Development of Al2O3/glass-based multi-layer composite seals for planar intermediate-temperature solid oxide fuel cells

In this paper, the novel multi-layer composite seals for planar solid oxide fuel cells are studied. The composite seals with sandwiched structure include Al₂O₃-based tape as support and glass-ceramic slurry as binder connecting the interface of the neighboring components. The result finds out that glass-ceramic slurry with 20 wt% Al₂O₃ has the suitable strength and deformability. The thermal cycle characteristics are greatly improved by using the multi-layer composite seals, and the corresponding leakage rates are lower than 0.025 sccm cm⁻¹ for 20 thermal cycles at the inlet pressure ranging from 0.5 psi to 2 psi. SEM investigations show a very compact and good adhesion between the neighboring components, which can minimize the leakage paths. Single cell testing is used to examine the performance of the seals. The value of open circuit voltage is 1.17 V. At the constant discharge current density of 0.37 A cm⁻², the voltage is stabilized at about 0.85 V for 50 h. The results demonstrate that the novel multi-layer composite seals are good candidate for SOFC application.     

Sr2Fe4/3Mo2/3O6 as anodes for solid oxide fuel cells

Sr₂Fe_4/3Mo_2/3O₆ has been synthesized by a combustion method in air. It shows a single cubic perovskite structure after being reduced in wet H₂ at 800 °C and demonstrates a metallic conducting behavior in reducing atmospheres at mediate temperatures. Its conductivity value at 800 °C in wet H₂ (3% H₂O) is about 16 S cm⁻¹. This material exhibits remarkable electrochemical activity and stability in H₂. Without a ceria interlayer, maximum power density (P_max) of 547 mW cm⁻² is achieved at 800 °C with wet H₂ (3% H₂O) as fuel and ambient air as oxidant in the single cell with the configuration of Sr₂Fe_4/3Mo_2/3O₆|La_0.8Sr_0.2Ga_0.83Mg_0.17O₃ (LSGM)| La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF). The P_max even increases to 595 mW cm⁻² when the cell is operated at a constant current load at 800 °C for additional 15 h. This anode material also shows carbon resistance and sulfur tolerance. The P_max is about 130 mW cm⁻² in wet CH₄ (3% H₂O) and 472 mW cm⁻² in H₂ with 100 ppm H₂S. The cell performance can be effectively recovered after changing the fuel gas back to H₂.     

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.     

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.     

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.     

Improved electrical conductivity of Ce0.9Gd0.1O1.95 and Ce0.9Sm0.1O1.95 by co-doping

Solid electrolytes are the most important and indispensable part of a solid oxide fuel cell (SOFC) where hydrogen is used as one of the fuels to obtain electricity. Ce_0.9Gd_0.1O_1.95 and Ce_0.9Sm_0.1O_1.95 were chosen to be the base electrolytes. The effects of MgO and Nd₂O₃ as co-dopants on the electrical conductivity were investigated, respectively. For 4 mol% Mg-doped Ce_0.9Gd_0.1O_1.95 or Ce_0.9Sm_0.1O_1.95, MgO phases were detected by FESEM micrographs, which showed a very low solubility of Mg²⁺ in ceria lattice. The existence of MgO phases was observed to have negligible effect on the grain conductivity, but improve the grain boundary conductivity measured by ac impedance spectroscopy. However, when Nd₂O₃ was used as a co-dopant, XRD patterns and FESEM both indicated a pure cubic phase. Ce_0.9Gd_0.05Nd_0.05O_1.95 and Ce_0.9Sm_0.05Nd_0.05O_1.95 were found to exhibit higher grain conductivity, comparing with single-doped ceria.     

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.     

