Studies of transport phenomena in hydrotalcite membranes, and their use in direct methanol fuel cells.

摘要

Currently, the humanity is encountering two major crises: energy deficiency and global warming. In order to resolve these crises, we should consider maximizing energy efficiency and minimizing its usage. Furthermore, we should develop alternative energy sources (e.g. wind, solar, biomass), instead of hydrocarbon products. Moreover, we need to commercialize well-known techniques such as fuel cells, which are environment-friendly and high efficiency systems for various applications, such as power generation and transportation. In addition, we need to continue research on CO2 capture and separation processes.;This study presents the synthesis and characterization of hydrotalcite (HT) membranes with several techniques. In addition, this study explores the possibility of using HT materials as inorganic fillers for conductive membranes in direct methanol fuel cells (DMFC). Due to their properties, hydrotalcites also known as layered double hydroixde compounds, are a potentially good candidate as CO2-selective membranes and inorganic filler of conductive membrane.;Chapter 1 presents a general Introdcution to the various topics discussed in this Thesis. Chapter 2 describes the use of electrophoretic deposition as a new method for the preparation of HT thin films. The films are deposited on macroporous alumina substrates and on alumina substrates, which were previously coated by conventional dip-coating techniques using slurries of HT colloidal particles. Their permeation properties are investigated by single and mixed-gas permeation tests. The films are shown to be permselective towards CO 2, consistent with the prior studies of these materials, which showed them to be effective CO2 adsorbents.;In Chapter 3 several methods are used for synthesis of effective CO 2-selective HT membranes. Single gases and mixtures of gases are tested and their permeation is studied. Unfortunately, the dip-coating method results in mesoporous membranes with Knudsen flow. But the vacuum-suction method shows that the He/CO2 separation factor for these membranes is significantly higher than the corresponding Knudsen values, despite the fact that these membranes are not CO2-permselective. In order to decrease voids and pinholes, a silicone layer is coated by vacuum suction on the HT membranes. The silicone coating appears to improve the separation characteristics of these membranes.;Chapter 4 describes preparation of a miniature-type micromembrane using silicon wafers and stainless steel (SS) foils as templates. Silicon-based micromembranes show the potential for application for microreactor systems, but their pressure resistance is not high enough to carry out the permeation test. HT micromembranes, prepared by coated HT colloid solution with 0.1∼0.2 mum diameter on SS substartes, are characterized by several analytical techniques and by single-gas permeation experiments. Most of the HT micro-membranes exhibit Knudsen transport behavior with He and N2-transport being favored when compared to CO2. Some of the HT micromembranes turned out to be CO2-selective, however.;Chapters 5 and 6 demonstrate how both hybrid and in-situ hydrotalcite-SPEEK (sulfonated polyetheretherketone) membranes are synthesized and investigated for the possibility of making a conductive membrane in direct methanol fuel cell. Our study's goal is to develop a new, cost-effective membrane with superior methanol barrier properties, and reasonable proton conductivity in order to replace commercial NafionRTM membranes. We prepare HT-SPEEK membranes by incorporating HT particles into SPEEK and by in-situ sulfonation polymerization from PEEK and HT. The hybrid HT-SPEEK membranes exhibit good resistance for methanol permeability and reasonable proton conductivity. Their properties depend strongly on the sulfonation degree of the polymer matrix, and on the fraction of the HT present in the hybrid membranes. Therefore, HT-SPEEK membranes are potentially viable candidates for replacing NafionRTM membranes. Moreover, the in-situ membrane's properties depend on the reaction time, and the fraction of hydrotalcite initially added to the PEEK materials prior to sulfonation. The MeOH permeability for the in-situ membranes is 3∼5 times smaller than the one for the commercial NafionRTM115 film.