Gd-doped Ceria Research Articles

Mutual Diffusion Occurring at the Interface between La0.6Sr0.4Co0.8Fe0.2O3 Cathode and Gd-doped Ceria Electrolyte during IT-SOFC Cell Preparation

The microstructure and local chemistry of the interface between the screen-printed La(0.6)Sr(0.4)Co(0.8)Fe(0.2)O(3) (LSCF) thin film cathode and Gd-doped ceria (GDC) electrolyte substrate have been investigated. Elemental distribution analyses, by energy-dispersive X-ray spectroscopy operated in scanning transmission electron microscopy (STEM) mode, illustrate that all constituent elements in GDC and LSCF mutually diffuse across the LSCF/GDC interface, with equal diffusion length. This leads to the formation of mutual diffusion zones at the LSCF/GDC interfaces, with the resultant mixture of diffusing ions being associated with specific valence state changes, as verified by STEM electron energy loss spectroscopy analyses. Moreover, this mutual diffusion can result in microstructural changes, where superstructure formation is accompanied by enhancement of oxygen vacancy ordering at this region. Such mutual diffusion and associated microstructure evolution is considered to be detrimental to fuel cell efficiency and should be suppressed by lowering cell fabrication temperatures.

Low temperature processed composite cathodes for Solid-oxide fuel Cells

Low temperature processed composite cathodes for solid-state fuel cell (SOFC) have been developed, consisting of La 0.6Sr 0.4Ti 0.1Fe 0.9O 3 and Ag, as low as 800 °C process temperature. Using micro-tubular design, the performances of the cathodes have been investigated at the operating temperatures of 650 and 700 °C. The cell consists of NiO–Y stabilized zirconia (YSZ) as an anode (support tube), Sc stabilized zirconia (ScSZ) as an electrolyte, Gd doped ceria (GDC) for an inter-layer between the electrolyte and the cathodes. The single performance has varied depending upon the types of cathodes, and the composite cathode with 50 wt% La 0.6Sr 0.4Ti 0.1Fe 0.9O 3 and 50 wt% Ag has shown comparable performance to the cell with standard LSCF–GDC cathode.

Water adsorption at the surface of pure and Gd-doped ceria

The presence of water at the surface of sputtered thin films of Ce 0.8Gd 0.2O 1.9, which display elastic anomalies as a function of thermal treatment, and of stoichiometric CeO 2 films, which do not, was monitored using X-ray photoelectron spectroscopy. We find that considerably more water is strongly bound at the surface of the Ce 0.8Gd 0.2O 1.9 films than of the CeO 2 films. This supports the theoretical prediction that water binds preferentially at the oxygen vacancy sites. In addition, all films were treated according to the protocol which has been shown to produce inelastic behavior in the doped films: annealed at 500 °C, exposed to ambient atmosphere for one month and then heated to 250 °C in ultra-high vacuum. Neither the Ce 0.8Gd 0.2O 1.9 nor the CeO 2 films show any change in the amount of adsorbed water. We therefore conclude that changes in the amount of surface adsorbed water do not play a role in the elastic anomalies observed as a function of thermal treatment of Ce 0.8Gd 0.2O 1.9 films.

Doped ceria powders prepared by spray pyrolysis for gas sensing applications

In the present study, ceria powder with and without gadolinium (Gd) or zirconium (Zr) dopants were synthesized by a spray pyrolysis (SP) process. The resulting powders (undoped CeO 2, Gd-doped ceria and Zr-doped ceria) were first mixed with organic binders, screen printed on alumina substrates, and then heat treated at 1200 °C for 2 h in air. Experimental results showed that the as-pyrolyzed powders were nanocrystalline and spherical in shape with uneven surfaces. After heat treatment, the powder coatings showed a 3-D network structure with interconnected pores exhibiting a high surface area. The electrical conductivity of CeO 2 was increased by the dope of Zr. The increasing rate of conductivity of ZDC (Zr-doped ceria) with decreasing oxygen pressure was also higher than that of undoped CeO 2. The CeO 2 and ZDC exhibited an n-type semiconductance in all the oxygen pressure regions, showing promise as candidates for sensor applications. The GDC (Gd-doped ceria) revealed a p-type and an n-type semiconductance in high- and low-oxygen pressure regions, respectively. The sensor applications of GDC are thus limited. Meanwhile, the ZDC exhibited a shorter response time due to its smaller grain size, showing a better oxygen sensing behavior.

