Thesis Type: Doctorate
Institution Of The Thesis: Gazi Üniversitesi, Fen Bilimleri Enstitüsü, Turkey
Approval Date: 2013
Student: ALPAY ŞAHİN
Consultant: İRFAN ARAbstract:
Fuel cells can be classified according to their electrolyte material as alkaline fuel cells, proton exchange membrane fuel cells (PEMFC), direct methanol fuel cells, phosphoric acid fuel cells, solid oxide fuel cells and molten carbonate fuel cells. PEMFC is the most attractive type of fuel cell due to its silent running, not having a residue other than pure water, low cost and high efficiency. The most important part of the PEMFC is its membrane. Currently, Nafion is the most widely used membrane in commercial PEMFC's. Although chemical, mechanical, proton conductivity and similar properties of Nafion are good, it is expensive and its proton conductivity decreases at temperatures higher than 90 °C due to the evaporation of its water content. Therefore, seeking alternative membranes is inevitable. In this doctoral study, firstly the main structure of the membrane, method of synthesis and additive agents are specified. After that, synthesis and characterization experiments and finally fuel cell performance tests were carried out. Although, initially it is proposed to synthesize membrane by using a combination of three different polymers, namely polyvinyl alcohol, chitosan and Teflon, depending on the results of the subsequent studies it was decided to use polyvinyl alcohol-formaldehyde-hypophosphorous acid combination. Polyvinyl alcohol based membranes containing different additives (TEOS, LUDOX and TiO2) in different ratios (5%, 10%, 15%, 20%, 25%, 30%, 40%) were synthesized by using conventional sol-gel method that used frequently in literature, and modified sol-gel method which is developed in this study. After synthesis of membranes, characterization experiments such as FT-IR analyses, water-holding capacity, swelling, ion exchange capacity (IEC), TGA-DSC analyses, fenton test, hydrophilicity test and electrochemical impedance spectroscopy (EIS) were performed. Afterwards, the fuel cell performance tests of these synthesized membranes were done and the performance analyses were studied. As a result of characterization experiments and performance tests, it was obviously seen that the properties of membranes synthesized by using modified sol-gel method developed in this study, was better than the membranes synthesized by using conventional sol-gel method. Results obtained by using the membranes containing nano-sized TiO2 as additive were better than the results obtained by using both nano sized SiO2 (LUDOX) and macro-sized SiO2 (TEOS) containing membranes. The best results were obtained with membranes having 15% titanium dioxide. This membrane has a proton conductivity of 0.03 S/cm, ion exchange capacity value of 1.04 meq/g and 45% water holding capacity and with many other properties, it come into prominence making itself promising for the usage in fuel cells.