SOFC Electrolyte Research Articles

Ionic conductivity and local structural features in Ce1-xSmxO2-x/2.

Sm-Doped ceria is one of the most promising materials to be used as electrolyte in solid oxide fuel cells due to its remarkable ionic conductivity values in the intermediate temperature range. Transport properties and local structural features of Ce1-xSmxO2-x/2 (0.1 ≤ x ≤ 0.7) were studied by an impedance/μ-Raman spectroscopy coupled approach up to 1073 K. Results suggest that C-based nanosized defect clusters are responsible for the drop in ionic conductivity observed even at x = 0.2, i.e. at a Sm content lower than necessary to allow C domains to reach the percolation threshold through crystallites. Moreover, within the fluorite-type compositional region, with increasing the Sm content, defect clusters undergo a rearrangement resulting in the enlargement of C-based domains rather than in the increase of their number; at higher x, on the contrary, both the size and amount of C domains increase in parallel.

Highly dense and novel proton conducting materials for SOFC electrolyte

Highly dense proton conducting materials of BCZYSZn (BaCe0.8−xZrxY0.15Sm0.05O3−δ (x = 0.15, 0.20) with 4 wt% ZnO as sintering additive), to be used as an intermediate temperature solid oxide fuel cells (IT-SOFCs) electrolyte, have been processed by the conventional solid state reaction method. The crystalline phase, microstructure, electrical properties, cell performance and chemical stability of the materials have been investigated. The ionic conductivity of BCZYSZn 3 (x = 0.20) material has been measured to be ∼2.56 × 10−3 S cm−1 and ∼8.32 × 10−3 S cm−1 at 600 °C and 850 °C, respectively in wet 5%H2 in Ar atmosphere. Microstructural characterizations of the zinc containing materials (BCZYSZn) show the formation of highly dense morphology with very large grains. The chemical stability test of BCZYSZn in pure CO2 shows that the material is very stable up to 1000 °C. The maximum power density for the BCZYSZn 3 electrolyte cell is found to be 0.42 W/cm2 at 700 °C under the testing atmosphere. The performed characterizations reveal that these are suitable proton-conducting candidate materials for efficient electrochemical devices.

High-temperature properties of (La,Ca)(Fe,Mg,Mo)O3-δ perovskites as prospective electrode materials for symmetrical SOFC

La1−yCayFe0.5+x(Mg,Mo)0.5−xO3-δ oxides with the orthorhombic GdFeO3-type perovskite structure have been synthesized at 1573K. Transmission electron microscopy study for selected samples shows the coexistence of domains of perovskite phases with ordered and disordered B-cations. Mössbauer spectroscopy studies performed at 300K and 573K show that while compositions with low Ca-content (La0.55Ca0.45Fe0.5Mg0.2625Mo0.2375O3-δ and La0.5Ca0.5Fe0.6Mg0.175Mo0.225O3-δ) are nearly oxygen stoichiometric, La0.2Ca0.8Fe0.5Mg0.2625Mo0.2375O3-δ is oxygen deficient with δ ≈ 0.15. Oxides are stable in reducing atmosphere (Ar/H2, 8%) at 1173K for 12h. No additional phases have been observed at XRPD patterns of all studied perovskites and Ce1−xGdxO2−x/2 electrolyte mixtures treated at 1173–1373K, while Fe-rich compositions (x≥0.1) react with Zr1−xYxO2−x/2 electrolyte above 1273K. Dilatometry studies reveal that all samples show rather low thermal expansion coefficients (TECs) in air of 11.4–12.7ppm K−1. In reducing atmosphere their TECs were found to increase up to 12.1–15.4ppm K−1 due to chemical expansion effect. High-temperature electrical conductivity measurements in air and Ar/H2 atmosphere show that the highest conductivity is observed for Fe- and Ca-rich compositions. Moderate values of electrical conductivity and TEC together with stability towards chemical interaction with typical SOFC electrolytes make novel Fe-containing perovskites promising electrode materials for symmetrical solid oxide fuel cell.

