoparticles of 3 nm as seed crystals. Detailed microstructural characterization by transmission electron microscopy with energy-dispersive X-ray spectroscopy revealed that many of the LSM/YSZ junctions in the cathode faced the homogeneously connected pore channels, indicating the formation of a considerable number of triple phase boundaries. The nanostructure served to reduce cathodic polarization. As a result, these anode-supported solid oxide fuel cells showed high power densities of 0.18, 0.40, 0.70 and 0.86 W cm⁻² at 650, 700, 750 and 800 °C, respectively, under the cell voltage of 0.7 V. Furthermore, no significant performance degradation of the cathode was observed during operation at 700 °C for 1000 h under a constant current density of 0.2 A cm⁻².     

La2O3-Al2O3-B2O3-SiO2 glasses for solid oxide fuel cell applications

La₂O₃-Al₂O₃-B₂O₃-SiO₂ glasses free of alkaline earth metals were prepared in this study for SOFC applications to relieve the poison caused by BaCrO₄ or SrCrO₄ formation. The apparent densities, coefficients of thermal expansion (CTE), and softening points of the La₂O₃-Al₂O₃-SiO₂-B₂O₃ glasses prepared in this study ranged respectively from 3.24 to 4.54 g/cm³, 4.1 to 8.1 ppm/°C, and 912 to 937 °C, depending on the glass composition. The CTE value dropped with the rise in SiO₂ content and escalated with increase in La₂O₃ content. Crystallization of La_9.51(SiO_1.0404)₆O₂ and La_4.67(SiO₄)₃O was observed in part of the glasses after soaking at 800 °C. Two CTE modifiers, MgO and SDC, effectively increased the CTE of La₂O₃-Al₂O₃-SiO₂-B₂O₃ glasses. The composites of selected La₂O₃-Al₂O₃-B₂O₃-SiO₂ glasses and SDC additive on the YSZ substrate were evaluated for use as sealing materials of solid oxide fuel cells (SOFCs). Results indicated that the leakage rates for the composites of A07 glass and 60-70 vol% SDC on the YSZ plate read less than 0.02 (sccm/cm)(kg/cm²) per min at 800 °C. This property seems highly promising for ensuring long-term stability of the sealing materials for SOFC applications.     

Preparation and electrochemical performance of Pr2Ni0.6Cu0.4O4 cathode materials for intermediate-temperature solid oxide fuel cells

Cathode material Pr₂Ni_0.6Cu_0.4O₄ (PNCO) for intermediate-temperature solid oxide fuel cells (IT-SOFCs) is synthesized by a glycine-nitrate process using Pr₆O₁₁, NiO, and CuO powders as raw materials. X-ray diffraction analysis reveals that nanosized Pr₂Ni_0.6Cu_0.4O₄ powders with K₂NiF₄-type structure can be obtained from calcining the precursors at 1000 °C for 3 h. Scanning electron microscopy shows that the sintered PNCO samples have porous microstructure with a porosity of more than 30% and grain size smaller than 2 μm. A maximum conductivity of 130 S cm⁻¹ is obtained from the PNCO samples sintered at 1050 °C. A single fuel cell based on the PNCO cathode with 30 μm Sm_0.2Ce_0.8O_1.9 (SCO) electrolyte film and a 1 mm NiO-SCO anode support is constructed. The ohmic resistance of the single Ni-SCO/SCO/PNCO cell is 0.08 Ω cm² and the area specific resistance (ASR) value is 0.19 Ω cm² at 800 °C. Cell performance was also tested using humidified hydrogen (3% H₂O) as fuel and air as oxidant. The single cell shows an open circuit voltage of 0.82 V and 0.75 V at 700 °C and 800 °C, respectively. Maximum power density is 238 mW cm⁻² and 308 mW cm⁻² at 700 °C and 800 °C, respectively. The preliminary tests have shown that Pr₂Ni_1−xCu_xO₄materials can be a good candidate for cathode materials of IT-SOFCs.     

