SOFC Electrolyte Research Articles

Single Cubic Phase Scandia-Stabilized Zirconia: Stabilization and Aging at 600oC

ZrO2:10 mol% Sc2O3 was mixed to MnO2 to evaluate its effect on cubic phase stabilization, oxide ion conductivity and aging at 600oC. Pressed ceramic pellets of zirconia stabilized with 10 mol% scandia with addition of 1.0, 2.0 and 2.5 mol% MnO2 were sintered at 1500 °C/2 h. In situ high temperature X-ray diffraction data were collected to follow the rhombohedral-to-cubic transition. Polished and thermally etched surfaces of the sintered pellets were observed in scanning electron microscope. Impedance spectroscopy measurements were carried out. The single cubic structure is achieved for the specimen with 2 mol% MnO2 nominal addition. The electrical conductivity of this specimen was monitored at 600 oC for 100 days. The ionic conductivity value remained constant, suggesting its use as solid electrolyte in SOFCs operating at that temperature.

Ln2NiO4+δ (Ln = La, Pr) As Suitable Cathodes for Metal Supported Cells

The chemical stability of lanthanide nickelates Ln2NiO4+δ (Ln = La or Pr) is studied in the temperature range 25 - 1400 °C, both in air and at low pO2 (down to 10-4 atm). Thermal gravimetry analysis (TGA) measurements coupled with thermal in situ X-ray diffraction (XRD) characterization show that all compounds retain their K2NiF4-type structure in these conditions, while remaining oxygen over-stoichiometric up to 1000 °C. For both nickelates, a structural transition occurs around 400 °C at pO2 = 10-4 atm in association with a variation in Dδ in the range 0.06 to 0.02 for Pr2NiO4+δ and La2NiO4+δ, respectively. Interestingly, their cell volume did not vary upon the variation of the oxygen partial pressure. Therefore, these materials do not exhibit a chemical expansion in these conditions, which is beneficial for a fuel cell application as cathode layers. EIS measurements carried out on Ln2NiO4+δ/Ce0.8Gd0.2O2-δ/8wt.%Y2O3-ZrO2 half cells revealed that the electrode are at least as efficient for the oxygen reduction reaction when sintered at pO2 = 10-4 atm, than when sintered in air. Finally, excellent matching was found between the thermal expansion coefficients of the lanthanide nickelates and SOFC electrolytes such as 8wt.% yttria stabilized zirconia (8YSZ) or Ce0.8Gd0.2O2-δ (GDC), up to 1200 °C either in air or at pO2 = 10-4 atm, which is particularly important to avoid delamination of the half cell layers during solid oxide fuel cell long-term experiments.

Effect of Bismuth Oxide as Sintering Aid for Gadolinia-doped Ceria at 1050°C

The addition of small amount of metal oxides can increase the sinterability of Gadolinia-doped Ceria, thus reducing the temperature necessary for its densification. Here, the effect of bismuth oxide as sintering aid is studied by means of dilatometry, porosity and density measurement on samples obtained by uniaxial pressing, with the aim of achieving a microstructure with no interconnected porosity, suitable for realizing a gas tight electrolyte for SOFC. The effect of Bi2O3 load and holding time at high temperature was studied. The most promising doping concentration (6 mol%) is identified and the densification was accomplished more than 400°C below the sintering temperature of GDC10, this allowing the cosintering with copper-based cermet anode.

Effect of Ceria Content of CeScSZ Powder on the Phase Stability and Electrical Conductivity of SOFC Electrolyte

The CeScSZ powders with ceria-doped content were synthesized by the co-precipitation process. The crystal lattice parameter and XRD peak pattern of powders were investigated with the increase of calcination temperature and the change of ceria-doped content at a certain calcination temperature. Moreover, ionic conductivities of the CeScSZ powders were also studied at the range of 500 to 900 °C. Electrolyte film was fabricated using the synthesized 1Ce10ScSZ powder and single cell of SOFC with anode-supported electrolyte was fabricated by a tape casting and co-sintering process. As a result, the SOFC cell showed power density of 1.2 W/cm2 at 2 A/cm2, 800 °C with low ohmic resistance of under 0.1 Ω·cm2.

