Browsing by Subject "Proton exchange membrane fuel cells"

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Development of new membranes for proton exchange membrane and direct methanol fuel cells
(2004-08) Yang, Bo, Ph. D.; Manthiram, Arumugam
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.
Synthesis and characterization of nanostructured electrocatalysts for proton exchange membrane and direct methanol fuel cells
(2004-08) Xiong, Liufeng; Manthiram, Arumugam
Proton exchange membrane fuel cells (PEMFC) and direct methanol fuel cells (DMFC) are attractive power sources as they offer high conversion efficiencies with low or no pollution. However, the most commonly used platinum electrocatalyst is expensive and the world supply of Pt is limited. In addition, the slow oxygen reduction and methanol oxidation kinetics as well as the poisoning of the Pt catalyst at the cathode resulting from methanol permeation from the anode through the Nafion membrane to the cathode lead to significant performance loss. Also, the electrocatalyst utilization in the electrodes also needs to be improved to reduce the overall cost of the electrocatalysts and improve the fuel cell performance. This dissertation explores nanostructured Pt alloys with lower cost and higher catalytic activity than Pt for oxygen reduction in PEMFC to understand the effect of synthesis and structure on the catalytic activity, methanol tolerant Pt/TiOx nanocomposites for oxygen reduction in DMFC, nanostructured Pt-Ru alloys for methanol oxidation in DMFC, and improvement in the utilization of Pt by optimizing the membrane-electrode assembly (MEA) fabrication. From a systematic investigation of a series of Pt-M alloys (M = Fe, Co, Ni, and Cu), the catalytic activity of Pt-M alloys is correlated with the extent of atomic ordering. More ordered Pt alloys exhibit higher catalytic activity than disordered Pt alloys. The higher activity of the ordered Pt alloys is found to relate to various factors including the Pt-Pt distance, Pt: 5d orbital vacancy, {100} planar density and surface atomic configuration. The catalytic activity of the Pt alloys is also influenced by the synthesis method. Low temperature solution methods usually result in smaller particle size and higher surface area, while high temperature routes result in larger particle size and lower surface area but with a greater extent of alloying. Pt/TiOx/C nanocomposites exhibit higher performance than Pt for oxygen reduction in DMFC. The nanocomposites show higher electrchochemical surface area, lower charge transfer resistance, and higher methanol tolerance than Pt. Pt-Ru alloy synthesized by a reverse microemulsion method exhibits higher catalytic surface area than the commercial Pt-Ru. The higher catalytic activity is attributed to a better control of the particle size, crystallinity, and microstructure. Membrane-electrode assemblies (MEAs) fabricated by a modified thin film method exhibit much higher electrocatalyst utilization efficiency and performance than the conventional MEAs in PEMFC. Power densities of 715 and 610 mW/cm2 are obtained at a Pt loading of, respectively, 0.1 and 0.05 mg/cm2 and 90 oC. The higher electrocatalyst utilization is attributed to the thin catalyst layer and a better continuity of the membrane/catalysts layer interface compared to that in the conventional MEAs.