Fabrication by Co-extrusion and electrochemical characterization of micro-tubular hollow fibre solid oxide fuel cells

A phase inversion process was used to co-extrude cerium–gadolinium oxide (Ce_0.9Gd_0.1O_1.95)/NiO–CGO dual-layer hollow fibres (HF), which were then sintered to form, respectively, the electrolyte and high porosity anode precursor of a solid oxide fuel cell (SOFC) with anode inner diameter of 0.8 mm. Graded CGO–lanthanum strontium cobalt ferrite (La_0.6Sr_0.4Fe_0.8Co_0.2O₃) cathode layers were then painted onto the CGO electrolyte to form a micro-tubular HF-SOFC. With a carefully designed anode current collector, this produced maximum power densities of 1186–5864 W m^? 2 at 450–570 °C. High magnification imaging analysis revealed large three-phase boundary regions within the anode, a dense electrolyte layer and clearly highlighted the multiple CGO–LSCF cermet and pure LSCF cathode layers. The performance of the HF-SOFC with a twenty millimetre active length showed no degradation after four thermal cycles between 300 °C and 570 °C.     

Development of micro-tubular SOFCs with an improved performance via nano-Ag impregnation for intermediate temperature operation

Micro-tubular solid-oxide fuel cell consisting of a 10-μm thick (ZrO₂)_0.89(Sc₂O₃)_0.1(CeO₂)_0.01 (ScSZ) electrolyte on a support NiO/(ScSZ) anode (1.8 mm diameter, 200 μm wall thickness) with a Ce_0.8Gd_0.2O_1.9 (GDC) buffer-layer and a La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF)/GDC functional cathode has been developed for intermediate temperature operation. The functional cathode was in situ formed by impregnating the well-dispersed nano-Ag particles into the porous LSCF/GDC layer using a citrate method. The cells yielded maximum power densities of 1.06 W cm⁻² (1.43 A cm⁻², 0.74 V), 0.98 W cm⁻² (1.78 A cm⁻², 0.55 V) and 0.49 W cm⁻² (1.44 A cm⁻², 0.34 V), at 650, 600 and 550 °C, respectively.     

Proton-conducting solid oxide fuel cell (SOFC) with Y-doped BaZrO3 electrolyte

Y-doped BaZrO₃ (BZY) electrolyte films are successfully fabricated by utilizing the driving force from the anode substrate, aiming to circumvent the refractory nature of BZY materials. The BZY electrolyte film on the high shrinkage anode becomes dense after sintering even though no sintering aid is added, while the BZY electrolyte remains porous on the conventional anode substrate after the same treatment. The resulting BZY electrolyte shows a high conductivity of 4.5 × 10^− 3 S cm^− 1 at 600 °C, which is 2 to 20 times higher than that for most of BZY electrolyte films in previous reports. In addition, the fuel cell with this BZY electrolyte generates a high power output of 267 mW cm^− 2 at 600 °C. These results suggest the strategy presented in this study provides a promising way to prepare BZY electrolyte films for fuel cell applications.     

Cobalt-free cathode material SrFe0.9Nb0.1O3−δ for intermediate-temperature solid oxide fuel cells

A cobalt-free cubic perovskite oxide, SrFe_0.9Nb_0.1O_3?δ (SFN) was investigated as a cathode for intermediate-temperature solid oxide fuel cells (IT-SOFCs). XRD results showed that SFN cathode was chemically compatible with the electrolyte Sm_0.2Ce_0.8O_1.9 (SDC) for temperatures up to 1050 °C. The electrical conductivity of SFN sample reached 34–70 S cm^?1 in the commonly operated temperatures of IT-SOFCs (600–800 °C). The area specific resistance was 0.138 Ω cm² for SFN cathode on SDC electrolyte at 750 °C. A maximum power density of 407 mW cm^?2 was obtained at 800 °C for single-cell with 300 μm thick SDC electrolyte and SFN cathode.     

