High sintering activity Cu-Gd co-doped CeO2 electrolyte for solid oxide fuel cells

Nano-sized Ce_0.79Gd_0.2Cu_0.01O_2−δ electrolyte powder was synthesized by the polyvinyl alcohol assisted combustion method, and then characterized by crystalline structure, powder morphology, sintering micro-structure and electrical properties. The results demonstrate that the as-synthesized Ce_0.79Gd_0.2Cu_0.01O_2−δ was well crystalline with cubic fluorite structure, and exhibited a porous foamy morphology composed of gas cavities and fine crystals ranging from 30 to 50 nm. After sintering at 1100 °C, the as-prepared pellets exhibited a dense and moderate-grained micro-structure with 95.54% relative density, suggesting that the synthesized Ce_0.79Gd_0.2Cu_0.01O_2−δ powder had high sintering activity. The powders made by this method are expected to offer potential application in intermediate-to-low temperature solid-oxide fuel cells, due to its very low densification sintering temperature (1100 °C), as well as high conductivity of 0.026 S cm⁻¹ at 600 °C and good mechanical performance with three-point flexural strength value of 148.15 ± 2.42 MPa.     

The heterogeneous electrolyte of CuFeO2 nano-flakes composited with flower-shaped ZnO for advanced solid oxide fuel cells

The flower-shaped ZnO was synthesized to form composite with the delafossite structure CuFeO₂. The composite heterojunction formed for the ZnO-CuFeO₂ composite material demonstrates a profound significance for exploring novel materials in solid oxide fuel cell (SOFC) field. At 550 °C, power outputs of 300 mW cm^?2 and 468 mW cm^?2 were achieved for SOFC devices using pure ZnO and composite with CuFeO₂ as the electrolytes, respectively. The composite showed a good performance at low temperatures, for instance, it showed a power output of 148 mW cm^?2 at 430 °C. The studies on photocurrent-time curves with visible light on/off irradiation provided an evidence for electron-hole separation. The heterojunctions separate holes and electrons, preventing short-circuiting while used in the SOFC device. These results demonstrate that introducing the heterojunctions in the electrolyte is an innovative approach for advanced SOFCs.     

Ni-Sm2O3 cermet anodes for intermediate-temperature solid oxide fuel cells with stabilized zirconia electrolytes

Ni-LnO_x cermets (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd), in which LnO_x is not an oxygen ion conductor, have shown high performance as the anodes for low-temperature solid oxide fuel cells (SOFCs) with doped ceria electrolytes. In this work, Ni-Sm₂O₃ cermets are primarily investigated as the anodes for intermediate-temperature SOFCs with scandia stabilized zirconia (ScSZ) electrolytes. The electrochemical performances of the Ni-Sm₂O₃ anodes are characterized using single cells with ScSZ electrolytes and LSM-YSB composite cathodes. The Ni-Sm₂O₃ anodes exhibit relatively lower performance, compared with that reported Ni-SDC (samaria doped ceria) and Ni-YSZ (yttria stabilized zirconia) anodes, the state-of-the-art electrodes for SOFCs based on zirconia electrolytes. The relatively low performance is possibly due to the solid-state reaction between Sm₂O₃ and ScSZ in fuel cell fabrication processes. By depositing a thin interlayer between the Ni-Sm₂O₃ anode and the ScSZ electrolyte, the performance is substantially improved. Single cells with a Ni-SDC interlayer show stable open circuit voltage, generate peak power density of 410 mW cm⁻² at 700 °C, and the interfacial polarization is about 0.7 Ω cm².     

CuFe2O4/CuO coating for solid oxide fuel cell steel interconnects

CuFe_0.8 (Fe:Cu = 0.8:1, atomic ratio) alloy layer is fabricated on both bare and pre-oxidized SUS 430 steels by direct current magnetron sputtering, followed by exposing at 800 °C in air to obtain a protective coating for solid oxide fuel cell (SOFC) steel interconnects. The CuFe_0.8 alloy layer is thermally converted to CuFe₂O₄/CuO coating, which effectively suppresses the out-migration of Cr. Pre-oxidation treatment not only initially accelerates the formation of CuFe₂O₄/CuO coating but also further inhibits the Cr and Fe outward diffusion. Suppressing outward diffusion of Cr could improve electrical property of oxide scale and decrease the risk of cathode Cr-poisoning. Blocking out-diffusion of Fe is beneficial to stabilize the CuO layer. After 2520 h oxidation, the scale ASR at 800 °C is 66.9 mΩ cm² for coated bare steel, 43.4 mΩ cm² for the coated pre-oxidized steel.     

