Gd-doped Ceria Thin Films Research Articles

Properties and Performance of Electrolyte Supported SOFCs with EB-PVD Gd-Doped Ceria Thin-Films

High temperature solid oxide fuel cell (SOFC) is regarded as a unique electrochemical device to convert the chemical energy of a large diversity of fuels into electricity and heat with high efficiency and low emission. The electrolyte-supported cell (ESC) design is proven to be one of the most feasible SOFC technologies for commercial applications. Thickness reduction of yttria stabilized zirconia (YSZ) electrolyte supports without sacrifice of mechanical strength is favorable for cell performance enhancement.In this work, dense gadolinium-doped ceria (GDC) thin-films with thickness in the range of 0.15-0.5 µm were fabricated as Sr-barrier layers on the cathode side, and as strength-balance and adhesion layer on the anode side by electron beam physical vapor deposition (EB-PVD) method. Replacing the state-of-the-art screen-printed GDC layers, the approach with YSZ substrate sandwiched in between two GDC thin-films was proposed not only to prevent the formation of interfacial secondary phase on the cathode side, but also to increase the overall robustness of ESCs. Investigated by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX), it was confirmed that 0.5 µm thick GDC thin-films were effective to suppress the formation of Sr-zirconate phase at the YSZ/GDC interface. In comparison to reference cells with screen-printed GDC layers, higher breaking loads were recorded on the cells with PVD-GDC thin-films by Ring-On-Ring bending tests, showing considerably improved mechanical properties. Current-voltage characteristics and electrochemical impedance spectroscopy demonstrated that the performance of the as-prepared ESCs with GDC thin-films was comparable to the reference cells. Operated at 860 °C, the ESC with 0.5 µm GDC thin-film outperformed other tested cells with 0.15 µm or 0.3 µm GDC layers, and delivered a power density of 0.55 W•cm-2 at a cell voltage of 0.7 V. The results achieved in this work can be considered as step stones for the further development of ESCs with reduced electrolyte thickness, sufficient robustness and improved performance.This work is financially supported by the German Ministry for Economic Affairs and Energy (BMWi) within the framework of the project “Kostenoptimierter Stack und verbessertes Offgrid-System (KOSOS)” with grant number of 03ETB005C.

Properties and Performance of Electrolyte Supported SOFCs with EB-PVD Gd-Doped Ceria Thin-Films

In this work, dense gadolinium-doped ceria (GDC) thin-films with thickness of 0.15 and 0.5 µm were fabricated on 90 µm thick 3YSZ electrolytes by electron beam physical vapor deposition (EB-PVD), and integrated into electrolyte supported solid oxide cells (ESC) as Sr-barrier layer. Operated at 860°C, the ESC with 0.5 µm GDC thin-film outperformed the cells with 0.15 µm GDC layers and delivered a power density of 0.54 W•cm-2 at a cell voltage of 0.7 V. Post mortem investigations by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) confirmed that 0.5 µm thick GDC thin-films were effective to suppress the formation of the strontium zirconate phase at the YSZ/GDC interface.

Electro-chemo-mechanical effect in Gd-doped ceria thin films with a controlled orientation

Enhanced Gd-doped ceria electrostriction: This study uncovers new anisotropic properties in thin films, enhancing the effect along with the 〈100〉 directions by one order of magnitude and proposing a new microscopic model to explain such findings.

Low oxygen partial pressure increases grain boundary ion conductivity in Gd-doped ceria thin films

Grain boundaries play an important role in the transport properties of oxide ion conducting electrolytes and mixed ionic electronic conductors. Nevertheless, very little is known about the electrical grain boundary properties in thin films. In these, the separation of in-plane grain and grain boundary conductivity is more complicated due to the large capacitive effect of the insulating substrate. This can be overcome by using interdigitating electrodes with separation of few micrometres. By comparing grain and grain boundary conductivities of Gd-doped Ceria (GDC) thin films with 5 and 10 mol % Gd content, we can show that the much lower conductivity of 5% doped GDC is almost exclusively caused by a significantly higher grain boundary resistance. In reducing atmosphere, GDC becomes mixed ion and electron conducting and in such conditions, the employed Pt thin film electrodes are virtually blocking for oxygen anions and reversible for electrons. With impedance spectroscopy we can therefore simultaneously measure ionic and electronic conductivities under reducing conditions. Although the bulk vacancy concentration remains dominated by the extrinsic acceptor doping, the ionic conductivity of the films increases by up to one order of magnitude when going from oxidising to reducing atmosphere. This result is—although in such a clear manner not observed or predicted before—in line with the widely accepted grain boundary space charge model. It is concluded that an accumulation of Ce3+ in the space charge zone weakens the oxygen vacancy depletion and therefore increases the grain boundary conductivity. The results are of high relevance for understanding and optimising the properties of GDC in anodes and electrolytes for solid oxide fuel cells, and potential new uses such as electrostrictive and memristive devices, for which oxygen partial pressure dependent ionic conductivity is an important new aspect.

