Browsing by Subject "Methanol as fuel"
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Item Activity of methanol electro-oxidation at PtRu materials at temperatures in the range of 23°C to 70°C(Texas Tech University, 2004-05) Xu, ShanhongThe electrochemical oxidation of 0.5 M methanol in 0.1 M HCIO4 on catalyst materials comprised of platinum and ruthenium (PtRu) was investigated. Cyclic voltammetry and constant potential amperometry were used to characterize the catalyst materials and study the methanol reaction kinetics. Measurements were performed at temperature in the range of 23°C to 70°C. The following catalyst materials were employed: PtRu black containing 50 at. % Ru supplied by Johnson Matthey of Ward Hill. MA (JM PtRu black); sonochemically prepared nanoparticles of PtRu containing either 50 at. % Ru (SC PtRu(50)) or 25 at. % Ru (SC PtRu(25)); and Pt black (supplied by Johnson Matthey) modified by spontaneous deposition of Ru via either two (JM Pt-Ru(2)) or four deposition cycles (JM Pt-Ru(4)). The rate of methanol oxidation was assessed through constant potential amperometry measurements. Current was recorded 20 min after stepping to the reaction potential. Mechanistic information was derived from Tafel plots (plot of the logarithm of the current versus the reaction potential).Item An electrochemical and spectroscopic investigation into carbon monoxide surface poisoning(Texas Tech University, 2000-05) Kardash Richardson, Dawn Jo-ElleAn electrochemical cell was constructed that allows surface infrared spectroscopy measurements to be made in situ at temperatures relevant to the operation of direct methanol fuel cells (ambient to 80°C). The cell was used to investigate temperature effects on the electrochemistry of water, CO and methanol at bulk Pt and Pt-Ru electrodes in 0.1 M HCIO4. Initially, the surface chemistry of CO on a polycrystalline Pt electrode was studied. An Adiayer of CO at saturation coverage was stable over a period of five hours in the range of 25 °C-50 °C. Above 60 °C, the adiayer became unstable. In the absence of CO in solution, only low CO coverages could be sustained between 60 °C and the high temperature limit of the experiments (75 °C). However, with CO or a source of CO such as methanol in solution, high CO coverages were sustained up to 75°C. In measurements of CO oxidation, the onset potential for the conversion of CO to CO2 decreased by 50 mV when the temperature was increased from 25 °C to 75 °C. In contrast, adsorbed CO formed through the dissociative chemisorption of methanol (1.5 x 10'^-1.0 M) was more oxidation resistant between 50 °C-75 °C. The in situ spectroscopic measurements provide molecular level evidence that the thermal activation of water dissociation can decrease the steady-state coverage of surface poisons and thereby increase the rate of methanol oxidation on Pt electrodes. In final studies, the surface chemistry of 0.1 M methanol on two bulk Pt-Ru alloy electrodes (10 atomic % Ru and 90 atomic % Ru) was investigated at 25 °C - 80 °C. High CO coverages were sustained on both alloys at all temperatures. However, CO2 evolved rapidly from CO covered surfaces above 0.4 V-0.5 V, suggesting that CO formed during methanol oxidation is more reactive and transient on the alloys than on Pt. The experiments reported in the dissertation provide a foundation for the in situ study of fuel cell reactions on new catalyst preparations with FTIR spectroscopy.Item Development of new membranes based on aromatic polymers and heterocycles for fuel cells(2007-05) Fu, Yongzhu, 1977-; Manthiram, ArumugamProton exchange membrane fuel cells (PEMFC) and direct methanol fuel cells (DMFC) have drawn much attention as alternative power sources for transportation, stationary, and portable applications. Nafion membrane is currently used in PEMFC and DMFC as electrolyte, but is confronted with a few difficulties: (i) high cost, (ii) limited operating temperature (< 100 o C), and (iii) high methanol permeability. With an aim to overcome some of the problems encountered with the Nafion membrane, this dissertation focuses on the design and development of new polymeric materials systems for use in PEMFC and/or DMFC. Sulfonated polysulfone (SPSf) membranes with various degrees of sulfonation were prepared and investigated in DMFC. With a degree of sulfonation of 50 - 70 %, the SPSf membranes exhibit low methanol permeability and electrochemical performance comparable to that of Nafion 115, making it an attractive low-cost alternative to Nafion. However, lower performance at higher current densities due to their low proton conductivities compared to Nafion is a disadvantage. It is found that the low methanol crossover is due to narrower hydrophilic channels, resulting in water/methanol confinement as in sulfonated poly(ether ether ketone) (SPEEK) membranes. Replacement of water by imidazole in Nafion helps to keep high proton conductivity at higher temperatures (> 100 o C) due to Grotthuss-type mechanism, but imidazole poisons the Pt catalyst. Interestingly, doping the Nafion-Imidazole composite membrane with H3PO4 partly suppresses the imidazole poisoning of the Pt catalyst. Employment of Pd-Co-Mo catalyst