Tez özetleri Astronomi ve Uzay Bilimleri Anabilim Dalı

Development of Polymer Electrolyte Membranes by Layer by Layer (LbL) Technique for Fuel Cell Applications

Recently, the fuel cells which directly convert chemical energy to electric energy have attracted a great attention due to a huge and increasing demand for clean and sustainable energy and there has been important development about this subject.

Nowadays, polymer electrolyte membrane fuel cells (PEMFC) and direct methanol fuel cells (DMFC) have been especially preferred in the applications required high power density such as vehicles, cell phones and notebooks. Perfluorosulfonic acid polymers such as Nafion are the most common membrane electrolytes used in these types of fuel cells owing to their high proton conductivity and good chemical and thermal stability. However, high methanol crossover through the Nafion membrane from the anode to the cathode causes a significant reduction in fuel cell performance in DMFC applications. That’s why, there has been extensive research activity in the modification of Nafion membranes to reduce the methanol crossover without reducing proton conductivity value.
In the thesis studies, it is aimed to prepare an effective multilayer methanol barrier thin film on Nafion membrane by Layer-by-Layer technique and to improve membrane selectivity value. For this purpose, the influences of deposition conditions such as concentration of polyelectrolyte solutions, pH value, dipping time, type of polyelectrolyte, presence of salt in polyelectrolyte solutions and type of membrane supports on proton conductivity (σ) and methanol permeability of the self-assembled LbL composite membranes were investigated in detail. In addition, the effects of the charge content and type of ionic form of polyelectrolyte layers on the membrane selectivity were also studied. The formation of the self-assembled multilayers of films on Nafion was followed by UV–vis spectroscopy and it was found that the multilayers growth linearly on the both sides Nafion membrane depending on the number of adsorbed multilayers. To characterize cell performance of the LbL self-assembled composite membranes and optimize the LbL deposition conditions, the proton conductivity measurements were carried out by using AC impedance analyzer at room temperature in water over a wide range of frequency. Methanol cross-over measurements were performed by using a home-made diffusion cell in 30 hours period. The thermal behavior of the composite membranes was investigated by TG-DTA analysis and compared with that of pristine Nafion. The results showed that the deposition conditions and surface properties significantly affect the proton conductivity and methanol blocking properties of the composite membranes and these properties can be improved by choosing the suitable deposition conditions. Upon the optimization study of polyallylamine hydrochloride/polystyrene sulfonic acid sodium salt (PAH/PSS) model system, we determined that the most important parameters, which affect the proton conductivity, methanol permeability and thermal behavior of the composite membrane, are the layer thickness, charge density and type. The multilayered membranes prepared from polyelectrolytes containing 0.1M NaCl with H⁺ form have exhibited 2-3 times higher σ values than that of pristine Nafion, while the methanol barrier property was improved in the range of 25-29%. Furthermore, it was found that improvement in methanol barrier property was 43% and nearly three times higher than that of the membranes obtained from highly charged polyvinyl sulfate potassium salt (PVS) as anionic polyelectrolyte. The membrane selectivity values (proton conductivity/methanol permeability ratio), which is important parameter for fuel cell performance, of the composite membranes prepared by LbL deposition technique was found to 3-4 times higher than that of pristine Nafion^®117.