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.     

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.     

Syngas reactivity over (LaAg)(CoFe)O3 and Ag-added (LaSr)(CoFe)O3 anodes of solid oxide fuel cells

The syngas, H₂ + CO gas mixture with various H₂/CO ratios, is used as the anode fuel of solid oxide fuel cell with La_0.7Ag_0.3Co_0.2Fe_0.8O₃ (LACF) and 2 wt% Ag-added La_0.58Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) as the anode, respectively, both being in composite with 50 wt% Ce_0.9Gd_0.1O_1.95 (GDC). Both the current-voltage and the fixed-voltage measurements are performed at 800 °C. The reactivity with H₂ as the fuel is larger than that with CO. The syngas reactivity increases with increasing H₂ content. The results of the current-voltage and the fixed-voltage measurements are in agreement with each other. Ag-added LSCF-GDC has better reactivity with H₂, CO and syngas and better stability in the H₂ atmosphere than LACF-GDC as the anode material.     

Use of La0.75Sr0.25Cr0.5Mn0.5O3 materials in composite anodes for direct ethanol solid oxide fuel cells

The perovskite system La_1−xSr_xCr_1−yM_yO_3−δ (M, Mn, Fe and V) has recently attracted much attention as a candidate material for the fabrication of solid oxide fuel cells (SOFCs) due to its stability in both H₂ and CH₄ atmospheres at temperatures up to 1000 °C. In this paper, we report the synthesis of La_0.75Sr_0.25Cr_0.5Mn_0.5O₃ (LSCM) by solid-state reaction and its employment as an alternative anode material for anode-supported SOFCs. Because LSCM shows a greatly decreased electronic conductivity in a reducing atmosphere compared to that in air, we have fabricated Cu-LSCM-ScSZ (scandia-stabilized zirconia) composite anodes by tape-casting and a wet-impregnation method. Additionally, a composite structure (support anode, functional anode and electrolyte) structure with a layer of Cu-LSCM-YSZ (yttria-stabilized zirconia) on the supported anode surface has been manufactured by tape-casting and screen-printing. Single cells with these two kinds of anodes have been fabricated, and their performance characteristics using hydrogen and ethanol have been measured. In the operation period, no obvious carbon deposition was observed when these cells were operated on ethanol. These results demonstrate the stability of LSCM in an ethanol atmosphere and its potential utilization in anode-supported SOFCs.     

Effect of substitution with Cr and addition of Ni on the physical and electrochemical properties of Ce0.9Sr0.1VO3 as a H2S-active anode for solid oxide fuel cells

Whereas Ce_0.9Sr_0.1Cr_0.5V_0.5O₃ is an active fuel cell anode catalyst for conversion of only the H₂S content of 0.5% H₂S-CH₄ at 850 °C, inclusion of 5 wt% NiO to form a composite catalyst enabled concurrent electrochemical conversion of CH₄. A fuel cell with a 0.3 mm thick YSZ membrane and Ce_0.9Sr_0.1Cr_0.5V_0.5O₃ as anode catalyst had a maximum power density of 85 mW cm⁻² in 0.5% H₂S-CH₄ at 850 °C, arising only from the electro-oxidation of H₂S. Using a same thick membrane, promotion of the anode with 5 wt% NiO increased the total anode electro-oxidation activity to afford maximum power density of 100 mW cm⁻² in 0.5% H₂S-CH₄. The same membrane provided 30 mW cm⁻² in pure CH₄, showing that the incremental improvement arose substantially from CH₄ conversion. Performance of each anode was stable for over 12 h at maximum power output. XPS and XRD analyses showed that an increase in conductivity of Ce_0.9Sr_0.1Cr_0.5V_0.5O₃ in H₂S-containing environments resulted from a change in composition and structure from the tetragonal oxide to monoclinic Ce_0.9Sr_0.1Cr_0.5V_0.5(O,S)₃.     

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.     

Electrochemical performances of solid oxide fuel cells based on Y-substituted SrTiO3 ceramic anode materials

Yttrium-substituted SrTiO₃ has been considered as anode material of solid oxide fuel cells (SOFCs) substituting of the state-of-the-art Ni cermet anodes. Sr_0.895Y_0.07TiO_3−δ (SYT) shows good electrical conductivity, compatible thermal expansion with yttria-stabilized ZrO₂ (YSZ) electrolyte and reliable stability during reduction and oxidation (redox) cycles. Single cells based on SYT anode substrates were fabricated in the dimension of 50 mm × 50 mm. The cell performances were over 1.0 A cm⁻² at 0.7 V and 800 °C, which already reached the practical application level. Although Ti diffusion from SYT substrates to YSZ electrolytes was observed, it did not show apparent disadvantage to the cell performance. The cells survived 200 redox cycles without obvious OCV decrease and macroscopic damage, but performance decreased due to the electronic properties of the SYT material. The influence of water partial pressure on cell performance and coking tolerance of the cells are also discussed in this study.     

