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Instability of a Noncrystalline NaO2 Film in Na–O2 Batteries: The Controversial Effect of the RuO2 Catalyst C

Fathi Tovini, Mohammad, Hong, Misun, Park, Jiwon, Demirtaş, Merve, Toffoli, Daniele, Ustunel, Hande, Byon, Hye Ryung, Yılmaz, Eda
Journal of physical chemistry 2018 v.122 no.34 pp. 19678-19686
batteries, carbon nanotubes, catalysts, catalytic activity, cellular microenvironment, chemical degradation, crystal structure, crystals, electrochemistry, electrodes, electrolytes, nanoparticles, oxygen, oxygen production, sodium
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.