Cobalt oxide co-doping effect on the sinterability and electrical conductivity of nano-crystalline Gd-doped ceria

Nano-crystalline 3 mol% CoO-doped GDC (Ce 0.9Gd 0.1O 1.95) powders of high sinterability at lower sintering temperature have been synthesized by deposition–precipitation method. The physical and chemical properties of the obtained powders were characterized by XRD, XPS, BET, EDS and TEM. From sintering studies, CoO-doped GDC can be fully densified (97% relative density) at a lower temperature of 850 °C. It has also been found that the sintered samples have a higher ionic conductivity of 2.38 × 10 −2 S cm −1 at 600 °C in air.

Growth of 20 mol% Gd-doped ceria thin films by RF reactive sputtering: The O 2/Ar flow ratio effect

The search for optimal materials and the utilization of proper manufacturing techniques to replace conventional electrolytes are our research objectives for the operation of solid oxide fuel cells under intermediate temperatures. Furthermore, understanding the effects of process parameters will be helpful for obtaining suitable materials for applications. In this study, we investigate the O 2/Ar flow ratio effect by employing RF reactive sputtering to fabricate 20 mol% Gd-doped ceria (20GDC) films on alumina substrates. The morphology of films was aggregated by nano-scale size of grains which gradually reduced in size from lower to higher O 2/Ar flow ratios. The microstructure of films was transferred from incomplete oxidized materials to well-crystallized cubic fluorite structures using an increased O 2/Ar flow ratio up to 0.30. The oxygen/metal ratio of films was increased gradually and saturated around 2.05 for O 2/Ar flow ratios over 0.25 and remained in uniform composition through whole films for each flow ratios.

High Performance Cathodes for Solid Oxide Fuel Cells Prepared by Infiltration of La0.6Sr0.4CoO3−δ into Gd-Doped Ceria

Cathodes prepared by infiltration of La0.6Sr0.4CoO3−δ (LSC40) into a porous Ce0.9Gd0.1O1.95 (CGO10) backbone have been developed for low temperature solid oxide fuel cells. The CGO10 backbone has been prepared by screen printing a CGO10 ink on both sides of a 180 μm dense CGO10 electrolyte-tape followed by firing. LSC40 was introduced into the CGO10 porous backbone by multiple infiltrations of aqueous nitrate solutions followed by firing at 350°C. A systematic study of the performance of the cathodes was performed by varying the CGO10 backbone firing temperature, the LSC40 firing temperature and the number of infiltrations. The cathode polarization resistance was measured using electrochemical impedance spectroscopy on symmetrical cells in ambient air, while the resulting structures were characterized by scanning electron microscopy (SEM) and high temperature X-ray diffraction (HT-XRD). The firing temperature of 600°C for the LSC40 infiltrate was found to provide a balance between LSC40 material formation and high surface area micro/nanostructure. The lowest polarization resistances measured at 600 and 400°C were 0.044 and 2.3 Ω cm2 in air, respectively. During degradation tests at 600°C, the cathode polarization resistance levels out after about 450 h of testing, giving a final polarization resistance of 0.07 Ω cm2.

Time-dependent performance changes in LSCF-infiltrated SOFC cathodes: The role of nano-particle coarsening

Symmetric solid oxide cells with Gd doped ceria (GDC) electrolytes and La 0.6Sr 0.4Co 0.2Fe 0.8O 3 – δ (LSCF)-infiltrated-GDC cathodes were maintained in air with no current at temperatures from 650 to 850 °C. Comparison of SEM images before and after aging showed inhomogeneous coarsening of LSCF nano-particles in the cathodes. The polarization resistance, measured periodically at 600 °C using electrochemical impedance spectroscopy, increased at a higher rate with increasing aging temperature. The resistance variation was fit using a model that relates resistance to time-dependent LSCF surface area, based on surface diffusion limited coarsening. Good agreement with the experimental data is obtained for a limited range of fitting parameters. The model predicts that the cathode resistance should increase from ~ 0.2 to 0.28–0.35 Ω cm 2, if maintained at the operating temperature of 600 °C for 40,000 h.

