Degree Sign Research Articles

Gas Phase Deposition and Characterization of (Si)-B-C Ceramics

B-C and Si-B-C coatings have been prepared by chemical vapour deposition (CVD) respectively from CH4/BCl3/H2 and MTS/BCl3/H2 precursor mixtures at low temperature (800-1050{degree sign}C) and reduced pressure (12kPa). The kinetic laws (including apparent activation energy and reaction orders) related to the deposition process were determined within the regime controlled by chemical reactions. A wide range of coatings, prepared in various CVD conditions, were characterized in terms of morphology (SEM), structure (TEM, Raman spectroscopy) and elemental composition (Auger electron spectroscopy), in the as-deposited state and after heat treatment. A correlation between the various experimental approaches has supported a discussion on the chemical steps involved in the deposition process.

Interface Resistance between FeCr Interconnects and La0.85Sr0.15Mn1.1O3

The long term oxidation behaviour and the electrical interface resistance between FeCr interconnects and La0,85Sr0,15Mn1,1O3 plates was studied by a DC four-point method in air at 750{degree sign}C for 10000 h. The tested FeCr alloys were: Crofer 22 APU, Sanergy HT, Plansee IT10, Plansee IT11, and Plansee IT14. The alloys were slurry sprayed with a dual layer coating consisting of an inner Co3O4 layer and an outer layer consisting of La0,85Sr0,15Mn1,1O3 with some Co3O4 added. The microstructure and composition of the oxide scales formed at the interfaces were investigated using SEM/EDX analysis. Low degradation rates of less than 1 mΩcm2/1000 h were measured on all interfaces. The microstructure analysis showed that a duplex Cr2O3-(Mn,Co,Cr)3O4 oxide scale with a thickness of 3-5 µm had evolved on the alloys.

Chemical Vapour Deposition of Metastable Ni3N

Metastable nickel nitride (Ni3N) has been chemically vapour deposited by the use of bis(2,2,6,6-tetramethyl-3,5-heptanedionato)Ni(II) (Ni(thd)2) and ammonia precursors. The growth on both non-etched Si(100) and amorphous SiO2 is polycrystalline at deposition temperatures between 200-290{degree sign}C. However, at the highest temperatures the impurity level of oxygen and carbon originating from the metal precursor ligand, is about 5%. The growth rate dependence of temperature is divided into three different regions with large differences in activation energies, interpreted as different factors controlling the growth. In addition the deposition rate as a function of precursor supply as well as the incubation time for the growth initiation are different at temperatures which are further indications of differences in reaction mechanism. By substitution of NH3 for H2 to the reactant gas the growth mechanism is shown to occur via surface -NHx groups.

Foturan® Glass Ceramic - a Substrate for Power Delivering Free-standing µ-SOFC Membranes

We investigated Foturan® glass ceramic as a promising substrate material for micro-solid oxide fuel cells (μ-SOFC), including processing and electrochemical characterization of free-standing μ- SOFC membranes. The Foturan® wet etching rate was determined to be 16 μm/min. This results in 31 min etching to fabricate a freestanding μ-SOFC membrane. The maximum power output of μ- SOFC membranes was 209 mW/cm2 at 550{degree sign}C with an open circuit voltage of 0.58 V.

IDEAL-Cell, Innovative Dual mEmbrAne fueL-Cell: Fabrication and Electrochemical Testing of First Prototypes

The IDEAL-Cell concept is based on the junction between a PCFC anode/electrolyte section and a SOFC cathode/electrolyte section, through a mixed proton and oxide ion conducting porous ceramic membrane, operating in the temperature range 600-700{degree sign}C. Recombination of ions takes place within the junction, or central membrane (CM), and water vapor is evacuated through open porosity. The first results on the fabrication of multilayered samples reproducing the IDEAL-Cell structure are reported here, together with electrochemical tests carried out on selected samples in order to evaluate the performances of the chosen materials and to demonstrate the feasibility of this innovative concept of fuel cell. Stable OCV and power generation were obtained in the multilayered structures. Anomalies on the I/V curves and impedance measurements under large perturbation could be considered as proofs of water formation inside the central membrane.

