Reoxidation behavior of Ni-Fe bimetallic anode substrate in solid oxide fuel cells using a thin LaGaO3 based film electrolyte

The reoxidation behavior of a Ni-Fe metal anode supported cell using a thin LaGaO₃ electrolyte film was investigated as a function of the reoxidation temperature. After oxidation and reduction treatments for 2 h, the voltage did not return to the initial voltage at higher temperatures (773-973 K); however, after reoxidation at 673 K, the cell exhibited almost the same OCV as the as-prepared cell. During reoxidation with air at the higher temperatures, the Ni-Fe metal substrate exhibited two different expansion behaviors by the different oxidation rates of Ni and Fe. On the other hand, the volumetric change of the oxidized substrate at 673 K was negligible. SEM-EDX results exhibited the reoxidation of Ni-Fe occurred only at the bottom part of the substrate and at the interface between the electrolyte and the substrate. In spite of temperatures as low as 673 K, the cell generated a power of 160 mW cm⁻², which hardly decreased after the redox cycle. The increasing anodic internal resistance accompanied with unreduced Fe.     

Intermediate temperature solid oxide fuel cells using LaGaO3 based oxide film deposited by PLD method

Dense La_0.9Sr_0.1Ga_0.8Mg_0.2O_3-δ (LSGM, 5 μm in thickness)/Ce_0.8Sm_0.2O_2-δ (SDC, 400 nm in thickness) bilayer films were deposited on a dense NiO (Fe₃O₄)-SDC anode substrate by a pulsed laser deposition (PLD) method. After in-situ reduction, the substrate turned to be porous and it can be used as a porous anode substrate. The power density was strongly affected by the oxide ion conductor combined with LSGM and it was found that SDC is the most useful for achieving the high power density. Preparation of Sm_0.5Sr_0.5CoO₃ cathode film by PLD method is also studied for decreasing the contact resistance of cathode. Preparation of SSC film by PLD is effective for decreasing cathodic overpotential and 400 nm thick film is the most effective for achieving the high power density. At 773 K, the maximum power density of the cell becomes higher than 500 mW/cm².     

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.     

Solution combustion synthesis of Ce0.6Mn0.3Fe0.1O2 for anode of SOFC using LaGaO3-based oxide electrolyte

This paper describes the potential of solution combustion synthesis (SCS) method for preparing Ce_0.6Mn_0.3Fe_0.1O₂ (CMF) as the anode material for solid oxide fuel cells (SOFC). The stability, crystallinity, morphology, and surface area of the products were depended on the fuel ratio used in SCS as investigated by TGA, XRD, SEM, and BET, which correspondingly influenced their electrochemical properties. The SCS-derived products were directly used for preparing anodes by sintering the screen-printed powders on the electrolyte membrane, and were evaluated from power generation performance, which were compared with the conventional solid-state-reaction (SSR) sample. Significantly, under configuration of the cell of CMF/La_0.8Sr_0.2Ga_0.8Mg_0.15Co_0.05O₃/Sm_0.5Sr_0.5CoO₃ using humidified hydrogen gas as a fuel and O₂ as an oxidizing agent, the maximum power densities obtained were 1.23 W/cm² at 1000 °C for the SCS product (CMF1) obtained at ? = 0.5. This value was higher than 1.09 W/cm² for the SSR-derived sample under the same evaluation conditions. The results appealed benefits of SCS method for preparing CMF as the anode material with high power generation performance for SOFC, due to its large surface area and nanosized grains, in which fuel ratio was a key parameter for its synthesis.     

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.     

Thermal cycle stability of Al2O3-based compressive seals for planar intermediate temperature solid oxide fuel cells

Al₂O₃-based compressive seals were fabricated by tape casting with Al₂O₃ and 0-30 wt% aluminum powders, and their sealing effectiveness, thermal cycle stability between 200 and 750 °C and applicability in planar intermediate temperature solid oxide fuel cells were evaluated. The results indicate that increasing the aluminum content from 0 to 30 wt% in the seals decreases the leakage rate and increases the thermal cycle stability under various inlet gas (N₂) pressures of 3.5, 7.0 and 10.5 kPa. Especially, with the seal containing 30 wt% of aluminum (ACS3), the initial leakage rate was below 0.03 sccm cm⁻¹ under an inlet pressure of 10.5 kPa, and the leakage rates during 96 thermal cycles were below 0.04 sccm cm⁻¹ under the same inlet gas pressure. The interfaces in the interconnect/seal/cell assembly with the ACS3 seal retained integrity after 50 thermal cycles, demonstrating the applicability of the Al₂O₃-based compressive seals in the planar intermediate temperature SOFCs.     