Samarium doped ceria-(Li/Na)2CO3 composite electrolyte and its electrochemical properties in low temperature solid oxide fuel cell

A composite of samarium doped ceria (SDC) and a binary carbonate eutectic (52 mol% Li₂CO₃/48 mol% Na₂CO₃) is investigated with respect to its morphology, conductivity and fuel cell performances. The morphology study shows the composition could prevent SDC particles from agglomeration. The conductivity is measured under air, argon and hydrogen, respectively. A sharp increase in conductivity occurs under all the atmospheres, which relates to the superionic phase transition in the interface phases between SDC and carbonates. Single cells with the composite electrolyte are fabricated by a uniaxial die-press method using NiO/electrolyte as anode and lithiated NiO/electrolyte as cathode. The cell shows a maximum power density of 590 mW cm⁻² at 600 °C, using hydrogen as the fuel and air as the oxidant. Unlike that of cells based on pure oxygen ionic conductor or pure protonic conductor, the open circuit voltage of the SDC-carbonate based fuel cell decreases with an increase in water content of either anodic or cathodic inlet gas, indicating the electrolyte is a co-ionic (H⁺/O²⁻) conductor. The results also exhibit that oxygen ionic conductivity contributes to the major part of the whole conductivity under fuel cell circumstances.     

The effect of Fe2O3 sintering aid on Gd0.1Ce0.9O1.95 diffusion barrier layer and solid oxide fuel cell performance

Solid oxide fuel cells (SOFCs) with the Gd_0.1Ce_0.9O_1.95 (GDC) diffusion barrier layer require the densification of GDC to improve the performance of the cells. In this work, the addition of 0.5 mol% Fe₂O₃ in GDC diffusion barrier layer as sintering aid is studied. The symmetrical cell and the fuel cell are fabricated by hot-pressing, co-sintering and screen-printing technologies. It is found that the addition of Fe₂O₃ can make GDC barrier layer denser at a reduced sintering temperature of 1250 °C and prevent diffusion of Sr to form ionic insulating interface between YSZ (Y₂O₃ stabilized ZrO₂) and La_0.6Sr_0.4Co_0.8Fe_0.2O_3-δ (LSCF). The fuel cell based on the GDC-Fe₂O₃ barrier layer achieves better maximum power density of 0.89 W cm⁻² at 800 °C than that of 0.68 W cm⁻² without Fe₂O₃ addition. No obvious degradation was observed on fuel cell based on the GDC-Fe₂O₃ diffusion barrier layer after a stability test at 750 °C for 100 h under 0.75 V and the thermal cycling between 750 and 400 °C. The results indicate that the addition of Fe₂O₃ sintering aid in GDC diffusion barrier layer can promote the densification of GDC and exhibit good long-term stability and thermal cycle stability.     

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.     

Interlayer-free electrodes for IT-SOFCs by applying Co3O4 as sintering aid

The influence of Co₃O₄ as a sintering aid for a series of cobalt-containing perovskite oxides on the microstructure and electrical properties have been investigated. X-ray diffraction and scanning electron microscopic results showed that well connected electrode particles with firm adhesion to the 8 mol% yttria-stabilized zirconia (YSZ) electrolyte surface were realized at a temperature free from interfacial phase reaction. Both ohmic and polarization resistances of symmetric cells by adopting YSZ electrolyte, measured by electrochemical impedance spectroscopy, were much lower than that without adding Co₃O₄. The peak power density of 1176 mW cm⁻² at 750 °C was achieved when La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ + Co₃O₄ was selected as a representative cobaltite cathode, which is much higher than a similar fuel cell with the cathode fabricated by a conventional way. Fabrication of interlayer-free electrodes by applying Co₃O₄ as a sintering aid is very simple and general, applicable for a wide range of cobalt-containing electrode materials.     

