Thesis (M.S.)--Worcester Polytechnic Institute. / Keywords: methanol; fuel cell; Nafion; membrane. Includes bibliographical references. (p.93-95)

Thesis (M.S.) -- Worcester Polytechnic Institute. / Keywords: electrode; membrane; methanol; fuel cell; Nafion. Includes bibliographical references (p. 93-95).

Thesis (M.Sc.)--Memorial University of Newfoundland, 2000. / Includes bibliographical references.

>Magister Scientiae - MSc / Fuel cells are widely considered to be efficient and non-polluting power source offering much higher energy density. This study is aimed at developing oxygen reduction reactions (ORR) catalysts with reduced platinum (Pt) loading. In order to achieve this aim, monometallic Pd and Pt nanostructured catalysts were electrodeposited on a substrate (carbon paper) by surface limited redox replacement using electrochemical atomic layer deposition (ECALD) technique. Pd:Pt bimetallic nanocatalysts were also deposited on carbon paper. Pd:Pt ratios were (1:1, 2.1 and 3:1). The prepared mono and bimetallic catalysts were characterized using electrochemical methods for the ORR in acid electrolyte. The electrochemical characterization of these catalysts includes: Cyclic Voltammetry (CV) and linear sweep voltammetry (LSV). The physical characterization includes: scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) for Morphology and elemental composition, respectively. The deposition of copper (Cu) on carbon paper was done by applying a potential of -0.05 V at 60s, 90s and 120s. 8x cycles of Pt or Pd showed better electrochemical activity towards hydrogen oxidation reaction. Multiples of eight were used in this work to deposit Pt: Pd binary catalyst. Cyclic voltammetry showed high electroactive surface area for Pt24Pd24/Carbon-paper while LSV showed high current density and positive onset potential. HRSEM also displayed small particle size compared to other Pt:Pd ratios.

A fuel cell is an electrochemical power source with advantages of both the combustion engine and the battery. Like a combustion engine, a fuel cell will run as long as it is provided fuel; and like a battery, fuel cells convert chemical energy directly to electrical energy. As an electrochemical power source, fuel cells are not subjected to the Carnot limitations of combustion (heat) engines. Fuel cells bear similarity to batteries, with which they share the electrochemical nature of the power generation process and to the engines that, unlike batteries, will work continuously consuming a fuel of some sort. A fuel cell operates quietly and efficiently and, when hydrogen is used as a fuel, it generates only power and water. Thus, a fuel cell is a so called ‘zero-emission engine’. In the past, several fuel cell concepts have been tested in the laboratory but the systems that are being potentially considered for commercial developments are: (i) Alkaline Fuel Cells (AFCs), (ii) Phosphoric Acid Fuel Cells (PAFCs), (iii) Polymer Electrolyte Fuel Cells (PEFCs), (iv) Solid Polymer Electrolyte Direct Methanol Fuel Cells (SPE-DMFCs), (v) Molten Carbonate Fuel Cells (MCFCs) and (vi) Solid Oxide Fuel Cells (SOFCs). Among the aforesaid systems, PEFCs that employ hydrogen as fuel are considered attractive power systems for quick start-up and ambient temperature operations. Ironically, however, hydrogen as fuel is not available freely in the nature. Accordingly, it has to be generated from a readily available hydrogen carrying fuel such as natural gas, which needs to be reformed. But, such a process leads to generation of hydrogen contaminated with carbon monoxide, which even at minuscule level is detrimental to the fuel cell performance. Pure hydrogen can be generated through water electrolysis but hydrogen thus generated needs to be stored as compressed/liquefied gas, which is cost-intensive. Therefore, certain hydrogen carrying organic fuels such as methanol, ethanol, propanol, ethylene glycol and diethyl ether have been considered for fueling PEFCs directly. Among these, methanol with hydrogen content of about 12.8 wt.% (specific energy = 6.1kWh kg-1) is the most attractive organic liquid. PEFCs using methanol directly as fuel are referred to as SPE-DMFCs. But SPE-DMFCs suffer from methanol crossover across the polymer electrolyte membrane, which affects the cathode performance and hence the fuel cell during its operation. SPE-DMFCs also have inherent limitations of low open-circuit-potential and low electrochemical-activity. An obvious solution to the aforesaid problems is to explore other promising hydrogen carrying fuels such as sodium borohydride (specific energy = 12kWh kg-1), which has a capacity value of 5.67Ah g-1 and a hydrogen content of about 11wt.%. Such fuel cells are called direct borohydride fuel cells (DBFCs). This thesis is directed to studies on SPE-DMFCs and DBFCs

Direct 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. / text

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. / text

>Magister Scientiae - MSc / The purpose of this study was to develop a high performance-lower cost catalyst to be applied in Direct Methanol Fuel Cells (DMFC). The study also aimed to prepare plurimetallic supported platinum (Pt), platinum-ruthenium (PtRu), platinum-ruthenium-vanadium (PtRuV) and platinum ruthenium-vanadium-iron (PtRuVFe) upon multi-walled carbon nanotube (MWCNT) as well as upon multiwalled carbon nanotube-titanium oxide (MWCNT/TiO2) supports. Platinum is very active but prone to poisoning by carbon monoxide (CO), which may be present in the fuel used in fuel cells. The focus on the use of methanol was because of its better reaction kinetics, and better performance in direct methanol fuel cells (DMFC) better than proton exchange membrane fuel cell (PEMFC). When Pt is alloyed with another platinum group metals (PGM) the alloying decreases the over-potential for reactions critical in the fuel cells. Proton exchange membrane fuel cell (PEMFC) performance may be improved at low metal loading, when supported pluri-metallic catalysts are applied since the trimetallic catalysts may promote high catalyst utilisation. In practice, DMFC require electrodes with a Pt loading to achieve acceptance fuel cell (FC) power performance. The aim of this study was therefore the reduction of the catalyst loading through further improvement of mass activity of Pt based catalysts by partial substitution of the noble metal/metals, and the use of a carbon support that will provide high surface area, good electrical conductivity and high stability. MWCNT supported pluri-metallic (PtRuVFe,) and bimetallic (PtRu) nanoparticles possessed characteristic of increased surface area, improved electron transfer rate, enhance electro-catalytic activity and promoted stability.

Carbon supported PtNi nanoparticles were prepared by hydrazine reduction of Pt and Ni precursor salts under different conditions, namely by conventional heating (PtNi-1), by prolonged reaction at room temperature (PtNi-2) and by microwave assisted reduction (PtNi-3). The nanocomposites were characterized by XRD, EDX, XPS and TEM and used as electrocatalysts in direct methanol fuel cell (DMFC) reactions. Investigations into the mechanism of PtNi nanoparticle formation revealed that platinum nanoparticle seeding was essential for the formation of the bimetallic nanoparticles. The average particle size of PtNi prepared by microwave irradiation was the lowest, in the range of 2.9 – 5.8 nm. The relative rates of electrooxidation of methanol at room temperature as measured by cyclic voltammetry showed an inverse relationship between catalytic activity and particle size in the following order PtNi-1 < PtNi-2 < PtNi-3. / Singapore-MIT Alliance (SMA)

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