Electrochemical characteristics of an La0.6Sr0.4Co0.2Fe0.8O3–La0.8Sr0.2MnO3 multi-layer composite cathode for intermediate-temperature solid oxide fuel cells

An La_0.6Sr_0.4Co_0.2Fe_0.8O₃–La_0.8Sr_0.2MnO₃ (LSCF–LSM) multi-layer composite cathode for solid oxide fuel cells (SOFCs) was prepared on an yttria-stabilized zirconia (YSZ) electrolyte by the screen-printing technique. Its cathodic polarization curves and electrochemical impedance spectra were measured and the results were compared with those for a conventional LSM/LSM–YSZ cathode. While the LSCF–LSM multi-layer composite cathode exhibited a cathodic overpotential lower than 0.13 V at 750 °C at a current density of 0.4 A cm⁻², the overpotential for the conventional LSM–YSZ cathode was about 0.2 V. The electrochemical impedance spectra revealed a better electrochemical performance of the LSCF–LSM multi-layer composite cathode than that of the conventional LSM/LSM–YSZ cathode; e.g., the polarization resistance value of the multi-layer composite cathode was 0.25 Ω cm² at 800 °C, nearly 40% lower than that of LSM/LSM–YSZ at the same temperature. In addition, an encouraging output power from an YSZ-supported cell using an LSCF–LSM multi-layer composite cathode was obtained.     

Preparation and characterization of silver-modified La0.8Sr0.2MnO3 cathode powders for solid oxide fuel cells by chemical reduction method

Herein a chemical reduction method is proposed in order to modify the solid oxide fuel cells (SOFC) traditional cathode material La_0.8Sr_0.2MnO_3−x (LSM). Silver nanoparticles were prepared by the reduction of ammoniacal silver nitrate with ascorbic acid in dilute aqueous solutions containing PVP. The obtained LSM–Ag composite powders were characterized by XRD, SEM, EDX, and STEM. The results showed that the LSM–Ag composite powder possess an elaborated fine structure with a homogeneous distribution of Ag and LSM, which effectively shortens the diffusion pathway for electrons and adsorbed oxygen. The electrochemical performance of the LSM–Ag cathode with different Ag loadings was investigated. A cathode loading with 1 wt.% Ag exhibited an area specific resistance as low as 0.45 Ω cm² at 750 °C, compared to around 1.1 Ω cm² for a pure LSM electrode. Similarly an anode-supported SOFC with 1 wt.% Ag in the cathode shows a peak power density of 1199 mW cm⁻¹, higher than the value of 717 mW cm⁻¹ achieved for a similar cell with a LSM cathode. Increasing the Ag loading is shown to have an insignificant effect on improving electrocatalytic performance at 750 °C, however it can increase output power at 650 °C.     

Compositional influence of LSM-YSZ composite cathodes on improved performance and durability of solid oxide fuel cells

The use of a dual-composite approach, in which both LSM and YSZ nanoparticles are placed on YSZ core particles, allows for the development of an ideal cathode microstructure with improved phase contiguity and durability for use in solid oxide fuel cells. The volume fraction of the conjugated YSZ phase plays a critical role in the optimization of the cathode microstructure for achieving good durability because it acts as an interconnecting bridge between YSZ core particles. However, the presence of excess conjugated YSZ phase interrupts the formation of sufficient triple phase boundary sites by disturbing the contacts between the LSM and YSZ phases. Impedance spectroscopy analysis and microstructural observations provide a better understanding of the influence of the composition on the electrochemical performance and durability of these dual composite cathodes.     

La0.84Sr0.16MnO3−δ cathodes impregnated with Bi1.4Er0.6O3 for intermediate-temperature solid oxide fuel cells

La_0.84Sr_0.16MnO_3−δ–Bi_1.4Er_0.6O₃ (LSM–ESB) composite cathodes are fabricated by impregnating LSM electronic conducting matrix with the ion-conducting ESB for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The performance of LSM–ESB cathodes is investigated at temperatures below 750 °C by AC impedance spectroscopy. The ion-impregnation of ESB significantly enhances the electrocatalytic activity of the LSM electrodes for the oxygen reduction reactions, and the ion-impregnated LSM–ESB composite cathodes show excellent performance. At 750 °C, the value of the cathode polarization resistance (R_p) is only 0.11 Ω cm² for an ion-impregnated LSM–ESB cathode, which also shows high stability during a period of 200 h. For the performance testing of single cells, the maximum power density is 0.74 W cm⁻² at 700 °C for a cell with the LSM–ESB cathode. The results demonstrate the ion-impregnated LSM–ESB is one of the promising cathode materials for intermediate-temperature solid oxide fuel cells.     