Barium carbonate nanoparticle as high temperature oxygen reduction catalyst for solid oxide fuel cell

BaCO₃ nanoparticles are demonstrated as outstanding electrocatalysts to enhance the high temperature oxygen reduction reaction (ORR) in solid oxide fuel cells (SOFCs). BaCO₃ nanoparticles are formed from thermal decomposition of barium acetate, Ba(Ac)₂ infiltrated to porous cathode skeleton and shows good chemical compatibility with cathode materials. BaCO₃ nanoparticles can greatly reduce the area specific resistance (ASR) of typical SOFC cathode materials, including La_0.8Sr_0.2FeO_3 -δ (LSF), La_0.6Sr_0.4Co_0.2Fe_0.8O_3 -δ (LSCF) and La_0.8Sr_0.2MnO_3 -δ (LSM). For example at 700 °C, ASR for LSF on yttria-stabilized zirconia (YSZ) electrolyte decreases from 2.95 Ω cm² to 0.77 Ω cm² when 12.9 wt.% BaCO₃ nanoparticles are deposited on the surface of the porous LSF electrode. Impedance spectra analysis shows that the decrease in ASR mainly comes from the reduction of the low frequency resistance. Furthermore, BaCO₃ nanoparticles are found to greatly enhance the oxygen chemical exchange coefficient. Most importantly, it has been found that the catalytic activity of BaCO₃ nanoparticles is even higher than those of the precious metals such as Pd, Rh, Pt and Ag, infiltrated into LSF, LSCF and LSM electrodes supported on YSZ electrolytes.     

A novel single phase cathode material for a proton-conducting SOFC

A novel single phase BaCe_0.5Bi_0.5O_3 ? δ (BCB) was employed as a cathode material for a proton-conducting solid oxide fuel cell (SOFC). The single cell, consisting of a BaZr_0.1Ce_0.7Y_0.2O_3 ? δ (BZCY7)-NiO anode substrate, a BZCY7 anode functional layer, a BZCY7 electrolyte membrane and a BCB cathode layer, was assembled and tested from 600 to 700 °C with humidified hydrogen (～3% H₂O) as the fuel and the static air as the oxidant. An open-circuit potential of 0.96 V and a maximum power density of 321 mW cm^?2 were obtained for the single cell. A relatively low interfacial polarization resistance of 0.28Ω cm² at 700 °C indicated that the BCB was a promising cathode material for proton-conducting SOFCs.     

Effects of CuO addition to anode on the electrochemical performances of cathode-supported solid oxide fuel cells

A cathode-supported electrolyte film was fabricated by tape casting and co-sintering techniques. (La_0.8Sr_0.2)_0.95MnO₃ (LSM95), LSM95/Zr_0.89Sc_0.1Ce_0.01O_2?x (SSZ), and SSZ were used as materials of cathode substrate, cathode active layer, and electrolyte, respectively. CuO–NiO–SSZ composite anode was deposited on SSZ surface by screen-printing and sintered at 1250 °C for 2 h. The effects of CuO addition to NiO–SSZ anode on the performance of cathode-supported SOFCs were investigated. CuO can effectively improve the sintering activity of NiO–SSZ. The assembled cells were electrochemically characterized with humidified H₂ as fuel and O₂ as oxidant. With 4 wt.% CuO addition, the ohmic resistance decreased from 3 to 0.46 Ω cm², and at the same time the polarization resistance decreased from 3.4 to 0.74 Ω cm². In comparison with the cell without CuO, the maximum power density at 850 °C increased from 0.054 to 0.446 W cm^?2 with 4 wt.% CuO addition.     

Cost-effective solid oxide fuel cell prepared by single step co-press-firing process with lithiated NiO cathode

A cost-effective cell fabrication process was developed for intermediate temperature solid oxide fuel cells (IT-SOFCs). Co-doped ceria Ce_0.8Gd_0.05Y_0.15O_1.9 (GYDC) was synthesized by carbonate co-precipitation method. Lithiated NiO was prepared by glycine-nitrate combustion method and adopted as cathode material for IT-SOFCs. Single cell was fabricated by one-step dry-pressing and co-firing anode, anode functional layer (AFL), electrolyte and cathode together at 1200 °C for 4 h. The cell presented decent performance and an overall electrode polarization resistance of 0.54 Ω cm² has been achieved at 600 °C. These results demonstrate the possibility of using lithiated NiO as cathode material for ceria-based IT-SOFCs and the development of affordable fuel cell devices is encouraged.     

