Nano-sized Sm0.5Sr0.5CoO3−δ as the cathode for solid oxide fuel cells with proton-conducting electrolytes of BaCe0.8Sm0.2O2.9

Nano-sized Sm_0.5Sr_0.5CoO_3−δ (SSC) was fabricated onto the inner face of porous BaCe_0.8Sm_0.2O_2.9 (BCS) backbone by ion impregnation technique to form a composite cathode for solid oxide fuel cells (SOFCs) with BCS, a proton conductor, as electrolyte. The electro-performance of the composite cathodes was investigated as function of fabricating conditions, and the lowest polarization resistance, about 0.21 Ω cm² at 600 °C, was achieved with BCS backbone sintered at 1100 °C, SSC layer fired at 800 °C, and SSC loading of 55 wt.%. Impedance spectra of the composite cathodes consisted of two depressed arcs with peak frequency of 1 kHz and 30 Hz, respectively, which might correspond to the migration of proton and the dissociative adsorption and diffusion of oxygen, respectively. There was an additional arc peaking at 1 Hz in the Nyquist plots of a single cell, which should correspond to the anode reactions. With electrolyte about 70 μm in thickness, the simulated anode, cathode and bulk resistances of cells were 0.021, 0.055 and 0.68 Ω cm² at 700 °C, relatively, and the maximum power density was 307 mW cm⁻² at 700 °C.     

Mechanical behavior of La0.8Sr0.2Ga0.8Mg0.2O3 perovskites

This paper examines the important mechanical properties of commercially purchased La_0.8Sr_0.2Ga_0.8Mg_0.2O₃ at room temperature and 800 °C. Sr and Mg-doped lanthanum gallates (LSGM) are strong candidates for use as solid electrolytes in lower temperature solid oxide fuel cells operating at or below 800 °C. The material was found to be phase pure with a Young's modulus value of ∼175 GPa. The four point bending strength of the LSGM samples remained almost constant from 121 ± 35 MPa at room temperature to 126 ± 20 MPa at 800 °C. The fracture toughness, as measured by the single edge V notch beam (SEVNB) method, was 1.22 ± 0.06 MPa√m at room temperature, 1.04 ± 0.09 MPa√m at 700 °C followed by a small increase 1.31 ± 0.16 MPa√m at 800 °C. We also report, for the first time, the static subcritical (or slow) crack-growth (SCG) behavior of natural cracks in LSGM performed in four point bending tests at room temperature. The exponent of a power-law representation in the SCG tests was found to be n = 15, a rather low value showing LSGM to be highly susceptible to room temperature SCG.     

Improvement of the electrochemical properties of novel solid oxide fuel cell anodes, La0.75Sr0.25Cr0.5Mn0.5O3−δ and La4Sr8Ti11Mn0.5Ga0.5O37.5−δ, using Cu-YSZ-based cermets

A Cu-metal-based cermet was used to improve the electrochemical properties of two novel oxide-based systems with intrinsic low electronic conductivity such as La_0.75Sr_0.25Cr_0.5Mn_0.5O_3−δ (LSCM) and La₄Sr₈Ti₁₁Mn_0.5Ga_0.5O_37.5−δ (LSTMG). The introduction of Cu results in a marked improvement of the polarisation resistance values and hence in the performance. The best results correspond to the addition of ∼15% of CuO. In both systems, the polarisation resistances were improved by a least a factor of 2. Despite there are reports claiming that the CuO-zirconia-based systems exhibit catalytic activity, such an improvement seems to be mainly related to the capability of CuO as a sintering agent, helping to bridge electrode particles together, creating new electronic paths and thus effectively increasing the triple phase boundary through the whole electrode material.     

La0.8Sr0.2Ga0.8Mg0.2O3 electrolytes prepared by vacuum cold spray under heated gas for improved performance of SOFCs

High impact velocity of particles has found its common way into the vacuum cold spray (VCS), but heating gas may further intensify this function, resulting in significantly higher impact velocity. That's the original design idea to realize denser ceramic deposition at low temperature in this paper. In this study, a ~ 10?µm thick La_0.8Sr_0.2Ga_0.8Mg_0.2O₃ (LSGM) electrolyte layer for SOFCs is prepared by VCS under heated gas. The effects of gas temperature on the deposition behavior, mechanical and electrical properties of the coatings are investigated. Results show improvements in coating density, hardness and ionic conductivity at elevated temperature. Additionally, the output performance of cell with LSGM electrolytes deposited at gas temperature of 400?°C achieved an open circuit voltage of ~ 1.0?V and the maximum power density of 855?mW/cm² at 750?°C. Overall, these findings testify of the promising aspects of VCS method for preparing solid electrolyte films for IT-SOFCs.     

