Gd-doped Ceria Research Articles

Preparation and characterization of Ce1−xGdxO2−δ (x=0.1–0.3) as solid electrolyte for intermediate temperature SOFC

Abstract The effect of Gd doping on the ionic conductivity of CeO2 for its use as a solid electrolyte material for the intermediate temperature solid oxide fuel cells (IT-SOFCs) has been explored here. Ce1−xGdxO2−δ (x = 0.1–0.3) samples are successfully synthesized by carbonate co-precipitation method. XRD, FT-IR, Raman spectroscopy, UV–Vis spectroscopy, SEM and impedance spectroscopy are used for structural and electrical characterization. From the XRD patterns, well-crystallite cubic fluorite structured solid solution is confirmed. As Gd3+ ions are doped into CeO2 lattice, the absorption spectrum exhibits a red shift when compared to CeO2. Raman spectra show a peaks at 463 cm−1, 546 and 600 cm−1, which are the characteristic peaks of doped ceria. Based on ac-impedance data, the total ionic conductivity is same for both Ce0.9Gd0.1O2–δ (GDC10) and Ce0.8Gd0.2O2–δ (GDC20) in the temperature range 473 K–623 K. Hence for Gd doped ceria solutions 10% and 20% of Gd, are proper doping level which leads to the maximum total conductivity. Above this doping level, the mobility of oxygen ions decreases the total conductivity due to blocking effect.

Solution Combustion Synthesis: Role of Oxidant to Fuel Ratio on Powder Properties

Solution combustion synthesis technique is one of the novel techniques used to prepare nanoparticles, multi-component ceramic oxides and nanocomposites with properties better than conventionally prepared one and these materials have been used for various applications such as sensors, catalysts, and materials for solid oxide fuel cell (SOFCs). In the present work, the method has been used to prepare nanoparticles of 10 mol% Gd doped ceria (GDC) and Cu and its oxides. The oxidant to fuel (O/F) ratio is found to affect the powder properties and even compositional homogeneity. In glycine-nitrate combustion synthesis of GDC, as revealed by XRD studies, phase pure nanoparticles with crystallite size in the range 9-12nm were obtained for all the O/F ratios. TEM measurements of calcined powder showed hexagonal shaped particles of roughly 20nm size. The exothermicity was increased with the oxidant to fuel ratio resulting in high surface area and soft agglomerates. A slightly lean O/F ratio gives surface area of 73 m2/g and soft agglomerates (D50= 5.34 mm), which eventually results into high sintering density at low temperature. Raman Spectra of GDC showed a sharp and intense peak at 467 cm−1which corresponds to CeO2due to F2gsymmetry of the cubic phase. In combustion synthesis of copper nitrate and citirc acid, the compositional homogenity and phase purity was affected by the oxidant to fuel ratio. The combustion at stoichiometric O/F ratio gives Cu nano particles, lean O/F ratio gives nanoparticles of Cu, CuO and Cu2O and rich ratio gives pure CuO nanoparticles. These nanoparticles have been studied with different characterization techniques like XRD, TG-DTA, SEM, TEM, FT-IR and Raman.

Acceptor-doped ceria deposited on a porous Ni film as a possible micro-SOFC electrolyte

Micro-SOFCs, miniaturized solid oxide fuel cells (SOFCs) for low temperature operation, are being developed as a power source for portable electronics. Reducing the thickness of the electrolyte and the adoption of acceptor-doped ceria as an electrolyte material are important to minimize the Ohmic resistance at low temperature. Acceptor-doped ceria thin-films are often deposited on nano-porous metal substrates to reduce cracking of the thin electrolyte. However, due to the difficulty of depositing a pore-free electrolyte on a porous medium, the cells often show the low open circuit voltages (OCVs). In this study, we have deposited ∼1 μm-thick Gd-doped ceria on a nano-porous nickel film to assess whether a thin-film, metal-supported GDC can be deposited as a pore-free layer and would thus be suitable as an electrolyte of micro-SOFCs. The Ni-supported GDC cell showed an OCV of ∼0.92 V at 450 °C under a hydrogen/air gradient. The high OCV verifies that the thin-electrolyte layer, deposited on porous Ni using the pulsed laser deposition (PLD) method, is dense enough to prevent gas leakage as also observed in its microstructure.

