Microstructural factors of electrodes affecting the performance of anode-supported thin film yttria-stabilized zirconia electrolyte (∼1 μm) solid oxide fuel cells

The effects of the microstructural factors of electrodes, such as the porosity and pore size of anode supports and the thickness of cathodes, on the performance of an anode-supported thin film solid oxide fuel cell (TF-SOFC) are investigated. The performance of the TF-SOFC with a 1 μm-thick yttria-stabilized zirconia (YSZ) electrolyte is significantly improved by employing anode supports with increased porosity and pore size. The maximum power density of the TF-SOFCs increases from 370 mW cm⁻² to 624 mW cm⁻² and then to over 900 mW cm⁻² at 600 °C with increasing gas transport at the anode support. Thicker cathodes also improve cell performance by increasing the active reaction sites. The maximum power density of the cell increases from 624 mW cm⁻² to over 830 mW cm⁻² at 600 °C by changing the thickness of the lanthanum strontium cobaltite (LSC) cathode from 1 to 2-3 μm.     

Bimetallic (Ni–Fe) anode-supported solid oxide fuel cells with gadolinia-doped ceria electrolyte

Anode-supported solid oxide fuel cells (SOFC) comprising nickel + iron anode support and gadolinia-doped ceria (GDC) of composition Gd_0.1Ce_0.9O_2−δ thin film electrolyte were fabricated, and their performance was evaluated. The ratio of Fe₂O₃ to NiO in the anode support was 3 to 7 on a molar basis. Fe₂O₃ and NiO powders were mixed in the desired proportions and discs were die-pressed. All other layers were sequentially applied on the anode support. The cell structure consisted of five distinct layers: anode support – Ni + Fe; anode functional layer – Ni + GDC; electrolyte – GDC; cathode functional layer – LSC (La_0.6Sr_0.4CoO_3−δ) + GDC; and cathode current collector – LSC. Cells with three different variations of the electrolyte were made: (1) thin GDC electrolyte (∼15 μm); (2) thick GDC electrolyte (∼25 μm); and (3) tri-layer GDC/thin yttria-stabilized zirconia (YSZ)/GDC electrolyte (∼25 μm). Cells were tested with hydrogen as fuel and air as oxidant up to 650 °C. The maximum open circuit voltage measured at 650 °C was ∼0.83 V and maximum power density measured was ∼0.68 W cm⁻². The present work shows that cells with Fe + Ni containing anode support can be successfully made.     

Ceria-based electrolyte reinforced by sol–gel technique for intermediate-temperature solid oxide fuel cells

High performance solid oxide fuel cells (SOFCs) based on gadolinia-doped ceria (GDC) electrolyte are demonstrated for intermediate temperature operation. The inherent technical limitations of the GDC electrolyte in sinterability and mechanical properties are overcome by applying sol–gel coating technique to the screen-printed film. When the quality of the electrolyte film is enhanced by the additional sol–gel coating, the OCV and maximum power density increase from 0.73 to 0.90 V and from 0.55 to 0.95 W cm⁻², respectively, at 650 °C with humidified hydrogen (3% H₂O) as fuel and air as oxidant. The impedance analysis reveals that the reinforcement of the thin electrolyte with sol–gel coating significantly reduces the polarization resistance. Elementary reaction steps for the anode and cathode are analyzed based on the systematic impedance study, and the relation between the structural integrity of the electrolyte and the electrode polarization is discussed in detail.     

YSZ electrolyte coating on NiO–YSZ composite by electrophoretic deposition for solid oxide fuel cells (SOFCs)

A thick dense film of YSZ has been fabricated on a porous NiO–YSZ substrate from the YSZ powders in the mixtures of absolute acetyl acetone–ethanol suspensions by electrophoretic deposition (EPD) method. Parameters affected on substrate porosity like pre-sintering temperature and percentage of starch and parameters affected on EPD process like applied voltage and time of deposition have been investigated. Linear dependence between weights of deposition, deposition time and applied voltage were observed. A crack-free dense thick film of YSZ was obtained on porous NiO–YSZ substrate. Adhesion between the two layers was observed by SEM. The ability of ionic transfer and permeability of the YSZ electrolyte were investigated by EIS, as well.     

