Due to escalating energy costs and limited fossil fuel resources, much attention has been given to polymer electrolyte membrane (PEM) fuel cells. Gas diffusion layers (GDLs) play a vital role in a fuel cell such as (1) water removal, (2) cooling, (3) structural backing, (4) electrical conduction and (5) transporting gases towards the active catalyst sites where the reactions take place. The power density of a PEM fuel cell in part is dependent upon how uniform the gases are distributed to the active sites. To this end, research is being conducted to understand the mechanisms that influence gas distribution across the fuel cell. Emerging PEM fuel cell designs have shown that higher power density can be achieved; however this requires significant changes to existing components, particularly the GDL. For instance, some emerging concepts require higher through-plane gas permeability than in-plane gas permeability (i.e., anisotropic resistance) which is contrary to conventional GDLs (e.g., carbon paper and carbon cloth), to obtain a uniform gas distribution across the active sites. This is the foundation on which this thesis is centered. A numerical study is conducted in order to investigate the effect of the gas permeability profile on the expected current density in the catalyst layer. An experimental study is done to characterize the effects of the weave structure on gas permeability in woven GDLs. Numerical simulations are developed using Fluent version 6.3.26 and COMSOL Multiphysics version 3.5 to create an anisotropic resistance profile in the unconventional GDL, while maintaining similar performance to conventional GDL designs. The effects of (1) changing the permeability profile in the in-plane and through-plane direction, (2) changing the thickness of the unconventional GDL and (3) changing the gas stoichiometry on the current density and pressure drop through the unconventional GDL are investigated. It is found that the permeability profile and thickness of the unconventional GDL have a minimal effect on the average current density and current density distribution. As a tradeoff, an unconventional GDL with a lower permeability will exhibit a higher pressure drop. Once the fuel cell has a sufficient amount of oxygen to sustain reactions, the gas stoichiometry has a minimal effect on increases in performance. Woven GDL samples with varying tightness and weave patterns are made on a hand loom, and their in-plane and through-plane permeability are measured using in-house test equipment. The porosity of the samples is measured using mercury intrusion porosimetry. It is found that the in-plane permeability is higher than the through-plane permeability for all weave patterns tested, except for the twill weave with 8 tows/cm in the warp direction and 4 tows/cm in the weft direction, which exhibited a through-plane permeability which was 20% higher than the in-plane permeability. It is also concluded that the permeability of twill woven fabrics is higher than the permeability of plain woven fabrics, and that the percentage of macropores, ranging in size from 50-400 µm, is a driving force in determining the through-plane permeability of a woven GDL. From these studies, it was found that the graduated permeability profile in the unconventional GDL had a minimal effect on gas flow. However, a graduated permeability may have an impact on liquid water transport. In addition, it was found that graduating the catalyst loading, thereby employing a non-uniform catalyst loading has a greater effect on creating a uniform current density than graduating the permeability profile.

Proton exchange membrane fuel cells (PEMFC) and direct methanol fuel cells (DMFC) are drawing much attention as alternative power sources for transportation, stationary, and portable applications. Nafion membranes are presently used in both PEMFC and DMFC as electrolytes, but are confronted with a few difficulties: (i) high cost, (ii) limited operating temperature of < 100 °C, and (iii) high methanol permeability. With an aim to overcome some of the problems encountered with the Nafion membranes, this dissertation focuses on the design and development of a few materials systems for use in PEMFC and/or DMFC. The incorporation of hydrous Ta₂O₅·nH₂O into Nafion membrane as well as the electrodes is shown to help the cell to retain water to higher temperatures. Membrane-electrode assembly (MEA) consisting of the composite membrane shows better cell performance at 100 and 110 °C than that with plain Nafion membrane, and a high power density of ~ 650 mW/cm² at 100 °C is obtained with H₂ - CO mixture as the fuel due to a significant alleviation of the CO poisoning of the catalysts. Sulfonated poly(etheretherketone) (SPEEK) membranes with various sulfonation levels are prepared and investigated in DMFC. With a sulfonation level of ~ 50 %, the SPEEK membranes exhibit low methanol permeability and electrochemical performance comparable to that of Nafion at around 60 °C, making it an attractive low-cost alternative to Nafion. From a comparative study of the structural evolutions with temperature in 2 M methanol solution, it is found that the lower methanol permeability of SPEEK membranes is related to the less connected and narrower pathways for water/methanol permeation. The dry proton conductor CsHSO₄ shows a high proton conductivity of ~ 10⁻³ S/cm at temperatures > 140 °C and water is not needed for proton conduction. However, it is found that CsHSO₄ decomposes to Cs₂SO₄ and H₂S at 150 °C in H₂ atmosphere in contact with the Pt/C catalyst. Thus, new catalyst materials need to be explored for CsHSO₄ to be used in practical high temperature PEMFC. Thin self-humidifying Nafion membranes with dispersed Pt/C catalyst powder are prepared and tested in PEMFC with dry H₂ and O₂. The Pt/C particles provide sites for catalytic recombination of H₂ and O₂ permeating from the anode and cathode, and the water produced at these sites directly humidifies the membrane. The performance of the cell with the self-humidifying membrane operated with dry reactants is ~ 90 % of that obtained with well humidified H₂ and O₂. / text

