Browsing by Subject "Proton exchange"
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Item Development of new membranes for proton exchange membrane and direct methanol fuel cells(2004-08) Yang, Bo, Ph. D.; Manthiram, ArumugamProton 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.Item The effect of nanocatalyst size on performance and degradation in the cathode of proton exchange membrane fuel cells(2011-12) Groom, Daniel Jeffrey; Ferreira, Paulo J. S. G.; Rabenberg, Llewellyn K.This thesis discusses the role of initial particle size on the mechanisms of surface area loss of carbon-supported platinum (Pt) electrocatalysts in the cathode of proton exchange membrane fuel cells. Electrocatalyst decay protocols were used to accelerate cathode performance loss for Pt catalysts. Four cathodes with mean platinum particle sizes of 2.1, 3.5, 6.7 and 11.3 nm were evaluated to elucidate the impact of particle size on initial performance and subsequent degradation, when subjected to identical potential cycles. The degradation of Pt electrochemically active surface area (ECA) was significantly greater for 2.1 and 3.5 nm initial sizes compared to 6.7 and 11.3 nm initial sizes. As expected, the ECA loss of the cathodes shows an inverse proportionality with initial particle size. However, the initial performance of the 11.3 nm initial particle size electrode was significantly lower than the three smaller sizes. Thus, an initial Pt particle size of 6.7 nm was identified to offer the ideal balance performance and durability. The current state of standardization in characterizing particle size by transmission electron microscopy (TEM) is also investigated. The result is a standardized protocol for image acquisition and analysis.