A new anode material for intermediate solid oxide fuel cells

A new anode material for intermediate temperature solid oxide fuel cells (IT-SOFCs) with a composite of La_0.7Sr_0.3Cr_1−xNi_xO₃ (LSCN), CeO₂ and Ni has been synthesized. EDX analysis showed that 1.19 at% Ni was doped into the perovskite-type La_0.7Sr_0.3CrO₃ and Ce could not be detected in the perovskite phases. Results showed that the fine CeO₂ and Ni were highly dispersed on the La_0.7Sr_0.3Cr_1−xNi_xO₃ substrates after calcining at 1450 °C and reducing at 900 °C. The thermal expansion coefficient (TEC) of the as-prepared anode material is 11.8 × 10⁻⁶ K⁻¹ in the range of 30–800 °C. At 800 °C, the electrical conductivity of the as-prepared anode material calcined at 1450 °C for 5 h is 1.84 S cm⁻¹ in air and 5.03 S cm⁻¹ in an H₂ + 3% H₂O atmosphere. A single cell with yttria-stabilized zirconia (YSZ, 8 mol% Y₂O₃) electrolyte and the new materials as anodes and La_0.8Sr_0.2MnO₃ (LSM)/YSZ as cathodes was assembled and tested. At 800 °C, the peak power densities of the single cell was 135 mW cm⁻² in an H₂ + 3% H₂O atmosphere.     

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.     

Fe-based perovskites as electrodes for intermediate-temperature solid oxide fuel cells

A solid-oxide fuel cell (SOFC) based upon Fe perovskites, has been designed and tested. Materials with nominal compositions Sr_0.9K_0.1FeO_3−δ (SKFO) and Sr_1.6K_0.4FeMoO_6−δ (SKFMO) with perovskite structure have been prepared and characterized as cathode and anode, respectively. The anode material exhibits high electrical conductivity values of 407-452 S cm⁻¹ at 750-820 °C in pure H₂. In the test cells, the electrodes were supported on a 300-μm-thick pellet of the electrolyte La_0.8Sr_0.2Ga_0.83Mg_0.17O_3−δ (LSGM). The single SOFC cells gave a maximum power density at 850 °C of 937 mW cm⁻² with pure H₂ as a fuel. Sizeable power densities were also observed with alternative fuels: 694 and 499 mW cm⁻² with H₂ containing 5 parts per million of H₂S and CH₄, respectively, at 800 °C. Moreover, only a slight degradation of about 3.6% of the power density has been obtained after 65 different cycles of fuel-cell test in H₂ at 750 °C and 14% at 850 °C in 50 cycles using H₂-H₂S. This remarkable behavior has been correlated to the structural features determined in a neutron powder diffraction experiment in the usual working conditions of a SOFC for a cathode (air) and an anode (low pO₂). On the one hand, the cubic Pm-3m Sr_0.9K_0.1FeO_3−δ cathode material is an oxygen deficient perovskite with oxygen contents that vary from 2.45(2) to 2.26(2) from 600 to 900 °C and high oxygen isotropic thermal factors (4.17(8) Å²) suggesting a high ionic mobility. On the other hand, the actual nature of the anode of composition Sr_1.6K_0.4FeMoO_6−δ has been unveiled by neutron powder diffraction to consist of two main perovskite phases with the compositions SrMoO₃ and SrFe_0.6Mo_0.4O_2.7. The association of two perovskites oxides, SrMoO₃ with high electrical conductivity, and SrFe_0.6Mo_0.4O_2.7 with mixed ionic-electronic conductivity has resulted in an extraordinarily performing anode material for SOFCs.     

Electrical conduction behavior of La,Co co-doped SrTiO3 perovskite as anode material for solid oxide fuel cells