Performance evaluation of combined solid oxide fuel cells with different electrolytes

The performance of a hybrid system of solid oxide fuel cells with different electrolytes, i.e., an oxygen-ion conducting electrolyte (SOFC-O²⁻) and a proton-conducting electrolyte (SOFC-H⁺) is evaluated in this study. Due to an internal reforming operation, SOFC-O²⁻ can produce electrical power as well as high-temperature exhaust gas containing remaining fuel, i.e., H₂ and CO that can be used for SOFC-H⁺ operation. The remaining CO can further react with H₂O via water gas-shift reaction to produce more H₂ within SOFC-H⁺ and thus, the possibility of carbon formation in SOFC-H⁺ can be eliminated and overall system efficiency can be improved. The simulation results show that the performance of the SOFC-O²⁻–SOFC-H⁺ system provides a higher efficiency (54.11%) compared with the use of a single SOFC. Further, the SOFC hybrid system performance is investigated with respect to important operating conditions, such as temperature, pressure, degree of pre-reforming, inlet fuel velocity, and cell voltage.     

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.     

Sintering and ionic conductivity of 8YSZ and CGO10 electrolytes with small addition of Fe2O3: A comparative study

Two typical electrolytes, i.e., 8YSZ (8 mol% yttria-stabilized zirconia) and CGO10 (10 mol% Gd-doped ceria), with Si contents of ∼30 ppm and ≥500 ppm were prepared, whose grain-boundary (GB) conductivities should be controlled by intrinsic (space-charge layer) and extrinsic (resistive siliceous films) effects, respectively. 1 at% FeO_1.5 was loaded into these materials via a conventional mixed-oxide method. A comparative study was carried out to demonstrate how 1 at% Fe addition affected these materials with different levels of SiO₂ impurities with respect to sintering, GB and GI (grain interior) conductivities. FeO_1.5 was found to be a sintering promoter for both 8YSZ and CGO10 ceramics, but it is more effective to enhance the densification of ceria-based electrolytes. A reduction in sintering temperature of ∼200 °C for 1 at% Fe-doped CGO10 was achieved compared with ∼110 °C reduction for the 8YSZ with the same amount of Fe loading. The effect of FeO_1.5 loading on the electrical conduction was found to be different, depending significantly on the impurity level and the types of electrolytes. In general, the loading of FeO_1.5 is positive for ceria-based ceramics since FeO_1.5 has a scavenging effect on SiO₂ impurity with little effect on the GI conduction. Although the scavenging behavior of FeO_1.5 was also found in the impure 8YSZ, it led to a significant reduction in the GI conductivity.     

The effect of current density on H2S-poisoning of nickel-based solid oxide fuel cell anodes

Sulphur-containing impurities can have a damaging impact on nickel-based SOFC anode performance even at sub-ppm concentrations, but the electrochemical mechanism of this interaction is not fully understood. In this work, three-electrode cells of Ni-Ce_0.9Gd_0.1O_1.95/YSZ/(La_0.8Sr_0.2)MnO_3−x have been used to obtain new electrochemical data on the sulphur poisoning behaviour of Ni-based SOFC anodes operating at different current densities in the temperature range 700-750 °C. The three-electrode arrangement enabled direct measurement of anode overpotential, with concurrent impedance measurement to provide detail into the electrochemical processes occurring at the anode during sulphur poisoning.The initial, stepwise degradation on exposure to 0.5 ppm H₂S caused an increase in anode polarization resistance, which was almost entirely recoverable on removal of H₂S. Operation at higher current density was found to result in a smaller increase in anode polarization resistance. It is proposed that this initial poisoning behaviour is caused by adsorbed sulphur inhibiting surface diffusion of H atoms to active sites.Exposure to 1 ppm and 3 ppm levels of H₂S led to an observed secondary degradation which was also recoverable on removal of sulphur. This degradation was caused by an increased ohmic resistance, and was more severe at higher temperatures. The authors discuss possible explanations for this behaviour.     