Cr Evaporation of Metallic Interconnects: A Novel Method for Quantification

Chromium evaporation from metallic interconnects in SOFCs is commonly poisoning the cathode of SOFCs. To reduce chromium evaporation, rather thick coatings have been deposited using various coating techniques. In this study a 22% Cr ferritic steel has been coated with thin films (<1µm) of metallic cobalt. The Cr evaporation measurements are carried out with a denuder technique which allows time resolved quantification of volatile chromium species. Downstream of the samples a denuder tube is placed which collects any CrO2(OH)2 evaporating from the samples. The experiments are carried out at 850{degree sign}C in a N2-20%O2-3%H2O atmosphere. The investigated Co coatings proved to be very efficient and reduce the Cr evaporation by approxiamatly 90%. Microscopic studies show that both coated and uncoated samples develop two-layered oxide scales with the bottom part consisting of Cr-rich corundum type oxide. The uncoated samples develop a top layer consisting of (Cr,Mn)3O4 oxide while the Co coated samples exhibit a top layer consisting of (Co,Mn)3O4.

In situ Reduction and Oxidation of Nickel from Solid Oxide Fuel Cells in a Transmission Electron Microscope

Environmental transmission electron microscopy was used to characterize in situ the reduction and oxidation of nickel from a Ni/YSZ solid oxide fuel cell anode support between 300-500{degree sign}C. The reduction is done under low hydrogen pressure. The reduction initiates at the NiO/YSZ interface, then moves to the center of the NiO grain. At higher temperature the reduction occurs also at the free NiO surface and the NiO/NiO grain boundaries. The growth of Ni is epitaxial on its oxide. Due to high volume decrease, nanopores are formed during reduction. During oxidation, oxide nanocrystallites are formed on the nickel surface. The crystallites fill up the nickel porosity and create an inhomogeneous structure with remaining voids. This change in structure causes the nickel oxide to expand during a RedOx cycle.

Progress in Metal-Supported SOFCs

Metal-supported SOFCs offer considerable cost and robustness advantages over traditional SOFC designs. In particular, the following topics will be addressed: - Advantages of the metal-supported design - Materials choice for electrolyte and metal support - Acceptable operating temperature range - Fabrication methods - Degradation mechanisms, including Cr migration, metal oxidation, anode-support interdiffusion, etc LBNL has demonstrated competitive power density and promising thermal shock and redox cycling tolerance for YSZ-based metal-supported cells. One cell design utilizes infiltrated anode and cathode catalysts coating porous, sintered YSZ electrode backbones. Our early work demonstrated high power density with infiltrated Ni anodes. Maximum power densities of >330mW/cm2 and >1000mW/cm2 were achieved with air and oxygen as oxidant, respectively, at 700{degree sign}C. Ni coarsens rapidly in fuel atmosphere, so we now use more stable oxide-based anode materials. Very promising stability at moderate power density (>200mW/cm2) will be reported, along with an overview of our cell scale-up efforts.

Exploring Highly Yttrium Doped Barium Zirconate Proton Conductor Electrolytes for Application in Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFCs)

High temperature proton conductor (HTCP) perovskites are gaining growing attention as electrolyte materials for application in intermediate temperature solid oxide fuel cells (IT-SOFCs). In particular, Y-doped barium zirconate (BZY) electrolyte has been recently the subject of several studies because of its good chemical stability. To investigate the influence of Y concentration on the electrical properties of BZY, BaZr1-xYxO3-δ (0.3 {less than or equal to} x {less than or equal to} 0.6) oxides were synthesized using a wet chemistry process to obtain powders with controlled compositional homogeneity and purity at low processing temperatures. The synthesized powders were characterized by X-ray diffraction (XRD) and inductively coupled plasma (ICP) analysis. Dense electrolytes were obtained pressing the calcined powders into cylindrical pellets and then sintering at 1600 {degree sign}C for 12 h. Electrical conductivity of the sintered pellets was measured as a function of the temperature and the Y concentration by electrochemical impedance spectroscopy (EIS) measurements.

Preparation of Iridium Thin Film by Pulse CVD from Ir(acac)(CO)2

Iridium thin films were deposited by pulse CVD with using dicarbonyl(acetylacetonato)iridium Ir(acac)(CO)2 as a precursor. Precursor was characterized by DTA and mass spectrometry; effusion Knudsen method was used for determination of thermodynamic parameters of sublimation process. By means of high temperature mass spectrometry precursor vapor decomposition was studied in vacuum, oxygen and hydrogen. Analysis of gaseous by-product composition has shown that the main products of reaction are acetylacetone, СО, СО2 and С2Н3О of presumably radical character. Mechanism of thermal transformations of precursor molecules at interaction with surface was supposed. Ir films deposited at substrate temperature between 240 and 300{degree sign}C were characterized by XPS and AFM. Ir in the films is in metal and chemically associated states, size of Ir particles is 4-20 nm.