Preparation and characterization of Ce0.8M0.2O2−δ (M = Y,Gd, Sm,Nd, La) solid electrolyte materials for solid oxide fuel cells

Characteristics, such as lattice parameter, theoretical densities, thermal expansion, mechanical properties, microstructure, and ionic conductivities, of Ce_0.8M_0.2O_2−δ (M = Y, Gd, Sm, Nd, La) ceramics prepared by coprecipitation were systematically investigated in this paper. The results revealed that the lattice parameter and density based on the oxygen vacancy radius generally agreed with experimental results. Ce_0.8Sm_0.2O_2−δ ceramic sintered at 1500 °C for 5 h possessed the maximum ionic conductivity, σ_800 °C = 6.54 × 10⁻² S cm⁻¹, with minimum activation energy, E_a = 0.7443 eV, among Ce_0.8M_0.2O_2−δ (M = Y, Gd, Sm, Nd, La) ceramics. The thermal expansion coefficients of Ce_0.8M_0.2O_2−δ (M = Y, Gd, Sm, Nd, La) were in the range of 15.176–15.571 ppm/°C, which indicates that the rare-earth oxide dopants have insignificant influence on the thermal expansion property. Trivalent, rare-earth oxide doped ceria ceramics revealed high fracture toughness, with the fracture toughness in the range of 6.393–7.003 MPa m^1/2. According to SEM observation, the cracks are limited to one grain diameter; therefore, the high fracture toughness of rare-earth oxide doped ceria may be due to the toughness mechanism of crack deflection at the grain boundary. Based on the results of grain size and mechanical properties, one may conclude that there is no significant dependence of fracture toughness and microhardness for Ce_0.8M_0.2O_2−δ ceramics on grain size. Correlation between the grain size of Ce_0.8M_0.2O_2−δ ceramics and the dopant species can be explained on the basis of the concept of the rate of grain growth being proportional to the boundary mobility M_b. This leads to a conclusion that the diffusion coefficient of La in Ce_0.8La_0.2O_2−δ>Nd in Ce_0.8Nd_0.2O_2−δ>Sm in Ce_0.8Sm_0.2O_2−δ>Gd inCe_0.8Gd_0.2O_2−δ>Y in Ce_0.8Y_0.2O_2−δ.     

Novel doped barium cerate–carbonate composite electrolyte material for low temperature solid oxide fuel cells

A composite of a perovskite oxide proton conductor (BaCe_0.7Zr_0.1Y_0.2O_3−δ, BCZ10Y20) and alkali carbonates (2Li₂CO₃:1Na₂CO₃, LNC) is investigated with respect to its morphology, conductivity and fuel cell performance. The morphology shows that the presence of carbonate phase improves the densification of oxide matrix. The conductivity is measured by AC impedance in air, nitrogen, wet nitrogen, hydrogen, and wet hydrogen, respectively. A sharp increase of the conductivity at certain temperature is seen, which relates to the superionic phase transition at the interface phases between oxide and carbonates. Single cell with the composite electrolyte is fabricated by dry-pressing technique, using nickel oxide as anode and lithiated nickel oxide as cathode, respectively. The cell shows a maximum power density of 957 mW cm⁻² at 600 °C with hydrogen as the fuel and oxygen as the oxidant. The remarkable proton conductivity and excellent cell performance make this kind of composite material a good candidate electrolyte for low temperature solid oxide fuel cells (SOFCs).     

