Preparation of proton conducting BaCe0.8Gd0.2O3 thin films

Thin films of BaCe_0.8Gd_0.2O₃ were prepared by solid state reaction of two screen-printed layers over porous substrates. The first layer consists of the oxygen ion conductor Ce_0.8Gd_0.2O₂ with a fluorite structure, whereas the top layer consists of BaCO₃. After decomposition of the carbonate, BaO reacts with Ce_0.8Gd_0.2O₂ forming the perovskite oxide BaCe_0.8Gd_0.2O_3−δ with protonic conductivity. The in-situ reaction and densification on the porous substrates results in gastight thin layers of 10 to 50 μm and allows overcoming the problems due to the poor sinterability of the proton conductor. Two different porous substrates prepared by warm-pressing were studied as membrane supports, i.e., (i) porous composite NiO–Zr_0.85Y_0.15O₂, commonly employed as solid oxide fuel cell anode and (ii) porous Ce_0.8Gd_0.2O₂ oxide. The structural properties of the layer, compositional gradients and occurring phases are described, as well as water uptake, gastightness (He leaking rate) and emf measurement. Protonic conducting membranes are particularly suited not only for hydrogen separation combined with reforming and water–gas-shift converters but also as a protonic fuel cell electrolyte.     

Spray pyrolysis of electrolyte interlayers for vacuum plasma-sprayed SOFC

The effects of gadolinia-doped ceria (CGO, Ce_0.8Gd_0.2O_1.9−x) and yttria-doped zirconia (8YSZ, Zr_0.92Y_0.08O_2−x) interlayers prepared by spray pyrolysis between vacuum plasma-sprayed 8YSZ electrolytes (8YSZ–VPS) and screen-printed (La_0.8Sr_0.2)_0.98MnO₃ cathodes (LSM) on the power output of solid oxide fuel cells (SOFC) are investigated. Amorphous thin films are deposited and then converted to nanocrystalline electrolyte–cathode interlayers during the first heat-up cycle of a SOFC to the operating temperature. CGO thin films between the YSZ plasma-sprayed electrolyte and the LSM cathode increased the power output by more than 20% compared to cells without interlayers, whereas YSZ films degraded the power output of cells. It is assumed that CGO improves the charge transfer at the electrolyte–cathode interface and that the CGO layer prevents the formation of undesirable insulation of La-zirconate at the interface 8YSZ/LSM.     

Structural,thermal and electrical properties of Bi and Y co-doped barium zirconium cerates

Bismuth- and yttrium-co-doped barium cerates were successfully synthesised by solid-state reactions followed by sintering between 1,400 and 1,500 °C for 1 to 6 h allowing densification above 98 % to be obtained. All samples were found to retain their original orthorhombic structure after treatment in either oxidising or reducing atmospheres (dry and wet). Mechanical strength was affected by structure upon reduction due in part to strains and stresses induced by bismuth ionic size variations. Conductivity values as high as 0.055 S/cm were obtained for sample BaCe_0.6Zr_0.1Y_0.1Bi_0.2O_3?δ and of 0.0094 S/cm for the Zr-free compound BaCe_0.7Y_0.2Bi_0.1O_3?δ at 700 °C in air. In all the investigated materials, sample BaCe_0.6Zr_0.1Y_0.1Bi_0.2O_3?δ exhibits the highest conductivity in both air and wet 5 % H₂/Ar with good mechanical strength. BaCe_0.6Zr_0.1Y_0.1Bi_0.2O_3?δ is a promising mixed H⁺/e^? conductor, a potential component of composite anode for solid oxide fuel cells.     

Chemical and redox stabilities of a solid oxide fuel cell with BaCe0.8Y0.2O3−α functioning as an electrolyte and as an anode

The operation of a solid oxide fuel cell (SOFC) based on BaCe_0.8Y_0.2O_3−α (BCY20) at 800 °C was studied without using an anode material. A porous, Ce-rich phase with a fluorite structure was formed at a depth of approximately 10 μm from the BCY20 surface by heat treatment at 1700 °C. This was due to the vaporization of BaO from the BCY20 surface. This treatment improved the cell performance and chemical stability to CO₂ because the Ce-rich phase functioned as an electrically conducting and protective layer. The heat-treated BCY20 also had better chemical and redox stabilities over a Ni–Ce_0.8Sm_0.2O_1.9 (SDC) cermet anode attached to the SDC electrolyte. The cell with the heat-treated BCY20 operated well on unhumidified methane, ethane, propane, and butane without carbon deposition, while the cell with the Ni–SDC cermet anode degraded within a few hours. Moreover, the BCY20 showed higher tolerance to 10 ppm H₂S and stability over 20 times redox cycling in comparison to the Ni–SDC cermet anode.     

