Instability of a noncrystalline NaO2 film in Na-O2 batteries: The controversial effect of the RuO2 catalyst

The unique electrochemical and chemical features of sodium-oxygen (Na-O2) batteries distinguish them from the lithium-oxygen (Li-O2) batteries. NaO2 as the main discharge product is unstable in the cell environment and chemically degrades, which triggers side products' formation and charging potential increment. In this study, RuO2 nanoparticles dispersed on carbon nanotubes (CNTs) are used as the catalyst for Na-O2 batteries to elucidate the effect of the catalyst on these complex electrochemical systems. The RuO2/CNT contributes to the formation of a poorly crystalline and coating-like NaO2 structure during oxygen reduction reaction, which is drastically different from the conventional micron-sized cubic NaO2 crystals deposited on the CNT. Our findings demonstrate a competition between NaO2 and side products' decompositions for RuO2/CNT during oxygen evolution reaction (OER). We believe that this is due to the lower stability of a coating-like NaO2 because of its noncrystalline 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-) because of the fast chemical degradation of the film NaO2 to the side products. Therefore, tuning the morphology and crystallinity of NaO2 by a catalyst is detrimental for the Na-O2 cell performance and it should be taken into account for the future applications.