Investigation of electrolyte-electrode components for high-performance direct methanol fuel cells

Energy is of critical importance for economic development, social welfare and environmental sustainability in modern industrial societies. However, the fact that most of the energy production is based on fossil fuels, which are argued to be depleted in the future, causes serious problems such as climate change, air pollution and energy security. Sources such as solar, wind, hydroelectricity and biomass attract attention as alternative energy sources because they provide clean, unlimited and environmentally friendly energy. However, these systems have some disadvantages such as variable energy production, production not meeting the demand, high initial investment costs, environmental impacts, and the need for certain geographical conditions. Fuel cells have a significant potential in terms of clean energy production and reducing dependence on fossil fuels. Unlike other energy production systems,they convert chemical energy directly into electrical energy and produce clean by-products such as water and heat. In portable applications, the polymer electrolyte membrane fuel cell (PEMFC) and the direct methanol fuel cell (DMFC), which is the methanol-operated type of this fuel cell, stand out. Increasing cell performance by developing DMFC components is a current and popular topic, and membrane electrode assembly (MEA) components have a significant effect on DMFC power output. For this reason, in addition to the synthesis of catalysts for the anode electrode, membranes with different structures were prepared and their properties were examined within the scope of the thesis. In the membrane studies that constitute the first part of the thesis, sulfonation reaction was carried out in order to add sulfonic acid groups to polyether ether ketone (PEEK) structure,and sPEEK-based blend membranes were obtained by blending the obtained sulfonated polyether ether ketone (sPEEK) with thermoplastic polyurethane (TPU) and polyvinylidene fluoride (PVDF) in different ratios. Additionally, Nafion 117 and Nafion 212 membranes were modified with cationic and anionic polyelectrolytes using the layer by layer (LbL) coating method. In addition to the thermal, structural and morphological properties of the obtained membranes, proton conductivity (σ) and methanol permeability (PM) values were examined. QCM-D analyzes were carried out to examine the interactions of the polymer electrolyte membranes (PEMs) prepared in the thesis with methanol used in DMFC applications. When the proton conductivity (4.7 mS cm-1) and methanol permeability (10-11 cm2 s-1) of the product coded sPEEK40-TPU60 were evaluated, it was determined as the most suitable membrane for DMFC application. It has been evaluated that N212-based composite membranes prepared with layer-by-layer coating approach can be an important membrane alternative due to their better proton conductivity and barrier properties against methanol compared to N117-based analog membranes. Similarly, it was determined that the membrane selectivity value, which is an important indicator for DMFC applications, was 4.7x108 S s cm-3 for the sPEEK40-TPU60 blended membrane and that N212-based composite membranes had a high selectivity value due to their very low methanol permeability. In the second part of the thesis studies, metal oxide-carbon supported PtRu catalysts were prepared as DMFC anode catalysts. In order to determine the effects of different metal oxide compounds used in the support material on methanol oxidation, cyclic voltammetry and chronoamperometry analyzes were applied to the catalysts and their electrochemical properties were compared and the best catalyst was determined to be PtRu/WO3-C. In the cyclic voltammetry analyzes performed with this catalyst, it was determined that the current density values were 67.4-112.6 mA cm-2, depending on the scanning rate used, and that it showed better results than the PtRu/C catalyst (16-38 mA cm-2) under the same experimental conditions. In addition, according to the results of the chronoamperometry analysis performed within the scope of the thesis, it was determined that the tolerance of the PtRu/WO3-C catalyst towards the species formed during methanol oxidation was superior to other catalysts.