Thermal, structural and electrical properties of samarium doped barium cerate electrolyte for SOFCs

In the present investigation Samarium doped barium cerate electrolyte powder of (1-x)Ce0.8Sm0.2O2-δ -xBaCe0.8Sm0.2O3-δ (where x = 0.1 mol fraction) stoichiometry formula is successfully synthesized through wet chemical route. A SDC-BCS nanocomposite electrolyte is premeditated with respect to its thermal, structural, morphological properties and conductivity performance. The crystal structure and microstructure of the SDC-BCS composite is investigated from XRD and SEM analysis. TG analysis reveals that the calcination temperature (900 °C) of the as prepared powder sample. The crystal structure of as prepared SDC-BCS composite powder is identified as cubic perovskite with orthorhombic distortions occurred at 900 °C. From TEM analysis, the particle size is found to be around 32 nm. SEM analysis reveals that the existence of dual phase microstructure in the composite at 1400 °C. The preparation of dense SDC-BCS nanocomposite with the development of dual phase microstructure is achieved for the better ionic conductivity in SDC-BCS nanocomposites for SOFC application.

A New Double Doped Bismuth Oxide with Stable Ionic Conductivity at Intermediate Temperatures

Pure bismuth oxide exhibits the highest oxygen ion conductivity of any known material from approximately 730°C up to the melting point. While the highly conductive FCC cubic phase of Bi2O3 can be stabilized to room temperature by doping the structure, the conductivity ultimately suffers at intermediate temperatures due to the oxygen subblattice ordering. Traditional bismuth oxide based electrolytes are not suitable in solid oxide fuel cells that operate below approximately 600°C because of the ordering phenomenon. Reported herein is a Bi2O3 electrolyte material that does not order below 600°C. At temperatures that range from 400°C up to 550°C the electrolyte material does not exhibit noticeable degradation of conductivity, even after annealing for hundreds of hours. Although the oxygen ion conductivity of the material is not equivalent to FCC cubic Bi2O3, it maintains superior conductivity to other prominent SOFC electrolytes such as GDC, SNDC, and YSZ. Furthermore the highly conductive and stable bismuth oxide material also works in a composite cathode alongside LSM. This dual bismuth based electrolyte and cathode design allows for a stable performing SOFC that can operate at intermediate temperatures.

A comparative study of aqueous tape and pellet of (La0.89Ba0.01) Sr0.1Al0.9Mg0.1O3-δ electrolyte material

Lanthanum aluminate (LaAlO3) is one of the promising materials as an electrolyte for SOFCs, which works close to 700°C. We investigated on Ba doped Lanthanum Strontium Magnesium Aluminate i.e., (La0.89 Ba0.01) Sr0.1Al0.9Mg0.1O3-δ (LSBAM01) tape which was obtained by tape casting technique and reaction sintering. In order to determine the conductivity, thickness of the tape was maintained in the range of 100–300µm using aqueous solution method. The thickness of the tape was set up for ~100µm. In order to compare the electrical conductivity of tape with the pellet, LSBAM01 powder was prepared by auto-combustion technique. A comparative study of the tape casted specimen and pellet was done in order to find out the effect on green as well as sintered densities of tape and pellet and their corresponding conductivities. The impedance plots showed that the tape was more resistive in comparison to pellet. The conductivity of the tape was found to be reduced in comparison to pellet having the low activation energy than that of tape sample.

Novel chemical reaction co-precipitation method for the synthesis of apatite-type lanthanum silicate as an electrolyte in SOFC

Apatite-type lanthanum silicates have promising performance as electrolyte materials for IT-SOFC. A novel co-precipitation method, based on chemical reaction, is employed in the synthesis process. The reaction products: La(OH)3 and SiO2 are used as precursors of apatite synthesis process. The precursors are mixed in molecular scale which provide much conducive to formation reaction. The pure apatite phase composition is obtained with quite moderate temperature of 700 °C for 6 h which shows considerable ionic conductivity performance. The conductivity decreases when you increase the calcination temperature in according with XRD measurement. The XRD measurement shows the presence of impurity phase when the calcination temperature increases. Furthermore, the activation energies of the apatite synthesized in this study are rather high in comparison with the literature data, which can be attributed to a presence of the impurity phase. The ionic conductivity is controlled by the grain boundaries contributions for low calcination temperature and by the grains contributions for the sample calcined at 1000 °C.