A general electrolyte–electrode-assembly model for the performance characteristics of planar anode-supported solid oxide fuel cells

A general electrode–electrolyte-assembly (EEA) model has been developed, which is valid for different designs of solid oxide fuel cells (SOFCs) operating at different temperatures. In this study, it is applied to analyze the performance characteristics of planar anode-supported SOFCs. One of the novel features of the present model is its treatment of electrodes. An electrode in the present model is composed of two distinct layers referred to as the backing layer and the reaction zone layer. The other important feature of the present model is its flexibility in fuel, having taking into account the reforming and water–gas shift reactions in the anode. The coupled governing equations of species, charge and energy along with the constitutive equations in different layers of the cell are solved using finite volume method. The model can predict all forms of overpotentials and the predicted concentration overpotential is validated with measured data available in literature. It is found that in an anode-supported SOFC, the cathode overpotential is still the largest cell potential loss mechanism, followed by the anode overpotential at low current densities; however, the anode overpotential becomes dominant at high current densities. The cathode and electrolyte overpotentials are not negligible even though their thicknesses are negligible relative to the anode thickness. Even at low fuel utilizations, the anode concentration overpotential becomes significant when chemical reactions (reforming and water–gas shift) in the anode are not considered. A parametric study has also been carried out to examine the effect of various key operating and design parameters on the performance of an anode-supported planar SOFCs.     

Ni–YSZ-supported tubular solid oxide fuel cells with GDC interlayer between YSZ electrolyte and LSCF cathode

Highly sinterable gadolinia doped ceria (GDC) powders are prepared by carbonate coprecipitation and applied to the GDC interlayer in Ni–YSZ (yttria stabilized zirconia)-supported tubular solid oxide fuel cell in order to prevent the reaction between YSZ electrolyte and LSCF (La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ) cathode materials. The formation of highly resistive phase at the YSZ/LSCF interface was effectively blocked by the low-temperature densification of GDC interlayer using carbonate-derived active GDC powders and the suppression of Sr diffusion toward YSZ electrolyte via GDC interlayer by tuning the heat-treatment temperature for cathode fabrication. The power density of the cell with the configuration of Ni–YSZ/YSZ/GDC/LSCF–GDC/LSCF was as high as 906 mW cm⁻², which was 2.0 times higher than that (455 mW cm⁻²) of the cell with the configuration of Ni–YSZ/YSZ/LSM(La_0.8Sr_0.2MnO_3−δ)–YSZ/LSM at 750 °C.     

Improvement of cathode–electrolyte interfaces of tubular solid oxide fuel cells by fabricating dense YSZ electrolyte membranes with indented surfaces

To improve cathode–electrolyte interfaces of solid oxide fuel cells (SOFCs), dense YSZ electrolyte membranes with indented surfaces were fabricated on tubular NiO/YSZ anode supports by two comparable methods. Electrochemistry impedance spectroscopy (EIS) and current–voltage tests of the cells were carried out to characterize the cathode–electrolyte interfaces. Results showed that the electrode polarization resistances of the modified cells were reduced by 52% and 35% at 700 °C, and the maximum power densities of cells were remarkably increased, even by 146.6% and 117.8% at lower temperature (700 °C), respectively. The indented surfaces extended the active zone of cathode and enhanced interfacial adhesion, which led to the major improvement in the cell performance.     

Electrical conductivity of Ce0.8Gd0.2−xDyxO2−δ (0 ≤ x ≤ 0.2) co-doped with Gd and Dy for intermediate-temperature solid oxide fuel cells

High-quality nano-sized Ce_0.8Gd_0.2−xDy_xO_2−δ (0 ≤ x ≤ 0.2) powders are synthesized by a solution combustion process. The calcined powders are composed of a ceria-based single phase with a cubic fluorite structure and are nanocrystalline nature, i.e., 15-24 nm in crystallite size. The addition of an intermediate amount of Dy³⁺ (0.03 ≤ x ≤ 0.16) for Gd³⁺ in Ce_0.8Gd_0.2O_2−δ decreases the electrical conductivity. On the other hand, the doping of a small amount of Dy³⁺ (0.01 ≤ x ≤ 0.02) and of a large amount of Dy³⁺ (0.17 ≤ x ≤ 0.19) leads to an increase in conductivity. The Ce_0.8Gd_0.03Dy_0.17O_2−δ shows the highest electrical conductivity (0.215 S cm⁻¹) at 800 °C.     