Surface modified Ni foam as current collector for syngas solid oxide fuel cells with perovskite anode catalyst

Cu surface modified nickel foam is obtained by heating copper coated nickel foam in a reducing atmosphere. La_0.75Sr_0.25Cr_0.5Mn_0.5O_3−δ (LSCM) perovskite oxide is prepared using a sol–gel combustion method. The modified foams and LSCM powders exhibit excellent resistance to carbon deposition in syngas at high temperatures. Furthermore, Cu modified foams show better mechanical strength compared to bare Ni foam, which readily cracks after exposure to syngas at high temperature. LSCM retains its perovskite structure during exposure to syngas or carbon monoxide at 900 °C for 10 h. Cu surface modified Ni foam current collector demonstrates good chemical compatibility with LSCM in syngas atmosphere at high temperature. Syngas solid oxide fuel cells (SOFCs) are assembled using Cu modified Ni foam anode current collector, LSCM anode catalyst, YSZ electrolyte, and porous Pt cathode. The present fuel cell provides similar power density to one with gold anode current collector and has excellent stability during operation at 900 °C.     

Upgrading the performance of La2Mo2O9-based solid oxide fuel cell under single chamber conditions

Various anode-supported solid oxide fuel cells (SOFC), based on 10 mol% Dy-doped La₂Mo₂O₉ (LDM) electrolyte, are prepared analytically and operated under single chamber conditions to explore the connections between electrode and power performance. The cathode of tested SOFCs is compositionally graded with three composites of samarium strontium cobaltite and Gd-doped ceria (GDC) to relax the thermal stress, because of sizable thermal expansion differences above 400 °C. We focus the research attention on varying the anode pore structure and composition to promote the power performance in methane/air mixture at 700 °C. For the one-layer support of GDC+NiO+LDM anode, addition of 10 wt% graphite minimizes its mass transport resistance through creating 8–5 μm long and ∼1 μm wide slit-shaped pores. The graphite pore former raises the peak power value by 80 mW cm⁻². Adopting a more porous and active outer layer, the double-layer support further enhances the cell power. The peak power was first raised by 48 mW cm⁻², using an outer layer that was prepared with 63 wt% NiO. Dosing 3% Pd on this outer layer uplifts another 59 mW cm⁻². In this study, with an improved anode, the peak power value reaches 437 mW cm⁻².     

Performance of a novel La(Sr)MnO3-Pd composite current collector for solid oxide fuel cell cathode

The electrochemical performance of LSM-Pd composite material as current collector of SOFC cathode is studied on (La_0.8Sr_0.2)_0.9MnO₃ (LSM90) cathode. The influence of Pd content on contact resistance is investigated. The investigation shows that the contact resistance of LSM-Pd is about 20 mΩ cm² at 750 °C when the composite contains 8 wt% Pd, and it could be comparable to pure Pt. The ohmic resistance of a single cell using LSM-Pd composite is about 255 mΩ cm² that contains 4 wt% Pd as current collector, this value is close to that of a cell using expensive Pt paste as current collector.     

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.     

Easy sintering of silver doped lanthanum strontium manganite current collector for solid oxide fuel cells

A new system, (La_0.8Sr_0.2)_1−xAg_xMnO_3+δ (LSAM, x ≤ 0.2), is developed as current collector for solid oxide fuel cell (SOFC). LSAM is prepared by a modified sol-gel method and presents a single phase. The shrinkage temperature reduces from 1150 °C to 800 °C with an addition of 15 mol% Ag to La_0.8Sr_0.2MnO_3+δ (LSM20). The contact resistance between the current collector and the cathode is measured, and the influence of Ag content on the contact resistance is investigated. The result shows that the contact resistance using (La_0.8Sr_0.2)_0.85Ag_0.15MnO_3+δ (LSAM15) as current collector is about 12 mΩ cm² at 750 °C, which is close to the value using expensive Pt paste as current collector. This new system is a promising current collecting material for the practical application of SOFC.     

