Samaria-doped Ceria Electrolyte Research Articles

Novel Ni–Ba 1+ xZr 0.3Ce 0.5Y 0.2O 3− δ hydrogen electrodes as effective reduction barriers for reversible solid oxide cells based on doped ceria electrolyte thin film

Samarium-doped ceria (SDC) is evaluated as electrolyte materials for intermediate temperature reversible solid oxide cells (RSOCs). The bulk resistances of cells with SDC thin-film electrolytes are 0.16 and 0.23 Ω cm 2 when operating in solid oxide electrolysis cell (SOEC) mode and in solid oxide fuel cell (SOFC) mode, respectively. This result suggests that the electrolyte is reduced to a greater extent in the SOEC mode. New Ni-Ba 1+ x Zr 0.3Ce 0.5Y 0.2O 3− δ ( x = −0.04, 0 and 0.04) composites are applied as hydrogen electrode for RSOCs to prevent SDC reduction. With the new hydrogen electrodes, the bulk resistances of the RSOCs are approximately 0.37 Ω cm 2 in both SOFC and SOEC modes, suggesting that reduction in the SDC electrolyte is successfully minimized. The energy dispersive X-ray mapping of the Ba element indicates that Ba cations partially transfer from the hydrogen electrode into the electrolyte layer.

Electrical and thermal characterization of Sm 3+ doped ceria electrolytes synthesized by combustion technique

Nanocrystalline samarium doped ceria electrolyte [Ce 0.9Sm 0.1O 1.95] was synthesized by citrate gel combustion technique involving mixtures of cerium nitrate oxidizer ( O) and citric acid fuel ( F) taken in the ratio of O/ F = 1. The as-combusted precursors were calcined at 700 °C/2 h to obtain fully crystalline ceria nano particles. It was further made into cylindrical pellets by compaction and sintered at 1200 °C with different soaking periods of 2, 4 and 6 h. The sintered ceria was characterized for the microstructures, electrical conductivity, thermal conductivity and thermal diffusivity properties. In addition, the combustion derived ceria powder was also analysed for the crystallinity, BET surface area, particle size and powder morphology. Sintered ceria samples attained nearly 98% of the theoretical density at 1200 °C/6 h. The sintered microstructures exhibit dense ceria grains of size less than 500 nm. The electrical conductivity measurements showed the conductivity value of the order of 10 −2 S cm −1 at 600 °C with activation energy of 0.84 eV between the temperatures 100 and 650 °C for ceria samples sintered at 1200 °C for 6 h. The room temperature thermal diffusivity and thermal conductivity values were determined as 0.5 × 10 −6 m 2 s −1 and 1.2 W m −1 K −1, respectively.

Low-temperature ceria-electrolyte solid oxide fuel cells for efficient methanol oxidation

Low temperature anode-supported solid oxide fuel cells with thin films of samarium-doped ceria (SDC) as electrolytes, graded porous Ni-SDC anodes and composite La 0.6Sr 0.4Co 0.2Fe 0.8O 3 (LSCF)–SDC cathodes are fabricated and tested with both hydrogen and methanol fuels. Power densities achieved with hydrogen are between 0.56 W cm −2 at 500 °C and 1.09 W cm −2 at 600 °C, and with methanol between 0.26 W cm −2 at 500 °C and 0.82 W cm −2 at 600 °C. The difference in the cell performance can be attributed to variation in the interfacial polarization resistance due to different fuel oxidation kinetics, e.g., 0.21 Ω cm 2 for methanol versus 0.10 Ω cm 2 for hydrogen at 600 °C. Further analysis suggests that the leakage current densities as high as 0.80 A cm −2 at 600 °C and 0.11 A cm −2 at 500 °C, resulting from the mixed electronic and ionic conductivity in the SDC electrolyte and thus reducing the fuel efficiency, can nonetheless help remove any carbon deposit and thereby ensure stable and coking-free operation of low temperature SOFCs in methanol fuels.

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

In this paper anode support was fabricated by tape casting method using SDC-50 wt.% NiO slurry, then printed the Ce 0.8Sm 0.2O 1.9 (SDC) electrolyte on the green piece which is cut out from the dried slurry piece. After at 90 °C drying for 14 h and co-sintered at 1350 °C for 10 h, get the Φ70 mm anode support and electrolyte planar bilayer. Based on the observation of photos and scanning electron microscopy (SEM) indicated that bilayer owns the flat anode support substrate, and the highly dense, crack free electrolyte film which is 12 μm in thickness. Small disks which were cut out from the Φ70 mm bilayer structure electrochemically were examined in a single button-cell mode incorporating a SDC-60 wt.% La 0.5Sr 0.5Co 0.8Fe 0.2O 3 composite cathode. The single cell was tested at 450 °C∼600 °C, an open-circuit voltage (OCV) of 0.94 V and the maximum power density of 797 mV cm −2 achieved with dry hydrogen as fuel gas and air as oxidant gas at 600 °C.

