Synthesis, structural analysis and electrochemical performances of BLSITCFx as new cathode materials for solid oxide fuel cells (SOFC) based on BIT07 electrolyte

BaIn_0.3Ti_0.7O_2.85 (BIT07) is a suitable electrolyte for Solid Oxide Fuel Cell (SOFC) but half cells based on La_0.58Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) as a cathode material show a degradation of the Area Specific Resistance (ASR) at 700 °C with time. This study deals with the characterization of alternative cathode materials showing a better compatibility with BIT07 than LSCF. A new solid solution, Ba_xLa_0.58(1−x)Sr_0.4(1−x)In_0.3xTi_0.7xCo_0.2(1−x)Fe_0.8(1−x)O_3−δ, with 0 ≤ x ≤ 1, also called BLSITCFx, with in this case x expressed in molar %, derived from BIT07 and LSCF, has been synthesized at 1350 °C in air using BIT07 and LSCF powders. Two compositions, BLSITCF12 and BLSITCF25, have been selected due to their thermal expansion and conductivity properties. Symmetrical half cells based on these two new materials deposited on BIT07 electrolyte have been studied by complex impedance spectroscopy in air versus temperature and time. Their behaviour is comparable to LSCF's, with ASR values never exceeding 0.2 Ωcm² at 700 °C, and moreover their less important Thermal Expansion Coefficient (TEC) mismatch with BIT07 lead to a better mechanical compatibility with time. These new compounds are therefore better candidates than LSCF as cathode materials for SOFC based on BIT07 electrolyte.     

The effect of porosity gradient in a Nickel/Yttria Stabilized Zirconia anode for an anode-supported planar solid oxide fuel cell

In this paper, a graded Ni/YSZ cermet anode, an 8 mol.%YSZ electrolyte, and a lanthanum strontium manganite (LSM) cathode were used to fabricate a solid oxide fuel cell (SOFC) unit. An anode-supported cell was prepared using a tape casting technique followed by hot pressing lamination and a single step co-firing process, allowing for the creation of a thin layer of dense electrolyte on a porous anode support. To reduce activation and concentration overpotential in the unit cell, a porosity gradient was developed in the anode using different percentages of pore former to a number of different tape-slurries, followed by tape casting and lamination of the tapes. The unit cell demonstrated that a concentration distribution of porosity in the anode increases the power in the unit cell from 76 mW cm⁻² to 101 mW cm⁻² at 600 °C in humidified hydrogen. Although the results have not been optimized for good performance, the effect of the porosity gradient is quite apparent and has potential in developing superior anode systems.     

Improved power generation performance of solid oxide fuel cells using doped LaGaO3 electrolyte films prepared by screen printing method II. Optimization of Ni-Ce0.8Sm0.2O1.9 cermet anode support

A Ni and Sm-doped ceria (Ce_0.8Sm_0.2O_1.9, SDC) cermet anode as a porous support of doped LaGaO₃ film prepared by a wet coating and co-firing process was investigated. Different preparation methods and compositions were used to improve the power density of intermediate temperature solid oxide fuel cells. NiO-SDC precursor powder with fine particles and a porous microstructure with high surface area was synthesized by a modified impregnation method and compared with that synthesized by a ball milling method. In addition, an open circuit voltage, which is almost equal to the theoretical value of 1.1 V, and maximum power densities of 835, 277, and 67 mW cm⁻² at 700, 600, and 500 °C, respectively, were achieved on a single cell supported by a 75 wt% Ni-SDC cermet anode when a 60 μm thick Sr- and Mg-doped lanthanum gallate (LSGM) electrolyte was used. The improved power density was explained by the enlarged reaction area for the anode as a result of the low polarization resistance of the anode by high porosity and uniform distribution of Ni and SDC particles. Although a small amount of Ni diffused to the interface between the La-doped ceria (LDC) buffer layer and the LSGM electrolyte film, an adverse reaction that deteriorates cell performance seemed to be suppressed, and thus, reasonably high power density was achieved on the cell using the LSGM film prepared by the screen printing method with optimization of the anode substrate structure and composition.     