Synthesis of GDC electrolyte material for IT-SOFCs using glucose & fructose and its characterization

Nano-powder of gadolinium-doped-ceria (GDC, Ce0.9Gd0.1O2) has been synthesized using a novel sol–gel method with glucose and fructose as organic additives. The main objective of the present study is to find the suitability of this synthesis method in synthesizing ceria-based SOFC electrolyte materials and evaluate its performance. The average crystallite/particle size obtained from XRD, TEM, BET surface area was found to be 4–12 nm. The phase was found to be cubic fluorite from XRD and further the structure and the nature of oxygen vacancies was confirmed using Raman spectroscopy. Dilatometer studies illustrated two shrinkage maxima (450 °C and 1450 °C). The ionic conductivity measurements were done using DC four-probe method on the GDC electrolyte sintered at 1500 °C. The sintered sample showed an ionic conductivity of 1.13E−02 Scm−1 at a temperature of 700 °C in the air, and the activation energy is 1.02 eV. The present study reveals that this synthesis method can be adaptable for synthesizing SOFC electrolyte materials.

A New Double Doped Bismuth Oxide with Stable Ionic Conductivity at Intermediate Temperatures

While the FCC cubic phase of doped Bi2O3 exhibits very high oxygen ion mobility at elevated temperatures, the conductivity ultimately suffers at intermediate temperatures due to the oxygen sublattice ordering. Traditional bismuth oxide based electrolytes are not suitable in solid oxide fuel cells that operate below approximately 600°C due to the ordering phenomenon. Reported herein is a Bi2O3 electrolyte material that does not order below 600°C. In the 550°C- 500°C temperature range the electrolyte material does not exhibit noticeable degradation of conductivity, even after annealing for one hundred hours. The oxygen ion conductivity of the material is comparable to notable FCC cubic Bi2O3, and has superior conductivity to other prominent SOFC electrolytes such as GDC, SNDC, and YSZ. This double doped bismuth based electrolyte would allow for a high performing and stable SOFC operating at intermediate temperatures.

Effect of Mg doping and sintering temperature on structural and morphological properties of samarium-doped ceria for IT-SOFC electrolyte

Samples of Sm and Mg co-doped ceria electrolyte of Ce1−xSmx−yMgyO2−δ (x = 0.2; y = 0.00, 0.05, 0.1, 0.15, and 0.175) were synthesized by sol–gel process. The prepared samples were sintered at 1100 and 1400 °C for 4 h. The bulk densities were measured by Archimedes method. XRD measurements indicate that the synthesized samples were in single-phase cubic fluorite structure (space group Fm3m). The cell parameters decrease with the concentration of Mg, and 2θ values slightly shift towards right. The particle sizes obtained were between 7.14 and 17.44 nm. The sintered sample achieved 95% of theoretical density. FTIR spectra of samples sintered at 1400 °C indicates weak interactions between 3550–3400 cm−1 and 1600–1300 cm−1 are attributed to O–H stretching modes and strong bonds 850–450 cm−1 are assigned to characteristic Ce–O vibrations. The surface morphology and chemical composition were analyzed by SEM and EDS, SEM micrographs show spherical faceted grains, and the samples were crack free, dense material with some pores on surface which are inconsistent with density results. The average grain size obtained was 0.5 μm. Particle size obtained by TEM was in agreement with that obtained by XRD. The high-density ceria co-doped ceramic can be used as electrolyte in SOFC.

Influence of deposition temperature on the microstructure of thin-film electrolyte for SOFCs with a nanoporous AAO support structure

