Solid oxide fuel cells (SOFCs) are power generation devices that offer many great advantages compared to lower temperature fuel cells; for example, they are able to operate at high efficiencies without the use of expensive precious metal catalysts, and are also able to directly utilize hydrocarbon fuels without the need of an external reformer. Unfortunately, the conventional high operating temperature of these devices (T ≈ 1000 °C) requires the use of expensive, specialized materials that can withstand these high temperatures. This issue has generated considerable interest in reducing the operating temperature of these devices to the intermediate-temperature (600 – 800 °C) to allow for the use of less-expensive materials, such as stainless steel. However, the conventionally utilized SOFC cathode materials exhibit poor electrochemical performance at these reduced temperatures. Currently considered alternative intermediate temperature cathodes, such as Ba₀.₅Sr₀.₅Co₀.₈Fe₀.₂O₃₋δ (BSCF), offer improved performance, but have a large thermal expansion coefficient (TEC), leading to cell failure. In light of these issues, this dissertation focuses on the development of low TEC cathodes for intermediate temperature SOFCS (IT-SOFCs). The primary focus of this dissertation is on the swedenborgite-type RBaCo₃MO₇₊δ (R = Y, In, and Ca; M = Zn and Fe) series of cathodes. Due to their tetrahedrally-coordinated M site, the cobalt ions in these materials do not undergo a spin-state transition, and have TECs similar to conventional SOFC electrolyte materials. The long-term phase stability of these materials was addressed, and it was discovered that a slight In substitution significantly promoted phase stability. In the Y₁₋[subscript x] In [subscript x] BaCo₃ZnO₇₊δ series, it was observed that x = 0.1 successfully stabilized the phase without observable degradation of performance. Similarly, a high-Ca content material (Y₀.₅In₀.₁Ca₀.₄BaCo₃ZnO₇₊δ) was successfully stabilized, though Ca is known to destabilize the phase; furthermore, this compound showed improved performance compared to YBaCo₃ZnO₇₊δ. Lastly, the replacement of the performance-inhibiting Zn with Fe was investigated, and the Y₀.₉In₀.₁BaCo₃Zn₀.₆Fe₀.₄O₇₊δ sample showed low temperature performance rivaling BSCF. Other work in this dissertation focuses on the application of functional silver materials for use in SOFCs, with good performance; these materials were easily manufactured, and they showed performance drastically greater than the conventionally utilized platinum. / text

This dissertation describes the synthesis and characterization of nitrogen doped carbon nanotube (NCNT) supported nanoparticles for catalysis, specifically, the cathodic oxygen reduction reaction (ORR) in fuel cells. Strategies for synthesis of mono- and bimetallic nanoparticle catalysts through dendrimer based templating techniques and with the aid of metal organic chemical vapor deposition (MOCVD) precursors and efficient assembly protocols of the catalysts with the NCNTs are discussed in detail. Physicochemical properties of the NCNTs and NCNT supported catalysts were characterized using a host of tools including scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, x-ray photoelectron spectroscopy (XPS), thermo gravimetric analysis, BET surface area and pore size analysis and electrochemical techniques including cyclic voltammetry, chronocoulometry, chronoamperometry and rotating disk electrode voltammetry. Chapter 1 serves as a general introduction and provides a brief overview of challenges associated with the synthesis, characterization and utilization of graphitic carbons and graphitic carbon supported catalysts in heterogeneous catalysis. Chapter 2 provides an overview of the synthesis and characterization of systematically doped iron and nickel catalyzed NCNTs in an effort to understand the effect of nitrogen doping on ORR. Chapter 3 describes the use of NCNTs as supports for dendrimer templated nanoparticle catalysts for ORR. A facile synthetic strategy for the immersion based loading of catalysts onto NCNTs by spontaneous adsorption to achieve specific catalyst loadings is explored. Chapter 4 details the loading of monodisperse Pt, Pd and PtPd catalysts on the as synthesized NCNTs using MOCVD precursors. The MOCVD route offers promise for direct dispersion and activation of ORR catalysts on NCNT supports and eliminates a host of problems associated with traditional solvent based catalyst preparation schemes. Chapter 5 details future directions on a few topics of interest including efficient electrodeposition strategies for preparing NCNT supported catalysts, studies on PtCu catalysts for ORR and finally prospects of using NCNT supported catalysts in fuel cell applications.

Not available / text

No description available.

Mathematical modeling helps researchers to understand the transport and kinetic phenomena within fuel cells and their effects on fuel cell performance that may not be evident from experimental work. In this thesis, a 2-D steady-state cathode model of a proton-exchange-membrane fuel cell (PEMFC) is developed. The kinetics of the cathode half-reaction were investigated, specifically the reaction order with respect to oxygen concentration. It is unknown whether this reaction order is one or one half. First- and half-order reaction models were simulated and their influence on the predicted fuel cell performance was examined. At low overpotentials near 0.3 V, the half-order model predicted smaller current densities (approximately half that of the first-order model). At higher overpotentials above 0.5 V, the predicted current density of the half-order model is slightly higher than that of the first-order model. The effect of oxygen concentration at the channel/porous transport layer boundary was also simulated and it was shown the predicted current density of the first-order model experienced a larger decrease (~10-15% difference at low overpotentials) than the half-order model. Several other phenomena in the cathode model were also examined. The kinetic parameters (exchange current density and cathode transfer coefficient) were adjusted to assume a single Tafel slope, rather than a double Tafel slope, resulting in a significant improvement in the predicted fuel cell performance. Anisotropic electronic conductivities and mass diffusivities were added to cathode model so that the anisotropic structure of the porous transport layer was taken into account. As expected, the simulations showed improved performance at low current densities due to a higher electronic conductivity in the in-plane direction and decreased performance at high current densities due to smaller diffusivities. Additionally, the concentration overpotential was accounted for in the model; however it had little influence on the simulation results. / Thesis (Master, Chemical Engineering) -- Queen's University, 2008-09-19 12:14:29.079

<p>The main objective of this work it to produce membrane electrode assemblies (MEAs) that have improved performance over MEAs produced by the conventional manner, by producing highly efficient, electroactive, uniform catalyst layers with lower quantities of platinum electrocatalyst. The catalyst coated membrane (CCM) method was used to prepare the MEAs for the PEM fuel cell as it has been reported that this method of MEA fabrication can improve the performance of PEM fuel cells. The MEAs performances were evaluated using polarisation studies on a single cell. A comparison of polarisation curves between CCM MEAs and MEAs produced in the conventional manner illustrated that CCM MEAs have improved performance at high current densities (&gt / 800 mA/cm2).</p>

This study investigated the applicability of various analytical tools for the qualitative and quantitative characterization of nanophase electrocatalysts.

Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

Thesis (Ph.D.)--Drexel University, 2008. / Includes abstract and vita. Includes bibliographical references (leaves 218-233).

Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2007. / Includes bibliographical references (leaves 178-195). Also available in electronic version.