Evaluation of Ba2(In0.8Ti0.2)2O5.2−n(OH)2n as a potential electrolyte material for proton-conducting solid oxide fuel cell

Electrochemical measurements of fuel cells based on proton conductor electrolyte Ba₂(In_0.8Ti_0.2)₂O_5.2−n(OH)_2n and prepared through a tape casting process and a co-pressing of anode-composite powder and electrolyte tape were performed at 500 °C under wet H₂. The varying parameter between the prepared cells was the thickness of the electrolyte that can be controlled during the tape casting process. The maximum power density was obtained for the cell with the thinnest electrolyte (35 μm) and was about 22 mW cm⁻² with an ohmic resistance about 2 Ω cm² at 500 °C.     

Mixed ionic electronic conducting perovskite anode for direct carbon fuel cells

The conversion of carbonaceous materials to electricity in a Direct Carbon Fuel Cell (DCFC) offers the most efficient process with theoretical electric efficiency close to 100%. One of the key issues for fuel cells is the continuous availability of the fuel at the triple phase boundaries between fuel, electrode and electrolyte. While this can be easily achieved with the use of a porous fuel electrode (anode) in the case of gaseous fuels, there are serious challenges for the delivery of solid fuels to the triple junctions. In this paper, a novel concept of using mixed ionic electronic conductors (MIEC) as anode materials for DCFCs has been discussed. The lanthanum strontium cobalt ferrite, La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) was chosen as the first generation anode material due to its well known high mixed ionic and electronic conductivities in air. This material has been investigated in detail with respect to its conductivity, phase and microstructural stability in DCFC operating environments. When used both as the anode and cathode in a DCFC, power densities in excess of 50 mW/cm² were obtained at 804 °C in electrolyte supported small button cells with solid carbon as the fuel. The concept of using the same anode and cathode material has also been evaluated in electrolyte supported thick wall tubular cells where power densities around 25 mW/cm² were obtained with carbon fuel at 820 °C in the presence of helium as the purging gas. The concept of using a mixed ionic electronic conducting anode for a solid fuel, to extend the reaction zone for carbon oxidation from anode/electrolyte interface to anode/solid fuel interface, has been demonstrated.     

Internal shorting and fuel loss of a low temperature solid oxide fuel cell with SDC electrolyte

A solid oxide fuel cell with Sm_0.2Ce_0.8O_1.9 (SDC) electrolyte of 10 μm in thickness and Ni–SDC anode of 15 μm in thickness on a 0.8 mm thick Ni–YSZ cermet substrate was fabricated by tape casting, screen printing and co-firing. A composite cathode, 75 wt.% Sm_0.5Sr_0.5CoO₃ (SSCo) + 25 wt.% SDC, approximately 50 μm in thickness, was printed on the co-fired half-cell, and sintered at 950 °C. The cell showed a high electrochemical performance at temperatures ranging from 500 to 650 °C. Peak power density of 545 mW cm⁻² at 600 °C was obtained. However, the cell exhibited severe internal shorting due to the mixed conductivity of the SDC electrolyte. Both the amount of water collected from the anode outlet and the open circuit voltage (OCV) indicated that the internal shorting current could reach 0.85 A cm⁻² or more at 600 °C. Zr content inclusions were found at the surface and in the cross-section of the SDC electrolyte, which could be one of the reasons for reduced OCV and oxygen ionic conductivity. Fuel loss due to internal shorting of the thin SDC electrolyte cell becomes a significant concern when it is used in applications requiring high fuel utilization and electrical efficiency.     

Fabrication and characterization of anode support low-temperature solid oxide fuel cell based on the samaria-doped ceria electrolyte

In this paper anode support was fabricated by tape casting method using SDC-50 wt.% NiO slurry, then printed the Ce_0.8Sm_0.2O_1.9 (SDC) electrolyte on the green piece which is cut out from the dried slurry piece. After at 90 °C drying for 14 h and co-sintered at 1350 °C for 10 h, get the Φ70 mm anode support and electrolyte planar bilayer. Based on the observation of photos and scanning electron microscopy (SEM) indicated that bilayer owns the flat anode support substrate, and the highly dense, crack free electrolyte film which is 12 μm in thickness. Small disks which were cut out from the Φ70 mm bilayer structure electrochemically were examined in a single button-cell mode incorporating a SDC-60 wt.% La_0.5Sr_0.5Co_0.8Fe_0.2O₃ composite cathode. The single cell was tested at 450 °C∼600 °C, an open-circuit voltage (OCV) of 0.94 V and the maximum power density of 797 mV cm⁻² achieved with dry hydrogen as fuel gas and air as oxidant gas at 600 °C.     