In this paper we investigate the influence of deposition temperature on the microstructure of yttria stabilized zirconia thin-films based on an anodized aluminum oxide (AAO) support structure. The yttria-stabilized zirconia (YSZ) electrolytes were deposited on a Pt anode/AAO support using DC magnetron reactive sputtering under deposition temperatures of RT and 500 °C. Elevating the deposition temperature led to enhanced surface mobility in the sputtered adatoms, which helped prevent pinhole generation and minimized the thickness of the electrolyte. A thin-film fuel cell with a YSZ electrolyte only 300 nm thick was successfully fabricated by elevating the deposition temperature. This cell exhibited an open circuit voltage (OCV) of 0.97 V, which is significantly higher than the OCV values of 0.3 V for a cell deposited at RT. However, in spite of the thin electrolyte, the performance of the cell deposited at the higher temperature showed limited value due to its relatively high polarization resistance. Through further investigation into the grain morphology, we verify that the increasing deposition temperature can affect not only the film density but also increase the grain size of the electrolyte, which is related to oxygen incorporation for ORR kinetics. Electrochemical impedance spectroscopy (EIS) results indicate that the grain size change caused by the elevated deposition temperature adversely affected the polarization resistance and the cell performance. These results indicate that careful adoption of elevated electrolyte deposition temperatures are required to optimize fuel cell performance.

A novel sol–gel coating method for fabricating dense layers on porous surfaces particularly for metal-supported SOFC electrolyte

This paper presents a study on a solution coating process for depositing layers on porous surfaces, i.e., metal-supported SOFC electrolyte. PVP and nanoparticles are used to achieve the study objectives. PVP, which possesses a type of coiled long chain structure, can increase the solution viscosity and relieve stress. However, PVP can react with nitrate ions, consequently generating excessive ignition, which causes an inhomogeneous microstructure and forms many defects. The application of nanoparticles can control the stress and reduce cracks. Subsequently, to densify and repair the cracks, an additional solution coating process is applied. YSZ is selected as a candidate, and OCV and SEM measurements are conducted to confirm the YSZ density. The YSZ solution is multi-coated to the GDC layer, and a fully dense layer can be deposited on the coated GDC surface. The proposed coating process can fabricate a dense electrolyte under oxidation environments at a relatively low temperature using a wet-chemical process. Furthermore, in terms of spin coating and heat treatment, it can be continuously and automatically performed. Therefore, the multi-coating process developed in this research can be readily commercialized.

Electrical and microstructural characterization of Gd-doped Ceria/Y-doped Zirconia bi-layer electrolyte prepared by tape casting

GDC-YSZ bi-layer SOFC electrolytes are an interesting manner to protect GDC electrolytes of the low oxygen partial pressure present at anode-electrolyte interface. This oxygen partial pressure condition and high temperatures during the operation of SOFC permit GDC electrolyte reduction injuring the performance of SOFC. In this work, GDC-YSZ bi-layer electrolytes were prepared using double tape casting technique. Single layers of YSZ and GDC electrolytes were also prepared by the same technique for comparison. All samples were sintered at 1600 °C/2 h in air and analyzed by SEM and impedance spectroscopy technique. A good adhesion was established between the layer GDC and YSZ and no cracks were detected despite the differential firing shrinkage. The electrical conductivity of GDC-YSZ bi-layer sample at 350 °C was 7.98x10-5 S.cm-1 and for GDC and YSZ single layer samples electrical conductivities were 1.89x10-4 S.cm-1 and 4.71x10-5 S.cm-1, respectively. These results show that GDC-YSZ bi-layer electrolyte is superior to YSZ single layer electrolyte.

Effects of Partial Substitution of CeO2 with M2O3 (M = Yb, Gd, Sm) on Electrical Degradation of Sc2O3 and CeO2 Co-doped ZrO2

Scandia-stabilized zirconia co-doped with CeO₂ is a promising electrolyte for intermediate temperature SOFC, but still shows rapid degradation during a long-term operation. In this study, CeO₂ (1 ㏖%) as a stabilizer is partially substituted with lanthanum oxides (M₂O₃, M=Yb, Gd, Sm) to stabilize a cubic phase and thus durability in reducing atmosphere. 0.5M0.5Ce10ScSZ electrolytes were prepared by solid state reaction and sintered at 1450℃ for 10 h to produce dense ceramic specimens. With addition of the lanthanum oxide, 0.5M0.5Ce10ScSZ showed lower degradation rates than 1Ce10ScSZ. Since Gd₂O₃ showed the highest ionic conductivity among the co-dopants, an electrolyte-supported cell with 0.5Gd0.5Ce10ScSZ was prepared to compare its longterm performance with that of 1Ce10ScSZ-based cell. Maximum power density of 0.5Gd0.5Ce10ScSZ-based cell was degraded by about 2.3% after 250 h, which was much lower than 1Ce10ScSZ-based cell (4.2%).