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.     

Potential oscillation of methane oxidation reaction on (La0.75Sr0.25)(Cr0.5Mn0.5)O3 electrodes of solid oxide fuel cells

Oscillation of open circuit potential (OCP) and potential is observed for the methane oxidation reaction on (La_0.75Sr_0.25)(Cr_0.5Mn_0.5)O₃ (LSCM) and LSCM/YSZ composite electrodes of solid oxide fuel cells (SOFCs) in weakly humidified methane (i.e., 97%CH₄/3%H₂O). In dry methane (i.e., 100%CH₄), the potential oscillation is reduced significantly. The oscillation behaviour of OCP is also found to be strongly related to the temperature, the microstructure of the composite electrode and the fuel composition. The results indicate that the potential oscillation is thermally activated and is most likely associated with the adsorbed oxygen species on the electrode surface.     

Ag decorated (Ba,Sr)(Co,Fe)O3 cathodes for solid oxide fuel cells prepared by electroless silver deposition

We reported nano-structured Ag modified Ba_0.5Sr_0.5Co_0.6Fe_0.4O_3−δ (Ag@BSCF) cathode for solid oxide fuel cells (SOFCs) that is prepared by vacuum assisted electroless deposition technique. We show that the concentration of Ag can be easily adjusted by tuning the deposition time without altering the perovskite structure of the pristine BSCF. The effect of Ag loading on the electrochemical performance of the material has been systematically studied by varying the Ag loading and the working condition (oxygen partial pressure). An optimized electrode performance is observed with an Ag loading of ∼2 wt%. We demonstrate that the presence of Ag significantly reduces the electrode ohmic resistance and enhances the catalytic O₂ reduction performance of the BSCF cathode.     

Phase stability of Sm0.5Sr0.5CoO3 cathodes for on-planar type, single-chamber, solid oxide fuel cells

The stability of Sm_0.5Sr_0.5CoO₃ (SSC) under reduction conditions is investigated to determine whether it can be used as a cathode material in on-planar type, single-chamber, solid oxide fuel cells. The techniques of X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy are used to reveal the reduction mechanism of SSC. Impedance spectroscopy analysis also provides a better understanding of the influence of decomposed SSC phases on cathode performance. Decomposition of SSC occurs on the surface by the formation of dot-shaped SrO, Co(OH)₂ and CoO on top of the reduced SSC layer at 250 °C in 4% H₂O-96% H₂. The SSC perovskite structure is destroyed at 350 °C in pure hydrogen. There is a catastrophic microstructural change in which SSC is completely decomposed to SrO and CoO that cover the surface of Sm₂O₃.     

Y-doped SrTiO3 based sulfur tolerant anode for solid oxide fuel cells

A solid oxide fuel cell (SOFC) anode with high sulfur tolerance was developed starting from a Y-doped SrTiO₃ (SYTO)-yttria stabilized zirconia (YSZ) porous electrode backbone, and infiltrated with nano-sized catalytic ceria and Ru. The size of the infiltrated particles on the SYTO-YSZ pore walls was 30–200 nm, and both infiltrated materials improved the performance of the SYTO-YSZ anode significantly. The infiltrated ceria covered most of the surface of the SYTO-YSZ pore walls, while Ru was dispersed as individual nano-particles. The performance and sulfur tolerance of a cathode supported cell with ceria- and Ru-infiltrated SYTO-YSZ anode was examined in humidified H₂ mixed with H₂S. The anode showed high sulfur tolerance in 10–40 ppm H₂S, and the cell exhibited a constant maximum power density 470 mW cm⁻² at 10 ppm H₂S, at 1073 K. At an applied current density 0.5 A cm⁻², the addition of 10 ppm H₂S to the H₂ fuel dropped the cell voltage slightly, from 0.79 to 0.78 V, but completely recovered quickly after the H₂S was stopped. The ceria- and Ru-infiltrated SYTO-YSZ anode showed much higher sulfur tolerance than conventional Ni-YSZ anodes.     

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.     

Electrical conductivity and structural stability of La-doped SrTiO3 with A-site deficiency as anode materials for solid oxide fuel cells

A-site-deficient (La_0.3Sr_0.7)_1−xTiO_3−δ materials were synthesized by conventional solid-state reaction. The A-site deficiency limit in (La_0.3Sr_0.7)_1−xTiO_3−δ was below 10 mol% in 5%H₂/Ar at 1500 °C. A-site deficiency level promoted the sintering process of (La_0.3Sr_0.7)_1−xTiO_3−δ. The ionic conductivity increased but the electronic conductivity decreased with increasing A-site deficiency level. The ionic conductivity of (La_0.3Sr_0.7)_0.93TiO_3−δ sample was as high as 0.2–1.6 × 10⁻² S/cm in 500–950 °C and 1.0 × 10⁻² S/cm at 800 °C, over twice of La_0.3Sr_0.7TiO_3−δ. Its electrical conductivity was in the range of 83–299 S/cm in 50–950 °C and 145 S/cm at 800 °C. A-site deficiency improved the thermal stability of (La_0.3Sr_0.7)_1−xTiO_3−δ and ensured the material with a stable electrical performance in different atmospheres.     