A promising Ni–Fe bimetallic anode for intermediate-temperature SOFC based on Gd-doped ceria electrolyte

Anode supported solid oxide fuel cells (SOFC) based on Ni–Fe bimetal and gadolinia-doped ceria (GDC) composite anode were fabricated and evaluated in the intermediate- and low-temperature range. Ni_0.75Fe_0.25-GDC anode substrate and GDC electrolyte bilayer were prepared by the multi-layered aqueous tape casting method. The single cell performance was characterized with La_0.6Sr_0.4Co_0.2Fe_0.8O₃-GDC (LSCF-GDC) composite cathode. The maximum power density reached 330, 567, 835 and 1333 mW cm⁻² at 500, 550, 600 and 650 °C, respectively. Good long-term performance stability has been achieved at 600 °C for up to 100 h. The improved single cell performance was achieved in the reduced temperature after the long-term stability test. The maximum power density registered 185 and 293 mW cm⁻² at 400 and 450 °C, respectively. The impedance spectra fitting results of the test cell revealed that the improved cell performance was attributed to the much lower electrochemical reaction resistance. XRD and SEM examination indicated that the outstanding performance of the single cell seemed to arise from the optimized composition and excellent microstructure of Ni_0.75Fe_0.25-GDC anode, as well as the improved stability of the anode microstructure with prolonged testing time.     

Intermediate temperature solid oxide fuel cell based on BaIn0.3Ti0.7O2.85 electrolyte

Electrolyte supported as well as anode supported single-cells based on BaIn_0.3Ti_0.7O_2.85 (BIT) electrolyte were developed. In these cells, Ni-BIT cermet was used as anode and La_0.8Sr_0.2MnO₃ as cathode. Electrolyte supported cells were fabricated by coating slurries of anode and cathode materials on the circular faces of sintered electrolyte discs. The maximum power (P_max) drawn was 15 mW cm⁻² at 30 mA cm⁻². Anode supported cells were fabricated by co-pressing and co-sintering anode and electrolyte powders. The thickness of electrolyte in anode supported cells was reduced to 80 μm and the area specific resistance decreased considerably. The value of P_max improved to ∼100 mW cm⁻².     

Co-sintering anode and Y2O3 stabilized ZrO2 thin electrolyte film for solid oxide fuel cell fabricated by co-tape casting

Fabricating a large-area unit cell is very important for the development of solid oxide fuel cell (SOFC) stack. In this study, details of sintering process of half cell with NiO-yttria stabilized zirconia (YSZ) anode-supported YSZ thin electrolyte film fabricated by co-tape casting have been discussed. The results demonstrates that the shrinkages and shrinking rates mismatches between the electrolyte and the anode can be controlled by the organic additive content in the anode slurry composition and heating rate. Low heating rate suppresses the cracks formation in the electrolyte films. A warp-free unit cell with size of 100 × 100 mm² and dense electrolyte has been successfully fabricated. A power of 22.2 W, with a power density of 0.27 W cm⁻² has been achieved at 0.7 V and 750 °C in O₂/humidified H₂ atmosphere. The area specific resistance of the cell is 1.20 Ω cm² at 0.7 V and 750 °C.     

