Browsing by Subject "Direct methanol fuel cells"
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Item Addition of platinum to palladium-cobalt nanoalloy catalyst by direct alloying and galvanic displacement(2010-12) Wise, Brent; Manthiram, Arumugam; Ferreira, PauloDirect methanol fuel cells (DMFC) are being investigated as a portable energy conversion device for military and commercial applications. DMFCs offer the potential to efficiently extract electricity from a dense liquid fuel. However, improvements in materials properties and lowering the cost of the electrocatalysts used in a DMFC are necessary for commercialization of the technology. The cathode electrocatalyst is a critical issue in DMFC because the state-of-the-art catalyst, platinum, is very expensive and rare, and its performance is diminished by methanol that crosses over from the anode to the cathode through the Nafion membrane. This thesis investigates the addition of platinum to a palladium-cobalt nanoalloy electrocatalyst supported on carbon black in order to improve catalyst activity for the oxygen reduction reaction (ORR) and catalyst stability against dissolution in acidic environment without significantly reducing the methanol-tolerance of the catalyst. Platinum was added to the palladium-cobalt nanoalloy catalyst using two synthesis methods. In the first method, platinum was directly alloyed with palladium and cobalt using a polyol reduction method, followed by heat treatment in a reducing atmosphere to form catalysts with 11 and 22 atom % platinum. In the second method, platinum was added to a palladium-cobalt alloy by galvanic displacement reaction to form catalysts with 10 and 22 atom % platinum. The palladium cobalt alloy was synthesized using a polyol method, followed by heat treatment in a reducing atmosphere to alloy the nanoparticles before the Pt displacement. It was found that both methods significantly improve catalyst activity and stability, with the displaced catalysts showing a higher activity than the corresponding alloy catalyst. However the alloy catalysts showed similar resistance to dissolution as the displaced catalysts, and the alloyed catalysts were more tolerant to methanol. The displaced catalyst with 22 atom % platinum (8 wt. % Pt overall) performed similar to a 20 wt. % commercial platinum catalyst in both RDE and single cell DMFC tests. The 10 and 22 atom % Pt displaced catalysts and 22 atom % Pt alloyed all showed higher Pt mass specific activities than a commercial Pt catalyst.Item Development and understanding of new membranes based on aromatic polymers and heterocycles for fuel cells(2009-08) Li, Wen; Manthiram, ArumugamDirect methanol fuel cells (DMFC) are appealing as a power source for portable devices as they do not require recharging with an electrical outlet. However, the DMFC technology is confronted with the high crossover of methanol fuel from the anode to the cathode through the currently used Nafion membrane, which not only wastes the fuel but also poisons the cathode platinum catalyst. With an aim to overcome the problems encountered with the Nafion membrane, this dissertation focuses on the design and development of new polymeric membrane materials for DMFC and a fundamental understanding of their structure-property-performance relationships. Several polymeric blend membranes based on acid-base interactions between an aromatic acidic polymer such as sulfonated ploy(ether ether ketone) (SPEEK) and an aromatic basic polymer such as heterocycle tethered poly(sulfone) (PSf) have been explored. Various heterochylces like nitro-benzimidazole (NBIm), 1H-Perimidine (PImd), and 5-amino-benzotriazole (BTraz) have been tethered to PSf to understand the influence of pKa values and the size of the hetrocycles. The blend membranes show lower methanol crossover and better performance in DMFC than plain SPEEK due to an enhancement in proton conductivity through acid-base interactions and an insertion of the heterocycle side groups into the ionic clusters of SPEEK as indicated by small angle Xray scattering and TEM data. The SPEEK/PSf-PImd blend membrane shows the lowest methanol crossover due to the larger size of the side groups, while the SPEEK/PSf-BTraz blend membrane shows the highest proton conductivity and maximum power density. To further investigate the methanol-blocking effect of the heterocycles, N,N’-Bis- (1H-benzimidazol-2-yl)-isophthalamide (BBImIP) having two amino-benzimidazole groups bonded to a phenyl ring has been incorporated into sulfonated polysulfone (SPSf) and