Olivera Kesler

Ex situ Evaluation of Tungsten Oxide as a Catalyst Support for PEMFCs

The oxidation of carbon catalyst supports causes acute degradation in catalyst performance in proton exchange membrane fuel cells (PEMFCs). Tungsten oxide is considered here as a candidate material as an alternative catalyst support. A new method has been developed that allows Pt deposition on tungsten oxide catalyst supports. The electrochemical stability of tungsten oxide was studied by applying a coating of a tungsten-oxide-supported Pt catalyst to a rotating disk electrode and to a gold mesh. Accelerated oxidation tests were performed in deoxygenated 0.5 M H 2 SO 4 at 30 and 80°C. The oxidation stability of Pt/tungsten oxide and of Hispec 4000, a commercial catalyst, each coated on a gold mesh, was measured at 80°C. The oxygen reduction reaction (ORR) activity of Pt/tungsten oxide remained high, even after accelerated oxidation tests, while the ORR activity was extremely poor after accelerated oxidation tests of Hispec 4000.

Ex Situ and In Situ Stability of Platinum Supported on Niobium-Doped Titania for PEMFCs

The stability and activity of 10 mol % niobium-doped titania (10Nb-TiO 2 ) were compared to those of commercial Pt/C catalyst (HiSpec 4000), both ex situ and in a proton exchange membrane fuel cell. The stability test involved holding the cell at 1.4 V for 20 h. Fuel cell polarization curves were obtained both before and after the potential holds. Cyclic voltammogram data showed that the Pt surface area was stable for Pt/10Nb-TiO 2 even after holding for 60 h at 1.4 V. A significant drop in the Pt surface area was observed, however, for Pt/C after 20 h at 1.4 V. The high potential hold test showed significantly higher stability of the membrane electrode assembly with Pt/10Nb-TiO 2 compared to that with Pt/C.

Connected Three-Phase Boundary Length Evaluation in Modeled Sintered Composite Solid Oxide Fuel Cell Electrodes

A numerical methodology for evaluating the three-phase boundary length (TPBL) in sintered composite solid oxide fuel cell electrodes is developed. Three-dimensional models of a representative volume element of sintered composite electrodes are generated for which the mean particle diameter, composition, and total porosity may be specified as input parameters. Tomographic methods are used to reconstruct the modeled electrode and the percolation for each phase is evaluated. The connected TPBL is calculated for a range of electrode designs and comparisons are made with calculated TPBL values available in the literature. The maximum connected TPBL occurred at a porosity of 0.21 and at equal solid volume fractions of ionic and electronic conducting phases for particles having the same mean diameter and particle size variance. A cubic envelope having a minimum length of 14 times the mean particle diameter was necessary to adequately represent the electrode structure.

Plasma Spray Processing of Solid Oxide Fuel Cells

Plasma spray processing is a low-cost, rapid manufacturing technique that is widely used industrially for fabrication of thermal barrier and wear- and corrosion-resistant coatings. Because the technique can be used to rapidly deposit coatings of high melting temperature materials with good substrate adhesion, it has also been applied to the production of individual component layers in tubular solid oxide fuel cells (SOFCs), and more recently, in planar SOFCs. The use of plasma spray processing for the fabrication of fuel cell components presents unique challenges, due to the high porosities required for the electrode layers and fully dense coatings required for electrolytes. Application of plasma spray processing for the manufacture of solid oxide fuel cells is discussed, with consideration of potential advantages of the technique compared to standard SOFC wet ceramic processing routes. Major challenges faced in the adaptation of the processing method to solid oxide fuel cell manufacture are discussed, along with current research approaches being used to overcome these challenges. Recent developments in the use of the technique for the rapid onestep manufacturing of direct oxidation SOFC anodes are discussed, for composite material combinations that cannot be co-sintered due to widely divergent melting points. The impacts of plasma sprayed coating properties on solid oxide fuel cell performance are considered, and implications of the use of the technique on overall stack and system manufacturing costs are discussed.

Transmission Electron Microscope Observation of Pt Deposited on Nb-Doped Titania

Pt nanoparticles supported on niobium (Nb)-doped titania (TiO 2 ) were prepared using the alcohol reduction method. The 10 mol % Nb-doped TiO 2 (10Nb-TiO 2 ) was subjected to various heat-treatments. Depending on the heat-treatment either anatase or rutile 10Nb-TiO 2 was produced. The change in Pt morphology when deposited on 10Nb-TiO 2 supports that were treated under hydrogen at different temperatures was investigated by high-resolution transmission electron microscopy. Platinum particles deposited on rutile 10NbTiO 2 and anatase 10NbTiO 2 have distinctly different morphologies. Those deposited on anatase 10Nb-TiO 2 are similar to Pt/C and are essentially spherical. Platinum particles on rutile 10Nb-TiO 2 , however, appear flattened, indicating a strong metal-support interaction.