Development of trimetallic Ni–Cu–Zn anode for low temperature solid oxide fuel cells with composite electrolyte

Trimetallic alloys of Ni_0.6Cu_0.4−xZn_x (x = 0, 0.1, 0.2, 0.3, 0.4) have been investigated as promising anode materials for low temperature solid oxide fuel cells (SOFCs) with composite electrolyte. The alloys have been obtained by reduction of Ni_0.6Cu_0.4−xZn_xO oxides, which are synthesized by using the glycine–nitrate process. Increasing the Zn content x decreases the particle sizes of the oxides at a given sintering temperature. Fuel cells have been constructed using lithiated NiO as cathode and as-prepared alloys as anodes based on the composite electrolyte. Peak power densities are observed to increase with the increasing Zn addition concentration into the anode. The maximum power density of 624 mW cm⁻² at 600 °C, 375 mW cm⁻² at 500 °C has been achieved for the fuel cell equipped with Ni_0.6Zn_0.4 anode. A.c. impedance results show that the resistances dramatically decrease with increasing temperatures under open circuit voltage state. Both cathodic and anodic interfacial polarization resistances increase with the amplitude of applied DC voltage. Possible reaction process for H₂ oxidation reaction at anode based on composite electrolyte has been proposed for the first time. The stability of the fuel cell with Ni_0.6Cu_0.2Zn_0.2 composite anode has been investigated. The results indicate that the trimetallic Ni_0.6Cu_0.4−xZn_x anodes are considerable for low temperature SOFCs.     

Low temperature electrochemical cells with sodium β″-alumina solid electrolyte (BASE)

Cells of Daniell-type with copper–zinc electrochemical couples and sodium β″-alumina solid electrolyte (BASE) were constructed. The cathode consisted of copper in contact with its ions (Cu/Cu²⁺) while zinc in contact with its ions (Zn/Zn²⁺) constituted the anode. Dimethyl sulfoxide (DMSO) containing 1 M NaBF₄ was used as the liquid electrolyte. The configuration of the cell constructed can be written as follows:

Zn(s)/ZnCl₂(DMSO)(0.1 M), NaBF₄(1 M)/BASE/NaBF₄(1 M), CuCl₂(DMSO)(0.1 M)/Cu(s)

A new nickel–ceria composite for direct-methane solid oxide fuel cells

Various Ni–La_xCe_1−xO_y composites were synthesized and their catalytic activity, catalytic stability and carbon deposition properties for steam reforming of methane were investigated. Among the catalysts, Ni–La_0.1Ce_0.9O_y showed the highest catalytic performance and also the best coking resistance. The Ni–La_xCe_1−xO_y catalysts with a higher Ni content were further sintered at 1400 °C and investigated as anodes of solid oxide fuel cells for operating on methane fuel. The Ni–La_0.1Ce_0.9O_y anode presented the best catalytic activity and coking resistance in the various Ni–La_xCe_1−xO_y catalysts with different ceria contents. In addition, the Ni–La_0.1Ce_0.9O_y also showed improved coking resistance over a Ni–SDC cermet anode due to its improved surface acidity. A fuel cell with a Ni–La_0.1Ce_0.9O_y anode and a catalyst yielded a peak power density of 850 mW cm⁻² at 650 °C while operating on a CH₄–H₂O gas mixture, which was only slightly lower than that obtained while operating on hydrogen fuel. No obvious carbon deposition or nickel aggregation was observed on the Ni–La_0.1Ce_0.9O_y anode after the operation on methane. Such remarkable performances suggest that nickel and La-doped CeO₂ composites are attractive anodes for direct hydrocarbon SOFCs and might also be used as catalysts for the steam reforming of hydrocarbons.     