The effects of La- and Co-doping into SrTiO₃ perovskite oxides on their phase structure, electrical conductivity, ionic conductivity and oxygen vacancy concentration have been investigated. The solid solution limits of La in La_xSr_1 − xTiO_3 − δ and Co in La_0.3Sr_0.7Co_yTi_1 − yO_3 − δ are about 40 mol% and 7 mol%, respectively, at 1500 °C. The incorporation of La decreases the band gap and thus increases the electrical conductivity of SrTiO₃ remarkably. La_0.3Sr_0.7TiO_3 − δ shows an electrical conductivity of 247 S/cm at 700 °C. Co-doping into La_0.3Sr_0.7TiO_3 − δ increases the oxygen vacancy concentration and decreases the migration energy for oxygen ions, leading to a significant increase in ionic conductivity but at the expense of some electrical conductivity. The electrical and ionic conductivities of La_0.3Sr_0.7Co_0.07Ti_0.93O_3 − δ are 63 S/cm and 6 × 10⁻³ S/cm, respectively, at 700 °C. Both La_0.3Sr_0.7TiO_3 − δ and La_0.3Sr_0.7Co_0.07Ti_0.93O_3 − δ show relatively stable electrical conductivities under oxygen partial pressure of 10⁻¹⁴–10⁻¹⁹ atm at 800 °C. These properties make La_0.3Sr_0.7Co_0.07Ti_0.93O_3 − δ a promising anode candidate for solid oxide fuel cells.     

In-situ Ni exsolution from NiTiO3 as potential anode for solid oxide fuel cells

Sample NiTiO₃ (NTO) is prepared by the molten salts synthesis route as a potential anode material for solid oxide fuel cell (SOFC) applications. An additional sample impregnated with 5 mol%Ni (N-NTO) is also presented. Structural characterization reveal a pure NiTiO₃ phase upon calcination at 850 °C and 1000 °C. Redox characterization by temperature programmed reduction tests indicate the transition from NiTiO₃ to Ni/TiO₂ at ca. 700 °C. Ni nanoparticles (ca. 26 nm) are exsolved in-situ from the structure after a reducing treatment at 850 °C. Catalytic activity tests for partial oxidation of methane performed in a fixed bed reactor reveal excellent values of activity and selectivity due to the highly dispersed Ni nanoparticles in the support surface. Time-on-stream behavior during 100 h operation in reaction conditions for sample N-NTO yield a stable CH₄ conversion. Electrolyte supported symmetrical cells are prepared with both materials achieving excellent polarization resistance of 0.023 Ω cm² in 7%H₂/N₂ atmosphere at 750 °C with sample N-NTO. The maximum power density achieved is of 273 mW cm⁻² at 800 °C with a commercial Pt ink used as a reference cathode, indicating further improvement of the system can be achieved and positioning the N-NTO material as a promising SOFC anode material.     

Co-free, iron perovskites as cathode materials for intermediate-temperature solid oxide fuel cells

We have developed a Co-free solid oxide fuel cell (SOFC) based upon Fe mixed oxides that gives an extraordinary performance in test-cells with H₂ as fuel. As cathode material, the perovskite Sr_0.9K_0.1FeO_3−δ (SKFO) has been selected since it has an excellent ionic and electronic conductivity and long-term stability under oxidizing conditions; the characterization of this material included X-ray diffraction (XRD), thermal analysis, scanning microscopy and conductivity measurements. The electrodes were supported on a 300-μm thick pellet of the electrolyte La_0.8Sr_0.2Ga_0.83Mg_0.17O_3−δ (LSGM) with Sr₂MgMoO₆ as the anode and SKFO as the cathode. The test cells gave a maximum power density of 680 mW cm⁻² at 800°C and 850 mW cm⁻² at 850 °C, with pure H₂ as fuel. The electronic conductivity shows a change of regime at T ≈ 350 °C that could correspond to the phase transition from tetragonal to cubic symmetry. The high-temperature regime is characterized by a metallic-like behavior. At 800 °C the crystal structure contains 0.20(1) oxygen vacancies per formula unit randomly distributed over the oxygen sites (if a cubic symmetry is assumed). The presence of disordered vacancies could account, by itself, for the oxide-ion conductivity that is required for the mass transport across the cathode. The result is a competitive cathode material containing no cobalt that meets the target for the intermediate-temperature SOFC.     