Permeability and Microstructure of Suspension Plasma-Sprayed YSZ Electrolytes for SOFCs on Various Substrates

Yttria-stabilized zirconia electrolyte coatings for solid oxide fuel cells were deposited by suspension plasma spraying using a range of spray conditions and a variety of substrates, including finely structured porous stainless steel disks and cathode layers on stainless steel supports. Electrolyte permeability values and trends were found to be highly dependent on which substrate was used. The most gas-tight electrolyte coatings were those deposited directly on the porous metal disks. With this substrate, permeability was reduced by increasing the torch power and reducing the stand-off distance to produce dense coating microstructures. On the substrates with cathodes, electrolyte permeability was reduced by increasing the stand-off distance, which reduced the formation of segmentation cracks and regions of aligned and concentrated porosity. The formation mechanisms of the various permeability-related coating features are discussed and strategies for reducing permeability are presented. The dependences of electrolyte deposition efficiency and surface roughness on process conditions and substrate properties are also presented.

Characterization of Porous Stainless Steel 430 for Low- and Intermediate-Temperature Solid Oxide Fuel Cell (SOFC) Substrates

One approach to lower the cost of solid oxide fuel cells (SOFCs) is to lower the operating temperatures below 1073 K to allow the use of robust and comparatively inexpensive stainless steels not only for interconnects but also for SOFC support structures. The metal supports must be sufficiently porous to facilitate gas flow toward the reactive sites in the electrodes. Gas flow and electrical conductivity must remain adequate during any oxidation that occurs during operation. In order to identify microstructures most suitable for use as SOFC supports, a series of gas permeation and surface profilometry experiments was conducted to determine the permeability and surface roughness of porous steels (AISI 430) having different pore structures. The materials were also characterized by a variety of porosity measurement methods, each yielding complementary information on the three-dimensional structures. A combination of moderately low surface roughness and high gas permeability was found to represent a good combination of properties for metal-supported SOFC application.

Deposition of Composite LSCF-SDC and SSC-SDC Cathodes by Axial-Injection Plasma Spraying

The performance of solid oxide fuel cell cathodes can be improved by increasing the number of electrochemical reaction sites, by controlling microstructures, or by using composite materials that consist of an ionic conductor and a mixed ionic and electronic conductor. LSCF (La0.6Sr0.4Co0.2Fe0.8O3−δ) and SSC (Sm0.5Sr0.5CoO3) cathodes were manufactured by axial-injection atmospheric plasma spraying, and composite cathodes were fabricated by mixing SDC (Ce0.8Sm0.2O1.9) into the feedstock powders. The plasma power was varied by changing the proportion of nitrogen in the plasma gas. The microstructures of cathodes produced with different plasma powers were characterized by scanning electron microscopy and gas permeation measurements. The deposition efficiencies of these cathodes were calculated based on the mass of the sprayed cathode. Particle surface temperatures were measured in-flight to enhance understanding of the relationship between spray parameters, microstructure, and deposition efficiency.

The Influence of Process Equipment on the Properties of Suspension Plasma Sprayed Yttria-Stabilized Zirconia Coatings

Suspension plasma-sprayed YSZ coatings were deposited at lab-scale and production-type facilities to investigate the effect of process equipment on coating properties. The target application for these coatings is solid oxide fuel cell (SOFC) electrolytes; hence, dense microstructures with low permeability values were preferred. Both facilities had the same torch but different suspension feeding systems, torch robots, and substrate holders. The lab-scale facility had higher torch-substrate relative speeds compared with the production-type facility. On porous stainless steel substrates, permeabilities and microstructures were comparable for coatings from both facilities, and no segmentation cracks were observed. Coating permeability was further reduced by increasing substrate temperatures during deposition or reducing suspension feed rates. On SOFC cathode substrates, coatings made in the production-type facility had higher permeabilities and more segmentation cracks compared with coatings made in the lab-scale facility. Increased cracking in coatings from the production-type facility was likely caused mainly by its lower torch-substrate relative speed.

Air Plasma Spraying of LSM/YSZ SOFC Composite Cathodes

Porous composite cathodes containing (La0.8Sr0.2)0.98MnO3 (LSM) and yttria stabilized zirconia (YSZ) for use in solid oxide fuel cells (SOFCs) have been produced by air plasma spraying. Deposition was carried out using axial powder injection for increased deposition efficiency and composition control. A number of composite cathodes were produced using different combinations of parameter values within the identified range. Successful coatings were then characterized for composition and porosity using EDX and SEM. As a result of these tests, combinations of input parameter values were identified that are best suited to the production of coatings with microstructures appropriate for use in SOFC composite cathodes.