Characterization of Cerium Dioxide Thin Films Prepared By Plasma-Enhanced Atomic Layer Deposition for Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) are considered to be the next generation of energy conversion devices because of their environmental friendliness and high efficiency. Particularly due to its high operating temperature, it has high fuel flexibility and does not require water management. However, the high operating temperature that causes these advantages accelerates the degradation of fuel cell components, thereby reducing the durability and limiting the usable materials. Therefore, in recent years, a lot of research has been conducted to lower the operating temperature of the SOFCs. One method of lowering the operating temperature of SOFCs is to apply an electrolyte having good ionic conductivity at low temperatures(400-600℃). Potential examples are doped ceria materials. One of the well-known ceria-based electrolytes is Gd-doped ceria (GDC), which exhibits ion conductivity 1-2 times higher than yttria stabilized zirconia, YSZ, which is the most commonly used electrolyte in SOFC. GDC has good ionic conductivity, but when exposed to a reducing environment, Ce4+ is reduced to Ce3+ and begins to have electronic conductivity, which causes a decrease of open circuit voltage (OCV). Therefore, in order to apply GDC as an electrolyte, an additional blocking layer must be added so that it is not exposed to the hydrogen environment of the anode. This additional layer increases not only the thickness of the electrolyte but also the interfacial resistance between the layers, thereby increasing the ohmic resistance. Therefore, the fabrication of GDCs with thinner and better surface properties is critical to improving the performance of low temperature SOFCs. In this study, CeO2 thin film was fabricated using atomic layer deposition (ALD) as a preceding step for the fabrication of GDC thin films. Oxygen plasma was used to oxidize the precursor to widen the ALD window and to produce the highly crystalline CeO2. The growth rate of CeO2 was determined by controlling precursor and oxidizer exposure time, and the thickness of thin film per cycle was confirmed. The surface morphology was observed by scanning electron microscope (SEM), and dense and pinhole-free thin film surface was confirmed. In addition, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses were performed and the characteristic crystalline peaks and chemical compositions were compared with those of the thermal ALD CeO2 thin films.

NiO-GDC10/GDC10/LSCF-GDC10 Manufactured By Tape Casting and Reactive Magnetron Sputtering Processes of Solid Oxide Fuel Cells

We developed a cost–effective a method to manufacture performance planar solid oxide fuel cells using composite electrode by tape casting and electrolyte by reactive magnetron sputtering. NiO and GDC (10% Gd), used for the anode-supported (as) and the anode functional layer (afl) powders were milled by using zirconia balls in a Turbula-(System Schatz from WAB) times up to 24 hours. Half-cell, 28 mm of diameter, Ni-GDC composite anodes have been fabricated by slurry based tape casting (Elcometer4340 Automatic Film Applicator) method for anode supported solid oxide fuel cell. Depending on the results, work on the support anode (AS) Ni0-GDC will continue in particular to avoid the presence of the residual phase of carbon resulting from the use of an agent poro formers with graphite . The decomposition of organic additives and the shrinkage of green tape is analyzed thermo-gravimetric and differential thermal analysis (TG) of anode. PVD by plasma emission monitoring (PEM) was employed to synthesize GDC (10%Gd doped ceria) thin films on anode-grade ceramic substrate (porous NiO-GDC10). An Alcatel SCM650 sputtering chamber was used for synthesizing the dense GDC layers. A Ce-Gd metallic target (90-10% at) was powered by a pinnacle + pulsed current generator from Advanced Energy under reactive atmosphere.The cathode, which is made of (LSCF-GDC10) was screen printed onto electrolyte film and sintered to form a complete SOFC. The obtained cell after sintering was seen without any deformation or crack. Pycnometry and porosimetry techniques were primarily used to determine pore size and pore volume by equation Washburn and Ideal gas law-method and the decomposition of organic additives and the shrinkage of green tape was analyzed thermo-gravimetric of NiO-GDC10 and LSCF-GDC10. The microstructural morphological surface features of the cell was by 3D profilometry and complete cell were analyzed by Scanning Electron Microscopy (SEM), XRD and was employed to nos-destructively quantify the craking and behavior observed before and after different annealing. The measurements will be carried out on a complete cell with the measuring bench of the company Fiaxell of 5*5 cm2 and on a cell of 28 mm of diameter in order to obtain curves I / V.