Plasma Sprayed LSM Cathode for Solid Oxide Fuel Cells

Lanthanum Strontium Manganate (LSM) and LSM/Yttria Stabilized Zirconia (YSZ) cathode materials were developed by Vacuum Plasma Spraying (VPS) and Air Plasma Spraying (APS). A parametric study on thermal spraying conditions was performed to manufacture LSM and LSM/YSZ coatings having a good cohesion, high enough porosity and a minimal decomposition of the LSM phase. Layers open porosity was measured by the Archimedes method and phase analysis was done by X-Ray Diffraction (XRD). Electrochemical characterization was performed by impedance spectroscopy at 900{degree sign}C under air on selected cathode materials. Different electrochemical performances were obtained following the spraying atmosphere and the cathode composition. The most efficient cathode developed in this work is composed of two layers: LSM/YSZ as active layer and LSM as collecting layer obtained by APS.

Influence of Operating Parameters on Start/Stop Induced Degradation in Polymer Electrolyte Fuel Cells

In order to investigate the influence of temperature, reactant gas humidification and the application of an auxiliary load on start/stop induced fuel cell degradation under application relevant conditions, alternating purging of the anode electrode with hydrogen and air was used to shut-down and start-up the fuel cell. It was found that loss of active catalyst surface area only significantly influences fuel cell performance at low temperatures and reactant gas humidifications. Under harsh conditions, performance decrease is dominated by mass transport losses which are caused by carbon corrosion induced changes in the structure of the catalyst layer. Measurements of the CO2 content in the cathode exhaust gas showed that corrosion rate depends linearly on water vapor partial pressure for temperatures up to 80{degree sign}C. Due to a strongly increased driving voltage and the longer duration of the detrimental potential, performance loss was dramatically increased for the cell cycled without auxiliary load.

Composite Proton Conductive Membranes for Elevated Temperature and Reduced Relative Humidity PEMFC

Nafion/inorganic (inorganic = S-ZrO2, SiO2-SO3H, SBA-15-SO3H, MCM-41-SO3H, and P-Si gels) composite membranes were synthesized with the goal of increasing the proton conductivity and water retention at elevated temperatures up to 120 {degree sign}C and reduced relative humidity (RH) down to 25%. Inorganic materials were incorporated within the Nafion by three different techniques. The addition of inorganic particles increased the membrane ion exchange capacity (IEC). The conductivity of composite membranes with sulfonated inorganic materials was in agreement with their IEC but not higher than the conductivity of the recast Nafion in the whole range of inorganic loading. Conductivity of the Nafion/P-Si gel membranes was affected by the surface of inorganic powders and was higher for membranes prepared by in situ formation techniques. Effect of the inorganic phase on membrane performance in a fuel cell was much stronger than on its conductivity due to substantial improvement in membrane water retention properties.

IR-assisted Sol-gel Synthesis of Ce0.8Gd0.2O2-δ for SOFCs Applications

Gadolinia-doped ceria (Ce0.8Gd0.2O2-δ, GDC) is an alternative electrolyte for Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFC). Nanostructured materials have recently attracted extensive attention because of their high reactivity that allowed milder sintering conditions. Although several methods were proposed for the synthesis of ultrafine powders, most of them generally require a subsequent calcination step. The aim of this work was to produce ultrafine, pure GDC powders using a standard chemical route (aqueous sol-gel) coupled with non-conventional heating processes (infrared heating). Nanocrystalline non-agglomerated GDC particles were successfully produced under mild conditions (170{degree sign}C, 1h) without the intermediate decomposition and/or calcining steps. Moreover the GDC obtained through the optimization of the IR-assisted sol-gel synthesis parameters exhibited values of ionic conductivity higher than the ones showed by commercial and conventional sol-gel produced powders of similar composition.

Atomic Layer Deposition of Quantum Confined Nanostructures on Particles

Atomic layer deposition (ALD) was used here to experimentally demonstrate the ability to deposit films that exhibit quantum confinement on spherical particle surfaces. Polycrystalline ZnO films ranging from ~1.5 to 15 nm in thickness were deposited via ALD using diethylzinc and hydrogen peroxide at 100{degree sign}C. Conformal, pinhole-free films were deposited on Si wafers and on nano-sized spherical SiO2 particles using an augmented central composite design strategy. A statistical design of experiments was performed to quantify the crystallite size of the primary peaks associated with polycrystalline ZnO, as well as the change in bandgap associated with the small crystal domains based on the number of ALD cycles and thermal annealing post-treatments. The shift in the optical bandgap of ZnO nanoshells was correlated to the domain size within the films at each point in the experimental matrix. The blue shift of 0.3 eV dissipated beyond crystallite sizes exceeding ~10nm, which was indicative of quantum confinement.