instead of Pt improves the fuel cell performance at 100 o C further due to a higher tolerance of the non-platinum Pd-Co-Mo catalyst to imidazole. Encouraged by this, benzimidazole group was then selected to promote proton conduction in the environment of sulfonic acid groups instead of imidazole (pKa = 7.0) due to its lower pKa value (5.5). Accordingly, 1,3-1H-dibenzimidazole-benzene containing two benzimidazole groups was synthesized and blended with SPSf. The blend exhibits higher proton conductivity under anhydrous conditions than plain SPSf and offers improved fuel cell performance and lower methanol crossover in DMFC. Polysulfones containing pendant N-heterocycles like benzimidazole, 2-amino-benzimidazole, or 3-amino-1,2,4-1H-triazole units were designed and synthesized. Blend membranes containing these polymers and SPEEK exhibit higher proton conductivities under anhydrous conditions as well as higher fuel cell performance due to acid-base interactions involving Grotthuss-type mechanism. They also lower methanol crossover further due to the insertion of the pendant N-heterocycles into the hydrophilic channels of SPEEK, improving the long-term stability in DMFC and reducing the Pt loading at the cathode side.Item Formaldehyde yields from methanol electrochemical oxidation on platinum and supported catalysts(Texas Tech University, 1999-12) Childers, Christina L.The formation of formaldehyde during methanol electrochemical oxidation is being measured with a fluorescence assay in order to assess the importance of formaldehyde as a reaction intermediate and source of efficiency loss in direct methanol fuel cells. Initial studies have focused on the oxidation of methanol on polycrystalline platinum. The formaldehyde yields approached 30% of the total electrolysis charge at 0.2-0.3 V (vs. a KCI saturated Ag/AgCI reference electrode) for methanol concentrations between 15 mM and 0.3 M in 0.1 M perchloric acid. The formaldehyde yields were lower at more positive potentials, as other oxidation pathways became dominant. However, the rate of formaldehyde production increased up to 0.5 V. These initial studies have demonstrated that formaldehyde, which is often not detectable with modern in situ spectroelectrochemical analysis techniques, can be produced in significant amounts during methanol electrochemical oxidation. More recent work has focused on the formation of formaldehyde during methanol electrochemical oxidation on supported platinum and platinumruthenium catalysts. Solid, polycrystalline platinum-ruthenium alloys have been considered. Other catalysts studied have been suspended in Nafion and supported on glassy carbon. Methanol oxidation on the catalysts has resulted in low formaldehyde yields, below 2% at all potentials studied. The low formaldehyde yields, which result from more complete methanol oxidation, are believed to arise from the ability of partial oxidation products to be transported to an array of active catalyst sites dispersed within the three dimensional Nafion film network. Efforts to eliminate these volume effects through techniques such as electrochemical depositions of catalyst crystallites by reduction of transition metal salts onto solid, glassy carbon electrodes; direct metal nanoparticle deposition onto solid, glassy carbon electrodes using a hydrogen tube furnace; and "sticky" carbon methods for metal/wax/carbon type electrodes have been under investigation.Item On-board hydrogen production for fuel cell vehicles via a membrane separator and an externally-fired methanol reformer(Texas Tech University, 2004-05) Mathakari, Sushil PrakashMany options exist for on-board hydrogen production from liquid fuels for fuel cell powered vehicles. This paper reports on design and construction of an on-board system for hydrogen production from methanol. Methanol reforming is accomplished using an externally fired catalytic reactor. Carbon monoxide, separated from the reactor effluent by a membrane separator, is consumed with the reactor fuel. The reactor is operated at 1950 kPa to supply gas to the membrane separator at its maximum design pressure, and at a temperature of 500°C, to minimize the amount of methanol remaining the reactor product. The design methanol feed rate of 0.32 1/min used for the prototype should be sufficient to supply a 10 kW fuel cell package, but the design can easily be expanded to larger sizes. The membrane separator is an off-the-shelf, polymer-based model, and it is not expected to reduce the carbon monoxide to below 10 ppm, required by proton exchange membrane fuel cells, being considered for vehicular power. For this reason it is necessary to include a selective oxidation reactor to remove the carbon monoxide as a contaminant. An adiabatic energy balance indicates that 30% excess energy is available from combustion of the separated carbon monoxide when used as fuel to the reformer. The thesis includes the design of methanol reformer system, calculation of heat transfer coefficients and heat transfer areas required for the heat exchange of the exhaust gases and the reaction mixture. It also includes the simulation of the entire process using ChemCAD as a chemical process simulator.