The proton exchange membrane is one key component of direct methanol fuel cells, which has double functions of conducting protons, separating fuels and oxidant. At present, the performance and price of sulfonic acid proton exchange membrane used in direct methanol fuel cells are deeply concerned. In order to reduce membrane 's cost and improve performance of Nafion membrane, three different kinds of membranes have been studied in this thesis. These membranes are SPEEK membranes, SPEEK/ZP composite membranes and Nafion/ZP composite membranes.

<p>For a direct methanol fuel cell (DMFC), the proton exchange membrane must conduct protons and be a good methanol barrier. In addition to the high methanol permeability achieved by these membranes, they are very expensive and contribute greatly to theoverall cost of fuel cell set up. The high cost of the DMFC components is one of the main issues preventing its commercialization. The main objective of this study was thus to produce highly proton conductive membranes that are cheap to manufacture and have low methanol permeability.</p>

Perhaps mankind's most urgent challenge at present is anthropogenic climate change, with the associated sea-level rise and desertification set to produce major losses of arable land and living space, as well as loss of life. The key to preventing the worst effects of AGW lies in limiting humanity's emissions of the greenhouse gas carbon dioxide, of which the vast majority comes from the burning of fossil fuels such as coal, oil and natural gas. However, fossil fuels are embedded in all of the world's economies, responsible for almost all of the provision of electrical power and transport, making the sizable reductions required in the timescale necessary somewhat impractical. One solution lies in Carbon Capture and Storage (CCS), which involves, in one incarnation, the combustion of fossil fuels in pure oxygen, simplifying the processing and storage of the carbon dioxide produced. There is potential for very high process efficiencies if oxygen is provided by Oxygen Transport Membranes (OTM). This thesis is concerned with the development of membranes and test procedures for mixed-phase OTM, which typically consist of a dense, gastight layer of perovskite and fluorite phases. An inactive support layer may also be present. The surface area, and therefore surface exchange of either side is improved by the addition of exchange layers to either side. Oxide ion migration is accomplished by applying a pO2 differential to the membrane at high temperature. Causes and mechanisms for degradation are not fully understood, and there is potential to improve oxygen flux. One way to achieve this is by the use of very thin, supported membranes, and this thesis demonstrates that such membranes can be fabricated with well-understood manufacturing processes. Another method of improving oxygen flux is by the use of catalysts on the exchange layers of the membrane. The most popular method of introducing 6 catalysts to an exchange layer or electrode involves impregnation of a metal salt into a ceramic backbone, followed by reduction to yield a catalytically active phase. However, this process is wasteful of catalyst, labour-intensive and control of the distribution of catalyst is difficult or impossible. An alternative exists, where metals doped into a perovskite migrate to the surface and form nanoparticles on exposure to a sufficiently high temperature and reducing atmosphere, and this thesis demonstrates the benefits of using such an approach. Improvements in oxygen flux of up to a factor of 7 over an undoped perovskite exchange layer have been demonstrated. The conductivity and crystal structures of (La0.8Sr0.2)0.95Cr0.5Fe0.5O3-δ and (Sc2O3)0.19(CrO2)0.01(ZrO2)0.789O1.94 under oxidising and reducing atmospheres at high temperatures have been evaluated using neutron powder diffraction and a novel in-situ rig, demonstrating that the OTM composition is a p-type conductor, and quantifying the effect of oxygen stoichiometry on conductivity and unit cell parameters.

This study focuses on the optimisation of submerged hollow fibre membrane performance by analysing the role of air sparging on the reduction of membrane fouling. In submerged hollow fibre membranes, rising bubbles have been shown to induce shear, liquid movement and fibre displacement. The interaction between fibre movement induced by bubbling and the microfiltration performance was assessed for various parameters (fibre tightness, fibre length, fibre diameter, air flowrate, nozzle size, and feed concentration). A model feed of yeast suspension and a series of isolated fibres were used. The fibre movement was assessed by monitoring the displacement using video recording. Bubble population parameters were also measured. The results suggest that bubbleinduced fibre movement plays an important role in controlling membrane fouling. Investigations of the critical flux at various operating conditions also supported these conclusions. Since energy consumption for aeration is a major contributor to the cost in submerged membranes, the potential to minimise the aeration cost has been tested by implementing intermittent aeration and different nozzle sizes. It was found that an optimum condition associated with a low fouling rate could be reached by combining various aeration intermittencies and nozzle sizes. An attempt to suppress fouling without aeration was made by incorporating vibrations into a submerged hollow fibre membrane system. The effects of vibration frequency, type of yeast (washed and unwashed) on the filtration performance were observed. The impact of coagulant addition on filtration enhancement was also analysed. The performance of microfiltration was evaluated based on its critical flux value. The findings in this preliminary study indicated potential fouling control by applying vibrations to submerged membranes. A semi-empirical model was developed to predict the filtration behaviour by taking into account the bubble-induced shear and fibre movement. The predicted critical flux values suggested that membrane fouling appears to be more prominent at low air flowrate, with tight fibres, and higher feed concentrations. The model fits the experimental data with discrepancies from approximately 0.3% to 20%. The predicted filtration profiles at different operating modes demonstrate the importance of bubble-induced shear and fibre movement in the improvement of filtration performance.