Fabrication and Characterisation of Cathode Support-tubes for Micro-tubular SOFC Application

Tubes of diameter <5mm of a nickelate cathode material of composition Nd1.95NiO4 were prepared by cold-isostatic pressing (CIP) and extrusion. These tubes are intended as cathode support structure for micro-tubular SOFCs at reduced working temperatures around 700{degree sign}C. The characteristics of these tubes - porosity, pore size, geometric dimensions and surface roughness - were measured by various techniques such as mercury intrusion, SEM-Imaging and with an in-house built diffusion setup. Diffusion measurements yielded average pore sizes of 0.12 μm for CIP-ed tubes and 0.22 μm for extruded tubes. Estimation of the loss due to oxygen diffusion has shown that the wall thickness of CIP-ed tubes must be reduced prior to coating processes in order to limit oxygen concentration overpotential loss during operation.

Effect of Air Contaminants on the Electrolyte Degradation in Polymer Electrolyte Membrane Fuel Cells

The effect of air contaminants (NH3, NO2, H2S, and SO2) on electrolyte degradation was evaluated in PEMFCs. 2 ppm of these contaminants were supplied, and the F− release rate in the cathode effluent water was analyzed under a current density of 1000 mA/cm2 at 80 {degree sign}C. In the case of sulfur species H2S and SO2, a significant decrease in cell voltage and an increase in F− release rate were observed. The electrolyte degradation appears to be accelerated by hydroxyl radicals generated from H2O2, which was produced by a sulfur-poisoned Pt catalyst. 19F-NMR analysis of the sulfur-poisoned MEA revealed that the degradation of the cathode ionomer was significant compared to the anode ionomer and the membrane. Degradation of the cathode ionomer was confirmed by experiment with the MEA composed of a hydrocarbon-based membrane and a perfluorosulfonic acid ionomer, in which an increase in the F− release rate was also observed.

Non-Ionic Conduction Phenomena in Yttria Stabilised Zirconia Electrolyte in SOFC operated in the Electrolysis Mode

Anode supported solid oxide fuel cells (SOFCs) with yttria stabilised zircona (YSZ) electrolytes were operated at ~800{degree sign}C as solid oxide H2O electrolyser cells (SOECs). Typical operation voltages were 1.0 to 1.3 V. When operation was extended to current densities j corresponding to a steam conversion rate above 100%, or when the steam supply was interrupted under constant current conditions, cell voltages saturated at about 1.8 to 2.0 V. Cells survived long-term current polarisation with j ~ -0.3 to -0.5 Acm-2 without steam supply. The mechanism limiting the cell voltage is attributed to electronic conduction in the YSZ electrolyte. Electrolyte conduction in the normal SOEC mode (with reasonable steam conversion rates) remains predominantly ionic in the investigated temperature range. Depending on the cell characteristics and the operation conditions (notably the temperature), however, the ionic transfer number may be significantly lower compared to the SOFC operation.

Pt/Ge Schottky-Barrier Reduction by Rapid Thermal Diffusion of P Dopants

Low platinum-germanide-Schottky barrier diodes are fabricated via the incorporation of dopants by diffusion. A phosphorous spin-on-dopant resist is used for the formation of a highly doped surface layer in a rapid thermal annealing process, prior to the formation of platinum-germanide Schottky barrier diodes. For rapid diffusion processes above 580 {degree sign}C ohmic contact behavior is found while evidence is given for a lowered effective Schottky-barrier height.

Design and Development of High-Performance Hybrid Inorganic-Organic Fuel Cell Membranes

We develop a sol-gel based strategy for the synthesis of hybrid organic-inorganic membranes for PEMFC. This approach consists in growing a functionalized (-SO3H) mesoporous SiO2 network in a hydrophobic fluorinated polymer (PVDF-co-HFP). We used different processes to fabricate these membranes including solvent casting and spray-coating. These different methods allow a control of the microstructure of the inorganic network in the polymer at different scales which in turn does influence proton transport throughout the inorganic mesostructured network. The interfacial area between the hydrophobic and hydrophilic domains can be tuned and conductivity values comparable to Nafion at 60{degree sign}C under 100% humidity were achieved.