Semiconductor Fe-doped SrTiO3-δ perovskite electrolyte for low-temperature solid oxide fuel cell (LT-SOFC) operating below 520 °C

High-temperature operation of solid oxide fuel cells causes several degradation and material issues. Lowering the operating temperature results in reduced fuel cell performance primarily due to the limited ionic conductivity of the electrolyte. Here we introduce the Fe-doped SrTiO_3-δ (SFT) pure perovskite material as an electrolyte, which shows good ionic conduction even at lower temperatures, but has low electronic conduction avoiding short-circuiting. Fuel cell fabricated using this electrolyte exhibits a maximum power density of 540 mW/cm² at 520 °C with Ni-NCAL electrodes. It was found that the Fe-doping into the SrTiO_3-δ facilitates the creation of oxygen vacancies enhancing ionic conductivity and transport of oxygen ions. Such high performance can be attributed to band-bending at the interface of electrolyte/electrode, which suppresses electron flow, but enhances ionic flow.     

Double-perovskites A2FeMoO6−δ (A = Ca, Sr, Ba) as anodes for solid oxide fuel cells

Double-perovskites A₂FeMoO_6−δ (A = Ca, Sr, Ba) have been investigated as potential anode materials for solid oxide fuel cells (SOFCs). At room temperature, A₂FeMoO_6−δ compounds crystallize in monoclinic, tetragonal, and cubic structures for A = Ca, Sr, and Ba, respectively. A weak peak observed at around 880 cm⁻¹ in the Raman spectra can be attributed to traces of AMoO₄. XPS has confirmed the coexistence of Fe²⁺-Mo⁶⁺ and Fe³⁺-Mo⁵⁺ electronic configurations. Moreover, a systematic shift from Fe^2+/3+-Mo^6+/5+ to Fe²⁺-Mo⁶⁺ configuration is seen with increasing A-site cation size. A₂FeMoO_6−δ samples display distinct electrical properties in H₂, which can be attributed to different degrees of degeneracy of the Fe²⁺-Mo⁶⁺ and Fe³⁺-Mo⁵⁺ configurations. Ca₂FeMoO_6−δ is unstable in a nitrogen atmosphere, while Sr₂FeMoO_6−δ and Ba₂FeMoO_6−δ are stable up to 1200 °C. The thermal expansion coefficients of Sr₂FeMoO_6−δ and Ba₂FeMoO_6−δ are very close to that of La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ (LSGM). The performances of cells with 300 μm thick LSGM electrolyte, double-perovskite SmBaCo₂O_5+x cathodes, and A₂FeMoO_6−δ anodes follow the sequence Ca₂FeMoO_6−δ < Ba₂FeMoO_6−δ < Sr₂FeMoO_6−δ. The maximum power densities of a cell with an Sr₂FeMoO_6−δ anode reach 831 mW cm⁻² in dry H₂ and 735 mW cm⁻² in commercial city gas at 850 °C, respectively.     

High performance Ca-containing La2-xCaxNiO4+δ(0≤x≤0.75) cathode for proton-conducting solid oxide fuel cells

In order to optimize the performance of La₂NiO_4+δ materials as H–SOFC cathodes, different Ca doping amounts (x = 0, 0.25, 0.5, 0.75) is adopted to find the best solution. Doping with Ca in La₂NiO_4+δ can effectively improve the electrical conductivity and the ORR activity of the cathode surface, which have good chemical stability and compatible TEC with the electrolyte. Electrochemical studies reveal that La_1.5Ca_0.5NiO_4+δ cathode shows the best electrochemical performance in single cell tests outputing the maximum power density (MPD) of 923 mW cm⁻² and the polarization resistance (R _p) of 0.053 Ω cm² at 700 °C. This work indicates that the single-phase R–P layered material La_1.5Ca_0.5NiO_4+δ could be a promising cathode for H–SOFC.     

