Investigating the effect of catalysts in sodium-oxygen batteries

Abstract

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 rst part of this thesis, RuO2 nanoparticles (NPs) dispersed on carbon nanotubes (CNTs) are used as a catalyst for Na-O2 batteries to elucidate the e ect of catalyst on this complex electrochemical system. RuO2/CNT contributes to the formation of a poorly crystalline and coating like NaO2 structure during oxygen reduction reaction (ORR) which is drastically di erent from the conventional micron sized cubic NaO2 crystals deposited on CNT. Our ndings 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 decomposition of lm NaO2 to side products. Even though the long term e ect of RuO2 catalyst during cycling and resting tests seems to be positive in terms of lower overpotential, no bene ts of catalyst is observed for stability and e ciency of the cell for the rst 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 nano ber (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 speci c 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 bene t of RuO2 and Mn2O3 synergistic e ect, 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.