Nanoscale bismuth oxide impregnated (La,Sr)MnO3 cathodes for intermediate-temperature solid oxide fuel cells

Bismuth oxide based oxygen ion conductors are incorporated into (La,Sr)MnO₃ (LSM), the classical cathode material for solid oxide fuel cells (SOFC), to improve the cathode performance. Yttria-stabilized bismuth oxide (YSB) is taken as an example and is impregnated into a preformed porous LSM frame, forming a highly active cathode for intermediate-temperature SOFCs (IT-SOFCs) with doped ceria electrolytes. X-ray diffraction indicates that YSB is chemically compatible with LSM at intermediate temperatures below 800 °C. The impregnated YSB particles are nanosized and are deposited on the surface of the framework. Significant performance improvement is achieved by introducing nanosized YSB into the LSM electrodes. At 600 °C, the interfacial polarization resistance under open-circuit conditions for electrodes impregnated with 50% YSB is only 1.3% of the original value for a pure LSM electrode. The resistance is further reduced dramatically when current is passed through. In addition, the YSB impregnated LSM electrodes has the highest electrochemical performance among those based on LSM. Single cell with 25% of YSB impregnated LSM cathode generates maximum power density of 300 mW cm⁻² at 600 °C, indicating the promise of using LSM-based electrodes for IT-SOFC.     

Co-sintering anode and Y2O3 stabilized ZrO2 thin electrolyte film for solid oxide fuel cell fabricated by co-tape casting

Fabricating a large-area unit cell is very important for the development of solid oxide fuel cell (SOFC) stack. In this study, details of sintering process of half cell with NiO-yttria stabilized zirconia (YSZ) anode-supported YSZ thin electrolyte film fabricated by co-tape casting have been discussed. The results demonstrates that the shrinkages and shrinking rates mismatches between the electrolyte and the anode can be controlled by the organic additive content in the anode slurry composition and heating rate. Low heating rate suppresses the cracks formation in the electrolyte films. A warp-free unit cell with size of 100 × 100 mm² and dense electrolyte has been successfully fabricated. A power of 22.2 W, with a power density of 0.27 W cm⁻² has been achieved at 0.7 V and 750 °C in O₂/humidified H₂ atmosphere. The area specific resistance of the cell is 1.20 Ω cm² at 0.7 V and 750 °C.     

LaNi0.6Fe0.4O3-Ce0.8Sm0.2O1.9-Ag composite cathode for intermediate temperature solid oxide fuel cells

LaNi_0.6Fe_0.4O₃ (LNF), LNF-Sm_0.2Ce_0.8O_1.9 (SDC), and LNF-SDC-Ag cathodes on SDC electrolytes were investigated at intermediate temperatures using AC impedance spectroscopy. Results show that adding 50 wt.% SDC into LNF yields a significant low area specific resistance (ASR) which was found to be 0.92 Ω cm² at 700 °C. Infiltrating 0.3 mg/cm² Ag into LNF-50 wt.% SDC can improve the electronic conductivity and oxygen exchange reaction activity, and thereby remarkably decrease the ASRs. The ASR value of the LNF-SDC-Ag cathode is as low as 0.18 Ω cm² at 700 °C, and 0.46 Ω cm² at 650 °C. The long-term test shows that the LNF-SDC-Ag cathode may be a promising candidate for solid oxide fuel cells operating at temperatures lower than 650 °C.     

Preparation and characterization of graded cathode La0.6Sr0.4Co0.2Fe0.8O3−δ

Perovskite oxide La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF6428), a wonderful electronic–ionic conductor could be used as cathode of solid oxide fuel cell (SOFC). Graded cathode with coarse layer and fine layer, could improve the diffusion rate and electrochemical reaction activity of oxidant. The fabrication and properties of graded LSCF6428 cathode were discussed in this paper. First, pure perovskite LSCF6428 powders were prepared by citrate–EDTA method (CEM), citrate method (CM) and solid phase synthesis (SPS). The powders with higher specific surface area and smaller grain size are easier to be sintered and densified. Single LSCF6428 cathode with thickness of 30 μm was prepared by SPS powders, the porosity of cathode was high about 30% and pore size was about 5 μm. Graded LSCF6428 cathode including 30 μm outer layer and 10 μm inner layer was prepared by SPS and CM powders, respectively. Clear double-layer cathode was observed by SEM, which combined tightly and transited gradually. Porosity of outer layer is high about 30% and pore size is about 1–5 μm; inner layer is finer and pore size is about 0.2–1 μm. Based on the above research, 300 μm yttria stabilized zirconia (YSZ) electrolyte supported cell with single LSCF6428 cathode and double-layer LSCF6428 cathode were prepared, and the properties of two type cells were tested in H₂. Power density of graded cell is 197 mW cm⁻² at 950 °C, and improved about 46% comparing that of single layer LSCF6428 cell (135 mW cm⁻²).     