A novel design of anode-supported solid oxide fuel cells with Y2O3-doped Bi2O3, LaGaO3 and La-doped CeO2 trilayer electrolyte

Anode-supported solid oxide fuel cells (SOFCs) with a trilayered yttria-doped bismuth oxide (YDB), strontium- and magnesium-doped lanthanum gallate (LSGM) and lanthanum-doped ceria (LDC) composite electrolyte film are developed. The cell with a YDB (18 μm)/LSGM (19 μm)/LDC (13 μm) composite electrolyte film (designated as cell-A) shows the open-circuit voltages (OCVs) slightly higher than that of a cell with an LSGM (31 μm)/LDC (17 μm) electrolyte film (designated as cell-B) in the operating temperature range of 500-700 °C. The cell-A using Ag-YDB composition as cathode exhibits lower polarization resistance and ohmic resistance than those of a cell-B at 700 °C. The results show that the introduction of YDB to an anode-supported SOFC with a LSGM/LDC composite electrolyte film can effectively block electronic transport through the cell and thus increased the OCVs, and can help the cell to achieve higher power output.     

Ceria-carbonate composite for low temperature solid oxide fuel cell: Sintering aid and composite effect

In this study, the effect of carbonate content on microstructure, relative density, ionic conductivity and fuel cell performance of Ce_0.8Sm_0.2O_1.9-(Li/Na)₂CO₃ (SDC-carbonate, abbr. SCC) composites is systematically investigated. With the addition of carbonate, the nano-particles of ceria are well preserved after heat-treatment. The relative densities of SCC pellets increase as the carbonate content increases or sintering temperature rises. Especially, the relative density of SCC2 sintered at 900 °C is higher than that of pure SDC sintered at 1350 °C. Both the AC conductivity and DC oxygen ionic conductivity are visibly improved compared with the single phase SDC electrolyte. Among the composites, SDC-20 wt% (Li/Na)₂CO₃ (SCC20) presents high dispersion, relative small particle size, and the dense microstructure. The optimized microstructure brings the best ionic conductivity and fuel cell performance. It is hoped that the results can contribute the understanding of the role of carbonate in the composite materials and highlight their prospective application.     

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.     

Comparative study of the nano-composite electrolytes based on samaria-doped ceria for low temperature solid oxide fuel cells (LT-SOFCs)

Ceria-based electrolyte materials have great potential in low and intermediate temperature solid oxide fuel cell applications. In the present study, three types of ceria-based nano-composite electrolytes (LNK-SDC, LN-SDC and NK-SDC) were synthesized. One-step co-precipitation method was adopted and different techniques were applied to characterize the obtained ceria-based nano-composite electrolyte materials. TGA, XRD and SEM were used to analyze the thermal effect, crystal structure and morphology of the materials. Cubic fluorite structures have been observed in all composite electrolytes. Furthermore, the crystallite sizes of the LN-SDC, NK-SDC, LNK-SDC were calculated by Scherrer formula and found to be in the range 20 nm, 21 nm and 19 nm, respectively. These values emphasize a good agreement with the SEM results. The ionic conductivities were measured using EIS (Electrochemical Impedance Spectroscopy) with two-probe method and the activation energies were also calculated using Arrhenius plot. The maximum power density was achieved 484 mW/cm² of LNK-SDC electrolyte at 570 °C using the LiCuZnNi oxide electrodes.     