Synthesis, characterization and evaluation of PrBaCo 2− xFe xO 5+ δ as cathodes for intermediate-temperature solid oxide fuel cells

Iron doped layered structured perovskites, PrBaCo 2− x Fe x O 5+ δ ( x = 0, 0.5, 1.0, 1.5 and 2.0), are evaluated as cathode materials for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The effects of dopant content are investigated on their structural and electrochemical properties including crystalline structure, oxygen nonstoichiometry, stability in presence of CO 2, compatibility with electrolytes, thermal expansion coefficient, electrical conductivity, and cathodic interfacial polarization resistance. The lattice parameter and oxygen nonstoichiometry content, δ, at room temperature increase, whereas the conductivity, thermal expansion coefficient, and cathodic performance decrease with increasing iron content, x. PrBaCo 2− x Fe x O 5+ δ exhibit excellent stability at 700 °C in atmosphere consisting of 3% CO 2 and 97% air, show good chemical compatibility with doped ceria electrolytes at 1000 °C, but react readily with yttria-stabilized zirconia at 700 °C. Even with a Co-free PrBaFe 2O 5+ δ as the electrode, a symmetrical cell demonstrates area specific resistance of 0.18 Ω cm 2 at 700 °C with samaria-doped ceria electrolyte. The resistance is lower than those for typical Co-free electrodes reported in the literatures, suggesting that PrBaCo 2− x Fe x O 5+ δ are potential promising cathode materials for IT-SOFCs.

Stabilization of a Complex Perovskite Superstructure under Ambient Conditions: Influence of Cation Composition and Ordering, and Evaluation as an SOFC Cathode

Ba1.6Ca2.3Y1.1Fe5O13 is an Fe3+ oxide adopting a complex perovskite superstructure, which is an ordered intergrowth between the Ca2Fe2O5 and YBa2Fe3O8 structures featuring octahedral, square pyramidal, and tetrahedral B sites and three distinct A site environments. The distribution of A site cations was evaluated by combined neutron and X-ray powder diffraction. Consistent with the Fe3+ charge state, the material is an antiferromagnetic insulator with a Neel temperature of 480−485 °C and has a relatively low d.c. conductivity of 2.06 S cm−1 at 700 °C. The observed area specific resistance in symmetrical cell cathodes with the samarium-doped ceria electrolyte is 0.87 Ω cm2 at 700 °C, consistent with the square pyramidal Fe3+ layer favoring oxide ion formation and mobility in the oxygen reduction reaction. Density functional theory calculations reveal factors favoring the observed cation ordering and its influence on the electronic structure, in particular the frontier occupied and unoccupied electronic states.

Fabrication and Characterization of Anode-Supported SDC Film Electrolyte and its Single Cell

NiO-YSZ (NiO-yttria stabilized zirconia, 3:2, wt.%) and samaria doped ceria (SDC) tapes were prepared by aqueous tape casting. NiO-YSZ anode-supported SDC film electrolyte half-cell was fabricated by laminating and co-sintering at 1400°C for 2 h. The single cell was prepared after LSCF-SDC (lanthanum strontium cobalt ferrite-SDC, 1:1, wt.%) cathode was coated on the electrolyte surface and sintered at 1300 °C for 2 h. The discharge performance of the single cell was tested from 500 °C to 800 °C at different H2 flow rate. Results showed that the relationship between current (I) of and H2 flow rate (ν) was I = 8 × 106 ν. Before reaching the threshold value of H2 flow rate, the current density of single cell increased with the increasing of H2 flow rate. However, the current density did not change with increasing of H2 flow rate over the threshold value. The open circuit voltage (OCV) of single cell at 500°C, 600°C, 700°C, 800°C was 0.978, 0.921, 0.861, 0.803 V, respectively. The maximum power density reached 93.03 mW/cm2 at 800°C. The resistance of interface layer between Ni-YSZ anode and SDC electrolyte was the key impact on the power density.