Characterization of Terbium and Samarium Co-Doped Ceria Films as SOFC Electrolyte Prepared by Using Ultrasonic Spray Pyrolysis Method

Tb and Sm cations binary co-doped ceria films were deposited using the ultrasonic atomizing spray pyrolysis method. The crack-free homogenous films with different dopant concentration were deposited and thereafter annealed at fixed temperatures 900, 1200 and 1300 °C, respectively. The Ce0.9Sm0.1-xTbxO2-δ films formed were analyzed using X-ray diffraction, scanning electron microscopy, high resolution transmission electron microscopy, atomic force microscopy and four probe dc technique. It was demonstrated that several microstructural parameters of oxide films are controlled by the sintering temperature and Tb dopant ions. Electrical properties like activation energies of ionic and electronic conductivity of the Ce0.9Sm0.1-xTbxO2-δ films were also influenced by the film composition.

Effect Of The Gadolinium Oxide Addition On The Electrical Properties Of Tetragonal Zirconium Dioxide

AbstractThe aim of this work was examination of gadolinium-doped 3YSZ electrical and structural properties. Such materials may be used as electrolytes in intermediate temperature solid oxide fuel cells (IT-SOFC). First step of the research was synthesis of 3YSZ with different contents of gadolinium (0.25; 0.5; 1.0 at %). Prepared materials were characterized by high porosity. No effect of gadolinium addition on grain size was observed. The experimentally determined values of grain interiors electrical conductivities for gadolinium doped samples are comparable to grain interior conductivities of gadolinium free tetragonal zirconia. On the other hand, a clear effect of gadolinium addition on electrical conductivity of grain boundaries was observed. It was found that 3YSZ containing 0.25 at. % Gd was the most promising from the investigated materials as a solid electrolyte for IT SOFC.

Chemical and structural changes in Ln2NiO4+δ (Ln=La, Pr or Nd) lanthanide nickelates as a function of oxygen partial pressure at high temperature

The chemical stability of lanthanide nickelates Ln2NiO4+δ (Ln=La, Pr or Nd) has been studied in the temperature range 25–1300°C, either in air or at low pO2 (down to 10−4atm). Thermal gravimetry analysis (TGA) measurements coupled with X-ray diffraction (XRD) characterization have shown that all compounds retain their K2NiF4-type structure in these conditions, while remaining over-stoichiometric in oxygen up to 1000°C. Only Nd2NiO4+δ starts to decompose into Nd2O3 and NiO above 1000°C, at pO2=10−4atm. In addition, a careful analysis of the lanthanide nickelates structural features has been performed by in situ XRD, as a function of temperature and pO2. For all compounds, a structural transition has been always observed in the temperature range 200–400°C, in air or at pO2=10−4atm. In addition, their cell volume did not vary upon the variation of the oxygen partial pressure. Therefore, these materials do not exhibit a chemical expansion in these conditions, which is beneficial for a fuel cell application as cathode layers. Additional dilatometry measurements have revealed that a temperature as high as 950°C for Pr2NiO4+δ or 1100°C for La2NiO4+δ and Nd2NiO4+δ has to be reached in order to begin the sintering of the material particles, which is of primary importance to obtain an efficient electronic/ionic conduction in the corresponding designed cathode layers. Besides, excellent matching was found between the thermal expansion coefficients of lanthanide nickelates and SOFC electrolytes such as 8wt% yttria stabilized zirconia (8YSZ) or Ce0.8Gd0.2O2−δ (GDC), at least from 400°C up to 1400°C in air or up to 1200°C at pO2=10−4atm.

