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

Author:
Fathi Tovini, Mohammad, Hong, Misun, Park, Jiwon, Demirtaş, Merve, Toffoli, Daniele, Ustunel, Hande, Byon, Hye Ryung, Yılmaz, Eda
Source:
Journal of physical chemistry 2018 v.122 no.34 pp. 19678-19686
ISSN:
1932-7455
Subject:
batteries, carbon nanotubes, catalysts, catalytic activity, cellular microenvironment, chemical degradation, crystal structure, crystals, electrochemistry, electrodes, electrolytes, nanoparticles, oxygen, oxygen production, sodium
Abstract:
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.
Agid:
6108062