Material Study of Micromachined Thin Film Solid Oxide Fuel Cells

Miniaturized, low temperature Solid Oxide Fuel Cells (SOFCs) that operate below 600 °C are promising for supplying electrical power to portable devices. Thin electrolyte membranes of less than one micrometer thickness have been shown to yield reasonable power densities. It remains, however, challenging to produce membranes with a high performance over a long time. While ohmic losses in the electrolyte can be reduced by reducing its thickness, polarization losses in cathode and anode layers are more difficult to get under control. It is generally found the oxygen reduction reactions (ORR) on the cathode are limiting the performance. The goal of this thesis is to realize the membrane fabrication based on silicon micromachining technology and improve the performance of micro-SOFC devices by studying the relevant properties of the materials in positive electrode-electrolyte-negative electrode (PEN) elements of generic cell configuration. Yttria-stabilized zirconia (YSZ) is the most favored material for the electrolyte in devices of all scales. It potentially meets the requirements for micro-SOFC's as the YSZ film thickness can be scaled down to optimal thickness, even though the membrane becomes very fragile. In this work, sputter deposition was used for YSZ thin films. This process leads to a considerable residual stress that is not compatible with thermal misfit strain, but must be assigned to defects created during sputter deposition or cool down in the sputter chamber. This stress can be a major factor to destabilize the membrane during operation. Indeed, a thermal post processing treatment was necessary for achieving YSZ films with thermal mismatch stress only. We thus studied the impact of deposition conditions and evaluated the impact on film microstructure and morphology, electronic conductivity and mechanical stress. The stress evolution with temperature demonstrates that the residual stress strongly correlated with oxygen gas pressure during deposition and cool down. The compressive stress of as-deposited YSZ films was relaxed by cooling in oxygen atmosphere. The excessive compressive stress is irreversibly removed by annealing at 600°C in air. The relaxation sets in at about 400°C. This phenomenon could be explained by the mobility of additional oxygen vacancies in the edge dislocation core. Therefore, excessive stress could be avoided by tuning deposition conditions and post-treatment. Durable electrolyte membranes were obtained by such processes. Moreover, PEN operation revealed a high concentration of electrochemically active sites on their surface. Since the polarization loss on the cathode is a key issue in cell performance limitation, we studied the simplest geometry with a defined, circular TPB line and a strong catalytic material like platinum as cathode metal in combination with 200 µm diameter electrolyte membranes obtained by silicon micromachining. The anode function was provided by a ceria doped gadolinia (CGO) layer, which is known to be electrically conductive at the reducing fuel gas atmosphere (one atmosphere of 25 % H2/ 75 % Ar). To our knowledge, we were the first to measure directly the current and power density per TPB length. An open circuit voltage of 0.93 V and an ionic current density per length of TPB of 1.24 mAm-1 were measured at 450°C at maximal power. Considering literature values for oxygen diffusion activation on Pt, the active zone supplying atomic oxygen to the triple phase boundary was calculated to be 22 nm wide at most. The active zone of metal is critical for designing and optimizing the microstructure of the composite electrodes with electronic and ionic conducting phases. Nano-porous Pt films thus look as ideal candidates for cathode layers. However, Pt films showed a strong dewetting on YSZ surfaces. We studied the possibility to stabilize a porous Pt structure within a composite film in order to obtain a temperature stable and high catalysis metal/ ionic conductor composite. The investigated materials cover combinations of (Pt, IrO2, Ni)-(YSZ, CGO). Nano-structured electrode films with metal nano-phases and amorphous/nano-crystalline oxide phases were designed and fabricated using a multi-source sputtering system. Metal formation and the presence of nano-phases in an amorphous oxide phase were confirmed by x-ray diffraction and transmission electron microscopy, respectively. The nano-structured composite electrodes showed improved electrochemical performance in oxygen reduction because of the presence of the metal nano-phases leading to longer TPB lines as compared to pure Pt electrodes. However, upon heating, segregation of metal phase in the composite was observed. Pt based composites in addition have shown strong compressive stresses, which promote recrystallization for stress release even more. It has been found that the microstructure and the amount of the metal phase are critical to conductivity. The segregation of IrO2 in the composite was also observed in the IrO2-YSZ and IrO2-CGO composite thin films. IrO2 based composites showed clearly less dewetting phenomena than Pt based composites. The Pt versions integrated onto electrolyte membranes showed all a too small in-plane conductivity to be efficient as PEN cathodes. The cells with a Pt-YSZ composite cathode layer covering the complete YSZ/CGO membrane showed that the cell peak power density was increased by a factor of 5 to 5 mWcm-2 as compared to the ring shaped Pt electrode at the border of the membrane. The open circuit voltage dropped to 0.68 V at 450°C, most likely by increased leakage either through the PEN structure, or through a path outside of the cell. Even though the Pt-YSZ composite film reveals improvements, it cannot be exploited as a good cathode to achieve a sufficient TPB lines due to Pt grains re-crystallization leading to a loss of electrical connectivity. The tested composite cathode layers were thus inadequate to contact the complete membrane area, leading to a too large area specific resistance (ASR) in the interior of the cell.