Need for In Operando Characterization of Electrochemical Interface Features

It has proven particularly difficult to determine the electrode reaction mechanisms in high temperature solid oxide cells (SOCs) that convert gases. The literature is full of contradictory statements and apparently contradictory findings. Often the same type of electrochemical kinetics that apply to low temperature aqueous systems are assumed valid for SOCs, but in our opinion this has not been fruitful as they do not describe the experimental findings properly. Classical room temperature wet electrochemistry has experienced a huge progress in understanding of the electrode reaction mechanisms during the recent 2 decades. This progress has to a large extent been based on combination of electrochemical characterization and in situ and in operando and in situ surface analysis techniques, which so far have been less developed for high temperature electrochemistry above 300 °C. In spite that such techniques have only recently started becoming available for SOC electrochemistry, they are strongly needed. The high temperature solid-solid and solid-gas interfaces tend to change a lot over time due to segregation of electrolyte and electrode constituents and unavoidable trace impurities on a level of few ppm. Furthermore, a porous electrode for solid-gas reactions has three phase boundaries (TPBs), where the electrolyte, the electrode and the gaseous reactants meet. The current density will be concentrated around the TPB. Also, the TPB seems particularly prone to collect trace impurities and minority components, probably because the TPB zone has many sites with higher free energy relative to the rest of the electrode and electrolyte surface. An example of the segregation is the enrichment of yttria to the yttria stabilized zirconia (YSZ – the common SOFC electrolyte) surface, which takes place during a few hours at operation temperature. Furthermore, most often a silica rich layer will form on top of the yttria enriched layer. These “interphase” (not interface) layers may grow and change over time and with changes in temperature and other test conditions. Such segregation seems to be equally pronounced for surfaces and interfaces of the popular perovskite structured metal oxide electrodes such as lanthanum strontium manganites or cobaltites on which a several nanometer thick skin of strontium rich oxide forms already during cell preparation and it is believed that this is changing significantly during electrode operation. However, our knowledge about the driving forces for and the kinetics of the formation of the interphases is very superficial. Thus, there is a strong need for in operando techniques that can characterize and monitor the development of the mentioned features as function of time and changing experimental conditions with respect to electrical, structural and chemical properties at the nano-scale. Going through the various known techniques, it becomes clear that there are not sufficient in operando techniques available to make a comprehensive electrode characterization, and therefore in situ techniques are usually employed, in which at least one of the operation conditions are fulfilled, e.g. temperature but not atmosphere is matching relevant operation conditions. Finally, our analysis of already published results points out the advantage of combining several different techniques such as electrochemical impedance spectroscopy with in operando scanning probe microscopy and surface sensitive chemical analysis methods. Examples of results will be presented.

Mechanical and electrochemical behavior of novel electrolytes based on partially stabilized zirconia for solid oxide fuel cells

In this study, 3mol% yttria stabilized zirconia (3YSZ) is investigated as a SOFC electrolyte alternative to 8mol% yttria stabilized zirconia (8YSZ). The mechanical and electrochemical properties of both materials are compared. The mechanical tests indicate that the thickness of 3YSZ can be reduced to half without sacrificing the strength compared to 8YSZ. By reducing the thickness of 3YSZ from 150µm to 75µm, the peak power density is shown to increase by around 80%. The performance is further enhanced by around 22% by designing of novel electrode structure with regular cut-off patterns previously optimized. However, the cell with novel designed 3YSZ electrolyte exhibits 30% lower maximum power density than that of the cell with 150µm-thick standard 8YSZ electrolyte. Nevertheless, the loss in the performance may be tolerated by decreasing the fabrication cost revealing that 3YSZ electrolyte with cut-off patterns can be employed as SOFC electrolyte alternative to 8YSZ.

Synthesis, thermal expansion and high-temperature electrical conductivity of Co-doped (Y,Са)FeO3−δ with orthorhombic perovskite structure

Orthorhombic perovskites Y1−xСаxFe1−yCoyO3−δ (0.1≤х≤0.2, 0.1≤y≤0.2 and х=0.1, y=0.3) were synthesized in air by the citrate route at 1150–1300°C. High-temperature X-ray powder diffraction (HT XRPD) data for Y0.9Са0.1Fe0.8Co0.2O3−δ at 25–800°C showed no phase transition with calculated thermal expansion coefficient (TEC) of 11.9ppmK−1. High-temperature electrical conductivity measurements revealed almost composition independent conductivity values of 22–27S/cm at 900°C. No chemical interaction of Y0.8Са0.2Fe0.9Co0.1O3−δ with (Zr,Y)O2−x (YSZ) or (Ce,Gd)O2−x (GDC) was observed up to 1000°C and 1100°С, respectively. Partial replacement of Y by Pr according to formula Y0.8−zPrzCa0.2Fe0.7Co0.3O3−δ, 0.1≤z≤0.35, leads to an increase of both electrical conductivity up to 50S/cm (z=0.3) at 900°C and dilatometry measured TEC up to 15.1ppmK−1. Moderate values of electrical conductivity in combination with low TEC and stability towards chemical interaction with typical SOFC electrolytes make Co-doped Y1−xCaxFeO3−δ promissing cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFC).

