Work

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

Year: 2018

Type: article

Abstract: The unique electrochemical and chemical features of sodium–oxygen (Na–O 2 ) batteries distinguish them from the lithium–oxygen (Li–O 2 ) batteries. NaO 2 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, RuO 2 nanoparticles dispersed on carbon nanotubes (CNTs) are used as the catalyst for Na–O 2 batteries to elucidate the effect of the catalyst on these complex electrochemical systems. The RuO 2 /CNT contributes to the formation of a poorly crystalline and coating-like NaO 2 structure during oxygen reduction reaction, which is drastically different from the conventional micron-sized cubic NaO 2 crystals deposited on the CNT. Our findings demonstrate a competition between NaO 2 and side products’ decompositions for RuO 2 /CNT during oxygen evolution reaction (OER). We believe that this is due to the lower stability of a coating-like NaO 2 because of its noncrystalline nature and high electrode/electrolyte contact area. Although RuO 2 /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 (NaO 2 → Na + + O 2 + e – ) because of the fast chemical degradation of the film NaO 2 to the side products. Therefore, tuning the morphology and crystallinity of NaO 2 by a catalyst is detrimental for the Na–O 2 cell performance and it should be taken into account for the future applications.

Source: The Journal of Physical Chemistry C

DOI: 10.1021/acs.jpcc.8b06024