Sodyum-oksijen pillerde katalizörlerin etkilerinin araştırılması

The unique electrochemical and chemical features of sodium oxygen (Na-O2) batteries distinguish them from the lithium-oxygen (Li-O2) batteries. NaO2, which is the main discharge product, is unstable in the cell environment and its dissolution in the electrolyte triggers side products formation and charging potential increment. In the first part of this thesis, RuO2 nanoparticles (NPs) dispersed on carbon nanotubes (CNTs) are used as a catalyst for Na-O2 batteries to elucidate the effect of catalyst on this complex electrochemical system. RuO2/CNT con- tributes to the formation of a poorly crystalline and coating like NaO2 structure during oxygen reduction reaction (ORR) which is drastically different from the conventional micron sized cubic NaO2 crystals deposited on CNT. Our findings demonstrate a competition among NaO2 and side products decompositions for RuO2/CNT during oxygen evolution reaction (OER). We believe that this is due to the lower stability of coating like NaO2 because of its non-crystalline nature and high electrode/electrolyte contact area. Although RuO2/CNT catalyzes the decomposition of side products at a lower potential (3.66 V) compared to CNT (4.03 V), it cannot actively contribute to the main electrochemical reaction of the cell during OER (NaO2 → Na+ + O2 + e – ) due to the fast chemical de- composition of film NaO2 to side products. Even though the long term effect of RuO2 catalyst during cycling and resting tests seems to be positive in terms of lower overpotential, no benefits of catalyst is observed for stability and efficiency of the cell for the first cycles. Therefore, tuning the morphology and crystallinity of NaO2 by catalyst is detrimental for Na-O2 cell performance and it should be taken into account for the future applications. In the second part of this thesis, a 3D RuO2/Mn2O3/carbon nanofiber (CNF) composite has been prepared as a bi-functional electrocatalyst towards oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in Na-O2 batteries. RuO2/Mn2O3/CNF exhibited higher specific capacity (9352 mAh.gcarbon-1) than CNF (1395 mAh.gcarbon-1), Mn2O3/CNF (3108 mAh.gcarbon-1) and RuO2/CNF (4859 mAh.gcarbon-1), which is believed to be due to its higher active surface area than its counterparts and its unique morphology. Taking the benefit of RuO2 and Mn2O3 synergistic effect, the decomposition of inevitable side products at the end of charge occurs at 3.838 V vs. Na/Na+ by using RuO2/Mn2O3/CNF, which is 388 mV more cathodic compared with CNF.