Relaxation studies of bulk samarium doped ceria electrolyte

The nanocrystalline samarium doped ceria powder of phase Ce0.8Sm0.2O1.9 was pelletized and sintered at about 1,623 K for the study of relaxation process. The impedance measurements were done within the frequencies from 1 Hz to 1 MHz for wide temperature range of 529–673 K. The temperature-dependent relaxation frequency found to be shifted towards higher frequency region from 17.26 to 707.96 KHz and the corresponding relaxation time for oxygen ions transport was varied from 9.22 to 0.22 μS. The temperature dependent conductivity and relaxation frequency revealed Arrhenius nature, showing activation energies 1.18 and 1.01 eV, respectively. This difference was attributed to pronounced interference due to the grain boundaries. The effect of sintering on microstructure of the sample was studied by using SEM and X-ray diffractometer (XRD). The structural and phase stability study was done by high-temperature (HT)-XRD. The thermal expansion coefficient of the samarium doped ceria powder determined from HT-XRD was found to be 13.9 × 10−6 K−1.

Effect of Ni content on the microstructure and electrochemical properties of Ni–SDC anodes for IT-SOFC

Ni–SDC (samaria-doped ceria) films for an IT-SOFC anode were fabricated by a slurry spin coating method. The microstructure and electrochemical performances of the Ni–SDC anodes were investigated with respect to the Ni vol.%. The connectivity of Ni–Ni grains, SDC–SDC grains and Ni–SDC grains, which is remarkably influenced by the Ni vol.%, is suggested to dominate the electrochemical behavior of the Ni–SDC anodes, including a hydrogen adsorption/diffusion process on the surface of Ni particles and a charge transfer process on the SDC electrolyte. A porous microstructure consisting of homogeneously distributed Ni and SDC phases together with well-connected grains was observed for the Ni–SDC anodes containing 50% and 60% volume content of Ni, which exhibited a polarization resistance around 0.1Ωcm2 together with an overpotential around 30mV under a current density of 200mAcm−2 at 800°C in a 60%N2+40%H2 atmosphere.

Hydrazine as efficient fuel for low-temperature SOFC through ex-situ catalytic decomposition with high selectivity toward hydrogen

Hydrazine is a promising fuel for portable fuel cells because it is a liquid, it is carbon free and it has a high energy density. In this work, hydrazine was investigated as an efficient fuel for low temperature solid-oxide fuel cells (SOFCs) with a traditional nickel anode. A catalytic system with high selectivity toward hydrogen was developed using Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3-δ (BSCF) as the main catalyst and potassium hydroxide as the promoter. The result of compositional analysis of the products showed that the hydrazine can be decomposed into hydrogen and nitrogen with 100% selectivity when an appropriate amount of KOH promoter is used. Acceptable power densities were achieved for a thin-film samaria-doped ceria (SDC) electrolyte cell operating on hydrazine decomposition products and hydrogen over a complete operation temperature range of 650–450 °C. In addition, a similar cell with ammonia as the fuel displayed a much lower performance.

Synthesis and properties of samaria-doped ceria electrolyte for IT-SOFCs by EDTA-citrate complexing method

An ultra-fine samaria-doped ceria (Ce 0.8Sm 0.2O 1.9, SDC) electrolyte prepared by a non-ion selective EDTA-citric complexing method is developed herein for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The rigid agglomerates due to organic compounds that exist in the SDC precursors during the EDTA-citrate complexing synthesis process inhibit crystalline growth and grain growth, leading to the generation of ultra-fine grain following the sintering procedure. Calcination is necessary above 500 °C for all precursors. The average grain size of the pellets after sintering at 1400 °C for 2 h is submicron in scale (from 200 nm to 600 nm) with various pH values, and the pellets are smaller than those obtained from other synthesis processes. Dense pellets with pH values of 10 (relative density of 99%) are obtained with precursor powder calcination at 900 °C for 3 h. Electrical conductivity is dependent on the calcination temperature and pH value of the solution, and the maximum electrical conductivity is 0.01 S cm −1 at 700 °C with a pH value of 10.