Specifics of the Electrical Properties of Composite Solid Oxide Membranes Based on SrTi<Subscript>0.5</Subscript>Fe<Subscript>0.5</Subscript>O<Subscript>3–δ</Subscript>

The electrical properties of dual-phase fluorite-pervoskite oxide systems based on strontium titanate- ferrite (SrTi_0.5Fe_0.5O_3–δ) are studied. We find that the oxygen ionic and ambipolar conductivities of strontium titanate-ferrite can be considerably improved by introducing the fluorite phase Ce_0.8(Sm_0.8Sr_0.2)_0.2O_2–δ. This is advantageous considering the prospects of applying these types of composite materials in different electrochemical devices, e.g., as membrane materials in electrochemical converters for the production of hydrogen and syngas and anode materials in solid oxide fuel cells.     

Influence of colloidal templates on the impedance spectroscopic behaviour of Pr0.7Sr0.3Fe0.8Ni0.2O3 for solid oxide fuel cell applications

The creation of porous materials with three-dimensional periodicity has been identified as being of potential interest for increasing the overall performance of solid oxide fuel cells (SOFC). In this work, we have investigated the formation of pore systems in the nanometer scale by replicating colloidal templates. Templating methods have been used to prepare iron-nickel-based perovskite Pr_0.7Sr_0.3Fe_0.8Ni_0.2O₃ material with nanoporous microstructure. Polymethyl methacrylate (PMMA), polystyrene (PS) and polycarboxylate (PC) microspheres with different diameters were used as pore formers. These samples were synthesized and characterized by thermogravimetric analysis, inductively coupled plasma atomic emission spectroscopy (ICP-AES), X-ray diffraction, transmission electron microscopy and field emission scanning electron microscopy. The polarization resistance of the materials was studied by Electrochemical Impedance Spectroscopy. The study demonstrated that templated porosity is maintained and highly influences on the impedance spectroscopic behaviour, being the material synthesized with policarboxylate microspheres the most interesting of the three used templates for SOFC applications.     

The chemical stability and conductivity improvement of protonic conductor BaCe0.8 − x Zr x Y0.2O3 − δ

A series of BaCe_0.8???x Zr _x Y_0.2O_3???δ (BCZYx) (x?=?0, 0.2, 0.4, 0.6, 0.8) powders were prepared by EDTA–citrate complexing sol–gel process in this paper. The electrical conducting behavior, as well as chemical stability, was investigated. X-ray diffraction (XRD) results reveal that all samples are homogenous perovskite phases. Observed from XRD patterns and thermogravimetric curves, the samples with x?≥?0.4 survive in the pure CO₂, while samples with various Zr contents all present structurally stable against steam at 800 °C. The Zr-free sample of BaCe_0.8Y_0.2O_3???δ possesses the maximum bulk conductivity, 4.25?×?10^?2 S/cm, but decomposes into Ba(OH)₂ and Ce_0.8Y_0.2O_3???δ in steam. A negative influence of increasing Zr content on the conductivity of BCZYx can be observed by impedance tests. Considering the effect of temperature on the bulk conductivity, BCZY0.4 is preferred to be applied in SOFC as a protonic conductor, ranging from 1.52?×?10^?4 to 1.51?×?10^?3 S/cm (500–850 °C) with E _a?=?0.859 eV, which is proved to be a good protonic conductor with t _H+?≥?0.9.     

Electrical properties of thin yttria-stabilized zirconia overlayers produced by atomic layer deposition for solid oxide fuel cell applications

Thin films of yttria-stabilized zirconia (YSZ) electrolyte were prepared by atomic layer deposition at 300 °C for solid oxide fuel cell (SOFC) applications. YSZ samples of 300-1000 nm thickness were deposited onto La_0.8Sr_0.2MnO₃ (LSM) cathodes. A microstructural study was performed on these samples and their electrical properties were characterised between 100 and 390 °C by impedance spectroscopy. A remarkable feature is that the as-deposited layers were already crystalline without any annealing treatment. Their resistance decreased when reducing the layer thickness; nevertheless, their conductivity and activation energy were significantly lower than those reported in the literature for bulk YSZ.     