A promising cathode for proton-conducting intermediate temperature solid oxide fuel cells: Y0.8Ca0.2BaCo4O7+δ

Y0.8Ca0.2BaCo4O7+δ (YCBC4) was synthesized and investigated as a cathode for proton-conducting intermediate temperature solid oxide fuel cells (IT-SOFCs). Low thermal expansion coefficient (TEC) (~9.9×10−6K−1) can provide good thermal expansion compatibility with the standard SOFC electrolyte materials. Laboratory-sized quad-layer cells, consisting of NiO–BaZr0.1Ce0.7Y0.2O3−δ (BZCY)/NiO–BZCY/BZCY/YCBC4, were fabricated by a one-step dry-pressing/co-firing process and tested from 550 to 700°C with humidified hydrogen (~3% H2O) as the fuel and static air as the oxidant, respectively. An excellent power density of 472mWcm−2 and a low electrode polarization resistance of 0.121Ωcm2 were obtained at 700°C. Preliminary results demonstrated that the YCBC4 can be a promising cathode material for proton-conducting IT-SOFCs.

Fundamentals of Synthesis, Sintering Issues, and Chemical Stability of BaZr0.1Ce0.7Y0.1Yb0.1O3-δ Proton Conducting Electrolyte for SOFCs

Fundamentals of synthesis, sintering issues, and chemical stability of BaZr0.1Ce0.7Y0.1Yb0.1O3-δ (BZCYYb) proton conducting electrolyte material were studied. Solid state reaction (SSR) and glycine nitrate process (GNP) methods were used to synthesize BZCYYb powders. The GNP method produced powders with higher purity and smaller particle size in reduced heat-treatment time and temperature. It was found that BZCYYb material, despite its high proton conductivity, has surprisingly high tendency to react with common substrate materials including alumina, zirconia, and even ceria during high temperature sintering. This led to significant barium loss and even disintegration of the perovskite phase. In addition, during sintering, BZCYYb also lost Ba through gas phase. The barium loss and unwanted side reactions with substrates could be suppressed either using enclosure of a platinum substrate and a cover or protective layers of BZCYYb powders. On the other hand, using metal oxide sintering aids such as NiO helps reducing sintering time and temperature which mitigates the extent of unwanted reactions. Finally, isothermal exposure experiments showed that at 750°C, chemical stability limits for BZCYYb in CO2 and H2S contaminated atmospheres are below ∼50 vol. % of CO2 and between 50 and 100 ppm of H2S, respectively, and they shift to lower values for Ba-deficient BZCYYb.

Proton trapping in Y and Sn Co-doped BaZrO3

Proton–dopant interactions in the exciting new SOFC electrolyte material, Y and Sn co-doped BaZrO3, are shown to be potentially detrimental to its electrochemical performance.

Synthesis and properties of CuO-doped Ce0.9Gd0.1O2−δ electrolytes for SOFCs

The effects of copper oxide (CuO) addition on the crystal structure, densification and microstructure of Gd-doped ceria (Ce0.9Gd0.1O2−δ, CGO) synthesized by the polymeric precursor method have been studied. Ce0.9−xGd0.1CuxO2−δ (0≤x≤0.01) precursor powders were calcined at 600°C for 1h and the phase formation was studied by powder X-ray diffraction combined with Rietveld refinement analysis. Relative density measurements and microstructural analysis were performed on pellets sintered in the temperature range 1000–1100°C in air. The effect of CuO as a sintering aid becomes more visible when its content is in the 0.5–1mol% range where the relative densities are found to be 98% for sintering temperature as low as 1000°C. A remarkable reduction (up to ~500°C) in the sintering temperature for 1mol% CuO-doped CGO ceramics is observed together with the formation of a dissolved phase of Gd2O3–CeO2–CuO. Ce0.89Gd0.1Cu0.01−yO2−δ sintered at 1000°C shows total electrical conductivity of 15.5mScm−1 at 600°C slightly higher than Ce0.9Gd0.1O2−δ sintered at 1500°C (12.4mScm−1).