Development of composite LaNi0·6Fe0.4O3-δ-based air electrodes for solid oxide fuel cells with a thin-film bilayer electrolyte

In this study the bilayer composite electrodes based on LaNi₀.₆Fe₀.₄O_3-δ (LNF) electronic conductor and Bi₂O₃-based electrolytes doped with Er (Bi₁.₆Er₀.₄O₃, EDB) and Y (Bi₁.₅Y₀.₅O₃, YDB) have been developed and their performance has been investigated in the dependence on the electrolyte content and sintering conditions. The polarization resistance of the optimized electrodes with electrolyte content of 50 wt % in the functional layer and with the LNF-EDB-CuO collector is in a range of 0.65–1.09 Ω cm² at 600 °C and 0.10–0.12 Ω cm² at 700 °C. The polarization characteristics of the Bi-based electrodes are compared with those for the composite electrodes based on LNF and Ce_0.8Sm_0.2O_1.9 (SDC). The developed electrodes have been tested in a SOFC mode in the anode-supported cells with a thin film electrolyte of YSZ/YDC (Y-doped zirconia/ceria). The single cells with such cathodes are shown to have performance characteristics that are several times higher than that for the cell with a standard platinum cathode. This is due to the optimized content and dispersity of the components; high conductivity of ionic and electronic constituents of the composite electrodes; greatly extended triple phase boundary (TPB) of the electrochemical reaction and advanced electrode design with collector providing uniform current distribution.     

Desirable performance of intermediate-temperature solid oxide fuel cell with an anode-supported La0.9Sr0.1Ga0.8Mg0.2O3 − δ electrolyte membrane

An anode-supported La_0.9Sr_0.1Ga_0.8Mg_0.2O_3 − δ (LSGM) electrolyte membrane is successfully fabricated by simple, cost-effective spin coating process. Nano-sized NiO and Ce_0.8Gd_0.2O_3 − α (GDC) powders derived from precipitation and citric-nitrate process, respectively, are used for anode support. The dense and uniform LSGM membrane of ca. 50 μm in thickness is obtained by sintering at relatively low temperature 1300 °C for 5 h. A single cell based on the as-prepared LSGM electrolyte membrane exhibits desirable high cell performance and generates high output power densities of 760 mW cm⁻² at 700 °C and 257 mW cm⁻² at 600 °C, respectively, when operated with humidified hydrogen as the fuel and air as the oxidant. The single cell is characterized by field-emission scanning electron microscope (FESEM), X-ray diffraction (XRD) and electrochemical AC impedance. The results demonstrate that it is feasible to fabricate dense LSGM membrane for solid oxide fuel cell by this simple, cost-effective and efficient process. In addition, optimized anode microstructure significantly reduces polarization resistance (0.025 Ω cm² at 700 °C).     

Cobalt-free LaNi0.4Zn0.1Fe0.5O3-δ as a cathode for solid oxide fuel cells using proton-conducting electrolyte

A Zn-doping strategy is employed to tailor the LaNi_0.5Fe_0.5O_3-δ material to improve its performance for proton-conducting solid oxide fuel cells (H–SOFCs). Zn can partially replace Ni in the LaNi_0.5Fe_0.5O_3-δ lattice to form LaNi_0.4Zn_0.1Fe_0.5O_3-δ material. In contrast, ZnO secondary phase can be detected if attempts are made to partially replace Fe with Zn, and the nominal composition LaNi_0.5Fe_0.4Zn_0.1O_3-δ cannot be obtained. First-principles calculations indicate that the Zn-doping method lowers the formation energy of oxygen vacancy and decreases the hydration energy, benefiting its application as the cathode for H–SOFCs. As a result, the H–SOFC with the LaNi_0.4Zn_0.1Fe_0.5O_3-δ cathode generates a peak power density of 1226 mW cm⁻² at 700 °C. In contrast, the peak power density for the cell using the Zn free LaNi_0.5Fe_0.5O_3-δ cathode only reaches 722 mW cm⁻² at the same testing temperature. The polarization resistance of the cell with the LaNi_0.4Zn_0.1Fe_0.5O_3-δ cathode is reduced to 0.043 Ω cm² at 700 °C, which is one of the smallest reported for H–SOFCs using cobalt-free cathodes. The high fuel cell performance coupled with the low polarization resistance for the Zn-modified LaNi_0.5Fe_0.5O_3-δ suggests that the Zn-doping strategy would be an interesting way to promote the performance of the cobalt-free LaNi_0.5Fe_0.5O_3-δ material for H–SOFCs.