Physically mixed LiLaNi-Al2O3 and copper as conductive anode catalysts in a solid oxide fuel cell for methane internal reforming and partial oxidation

Different concentrations of copper are added to LiLaNi-Al₂O₃ to improve the electronic conductivity property for application as the materials of the anode catalyst layer for solid oxide fuel cells operating on methane. Their catalytic activity for the methane partial oxidation, steam and CO₂ reforming reactions at 600-850 °C is systematically investigated. Among the three catalysts, the LiLaNi-Al₂O₃/Cu (50:50, by weight) catalyst presents the best catalytic activity. Thus, the catalytic stability, carbon deposition and surface conductivity of the LiLaNi-Al₂O₃/Cu catalyst are further studied in detail. O₂-TPO results indicate that the coking resistance of LiLaNi-Al₂O₃/Cu is satisfactory and comparable to that of LiLaNi-Al₂O₃. The surface conductivity tests demonstrate it is extremely improved for LiLaNi-Al₂O₃ catalyst due to the addition of 50 wt.% copper. A cell with LiLaNi-Al₂O₃/Cu (50:50) catalyst layer is operated on mixtures of methane-O₂, methane-H₂O and methane-CO₂, and peak power densities of 1081, 1036 and 988 mW cm⁻² are obtained at 850 °C, respectively, comparable to the cell with LiLaNi-Al₂O₃ catalyst layer. In summary, the results of the present study indicate that LiLaNi-Al₂O₃/Cu (50:50) catalysts are highly coking resistant and conductive catalyst layers for solid oxide fuel cells.     

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.     

Layered GdBa0.5Sr0.5Co2O5+δ as a cathode for intermediate-temperature solid oxide fuel cells

The layered GdBa_0.5Sr_0.5Co₂O_5+δ (GBSC) perovskite oxides are synthesized by Pechini method and investigated as a novel cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The single cell of NiO–SDC (Sm_0.2Ce_0.8O_1.9)/SDC (20 μm)/GBSC (10 μm) is operated from 550 to 700 °C fed with humidified H₂ as fuel and the static air as oxidant. An open circuit voltage of 0.8 V and a maximum power density of 725 mW cm⁻² are achieved at 700 °C. The interfacial polarization resistance is as low as 0.88, 0.29, 0.13 and 0.05 Ω cm² at 550, 600, 650 and 700 °C, respectively. The ratio of polarization resistance to total cell resistance decreases with the increase in the operating temperature, from 60% at 550 °C to 21% at 700 °C, respectively. The experimental results indicate that GBSC is a promising cathode material for IT-SOFCs.     

The presence of nanostructured Al2O3 in PMMA-based gel electrolytes

Gel polymer electrolytes (GPEs)-based on poly(methyl methacrylate) PMMA and propylene carbonate (PC) with LiClO₄ or NaClO₄ salt are prepared using either the commercial product Superacryl^® or directly from the monomer and AIBN (2,2′-azobis(isobutyronitrile)) initiator. The nanostructured aluminum oxide is added to the mentioned systems in various ratios. Solutions of liquid PC–perchlorate and polymer electrolytes are compared with focus on ionic conductivity. The ionic conductivity of polymer-based electrolytes is significantly influenced (of almost by one half order of magnitude at room temperature) by the addition of nanosized Al₂O₃. On the contrary, the conductivity of liquid electrolytes is decreased by the addition of alumina in the blend. A slight enhancement of mechanical properties is observed.     

Preparation and characterization of Sm0.2Ce0.8O1.9/Na2CO3 nanocomposite electrolyte for low-temperature solid oxide fuel cells

Sm_0.2Ce_0.8O_1.9 (SDC)/Na₂CO₃ nanocomposite synthesized by the co-precipitation process has been investigated for the potential electrolyte application in low-temperature solid oxide fuel cells (SOFCs). The conduction mechanism of the SDC/Na₂CO₃ nanocomposite has been studied. The performance of 20 mW cm⁻² at 490 °C for fuel cell using Na₂CO₃ as electrolyte has been obtained and the proton conduction mechanism has been proposed. This communication demonstrates the feasibility of direct utilization of methanol in low-temperature SOFCs with the SDC/Na₂CO₃ nanocomposite electrolyte. A fairly high peak power density of 512 mW cm⁻² at 550 °C for fuel cell fueled by methanol has been achieved. Thermodynamical equilibrium composition for the mixture of steam/methanol has been calculated, and no presence of C is predicted over the entire temperature range. The long-term stability test of open circuit voltage (OCV) indicates the SDC/Na₂CO₃ nanocomposite electrolyte can keep stable and no visual carbon deposition has been observed over the anode surface.     