Cathode overpotential investigation by means of “Built-in” potential electrode

The objective of the present work is the development of a “built-in” potential electrode method for direct measurements of the cathode and anode overpotentials and the corresponding interface resistances of solid oxide fuel cells (SOFC). The studies were performed on a yttria-stabilised zirconia (YSZ) electrolyte-supported SOFC using La_0.8Sr_0.2MnO₃ as cathode, GDC as protecting layer and Ni-ScSZ cermet as anode. The mesh potential electrode was placed inside the YSZ membrane near the cathode side. Using the combination of the I-U and the impedance measurements with the built-in potential electrode technique, the temperature dependencies of the electrodes and electrolyte contributions to the total cell resistance were determined.     

Recent progress in solid oxide and lithium ion conducting electrolytes research

Recent material developments of fast solid oxide and lithium ion conductors are reviewed. Special emphasis is placed on the correlation between the composition, structure, and electrical transport properties of perovskite-type, perovskite-related, and other inorganic crystalline materials in terms of the required functional properties for practical applications, such as fuel or hydrolysis cells and batteries. The discussed materials include Sr- and Mg-doped LaGaO₃, Ba₂In₂O₅, Bi₄V₂O₁₁, RE-doped CeO₂, (Li,La,)TiO₃, Li₃La₃La₃Nb₂O₁₂ (M=Nb, Ta), and Na super-ionic conductor-type phosphate. Critical problems with regard to the development of practically useful devices are discussed.     

Zirconia stabilized by Y and Mn: A microstructural characterization

Cubic stabilized ZrO₂ with 8 mol% Y₂O₃ (YSZ) is commonly used as an electrolyte in solid oxide fuel cells (SOFC). One of the most promising cathode materials is La,Sr-manganite (LSM). During manufacture and operation of the SOFC, Mn diffuses from the LSM into YSZ. The structural changes caused by the presence of Mn in the electrolyte under various oxygen partial pressures have been examined by X-ray diffraction. Microstructures of YSZ containing Mn have been examined by transmission electron microscopy and surface changes of the electrolyte due to Mn diffusion were studied by scanning electron microscopy. The results are discussed with respect to properties and stability of the electrolyte and the SOFC. Paper presented at the 2nd Euroconference on Solid State Ionics, Funchal, Madeira, Portugal, Sept. 10–16, 1995     

H2S Solid oxide fuel cell based on a modified Barium cerate perovskite proton conductor

Urea combustion method was adopted to prepare precursor powder, MCeO₃ doped with Zr (M is alkaline earth element, such as barium, strontium, and calcium). The precursor powder has typically perovskite structure after being calcined at 873 K. In 773 K∼1,273 K, BaCe_0.425Zr_0.475Y_0.1O₃ has the highest conductivity above 10⁻² S cm⁻¹ and good chemical stability, while the phase transition may exist in H₂S atmosphere for the proton conductors. In the single fuel cell composed of MoS₂-BaCe_0.425Zr_0.475Y_0.1O_3-σ-Ag with BaCe_0.425Zr_0.475Y_0.1O_3-σ as electrolyte, the best performance is obtained. The open circuit voltage of fuel cell is all about 0.72 V, the max power density, 1.55 mW cm⁻². The performance drop is attributed to ohmic loss resulting from the separation of electrolyte and electrode, and improvement is required to bring out new anode materials compatible to the proton conductor, BaCe_0.425Zr_0.475Y_0.1O_3-σ, as electrolyte.     

Electrical conductivity and thermal expansion of La0.8Sr0.2(Mn,Fe,Co)O3-δ perovskites

La_1−xSr_xMeO₃ (Me = Mn, Co, Fe) perovskites are used as cathodes and are also attractive materials for application as the contact layer between cathode and interconnect in solid oxide fuel cells. In this contribution, three perovskite series, La_0.8Sr_0.2Mn_1−xCo_xO_3-δ (series 1), La_0.8Sr_0.2Fe_1−xCo_xO_3-δ (series 2) and La_0.8Sr_0.2Mn_1−x/2Fe_(1−x)/2Co_xO_3-δ (series 3) with x = 0, 0.25, 0.5, 0.75 and 1 were re-investigated under identical synthesis and measurement conditions with the aim of obtaining a full overview of the quasi-ternary system La_0.8Sr_0.2MnO_3-δ–La_0.8Sr_0.2FeO_3-δ–La_0.8Sr_0.2CoO_3-δ. The distribution of the different crystallographic phases in the selected series, the DC electrical conductivity and the thermal expansion coefficients are presented.     