The aim of this project was to prepare a series of silica materials based on sol-gel processing of alkoxysilanes using glucose and glycerol as templates for potential applications in membrane design for pervaporation. The materials were characterized using structural and dynamic techniques to gain information about the effect of the templates on the formation of micro- and mesoporous silicates. The interaction between templates and silica matrices were investigated using FTIR, Raman Spectroscopy, Solid State NMR Spectroscopy, Physisorption and SEM. Close contact between templates and silica networks was observed by NMR cross polarization studies. The chemistry was then extended to prepare hybrid organic-inorganic silica materials by introducing organic ligands, with glycerol as a template to control the porosity of the hybrid materials. By varying the ligand as well as the template, the physical properties of the gel can be controlled. Composites of hydroxypropylcellulose, HPC, and silica were also prepared and characterized. There was no phase separation during sol-gel processing suggesting HPC was dispersed homogenously in the silica matrices. This was also confirmed by solid state NMR. Temperature dependence showed some indications of conformational change in the HPC within the silicate, above 308K. The transport properties of the hybrid materials were observed by monitoring the diffusion behaviour of water and several selected solvents using Pulsed Field Gradient NMR. The self-diffusion of water and the organic solvents in the hybrid silica materials were two to three orders of magnitude smaller than in the liquid bulk suggesting restricted diffusion at the pore surface. The effect of surface polarity also contributed to water and solvents diffusivities. The temperature dependence of diffusion was useful to derive the activation energy whereas the dependence on NMR observation time provided information on both tortuosity and pore connectivity of the hybrid silica materials. The hybrid silica membranes were prepared by spin coating of polymeric silica sol on top of a macroporous alumina support after being occluded by colloidal silica. It was then used for pervaporation of water ethanol mixtures. The results implied that separation factor increased as the temperature increased. However permeate fluxes were less affected.

Thesis (M. Phil.)--Hong Kong University of Science and Technology, 2003. / Includes bibliographical references. Also available in electronic version. Access restricted to campus users.

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The goal of this research was to determine relationships between post-coarsening processing conditions and the microporous morphology of membranes. Specifically, the processes of matrix solidification in liquid -- liquid thermally induced phase separation (L−L TIPS), the drying of the microporous structure, and the uni-axial elongation of a simple microporous structure were examined. Additionally, the effect uni-axial elongation has on pore shape was included in a sieve filtration model to look at the impact on performance. A deterministic approach was taken to predict membrane morphologies resulting from the matrix solidification step that occurs in L–L TIPS. Many studies have examined the growth rate of droplets in the coarsening stage of membrane formation, but few have attempted to extend this information into the subsequent processing steps of matrix solidification, diluent extraction/exchange, and drying. The modeling of matrix solidification utilized Monte-Carlo routines to provide quantitative information on cell size and cell size distribution for a representative polymer -- diluent system. The predicted structures were in agreement with experimentally formed membranes. The information gained from matrix solidification modeling was used to make finite element (FE) simulations in ABAQUS CAE to model the drying of the microporous morphology, with capillary forces being the dominant force driving shrinkage and collapse of the structure. These FE simulations predicted no permanent deformation arising from only capillary forces, which was confirmed through experimental evidence showing no correlation to surface tension. For polar polymers an additional heuristic was proposed: use extractants that are more alkane-like, regardless of surface tension, to reduce the collapse of the structure. FE simulations were used to model the uni-axial elongation of track-etch membranes in an effort to change performance characteristics. The FE simulations accurately predicted pore shape changes comparable to experimental values. The pore shape change information was used to modify standard sieve filtration models. The modified sieve filtration models show that a relatively modest strain of 35% can double the initial flux of track-etch membranes. / text

A fuel cell is an energy device that converts chemical energy to electrical energy. Low temperature fuel cells, namely the hydrogen fuel cell and the direct methanol fuel cell are preferred amongst other fuel cell types for stationary and vehicular applications, due to their small size and their low operating temperature. The direct methanol fuel cell has several advantages over the hydrogen fuel cell including ease of transport and storage since methanol is a liquid. Since methanol is used directly in the cell there is no need for a reforming process, which results in a less complicated system. However, direct methanol fuel cell are in their infancy and many problems need to be overcome before reaching commercialization. The direct methanol fuel cell has several disadvantages, namely, the sluggish methanol oxidation reaction, the high cost of state-of-the-art proton exchange membranes, the high methanol permeability from anode to cathode and the dependence on the conductivity on membrane water content, which limits their use to temperatures below the boiling point of water, while the need is to work at high temperatures. Attempts to overcome the disadvantages of the state-of-the-art membrane were made in this study, including the development on novel proton exchange membranes and also the modification of existing state-of-the-art membranes.

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