Effect of anode structure on performance of cone-shaped solid oxide fuel cells fabricated by phase inversion

Anode-supported cone-shaped tubular solid oxide fuel cells (SOFCs) are successfully fabricated by a phase inversion method. During processing, the two opposite sides of each cone-shaped anode tube are in different conditions--one side is in contact with coagulant (the corresponding surface is named as “W-surface”), while the other is isolated from coagulate (I-surface). Single SOFCs are made with YSZ electrolyte membrane coated on either W-surface or I-surface. Compared to the cell with YSZ membrane on W-surface, the cell on I-surface exhibits better performance, giving a maximum power density of 350 mW cm⁻² at 800 °C, using wet hydrogen as fuel and ambient air as oxidant. AC impedance test results are consistent with the performance. The sectional and surface structures of the SOFCs were examined by SEM and the relationship between SOFC performance and anode structure is analyzed. Structure of anodes fabricated at different phase inversion temperature is also investigated.     

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.     

B-site doping of lanthanum strontium titanate for solid oxide fuel cell anodes

In this study the properties of the compounds La_0.33Sr_0.67Ti_0.92X_0.08O_3+δ where X = Al³⁺, Ga³⁺, Feⁿ⁺, Mg²⁺, Mnⁿ⁺ and Sc³⁺, have been investigated in the search for alternative solid oxide fuel cell anodes. The choice of dopant controls the structure, redox properties, conductivity and electrocatalytic properties of the compound.     

Investigation of compatible anode systems for LaNbO4-based electrolyte in novel proton conducting solid oxide fuel cells

In the current manufacturing process of novel LaNbO₄-based proton conducting fuel cells a thin layer of the electrolyte is deposited by wet ceramic coating on NiO-LaNbO₄ based anode and co-sintered at 1200-1300 °C. The chemical compatibility of NiO with acceptor doped LaNbO₄ material is crucial to ensure viability of the cell, so potential effects of other phases resulting from off-stoichiometry in acceptor doped LaNbO₄ should also be explored. Compatibility of NiO with Ca-doped LaNbO₄ and its typical off-set compositions (La₃NbO₇ and LaNb₃O₉) are investigated in this work. It is shown that while NiO does not react with Ca-doped LaNbO₄, fast reaction occurs with La₃NbO₇ or LaNb₃O₉. La₃NbO₇ and NiO form a mixed conducting perovskite phase LaNi_2/3Nb_1/3O₃, while LaNb₃O₉ and NiO form either NiNb₂O₆ or Ni₄Nb₂O₉ depending on the annealing temperature. This implies that manufacturing LaNbO₄-based proton conducting fuel cells requires a strict control of the stoichiometry of the electrolyte.     

Ink-jet printing of electrolyte and anode functional layer for solid oxide fuel cells

In this work, solid oxide fuel cells were fabricated by ink-jet printing. The cells were characterized in order to study the resulting microstructure and electrochemical performance. Scanning electron microscopy revealed a highly conformal 6–12 μm thick dense yttria-stabilized zirconia electrolyte layer, and a porous anode-interlayer. Open circuit voltages ranged from 0.95 to 1.06 V, and a maximum power density of 0.175 W cm⁻² was achieved at 750 °C. These results suggest that the ink-jet printing technique may be used to fabricate stable SOFC structures that are comparable to those fabricated by more conventional ceramics processing methods. This study also highlights the significance of overall cell microstructural impact on cell performance and stability.     

Effect of the anode microstructure on the enhanced performance of solid oxide fuel cells

Anode microstructure has a vital effect on the performance of anode supported solid oxide fuel cells. High electrical conductivity, gas permeability and low polarization are the required features of anodes to achieve high power densities. The desired properties of the anodes were obtained by modifying their microstructural development using pyrolyzable pore former particles without introducing any functional layers and compositional modifications. The microstructures of fabricated anodes were characterized using scanning electron microscopy and mercury intrusion porosimetry techniques while their electrochemical properties were identified using impedance spectroscopy and voltammetric measurements. Detailed investigations demonstrated that the pore structure has a major impact on the electrical conductivity, polarization and gas permeability of the anodes. Through tailoring of conventional anode microstructures, a significantly high power density of 1.54 W/cm² was achieved at 800 °C using diluted hydrogen (10% H₂ in argon) as fuel.     