Preparation of an extremely dense BaCe0.8Sm0.2O3−δ thin membrane based on an in situ reaction

The thin membrane of BaCe_0.8Sm_0.2O_3−δ (BCS) with high quality was successfully fabricated on porous NiO–BCS anode substrate through a novel in situ reaction method. The key part of this method is to directly spray well-mixed suspension of BaCO₃, CeO₂ and Sm₂O₃ instead of pre-synthesized BCS ceramic powder on the anode substrate. After sintering at 1400 °C for 5 h, the extremely dense electrolyte membrane in the thickness of 10 μm is obtained. A single cell was assembled with La_0.7Sr_0.3FeO_3−σ as cathode and tested with humidified hydrogen as fuel at 650 °C. The open circuit voltage (OCV) and maximum power density respectively reach 1.04 V and 535 mW/cm². Interface resistance of cell under open circuit condition was also investigated.     

Enhancement of electrochemical performance and thermal compatibility of GdBaCo2/3Fe2/3Cu2/3O5+δ cathode on Ce1.9Gd0.1O1.95 electrolyte for IT-SOFCs

Transition-metal doped double-perovskite structure oxides GdBaCo_2/3Fe_2/3Ni_2/3O_5+δ (FN-GBCO), GdBaCo_2/3Fe_2/3Cu_2/3O_5+δ (FC-GBCO), GdBaCoCuO_5+δ (C-GBCO) and pristine GdBaCo₂O_5+δ (GBCO) were synthesized via a citrate combustion method. The thermal-expansion coefficient (TEC) and electrochemical performance of the oxides were investigated as potential cathodes for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The TEC exhibited by the FC-GBCO cathode up to 900 °C is 14.6 × 10^?6 °C^?1, which is lower than the value of GBCO (19.9 × 10^?6 °C^?1). Area specific resistances (ASR) of 0.165 Ω cm² at 700 °C and 0.048 Ω cm² at 750 °C were achieved for the FC-GBCO cathode on a Ce_0.9Gd_0.1O_1.95 (CGO) electrolyte. An electrolyte supported (300 μm thick) single-cell configuration of FC-GBCO/CGO/Ni-CGO attained a maximum power density of 435 mW cm^?2 at 700 °C. The unique composition of GBCO co-doped with Fe and Cu ions in the Co sites exhibited reduced TEC and enhancement of electrochemical performance and good chemical compatibility with CGO, and this composition is proving to be a potential cathode for IT-SOFCs.     

Electrochemical performance of La-doped Sr2MgMoO6−δ in natural gas

Modification of the double perovskite Sr₂MgMoO_6−δ by La substitution has shown that Sr_2−xLa_xMgMoO_6−δ with 0.6 ⩽ x ⩽ 0.8 has better performance as the anode of a solid oxide fuel cell. With a Sr_1.2La_0.8MgMoO_6−δ anode, LSGM electrolyte, SrCo_0.8Fe_0.2O_3−δ cathode, and a La_0.5Ce_0.4O_1.7−δ buffer layer between the anode and the electrolyte, a maximum power density of 550 mW/cm² has been obtained for a SOFC operating on wet methane (3%H₂O) at 800 °C. The performance of the SOFC using C₂H₆ fuel, like that of CH₄, changes little on switching from dry C₂H₆ to 3% H₂O/C₂H₆, but improvement with wet C₃H₈ shows that some steam will need to be added to a moderately desulfurized natural-gas fuel.     

Fabrication of a large area cathode-supported thin electrolyte film for solid oxide fuel cells via tape casting and co-sintering techniques

A large area cathode-supported electrolyte film, comprising porous (La_0.8Sr_0.2)_0.95MnO₃ (LSM95) cathode substrate, LSM95/Zr_0.89Sc_0.1Ce_0.01O_2?x (SSZ) cathode active layer, and SSZ electrolyte, has been successfully fabricated by tape casting and co-sintering techniques. The interface reaction between cathode and electrolyte was inhibited by using A-site deficient LSM. A dense enough SSZ thin film with a thickness of ～26 μm was obtained at 1250 °C. By using Pt as anode, the obtained single cell reached the maximum power density of 0.54 W cm^?2 at 800 °C in O₂/humidified H₂, with open circuit voltage (OCV) value of 1.08 V.     