Neutron structural characterization and transport properties of the oxidized and reduced LaCo0.5Ti0.5O3 perovskite oxide

Polycrystalline oxygen-stoichiometric LaCo0.5Ti0.5O3 perovskite oxide has been prepared by soft-chemistry procedures followed by annealing in air at 800°C. A new reduced LaCo0.5Ti0.5O3-δ specimen has been obtained by topotactical oxygen removal in an H2/N2 (5%/95%) flow at 600°C. The structural characterization has been conducted from neutron powder diffraction (NPD) data, very sensitive to the contrast between Co and Ti and the oxygen stoichiometry. Both perovskites (oxidized and reduced) crystallize in the orthorhombic Pbnm, space group. The partial reduction of Ti4+ to Ti3+ in the reduced phase is accompanied with the occurrence of oxygen vacancies, located at the axial octahedral sites, and it is expected to support the ionic conductivity, as usually observed in oxygen-defective perovskites. Thermogravimetric analysis (TGA) substantiates the oxygen stoichiometry and the stability range of the reduced sample. All the samples in study display a semiconductor-like behavior with values that not reach below to 0.5 Scm−1 for all the phases. Moreover, the measured thermal expansion coefficients perfectly match with the values usually displayed by SOFC electrolytes.

Synthesis and Characterization of Double-Doped Bismuth Oxide Electrolytes for Lower Temperature SOFC Application

Introduction A critical hurdle to the commercialization of SOFCs is the ability to reduce system cost. A lower temperature would allow for the use of less expensive interconnect materials, reduction of the start-up time, longer mechanical and chemical stability, thus reducing total system cost. However, lowering the operation temperature of SOFCs is difficult due to ionic conductivity limitations of electrolyte materials. For example, yttria-stabilized zirconia (YSZ) is a widely used electrolyte material but its ionic conductivity is very low at temperatures below 700⁰C. It has been demonstrated that erbium stabilized bismuth oxide (ESB) has one to two orders of magnitude higher ionic conductivity than YSZ due to ESB's high oxygen vacancy concentration. Furthermore, we have developed dysprosium-tungsten doped bismuth oxide (DWSB) which has an even higher conductivity and stability at lower temperatures as compared to ESB. In this presentation, we report our recent progress in developing DWSB and other high performance double doped bismuth oxide (DD-SB) electrolytes for SOFCs. In order to further enhance the performance we developed nano-sized DD-SB powder using wet chemical methods and investigated the effect of powder morphology on the performance of DD-SBs using AC impedance spectroscopy. Experimental Procedure DD-SB powders were prepared via conventional solid state and wet chemical methods. Solid state (SS) powder was fabricated by milling Bi2O3 and MOx dopants in a HDPE mill for 24h. The milled powder was calcined at 800⁰C for 16 hours to achieve the desired phase. To make coprecipitated (CP) powder, appropriate amounts of Bi(NO3)3-xH2O and dopant metal nitrate salts were dissolved in nitric acid and for DWSB (NH4)6W12O36-xH2O was dissolved in NH4OH. The acid solution is added drop wise to the base solution while stirring with a magnetic stir bar. The precipitate was washed in DI H2O, filtered, dried at 100⁰C, and fired at 550⁰C to achieve the appropriate phase. Additionally, glycine nitrate combustion is used. Appropriate amounts of the metal salts and NH2CH2COOH (glycine) are dissolved in DI H2O and 10 vol% HNO3. The solution is evaporated at 100 °C. The resulting gel is heated to 350 °C until auto-ignition occurs. The resulting ash is collected and fired at 550 - 650 °C to achieve the desired phase.The phase analysis of the powders was performed by means of X-ray diffraction (XRD). BET surface area, and particle size calculations were used to quantify the geometric powder properties. SEM images were taken to observe the morphology of the powder. Symmetric cells were fabricated using uniaxial pressing. Thin DD-SB electrolytes were also colloidally deposited onto NiO-GDC anodes and sintered at 750⁰C. Conductivity measurements using electrochemical impedance spectroscopy (EIS) were performed. Results Fig.1 shows an example of a deposited thin film of wet chemically synthesized DWSB onto a pre-fabricated anode supported NiO-GDC with a GDC electrolyte.