High-Performance composite cathodes for solid oxide fuel cells

The composite cathodes with lanthanum-based iron and cobalt-containing perovskite La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) and Ce_0.9Gd_0.1O_1.95 (GDC) are investigated for solid oxide fuel cell (SOFC) applications at relatively low operating temperatures (700-800 °C). LSCFs with high surface areas of 55 m²g⁻¹ are synthesized via a complex method with inorganic nano dispersants. The fuel cell performances of composite cathodes on anode supported SOFCs are characterized with GDC materials of surface areas of 5 m²g⁻¹ (ULSA-GDC), 12 m²g⁻¹ (LSA-GDC), and 23 m²g⁻¹ (HAS-GDC). The maximum power density of the SOFCs increases from 0.68 Wcm⁻² to 1.2 Wcm⁻² at 780 °C and 0.8 V as the GDC surface area increases from 5 m²g⁻¹ to 23 m²g⁻¹. The area specific resistance of the porous composite cathodes with a HAS-GDC are 0.467 ohmcm² at 780 °C and 1.086 ohmcm² at 680 °C, while these values with an LSA-GDC are 0.543 ohmcm² and 0.945 ohmcm², respectively. The best compositions of the porous composite cathodes result from the morphologies of the GDC materials at each temperature due to the formation of an electron-oxygen ion-gas boundary.     

Proton-conducting solid oxide fuel cells prepared by a single step co-firing process

Proton-conducting solid oxide fuel cells (SOFCs), consisting of BaCe_0.7In_0.3O_3−δ (BCI30)-NiO anode substrates, BCI30 anode functional layers, BCI30 electrolyte membranes and BCI30-LaSr₃Co_1.5Fe_1.5O_10−δ (LSCF) composite cathode layers, were successfully fabricated at 1150 °C, 1250 °C and 1350 °C respectively by a single step co-firing process. The fuel cells were tested with humidified hydrogen (∼3%H₂O) as the fuel and static air as the oxidant. The single cell co-fired at 1250 °C showed the highest cell performance. The impedance studies revealed that the co-firing temperature affected the interfacial polarization resistance of a single cell as well as its overall electrolyte resistance.     

Ag current collector for honeycomb solid oxide fuel cells using LaGaO3-based oxide electrolyte

Honeycomb type solid oxide fuel cell (SOFC) using a Ag mesh as a current collector and La_0.9Sr_0.1Ga_0.8Mg_0.2O₃ (LSGM) as an electrolyte was studied for reducing production cost. When an Ag mesh was used as a current collector, the power density of the cell became lower than that of a cell using a Pt mesh due to the relatively worse contact caused by the lower calcination temperature, particularly in the case of the anode. The preparation method and the electrode structure were investigated for the purpose of increasing the power density of the cell using the Ag current collector. It was found that an interlayer of Ni–Sm_0.2Ce_0.8O_1.9 (1:9) between the NiFe–LSGM cermet anode and the LSGM electrolyte was effective for decreasing the pre-calcination temperature for anode fabrication. Much higher power densities of 300 mW cm⁻² and 140 mW cm⁻² at 1073 K and 973 K, respectively, were achieved by inserting an interlayer. These results for the power density of the cell using the Ag mesh current collector after the optimization of the electrode structure and the preparation procedure are close to those of the cell using the Pt mesh current collector cell presented in our previous work.     

Ni and metal aluminate mixtures for solid oxide fuel cell anode supports

Mixtures of nickel and metal aluminate (Ni–MAl₂O₄ [M = Fe, Co, Ni and Cu]) were fabricated, and their electrical conductivities, microstructures and thermal expansions were measured. During the sintering of these mixtures, MAl₂O₄ reacts with NiO to form NiAl₂O₄ and MO_x which are thought to be the reasons for the differences in the microstructures and electrical properties. Except for FeAl₂O₄, Ni–MAl₂O₄ mixtures show metallic conductivity behavior and their electrical conductivities are sufficient for cell operation. Their thermal expansion coefficients are much lower than conventional Ni-YSZ mixtures and closer to the 8YSZ electrolyte. The peak power densities of single cells supported with Ni–NiAl₂O₄ and Ni–CoAl₂O₄ are 410 and 440 mW cm⁻² at 850 °C, respectively, which are lower than 490 mW cm⁻² of Ni-YSZ. This is due to the polarization resistances of functional anode layer. The Ni–CuAl₂O₄-supported cell has no electrical performance because of Cu migration and segregation.     