Catalytic steam reforming of methane over La0.8Sr0.2CrO3 based Ru catalysts

La_0.8Sr_0.2CrO₃ based Ru catalysts were studied as potential new anodic materials for Solid Oxide Fuel Cells directly fed with methane and operating at intermediate temperature under water deficient conditions. Two kinds of materials very close in composition were obtained following two different preparation procedures. Catalyst samples were characterized by physicochemical methods (XRD, SEM, BET and Chemical Analysis) and studied in methane steam reforming under water deficient conditions. Carbon formation during catalytic testing was studied by temperature programmed oxidation (TPO). Both types of catalysts were found very active and resistant to carbon formation. The unusual oscillatory behavior of the catalytic activity observed for one type of catalyst was discussed.     

A bi-layered composite cathode of La0.8Sr0.2MnO3-YSZ and La0.8Sr0.2MnO3-La0.4Ce0.6O1.8 for IT-SOFCs

A bi-layered composite cathode of La_0.8Sr_0.2MnO₃ (LSM)-YSZ and LSM-La_0.4Ce_0.6O_1.8 (LDC) was fabricated for anode-supported solid oxide fuel cells with a thin YSZ electrolyte film. The cell with the bi-layered composite cathode displayed better performance than the cell with the corresponding single-layered composite cathode of LSM-LDC or LSM-YSZ. At 650 °C, the cell with the bi-layered composite cathode gave a higher maximum power density than the cells with the single-layered LSM-LDC and LSM-YSZ composite cathodes, by 52% and 175%, respectively. The impedance spectra results show that the thin LSM-YSZ interlayer not only improves the cathode/electrolyte interface but also reduces the polarization resistance of the cathode. The activation energy for oxygen reduction on the bi-layered composite cathode is much smaller than that on LSM-YSZ composite cathode, and it is suggested that the special redox property of Ce⁴⁺/Ce³⁺ in LDC facilitates the oxygen reduction process on the bi-layered composite cathode. The cell with the bi-layered composite cathode operated quite stably during a 100 h run.     

Oxidative coupling of methane in Ba0.5Sr0.5Co0.8Fe0.2O3−δ tubular membrane reactors

A dense membrane tube made of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) was prepared by plastic extrusion from BSCF oxide synthesized by the complexing EDTA-citrate method. The membrane tube was used in a catalytic membrane reactor for oxidative coupling of methane (OCM) to C₂ without an additional catalyst. At high methane concentration (93%), about 62% C₂ selectivity was obtained, which is higher than that achieved in a conventional reactor using the BSCF as a catalyst. The dependence of the OCM reaction on temperature and methane concentration indicates that the C₂ selectivity in the BSCF membrane reactor is limited by high ion recombination rates. If an active OCM catalyst (La-Sr/CaO) was packed in the membrane tube, C₂ selectivity and CH₄ conversion increased compared to the blank run. The highest C₂ yield in the BSCF membrane reactor in presence of the La-Sr/CaO catalyst was about 15%, similar to that in a packed-bed reactor with the same catalyst under the same conditions. However, the ratio of C₂H₄/C₂H₆ in the membrane reactor was much higher than that in the packed-bed reactor, which is an advantage of the membrane reactor.     