Investigation of chemisorbed oxygen, surface segregation and effect of post-treatments on La0.8Sr0.2MnO3 powder and screen-printed layers for solid oxide fuel cell cathodes

In order to better understand the mechanism of the reaction of oxygen reduction at the surface of strontium doped lanthanum manganites (LSM) cathodes in solid electrolyte fuel cells (SOFC), the surface properties of La_0.8Sr_0.2MnO₃ powders and screen-printed layers have been characterised by various techniques.Strontium enrichment at the surface has been evidenced by X-ray photoelectron spectroscopy according to the conditions of annealing (temperature, oxygen pressure) and polarisation treatments of the samples.The interaction between oxygen and La_0.8Sr_0.2MnO₃ for SOFC cathodes has been studied by thermo-programmed desorption, in situ infrared spectrometry and calorimetry. The results indicate that various adsorbed oxygen species may exist on the surface of LSM depending on temperature.The presence of various adsorbed oxygen species and the surface Sr segregation are important factors to consider in the mechanism of oxygen reduction at LSM SOFC cathodes since they could be responsible for many discrepancies between the interpretations that can be found in the literature data.     

An improved method for preparation of Ce<Subscript>0.8</Subscript>Pr<Subscript>0.2</Subscript>O<Subscript>Y</Subscript> solid solutions with nanoparticles smaller than 10 nm

Ce_0.8Pr_0.2O_Y solid solutions with ultrafine crystalline sizes and high specific surface area were prepared by an improved citrate precursor method, where a nitrogen treatment was added prior to calcinations in air. The samples were characterized by TG-DSC, Raman spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM) and BET nitrogen adsorption. XRD and Raman results show that the formation of Ce_0.8Pr_0.2O_Y solid solutions typical of the fluorite-like cubic structure with oxygen vacancies occurs when the Ce–Pr citrate precursors are heated at high temperature in the nitrogen atmosphere. Subsequent calcinations at a low temperature in air to remove carbon species have no apparent effects on the formed solid solutions. Ce_0.8Pr_0.2O_Y solid solution prepared by the improved citrate precursor method at 800°C has ultrafine nanoparticles of less than 10 nm and high specific surface area of 92.1 m²/g, while the sample prepared by the conventional citrate precursor method has mean particle size of 62.1 nm and specific surface area of 18.1 m²/g. Furthermore, Ce–Pr solid solution by the improved method have the mesoporous structure, more lattice defects and oxygen vacancies, which will have a promising application in the catalyst region as well as SOFC field.     

Sol-gel preparation of selected high temperature proton conductors

The sol-gel preparation of the compounds BaCe_0.8Y_0.2O_2.9 (BCY20), BaCe_0.8Gd_0.2O_2.9 (BCG20), BaTiO₃, BaTi_0.9Y_0.1O_2.95 and BaTi_0.9Er_0.1O_2.95 using the acrylamide route is described. The advantages lie in the short reaction time leading to the gels and the large batch sizes possible. In the case of the titanates there was the additional advantage that one of the starting compounds was metallic Ti powder which was dissolved in H₂O₂ and NH₃. Basically, all 5 compounds could be synthesized phase pure. The cerates were well described by PDF 35-1318 and absorb large quantities of water vapor in TG experiments. The doped titanates were cubic and were well characterized by PDF 31-0174. They only absorb relatively small amounts of water vapor but still may be termed proton conductors.     