Direct evidence of dopant segregation in Gd-doped ceria

Microstructures and segregations of dopants and associated oxygen vacancies in gadolinium-doped ceria (GDC) have been characterized by high-resolution transmission electron microscopy (HRTEM) and scanning TEM (STEM). Diffuse scattering was detected in 25 at. % GDC (25GDC) in comparison to 10GDC, which is ascribed to nanodomain formation in 25GDC. HRTEM, dark-field, and STEM Z-contrast imaging investigations all provide direct evidence for dopant segregation in doped ceria. It is illustrated that dopant cations cannot only segregate in grain interior forming larger nanodomains but also at grain boundary forming smaller ones. Detailed analyses about nanodomain formation and related dopant segregation behaviors are then elucidated.

Processing and performance of solid oxide fuel cell with Gd-doped ceria electrolyte

Fuel Cell performance was measured at 792–1095 K for Ni-GDC (Gd-doped ceria) anode-supported GDC film (60 μm thickness) with a (La 0.8Sr 0.2)(Co 0.8Fe 0.2)O 3 cathode using H 2 fuel containing 3 vol% H 2O. A maximum power density, 436 mW/cm 2, was obtained at 1095 K. The electrical conductivity of GDC electrolyte in N 2 atmosphere of 10 −15–10 0 Pa oxygen partial pressures ( Po 2) at 773–1073 K was independent of Po 2, which indicated the diffusion of oxide ions. The conductivity of GDC in H 2O/H 2 atmosphere increased because of the further formation of electrons due to the dissociation of hydrogen in GDC (H 2 → 2H + + 2e −). The hole conductivity was observed at 873 K in Po 2 = 10 0–10 4 Pa. The key factors in increasing power density are the increase of open circuit voltage and the suppression of H 2 fuel dissolution in GDC electrolyte. These are controlled by the cathode material and Gd-dopant composition.

Synthesis and characterization of 10%Gd doped ceria (GDC) deposited on NiO-GDC anode-grade-ceramic substrate as half cell for IT-SOFC

In the present research work spray pyrolysis technique (SPT) is employed to synthesize GDC (10%Gd doped ceria) thin films on anode-grade-ceramic substrate (porous NiO-GDC). The film/substrate structure was characterized for their micro-structural and electrical properties along with their interfacial-quality. By optimization of preparative parameters of SPT and modification of surface of anode-grade ceramic substrate, we were able to prepare the GDC films having thickness of the order of 13 μm on NiO-GDC substrate. Further to improve the interfacial quality and densification of film, annealing of structure at 1000 °C for 8 h was carried out which leads to fully dense (>96%) GDC films, forming a gas-tight interface with substrate. Impedance measurements revealed that grain interior conductivity for GDC/NiO-GDC half-cell was of the order of 0.1S/cm at 500 °C which is the desired conductivity for successful operation of IT-SOFC. The activation energy for grain interior and grain-boundary conduction estimated for GDC/NiO-GDC was 1.07 eV and 0.93 eV, respectively.

First-principles Study of Defect Migration in RE-doped Ceria (RE = Pr, Gd)

ABSTRACTOxygen vacancy formation and migration in ceria is central to its performance as an ionic conductor. Ceria doped with suitable aliovalent dopants has enhanced oxygen ion conductivity – higher than that of yttria stabilized zirconia (YSZ), the most widely used electrolyte material in solid oxide fuel cells (SOFC). To gain insight into atomic defect migration in this class of promising electrolyte materials, we have performed total energy calculations within the framework of density functional theory (DFT+U) to study oxygen vacancy migration in ceria, Pr-doped ceria (PDC) and Gd-doped ceria (GDC). We report activation energies for various oxygen vacancy migration pathways in PDC and GDC. Results pertaining to the preferred oxygen vacancy formation sites and migration pathways in these materials will be discussed in detail. Overall, the presence of Pr and Gd ions significantly affects oxygen vacancy formation and migration, in a complex manner requiring the investigation of many different migration events. We propose a relationship that explains the role of additional dopants in lowering the activation energy for vacancy migration in PDC and GDC.