SPEEK membranes. With two 2-amino-benzimidazole groups, which could greatly increase the proton transfer sites, and three phenyl rings, which are compatible with the aromatic polymers, the BBImIP/SPSf and BBImIP/SPEEK blend membranes show suppressed methanol crossover and increased fuel cell performance in DMFC. Novel sulfonated copolymers based on poly(aryl ether sulfone) (SPS-DP) that exhibit low methanol crossover have been synthesized and explored as a methanol-barrier center layer in a multilayer membrane configuration having SPEEK as the outer layers. These multilayer membranes exhibit better performance in DMFC than plain SPEEK and Nafion 115 membranes due to suppressed methanol crossover. To address the issue of incompatibility between the new hydrocarbon-based membranes synthesized and the Nafion ionomer used in the catalyst layer in fabricating membrane-electrode assemblies (MEAs), the MEAs have been fabricated with the SPEEK membranes and 10 to 30 % SPEEK ionomer in the catalyst layer. These MEAs exhibit better performance in DMFC compared to the MEAs fabricated with the SPEEK membranes and Nafion ionomer in the catalyst layer due to lower interfacial resistance.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 Synthesis and characterization of nanostructured electrocatalysts for proton exchange membrane and direct methanol fuel cells(2004-08) Xiong, Liufeng; Manthiram, ArumugamProton 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.Item Synthesis of cross-linked sulfonated polysulfone and mechanical properties of SPEEK-based membranes for direct methanol fuel cells(2011-05) Zieren, Shelley Marie; Manthiram, Arumugam; Meyers, Jeremy P.Direct methanol fuel cells (DMFC) are being investigated for use as low-power electrochemical energy conversion devices. These types of fuel cells can be useful for portable electronics. The polymer electrolyte membrane plays a critical role in the overall performance of DMFC. The commercially available membrane, Nafion, suffers from high methanol permeability and a resulting methanol crossover from the anode to the cathode; it is also expensive. Accordingly, alternative membrane materials, such as sulfonated hydrocarbons, are intensively pursued for DMFC. For example, sulfonated poly (ether ether ketone) (SPEEK) and sulfonated polysulfone (SPsf) are two such candidates. This thesis focuses first on a simple synthesis method for a cross-linked sulfonated polysulfone membrane. Sulfonated polysulfone (Psf) membranes, with high IEC (1.4 - 2.2 meq/g), were characterized by nuclear magnetic resonance spectroscopy (NMR), proton conductivity, and water uptake. The degree of sulfonation was calculated by NMR and verified by acid-base titration analysis. Although the membranes showed good proton conductivity, they suffered from excessive swelling at high temperatures. Furthermore, the post-sulfonation of a carboxyl-substituted polysulfone (Psf-COOH) was carried out with trimethylsilyl chlorosulfonate, and solubility issues of the Psf-COOH in chlorinated solvents led to difficulty in controlling the degree of sulfonation (DS) and in purification. Accordingly, this approach to cross-linking sulfonated polysulfone was rejected as a viable method. This thesis then focused on the investigation of the mechanical properties of acid-base blend membranes based on SPEEK and heterocycle-tethered Psf and cross-linked membranes based on SPEEK that were previously reported by our group; these membranes were known to exhibit good performance in DMFC. However, the assessment of the mechanical stability of any new membranes developed is critical for their practical viability in DMFC. Accordingly, the mechanical strength and ductility of these membranes were investigated and compared for various membrane compositions. The acid-base blend membranes investigated consisted of SPEEK (acidic polymer) and a heterocycle-tethered Psf (basic polymer); for example, blends consisting of SPEEK and amino-benzimidazole-tethered Psf (SPEEK/Psf-ABIm) and SPEEK and benzotriazole tethered Psf (SPEEK/Psf-Btraz) were investigated. The cross-linked SPEEK was made by Friedel-Craft acylation with Psf-COOH (DS = 1 or 2). The two blend membranes showed superior mechanical properties compared to Nafion 115 and comparable to plain SPEEK. The crosslinked membranes showed good mechanical properties and better strength than Nafion 115, but they were more brittle than both Nafion 115 and plain SPEEK. Further optimization of cross-linking conditions is necessary to produce the best performing membrane.