Sr2CoMoO6 anode for solid oxide fuel cell running on H2 and CH4 fuels

The double perovskite Sr₂CoMoO_6−δ was investigated as a candidate anode for a solid oxide fuel cell (SOFC). Thermogravimetric analysis (TGA) and powder X-ray diffraction (XRD) showed that the cation array is retained to 800 °C in H₂ atmosphere with the introduction of a limited concentration of oxide-ion vacancies. Stoichiometric Sr₂CoMoO₆ has an antiferromagnetic Néel temperature T_N ≈ 37 K, but after reduction in H₂ at 800 °C for 10 h, long-range magnetic order appears to set in above 300 K. In H₂, the electronic conductivity increases sharply with temperature in the interval 400 °C < T < 500 °C due to the onset of a loss of oxygen to make Sr₂CoMoO_6−δ a good mixed oxide-ion/electronic conductor (MIEC). With a 300-μm-thick La_0.8Sr_0.12Ga_0.83Mg_0.17O_2.815 (LSGM) as oxide-ion electrolyte and SrCo_0.8Fe_0.2O_3−δ as the cathode, the Sr₂CoMoO_6−δ anode gave a maximum power density of 1017 mW cm⁻² in H₂ and 634 mW cm⁻² in wet CH₄. A degradation of power in CH₄ was observed, which could be attributed to coke build up observed by energy dispersive spectroscopy (EDS).     

Novel electrolytes for solid oxide fuel cells with improved mechanical properties

The improvement of the mechanical properties of novel structured electrolytes with triangular cut off geometry in the active region is presented by filleting the tips of triangles. The effect of fillet radius on the bending strength of the yttria stabilized zirconia electrolyte was investigated with a commercial finite element code implementing the calculated Weibull stress through the experimental stress–strain curve determined via tensile tests. The model was verified with the experimental three point bending test results for the electrolyte with unfilleted triangular cut off patterns. Ten different fillet radii ranging from 0.05 mm to 0.5 mm were considered in the simulations. The fracture displacement was found to increase with increasing fillet radius as expected. Since the electrolyte with fillet radius of 0.5 mm was found to show the highest flexural strength, single cell based on this electrolyte was fabricated and the cell performance was measured. It was found that the strength of the novel electrolyte with partly reduced thickness can be increased by 26.2% with sacrificing only 10.2% decrease in the performance. Since the final cell still showed 22.2% higher peak performance than the standard electrolyte supported cell, 10.2% decrease in the cell performance compared to the cell having unfilleted triangular cut off patterns is acceptable.     

CeO2 coated NaFeO2 proton-conducting electrolyte for solid oxide fuel cell

Nowadays, the low-temperature operation has become an inevitable trend for the development of SOFCs. Transition metal layered oxides are considered as promising electrolyte materials for low-temperature solid oxide fuel cells (LT-SOFCs). In this work, we report the CeO₂ coated NaFeO₂ as an electrolyte material for LT-SOFC. The study results revealed that the piling of CeO₂ significantly influenced the open-circuit voltage (OCV) as well as the power output of the fuel cells. In comparison with pure NaFeO₂, the denser structure of CeO₂ coated NaFeO₂ leads to higher OCV (1.06 V, 550 °C). The electrochemical impedance spectrum (EIS) fitted results showed that NaFeO₂–CeO₂ composites possessed higher ionic boundary conductivity. This is because that the hetero-interfaces between NaFeO₂ and CeO₂ provide fast ion conducting path. The high ionic conductivity of CeO₂ coated NaFeO₂ lead to admirable fuel cell power output of 727 mW cm^?2 at 550 °C.     

Investigation on silver electric adhesive doped with Al2O3 ceramic particles for sealing planar solid oxide fuel cell

The silver electric adhesive doped with Al₂O₃ ceramic particles is used as sealing material for planar solid oxide fuel cell (SOFC). The sealing temperature of this sealing material is 600 °C with the heating rate of 2 °C min⁻¹, and the minimal leak rate ranges from 0.030 sccm cm⁻¹ to 0.040 sccm cm⁻¹. When doping 15 mass% Al₂O₃ ceramic particles into this sealing material, the thermal expansion coefficient of this material decreases from 20 ppm K⁻¹ to 15 ppm K⁻¹, which improves the thermal matching performance and the long-term stability of the material significantly. When using the gradient sealing method with the pure silver electric adhesive and the silver electric adhesive doped with Al₂O₃ ceramic particles to seal the interface of Ni-YSZ/SUS430 in the simulating cell, the minimal leak rate of 0.035 sccm cm⁻¹ is obtained for the cell. Furthermore, the simulating cell sealed with the compound silver electric adhesive presents good heat-resistant impact ability. Therefore, this compound sealing technique is a very promising sealing method for SOFC.