Bi0.5Sr0.5MnO3 as cathode material for intermediate-temperature solid oxide fuel cells

Bi_0.5Sr_0.5MnO₃ (BSM), a manganite-based perovskite, has been investigated as a new cathode material for intermediate-temperature solid oxide fuel cells (SOFCs). The average thermal-expansion coefficient of BSM is 14 × 10⁻⁶ K⁻¹, close to that of the typical electrolyte material. Its electrical conductivity is 82-200 S cm⁻¹ over the temperature range of 600-800 °C, and the oxygen ionic conductivity is about 2.0 × 10⁻⁴ S cm⁻¹ at 800 °C. Although the cathodic polarization behavior of BSM is similar to that of lanthanum strontium manganite (LSM), the interfacial polarization resistance of BSM is substantially lower than that of LSM. The cathode polarization resistance of BSM is only 0.4 Ω cm² at 700 °C and it decreases to 0.17 Ω cm² when SDC is added to form a BSM-SDC composite cathode. Peak power densities of single cells using a pure BSM cathode and a BSM-SDC composite electrode are 277 and 349 mW cm² at 600 °C, respectively, which are much higher than those obtained with LSM-based cathode. The high electrochemical performance indicates that BSM can be a promising cathode material for intermediate-temperature SOFCs.     

Optimization of La0.6Ca0.4Fe0.8Ni0.2O3-Ce0.8Sm0.2O2 composite cathodes for intermediate-temperature solid oxide fuel cells

Sample of nominal composition La_0.6Ca_0.4Fe_0.8Ni_0.2O₃ (LCFN) was prepared by liquid mix method. The structure of the polycrystalline powder was analyzed with X-ray powder diffraction data. This compound shows orthorhombic perovskite structure with a space group Pnma. In order to improve the electrochemical performance, Sm-doped ceria (SDC) powder was added to prepare the LCFN-SDC composite cathodes. Electrochemical characteristics of the composites have been investigated for possible application as cathode material for an intermediate-temperature-operating solid oxide fuel cell (IT-SOFC). The polarization resistance was studied using Sm-doped ceria (SDC). Electrochemical impedance spectroscopy measurements of LCFN-SDC/SDC/LCFN-SDC test cell were carried out. These electrochemical experiments were performed at equilibrium from 850 °C to room temperature, under both zero dc current intensity and air. The best value of area-specific resistance (ASR) was for LCFN cathode doped with 10% of SDC (LCFN-SDC9010), 0.13 Ω cm² at 850 °C. The dc four-probe measurement exhibits a total electrical conductivity over 100 S cm⁻¹ at T ≥ 600 °C for LCFN-SDC9010 composite cathode.     

Study of oxygen reduction mechanism on Ag modified Sm1.8Ce0.2CuO4 cathode for solid oxide fuel cell

Different amount of metal silver particles are infiltrated into porous Sm_1.8Ce_0.2CuO₄ (SCC) scaffold to form SCC–Ag composite cathodes. The chemical stability, microstructure evolution and electrochemical performance of the composite cathode are investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and AC impedance spectroscopy respectively. The composite cathode exhibits enhanced chemical stability. The metal Ag remains un-reacted with SCC and Ce_0.9Gd_0.1O_1.95 (CGO) at 800 °C for 72 h. The polarization resistance of the composite cathode decreases with the addition of metal Ag. The optimum cathode SCC-Ag05 exhibits the lowest area specific resistance (ASR, 0.43 Ω cm²) at 700 °C in air. Investigation shows that metal Ag accelerates the charge transfer process in the composite cathode, and the rate limiting step for electrochemical oxygen reduction reaction (ORR) changes to oxygen dissociation and diffusion process.     

Nanostructured La0.6Sr0.4Co0.8Fe0.2O3/Y0.08Zr0.92O1.96/La0.6Sr0.4Co0.8Fe0.2O3 (LSCF/YSZ/LSCF) symmetric thin film solid oxide fuel cells

Functional all-oxide thin film micro-solid oxide fuel cells (μSOFCs) that are free of platinum (Pt) are discussed in this report. The μSOFCs, with widths of 160 μm, consist of thin film La_0.6Sr_0.4Co_0.8Fe_0.2O₃ (LSCF) as both the anode and cathode and Y_0.08Zr_0.92O_1.96 (YSZ) as the electrolyte. Open circuit voltage and peak power density at 545 °C are 0.18 V and 210 μW cm⁻², respectively. The LSCF anodes show good lattice and microstructure stability and do not form reaction products with YSZ. The all-oxide μSOFCs endure long-term stability testing at 500 °C for over 100 h, as manifested by stable membrane morphology and crack-free microstructure.     

