Yttria-stabilized Zirconia Thin Film Electrolyte Research Articles

Nanocrystal Engineering of Thin-Film Yttria-Stabilized Zirconia Electrolytes for Low-Temperature Solid-Oxide Fuel Cells.

To overcome significantly sluggish oxygen-ion conduction in the electrolytes of low-temperature solid-oxide fuel cells (SOFCs), numerous researchers have devoted considerable effort to fabricating the electrolytes as thin as possible. However, thickness is not the only factor that affects the electrolyte performance; roughness, grain size, and internal film stress also play a role. In this study, yttria-stabilized zirconia (YSZ) was deposited via a reactive sputtering process to fabricate high-performance thin-film electrolytes. Various sputtering chamber pressures (5, 10, and 15 mTorr) were investigated to improve the electrolytes. As a result, high surface area, large grain size, and residual tensile stress were successfully obtained by increasing the sputtering pressure. To clarify the correlation between the microstructure and electrolyte performance, a YSZ thin-film electrolyte was applied to anodized aluminum oxide-supported SOFCs composed of conventional electrode materials which are Ni and Pt as the anode and the cathode, respectively. The thin-film SOFC with YSZ deposited at 15 mTorr exhibited the lowest ohmic resistance and, consequently, the highest maximum power density (493 mW/cm2) at 500 °C whose performance is more than five times higher than that of the cell with YSZ deposited at 5 mTorr (94.1 mW/cm2). Despite the basic electrode materials, exceptionally high performance at low operating temperature was achieved via controlling the single fabrication condition for the electrolyte.

Porous Nickel Based Half-Cell Solid Oxide Fuel Cell and Thin-Film Yttria-Stabilized Zirconia Electrolyte

In this work, a porous nickel anode for thin-film solid oxide fuel cell prepared by the simple powder hot-pressing method is investigated. Powders of Ni and pore-forming agent (PFA) were thoroughly mixed in different ratios, pressed in a mold and further sintered. The polishing technique with Yttria-Stabilized Zirconia (YSZ) powder has been developed to decrease the surface roughness of Ni-based anode in order to deposit a crack-free electrolyte layer. The 3 μm YSZ thin-film electrolyte was deposited by the pulsed laser deposition technique on the surface of the anode. Morphological and elemental analyses of the samples were characterized by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analyses. X-ray diffraction was used for phase analysis and structural characterization. The specific surface areas of the resulting anodes were calculated from their isotherms of N2 adsorption and desorption using the Sorbtometer and calculated by Brunauer Emmett-Teller (BET) method. As a result, the highest mechanical strength and specific surface area (15.42 m2g-1) possessed a sample with the content of PFA equal to 40%, while its ionic conductivity at 800 °C reached 6. 4∙10-2 S/cm.

Optimization of Y2O3 dopant concentration of yttria stabilized zirconia thin film electrolyte prepared by plasma enhanced atomic layer deposition for high performance thin film solid oxide fuel cells

In this study, the Y2O3 doping concentration of yttria stabilized zirconia (YSZ) thin film electrolyte deposited by plasma enhanced atomic layer deposition (PEALD) is optimized to maximize the performance of thin film solid oxide fuel cells (TF-SOFCs). The PEALD YSZ thin films are highly crystalline, and the Y2O3 concentration is controlled by changing the ratio between ZrO2 and Y2O3 ALD cycles. Electrochemical performances of TF-SOFCs are strongly dependent on the Y2O3 doping concentration in electrolytes. The cell with 10.7 mol% doped YSZ achieves the best performance (180 mW/cm2) at 450 °C due to decreased polarization loss because of its higher density of oxygen vacancies. These results demonstrate the effectiveness of PEALD process to deposit crystalline YSZ thin film electrolytes with optimal doping for high performance TF-SOFCs.

Yttria-stabilized zirconia thin film electrolyte deposited by EB-PVD on porous anode support for SOFC applications

Electron beam physical vapor deposition (EB-PVD) has been used to deposit thin film (YSZ) electrolytes on NiO-YSZ composite substrates pre-sintered at 1100°C, followed by further sintering at 1400°C in order to achieve dense films. Surface morphology of deposited layers has been investigated by AFM and FE-SEM. Increasing the deposition time and YSZ films thickness results in higher roughness and mean particle size of the thin film layer. FE-SEM observations of fresh fractured anodic half cells confirmed dense, crack-free, homogeneous and intensely adhered YSZ electrolyte layers. Sintering behavior of NiO and YSZ grains has been also investigated by the micrographs of cross sections of NiO-YSZ substrates.

Effect of internal and external constraints on sintering behavior of thin film electrolytes for solid oxide fuel cells (SOFCs)

The effect of internal and external constraints on the sintering behavior of yttria-stabilized zirconia (YSZ) thin film electrolytes is investigated for intermediate-temperature solid oxide fuel cells (SOFCs). The external constraints imposed by rigid substrates result in major process flaws, such as vertical and delamination cracks during film densification, in a chemical solution deposition (CSD) process. In this study, YSZ nanoparticles are incorporated into the precursor solution to impose internal constraints and control the shrinkage rate of the sol–gel matrix. Because the structural stability of the thin film is determined by the grain size distribution, it is critical to optimize the content and dispersion structure of the nanoparticles and the sintering temperature to obtain a gas-tight thin film electrolyte for SOFCs. A clustering model is proposed to obtain a comprehensive understanding of the sintering behavior of the YSZ thin-film electrolyte containing the nanoparticles in the CSD process.