Fe-substituted (La,Sr)TiO3 as potential electrodes for symmetrical fuel cells (SFCs)

In the work presented herein, the potential use of La₄Sr₈Ti_12−xFe_xO_38−δ (LSTF) materials as electrodes for a new concept of solid oxide fuel cells, symmetrical fuel cells (SFCs), is considered. Such fuel cells use simultaneously the same material as anode and cathode, which notably simplifies the assembly and further maintenance of the cells. Therefore, we search for materials showing high conductivity in a wide range of oxygen partial pressures in addition to certain degree of catalytic activity for the oxidation of the fuel and reduction of the oxidant, respectively. The preliminary electrochemical experiments performed reveal that the overall conductivity increases notably upon Fe substitution, being the main contribution electronic n-type. The fuel cell tests indicate that LSTF composites with YSZ and CeO₂ perform reasonably well under H₂ conditions, although the performance in methane is rather modest and require further optimisation.     

Micro modeling of solid oxide fuel cell anode based on stochastic reconstruction

A novel modeling scheme of SOFC anode based on the stochastic reconstruction technique and the Lattice Boltzmann Method (LBM) is proposed and applied to the performance assessment and also to the optimization of anode microstructures. A cross-sectional microscopy image is exploited to obtain a two-dimensional phase map (i.e., Ni, YSZ and pore), of which two-point correlation functions are used to reconstruct a three-dimensional model microstructure. Then, the polarization resistance of the reconstructed anode is obtained by the LBM simulation. The predicted anodic polarization resistance for a given microstructure and its sintering temperature dependence are in good agreement with the literature data. Three-dimensional distributions of potential and current can be obtained, while and the effect of working temperature is discussed. The proposed method is considered as a promising tool for designing SOFC anodes.     

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.     

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.     

Effect of O2 concentration on performance of solid oxide fuel cells with V2O5 or Cu added (LaSr)(CoFe)O3-(Ce,Gd)O2−x cathode with and without NO

A solid oxide fuel cell unit is constructed with Ni-(Ce,Gd)O_2−x (GDC) as the anode, yttria-stabilized zirconia as the electrolyte, and V₂O₅ or Cu added (LaSr)(CoFe)O₃-GDC as the cathode. The effect of the O₂ concentration on the open circuit voltage (OCV) is studied and a mass-transfer limited OCV is observed. The power density with Cu addition can be much higher than that with V₂O₅ addition but the effect of the O₂ concentration with Cu addition is larger than that with V₂O₅ addition. Without the presence of NO, both the power density and the OCV decrease with decreasing O₂ concentration. The OCV variation can be substantial with the variation of the flow rate, the O₂ concentration and the NO concentration. The presence of CO₂ can increase the OCV while that of NO can decrease the OCV; however, a synergistic effect can occur on the OCV when NO is present at a very low O₂ concentration which results in a sudden drop of the OCV.     

Synthesis and electrochemical performance of La0.6Ca0.4Fe1−xNixO3 (x = 0.1, 0.2, 0.3) material for solid oxide fuel cell cathode

Polycrystalline samples of La_0.6Ca_0.4Fe_1−xNi_xO₃ (x = 0.1, 0.2, 0.3) (LCFN) are prepared by liquid mix method. The structure of the polycrystalline powders is analyzed with X-ray powder diffraction data. The XRD patterns are indexed as the orthoferrite similar to that of LaFeO₃ having a single phase with orthorhombic perovskite structure (Pnma). The morphological characterization is performed by scanning electron microscopy (SEM) obtaining a mean particle size less than 300 nm.Polarization resistance is studied using two different electrolytes: Y-stabilized zirconia (YSZ) and Sm-doped ceria (SDC). Electrochemical impedance spectroscopy (EIS) measurements of LCFN/YSZ/LCFN and LCFN/SDC/LCFN test cells are carried out. These electrochemical experiments are 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) obtained is 0.88 Ω cm², corresponding to the La_0.6Ca_0.4Fe_0.9Ni_0.1O₃ material using SDC as electrolyte. The dc four-probe measurement indicates that La_0.6Ca_0.4Fe_0.9Ni_0.1O₃ exhibits fairly high electrical conductivity, over 300 S cm⁻¹ at T > 500 °C.