Polycrystalline nanowires of gadolinium-doped ceria via random alignment mediated by supercritical carbon dioxide

This study proposes a seed/template-free method that affords high-purity semiconducting nanowires from nanoclusters, which act as basic building blocks for nanomaterials, under supercritical CO2 fluid. Polycrystalline nanowires of Gd-doped ceria (Gd-CeO2) were formed by CO2-mediated non-oriented attachment of the nanoclusters resulting from the dissociation of single-crystalline aggregates. The unique formation mechanism underlying this morphological transition may be exploited for the facile growth of high-purity polycrystalline nanowires.

Influence of the precursor pyrolysis temperature on the microstructure and conductivity of Gd-doped ceria materials

Abstract Gd-doped ceria nanopowders have been synthesized via a modified sol–gel technique using different pyrolysis temperatures to produce a range of particle sizes. Such nanocrystalline oxides have been sintered at 1400 °C for 24 h to produce fully dense disks. The microstructural characterization reveals that the pyrolysis temperature notably affects the grain size distribution in the sintered ceramics, e.g. powders treated at 700 °C render the narrowest grain size distribution. The electrochemical characterisation performed by electrochemical impedance spectroscopy shows that the distribution of grain sizes in the dense electrolytes rules the electrical conductivity of CGOs rather than the average grain size. Narrower grain size distributions render electrolytes exhibiting higher overall conductivity, independent of the average grain size.

Electrical conductivity of Gd-doped ceria film fabricated by aerosol deposition method

Gd-doped ceria (GDC) films were deposited on various substrates using aerosol deposition (AD) method in order to determine whether this process is useful for the fabrication of thin (~1μm) electrolyte layers. The AD method is attractive because no vacuum equipment or target material is required. Although the AD method utilizes pre-calcined powder for the deposition of film, the particle size and microstructure of films are largely dependent upon the choice of substrate (sapphire, glass, Si–Pt). The film deposited on sapphire substrate had a relatively dense microstructure and the smallest particle size. Total conductivity of film was measured using either in-plane or across-plane mode depending upon the substrate. Across-plane measurement gave a lower conductivity than in-plane measurement, possibly due to the difficulty of current collection or anisotropic microstructure. The magnitude of in-plane conductivity was comparable to the conductivity of a bulk sample, i.e., ~1.2×10−3S/cm at 450°C for the film deposited on sapphire. The activation energy of electronic conductivity at 300–450°C was also similar, ~2.23eV. However, the activation energy of ionic conductivity differed depending upon the substrates: ~0.71eV for the film on sapphire substrate and 0.93–0.94eV for other films. The Po2 dependence had a −1/4 slope, similar to that of bulk or film form of GDC at higher (>500°C) temperature. Overall, the AD method may be useful for the deposition of thin-film GDC since the conductivity was similar with that of film or bulk samples fabricated by conventional methods.

Fe-substituted SrTiO3−δ–Ce0.9Gd0.1O2 composite anodes for solid oxide fuel cells

A new composite solid oxide fuel cell anode material, SrTi1−xFexO3−δ mixed with Gd-doped ceria, was tested in La0.9Sr0.1Ga0.8Mg0.2O3−δ electrolyte-supported cells with La0.4Ce0.6O2 barrier layers and La0.6Sr0.4Fe0.8Co0.2O3 cathodes. The x = 0.7 composition had an anode polarization resistance of 0.17 Ω cm2, at 800 °C in humidified H2, much lower than the value 0.39 Ω cm2 measured for x = 0.4 and 3.14 Ω cm2 for x = 0. The reduced polarization resistance correlated with increased oxygen non-stoichiometry δ, which suggests that it may be related to increased ionic conductivity. Electrochemical impedance spectroscopy (EIS) measurements at 800 °C on the x = 0.7 anode cell showed a main response centered at ∼1 Hz that increased with decreasing hydrogen partial pressure. The maximum power density observed was for the x = 0.7 cell – 337 mW cm−2 at 800 °C in air and humidified hydrogen – and was limited mainly by the thick electrolyte.

Effect of Ca on the Properties of Gd-Doped Ceria for IT-SOFC

Ceria based electrolyte materials are very useful in intermediate-temperature solid oxide fuel cell (IT-SOFC). The compositions Ce0.85Gd0.15 - xCaxO2 - δ (x = 0.0 - 0.075) were prepared through sol-gel method. Their structure was studied by X-ray diffraction. Dense ceramic Ce0.85Gd0.15 - xCaxO2 - δ samples were prepared by sintering the pellets at 1300°C. The lattice parameter was calculated by Rietveld refinement of XRD patterns. Four probe A.C. impedance spectroscopy was used to study the total ionic conductivity of doped and co-doped ceria ceramics in the temperature range 200°C - 700°C. The Ce0.85Gd0.15 -xCaxO2 -δ composition showed maximum ionic conductivity with less activation energy.