Tolerance tests of H2S-laden biogas fuel on solid oxide fuel cells

Biogas is a variable mixture of methane, carbon dioxide and other gases. It is a renewable resource which comes from numerous sources of plant and animal matter. Ni-YSZ anode-supported solid oxide fuel cell (SOFC) can directly use clean synthesized biogas as fuel. However, trace impurities, such as H₂S, Cl₂ and F₂ in real biogas can cause degradation in cell performance. In this research, both uncoated and coated Ni-YSZ anode-supported cells were exposed to three different compositions of synthesized biogases (syn-biogas) with 20 ppm H₂S under a constant current load at 750-850 °C and their performance was evaluated periodically using standard electrochemical methods. Postmortem analysis of the SOFC anode was performed using X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The results show that H₂S causes severe electrochemical degradation of the cell when operating on biogas, leading to both complete electrochemical and mechanical failure. The Ni-CeO₂ coated cell showed excellent stability during CH₄ reforming and some tolerance to H₂S contamination.     

Electrochemical performances of spinel oxides as cathodes for intermediate temperature solid oxide fuel cells

The spinel-type oxides of (Mn, Co, Cu)₃O₄ prepared via a citric–EDTA acid process were investigated as candidate cathodes of intermediate temperature solid oxide fuel cells (IT-SOFCs). (Mn, Co)₃O₄ spinel oxide shows a phase transition from tetragonal to cubic when the doping amount of cobalt element increases. Their electric conductivities increase with the cobalt content and are enough high for them used as cathodes of IT-SOFCs. A fuel cell with (Mn, Co)₃O₄ spinel cathode was successfully evaluated based on YSZ electrolyte. (Mn, Co)₃O₄ spinel cathodes show good electrochemical activities, demonstrating the feasibility of the spinel oxide being a cathode of IT-SOFC. As copper doped into (Mn, Co)₃O₄ spinel, the P_peak for Cu_0.5MnCo_1.5O₄ cathode rise to 343, 474 and 506 mW cm⁻² at 700, 750 and 800 °C, respectively. The results reveal that the spinel-type oxides are promising cathodes for IT-SOFCs, especially for Cu_0.5MnCo_1.5O₄.     

Evaluation of Sr substituted Nd2CuO4 as a potential cathode material for intermediate-temperature solid oxide fuel cells

The Nd_1.7Sr_0.3CuO₄ (NSCu) material with perovsikite-related structure was synthesized and evaluated as a new cathode for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The crystal structure, thermal expansion, electrical conductivity and electrochemical performance of NSCu have been investigated by X-ray diffraction, a dilatometer, DC four-probe method, AC impedance and cyclic voltammetry (CV) techniques. The polarization resistances of NSCu cathode on Sm-doped ceria (SDC) electrolyte in air were 0.07 Ω cm², 0.24 Ω cm² and 1.60 Ω cm² at 800 °C, 700 °C and 600 °C, respectively. The results demonstrated that both impedance and CV methods are consistent with high exchange current density i₀ (390.7 mA/cm² and 76.1 mA/cm² at 800 °C and 700 °C.), making NSCu a promising cathode material for the IT-SOFCs based on doped ceria electrolytes.     

Durability of solid oxide fuel cells using sulfur containing fuels

The usability of hydrogen and also carbon containing fuels is one of the important advantages of solid oxide fuel cells (SOFCs), which opens the possibility to use fuels derived from conventional sources such as natural gas and from renewable sources such as biogas. Impurities like sulfur compounds are critical in this respect. State-of-the-art Ni/YSZ SOFC anodes suffer from being rather sensitive towards sulfur impurities. In the current study, anode supported SOFCs with Ni/YSZ or Ni/ScYSZ anodes were exposed to H₂S in the ppm range both for short periods of 24 h and for a few hundred hours. In a fuel containing significant shares of methane, the reforming activities of the Ni/YSZ and Ni/ScYSZ anodes were severely poisoned already at low H₂S concentrations of ∼2 ppm H₂S. The poisoning effect on the cell voltage was reversible only to a certain degree after exposure of 500 h in the state-of-the-art cell, due to a loss of percolation of Ni particles in the Ni/YSZ anode layers closest to the electrolyte. Using SOFCs with Ni/ScYSZ anodes improved the H₂S tolerance considerably, even at larger H₂S concentrations of 10 and 20 ppm over a few hundred hours.