Understanding the Conductivity Degradation Mechanism of Er2O3-Stabilized Bi2O3

Rare earth stabilized bismuth oxides have attracted much attention due to their high oxygen ionic conductivity. Among singly-doped stabilized bismuth oxides, the Er2O3-stabilized Bi2O3, which is generally abbreviated to ESB, is particularly known for its superior ionic conductivity to even other stabilized Bi2O3s, not to mention conventional oxygen ion conductors such as doped ceria and stabilized zirconia[1]. In spite of its remarkable ionic conductivity (≈ 0.3S/cm at 700℃), ESB has not been practically employed as a SOFC electrolyte. This is not only because ESB has thermodynamic instability under reduction conditions, but also because it experiences time-dependent conductivity degradation on extended annealing below 600℃. In this study, the crystallographic structure and conductivity of ESB were analyzed using X-ray diffraction and electrochemical impedance spectroscopy, respectively, during its prolonged annealing at 600℃ over 400h in order to investigate the mechanism of the time-dependent conductivity degradation of ESB. It was found that the initial cubic phase of ESB was mostly transformed to the rhombohedral phase within first 100h. At the same time, the discrepancy in time constant of the phase transformation(65h) and the conductivity degradation(52h) was observed. In order to elucidate this time constant discrepancy, the activation energies of low temperature domain before and after annealing were evaluated. The activation energy of as-sintered ESB was approximately 1.2eV, whereas that of the annealed ESB was approximately 1.01eV. Such a decrease in the activation energy can be attributed to order-disorder transition of the oxygen sub-lattice in ESB[2]. Therefore, the conductivity degradation of ESB is probably the combined effect of the phase transformation from cubic to rhombohedral as well as the order-disorder transition. Acknolwdge This research was supported by (1) Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2014R1A1A2057681), (2) the DGIST R&D Program of the Ministry of Science, ICT and Future Planning of Korea (1501- HRLA-01) and (3) the Global Frontier R&D on Center for Multiscale Energy System funded by the National Research Foundation under the Ministry of Science, ICT & Future Planning, Korea (2014M3A6A7074784).

Materials Diffusion Stability at Interface Between Stabilized ZrO2 Electrolyte and CeOx Interlayer in Model SOFC Cell

We observed the solid state reaction at the interface between ZrO2 electrolyte and CeOx interlayer in a model SOFC. At that interface, ZrOx and CeOx diffuse each other and the Zr-CeO solid solution lowers the cell performance. In this report, diffusion coefficient which enables to predict how much the reaction occurs is derived by using the ZrO2 / CeOx diffusion couples, simulating the long time operation. Stabilized ZrO2 powder and Gd-added CeO2 powder were laminated into two layers and pressed. To measure more practical interface, we chose the material powders used for cell production. Then the pressed powder was sintered at tempertures of 1350 to 1600 oC for 24 hours. The sintered blocks were cut into 3mm thick samples and polished. The ZrOx / CeOx interface were observed by scanning electron microscope (SEM) and analyzed about composition and their amount by Energy-dispersive X-ray spectroscopy (EDS).

Physicochemical properties of proton-conductive Ba(Zr0.1Ce0.7Y0.1Yb0.1)O3−δ solid electrolyte in terms of electrochemical performance of solid oxide fuel cells

Previously, most studies of proton-conductive electrolytes for SOFCs were conducted to achieve lower-temperature operation. In this study, we investigate a proton-conductive electrolyte to realize high-efficiency SOFCs. To this end, the dependencies of the total conductivity of Ba(Zr0.1Ce0.7Y0.1Yb0.1)O3−δ on the oxygen partial pressure and temperature under wet and dry conditions were measured. Based on the measurement data, we analyzed the ratio of ionic current density to electronic current density in the temperature range of 550–900 °C. Assuming that the area-specific resistance of the electrolyte and the external current density were 0.383 Ω cm2 and 0.25 A cm−2, respectively, the leakage current densities caused by the minority carriers were calculated to be 5.4% and 9.7% of the external current density at 550 °C and 600 °C, respectively. This study developed a method to evaluate proton-conductive electrolyte materials and established guidelines for the development of new materials for high-efficiency SOFCs.