Reverse micelle synthesis of perovskite oxide nanoparticles

La_0.6Sr_0.4Co_xFe_1−xO_3−δ (LSCF), La_0.6Sr_0.4Cu_0.2Fe_0.8O_3−δ, Ba_0.5Sr_0.4Co_0.8Fe_0.2O_3−δ and LaFeO_3−δ nanoparticles were synthesized by a reverse micelle procedure. Controlling the size of the micelles through the water:oil phase ratio enabled synthesis of phase pure perovskite particles with average sizes from 14 nm to 50 nm. Small amounts of an impurity phase, likely cobalt oxide, were detected in the XRD spectrum of high cobalt content samples of LSCF (x = 0.8). La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ nanoparticles were utilized to coat the surface of a dense thin-film La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ solid oxide fuel cell cathode. The polarization resistance of the nanoparticle coated electrode, measured at open circuit in air at 973 K, was 20% lower than an equivalent un-coated electrode.     

Oxy-combustion of coal in liquid-antimony-anode solid oxide fuel cell system

A novel system based on the indirect oxy-combustion of coal in a liquid Sb anode solid oxide fuel cell (SOFC) has been used to produce electricity for over 48?h. Pulverized anthracite was fed to the liquid-antimony-anode of the fuel cell, and a peak power density of 47?mW cm^?2 was reached at 1023?K and 35?mW cm^?2 at 973?K. The fuel cell was prepared using a porous stainless-steel tube as a support for an LSM cathode, antimony oxide (Sb₂O₃)/yittria stabilized zirconia (YSZ, Y_0.08Z_0.92O_1.96) composite electrolyte (membrane), while liquid antimony acted as the anode. Liquid antimony/antimony oxide served as the intermediate medium for coal oxidation producing mainly carbon dioxide, which evolved as a separate gas stream. The fuel cell will facilitate carbon capture process, and simultaneously convert the chemical energy of coal directly to electricity. The experiment showed that while the fabricated electrolyte was porous, it became dense during the actual operation, preventing nitrogen leakage into the Sb/C side and producing reasonable open circuit voltage. Analysis of the experimental EIS data illustrates that the Ohmic resistance was the primary loss mechanism in the system. It further suggests approaches to improve the design. Continuous operation of this coal fueled oxy-combustion/fuel cell system achieved an overall efficiency of 28.2% despite of its tiny scale. Simple technologies can be employed to scale up this system at relatively low cost of fabrication and materials.     

High oxide ion conductivity in Fe and Mg doped LaGaO3 as the electrolyte of solid oxide fuel cells

La(Sr)Ga(Fe,Mg)O₃ exhibited the high oxide ion conductivity and the electrical power generating property of SOFC single cell using La_0.7Sr_0.3Ga_0.7Fe_0.2Mg_0.1O_3-δ (LSGFM) electrolyte was investigated in this study. The transport number of oxide ion is almost 0.8 in LSGFM and so open circuit potential (OCV) is as low as 0.8 V. OCV was strongly affected by anode materials and the highest OCV was achieved on Ni–Fe bimetallic anode. The extremely high power density was achieved by using LSGFM for electrolyte of SOFC. The maximum power densities of the cells can be elevated by coating with oxide ion conductor film at anode side. The maximum power density increased in the following order for coating film: LSGM > SDC > YSZ. The maximum power density of 197 and 100 mW/cm² can be achieved at 873 and 773 K, respectively, when LSGM film deposited on the anode side of LSGFM. Therefore, LSGFM can be used as electrolyte of SOFC operating at intermediate temperature.     

Low energy ion scattering (LEIS) applied to ionic materials

Surfaces and interfaces play a dominant role in the performance of devices like fuel cells and membranes based on ionic materials. The LEIS technique offers a unique possibility to determine the composition of the outermost atomic layer, which is often quite different from that of deeper layers. For instance impurities that are present in minute concentrations −1 ppb or even less- in the bulk may segregate to the surface and there become the dominant species. By carefully sputtering away the top layers also a compositional depth profile can be made of the first 10 to 20 layers. The basic principles of the LEIS technique will be explained, answering questions like: why is it so extremely surface sensitive; what does the instrumentation look like; what can you learn from the results. Its applicability to ionic materials will be demonstrated with results from measurements on various substrates used in fuel cells. The oxygen exchange across the surface of Sm_0.8Sr_0.2CoO₃ can be studied, because it is possible to differentiate between the¹⁸O and¹⁶O isotopes. One can determine which elements are on top in La_0.8Sr_0.2Ga_0.8Mg_0.2O_3±δ. The top layer of Ce_0.8Gd_0.2O_1.9 proves to be enriched in Gd oxide. The Y segregation in isotopically enriched YSZ is being studied. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.