Electron-hole transport in (La0.9Sr0.1)0.98Ga0.8Mg0.2O3−δ electrolyte: effects of ceramic microstructure

The oxygen ion transference numbers of a series of (La_0.9Sr_0.1)_0.98Ga_0.8Mg_0.2O_3−δ (LSGM) ceramics with different microstructures, prepared by sintering at 1673 K for 0.5-120 h, were determined at 973-1223 K by a modified Faradaic efficiency technique, taking electrode polarization into account. In air, the transference numbers vary in the range 0.984-0.998, decreasing when temperature or oxygen partial pressure increases. Longer sintering times lead to grain growth and to the dissolution of Sr-rich secondary phases and magnesium oxide, present in trace amounts at the grain boundaries, into the major perovskite phase. This is accompanied with a slight decrease of the total grain-interior resistivity and thermal expansion, while the boundary resistance evaluated from impedance spectroscopy data decreases 3-7 times. The electron-hole transport in LSGM ceramics was found to decrease when the sintering time increases from 0.5 to 40 h, probably indicating a considerable contribution of acceptor-enriched boundaries in the hole conduction. Due to reducing boundary area in single-phase materials, further sintering leads to higher p-type conductivity. The results show that, as for ionic conductivity, electronic transport in solid electrolytes significantly depends on ceramic microstructure.     

Mechanical characterization of porous Ba0.5Sr0.5Co0.8Fe0.2O3−d

The mechanical stability of porous Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−d (BSCF) material was investigated using depth-sensitive microindentation and ring-on-ring biaxial bending tests. The porous BSCF was characterized as potential substrate material for the deposition of a dense membrane layer. Indentation tests yielded values for hardness and fracture toughness up to a temperature of 400 °C, while bending tests permitted an assessment of elastic modulus and fracture stress up to 800 °C. In addition the fracture toughness was evaluated up to 800 °C measuring in bending tests the fracture stress of pre-indented specimens. The results proof that the indentation-strength method can be applied for the determination of the fracture toughness of this porous material. In comparison to dense material the values of the mechanical parameters were as expected lower but the temperature dependences of elastic modulus, fracture strength and toughness were similar to those reported for dense BSCF.     

Micro- and macro-indentation behaviour of Ba0.5Sr0.5Co0.8Fe0.2O3−d perovskite

Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−d (BSCF) is a candidate material for the application as oxygen separation membrane. However, the requisite mechanical reliability needs to be warranted. Indentation tests on dense BSCF yielded data for hardness, stiffness and fracture toughness up to a temperature of 340 °C. Complementary to this, the fracture toughness was also evaluated up to 800 °C based on an indentation-strength method.Up to 200 °C, the values of all characteristic mechanical parameters decreased. At high temperatures they increased. The morphology of the indentation cracks depended on the applied indentation load. This was taken into account while selecting suitable expressions for calculating indentation toughness. The temperature dependence of the normalised fracture toughness as determined by indentation technique and indentation-strength method matched quite well. They revealed a good agreement with the temperature dependence of previously reported normalised fracture stresses. In addition to this, the effect of annealing on the mechanical properties of the material was also studied.     

Partial oxidation of methane in Ba0.5Sr0.5Co0.8Fe0.2O3−δ membrane reactor at high pressures

Oxygen permeation fluxes through dense disk-shaped Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCFO) membranes were investigated as a function of temperature (973–1123 K), pressure (2–10 atm), and membrane thickness (1–2 mm) under an air/helium gradient. A high oxygen permeation flux of 2.01 ml/cm² min was achieved at 1123 K and 10 atm under an air/He oxygen partial pressure gradient. Based on the dependence of the oxygen permeation flux on the oxygen partial pressure difference across the membrane and the membrane thickness, it is assumed that bulk diffusion of oxygen ions was the rate-controlling step in the oxygen transport across the BSCFO membrane disk under an air/He gradient. The partial oxidation of methane (POM) to syngas using LiLaNiO_x/γ-Al₂O₃ as catalyst in a BSCFO membrane reactor was successfully performed at high pressure (5 atm). Ninety-two percent methane conversion, 90% CO selectivity, and 15.5 ml/cm² min oxygen permeation flux were achieved in steady state at a temperature of 1123 K and a pressure of 5 atm. A syngas production rate of 79 ml/cm² min was obtained. Characterization of the membrane surface by SEM and XRD after reaction showed that the surface exposed to the air side preserved the Perovskite structure while the surface exposed to the reaction side was eroded.     