Reforming of biogas using electrochemical cell

This review reports the recent research results of solid oxide fuel cells using biogas (typical composition, 60% CH4–40% CO2), reforming of CH4 with CO2 (CH4 + CO2 → 2CO + 2H2) and shift reaction of CO with H2O (CO + H2O → H2 + CO2). Solid oxide fuel cells using dense yttria-stabilized zirconia and gadolinium-doped ceria electrolytes were operated with biogas containing CH4 of 7–63 vol % at 630–875°C. A maximum power density of 20–160 mW/cm2 was measured and this value was 1/10–1/3 of that for the cells with a H2 fuel. The reforming of CH4 with CO2 proceeds over a Ni-based catalyst at 700–900°C to produce a H2–CO fuel. The carbon deposition due to the pyrolysis of CH4 as a parallel reaction is suppressed by addition of second phase such as K2O or Ru to Ni catalyst. Electrochemical reforming of CH4 with CO2 using a porous Gd-doped ceria (GDC) cell is an attractive process to produce a H2–CO fuel at 400–800°C. The supplied CO2 is changed to CO and O2− ions by the reaction with electrons at the cathode (CO2 + 2e− → CO + O2−). The produced CO and O2− ions are transported to the anode through a porous mixed conductor GDC film. In the anode CH4 reacts with O2− ions to produce CO, H2 and electrons (CH4 + O2− → CO + 2H2 + 2e−). This process suppresses the carbon deposition from CH4. Ni in the anode accelerated the pyrolysis of CH4 but Ru promoted the oxidation of CH4 with transported O2− ions. Ni and Ru in the cathode worked well to change CO2 to CO plus O2−. Furthermore, shift reaction of reformed gas with H2O vapor proceeded to produce a H2 fuel using a porous GDC electrochemical cell with Co2O3 catalyst at 400–500°C.

A DFT+U study of defect association and oxygen migration in samarium-doped ceria

A specialized genetic algorithm (GA) is used to search the structural space of samarium-doped ceria (SDC) for the most energetically stable configurations which will predominate in low temperature fuel cells. A systematic investigation of all configurations of 3.2% SDC and a GA investigation of 6.6% SDC are presented for the first time at the DFT+U level of theory. It was found that Sm atoms prefer to occupy the nearest neighbor (NN) position relative to the oxygen vacancy at 3.2%, while at 6.6%, a balance exists between various Sm-vacancy interactions and the vacancies prefer to be separated by ∼6 Å. Also, the migration barriers for oxygen diffusion are calculated amongst the best structures in 3.2% and 6.6% SDC and are found to be comparable to the barriers in Gd-doped ceria at the DFT+U level of theory. While the migration calculations provide insight on the oxygen diffusion mechanism in this material, the favored configurations from our GA enable future research on concentrated SDC and contribute to the atomistic understanding of the influence of dopant-vacancy and vacancy-vacancy interactions on ionic conductivity.

Electrochemical reforming of CH4-CO2 mixed gas using porous yttria-stabilized zirconia cell

Flow rate and compositions of outlet gas and carbon deposition in the cell were studied upon the electrochemical reforming of 50% CH4–50% CO2 mixed gas using a cell of Ru–yttria-stabilized zirconia (YSZ) anode/porous 8 mol %-YSZ electrolyte/Ni–YSZ cathode system at 400–800°C under 1.0 V/cm of electric field strength. The supplied mixed gas of 95–97% changed to H2 and CO fuels at 800°C (CH4 + CO2 → 2H2 + 2CO). As a parallel reaction, pyrolysis of CH4 occurred over a Ni cathode to produce carbon powder in the cell (CH4 → C + 2H2), which decreased the flow rate of outlet gas at a high temperature. The above results were compared with the previously measured results with a porous Gd-doped ceria cell (mixed conductor of O2− ions and electrons). In the GDC cell, a part of deposited carbon was removed as CO gas by the reaction with O2− ions formed from CO2 gas in the cathode (CO2 + 2e− → CO + O2−, C + O2− → CO + 2e−) and the electrons produced were transported to the anode through the mixed conductor electrolyte film.

Correlation between microstructure and electrical conductivity in composite electrolytes containing Gd-doped ceria and Gd-doped barium cerate

Composite electrolytes with nominal compositions, Ce 0.8Gd 0.2O 1.9 + xBaO ( x = 0.2 and 0.3), have been synthesized through the citrate route. Formation of two phases, namely Gd-doped ceria and Gd-doped barium cerate, has been confirmed through XRD and SEM studies. The impedance spectra show three distinct semi-circles, all originating from the composite electrolytes. In the temperature range 175–350 °C, the activation energies for the conductivity values extracted from the high frequency and intermediate frequency parts of the impedance spectra remains the same, irrespective of compositional and micro-structural variation. On the other hand, the activation energies for the conductivity values associated with the low frequency impedance spectra show a significant change with micro-structural variation. Solid oxide fuel cells constructed using these composite electrolytes exhibit a higher open circuit voltage compared to those based on single phase 20 mol% Gd-doped ceria.

