Concerns on the Peroxo-Based Reaction Process in Aqueous Zn–O2 Battery

The aqueous Zn–O2 battery, operating based on the zinc peroxide and oxygen (O2/ZnO2) redox cycle in the zinc trifluoromethanesulfonate (Zn(OTf)2) aqueous electrolyte solution, can alleviate a series of issues relating to working in an alkaline environment, such as the sluggish kinetics of the O2 reduction/evolution reaction (ORR/OER), corrosion of the electrolyte toward Zn, and Zn dendrite growth. However, the specific reaction pathways of aqueous Zn–O2 batteries based on the redox cycle of O2/ZnO2 are not fully understood at the experimental level. Herein, we employ the in situ surface-enhanced Raman spectroscopy (SERS) method to probe the O-related intermediate species during the ORR process of the aqueous Zn–O2 battery with Zn(OTf)2 electrolyte. The in situ SERS measurements demonstrate that the gold (Au) electrode surface is hydrophobic in the Zn(OTf)2 electrolyte but hydrophilic in the ZnSO4 electrolyte. Furthermore, the SERS observations show the inhomogeneity of the O-related intermediate species on the Au electrode surface in the Zn(OTf)2 electrolyte during ORR. On the other hand, it was found that the cell voltage of the Zn–O2 battery in the Zn(OTf)2 electrolyte continuously drops. These features should be ascribed to the local pH changes in the cells assembled with the Zn(OTf)2 electrolyte since there is no pH buffer ability. Finally, the irreversible potential gap during the ORR/OER process of an aqueous Zn–O2 battery with Zn(OTf)2 electrolyte is tuned via electrolyte and solid catalyst modifications. We call for employing more in situ/operando spectroscopy and microscopy characterization to unravel the reaction mechanism of the aqueous Zn–O2 battery in a nearly neutral environment.