Electrochemical characterization of YBaCo3ZnO7 + Gd0.2Ce0.8O1.9 composite cathodes for intermediate temperature solid oxide fuel cells

YBaCo₃ZnO₇ + Gd_0.2Ce_0.8O_1.9 (GDC) composites with various GDC contents (0-70 wt.%) have been investigated as cathode materials for intermediate temperature solid oxide fuel cells (SOFC). The effect of GDC incorporation on the microstructure, electrochemical properties, and thermal expansion behavior of the YBaCo₃ZnO₇ + GDC composites has been studied. The composite cathodes consist of smaller particles with larger surface area compared to the pure YBaCo₃ZnO₇ cathode, which is beneficial for providing extended triple-phase boundary (TPB) where the oxygen reduction reaction (ORR) occurs. Among the various compositions investigated, the YBaCo₃ZnO₇ + GDC (50:50 wt.%) composite is found to be optimum with the lowest polarization resistance (0.28 Ω cm² at 600 °C) compared to that of pure YBaCo₃ZnO₇ (0.62 Ω cm² at 600 °C). Anode-supported single cell SOFC fabricated with the YBaCo₃ZnO₇ + GDC (50:50 wt.%) composite cathode also exhibits excellent performance with a maximum power density of 743 mW/cm² at 750 °C. Additionally, the YBaCo₃ZnO₇ + GDC (50:50 wt.%) composite shows a low thermal expansion coefficient (TEC) of 10.7 × 10⁻⁶ °C⁻¹, which provides good compatibility with those of standard SOFC electrolytes.     

Investigation of a Ba0.5Sr0.5Co0.8Fe0.2O3−δ based cathode SOFC: II. The effect of CO2 on the chemical stability

The effect of carbon dioxide on the chemical stability of a Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ cathode in the real reaction environment at 450 °C was investigated by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), temperature programmed desorption (TPD), X-ray diffraction (XRD) and electrochemical impedance spectra (EIS) techniques. It was found that the presence even of very small quantities of CO₂ seriously deteriorates the fuel cell performance at 450 °C. XPS, TPD and XRD results strongly evidenced the formation of carbonates involving strontium and possibly barium after the BSCF cathode was operated in 1% CO₂/O₂ gas mixture at 450 °C for 24 h. SEM-EDX analysis of the BSCF cathode surface, after treatment in CO₂/O₂ environment at 450 °C, showed small particles on the surface probably associated with a carbonate phase and a segregated phase of the perovskite. The corresponding EDX spectra confirmed the presence of a carbonate layer and also revealed the surface enrichment of strontium and barium elements. EIS results indicated that both ohmic and polarization resistances increased gradually with the introduction of carbon dioxide in the oxidant stream, which could be interpreted by the decreased oxygen reduction kinetics and the formation of carbonate insulating layer.     

Mixed reforming of heptane to syngas in the Ba0.5Sr0.5Co0.8Fe0.2O3 membrane reactor

Fuel cells are recognized as the most promising new power generation technology, but hydrogen supply is still a problem. In our previous work, we have developed a LiLaNiO/γ-Al₂O₃ catalyst, which is excellent not only for partial oxidation of hydrocarbons, but also for steam reforming and autothermal reforming. However, the reaction needs pure oxygen or air as oxidant. We have developed a dense oxygen permeable membrane Ba_0.5Sr_0.5Co_0.8Fe_0.2O₃ which has an oxygen permeation flux around 11.5 ml/cm² min at reaction conditions. Therefore, this work is to combine the oxygen permeable membrane with the catalyst LiLaNiO/γ-Al₂O₃ in a membrane reactor for hydrogen production by mixed reforming of heptane. Under optimized reaction conditions, a heptane conversion of 100%, a CO selectivity of 91–93% and a H₂ selectivity of 95–97% have been achieved.     

On the simultaneous use of La0.75Sr0.25Cr0.5Mn0.5O3−δ as both anode and cathode material with improved microstructure in solid oxide fuel cells

A new concept of a solid oxide fuel cell (SOFC) using simultaneously the same electrode material at the anode and cathode sides with improved microstructure is proposed. We have found that La_0.75Sr_0.25Cr_0.5Mn_0.5O_3−δ (LSCM) can be considered as a good candidate for such configuration, symmetrical fuel cells (SFCs), due to its enhanced electrochemical properties in both reducing and oxidising conditions. LSCM-based SFCs offer promising performances, e.g., 0.5 and 0.3 W cm⁻² at 950 °C using H₂ and CH₄, respectively as fuels. Finally, the optimisation of the microstructure has been achieved via a novel facile procedure, using poly(methyl methacrylate) PMMA microspheres as templates.     