Measurement of oxygen chemical potential in Gd2O3-doped ceria-Y2O3-stabilized zirconia bi-layer electrolyte, anode-supported solid oxide fuel cells

Solid oxide fuel cells (SOFC) were fabricated with gadolinia-doped ceria (GDC)-yttria stabilized zirconia (YSZ), thin bi-layer electrolytes supported on Ni + YSZ anodes. The GDC and YSZ layer thicknesses were 45 μm, and ∼5 μm, respectively. Two types of cells were made; YSZ layer between anode and GDC (GDC/YSZ) and YSZ layer between cathode and GDC (YSZ/GDC). Two platinum reference electrodes were embedded within the GDC layer. Cells were tested at 650 °C with hydrogen as fuel and air as oxidant. Electric potentials between embedded reference electrodes and anode and between cathode and anode were measured at open circuit, short circuit and under load. The electric potential was nearly constant through GDC in the cathode/YSZ/GDC/anode cells. By contrast, it varied monotonically through GDC in the cathode/GDC/YSZ/anode cells. Estimates of oxygen chemical potential, μO₂, variation through GDC were made. μO₂ within the GDC layer in the cathode/GDC/YSZ/anode cell decreased as the current was increased. By contrast, μO₂ within the GDC layer in the cathode/YSZ/GDC/anode cell increased as the current was increased. The cathode/YSZ/GDC/anode cell exhibited maximum power density of ∼0.52 W cm⁻² at 650 °C while the cathode/GDC/YSZ/anode cell exhibited maximum power density of ∼0.14 W cm⁻² for the same total electrolyte thickness.     

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.     

Structural,electrical, and electrochemical properties of cobalt-doped NiFe2O4 as a potential cathode material for solid oxide fuel cells

In this work, Co-doped NiFe₂O₄ spinels (NFCO-x) are successfully fabricated and characterized as possible cathode materials for the intermediate-temperature solid oxide fuel cells (SOFC). Results of the binding energy calculations using the density functional theory suggest that the reverse spinel structure is stable when Co³⁺ occupies the octahedral interstitial sites. Total and ionic-only conductivities indicate that NFCO-x are a kind of mixed electronic-ionic conductors. Ionic transferring numbers are approximately 0.049 and 0.006 for NFCO-0.1 and NFCO-0.5, respectively, measured at 700 °C in air. Co dopant in the NFCO-x improves the electronic conductivity at the expense of the ionic conductivity. For NFCO-0.5, electronic and ionic conductivities are approximately 0.24 and 9.6 × 10⁻⁴ S cm⁻¹, respectively, measured also at 700 °C in air. Unlike behaviour of the conductivities, the polarization resistance of symmetric cells with NFCO-x electrodes decreases when increasing the Co content (x) to a certain level, and then increases. The cell containing the NFCO-0.5 electrode exhibits the lowest polarization resistance (R_p), which is approximately 1.51 Ω cm² at 650 °C. For single cells, the maximum power density is 320 mW cm⁻² measured at 650 °C using a 38-μm-thick SDC electrolyte and an NFCO-0.5 cathode.     

Performance of double-proveskite YBa0.5Sr0.5Co2O5+δ as cathode material for intermediate-temperature solid oxide fuel cells

Double-proveskite YBa_0.5Sr_0.5Co₂O_5+δ (YBSC) was investigated as potential cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs). YBSC material exhibited a good chemical compatibility with the La_0.9Sr_0.1Ga_0.8Mg_0.115Co_0.085O_2.85 (LSGMC) electrolyte up to 950 °C for 2 h. The substitution of Sr for Ba significantly enhanced the electrical conductivity of the YBSC sample compared to undoped YBaCo₂O_5+δ, and also slightly increased the thermal expansion coefficient. At 325 °C a semiconductor-metal transition was observed and the maximum electrical conductivity of YBSC was 668 S cm⁻¹. The maximum power densities of the electrolyte-supported single cell with YBSC cathode achieved 650 and 468 mW cm⁻² at 850 and 800 °C, respectively. Preliminary results suggested that YBSC could be considered as a candidate cathode material for application in IT-SOFCs.     