Fabrication of multi-layered solid oxide fuel cells using a sheet joining process

Ceria-based electrolytes can be used in solid oxide fuel cells (SOFCs) that operate at intermediate-temperature due to their high ionic conductivity. However, the difficulty in fabricating a thin and dense ceria-based electrolyte on an anode support is well-known. In this study, a new sheet joining process is suggested to produce a thin and dense ceria-based electrolyte for anode-supported SOFCs. The main idea used with the sheet joining process is a combination of a sheet cell fabricated by tape casting, and an anode pellet, fabricated by pressing. The maximum power density of a single cell, fabricated by the sheet joining process, is 0.315 W cm⁻² at 600 °C in a power generation test when Pr_0.3Sr_0.7Co_0.3Fe_0.7O_3−δ was used as the cathode material. The performance durability of a single cell is tested over 1000 h of operation in which a dense electrolyte was observed to survive.     

Fabrication and characterization of Ni/ScSZ cermet anodes for IT-SOFCs

In this study the fabrication and characterization of Ni/10ScSZ (Ni/10 mol% Sc₂O₃-90 mol% ZrO₂) and Ni/10Sc1CeSZ (Ni/10 mol% Sc₂O₃-1 mol% CeO₂-89 mol% ZrO₂) cermet anode films was studied and compared. Both 10ScSZ and 10Sc1CeSZ electrolyte powders showed tetragonal and cubic phases at room temperature, respectively. The NiO/10ScSZ and NiO/10Sc1CeSZ composites with 10-60 vol% of Ni content were prepared by mixing as-received commercial powders of NiO, 10ScSZ and 10Sc1CeSZ followed by ink preparation. Samples were sintered for 1 h at temperatures of 1250-1350 °C. All the cermet films were then reduced under a mixture of hydrogen (10%) and nitrogen (90%) at 800 °C for 2 h. The effect of Ni content and sinter temperature on the DC electrical conductivity were investigated, and the results showed a sharp change in conductivity at around 30 vol% Ni, corresponding to continuity/discontinuity of the Ni-Ni contact network, and the conductivity increased as the sinter temperature increased from 1250 to 1350 °C. An acceptable electrical conductivity at 700 °C for these cermet films was obtained at >40 vol% Ni, consistent with behaviour reported for more conventional Ni/YSZ cermets. The effect of sinter temperature on the microstructure and porosity of Ni/10Sc1CeSZ and Ni/10ScSZ cermet films was also investigated. This revealed that the porosity of the cermet films with the same Ni content decreased as the sinter temperature increased and that, for a given sinter temperature, the porosity of the cermet films increased with Ni content. The porosities of 40Ni/60ScCeSZ (40 vol% Ni/60 vol% 10Sc1CeSZ) and 40Ni/60ScSZ (40 vol% Ni/60 vol% 10ScSZ) anodes sintered at 1250, 1300 and 1350 °C for 1 h were in the range of 30-45%. Electrochemical measurement of symmetrical cells using an 8YSZ electrolyte at 700 °C revealed that the lowest electrode polarization resistance of 40Ni/60ScCeSZ and 40Ni/60ScSZ anodes was obtained at sinter temperatures of 1350 °C and 1300 °C respectively. Carbon deposition over 40Ni/60ScCeSZ, 40Ni/60ScSZ and 40Ni/60YSZ catalysts was evaluated at 700 °C for 1 h at S/C = 0.8 and the results showed that the ratio of deposited carbon was lower in the case of Ni/10ScSZ and Ni/10Sc1CeSZ compared to Ni/YSZ (0.35). Overall, Ni/10Sc1CeSZ and Ni/10ScSZ cermets having 40 vol% Ni were found to be optimum, with the 40Ni/60ScCeSZ cermet proving to be better than 40Ni/60ScSZ cermet in terms of both electronic conductivity and electrode polarization resistance, with both materials exhibiting improved tolerance towards carbon deposition compared to Ni/YSZ.     