Preparation and characterization of nanocrystalline Ce0.8Sm0.2O1.9 for low temperature solid oxide fuel cells based on composite electrolyte

Nanocrystalline Ce_0.8Sm_0.2O_1.9 (SDC) has been synthesized by a combined EDTA–citrate complexing sol–gel process for low temperature solid oxide fuel cells (SOFCs) based on composite electrolyte. A range of techniques including X-ray diffraction (XRD), and electron microscopy (SEM and TEM) have been employed to characterize the SDC and the composite electrolyte. The influence of pH values and citric acid-to-metal ions ratios (C/M) on lattice constant, crystallite size and conductivity has been investigated. Composite electrolyte consisting of SDC derived from different synthesis conditions and binary carbonates (Li₂CO₃–Na₂CO₃) has been prepared and conduction mechanism is discussed. Water was observed on both anode and cathode side during the fuel cell operation, indicating the composite electrolyte is co-ionic conductor possessing H⁺ and O²⁻ conduction. The variation of composite electrolyte conductivity and fuel cell power output with different synthesis conditions was in accordance with that of the SDC originated from different precursors, demonstrating O²⁻ conduction is predominant in the conduction process. A maximum power density of 817 mW cm⁻² at 600 °C and 605 mW cm⁻² at 500 °C was achieved for fuel cell based on composite electrolyte.     

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.     

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.     

Plasma spray synthesis of La10(SiO4)6O3 as a new electrolyte for intermediate temperature solid oxide fuel cells

The apatite-type lanthanum silicate films were successfully synthesized by modified atmosphere plasma spraying using lanthanum oxide and silicon oxide mixed powders and precalcined hypereutectic powders in the size range 1–3 μm and 5–8 μm, respectively, as starting feedstock materials. The films differed not only in microstructural scale, but also in the characteristic of the degree of film densification. A detail describing the evolution of microstructure has been discussed. A considerable improvement in densification of the La₁₀(SiO₄)₆O₃ electrolyte films has been observed.     

MoO3 nanorods/Fe2(MoO4)3 nanoparticles composite anode for solid oxide fuel cells

MoO₃ nanorods/Fe₂(MoO₄)₃ nanoparticles composite has been prepared by a hydrothermal method combined with an in situ diffusion growth process. Single cells based on 300 μm LSGM electrolyte have been fabricated with the MoO₃ nanorods/Fe₂(MoO₄)₃ nanoparticles composite anode and a composite cathode consisting of Sr_0.9Ce_0.1CoO_3−δ and Sm-doped ceria (SDC). The peak power densities reach 225, 50, 75 mW cm⁻² at 900 °C in H₂, CH₄ and C₃H₈, respectively. The cell shows excellent long-term stability at 850 °C. The preliminary results demonstrate that the MoO₃ nanorods/Fe₂(MoO₄)₃ nanoparticles composite is a promising alternative anode for solid oxide fuel cells.     

Mechanical strengthening of Sm-doped CeO2 ceramics by 1 mol% cobalt oxide for solid oxide fuel cell application

Combustion processed nano-sized Ce_0.79Sm_0.20Co_0.01O_2−δ (CSCoO) and Ce_0.80Sm_0.20O_2−δ (CSO) were sintered at various temperatures, and mechanical properties and microstructures were characterized. The results demonstrate that besides a lowering of sintering temperature to achieve sufficient densification, the doping of very minor amounts of CoO (1 mol%) to CSO significantly enhanced mechanical properties. The CSCoO sintered at 1100 °C exhibits a biaxial flexural strength of 354 ± 42 MPa which compared with 194 ± 57 MPa for CSO with a similar relative density sintered at 1400 °C. This slight modification in chemical composition also resulted in a slight improvement in micro-hardness.     

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.     

Novel Al2O3-based compressive seals for IT-SOFC applications

Novel compressive Al₂O₃-based seals were developed and characterized under simulated intermediate temperature solid oxide fuel cell (IT-SOFC) environment. The seals were prepared by tape casting, mainly composed of fine Al₂O₃ powder with various contents of fine Al powder addition. The leakage rates were determined at 800 °C under 0.14–0.69 MPa compressive stresses, and the stabilities were evaluated at 750 °C under constant 0.35 MPa compressive stress. The leakage rates at 800 °C were in range of 0.2–0.01 sccm cm⁻¹, decreasing with increasing the compressive stress and Al content; Al addition significantly improved the stability, the leakage rate with 20 wt% Al addition was as low as 0.025 sccm cm⁻¹ at 800 °C under 0.35 MPa compressive stress with a gauge pressure of 6.9 kPa, and exhibited good stability at 750 °C. Single cell test also confirmed the effectiveness of the tape cast Al₂O₃-based seal for planar IT-SOFC applications.