Cobalt-free layered perovskite GdBaFe2O5+x as a novel cathode for intermediate temperature solid oxide fuel cells

A cobalt-free layered perovskite oxide, GdBaFe₂O_5+x (GBF), was investigated as a novel cathode for intermediate temperature solid oxide fuel cells (IT-SOFCs). Area-specific resistance (ASR) of GBF was measured by impedance spectroscopy in a symmetrical cell. The observed ASR was as low as 0.15 Ω cm² at 700 °C and 0.39 Ω cm² at 650 °C, respectively. A laboratory sized Sm_0.2Ce_0.8O_1.9 (SDC)-based tri-layer cell of NiO-SDC/SDC/GBF was tested under intermediate temperature conditions of 550-700 °C with humidified H₂ (∼3% H₂O) as a fuel and the static ambient air as an oxidant. A maximal power density of 861 mW cm⁻² was achieved at 700 °C. The electrode polarization resistance was as low as 0.57, 0.22, 0.13 and 0.08 Ω cm² at 550, 600, 650 and 700 °C, respectively. The experimental results indicate that the layered perovskite GBF is a promising cathode candidate for IT-SOFCs.     

Constrained sintering of Y2O3-stabilized ZrO2 electrolyte on anode substrate

Understanding the sintering processes extensively is critical in fabricating a flat cell for solid oxide fuel cell stacks, but few have reported the sintering process and stress development during the constrained sintering of the electrolyte layer on anode substrate. In this study, we show that the green tape of half cell fabricated by co-tape casting cracks into several pieces when it is heated directly to 1400 °C of profile I, while it remains flat and complete when the green tape is sintered with additional pre-sintered profile at 1300 °C of profile II. The strain rate characteristics indicate that the difference of 2.43 × 10⁻⁶ s⁻¹ between the electrolyte and the anode layer leads to the stress development in the directly sintered cell, while it reduces to 6.7 × 10⁻⁸ s⁻¹ for the pre-sintered cell, which is only 3% of that without pre-sintering. The stress based on continuum model calculated results in the sintered cell demonstrates that the stress increases from 0 at about 1000 °C to 2.60 MPa at 1300 °C, and increased from 2.60 MPa to 6.54 MPa in temperature range of 1300–1400 °C. But it was lower than half of the stress for the pre-sintered cell according to profile II. The SEM images, together with a circuit voltage of 2.22 V for two cell stack, indicate that the electrolyte of the unit cell is dense. The power is 41.7 W, with a power density of 0.26 W cm⁻² at 1.4 V and 750 °C for a two tells stacks sintered according to profile II. The ASR of the two cells stack is 2.50 Ω cm².     

Infiltrated Pr2NiO4 as promising bi-electrode for symmetrical solid oxide fuel cells

In-situ growth of nanoparticles on electrode surface for high temperature energy conversion devices is one of efficient ways for new electro-catalyst design. Here, perovskite-related Pr₂NiO₄ (PNO) has been evaluated as a novel symmetrical electrode for intermediate-temperature solid oxide fuel cells (IT-SOFCs). Nickel nanoparticles are exsolved and dispersed on the surface of PNO after exposed in H₂ at 800 °C as an efficient electrocatalyst for hydrogen oxidation reaction (HOR) as anode. The electro-activity towards oxygen reduction reaction (ORR) at cathode side is further improved by infiltration process. A synergetic effect of in-situ exsolved nanoparticles as well as infiltrated ionic conducting particles have improved HOR and ORR activity at anode and cathode side, respectively. Furthermore, symmetrical SOFC (SSOFC) with infiltrated Pr₂NiO₄ as bi-electrode exhibits excellent short-term stability and reliable redox stability in repeated H₂/air cycles.     

High performance layered SmBa0.5Sr0.5Co2O5+δ cathode for intermediate-temperature solid oxide fuel cells

The layered SmBa_0.5Sr_0.5Co₂O_5+δ (SBSC) perovskite oxide is synthesized by the Pechini method and investigated as a novel cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs). A laboratory-sized Sm_0.2Ce_0.8O_1.9 (SDC)-based tri-layer cell of NiO–SDC/SDC/SBSC is operated from 500 to 700 °C fed with humidified H₂ (3% H₂O) as a fuel and the static ambient air as oxidant. A maximum power density of 1147 mW cm⁻² is achieved at 700 °C. The interfacial polarization resistance is as low as 1.01, 0.38, 0.16, 0.06 and 0.03 Ω cm² at 500, 550, 600, 650 and 700 °C, respectively. The experimental results indicate that SBSC is a very promising cathode material for IT-SOFCs.