Synthesis and sintering behavior of La0.8Sr0.2CrO3 by a glycine nitrate process

La_0.8Sr_0.2CrO₃ powder was synthesized by a glycine nitrate process from an aqueous solution of lanthanum, strontium, and chromium nitrates, and glycine. The apparent density, size and morphology of the La_0.8Sr_0.2CrO₃ powder depended on the glycine-to-nitrate ratio. However, the pH value of the precursor solution had no significant effect on these properties. It was found that a single-phase perovskite, La_0.8Sr_0.2CrO₃, was synthesized when the dried ash was calcined at 1200 °C for 5 h. A secondary minor phase, SrCrO₄, was observed in the powder calcined at temperatures lower than 1100 °C. The presence of the SrCrO₄ phase has a significant effect on the sinterability and microstructural evolution of the La_0.8Sr_0.2CrO₃. A relative density higher than 90% could be achieved when the 1000 °C-calcined La_0.8Sr_0.2CrO₃ powder was sintered at 1450 °C.     

LaNi0.9Ru0.1O3 as a cathode catalyst for a direct borohydride fuel cell

LaNi_0.9Ru_0.1O₃ as cathode catalyst for a direct borohydride fuel cell (DBFC) was synthesized and investigated for the first time. The electrochemical experiments indicated that perovskite-type oxide LaNi_0.9Ru_0.1O₃ exhibited higher electrochemical performance compared with LaNiO₃, which suggested incorporation of element Ru into LaNiO₃ could further improve the catalytic ability for oxygen reduction reaction (ORR) in alkaline solution. LaNi_0.9Ru_0.1O₃ catalyst was found to have good tolerance of BH₄⁻. Meanwhile the maximum power density of 171 mW cm⁻² was obtained at 65 °C without using any precious ion exchange membrane. A life test indicated that the DBFC displayed no significant degradation for about 70 h testing. The electrochemical data suggested that LaNi_0.9Ru_0.1O₃, which provided a simple way to construct DBFCs without using any ion exchange membrane, might be promising cathode catalyst with high performance and low cost for DBFCs.     

Rapid degradation phenomenon of NiCu–Ce0.8Gd0.2O1.9 anode at high p(H2O) in different concentrations of dry methane

Degradation phenomena of Ni_0.5Cu_0.5–CGO (Ce_0.8Gd_0.2O_1.9) bimetallic anode in different concentrations of dry methane were studied in LSM ((La_0.75Sr_0.25)_0.95MnO_3−δ)–CGO cathode supported SOFCs. According to XRD, SEM and EIS analyses, the degradation of the Ni_0.5Cu_0.5–CGO bimetallic anode could be mainly attributed to re-oxidation of Ni by H₂O and/or by rapid sintering of Ni due to a high p(H₂O) appeared at a high current density. Furthermore, it is found that the degradation of anode resulted by the re-oxidation of Ni and/or by rapid sintering of Ni in the as-prepared cells generally occurred when the partial pressure ratio of the produced H₂O and CH₄ at the outlet of anode increased above a threshold value in different concentrations of dry methane.     

Structural and electrical properties of La0.7Sr0.3Co0.5Fe0.5O3 powders synthesized by solid state reaction

This work investigates the effect of synthesis parameters (calcination temperature, milling conditions and sintering temperature) on the structural, morphological and electrical properties of La_0.7Sr_0.3Co_0.5Fe_0.5O₃ (LSCF) powders prepared by the solid state reaction. The thermogravimetric profile showed that the minimum temperature needed for the carbonate decomposition and formation of perovskite phase is 800 °C. SEM analysis revealed the loose and porous structure of the powder materials. The XRD patterns demonstrate that milling parameters such as grinding balls:sample ratio, rotational speed, and milling time influence the structural properties. The results revealed that powders synthesized with grinding balls:sample ratio of 8:1, 500 rpm and 4 h of milling present pure LSCF phase. Porosity of the pellets decreased with increasing sintering temperature from 950 to 1100 °C. Electrical conductivity was measured at 400–1000 °C and correlated with sintering temperature.