Thin Film Solid Oxide Fuel Cells Deposited by Spray Pyrolysis

Two techniques of spray pyrolysis, namely, electrostatic and pneumatic spray deposition, were used to deposit samaria-doped ceria (SDC) electrolyte and lanthanum strontium cobalt ferrite (LSCF) cathode on cermet or metal supported anodes for solid oxide fuel cells (SOFCs) operated at reduced temperature. The deposition processes, the properties of the deposited films, and the electrochemical performances of the fabricated cells are reported in this paper. The deposited SDC electrolytes were dense and gas-tight, and had good adhesion to the underlying anodes. The deposited LSCF cathode had a preferred morphology to facilitate the transport of oxygen gas and effective contact with the electrolyte. Button cell testing indicated that the SOFCs with electrolyte or cathode deposited by spray pyrolysis had good electrochemical performance. This study demonstrated that spray pyrolysis is a cost-effective process for fabricating thin film SOFCs, especially metal supported SOFCs.

Methane Oxidation at the Ni1-XCoX-Samaria-doped Ceria Cermet Anode

Substitution of Co atoms for Ni atoms in the Ni-based cermet anode resulted in a decrease in the anodic overvoltage and interfacial resistance between the anode and electrolyte. This effect has been confirmed for a samaria-doped ceria (SDC) electrolyte and a scandia-stabilized zirconia (ScSZ) electrolyte. The results of electrochemical measurements revealed that the enhancement of the direct oxidation of methane was caused by the modification of microstructure in the cermet anode.

Novel layered perovskite oxide PrBaCuCoO 5+ δ as a potential cathode for intermediate-temperature solid oxide fuel cells

A novel layered perovskite oxide PrBaCuCoO 5+ δ (PBCCO) is employed as a potential cathode for intermediate-temperature solid oxide fuel cells (IT-SOFCs). Thermal expansion and electrochemical performance on samarium-doped ceria (SDC) electrolyte are evaluated. The thermal expansion coefficient (TEC) of PrBaCuCoO 5+ δ (PBCCO) is close to that of SDC electrolyte and electrical conductivity of PrBaCuCoO 5+ δ (PBCCO) reaches the general required value of cathode material. Symmetrical electrochemical cell with the configuration of PrBaCuCoO 5+ δ (PBCCO)/SDC/PrBaCuCoO 5+ δ (PBCCO) applied for the impedance studies, the area specific resistance of PrBaCuCoO 5+ δ (PBCCO) cathode is as low as 0.047 Ω cm 2 at 700 °C. A maximum power density of 791 mW cm −2 is obtained at 700 °C for the single cell consisting of PrBaCuCoO 5+ δ (PBCCO)/SDC/NiO–SDC. Preliminary results indicate that PrBaCuCoO 5+ δ (PBCCO) is especially promising as a cathode for IT-SOFCs.

Studies on electrophoretic deposition of nanocrystalline SDC electrolyte films

Thin films of nanocrystalline samarium doped ceria (SDC) were electrophoretically deposited on stainless steel substrate using Sm 0.2Ce 0.8O 1 .9 nanopowder prepared by polyvinyl alcohol (PVA) assisted combustion method. The thermal behavior of the combusted powder was studied by thermogravimetric and differential thermal analysis (TG-DTA). Nanocrystallinity and phase purity of the calcinated SDC powder were confirmed from transmission electron microscope (TEM) and X-ray diffraction (XRD). In electrophoretic deposition (EPD), the parameters related to the suspension and deposition process were optimized in acetone. The film formation during deposition was found to be diffusion controlled process. The surface morphology of the deposited films and its cross-section were studied by scanning electron microscope (SEM). The preparation of nanocrystalline SDC electrolyte films from acetone-based suspension by EPD is reported for the first time in this paper.

Intermediate-temperature electrochemical performance of a polycrystalline PrBaCo 2O 5+ δ cathode on samarium-doped ceria electrolyte

A-site cation-ordered PrBaCo 2O 5+ δ (PrBC) double perovskite oxide was synthesized and evaluated as the cathode of an intermediate-temperature solid-oxide fuel cell (IT-SOFC) on a samarium-doped ceria (SDC) electrolyte. The phase reaction between PrBC and SDC was weak even at 1100 °C. The oxygen reduction mechanism was investigated by electrochemical impedance spectroscopy characterization. Over the intermediate-temperature range of 450–700 °C, the electrode polarization resistance was mainly contributed from oxygen-ion transfer through the electrode–electrolyte interface and electron charge transfer over the electrode surface. An area-specific resistance as low as ∼0.4 Ω cm 2 was measured at 600 °C in air, based on symmetric cell test. A thin-film SDC electrolyte fuel cell with PrBC cathode was fabricated which delivered attractive peak power densities of 620 and 165 mW cm −2 at 600 and 450 °C, respectively.