Electrophoretic deposition of dense BaCe0.9Y0.1O3−x electrolyte thick-films on Ni-based anodes for intermediate temperature solid oxide fuel cells

Proton conducting BaCe_0.9Y_0.1O_3−x (BCY10) thick films are deposited on cermet anodes made of nickel–yttrium doped barium cerate using electrophoretic deposition (EPD) technique. BCY10 powders are prepared by the citrate–nitrate auto-combustion method and the cermet anodes are prepared by the evaporation and decomposition solution and suspension method. The EPD parameters are optimized and the deposition time is varied between 1 and 5 min to obtain films with different thicknesses. The anode substrates and electrolyte films are co-sintered at 1550 °C for 2 h to obtain a dense electrolyte film keeping a suitable porosity in the anode, with a single heating treatment. The samples are characterized by field emission scanning electron microscopy (FE-SEM) and energy dispersion spectroscopy (EDS). A prototype fuel cell is prepared depositing a composite La_0.8Sr_0.2Co_0.8Fe_0.2O₃ (LSCF)–BaCe_0.9Yb_0.1O_3−δ (10YbBC) cathode on the co-sintered half cell. Fuel cell tests that are performed at 650 °C on the prototype single cells show a maximum power density of 174 mW cm⁻².     

Stable BaCe0.5Zr0.3Y0.16Zn0.04O3−δ thin membrane prepared by in situ tape casting for proton-conducting solid oxide fuel cells

Stable BaCe_0.5Zr_0.3Y_0.16Zn_0.04O_3−δ (BCZYZ) thin membrane was successfully prepared by in situ tape casting/co-firing method for proton-conducting solid oxide fuel cells. The starting powders were BaCO₃, CeO₂, ZrO₂, Y₂O₃, ZnO for electrolyte and BaCO₃, CeO₂, ZrO₂, Y₂O₃, ZnO, NiO, graphite for anode. The anode/electrolyte bi-layers were prepared by a simple multi-layer tape casting/co-firing method. The phase characterizations and microstructures were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The anode–electrolyte bi-layers were sintered at 1450 °C. The electrolytes were extremely dense with pure perovskite phase and the thickness was about 25 μm. The anodes were porous and no obvious reaction was found between NiO and BCZYZ. With LaSr₃Co_1.5Fe_1.5O_10−δ (LSCF)/BCZYZ as cathode, the open current voltage and maximum power density respectively, reached 1.00 V and 247 mW cm⁻² at 650 °C.     

Metal-supported solid oxide fuel cells with in-situ sintered (Bi2O3)0.7(Er2O3)0.3–Ag composite cathode

Metal-supported solid oxide fuel cells (SOFCs) containing porous 430L stainless steel support, Ni-YSZ anode and YSZ electrolyte were fabricated by tape casting, laminating and co-firing in a reduced atmosphere. (Bi₂O₃)_0.7(Er₂O₃)_0.3–Ag composite cathode was applied by screen printing and in-situ sintering. The polarization resistances of the composite cathode were 1.18, 0.48, 0.18, 0.09 Ω cm² at 600, 650, 700 and 750 °C, respectively. A promissing maximum power density of 568 mW cm⁻² at 750 °C was obtained of the single cell. Short-term stability was measured as well.     

A cost-effective process for fabrication of metal-supported solid oxide fuel cells

Metal-supported SOFC cells with Y₂O₃ stabilized ZrO₂ as the electrolyte were prepared by a low cost and simple process involving tape casting, screen printing and co-firing. The interfaces were well bonded after the reduction of NiO to Ni in the support and the anode. AC impedance was employed to estimate the cell polarizations under open circuit conditions. It was found that the electrode polarization resistance was high at low temperatures and became equivalent to the ohmic resistance at higher temperatures near 800°°C. The cell performance was evaluated with H₂ as the fuel and air as the oxidant, and maximum power density between 0.23 and 0.80  W/cm² was achieved in the temperature range of 650–800°C, which confirms the applicability of the cost-effective process in fabrication of metal-supported SOFC cells.     

Performance and stability of SOFC anode fabricated from NiO–YSZ composite particles

Ni–YSZ cermet anodes for solid oxide fuel cells (SOFCs) were fabricated at various sintering temperatures from NiO–YSZ composite particles made by spray pyrolysis (SP) technique. NiO particles covered with fine YSZ (Y₂O₃ stabilized ZrO₂) particles were used as the composite particles, and the initial ratio of Ni and YSZ was set at 75:25 (mol%). As a result, the cermet anode sintered at 1350 °C showed the morphology in which fine YSZ grains were uniformly dispersed on the surface of Ni grain network. Electrical performance such as electrochemical activity and internal resistance of a Ni–YSZ cermet anode changed with sintering temperature. The anode fabricated at 1350 °C showed the highest electrical performance. Especially, a single cell voltage with the Ni–YSZ cermet anode kept very stable for 8000 h at 1000 °C in the SOFC operation condition of H₂—3% H₂O and air. The cermet anode after a long-term test had its initial morphology. It indicates that the Ni–YSZ cermet anode fabricated from NiO–YSZ composite particles is a very promising material for its practical use as SOFCs.     