Composite electrodes for ceria-carbonate intermediate temperature electrolytes

Composite electrodes based on several ceramic oxides were tested for their chemical compatibility and electrochemical performance in systems based on composite Gd-doped ceria+alkaline carbonate electrolytes. Reactivity tests performed at 700°C (accelerated testing conditions), involving the ceramic electrode materials and one mixture of Na and Li carbonates, identified two promising perovskyte-type ceramics, LaCoO3 and La0.8Sr0.2Co0.2Fe0.8O3, as the most stable. Co-pressed and co-sintered cell assemblies (electrode/electrolyte/electrode) involving composite electrodes (perovskytes mixed with alkaline carbonates) were studied in detail by scanning electron microscopy and tested by impedance spectroscopy in air in the temperature range 300–600°C. These tests showed the potential of these composite electrodes for intermediate temperature applications, clearly surpassing in performance standard Au electrodes, here used as reference.

Cryogel synthesis and solid state reactivity of NdCaCoO4

The synthesis of NdCaCoO4 precursors was performed by coprecipitation followed by freeze drying. The thermal decomposition of the freeze dried product at T=800°C results in the formation of finely grained NdCaCoO4 while at higher temperatures the appearance of perovskite-like (Nd,Ca)CoO3-δ among reaction products is observed. Reaction of NdCaCoO4 with Al2O3 and ZrO2 at 900–1000°C results in the complete disappearance of the initial phases while thermal processing of NdCaCoO4 mixtures with Gd-doped ceria or with MgO is not accompanied by the appearance of new reaction products. The doping of NdCaCoO4 with 2–10mass.% of MgO by means of cryoimpregnation method causes a substantial decrease in NdCaCoO4 grain growth rate at 1000°C and promoting its thermal stability.

Giant Electrostriction in Gd‐Doped Ceria

Gd-doped CeO(2) exhibits an anomalously large electrostriction effect generating stress that can reach 500 MPa. In situ XANES measurements indicate that the stress develops in response to the rearrangement of cerium-oxygen vacancy pairs. This mechanism is fundamentally different from that of materials currently in use and suggests that Gd-doped ceria is a representative of a new family of high-performance electromechanical materials.

Porous Gd-doped ceria barrier layer on solid oxide fuel cell with Sm0.5Sr0.5CoO3−δ Cathodes

Cobaltite based perovskites, such as Sm0.5Sr0.5CoO3-δ (SSC), are attractive as a cathode for solid oxide fuel cell (SOFC) due to their high electrochemical activity and electrical conductivity. In this study, nano-sized SSC powders were synthesized by a complex method. To prevent reactions between cathode and an yttria-stabilized zirconia (YSZ) electrolyte, we propose a Gd-doped ceria (GDC) porous barrier layer and formed barrier layers, screen-printed to 5, 10, and 15μm thicknesses. The performance of SOFCs without a GDC layer decrease by 23% from 0.78Wcm−2 to 0.6Wcm−2 after 12h of operation at 780°C. A SOFC with a screen-printed GDC of 5μm shows a performance decrease of only 4% from 0.75Wcm−2 to 0.72Wcm−2 after 12h of operation at 780°C. Fuel cell performance decreases with decreasing thickness of the porous GDC layer, which may result from the lowered SSC active area due to its reaction with the YSZ electrolyte. The increase in ohmic resistance due to a thicker porous GDC layer is mitigated by a decrease in ohmic resistance due to reduced reaction between the SSC and YSZ. To use SSC in SOFCs, a barrier layer should be applied to obtain grater fuel cell performance and stability.

