To investigate the role of O2 at the anodic process, a Zn-Zn symmetric cell is devised (split cell setup is given in SI Appendix, Fig. ). Zinc metal discs (1 cm diameter) are used as both, the working and counter electrodes, and 6 M KOH as the electrolyte. Cyclic voltammogram (CV) are acquired in N2 and O2 supplied environments (fixed flow rate, Fig. A and 1 B) (comparison to Ar in SI Appendix, Fig. and). For the CV, the potential is swept from -0.4 V to 0.4 V at a scan rate of 1 mV/s. During the positive sweep, the current first increased rapidly, reaching a maximum indicating the Zn to ZnO conversion. Once the Zn surface gets passivated completely with ZnO, the current immediately drops. During the reverse scan, the ZnO layer gets cracks which causes sudden jump in the current. The observed reduction current is due to ZnO to Zn conversion. Though the oxidative and reductive current values are comparable in the initial (say in 5 th , Fig. ) cycles, both gradually decrease with the cycle number in O2 environment when compared to that in N2 (O2 free environment). Similar trend, like in N2, is also observed with Ar, SI Appendix, Fig. and. This symmetric cell study indicates that the Zn surface is becoming electrochemically inactive (passivating) at a much higher rate in the O2 environment compared to the N2 and Ar environments.