Electrochemical characterizations of microtubular solid oxide fuel cells under a long-term testing at intermediate temperature operation

We report the long-term stability of a microtubular solid oxide fuel cell (SOFC) operable at ∼500°C. The SOFC consists of NiO–Gd doped ceria (GDC) as the anode as well as the tubular support, GDC as an electrolyte and La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF)–GDC as the cathode. A single tubular cell with a diameter of approximately 1.8mm and an effective electrode length of approximately 20mm generated 150mWcm−2 and 340mWcm−2 at 500°C and 550°C, respectively, under the operation conditions of 0.7V and humidified H2 fuel flow. The cell exhibited good stability with a degradation rate of 0.25%/100h under operation conditions of 200mA and 0.75V.

Tensile Stress Evolution During the Early-Stage Constrained Sintering of Gadolinium-Doped Ceria Films

In situ measurements during the constrained sintering of Gd-doped ceria reveal tensile stresses up to ∼250 MPa. These large tensile stresses are likely to contribute to the reduced densification (compared with freely sintered material) typically observed during constrained sintering. While existing models postulate that the tensile stress in a densifying constrained film cannot exceed the “sintering stress,”σS, the observed tensile stresses are significantly larger than the estimated σS for these materials. To explain this observation, we propose that the formation and extension of interparticle grain boundaries induce substantial tensile stresses in constrained films. A model of this phenomenon shows that converting excess surface energy to elastic strain energy can produce stresses that are comparable to the measured values. Further, if these “cohesive” stresses exceed σS, grain-boundary diffusion should initially move material from the neck regions into the grain boundaries, not out of the grain boundaries as described by traditional sintering models.

La2Mo2O9-Based Electrolyte: Ion Conductivity and Anode-Supported Cell under Single Chamber Conditions

We demonstrate the applicability of LAMOX as an electrolyte for solid oxide fuel cells (SOFC) by preparing two anode-supported cells based on La0.9Dy0.1Mo2O9 and La0.9Dy0.1Mo1W1O9, and operating at 625°–700°C under single chamber conditions. The two cells, supported by a composite of NiO and Gd-doped ceria (GDC), are initialized with a porous Ni+GDC disk to catalyze the partial oxidation reaction of methane. The partially oxidized methane reduces NiO to Ni catalyst and activates the cell at 650°C. A typical membrane electrode assembly of the single chamber SOFC cell consists of a 60-μm-thick La0.9Dy0.1Mo2O9 electrolyte, an anode of Ni+GDC, and a cathode of lanthanum strontium cobalt ferrite. Its peak power measures 220 mW/cm2 at 700°C in a tubular chamber of 15 mm in diameter, with the total methane/air flow rate of 350 mL/min and CH4:O2=2:1.

Effect of electrolyte thickness on the performance of anode-supported ceria cells

Anode-supported ceria cells are promising as SOFCs (solid oxide fuel cells) to be used at intermediate temperatures. However, their performance varies widely depending on the experimental conditions due possibly to the partial electronic conductivity of the acceptor-doped ceria electrolyte that is observed in reducing atmosphere. In this study, the effect of electrolyte thickness on the performance of anode-supported ceria cells was studied. 20 mol% Gd-doped ceria (Gd 0.2Ce 0.8O 2 − δ , GDC20) electrolyte films were coated on the porous Ni-GDC20 anode supports by screen printing and co-firing. BSCF (Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3 − δ ) was used for the cathode. The thickness of the GDC20 electrolyte was found to have a significant effect on the OCV (open circuit voltage), impedance spectra, I–V (current–voltage) and I–P (current–power) curves. When the thickness of the electrolyte decreased from 40 μm to 12 μm in the anode-supported cell, the maximum power densities (P max) increased while the OCV values decreased. The OCV and P max values of the cell with 12 μm of GDC20 electrolyte were 0.772 V and 1.16 Wcm − 2 , respectively, when 97% H 2 + 3% H 2O was used as fuel and open air was used as the oxidant gas at 600 °C. Although a thin electrolyte layer reduces the OCV value, the reduced Ohmic and polarization resistance are advantageous for the cell performance in the tested thickness range because they increased the power density of cell.