Novel SrSc0.2Co0.8O3−δ as a cathode material for low temperature solid-oxide fuel cell

The SrSc_0.2Co_0.8O_3−δ (SSC) perovskite was investigated as a cathode material for low temperature solid-oxide fuel cell. The material showed an almost linear thermal expansion from room temperature to 1000 °C in air with the average thermal expansion coefficient of only 16.9 × 10⁻⁶ K⁻¹. The Sc-doping made the absence of Co⁴⁺ in SSC, which resulted in not only dramatically reduced thermal expansion coefficient but also extremely high oxygen vacancies concentrations in the lattice at low temperature. The area specific polarization resistance was 0.206 Ω cm² for SSC at 550 °C, which is about 52% lower than the value of a Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ-based cathode. A peak power density as high as 564 mW cm⁻² was obtained at 500 °C based on a 20 μm thick Sm_0.2Ce_0.8O_1.9 electrolyte by adopting SSC cathode.     

High performance nanostructured IT-SOFC cathodes prepared by novel chemical method

The electrochemical performance of La_0.4Sr_0.6Co_0.8Fe_0.2O_3−δ (LSCF) cathodes with different nano/microstructures is compared using the area specific resistance (ASR). Cathodes are prepared using two chemical routes, including a novel method to obtain nanosized LSCF oxide. The results clearly point that the intermediate temperature solid oxide fuel cells (IT-SOFC) cathode performance strongly depends on microstructure and that ASR can vary more than two orders of magnitude for identical composition and different morphologies, reaching values as low as 0.05 Ω cm² at 600 °C and 0.4 Ω cm² at 450 °C using the novel chemical route, which are even lower than the best known cathodes for IT-SOFC.     

A mixed-conducting BaPr0.8In0.2O3 − δ cathode for proton-conducting solid oxide fuel cells

A mixed ionic and electronic conductor, BaPr_0.8In_0.2O_3 − δ (BPI), was synthesized and examined as a cathode material for proton-conducting solid oxide fuel cells (H-SOFCs). X-ray diffraction analysis revealed that BPI had a perovskite structure and showed satisfactory tolerance to CO₂ and H₂O and good chemical compatibility with BaZr_0.1Ce_0.7Y_0.1 Yb_0.1O_3 − δ (BZCYYb) electrolyte. Test cells with a single-phase BPI cathode exhibited excellent electrochemical performances, demonstrating a peak power density of ~ 688 mW cm^− 2 at 750 °C. Furthermore, the cells with a BPI cathode showed very stable power output at a cell voltage of 0.7 V at 600 °C over 100 h, suggesting that BPI is a promising alternative cathode for H-SOFCs.     

Prediction of infiltrated solid oxide fuel cell cathode polarization resistance

The polarization resistance of La_0.6Sr_0.4Co_0.2Fe_0.8O_3?δ (LSCF)-infiltrated Ce_0.9Gd_0.1O_1.95 cathodes was quantitatively explained using a simple model where the resistance scaled directly with the LSCF surface area, as estimated from cross-sectional fracture surfaces. The Tanner, Fung, Virkar composite cathode model was also applied and showed that ionic transport in these 25-μm-thick cathodes was not a significant limitation at 600 °C, but became more limiting at 700 °C. Calculated polarization resistances were within ～40% (without fitting parameters) of reported values.     