Effect of oxygen partial pressure on the microstructural, optical and gas sensing characterization of nanostructured Gd doped ceria thin films deposited by pulsed laser deposition

Microstructural properties of 10 mol% gadolinium doped ceria (CeO2) thin films that were deposited on quartz substrate at substrate temperature of 1023 K by using pulsed laser deposition with different oxygen partial pressures in the range of 50–200 mTorr. The influence of oxygen partial pressure on microstructural, morphological, optical and gas sensing characterization of the thin films was systematically studied. The microstructure of the thin films was investigated using X-ray diffraction, atomic force microscopy and Raman spectroscopy. Morphological studies have been carried out using scanning electron microscope. The experimental results confirmed that the films were polycrystalline in nature with cubic fluorite structure. Optical properties of the thin films were examined using UV–vis spectrophotometer. The optical band gap calculated from Tauc’s relation. Gas sensing characterization has been carried at different operating temperatures (room temperature to 523 K) for acetone gas. Response and recovery times of the sensor were calculated using transient response plot.

Relaxation and saturation of electrostriction in 10 mol% Gd-doped ceria ceramics

10 mol% Gd-doped ceria (10GDC) ceramics, with grain size in the single micron range, display electrostrictive behavior under ambient conditions of temperature and pressure. In weak, quasi-static electric fields, i.e. <1 kV/cm, frequency <1 Hz, the longitudinal strain is measured to be proportional to the square of the applied electric field, albeit with the corresponding electrostrictive strain coefficient (M33) displaying large variability between samples: −(2-20)·10−17 (m/V)2. Nevertheless, |M33| of all samples exceeds the values expected on the basis of the classical (Newnham) electrostriction scaling law by up to two orders of magnitude. A systematic study reveals the functional dependence of M33 on frequency: above 10 Hz, |M33| decreases to ≈10−18 (m/V)2, which may be characterized as non-Debye relaxation with non-ideality factor 0.35–1.13. For frequencies ≤1.5 Hz, increasing the field strength beyond 1 kV/cm results in an exponential decrease in |M33|: the longitudinal strain saturates at 1-4 ppm. Dielectric impedance spectra suggest that partitioning of the applied voltage between grain boundaries and grain cores may be a factor contributing both to the large variability in the electrostriction parameters, and to the strong dependence on electric field amplitude. The frequency dependence may have two sources: the slow electric field-driven reorganization of the Ce-containing active complexes in the electrostrictive medium as well as the influence of the grain boundaries. 10GDC ceramics may therefore be added to the list of non-classical electrostrictors which includes reduced and Gd-doped ceria thin films and (Nb,Y)-doped bismuth oxide ceramics.

Effect of Gd-Doped Ceria Interlayer Microstructure on the Interdiffusion Behavior Between La0.6Sr0.4Co0.2Fe0.8O3-δ Cathodes and Yttria-Stabilized Zirconia Electrolytes

Formation of secondary phases at the interface between cathode and electrolyte is one of the issues affecting the long-term stability of solid oxide fuel cells. La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) perovskite cathode is known to react with yttria-stabilized zirconia (YSZ) forming SrZrO3 at the interface. To reduce secondary phase formation, a reaction barrier layer such as Gd-doped ceria (GDC) is inserted between LSCF and YSZ. However, the stability of the reaction barrier layer after long-term operation may be directly affected by the microstructure and cation diffusion through GDC. In this work, the microstructure of the reaction barrier layer is controlled by growing the GDC thin films on polycrystalline and single crystal YSZ (100) electrolytes using pulsed laser deposition technique. Electrochemical impedance spectroscopy (EIS) and microstructural analyses were performed on LSCF|GDC|YSZ|Pt cells with different GDC interlayers under an open-circuit condition at 800⁰C for 300 h in air. It is observed that SrZrO3 formed primarily at the LSCF/GDC interface of cells with polycrystalline GDC interlayers resulting to a rapid increase in area-specific resistance (ASR) with time. In the case of single-crystalline GDC interlayer, no apparent formation of SrZrO3 but a possible formation of GdFeO3 or LaMO3 (M= Co, Fe) at the LSCF/GDC interface was observed. EIS analysis of cells with single-crystalline GDC interlayer reveals a slow increase in the ASR with time. Microstructural evolution of the interfaces and type of secondary phase formation at the interface gives rise to a different rate of ASR and is shown to be directly related to the microstructure of GDC interlayers.