Terbium and Tungsten Co-doped Bismuth Oxide Electrolytes for Low Temperature Solid Oxide Fuel Cells

We developed a novel double dopant bismuth oxide system with Tb and W. When Tb was doped as a single dopant, a Tb dopant concentration more than 20 mol% was required to stabilize bismuth oxides with a high conductivity cubic structure. High temperature XRD analysis of 25 mol% Tb-doped bismuth oxide (25TSB) confirmed that the cubic structure of 25TSB was retained from room temperature to 700oC with increase in the lattice parameter. On the other hand, we achieved the stabilization of high temperature cubic phase with a total dopant concentration as low as ~12 mol% with 8 mol% Tb and 4 mol% W double dopants (8T4WSB). Moreover, the measured ionic conductivity of 10T5WSB was much higher than 25TSB, thus demonstrating the feasibility of the double dopant strategy to develop stabilized bismuth oxide systems with higher oxygen ion conductivity for the application of SOFC electrolytes at reduced temperature. In addition, we investigated the long-term stability of TSB and TWSB electrolytes.

Interaction Study of Yttria‐Based Glasses with High‐Temperature Electrolyte for SOFC

AbstractThe chemical interaction study of AO–SiO2–B2O3–Y2O3 (A = Ba, Sr) (BaY, SrY) glass with high‐temperature electrolyte yttria‐stabilized zirconia (YSZ 8 mol%) is reported as a function of different heat treatment durations. The as‐prepared glass with 10 mol% of yttria shows limited amount of crystallization at 800 °C. Due to this yttria‐based glasses BaY and SrY have been chosen to make diffusion couples with high‐temperature electrolyte and interconnect material. These diffusion couples have been heat treated at 850 °C, for 100, 200, and 500 h. The heat‐treated diffusion couples have been characterized using X‐ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). Microstructural analysis of diffusion couples shows absence of any undesired oxides and detrimental reaction products at the interface. The glass has shown good bonding characteristics and absence of cracks, pores, or any kind of delamination from YSZ. Apart from this, SrY and BaY glass seals have also shown good adhesion characteristics with Crofer 22 APU, even after 500 h at 850 °C. The morphology and microstructure of the glass matrix suggest limited amount of devitrification in the glass.

Electrical and structural properties of Ce0.8(Sm0.5−xY0.5Alx)0.2O2−δ ceramics as an electrolyte for SOFC

The effects of various contents of alumina co-doping on a ceria-based electrolyte were investigated. All samples were sintered at 1500°C for 10h. The X-ray diffraction (XRD) patterns showed a fluorite cubic structure. To investigate the surfaces of the Ce0.8(Sm0.5−xY0.5Alx)0.2O2−δ (CSYA) ceramics by scanning electron microscopy (SEM), the electrical properties of the CSYA ceramics were measured by AC impedance. The optimal values of ionic conductivity and activation energy were reached at x=0.4 measured at 600°C. This result indicated that the alumina co-doping improved the electrical and microstructural properties of the ceria-based electrolyte.

Citrate-complexation synthesized Ce0.85Gd0.15O2−δ (GDC15) as solid electrolyte for intermediate temperature SOFC

A typical Ce0.85Gd0.15O2−δ (GDC15) composition of CeO2−Gd2O3 system is synthesized by modified sol−gel technique known as citrate-complexation. TG–DTA, XRD, FT-IR, Raman, FE-SEM/EDX and ac-impedance analysis are carried out for structural and electrical characterization. XRD pattern confirmed the well crystalline cubic fluorite structure of GDC15 after calcining at 873K. Raman spectral bands at 463, 550 and 600cm−1 are also in agreement with these structural features. FE-SEM image shows well-defined grains separated from grain boundary and good densification. Ac-impedance studies reveal that GDC15 has oxide ionic conductivity similar to that reported for Ce0.9Gd0.1O2−δ (GDC10) and Ce0.8Gd0.2O2−δ (GDC20). Ionic and electronic transference numbers at 673K are found to be 0.95 and 0.05, respectively. This indicates the possible application of GDC15 as a potential electrolyte for IT-SOFCs.