Experimental study of single triple-phase-boundary and platinum-yttria stabilized zirconia composite as cathodes for micro-solid oxide fuel cells

Janine Conde, Paul Muralt, Silviu Cosmin Sandu, Yan Yan

Micro-solid oxide fuel cell (mu SOFC) structures with annular Pt electrodes have been fabricated by means of silicon micromachining. The annular cathodes contained a defined triple phase boundary (TPB) length, which allowed the derivation of the ionic current per length of TPB as 1.24 mA m(-1), measured at 450 degrees C at maximal power. Considering literature values for oxygen diffusion activation on Pt, the active zone supplying atomic oxygen to the triple phase boundary was calculated to be 22 nm wide at most. The anode function was provided by a CGO layer, known to be electrically conductive at reducing conditions, with annular Pt current collector. The TPB length was increased by adding a Pt-YSZ composite cathode layer covering the complete YSZ/CGO membrane. The peak power density was increased by a factor of 5 to reach 5 mW cm(-2) with an open circuit voltage of 0.68V at 450 degrees C. It was observed that the Pt grains re-crystallized to large grains, leading to a loss of electrical connectivity in the composite layer. The composite cathode layer was thus inadequate to contact the complete membrane area, leading to a too large area specific resistance (ASR) in the interior of the cell. (C) 2012 Elsevier B.V. All rights reserved.

2012

Free carriers in nanocrystalline titanium dioxide thin films

Simon Springer

This thesis analyzes the properties of heavily doped nanocrystalline titanium dioxide. Thin films were deposited by magnetron sputtering with either water vapor as reactive gas or a periodically interrupted oxygen supply. The samples were at the same time electrically conducting and transparent. They consisted of mixtures of rutile, anatase and amorphous phases. Titanium dioxide is chemically stable, hard, non-toxic, transparent, and inexpensive. Due to a high refractive index it is often found in optical applications as a thin film coating. For many purposes it is interesting to take advantage of a combination of good transparency and electrical conductivity. An even larger field of applications would open if the electric conductivity of TiO2 could be increased in a controlled manner. Conventional bulk doping is limited, however, by low solubility limits. Highly conducting nanocrystalline TiO2 thin films have been obtained recently by sputtering in presence of water vapor. The present project intends to elucidate the doping mechanisms at work in such films. In addition, an alternative process to achieve grain-boundary doping of TiO2 was explored. Spectrophotometry over a wide spectral range (0.05 to 5 eV) was the main tool in probing the mobile electrical charge carriers. The interpretation of these optical measurements required appropriate models. The models developed took many structural properties into account, such as thickness, surface roughness, density, anisotropy and crystalline phase composition. The phase and morphology of the films were analyzed by X-ray diffraction and reflection, scanning probe microscopy, and electron microscopy. The chemical analyses included electron-probe microanalyses, Rutherford backscattering and elastic recoil detection analysis. The DC electrical properties were measured between room temperature and 250 °C. Most water-deposited samples showed semiconducting behavior with an activation energy of the order of 100 meV. The optical measurements revealed a broad absorption band around 1 eV. The absorption coefficient scaled with conductivity. It was successfully modelled as a Lorentzian. Similar absorption bands have been reported for reduced rutile and anatase single crystals. Water-deposited samples containing anatase exhibited a metallic behavior. In these samples the thermal activation energy was reduced to about 30 meV and the charge carriers displayed a high mobility (3 cm2V-1s-1). The infrared absorption band was about 10 times less important than in the samples devoid of anatase. Thin films of TiO2 intercalated with the equivalent of nanometer-thick layers of Ti were prepared. By changing the period in the oxygen supply the grain size, and thereby the film properties, could be modified. The films obtained in this way had many properties in common with water-deposited films. They showed the same absorption band in the infrared, had comparable charge carrier densities and were electrically conductive (102 to 104 Sm-1 at room temperature). To our knowledge, this was the first time that such high electrical conductivities were achieved by sputtering in an oxygen plasma.