Oxidation behavior of (Co,Mn)3O4 coatings on preoxidized stainless steel for solid oxide fuel cell interconnects

Ceramic coatings are being explored to extend the lifetime of stainless steel interconnects in planar Solid Oxide Fuel Cells (SOFCs). One promising coating is Co_1.5Mn_1.5O₄ spinel, which is deposited using various techniques, resulting in different coating thicknesses, compositions and microstructures. In this study, stainless steel 441HP samples were subjected to three levels of preoxidation (0, 3, 10 and 100 h in 800 °C lab air) prior to coating. Samples were coated with 2 μm CoMn alloy using magnetron sputtering and were subsequently annealed in 800 °C air for 0, 10, 100 or 1650 h. Oxidation behaviors were evaluated as a function of these exposures, as well as in dual atmospheres and during area specific resistance (ASR) measurements in 800 °C lab air. Preoxidation was found to inhibit Fe and Cr transport from the stainless steel into the coating and preoxidized samples exhibited a substantially thinner surface layer after oxidation. After ASR testing for 1650 h in 800 °C air, the trend of the preoxidized sample values remained level while trend of the non-preoxidized sample values showed an increase. Observed oxidation behaviors, their possible mechanisms, and implications for SOFC interconnects are presented and discussed.     

Solid oxide fuel cell cathodes prepared by infiltration of LaNi0.6Fe0.4O3 and La0.91Sr0.09Ni0.6Fe0.4O3 in porous yttria-stabilized zirconia

SOFC composite electrodes of yttria-stabilized zirconia (YSZ) and either LaNi_0.6Fe_0.4O₃ (LNF) or La_0.91Sr_0.09Ni_0.6Fe_0.4O₃ (LSNF) were prepared by infiltration to a loading of 40 wt% of the perovskite into porous YSZ using aqueous solutions of the nitrate salts. XRD measurements indicated that the perovskite structures were formed following calcination at 850 °C, at which temperature the LNF and LSNF form small particles that coat the YSZ pores. Heating to 1100 °C causes the particles to form a dense film over the YSZ but caused no solid-state reaction. Calcination of an LNF-YSZ composite to 1200 °C led to an expansion of the LNF lattice, suggesting introduction of Zr(IV) into the perovskite; further heating to 1300 °C caused the formation of La₂Zr₂O₇. For 850 °C calcination, the electrode performance of both LNF-YSZ and LSNF-YSZ composites was similar to that reported for composites of YSZ and La_0.8Sr_0.2FeO₃ (LSF), with a current-independent impedance of approximately 0.1 Ω cm² at 700 °C in air. For 1100 °C calcination, both LNF-YSZ and LSNF-YSZ composites exhibited impedances that decreased strongly under both anodic and cathodic polarization. The implications of these results for preparing electrodes based on LNF and LSNF are discussed.     

Effect of additives on the thermal properties and sealing characteristic of BaO-Al2O3-B2O3-SiO2 glass-ceramic for solid oxide fuel cell application

The apparent densities, coefficients of thermal expansion (CTE), and softening points of the BaO-Al₂O₃-SiO₂-B₂O₃ glasses prepared in this study range from 2.61 to 3.92 g/cm³, 4.92 to 10.98 ppm/°C, and 656 to 854 °C, respectively, depending on the glass composition. The softening point decreases with increasing BaO or B₂O₃ content in the glass. The feasibility of using MgO, KAlSi₂O₆, and KAlSiO₄ to tune the CTE of BaO-Al₂O₃-SiO₂-B₂O₃ glass is investigated. Composites containing MgO additive show little change in CTE at high temperature and report a high structural stability with the change in time. Wetting angle of the glass on the YSZ substrate depends to a great extent on the soaking temperature and the BaO content in the glass. Composites of selected BaO-Al₂O₃-SiO₂-B₂O₃ glasses and MgO additive on the YSZ substrate are evaluated for use as sealing material of solid oxide fuel cells (SOFCs). Results indicate that leakage rates for the composites of L06-20 vol% MgO, L08-30 vol% MgO, and L09-40 vol% MgO are lower than the detectable limit in this study.