Composite cathodes based on Sm0.5Sr0.5CoO3−δ with porous Gd-doped ceria barrier layers for solid oxide fuel cells

Abstract Cobaltite-based perovskites based on Sm0.5Sr0.5CoO3−δ (SSC) are attractive as a cathode material with a barrier layer for solid oxide fuel cells (SOFC) due to their high electrochemical activity and electrical conductivity. SSC, synthesized by a complex method, is used as a cathode material in a composite cathode with Gd-doped ceria (GDC). A porous GDC layer is fabricated as a barrier to resist reactions of SSC with yttria-stabilized zirconia (YSZ). The effects of the ratio of SSC on GDC in composite cathodes and the thickness of the GDC barrier are characterized in this study. An SOFC with an SSC7–GDC3 composite cathode on a 4 μm GDC layer at 0.8 V yields the highest fuel cell performance: 1.24 W cm−2 and 0.61 W cm−2 at 780 °C and 680 °C, respectively. Impedance analysis indicates that the ohmic resistances are more dependent upon the GDC barrier thickness than the cathode composition. The polarization resistances at 780 °C and 730 °C exhibit similar values, but with decreasing temperature, the polarization resistances change dramatically according to the composition and barrier thickness. The ohmic and polarization resistances show different trends in different temperature ranges, due to the different charge transfer mechanisms of SSC and GDC within those temperature ranges. To obtain higher fuel cell performance, the addition of GDC into the porous SSC is effective, and the compositions of the composite cathode as well as the thickness of the barrier layer need to be optimized.

Study of Raman peak shift under applied isostatic pressure in rare-earth-doped ceria for evaluation of quantitative stress conditions in SOFCs

We studied the relation between the Raman peak shift of the ceria F2g peak and isostatic pressure in rare-earth-doped ceria, which is used as interlayer in anode-supported SOFCs, to evaluate the stress in operational anode-supported SOFCs. First, 10 and 20mol% Sm- and Gd-doped ceria, and pure ceria were isostatically compressed up to 10GPa in a diamond anvil cell and the Raman spectra were measured at each pressure. Based on the results and reported elastic properties, the relation was retrieved for rare-earth-doped ceria under anisotropic stress. Although rare-earth-doped ceria showed different behavior than pure ceria, the difference due to the dopant and its concentration was small. The obtained ratio between anisotropic pressure and Raman peak shift in rare-earth-doped ceria was 0.441GPa/cm−1, and this value indicates that a change of 1GPa in anisotropic stress will shift the Raman peak by approximately 2.3cm−1. Hence, a Raman system, which can measure the Raman peak shift with an accuracy of 0.1cm−1, can recognize differences in stress conditions in the interlayer to an accuracy of approximately 40–50MPa.

Grazing incidence X-ray diffraction for the study of polycrystalline layers

Different diffraction patterns measured above and below the critical angle for total reflection were recorded on a Gd doped ceria layer deposited on a Si(100) substrate in asymmetric coplanar diffraction conditions. From the analysis of diffraction patterns, it was possible to point out peculiar optical aberrations responsible for the shifts and the broadenings of Bragg peaks induced by this setup. Corrections for all these aberrations were implanted in a Rietveld programme. Using these corrections, it becomes possible to follow the evolution of the structure (chemical composition, phases) and the micro structure (strain field, micro strain due to heterogeneities between grains, size of coherent diffracting domains) versus the depth on thin films.

Melting and superionic transition of Gd-doped ceria nanocrystals: Molecular dynamics study

Molecular dynamics program for simulation of nanocrystals in vacuum, with ionic model approximation, was developed. The use of graphics processor allows us to speed up calculation by two orders of magnitude in comparison with CPU realization. For Ce1−XGdXO2−X/2 system by fitting to experimental data pair interaction potentials were obtained, comparison with existing potential set was carried out. Melting temperature of CeO2 nanocrystal depends on its octahedron diagonal size as Тm(L,nm)=Тm(∞)−(5534±153)⋅L−2.Structural disorder of Ce1−XGdXO2−X/2 nanocrystals (X=0–0.3) in superionic transition and melting regimes was investigated. Dependences of melting temperature and specific volume change at melting from gadolinium dopant quantity have downward trend. It is shown, that oxygen sublattice disorder (“melting”) in crystallites takes place at about 0.7Tm. The contribution of intrinsic disorder to anion diffusion coefficient in systems with gadolinium dopant was extracted and the presence of superionic transition is shown. Comparison with high-temperature ionic conductivity data was carried out.

Influence of Composition of Gd-Doped Ceria Electrolyte on Performance of Solid Oxide Fuel Cell

Cell performance was measured for four types of Ni (40 vol%)-Gd-doped ceria (GDC) anode-supported solid oxide fuel cells with GDC electrolyte (40-120 μm thickness) of Ce1-xGdxO2-x/2 compositions (x = 0.05, 0.1, 0.15 and 0.2) at 773-1073 K using a H2 fuel. (La0.8Sr0.2)(Co0.8Fe0.2)O3 cathode was printed on the GDC films. The open circuit voltage and maximum power density at 873-1073 K showed a maximum at x = 0.1. The maximum power density at x = 0.1 was 166 and 506 mW/cm2 at 873 and 1073 K, respectively. The excess oxygen vacancy at x = 0.1-0.2, which does not contribute to the oxide ion conductivity, reacts with a H2 fuel to form electrons (H2 + VO 2H+ + VO×, VO× VO + 2e-). This reaction reduces the cell performance.