Effect of Gd-Doped Ceria Interlayer Microstructure on the Interdiffusion Behavior Between La0.6Sr0.4Co0.2Fe0.8O3-δ Cathodes and Yttria-Stabilized Zirconia Electrolytes

Formation of secondary phases at the interface between cathode and electrolyte is one of the issues affecting the long-term stability of solid oxide fuel cells. La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) perovskite cathode is known to react with yttria-stabilized zirconia (YSZ) forming SrZrO3 at the interface. To reduce secondary phase formation, a barrier layer such as Gd-doped ceria (GDC) is inserted between LSCF and YSZ. However, the stability of the interlayer may be directly affected by the microstructure and cation diffusion through GDC. In this work, the microstructure of the reaction barrier layer is controlled by growing the GDC thin films on polycrystalline and single crystal YSZ (100) electrolytes using pulsed laser deposition technique. Electrochemical impedance spectroscopy and microstructural analyses were performed on LSCF|GDC|YSZ|Pt cells with different GDC interlayers under an open-circuit condition at 800ºC for 300 h in air. It is observed that SrZrO3 formed at the LSCF/GDC interface of cells with polycrystalline GDC interlayers resulting in a rapid increase in area-specific resistance (ASR) with time. In the case of GDC on YSZ(100), the LSCF/GDC interface showed no SrZrO3. Instead, a significant interdiffusion between LSCF cathode and GDC interlayer occurred. The active diffusion of Gd into LSCF and possible formation of perovskite phases at the LSCF/GDC interface may have contributed to slow increase in the ASR with time. Microstructural changes and type of secondary phase formation at the LSCF/GDC interface give rise to a different rate of increase in ASR and is shown to be directly related to the microstructure of GDC interlayer.

Chemical solution deposition of pure and Gd-doped ceria thin films: Structural, morphological and optical properties

High quality smooth, uniform and crack-free ceria and gadolinium doped ceria (GDC) thin films were prepared on Si and Si/YSZ substrates by chemical solution deposition. The thermal behavior of Gd-Ce-O precursor was investigated by TG-DSC measurements. The phase purity and structure of deposited films were evaluated using X-ray diffraction (XRD) analysis and Raman spectroscopy. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were employed for the estimation of surface morphological features. Oxidation state of Ce ions in fabricated films was analyzed by X-ray photoelectron spectroscopy (XPS). Optical properties were evaluated by diffuse reflectance UV–vis spectrometry. Thickness of the films can be controlled by applying a certain number of spin coating cycles. A linear relation between the thickness of the films and the number of deposited layers was observed. The single-layer thickness was determined to be approximately 20nm. The influence of annealing temperature and Gd content on the film structure, morphology and optical properties was studied and discussed. The dependence of an optical band gap as a function of grain size was demonstrated.

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.

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.

Electrical conductivity of thin film ceria grown by pulsed laser deposition

Gd-doped ceria (GDC) thin films, 0.5–1.8 μm thick, were deposited on Pt and sapphire substrates by pulsed laser deposition (PLD) method. The in-plane and the perpendicular-to-plane conductivities (hereafter, “across-plane” conductivity) of thin films were measured and compared to that of bulk sample. X-ray diffraction and electron microscopy results showed that the films on Pt and sapphire were polycrystalline cubic with a columnar structure. The grain conductivities of the GDC films, obtained from the impedance spectra of in-plane and across-plane measurements, were slightly lower than that of the GDC bulk. However, very little difference in conductivity was shown between films, regardless of substrates and measurement modes. The activation energies and the P O 2 dependence of the GDC thin films were similar to that of bulk GDC. In the low temperature range, the GDC films exhibited conductivities about one order of magnitude higher than that of the YSZ (Yttria-stabilized zirconia) film.