Low-temperature solid oxide fuel cells with La 1− xSr xMnO 3 as the cathodes

La 1− x Sr x MnO 3 (LSM) has been widely developed as the cathode material for high-temperature solid oxide fuel cells (SOFCs) due to its chemical and mechanical compatibilities with the electrolyte materials. However, its application to low-temperature SOFCs is limited since its electrochemical activity decreases substantially when the temperature is reduced. In this work, low-temperature SOFCs based on LSM cathodes are developed by coating nanoscale samaria-doped ceria (SDC) onto the porous electrodes to significantly increase the electrode activity of both cathodes and anodes. A peak power density of 0.46 W cm −2 and area specific interfacial polarization resistance of 0.36 Ω cm 2 are achieved at 600 °C for single cells consisting of Ni-SDC anodes, LSM cathodes, and SDC electrolytes. The cell performances are comparable with those obtained with cobalt-based cathodes such as Sm 0.5Sr 0.5CoO 3, and therefore encouraging in the development of low-temperature SOFCs with high reliability and durability.

Novel copper-based anodes for solid oxide fuel cells with samaria-doped ceria electrolyte

A novel copper-based anode for low-temperature solid oxide fuel cells was prepared through the conventional ceramic technology and using CuO and SDC (Ce 0.8Sm 0.2O 1.9) powders with controlled particle size. The new Cu–SDC anode also contained highly dispersed CeO 2 and Ni particles to increase its surface area and fuel cell performance. The specific surface area of the Cu–SDC bare anode, CeO 2 and Ni-dispersed phases were estimated to be 1.53, 39.4 and 86.4 m 2 g −1, respectively. Solid oxide fuel cells having the new anode were tested for both humid hydrogen and methane. Power densities of ca. 250 mW cm −2 were achieved in H 2 at 600 °C and in CH 4 700 °C, even if the SDC–electrolyte supporting membrane was 250-μm thick. Short term stability tests (maximum 64 h) showed an initial impairment, but not dramatic, of the new anode performance and the formation of carbon deposits. The addition of MoO x to the new anode did not prevent the formation of carbon deposits.

Characterization and electrochemical performances of Ba 2− xSr xFeO 4+ δ as a novel cathode material for intermediate-temperature solid oxide fuel cells

Novel cathode materials, Ba 2− x Sr x FeO 4+ δ ( x = 0.5, 0.6, 0.7, 0.8, 1.0), for intermediate-temperature solid oxide fuel cells on a samaria-doped ceria (SDC) electrolyte were prepared by the glycine–nitrate route and characterized by X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), thermogravimetric (TG) analysis, electrochemical impedance spectroscopy and steady-state polarization measurement. SEM results showed that the electrode formed a good contact with the SDC electrolyte after sintering at 1000 °C for 2 h. The value of δ in Ba 1.0Sr 1.0FeO 4+ δ materials was calculated from the TG results. The electrochemical impedance spectra revealed that Ba 2− x Sr x FeO 4+ δ had a better electrochemical performance than that of Ln 2NiO 4 (Ln = La, Pr, Nd, Sm). In the Ba 2− x Sr x FeO 4+ δ ( x = 0.5, 0.6, 0.7, 0.8, 1.0) family, the composition Ba 1.0Sr 1.0FeO 4+ δ exhibited the best electrochemical activity for oxygen reduction. The polarization resistance of Ba 1.0Sr 1.0FeO 4+ δ on SDC electrolyte was 1.11 Ω cm 2 at 700 °C, which was less than half that reported for Ln 2NiO 4 at the same temperature.

Spray Pyrolysis Deposition of Electrolyte and Anode for Metal-Supported Solid Oxide Fuel Cell

Metal-supported solid oxide fuel cells (SOFCs) offer many advantages, including increased robustness, improved thermal shock resistance, and decreased cost. However, fabricating metal-supported SOFCs using conventional techniques is both very difficult and very costly. In this study, two processes of spray pyrolysis deposition, pneumatic spray deposition and electrostatic spray deposition, were used to deposit samaria-doped ceria (SDC) electrolytes on different substrates and NiO–SDC anodes on porous stainless steel substrates. A cathode layer was subsequently applied on the electrolyte by stencil printing for electrochemical testing. The test results indicated that the electrolyte had reasonable cell performance, but the topography of the anode needed optimization. It was also discovered that the porous ferritic stainless steel 430 substrate used in this study did not have sufficient oxidation resistance as the substrate of a metal-supported SOFC.