EPFL2004

Microfabricated solid oxide fuel cells

Samuel Rey-Mermet

Micro-fabricated solid oxide fuel cells (µSOFCs) are finding an increasing interest as potential power sources for portable devices such as MP3 players or laptops. The aim of this work was to fabricate a µSOFC demonstrator that works at 500°C and is fuelled by hydrogen. This thesis was divided into two parts. The first one was devoted to the development of an electrolyte and electrodes in form of sputtered thin films with electrical and mechanical properties suitable for the implementation in a real cell. YSZ and CGO electrolyte have been reactively sputtered from metallic targets. Both films are dense and have a columnar microstructure. The ionic conductivity of these films was of 0.5 S/m at 550°C for the CGO and of 5.5 x 10-3 S/m at 500°C for the yttria stabilized zirconia (YSZ) . Albeit the ceria doped gadolinia (CGO) was a better ionic conductor at low temperature, it was not possible to obtain an open circuit voltage (OCV) with a CGO electrolyte film. Most likely, the reduction of the Ce+4 ion into Ce+3 in a hydrogen atmosphere creates an electrical leakage. Better results were obtained with YSZ layers. Single, (111) textured columnar films showed OCV's of 200 mV. Best results were obtained with a double layer of two different microstructures. The first one exhibited a dense, columnar microstructure with (111) texture. The second layer was porous with nanocrystalline grains and preferential (200) orientation. The improved properties are ascribed to the absence of film crossing grain boundaries. Of special interest is the mechanical stress behaviour upon heating to the operation temperature. The stress was investigated as a function of temperature up to 700°C. An anomalous, hysteretic behaviour was found during the first heating cycle in YSZ as well as CGO thin films. This phenomenon could be modelled as an oxygen uptake to fill up excess oxygen vacancies created during the sputtering process. The model allowed to derive a diffusion activation energy of 0.6 eV for these excess vacancies in YSZ. Annealing in air at 700 °C permits to reduce stress and to stabilize the YSZ membrane. As electrode materials, sputter deposited, porous platinum, porous Ni-CGO composites and dense LaxSr1-xCoO3-y (LSCO) thin films were developed and characterized. The PEN (Positive electrode-Electrolyte-Negative electrode) layer processes were combined with Micro Electro Mechanical System (MEMS) process technology to fabricate µSOFC test devices. The PEN membranes were liberated by deep silicon dry etching. The cell diameter was varied between 0.5 and 5 mm, the electrolyte thickness between 500 and 700 nm. A nickel grid grown by electroplating was used to support the electrolyte layer and to serves as current collector for the anode. The cell with a 5 mm diameter shows a very good mechanical stability up to 600°C in SOFC operating conditions and for several heating cycles. The functionality of the fuel cell with two 20 nm thick porous platinum electrodes and a YSZ bilayer electrolyte (500 nm) has been demonstrated. An OCV of 850 mV was measured at 500°C with hydrogen fuel. Unfortunately, a too high cathode contact resistance reduced the current to very low values. The achieved maximal power density was only 19 µW/cm2. A simple design change should remedy the problem.