A novel anode supported BaCe0.7Ta0.1Y0.2O3−δ electrolyte membrane for proton-conducting solid oxide fuel cell

A novel composition of BaCe_0.7Ta_0.1Y_0.2O_3−δ (BCTY10) electrolyte membrane was successfully fabricated on porous NiO-BCTY10 anode substrate. The anode was prepared through a route combining a solid state reaction and a wet chemical method. After sintering at 1450 °C for 5 h, the BCTY10 membrane showed adequate chemical stability against CO₂ and H₂O. With a mixture of La_0.7Sr_0.3FeO_3−δ (LSF) and BaCe_0.7Zr_0.1Y_0.2O_3−δ (BZCY7) as cathode, a single fuel cell with 25 μm thick BCTY10 electrolyte generated maximum power densities of 195, 137, 84, 44 mW/cm² at 700, 650, 600 and 550 °C, respectively. The interface resistance of the cell under open circuit condition was also investigated.     

Composite cathode for IT-SOFC: Sr-doped lanthanum cuprate and Gd-doped ceria

Composite cathodes were synthesized via a citrate combustion method followed by an organic precipitation method. The cathodes were of K₂NiF₄-type crystal structure with x wt.% Ce_0.9Gd_0.1O_1.95 (CGO)–(100 ? x) wt.% La_1.96Sr_0.04CuO_4 + δ (LSC), where x = 0, 10, 20 and 30. The individual structural phases of the composite cathodes were characterized using a third-generation synchrotron source beamline powder X-ray diffractometer (XRD). The porous grain morphology of the CGO–LSC cathode composite for a symmetrical half-cell was determined from cross-sectional scanning electron microscopy images and elemental line profiles. The composite cathode was made of 20 wt.% CGO–80 wt.% LSC (CL20–80) and was coated onto a Ce_0.9Gd_0.1O_1.95 electrolyte. It showed the lowest area specific resistance (ASR) of 0.07 Ω cm² at 750 °C. An electrolyte-supported (300 μm thick) single-cell configuration of CL20–80/CGO/Ni-CGO attained a maximum power density of 626 mW cm^? 2 at 700 °C. The unique composite composition of CL20–80 demonstrates enhanced electrochemical performance and good chemical compatibility with the CGO electrolyte, as compared with the pure LSC (CL0–100) cathode for IT-SOFCs.     

Coal syngas reactivity over Ni-added LSCF–GDC anode of solid oxide fuel cells

Coal syngases of various CO/H₂ ratios were used as the fuel over anode of solid oxide fuel cells. Anode materials of La_0.58Sr_0.4Co_0.2Fe_0.8O_3?δ (LSCF)–gadolinia-doped ceria (GDC) composites and Ni-added LSCF–GDC were tested. The maximum power density of CO is higher than that of H₂ and increases as the CO concentration in the CO + H₂ mixture increases. As the LSCF content in the LSCF–GDC composites increases, the H₂ reactivity increases but the CO reactivity decreases. The maximum power density of Ni-added LSCF–GDC is highest but the steady-state current density of LSCF–GDC can be higher than that of Ni-added LSCF–GDC via an induction period of activation.     

Novel BaCo0.7Fe0.3−yNbyO3−δ (y = 0–0.12) as a cathode for intermediate temperature solid oxide fuel cell

The BaCo_0.7Fe_0.3?yNb_yO_3?δ oxides (BCFNy, y = 0.00–0.12) were synthesized by the conventional solid state reaction process and investigated as a novel cathode for intermediate temperature solid oxide fuel cells(IT-SOFCs). Cubic perovskite, with enhanced phase stability at higher Nb concentration, was obtained at y ? 0.04. The unit cell volumes increased with y, reached a maximum at y = 0.10, and then decreased. The niobium doping concentration also had a significant effect on the electrochemical performance of BCFNy materials. Among the various BCFNy oxides tested, BCFN0.10 possessed the smallest interfacial polarization resistance (R_p). The R_p was as low as 0.9406, 0.1300, 0.0211, and 0.0082 Ω cm² at 500, 600, 700, and 800 °C, respectively. With a 220 μm-thick Sm_0.2Ce_0.1O_1.9 (SDC) as electrolyte and BCFN0.10 as the cathode, a fuel cell provides maximum power densities of 202, 350, 569, 820, and 1006 mW cm^?2 at 600, 650, 700, 750, and 800 °C, respectively. The encouraging results suggested that BCFN0.10 was a very promising cathode material for IT-SOFCs.