Sodium oxygen batteries: one step further with catalysis by ruthenium nanoparticles
Sodium-oxygen batteries have received much attention recently due to their possible higher energy efficiency and lower cost than lithium-oxygen batteries. Na+ ions are less electrophilic than Li+ ions (softer Lewis bases). Thereby, oxygen reduction in the presence of Na+ ions may undergo a highly reversible one electron process to form sodium superoxide as a main product. However, sodium superoxide may have adverse effects on the stability and cycle life of sodium-oxygen batteries because of its high reactivity towards all kinds of relevant solvents. Therefore, sodium-oxygen batteries, in which the major oxygen reduction products are sodium peroxide moieties, may have an advantage in terms of better stability. This paper reports for the first time on sodium-oxygen batteries in which the cathodes comprise carbon nanotubes (CNTs) decorated with nanoparticles of ruthenium serving as a stationary catalyst. With these cathodes both oxygen reduction and evolution reactions are effectively catalyzed. The main oxygen reduction product on these CNT/Ru containing cathodes was deficient sodium peroxide, analyzed by XRD, XPS and SEM. Sodium-oxygen cells with Ru decorated CNT cathodes exhibited stable cycling performance over 100 cycles, while similar cells having CNT based cathodes showed much lower stability. It was clear that the limiting factor in the sodium-oxygen batteries containing CNT/Ru cathodes was the sodium anodes. Thereby it is believed that the present study is a step forward in the efforts to develop sodium-oxygen batteries.