In-situ imaging electrocatalysis in a Na-O2 battery with Au-coated MnO2 nanowires air cathode

Metal-air batteries have much higher theoretical energy density than metal ion batteries, however, their application is hampered by the sluggish oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). As such, electrocatalysis is employed to facilitate the ORR and OER. Despite enormous efforts, real time imaging of the electrocatalysis during the ORR and OER processes remain elusive. Furthermore, it is controversial whether or not electrocatalysis is necessary in the Na-O-2 battery. Here we show the first in-situ imaging of the operation of the electrocatalysis in a Na-O-2 battery in an advanced aberration corrected environmental transmission electron microscopy (ETEM). In the Au-coated MnO2 nanowire air cathode, the ORR is characterized by the formation of NaO2 nano bubbles nucleated from the Au catalysts, causing an 18 times volume increase on the surface of the MnO2 nanowires; the NaO2 quickly disproportionated to Na2O2 and O-2, causing collapse of the NaO2 nano bubbles. In contrast, no ORR took place in the bare MnO2 nanowire cathode; instead, the MnO2 nanowires only swelled 217% as a result of the Na+ intercalation. The results provide not only new insight into the Au-catalyzed oxygen chemistry in the Na-O-2 battery, but also an atom-level characterization technique to evaluate the electrocatalysis in the metal air batteries.