Operational characteristics of thin film solid oxide fuel cells with ruthenium anode in natural gas

Direct utilization of hydrocarbons in low temperature solid oxide fuel cells is of growing interest in the landscape of alternative energy technologies. Here, we report on performance of self-supported micro-solid oxide fuel cells (μSOFCs) with ruthenium (Ru) nano-porous thin film anodes operating in natural gas and methane. The μSOFCs consist of 8 mol% yttria-stabilized zirconia thin film electrolytes, porous platinum cathodes and porous Ru anodes, and were tested with dry natural gas and methane as fuels and air as the oxidant. At 500 °C, comparable power densities of 410 mW cm−2 and 440 mW cm−2 were obtained with dry natural gas and methane, respectively. In weakly humidified natural gas, open circuit voltage of 0.95 V at 530 °C with peak power density of 800 mW cm−2 was realized. The μSOFC was continuously operated at constant voltage of 0.7 V with methane, where quasi-periodic oscillatory behavior of the performance was observed. Through post-operation XPS studies it was found that the oxidation state of Ru anode surfaces significantly differs depending on the fuel used, oxidation being enhanced with methane or natural gas. The nature of the oscillation is discussed based on the transition in surface oxygen coverage states and electro-catalytic activity of Ru anodes.

Nanostructured ruthenium – gadolinia-doped ceria composite anodes for thin film solid oxide fuel cells

Ruthenium and gadolinia-doped ceria (Ru–CGO) composite nano-crystalline thin film anodes are demonstrated for the first time in self-supported micro-solid oxide fuel cells (μSOFCs) for direct methane utilization, targeting development of microstructurally stable anodes under operation. Nano-composite thin film anodes are synthesized by co-sputtering from Ru metal and CGO oxide targets. Detailed electrical, crystalline and microstructural analyses were carried out on composite films to characterize properties as a function of Ru/CGO relative fraction. Microstructural stability of the composite film is compared to Ru metal film from conductivity and grain size behavior during 500°C thermal treatments. Stress-relaxed composite thin film anodes were developed on self-supported μSOFCs with yttria-stabilized zirconia thin film electrolytes and porous platinum cathodes. μSOFCs were tested with room temperature humidified methane as fuel and air as the oxidant, exhibited an open circuit voltage of 0.97V and a peak power density of 275mWcm−2 at 485°C. Microstructural stability of Ru–CGO composite anodes relative to metal electrodes is discussed. The results suggest a general approach to synthesis of functional metal-oxide nano-composite thin films for electrodes in fuel cells and related energy conversion devices.

Preparation and Characterization of Anode-Supported YSZ Thin Film Electrolyte by Co-Tape Casting and Co-Sintering Process

In this study, a co-tape casting and co-sintering process has been developed to prepare yttria-stabilized zirconia (YSZ) electrolyte films supported on Ni-YSZ anode substrates in order to substantially reduce the fabrication cost of solid oxide fuel cells (SOFC). Through proper control of the process, the anode/electrolyte bilayer structures with a size of 7.8cm × 7.8cm were achieved with good flatness. Scanning electron microscopy (SEM) observation indicated that the YSZ electrolyte film was about 16 μm in thickness, highly dense, crack free and well-bonded to the anode support. The electrochemical properties of the prepared anode-supported electrolyte film was evaluated in a button cell mode incorporating a (LaSr)MnO3-YSZ composite cathode. With humidified hydrogen as the fuel and stationary air as the oxidant, the cell demonstrated an open-circuit voltage of 1.081 V and a maximum power density of 1.01 W/cm2 at 800°C. The obtained results represent the important progress in the development of anode-supported intermediate temperature SOFC with reduced fabrication cost.

Yttria-stabilized zirconia thin film electrolyte produced by RF sputtering for solid oxide fuel cell applications

Thin film (40–600 nm) yttria-stabilized zirconia (YSZ) electrolytes for solid oxide fuel cells (SOFC) were deposited on NiO-YSZ anodes and fused silica substrates by RF sputtering, using low applied power without the use of post deposition annealing heat treatment. YSZ film showed a nanocrystalline structure and consisted of the Zr .85Y .15O 1.93 (fcc) phase. The film was dense and the YSZ/anode interface was continuous and crack free. According to preliminary in-plane conductivity measurements (temperature range 550–750 °C) on the YSZ film, the activation energy for ionic conduction was found to be 1.18 ± 0.01 eV.

Preparation of Yttria‐Stabilized Zirconia Thin Films on Strontium‐Doped LaMnO3 Cathode Substrates via Electrophoretic Deposition for Solid Oxide Fuel Cells

A yttria‐stabilized zirconia (YSZ) thin film on an La0.8Sr0.2MnO3 porous cathode substrate was prepared, using electrophoretic deposition (EPD) to fabricate a solid oxide fuel cell (SOFC). The electrical conductivity of an La0.8Sr0.2MnO3 substrate is satisfactorily high at room temperature; therefore, YSZ powder could be deposited electrophoretically onto an La0.8Sr0.2MnO3 substrate without any extra surface treatment, such as a metal coating. Successive repetition of EPD and sintering was required to obtain a film without gas leakage, because of the thermal expansion coefficient mismatch between the YSZ and the La0.8Sr0.2MnO3 substrate. On the other hand, the electromotive force of the oxygen concentration in the cell that used YSZ film prepared via EPD increased and attained the theoretical value when the number of deposition and calcination cycles was increased. Six or more successive repetitions were required to obtain a YSZ film without gas leakage. A planar‐type SOFC was fabricated, using nickel as the anode and YSZ film (∼10 μm thick) that had been deposited onto the La0.8Sr0.2MnO3 substrate as the electrolyte and cathode. The cell exhibited an open circuit voltage of 1.0 V and a maximum power density of 1.5 W/cm2. Thus, the EPD method could be used as a colloidal process to prepare YSZ thin‐film electrolytes for SOFCs.