New ionic diffusion strategy to fabricate proton-conducting solid oxide fuel cells based on a stable La2Ce2O7 electrolyte

A composite of NiO–BaZr_0.1Ce_0.7Y_0.2O_3−δ (NiO-BZCY) was successfully prepared by a simple one-step-combustion process and applied as an anode for solid oxide fuel cells based on stable La₂Ce₂O₇ (LCO) electrolyte. A high open circuit voltage of 1.00 V and a maximum power density of 315 mW cm⁻² were obtained with NiO-BZCY anode and LCO electrolyte when measured at 700 °C using humidified hydrogen fuel. SEM-EDX and Raman results suggested that a thin BaCeO₃-based reaction layer about 5 μm in thickness was formed at the anode/electrolyte interface for Ba cations partially migrated from anode into the electrolyte film. Impedance spectra analysis showed that the activation energy for LCO conductivity differed with the anode materials, about 52.51 kJ mol⁻¹ with NiO-BZCY anode and 95.08 kJ mol⁻¹ with NiO-LCO anode. The great difference in these activation energies might suggest that the formed BaCeO₃ reaction layer could promote the proton transferring numbers of LCO electrolyte.     

A novel way to improve performance of proton-conducting solid-oxide fuel cells through enhanced chemical interaction of anode components

We proposed a novel way to improve the cell performance of proton-conducting solid-oxide fuel cells by increasing the chemical interaction between the anode components using BaZr_0.4Ce_0.4Y_0.2O_3−δ (BZCY4) as the ionic conducting phase of anode for a fuel cell with a BaCe_0.8Y_0.2O_3−δ (BCY) electrolyte. The strength of the chemical interaction between NiO and the ionic conducting phase (BZCY4 or BCY) was analyzed by the hydrogen temperature-programmed reduction (H₂-TPR) technique. The effect of chemical interaction between NiO and the ionic conducting phase on the NiO diffusivity was investigated by SEM-EDX. The results demonstrated NiO had a much stronger interaction with BZCY4 than with BCY, thereby resulting in suppressed diffusivity of NiO into the BCY electrolyte. Using BZCY4 as the ionic conducting phase of the anode, a cell with an ohmic resistance of 0.65 Ω cm² at 700 °C was obtained. In contrast, a cell with BCY as the ionic conducting phase of the anode had an ohmic resistance of 0.82 Ω cm² at 700 °C. Therefore, the single cell with NiO + BZCY4 anode showed a peak power density higher than that of the cell with the NiO + BCY anode.     

High performance Ni–Fe alloy supported SOFCs fabricated by low cost tape casting-screen printing-cofiring process

Novel Ni–Fe alloy supported solid oxide fuel cells, with Ni cermet as functional anode, La_0.8Sr_0.2MnO₃ coated Ba_0.5Sr_0.5Co_0.2Fe_0.8O₃ as cathode and Gd-doped Ce₂O₃ as electrolyte, are successfully fabricated by the cost effective method of tape casting-screen printing-cofiring. The Ni–Fe porous substrate is obtained by reduction (in H₂ at 650 °C for 2 h) of sintered NiO-10 wt% Fe₂O₃ consisting of NiO and NiFe₂O₄. The cell is subjected to evaluation in the aspects of electrochemical performance and redox capability at temperatures between 500 and 650 °C. The result shows a peak power density of 1.04 W cm⁻² at 650 °C. Furthermore, the metal support cell exhibits excellent tolerance to redox cycles. Five redox recycles for cells are operated at 600 °C, which shows no significant degradation in open circuit voltage and power density.