Proton exchange membrane (PEM) fuel cells are competitive with other emerging technologies that are being considered for automotive transportation. Commercialization of PEM fuel cells would decrease emissions of criteria pollutants and greenhouse gases and reduce US dependence on foreign oil. However, many challenges exist that prevent this technology from being realized, including power requirements, durability, on-board fuel storage, fuel distribution, and cost. This dissertation focuses on fuel-cell durability, or more specifically catalyst stability. New techniques to comprehensively observe and pin-point degradation mechanisms are needed to identify stable catalysts. In this text, an in operando method to measure changes in catalyst particle size at the cathode of a PEM fuel cell is demonstrated. The pair distribution function analysis of X-ray diffraction patterns, generated from an operating fuel cell exposed to accelerated degradation conditions, was used to observe the growth of catalyst particles. The stability of Pt/C and PtCo/C electrodes, with different initial particle sizes, was monitored over 3000 potential cycles. The increase in particle size was fit to a linear trend as a function of cycle number for symmetric linear sweeps of potential. The most stable electrocatalyst was found to be alloyed PtCo with a larger initial particle size. A better understanding of oxide growth kinetics and its role in platinum dissolution is needed to develop a comprehensive fuel-cell performance model. There is an ongoing debate present in the current literature regarding which oxide species are involved in the oxide growth mechanism. This dissertation discusses the results of in operando X-ray absorption spectroscopy studies, where it was found that PtO2 is present at longer hold times. A new method to quantify EXAFS data is presented, and the extent of oxidation is directly compared to electrochemical data. This comparison indicated that PtO2 was formed at the expense of an initial oxide species, and these steps were included in a proposed mechanism for platinum oxidation. Simulations of platinum oxidation in literature have yet to fully replicate an experimental cyclic voltammogram. A modified Butler-Volmer rate equation is presented in this thesis. The effect of including an extra parameter, χ, in the rate equations was explored. It was found that while the χ-parameter allowed the cathodic peak width to be decoupled from the Tafel slope for the platinum-oxide reduction, its inclusion could not address all observed experimental characteristics. Exploration of this concept concluded that current is not a function of only potential and coverage. To that end, a heterogeneous oxide layer was introduced. In this model, place-exchanged PtO2 structures of varying energy states are formed through a single transition state. This treatment allowed, for the first time, the simulation of the correct current-potential behavior under varying scan rates and upper potential limits. Particle size plays a critical role in catalysts stability. The properties of nanoparticles can differ significantly from bulk values, yet few tools exist to measure these values at the nanoscale. Surface stress and surface energy are diagnostic criterion that can be used to differentiate nano from bulk properties. The pair distribution function technique was used to measure lattice strain and particle size of platinum nanoparticles supported on carbon. The effect of adsorbates on surface stress was examined and compared to previous literature studies. Furthermore, a methodology for measuring the surface energy of supported platinum nanoparticles has been developed. While the results of this work are significant, many more challenges need to be addressed before fuel-cell vehicles are marketed. Recommendations for future work in the field of catalyst durability are addressed.

Thesis (MS)--Montana State University--Bozeman, 2009. / Typescript. Chairperson, Graduate Committee: Steven R. Shaw. Includes bibliographical references (leaves 55-56).

Thesis (Ph.D.)--Mississippi State University. Bagley College of Engineering. / Title from title screen. Includes bibliographical references.

Thesis (M.S.)--Humboldt State University, 2009. / Includes bibliographical references (leaves 56-58). Also available via Humboldt Digital Scholar.

Includes bibliographical references (p. 115-129).

Thesis (Ph.D)--Chemical Engineering, Georgia Institute of Technology, 2010. / Committee Chair: Meredith, Carson; Committee Member: Bucknall, David; Committee Member: Fuller, Tom; Committee Member: Griffin, Anselm; Committee Member: Koros, William. Part of the SMARTech Electronic Thesis and Dissertation Collection.

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

Made available in DSpace on 2014-10-09T12:53:45Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:58:52Z (GMT). No. of bitstreams: 0 / Dissertação (Mestrado) / IPEN/D / Instituto de Pesquisas Energéticas e Nucleares - IPEN-CNEN/SP