Synthesis and preliminary study of pure and Zr-doped YMnO3 compounds as Solid Oxide Fuel Cells electrode

The Y1−xZrxMnO3 series (0 ≤ x ≤ 0.30) has been studied to be used as possible SOFC electrode material. These compounds adopt a layered structure of hexagonal symmetry in which the manganese cations are located in trigonal bi-pyramidal coordination of oxygen atoms, different from that of the classical perovskite. The synthesis of the materials has been carried out by solid state reaction and X-ray diffraction technique reveals that pure phases can be obtained until x∼0.10. For x ≥ 0.10, an additional YSZ phase is formed, similar to the SOFC electrolyte material. HT-XRD technique and thermogravimetric analysis of pure or Zr-doped YMnO3 indicate that, in diluted dry H2 for T > 600 °C, these compounds are unstable, what precludes their use as anode material. On the other hand, reactivity studies at high temperature (T = 1300 °C) between Y1−xZrxMnO3 (x = 0, 0.05 and 0.1) and 8YSZ show a chemical compatibility in which the formation of an electrically insulating phase does not take place, but a crossed diffusion of Y3+/Mn3+ and Zr4+ at the interface between both materials. Thermomechanical compatibility in air between YSZ and Y1−xZrxMnO3 is also demonstrated from RT to 850 °C.

Preparation and conductivity measurement of 7-8 mol % YSZ and 12 mol % CSZ for electrolyte SOFC

The study of 7YSZ (93% ZrO2-7%Y2O3), 8YSZ (92% ZrO2-8%Y2O3), and CSZ (88% ZrO2-12% CaO) as SOFC electrolytes have been carried out successfully. 7YSZ and 8YSZ powders were prepared by solid state reaction method of mixed Y2O3 and ZrO2 followed by calcination at 1350 °C for 1 hour, while CSZ was commercial products. Pellets of 7YSZ, 8YSZ, and CSZ were prepared by 1.2 gr, pressed at 40 MPa and sintered at 1550 °C for 4 hours. Rietveld refinement revealed that 7YSZ comprised 47.27% monoclinic, 52.65% cubic, and 0.008% Y2O3 cubic, while 8YSZ comprised 48.45% monoclinic, 49.32% cubic, 2.23% Y2O3 cubic and CSZ has 88% ZrO2 and 12% CaO. Ionic conductivity and activation energy were obtained from Cole- Cole Plot of impedance, the activation energy of 7YSZ=1.03eV, 8YSZ=0.96eV and CSZ=0.78eV.

Ionic transport mechanism of La0.9Sr0.1Ga0.8Mg0.2O2.85-(Li/Na)2CO3 composite electrolyte for low temperature SOFCs

La0.9Sr0.1Ga0.8Mg0.2O2.85 (LSGM)-(52 mol% Li2CO3: 48 mol% Na2CO3) composite was investigated as a novel electrolyte for low temperature solid oxide fuel cells (LT-SOFCs) operated at 400–600 °C. In this study, the stability of conductivity of LSGM-30 wt% (Li/Na)2CO3 composite electrolyte was been investigated by AC impedance measurements. The composite sample showed a relatively stable conductivity (0.098–0.12 S cm−1) over 120 h measurement in air at 600 °C. As the experiment time elapsed, the conductivity had a slight decline (0.085 S cm−1). Further, it was also the first time to discuss the conduction behaviours of LSGM-30 wt% (Li/Na)2CO3 composite electrolyte in various dry and wet atmospheres (nitrogen, hydrogen, oxygen, carbon dioxide and air) by electrochemical impedance spectroscopy. It was found that wet atmosphere could lower transition temperature at low temperature environment (450 °C), carbon dioxide was not inert gas for composite conduction but could improve the ionic conductivity. The value of O2− conductivity in oxygen and H+ conductivity in hydrogen was obtained by DC polarization method. Based upon the performances, the possible conduction mechanism of the composite electrolytes was discussed. The results indicated that the mixed O2−/H+/CO32− conduction happened in LSGM-(Li/Na)2CO3 composite electrolyte with the increasing dominance of CO32− conduction at high temperatures (600 °C).