Electrochemical reforming of CH4−CO2 mixed gas using porous Gd-doped ceria electrolyte with Cu electrode

This paper reports on the composition and flow rate of outlet gas and current density during the reforming of CH4 with CO2 using three different electrochemical cells: cell A, with Ni−GDC (Gd-doped ceria: Ce0.8Gd0.2O1.9) cathode/porous GDC electrolyte/Cu−GDC anode, cell B, with Cu−GDC cathode/ porous GDC electrolyte/Cu−GDC anode and cell C, with Ru−GDC cathode/ porous GDC electrolyte/ Cu−GDC anode. In the cathode, CO2 reacts with supplied electrons to form CO fuel and O2− ions (CO2+2e−→CO+O2−). Too low affinity of Cu cathode to CO2 in cell B reduced the reactivity of the CO2 with electrons. The CO fuel, O2− ions and CH4 gas were transported to the anode through the porous GDC mixed conductor of O2− ions and electrons. In the anode, CH4 reacts with O2− ions to produce CO and H2 fuels (CH4+O2−→2H2+CO+2e−). The reforming efficiency at 700−800°C was lowest in cell B and highest in cell A. The Cu anode in cells A and C worked well to oxidize CH4 with O2− ions (2Cu+O2−→Cu2O+2e−, Cu2O+CH4→2Cu+CO+2H2). However, a blockage of the outlet gas occurred in all the cells at 700−800°C. The gas flow is inhibited due to a reduction in pore size in the cermet cathode, as well as sintering and grain growth of Cu metal in the anode during the reforming.

Upgrading the performance of La2Mo2O9-based solid oxide fuel cell under single chamber conditions

Various anode-supported solid oxide fuel cells (SOFC), based on 10 mol% Dy-doped La2Mo2O9 (LDM) electrolyte, are prepared analytically and operated under single chamber conditions to explore the connections between electrode and power performance. The cathode of tested SOFCs is compositionally graded with three composites of samarium strontium cobaltite and Gd-doped ceria (GDC) to relax the thermal stress, because of sizable thermal expansion differences above 400 °C. We focus the research attention on varying the anode pore structure and composition to promote the power performance in methane/air mixture at 700 °C. For the one-layer support of GDC+NiO+LDM anode, addition of 10 wt% graphite minimizes its mass transport resistance through creating 8–5 μm long and ∼1 μm wide slit-shaped pores. The graphite pore former raises the peak power value by 80 mW cm−2. Adopting a more porous and active outer layer, the double-layer support further enhances the cell power. The peak power was first raised by 48 mW cm−2, using an outer layer that was prepared with 63 wt% NiO. Dosing 3% Pd on this outer layer uplifts another 59 mW cm−2. In this study, with an improved anode, the peak power value reaches 437 mW cm−2.

Performance and long term stability of large area anode supported solid oxide fuel cells (SOFCs)

A long term stability test was conducted on a large area anode supported cell. The cell consisted of anode support, NiO (Nickel Oxide)+YSZ (Yttira Stabilized Zirconia); an anode functional layer, NiO+YSZ; electrolyte, YSZ; a cathode functional layer, LSCF (La0.6Sr0.4Co0.2Fe0.8O3-d)+GDC (Gd doped ceria); and a cathode current collector, LSCF. For a single cell stack test set-up, FeCr alloy end plates were machined for a co-flow of fuel and oxidant gases through internal manifolds and gas channels. The cell voltage was measured in a constant current mode (0.4A/cm2) for ~4400h at 750°C. During the long term test, I–V curve and impedance spectra were frequently measured to trace an increase in ohmic and non-ohmic resistance as a function of time. It was found that the dominant factor in the single cell stack degradation was an increase of non-ohmic resistance (activation polarization) rather than ohmic resistance. After the long term test, a post-material analysis was conducted by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), scanning Auger microscopy (SAM), energy dispersive spectroscopy (EDS), etc. The main degradation mechanism was determined to be the change of anode microstructure, which was caused by the high pressure of water vapor, especially at the gas outlet region.